Triplet enone

Mechanistic evidence indicates 450,451> that the triplet enone first approaches the olefinic partner to form an exciplex. The next step consists in the formation of one of the new C—C bonds to give a 1,4-diradical, which is now the immediate precursor of the cyclobutane. Both exciplex and 1,4-diradical can decay resp. disproportionate to afford ground state enone and alkene. Eventually oxetane formation, i.e. addition of the carbonyl group of the enone to an olefin is also observed452. Although at first view the photocycloaddition of an enone to an alkene would be expected to afford a variety of structurally related products, the knowledge of the influence of substituents on the stereochemical outcome of the reaction allows the selective synthesis of the desired annelation product in inter-molecular reactions 453,454a b). As for intramolecular reactions, the substituent effects are made up by structural limitations 449). 

The [2 + 2] photocycloaddition reaction of enones with allenes was first reported in 1966. A diradical intermediate is formed from a triplet enone via an exciplex. The triplet diradical cyclizes to the product after spin inversion to the singlet state [31,32]. 

The proposed exciplex orientation, suggested to dictate the regioselectivity of the photoaddition, is consistent with the orientation of the dipolar attraction between the electronically n,7r excited triplet enone and the ground state alkene. Generally, electron acceptor substituents on the alkene provide preferential formation of the head-to-head (H,H) products, whereas electron donor substituents provide preferential formation of the head-to-tail (H,T) photoproducts55 (Scheme 16). 

The mechanism of the enone-aikene [2+2] photocycloaddition presumably follows the scheme below. Upon irradiation 1) a triplet exciplex is irreversibly formed from the triplet enone and ground state alkene 2) the triplet exciplex collapses to one or more 1,4-biradicals. 3) the biradicals either cyclize to the cyclobutane or revert to starting materials and 4) the biradical reversion decreases the overall efficiency of the process. 

A single electron transfer mechanism is involved in the phototransformation of the enones (38) into (39) in the presence of phosphites. The reactions are carried out in acetonitrile and proceeded by the triplet enone to which an electron is transferred from the phosphite to give the radical cation/radical anion pair (A). Collapse of radical cation component of (A) gives (B) which then reacts by addition to the enone radical anion. The products (39) are isolated after hydrolysis of the corresponding silyl ethers. The influence of ring size and substituents was also examined and these results are given in Scheme... 

In fact, as shown by the quantum yields observed in fert-butyl alcohol, these reactions appear to be quite inefficient, e.g. formation of 5 and 6. From quenching and sensitization studies, it was shown that the triplet excited state is responsible for these rearrangements. The rate of rearrangement of triplet enone to product (or some precursor of product) was found to be about 10 s F... 

A common theme in enone cycloadditions is the involvement of a triplet enone excited state. The regioselectivity of triplet photocycloadditions is typically explained by formation of the most stable intermediate biradical species (see Scheme 1). Schuster et al. [22] and Weedon et al. [23] have made significant contributions to the understanding of enone photoreactivity. Corey et al. [24] originally suggested that the [2 + 2] reaction involved a polar Jt-complex, and this justification for the observed regioselectivity continues to appear in the literature as will be described below. 

Table 4 Values of the density of the a HOMO, the a LUMO, the HOMO and /J LUMO orbitals, condensed to the O, C and atoms of the triplet enones considered in this work, using...

According to DFT calculations, the photoreaction of cycloalkenones with alkenes generally occurs through interaction between the LSOMO of the triplet enone and the HOMO of the alkene. The product distribution is explained by coupling of the LSOMO and HSOMO in the biradical intermediates. ... 

These latter results, combined with the older ones reported, were interpreted as follows. Excitation of 57, followed by intersystem crossing, gives triplet enone 63, which reacts with furan to give two diastereomeric biradicals, 64 and 65, the precursors of 58 and 60, and 59, respectively. In the absence of furan, 63 affords -enone 64, which either re-isomerizes to 57 or is trapped by methanol to give 61 and 62. In contrast to (carbocyclic) cyclohexenones, two regioisomers are formed from 57 as both the carbonyl group and the S-atom stabilize an adjacent carbanionic center (Scheme 20). 

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