Flash vacuum pyrolysis of stabilised phosphorus ylides. Part 14.1 Tandem cyclisation of intermediate aryloxy and arylthio radicals leading to tri- and tetra-cyclic aromatic heterocycles

Abstract

Seven new stabilised phosphorus ylides 8–14 designed to undergo thermal tandem cyclisation have been prepared. Upon flash vacuum pyrolysis at 850 °C, loss of Ph₃PO and Me˙ results in tandem cyclisation to give benzothieno[3,2-b]benzothiophene 16 in the case of 11 while for 8 an abstraction–rearrangement–extrusion sequence leads to 2-phenylbenzofuran 18. For 9 and 10 both types of process occur to give respectively benzothieno[3,2-b]benzofuran 27, prepared here for the first time, and 2-phenylbenzothiophene 29. For 12 and 13, initial cyclisation is followed by intramolecular addition and aromatisation to give the tricyclic products 31 and 32, while for 14 intramolecular homolytic substitution leads to the tetracyclic benzonaphthofuran 34 and, by rearrangement, to the isomer 37. Fully assigned ¹³C NMR spectra are presented for all seven ylides.