Cyclohex-2 -enones photorearrangement

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). 

Despite the fact that synthetically relevant reactions of cyclohex-2-enones are more often bimolecular ones, for example, wherein the excited enone undergoes cycloaddition to an alkene, (monomolecular) photorearrangements of cyclohex-2-enones have been extensively investigated in recent decades and appropriately reviewed ten years ago. This chapter presents and discusses both novel preparative results as well as alternative mechanistic interpretations to apparently well-known reactions. 

Cyclohex-2-enones bearing at least one aryl group (in addition to an alkyl group) on C(4) undergo photorearrangements different from that presented in Section 76.1. The so-called di-Ji-methane rearrangement mode — typical of molecules containing a 3-phenylprop-l-ene substructure which isomer-izes to a phenylcyclopropane moiety — now becomes predominant, as illustrated for the photoisomerization of 4,4-diphenylcyclohex-2-enone (21) to a mixture of diastereomeric 5,6-diphenyl-bicyclo[3.1.0]hexan-2-ones 22 (Scheme 8). ... 

Schuster, D.I., Brown, R.H., and Resnick, B.M., Stereospecific Triplet-State Photorearrangements of Chiral Cyclohex-2-enones, /. Am. Chem. Soc., 100,4504, 1978. 

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