Metallacarboranes in Catalysis. 8. I: Catalytic Hydrogenolysis of Alkenyl Acetates. II: Catalytic Alkene Isomerization and Hydrogenation Revisited

Article abstract of DOI:10.1021/ja00201a031

Part I of this study describes the facile hydrogenolysis (and deuteriolysis) of alkenyl acetates, such as isopropenyl acetate (D) and 1-phenylvinyl acetate (E), with rhodacarborane catalyst precursors to yield acetic acid and the corresponding alkene.The catalyst precursors employed were (I), (II), and (III).The hydrogenolysis of alkenyl acetates D and E produced propene and styrene, respectively, along with acetic acid in essentially quantitativeyields.Deuterium at Rh was demonstrated not to enter the hydrogenolysis reaction.Use of D2 as the reducing agent with I and E resulted in the incorporation of deuterium into reactant E and products.Styrene produced in these reactions was predominantly d1 with appreciable quantities of d0 and d2.Ethylbenzene, a byproduct resulting from the hydrogenation of styrene, contained only traces of d0 species and was largely d1, d2, and d3.The acetic acid formed in these reactions was isotopically pure CH3COOD.The rate law for E hydrogenolysis with I contained no term showing hydrogen dependence.These results suggest a reaction mechanism for hydrogenolysis that is based upon the relatively slow formation and decomposition of a very reactive rhodium(III) monohydride formed through the regioselective oxidative addition of Rh(I) (in the exo-nido tautomer of the rhodacarborane) to terminal B-H bonds.The monohydride produced in this fashion then enters a cyclic heterolysis process with H2 which leads to rapid product formation.This mechanism suggests that slow B-D/C-H exchange should occur between I-d9 (B-D at all vertices of I) and anisotopically normal alkane, such as 1-hexene (B), during alkene isomerization.Such exchange was observed and shown to be regioselective.This new information predicated part II of this study, which is devoted to a modification of previously advanced proposals for the mechanisms of alkene isomerization and hydrogenation with rhodacarborane precursors.The facile and regioselective exchange of B-H in (IV) with D2 was examined and shown to be electrophilic in character and to apparently proceed through very reactive monohydride intermediates.These new data coupled with previously reported results allow the formulation of unified mechanisms for B-H/D2 and B-H/C-D exchange, alkenyl acetate hydrogenolysis, alkene isomerization, and alkene hydrogenation based upon the key B-Rh(III)-H species formed by the regioselective oxidative addition of terminal B-H bonds to Rh(I) centers.Thus, the effective catalytic sites in all of these reactions appear to be an array of B-Rh(III)-H centers formed reversibly from the Rh(I) present in exo-nido-rhodacarborane tautomers which are, in turn, in equilibrium ...