Benzophenone oxetane formation

Yang144 has reported a study of 3-methyl-2-pentene (30), which undergoes isomerization and oxetane formation when irradiated with benzophenone or benzaldehyde. The quantum yields for these processes are shown in Table V. 

In the presence of benzophenone sensitizer, dimerization is observed accompanied by oxetane formation (67MI1). U V irradiation of 6-acetylura-cil in water gives a tetracyclic oxetane (79MI1 83MI2) (Scheme 25). 

Since most simple alkenes have a high triplet energy ( t = 75-78 kcal/ mol), triplet sensitizers have to be chosen accordingly to prevent oxetane formation (see Section 7.4.4), as shown in Scheme 14 for norbornene with acetophenone (E = 75 kcal/mol) and benzophenone ( t = 69 kcal/mol), respectively, as sensitizers (Arnold et al., 1965) ... 

Photolysis of benzophenone (23.) with 1-methyl-2,4,5-triphenyl imidazole ( 2) in either acetonitrile solution or in the solid state (33) produces oxetane 21 (eq 6) (30). Irradiation of 4,4 -dimethylbenzophenone (2), 4-methylbenzophenone ( ), and 4-benzoylpyridine (15) in solution in the presence of 22, gives likewise oxetane adducts. In the solid state (33), however, no oxetane is formed from these three ketones. It appears that only 13 can take a relative geometric arrangement with 21 that is suitable for the oxetane formation in the solid state. 

Cycloaddition to five-membered heterocyclic aromatic compounds is well documented. Examples reported this year include the formation of azetidine adducts (67) by irradiation of 3-(p-cyanophenyl)-2-isoxazoline with furan or thiophen (and also with benzene) benzophenone-sensitized reaction of selenophen with dimethylmaleic anhydride (68) to give 1 1 and 1 2 adducts and oxetan formation from benzophenone and 1-acylimidazoles (69), thiazoles, or isoxazoles. ... 

Reaction of triplet benzophenone with 5-methylenenorborn-2-ene is regiospecific since oxetan formation occurs only by addition to the exocyclic double bond the oxetans (324) and (325) are formed in 55 and 15% yields respectively. The related dihydro-oxetans are formed from 2-methylenenorbornane (57 and 10% respectively), and norborn-2-ene affords the exo-oxetan in 68 % yield. Explanations for the regio-specificity in the case of 5-methylenenorborn-2-ene, based on homoconjugative interactions between the tc-bonds and on the formation of the most stable biradical... 

Past attempts to achieve the photoaddition of carbonyl compounds to pyrroles (Paterno-Buchi reaction) have failed. Benzoylation on nitrogen decreases the electron density to a point where oxetane formation is feasible, and adducts from benzophenone and 3- and 4-benzoylpyridine have now been prepared (Scheme... 

The photocycloaddition of triplet benzophenone to norbornene was originally reported by Scharf and Korte. The photoproduct 101 that is formed in high exo-selectivity could be thermally cleaved to the 5,e-unsaturated ketone 102, an appHcation of the carbonyl-olefin metathesis (COM) concept. The 1,4-biradical formed in the interaction of norbornene with o-dibenzoyl-benzene was trapped in an intramolecular fashion by the second carbonyl moiety. A highly regioselective reaction of triplet benzophenone was reported with 5-methylenenorborn-2-ene, with preferential attack toward the exo CC double bond. A number of publications have discussed the photocycloaddition reactions of triplet carbonyl compounds to norbornadiene and quadricyclane, as weU as the competition between the Paterno-Biichi reaction and the sensitized norbornadiene/quadricyclane interconversion. Oxetane formation has also been reported for the photoreaction of biacetyl and para-quinones with benzvalene. ... 

Gorman, A. A., Leyland, R. L., Parekh, C. T, and Rodgers, M. A. J., The reaction of triplet benzophenone with 5-methylenenorborn-2-ene regiospecific oxetane formation, Tetrahedron... 

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

The photocycloaddition of a carbonyl compound to an olefin with the formation of an oxetane 3 was first observed by Paterno and Chieffi in 1909.1 They reported obtaining a compound in good yield from the irradiation of benzophenone in a petroleum mixture rich in 2-methyl-2-butene, for which they proposed the oxetane structure. Although Paterno and Chieffi had suggested the correct structure, it was not until 1954 that Buchi and his collaborators reinvestigated this reaction and... 

The photocycloaddition of carbonyl compounds to unsymmetrical olefins (electron rich) can give two products however, usually one predominates. For example, the photocycloaddition of benzophenone to isobutylene gives a mixture of the two oxetanes 30 and 31 in the ratio of 9 1.17 This ratio is consistent with the preferential formation and/or closure of the intermediate 30a relative to 31a. The diradical 30a is more stable than 3la since a tertiary radical is more stable than a primary radical by about 8 kcal.62 Many of the examples listed in Section VII are consistent with this apparent generalization there are, however, exceptions. 

When benzophenone is irradiated in either cis- or trans-2-butene solution, the photocycloaddition yields essentially the same mixture of both isomeric oxetanes, 40 and 41, before significant isomerization of the starting olefin occurs.37 This result strongly supports a mechanism involving the diradical 39. Since 39 was undoubtedly formed initially in the triplet state (from the n,ir triplet) spin inversion was necessary before bonding. There is no evidence pertaining to the formation of this intermediate from the n,n singlet. 

Another characteristic of photocycloaddition to electron-rich alkenes is the loss of any stereochemistry of the starting alkene in the oxetane structure. An example is the formation of practically the same mixture of geometric isomers of 2,2-diphenyl-3,4-dimethyloxetane from benzophenone and either cis- or trans- 2-butene (equation 103). This is understandable on the basis of the diradical intermediate having a sufficiently long lifetime for bond rotations to occur. 

The formation of 3-pyrrolylcarbinols (280) from the photochemically induced reaction of pyrrole, or its 1-alkyl derivatives, with aliphatic aldehydes and ketones is thought to proceed via an oxetane intermediate (279) (79JOC2949). In contrast, the analogous reaction of 1 -phenylpyrrole with benzophenone leads to the formation of the diphenyl(2-pyrrolyl)car-binol, whilst the oxetane (281) has been isolated from the photoaddition of 1-benzoylpyrrole and benzophenone (76JHC1037, B-77MI30500). 2-Benzoyl-1-methylpyrrole undergoes a normal Paterno-Buchi photocyclization with 2,3-dimethylbut-2-ene, via the n -> v triplet... 

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 kinetics of the reaction is in agreement with a mechanism involving the formation of the biradical 6 following Scheme 3.6, where B is benzophenone, F is furan, (B-F) is the biradical 6, and Ox is the oxetane [28]. Subsequently, more complex carbonyl compounds were used to give the corresponding... 

The photochemical reaction of carbonyl compounds and alkenes, which is referred to as the Paterno-Buchi (PB) reaction, was developed in 1909 [13], and is currently one of the most widely used methods for oxetane synthesis (Scheme 7.4). As exemplified in the PB reaction of benzophenone with 2-methylpropene [14], a selective formation of the oxetane is possible even when the photochemical reaction involves highly unstable molecules that is, the excited state of carbonyls. Due to its synthetic importance and mechanistic interest, the PB reaction is the most extensively studied synthetic method for oxetanes. Thus, several extensive reviews describing the PB reaction have been published since 1968, and the reader is directed towards these for further information [15]. In this chapter, methods that allow for the control of the regioselective and stereoselective formation of synthetically important oxetanes will be described. 

Recently, a notable temperature-related effect was reported for site-selectivity (double-bond selectivity or chemoselectivity) in the PB reaction of unsymmetrically substituted furans (Scheme 7.14) [30]. For example, the selective formation of the more substituted oxetane, OX1, was observed during the PB reaction of 2-methyl-furan with benzophenone at a high temperature (61 °C). However, a 58 42 mixture of the oxetanes, 0X1 and 0X2, was reported at low temperature (—77 °C). This notable effect of temperature could be explained by the relative population of conformers of the intermediary triplet 1,4-biradicals, T-BR1 andT-BR2. The excited benzophenone was considered to attack the double bonds equally so as to produce a mixture of the conformers of T-BR1 and T-BR2 however, at low temperature the conformational change was suppressed. Thus, the site-random formation of oxetanes 0X1 and 0X2 was observed after the ISC process. Nonetheless, at high... 

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