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

Abstract

AbstractUltraviolet 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 ωm+ 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 Lb 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.