Cyclic ethers protonation

Molecular orbital calculations predict that oxirane forms the cyclic conjugate acid (39), which is 30 kJ moF stabler than the open carbocation (40) and must surmount a barrier of 105kJmoF to isomerize to (40) (78MI50500). The proton affinity of oxirane was calculated (78JA1398) to be 807 kJ mol (cf. the experimental values of 773 kJ moF for oxirane and 777-823 kJ moF for dimethyl ether (80MI50503)). The basicity of cyclic ethers is discussed in (B-67MI50504). 

Hodgson and coworkers extended this concept to epoxides of unsaturated cyclic ethers 128 [5] and amines 130 [46, 47] (Scheme 5.28). It is interesting that the use of trimethylsilylmethyllithium as the organolithium in this case resulted in substituted allylsilanes 129 and 131 (R = CH2SiMe3) presumably the epoxide ring protons of 128 and 130 are more acidic than those of a simple terminal epoxide (see Scheme 5.26). 

Lewis acids such as BF3 and SbCl5, almost always in conjuction with water or some other protogen, initiate polymerization of cyclic ethers. The initiator and coinitiator form an initiator-coinitiator complex [e.g., BF3 H20, H+(SbCl6) ], which acts as a proton donor (Sec. 5-2a-2). Cationic photopolymerizations are achieved when similar proton donors are formed by the photolysis of diaryliodonium and triarylsulfonium salts (Sec. 5-2a-4). 

Cationic ROP of lactones in the presence of an alcohol proceeds by an activated monomer mechanism similar to that for cyclic ethers (Sec. 7-2b-3-b) [Endo et al., 2002 Lou et al., 2002]. Propagation proceeds by nucleophilic attack of the hydroxyl end group of a propagating chain on protonated (activated) monomer ... 

This is a special case of 0-12. The base removes the proton from the OH group and the epoxide then attacks in an internal Sn2 reaction.589 Many epoxides have been made in this way.590 The method can also be used to prepare larger cyclic ethers five- and six-membered rings. Additional treatment with base yields the glycol (0-7). 

Chemical shifts of protons of 1,2-dithiin (Table 4) show no evidence of a ring current, but borderline aromaticity of a 1,2-oxathiin 2,2-dioxide derivative has been inferred from the chemical shift of H-6 which falls midway between that for a proton in a nonaromatic cyclic ether and for H-2 of furan (64JOC1110). 13C shifts of 1,2-oxathiin 2,2-dioxides at the C-4 and C-6 positions have been considered anomalous but may be rationalized by invoking mesomeric withdrawal of electron density from these positions by the SO2 moiety (770MR(10)208>. 

In this paper, we try to review results obtained from anionic copolymerization of cyclic ethers with cyclic anhydrides. For a better understanding data and theoretical views on non-catalyzed copolymerizations are also included. We concentrate mainly on the kinetics and mechanism of copolymerization and the effect of the type and character of the initiator used. The influence of the epoxide and anhydride structure on copolymerization, of proton donors on the rate and course of copolymerization, and on the molecular weight of the resulting polyesters are also discussed. 

Alko xy ally 1)stannane aldehydes (57) can cyclize either thermally or with Lewis or protic acid catalysis to give cyclic ethers (58).85 The interrelationship of the reactant and product stereochemistries has been investigated, as have the methods used to promote the reaction. For both thermal and proton-promoted reactions, [(Z)-57 gave (cis-58), and [(E)-57] gave trans-58), whereas trans-58) was the predominant or exclusive product of Lewis acid mediation, regardless of the double bond geometry of... 

Preparation Of Oxacycloalkanes. Unsaturated alcohols can be cyclized under superacid conditions to yield oxolane derivatives. Laali et al.685 have studied the protonation of homoallylic adamantylideneadamantyl alcohols. The pseudo-axial alcohol 157 was protonated in HSO3F-SCUOF to give the intermediate protonated cyclic ether observed by 1H and 13C NMR spectroscopy, which, upon quenching, furnished the corresponding ether [Eq. (5.243)]. 

The difference between the most kinetically favoured cyclisations is easily seen in the type of base needed to cyclise chloro-alcohols to three- and five-membered cyclic ethers (epoxides and THFs). Chloroethanol 19 cyclises only as the oxyanion specific base is needed, i.e. a strong enough base to remove the OH proton completely. By contrast, 4-chlorobutanol cyclises by general base catalysis the proton is removed during the cyclisation 22 and weak bases will do.1, 2... 

The cyclic ether (25) has been prepared via the cyclization of 26 in monoglyme with potassium amide.14,15 The magnetic nonequivalence of the methylene group protons in 25 have been compared with other 1,2-disubstituted ferrocenes.16... 

Elimination reactions are facile processes as far as they have been studied in the gas phase. It is, however, often difficult to distinguish them from SN2 substitution reactions since both reactions mostly lead to the same product ions, but not to the same neutral products which in most experiments are not known (Smith et al., 1980 Jones et al., 1985). In that respect the reactions of cyclic compounds, such as cyclic ethers, are good probes for the study of elimination reactions because the leaving group remains with the anion. For example, the reaction of NH2 with tetrahydrofuran leads to (M — H) ions. Deuterium labelling has shown that the proton is abstracted exclusively from the P-position (DePuy and Bierbaum, 1981b DePuy et al., 1982b). 

Here is a simple example, the four-membered cyclic ether oxetane. Its NMR spectrum has a 4H triplet for the two identical CH2 groups next to oxygen and a 2H quintet for the CH2 in the middle. Each proton Hx sees four identical neighbours (HA) and is split equally by them all to give a... 

Protonation of the cyclic ether creates a carbocation intermediate that can react in a Friedel-Crafts alkylation. 

Polymerization of several representative monomers of vinyl and cyclic ethers was shown to be initiated efficiently by this particular two component system in which o-phthalaldehyde was used as promoter (Table 4). The proposed mechanism was further supported by the polymerization experiments in the presence of a strong scavenger, 2,6,6-di-t-butyl-4-methylpyridine (DBMP). Experiments in the presence of DBMP failed to produce any precipitable polymer indicating that initiation involves protons generated according to the above reaction. 

An NOE experiment of cyclic ether 40 with irradiation at the methyl group on C-3 showed 3% enhancement in the signal of the vinyl proton at C-8. This result along with the molecular modeling suggests that the C(3)-C(4) and C(7)-C(8) olefinic moieties of 40 form stereogenic planes in the most stable conformation, and proves its planar chiral nature <2005JA12182>. 

Esters of very strong protonic acids (trifluoromethanesulfonic, fluoro-sulfonic, perchloric), however, are sufficiently strong alkylating agents to initiate the polymerization of even weakly nucleophilic monomers (cyclic acetals, ethers) [2-6], Also their anhydrides (e.g., triflic anhydride) are efficient initiators. This last compound is especially interesting, because in the polymerization of cyclic ethers it leads to macromolecules with two identical growing chain ends (difunctional initiator) [30] ... 

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