Cations silyl

Another milestone discovery in the held of silyl cations chemistry was achieved by Reed and co-workers in the same year, 1993, when Lambert published his Et3Si study. Reed synthesized his t-PrjSi (CBuHsBrg), (2+ (CBnH5Br5), by the hydride transfer reaction of /-PrjSiH and Ph3C (CBnH6Br6) in toluene,taking advantage of the very low nucleophilicity of the carborane anion" (Scheme 2.9). 

While related to its carbon analogs, the existence of the RsSi species as a free ion in condensed phases had been doubted for a long time. However, NMR characterization using bulky aryl substituents has provided evidence for the triply coordinated silicon cation. " However, definitive evidence was recently reported by the groups of Reed and Lambert with a silyl cation species bound to three mesityl groups and a carborane [HCBnMesBrg] counterion (Eig. 7.5). It was suggested that... 

These reactions presumably proceed by catalytic cycles in which the carbonyl component is silylated. The silyl ether can then act as a nucleophile, and an oxonium ion is generated by elimination of a disilyl ether. The reduction of the oxonium ion regenerates the silyl cation, which can continue the catalytic cycle. 

In these studies (291, 292), the ring-chain tautomerism of silylated cations A, prepared by reversible silylation of the above mentioned nitronates (96) was suggested as an explanation for the observed transformations of y-keto-functionalized SENAs 96 (Scheme 3.80). 

The results indicate that the formation of long-lived trimethyl substituted silyl cations, in the presence of aromatic solvents, as claimed by Lambert et al.95 is not feasible under these conditions. Persistent silicenium ions require sterically more shielding substituents at silicon or hypercoordinative stabilization.96 98 13C and 29Si NMR chemical shifts were calculated for a series of disilylated arenium ions 85 using density functional theory (DFT). The calculations predict consistently the unsaturated carbon atoms to be too deshielded by 8-15 ppm. Applying an empirical correction, the deviation between experiment and theory was reduced to -0.4 to 9 ppm, and the 13C NMR chemical shift of the highly diagnostic cipso is reproduced by the calculations (Ad = -3.8 to 2.7 ppm).99... 

Silyl Radicals, Silyl Cations and Silyl Anions 418... 

It is an intriguing idea to stabilize low-valent silicon species, such as silyl cations, silylenes, silenes, and disilenes using intramolecularly coordinating ligands. Corriu et al. succeeded in the preparation of the first hypervalent silyl cation [(8-Me2NCioH6)2SiH]+l/2[l8]z 782 by the reaction of the hexacoordinated diorganosilane... 

With the potentially bis-chelating 2,6-bis(/V,V-dimethylaminomethyl)phcnyl ligand, which can be attached to silicon via its lithium salt (Figure 4), Corriu et al. have prepared a number of trivalent silyl cations as their chloride salts 787-790 (Scheme 103).811... 

Silyl cations are formed from silyltetracarbonyl cobalt, H3 SiCo(C0)4, or silylpentacarbonyl manganese42) ... 

The photochemical reduction of 1-methylquinolinium ions by (TMS)3SiH proceeds regioselectively to afford the corresponding 1,4-dihydroquinones in a water-acetonitrile solvent system (Reaction 4.47) [83]. Mechanistic studies demonstrated that the reactions are initiated by photoinduced electron transfer from the silane to the singlet excited states of 1-methylquinolinium ions to give the silane radical cation-quinolinyl radical pairs, followed by hydrogen transfer in the cage to yield 1,4-dihydroquinones and silicenium ion. Silyl cations are quenched by water. 

On the basis of the above results, a possible mechanism for the allylsilylation of cyclohexene with la has been proposed as illustrated in Scheme 1. A silyl cation or a complex intermediate I is directly formed at the beginning stages of the reaction... 

The cleavage of a silicon-methyl bond by the silylated benzenium ion [EtsSi/ CgHg] and formation of the silylium ion 14 is the first step in the unexpected synthesis of the homoaromatic silyl cation 3 from trisilacyclopropene 15 (Scheme 6). ... 

The intermediate formation of the ferrocenyl-substituted silylium ion 16 by protonation of the ansa-ferrocenyl silane 17 can be regarded as a special case of electrophilic cleavage of an activated C-Si bond (see Scheme 7). The driving force for this reaction is the release of a strain by formation of the silyl cation. In a... 

There is no general theoretical study for trialkyl-substituted cations R3E, which investigates the relationship of the classical planar trigonal structure to isomeric complexes RE /R2 and its relative energy compared to the dissociation products, the singly coordinated four-valence-electron species R E and the hydrocarbon R2. The only exceptions are 7-norbornadienyl cations 37 for which the germyl and silyl cation has been intensively studied theoretically by Radom and Nicolaides. ... 

Silyl cations like 3 and 7 in which the positively charged silicon is part of a n-conjugated system attracted particular interest. The marginally stable silatropylium ion 7, is characterized by a Si NMR resonance at 8 Si = 149 in CD2CI2 at —50 °C, downfield-shifted by 192 ppm compared to the precursor silane.This experimental value is in fair agreement with the calculated silicon NMR chemical shift for the optimized gas phase structure of 7 (8 Si = 159.9, at GIAO/HF/6-311 + G(2df,p)(Si), 6-31G(d) (C,H)). This indicates only small interactions between the cation and dichloromethane, the solvent used for the NMR investigations. 

The reported Si NMR data for silyl cation 3 demonstrate its homoaromatic character and characterize 3 as a free silyl cation in solution. The four-membered ring in 3 is identified by three Si NMR signals 8 Si = 77.3 (Si Si ), 315.7 (Si ) and 34.3 (Si ). Remarkably, the most deshielded silicon atom in 3 is the central tricoordinated silicon. This is in agreement with some homoaromatic nature of the cation 3 with charge localization at Si (see Fig. 2a) and it discards the possibility of a classical... 

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