New approaches to olefin cross-metathesis

New methodology for the selective cross-metathesis (Chl) of terminal olefin s employing ruthenium benzylidene 1 is described.(1) CM with symmetric inte rnal olefins was found to provide a useful means for homologating terminal olefins to protected allylic alcohols, amines, and eaters. Due to the limit ed commercial availability of symmetric internal olefins, a two-step CM pro cedure was developed in which terminal olefins were first homodimerized pri or to the CM reaction. Terminal olefins with allylic methyl substituents we re observed to provide CM products in diminished yield albeit with markedly improved trans-selectivity, Reaction rates were measured for CM reactions utilizing butenediol and allyl alcohol derivatives, and the results demonst rated distinct advantages in reaction rate and stereoselectivity for reacti ons employing the disubstituted olefins. In the course of studies of substr ates with allylic oxygen substituents, a new CM application was discovered involving the metathesis of acrolein acetal derivatives with terminal olefi ns. Acrolein acetals, including asymmetric variants derived from tartaric a cid, proved to be exceptionally robust and trans-selective CM substrates. I n related work, a pinacol-derived vinyl boronate was also found to be a rea ctive CM partner, providing a novel means for converting terminal olefins i nto precursors for the Suzuki coupling reaction.