Carbocations silyl cations

There has been much dispute on the question whether triorganosilicenium cations, (R3Si+), can be generated in solution and whether some of the features found in the rich carbocation chemistry can also be found in silyl cation chemistry39 (See Schleyer, Chapter 10, and Lickiss, Chapter 11 in this book). 

These results suggest that the tertiary a-silyl carbocation 105 has a higher stability than the primary /J silyl cation 107, formed by a-cyclization. This shows that the /1-effect does not always dominate the regioselectivity of reactions of vinylsilanes. 

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. 

The major reaction product formed from the solvolysis of the trans y-silyl ester was cyclohexene formed from the carbocation 138 (Scheme 18) by 1,2-hydrogen migration to give the p-silyl cation 139 followed by loss of the silicon substituent. In contrast to the behavior shown by 136, the cis y-silyl ester 134 exhibited a small inverse p-d4 isotope effect (kH/kn = 0.97) attributed to the inductive effect of the P-deuteriums and implies that there is very little hyperconjugative stabilization of... 

It is justified to ask why so much attention has been paid to a problem that may be only interesting within Si chemistry. Why did the silylium cation problem lead to dozens of investigations, publications, and several review articles just within the last years The answer to this question has to be given in three parts. First of all, there is of course the question whether a silyl cation chemistry can be established in solution phase in a similar way as this was done in the case of carbocations. There is a general chemical interest to see how... 

Since the silicon atom is less electronegative than carbon, it should carry a positive charge more readily . Trivalent silyl cations are energetically similar to the corrraponding trivalent carbocations but are substantially less stable than... 

The elusiveness of "free" silyl cations in condensed phases stands in remarkable contrast to the behavior of the isoelectronic carbocations [11] which Olah s school has explored so extensively in super acid media [12]. Laube s splendid recent series of carbocation X-ray structures [13, 14] illustrate what to expect truly "free" cations will be separated from the nearest atoms of the counteranions by 3 A or so. Despite the larger size of silicon, the Si-ligand distances in the Lambert-Reed structures are only modestly longer than typical covalent bonds, and are in the range expected for dative bonds [IS]. 

Besides being stabilized to a smaller extent by substituents, silyl cations are far less shielded sterically by bulky groups than carbocations (Fig. 7). 

The much more highly charged silicon atom can interact far more readily with nucleophiles. Silyl cations may even be complexed simultaneously and symmetrically by two electron pair donors (hypercoordination), in contrast to carbocations. With ammonia, the methyl cation gives the very stable protonated methyl amine, H3C-NH3 a second ammonia molecule is only weakly bound to this complex. If both NH3 groups are forced to be equidistant from carbon, a Sn2 transition state results, 20 kcal mol" higher in energy than the minimum. 

MP2-FC/6-31G calculations reveal the symmetrically bridged 6-sila-2-norbomyl cation 1 not only to be a local minimum (Fig. 18), but also to be 17.2 kcal mof more stable than the 2-norbomyl cation (Eq. 3) at MP2-FC/6-31G + AZPE(SCF/6-31G 0.89). However, the inherently greater stability of silyl cations contributes to this difference. The Si NMR chemical shift of the bridging silicon atom, ca 1 ppm vs TMS (IGL0/H //MP2-FC/6-31G ), is very strongly shielded in comparison with ca 300 ppm expected for a free RSiH2 species [38]. Thus, the sila congener of the 2-norbomyl carbocation also possesses a nonclassical stracture which is reflected by its stracture as well as its NMR properties. 

Competition studies reported by Kuwajima, " which also complement the results of Nakai," illustrate the limitations of the 3-effect as a tool for predicting the outcome of vinylsilane-terminated cyclizations (Scheme 4). Acylium ion initiated cyclizations of (7a) and (7b) gave the expected cyclopentenones (8a) and (8b). However, compound (7c), upon treatment with titanium tetrachloride, gave exclusively the cyclopentenone proiduct (8c) arising fr the chemoselective addition on the 1,1-disubstituted alkene followed by protodesilylation of the vinylsilane. The reversal observed in the mode of addition may be a reflection of the relative stabilities of the carbocation intermediates. The internal competition experiments of Kuwajima indicate that secondary 3-silyl cations are generated in preference to secondary carbocations (compare Schemes 3 and 4), while tertiary carbocations appear to be more stable than secondary 3-silyl cari ations, as judged by the formation of compound (te). 

However we rationalized that using silicon as a migrating group could result in a unique stabilization of the energy surface of olefin isomerization. This rationalization was based on silicon s well known ability to stabilize both a-carbanions and p-carbocations. Thus a hypothetical "dual-stabilized" zwitterion would be produced by a 90° twist of a vinyl silane, and a following 1,2-shift of Silicon would produce a singlet carbene possessed of the same hyperconjugative stabilization as in a P-silyl cation (Eq. 5). 

Figure 3.19. Weakly coordinating anions [HCBh R X,] (R = H, Me, Cl X = Cl, Br, I) that have been used " - 5.i85.i86,i89-i97 prepare salts of silyl cations (e.g., mesityUSL), carbocations (e.g., McsC ), hydronium ions (e.g., 119O4 ), protonated arenes (e.g., CJl/), and HCeoL...

In carbon chemistry, the term carbenium for trivalent, sextet, positively charged carbon was derived from carbene, for divalent, sextet, neutral carbon. Because divalent silicon is commonly called silylene, the appropriate term for trivalent, sextet, positively charged silicon is silylenium. The alternative but commonly used term carbocation for all positively charged carbon systems finds analogy in silyl cation (much less often, silico-cation). We will use both these naming systems in this discussion. The term siliconium, like carbonium, denotes the highest valency (pentavalency), as in SiH3. The term silicenium for the trivalent system has been discarded since silylene rather than silicene has been accepted as the appropriate term for the divalent state. 

Presumably because of the longer bonds to silicon, silyl cations are much more reactive than analogous carbocations. They are extraordinarily powerful Lewis acids and have recently been applied as such to novel synthetic problems. One such application that we have discussed is fluoride ion abstraction. 

AllylsUanes are reactive towards 1,3-dithienium tetrafluoroborate. The intermediate -silyl carbocation typically suffers elimination of the silyl cation to furnish a 2-allyl-1,3-dithiane (eq 5). This protocol was employed to homologate allylsilane 3 to produce dithiane 4, an intermediate in the synthesis of epiantillatoxin (eq 6). A fert-butyldimethylsilyl ether used as a protecting group was cleaved from 3 during this reaction. 

A general mechanism of this reaction involves globally a regiospecifictranspositionoftheallylicmoiety (Scheme 12.12). The reaction is initiated by the nucleophilic addition of an allylsilane 39 over an electrophilic carbon, which is normally activated by a strong Lewis acid. Thus, a secondary carbocation intermediate (p-silyl cation, 46) stabilized by hyperconjugation is formed. Then it evolves into a compound with a new double bond by elimination of the silyl group. 

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