Patemo triplet 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. 

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... 

Oxetane Formation—The Patemo-Bnchi Reaction. A large number of carbonyl compounds, primarily aldehydes, ketones, and quinones, form oxetanes by photocycloadditions to olefins.61-63 In general, it is observed that (/) carbonyl compounds which have low-lying (77, ) triplet states and which are photoreduced in isopropyl alcohol form oxetanes most readily, and (2) oxetane formation takes place when energy transfer from the carbonyl compound to the olefin is unfavorable because of the relative location of their triplet levels.64,65 Hence, oxetanes are most readily formed from simple olefins and allenes63,66 but are seldom formed from dienes.67 An extensive review by Arnold63 covers the mechanism and scope of this reaction. 

The use of dienes such as piperylene in the quenching of triplet states of carbonyl compounds is well known. However, in many photochemical studies, attempts to quench the carbonyl Ti state are often complicated by the formation of oxetane photoproducts.Since the photosensitized dimerization of diene triplets is well known, synthetic applications utilizing dienes in the Patemo-BUchi photocycload dition are rather limited. However, the diene addend not only allows for rapid assemblage of functionality but remains a continuing challenge in terms of experimental efficiency and generality. 

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... 

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

Dimerizations between adjacent pyrimidine bases are by far the most prevalent photoreactions resulting from UVC or UV-B irradiation of DNA (Table 140.1), with the relative induction dependent on wavelength, DNA sequence, and protein-DNA interactions. The major photoproducts in DNA are the cydobutyl pyrimidine dimer (CPD), formed through the exdted triplet state, and the pyrimidine-(6-4)pyrunidone dimer [(6-4)PD], formed from a Patemo-Biichi reaction. The (6-4) photoproduct undergoes further UV-B-dependent conversion with its valence photoisomer, the Dewar pyrirnidinone. In addition to the major photoproducts, rare dimers may also form, such as the adenine-thyrnine heterodimer or the 2-imida-zolone(5 )pyrimidone product, another photoisomerization product of the (6—4)PD. 

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. 

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. 

The Patemo-Biichi reaction of carbonyl compounds to alkenes fails when the energy difference between the triplet and groimd states of carbonyl compound is greater than that between the corresponding states of the alkene. In such case, the... 

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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