Transition state ether ions

Evidence that the actual methylation of the anion can be divided into SnI, Eq. (3), and Sx2 types, Eq, (4), is provided by a whole series of investigations. " The terms S l and 8 2 must be taken to mean reactions with, respectively less or greater nucleophilic participation of the anion in the transition state. The importance of oriented ion pairs" in the solvents of low polarity frequently used in reactions involving diazomethanc, e.g., the ions formed by a diazoalkane and benzoic acid in ether, should be emphasized. The expression oriented ion pair means that, because of insufficient solvation, the ions are not individually solvated but exist as ion pairs within a solvent cage. The orientation within the ion pair is defined electrostatically, and this orientation fixes the path for the productdetermining step. Several indications (cf, foo otes 22-24) in the literature indicate the occurrence of carbonium ions and oriented ion pairs in Broensted-type equilibria of the type of Eq. (2). 

Polymerization of t-butyl methacrylate initiated by lithium compounds in toluene yields 100% isotactic polymers 64,65), and significantly, of a nearly uniform molecular-weight, while the isotactic polymethyl methacrylate formed under these conditions has a bimodal distribution. Significantly, the propagation of the lithium pairs of the t-Bu ester carbanion, is faster in toluene than in THF. In hydrocarbon solvents the monomers seem to interact strongly with the Li+ cations in the transition state of the addition, while the conventional direct monomer interaction with carbanions, that requires partial dissociation of ion-pair in the transition state of propagation, governs the addition in ethereal solvents. 

The observation of a primary solvent deuterium isotope effect (kH/fa>) = 2-4 on the specific acid-catalyzed hydrolysis of vinyl ethers provides evidence for reaction by rate-determining protonation of the alkene.69 Values of kHikD 1 are expected if alkene hydration proceeds by rate-determining addition of solvent to an oxocarbenium ion intermediate, since there is no motion of a solvent hydron at the transition state for this step. However, in the latter case, determination of the solvent isotope effect on the reaction of the fully protonated substrate is complicated by the competing exchange of deuterium from solvent into substrate (see above). 

In the propagation by the Jaacks mechanism the transition state (XIV) would resemble closely that in the polymerisation of cyclic ethers, such as THF for this the transition state is (XV) which is evidently very similar to (XIV). It is reasonable to suppose that the AS corresponding to (XIV) would have a value close to that for (XV) however, the AS1 for (XV) shown in Table 2 is much less negative than that for the polymerisation of DXL by (VII). It is evident therefore that whatever may be the growth mechanism when initiation is by organic cations, it is very unlikely to involve the microcyclic tert.-oxonium ion growth centre of Jaacks. Thus all the evidence taken together indicates that in the... 

The effects of metal ions on the alcoholysis and hydrolysis of esters containing crown ether functionalities have also been studied (Cacciapaglia et al., 1989, 1992 Hedderwick et al., 1991a,b). In the former case, transition state binding was considered explicitly, in the same manner as did Ercolani and Mandolini (1990). 

An ab initio study of the energetics of deprotonation of cyclic vinyl ethers by organolithium reagents has clarified the ring-size-dependent competition between vinylic and allylic deprotonation.The respective transition states involve preequilibrium complexation of lithium to the electron-rich vinyl ether oxygen, prior to deprotonation via a multi-centre process free ions are not formed during the lithiation. 

Also inconclusive are the results of the quenching experiments shown in Table 12. Thus, reaction of the ion with pyridine has been reported to yield nortricyclene, while water and methanol yield mainly 2-exo-norbomyl derivatives. If the ion is classical, the production of alcohols and methyl ethers is regarded as due to condensation of it and water, methanol, or methoxide. The production of nortricyclene would then be regarded as due to a deep seated reaction of pyridine with ion [2] leading to a transition state resembling pyridine, a proton and nortricyclene. 

In the third transition state (TS3), the neutral catalyst is recovered by transferring the proton back from the catalyst to the substrate. In other words, the (former) azlactone ether oxygen atom deprotonates the tertiary ammonium ion. For proton transfer, again an LBHB is formed (N-0 distance 2.479 A, <(0,H,N)=166.2°). In the product complex, the catayst is neutral and the A-acylamino acid ester is bound in its iminol form to the catalyst (Product(iininol)). Finally, an additional 66.6 kJ moF are gained by the subsequent iminol-amide tautomerization (Product(ainide)) (Fig. 1). 

The MNDO structures of intermediates and transition states for the cleavage of benzyl and cinnamyl ethers through anion-radical routes and dianion routes involving CIPs (contact ion pairs) and IIPs (isolated nonsolvated ion pairs) are given in Figure 18. 

Cacciapaglia, R., van Doom, A.R., Mandolini, L., Reinhoudt, D.N. and Verboom, W. (1992) Differential metal ion stabilization of reactants and transition states in the transacylation of crown ether aryl acetates./. Am. Chem. Soc.. 114, 2611. 

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