Organic reducing agents featuring iminophosphorano substituents
University of New Brunswick
A new family of potent organic reducing agents incorporating two and four strongly π-donating iminophosphorano (−N=PR3; R= Ph, Cy) groups on a bispyridinylidene skeleton have been synthesized in moderate to high yield, via simple deprotonation of the respective pyridinium ion precursors with KHMDS, and characterized by 1H, 13C and 31P NMR spectroscopies. Electrochemical studies on the electron donors featuring two π-donating substituents show that the reduction potentials of the redox couples range from −1.17 to −1.51 V vs SCE with bispyridinylidenes featuring the tricyclohexyliminophosphorano group emerging as the strongest of the donors synthesized. The redox properties of these compounds are sensitive not only to the substituent (R) on phosphorus but also on the position of the substituents on the pyridine ring. Furthermore, the C4 position of the pyridine ring also showed enhanced electron donating abilities over the C2 position with propylene-bridged bispyridinylidene derived from the former being 130 mV better than the one derived from the latter. Methyl groups on the pyridyl nitrogen lead to slightly stronger donors than those featuring a propylene-bridge between the two pyridyl nitrogen. The N-methyl derived donors with two triphenyliminophosphorano groups effectively reduced 1-bromonaphthalene to naphthalene under milder reaction conditions than could be achieved with the dimethylamino derivative. This experiment demonstrates that the superior reduction potential, which results from the superior π-donating properties, allows for more mild reaction conditions. A N, N’-dimethyl-derived bispyridinylidene featuring four triphenyliminophosphorano substituents was synthesized via simple deprotonation in moderate yield with the redox couple shown by cyclic voltammetry to have a significantly low reduction potential (−1.70 V vs SCE), thereby making it the strongest ground state organic electron donor yet synthesized. This donor, which can be generated in situ, was used to reduce a variety of aryl halides, and was found to be the first organic donor to be effective in the thermally-induced reductive S-N bond cleavage of N, N-dialkylsulfonamides, and reductive decyanation of malononitriles.