Oxetanes oxygen

CHEC-II(1996) discusses reaction between carbenes and the oxetane oxygen, and subsequent Stevens rearrangement or /3-elimination of the oxygen ylide formed in this reaction <1996CHEC-II(1)721>. However, no relevant literature on the reactions of substituents attached to the oxetane ring heteroatom have been published since 1995. 

The reaction of oxetane with carbenes follows two major pathways carbonhydrogen insertion or the formation of an oxygen ylide by reaction of the carbene and the oxetane oxygen. The oxygen ylide can produce a tetrahydrofuran by a Wittig rearrangement or generate an allyl ether by an intermolecular -elimination process (Scheme 61). 

Neighboring group effects can compete with steric effects, as illustrated by treatment of oxetane 142 with diethylaluminum-At-methylaniline. This elimination reaction gave a 99% yield of (E)-alkene 143 in a synthesis of humulene. The aluminum coordinated to the oxetane oxygen on the less hindered p face in 144 and 145, the two key rotamers. Rotamer 144 is lower in energy due to the severe nonbonded interaction... 

A computational study of the concerted C(sp )-0 reductive elimination of olefin oxides from complexes 19a and 19b showed that the Gibbs activation energy of epoxide elimination increases in the following order of 19a 19b 20 [33]. In other words, migration of a secondary aUcyl from the Pt center to the oxetane oxygen atom is a more facile process than that involving a primary alkyl group. 

Keywords Configurational stability Deep eutectic solvents Direct metalation group Epoxides Natural products Organolithiums Oxetanes Oxygen heterocycles Stereoselective synthesis Tetrahydrofurans... 

Preliminary activation of the Lewis basic oxetane oxygen with Bp3 Et20 of ori/jo-hydroxyalkylated phenyl oxetane (i .)-(+)-106, obtained by subjecting (il)-(+)-105 to ortho-lithhtion followed by trapping with acetone, induced an almost quantitative 5-exo-tet cycHzation at the phenyl-substituted carbon atom to give the enantiomericaUy enriched (er 96 4) l,3-dihydrobenzo[c]furan (—)-107 (Scheme 29). 

Both the carbon-carbon and carbon-oxygen double bonds of fluoroketenes can take part in [2+2] cycloadditions, but with cyclopentadiene, only cyclo butanones are produced via concerted [2 +2 ] additions [J34] (equation 58) Cycloadditions involving the carbon-oxygen double bonds to form oxetanes are discussed on page 855 Difluoroketene is veiy short lived and difficult to intercept but has been trapped successfully by very electron rich addends to give 2 2 di fluorocyclobutanones m moderate yields [/55] (equation 59)... 

The dimerization of the parent ketene gives the P-lactone. One molecule of ketene reacts across the C=C bond as a donor and the other molecule reacts across the C=0 bond as an acceptor. This is similar to the concerted [2+2] cycloaddition reaction between bis(trifluoromethyl)ketene and ethyl vinyl ether to afford the oxetane (Scheme 26) [127], A lone pair on the carbonyl oxygen in the ketene molecule as a donor activates the C=C bond as the alkoxy group in vinyl ether. 

The domino reaction is initiated by the chemoselective attack of the carbanion 2-458 on the terminal ring carbon atom of epoxyhomoallyl tosylate 2-459 to give the alkoxides 2-460 after a 1,4-carbon-oxygen shift of the silyl group. The final step to give the cyclopentane derivates 2-461 is a nucleophilic substitution. In some cases, using the TBS group and primary tosylates, oxetanes are formed as byproducts. 

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>. 

The first study with an oxygen compound which was sufficiently rigorous to provide evidence on the question of co-catalysis was that of Farthing and Reynolds [61]. They showed that 3,3-bischloromethyl oxetan could be polymerised in methyl chloride solution by boron fluoride only in the presence of water. Tater, Rose [62] obtained kinetic evidence for the need for a co-catalyst in the system oxetan—boron fluoride—methyl chloride, and he interpreted the low reaction rate when no water was added as due to residual water he also showed that water and a hydroxyl-terminated polymer could act as co-catalysts. 

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]. 

The strained rings of epoxides and oxetanes are susceptible to nucleophilic attack. In this section we discuss the reactions of these oxygen heterocycles with nitrogen oxides and other... 

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