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 (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.