Carbonium ions benzhydryl

O-Alkyl thionesters rearrange easily to thiolesters when the alkyl group can form relatively stable carbonium ions. Benzhydryl thioncarbonate (87) yields 88 on heating in ethanol and the suggested mechanism is a dissociation to a ion pair, the return of which occurs via the sulphur because of the greater nucleophility of the sulphur compared to the oxygen. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

According to the mechanism suggested by Huisgen and Ruchardt [217] and by O Ferrall et al. [216], an ion pair is formed in a rate-determining proton transfer step. It subsequently splits off N2 to form an ion pair containing the benzhydryl cation. The carbonium ion either undergoes covalent bond formation with the anion or diffuses out of its solvent cage and then reacts with ethanol, viz. 

A more critical examination of scheme 5 and of the data for the DDM reaction suggests alternatives to the proposed paths to the products. Thus, the carbonium ion precursors of the benzhydryl ether might be formed not from dissociation of carbonium carboxylate ion pairs but by nitrogen loss from already dissociated diazonium ions. Whether or not... 

A further possibility is bimolecular displacement of nitrogen from the benzhydryldiazonium ion. Again, this may reasonably be ruled out on the basis of the stability of the carbonium ion. Since benzhydryl chloride is known to solvolyse exclusively by an S l mechanism in ethanol, the benzhydryldiazonium ion with its superior leaving group should, a fortiori, do so. This conclusion is confirmed by Bethell and Callister s finding that in aqueous acetonitrile the reaction of DDM with dissociated toluenesulphonic acid leads to the same product distribution as the solvolysis of benzhydryl toluenesulphonate (p. 354). 

Present information on the DDM reaction sheds little light on the details of reaction between benzhydryl cations and the solvent. Recent studies (Bethell and Howard, 1966 White and Elliger, 1967) suggest the intermediacy of more than one type of carbonium ion pair, each susceptible to solvent attack, but to simplify the present discussion this and the other possible elaborations of scheme 5 are omitted. 

Whiting and Huisgen found identical products from diazonium ions formed in different reaction paths when ion pair dissociation or exchange could reasonably be supposed to have occurred prior to formation of the carbonium ion. A rather different type of study has been carried out by Miller and Stedronsky (1966) (ef. White and Elliger, 1967) who have sought to generate the same diazonium ion pair in the reaction of diphenyldiazomethane with jj-nitrobenzoic acid and in the rearran -ment of N-benzhydryl-N-nitroso-p-nitrobenzamide. 

The most detailed comparison of carbonium ions from diazonium ion and ester precursors is due to Diaz and Winstein (1966) who measured the relative rates of ion pair collapse and dissociation in the reaction of diphenyldiazomethane, DDM, with benzoic acids and in the solvolysis of benzhydryl benzoates (scheme 9). 

Halides. Sodium borohydride reduces secondary and tertiary alkyl halides capable of forming relatively stable carbonium ions, for example benzhydryl... 

With primary halides, the first step is probably normally SN2, as commonly assumed, although the existing kinetic and stereochemical data do not establish this with certainty. Secondary halides are considerably less reactive and olefins are frequently found among the products, suggestive of greater importance for an SNl mechanism (260). A rather clear-cut example is seen in the exceptional reactivity of benzhydryl and 9-fluorenyl bromide with triethyl phosphite for which Smith and Burger (290) proposed prior formation of a carbonium ion ... 

In the case of triphenylsilane and trityl chloride, this reaction is first order in both reactants in benzene (233). The reaction rate for modified substrates indicates that with respect to R3CX, rate is a function of proclivity to ionize, trityl halides being more rapid than benzhydryl or allyl or tcr/-butyl halides. Indeed the latter required the presence of BBr3 as a catalyst. Within a given series. Cl > Br > I, phenylsilanes react more rapidly than alkyl silanes. A mechanism involving simultaneous nucleophilic attack of halide on silicon, and electrophilic attack on hydrogen by the carbonium ion is reasonable,... 

As predicted from the pKr+ values (Table 7.4) the S-di-p-methoxybenzhydryl group [18] is cleaved much more rapidly than other S-benzhydryl thioethers. Konig et al. [55] report complete removal in 2 hr with trifluoroacetic acid (70 ) or in 10 min with added anisole. Photaki et al. [58] report 80% removal in 2 hr (20-25°) with trifluoroacetic acid and 15% phenol. The pKr value of this cation predicts it to be more stable than the S-trityl cation which in turn su ests a more rapid conversion of the conjugate acid of the thioether to the carbonium ion and thiol and a selectivity comparable with trityl in the reaction with nucleophiles. This prediction is not entirely borne out by facts and indeed the S-di-p-methoxybenzhydryl group falls between the benzhydryl and trityl in reactivity. This lowered reactivity is probably due to the presence of significant concentrations of the less reactive dication. 

Sodium borohydride efficiently reduces organic halides, especially if they can form stable carbonium ions. For example, icrl-cumyl chloride (2-phenyl-2-chloropropane) and benzhydryl chloride are converted into cumene (82 /o) and diphenylmethane (72" ), respectively. The following conditions are used for the reduction of these compounds 50°C reaction temperature, 1-2 hr reaction time, 65 vol% diglyme, 0.5 mole of the organic halide, 4.0 mole of sodium borohydride, and 1.0 mole of sodium hydroxide (added in order to minimize the hydrolysis of sodium borohydride). The use of DMSO as a solvent has also been reported. 

The very small secondary isotope effect arising from replacement cf p-CHs by p-CDs is instructive largely because it shows that full development of carbonium ion character is not necessarily associated with a substantial benzylic deuterium secondary isotope effect. It must be recalled in this connection that the Hammett rho for Ki value is —4.410 and that the effect of one p-methyl group on the equilibrium corresponds to a typical cr+ value, i.e., the reaction is quite sensitive to stabilization of charge and p-methyl shows enhanced participation in stabilization. The observed effect is consistent with the analogous very small effects which have been observed in aromatic substitution (105), including bromination, for which p is —12, but smaller than those which have been reported for solvolyses of benzylic (106) and benzhydryl (107) compounds. It does not appear to be possible to draw a general conclusion from these data. 

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