ORGANOPHOSPHORUS COMPOUNDS .121. PHOSPHAALKYNES IN ENE REACTIONS WITHALKYLIDENECYCLOPROPANES AND ALLENES

Phosphaalkynes readily participate in ene reactions both with alkylide necyclopropanes and with allenes to furnish phosphaalkenes (the produc ts of a simple ene reaction) and phosphanes (the products of a tandem ene reaction). Thus, irrespective of the stoichiometry, the thermal re actions of the phosphaalkynes la-e with isopropylidenecyclopropane (8) or cyclopropylidenecyclopentane (11) proceed by a tandem ene process with retention of the three-membered ring unit to furnish the correspo nding bis(cyclopropyl)phosphanes 10a-c or 12. In contrast, with cyclop ropylidenecyclohexane (13) as the H-donor, the specific formation of e ither the monoadducts 14a,c or of the bisadducts 15a,c can be controll ed. A crystal structure analysis of 15a confirmed the structures of th e tandem ene products while the constitutions and configurations of th e novel P-cyclopropylphosphaalkenes 14a,c were unequivocally elucidate d by an X-ray crystal structure analysis of the corresponding W(CO)(5) -complexed system 16. Alkylated allenes such as 1,1-dimethylallene (17 ), vinylidene-cyclohexane (20), tetramethylallene (22), or 1,2-cyclono nadiene (24) undergo thermal addition by way of a double ene process t o afford the phosphaalkynes 1. In each case, regiospecific attack of t he phosphorus atom at the central C-atom of the cumulene system result s in the formation of the structurally unique phosphanes 21, 23a,b, an d 25a,b. On the other hand, thermal reactions of la,b in the presence of 1,1-diethylallene (26) come to a standstill after the first ene add ition and furnish the cross-conjugated phosphatrienes 27a,b as the fir st representatives of the previously unknown phospha[3]dendralene syst em. The thermally initiated tandem ene reactions of the phosphaalkynes 1 with propadiene (28) or tert-butylallene (30) each proceed through transfer of a vinylic hydrogen atom from the ene to the enophile to af ford the bis(propargyl)phosphanes 29a,b and 31, respectively, as addit ion products.