Synthesis, structure determination and immobilization of some dirhodium complexes with chiral binding thiolato ligands. Investigation of their catalytic activity for enantioselective hydrogenation

Abstract

(−)- cis[Dicarbonyl-μ-chloro-[μ-6,6-dimethylbicyclo[3.1.1]heptane-2-methanethiolato- S:S)]]bis[tris(1,1-dimethylethyl)phosphine]dirhodium ( 5a) and (+)- cis-[dicarbonyl-μ-chloro-[μ-5β-methyl-2α-(1-methyl-ethyl)cyclohexanethiolato- S:S]]bis[tris(1,1-dimethylethyl)phosphine]dirhodium ( 5b) were prepared from [Rh(CO) 2] 2(μ-Cl) 2, P(t-Bu) 3 and the corresponding sulfides (−)- cis-(myrtanethio)trimethylsilane ( 4a) and (+)-(neomenthanethio)trimethylsilane ( 4b). The molecular structures of 5a and 5b were determined by single-crystal X-ray diffraction ( 5a: C222 1, a = 13.749(4), b = 23.509(9), c = 27.271(9) Å, Z = 8, R = 0.0443, R w = 0.0491. 5b: P 1 , a = 16.209(6), b = 14.150(5), c=9.899(3) Å, Z = 2, R = 0.058, R w = 0.094). Complex 5b was found to exist in the crystal as a pair of 1 R,2R,5S- and 1S,2R,5S-epimers. Both chiral complexes have been immobilized by attachment to divinylbenzene-crosslinked polystyrene resins. Application of the chiral dirhodium complexes as catalysts for hydrogenation of methyl α-acetamidocinnamate revealed that while 5b leads to optically active N-acetylphenylalanine methyl ester (up to 50% ee) 5a gives only the racemic product. The immobilization of the complexes proved to improve the enantioselectivity of 5a but decreases the ability of 5b to induce asymmetric reduction.