Substituent effects olefin carbonylation

This article will only discuss two particular kinds of photocycloaddition reactions, the photodimerization or cross-cycloaddition of two olefins to yield a cyclobutane derivative, and the photoreaction of an olefin with a carbonyl compound to give an oxetane, Eq. 1 and Eq. 2. The inportance of substituent effects in reactions of these types is pointed... 

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). 

Cyclization reactions utilizing a vinyl sulfide group were also examined (Scheme 34) [46], This substrate was chosen for study because, like the methyl substituents used earlier, the sulfide would have a favorable conformational effect on the substrate and would serve as an electron-donating group for making the olefin more nucleophilic. Unlike the methyl substituent, the use of the sulfide led to a carbonyl product that could then be used to further elaborate the cyclized product. Hence, the success of the vinyl sulfide-based cyclization reaction served to extend the synthetic scope of these reactions. 

The orientation of the two molecules in the CT complex is not well understood. Turro has presented evidence that substituents on the ketone in the nodal plane of the carbonyl -system stoically hinder CT quenching of the triplet ketone 133). The effect is not large but given that the approach between donor and acceptor in CT complexes is not as close as required for covalent bonding, a large effect is not needed for one to infer that the donor orbital probably overlaps with the half-vacant w-orbital. Inasmuch as the olefin has radical-catonic... 

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