Trimethylsilyl cation

The strong affinity of the hard potential trimethylsilyl cation for the hard N-oxide moiety and of the soff cyanide anion (or the soff iodide anion) for the ad-... 

Because allyltrimethylsilane 82 or benzyltrimethylsilane 83 can be regarded as combinations of the hard trimethylsilyl cation and the soff allyl or benzyl anions, pyridine N-oxide 860 reacts with excess 82 or 83 in the presence of catalytic amounts of tetrabutylammonium fluoride di- or trihydrate in THF to give 2-allyl-or 2-benzylpyridines 948 and 950 [60]. The general reaction of silicon reagents such as 82 and 83 or of trimethylsilyl cyanide 18 with fluoride to generate allyl or... 

These reactions may operate by providing a source of trimethylsilyl cations, which serve as the active catalyst by a cycle similar to that for Lewis acids. 

Subsequently, the rapid unimolecular (mesolytic) fragmentation of the resulting pinacol cation radical followed by proton (or trimethylsilyl cation) transfer to quinone anion radical (within the solvent cage) yields the retropinacol products in equations (55) and (56) (equation 58). 

Bakhtiar and coworkers also studied the isomerization of SiMe2R+ (80) to SiHEtR+ (81) upon collisional activation239. As already described for the trimethylsilyl cation, this isomerization seems to proceed via a concerted [crs2 + crs2] process (equation 36). 

The structure of diphosphallenic radical cations, generated from the allene ArP=C=PAr by electrochemical oxidation, has been examined using EPR spectroscopy. Ab initio calculations including correlation effects at the MP2 and MCSCF levels have determined that two rotamers exist compatible with Jahn-Teller distortion of the allene.146 Anodically generated radical cations of alkyl phosphites [(RO P] and silylphosphites [(RO)2POSiMe3] reacted with alkenes by initial attack at the C=C bond followed by electron transfer, deprotonation, and elimination of an alkyl or trimethylsilyl cation to form identical alkyl phosphate adducts.147 The electron ionization-induced McLafferty rearrangement of n-hexylphosphine afford the a-distonic radical cation CTEPH, the distinct reactivity of which suggests there is no... 

Employing the lactone 279, the trimethylsilyl cation generates a positive charge at the anomeric carbon position of 280. The corresponding triflate anion recombines with one of the silyl ethers in 281 and yields the alk oxide 282 and a molecule of catalyst. After nucleophilic attack, 283 is formed, and by a subsequent intramolecular reaction, the orthoester 284 is obtained simultaneously releasing a molecule of hexamethyldisil-oxane. 

A different approach must be used for the photochemical hydrophosphination of electron-poor olefins, and this involves a PET reaction. Silyl phosphites (e.g., 30) were used as electron donors, whereas conjugated ketones have the double role of electron acceptors and absorbing species. Thus, the irradiation of a mixture containing 2-cydohexenone and 30 generated an ion pair. The phosphoniumyl radical cations decomposed to give trimethylsilyl cations (which in turn were trapped by the enone radical anion) and phosphonyl radicals. A radical-radical combination afforded the 4-phosphonylated ketones in yields ranging from 78% to 92% (Scheme 3.20) [49]. This reaction was exploited for the preparation of substituted phosphonates, which serve as key intermediates in the synthesis of a class of biologically active compounds. 

Electron-impact ionization of trimethylchlorosilane in an ICR mass spectrometer resulted in the formation of the trimethylsilyl cation, which could be detected for as long as a 800 mseconds (33). On collision it... 

In the presence of A1C13 and Me3SiCl, 1,3-dienes react with 10 to afford trans-[3 + 2]-cycloadducts (Equation (46)). A trimethylsilyl cation-mediated mechanism involving 1,2-silyl migration has been proposed.183... 

The Lewis acid-catalyzed reactions of allylsilanes with simple alkenes and alkynes introduce both allyl and silyl groups into the unsaturated bonds (Equations (53) and (54)).209 210b These allylsilylations are 7-regiospecific with respect to allylsilanes. The /raor-addition of allylsilanes occurs in the reaction with alkynes. There are two possible mechanisms for the Lewis acid-promoted allylsilylations, that is, trimethylsilyl cation- and Lewis acid-mediated pathways. 

Reaction of benzaldehyde and its 4-nitro derivative with the enantiopure diene 796 in DCM and in presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) afforded the condensed enantiopure oxathiocanes 777 and 778 (50% yield). The reaction path began with the ionic addition of the diene 796 onto the carbon atom of the aldehyde function in 797a,b activated by trimethylsilyl cation attack to the carbonyl oxygen. 

B3LYP/6-31G(d)] calculations of cr-complexes derived from the substrates with trimethylsilyl cation (Scheme 25). All three methods predict... 

Table 3 Total energies of cationic tr-complexes formed from pyrrole, N-methylpyrrole, and N-(tert-butyl)pyrrole on addition of trimethylsilyl cation at a- ( j and -positions ( ) as well as values characterizing preferable site of electrophilic attack... 

The reaction was suggested to proceed by a carbocationic mechanism followed by ligand coupling. The electrophilic attack of the iodine atom on the double bond leads first to an intermediate cation (129) which loses either a proton or a trimethylsilyl cation. The tricoordinate iodane intermediates of types (130), (131) or (132) eventually undergo ligand coupling to afford the final reaction products.233 (Scheme 5.14)... 

Detailed studies on the reactivity of the trimethylsilyl cation with nucleophiles were performed by Bowie and coworkers. Out of the many systems investigated the following should be discussed briefly. 

Trimethyhilylfluorosulfonate. The compound is comparable to trimethylsilyl triflate as a source of trimethylsilyl cation. It is formed by treatment of allyltrimethylsilane (or tetramethylsilane) with fluorosulfonic acid in CH2CI2 at —78°C. 

Apparently, the intermediate carbocation eliminates a trimethylsilyl cation. The cation derived from oxalyl chloride, however, did not eliminate a trimethylsilyl cation and the dihydrofuranone 9 was obtained (equation 12). 

A similar reaction may also be accomplished under nonaqueous conditions with trimethylsilyl cyanide. Though exceedingly unstable by themselves, trimethylsilyl cations in complexed form may be profitably viewed as fat protons and are excellent Lewis acids ... 

One electron oxidation of silyl enol ether 44 leads to the formation of a radical ion pair of 45 and the xenon difluoride radical anion. Subsequent transfer of a fluoride radical yields cation 46, which reacts by loss of the trimethylsilyl cation to yield the fluoroketone 47. The formation of ketone 49 is explained by a [1,5] -hydrogen migration from the trimethylsilyl group to the radical cationic moiety of 45, leading to the formation of 48. 

The corresponding trimethylsilyl enol ether 65, which is unstable under CAN oxidation conditions, has been successfully cyclized by Mattay and co-workers via PET with 9,10-dicyanoanthracene (DCA) as sensitizer (Scheme 15). The selective formation of the 6-endo cyclization products was explained through the radical cation 66 as the reactive intermediate in the ring closure step. Loss of the trimethylsilyl cation is assumed to occur from radical cation 67 after cycHzation. However, the source of the required hydrogen transfer step has not been clarified in detail. 

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