ABSTRACT
Flash vacuum pyrolysis (FVP) of aryl 2-(allyloxy)benzoates 5 and of the corresponding aryl 2-(allylthio)benzoates 6 at 650 degrees C, gives dibenzofurans 19 and dibenzothiophenes 20, respectively. The mechanism involves generation of phenoxyl (or thiophenoxyl) radicals by homolysis of the O-allyl (or S-allyl) bond, followed by ipso attack at the ester group, loss of CO2 and cyclisation of the resulting aryl radical. Synthetically, the procedure works well for p-substituted substrates, which lead to 2-substituted dibenzofurans 19b-f (73-90%) and dibenzothiophenes 20b-c (90-94%). Little selectivity is shown in the cyclisation of m-substituted substrates and competing interactions of the radical with the substituent--and ipso-attack--complicate the pyrolyses of o-substituted substrates. FVP of related radical precursors including 2-(allyloxy)phenyl benzoates 43 gave no dibenzofurans, whereas 2-(allyloxy-5-methyl)azobenzene 44 gave a much reduced yield. No carbazoles were obtained by FVP of 4-methylphenyl 2-(allylamino)benzoate 42.
ABSTRACT
2-(Pyrrol-1-yl)phenoxyl, aminyl, thiophenoxyl and benzyl radicals 2a-2d, respectively, were generated in the gas-phase under flash vacuum pyrolysis conditions. In all cases except the phenoxyl, cyclisation took place providing acceptable synthetic routes to the fused heterocycles 11, 14 and 15, respectively. Only sigmatropic rearrangement products were isolated, in low yields, from the phenoxyl 2a. The pyrrolo[1,2-a]benzimidazole 11 adopts the 1H-tautomer exclusively in chloroform solution. Electrophilic substitution reactions of pyrrolo[2,1-b]benzothiophene 14 were studied, including protonation, deuterium exchange, Vilsmeier formylation and reaction with dimethyl acetylenedicarboxylate. 2-(2,5-Diarylpyrrol-1-yl)thiophenoxyl, phenoxyl and aminyl radicals 23a-f, were also generated in the gas-phase under similar conditions. The thiophenoxyls 23a/b gave extremely complex pyrolysate mixtures in which primary cyclisation products were formed by attack of the radical at the pyrrrole ring and attack at the ipso-, ortho- and meta- positions of the aryl ring. Secondary pyrolysis products were obtained by specific sigmatropic shifts of the N-aryl group. The 2,5-di(thien-2-yl)thiophenoxyl radical 23c gave the pyrrolobenzothiazole 31c as the only cyclisation product in low yield. FVP of the phenoxyl and aminyl radical generators 26d and 26f, respectively, gave 3-arylpyrrolo[1,2-f]phenanthridines 46d and 46f, respectively, by a hydrogen transfer-cyclisation mechanism.
ABSTRACT
The pyrrolylacrylates 9 and 10 were synthesised and subjected to flash vacuum pyrolysis (FVP) at 650-700 degrees C to generate the radicals 11 and 18, respectively. The phenoxyl 11 underwent hydrogen capture to give a mixture of the phenol 12 and the pyrrolobenzoxazine 13 in low yields, which were also obtained by a Wittig reaction of the 2-formylpyrrole 14. The thiophenoxyl 18 gave a single major product in 41% yield which was identified as the pyrrolo[1,2-a]quinoline 17 by a sequence of NMR experiments. A mechanism for the formation of 17 by a rearrangement-sulfur extrusion sequence is proposed.