1235997-91-7Relevant academic research and scientific papers
Investigation of substituent effects on the selectivity of 4π-electrocyclization of 1,3-diarylallylic cations for the formation of highly substituted indenes
Smith, Chris D.,Rosocha, Gregory,Mui, Leo,Batey, Robert A.
supporting information; experimental part, p. 4716 - 4727 (2010/09/05)
(Figure Presented) Differentially substituted 1,3-diaryl-substituted allylic cations generated by ionization of the corresponding allylic alcohols in the presence of a Lewis acid undergo chemoselective and regioselective electrocyclization reactions to generate 1-aryl-1H-indenes. Electrocyclization only occurs for allylic cations bearing a 2-substituent, with 2-ester and 2-alkyl substituents both tolerated. In general, the presence of electron-withdrawing substituents deactivates the ring and disfavors cyclization. In contrast, the selectivity of cyclization of systems containing electron-donating substituents depends on the nature and position of the electron-donating group. Electron-donating substituents at the meta position particularly favor cyclization. There was no obvious correlation of cyclization selectivity with calculated electron densities as has been suggested for electrophilic aromatic substitution reactions. However, the calculated selectivities determined by a gas-phase (B3LYP/6-31G* + ZPVE) comparison of the relative rates of cyclization were in remarkably good agreement with the observed selectivities. Calculated transition-state structures for cyclization are consistent with a cationic π4a conrotatory electrocyclization mechanism. In some cases involving more electron-deficient systems, the initially formed 1H-indene underwent subsequent alkene isomerization to the 3H-indene. In one example, an unusual dimerization reaction occurred to give a cyclopenta[a]indene via an unusual formal cationic 2π+2π cycloaddition of the allylic cation with the intermediate indene.
