Triorganotin halides

Table 4. Physical Properties of Typical Triorganotin Halides ...

In 1935, Naumov and Manulkin 7) tried without success to isolate (—)-methyl-ethylpropyltin iodide, (—)-(/). In fact, triorganotin halides are generally configurationally unstable, as shown in 1968 by Peddle and Redl8), whereas tetraorganotin compounds appeared to be configurationally stable within the NMR time-scale (see below). 

A triorganotin halide, bis(2-butyl)methyltin chloride14>, with more than one asymmetric atom has been used to show the configurational instability of such compounds in the presence of nucleophiles. It can exist as four different isomers. 

The determination of the Concentrations in pyridine [N] which cause the coalescence of the signals of the diastereotopic groups of a 0.262 M solution methylneophyl-t-butyltin bromide (6) and of 0.332 M solution methylneophylphenyltin chloride (3) at 22 °C at respectively 60, 100 and 270 MHz shows12) that the k2 term is much smaller than the k3[N] term. From these results, it is clear that the inversion of the configuration of the metal atom of triorganotin halides is second-order in the nucleophile pyridine. An analogous rate equation has been found for the racemiza-tion of triorganosilicon halides 29), for which the activation entropy AS is about —50 e.u. Several mechanisms with increase of coordination number 30) can be proposed to account for this second order in the nucleophile 31) ... 

Fig. 2. Possible mechanism for the optical instability of triorganotin halides catalyzed by the nucleophile N...

The first step of both mechanisms is the same, namely the addition of pyridine at the electrophilic metal atom of the triorganotin halide to give a pentacoordinate adduct. 

The reaction of configurationally unstable triorganotin halides with the chiral reducing agent introduced by Vigneron and Jacquet45H-Al(0-3,5-Me2Ph)2(0-... 

In the presence of nucleophilic solvents, a racemization is observed too in less than one hour, in the presence or in the absence of hydroquinone, compound (12) is fully racemized in methanol44. A mechanism analogous to that proposed to rationalize the optical instability of triorganotin halides can be given here 44). 

The configurational stability of triorganotin halides is considerably enhanced by the presence of an amine ligand that can coordinate intramolecularly with the tin atom. This was demonstrated by analysis of the 111 NMR spectrum of the stannyl bromide 47 depicted in Figure 626. Below 30 °C, both theN-methyl and the benzylic protons are diastereotopic ... 

FIGURE 6. Inversion of pentacoordinated triorganotin halides 47 and ent-47 according to 1H NMR analysis... 

The main method for the preparation of different types of organostannanes is the ancient Wilrtz reaction between triorganotin halides or diorganotindihalides. Reactions proceed with or without solvent. Usual solvents are benzene, toluene, xylene, diethyl ether and ethanol. Occasionally hquid ammonia is also used. The preferred metals for this reaction are sodium and lithium. In the case of triorganotin halides, the reaction leads to symmetric hexaorganoditin compounds72 ... 

Triorganotin chlorides, in triorganotin preparation, 24 815-816 Triorganotin halides, physical properties of, 24 814t... 

The results are relevant to the mechanism of the isomerization and racemization of triorganotin halides, which may include the selective cleavage of the bridging halogen bonds in unsymmetrical intermediates X—SnR3—Y—SnR3—D (D = donor) like 53. 

SCHEME 12. Triorganotin halides containing one five-membered C,N-chelating ligand... 

In the crystal structure of triorganotin halide 555a, the two C(Ar) atoms are in trans position. For hypervalent bonds, the length of the N — Sn bond trans to the iodine atom is considerably longer (2.53 A) than that trans to the methyl group (3.10 A). The 1H, 13C... 

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