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

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 (C 11 H 11 N 2 + ) for all DCB isomers at comparable intensities. The C 11 H 11 N 2 + 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 (C 11 H 10 N 2 ) 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 C 11 H 11 N 2 + 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.