Silylium bonds

When the counterion is complex, for example metal-halogen anions such as BF4-, the most electronegative portion of the counterion becomes attached to the silicon center. Because of this attachment, it is natural to consider the intermediacy of a silicenium cation (silylium or silylenium ion) intermediate in such reactions (Eq. 4). Bond energies derived from electron impact studies indicate that Eq. 4 is exothermic in the gas phase by about 8 kcal/mol.26,29 There seems little doubt that trivalent silicon-centered cationic species do exist in the gas phase30,31 or that processes similar to that shown in Eq. 4 do occur there.32,33... 

Recent results on the chemistry of persistent vinyl cations are summarized. / , / -Disilyl-substituted vinyl cations were synthesized by intramolecular addition of transient silylium ions to alkynes. The vinyl cations are stable at ambient temperature and were isolated in the form of their tetrakispentafluorophenylborate and hexabromocarboranate salts. The vinyl cations were characterized by IR and NMR spectroscopy and by X-ray crystallography. The experimental results for the a-alkyl- and a-aryl-substituted vinyl cations confirm their Y-shape structures, consisting of a linear dicoordinated, formally positively charged a-carbon atom and a trigonal planar coordinated /f-carbon atom. In addition, the spectroscopic data clearly indicate the consequences of, / -silyl hyperconjugation in these vinyl cations. Scope and limitations of the synthetic approach to vinyl cations via addition of silylium ions to C=C triple bonds are discussed. 

On the basis of these results we embarked on a systematic study on the synthesis of vinyl cations by intramolecular addition of transient silylium ions to C=C-triple bonds using alkynyl substituted disila alkanes 6 as precursors.(35-37) In a hydride transfer reaction with trityl cation the alkynes 6 are transformed into the reactive silylium ions 7. Under essentially nonHnucleophilic reaction conditions, i.e. in the presence of only weakly coordinating anions and using aromatic hydrocarbons as solvents, the preferred reaction channel for cations 7 is the intramolecular addition of the positively charged silicon atom to the C=C triple bond which results in the formation of vinyl cations 8-10 (Scheme 1). 

Interestingly, reaction of alkynyl disilane 14 with trityl cation did not result in the formation of stable vinyl cations. Obviously, the formation of the four-membered disilacyclobutane ring is unfavorable. Similarly, treatment of alkyne 15 with the pre-formed triethylsilylarenium ion 1 derived from toluene did not give the expected intramolecular transfer of the silylium ion to the triple bond. Instead, only a complex product mixture was obtained. 

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... 

In Eq. (2), very reactive silylium ions are effectively eliminated or, more likely as will be discussed in Section III, the bound silicon is attacked by nucleophiles such as adventitious water or chloride from solvents such as dichloromethane. Similarly, a coordinated B H bond in a BH3PMe3 ligand in [Mn(CO)4(PR3)(BH3 - PMe3)]+ has recently... 

This occurs because the Si-H bond in free silanes is already polarized in the sense Si8+ Hs and coordination to an electrophilic M increases the positive charge on Si. This favors its effective elimination as a silylium cation, R3Si +, a powerful electrophile that can abstract OH from trace water to give R3SiOH and also extract fluoride from counteranions such as SbF6 [Eq. (28)] and even the BArf anion (104). As initially proposed by Crabtree (103) and supported by calculations discussed below, the cleavage in most cases is likely to be a concerted process, i.e., the nucleophile attacks the bound Si [Eq. (29)]. 

The Bartlett Condon Schneider hydride transfer reaction,22 23 first employed in silicon chemistry by Corey in 1975,24 developed since then to be the most popular synthetic approach to silylium ions in the condensed phase.10 Subsequently, it was also used for the generation of germylium22,56 and stannylium compounds.4,17,26 29 This method exploits the relative weakness of the E-H bond and involves the transfer of the hydride from the element to a strong Lewis acid, in most cases to trityl cation. The easy access of trityl salts with a wide variety of weakly coordinating counteranions is a clear advantage of this method. The reaction can be applied in polar solvents such as sulfolane, ethers and nitriles but also in chlorinated... 

---