Chlorohydrins Wacker process

The essence of the Wacker process is the invention of the reoxidation process for Pd° by using CuCh as a cocatalyst. Cu" salts are good reoxidants, but chlorination of carbonyl compounds takes place with CuCh. For example, chloroacetaldehyde is a by-product of the Wacker process. Chlorohydrin is another by-product from the reaction of ethylene with PdCh and CuCb. - Thus, a number of other reoxidants were introduced. When CuCl, pretreated with oxygen, is used, no chlorination of ketones takes place and the rate of the reaction is higher. - Also Cu(N03)2 and Cu(OAc)2 have been used. Oxidation of cy-clopentene with PdCl2/Fe(C104)3 combined with electrochemical oxidation was carried out. Benzoqui-none was used at first by Moiseev et al and later by many other researchers as a good reoxidant, but a stoichiometric amount is necessary. The oxidation of alkenes can be carried out smoothly with catalytic... 

Ethylene chlorohydrin (38) is formed in the Wacker process as a byproduct. Chlorohydrins are obtained as main products when PdCl3 (pyridine) is used [29]. The optically active chlorohydrin 40 with high ee was obtained without forming the regioisomer 41 from allyl phenyl ether (39) and other substituted alkenes when the bimetallic Pd complex 42 coordinated by water soluble chiral BINAP-based ligand 43 (II-9) was used [30]. 

Stoichiometric amounts of Pd(II) and exchange of the CuCl by CaClj led to no chlorohydrin formation. Jira proposed a reaction sequence of oxypalladation to AJ as observed in the Wacker process and oxidative-induced reductive C-Cl bond formation of the Pd alkyl intermediate AK (Scheme 16.41). 

Optically active, vicinal chlorohydrins can serve as building blocks in much the same capacity as epoxides, azido alcohols, or diols. Enantioselec-tive access to chlorohydrins such as 165 was made possible through Henry s discoveiy of a Pd-catalyzed interrupted Wacker oxidation of olefins (Equation 28) [136, 137]. The process employs tetrasulfonated BINAP 166 as a chiral ligand embedded within the bimetallic triketone complex 164 [136]. 

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