ChemInform Abstract: PHOTOISOMERISIERUNG VON NOPINON

Abstract

Nopinon (I) reagiert bei der Photolyse zu den beiden Spaltprodukten (II) und (III), bei längerer Bestrahlung entsteht, auch unabhängig aus (II), lediglich 4‐ Isopropenyl‐5‐hexenal (III).

Photoisomerization of Bicyclo[3.1.1]heptan-2-ones in Methanol: Chemical Transformations of 'Verbanones'

“…The spectrum of the minor isomer was consistent with structure 12: the aldehydic hydrogen gave rise to a triplet at 6 9.78, J = 2 Hz, the olefinic protons appeared as a multiplet 6 6.10, and the saturated alcohol derived from 12 has been compared with an authentic sample by others (7).…”

“…spectrum of 8 displayed a doublet at 6 1.1 1, J = 6.5 Hz due to the saturated methyl substituent, the gem-dimethyl group appeared as a six-proton singlet at 6 1.73 and the aldehydic hydrogen as a triplet at 6 9. The preferential photolytic formation of 2 was previously rationalized on the basis of the efficient orbital overlap felt to be provided by the fixed geometry of this ring system and the suggestion that the P-hydrogen atom migration was probably concerted with ring cleavage to give the cyclobutane product directly (3,7). However, in the light of evidence that diradicals are discrete intermediates for Norrish Type I reactions (8) this may be an oversimplification.…”

“…spectrum of the major isomer contained a doublet J = 2 Hz at 6 9.74 for the aldehydic hydrogen while the vinyl proton appeared as a multiplet at 6 5.5-5.9, one of the terminal methylene hydrogens as a singlet at 6 5.04, and the remaining one as a doublet of multiplets J = 6 Hz at 6 4.91. In addition, a synthesis of the cis-and trans-methyl ketones derived from 11 has been 3While this work was in progress, we learned of parallel studies on nopinone by Professor N. J. Turro, Columbia University, whom we thank for exchange of information and subsequently another report on nopinone has appeared (7 reported by Shaffer and co-workers (7) who have noted, as have we, that further irradiation gave rise to 4-isopropenyl-5-hexenal (13) a Norrish Type I1 product.…”

“…Unfortunately a referee's suggestion that "quantum yields might be conclusive" is overly optimistic, although they are indicative. The quantum yields (&,) (7,11) (8) have estimated that 33"7, of the biradical formed on photolysis of 2,3-dimethylcyclohexanone reverts to ketone rather than product. Thus these data support the conformational and radical stability arguments enunciated above to rationalize the product distributions in these bicyclo[3.1. l Iheptan-2-ones.…”