Cationic interactions rhodium

The solvents used in these rhodium-catalyzed reactions may also act as complexing agents for counterions of the anionic rhodium complexes. For example, tetraglyme is known to coordinate alkali metal cations. Such solvation decreases the possibility of the cation interacting with the anionic rhodium catalyst and lowering its activity or solubility. The crown ethers, such as [18]-crown-6... 

As would be expected for a highly electrophilic species, rhodium-catalyzed carbenoid additions are accelerated by aryl substituents, as well as by other cation-stabilizing groups on the alkene reactant.205 When applied to 1,1-diarylethenes, ERG substituents favor the position trans to the ester group.206 This can be understood in terms of maximizing the interaction between this ring and the reacting double bond. 

In these reactions, the major diastereomer is formed by the addition of hydrogen syn to the hydroxyl group in the substrate. The cationic iridium catalyst [Ir(PCy3)(py)(nbd)]+ is very effective in hydroxy-directive hydrogenation of cyclic alcohols to afford high diastereoselectivity, even in the case of bishomoallyl alcohols (Table 21.4, entries 10-13) [5, 34, 35]. An intermediary dihydride species is not observed in the case of rhodium complexes, but iridium dihydride species are observed and the interaction of the hydroxyl unit of an unsaturated alcohol with iridium is detected spectrometrically through the presence of diastereotopic hydrides using NMR spectroscopy [21]. 

In the case of cyclopentenyl carbamate in which a directive group is present at the homoallyl position, the cationic rhodium [Rh(diphos-4)]+ or iridium [Ir(PCy3)(py)(nbd)]+ catalyst cannot interact with the carbamate carbonyl, and thus approaches the double bond from the less-hindered side. This affords a cis-product preferentially, whereas with the chiral rhodium-duphos catalyst, directivity of the carbamate unit is observed (Table 21.7, entry 7). The presence of a hydroxyl group at the allyl position induced hydroxy-directive hydrogenation, and higher diastereoselectivity was obtained (entry 8) [44]. 

To summarize, chiral heterogeneous catalysts were prepared from rhodium-diphosphine complexes and aluminum-containing mesoporous materials. The bonding occurred via an ionic interaction of the cationic complex with the host. These catalysts were suitable for asymmetric hydrogenation of functionalized olefins. The catalysts can be recycled easily by filtration or centrifugation with no significant loss of activity or enantioselectivity. 

Figure 4 (A) A spherical reversed micelle of a negatively charged micro droplet of water stabilised by cationic surfactant molecules. (B) Schematic representation of the steric interactions in the reversed micelle which favors the formation of linear alkyl rhodium intermediates.

Other, less well-defined examples of suspected M — H-C interactions have appeared in the literature 134-147) One is illustrated by the structure of the T-shaped [Rh(PPh3)3]+ cation 14°). The rhodium atom in this formally 14-electron complex is apparently trying to compensate for its extreme electron deficiency by interacting with the C-C-H portion of one of its nine phenyl rings (Fig. 31). This results in one of the Rh—P-C angles being severely distorted [75.6(5)°] from a normal tetrahedral value. In this case, the Rh — H—C interaction is augmented by... 

Fig. 31. A close-up of the interaction region between the rhodium atom and the unique phenyl group in the [Rh-(PPh3)3]+ cation (Ref. 140). There are weak but perceptible interactions between Rh and the Cj and C2 atoms, causing the Rh-Pi-Cj angle to assume a highly distorted value of 75.6 (5)°. The ortho hydrogen atom, whose calculated position is indicated in this diagram, also appears to be weakly interacting with the metal...

The Goodenough model (10) can account for the electrical properties of ternary rhodium chalcogenides of the type ARh2X4. The octa-hedrally coordinated Rh3+(4d6) has the low-spin configuration, and no contribution to the conductivity is made either by direct interaction of the cation t2g orbitals or by indirect e -anion s,pa interaction. This is not the case, however, for the M cations in the MRh2X4 compounds. For example, Ni2+(f2/e/) may contribute to metallic conductivity via formation of partially-filled or bands as a result of nickel e -anion s,pa interactions. Similar considerations apply to the other ternary rhodium chalcogenides. 

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. 

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