DIELS-ALDER REACTIONS OF TRICHLOROPHOSPHAETHENE

The Diels-Alder reactions of trichlorophosphaethene (3), generated in situ from dichloro(dichloromethyl)phosphine (2), with 1-vinylnaphthale ne (6), 1-(1-methylethenyl)cyclohexene (4), and 1-[1-(trimethylsiloxy) ethenyl]cyclohexene (13) at 60-75 degrees C, are described. The reacti ons of 3 with 4 and 6 afforded, after aromatization of the primary Die ls-Alder adducts 5 and 8 under the influence of triethylamine, the 2-c hlorophosphinines 9 and 11, respectively. The reaction with 13, howeve r, led to the formation of the ''double adduct'' 16, which could not b e isolated in pure form. The formation of the ''double adduct'' is exp lained by a subsequent Diels-Alder reaction of 13 with dihydrophosphin ine 15, which is formed after HCl elimination from the primary Diels-A lder adduct 14. The reaction of 13 with 23, the pentacarbonyltungsten complex of 2, furnished 27, the pentacarbonyltungsten complex of 16, w hich was isolated in pure form. The regiochemistry of the Diels-Alder reactions described above is discussed on the basis of MNDO/PM3 calcul ations of the frontier molecular orbital coefficients. The Diels-Alder reactions proceed with normal electron demand, and the experimentally observed regiochemistry is in accordance with theoretical predictions . The formation of the ''double adducts'' 16 and 27 is rationalized as a consequence of the high HOMO energy of 13 compared with that of 4.