Photolysis enol formation

Laser flash photolysis of 46 showed results similar to those obtained for 45. The lifetimes and yields of Z and E photoenols from 46 are comparable to those obtained for 56. Similarly, laser flash photolysis of 47 reveals that the major reactivity pattern of 47 is intramolecular H-atom abstraction to form Z-58 and E-58 even though no products were observed that can be attributed to the formation of photoenol 58. Laser flash photolysis of 47 in methanol showed formation of biradical 57 ( max 330 nm, r = 22ns), which was efficiently quenched with oxygen (Scheme 32). Biradical 57 intersystem crosses to form Z-58 and E-58, which have maximum absorption at 400 nm. Enols Z-58 to E-58 were formed in the approximate ratio of 1 4. Enol Z-58 had a lifetime of 6.5)0,s in methanol, but its lifetime in dichloro-methane was only 110 ns. The measured lifetime of E-58 in methanol was 162)0,s, while it was 44 ms in 2-propanol. Thus, E-58 is considerably shorter-lived than E-56. Furthermore, E-58 is also shorter-lived than the analogous E-59 (Scheme 33), which cannot decay by intramolecular lactonization and has a lifetime of 3.6 ms in methanol. Thus, we proposed that E-58 undergoes solvent-assisted reketonization that is facilitated by the intramolecular H-atom bonding, as shown in Scheme 34. 

The yield is invariant with changes in pressure, temperature, intensity, and added inert gases.26 The mechanism favored involves the formation of ethylene and the enolic form of acetone by way of a six-membered ring intermediate,46 and has recently been substantiated by results of studies of the photolysis of deuterated ketones.112 The mechanism is given below... 

For a few other ketones with more remote cyclopropyl groups, photoexcitation led to the formation of a diradical which was transformed into a cyclopropylmethyl-containing diradical by a preliminary rearrangement such as a 1,5-hydrogen migration or a cyclization. 1-Phenyl-hept-4-en-l-one (22) and l-phenylcyclohept-4-enol (24) formation in the photolysis of 4-cyclo-propyl-l-phenylbutan-l-one (21) was accounted for by means of such a 1,5-hydrogen migra-tion. ... 

Enolization of triplet 5,8-dimethyl-1-tetralone by hydrogen transfer effects" ", spin-lattice relaxation of pentacen-6,13-quinone triplets in a benzoic acid host crystal involves proton tunnelling", and photoreduction of benzophenone and thioxanthen-9-one by amines" are other researches of photochemical significance. Interactions of formate ions with triplet states of benzophenone-4-carboxylate and -4-sulphonate, 1,4-naphthoquinone, and anthraquinone-2-sulphonate have been examined systematically by laser flash photolysis". ... 

Further experimental support for the apparent facile 1,6-ECRC of some extended enolates has been reported105. Photolysis of the unsaturated keto ester 57 in basic methanol affords a 70% yield of / ,y- and a,/)-unsaturated decalones 59. 1,6-ECRC of the intermediate enolate 58 accounts for the formation of 59. Similar observations had been reported earlier106, and the notion that electron-rich trienes could be useful substrates in 1,6-ECRCs was cemented in a concept paper 07. 

Matrix photolysis of 2i/-pyran-2-one at 8 K leads to electrocyclic ring opening of the bond O/C-2 and formation of the aldoketene 8 which, by addition of CH3OH, can be intercepted as ester 10. Photolysis at higher temperatures or in ether leads to electrocyclization of the 1,3-diene system and formation of the p-lactone 9 which, by addition of CH3OH, yields the enol ether 12. Further photolysis of the P-lactone 9 causes decarboxylation resulting in cyclobutadiene 11 and its products, e.g. the dimer 14 (CHAPMAN) [12] ... 

Fig. 3.8 Photolysis of the ketonyl oxirane 11 led to formation of, amongst other products, phenylketene (isolated as the phenylaeetie aeid). The reaction was rationalized as a Norrish type II process abstraction of a y-hydrogen by a ketone carbonyl group, giving a 1,4-diradical which fragments to an alkene (in this case phenyloxirene) and the enol of a ketone. However, photolysis of 12 gave 13, presumably derived from the oxo carbene, but no 14, which argues against the (pseudo)symmetrical ethylpropyloxirene and suggests that in the diradical bonds a and b cleave simultaneously...

A combined study on 2-acetyltetralone using laser flash photolysis and time resolved EPR shows formation of triplet excited state. From these results, it is concluded that the molecule is in the enol form, with its double bond in endocyclic position. ... 

The formation of the intermediate enol-form of carbonyl compounds in aldehyde and ketone photolysis was first suggested by Davis and Noyes (1947) and was invoked by Gruver and Calvert (1958) to explain some unexpected results in the deuterium... 

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