Transition-metal-promoted reactions of boron hydrides. 16. Platinum- and palladium-catalyzed olefin addition and olefin dehydrogenative borylation reactions of arachno-6,8-C2B7H13: Syntheses and structural characterizations of 7-R-arachno-6,8-C2B7H12 and 7-(trans-R-CH=CH)-arachno-6,8-C2B7H12

The chloroplatinic acid or platinum(II) bromide-catalyzed reaction of arach no-6,8-C2B7H13 with ethylene results in hydroboration to yield the single p roduct 7-(C2H5)-arachno-6,8-C2B7H12. Analogous reactions with l-pentene or styrene yield a mixture of both hydroboration, 7-R-arachno-6,8-C2B7H12, and dehydrogenative borylation, 7-(trans-R-CH=CH)-arachno-6,8-C2B7H12, product s. On the other hand, the palladium(II) bromide-catalyzed reaction of arach no-6,8-C2B7H13 with ethylene yields predominantly the dehydrogenative boryl ation product (7-(CH2=CH)-arachno-6,8-C2B7H12 along with smaller amounts of 7-(C2H5)-arachno-6,8-C2B7H12. Palladium(II) bromide-catalyzed reactions wi th l-pentene or styrene result in only dehydrogenative borylation to produc e 7-(trans-R-CH=CH)-arachno-6,8-C2B7H12. The hydroboration and dehydrogenat ive borylation products observed in the platinum- and palladium-catalyzed r eactions are related to those that have been observed in recently reported metal-catalyzed hydrosilations and catecholborane or pinacolborane hydrobor ations and are consistent with a reaction mechanism involving competitive h ydride-migration/reductive-elimination and boryl-migration/beta -hydride-el imination steps. Crystallographic and DFT computational studies of the alke nyl-carbaboranes have also revealed unusual cage-bonding features that sugg est pi -bonding interactions of the B7 cage boron with its olefinic substit uent.