Expanding Deoxygenative Transformations of Alcohols by Phosphorus Compounds Through Geometric Deformation

Abstract

Tricoordinate phosphorus (σ³-P) compounds act as strong O-atom acceptors in many deoxygenative transformations by forming strong P=O bonds in phosphine oxides. While typical trigonal σ³-P compounds are nucleophilic, it has been demonstrated that lowering the C₃ᵥ-symmetry of σ³-P compounds to Cₛ-symmetry leads to enhanced electrophilicity of the P-center. Such electrophilic character of the nontrigonal σ³-P compounds has enabled unprecedented reactivity toward alcohols, forming hydridoalkoxyphosphoranes through a formal O–H oxidative addition. Hypothetically, these resulting phosphoranes can undergo deoxygenative reactions of alcohols via C–O cleavage driven by P=O formation through accessing alkoxyphosphonium and alkoxyphosphoranyl species. With the approach converting hydridoalkoxyphosphoranes into phosphine oxides, this thesis details the development of a new deoxygenative transformation of alcohols mediated by nontrigonal σ³-P compounds.

As a point of departure, the existing literature precedent involving C–O cleavage via alkoxyphosphonium and alkoxyphosphoranyl intermediates is reviewed in Chapter 1. Given that these intermediates can be potentially generated by cleaving P–H bond in hydridoalkoxyphosphoranes, understanding the factors controlling P–H thermochemistry is illustrated. In Chapter 2, proximal substituent effects on P–H thermochemical parameters (pKₐ, bond dissociation energy, and thermodynamic hydricity) are described. Specifically, it is shown that hydridophosphoranes derived from O–H oxidative addition to a nontrigonal σ³-P compound with an O,N,O-chelate results in an acidic P–H bond; by complement, hydridophosphoranes derived from O–H oxidative addition to a nontrigonal σ³-P compound with N,N,N-chelate affords an hydridic P–H moiety. In Chapter 3, ancillary substituent effects on the stability of hydridophosphoranes is shown. By introducing an ethylene linker in the ligand of a nontrigonal σ³-P compound, the (σ5-P)→(σ³-P) tautomerism is considerably suppressed upon E–H bond (E = OR, OOCR, and SR) oxidative addition without significant changes in the geometry and electronic structure of the nontrigonal P-compound. Employing the knowledge gained from the aforementioned fundamental studies, in Chapter 4, the development of a new deoxyfluorination of alcohols via alkoxyphosphonium intermediates is detailed. This method has shown that access to alkoxyphosphonium cations sufficiently activates alcohols to undergo C–O cleavage. Combined with the use of fluoroborate as a fluoride donor, a nontrigonal σ³-P compound has enabled fluorination of tertiary and secondary alcohols with stereoinversion.