Cobalt-mediated reactions: inter- and intra-molecular additions of carbamoyl radicals to alkenes in the synthesis of amides and lactams

Abstract

Carbamoylcobalt(III) salophen compounds, such as 4, are sources of carbamoyl radicals under either thermal or photolytic conditions. Generation of N,N-dimethylcarbamoyl radicals, e.g.5, in the presence of trapping agents such as diphenyl disulfide or 2,2,6,6-tetramethylpiperidin-1-yloxy radical 7 afforded the expected amido derivatives, e.g.6 and 8, respectively. These radicals 5 also underwent oxidative intermolecular addition to styrene; the thermal process gave the E-cinnamide 10 exclusively, whereas under photochemical conditions a 1:1 mixture of E- and Z-cinnamides, 10 and 11, respectively, was obtained. A series of N-alkenyl-N-alkylcarbamoylcobalt(III) salophens have also been prepared (i.e.14, 33 and 43), and are shown to be suitable precursors for the synthesis of β-,γ- and δ-lactams through the 4-, 5- or 6-exo-trig-modes of cyclisation of the corresponding intermediate carbamoyl radicals. The products of these cyclisations, N-alkyl-3-lactamidomethyl radical (i.e.16, 34 or 46) are trapped by the cobalt(II) salophen to give the corresponding alkylcobalt(III) salophens (e.g.16→17). Although these alkylcobalt salophens appeared to be stable at temperatures ⩽40 °C, under the thermolytic conditions for radical generation that were usually employed (110 °C) dehydrocobaltation occurred, and the unsaturated lactam was formed. Results concerning the introduction of side-chain oxygen functionality at the product radical centre in tandem with the carbamoyl radical cyclisation are also presented. Computer-generated molecular modelling calculations supporting the novel 4-exo-trig-cyclisation of 15 are discussed.