Stannyl cations

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, ... 

Dialkynylstannanes were synthesized and characterized by their NMR spectra. These compounds react with boranes yielding l-stanna-4-bora-2,5-cyclohexadienes (97), stan-noles (98), as shown in reaction 24, and stannolines (99), as shown in reaction 25. When R 4 H it is possible to isolate at low temperature the tautomeric intermediates 95 and 96 of reaction 23, one of which is a TT-stabilised stannyl cation complex279. Application of the same borane derivatization scheme to tetraalkynylstannanes leads to formation of 1-stannaspiranes where the rings are combinations of 97, 98, and 99 structures116,280. 

Addition reactions at the alkyne bonds are dealt with in the section on alkenylstannanes that are produced. The alkynyl-tin bond is more readily cleaved by both electrophiles and nucleophiles than is the alkenyl- or alkyl-tin bond. Strong electrophiles such as halogens or halogen acids attack at the z/Mzi-position of the triple bond to give a /3-stannyl cation that is stabilized by C-Sn hyperconjugation, but this is followed by cleavage of the C-Sn bond (Equation (83)). 

Similarly, trityl cation in aromatic hydrocarbons initiates the fragmentation of simple tetraalkyl plumbanes and stannanes yielding the plumbyl or stannyl cationic species, e.g. 11, and alkenes. The reaction is thought to proceed via plumbyl-or stannyl-substituted carbocations 12, which in a second step eliminate the al-kene. This approach was used in the synthesis of norbornyl cations of the elements tin and lead, e.g. 13, (Scheme 5). ... 

The stannyl cation in zwitterion 80 is only stabilized by side-on coordination with the remote C = C triple bond. The crystal structure of 80 revealed a pyramidalized tin center with the sum of valence angles of 351.1° (see Table XIV). The coordination of the tin atom to the C = C triple bond is unsymmetrical and the SnC distances... 

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 ... 

Nitrile complexes of stannyl cations with a stoichiometry of staimyl cation/nitrile = 1 1 are described in the literature however no structural or spectroscopic data are given. 

While germyl cations are not known in aqueous solutions, reports on the formation of stannyl cations appeared as early as 192324. Since then, numerous investigations25-27 established that ions of the type R3M(OH2)n+ (M = Sn, Pb) can be prepared in aqueous solutions. The early literature on this subject was reviewed in 1966 by Tobias28. Among earlier studies one should mention a series of publications by the group of Rabenstein29,30 who studied plumbyl cation complexes in aqueous solutions by H NMR. 

Tetrastannylammonium and -phosphonium cations 5 were prepared by the reaction of (Me3Sn)3E (E = N, P) with MejSnOTf or MejSnF/NaBPln in toluene74. These cations dissociate slightly in THF and can be viewed as base-stabilized stannyl cations. 

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... 

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 + [Me3Sn(H20)2]+ 84178 has its counterpart in the tetrahedral structure of the pro-tonated silanol 85.116 The structure of the bis(acetonitrile)tricyclohexyltin hexa-fluoroantimonate 86 SbF6179 reveals pentacoordination for the tin atom,180 while all... 

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