Wacker-Type Carbonylative Processes

The mechanism of this reaction involves anfi-aminopalladation of the alkene to afford 163, which undergoes CO insertion to form 164. Capture of the acylpalladium intermediate 164 with methanol (formed in situ from trimethyl orthoacetate) gives the heterocyclic product. 

A number of Pd-catalyzed carbonylative processes have employed allenes as substrates for the synthesis of nitrogen heterocydes [108], For example, subjection of substituted allene 170 to reaction conditions similar to those employed in related reactions of alkenes led to the formation of pyrrolidine 171 in 68% yield with 2 1 dr (Eq. (1.67)) [108d], Modest asymmetric induction has been achieved in these transformations using simple chiral auxiliaries [108b,d]. This strategy was employed in an asymmetric synthesis of pumiliotoxin 251 D, which involved the aminocarbo-nylation of allene 172 to pyrrolidine 173 as a key step (Eq. (1.68)) [108b]. 

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Several Pd-catalyzed domino processes start with a Tsuji-Trost reaction, a pal-ladation of alkynes or allenes [5], a carbonylation [6], an amination [7] or a Pd(II)-cat-alyzed Wacker-type reaction [8]. A novel illustrious example of this procedure is the efficient enantioselective synthesis of vitamin E [9]. 

Carbonylations involving catalytic amounts of Pd(n) and substoichiometric Cu(n) have been developed for the synthesis of lactams. This Wacker-type process is assumed to proceed via C-H functionalization involving a palladacycle intermediate such a hypothesis is backed by the high degree of regioselectivity attained, when using a bis-chelating substrate (Equation (114)).104... 

For a long time, electrocatalysis (cf. Section 3.2.8) has not been considered important in organometallie chemistry although numerous known processes depend on redox reactions (e. g., Wacker-type catalysis, oxidative olefin carbonylation. 

Unlike the cases of alkenes, Wacker-type intermolecular oxypalladation reactions of alkynes have not been extensively investigated, although their intramolecular cyclization reactions have been developed into synthetically useful procedures (Sects. V3.2). In principle, they can proceed by a few alternative paths shown for the cases of terminal alkynes in Scheme 14. In reality, however, alkynyl C—H activation by Pd to give alkynylpalladium derivatives shown in Scheme 3 may well be the dominant path, as suggested by the carbonylative oxidation of terminal alkynes to give alkynoic acid esters shown in Scheme 15. Oxidative dimerization of alkynes is a potentially serious side reaction. Further systematic investigation of this fundamentally important process appears to be highly desirable. 

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