Mechanism oxetane formation

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 mechanism given by Eqs. 12—18 is a minimum mechanistic scheme for oxetane formation derived mainly by combining suggestions... 

The mechanism of oxetane formation is similar to the one discussed for cyclobutane formation in chapter 4.3.3. The 1,4-diradicals can be efficiently trapped with molecular oxygen. The resulting 1,2,4-trioxanes are interesting synthetic intermediates (4.81) 495>. 

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. 

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... 

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. 

Dichlorine heptoxide, which can be a powerful oxidizing agent, reacts with oxetane in carbon tetrachloride solution at 0°C to give a fair yield of propane-1,3-diperchlorate (equation 38). It was suggested that the mechanism involved formation of a per-chlorooxetanium ion, which subsequently reacted with perchlorate ion (75JOC81). 

Figure 4.53 Mechanism of oxetane formation between going through a biradical intermediate...

Molecules of 43c adopt chiral packing (space group P2 ) and a helical molecular conformation, and crystallize in (E,Z) conformation which is unfavorable for the oxetane formation. The solid-state irradiation of 43c was found to give the oxetane 44c and a 3-lactam derivative 45c. The (3-lactam 45c was revealed to be enantiomerically enriched to 88% ee, whereas the other photoproduct 44c was racemic. The occurrence and the mechanism of transformation of 43c to 45c involve hydrogen abstraction by the alkenyl carbon atom. 

One case of apparent defiance of the trans requirement for oxetane formation has recently been reported in the solvolysis of the 3"tosylate (i6) of 3j5,5jS dihydroxycholestan"6"One [ 138]. Under strictly anhydrous conditions the major product was the 3 ff,5j5 oxetane (17), although aqueous solvents gave mainly the 3a,5jS-diol. The mechanism of internal cis substitution at C(3) is not known. Various possibilities include the formation of an internally "solvated ion-pair (18), as a consequence of the extra acidity imposed upon the 5i -hy-droxylic proton by the 6 oxo group this could be followed by oxetane formation with extrusion of a molecule of toluene- -sulphonic acid. 

Scheme 14. Cation radical chain mechanism for oxetane formation.

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. 

The scheme above resembles the two-step mechanism for oxetane formation by cycloaddition of olefins to monoketones which is generally accepted 1-4> to proceed via the n,7t triplet state of the ketone. It is assumed, by analogy, that the same excited state is involved in dione reactions. Additional support for intermediacy of the n,n triplet and concomitant stepwise formation of the new bonds derives from a kinetic study 30> of the reaction of PQ with cis- and 7m s-stilbeneb> where the... 

The mechanisms of two particular classes of photochemical (2 + 2)-cycloaddition reactions have been studied extensively, viz. (i) of reactions in which either the heterocyclic or the substrate contains an enone moiety,1231,24 which are believed to occur via the excited triplet state of the enone, and (ii) of oxetane formation, which is thought to proceed via n — 7T excitation of the carbonyl group, followed by intersystem crossing to the triplet state and addition to the carbon-carbon double bond in its ground state6 in some cases, however, an oxetane has been reported to be formed from a ground-state ketone and an excited-state olefin (Section III,C,2). 

In parallel with these experiments, studies on the mechanism of formation of TXBj were carried out [19]. It appeared conceivable that more than one reaction would be needed for the formation of the non-prostanoic acid structure of TXB2 from PGG2/PGH2. An attractive sequence of reactions consisted of enzymatic formation of a bicyclic oxetane-oxane derivative followed by non-enzymatic hydrolysis of the oxetane ring to yield TXB2. In order to test this possibility, short time incubations (30-60 s) of arachidonic acid with platelets were carried out. The... 

Two recent reports have appeared of oxetan formation via an exciplex derived from singlet-state olefin and ground-state carbonyl, rather than the much more usual excited-state carbonyl and ground-state olefin. In one of these the oxetan (31) and the cyclobutane (32) were formed from phenan-threne and dimethyl fumarate, whereas in the other the oxetan (33) and olefin dimers were obtained on irradiation of methyl coumarilate and benzo-phenone. In the latter case, the increase in the quantum yield of oxetan at the expense of the dimers on increasing the concentration of ground-state benzo-phenone relative to olefin is compelling evidence for the proposed mechanism. 

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