Biphilic Organophosphorus Catalysis: Regioselective Reductive Transposition of Allylic Bromides via P(III)/P(V) Redox Cycling

Abstract

We report that a regioselective reductive transposition of primary allylic bromides is catalyzed by a biphilic organophosphorus (phosphetane) catalyst. Spectroscopic evidence supports the formation of a pentacoordinate (σ(5)-P) hydridophosphorane as a key reactive intermediate. Kinetics experiments and computational modeling are consistent with a unimolecular decomposition of the σ(5)-P hydridophosphorane via a concerted cyclic transition structure that delivers the observed allylic transposition and completes a novel P(III)/P(V) redox catalytic cycle. These results broaden the growing repertoire of reactions catalyzed within the P(III)/P(V) redox couple and suggest additional opportunities for organophosphorus catalysis in a biphilic mode.

Figure 1

Modes of organophosphorus catalysis.

Figure 2

In situ ³¹P NMR spectra for the reaction shown in eq 5. (A) Initial spectrum of phosphetanium 12. (B) Spectrum recorded at −70 °C immediately following addition of LiAlH₄ showing formation of 17. (C) Spectrum recorded after warming reaction to −30 °C showing formation of 10. Units are ppm relative to 85% H₃PO₄ external standard.

Figure 3

Computational modeling of the conversion σ⁵-P → σ³-P [M06-2X/6-311++G(2d,2p)]. (A) Enthalpy (free energy) for pathway in kcal/mol. (B) Optimized σ⁵-P hydridophosphorane structure 21. (C) Optimized transition state structure TS.