关键词: 四取代烯烃, 偶联反应, 脱氧反应, 金属催化, 铬

Tetrasubstituted olefins are industrially important building blocks, which have been widely used in material and medicinal chemistry. Strategies that enable access to olefins have been developed, including the classic McMurry reactions of ketones using organotitanium reagents. The development of McMurry-type reactions using earth-abundant metals as catalysts, by the homo- and cross-couplings of two carbonyls in the deoxygenative fashion, would be a value-addition to tetrasubstituted olefins. Herein, we report the deoxygenative couplings of ketones that are enabled by cost-effective chromium catalysis. These couplings occur with low-cost Cr salt as precatalyst, 4,4'-di-tert-butyl-2,2'-bipyridyl (dtbpy) as ligand, metallic Mg as reductant, and trimethylchlorosilane (TMSCl) as additive. The protocols can be applied to achieve both homo- and cross-coupling of ketones, allowing for the efficient formation of sterically demanding tetrasubstituted olefins in one-step operation. Based on our experimental results and known reports, a plausible mechanism involving in the Cr-catalyzed deoxygenative couplings of ketones was proposed, wherein the reactive silachromate(I) might be formed by reaction of low-valent Cr species with Mg and trimethylchlorosilane, and is detected by high-resolution mass spectrometry analysis. It adds to unsaturated carbonyl of ketone in giving silyloxy-substituted (alkyl)Cr intermediate, followed by process of carbometalation by the addition of C—Cr bond to carbonyl of another ketone molecule. The C=C bond is formed by following deoxygenation in giving tetrasubstituted olefin product. Trimethylchlorosilane plays important roles in the ketone deoxygenative coupling, by the formation of reactive silachromate in responsibility for the addition and deoxygenation processes. The general procedure for the deoxygenative homocoupling is as following: in a dried Schlenk tube are placed ketone 1 (0.4 mmol), CrCl₂ (5 mg, 0.04 mmol), dtbpy (10 mg, 0.04 mmol), Mg (20 mg, 0.8 mmol) and TMSCl (50 µL, 0.4 mmol) under atmosphere of nitrogen. After the addition of freshly distilled tetrahydrofuran (THF, 2 mL) by syringe, the resulting mixture was stirred at 90 ℃ for 12 h. The volatiles were removed under reduced pressure, and the crude product was purified by silica gel chromatography to give the corresponding tetrasubstituted olefin.

Key words: tetrasubstituted olefin, coupling reaction, deoxygenation, metal catalysis, chromium