Photochemical α‐Cleavage of β, γ‐Unsaturated Aryl Ketones: Competition between recombination/disproportionation and dissociation in geminate singlet and triplet radical pairs

Abstract

Summary. The photolysis of (I?)-( + )-phenyl and (R)-(+ )-p-anisyl 1,2,3-triinethylcyclopent-2-enyl ketone (1, 2) and the corresponding rac-1-and 3-desmethyi analogs (3, 4) led t o isomerization due to formal 1 , 3 aroyl migration and to formation of aryl aldehydes (7, 8), dienes (9,lO) and dimers (5, 6) of the cyclopentenyl radical. Evidence obtained from a chiroptical and mass spectrometric analysis of a crossing experiment and from photolytic CIDNP measurements including the use of CCl, as a free radical scavenger, supports the conclusion ( I ) : that the ketones undergo photochemical cc-cleavage predominantly in the triplet state ; (2) : that recombination and disproportionation reactions within the geminate singlct and triplet aroyllallyl radical pairs (11) compete with the dissociation into free radicals (12) ; (3) : that ketone isomerization by paths not involving polarizable radical intermediates is unimportant; (4) : that no triplet oxa-di-nmethane type rearrangement products are formed.We have previously shown that cyclopent-2-enyl methyl ketones undergo 1,3 acetyl shifts and a-cleavage to radicals in the excited singlet state faster than intersystem crossing, and oxa-di-n-methane type rearrangements to 5-acetyl bicyclopentanes (endo > exo) in the triplet state [2]2). Recent CIDNP investigations [4] revealed, furthermore, that radical recombinations are very minor paths for these reactions.We now report (7): that the corresponding aryl ketones, e.g. (R)-(+)-l, undergo photochemical a-cleavage predominantly in the triplet state; (2) : that recombination and disproportionation reactions within the geminate singlet and triplet aroyl/allyl radical pairs compete with the dissociation into free radicals; (3): that ketone isomerization by paths not involving polarizable radical intermediates is unimportant, and (4) : that no triplet oxa-di-%-methane type rearrangement products are formed. Part of these results were presented at the IVth International IUPAC Conference on Photochemistry, Baden-Baden, July 1972 [l]. Such divergent behavior of excited singlets and triplets is often observed with /3,y-unsaturated ketones; see [3] for references. Irradiations were carried out in iso-octane, benzene, or acetone at wavelengths 254 nm and > 340 nm.

[3+2] Cycloaddition Reaction of Cyclopropyl Ketones with Alkynes Catalyzed by Nickel/Dimethylaluminum Chloride

“…The expected [3+2] cycloaddition reaction occurred to give 3 aa in 80 % yield. [10] In the absence of AlMe 3 , 3 aa was not obtained at all, thus indicating that AlMe 3 is crucial for the reaction (entry 2). In the absence of alkynes, the nickel catalyst was immediately decomposed to give a black precipitate.…”

S1 vs. T2 photoreactivity of .beta.,.gamma.-unsaturated ketones. Temperature dependent photo-CIDNP of 2-cyclopentenyl methyl ketones

“…So far the reported photoinduced a-cleavage reactions of acyclic imides14 and lactams1-'' were primarily related to the fission of the C(0)-N bond, and that of the C(0)-C bond was significant in a few exceptional imides. 3 The present example shows the generality and importance of -cleavage reaction, especially the C(0)-C bond fission of alicyclic imides. Its scope and limitation are being studied.…”

[3+2] Cycloaddition Reaction of Cyclopropyl Ketones with Alkynes Catalyzed by Nickel/Dimethylaluminum Chloride

“…In the presence of 10 mol % of [Ni(cod) 2 ] and 100 mol % of AlMe 3 , the reaction of cyclopropyl phenyl ketone ( 1 a ) with 2‐butyne ( 2 a ) was conducted in THF at room temperature (Table 1, entry 1). The expected [3+2] cycloaddition reaction occurred to give 3 aa in 80 % yield 10. In the absence of AlMe 3 , 3 aa was not obtained at all, thus indicating that AlMe 3 is crucial for the reaction (entry 2).…”