Reactions of Alkynyldihaloboranes with 1,3-Dienes. 1,4-Alkynylborations and Stepwise Diels-Alder Reactions.
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Alkynyldihaloboranes 1-6 are readily generated in situ from the boron-tin exchange reaction of BCl(3) or BBr(3) with the corresponding alkynylstannanes. The Diels-Alder reactions of 1-4 with isoprene in hexanes proceed rapidly at 25 degrees C, affording 1,4-cyclohexadiene products in high yield with high regioselectivity. Reactions carried out in CH(2)Cl(2) exhibited an alternative product that results from the formal 1,4-alkynylboration of the diene. The alkynylboration intermediates can undergo further conversion to the Diels-Alder adducts under the reaction conditions. The mechanism of these reactions is discussed.