Radical-chain addition of aldehydes to alkenes catalysed by thiols

Abstract

Thiols catalyse the radical-chain addition of primary aliphatic aldehydes R¹CH₂CHO to terminal alkenes H₂C[double bond, length half m-dash]CR²R³ to give ketonic adducts R¹CH₂C(O)CH₂C(H)R²R³ in moderate to good yields. The reaction takes place under mild conditions (dioxane solvent, 60 °C) and is initiated by di-tert-butyl hyponitrite (TBHN). Thiol catalysis is effective for hydroacylation of electron-rich, -neutral and -deficient alkenes, but is most efficient for addition to electron-rich double bonds. For example, the addition of butanal (2 equiv.) to isopropenyl acetate [H₂C[double bond, length half m-dash]C(Me)OAc] in the presence of TBHN (2 × 2.5 mol%) and methyl thioglycolate (MeO₂CCH₂SH; 2 × 5 mol%) gives the adduct in 80% yield, whilst a similar reaction in the absence of thiol catalyst affords only an 8% yield. Other enol acetates, silyl enol ethers, an enol phosphate and butyl vinyl ether react similarly. For comparison, the reaction of butanoyl phenyl selenide with isopropenyl acetate, in the presence of tributyltin hydride and azoisobutyronitrile initiator in benzene at 80 °C, gives the adduct in only 7% yield. Methyl thioglycolate is generally the most efficient catalyst for hydroacylation of electron-rich alkenes, whilst tert-dodecanethiol is more effective for addition of aldehydes to electron-deficient alkenes. Triorganosilanethiols also function as catalysts, as does the arenethiol 2,4,6-tris(trifluoromethyl)thiophenol. The role of the thiol is to act as a polarity-reversal catalyst that promotes the overall hydrogen-atom transfer from the aldehyde to the carbon-centred radical produced by addition of the acyl radical to the alkene. Intramolecular hydroacylation is also subject to thiol catalysis and the radical-chain cyclisation of citronellal to a mixture of menthone and isomenthone is effectively promoted in the presence of triphenylsilanethiol.

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