Germanes chiral—

The first chiral nonracemic germanes were prepared from the tetraphenyl derivative through a series of successive electrophilic and nucleophilic substitutions as illustrated in Scheme 1. Brook and Peddle were able to separate the diastereomeric (—)-menthyloxy derivatives 1 and 2 by fractional crystallization1. Treatment of each diastereomer with LiAlFLt afforded the (+) and (—) enantiomeric hydrides R-3 and S-3, respectively,... 

In a report describing the first enzymatic synthesis of a chiral nonracemic tetraorgano germane, Tacke and coworkers subjected the prochiral cis-hydroxymethyl derivative (10) to acetylation catalyzed by pig liver esterase (Scheme 3)6. The resulting monoacetate (11) was shown to be of 55% ee through 11 NMR analysis of the Mosher ester derivative. 

A. S. Bommarius, Development of enzymatic process for the synthesis of chiral building blocks with emphasis on protein stability (in German), Habitation thesis, RWTH Aachen, Aachen, Germany, 2000. 

Asymmetric Free-Radical Reductions Mediated by Chiral Stannanes, Germanes, and Silanes... 

Free-radical reductions mediated by chiral stannanes, germanes, and silanes may occur with enantioselectivities in excess of 99% ee. Owing to the involvement of radical intermediates and the mild reaction conditions, this process is applicable for a large variety of simple or even complex target molecules that are incompatible to asymmetric reductions that require ionic reaction conditions. 

New methods for the preparation of germanes and stannanes reported since 1995 are dealt with in Section n. In Section III, radical chain chemistry involving trialkyltin hydrides is examined. In particular, the synthetic utility of tributyltin hydride will be reviewed, as well as that of other stannanes. Recent advances in the area of asymmetric radical chemistry involving chiral non-racemic stannanes are also included. Section IV details a limited number of examples of non-radical stannane chemistry, while Section V covers recent advances in germane and plumbane chemistry. While we have restricted ourselves largely to the literature since the beginning of 1996, some salient features of earlier work are included when relevant to the discussion. 

Very recently, Gualtieri reported the preparation of chiral, C2-symmetric binaphthyl-substituted germanes (8, 9) containing S—Ge bonds (equation 9) and their application to enantioselective radical chemistry26. These compounds are reported to exhibit superior stability properties than the corresponding tin compounds which could not be isolated (see later)26. 

Lastly, Gualtieri reported the use of binaphthyl-substituted germanes (8, 9) in enan-tioselective radical chemistry26. For example, an enantioselectivity of 59% was reported for the reaction of 126 with 8 at —60° (equation 127). To the best of our knowledge, this represents the first account of the use of a chiral germanium hydride in free-radical reduction chemistry. 

Chiral adamantyl cages containing C-Br bonds at the bridgehead carbons will undergo oxidative-addition reactions with GeCl2 dioxane to yield chiral germanes. ... 

Halogermane reductions by complex hydrides are efficient, preferred methods for germane synthesis " . The complex hydrides used are MBH (M = Li, Na, K) and LiAlH.,. Reduction of Ge—X bonds by this method can be used for any molecule that otherwise is unsusceptible to complex hydride reduction or reaction. Lithium tetrahydroaluminate reduction of chiral halogermanes and alkoxygermanes results in inversion and retention of configuration, respectively. The LiBH., and LiAlH reactions require aprotic solvents, such as EtjO, THF, n-Bu O or glyme ethers. Sodium and K... 

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