Allenylpalladium chloride

The Pd(0)-catalyzed reactions of propargylic compounds can be understood by the following mechanistic considerations. The first step in the catalytic reactions is the oxidative addition of a propargylic compound to Pd(0) species to form an inteimediate complex. By this oxidative addition, Pd(0) is oxidized to Pd(II). The intermediate Pd(II) complex undergoes further reactions with other reactants. Complex formation by stoichiometric reactions of propargylic chlorides 2 and 4 with Pd(Ph3P)4 has been studied, and the o-allenylpalladium 3 and the propargylpalladium (or c7-prop-2-ynylpalladium) 5 were isolated as yellow powders (Scheme 11.2) [2]. The allenylpalladium chloride 3 is... 

Some synthetically important allenylmetallics, such as allenylzinc and allenylin-dium reagents, are prepared from allenylpalladium intermediates. These reactions are discussed in appropriate sections of this chapter. This section covers the reactions of allenylpalladium compounds without further transmetallation. Allenylpalladium complexes can be prepared from propargylic halides, acetates, carbonates, mesylates, alcohols and certain alkynes [83-87], The allenylpalladium compound prepared from 3-chloro-3-methyl-l-butyne has been isolated and characterized spectroscopically (Eq. 9.106) [83], It was found to couple with organozinc chlorides to produce homologated allenes quantitatively (Eq. 9.107). 

Chiral allenylpalladium complexes have been prepared from non-racemic propargylic chlorides and Pd2(dba)3-PPh3 [88]. Although nominally stable, these complexes were found to racemize under certain conditions (Eq. 9.108). Reagents prepared under oxygen-free conditions racemize more slowly than those prepared in... 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-I,2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5]. The reaction can be explained by the transmetallation of the c-allenylpalladium methoxide 4 with copper acetylides to form the allenyl(alk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCN) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7],... 

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