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

Abstract

Photolysis of Bicyclo[3.3.l]nonan-2-one. -Summary. Disproportionations to the unsaturated aldehydes 2 and 3 or the ketene 4, the secondary processes available to the acyl-alkyl hiradical b[X(9) = CHz] formed from bicyclo[3.3.l]nonan-2-one (1) in a primary photochemical process by cc-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 ) (l+ kctene 4) > a t 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 a t 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.l]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.l]nonan-2-ones 24 and 25 in methanol. I n 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 y-H abstraction followed by Norvish type I1 reaction; the spiro-alcohols 45 and 46 from 2 by intramolecular y-H abstraction followed by ring closure to cyclobutanols; the oxetanes 49-52 by cycloaddition (Patenzo-Biichi 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.Einleitung . -Bei n,n*-angeregten Cycloalkanonen ist die sog. a-Ketonspaltung