Optimization or breakthrough?: The first highly cis- and enantioselective asymmetric cyclopropanation of 1-octene by electronic and counterion tuning of [RuCl(PNNP)]+ catalysts

The dichloro complexes [RuCl2(1a)] (2a) and [RuCl2(1b)] (2b) with the chiral tetradentate PNNP ligands (1S,2S)-N,N'-bis[o-(diphenylphosphino)benzylidene]cyclohexane-1,2-diamine (1a) and (1S,2S)-N,N'-bis[o-(bis(4-trifluoromethylphenyl)phosphino)benzylidene]cyclohexane-1,2-diamine (1b) react with AgSbF6 (1 equiv) to give the highly reactive five-coordinate complexes [RuCl(PNNP)]SbF6 (3aSbF(6) or 3bSbF(6)), which were characterized in solution. The reaction. of 2a or 2b with AgBF4 gives [RuCl(PNNP)]BF4 along with unidentified products. Complex [RuCl(1b)]SbF6 (3bSbF(6)) cyclopropanates 1-octene to ethyl 2-hexylcyclopropane-1-carboxylate (up to 66% overall isolated yield) in the presence of ethyl diazoacetate with unprecedented cis-diastereoselectivity (85:15 cis:trans ratio) and with excellent enantioselectivity (up to 99 and 98% ee for the cis and trans isomers, respectively). Complex 3bSbF(6) is also an excellent cis-selective cyclopropanation catalyst for styrene (96% ee for the cis isomer, 99:1 cis:trans ratio) and alpha-methylstyrene (96% ee for the cis isomer, 92:8 cis:trans ratio). The catalyst activity depends on the counterion and increases in the order [BF4](-) < [SbF6](-) < [PF6](-), whereas the best stereoselectivity is observed with [SbF6](-). Molecular modeling calculations are extended to 1-octene and alpha-methylstyrene and revised to account for the absolute configuration of ethyl cis-2-phenylcyclopropane-1-carboxylate (1S,2R, incorrectly reported as 1R,2S in a former paper).