Alkenes oxetane formation

When one reactant is electronically excited ketone and other is ground state alkene-Oxetane formation. 

Mechanistic evidence indicates 450,451> that the triplet enone first approaches the olefinic partner to form an exciplex. The next step consists in the formation of one of the new C—C bonds to give a 1,4-diradical, which is now the immediate precursor of the cyclobutane. Both exciplex and 1,4-diradical can decay resp. disproportionate to afford ground state enone and alkene. Eventually oxetane formation, i.e. addition of the carbonyl group of the enone to an olefin is also observed452. Although at first view the photocycloaddition of an enone to an alkene would be expected to afford a variety of structurally related products, the knowledge of the influence of substituents on the stereochemical outcome of the reaction allows the selective synthesis of the desired annelation product in inter-molecular reactions 453,454a b). As for intramolecular reactions, the substituent effects are made up by structural limitations 449). 

Initially, it was thought more likely that the electron poor metal atom would be involved in the electrophilic attack at the alkene and also the metal-carbon bond would bring the alkene closer to the chiral metal-ligand environment. This mechanism is analogous to alkene metathesis in which a metallacyclobutane is formed. Later work, though, has shown that for osmium the actual mechanism is the 3+2 addition. Molecular modelling lends support to the 3+2 mechanism, but also kinetic isotope effects support this (KIEs for 13C in substrate at high conversion). Oxetane formation should lead to a different KIE for the two alkene carbon atoms involved. Both experimentally and theoretically an equal KIE was found for both carbon atoms and thus it was concluded that an effectively symmetric addition, such as the 3+2 addition, is the actual mechanism [22] for osmium. 

Both the reduction potential of 6-substitutcd cyclohex-2-enones and the ionization potential of the alkene control the product ratio of photocycloadducts vs. photoadducts on the one hand,90 and cyclobutane vs. oxetane formation, on the other91 (a similar dependence of the site of reaction as a function of the ionization potential of the alkene has been observed for 1,4-naphthoquinone92). 3- and 2-Alkynylcyclohex-2-enones on irradiation in the presence of alkenes afford both cyclobutane and cyclopentane derivatives.93-94 The regiochemistry of the photoadducts of simple cyclohex-2-enones to cycloalkenes seems to be dependent on the ring size of this latter species.95... 

M425>. Here the selectivity was only low with respect to the site of addition, which could be either the benzoyl or 4-substituted benzoyl group. Phenyl glyoxylates can also be successfully utilized as reactive carbonyls in the Paterno-Btichi reaction as demonstrated by Hu and Neckers <1997JOC564>. Oxetanes were formed in very high yields with electron-rich (e.g., polyalkylated) alkenes, but with monosubstituted alkenes there was no oxetane formation due to the prevalence of Norrish II type hydrogen abstraction (Scheme 22). 

When the nucleophilicities of the two carbons in alkenes differ significantly, the regioselective formation of 1,4-biradicals results. In fact, the regioselective oxetane formation was reported for the PB reaction of both furans [22] and vinyl ethers [23] (Scheme 7.8). Thus, 2-alkoxyoxetanes are formed exclusively during the PB reaction of furan derivatives. In contrast, 3-alkoxyoxetanes were selectively prepared in the PB reaction of vinyl ethers. This dramatic change in regioselectivity can be explained by the difference in the HOMO coefficient. Thus, in a furan ring, the C-2 carbon is known to be more nucleophilic, whereas the (3-carbon is the nucleophilic site in vinyl ethers. 

A highly diastereoselective oxetane formation was identified in the PB reaction of dihydropyridone with a m-hydroxybenzaldehyde derivative (Scheme 7.33). The chiral auxiliary, when bound to the aldehyde, offered a binding site to which the reaction partner could attach by two hydrogen bonds. In the hydrogen-bonded complex that was produced, the two enantiotopic faces of the alkene could be differentiated [52]... 

Photooxetane formation is quite inefficient, a fact which usually points to the presence of an intermediate which can partially revert to ground state reactants. Cleavage of the diradical must be responsible for some of the inefficiency in oxetane formation 129>. However, in the past few years convincing evidence has appeared that a CT complex precedes the diradical iso.isi). The two most telling pieces of evidence are the relative reactivities of different alkenes 130> and the absence of any measurable secondary deuterium isotope effect on quenching rate constants 131>. Relative quenching rates of sterically un crowded olefins are proportional both to the ionization potentials of the donor olefins 130> and to the reduction potentials of the acceptor ketones 131>, as would be expected for a CT process. Inasmuch as n,n triplets resemble electron-deficient alkoxy radicals, such substituent effects would also be expected on direct radical addition of triplet ketone to olefin. However, radical addition would yield an inverse isotope effect (in, say, 2-butene-2,3-d2) and would be faster to 1,1-dialkylethylenes than to 1,2-dialkylethylenes, in contrast to the actual observations. 

Sensitization of alkenes by carbonyl compounds, which is likely to proceed via an exciplex, can be accompanied by oxetane formation. (See Section 7.4.4.)... 

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) ... 

During the first total synthesis of taxol , R. Holton and co-workers installed an exo-methylene group on the C ring in order to set the stage for the D ring (oxetane) formation. The Burgess dehydration reaction was applied to a complex tricyclic tertiary alcohol intermediate (ABC rings) and the desired exocyclic alkene was isolated in 63% yield. 

Funke, C. W., Cerfontain, H. Photochemical oxetane formation the Paterno-Biichi reaction of aliphatic aldehydes and ketones with alkenes and dienes. J. Chem. Soc., Perkin Trans. 2 1976, 1902-1908. 

The glyoxalate (64) undergoes oxetane formation with electron-rich alkenes. The reactions, as expected for an aryl ketone, take place from the triplet state and occur with high regio- and stereo-selectivity. Some examples of the alkenes and the yields of the oxetanes are illustrated in Scheme 4. A study has described the results from the addition of a variety of ketones and diketones to the alkene (65). 

Photo-addition of alkenes to A methylnaphthalene dicarboxamides in benzene has been studied. The structure of the arene moiety in the imide was important in determining the reaction path. Mainly cyclobutane and oxetan formation occurred. The dicarboximide (342) undergoes photochemical cyclization with incorporation of methanol to yield the two products (343) and (344) in 55 and 16% respectively. This type of cyclization appears to be quite general for such systems and is also reported for the imides (345) and (346). A variety of products resulting from aminolysis, reduction, and radical coupling is produced on irradiation of the phthalimide (347) in diethylamine. ... 

The lowest excited singlet states of aliphatic aldehydes and ketones have lifetimes on the order of nanoseconds, but they can be trapped by alkenes in a diffusion-controlled bimolecular oxetane formation. According to a theoretical study, a C-atom attack mechanism is either a concerted process producing oxetane directly or it involves a C—C bonded transient singlet biradical intermediate that rapidly cyclizes.896 The O-atom attack, in contrast, represents a nonconcerted path, allowing conformational motion of the shortlived intermediate thereby formed. 

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