Optimization or Breakthrough? The First Highly cis- and Enantioselective Asymmetric Cyclopropanation of 1-Octene by “Electronic and Counterion” Tuning of [RuCl(PNNP)]+ Catalysts

Abstract

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 (3aSbF6 or 3bSbF6), 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 (3bSbF6) 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 3bSbF6 is also an excellent cis-selective cyclopropanation catalyst for styrene (96% ee for the cis isomer, 99:1 cis:trans ratio) and α-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 α-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).