Photochemistry of cyclobutanones

The photochemistry of cyclobutanones differs from that of less strained larger cycloalkanones. Fragmentation to ethylene and ketene (derivatives), decarbonylation and rearrangement to oxacarbenes predominate here. The oxacarbene formation, which occurs with retention of the configuration of the... 

Photochemical transformations of conjugated cyclohexenones, 317 Photochemical transformations of non-conjugatea ketones, 292 Photochemistry of cyclobutanones, 293 Photolysis of nitrites, 253... 

The photochemistry of cyclobutanone presents a special case since the Norrish type-I cleavage to give an acylalkyl diradical intermediate releases ring-strain energy. Thus the energy available for subsequent reactions is reduced correspondingly, compared to the energy retained in an acyl radical from an acychc ketone, or less strained cyclic ketones. 

Photochemistry of cyclobutanones in the presence of HCN or malononitrile gave photoadducts derived from an oxacarbene insertion into a C-H bond <07TL2787>. 

We shall see that a discussion of the photochemistry of cyclobutanones in solution will help unravel the nature of the rapid new process which makes of cyclobutanones so much more reactive than other alkanones. The inherent ring strain of cyclobutanone probably triggers the pre-dissociative behavior of the molecule in the state. 

To date, the photochemistry of cyclobutanone has been most extensively studied in the gas phase (14), where the two processes, 3-cleavage (or cycloelimination) and decarbonylation, are the only observable reactions. The solution-phase photochemistry of this parent ketone, on the other hand, has been less extensively studied. Turro and Southam (15) investigated the photochemical transformations of cyclobutanone [21] in methanol and observed products attributable to the three expected reactions. 

Klemm (16) and Lee and coworkers (17) have examined the effect of various solvents on the photochemistry of cyclobutanone. By monitoring the quantum yields for formation of ethylene (B-cleavage product) and cyclopropane (decarbonylation product) in different solvents, they were able to demonstrate a significant reduction in the quantum yields for product formation in methanol as compared to other hydrocarbon solvents. Whereas the quantum yield of ethylene formation was found to be essentially solvent insensitive, that for cyclopropane formation was found to be somewhat solvent sensitive. This suggested that B-cleavage and decarbonylation do not result from the same immediate precursor. Since ring-expansion derivatives have not been isolated from photolyses carried out in saturated hydrocarbon solvents, the importance of this process under these conditions remains to be determined. Irradiation of cyclobutanone in the presence of 1,3-penta-diene (17,59) or 1,3-cyclohexadiene (16) did not appear to affect the quantum yields for ketone disappearance or product appearance. 

The photochemistry of cyclobutanones possessing a spiro-fused three-membered ring at the a-position has also been reported (15b). Spiro[2.3]hexan-4-one[64a], for example, undergoes photolysis in methanol solution to cyclic acetals [65a] and [66a] in addition to 3-cleavage ester [67]. The dialkylated derivative [64b], on the other hand, affords ring-expanded acetals [65b] and [66b]... 

We can now consider, simply from the empirical standpoint, some "rules" which pertain to the photochemistry of cyclobutanones. (1) Three reactions are commonly observed B-cleavage, decarbonylation, and ring expansion... 

To obtain a detailed understanding of the photochemistry of cyclobutanones in solution, we must understand the cause of the general quantum inefficiency exhibited by cyclobutanones. As we have seen, there is substantial evidence to support the idea that deactivates very efficiently (for saturated cyclobutanones). We can identify four processes which might be attributed as the cause of inefficiency (a) intersystem crossing to T ... 

Lee and coworkers (17) have also investigated the solution-phase photochemistry of cyclobutanone and have found (X - 310 nm) to be l/20th of that obtained in the vapor phase. This reduction has been attributed to enhanced predissociation by the interaction of vibrationally excited with neighboring solvent molecules. If this solvent interaction in addition to ring strain already present in the cyclic ketone both facilitate a-cleavage from the vibrationally excited state, relative to intersystem crossing to T, then it is not unreasonable to propose the cyclobutanone state as the precursor to some, if not all, of the observed photoproducts. 

From this state, ring strain facilitated predissociation to a "biradical-like" transition state [135] or vibrational relaxation (k ) to S may occur. It is also conceivable that transition state [135] could be produced directly from S °. Alternatively, molecules in the S ° state could intersystem cross (kST) to the triplet manifold (T ). For 2-alkylidenecyclobutanones, reactivity is manifested in isomerization about the exocyclic carbon-carbon double bond, while for the saturated cyclobutanone derivatives studied, definitive evidence for solution-phase reactivity is not available. If analogy is again made to the vapor-phase photochemistry of cyclobutanone [21], reactivity could conceivably result in decarbonylated products. Indeed, preliminary evidence has been obtained from sensitization experiments employing m-xylene as triplet sensitizer that decarbonylation of a saturated cyclobutanone is enhanced by selective population of its state (35). ... 

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