Chiral dicationic bis(aqua) complexes [Ru(OH2)2(PNNP)]2+:: The effect of double chloride abstraction on asymmetric cyclopropanation

The dichloro complexes [RuCl2(PNNP)] (2a-c) undergo double chloride abstraction when treated with AgSbF6 (2 equiv) (PNNP is one of (1S,2S)-N,N'-bis[o-(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine, 1a; (1S,2S)-N,N'-bis[o-(bis(4-trifluoromethylphenyl)phosphino)benzylidene]cyclohexane-1,2-diamine, 1b; (1S,2S)-N,N'-bis[o-(bis(3,5-bis(trifluoromethyl)phenyl)phosphino)benzylidene]cyclohexane-1,2-diamine, 1c). The resulting elusive species form the corresponding bis(aqua) complexes [Ru(OH2)(2) (pNNp)](2+) by reaction with water. The bis(aqua) complexes [Ru(OH2)(2)(PNNP)](SbF6)(2) (6ac) were fully characterized, including an X-ray structure of 6c. The X-ray structure of 2c with the new electron-poor chiral tetradentate PNNP ligand 1c is also reported. Complexes 6a and 6c catalyze the cis-selective asymmetric cyclopropanation of styrene, albeit with low yield. The best cis:trans ratio and enantioselectivity were obtained with 6c (87:13 and 92% ee for the cis isomer, respectively). The products of double chloride abstraction from [RuCl2(PNNP)] gave moderate to high conversion of styrene (37-70%), but the yield of the cyclopropane product was generally modest (13-64%). The best cis- and enantioselectivity were 86:14 and 81% ee, respectively.