Acetone triplet, reaction

The decarbonylation of dibenzyl ketone has been shown to result from the carbonyl triplet state by its ability to be quenched by 1,3-cyclohexadiene or l,3-pentadiene.<66) Using 1,3-cyclohexadiene as quencher, photodimers of the cyclohexadiene were obtained. Since these are formed only by triplet sensitization,<66) the quenching of ketone triplet states, rather than their excited singlets, was assured. Further evidence for a triplet reaction follows from the fact that decarbonylation could be sensitized by acetone under conditions where the sensitizer absorbed 93% of the light. 

It has been established that the olefin elimination originates from the triplet state in the acetone-sensitized photolysis of 3-methyl pentanal. The distribution of the butene isomers, formed in the acetone-sensitized reaction, is independent of the wavelength between 2537 and 3130 A, thus, the butenes must have been formed from low-lying vibrational levels of the triplet state. 

Irradiation of the 5 -methyl-9-en-6-one (496 Westphalen structure) in acetone gave the 5a-isomer (497), presumably via the biradical (498) in a triplet reaction. The 5a-methyl-3,6-diketone rearranged on silica gel, or with acid, to give the C-lO-spirocyclopentanone (500). A probable mechanism involving the enol (499) is illustrated. Further reaction with acid gave a compound formulated as the 4,10-bridged structure (501). ... 

The parent benzobarrelene upon direct irradiation produces benzocyclooctatetraene exclusively, whereas acetone-sensitized reaction affords the DPM rearrangement product benzosemibullvalene via a triplet-state pathway. Zimmerman s elegant studies on deuterated benzobarrelene (8) revealed that the benzo-vinyl bridging leading to 9a is the predominant route on direct irradiation, and vinyl-vinyl bridging leading to 10a is the predominant route in the sensitized reaction " (Scheme 3). 

Quantum yields of acetone and the corresponding alkene product of the Norrish type-II reaction for the 2-alkanones were found to be near equal in preliminary experiments of Golemba and Guillet (1972). In most experiments, they used the alkene to monitor the extent of the type-11 reaction. The results showed a small decrease in (pii with increase in chain length of the ketone, which is shown in parentheses 0II = 0.25(6) 0.20 (7) 0.20 (8) 0.20 (9) 0.20 (11) 0.15 (19). Solution-phase photolyses of the ketones were made with added triplet molecule quencher, cis-1,3-cyclooctadiene, at concentrations sufficient to quench virtually all of the triplet reactions. From such experiments, Golemba and Guillet (1972) found the ratios of the quantum yields of the triplet to that of the singlet participation in reaction (II) (fii/(pii = 0.65 with 2-hexanone 0.61 with 2-octanone 0.64 with 2-nonadecanone. 

Excitation appears to be general for this reaction but yields of excited products vary substantially with the substituent R. The highest yield reported is from tetramethyl-l,2-dioxetane [35856-82-7] (TMD) where the yield of triplet acetone is 50% of total acetone formed (18,19). Probably only one carbonyl of the two produced can be excited by the thermal decomposition, and TMD provides 100% of the possible yield of triplet acetone. Singlet excited acetone is also formed, but at the low yield of 0.1—0.3% (17—21). Other tetraaLkyldioxetanes behave similarly to TMD (22). 

Catalytic oxidation of isobutyraldehyde with air at 30—50°C gives isobutyric acid [79-31-2] ia 95% yield (5). Certain enzymes, such as horseradish peroxidase, cataly2e the reaction of isobutyraldehyde with molecular oxygen to form triplet-state acetone and formic acid with simultaneous chemiluminescence (6). 

Oxepin has also been converted photochemically to phenol in 74% yield. This reaction occurs under irradiation conditions by which benzene oxide is excited to a triplet state, e.g. by irradiation in acetone as solvent.207 A rare example for a nucleophilic catalysis of the aromatization of an oxepin/benzene oxide to a phenol has been reported for /err-butyl oxepin-4-carboxylate which undergoes a rearrangement reaction in the presence of trimethylamine to give a mixture of /m-butyl 3-hydroxybenzoate (94%) and 4-hydroxybenzoate (6%).243... 

As it pertains to the solid state photodecarbonylation reaction, the model assumes that most aliphatic ketones have similar excitation energies, that reactions are more likely along the longer-lived triplet excited state, and that each reaction step must be thermoneutral or exothermic to be viable in the solid state. " Using acetone and its decarbonylation intermediates as a reference reaction (dashed lines in Fig. 7.24), we can analyze the energetic requirements to predict the effects of substituents on the stability of the radical intermediates. The a-cleavage reaction of triplet acetone generates an acetyl-methyl radical pair in a process that is 3.5 kcal/mol endothermic and the further loss of CO from acetyl radical is endothermic by 11.0... 

The higher degree of stereospecificity in the singlet reaction relative to that of the triplet is not unexpected in light of the greater lifetime of the triplet bira cal allowing bond rotation or bond breaking to yield acetone and an isomerized olefin. 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

We can illustrate the application of PAC to a simple photochemical reaction. Acetone is readily excited to its singlet excited state which rapidly undergoes efficient intersystem crossing to its triplet state. The triplet state decays in solution primarily by radiationless decay. The PAC experimental waveforms obtained from the photoexcitation of acetone in air and argon-saturated cyclohexane are shown in Fig. 1. In addition, the waveform obtained from the calibration compound 2-hydroxybenzophenone is also shown. 

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