Photorearrangements enones

Another example of acyclic enone photorearrangements has been provided by Dauben and co-workers(69> ... 

They argue that this and related enone photorearrangements do not take place via an a-cleavage (8.67), inasmuch as bonding to the central carbon atom of the allylic radical (47) or the rearrangement of an allyl radical to a cyclopropyl radical is the least favorable path available to the radical species ... 

Enones undergo a variety of photorearrangements 333). In contrast to a,P-unsaturated ketones which react in many different ways, p,y-unsaturated ketones undergo the oxadi-7c-methane rearrangement in sensitized irradiations in complete analogy the products formed are cyclopropyl ketones... 

Diarylcyclohex-2-enones undergo a different photorearrangement to bicydol3.1.0]hexan-2-ones, in which an aryl substituent migrates from C-4 to C-3 (4.641. This reaction finds a parallel in the di-n-methane rearrangement of 3-phenylalkenes (see p. 54). It is usually efficient (<> — 0.1-0.2), it occurs by way of the (n.Jt ) triplet... 

Over the last decade, a large number of examples (in the crystalline state) have corroborated the reaction cavity model due to Cohen and have brought out elegantly the need to have free volume within reaction cavities for the occurrence of solid state reactions. Even quantitative correlations have been attempted. Scheffer, Trotter, and co-workers have examined free volumes within reaction cavities to gain insight into the mechanism of intramolecular photorearrangements of enones [126,127]. They have shown that the course of a solid state reaction is influenced profoundly by certain specific... 

The observed quenching effects suggest that the excitation energy of the reactive triplet may be close to the 71 kcal Zimmerman estimated from the phosphorescence spectrum of 35. On the other hand, lumi-santonin, also an enone, must have a relatively low-energy reactive triplet, since its rearrangement is sensitized by Michler s ketone.401 From a Stern-Volmer plot of the effect of naphthalene on the quantum yield for photorearrangement of 35, kjkr was measured to equal 7000.416 Thus if naphthalene quenches triplet 35 at the diffusion-con-... 

Enone Photorearrangements. As will be seen, the steric compression is associated with the substituents attached to the... 

The preceding discussion reveals a few of the complexities of the photochemistry that is typical of carbonyl and azo compounds. Results on the olefin-carbonyl Paterno-Buchi system,30 and on the photorearrangements of a, (i-enones,31 P, y-enones,32 azo compounds (diazomethane33 and cyclic diazoalkenes104), and acylcyclopropenes34 show similar features. In these examples, one encounters fourfold intersections as well as conical intersections and singlet-triplet crossings. Thus the potential surfaces are more complex than in hydrocarbon photochemistry. 

Similarly, the photorearrangement of p,y-unsaturated e-diketone (75) (c/. Table 1) has been probed in view of its potential application as a key step for the synthesis of cedranoids." Although the tricyclooc-tanedione (76) is cleanly accessible, the scheme has been abandoned in view of difficulties in assembling the ultimately required functionalities routine p,7-enone photochemistry has been employed instead. 

Cyc ic enones and dienones undergo a number of photorearrangements. As in in Scheme 49, both ring contraction via a [1,2] shift (Zimmer-... 

Table 16. Enones and Other Products from the Photorearrangements of Cyclic Conjugated Cyclopropyl Ketones...

Cyclic enones, such as substituted cyclohex-2-enones or cyclohexa-2,5-diones, also undergo sigmatropic photorearrangement to form bicyclo[3.1.0]hexanones (lumiketones) or bicyclo[3.1.0]hex-3-en-2-ones, respectively, for which both concerted and stepwise (biradical) reaction mechanisms have been proposed.640,641,770 For example, a [l,2]-shift concurrently with the ring contraction (termed the type A reaction) is observed upon irradiation of the methylphenyl derivative 159 in polar solvents, whereas phenyl migration (termed the type B reaction) predominates in nonpolar solvents (Scheme 6.70).771,772 The reactions are believed to proceed via both the n,n and n,Tt triplet ketone states. In the presence of alkenes, cyclic enones may readily undergo a competitive photocycloaddition reaction (Section 6.1.5). 

Photorearrangements in the crystalline state773 usually afford products with very high selectivity (Special Topic 6.5). Whereas irradiation of 4,4,5-triarylcyclohex-2-enone (160) in benzene solution affords a 1 1 ratio of phenyl to /)-cyanophenyl migration to form 161 and 162, the former product is produced exclusively in the solid state (Scheme 6.71).755... 

The photorearrangement of chiral cyclohexadienones and cyclo-hexenones has been covered in other reviews and is not examined here [173]. However, it is now well established that the formation of lumiketones from rigid enones and dienones (226, 228, and 231), or from monocyclic, chiral cyclohexenones having C-4 as the only asymmetric center, occurs with more than 99% selectivity [174] (Scheme 36). 

Kinetic studies demonstrated that adducts 30 and 31 and lumiketone 29 arise from the same enone excited state. Abundant data indicates that the photorearrangement arises from the enone (Jt,Jt ) state. When DMCH is irradiated in neat alkene, oxetane 32 is formed in addition to the other products. The... 

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