Metallacarboranes in Catalysis. 7. Kinetics and Mechanism of Acrylate Ester Hydrogenation Catalyzed by closo-Rhodacarboranes

Article abstract of DOI:10.1021/ja00336a025

In a search for reactions catalyzed by rhodacarborane clusters, the species I, , II, , III, , and IV, , were examined as catalyst precursors in the hydrogenation of 1-butyl acrylate (C) in THF solution at 40.8 deg C.Only catalyst precursors I and III gave results that were free of complications and suitable for detailed kinetic analyses.Both I and III reversibly hydridometalate C to produce the chelated closo adducts VI and VII, respectively, and an equivalent quantity of PPh3.The slow attainment of equilibria in these hydrometalation reactions accounts for the appearance of a lengthy induction period at the outset of C hydrogenations which employed either I or III as catalyst precursor.The rate law for C hydrogenation using I or III was elucidated and found to be identical in form with that previously observed using closo- or exo-nido-rhodacarboranes in 3-metyl-3-phenylbutene-1 (A) hydrogenation.As in the case of A, it was shown by deuterium labeling that C hydrogenation did not involve the hydride ligand at the RhH vertex of either I of III.The BH vertices of these same species were similarly shown to not be involved.Reduction of C with D2 using I as the catalyst source gave a moderate amount of D scrambling into recovered C and produced 1-butyl propionate-d₀-d₄.The above results led to a proposed catalytic cycle that culminates in the slow, but probably not rate-limiting, elimination of 1-butyl propionate from a reversibly formed exo-nido alkyl hydride intermediate.These kinetic characteristics may have their origin in the weak electron donor properties of the chelated ^- ligands that are attached to Rh(1+) or Rh(3+) in exo-nido intermediates by a pair of B-H-Rh three-center bonds.