An Experimental and Theoretical Evaluation of the Intramolecular Reactions of Cyclohexyne

Article abstract of DOI:10.1021/ja00252a028

The intramolecular reactions of cyclohexyne (1a) have been explored theoretically by using ab initio calculations at the Mp2/6-31G^* level and experimentally by examining the ring expansion of cyclopentylidenecarbene (2a) at elevated temperatures.The calculations indicate that 1a is more stable than 2a by 19.0 kcal/mol and that the barrier for 1a -> 2a is 26.4 kcal/mol.Carbene 2a can rearrange to bicyclo<3.1.0>hex-5-ene and thence to cyclohexadiene with a barrier of 38.9 kcal/mol.A higher energy reaction of 1a is cleavage to ethylene and butatriene in a retro-Diels-Alder reaction, which is calculated to have a barrier of 46.8 kcal/mol.This retro-Diels-Alder reaction is observed experimentally when 2a is generated by the pyrolysis of the cyclopentylidene adduct of Meldrum's acid.