2-Cyclohexenone photodimerization

The photodimerization of 2-cyclohexenone to yield cyclobutanes (70) and (71) has also been reported 84 ... 

More recent investigations of the photodimerizations of cyclopentenone and cyclohexenone<134) have revealed that both reactions result from the lowest triplet states of the enones (Et > 70 kcal/mole), which is probably 77- 77 in character/1331 Both photodimerizations occur via reversibly... 

Effect of Solute-solute Clustering on the Photodimerization of Cyclohexenone... 

Experimental. All photodimerizations were carried out in a stainless steel fixed volume cell (1.75 cm ID with a 1.0 cm path length) with sapphire windows under the irradiation of a Hanovia medium pressure mercury lamp filtered through water and Pyrex for a 13.5 hour exposure. The cell and lamp assembly have been described previously (31). For selected runs a custom built 0.9 mL variable-volume pump was connected to the cell and the pressure was varied to determine the exact location of the phase boundary, based on light scattering measured in a Cary 2290 UV-Vis spectrophotometer (Varian Inst.). The spectrophotometer was also used to measure the concentrations of the monomeric cyclohexenone before and after reaction. 

In order to verify that higher cyclohexenone concentrations increase the HH/HT ratio in SCF solvents as it does in liquids, the photodimerization was performed in high density SCF ethane at different concentrations. In this regime (P = 124 bar and T = 35°C) clustering is presumably minimal. The HH/HT ratio increases with monomer concentration, which is consistent with the trend observed in liquid solvents (30), see Table I. As bulk solute concentration increases by a factor of 3, the increase in HH/HT is relatively small compared with the HH/HT increases portrayed in Figure 7. This comparison suggests that the local solute concentrations are increased dramatically, perhaps by even an order of magnitude or more, in solute-solute clusters. 

The unusual increase in regioselectivity with a decrease in pressure is not present for the photodimerization of a similar enone, isophorone, in SCF CO2 (31). The difference for the two enones is a result of differences in their phase diagrams and differences in the phase region in which the experiments were performed. As shown in Figure 8, the critical point for the cyclohexenone-ethane mixture is close to that of pure ethane. Hence the compressibility is large. In contrast, the... 

The first such reaction published in 1908 by Ciamician and Silber was the light induced carvone —> carvonecamphor isomerization, corresponding to type b [1]. Between 1930 and 1960 some examples of photodimerizations (type c) of steroidal cyclohexenones and 3-alkylcyclohexenones were reported [2-5]. In 1964, Eaton and Cole accomplished the synthesis of cubane, wherein the key step is again a type b) photocycloisomerization [6]. The first examples of type a) reactions were the cyclopent-2-enone + cyclopentene photocycloaddition (Eaton, 1962) and then the photoaddition of cyclohex-2-enone to a variety of alkenes (Corey, 1964) [7,8]. Very soon thereafter the first reviews on photocycloaddition of a,(3-unsaturated ketones to alkenes appeared [9,10]. Finally, one early example of a type d) isomerization was communicated in 1981 [11]. This chapter will focus mainly on intermolecular enone + alkene cycloadditions, i.e., type a), reactions and also comprise some recent developments in the intramolecular, i.e., type b) cycloisomerizations. 

Stable cis- -phenyl-1 -cyclohexene 24 photodimerizes via Diels-Alder cycloaddition to trans adduct 25 (Equation 1.33) [66] and the photoexcitation of dihydrobenzofuran-fused cyclohexenone 26 in net furan gives the trans fused Diels-Alder adduct 27 (Equation 1.34) [67]. 

Additions to Cyclohexenones and Related Systems - A re-investigation of the photodimerization of isophorone (37) has been reported. The study examined the influence of solvent and of the concentration of the enone. Some of the results and the yields of dimers obtained are shown in Scheme 4. From this detailed study the authors suggest that supramolecular structures are involved in the dimerization. These apparently take part even at low concentrations of enone. The photocycloaddition of enones such as (39) to buckminsterfullerene (Ceo) has been studied. The outcome of the addition is the formation of low yields of furanylfullerenes. This addition occurs to the exclusion of de Mayo type of addition. Photocycloaddition of the cyanocyclohexenone derivative (40) to alkenes has been reported. ... 

Yang J, Dewal MB, Shimizu LS (2006) Self-assembling bisurea macrocycles used as an organic zeolite for a highly stereoselective photodimerization of 2-cyclohexenone. J Am ChemSoc 128(25) 8122-8123... 

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