Chiral organometallic cations

Owing to the high Lewis acidity the group 14 organometallic cations are polymerization catalysts par excellence. so Silanorbonyl cations and triethylsilyl arenium have been shown to be efficient catalysts for metal-free hydrosilylation reactions. Chiral silyl cation complexes with acetonitrile have been applied as cata -lysts in Diels Alder-type cyclization reactions °792 intramolecularly stabilized tetracoordinated silyl cations have been successfully used as efficient catalysts in Mukaiyama-type aldol reactions. 

A-TRISPHAT anion intercalates between two cationic complexes providing a onedimensional supramolecular chain. To our knowledge this example represented the first direct observation of chiral recognition between the A-TRISPHAT and any chiral organometallic species at least in the solid state. We also feel that this resolution originates from supramolecular control. 

TRISPHAT anion 8 seems to be more particularly efficient with cationic metallo-organic and organometallic substrates. BINPHAT 15 has often-supe-rior chiral shift properties than 8 when associated with organic cations such as ammonium cation 68 (Fig. 24). In all these examples, solvent polarity influences the quality of the separation since ion association is crucial. Solvent or solvent mixtures of low polarity are preferred for these experiments. 

In conclusion, chiral heterogeneous catalysts are prepared from chiral Rhodium diphosphine complexes and Al-MCM-41. The bonding supposedly occurs via an ionic interaction of the cationic complex with the host. Also a slight reduction of weak acidic sites of Al-MCM-41 has been observed. These catalysts are suitable for the hydrogenation of functionalised olefins. The organometallic complexes remain stable within the mesopores of the carrier at reaction conditions. The catalyst can be recycled by filtration or centrifugation. 

Many organometallic catalysts, and especially many chiral catalysts, are cationic and modification of the anion has been found to be very effective for enhancing their solubility in scC02. Tris(3,5-dimethylphenyl)borate (BARF) has proven extremely useful for this purpose early on [96], and very pronounced anion effects on the activity and selectivity of the catalysts are observed in many cases [48, 87,... 

As presented in the previous section, the chiral self-assembled M4L6 supramolecular tetrahedron can encapsulate a variety of tetra-alkyl ammonium guests and presents high configurational stability. Raymond, Bergman and coworkers, used the tetrahedral [Ga4(23)6] (5.32) (H4(23) = l,5-bis(2,3-dihydroxybenzamido)naphthalene) (Figure 5.11) as a host to encapsulate reactive cationic organometallic complexes and studied some catalytic transformations within the chiral environment of the cavity of the supramolecular tetrahedron. 

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