Design and transformation of versatile ruthenium catalysts

Abstract

Ruthenium metathesis catalyst RuCl2(PCy3) 2(CHPh) (1a) was transformed into hydrogenation-active species via hydrogenolysis of the alkylidene ligand. Several different hydride products were identified, including novel dihydrogen complex RUCl2(H 2)(PCy3)2 (3), and hydridochloro species RuHCl(H2)(PCy3)2 (4). In related work, an optimized protocol for tandem ring-opening metathesis polymerization (ROMP)-hydrogenation was developed. Thus, hydrogenation of ROMP derived poly(octene) was accomplished under the very mild conditions of 1 atm H2 and 60°C. Strategic regeneration of a metathesis catalyst following hydrogenation allowed implementation of a "double tandem" sequence of ROMP-hydrogenation-ROMP, whereby bimodal polymer blends were generated in a one-pot process starting from a single catalyst precursor. The potential of perfluorophenoxide ligands for design of metathesis catalysts was investigated via synthesis of vinylidene complex Ru(OC 6F5)2(=C=CHBut)(dcypb) (18), where dcypb = bis(dicyclohexyl)-1,4-phosphinobutane. Protonolysis of the perfluorophenoxide ligands in 18 by HCl gave the dichloro analogue [{Ru(=C=CHBut)(dcypb)}(mu-Cl) 3]Cl (19•Cl). The structure of 19•Cl illustrates a common motif in Ru chemistry: the tendency to form trichloride-bridged dimers. By contrast, replacement of chloride ligands in 19•Cl by perfluorophenoxides yields a stable mononuclear complex (18). While both 18 and 19•Cl catalyze the ROMP of norbornene, preliminary results suggest that the mononuclear 18 is a more efficient initiator than 19•Cl. Replacement of each vinylidene in 19•Cl by a CO ligand gave [{Ru(CO)(dcypb)}(mu-Cl)3]Cl (21a), which is a direct structural analogue of 19•Cl. While 21a is stable in air, reaction with hydride reagent KBHSBu 3 gave a highly reactive trihydride K[RuH3(CO)(dcypb)] ( 28). Complex 28 was found to be moderately active for Murai catalysis and extremely active for the hydrogenation of benzophenone. A catalytic intermediate in the hydrogenation chemistry was isolated and identified as orthometallated RuH[o-C(O)(Ph)C6H4](CO)(dcypb) (30). Interestingly, the Murai activity of complex 30 is lower than that of 28, suggesting that the orthometallated species is not on the reaction path for the coupling reaction. The potentially stabilizing effect of a chelating iminopyrrolato ligand was investigated via synthesis of novel metathesis catalysts RuCl(NN ')(PCy3)(CHPh) (34) and RuCl(NN ')(py)2(CHPh) (35), where NN ' is (2, 6-iPr2-C 6H3N=CH)-2-C4H3N-. The activity of 35 for RCM was found to be very low. However, phosphine derivative 34 is an extremely robust metathesis catalyst, capable of effecting RCM in air in non-distilled, non-degassed solvent. (Abstract shortened by UMI.)