Oxetane, basicity

Cyclobutane has not been polymerised cationically (or by any other mechanism). Thermochemistry tells us that the reason is not thermodynamic it is attributable to the fact that the compound does not possess a point of attack for the initiating species, the ring being too big for the formation of a non-classical carbonium ion analogous to the cyclopropyl ion, so that there is no reaction path for initiation. The oxetans in which the oxygen atom provides a basic site for protonation, are readily polymerizable. Methylenecyclobutane polymerises without opening of the cyclobutane ring [72, 73]. 

On the other hand, in cyclic ethers (alkene oxides, oxetans, tetrahydrofuran) and formals the reaction site is a carbon-oxygen bond, the oxygen atom is the most basic point, and, hence, cationic polymerization is possible. The same considerations apply to the polymerization of lactones Cherdron, Ohse and Korte showed that with very pure monomers polyesters of high molecular weight could be obtained with various cationic catalysts and syncatalysts, and proposed a very reasonable mechanism involving acyl fission of the ring [89]. 

More recently, Kim attempted unsuccessfully to anionically polymerize 2-hydroxymethyloxetane [17]. The failure of such a reaction is most likely due to the fact that oxetanes are not known to ring-open under basic conditions. The... 

The instability of protonated oxetane has prevented the determination of p/sfa or gas-phase basicity values for oxetane itself, whereas these have been obtained for tetrahydrofuran and tetrahydropyran. 3,3-Dialkyl substitution, however, sufficiently reduces the reactivity that pKa values can be obtained. The p/sTa for 3,3-dimethyloxetane has been determined to be -2.56, which is less than that for tetrahydrofuran (-2.08) but more than that for tetrahydropyran (-2.79). Since the 3,3-dialkyl substitution markedly reduces the basicity of oxetane towards iodine, the p/sfa of the parent oxetane must be greater than these values (see equation 8) (71BSF4576,80JOC1166). 

The overall course of reaction depends on the relative rate constants for the various secondary radical processes. Aliphatic ketones are often photoreduced to secondary alcohols (4.121, but although there are interesting features in the stereochemistry of the reduction, the method is not a worthwhile alternative to thermal reduction using hydride reagents, except in cases where the substrate is sensitive to basic conditions. Photoaddition of methanol is promoted in the presence of titaniurnfiv) chloride, both for acyclic and cyclic (4.33) ketones the titanium involvement probably starts in the early steps of the reaction, but the detailed mechanism is not known. Addition may also be a major pathway when cyclohexene is used as hydrogen source (4.341 unlike many other simple alkenes, cydohexene does not readily give oxetanes by photocycloaddition (see p. 126). 

Mildly basic to neutral conditions for the ring closure of 1,3-halohydrins include tetra-phenylantimony methoxide as an effective non-basic reagent for oxetane synthesis from 1,3-bromohydrins (90S106). The salts of (3-halo acids cyclize in ionizing media to oxetan-2-ones, as do (3-diazonium carboxylates (64HC(l9-2)787). Thietanes are obtained analogously. 

All of these findings clearly indicate that the demand for synthetic oxetanes is high. There are basically three methods for preparing oxetanes ... 

Similarly, when the polymer produced by cationic polymerization is more basic than the monomer, the centres can lose their reactivity by interaction with the nucleophilic sites on the chains. This situation can be demonstrated by the observation of Penczek and Kubisa during polymerization of 3,3-bis-(chloromethy 1 )-oxetane [113]... 

Oxetanes are synthesized from /1-halo alcohols under basic conditions. Chlorides, iodides,mesylates," " or tosylatcs" are used. Though different bases give good results,... 

Oxetanes are synthesized from /(-halo alcohols under basic conditions. Chlorides,426-429 iodides,430 mesylates,431,432 or tosylatcs433 are used. Though different bases give good results, 1,8-diazabicyclo[5.4.0]undcc-7-ene seems to work best for the cyclization of alcohol 1 to oxetane 2.430 Higher yields are obtained with chlorides, especially if no elimination to form alkenes can occur.426,427... 

As a consequence of the selenoxide instability, the oxidation into selenones is only possible if the syn-elimination reaction becomes difficult, as in the case of strained structures. Under these conditions, the selenone is deprotonated and the a-selenonylalkyllithiums formed can react as other selenium-stabilized car-banions. It must be added that the seleninyl [PhSe(O)-] and selenonyl [PhSe(02)-j substituents behave as very good leaving groups. Vinylic selenoxides and selenones can undergo an intramolecular displacement of the selenium moiety after nucleophilic addition to the double bond under basic conditions. Cyclopropanes and oxetanes have been synthesized in this way [1,2]. 

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