Dialkylstannylenes

Mossbauer spectroscopy has also been widely used to investigate the structures of dialkylstannylene derivatives of carbohydrates in the solid state. The usual magnitude of A = 2.78-3.07 mms indicated a coordination number larger than four, with Sn centers in a penta- or hexacoordinated environment. 

Transient dialkylstannylene intermediates R2Sn (reviewed in ref. 333) can be prepared by the thermolysis of distannanes ClR2SnSnR2Cl, R3SnSnR2Cl, or HR2SnSnR2H, e.g., (334, 335)... 

Dialkoxydialkyl stannanes cross-couple with derivatives of chiral tartaric acid, preserving the original chirality, as shown in reaction 32. The dialkylstannylene acetals produced in this reaction are useful reagents for synthesis of chiral compounds28611. 

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. 

M. F. Lappert, The Coordination Chemistry of Bivalent Group IV Donors Nucleophilic-Carbene and Dialkylstannylene Complexes, J. Organomet. Chem. 100, 139-159 (1975). 

This indicates that the stannylene ligand is strongly held. Additionally, attempts at coordinating other bases to the vacant orbital of the stannylene failed with this and related complexes of Sn[CH(SiMe3)2 (78). That is noteworthy, since analogous dialkylstannylene-chromiumpenta-carbonyl complexes with smaller alkyl groups attached to the tin are unstable in the absence of coordinated base (101). 

Typical 119Sn NMR Chemical Shifts for Dialkylstannylene Acetals... 

When the two oxygen atoms involved in the stannylene acetal are diaste-reotopic, three dimers, two with C2 symmetry, can be formed, as shown in Fig. 7 for those obtained from 1,2-propanediol. Dimers are named by means of the numbers of the tricoordinate oxygen atoms.44 Steric effects appear to be the most important factor in determining the relative populations of the three dimers. In particular, stannylene acetals derived from fram-diols with one adjacent axial substituent exist in solution, to the level of detection of 119Sn NMR spectroscopy, as the symmetric dimer in which the tricoordinate oxygen atom is not adjacent to the axial substituent.44 Similarly, dialkylstannylene acetals from carbohydrate-derived terminal 1,2-diols exist predominantly as symmetric dimers with the primary oxygen atoms tricoordinate (Fig. 8).19,37... 

Although the factors that influence the regioselectivities obtained from dialkylstannylene acetals are considerably more complicated than with tributylstannyl ethers, the outcome in most cases is surprisingly similar. The factor influencing regioselectivity in the absence of added nucleophiles will be discussed first. 

If p-toluenesulfonylation occurred under these conditions, the product distribution would depend on [R X]. Changing the concentration of p-toluenesulfonyl chloride from 0.10 M to 1.2 M did not affect the product ratio in a reaction with a dialkylstannylene acetal of methyl 2,3-0-isopropylidene-D-mannofuranoside. This result provides additional evidence that this reaction and all slower reactions occur under conditions where equilibration is faster than reaction. 

Dialkylstannylenes insert into Sn—H bonds forming new Sn—H bond-containing products ... 

Trialkyltin alkoxides (86) and dialkylstannylene (87) have recently attracted attention because a tin atom attached to an oxygen atom enhances the nucleophilicity of the latter without increasing its basicity. The synthetic applications of tin alkoxides manifest most remarkably in the case of polyols. For example, 1,2,6-hexanetriol was first stannylated with 1 equiv. of (Bu3Sn)20, followed by treatment with benzoyl chloride giving (88) in 67% yield. Intermediacy of the tributyltin ether (89) was proposed for the regioselective benzoylation (Scheme 35). ... 

Two types of tin derivatives have been used mainly to achieve these reactions dialkylstannylene acetals are formed by reaction of dialkyltin oxides with diols tributyltin ethers are formed by reaction of bis(tributyltin) oxide with alcohols. Some aspects of the regioselectivity achieved with these two types of intermediates are the same, while others are different, as will be outlined in the sections to follow. Where the reaction outcomes are similar, the dialkyltin reagents are strongly preferred because of their lower toxicity. As these reagents are heavily employed, the topic has been reviewed several times, but siguilicant uew observatious continue to be made. This chapter is not intended to be a comprehensive summary of all applicatious of these intermediates, but will highlight examples of each type of selectivity. 

---