Configurational Stability of 2,7-Dihydro-1-Phenyl-Phosphepine-Oxides and Electronic Properties of the Phosphanes

The configurational stability of four chiral 2,7-dihydro-1-phenyl-phosphepine-oxides 1a-4a (Figure 1), differing for the nature of the aromatic rings (carbocyclic or heterocyclic, six- or five-membered) constituting the stereogenic scaffold have been investigated. Unknown compound 4a was fully characterized by single crystal X-ray analysis. The tendency to enantiomerize has been determined by circular dichroism and on-column enantioselective HPLC methods. Biphenine-oxide 1a was found the only unstable compound at room temperature, displaying an enantiomerization barrier of about 20 kcal mol-1. Also the corresponding phosphanes 1b-4b have been considered in order to perform a comparative evaluation of their electronic availability, which is a crucial parameter affecting both the catalytic activity and stereoselection ability of their metal complexes. Unknown bithienophosphepine 4b was fully characterized by single crystal X-ray analysis. The quantitative evaluation of the electronic properties was performed by voltammetry, using the electrochemical oxidative peak potential as probe: the higher the value, the electron-poorer the phosphane. The electronic availability was found to decrease along to the sequence: 2b, 4b, 3b, 1b.