Photochemical Reactions. Part 63 [1]. The photodecarbonylation of α-aryl aldehydes

Ultraviolet irradiation of the aldehydes 6 – 11 in degassed solutions results exclusively in decarbonylation to the major products 34, 35 and 37 – 40 , and to small amounts of 2, 3-diphenyl-2, 3-dimethyl-butanes 36 from the phenyl aldehydes 6 and 7 . In the presence of tri-n-butylstannane, incorporation of stannane hydrogen competes, to substrate-specific limits, with the intramolecular deuterium transfer in 7 → 35 and 11 → 40 . The quantum yields for decarbonylation are Φ ～ 0.4–1.0 for the phenyl aldehydes 6 and 9 , and 0.02 for 8. Hammett correlations of Φ with resonance constants ( R ) for 6 (X = H, p-CH₃, ? OCH₃) and (? CF₃) and with ω_m⁺ values for the meta-substituted isomers are in agreement with the proposed α-cleavage to an associated radical pair with only moderate free radical character as the primary photochemical step. Φ for 10 (X = H) is 0.11, and for 10 (X = OCH₃) 0.065. It is noteworthy that decarbonylation of 10 (X = OCH₃) occurs also at 3340 Å (Φ_{? CO} = 0.11) i.e., upon excitation in an absorption band which is presumably lower in energy than the n → π* transition and corresponds to the aromatic L_b transition of 2-methoxynaphthalene. Singlet multiplicity of the reactive excited states is probable on the basis of the fact that the decarbonylation of 6 (X = H) and 10 (X = H and OCH₃) could be sensitised neither by acetone nor acetophenone, and could be quenched neither by naphthalene nor by cis-1, 3-pentadiene and nor by 1, 3-cyclohexadiene.