Complexation of a Singlet Carbene by a Fluoroalkane. Modification of 1:CHCO2Et Selectivity in Perfluorohexane Solution and ab Initio Calculations

Article abstract of DOI:10.1021/jo00090a020

We have compared product distribution from reactions of ¹:CH2 with 2,5-dimethylhexane and ¹:CHCO2Et with 2,3-dimethylbutane in alkane vs 1:1 pentane-perfluorohexane solution.A solvent effect was observed in the reaction of ¹:CHCO2Et, but not ¹:CH2.The preference for tertiary/primary C-H insertion by ¹:CHCO2Et is 3.97 +/- 0.04 in 2,3-dimethylbutane solution and 4.48 +/- 0.09 in 1:1 pentane-perfluorohexane solution.There is no change in the stereospecificity of ¹:CHCO2Et addition to cis-2-pentene in pentane vs 1:1 pentane-perfluorohexane solution, indicating that the observed change in selectivity is probably not due to intersystem crossing to ³:CHCO2Et.Ab initio calculations provide evidence for a possible origin of the solvent effect via formation of carbene-fluoroalkane complexes.At the RHF/3-21G level, ¹:CHCO2Me forms bound complexes with CH3F and CF4 with binding energies ranging from 5.7-19.6 kcal/mol.These complexes are 1.3-1.7 times more strongly bound than complexes formed between ¹:CH2 and the same fluoroalkanes.