Oxetane kinetic scheme

The first complete kinetic treatment of an intermolecular interaction was given in the studies of the polymerization of 3,3-bis-(chloromethyl)oxetane % The following kinetic scheme describes the nothstationary process in which termination on polymer competes with drain propagation (initiation is assumed to be fast) ... 

Figure 7.34. A simplifled kinetic scheme for the diastereoselective oxetane formation in the Paterno-Bdchi reaction (Aj and 2 as A, and include intersystem crossing steps) (by permission from Buschmann et al., 1989).

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 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 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 fact, that the difference between these two systems is quantitative rather than qualitative (i.e. the same mechanism but different kinetics) is not always appreciated. For example in Ref.26) it is claimed, that cyclization in oxetane ind THF polymerization proceed by different mechanisms. This is based on the observation that in oxetane polymerization cyclization is concurrent with propagation but in THF polymerization macrocycles appear after the monomer-polymer equilibrium has already been reached. This assumption of two different mechanisms is not necessary, however as we have shown the difference comes only from the differences in kp/kb in Scheme 3.6, conforming to the polymerization of both monomers. 

Recently, Bouchekif et al. published a paper claiming that they observed pseudoperiodic living and/or controlled nature of cationic copolymerization of oxetane (OXT) with tetrahydropyran (THP). Their kinetic interpretation of the results of the copolymerization is, however, not correct. The authors considered the formation of a copolymer as a pseudoperiodic living and/or controlled cationic RO copolymerization of OXT with THP, as described by Scheme 5. 

---