Vinyl cations destabilized

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

The formation of any vinyl products in electrophilic additions to RCH=C=CH2 and RCH=C=CHR is surprising, since central protonation should yield a secondary carbonium ion compared to terminal protonation and formation of a vinyl cation. Perhaps a secondary carbonium ion destabilized by... 

If a carbocation or a dication at the same time is also a Hiickeloid An + 2)jt aromatic system, resonance can result in substantial stabilization. The simplest 2jt aromatic system is the Breslow s cyclopropenium ion 206.434 439 Recently, electronic and infrared spectra of the parent ion cyclo-C3H3+ (206, R = H) in neon matrices440 and the X-ray characterization of the tris(trimethylsilyl) derivative were reported.441 The destabilizing effect of the silyl groups was found to be significantly smaller than in vinyl cations. The ion was computed to be more stable than the parent cyclopropenium ion by 31.4 kcal mol1 [MP3(fc)/6-31 lG //6-31G +ZPVE level]. The alkynylcy-clopropenylium ions 207 have been reported recently.442... 

Starting with the knowledge of carbenium ion stability and the understanding that it is necessary to decrease this stability enabling the reaction of these ions with nitriles102 (Section III.B.l), it should be assumed that acetals 209, oc-haloethers 211, enols 217 and vinyl ethers 218 are ineffective precursors since they are the sources of the highly stable hydroxy- (223) and alkoxycarbenium ions 224. In contrast, the carbonyl compound derivatives, which can produce destabilized ions (in comparison to ions 223 and 224) are the most interesting for reactions with nitriles. This includes acyloxycarbocations 225 (see Section III.B.3), halocarbocations 226, A-acyliminium ions 228 and vinyl cations 229. 

Netherlands This study compared MP2/6-311++G(cf,/7), CBS-Q, and B3LYP/6- ill+G d,p) calculations and found that the CBS-Q method gave the best correlation with experimental data. There was also good correlation with MP2 and B3LYP results, which could be used to obtain CBS-Q-like results when the CBS-Q method was not applicable. Table 3.12 gives some of the computed stabilization (-h) and destabilization (-) relative to ethyl and vinyl cations found using isodesmic reactions ... 

Table 4.P26 shows stabilization (+) and destabilization (-) of a-substituted ethyl and vinyl cations as determined by the isodesmic reactions shown below. Comment on the following trends in the data. 

The positive charge in a vinyl carbocation resides on a carbon that is sp hybridized, while in (CH3)2CH, the positive charge is located on an sp hybridized carbon. The higher percent -character on carbon destabilizes the positive charge in the vinyl cation. Moreover, the positively charged carbocation is now bonded to an sp hybridized carbon, which donates electrons less readily than an sp hybridized carbon. 

To study the possible stabilizing effect of [3-silyl cations, Olah and co-workers334 prepared the 2- [(1 -trimethylsilyl)vinyl]-2-adamantyl cation 132 [Eq. (3.43)] as well as the parent silicon-free carbocation. In contrast to the above observations, NMR data [the (Cl ), (C2), and (C2 ) carbons are more deshielded in 132 than in the parent ion] showed that cation 132 is destabilized compared with the silicon-free analog. Furthermore, at — 100°C the C(l) and C(3) carbons were found to be equivalent, whereas in the parent ion they were nonequivalent. This indicates a rapid rotation about the C(l)-C(3) bond in 132, which can be rationalized by assuming the intermediacy of the [3-silyl-stabilized cation 133. The difference between cation 132 and those having [3-silyl-stabilization discussed above may be the orthogonal arrangement of the [3-C-Si bond and the p-orbital of the carbocation center. 

With vinyl chloride and acrylonitrile, the cations at the end of the growing chain wonld be destabilized by electron-withdrawing gronps. 

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