Chiral organotin

Gielen, M. Chirality, Static and Dynamic Stereochemistry of Organotin Compounds. 104, 57-105... 

Contents Structure and reactivity of monomeric, molecular tin(ll) compounds / M. Veith, O. Recktenwald — Chirality, static and dynamic stereochemistry of organotin compounds / M. Gielen — Coordination effects in formation and cross-linking reactions of organotin macromolecules / Z. M. O. Rzaev. 1. Organotin compounds — Addresses, essays, lectures. I. Gielen, M. (Marcel), 1938 — II. Series. 

Chirality, Static and Dynamic Stereochemistry of Organotin Compounds... 

Several methods can be used to prepare optically active organotin compounds in which the tin atom is the only chiral center. 

The classical method, which was followed to prepare the first example of an optically pure chiral organotin compound, is characterized by the use of a auxiliary chiral group necessary to convert the racemic mixture of enantiomers into a mixture of diastereomers which are then separated by a suitable physical method and converted back into the separated enantiomers by splitting off the chiral auxiliary group. This last step is sometimes difficult to achieve 34 ). 

A second method is the replacement of a chiral leaving group of an (optically unstable) organotin compound (a triorganotin menthoxide for instance) by a more nucleophilic reagent (a Grignard reagent or lithium aluminum hydride for instance). 

A fourth method is a chromatographic resolution of a racemic mixture of organotin compounds for instance on a chiral matrix such as microcrystalline cellulose triacetate. 

Separation of Diastereomeric Organotin Compounds Followed by the Cleavage of the Auxiliary Chiral Group... 

Table 3. Synthesis of optically active organotin compounds RR R"SnL from one of the diastereomeric compounds bearing the auxiliary chiral group R RR R SnLR -> RR R Sn—L ( optically pure optical purity unknown)... 

If organotin molecules can be made which contain, besides the chiral tin atom, another asymmetric atom which is optically stable, then they can exist as two diastereomeric racemic mixtures if the other atom is not resolved, or as two diastereomers if the other atom is. If these two diastereomeric mixtures (or diastereomers) can be separated (or at least if two mixtures of different compositions, i.e. diastereomeric ratios, can be obtained), then the rate at which these two mixtures (or compounds) are transformed into the equilibrium mixture is a measure for the rate of inversion at the tin atom. 

Section 3 shows that many optically active organotin compounds with an asymmetric tin atom as only chiral center can be made. This fact is already strong evidence for the optical stability of those compounds. Furthermore, their optical rotation does... 

Pentadienyltrimethylstannanes undergo regioselective conjugate additions to aldehydes, catalysed by Lewis acids. The dominant product obtained depends on the catalyst used, as shown in reaction 46. In the case of titanium tetrachloride catalysis the reaction is also stereoselective and only one diasteroisomer is obtained297. Reaction with chiral aldehydes leads to asymmetric induction with similar organotin compounds298. 

The first optically active organotin, where the metal atom is the only chiral center, was synthesized132 using tetra-p-anisyltin as starting material. A racemic triorganotin hydride was first prepared by the sequence of reactions... 

Besides the silyl enolate-mediated aldol reactions, organotin(IY) enolates are also versatile nucleophiles toward various aldehydes in the absence or presence of Lewis acid.60 However, this reaction requires a stoichiometric amount of the toxic trialkyl tin compound, which may limit its application. Yanagisawa et al.61 found that in the presence of one equivalent of methanol, the aldol reaction of an aldehyde with a cyclohexenol trichloroacetate proceeds readily at 20°C, providing the aldol product with more than 70% yield. They thus carried out the asymmetric version of this reaction using a BINAP silver(I) complex as chiral catalyst (Scheme 3-34). As shown in Table 3-8, the Sn(IY)-mediated aldol reaction results in a good diastereoselectivity (,anti/syn ratio) and also high enantioselectivity for the major component. 

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