Carbocation hydrogen-bridged

A mercurinium ion has both similarities and differences as compared with the intermediates that have been described for other electrophilic additions. The proton that initiates acid-catalyzed addition processes is a hard acid and has no imshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron-deficient and highly reactive. 

Olah and coworkers have carried out reactions of alkanes in superacid solution (SbFs-HF or SbFs-FSOsH), leading to observable (by NMR) tertiary alkyl carbocations (17, 18, 19). This reaction is pictured as a protonation of the alkane to give a pentacoordinated intermediate which then loses H2. Thus, a plausible mechanism for the cyclodecyl -> 9-decalyl cation reaction could involve loss of the bridging hydrogen (as H ) to form decalin, and then reprotonation of the decalin to eventually form H2 + 9-decalyl cation. However, when the cyclodecyl cation 1 was prepared in SbFs-FSOsD solution, there was no evidence of H-D formation in the hydrogen gas that was produced. It was concluded therefore that the H2 gas must come directly from the carbocation hydrogens without any involvement by the superacid system. 

The hydrogen-bridged carbocation (B) contains a two-coordinate hydrogen atom. Hypercoordination— which includes two-coordination for hydrogen and at least five-coordination for carbon—is generally... 

TABLE 5.2. NMR Shifts and Calculated Data for Hydrogen-Bridged Carbocations... 

The unusual carbocation structure 53, a linear 5-center 4-electron C-H-C-H-C array can be derived from three anthracenes joined up around a C-H-C-H-C core.73 The GIAO-DFT computed H and 13C NMR chemical shifts for the bridging hydrogens (2.9 ppm) and carbons (112 and 182 ppm) of the 5-center array differ considerably from those found in typical 3-center 2-electron systems. 

Recently, we have shown that iminium ions can induce a hydride shift to form a new carbocation which then reads with a nucleophile. By this way the novel unusual bridged steroid alkaloids 25 were prepared from the secoestron derivative 19 (scheme 5) M Treatment of 20 obtained from 19 by hydrogenation with aniline or an aniline derivative 21 containing an dedron-withdrawing group in the presence of the Lewis add BF3-OEt2 leads to the iminium... 

Further calculations39 on secondary and tertiary systems at the MP2/6-31G(d) level showed that for the 2-propyl cation, a /1-silyl group in the bisected orientation provides 22.1 kcalmol-1 of stabilization compared to a /1-hydrogen, as opposed to 6.6 kcalmol-1 provided by a /)-methyl group. For the secondary system, the bridged carbocation was found to have a stabilization of 18.4 kcalmol-1, i.e. less than the open form. 

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