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

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) = CH 2 ] 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) = CH 2 ] 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.