Backuall, Akermark, Ljunggren
/
241 1
Pd(II)-Catalyzed Oxidation of Ethene in Water
Stereochemistry and Mechanism for the Palladium( 11)-Catalyzed Oxidation of Ethene in Water (the Wacker Process) J. E. Backvall,*'a B. Akermark,la and S. 0. Ljunggrenlb Contributionfrom the Department of Organic Chemistry and the Department of Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden. Received December 4, 1978
Abstract: Palladium(I1)-catalyzed oxidation of specifically deuterated ethenes in water in the presence of cupric chloride and chloride ion has been studied. Stereospecific formation of threo- 1,2-dideuterio-2-chloroethanol (threo-3)from ( E ) -1,2-dideuterioethene ( ( E ) - 2 ) indicates that the hydroxypalladation step in the Wacker oxidation is a trans process. Generation of an erythro-P-hydroxyethylpalladiumcomplex ( 6 ) from erythro-P-hydroxyethylmercuric chloride (5) in the presence of CuC12LiCl also results in the formation of rhreo-3. Analyses of the deuterated products were done by microwave spectroscopy. A new (modified) mechanism is proposed, which involves reversible formation of 11 by external attack of water on 8, followed by a rate-determining dissociation of a chloride ligand from 11.
Introduction The success of the Wacker process2 as an efficient industrial method for oxidation of ethene to acetaldehyde (eq 1) has inCH2 = CH
2
+
12. 02
Scheme I
PdC12
x q CH3CHO
( e q 1)
II-
I
y-
9" -+m
- ::-
I itiated an intense study of palladium-catalyzed organic reac/ - Pdt i o n ~ .Special ~ , ~ interest has been devoted to reactions in which Nu palladium(I1) promotes nucleophilic addition to olefins and E related unsaturated systems. This has led to the development of a number of palladium-promoted or -catalyzed reactions Scheme I1 of olefin^.^-^ It is generally agreed that, in most reactions of acetaldehyde- 62 this type, a u complex 1, usually unstable, is an intermediate. An important question concerning the mechanism of such reactions is the steric course of the addition of the metal and the nucleophile to the double bond (path A or B, Scheme I). HZO-td - CI Several studies have considered this question and, because (21 ti different nucleophiles can add with different stereochemistry, there was originally some confusion and much speculation. More recently, however, the stereochemistry of such reactions has been studied directly. Thus palladium and nucleophiles like a ~ e t a t em, ~e t h a n ~ l ,or~ ?aminel0 ~ have been shown to add trans to olefins (path A), whereas palladium and nucleophiles such as aryl,7b or methoxycarbony18 have been shown to add cis (path B). Although the mechanism of the Wacker oxidation of ethene to acetaldehyde has attracted considerable a t t e n t i ~ n , ~ ? ~ , ~ - l ~ there is still disagreement about the steric course of the hydroxypalladation step (path A or path B, Scheme I). Kinetic main product by increasing the copper(I1) and chloride ion studies suggest that a cis addition of Pd(I1) and coordinated concentration^.'^ It is reasonable to assume that the acetalOH takes p l a ~ e ,1-13 ~ ~implying ~ ~ ~ ' that the hydroxylation has dehyde and the 2-chloroethanol are formed from the same a different steric course from the addition of other related intermediate, a P-hydroxyethylpalladium complex (1, Nu = nucleophiles. However, more recent studies by uslo and others8 OH). &Elimination gives the aldehyde and oxidative cleavsuggest that also the Wacker hydroxylation might be a trans age16 the chlorohydrin (Scheme 11). By determining the conprocess. We therefore undertook a study on the mechanism of figuration of the chlorohydrin obtained from specifically the Wacker hydroxylation with the aim to obtaining conclusive deuterated ethene, we have thus been able to establish the steric evidence for the steric course of the reaction. In a preliminary course of the hydroxypalladation step. c ~ m m u n i c a t i o nwe ' ~ reported that trans-hydroxypalladation Results and Discussion takes place, and on the basis of this finding we proposed a new mechanism. In the present paper we give a complete account The oxidation of ( Z ) - and (E)-ethene-dz was performed in of these results, discuss the new mechanism, and also present aqueous solution of PdClz-CuC12-LiCl under pressure ( N 3-5 additional results which support the conclusions drawn in the kg/cm2) (Scheme 11). The LiCl concentration used was 3.3 earlier communication. M. The main products from these reactions were chloroethaIn the Wacker oxidation of ethene, the formation of chlornol-dz (3) and acetaldehyde. The configuration of 3 was deohydrin has been observed2>l5which can be obtained as the termined by microwave spectroscopy. The rotational transi-
0 1979 American Chemical Society
241 2
Journal of the American Chemical Society
/ 101:9 / April 25, 1979
Table I. Rotational Transitions of 3a Used for Analysisbte transition
--
725 7 1 6 826 817 143,11 1 4 2 ~ 2 1 5 3 , 1 2 - 152.13 183,15 182,16
+-
thre0-(A)-3~
threo-(B)-3
erythro-(A)-3
erythro-(B)-3
20607.02 (0.58) 20354.1 1 31617.31 (0.71) 3 1053.84 32166.13 (1.72)
20483.14 20595.77 (1.19) 3 1 100.91 31525.59 (1.02) 37348.59 (3.16)
21 106.39 20940.21 32155.69 (-0.3) 3 1822.50 34249.85 (2.21)
19966.80 19975.03 30285.76 30418.84 (0.63) 34849.36 (2.78)
Authentic samples were used for the recording of the spectra (see Experimental Section). Frequencies of transitions are given in megahertz. Only the gauche conformation of chloroethanol has been observed in the gas phase.]' Owing to two energetically equivalent gauche confor-
mations in each of threo-3 and erythro-3, two different spectra were observed in each case. d The figures in parentheses indicate quadropole splitting. All of the lines chosen were either singlets or doublets. No figure means singlet.
tions used for this analysis are given in Table I. By treatment with hydroxide ion 3 was converted into the corresponding epoxide 4, which was also analyzed by microwave spectroscopy. This conversion, which results in one inversion, was carried out in order to facilitate a more accurate quantitative analysis, the rotational transitions of 4 being much stronger than those of 3. Transitions used for quantitative analysis of 4 are given in Table 11. Microwave spectroscopy proved to be a convenient and powerful method for the analysis of 3 and 4. The results from such an analysis are given in Table 111. The chlorohydrin formed from ( E ) - 2 was found to be threo-3 and the corresponding epoxide was ( 2 ) - 4 . Similarly ( 2 ) - 2 gave erythro- 3. The extent of 2 . E isomerization of ethene-d2 was
