Carbon–carbon bond formation by radical addition–fragmentation reactions of O-alkylated enols

Abstract

α-tert-Butoxystyrene [H₂CC(OBu^t)Ph] reacts with α-bromocarbonyl or α-bromosulfonyl compounds [R¹R²C(Br)EWG; EWG = –C(O)X or –S(O₂)X] to bring about replacement of the bromine atom by the phenacyl group and give R¹R²C(EWG)CH₂C(O)Ph. These reactions take place in refluxing benzene or cyclohexane with dilauroyl peroxide or azobis(isobutyronitrile) as initiator and proceed by a radical-chain mechanism that involves addition of the relatively electrophilic radical R¹R²(EWG)C˙ to the styrene. This is followed by β-scission of the derived α-tert-butoxybenzylic adduct radical to give Bu^t˙, which then abstracts bromine from the organic halide to complete the chain. α-1-Adamantoxystyrene reacts similarly with R¹R²C(Br)EWG, at higher temperature in refluxing octane using di-tert-amyl peroxide as initiator, and gives phenacylation products in generally higher yields than are obtained using α-tert-butoxystyrene. Simple iodoalkanes, which afford relatively nucleophilic alkyl radicals, can also be successfully phenacylated using α-1-adamantoxystyrene. O-Alkyl O-(tert-butyldimethylsilyl) ketene acetals H₂CC(OR)OTBS, in which R is a secondary or tertiary alkyl group, react in an analogous fashion with organic halides of the type R¹R²C(Br)EWG to give the carboxymethylation products R¹R²C(EWG)CH₂CO₂Me, after conversion of the first-formed silyl ester to the corresponding methyl ester. The silyl ketene acetals also undergo radical-chain reactions with electron-poor alkenes to bring about alkylation–carboxymethylation of the latter. For example, phenyl vinyl sulfone reacts with H₂CC(OBu^t)OTBS to afford Bu^tCH₂CH(SO₂Ph)CH₂CO₂Me via an initial silyl ester. In a more complex chain reaction, involving rapid ring opening of the cyclopropyldimethylcarbinyl radical, the ketene acetal H₂CC(OCMe₂C₃H₅-cyclo)OTBS reacts with two molecules of N-methyl- or N-phenyl-maleimide to bring about [3 + 2] annulation of one molecule of the maleimide, and then to link the bicyclic moiety thus formed to the second molecule of the maleimide via an alkylation–carboxymethylation reaction.