Dienes from vinyl boranes

R and R" are cis in the starting compound, they will be trans in the product (2) there is retention of configuration within the migrating group R. " " Since vinylic boranes can be prepared from alkynes (15-16), this is a method for the addition of R and H to a triple bond. If R = H, the product is a (Z)-alkene. The mechanism is believed to involve an iodonium intermediate, such as 92, and attack by iodide on boron. When R is vinylic, the product is a conjugated diene." " ... 

The chemistry of unsaturated organoboranes often differs markedly from that of their saturated analogues. Both vinylic and allylic boranes react readily with many substrates toward which trialkylboranes are inert90 . Allylic boranes can be synthesized selectively via direct hydroboration of an appropriate allene or conjugated diene and are of immense synthetic importance 91 93). Mikhailov in his book and review 7,94) has documented the synthetic applicability of allylic boranes with caution of high thermal reactivity with respect to allylic rearrangement. For example, (l-methyl-2-propenyl)dialkylboranes rearrange spontaneously to the 2-butenyl isomer even at -78 °C (Eq. 40). 

The allylborations of vinyl and silyl enol ethers affords 1,4-dienes (Scheme 23.34). The dienes apparently arise from an initial allylboration followed by elimination of the 3-alkoxy- or P-(silyloxy) borane. 

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