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

Abstract 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 ‐CH 3 , OCH 3 ) and (CF 3 ) 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 = OCH 3 ) 0.065. It is noteworthy that decarbonylation of 10 (X = OCH 3 ) 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 OCH 3 ) 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.