METALLACYCLES WITH STEREOGENIC METAL CENTERS - SYNTHESIS AND CHARACTERIZATION OF DIASTEREOMERIC CYCLORUTHENATED CHIRAL AMINES

The transmetalation reaction of enantiomerically pure (R)(C)- or (S)(C)-(HgCl[C6H4CH(Me)NMe(2)]} with [(eta(6)-arene)RuCl2](2) dimers, where eta(6)-arene (toluene, 2), and 1-Me-4-Pr-i-C6H4 (cymene, 3), in CH3CN at ambient temperatures forms in each case a mixture of two diastereomeric ruthenacycles [(R)(C)(S)(Ru)-a, major, and (R)(C)(R)(Ru)-a', minor, or (S)(C)(R)(Ru)-b', major, and (S)(C)(S)(Ru)-b', minor] of the type ((eta(6)-arene)RuCl[C6H4CH(Me)NMe(2)]} (1a-a', 2a-a',3a-a' or 1b-b', 2b-b', 3b'-b) in good chemical and optical yields. The diastereoselectivity of these reactions, which molecular mechanics calculations suggest to be under kinetic control, varies as a function of the nature of the arene ligand (de is the diastereomeric excess): benzene(90% de) > toluene (87% de) > cymene (67% de). These complexes are characterized in solution by a variety of spectroscopic techniques (H-1- and C-13-NMR, UV, CD) and in the solid-state by single-crystal X-ray crystallography and far-IR. X-ray crystallographic studies, carried out on 1b', 2b', and 3a, establish the absolute configuration at the chiral Ru center in each. Both 1b' and 2b' crystallize in the orthorhombic space group P2(1)2(1)2(1), Z = 4, but 3a crystallizes in the monoclinic space group P2(1), Z = 2. Unit cell parameters for: 1b': a = 12.886(3) Angstrom, b = 17.254(5) Angstrom, c = 6.726(2) Angstrom, V = 1495.4 Angstrom(3). For 2b': a 12.523(3) Angstrom, b = 18.679(5) Angstrom, c 6.765(2), V = 1582.6 Angstrom(3). For 3a: a = 14.050(4) Angstrom, b = 6.633(1) Angstrom, c = 10.131(3), V = 942.5 Angstrom(3). The latter data show that of the two possible diastereomeric envelope conformations of the five-membered chelate ring in each compound only the one with a minimum C-CH3 ...eta(6)-arene interaction is formed. NMR (CDCl3) studies show that the Ru-N bond is preserved in solution, leading to configurationally-stable (at Ru), optically-active species. Moreover, H-1 difference NOE spectroscopy reveals the existence of a dynamic equilibrium between the two limiting conformations of the five-membered chelate ring in each diastereomeric ruthenacycle.