Stannylenes dimerization

The second stable biradicaloid, l,3-diaza-2,4-distannacyclobutane-l,3-diyl, 54, was unexpectedly obtained by the reaction of chloro(amino)stannylene dimer [Sn N(SiMe3)2 (n-Cl)]2 and AgOCN in diethyl ether (Scheme 2.41). ... 

The 3- and 4-coordinate stannylenes are protected against self reaction, and show the same structures in solution and the solid state. Bis[2,4,6-tris(trifluoromethyl)phenyl] stannylene (21-10) is classied here as 3-coordinate because it and related ortho-trifluoromethylphenylstannylenes, in the solid state, show a weak interaction between tin and a trifluoromethyl group, and in the crystal the Sn..Sn distance is 364 pm,34 much longer than that in authentic stannylene dimers (ca. 277-291 pm). 

It is interesting to note that no examples are known for a retro-reaction of this dimerization. Such a reaction has been observed, however, for germylene complexes and for stannylene complexes, in some cases an equilibrium between uncomplexed and base-stabilized compounds has been found. 

Reactions of stannylenes Sn[N(SiMe3)2][(NCy)2CR] (R=Me, t-Bu) stabi-hzed by bulky trialkyl amidinates with elemental sulfur proceeded efficiently to afford the corresponding tetrathiastannolane Sn[N(SiMe3)2][(NCy)2CR]S4 [23]. This is in sharp contrast to the reaction of M[N(SiMe3)2]2 (M=Ge, Sn) with elemental sulfur giving the bridged dimer [M N(SiMe3)2 2S]2 [24, 25] (Scheme 2). 

The low-temperature photolysis of hexakis(2,4,6-triisopropylphenyl) cyclotristannane leads quantitatively to the corresponding distannene (Eq. 30).90 The distannene was found to be in equilibrium with the cyclotristannane over a wide range of temperatures. The simplest explanation for this is that the stannylene is formed as an intermediate from either the cyclotristannane or the distannene, but quickly adds to the distannene or dimerizes. This equilibrium can be followed by ll9Sn NMR spectroscopy at elevated temperatures.96... 

Actually, 64 is known to be dimeric in the solid state but monomeric in dilute solution or in the gas phase. The first monomeric dialkyl- and diarylstannylenes are 2-pyridylbis[(tri-methylsilyl)methyl]-substituted stannylenes and bis[2,4,6-tris(tiifluoromethyl)phenyl]stan-nylene it should be stressed, however, that the coordination number around Sn in the solid state is not 2 in these compounds. The first actual monomer with coordination number 2 in the solid state was found to be 2,2,5,5-tetrakis(trimethylsilyl)cyclopentane-l-stannylene, 65, prepared by the following reaction141 ... 

The compounds that are two coordinate in solution usually form SnSn-bonded dimers in the solid state, but the three- and four-coordinate stannylenes do not change their structures, and remain as monomers when they crystallize. Bis(2,4,6-tri-/-butylphenyl)stannylene is exceptional in that it is stable in the crystal, but in solution it rearranges to... 

The NMR spectra of the two-coordinate stannylenes in solution show values of Sn ranging from about 1150 (e.g., in ArSnl) to 3750 (in (Ar3Sn)Sn ), with a large anisotropy. The stannylenes behave as Lewis acids, for example, in the three- or four-coordinate complexes (e.g., 78, 79, and 80), which are formed when the molecule carries an intramolecular ligand, and as Lewis bases, particularly in complexing to transition metals (e.g., 81, 82, and 83). The dimerization of stannylenes to give distannenes can be regarded as a result of this amphoteric character (Equation (179)). 

Eleterostannenes can also be stabilized by intramolecular coordination by a nitrogen substituent. The four-coordinate stannylene shown in Equation (191) reacts with sulfur to give the dimeric thione, but with selenium or tellurium to give the five-coordinate monomeric yellow-orange selenone or red tellurone. The Sn=Se and Se=Te bonds, at 239.8 and... 

Soon after the isolation of 136, Tokitoh et described the synthesis of the first kinetically stabilized diarylstannylene stable in solution, that is, Tbt(Tip)Sn (169), by treatment of TbtLi with stannous chloride followed by addition of TipLi (Scheme 14.74). Under an inert atmosphere, stannylene 169 was found to be quite stable even at 60 °C in solution, and it showed a deep purple color (A,max =561 nm) in hexane. The Sn NMR spectrum of 169 showed only one signal at 2208 ppm, the chemical shift of which is characteristic of a divalent organotin compound as in the case of a monomeric dialkylstannylene (136). The bandwidth and the chemical shift of 169 were almost unchanged between —30 and 60 °C, indicating the absence of a monomer-dimer equilibrium. 

Monoetherification of polyols.12 Monobenzylation and monoallylation of polyols can be conducted conveniently under mild conditions by conversion to the stannylene derivative (dimeric) by di-n-butyltin oxide (5, 189 9, 141). The stannylene is then treated with benzyl bromide or allyl bromide and tetra-n-butylammonium iodide (1 equiv.) in benzene. The same conditions can be used to prepare monomethoxymethyl ethers. Quaternary ammonium bromides are less efficient catalysts than the iodides. These salts also accelerate reaction of stannylenes with acid anhydrides. The mechanism for this activation is not clear it may involve coordination of I" to tin. 

X. Kong and T. B. Grindley, An improved method for the regioselective oxidation of stannylene acetals and dimerization of the a-hydroxyketone products, J. Carbohydr. Chem., 12 (1993) 557-571. 

It was found that treatment of the corresponding dibromostannane with lithium naphthalenide afforded stannylene <1995OM3620>. The reaction of the latter with an excess of carbon disulfide resulted in the formation of the unsymmetrical ethene 24 (Scheme 53). Although the details of the mechanism are not clear, it is considered that the reaction proceeds via the formation of dithiocarbene 142 followed by dimerization to give a product that is thermally unstable and easily loses carbon disulfide. Thermolysis of 24 afforded symmetrically substituted ethene 143, in a quantitative yield via extrusion of carbon disulfide. 

FIGURE 5. The dimer of polycyclic stannylene 23 in the crystal short Sn Sn contacts are given as dashed lines (- -)... 

FIGURE 13. Molecular structure of the chloro(alkyl)stannylene 61a (left) and of chloro(alkyl)plum-bylene 62b (right) in the crystal, both forming asymmetric halogen-bridged dimers due to competitive intramolecular Lewis acid/Lewis base interactions. These are depicted as dashed lines (----) and the longer E—Cl bonds as thin solid lines... 

This difficulty arises also from another problem in those compounds the fact that equilibria exist between stannylenes in their true state as R2Sn and their dimeric counterparts, the distannenes R2Sn=SnR2. For this reason, the separation of compounds belonging to Tables 3 and 4 is probably open to debate. Recent reviews about the chemical behavior of stannylenes, distannenes and their relatives in group 14 are available123,125,127-130. 

Quantum-chemical calculations have been used to probe all the characteristic chemical reactions of CAs (at least in the case of silylenes and germylenes). The theoretical studies cover intramolecular rearrangements, insertions into 0-bonds, additions to double and triple bonds and dimerizations. Note that experimental data on the mechanisms of these reactions are still scarce and the results of theoretical studies are needed to understand the main trends in the reactivity of germylenes, stannylenes and plumbylenes. 

Experimental aspects of the dimerization of germylenes, stannylenes and plumbylenes were discussed in a review363. Relations between the characteristics of ER2 (E = C, Si, Ge, Sn, Pb R = H, F) and the structure of their dimers E2R4 were studied theoretically by Trinquier and coworkers364-367. 

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