Photoallylation Mechanism of Dicyanobenzenes with Allyltrimethylsilane in Solution Investigated by Cold UV and IR Spectroscopy in the Gas Phase

The structure of the chemical intermediate in photoallylation of dicyanobenzenes (DCBs) with allyltrimethylsilane (ATMS) in acetonitrile was determined by cold UV and IR spectroscopy in the gas phase. The reaction mechanism was discussed on the basis of the intermediate structure. It was found that the elimination of a CN group from the intermediate is a rate-limiting step that governs the reactivity of DCBs.

Abstract

The photoallylation of three structural isomers of dicyanobenzene (DCB), 1,4-DCB, 1,3-DCB, and 1,2-DCB, was investigated. Reactivity dependence on the relative position of the cyano groups is acknowledged; 1,3-DCB exhibits lower reactivity. The protonated reaction intermediates were identified at m/z 171 (C₁₁H₁₁N₂ ⁺) for all DCB isomers at comparable intensities. The C₁₁H₁₁N₂ ⁺ intermediates are produced by adding an allyl group to DCB, implying that (1) the reaction is initialized via allyl-group addition to DCB and the formation of neutral (C₁₁H₁₀N₂) intermediates is followed by cyano-group elimination and (2) the initial allyl-group addition occurs effectively regardless of the cyano-group position. UV photodissociation (UVPD) and IR-UV double resonance (DR) spectra of the C₁₁H₁₁N₂ ⁺ intermediates were measured under cold (∼10 K) gas-phase conditions and compared with theoretical spectra. The intermediate structures identified from 1,2-DCB and 1,4-DCB appear compatible with the subsequent β-elimination of the cyano group, which is probably the final step of photoallylation. Conversely, the intermediate structure from 1,3-DCB comprises β hydrogens between the cyano and allyl groups, which can induce the CH/π hydrogen bonds with the cyano and allyl groups. This conformation restricts nucleophilic access to the β hydrogens and suppresses cyano-group elimination, accounting for the low reactivity of 1,3-DCB in solution.

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