[10]annulene: Electrocyclization mechanisms

The mechanism of 6‐π electrocyclization of all‐cis, mono‐trans, and double‐trans [10]annulene to yield 4a,8a‐dihydronaphthalene has been explored at various quantum‐chemical methods. The mono‐trans configuration cyclizes preferentially to trans‐4a,8a‐dihydronaphthalene, in agreement with the experimental results. The cyclization of the all‐cis configuration requires firstly a bond‐shifting to the naphthalene‐like conformation of double‐trans [10]annulene, which is the rate‐limiting step, and finally its azulene‐like conformation electrocyclizes quickly to cis‐4a,8a‐dihydronaphthalene. Its experimental rate coefficient is consistent with the computed one for the cyclization of the all‐cis configuration, unlike the calculated one for the double‐trans configuration. These results confirm the configurations assigned by Masamune et al. to the two isomers which they isolated. The 6‐π electrocyclization pathways of all‐cis, mono‐trans and double‐trans [10]annulene are investigated using various quantum‐chemical methods. The infered kinetics is consistent with the experimental data. Unlike double‐trans and mono‐trans configurations, the disrotatory electrocyclization of all‐cis [10]annulene, though thermally allowed regarding Woodward–Hoffman rules, is too slow to explain the experimental kinetics. Instead, an alternative stepwise route accounting for the experimental results has been found.