Chloro(4-methylpent-3-en-1-ynyl)carbene: IR spectrum, structure, photochemical transformations, and reactions with alkenes

A comprehensive study of previously unknown chloro(4-methylpent-3-en-1-ynyl)carbene generated by photolysis of newly synthesized 5-chloroethynyl-3,3-dimethyl-3 H -pyrazole was carried out both in low-temperature argon matrix and in solution at room temperature. Irradiation at 330 nm < λ < 380 nm causes the pyrazole to undergo a selective transformation to 1-chloro-3-diazo-5-methylhexa-4-en-1-yne. Photolysis of the latter at λ >520 nm leads to elimination of dinitrogen, thus producing the title carbene. Key structural parameters of this species were determined using matrix IR spectroscopy and quantum chemical calculations. It was established that the more stable state of the carbene is a singlet one. Further phototransformations of chloro(4-methylpent-3-en-1-ynyl)carbene lead to formation of isomeric 6-chloro-2-methylhexa-1,3-diene-5-yne. A preparative method for the synthesis of alkynylchlorocyclopropanes with yields up to 76% was proposed. It is based on photolysis of 5-chloroethynyl-3,3-dimethyl-3 H -pyrazole in benzene in the presence of excess amounts of various alkenes. The possibility of using this approach for selective cyclopropanation of double bonds in the presence of hydroxyl groups in the substrate molecule was demonstrated taking 3-methylbut-2-enol as an example.