Silyl cations preparation

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). 

Intramolecular Trapping of Bis-N,N-Oxyiminium Cations Special y-functionalized nitro compounds (372) were constmcted with the aim of performing intramolecular trapping of bis-7V,7V-siloxyiminium cations prepared in process of double silylation of (372). Monosilylation of the latter compounds can afford different silyl derivatives (373a-c) (Scheme 3.212) (486, 487). 

This approach allows one to functionalize the j3-C atom of AN. For this purpose, AN are initially subjected to double silylation to prepare BENAs, which are then coupled with various stabilized carbocations (512, 513), as well as with sulfenyl and episulfonium cations (514). Molecules containing good leaving groups (e.g., arenesulfenyl chlorides (514)) are used as sources (or precursors) of... 

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... 

In 1998 the groups of Jprgensen and Helmchen reported the preparation of the chiral silyl cationic salt 2 (Scheme 3) [30]. This was the first time that a chiral silyl cation was used as a catalyst in an enantioselective reaction, hi order to ensure that the silyl salt had a high reactivity, the almost chemically inert and non-coordinating anions tetrakis[pentafluorophenyl]borate [TPFPB] and tetrakis[3,5-bis (trifluor-omethyl)phenyl]borate [TFPB] were chosen as counter anions. 

Since silyl cations are highly reactive and moisture sensitive, the salts (S)-2a and (S)-2b were prepared in situ from the air and moisture stable precursor (S)-5 via a hydride transfer [34, 35] with trityl tetrakis[3,5-bis(trifluoromethyl)phenyl]borate [Tr][TFPB] or trityl tetrakis[pentafluorophenyl]borate [Tr][TPFPB], The authors showed by Si-NMR studies that the desired salts were formed. The silyl salt (5)-2a was then tested in the Diels-Alder reaction as shown in Scheme 5. A good reactivity was found, and the product was obtained in 95% yield with higher than 95% endo selectivity at -40 °C in 1 h. However, only 10% ee was achieved. 

J. Lambert, Preparation of the First Tricoordinate Silyl Cation J. Phys. Org. Chem. 2001, 14, 370. 

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. 

Several major advances have been made in the field of R3M+ cations (M = Ge, Sn, Pb) in recent years, including the preparation of free cyclotrigermenium cations and possibly close to free trimesitylgermyl and trimesitylstannyl cations. However, many challenges remain, for example, a structural characterization of free R3M+ cations (M = Ge, Sn, Pb). In many respects these cations appear to be even more challenging than the silyl cations, due to their weaker and longer bonds to carbon. Presently inadequate tools which need to be improved are computational methods for the NMR chemical shifts of the Sn and Pb nuclei. 

Summary 2-(Trimethylhydrazino)phenyl substituted organosilicon compounds 3, 4, and 5 were prepared. Their NMR spectra reflect the pentacoordination of the silicon center in solution at room temperature. Compound 5 is hexacoordinated in the solid state, whereas 4 shows pentacoordination. The phenyltrimethylhydrazino as well as the dimethylbenzylamino substituent proved to be highly suitable for the intramolecular stabilization of silyl cations. 

A third possibility of chemical modification is conversion into an acylsilane which reduces the oxidation potential of the corresponding ketone by approximately 1 V. A peak potential of 1.45 V (relative to Ag/AgCl) for the oxidation of undecanoyltrimethylsilane has been reported. Preparative electrochemical oxidations of acylsilanes proceed in methanol to give the corresponding methyl esters. A two-step oxidation process must be assumed because of the reaction stoichiometry —oxidation of the acylsilane results in the carbonyl radical cation which is meso-lytically cleaved to give the silyl cation and the acyl radical, which is subsequently oxidized to give the acyl cation as ultimate electrophile which reacts with the solvent. A variety of other nucleophiles have been used and a series of carboxylic acid derivatives are available via this pathway (Scheme 49) [198]. 

Intramolecularly coordinated silyl cations are known [1, 2], but the factors favoring the formation of these complexes are not fully understood. We have prepared a series of monohalogenated silanes bearing the 2-(dimethylaminomethyl)phenyl (R = 2-(Me2NCH2)CjH4) substituent in order to obtain more insight into this question. 

Much research has been carried out in many contexts to create anions with low nucleophilicity. In the specific context of low valent silyl cations, the anion families that have proved most successful have been perfluoroaromatics introduced by Lambert and Zhang and carboranes introduced by Reed and coworkers, reported in adjacent publications in 1993. Prior to use of these anions, unsuccessful investigations were reported with anions such as perchlorate, tetrafluoroborate, tetraphenylborate, and hexachloroantimonate. All successful preparations of stannylium cations have used perfluoroaromatic anions, particularly tetrakis(pentafluorophenyl)borate (TPFPB). 

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...

Attempts have been made to prepare cyclic and halo-bridged organosilicenium ions, but the novel silyl cation Cp 2SiH+ can be made as a salt from Cp 2Si and catechol R3Sn+ can be prepared in nonnucleophilic solvents S. 

MeOH and Si react directly on a fluid bed to give (MeO)3SiH and (MeO>4Si, M(CO)2(dmpe)2Cl on reductive silylation gives the bissiloxy alkyne derivative which acidifies to the first dihydroxyacetylene complex, and silyl acetylenes prepared regioselective. The relative stability of ketene and silaketene radical cations are compared and ketene thermally eliminated from ethyl silyl acetates. ... 

Abstract The chemistry of silylium ions and related silyl cations is reviewed. The discussion is focused on work since 2005 with recourse on investigation since the early 1990s. Stmctural and spectroscopic aspects of silylium ions are emphasized. Included is also survey of methods for their preparation and of their applications in organic synthesis and catalysis. 

The oxidation of sterically hindered disilanes by trityl cation in pivaloyl nitrile was shown to yield silylium ions which form with the solvent silylnitrilium ions, e.g., 38 (Scheme 14) [53]. The generatitMi of silyl cations was, however, only observed for sterically overloaded disilanes, such as hexa-tcrt.-butyldisilane. Hexaethyldisilane for example was found to be inert under the applied reaction conditions [53], In view of the problems with bulky silanes in the standard hydride-transfer reaction, the oxidation of disilanes is a complementary addendum to the synthetic methodology. The very limited preparative access to stable radicals restricts the oxidation of silyl radicals to give silylium ions to only a few selected examples. The most prominent example is the oxidation of the stable radical 39 to the homoaromatic silylium ion 40 by trityl cation (Scheme 15) [23]. 

N-heterocyclic carbenes (NHCs) are strong donors and they form stable complexes with silylium ions which are accurately described as being 2-silylimidazolium ions 89. A remarkable NHC-stabilized silyl cation was recently prepared by the Sekiguchi group [81]. Addition of methyl triflate to the NHC/disilyne complex 90 yields the triflate of disilenyl cation, 91, the persila-variant of a vinyl cation, in good yields (Scheme 28). 

Lambert JB, Zhao Y, Zhang SM (2001) Preparation of the first tricoordinate silyl cation. J Phys Org Chem 14 370... 

Other Reactions. Trityl fluoroborate has been used often to prepare cationic organometallic complexes, as in the conversion of dienyl complexes of iron, ruthenium, and osmium into their cationic derivatives. It alkylates pyridines on the nitrogen atom in a preparation of dihydropyridines and acts as a tritylating agent. It has also been used in attempts to form silyl cations and silyl fluorides from silanes. Finally, it has been reported to be a usefiil desiccant. ... 

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