Organotin compounds chiral

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 ). 

In the present work, some promising results obtained with this kind of asymmetric heterogeneous catalyst, based on silica-supported Ni, Rh and Pt, chemically modified with chiral organotin compounds, are presented. The systems were tested in the enantioselective hydrogenation of ethyl pymvate, acetophenone and 3,4-dimethoxyacetophenone. The stabiUty of these catalysts was also studied to check if they could be reused. 

Table 6.11 Composition, activity, selectivity and enantioselectivity in the hydrogenation of acetophenone over different Pt and Rh-based catalysts promoted with different chiral organotin compounds (PE = 1-phenylethanol ...

The enantioselective hydrogenation of acetophenone afforded an excess of the (S)-phenylethanol isomer. An enantiomeric excess (e.e.) of around 20% was obtained with all the chiral organotin compounds tested, which is a good result for acetophenone, a non-activated ketone, and especially because of the high selectivity to PE (over 97%). Previously published results with the classical Pt/cinchoni-dine system showed an e.e. of 17%, corresponding to a yield of 4.7% [133]. The similar e.e.s obtained for the three rather different modifiers employed in this work seems to indicate that chiral induction must be assigned to the presence of at least one menthyl group attached to the surface. 

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. 

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)... 

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... 

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