hide
Free keywords:
photoisomerisation; 1;2-Shift; cyclopropane; electron-deficient olefin; excited singlet state
Abstract:
The chemical behaviour of 32 selected 1,1-dicyano-1-alkenes (DCNA) that are devoid of additional unsaturation and of additional hetero-atoms, upon direct excitation by continuous irradiation with light of 253.7 nm wavelength into the long- wavelength flank of their longest wavelength UV absorption band has been studied in solvents ranging from cyclohexane to methanol. The predominant reaction products in the majority of cases were 1,1-dicyano-cyclopropanes formed via 1,2-migration of either hydrogen or methyl/alkyl from C-3 to C-2 (olefin to cyclopropane photorearrangement, OCPR). Photoreactions competing with OCPR were hydrogen atom abstraction from solvent by the C-2 of the DCNA and, in characteristically favourable cases only, 3,4-C-C bond cleavage. In cases of low OCPR quantum yields, hydrogen abstraction from solvent was dominant in cyclohexane or methanol but it could be suppressed by the choice of a solvent (methylene chloride, acetonitrile, tert- butanol) that more strongly resisted hydrogen abstraction. Further minor by-products were isomeric DCNA and 1, 1 -dicyano- 3-alkenes. No carbene-derived products were observed. Supplementary experiments included quenching experiments and an investigation of the DCNA triplet state. The DCNA triplet state was formed at only ca. 1% on direct irradiation but it could be efficiently produced by sensitisation with benzophenone; in the absence of olefins as inter- or intramolecular substrates, it was fairly unreactive. All observed reactions occur from the lowest excited DCNA singlet state. According to the quenching experiments, this state is short-lived as compared to diffusional movements. Other than OCPR which appears to be due to cationic reactivity at C-2 exhibited by the perpendicular geometry of the excited double bond, hydrogen abstraction and 3,4-C-C bond cleavage appear to be due to radical reactivity at C-2 exhibited by geometries of the excited double bond that are intermediate between planar and perpendicular and are due to vibration about the perpendicular conformation. (C) 2002 Elsevier Science B.V. All rights reserved.
