Stannylenes

A redox process also occurs in the reaction of selenium diimides with bis(amino)stannylenes. Eor example, the cyclic stannylene McaSi)//-N Bu)2Su reacts in a 1 1 molar ratio with BuN=Sc=N Bu to give a spirocyclic tin complex, which reacts with a second equivalent of the stannylene to generate a Sn-Sn bond [d(Sn-Sn) = 2.85 A, /( Sn- Sn) = 13,865 Hz)] (Scheme 10.6). ... 

Scheme 10.6 Reaction of a selenium diimide with a bis(amino)stannylene...

SMOMCl, /-PrEt2N, CH3CN, 3 h, 40°, 87-91% yield. Diols are selectively protected using the stannylene methodology. 

Bu2SnO, benzene BnBr, DMF, heat, 80% yield. This method has also been used to protect selectively the anomeric hydroxyl in a carbohydrate derivative. The replacement of Bu2SnO with Bu2Sn(OMe)2 improves the process procedurally. The use of stannylene acetals for the regioselective manipulation of hydroxyl groups has been reviewed. ... 

Bu3Sn)20 BzCl. The use of microwaves accelerates this reaction. Bu2Sn(OMe)2 is reported to work better than Bu2SnO in the monoprotection of diols. The monoprotection of diols at the more hindered position can be accomplished through the stannylene if the reaction is quenched with PhMe2SiCl (45-77% yield).Microwave heating has been found to be effective for this transformation in some cases. ... 

Treatment of the boronate with Bui, AgO affords the monoalkylated diol in a manner similar to stannylene-directed monoalkylation and acylation. ... 

Considering the interesting bissilylene complexes, the question arises how many silylenes can be coordinated to a transition metal. In the series of stannylene complexes, a maximum coordination number of at least 3 could be established. 

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. 

Another synthesis, described in detail in Scheme 12, was devised specifically for the introduction of deuterium at both ends of l-deoxy-D-t/ireo-pentulose.21,22 Stannylene methodology was used twice, first for glycol splitting with phenyliodonium diacetate, under strictly neutral conditions (necessary to preserve the benzylidene acetal), and secondly to convert the sequence -CHOH-CD3 to CO-CD3 by brominolysis. The final, labeled pentulose was l-deo y-D-threo-(1-2H3, 5-2H)pentulose. 

Tin amidinates display a rich coordination chemistry with the metal in both the di- and tetravalent oxidation states. The first results in this area were mainly obtained with N-silylated benzamidinate ligands. Typical reactions are summarized in Scheme 48. A stannylene containing unsymmetrically substituted amidinate ligands, [o-MeC6H4C(NSiMe3)(NPh)]2Sn, has been prepared accordingly and isolated in the form of colorless crystals in 75% yield. ... 

Like the Sn(III) radicals, the Sn(II) stannylenes are familiar both as persistent species of unlimited life and as transient, highly reactive intermediates. Many of the compounds that, in the older literature, were... 

The stannylenes from either source will insert into the Sn- Sn, Sn-R, or Sn-H bonds of organotin compounds, and react with alkyl halides, disulfides, or peroxides as shown in the reaction scheme below, but only the stannylenes that are generated photolytically will react with carbonyl compounds, and it appears that the stannylenes may exist in two forms, perhaps related as singlet and triplet, or a com-plexed and uncomplexed species. 

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). 

Scheme 42 Synthesis of a stannylene by desulfurization of the corresponding tetrasul-fide...

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). ... 

Compounds containing doubly bonded tin are highly colored due to a strong absorption in the visible spectrum (77-77 transition of the double bond). The extinction coefficients of the distannene [[(Me3Si)2CH]2Sn]2 (entry 3, Table VII) were found to depend on concentration and temperature5 that for the absorption at 332 nm increases with dilution while that for the 495 nm absorption decreases. This suggests that the absorptions arise from two different species, presumably the distannene and the stannylene. By comparison with the other absorption maxima of doubly bonded tin compounds, the absorption at 495 nm is probably that of the distannene. 

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... 

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... 

Two stannenes have been synthesized by the reaction of a stannylene with a boranediylborirane (Eq. 34).85 The boranediylborirane has been shown to react toward suitable reagents as though it were the carbene,101 which is only slightly higher in energy than the boranediylborirane.102 The reaction occurs at room temperature in pentane solution. The resulting stannene has a considerable contribution from the ylide resonance structures. The carbene arising from the boranediylborirane is extremely electrophilic, and therefore the stannenes can be considered formally to be adducts of the stannylene as Lewis base and the carbene. 

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. 

Distannenes are in equilibrium with the corresponding stannylenes in solution therefore, reaction with alkynes can lead either to stannacyclo-propene or l,2-distannacyclobut-3-ene derivatives. Reaction of tetra-... 

---