Thermal Ring Contraction of Dibenz[b,f]azepin-5-yl Radicals: New Routes to Pyrrolo[3,2,1-jk]carbazoles

• Published in: Journal of organic chemistry Vol. 73; no. 17; pp. 6642 - 6646
• Main Authors: Crawford, Lynne A, McNab, Hamish, Mount, Andrew R, Wharton, Stuart I
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 05.09.2008; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Organic; Exact sciences and technology; Free radicals chemistry; Heterocyclic compounds; Heterocyclic compounds with only one n hetero atom and condensed derivatives; Heterocyclic compounds with several n hetero atoms in the same ring, in separated rings or in fused rings; Kinetics and mechanisms; Organic chemistry; Physical Sciences; Preparations and properties; Reactivity and mechanisms; Science & Technology; N-ALKYLATION; GENERATION; PYROLYSIS; BENZYNE; Nitro compound; Under vacuum; Nitrogen heterocycle; Carbazole derivatives; Flash pyrolysis; Thermal reaction; Allylic compound; Cyclization; Organic free radical; Reaction mechanism; Transannular reaction; Ring contraction; Indole derivatives; Chemical synthesis; Electrophile
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo800637u

Flash vacuum pyrolysis (FVP) of N-allyl- or N-benzyldibenz[b,f]azepine at temperatures from 750 to 950 °C gives pyrrolo[3,2,1-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 pyrrolo[3,2,1-jk]carbazole (54% overall yield in two steps from indole). The first substituted pyrrolo[3,2,1-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.