- Hydroacylation of α,β-unsaturated esters via aerobic C-H activation
-
The development of methods for carbon-carbon bond formation under benign conditions is an ongoing challenge for the synthetic chemist. In recent years there has been considerable interest in using selective C-H activation as a direct route for generating reactive intermediates. In this article, we describe the use of aldehyde auto-oxidation as a simple, clean and effective method for C-H activation, resulting in the generation of an acyl radical. This acyl radical can be used for carbon-carbon bond formation and herein we describe the application of this method for the hydroacylation of α,β-unsaturated esters without the requirement of additional catalysts or reagents. This methodology generates unsymmetrical ketones, which have been shown to have broad use in organic synthesis.
- Chudasama, Vijay,Fitzmaurice, Richard J.,Caddick, Stephen
-
scheme or table
p. 592 - 596
(2010/10/04)
-
- Radical-chain addition of aldehydes to alkenes catalysed by thiols
-
Thiols catalyse the radical-chain addition of primary aliphatic aldehydes R1CH2CHO to terminal alkenes H2C=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 deg 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 in the presence of TBHN (2x2.5 molpercent) and methyl thioglycolate (MeO2CCH2SH; 2x5 molpercent) gives the adduct in 80percent yield, whilst a similar reaction in the absence of thiol catalyst affords only an 8percent 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 deg C, gives the adduct in only 7percent 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.
- Dang, Hai-Shan,Roberts, Brian P.
-
-
- Photochimie en solution. XXIV. Mecanisme de l'addition des aldehydes aux double liaisons ethyleniques activees par des groupements attracteurs d'electrons
-
The mechanism of the photochemical addition of aliphatic aldehydes to electron-deficient olefins has been rationalized.No addition to α,β-unsaturated ketones was observed when the latter were excited.The reaction can be explained by a first self-quenching step of the aliphatic aldehyde involving its lowest triplet state, which is reached with a moderate quantum yield (0.5).The radical R-C.=O produced in the self quenching process adds to the olefin at the more positively-charged carbon atom.The reaction ends by a hydrogen atom exchange between the radical produced by this addition and the radical R-C.HOH.The quantum yield of the product formation is much higher with benzaldehyde owing to the almost unity value of the intersystem crossing, but it competes with the formation of benzile.
- Kawenoki, Isabelle,Maurel, Daniele,Kossanyi, Jean
-
p. 385 - 390
(2007/10/02)
-