Electron‐Rich Oxycarbenes: New Synthetic and Catalytic Applications beyond Group 6 Fischer Carbene Complexes

Abstract

Oxycarbenes have emerged as useful intermediates in synthetic chemistry. Compared to the widely studied oxycarbene metal complexes bearing Group 6 metals, the synthetic and catalytic applications of oxycarbenes beyond Group 6 Fischer carbene complexes are less explored because of the difficulty in controlling their reactivity and the need to use a stoichiometric amount of a presynthesized Group 6 metal carbene complex as the starting material. This Minireview summarizes early synthetic and catalytic applications of late-transition-metal oxycarbene complexes and highlights recent advances in free oxycarbene reactions and transition-metal-catalyzed reactions involving oxycarbenes. We hope this Minireview will inspire further developments in this emerging area.

A Combination of Computational and Experimental Studies to Correlate Electronic Structure and Reactivity of Donor–Acceptor Singlet Carbenes

“…Aryl acetates were procured commercially from Sigma-Aldrich, TCI Chemicals, and Acros Organics. All 1 H and 13 C NMR spectra were recorded taking tetramethylsilane (TMS) as an internal standard at ambient temperature unless otherwise indicated with Bruker 400 MHz instruments at 400 MHz for 1 H and 100 MHz for 13 C NMR spectroscopy. Splitting patterns are designated as singlet (s), broad singlet (br s), doublet (d), triplet (t), quartet (q), doublet of doublets (dd) and triplet of doublets (td).…”

“…After completion solvent was removed under reduced pressure, collected for re-use, followed by column chromatography in silica gel (100-200 mesh size) with ethyl acetate (15-30%) in hexane to afford the desired product, 7r, as colourless oil, (150.52 mg, 91%); 1 H NMR (CDCl3, 400 MHz) δH 7.39-7.37 (m, 1H), 7.30 -7.28 (m, 1H), 6.96 -6.86 (m,2H), 5.30 (s, 1H), 5.26 (s, 1H), 3.95-3.87 (m, 2H), 3.82 (s, 3H), 3.63-3.46 (m, 2H), 2.22(s, 2H), 2.18(s, 2H), 1.86-1.69 (m, 5H), 1.65-1.55 (m, 6H), 1.24 (s, 1H), 1.04 (s, 6H), 0.86-0.80 (m, 3H) ppm. 13…”

“…After completion solvent was removed under reduced pressure, collected for re-use, followed by column chromatography in silica gel (100-200 mesh size) with ethyl acetate (15-30%) in hexane to afford the desired product, 7t, as colourless oil, (159.04 mg, 89%); 1 H NMR (CDCl3, 400 MHz) δH 7.38 (dd, J =7.6, 1.6 Hz, 1H), 7.32-7.28 (m, 1H), 6.98-6.93 (m, 1H), 6.90-6.87 (m, 1H), 6.73 (s, 3H), 5.93 (s, 2H), 5.31 (s, 1H), 5.30 (s, 1H), 5.05 (s, 2H), 3.83 (t, J =6.2 Hz, 2H), 3.78 (s, 3H), 3.64-3.47 (m, 2H), 2.24 (s, 2H), 2.20 (s, 2H), 1.85-1.78 (m, 2H), 1.76-1.69 (m, 2H), 1.06 (s, 6H) ppm. 13 43, 1744.28, 1646.36, 1598.43, 1448.20, 1370.28, 1211.90, 1106.87, 1006.49, 820.62, 724.56, 628.24, 546.40, 481.91.…”

Visible-light-promoted defluorinated alkylation of trifluoromethyl alkenes initiated by radical [1,2]-Brook rearrangement: facile synthesis ofgem-difluoro homoallylic alcohol derivatives

“…8 As a synthetic application of such rearrangement, a variety of valuable alcohols with a silyl protection-group can be obtained by leveraging the in situ generated siloxycarbene intermediates from acylsilanes for further insertion of C–H, B–H, C–B, and unsaturated C–C bonds. 9,10 Despite the tremendous progress, the radical Brook rearrangements via single-electron transfer (SET) remain significantly less explored, due to the difficulty associated with the formation of an alkoxy radical. 11,12 In 2017, Smith developed an elegant oxidative [1,2]-Brook rearrangement via a hypervalent silicon intermediate induced by photoredox-catalyzed single-electron transfer, in which hypervalent organosilicates as alkyl radical precursors provided an alternative to the formation of alkoxyl radicals.…”

Radical Conjugate Addition of Acylsilane Enabled by Synergistic Photoredox and Lewis Acid Catalysis

“…With the rapid development of photocatalysis, [4] the photochemical reactivity of acylsilanes has attracted much attention due to the long n‐π* absorption [5] . Siloxycarbenes are prone to be formed through 1,2‐Brook rearrangement of acylsilanes under the condition of light or heat, [1c–e] which are able to serve as sought‐after nucleophiles to undergo X−H insertion, [6] nucleophilic addition, [7] cross‐coupling [8] and cyclization reactions [9] . Alternatively, umpolung of siloxycarbenes would arise upon combination with transition metals, such as copper and palladium, [10] generating electrophilic metal‐siloxycarbenes to trigger the following cyclization or acylation.…”