Oxonium ion pair

When all monomer is consumed in polymerisation then the positive charge almost certainly resides on some small molecular species, not a polymer molecule. This is not necessarily so however, in the case of the polymerisation of tetra-hydrofuran (THF), where the presence of living polymeric cations was demonstrated originally by Dreyfuss and Dreyfuss (50). Sangster and Worsfold (27) have since been able to make direct conductance measurements, and evaluate Kd for the living oligomeric oxonium ion pair (BFJ counterion) derived from THF in methylene chloride at —0.5° C. 

In order to explain the observed substituent and solvent effects on the rate of the Ciaisen rearrangement, Coates et al. [28] suggested a dipolar character for the transition state "... partial delocalization of a non-bonded electron pair from the donor substituent generates a significant degree of enolate-oxonium ion-pair character that stabilizes the TS more than the ground state,... (Scheme 11.22). 

Notably, optimization studies exposed the critical influence of phenol on the reactivity and enantioselectivity within this manifold, suggesting a two-step pathway as illustrated in Figure 9. Initially, enantioselective protonation takes place from the chiral Brdnsted acid 57 or oxonium ion pair 60, generated by rapid proton transfer between 57 and phenol, to silyl enol ether 61 to form chiral ion pair 62. This is followed by desilylation with phenol to form the corresponding ketone 63, silylated phenol, and catalyst 57 for further turnover. 

Further control experiments gave some insights into the mechanism of this organocataly tic process. Informatively, the reaction did not occur in the absence of the phenol, suggesting that the active catalytic species would result from a preassociation between the A -ttiflyl thiophosphoramide 82a and phenol. The authors speculated that the formation of a chiral oxonium ion pair would promote the proto-desilylation of the silyl enolate 80 through a two-step mechanism. Initial enantioselective protonation of the silyl enolate 80 would then be followed by desilylation of the resulting intermediate to furnish the desired ketone 81 along with silylated phenol (Scheme 3.43). 

Studies have shown that, in marked contrast to carbanionic polymerisation, the reactivity of the free oxonium ion is of the same order of magnitude as that of its ion pair with the counterion (6). On the other hand, in the case of those counterions that can undergo an equiUbrium with the corresponding covalent ester species, the reactivity of the ionic species is so much greater than that of the ester that chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerisation process. The relative concentration of the two species depends on the dielectric constant of the polymerisation medium, ie, on the choice of solvent. 

The protonated azirine system has also been utilized for the synthesis of heterocyclic compounds (67JA44S6). Thus, treatment of (199) with anhydrous perchloric acid and acetone or acetonitrile gave the oxazolinium perchlorate (207) and the imidazolinium perchlorate (209), respectively. The mechanism of these reactions involves 1,3-bond cleavage of the protonated azirine and reaction with the carbonyl group (or nitrile) to produce a resonance-stabilized carbonium-oxonium ion (or carbonium-nitrilium ion), followed by attack of the nitrogen unshared pair jf electrons to complete the cyclization. 

Entry 4 shows that reaction of a secondary 2-octyl system with the moderately good nucleophile acetate ion occurs wifii complete inversion. The results cited in entry 5 serve to illustrate the importance of solvation of ion-pair intermediates in reactions of secondary substrates. The data show fiiat partial racemization occurs in aqueous dioxane but that an added nucleophile (azide ion) results in complete inversion, both in the product resulting from reaction with azide ion and in the alcohol resulting from reaction with water. The alcohol of retained configuration is attributed to an intermediate oxonium ion resulting from reaction of the ion pair with the dioxane solvent. This would react until water to give product of retained configuratioiL When azide ion is present, dioxane does not efiTectively conqiete for tiie ion-p intermediate, and all of the alcohol arises from tiie inversion mechanism. ... 

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

It would be reasonable to assume that, in a solvent of high dielectric constant, such as nitromethane, the nitrile (84) is formed by direct attack of cyanide ion on an ion-pair (86) in which the bromide ion has the a-D orientation. Departure, assisted by metal ions, of the halide ion from 83 or 86, with possible assistance by the lone pair of the ring-oxygen atom, would lead to an oxonium ion (87) that could... 

Oxygen-containing solvents with a strong coordinating ability, such as diethyl ether, methyl /.so-butyl ketone and /.so-amyl acetate, form oxonium cations with protons under strongly acidic conditions, e.g. (R20) H. Metals which form anionic complexes in strong acid can be extracted as ion pairs into such solvents. For example, Fe(III) is extracted from 7 M hydrochloric acid into diethyl ether as the ion pair... 

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

Racemization of chiral a-methyl benzyl cation/methanol adducts. The rate of exchange between water and the chiral labeled alcohols as a function of racemization has been extensively used as a criterion for discriminating the Sn2 from the SnI solvolytic mechanisms in solution. The expected ratio of exchange vs. racemization rate is 0.5 for the Sn2 mechanism and 1.0 for a pure SnI process. With chiral 0-enriched 1-phenylethanol in aqueous acids, this ratio is found to be equal to 0.84 0.05. This value has been interpreted in terms of the kinetic pattern of Scheme 22 involving the reversible dissociation of the oxonium ion (5 )-40 (XOH = H2 0) to the chiral intimate ion-dipole pair (5 )-41 k-i > In (5 )-41, the leaving H2 0 molecule does not equilibrate immediately with the solvent (i.e., H2 0), but remains closely associated with the ion. This means that A inv is of the same order of magnitude of In contrast, the rate constant ratio of... 

In decomposing oxonium ions the situation is quite different, i.e., the preference for alkene loss is much less emphasized and aldehyde (ketone) loss is gaining importance. The observed changes are in good agreement with the postulated mechanism of the onium reaction. [143,167] The alternative pairs of oxonium ion plus alkene and aldehyde plus carbenium ion may be formed with a preference for the first one, because APA is comparatively small (20-60 kJ mol" ) or even zero, e.g., for the acetone/isobutene pair [143,166,167]... 

Alkyl halides react with diazines less readily than with pyridines. All the diazines are, nevertheless, more reactive toward methyl iodide than predicted by their pKa values and the Bronsted relationship. The significant although modest rate enhancements found are considered to arise from interactions between the two lone pairs on the nitrogen atoms this interaction is largest in pyridazine. Use of oxonium ions can convert the diazines into diquatemary salts. Quinoxalines and phenazines similarly yield diquatemary salts under forcing conditions. 

The capture of carbocations by alcohols involves a similar donation of a lone pair of electrons on oxygen to the vacant 2p atomic orbital of the sp2-hybridized, sextet carbocation. Note that charge must be conserved so the first formed product is a positively charged oxonium ion. 

Resonance stabilization can also make n -electron donation much more effective by avoiding the formation of a sextet carbocation. Lone-pair donation from tire oxygen of enol derivatives is very important to the good donor ability of these compounds. The resulting oxonium ion has all valence octets (although positively charged) and is thus stabilized over sextet canonical forms. 

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