Patemo singlet state

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

It is evident from the exceptions noted that the mechanism proposed above does not fully capture the pathways open to the Patemo-Biichi reaction. A great deal of effort has been devoted to deconvoluting all of the possible variants of the reaction. Reactions via singlet state carbonyls, charge-transfer paths, pre-singlet exciplexes, and full electron transfer paths have all been proposed. Unfortunately, their influence on product... 

These adducts were thought to occur via attack of the first excited singlet state of propanal on the diene. Evidence from subsequent studies point to the fact that it is likely the excited singlet state of the aldehyde that is responsible for the Patemo-Biichi reaction with 1,3-dienes. This comes from a number of sources and includes evidence from a number of studies . The synthesis of ( )-6-nonen-l-ol, a component of the sex pheromone of the Mediterranean fruit fiy, applied this process as the first step. Hydrogenation and metal catalyzed [2 + 2] cycloreversion gave 165, which was then easily converted to the target by reduction (Scheme 37). 

One well-known class of photocycloadditions is that of aldehydes and ketones with olefins to give oxetanes375 (e.g. the reaction of 462 with 463 to give 464 and 465).376 This kind of reaction is known as the Patemo-Biichi reaction. The excited state of the ketone is the n-rc one, and it is the orbitals of this state which interact with the ground-state orbitals of the olefin. Often it is the triplet state which is involved, but occasionally the singlet state is important. The orientation usually observed is shown in the following examples, where C- and X-substituted olefins are involved.377,376... 

By a detailed CIDNP investigation [117a] of the Patemo-Biichi reactions of anetholes 31 with quinones 30 in polar medium earlier mechanistic hypotheses were disproved. Stationary and time-resolved experiments showed the mechanism to have the following novel features (cf. Chart XIV) Spin-correlated radical ion pairs (i.e., 30 31,+) are key intermediates for cycloadduct formation free radical ions do not play a significant role. In the singlet state, these pairs undergo back electron transfer geminate reaction of triplet pairs leads to triplet biradicals, which are the precursors to the photoproducts. 

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

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. 

Kutateladze, A. G., Conformational Analysis of Singlet Triplet State Mixing in Patemo Biichi Diradicals, J. Am. Chem. Soc. 2001, 123, 9279 9282. 

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. 

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