Patemo 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. 

FMO calculations using PM3-C1 were used to investigate the regioselectivities obtained by the photochemical reactions between 2-pyridone and pcnta-2,4-dienoate.46 The hard and soft acid-base principle has been successfully used to predict product formation in Patemo-Buchi reactions.47 The 2 + 2-photo-cycloaddition of homobenz-valene with methyl phenylglyoxylate, benzyl, benzophenone, and 1,4-benzoquinone produced the corresponding Patemo-Buchi products.48 The photo-cycloaddition of acrylonitrile to 5-substituted adamantan-2-ones produces anti- and svn-oxetanes in similar ratios irrespective of the nature of the 5-substituent49... 

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. 

Scheme 22 Regioselectivity/diastereoselectivity in the Patemo-Bttchi reaction.

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. 

A further utilisation of the Patemo-Buchi enone photoannulation reaction has been performed by Chaudhuri et al. ( ref 30 ) who, by employing 5-(trimethylsilyl)-l,3-cyclopentadiene 77 in acetonitrile at -50 °C ( see scheme 15 ), obtained in a regioselective way only the photoadduct 78, by reaction with methyl-diformyl acetate 48. 

The Patemo-Buchi reaction is one of the more predictable photocycloaddition reactions. Regiocontrol of the photoproduced oxetane is a function of the stepwise addition of the carbonyl chromophore to the alkene [30]. In the case of electron-rich alkenes, excitation of the carbonyl group produces a triplet species that adds to the alkene. The product regioselectivity is a result of addition that generates the most stable biradical, and the triplet lifetime of the intermediate biradical allows for substantial stereoselectivity prior to closing (see Scheme 2). Electron poor alkenes are more likely to undergo cycloaddition with carbonyl groups directly from an exciplex [31]. 

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. 

Abe, M., Torii, E., and Nojima, M., Patemo-Buchi photocycHzation of 2-slloxyfurans and carbonyl compounds notable substituent and carbonyl (aldehyde vs. ketone and singlet- triplet-excited state) effects on the regioselectivity (double-bond selection) in the formation of bicychc exo-oxetanes, J. Org. Chem., 65, 3426, 2000. 

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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