Experimental and Computational Evidence Indicating That the Initial Carbene Conformation Is Product Determining in the Reactions of Cyclopropylmethylene

Article abstract of DOI:10.1021/jo00297a057

The decomposition of cyclopropyldiazomethane, 2, to generate cyclopropylmethylene, 1, was carried out from 573 to 773 K under flash vacuum pyrolysis (FVP) conditions and from 77 to 303 K photolytically in hydrocarbon solvents.An examination of the ratio of ring-expanded products (RE, butadiene and cyclobutene) to those of cleavage (C, ethylene and acetylene) as a function of temperature leads to the conclusion that RE is favored over C by 2.3 kcal/mol in the FVP of 2.Ab initio calculations (MP2/6-31G*//3-21G) predict that decomposition of 2 to give the conformer of 1in which the ring methyne hydrogen and the α-hydrogen are cis (1a) is more favorable than formation of the trans conformer (1b) by 1.1 kcal/mol.The calculated activation parameters for the formation of 1a and 1b have been combined with those calculated previously for the reactions of 1a and 1b, which indicate the RE is favored from 1a while 1b gives C, to give theoretical values of RE:C as a function of temperature which lead to a computed difference in E_a between RE and C of 2.7 kcal/mol, in good agreement with the 2.3 kcal observed experimentally.Photolysis of 2 gives a higher ratio of C to RE than does FVP.This may be a consequence of the fact that a more energetic carbene is produced in the photolysis than in the thermolysis.