Mirzaei M Saeed, Wentrup Curt
Department of Chemistry, Rice University, Houston, Texas 77005, United States.
School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia.
J Org Chem. 2024 Jan 5;89(1):553-564. doi: 10.1021/acs.joc.3c02289. Epub 2023 Dec 12.
Flash vacuum pyrolysis (FVP) of pyrazoles and indazoles constitutes a valuable route to carbenes and nitrenes. In this study, we employed M062X and CCSD(T) calculations to provide a mechanistic rationale for the formation of fulvenallene and fluorenes from indazoles and the corresponding formation of azafulvenallene , cyanocyclopentadiene , and azafluorenes, e.g. , from azaindazoles, e.g. , and from homoquinolinic anhydride. The results reveal the importance of initial tautomerization in the pyrazole moiety of 7-azaindazole , which drives the mechanism toward 2-diazo-3-methylene-2,3-dihydropyridine and hence 3-methylene-2,3-dihydropyridin-2-ylidene , followed by Wolff-type ring contraction to 1-azafulvenallene . This path has a calculated activation energy ∼10 kcal/mol lower than that for an alternate route involving ring opening to 3-diazomethylpyridine, dediazotization, and rearrangement of 3-pyridylcarbene to azacycloheptatetraene and phenylnitrene . FVP of 2,5-diphenyltetrazoles and phenyl(pyridyl)tetrazoles leads to nitrile imines, which cyclize to 3-phenylindazoles and -azaindazoles. Nitrogen elimination from these (aza) indazoles results in the formation of (aza) fluorenes, for which two alternate mechanisms are described: route A by rearrangement of (aza) indazoles to diazo(aza)cyclohexadienes and (aza)cyclohexadienylidenes and route B by rearrangement to diaryldiazomethanes and diarylcarbenes. Both paths are energetically feasible, but path A is preferred and corresponds to the azafluorenes obtained experimentally.
吡唑和吲唑的快速真空热解（FVP）是生成卡宾和氮宾的一条重要途径。在本研究中，我们采用M062X和CCSD(T)计算方法，为从吲唑生成富烯并苯和芴以及从氮吲唑（例如）和高喹啉酸酐相应生成氮杂富烯并苯、氰基环戊二烯和氮杂芴提供了机理依据。结果揭示了7-氮吲唑吡唑部分初始互变异构的重要性，它推动反应机理朝着生成2-重氮-3-亚甲基-2,3-二氢吡啶进而生成3-亚甲基-2,3-二氢吡啶-2-亚基的方向进行，随后通过沃尔夫型环收缩生成1-氮杂富烯并苯。该路径的计算活化能比另一条涉及开环生成3-重氮甲基吡啶、脱重氮化以及3-吡啶基卡宾重排为氮杂环庚四烯和苯基氮宾的替代路径低约10 kcal/mol。2,5-二苯基四唑和苯基（吡啶基）四唑的FVP生成腈亚胺，腈亚胺环化生成3-苯基吲唑和氮杂吲唑。这些（氮杂）吲唑消除氮后生成（氮杂）芴，对此描述了两种替代机理：路径A是（氮杂）吲唑重排为重氮（氮杂）环己二烯和（氮杂）环己二烯亚基，路径B是重排为二芳基重氮甲烷和二芳基卡宾。两条路径在能量上都是可行的，但路径A更受青睐，且与实验得到的氮杂芴相对应。
