Thermal ring contraction of dibenz[b,f]azepin-5-yl radicals: new routes to pyrrolo[3,2,1-jk]carbazoles |
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Authors: | Crawford Lynne A McNab Hamish Mount Andrew R Wharton Stuart I |
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Institution: | School of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UK. |
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Abstract: | Flash vacuum pyrolysis (FVP) of N-allyl- or N-benzyldibenzb,f]azepine at temperatures from 750 to 950 degrees C gives pyrrolo3,2,1-jk]carbazole as the major product. The mechanism of the ring contraction involves dibenzazepin-1-yl radical formation, followed by transannular attack and formation of a 2-(indol-1-yl)phenyl radical which cyclizes. The mechanism is supported by independent generation of 2-(indol-1-yl)phenyl radicals by two different methods, and the use of 1-(2-nitrophenyl)indole as a radical generator gives an optimized synthetic route to pyrrolo3,2,1-jk]carbazole (54% overall yield in two steps from indole). The first substituted pyrrolo3,2,1-jk]carbazoles have been synthesized by FVP methods and also by reactions of the parent compound with electrophiles, leading to a range of 4-substituted pyrrolocarbazoles. |
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