Stannylium cations

Free stannylium cations and stannylium ions stabilized by side-on coordination with C=C... 

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). 

In the field of stannylium ions, n-BusSn B (C6F5)3H , synthesized by the oxidation of stannyl hydride with B(CgF5)3 (Section 2.1.2.2, Scheme 2.2, D), was characterized by Lambert et al. as a tricoordinate stannyl cation becanse of its low-field Sn NMR resonance of 360ppm. The same n-BusSn cation, having a TPFPB connterion, exhibited a less deshielded valne of +263 ppm (at room temperature), which, however, was corrected in snbseqnent stndies to +434ppm (at -Bn3Sn+ TFPB, ... 

In recent years, examples of stannylium ions, R3Sn+, and of stannylalkyl cations, R3SnCC+, have been isolated and characterized. 

The straightforward access to stable radicals opened the avenue for the synthesis of the corresponding cations by one-electron oxidations. Thus, the stable radicals (t-Bu2MeSi)3E (E = Si, Ge, Sn) can be oxidized by trityl cation in benzene, and for germanium and tin the free germylium, 22, and stannylium ion, 23, could be... 

The intramolecular n-electron donation from the remote double bond to the stannylium ion center in the stannanorbornyl cation, 62 (8 Sn = 336 in CeDg), also leads to a significant shielded tin nucleus compared to what is expected for a free trialkylstannylium ion. As already shown for the sila compounds 55-58, the coordination of the C = C double bond to the electron-deficient tin center in 62 is indicated by a low-field shift of the resonance of the vinylic carbon atoms by 11.1 ppm. Additionally, the small scalar Sn-C coupling constant J(SnC) = 26 Hz gives clear evidence for the intramolecular Ti-complexation." ... 

Similarly, the cationic character of the germylium ion MessGe", 76, was estimated by comparison of the low-field shift of the C NMR resonances of the aryl carbon atoms upon ionization with those low-field shifts found for the homologous silylium and stannylium ions." The deshielding of the aryl carbon atoms in 76 is comparable to that found for 1 and 65. This is consistent with charge development on germanium in 76 that is comparable to that on silicon in MessSi, 1, and tin in Mes3Sn, 65. 

The next landmark was the synthesis of the germylium, 22, and the stannylium ion, 23, by one-electron oxidations from the corresponding stable radicals with trityl TPFPB by Sekiguchi and co-workers. As in the case of the allyl cleavage to generate the mesityl-substituted cations, the reaction, in this case the oxidation, occurs at the periphery of the molecule and gives the possibility for efficient steric protection of the incipient cation. Both trivalent cations were obtained as their TPFBP salts and the crystal structure show well separated anions and cations. 

While for silyl and germyl cation complexes tetracoordination of the element prevails, stannyl cations favor pentacoordination. That is, the trigonal-bipyramidal environment of the Sn atom in the diwater complex of the stannylium ion Me3Sn ... 

Recently, catalytic amounts of Lewis acids have been used in the reaction of allyllic tri- -butyltins with carbonyl compounds. Maruoka et al. report the remarkably chemoselective allylstannation of o-anisaldehyde over/>-anisaldehyde, catalyzed by BlCgFsL.1 z Piers et al. report that the chemoselectivity observed does not rely on classical chelation control. They conclude that stannylium ion pair [Bu3Sn(o-anisaldehyde)2]+[o-ArCH(allyl)OB(G6Fs)3] is the active species which is preferentially formed over the complex of/>-anisaldehyde with BlCgFsL (Equation (44)).143 Lambert et al. report a similar formation of stannyl cation from allyltri- -butyltin and trityl (CgFsLB-.144... 

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... 

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