Biradical intermediates, Patemo-Buchi reaction

Recently, studies were carried out to explain the exo/endo selectivity the Patemo-Buchi reaction [30]. These studies were carried out mostly achiral or racemic substrates. Excited monocyclic aromatic aldehydes 33 re in their 3n,/rr state with cyclic enol ether derivatives like 2,3-dihydrofuran (Scheme 8) [31]. In these cases, the sterically disfavored endo isomer 35a obtained as major product. This result was explained by the fate of the trip biradical intermediate G. In order to favor cyclization to the oxetanes 35a,b, radical p-orbitals have to approach in a perpendicular fashion to increase spin-orbit coupling needed for the triplet to singlet intersystem crossing [32]. sterically most favored arrangement of this intermediate is depicted as G. encumbering Ar substituent is orientated upside and anti to the trihydrofur moiety. Cyclization from this conformation yields the major isomer 35a. 

The photo-Diels-Alder reaction of a-acetoTTapfithone 145 with the chiral a-enaminonitrile 146 yielded the cycloadduct 147 with almost complete diastere-oselectivity (Scheme 37) [91,92]. The intermediately formed biradical R is particularly stable owing to delocalization of the radical on the aromatic moiety and to a captodative effect on the enamine moiety. In analogy to the Patemo-Buchi reaction (see Scheme 7), the chiral induction occurred in two steps. In the first step, a chiral center is created at the a-position of the acyl group. In the second step of the diastereoselection, one of the two diasteromeric intermediates undergoes preferential cyclization to yield the final product 147, while the other one is decomposed to form the starting material [92]. 

The Patemo-Buchi reaction is the photocycloaddition of an alkene with an aldehyde or ketone to form oxetanes. This transformation has been shown to proceed through a biradical intermediate, and up to three new stereocenters can be formed as a result of this reaction. A general mechanism for the reaction between an aldehyde and a chiral enol silyl ether is shown in Eq. (13.7) [18]. Allylic 1,3-strain is cited as the control element in reactions of this type, and diastereomeric ratios of >95 5 are reported for products 30 containing four contiguous stereocenters. Examples of photocyclizations of amino acid derivatives proceeding through biradical intermediates have been repotted [19]. 

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]. 

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