Enhancing the dienic reactivity of phospholes: An improved access to trivalent 7-phosphanorbornenes

A structural comparison of 1-cyano- and 1-alkoxy-3,4-dimethylphospholes wit h 1-benzylphosphole has led to some unexpected conclusions. There is no uni vocal relationship between phosphole aromaticity and pyramidality at phosph orus. It has been found that both the highly pyramidal 1-cyanophosphole 1 ( Sigma (CPC angles)= 290 degrees), and the much less pyramidal 1-alkoxyphosp hole 6 (Sigma (CPC angles) = 310 degrees) have a low Bird aromaticity index (27 for both molecules), when compared to 1-benzylphosphole (Sigma (CPC an gles) = 303 degrees, BI = 35.5). This low aromaticity is correlated with a high reactivity of the diene in both 1 and 7 (similar to 6) toward acryloni trile. Good stereochemical control is observed with 7, which gives exclusiv ely the anti,endo [4 + 2] cycloadducts with acrylonitrile and diethyl vinyl phosphonate.