Stannylenes reactions

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...

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. ... 

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. 

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. ... 

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). 

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... 

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-... 

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. 

By far the most important reaction is the salt elimination reaction (a). Most of the known monomeric stannylenes have been synthesized via this route which is unaffected by the bulkiness of the substituent. In all cases, tin(II) chloride is taken as the tin component, because it dissolves quite well in ethers. The yields of the stannylenes are relatively high and may attain 95 %. 

Ligand transfer from tin(IV) to tin(II) compounds with appropriate ligands such as chlorine (d) has been used infrequently in the synthesis of stannylenes. The only reactions reported so far have been performed with oxygen as substituents at tin (see also Sect. 6.4.3). 

Bulky substituents are necessary for the stabilization of stannylenes which may have different structures in solution and in the solid state. The substituents envelope the tin atom from the back side, thus restricting its reactivity. Reactions should therefore occur predominantly from the front side ... 

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. 

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) ... 

Another very interesting reaction involving insertion of dicyclopentadienyl-stannylene into metal-hydrogen bond with displacement of its ligands has been described by J. G. Noltes et al.16S). The resulting product was identified by X-ray structural analysis. 

Insertion reactions of stannylenes, even of unstable ones, into metal-metal bonds have attracted considerable attention 156 158>. In this context, it is very astonishing that the reaction (28) between the alkyl-substituted stannylene 14 and Fe2(CO)9 does not lead to a product of type 66 (X2Sn Fe(CO)4)2 (an X-ray structural analysis indicates an Sn2Fe2-ring167)) but to a three-membered ring, as determined by elemental analysis and from IR-spectral data133). 

Nevertheless, not every reacts with any stannylene. In principle, the Y—Z bond in Eq. (25) should be rather polar or, if it is not, the elements involved in bonding should be rather heavy (high polarizability). Up to now, no reactions involving a cr—C—C- or - N- N-bond have been reported. 

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)... 

It should be noted that dicyclopentadienyltin does not give any detectable reaction with 2,3-dimehtyl-1,3-butadiene, in contrast to the dialkyltin compound 154). According to Eq. (31) 6- or 9-membered ring compounds are formed. The dicyclopentadienyl compound in Eq. (31) can also be replaced by the unstable stannylens 154). 

More studies have been concentrated on the reaction of stannylenes with molecular oxygen (which is of course a limiting-case with respect to the classification as a double bond). Eq. (33) reflects the data of a variety of experiments 86,87 93 ... 

The dispiro compound A reacts with 2 cage molecules B to form the complex molecule 77 displayed in Fig. 13. The intermediate in brackets cannot be isolated. In contrast to the reaction of the same stannylene with sulfur (Eq. (26)) the dispiro compound A cannot be isolated seperately. The mechanism of reaction (34) may of course be more complicated. The cage molecule B is discussed in more detail in Section 6.5. It should be noted that in 77 six tin atoms of two different oxidation states are combined. 

The synthesis of asymmetrically substituted stannylenes is most efficiently achieved by ligand exchange reactions between two stannylenes, SnXj and SnY2 (see Chapter 4). For example, the stannylenes [(Me3Si)2NJClSn and (C5H5)ClSn can be synthesized according to Eqs. (36) and (37) 86,93 94). 

In both examples the reaction is shifted to the right side, because the un-symmetrical compound is highly associated and poorly soluble. If the diazastannylene and dicyclopentadienyltin are mixed in 1 1 ratio, an adduct is formed which is unstable and thus cannot be isolated it decomposes to the original stannylenes (Eq. (38)) 87). 

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