Efficient photocyclization of o-alkylbenzaldehydes in the solid state: direct observation of E-xylylenols en route to benzocyclobutenols |
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Authors: | Moorthy J N Mal P Natarajan R Venugopalan P |
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Institution: | Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India. moorthy@iitk.ac.in |
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Abstract: | The photocyclization to benzocyclobutenols of o-alkyl aromatic aldehydes that are predestined for gamma-hydrogen abstraction is found to occur efficiently in the solid state; in contrast, solution-phase photolysis is known to afford a mixture of several products. It is shown that mesitaldehyde, which is a liquid, also undergoes efficient cyclization when subjected to photolysis as a solid inclusion complex. The marginal energy differences in the relative energies of the E-enols and the corresponding cyclobutenols in the case of cyano-substituted mesitaldehydes has permitted direct observation, for the first time, of the E-enols en route to benzocyclobutenols. The AM1 calculations suggest that the cyano-substitution causes intrinsic stabilization of the E-enols relative to the corresponding cyclobutenols, while the bromo groups do the opposite. The lack of observation of the red color in bromo- and formyl-substituted aldehydes is attributed to rapid cyclization of the E-enols to the their respective cyclobutenols even at low temperatures. |
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