Patemo-Biichi reaction regioselectivity

In the context of photoinduced [2 -I- 2] cycloadditions, the reaction of an olefin with a carbonyl center, known as the Patemo-Biichi reaction, has to be mentioned. The resulting oxetanes are thereby produced with high regioselectivity and stereoselectivity. 

Similar to the regioselectivity of the Patemo Biichi reaction, there is no general rule for stereoselectivity covering all reported examples. Several features must be considered 1) multiplicity, i.e. concerted or nonconcerted, 2) stability of 1,4-diradicals and their competing decay processes, 3) hyperconjugation effects, 4) organized medium etc. 

The reaction is stereospecific for at least some aliphatic ketones but not for aromatic carbonyls. This result suggests that the reactive excited state is a singlet for aliphatics and a triplet for aromatics. With aromatic ketones, the regioselectivity of addition can usually be predicted on the basis of formation of the more stable of the two possible radicals by bond formation between oxygen and the alkene. Some examples of Patemo-Biichi reactions are given in Scheme 6.9. 

The synthesis of oxetanes from alkenic precursors has been demonstrated and the range and scope of Patemo-Biichi photocycloadditions are broad. In general, both the reaction regioselectivity and stereoselectivity can often be predicted by considering the reaction mechanism (Section 2.4.2) for example, the directing effects of alkene substituents are readily imderstood. Synthetic applications are numerous owing to the rtq>id stereocontrolled assembly of multifimctional targets. 

FMO calculations using PM3-C1 were used to investigate the regioselectivities obtained by the photochemical reactions between 2-pyridone and penta-2,4-dienoate." The hard and soft acid-base principle has been successfully used to predict product formation in Patemo-Buchi reactions." The 2 - - 2-photo-cycloaddition of homobenz-valene with methyl phenylglyoxylate, benzyl, benzophenone, and 1,4-benzoquinone produced the corresponding Patemo-Biichi products." The photo-cycloaddition of acrylonitrile to 5-substituted adamantan-2-ones produces anti- and 5y -oxetanes in similar ratios irrespective of the nature of the 5-substituent." ... 

Photochemical Cycloadditions. Isobutene has been widely used in intermolecular [2 + 2] photocycloaddition reactions with enones. The weakly polarized isobutene is often used to study the regioselectivity of the photocycloaddition. Cyclohexenones (eq 9), cyclopentenones (eq 10), and functionalized enones (eq 11 ) undergo cycloaddition with isobutene. Patemo-Biichi... 

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