Of oxiranes and thiiranes

In general, the methods of synthesis and reactions of oxiranes and thiiranes fused to ordinary or large rings are not particularly affected by the ring fusion. The reader is referred to Chapters 5.05 and 5.06, and reviews cited there, many of which include fused-ring examples. 

Stereoelective polymerization of enantiomerically unbalanced (i.e., partly resolved) mixtures of oxiranes and thiiranes has given suq)rising results (277, 318, 322). The stereoelectivity ratio, r, is greatly dependent on the enantio-... 

Another possibility in the method is the reaction via a 3-hydroxydiphenylphosphine oxide. Deoxygenation takes plaee with retention by means of triphenylphosphine derivatives ArsP=X (X = S, Se). (Eq. 103). Triphenylphosphine itself has often been employed in deoxygenation of oxiranes and thiiranes. 

Laser flash-photolysis investigations have been carried out for benzylchloro-carbene, phenylchlorocarbene, methoxyphenylcarbene and fluorenylidene, and some other carbenes, generated from diazirine, diazo and other precursors, in the presence of oxiranes and thiiranes. These carbenes abstract oxygen or sulfur atoms with bimolecular rate constants in the range 10 to 10 s ... 

Spassky, N. Stereoselective and stereoelective polymerization of oxiranes and thiiranes. ACS Symposium Series 1977, 59, 191-209. 

Ionic polymerization of oxiranes and thiiranes is usually divided in three main types anionic, cationic and stereospecific-"coordinated". While anionic and cationic polymerization produce random amorphous polymers, at least when starting from racemic monomers, stereospecific initiators may give isotactic crystalline polymers. 

Most of the work in the field of stereoselective and stereoelective polymerization of oxiranes and thiiranes was carried out on monosubstituted monomers and was reviewed in some publications... 

Influence of the nature of the initiator Initiators resulting from the reaction between an organometallic derivative and a chiral compound containing an acidic hydrogen were the most usually employed in the stereoelective polymerization of oxiranes and thiiranes. 

Up to now we have found that the best stereoelective initiator system for polymerization of oxiranes and thiiranes resulted from the reaction of diethylzinc and (-)3,3 dimethyl 1,2 butane diol (DMBD) taken in (1 1) proportion. 

The interest for such stereospecific polymerization started beginning of 50 S when crystalline polypropylene oxide of h.m.w. was synthesized. Numerous researches performed and still under investigation in the field of oxiranes were summarized in several reviews (1-5). The study of stereospecific polymerization of thi-iranes started later, early 60 S, and the main results were also recently reviewed (6-8). The comparison of different aspects of polymerization of oxiranes and thiiranes were examined in few publications (9-12). Practically no work have been succeeded in the field of coordination type polymerization of aziridines, although the cationic polymerization was extensively studied. 

The aim of this paper is to present the main aspects of the stereospecific polymerization of oxiranes and thiiranes with emphasis on the more recent results obtained in this field. Unpublished data are included and new possible ways are discussed. 

The kinetics of the reaction with different classes of alcohols was studied by Tsuruta (2) using IR spectroscopy. Products with predominance of dialkoxide species (y y x) are excellent initiators for the polymerization of oxiranes and thiiranes, while products with predominance of alkylalkoxide species are generally not polymerizing oxiranes but polymerizing thiiranes. 

Another initiator which was extensively used in polymerization of oxiranes and thiiranes is diethylzinc-water system. 

For example, in the case of oxiranes and thiiranes one must prepare first the corresponding chiral 1,2 diols, and the overall yields from starting materials to final cyclic monomers are sometimes less than 5%. Price and Osgan [1] were the first to polymerize with different initiators the dextrorotatory propylene oxide. Several other optically active oxiranes and thiiranes were successfully prepared and polymerized [2, 3]. 

Owing to available experimental data only the case of oxiranes and thiiranes will be considered in this review. 

If during the polymerization of oxiranes and thiiranes the ring opened exclusively in the p position, that is at the X—CH2 bond, the configuration of the asymmetric carbon atom would remain unchanged. 

Fig. 18. Mechanistic scheme of stereoelectrve polymerization of oxiranes and thiiranes using ZnEtj-chiral-l,2-diol initiator system.

The stereoelective polymerization of oxiranes and thiiranes allows us to prepare optically active polymers starting from racemic monomers by preferential incorporation of one of the enantiomers in the polymer chain. The unreacted monomer is enriched in the opposite antipode. 

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