AbstractDiazodiphenylmethane (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.