Cycloadditions between 2H-Phospholes and Alkenes. Synthesis and Properties of 1-Phosphanorbornenes

Article abstract of DOI:10.1021/jo00281a013

2H-Phospholes, obtained by isomerization of 1H-phospholes at ca. 150 deg C, achieve <4 + 2> cycloadditions with disubstituted alkenes to yield 1-phosphanorbornenes.In the case of symmetrical (E)-alkenes, α-exo,β-endo compounds are formed predominantly or even exclusively.Spectral assignments of these compounds are checked by an X-ray structure determination.The 1-phosphanorbornenes formed by reacting 2H-phospholes and (Z)-alkenes undergo an epimerization and then give the same products as (E)-alkenes do.The study of the evolution of the reaction mixture shows that the <4 + 2> cycloaddition should proceed via a concerted mechanism followed by a cleavage of the newly formed P-C single bond.Regioselectivity of nonsymmetrical alkenes is controlled by the steric hindrance of the phosphole moiety.The transient phosphadiene exhibits a rather high reactivity toward dienophiles and the presence of an alcoholic function does not disturb the cycloaddition.