Theoretical Investigation of the Mechanisms and Stereoselectivities of Reductions of Acyclic Phosphine Oxides and Sulfides by Chlorosilanes

Abstract

Computational studies were performed to explain the highly varied stereoselectivities obtained in the reductions of acyclic phosphine oxides and sulfides by different chlorosilanes. The reductions of phosphine oxides by HSiCl(3), HSiCl(3)/Et(3)N, and Si(2)Cl(6) and the reductions of phosphine sulfides by Si(2)Cl(6) (all in benzene) were explored by means of B3LYP, B3LYP-D, and SCS-MP2 calculations. For the reductions of phosphine oxides by HSiCl(3), the calculations support the mechanism proposed by Horner in which a hydride is transferred from silicon to phosphorus through a four-centered, frontside transition state. This mechanism leads to retention of stereochemistry at phosphorus. For the other three reductions, two classes of mechanisms were explored. Phosphorane-based mechanisms that were previously proposed by Mislow and involve SiCl(3)(-) were compared with novel alternative mechanisms that involve nonionic rearrangement processes. In one of these, donor-stabilized SiCl(2) is formed as an intermediate. The calculations support a phosphorane-based mechanism for the reductions of phosphine oxides by HSiCl(3)/Et(3)N and Si(2)Cl(6) (which proceed with inversion) but favor the rearrangement pathways for the reductions of phosphine sulfides by Si(2)Cl(6) (which proceed with retention).

The Deoxygenation of Phosphine Oxides under Green Chemical Conditions

“…Cl 3 SiH is widely used in synthetic organic chemistry [21,22]. Stereochemical retention was also justified by theoretical calculations [23]. In the latter case, the P-configuration was preserved [22].…”

A study on the deoxygenation of trialkyl‐, dialkyl‐phenyl‐ and alkyl‐diphenyl phosphine oxides by hydrosilanes

“…Many silanes were described as reducing agents. Trichlorosilane is the most widespread reagent; however, its volatile (bp: 32°C) and corrosive properties mean disadvantage. For this, Cl 3 SiH is, in most cases, applied together with three equivalents of pyridine or triethylamine that has also an impact on the stereochemistry .…”

Chemoselective Reduction of Phosphine Oxides by 1,3‐Diphenyl‐Disiloxane

“…While decreasing the electron density of the silicon atom has been postulated to improve the rate of reduction, our disiloxane reagent would still not be as Lewis acidic as trichlorosilane. [13] The drastic differences between experimental and calculated activation energies suggests that DPDS may reduce phosphine oxides through an alternative mechanism than the ones presented by Horner for trichlorosilane and Marsi for phenylsilane. [6] We also believe that silanes are not simple reducing agents, as DPDS seems to react similarly to the combination of phenylsilane and a Brønsted acid catalyst.…”