Stannylenes stable

The first stable group 14 heteroallene, a l-stannaketenimine (99), was reported by Griitzmacher et al. in 1992." Compound 99 was synthesized in 9 % yield by adding diarylstannylenc 97 and mesityl isocyanide 98 in hexane (Eq. (12)). The bonding in 99 can be described as a stannylene-isocyanide adduct rather than a... 

The first landmark syntheses of persistent radicals of the heavier group 14 elements were reported nearly 30 years ago by Lappert s group. ° The first radical of this series, [(Me3Si)2CH]3Sn , was prepared by photolysis of the stable stannylene [(Me3Si)2CH]2Sn with visible light in benzene (Scheme 2.26). ° =... 

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 synthesis of doubly bonded tin compounds by the coupling of stannylenes, however useful, is limited by the need for a stable stannylene and often a second divalent species (for example, a carbene or isonitrile). The simplest example of this reaction is the formation of tetrakis[bis(tri-methylsilyl)methyl]distannene from two molecules of the corresponding stannylene,83 with which it is in equilibrium in solution as evidenced by NMR spectroscopy.91... 

A stannaketenimine has been synthesized by the low-temperature reaction of a diarylstannylene with mesityl isocyanide.87 The stannaketenimine is thermally stable, crystals being obtained by sublimation at 40°C/0.01 mmHg. In this case, the stannylene acts as the Lewis acid for formation of the adduct and, along with the stannenes described above, covers the full range of bonding modes available to the stannenes. 

A stable diarylstannanethione has been synthesized by the reaction of an extremely bulky stannylene with either styrene episulfide95 or less than one equivalent of elemental sulfur.9 If the 2,4,6-tris[bis(trimethylsilyl) methyl]phenyl group (Tb) is replaced by the smaller mesityl group, no monomeric products are observed thus the stability of the stannanethione is attributed to the demand of the substituents. 

Until now, none of these reactions has ever lead to stable monomolecular stannylenes. Nevertheless, trapping reactions have confirmed the presence of intermediate Sn(II) species 7,80,81). 

Stable stannylenes can be synthesized via routes which are combined in Scheme 1 whereas typical examples for these reactions are listed in Table 12. 

In contrast to carbenes the singlet electron configuration in stannylenes SnX2 is much more stable this implies that the non-bonding electron pair can remain unchanged during a reaction. Consequently, this reaction center and other centers must be considered in a reaction pathway multiplying the reaction possibilities compared with the isoelectronic carbenes. 

Whereas adducts of instable stannylenes with bases are stable, easily isolatable compounds, stable stannylenes form adducts which are difficult to handle and to characterize. Taking [(Me3Si)2CH]2Sn (14) and Me2Si(NCMe3)2Sn (1) as examples 133 134), it is found that these two compounds can be coordinated with a base like pyridine (and also with other bases, see Ref.133)). However, the 1 1 adduct is only stable at —30 °C and decomposes at room temperature. The base attached to the stannylene can easily be removed in a vacuum at reduced pressure. When allowed to recondensate, it again forms a complex with stannylene 134). The equilibrium formulated by Eq. (10) can thus be shifted to the desired side. 

Generally speaking, the Lewis acidity of the tin atom and, as a consequence, the stability of the formed base adduct can easily be deduced from the structure of the stannylene under consideration. If it is stable and monomolecular in non-polar solvent, it will form weak adducts. If it is not, it will form strong and stable adducts. 

In 1970 Harrison and Zuckerman 136) reported for the first time the existence of a stable adduct of a stannylene (dicyclopentadienyltin(II)) with a Lewis acid (tri-fluoroborane). In the meantime, P. G. Harrison was able to demonstrate that these Lewis-acid base interactions can be extended to a variety of main group acidsl37,138) (Eqs. (15) and (16)). 

As heavier analogs of carbenes141) stannylenes can be used as ligands in transition-metal chemistry. The stability of carbene complexes is often explained by a synergetic c,7t-effect cr-donation from the lone electron pair of the carbon atom to the metal is compensated by a a-backdonation from filled orbitals of the metal to the empty p-orbital of the carbon atom. This concept cannot be transferred to stannylene complexes. Stannylenes are poor p-a-acceptors no base-stabilized stannylene (SnX2 B, B = electron donor) has ever been found to lose its base when coordinated with a transition metal (M - SnXj B). Up to now, stannylene complexes of transition metals were only synthesized starting from stable monomoleeular stannylenes. Divalent tin compounds are nevertheless efficient cr-donors as may be deduced from the displacement reactions (17)-(20) which open convenient routes to stannylene complexes. 

Base-stabilized stannylenes have been known to form complexes with transition metals before stable stannylenes were detected. They are synthesized by a reaction similar to process (17) or by reduction of Sn(IV) compounds according to Eq (23) 146) ... 

Stable stannylenes, base-stabilized stannylenes and unstable stannyles are known to react easily with two-electron bonds, e.g. with molecule Y—Z ... 

Meehanistieal studies on the reaction of stable stannylenes with organic halides have been performed by M. F. Lappert and his group161 163>. On the basis of ESR spectroscopic data they proposed a radicalic pathway for this reaction. Initially, one electron of the stannylene is transferred to the organic halide the... 

In contrast to the numerous reactions involving single bonds, interactions of stannylenes with double bonds have not extensively been studied. There are only two cases known where addition of a monomolecular stable stannylene to a double bond system takes place (Eqs. (31) 154) and (32)133) see also Ref.169)... 

Besides these electronic phenomena, chemical reactions can be induced when stannylenes interact with light. M, F. Lappert et al. have described that certain stable stannylenes form radicals when irradiated 179,180). 

A remarkably stable, deep red Ni° stannylene complex, [Ni(1068)4l, has been prepared by the reaction of [Ni(l,5-cyclooctadiene)2] with (1068) in toluene at —78 °C. 70 In spite of the bulkiness of (1068) and the known tendency of analogous Ni° phosphine complexes to dissociate in solution, [Ni(1068)4] remains intact in solution and, moreover, melts at 178-180 °C without decomposition. X-ray crystallography shows tetrahedral geometry about the nickel atom, with Ni—Sn bond lengths of 2.3898(2)-2.399(2) A. 

Stannylenes, the analogues of carbenes, are stabilized by sterically hindered groups such as mesitylene. Dimethylstannylene, a less stable species, was trapped in an argon matrix at 5 K and analysed by IR spectroscopy129. 

Unsubstituted or 2,3-disubstituted stannoles can be prepared from the palladium-catalyzed reaction of an alkyne with the stable stannylenes [(Me3Si)2CH]2Sn and CH2C(SiMe3)2 2Sn . The mechanism has been traced by the isolation of intermediates,235 and by ab initio calculations (Equation (78)).236... 

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... 

Stannenes, R2Sn=CR2, are usually prepared either by coupling of a stable stannylene with a stable carbene, or by an elimination reaction (Equation (185)). 

The structures of some stable stannylenes, such as several amino-, " alkoxy-, and arylthio-substituted intermediates, have been revealed by X-ray crystallography. They are monomeric crystals and the tin atom has the coordination number 2. The divalent tin in such compounds is stabilized by the effects of electronegativity of the ligand atoms and by the donation of the lone-pair electrons to the vacant 5p orbital of the tin. Although the first monomeric dialkyl- and diaryl- stannylenes in... 

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. 

Stannylenes have been prepared efficiently from some fairly dilute (20-40 mM) solutions of nucleotides in methanol, by refluxing for 30 min in the presence of an equimolar amount of BUjSnO, without removal of water [9]. In these conditions, the soluble methyl ether CHj-O-SnBu -O-SnBu -OCHj may be the reactive species. This is a useful technique for compounds insoluble in benzene or toluene, provided die stannylene is stable in methanol, which is not always so. Problems may arise with rnmr-diequatorial 1,2-diols. 

In this tetrol, a single secondary alcohol is oxidized with 88% yield thanks to the formation of the most stable cyclic stannylene intermediate by the regioselective reaction of BU2S11O with one of the 1,2-diol moieties in the molecule. 

This section is devoted to the electronic spectra of germylenes, stannylenes and plumby-lenes carrying polyatomic substituents. The available data on the absorption maxima of these species are collected in Table 4, from which it can be seen that only stable polyatomic stannylenes and plumbylenes have been characterized by UV spectroscopy. 

UV spectra of stable germylenes, stannylenes and plumbylenes were recorded in solutions or in the solid phase under inert atmosphere in order to prevent access of oxygen and moisture. Thermochromic transitions have been revealed for many stable solid germylenes, stannylenes and plumbylenes. Their nature has not been studied. 

---