Selective Enzymatic Oxidation of Silanes to Silanols

Abstract

Compared to the biological world's rich chemistry for functionalizing carbon, enzymatic transformations of the heavier homologue silicon are rare. We report that a wild-type cytochrome P450 monooxygenase (P450_BM3 from Bacillus megaterium, CYP102A1) has promiscuous activity for oxidation of hydrosilanes to give silanols. Directed evolution was applied to enhance this non-native activity and create a highly efficient catalyst for selective silane oxidation under mild conditions with oxygen as the terminal oxidant. The evolved enzyme leaves C-H bonds present in the silane substrates untouched, and this biotransformation does not lead to disiloxane formation, a common problem in silanol syntheses. Computational studies reveal that catalysis proceeds through hydrogen atom abstraction followed by radical rebound, as observed in the native C-H hydroxylation mechanism of the P450 enzyme. This enzymatic silane oxidation extends nature's impressive catalytic repertoire.

Keywords: P450 enzymes; biocatalysis; directed evolution; monooxygenation; silanols.

Figure 1.

Activity of P450_BM3 variants in whole cells (green) or lysate (blue) over the course of directed evolution and under optimized conditions (red), given as total turnover number for silanol production. See the Supporting Information for experimental details. B) Structure of wild-type cytochrome P450_BM3 (PDB:1JPZ)^[22] showing bound heme cofactor. Amino acid residues mutated during evolution are highlighted in orange (A = alanine, F = phenylalanine, L = leucine; NADPH = nicotinamide adenine dinucleotide phosphate).

Figure 2.

A) Computed mechanism for Fe^IV–oxene-catalyzed oxidation of 1a to 2a. B) DFT-optimized, lowest energy and rate-determining H atom abstraction, transition state TS1 and radical intermediate INT1 (quartet electronic state). The spin density localized at the Si atom (ρ_spin(Si)) in INT1 is shown. C) The endergonic (ΔG = 10.2 kcal·mol^–1) electron transfer for the silyl cation formation pathway reinforces the conclusion that the radical oxidation pathway is the most plausible. Key distances are given in Å and angles in degrees.

Scheme 1.

Synthesis of silanols (top) and native vs. evolved activity of P450s (bottom).

Scheme 2.

Substrate scope of P450_SiOx3. GC yields are given as average of triplicate runs (see the Supporting Information for further details). ^a Isolated yield after 72 h at 37 °C in parentheses. ^b No conversion to the silanol observed.