Oxetane formation reaction

Oxetane Formation. Reactions of carbonyl compounds such as ketones and aldehydes with electron-rich olefins results in the formation of oxetanes. The oxetane formation involves the addition of the carbonyl oxygen to the olefinic n -system to produce a biradical intermediate, which then undergoes spin inversion to produce the oxetane (Hor-spool, 1976). Typical examples of oxetane formation include the photoreaction of furan compounds with aldehyde or alkene compounds as shown in Scheme 4 (Cantrell, 1977 Whipple and Evanega, 1968). 

TABLE 11.3 Energy Parameters of Oxetane Formation Reaction, kJ/mol... 

The transition states of the oxetane formation reaction are shown in Fig. 11.7. 

With aromatic carbonyls, oxetane formation appears to arise from the carbonyl triplet state, as evidenced by quenching studies. For example, benzaldehyde irradiated in the presence of cyclohexene yields products indicative of hydrogen abstraction reactions and an oxetane ... 

The reaction of carbonyl compounds to olefins often yields products difficult to obtain synthetically by other routes. The excellent yields obtainable under proper conditions make this reaction of definite preparative interest. Examples of some synthetic applications of oxetane formation follow ... 

For oxetane formation from formaldehyde and ethylene, we should consider the following four transition states and intermediates for the reaction<181) ... 

The other photochemical reactions of simple carbonyls mentioned earlier in this chapter—type I cleavage (a-cleavage) and oxetane formation—will be discussed in Chapter 4. 

In Chapter 3 we discussed two photochemical reactions characteristic of simple carbonyl compounds, namely type II cleavage and photoreduction. We saw that photoreduction appears to arise only from carbonyl triplet states, whereas type II cleavage often arises from both the excited singlet and triplet states. Each process was found to occur from discrete biradical intermediates. In this chapter we will discuss two other reactions observed in the photochemistry of carbonyls, type I cleavage and oxetane formation. 

That oxetane formation results from a singlet state reaction follows from the following evidence (a) Acetone fluorescence is quenched by addition of the olefin, (b) oxetane formation is relatively insensitive to piperylene, and (c) cis-trans isomerization of the olefin is quenched at high olefin concentrations but oxetane formation is not affected. Since oxetane formation was... 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

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). 

Oxetanes are the cycloadducts from a carbonyl compound and an olefin. This one step photochemical formation of a four membered ring heterocycle has been named the Paterno-Buchi reaction 489a> b). Oxetanes are important synthetic intermediates as they can fragment into the carbonyl-olefin pair by which they were not formed (a so termed carbonyl-olefin metathesis). Two examples of such oxetan cracking reactions are shown below in (4.76)490) and in (4.77)491) in this last example the oxetane was used as a precursor for the pheromone E-6-nonenol,... 

The formation of thietanes from thiones and olefins has been less exploited for synthetic purposes than the corresponding oxetane-forming reaction. It should be remarked that thiocarbonyl compounds very often undergo efficient photoreactions from the second excited singlet state S2 U4). One interesting synthetic application is found in the photochemical preparation of quinolines from N-thioamides (4.84)498). The primary photochemical step is assumed to be the intramolecular thietane formation. 

With many other ketones and aldehydes, reaction between the photoexcited carbonyl chromophore and alkene can result in formation of four-membered cyclic ethers (oxetanes). This reaction is often referred to as the Paterno-Biichi reaction.127 128 129... 

One important reaction which competes with photocycloaddition is hydrogen abstraction. The n,ir state can both abstract hydrogen from, and add to olefins. If the starting olefin or carbonyl compound has easily abstractable hydrogens, this competing reaction can become so important that oxetane formation is excluded. 

If the olefin bears easily abstractable hydrogens, this reaction can also compete with oxetane formation and the resulting radical coupling products will predominate. This competition can be expressed as a ratio of yields of addition products to abstraction products (A add/A abs), and will be different for each pair of reactants. This ratio is delicately balanced, and seemingly small changes in the structure of either starting material can shift the predominant reaction. 

A third type of cycloaddition reaction has recently been reported.74 When 5,5,6-trimethyl-3,6-heptadien-2-one 58 was irradiated, two intramolecular cycloaddition products 59 and 60 were obtained, affording the first example of dihydropyran formation from this reaction. Although a reasonable mechanism, analogous to that leading to oxetane formation, has been proposed, it was recognized that 58 is a special type... 

For carbonyl compounds that ordinarily would react in the n,n triplet state, an important reaction which can compete with photocycloaddition is energy transfer to the unsaturated system. When the triplet energy of the unsaturated system is below that of the carbonyl compound, triplet-triplet transfer may take place to the exclusion of oxetane formation. Such is the case when irradiation of a carbonyl compound in the presence of a diene results in isomerization and dimerization of the diene, while no direct photocycloaddition takes... 

When acetone is irradiated in the presence of cyclobutene, the same competition between photocycloaddition and olefin dimerization is observed, with transfer being about ten times more efficient than oxetane formation. In this case, both reactions involve the triplet, since no reaction occurs upon addition of biacetyl.83... 

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. 

Further work by Turro and coworkers70 led to the discovery of a singlet complex in this reaction. A plot of the reciprocal of the quantum yield for oxetane formation versus the reciprocal of the concentration of 12 according to Eq. (34) p. 275 gave a slope of 2.6 but an intercept of 13.2. This suggests that oxetane formation proceeds via an intermediate (complex) which is frequently deactivated. Since the value of Ks obtained from this plot agreed... 

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... 

At high concentrations of t-1, irradiation with dimethyl fumarate yields 34 and a mixture of isomeric oxetanes, 37 (76). Based on the concentration dependence of truxinate and oxetane formation, it was proposed that oxetane formation occurs upon reaction of the t-1 excimer with dimethylfumarate (eq. 18). 

Oxetane formation is presumed to occur via the singlet exciplex however, excitation of the ground state charge-transfer complex may be necessary in order for the formation of 39 to compete with the rapid isomerization of c-1. The factors which favor oxetane versus cyclobutane formation in this reaction are not understood. 

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). 

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