TUNING OF VINYLBORANE DIENOPHILICITY - OPTIMIZATION OF REACTIVITY, REGIOSELECTIVITY, ENDOSTEREOSELECTIVITY, AND REAGENT STABILITY

Simple syntheses of some vinylboranes are reported, and their properties in Diels-Alder reactions are compared. Vinyl-3,6-dimethylborepane was the most stable simple vinylborane examined, and appears to be indefinitely stable at 25-degrees-C. Surprisingly, trivinylborane is the most reactive, and reacts about 18 times faster than the vinyldialkylboranes with cyclopentadiene. Vinyl-9-BBN is the most regioselective dienophile, in keeping with principally steric control of regioselectivity in Diels-Alder reactions of vinylboranes. All the dienophiles display high endo-stereoselectivity with piperylene, but vinyl-3,6-dimethylborepane displays significantly improved stereoselectivity with cyclopentadiene. In general, by choice of alkyl-substituents on boron, the reactivity, regioselectivity, endo-stereoselectivity, and stability of vinylporanes can be optimized.