The quantification and exploitation of the strong π-donor ability of iminophosphorano substituents

Abstract

This study was focused on the preparation of new iminophosphorano substituted bispyridinylidenes (and related bispyridinium ions) as potent organic electron donors and new iminophosphorano substituted pyridines as potential powerful acylation catalysts. Correlations for the determination and prediction of the substituent constants of iminophosphorano groups has also been developed. New iminophosphorano substituted bipyridinium ions featuring phenoxy, 4-methylphenoxy, and pyrrolidinyl groups on the phosphorus were found to have redox potentials of -1.13, -1.14, and -1.50 V vs SCE, respectively. The tris(pyrrolidinyl)iminophosphorano derivative was found to be on par in terms of reducing power with the previously prepared tricyclohexyliminophosphorano derivative (-1.51 V), which is the strongest 4,4’-substituted bispyridinylidene based organic electron donor developed to date. The catalytic activity of iminophosphorano substituted pyridines towards the acylation of 1-ethynlcyclohexanol was assessed, with iminophosphorano substituted (R3P=N-) pyridines with R= O(p-tolyl), Ph, N(CH2)4, and Cy being found to catalyze the acylation of 1-ethynylcyclohexanol to 50% conversion in 798.9, 27.6, 7.6 and 5.7 minutes, respectively, making the tris(pyrrolidinyl)iminophosphorano and tricyclohexyliminophosphorano substituted pyridines the most active pyridine based acylation catalysts developed to date. By establishing a relationship between Hammett’s σp+ values and the redox potential of bipyridinium ions it was possible to determine the σp+ values of iminophosphorano substituents from the redox potentials of iminophosphorano substituted iii bipyridinium ions, with the substituent constants of iminophosphorano substituents (–N=PR3), with R= OPh, O(p-tolyl), Ph, p-tolyl, N(CH2)4, and Cy being -1.30, -1.33, -1.82, -1.92, -2.20, and -2.23, respectively. It was found that iminophosphorano substituents with R= N(CH2)4, and Cy are the strongest neutral π-donating substituents reported to date. Two other linear correlations were found, which utilized the Tolman electronic parameter (TEP) of the phosphine component of the R3P=N- substituent, with the correlations being: TEP and the Hammett constants (σp+) as well as TEP and the redox potentials of iminophosphorano substituted bipyridinium ions. These can be used for the prediction of new σp+ values and redox potentials for iminophosphorano substituents and bispyridinylidenes, respectively.