Photochemische Reaktionen. 86. Mitteilung [1]. Zur Photolyse von Bicyclo[3.3.1]nonan‐2‐on

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Abstract
Photolysis of Bicyclo[3.3.1]nonan‐2‐one.Disproportionations to the unsaturated aldehydes 2 and 3 or the ketene 4, the secondary processes available to the acyl‐alkyl biradical b[X(9) = CH2] formed from bicyclo[3.3.1]nonan‐2‐one (1) in a primary photochemical process by α‐cleavage (Norrish type I cleavage), were studied.If the acyl‐alkyl biradical b [X(9) = CH2] has a lifetime sufficient to permit rotation around one or several bonds before H‐transfer occurs, considerations about the energetically most favored conformations of b allow to deduce the following preference for intramolecular H‐abstraction: at C(9) (f → unsaturated aldehyde 2) > at C(3) (1 → ketene 4) > at C(8) (i → unsaturated aldehyde 3). If H‐transfer takes place very quickly, following an energetically favored small conformational change of the acyl‐alkyl biradical, one expects H‐abstraction at C(9) and/or C(3) to be preferred over that at C(8). These predictions were fully confirmed by the experiments.UV. irradiation of bicyclo[3.3.1]nonan‐2‐one (1) in n‐pentane gave a complex mixture of the unsaturated aldehyde 2 and products of further photochemical reactions of 2:43 and 44, 45 and 46, 49–52, and 54–57 (see below). Photolysis of 1 in methanol, however, yielded 82% of 2 and 1,2% of ester 23, the final product from ketene 4 formed by trapping of a solvent molecule.UV. irradiation of the 9,9‐dideuterio‐ketone 20 confirmed the intramolecular D‐abstraction at C(9) (→ unsaturated aldehyde 21) and consequently the H‐abstraction at C(9) in the sequence 1 → 2. That either of the two H‐atoms at C(9) in 1 can be abstracted was demonstrated by the photolysis of the C(9)‐epimeric 9‐acetoxy‐bicyclo[3.3.1]nonan‐2‐ones 24 and 25 in methanol. In both cases the unsaturated aldehyde 26 was formed, which corresponds to abstraction of H—C(9)c(7) and H—C(9)C(3)15, respectively.In separate UV. irradiation experiments it was shown that the products accompanying the unsaturated aldehyde 2 (photolysis of 1 in n‐pentane) were formed in the following ways: diene 43 and acetaldehyde (44) from 2 by intramolecular γ‐H abstraction followed by Norrish type II reaction; the spiro‐alcohols 45 and 46 from 2 by intramolecular γ‐H abstraction followed by ring closure to cyclobutanols; the oxetanes 49–52 by cycloaddition (Paterno‐Büchi reaction) of acetaldehyde (44) to 3‐methylidene‐cyclohexene (43); the aliphatic alcohols 54–57 by addition of acetaldehyde (44) to n‐pentane, i.e. H‐abstraction from the hydrocarbon solvent followed by combination of the radicals thereby formed.

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