Organophosphorus Compounds, 121. Phosphaalkynes in Ene Reactions with Alkylidenecyclopropanes and Allenes

Phosphaalkynes readily participate in ene reactions both with alkylidenecyclopropanes and with allenes to furnish phosphaalkenes (the products of a simple ene reaction) and phosphanes (the products of a tandem ene reaction). Thus, irrespective of the stoichiometry, the thermal reactions of the phosphaalkynes 1a‐c with isopropylidenecyclopropane ( 8 ) or cyclopropylidenecyclopentane ( 11 ) proceed by a tandem ene process with retention of the three‐membered ring unit to furnish the corresponding bis(cyclopropyl)phosphanes 10a‐c or 12 . In contrast, with cyclopropylidenecyclohexane ( 13 ) as the H ‐donor, the specific formation of either the monoadducts 14a,c or of the bisadducts 15a,c can be controlled. A crystal structure analysis of 15a confirmed the structures of the tandem ene products while the constitutions and configurations of the novel P ‐cyclopropylphosphaalkenes 14a,c were unequivocally elucidated by an X‐ray crystal structure analysis of the corresponding, W(CO) 5 ‐complexed system 16 . Alkylated allenes such as 1,1‐dimethylallene ( 17 ), vinylidenecyclohexane ( 20 ), tetramethylallene ( 22 ), or 1,2‐cyclononadiene ( 24 ) undergo thermal addition by way of a double ene process to afford the phosphaalkynes 1 . In each case, regiospecific attack of the phosphorus atom at the central C‐atom of the cumulene system results in the formation of the structurally unique phosphanes 21, 23a,b , and 25a,b . On the other hand, thermal reactions of 1a,b in the presence of 1,1‐diethylallene ( 26 ) come to a standstill after the first ene addition and furnish the cross‐conjugated phosphatrienes 27a,b as the first representatives of the previously unknown phospha[3]dendralene system. 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 afford the bis(propargyl)phosphanes 29a,b and 31 , respectively, as addition products.