Proton bridged intermediate

The main reaction path is stereospecific, with the trifluoroacetate being added syn to the proton. This implies that the reaction proceeds through a discrete Itydride-bridged intermediate rather than a conformationally mobile cyclooctyl cation. 

Fig. 4.7 shows the titration with perchloric acid of a mixture of piperidine, ethylenediamine and p-toluidine. Fig. 4.8 illustrates the effect of different chain lengths on the titration (with perchloric acid) of diamines in nitrobenzene containing 2.5% (v/v) of methanol once the first amino group of EDA has been protonated, the resulting proton bridge with the lone pair of electrons of the second amino group lowers the basicity of the latter considerably the effect decreases on the introduction of more intermediate CH2 groups until complete disappearance when six are present. 

The [H + ] dependence of kobs has been interpreted in terms of an acid-catalyzed and an uncatalyzed reaction path, as shown in Scheme 6. The acid-catalyzed path has been proposed to involve protonation of one hydroxo bridge to give a labile aqua-bridged intermediate. The aqua-bridged complexes have in no case been identified, but spectroscopic results indicate that such species are very strong acids with Ka3 1. The kinetic parameters calculated from the rate expression, Eq. (55), are... 

Decarboxylation of carbonate complexes is usually effected by acid hydrolysis with the formation of a C02 free oxide or hydroxide complex.128 All such reactions involve a protonated (bicarbonate) intermediate but there are some useful deferences which, in many instances, may be reconciled with the three main structural types of carbonate complexes. Both unidentate and chelate carbonates readily yield C02 on acidification, while there is a greater resistance to C02 loss when the carbonate is a bridging ligand. Unidentate carbonate complexes decarboxylate with the initial formation of a bicarbonate intermediate and subsequent loss of C02 without rupture of the M—O bond, viz. structure (3). By contrast, in chelate carbonate complexes, cleavage of the M—O bond occurs (with ring opening) with the formation of a bicarbonate aqua ion before the loss of C02, viz. equation (5).29... 

This ion appears to have a classical structure, which is in equilibrium with a bridged ion under conditions of low nucleophilicity, but undergoes extremely fast 1,2-hydride shifts which have not been frozen out. It also shows complete hydrogen and carbon degeneracies which most probably involve corner protonated cyclopropane intermediates. At higher temperatures it rearranges to t-butyl cation. 

The E/Z substrate-dependent absolute stereochemistry and the steric influence of tin-substituents on the enantioselectivity observed in these reactions suggest that the mechanism is essentially different from that of sily 1 enol ethers. Although the detailed stereochemical course is not ascertained, it is possible that the protonation may occur via a two chlorine-bridged intermediate involving allyltrimethyltin and LBA. 

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