Introduction and Classification of Reactions

Pd(0)-catalyzed reactions of various propargyl compounds proceed by the formation of either the cr-allenylpalladiums 1 or the propargylpalladiums (or cr-prop-2-ynylpalladiums) 2 as intermediates. 

The Pd(0)-catalyzed reactions of propargyl compounds so far discovered can be classified into several types from a mechanistic viewpoint [1], The a-allenylpalla-dium complexes 1 as intermediates undergo three types of transformations depending on reactants. 

The first one is insertion of unsaturated bonds to the cr-bond. The insertions of alkenes, alkynes, and CO generate the alkylpalladium 3, 1,3,4-trienylpalladium 4, and acylpalladium compounds 5, respectively. These intermediates undergo further transformations. 

Palladium Reagents and Catalysts—New Perspectives for the 21st Century J. Tsuji 2004 John Wiley Sons, Ltd ISBNs 0-470-85032-9 (HB) 0-470-85033-7 (PB) 

The second type of reactions proceed by transmetallation of the complexes 1. MR (M = main group metals) and metal hydrides MH undergo the transmetallation with 1 to generate 6. Subsequent reductive elimination gives the allene derivative 7. Also reactions of 1 with 1-alkynes in the presence of Cul to afford allenylalkynes belong to this type. 

---