Interplay of ortho- with spiro-cyclisation during iminyl radical closures onto arenes and heteroarenes

Abstract

Sensitised photolyses of ethoxycarbonyl oximes of aromatic and heteroaromatic ketones yielded iminyl radicals which were characterised by EPR spectrscopy. Iminyls with suitably placed arene or heteroarene acceptors underwent cyclisations yielding phenanthridine type products from ortho-additions. For benzofuran and benzothiophene acceptors, spiro-cyclisation predominated at low temperatures but thermodynamic control ensured ortho-products, benzofuro- or benzothieno-isoquinolines, formed at higher temperatures. Estimates by steady state kinetic EPR established that iminyl radical cyclisations onto aromatics took place about an order of magnitule more slowly than prototypical C-centred radicals. Cyclisation energetics were investigated by DFT computations which gave insights into factors influencing the two cyclisation modes.