Abstract

I. 74)Swmzary. UV.-Irradiation of methyl t-butyl ketone, di-t-butyl ketone, pivalaldehyde and isobutyraldehyde in benzenc solution leads to chemically induced dynamic nuclear polarization (CIYNP.) of the parent compounds and of various reaction products. C1DNP.-effects and product distributions establish type I a-cleavage predominantly of triplet state molecules as 'the major primary photochemical step for all the four a-branched carbonyl compounds. I n chlorinated solvents singlet reactions interfere with the triplet processes.

Vinylalcohol, formed by disproportionation of a-hydroxyethyl radicals, is detected by NMR. spectroscopy during photoreactions of acetaldchyde and acetoin in solution and slowly tautomerizes to acctaldehyde.At room temperature, the equilibrium concentration of vinyl alcohol 1 has been estimated to be at least seven orders of magnitude lower than that of the keto isomer, acetaldehyde 2, i.e. to be lower than the detection limit of most physical techniques [2] 131. This has impeded the determination of spectroscopic properties of this most simple enol. 1 2 During photolysis of 2 and of 2-hydroxybutan-3-one (acetoin, 3) in the probe of a modified HA-100 lH-NMR. spectrometer, we detect 1 as thermally unstable radical reaction product, and observe its vinyl proton spectrum highly enhanced by chemically induced dynamic nuclear polarization (CIDNP.) [4] [S] [6]. Fig. 1 shows lH-NMR.-spectra taken at room temperature during irradiation of 2 (a) and 3 (b), 0 . 2~ in benzene, with the beam of a high pressure mercury capillary lamp filtered to transmit radiation of wavelength 300 nm < 2 < 350 nm. During the periods of observation conversion was less than 10%. The same CIDNP. effects were found with benzene-h,, benzene-$, cyclohexane-h,,, cyclohexane-d,,, and octamethyltetrasiloxane as solvents.The Table gives the assignments of the transitions 1 through 10 of Fig. 1 to the various reaction products together with the description of the C1DNP.-effects in the 1)For part X of this series : see ill.

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.

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