Diazodiphenylmethane and Monosubstituted Butadienes: Kinetics and a New Chapter of Vinylcyclopropane Chemistry

Diazodiphenylmethane (DDM) undergoes cycloadditions to 1-substituted buta-1,3-dienes exclusively at the C(3)C(4) bond. At room temperature, the N2 loss from the initially formed 4,5-dihydro-3H-pyrazoles 2 is faster than the cycloaddition and furnishes the vinylcyclopropane derivatives 7 and 9 with structural retention at the C(1)C(2) bond. 2-Substituted butadienes react with DDM at the C(3)C(4) bond to give 12; isoprene, however, affords 3,4/1,2 products in the ratio of 86 : 14. DDM is a nucleophilic 1,3-dipole: 1-Cyanobutadiene reacts 400 times faster than 1-methoxybuta-1,3-diene (DMF, 40°). The log k2 for the additions to six 1-substituted butadienes show a linear correlation with σp (Hammett) and ϱ=+2.9; the log k2 of five 2-substituted butadienes are linearly related to Taft's σI (ϱ=+1.7). The structures of the vinylcyclopropanes 7, 9, and 12 are established by NMR spectra and oxidation. A cyclopropyl carbinyl cation is made responsible for the isomerization of 12, R=Ph, Me, by acetic acid to 4-substituted 1,1-diphenylpenta-1,3-dienes 25 and 29; TsOH at 200° converts 25 further to 9,10-dihydro-9-methyl-10-phenyl-9,10-ethanoanthracene (27). Thermal rearrangement of 7, 9, and 12 at 200–300° produces the 3- or 1-substituted 4,4-diphenylcyclopentenes 30 and 31. These give the same mass spectra as the vinylcyclopropanes, and an open-chain distonic radical cation is suggested as common intermediate. Besides spectroscopic evidence for the cyclopentene structures, hydrogenation and epoxidation are described; NMR data support the trans-attack by perbenzoic acid.