Newer Investigations on Oxidation with Lead Tetraacetate1

Lead Tetraacetate

R. CRIEGEE

institute fur Organische Chemie der Technischen Hochschule Karlsruhe

The lead tetraacetate discovered by Jacquelin (2) in 1851 can be thought of as being a salt of quadrivalent lead, or better yet, as being a mixed anhydride of orthoplumbic and acetic acids, because it is soluble in many organic solvents. As is well known, hydrolysis to brown P b 02

takes place instantaneously.

Dimroth, who introduced lead tetraacetate as a reagent into the organic laboratory in 1920 (3), looked upon the compound chiefly as being only a soluble lead dioxide and used it as such—in homogeneous solution, however,—for dehydrogenations. N o t until three years later did he show in a fundamental paper with Schweizer (4) that with its help substitution of Η by O H (protected by acetyl) as well as the addi­

tion of two O H groups (also protected by acetyl) to a double bond are also possible. These three modes of action correspond to those of the halogens and Dimroth looked upon lead tetraacetate as a mild halogen;

the two acetoxyls released m a y indeed be considered pseudohalogens according to Birckenbach. Yet, the acetoxyl groups are not "ατο/xo?,"

indivisible, in contrast to the halogen atoms; on this fact is based one of the differences in behavior.

T h e statement that lead tetraacetate yields two acetoxy groups in oxidations and thus is transformed into lead acetate is only a general description of the oxidation process, for this process can proceed in various ways. In the first place, two acetoxy radicals can be released;

in the second place, an acetoxy anion and an acetoxy cation (in which case the latter is then the oxidative species) can be released; finally, two acetate anions can be given off by the addition of two electrons

(which bring about the o x i d a t i o n ) :

1. P b ( 0 A c )4 - Pb(OAc)2 + 2 AGO' 2. Pb(OAc)4 = Pb(OAc)2 + AcO® + A c Oe

3. Pb(OAc)4 + 2Θ = Pb(OAc)2 + 2 A c O0

All of these reactions can take place in one or two steps.

Certainly not all oxidations which will be discussed proceed accord­

ing to one mechanism; the nature of the substrate and reaction condi- 367

368 R. CRIEGEE

tions are much too different. Until now a deeper insight was gained in only a few cases; in many reactions conjectures were made or conclusions were drawn from analogy to similar systems.

In the following the reactions of lead tetraacetate with three different groups will be discussed: (1) with the hydroxyl group; (2) with certain C H groups; and (3) with the carbon-carbon double bond. This classi­

fication coincides partly, but not fully, with dehydrogenation, substitu­

tion, and addition.

R e a c t i o n s w i t h H y d r o x y l C o m p o u n d s

Alcohols, phenols, carboxylic acids, and hydroperoxides can be at­

tacked by lead tetraacetate. In many cases the reaction results in a dehydrogenation. This occurs most easily when a suitable second h y ­ droxyl group is still available and the molecule possesses the possibility of stabilization after the removal of the two hydrogen atoms bound to the oxygen.

This is particularly the case in compounds of the hydroquinone and pyrocatechol type. Quinones, diquinones ( 5 ) , quinone imines ( 5 ) , and quinoid dyestuffs are formed quantitatively in all cases under the mildest conditions. The high redox potential of the quadrivalent lead produces an equilibrium which always lies completely on the side of the quinoid compounds.

HO-^y~OH

Similar also in its course and its nature is the dehydrogenative split­

ting of α-glycols (6) and other classes of compounds which carry either two hydroxyl groups, one hydroxyl and one amino, or two amino groups in the α-position. T h e reaction is also carried out in dilute solution at low temperatures and occurs quantitatively and irreversibly, although with a different velocity dependent upon the structure of the glycol and the conditions of reaction.

/ - O H ) c = 0

I + Pb(OAc)4 —-> + Pb(OAc)2 + 2 HOAc

) c OH ) o = 0

The characteristic feature of splitting a single carbon-carbon bond has lead to a comprehensive investigation of the mechanism.

The dehydrogenation of monohydroxy primary and secondary alco­

hols to aldehydes and ketones occurs on the other hand only at higher temperatures, where the reaction products themselves are readily oxi­

dized further. Sometimes, however, the corresponding aldehydes or

ketones can be obtained in good yields, as M i c o v i e ( 7 ) , for example, has shown with 3-pyridylcarbinols. In other cases, as with benzpinacolyl alcohol, Mosher (8) found rearrangements and the cleavage of molecules, which he explained with an ionic mechanism.

Whereas most carboxylic acids are, for the most part, stable towards lead tetraacetate (apart from the reversible formation of the correspond­

ing tetravalent lead salts), formic acid is dehydrogenated smoothly and quantitatively to carbon dioxide.

Monohydric phenols do not possess a second hydrogen atom which can be given up. Consequently stabilization of the phenol dehydrogenated by one hydrogen atom is possible only through the reception of one acetoxy group. As Wessely (9) and others have shown, the reaction products are quinols, or in case of further oxidation, quinones respectively:

OH Ο H3C I C H3 H3C UI CH,

Y Y + Pb(OAc)⁴ Υ Y + P b ( O A c )²

\J \J OAc + HOAc

C H3 C H3

The common primary step in the preceding reactions seems to be a rapid, reversible alcoholysis of lead tetraacetate:

Pb(OAc)4 + R - O H T ± R O - P b ( O A c )3 + HOAc

/ O C H , Such an alcoholysis product, ((AcO)2Pb x )

Ο Η

could be isolated in one case (6). The rest of the reaction (in other words the real oxidation) takes place by the acceptance of the P b O electron pair by lead (caused by the strong electron affinity of quad­

rivalent l e a d ) , which in so doing is transformed into P b ( O A c )3" and then into P b ( O A c )2 + O A c " :

RO £ p b ( O A c )3 —r RO® + Pb(OAc)3® — > P b ( O A c )2 + OAc®

This electron shift is possible, however, only if the electron sextet of oxygen can be filled up at the same time. This becomes possible in the case of hydroquinone and α-glycols—ignoring the details—by a shift of the electron pair of the second O H bond. In monohydroxyl alcohols (more difficultly) and in formic acid it is realized by a shift in the electron pair of a C H bond. In monohydric phenols the sextet is restored by electrons from the ring:

370 R. CRIEGEE

0 - i ° P b ( O A c )3 0 II

% 0 : - P b ( O A c )3

R _ C H ^ O- C p b( O A c )³ R - C H = 0

0 = C ^ 0- C p b( 0 A c )³ 0 = C = 0

H,C.

0-L-Pb(OAc)3

CH3 H3C

\ C H3

. C H3

OAc C H3

That a radical mechanism can play a part in the last named instance also is shown by the occasional appearance of dimerization products.

The oxidation of hydroperoxides is based on dehydrogenation with participation of an oxygen-bonded hydrogen atom (10). In contrast to all other classes of peroxides, these are attacked in the cold b y lead tetraacetate, mostly with violent evolution of oxygen. Along with other methods this has become of importance for identification. T h e course of reaction is relatively smooth with primary and secondary m o n o h y d r o - peroxides as well as with ditertiary bishydroperoxides, while tertiary monohydroperoxides furnish various reaction products. Examples of the first two subgroups are the following reactions:

H ^/O O H

Ο Ο Η

Pb(OAc)4

Pb(OAc)4

, Ο Ο Η

Pb(OAc)2

2 HOAc o²/²

xO ( Pb(OAc), I + I 2 HOAc

A mechanism which must explain the different behavior of sporadic hydroperoxides is not yet known, although the primary step appears to be similar to that for hydroxyl compounds (11).

R e a c t i o n s w i t h C H G r o u p s

Some hydrogen atoms bound to carbon can be substituted b y ace­

tylated hydroxyl groups. Such acetoxylations are then—and only then—

possible, when the carbon-hydrogen bond is adjacent to at least one

activating group. Carbonyl groups of all kinds can function as such activating groups. There activity of activated groups increases in order from carboxylic acids (which accept an acetoxy group in the a-position only under extreme conditions) to acid anhydrides, to ketones.

T h e β-diketones and the β-ketoacid esters react especially easily, as Dimroth (4) already found. The reaction is a first order reaction, just as is the halogenation of ketones, and therefore should occur via the enol form, according to Ichikawa (12) and Cavill (13). T h a t the free enol of dimesitylacetaldehyde furnishes the corresponding oxidation product according to Fuson (14), was in agreement with this hypothesis. The mechanism is then the same as that for monohydric phenols. Occasion­

ally, however, dimerization products appear here also (15).

Aromatic hydrocarbons with side chains also react in the same m a n ­ ner. Thus, toluene furnishes benzyl acetate (4), tetralin the acetate of ac-tetrahydronaphthol (11).

Since substituents in the benzene ring exert a strong influence on the yields (between 6 and 6 0 % ) and dimerization products were never

isolated, Cavill (16) rejects a radical mechanism (17).

T h e activating group, however, can also be a carbon-carbon double bond. Thus, alkenes of the allyl type, of which cyclohexene and a-pinene are examples (11, 18, 18a), can take on an acetylated hydroxyl group.

In both cases yields of 6 0 - 7 0 % can be obtained; in other cases they are poorer, because alkenes can react quite differently as will be shown later on. T h e double bond can shift in the oxidation (18a); the stereo- specific course argues against free radical formation. Acetylenes (19) of suitable structure can react in a similar manner under more strenuous conditions. T h e direct substitution of aromatically bound hydrogen b y acetoxy succeeds only in unusual cases with exceptionally reactive ring hydrogen atoms, as is the case in benzanthracene or benzpyrene (20).

An acetoxy group can also be introduced in the para position in anisole and other phenol ethers with moderate yields, according to Cavill (21).

In all of the examples cited the reactions proceed more smoothly R - C O - C H3 + Pb(OAc)4 =» R - C O - C H2O A c + P b ( 0 A c )2 + HOAc

3 7 2 R. CRIEGEE

and more uniformly and are therefore more efficient as preparative methods, the lower the temperature at which they can be carried out.

Substances which are attacked by lead tetraacetate solution only with boiling, give mostly an abundance of oxidation products difficult to separate. All the more remarkable was the discovery of Fieser {22), that preparatively worthwhile methylations can be obtained in some cases under these conditions. Examples are the methylation of methyl- naphthoquinone to dimethylnaphthoquinone or of trinitrobenzene to tri­

nitrotoluene. With other aliphatic quadrivalent lead salts the corres­

ponding alkylations are possible; with lead tetrabenzoate, phenylation occurred (23).

There is no doubt that here a pure radical reaction is involved.

ο

P b ( Q A c )4 ) j / Y Y ^{C H l}

N 02 N 02

λ λ ^CH3

κ \ Pb(OAc>4 / y

02N N 02 02N N 02

Pb(OBz)⁴

- C^eH⁵, o, m, ρ

Lead tetraacetate decomposes under the reaction conditions into lead acetate and acetoxy radicals, which in turn furnish carbon dioxide and methyl radicals. In support of this mechanism is the completely similar action (not found with other lead tetraacetate oxidations) of diacyl peroxides, as well as the observation of an induction period which can be shortened in various ways.

R e a c t i o n a t the C a r b o n - C a r b o n D o u b l e B o n d

A d d i t i o n of T w o A c e t o x y G r o u p s

The " n o r m a l " reaction course of unsaturated compounds with lead tetraacetate consists in the addition of two acetoxy groups with the formation of the diacetate of ^-glycols (4). However, this reaction with simple aliphatic or cyclic alkenes is less important than the above mentioned substitution in the alkyl position. In contrast, enol ethers (24), which are readily attacked at room temperature owing to the electron donating alkoxy groups, furnish the diacetates smoothly:

OR C H . - C H - O R £ ^ 2 ^ A c O - C H2- C H (

OAc H O C H2- C H O

In enol esters (25) the original acyl group present is frequently split off in the course of the reaction, so that free acetoxy ketones are formed;

the course can be represented in the following manner:

C H2 CH2OAc CH2OAc

1 1 ff,l OAc® ¹

C - O- C O C H 3 — > ® C - 0 - C O C H³ - > C = 0 + AcOAc

1 1 ^J · I

C H3 C H3 C H3

in which the formation of the cationic intermediate m a y proceed in such a manner as will be shown below with other alkenes.

With dienes, 1,2- or 1,4-addition can take place (11,26). T o all appearances the latter is exclusively the case with furan and a-acetoxy- furan (27), whose oxidations have found preparative applications. The oxidation of anthracene, according to Fieser and Putnam (28), is also a 1,4-addition in the first stage.

A d d i t i o n of M e t h y l A c e t a t e

W e observed a combination of methylation and addition of acetoxy groups with styrene and with unsymmetrical diphenylethylene (18).

The homologous alcohols, esterified with acetic acid, are formed in benzene solution in good yield under relatively mild conditions, at around 60°, thus, far below the decomposition point of lead tetraacetate.

C⁶H⁶- C H = C H² — > C^CH⁵- C H - C H²- C H³; I

OAc CeH6 CeH5

\ \ C = C H2 — > C - C H2- C H . / / \

CeH6 CeH6 OAc

T h e reaction, usually not encountered, is also applicable to lead tetrabenzoate (29): in this case phenyl and benzoxy are added.

In the case of styrene, polymerization of the hydrocarbon does not occur as a side reaction; the action of lead tetraacetate is thus c o m - pletely different from that of diacetyl peroxide. Nevertheless, and in spite of the realtively mild conditions, a radical chain mechanism cannot be excluded with certainty; it could occur in somewhat the following manner:

C^eH⁵- C H = C H² + C H³' = C^eH⁵- C H - C H²- C H³ C⁶H⁵- C H ' - C H²- C H³+ Pb(OAc)⁴ = C^eH⁵- C H ( O A c ) - C H²- C H³

+ Pb'(OAc)³ Pb*(OAc)³ = Pb(OAc)² + C 0² + CH³*

374 R. CRIEGEE

O x i d a t i o n with R e a r r a n g e m e n t

The investigation of a styrene derivative, namely p-methoxystyrene (24), furnishes a deeper insight into the mechanism of the reaction.

This example shows at the same time how seemingly very small changes in the structure of the substrate can lead to a fully altered course of reaction. p-Methoxystyrene was investigated in order to check if the activating influence of a methoxy group on a double bond (already men­

tioned) is transmitted through a benzene ring. Actually p-methoxy- styrene reacts violently with the oxidizing agent even at room tempera­

ture; the expected reaction product was not obtained, instead the crystal­

line isomeric aldehyde diacetate of p-methoxyphenylacetaldehyde was obtained in 9 0 % yield:

The same reaction took place also with o-methoxystyrene. T h a t there is present in the reaction products a carbon skeleton altered in relation to the starting material is shown from the behavior of the styrene deriva­

tives "tagged" in the α-position with a methyl group or with a second methoxyphenyl group: the " t a g g e d " group was found in the reaction products in the /2-position. Plainly then in all cases a methoxyphenyl group has shifted from the a- to the ^-carbon atom.

Similar rearrangements in oxidations with lead tetraacetate were occasionally observed at one time or another. Thus a homocamphenilone is formed from camphene by ring enlargement, according to Huckel (30), while according to Hurd (31), tetrahydro-2-furanmethanediol di­

acetate is obtained from dihydropyran through ring contraction (32):

A hypothesis concerning the course of these rearrangement reactions as well as other oxidation reactions of lead tetraacetate was developed by Weis and Dimroth (24). According to them, lead tetraacetate acts in the first phase of the reaction by adding the fragments P b ( O A c )^{3 +} O A c^- to the double bond, similar to the long-known case of mercuric acetate. T h e expected lead organic compound is not stable because of the great electron affinity of quadrivalent lead, but decomposes instantly with the extraction of an electron pair from the substrate into P b ( O A c) 3 ~

(which in turn decomposes into P b ( O A c )² and O A c^-) and an organic cation in which the migration of the methoxyphenyl group takes place

p - C H3O CeH4C H : C H2 —1|-> p - C H3O CeH4C H ( O A c ) C H2O A c

— > p - C H8O CeH4C H2C H ( O A c )2

Ο Ο

immediately. Final stabilization is achieved b y the addition of an acetate anion:

C H30 CeH4C H = C H2 — > C H30 - CeH4- C H - C H2J ? P b ( 0 A c )3

OAc I

— * C H j O C^eH⁴- C H - C H^t® II OAc

— > C H³O C⁶H⁴- C H²- C H O A c — * C H³O C^eH⁴- C H²C H ( O A c )²

Consideration should be given to the fact that the intermediate I I could arise directly from the styrene derivative by the action of an O A c⁺ cation split out from lead tetraacetate, such as Mosher (33) has accepted as the reactive agent in certain oxidations. In this case, h o w ­ ever, the OAc group must add to the β-carbon and not to the α-carbon atom (33a).

R e a c t i o n s w i t h M e r c u r y C o m p o u n d s

The above described mechanism utilizes organolead compounds of the type R P b ( O A c )3 as intermediates. Such compounds were first re­

cently discovered b y Panov and Kocheshkov (34) in which R was an aromatic group. Since I must be very short-lived—it never could be isolated—a great difference in the stability of the compound, R P b ( O A c )³ should exist depending upon the nature of R . W e found in the reaction of diarylmercury compounds with lead tetraacetate (35) a w a y to prepare compounds of this type under mild conditions. In agreement with the presentation of the Russian authors the compounds proved to be completely stable.

R²H g + Pb(OAc)⁴ RHgOAc + RPb(OAc)³

If the reaction is applied to dialkylmercury compounds, then in place of R P b ( O A c )3 only its decomposition products, namely lead acetate and alkyl acetate, are obtained.

This decomposition should occur ionically and therefore furnish R+ as intermediates. In agreement with this the neopentyl radical (known since the time of Whitmore (36) that it is unstable as a cation) undergoes a rearrangement to the ieri-amyl group in the reaction of dineopentylmercury with lead tetraacetate and tert-amyl acetate is obtained instead of neopentyl acetate.

In order to decide whether the formation of the triacetoxylead c o m ­ pounds (stable aromatic or unstable aliphatic) takes place through the P b ( O A c )³" ion or the P b ( O A c )^{3 +} ion, the mixed mercury compound, benzylphenylmercury, was oxidized. I t is known from the work of

376 R . C R I E G E E

Kharasch (37), that polar reagents cleave these compounds in such a way that the positive fragment adds to the phenyl, the negative to the benzyl. Thus, with hydrogen chloride, benzene and benzylmercuric chlo­

ride are obtained. Lead tetraacetate furnished phenyllead triacetate and benzylmercuric acetate which is explainable only by the appearance of the lead triacetate cation. This reacts as an electrophilic reagent at least in the case under discussion and also with respect to methoxystyrene.

M a n y of the reactions previously considered can be explained by an electrophilic attack of the P b ( O A c )^{3 +} cation (or also of the undissociated lead tetraacetate; in this case the OAc~ anion is removed in an inter­

mediary step).

I n v e s t i g a t i o n s w i t h Thallium(lll) A c e t a t e

W e insert here some comment concerning the action of thallium ( I I I ) salts (38) which have not been investigated extensively. As is well known thallium stands between lead and mercury in the periodic system. Thus it was expected that thallium ( I I I ) acetate would react in a manner intermediate between that of mercuric acetate (which only adds on to double bonds under the mildest conditions) and lead tetraacetate (with which the corresponding adducts are assumed, but have never been isolated). Actually it was possible with the new reagent under mild conditions to isolate adducts which are transformed into the oxidation product of the substrate with the splitting off of thallium (I) acetate at higher temperature. An example of this is the reaction of styrene with thallic acetate in methanol:

C⁶H⁵- C H = C H^{a T i}<^{O A c}> » Ο , Η β - Ο Η - Ο Η , - Τ Κ Ο Α Ο , ^{1 3 0} °°>

C H . O H I O C H3

/ O C H³

C^eH^e- C H - C H⁸O A c + C^eH^e- C H²- C H ^ + Tl(OAc)

O C H , OAc

The decomposition of the adduct occurs in part with, in part without, rearrangement in the organic portion.

F o r m a t i o n of Esters of G l y c o l i c A c i d a n d of G l y o x y l i c A c i d As was indicated above, C H3, C H2, and C H groups are capable of undergoing substitution of a hydrogen atom by an acetoxy group under the influence of an activating neighboring group. A free or esterified carboxyl group does not bring about such activation below 100° (89).

Quite astonishing was the discovery that during the addition of two

acetoxy groups to some alkenes, these groups themselves were substituted into the C H3 group so that esters of glycolic acid or glyoxylic acid were obtained as final products depending upon whether one or two acetoxy groups entered the methyl group. These coupled oxidations occur gen­

erally at room temperature and often with excellent yields.

Cyclopentadiene (11) was a case known for some time; here, however, the results are somewhat complicated, since cis- and irans-addition occur as does addition in the 1,2- and 1,4-positions as well. A better example is furnished by isobutylene (40). If this is introduced into a suspension of lead tetraacetate in glacial acetic acid, then a glyoxylic acid deriva­

tive probably having the following structure is obtained:

( C H3)2C ~ C H2 + 3 Pb(OAc)4 -> ( C H3)2C - C H2

AcO O C O C H ( O A c )2

Brutcher (41) supplied the key to the understanding of this unex­

pected reaction with the discovery that the course of the reaction is completely altered by the addition of some water, and the monoacetate of a diol is formed in place of the complicated reaction product. With the use of his explanation, as well as the previous statements concerning the primary steps of the oxidation of methoxystyrene, the reaction sequence with isobutylene appears to be the following:

P b ( O A c )4 Pb(OAc)4

( C H³)²C = C H² - 1> ( C H³)²C C H²: - L P b ( 0 A c )³_ > ( C H³)²C — C H² — - - A

\ τ • I I

ο ( ο ο ο

C C

ι in ι

C H3 C H3

\ H²0 OAc

OAc® I

( C H3)2C C H2 > ( C H3)2C C H2 ( C H3)2C C H2 ( C HS)2C C H2

l l I I I —> 1 I

Ο Ο Ο Ο Ο Ο OH OAc

\ ® / \ / \ /

C C C

I I

IV C H ( O A c )² C H ( O A c )^{2 H3C ο η}

T h e crucial step is the " b r i d g e d " cation I I I in which the hydrogen atoms of the acetate-methyl group are substituted because of the positive charge on the acetate-carbon atom. T h e latter plays the part here of a strong activating group; much stronger than the grouping

- C - C H j <—• - C ® C H3

ο ο®

in methyl ketones. The stabilization of the newly formed cation I V occurs through the attachment of an acetate anion to the α-carbon of isobutylene.

378 R. CRIEGEE

F o r m a t i o n o f L a c t o n e s f r o m A l k e n e s

In conclusion an unusual side reaction should be mentioned which occurs in the oxidation of simple aliphatic alkenes. In the higher boiling fractions γ-lactones (40) are found which are formed b y the addition of the group O C O C H2 to the double bond. In the case of 4-octene the reaction product has the following structure confirmed through synthesis:

C³H⁷- C H = C H - C³H⁷ — > C³H^V- C H - C H - C³H⁷ Ο ^XC H²

CO

Here also—as in the formation of esters of glycolic acid mentioned in the preceding section—the reactive O C O C H3 group is altered in its methyl portion. Several mechanisms can be formulated for this reaction, but a positive decision is not yet possible.

C o n c l u d i n g R e m a r k s

Once again an attempt should be made to bring order to the large body of factual material through a mechanistic approach. The following statements appear possible:

(1) In all cases in which lead tetraacetate reacts under mild condi­

tions, the primary step of the reaction is an electrophilic attachment of P b ( O A c )3 + (or of P b ( O A c )4 itself, followed or accompanied by the expulsion of an acetate anion) to the substrate. This can b e :

(a) a compound with a hetero atom to whose electron pair P b ( O A c )^{3 +} attaches itself,

(b) an alkene which adds P b ( O A c )3 . . . O A c ,

(c) an aromatic compound with strong electron donating groups, (d) an organometallic compound such as diphenylmercury.

(2) The resulting compounds are of the type R P b ( O A c )3 or R O P b ( O A c )3. The first of these types (with P b C bond) is stable only when R is an aromatic group (or perhaps also a vinyl t y p e ) ; the second (with PbO bond) only when R is part of a carboxylic acid (except formic acid) or a tert alkyl group. In all other cases the P b ( O A c )3 group takes possession more or less easily, of the electron pair previously belonging to the substrate. In this frequently occurring case then, the oxidation with lead tetraacetate takes place only in the (indirect) removal of an electron pair. Consequently it falls into the large group of oxidation reactions of organic compounds, which Levitt (42) has assembled recently, having this feature in common.

(3) This acceptance of the electron pair by the O P b bond can occur

only if the oxygen atom simultaneously receives electrons from the substrate. The ease with which this is possible is crucial for the ease of oxidation.

( 4 ) The carbonium ion formed from the C P b bond must be stabil­

ized, and this is possible in several w a y s :

(a) b y accepting an acetate anion, either from the P b ( O A c )3 group or from the solvent with the formation of a " n o r m a l " diacetate, (b) by forming a dioxolenium cation by ring closure with a

— Ο — C ( C H³) = 0 group already present. With sufficient stability of the new cation this can lead to the formation of esters of glycolic and glyoxylic acids.

(c) by rearrangement, followed by the addition of an acetate anion.

The occurrence of the rearrangement depends upon the "ease of migration" of the substituents present.

( 5 ) The methylation of aromatic compounds is a typical radical reaction. I t requires temperatures at which lead tetraacetate begins to decompose, thus is observed only with substances which react only with difficulty with lead tetraacetate owing to a lack of nucleophilic positions.

(6) A number of oxidation reactions of lead tetraacetate are not y e t classified in this scheme. Among these are: (a) substitution in the allyl position; (b) the addition of methyl acetate; (c) the formation of γ-lactones.

Predictions concerning the behavior of definite compounds with lead tetraacetate have only limited validity for the time being. The possibil­

ity of influencing the course of a reaction through choice of solvents or catalysts (43) e.g., B F3 exists, but it has not yet been systematically investigated in most cases.

Otto Dimroth has demanded the renunciation of the empirical as a goal of organic chemistry. He wished to predict the course of a reaction from a knowledge of the energy parameters, from kinetic data, and from reaction mechanisms (all three of which are not independent of each other), and thus to be able to choose suitable reactants. The pre­

ceding view of quite a small branch of organic chemistry shows how far we still are today from this possibility. T h e close cooperation of experimenter and theoretician will be necessary in order to reconcile the disparity of data.

S u p p l e m e n t

Since the preceding article was written, some important contributions have appeared. T h e y will be discussed in the same order as in the main article.

380 R. CRIEGEE

Reactions with H y d r o x y l C o m p o u n d s

W . von E. Doering (44) had shown in numerous investigations that 1,2-dicarboxylic acids could be transformed by decarboxylation with lead dioxide into alkenes. As Grob (45a, 45b) since then has discovered, the same degradation may be carried out with better reproducibility and with better yields using lead tetraacetate. T h e dicarboxylic acid is heated with lead tetraacetate in benzene, acetonitrile or, best, dimethylsulfoxide (46) solution with the addition of pyridine until the reaction begins with the evolution of carbon dioxide. The yields generally amount to 5 0 - 7 0 % . Several examples m a y illustrate the application of the method:

C02R C02R C02R C02R

CH3 CH3

C02H

C02H

The following mechanism best satisfies the data:

V

i i o i e , c I ο

ο

ι

χ

C OaH O - P b ( 0 A c )3

C

/ c

According to Jacques (47) β, γ-unsaturated acids are not only oxi­

dized with lead tetraacetate but decarboxylated as well, in which case the double bond can be shifted at the same time. Probably the oxidizing agent attaches itself to the carboxyl group and not to the double b o n d :

H³C . C H^{3 H>C}\ jj H³C C H^a

R - C C-COAH

J

<!:H3

^ ^ P b i O A c ) , R-c) 1 ° A C

(!:H3

+ Pb(OAc)a

C H - O A c

C H30

Reaction with C H G r o u p s

Whereas normally only such C H groups which are activated by neighboring groups such as aryl radicals, carboxyl groups, or double bonds, are attacked by lead tetraacetate, there was discovered recently by a group of Swiss chemists (48) that a nonactivated methyl group can be oxidized under special steric conditions. The reaction product, a saturated cyclic ether, is formed in greater than 3 0 % yield. The authors assume the formation of a tetravalent lead alkoxide as an intermediate and an electrophilic attachment to the methyl group after the splitting off of P b ( O A c )³- :

OH H3C C H - C H , ι

/ \ | I. ^H Pb(OAc)⁴ AcO

a

Benzene 40 °C

Pb(OAc)³

H - C H2 C H - C H3

I |···Η

Η®

Ο / \

H C C H - C H , ^{+P 5(O A} c)3 e

A new example of the dehydrogenation of a hydrocarbon by lead tetraacetate is furnished by Gardner and Thompson (49). I t seems note­

worthy that the dehydrogenation stops after the introduction of one double bond, whereas chloranil introduces the second double bond into the seven membered ring (although with poorer y i e l d ) :

1

Pb(OAc)⁴ HOAc Heat 1 hr

Ο

^ \ / ^ (51 % )

R e a c t i o n s a t the C a r b o n - C a r b o n D o u b l e B o n d

Alder et at. (50) in a very careful piece of work investigated the reaction of lead tetraacetate on the double bond of the bicyclo [2.2.1]

heptene system. In all cases the addition results in rearrangement.

Noticeable is the strong influence of the solvent upon the nature of the reaction product. All reactions are in agreement with the view of an electrophilic attack of a P b ( O A c )3 + cation on the strained double bond as being the first reaction step. T h e following scheme gives a survey of the results (parentheses around the group denotes unknown steric arrangement):

382 R. CRIEGEE

Pb(OAch

AC

°\

+ Pb(OAc)4 < CM

A (59%)

CH30- CHjO CH³0^

r ACO- HOAc ACO*

Ac0<

AcO<

/) HOAc L^^l //

a 40°C

A C° \

HOAc ^

AcOy^l^^

A

(together,

\ 85%)

(85%)

AcO- (OAc)

(25%) + ffl 1(5%)

II

(43%)

(25%)

CH30*

AcO*

(33%)

(OAc) (AcO)

(65%)

C6H6 . ^Β

+Pb(OAcL <

6(75-80%) + C( 10-15%) (OAc)

CH³0H (CH³0)

(OAc)

β (50-55%) + (25%)

M i s c e l l a n e o u s

Lead tetraacetate not only attacks organomercuric compounds as an electrophile (51) but in certain cases it is possible also to convert aromatic compounds into organolead compounds through a "plumbation reaction." Panov and Kocheshkov (52) used lead tetraisobutyrate for this, with thiophene as the substrate. Dependent upon the proportion of reactants and the reaction conditions, compounds containing one or two thiophene radicals on the lead m a y be obtained:

Pb(isobutyrate)4 Λ, \

ll I) > 1 ^ l'-Pb(isobutyrate),or \\\ } - Pb(isobutyrate)2

S S \ S /²

According to Field and Lawson (53) mercaptans and thiophenols can be converted more or less smoothly into disulfides with lead tetraacetate.

The speed of this reaction is greater than that of the glycol cleavage of pinacol. Accordingly, monothioglycol is not cleaved oxidatively by lead tetraacetate, but on the contrary is converted to the corresponding disulfide (54). A lead tetramercaptide is considered as the intermediate.

Noteworthy and of preparative importance is the discovery of Horner (55) 7 that tertiary amines can be dealkylated on treatment with lead tetraacetate in acetic anhydride (just as with N-bromosuccinimide) with excellent yields. Thus, dimethylaniline gives 8 3 % of acetylmethylaniline and diethylaniline 9 0 % of the corresponding acetyl compound. T h e mechanism of this reaction is still unknown.

S e l e c t e d P r e p a r a t i o n s

P y r i d i n e a l d e h y d e (7)

In a 1 liter three-necked flask fitted with a dropping funnel, reflux condenser, and stirrer, 53.2 gm (0.12 mole) of lead tetraacetate (dried over P205) and 200 ml of absolute benzene are placed. The mixture is heated to boiling with stirring; then—after removing the source of heat—

a solution of 13.1 gm (0.12 mole) of freshly distilled pyridinecarbinol in 50 ml of absolute benzene is added dropwise from the dropping funnel in 3-5 min. After the brown solution boils for several minutes, it becomes light yellow or colorless and the resulting lead acetate precipitates.

Heating is continued for another 45 min and then any excess lead tetraacetate is removed by the addition of a few drops of glycol. The cooled solution is suction filtered, and the lead salt is then washed twice with benzene. The combined benzene solutions are shaken with a solution of 20 gm of potassium carbonate in 200 ml of water to remove acetic acid, the aqueous layer is then extracted five times with chloroform or ether.

The combined solutions are dried and the solvent removed. The aldehydes remaining are fractionated in vacuo.

2-Pyridinealdehyde b.p. 7 0 - 7 3 ° / 1 3 m m , yield 65.4%

3-Pyridinealdehyde b.p. 8 6 - 8 9 ° / 1 3 m m , yield 77.8%

4-Pyridinealdehyde b.p. 90-91 ° / 2 0 m m , yield 6 8 . 4 % 3 , 3 , 6 , 6 - T e t r a m e t h y l - o - d i o x a n e ( 7 0 )

A 14.6 gm portion of technical l,l,4,4-tetramethyl-l,4-butanediol is stirred with 57 ml of 5 0 % hydrogen peroxide ( d1 8 = 1.200) and after cooling with vigorous stirring is decomposed with 45.5 ml of 7 0 % sulfuric acid. Stirring is continued for 40 min at 2 5 - 3 0 ° , and a volume of ice water three times the total volume of the mixture is added. The mixture is filtered by suction and washed well with ice water and some sodium

384 R. CRIEGEE

bicarbonate solution; yield, after drying in desiccator, is 10.5 gm ( 5 9 % ) , m.p. 105° (from benzene).

A 17.8 gm portion of bishydroperoxide in 150 ml of glacial acetic acid is added dropwise to a vigorously stirred suspension of 45 gm of lead tetraacetate in 200 ml of glacial acetic acid at 2 5 - 3 0 ° . When oxygen is no longer evolved the mixture is neutralized with 2 Ν N a O H and extracted with ether. Vacuum distillation gives 6.8 gm ( 4 7 % ) of tetra- methyl-o-dioxane with a b.p. 4 8 - 4 9 ° / 1 4 m m : nD 2 0 1.4251.

Trans-Verbenol A c e t a t e ( 1 8 )

A 20 gm portion of a-pinene in 450 ml of absolute benzene is oxidized with 64 gm of lead tetraacetate with vigorous stirring at 70° for 2 hr.

The cooled solution is filtered from the lead acetate and the residue washed with some benzene. The benzene is distilled off from the filtrate at about 100 mm at a bath temperature of 30-40° and utilizing a small column; some more lead acetate separates. Once again it is filtered and the reaction product is distilled in vacuo. After a forerun consisting of glacial acetic acid and 3 gm of unreacted pinene is obtained, 18.5 gm ( 7 4 % ) of verbenol acetate is obtained, b.p. 70-71 ° / 2 . 5 m m ; nD 2 3 1.4730;

[ « ] D^{2 0} —11.2°. Saponification gives irans-verbenol, b.p. 5 6 - 5 7 ° / 0 . 7 m m , p-nitrobenzoate, m.p. 82°. If the oxidation of α-pinene takes place in glacial acetic acid as solvent, then only 3 4 % of verbenol acetate is obtained, along with verbenes and sorbrerol diacetate.

2 , 3 - D i m e t h y l - l , 4 - n a p h t h o q u i n o n e (22)

A solution of 0.86 gm of 2-methyl-l,4-naphthoquinone and 0.6 gm of malonic acid (as catalyst) in 15 ml of glacial acetic acid is treated with 5 gm of lead tetraacetate and warmed in the water bath first for 1 hr at 5 0 - 6 0 ° , then for an additional hour at 70°. A precipitate (a lead malonate?) which first forms then disappears. A t the same temperature 2 gm portions of lead tetraacetate are added 2-3 times, until upon further addition gas is no longer evolved. The excess oxidizing agent is destroyed with several drops of glycerol. The reaction mixture is poured into water and the yellow precipitate is crystallized from methanol;

m.p. 122-124°; yield 0.45 gm ( 4 9 % ) .

E t h o x y - l , 2 - e t h a n e d i o l D i a c e t a t e (18,24)

A 30 gm portion of freshly distilled ethyl vinyl ether is added d r o p - wise over a 40 min period to a vigorously stirred suspension of 185 gm of lead tetraacetate in 600 ml of absolute benzene. After cooling the temperature is kept at 30°. The workup is like that for verbenol acetate.

Distillation gives 72'gm ( 8 9 % ) of the diacetoxy compound, b.p. 7 6 - 7 7 ° / 2mm, nD 2 5 1.4133.

On shaking with an aqueous solution of semicarbazide hydrochloride the semicarbazone of acetylglycolaldehyde is obtained, m.p. 170°.

2 , 5 - D i h y d r o - 2 , 5 - f u r a n d i o l D i a c e t a t e (27)

In a 1 liter three-necked flask fitted with a stirrer, thermometer, and reflux condenser are placed 580 ml of glacial acetic acid and 230 ml of acetic anhydride and with vigorous stirring there is added in 10-20 gm portions a total of 300 gm of lead oxide ( P b304) . The temperature is maintained at 50°. After about 3.5 hr the reaction is finished. Then 29.8 gm of furan is added, the temperature rises to 60° and the heating bath is removed. In the next 10 min the temperature rises to 65°. A temperature of 60-65° is maintained for 75 min, first through cooling, then by heating; then the major portion of the solvent is removed at 10 m m with the water bath at 6 0 - 6 5 ° , and 400 ml of absolute ether is added to the paste-like residue. After shaking and stirring the precipi­

tated lead acetate (415 gm) becomes filterable. The yellow-brown filtrate is freed from ether and distilled in vacuo. The major portion (56 gm, 6 9 % ) has a b.p. 8 9 - 9 3 ° / 0 . 5 m m , n^{D 2 5} 1.4536. The substance is a mixture of the cis and trans forms, from which the isomer of m.p. 51-52° m a y be obtained by crystallization from methanol at — 2 0 ° . Hydrolysis of both forms leads to malealdehyde.

A c e t a t e o f E t h y l p h e n y l m e t h a n o l (Ί8)

A 23.5 gm portion of freshly distilled styrene in 500 ml of absolute benzene is oxidized with 100 gm of lead tetraacetate with stirring at a bath temperature of 7 5 - 8 0 ° ; workup as for verbenol acetate. On distilla­

tion 30 gm ( 8 2 % ) of the acetate of ethylphenylmethanol is obtained, b.p. 103-110°/12mm (redistillation: b.p. 7 7 - 7 8 ° / 2 m m ) . Alkaline saponi­

fication furnishes a 9 0 % yield of ethylphenylmethanol, b.p. 1 0 5 - 1 0 6 ° / 16mm, nD 2 0 1.5208; p-nitrobenzoate, m.p. 56°.

p- M e t h o x y p h e n y l a c e t a l d e h y d e D i a c e t a t e (24)

A 15 gm portion of freshly distilled p-methoxystyrene is added with vigorous stirring to a suspension of 52 gm of lead tetraacetate in 80 ml of glacial acetic acid; the temperature is maintained at 25° by external cooling. When the reaction is finished (verified with leucomalachite green), the mixture is poured into 400 ml of water. The precipitated yellow oil solidifies after a time and after filtration is dried in the desiccator. The crude product (23.5 g m ; 9 4 % ) is recrystallized from petroleum ether (b.p. 4 0 - 6 0 ° ) ; m.p. 52°. The substance is not infinitely stable in the laboratory atmosphere ( a c i d i c ) , therefore it is sealed up in an appropriate fashion.

386 R. CRIEGEE

1 - C a r b e t h o x y- A2- b i c y c l o [ 2 . 2 . 2 ] o c t e n e (45b)

In a 1 liter flask fitted with stirrer, condenser, and gas inlet tube 27 gm (0.1 mole) of l - c a r b e t h o x y- A²- b i c y c l o [2.2.2] octane-2,3-dicar- boxylic acid is treated with 150 ml of absolute benzene and 12 ml (0.15 mole) of absolute pyridine. After the addition of 46.6 gm (0.1 mole) of about 9 5 % lead tetraacetate the mixture is warmed on the steam bath in a current of nitrogen and with stirring until a clear solution is formed and a somewhat vigorous reaction sets in. After the evolution of carbon dioxide has subsided, the mixture is refluxed for 2 hr longer. Then the precipitated lead acetate is filtered and the benzene solution is washed successively with water, 2 Ν sodium carbonate, 2 Ν hydrochloric acid, and finally again with water. After drying over sodium sulfate the solu­

tion is concentrated using a column. Distillation of the oily residue in a stream of nitrogen furnishes 11.3 gm of a bicyclooctene derivative of b.p. 9 5 - 9 6 ° / 1 0 m m .

The crystalline distillation residue furnishes 3 gm of the anhydride of the starting material after two recrystallizations from ether-petroleum ether. Considering this amount of recovered material, the total yield of the unsaturated compound is 7 1 % .

D e m e t h y l a t i o n o f D i m e t h y l a n i l i n e ( 5 5 )

T o a mixture of 6.0 gm (50 mmole) of dimethylaniline in 25 ml of chloroform and 10 ml of acetic anhydride, which is contained in three- necked flask fitted with a stirrer, a solution of 22.15 gm (50 mmole) of lead tetraacetate in 50 ml of chloroform is added dropwise over a period of 30-40 min at room temperature in an atmosphere of nitrogen. The reaction, after an initial green coloration, proceeds with a strong evolu­

tion of heat. With occasional cooling with water, stirring is continued for 1 hr. The precipitated lead acetate is filtered, the chloroform solution is extracted with 200 ml of water, and the layers are separated. The chloroform layer is concentrated, the acetic acid and acetic anhydride removed in vacuo and the remaining N-methylacetanilide is recrystal­

lized from water; m.p. 102°; yield 6.1 gm ( 8 3 % ) .

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(2) A. Jacquelin, J. prakt. Chem. 53, 151 (1851).

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(5) R. Adams and co-workers, numerous publications in Λ Am. Chem. Soc. (1950- 1957).

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599, 81 (1956).

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(10) R. Criegee, H. Pilz, and H. Flyare, Ber. deut. chem. Ges. 72, 1799 (1939); R.

Criegee and G. Paulig, ibid. 88, 712 (1955).

(11) Concerning the dehydrogenation of hydrocarbons and heterocyclics to aro­

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Liebigs 481, 263 (1930) as well as reference ( 1 ) ; see also H. Meerwein, Ber.

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(12) K. Ichikawa and Y . Yamaguchi, J. Chem. Soc. Japan 73, 415 (1952); Chem.

Abstr. 47, 10474 (1953); for other preparative examples of ketone oxidation see also R. Criegee and K. Klonk, Ann. Chem. Liebigs 564, 1 (1949); L. F.

Fieser and R. Stevenson, Λ Am. Chem. Soc. 76, 1728 (1954).

(13) G. W . K. Cavill and |D. H. Solomon, J. Chem. Soc. p. 4426 (1955).

(14) R. C. Fuson and co-workers, J. Am. Chem. Soc. 79, 1938 (1957).

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Chem. Abstr. 46, 435 (1952).

(16) G. W . K. Cavill and D. H. Solomon, J. Chem. Soc. p. 3943 (1954); see also G. W . K. Cavill, A. Robertson, and W . B. Whalley, ibid. p. 1567 (1949) (17) Further examples: W . S. Johnson and co-workers, J. Am. Chem. Soc. 66, 218

(1944); ibid. 78, 6312 (1956).

(18) C. Weis, Dissertation, Karlsruhe, 1953.

(18a) G. H. Wilham, J. Chem. Soc. p. 2232 (1961).

(19) V. Franzen, Chem. Ber. 87, 1478 (1954); see also Marktscheffel, Diplomarb., Karlsruhe, 1952.

(20) L. F. Fieser and Ε. B. Hershberg, / . Am. Chem. Soc. 60, 1893 (1938); ibid.

61, 1565 (1939).

(21) G. W . K. Cavill and D. H. Solomon, J. Chem. Soc. p. 1404 (1955); Η. E. Bar­

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(22) L. F. Fieser and F. C. Chang, J. Am. Chem. Soc. 64, 2043 (1942); L. F. Fieser, R. C. Clapp, and W . H. Daudt, ibid. p. 2052; see also J. W . Cornforth and E. Cookson, / . Chem. Soc. p. 1085 (1952).

(23) D. H. Hey, J. M. Stirling, and G. H. Williams, J. Chem. Soc. p. 2747 (1954);

ibid. p. 3963 (1955).

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Abstr. 48, 1243 (1954).

(25) R. Simon, Dissertation, Karlsruhe, 1951; W . S. Johnson, B. Gestambide, and R. Pappo, / . Am. Chem. Soc. 79, 1991 (1957).

(26) T. Posternak and H. Friedli, Helv. Chim. Acta 36, 251 (1953).

(27) N. Elming and N. Clauson-Kaas, Acta Chem. Scand. 6, 535 565 (1952); the oxidation of 2,5-diarylfurans goes quite differently: Ch.-K. Dien and R. E.

Lutz, / . Org. Chem. 22, 1355 (1957).

(28) L. F. Fieser and S. T. Putnam, J. Am. Chem. Soc. 69, 1038 (1947).

388 R. CRIEGEE

(29) S. Goldschmidt and E. Stockl, Chem. Ber. 85, 630 (1952).

(30) W . Huckel and H. G. Kirschner, Chem. Ber. 80, 41 (1947).

(31) C. D. Hurd and Ο. E. Edwards, J. Org. Chem. 19, 1319 (1954).

(32) A further interesting case is that of longifolen: P. Naffa and G. Ourisson, Bull. soc. chim. France p. 1115 (1954).

(33) W . A. Mosher and C. L. Kehr, J. Am. Chem. Soc. 75, 3172 (1953).

(33a) Remark by the referee: the lead tetraacetate oxidation of bicyclo[1.2.2]

heptene and -heptadiene occurs in complete agreement with this concept:

K. Alder, F. H. Flock, and H. Wirtz, Chem. Ber. 91, 609 (1958).

(34) Ε. M . Panov and K. A. Kocheshkov, Doklady Akad. Nauk S3.S.R. 85, 1037 (1952); Chem. Abstr. 47, 6365 (1953).

(35) R. Criegee, P. Dimroth, and R. Schempf, Chem. Ber. 90, 1337 (1957). Com­

pounds of the type R2Pb(OAc)2 were prepared already in this manner by Μ . M. Nad' and K. A. Kocheshkov, Zhur. Obshchei Khim. 12, 409 (1942) ; Chem. Abstr. 37, 3068 (1943); as well as by A. N. Nesmeyanov, R. K. Fried- lina, and A. Kochetkov, Izvest. Akad. Nauk S.S.S.R. Otdel Khim. Nauk p. 127 (1948); Chem. Abstr. 43, 1716 (1949).

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(41) F. V. Brutcher, Jr. and F. J. Vara, J. Am. Chem. Soc. 78, 5695 (1956).

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(43) M. Finkelstein, Chem. Ber. 90, 2097 (1957).

(44) W . von E. Doering, M. Farber, and A. Sayigh, Λ Am. Chem. Soc. 74, 4370 (1952); W . von E. Doering and M . Finkelstein, J. Org. Chem. 23, 141 (1958).

(45a) C. A. Grob, M . Ohta, and A. Weiss, Angew. Chem. 70, 343 (1958).

(45b) C. A. Grob, M . Ohta, E. Renk, and A. Weiss, Helv. Chim. Acta 41, 1191 (1958).

(46) Private communication from Prof. C. A. Grob.

(47) J. Jacques, C. Weidmann, and A. Horeau, Bull. soc. chim. France p. 424 (1959).

(48) G. Cainelli, M . L. Mihailovic, D. Arigoni, and O. Jeger, Helv. Chim. Acta 42, 1124 (1959).

(49) P. D. Gardner and R. J. Thompson, J. Org. Chem. 22, 36 (1957).

(50) K. Alder, F. H. Flock, and H. Wirtz, Chem. Ber. 91, 609 (1958).

(51) The reaction, Ar²Hg + Pb(OAc)⁴ - > ArPb(OAc)³ + ArHgOAc, which was not known to us at the time, had already been carried out by Ε . M . Panov, V. J.

Lodochnikova, and K. A. Kocheshkov, Doklady Akad. Nauk SJ3JS.R. I l l , 1042 (1956); Chem. Abstr. 51, 9512 (1957).

(52) Ε. M . Panov and K. A. Kocheshkov, Doklady Akad. Nauk S.S.S.R. 123, 295 (1958); Chem. Abstr. 53, 7133 (1959).

(53) L. Field and J. E. Lawson, J. Am. Chem. Soc. 80, 838 (1958).

(54) How lead teraacetate acts upon such thioglycols, which contain the OH and SH group in small rings in the cis-position, is not known. It is quite con­

ceivable that in such cases the glycol cleavage is the quicker reaction.

(55) L. Horner, E. Winkelmann, Κ. H. Knapp, and W . Ludwig, Chem. Ber. 92, 288 (1959).