Reaction mechanism vinyl halide cross-coupling

The Pd-catalyzed cross-coupling reaction of vinyl(2-pyridyl)silanes 5.48 with organic halides gave substituted vinyl(2-pyridyl)silanes 5.49 in high yields. The mechanism of this reaction involves the carbometallation pathways (Scheme 5.18). 

The mechanism of the Sonogashira cross-coupling follows the expected oxidative addition-reductive elimination pathway. However, the structure of the catalytically active species and the precise role of the Cul catalyst is unknown. The reaction commences with the generation of a coordinatively unsaturated Pd species from a Pd " complex by reduction with the alkyne substrate or with an added phosphine ligand. The Pd " then undergoes oxidative addition with the aryl or vinyl halide followed by transmetallation by the copper(l)-acetylide. Reductive elimination affords the coupled product and the regeneration of the catalyst completes the catalytic cycle. 

The dotted arrows on the transmetallation step 243 show only what joins to what and are not intended as a serious mechanism. Indeed a better mechanism might involve addition of RO to the boron atom before transmetallation. This process can be used to couple aryl to aryl, vinyl to vinyl, and aryl to vinyl (either way round ). As boron compounds are much less toxic than tin compounds, the Suzuki coupling is often preferred industrially. Because each partner in the coupling reaction is marked in a different way - one with a boron atom and one with a halide - we can be sure that we shall get cross coupling reactions only. 

Arylboronic and alkenylboronic acids undergo transition metal complex-catalyzed synthetic organic reactions such as cross-coupling with organic halides [58-60], 1,4-addition to a,/I-unsaturated ketones [61-63], and ring-opening addition to vinyl oxirane [64]. Scheme 5.8 depicts the mechanism proposed for the... 

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