Omniphilic dienophile

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

The frans-metalation process has been employed for the synthesis of 9-vinyl-9-BBN, by the reaction of B-Br-9-BBN either with vinyltributyltin or tetravinyl-tin (Eq. 28.3) [14]. Singleton and his group have extensively used 9-vinyl-9-BBN as a dienophile for Diels-Alder reactions vide supra) and have described it as omniphilic dienophile [15] because of its high reactivity. 

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