Chiral compounds metallocenes

On the basis of this definition (Fig. 1) the following classes of chiral compounds will be treated in this article cyclophanes, bridged anulenes and [8]anulenes, and ( )-cyclooctene and related structures. As mentioned above, metallocenes will be excluded. 

The first assignment of the chirality (absolute configuration) to a planar chiral phane ([2.2]paracyclophanecarboxylic acid 23, in 1968),04) was deduced from the results of a kinetic resolution of its (racemic) anhydride with (—)-l-phenylethyl-amine and is based on the related topology of 23 and 2-methyl-metallocene-l-carb-oxylic acids 19). For these chiral compounds, this method had given (correct) results, as confirmed afterwards by the Bijvoet method 109). Since this method has been reviewed in some detail19,100) it will not be discussed in this survey. 

Planar chiral compounds usually (and for the purpose of this review, always) contain unsymmetrically substituted aromatic systems. Chirality arises because the otherwise enantiotopic faces of the aromatic ring are differentiated by the coordination to a metal atom - commonly iron (in the ferrocenes) or chromium (in the arenechromium tricarbonyl complexes). Withdrawal of electrons by the metal centre means that arene-metal complexes and metallocenes are more readily lithiated than their parent aromatic systems, and the stereochemical features associated with the planar chirality allow lithiation to be diastereoselective (if the starting material is chiral) or enantioselective (if only the product is chiral). 

Achiral metallocenes with a Cg-symmetry instead of a C2-symmetry have been described by Ewen et al. as syndiospecific catalysts. These are bridged fluorene-cyclopentadienyl-systems. Chiral compounds of the fluo-rene-cyclopentadienyl-type can be obtained by suitable substituition. Seemingly trivial examples of achiral bridged systems are the meso compounds of the bisin-denyl type. 

BF4] or [bmim][PF6] (Scheme 22.8). Bis-amide 19-catalyzed aldol reactions performed in [bmimllBFJ required a much lower excess of donor ketone 21 (3 equiv. instead of 30 equiv. in proline-catalyzed reactions) and allowed a synthesis of chiral compounds 22 bearing heterocyclic, prenyl, or metallocene units [43], The improved catalytic performance of prolinamide derivatives in ionic liquids might be due to a stabihzation of the iminium intermediate formed from the ketone and the catalyst or because of the enhanced nucleophilicity of the enamine [42]. Notably, IL dilution with water (1 1 by volume) accelerated the enamine/iminium ion hydrolysis and raised reaction rates and product yields, with the enantioselec-tivity being retained or even becoming somewhat higher than under water-free conditions [45], Furthermore, the catalyst/lL/water system could be easily recycled five times without aldol yield, dr, and ee losses. 

Appllca.tlons. The first widely appHcable Ic separation of enantiomeric metallocene compounds was demonstrated on P-CD bonded-phase columns. Thirteen enantiomeric derivatives of ferrocene, mthenocene, and osmocene were resolved (7). Retention data for several of these compounds are listed in Table 2, and Figure 2a shows the Ic separation of three metallocene enantiomeric pairs. P-Cyclodextrin bonded phases were used to resolve several racemic and diastereomeric 2,2-binaphthyldiyl crown ethers (9). These compounds do not contain a chiral carbon but stiU exist as enantiomers because of the staggered position of adjacent naphthyl rings, and a high degree of chiral recognition was attained for most of these compounds (9). 

Chiral, Ci-symmetric (asymmetric) bridged metallocenes, 16 108-109 Chiral, C2-symmetric bridged metallocenes, 16 104-108 Chiral, C2-symmetric catalysts, racemic mixture of, 16 106 Chiral, C2-symmetric unbridged metallocenes, 16 108 Chiral catalysts, 16 395 Chiral centers, in biochemical compounds, 17 402... 

Reaction of the anion 21 with Cp or Cp metal fragments provides further metallocene-type complexes with a pendant phosphaferrocene side-chain. For example, the reaction of the thallium derivative T1 21 with [Cp RhCl2]2 yields the cationic pentamethylrhodocenium 24 as its chloride (Scheme 1.5.10). This is an interesting species because it is a chiral water-soluble P ligand. The chloride anion can be exchanged by PF,s to make the compound more soluble in organic solvents. 

Besides other intriguing properties, such as inherent planar chirality, metallocenes are of interest due to their considerable Lewis basicity. Their direct or conjugative attachment to a polymer chain should enhance the electron density along the main chain and therefore lower the HOMO-LUMO band gap. In addition, organometallic compounds and metallocene-based monomers and polymers represent interesting potential nonlinear optical materials, useful for frequency doubling, modulation, and switching, for three reasons ... 

A separation of the racemic mixture of chiral zirconocene compounds into the optically active pure enantiomers is performed using 0-acetyl-(/ )-mandelic acid as chiral auxiliary (Structure 28). Using this enantiomerically pure metallocene in oligomerization experiments confirms the mechanistic hypothesis of stereospecificity predicting the topicity of insertion [69]. 

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