Rhodium cationic

While metalloporphyrin carbene complexes are well established for ruthenium and osmium, they are less well known for rhodium. Cationic rhodium porphyrin carbene intermediates were implicated in a report by Callot et al. in w- hich... 

The method of catalyst immobilisation appeared to affect its performance in catalysis. Catalyst obtained by method II showed a low selectivity in the hydroformylation of 1-octene (l b aldehyde ratio was even lower than 2) at a very high rate and high yields of isomerised alkenes (Table 3.2, entry 2), whereas procedure IV resulted in a catalyst that was highly selective for the linear aldehyde (with a l b ratio of 37) (entry 5). In accordance with examples from literature it is likely that procedure II gave rise to the ionic bonding of ligand-free rhodium cations on the slightly acidic silica surface [29],... 

It is interesting to note that using the sol-gel procedure (I) the pre-formation of the rhodium diphosphine complex suppressed the formation of ligand free rhodium-cations on the silica surface. This approach gave rise to a well-defined, very selective hydroformylation catalyst. All immobilised catalysts were 10 to 40 times slower than the homogeneous catalyst under the same conditions, the sol-gel procedure yielding the fastest catalyst of this series. 

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

There are a few exceptions amongst the cationic complexes that also undergo oxidative addition of dihydrogen prior to alkene complexation. Alkylphosphines, raising the electron density on the rhodium cation, have been shown to belong to these exceptions, which seems logical [16] electron-rich phosphine complexes can undergo oxidative addition of H2 before the alkene coordinates to the rhodium metal. 

The chiral bidentate amino-arsine 35 has been used to form chiral rhodium cationic species and the mixed arsine-phosphines Ph2As(CH2)2 or sPPhj have found extensive... 

Rhodium cationic and zwitteiionic complexes proved to be superior catalysts for the hydroformylation of vinylsilanes, producing either a- or ff-silyl aldehydes depending on the reaction conditions [162], On the other hand, carbonylation of vinylsilanes in the reaction related to hydrocarboxylation and hydroesterification afforded P- and a-silyl esters in high yields (eq. (14) [163]). 

A highly active catalytic system for direct arylation reactions of nonactivated arenes relied on a homobimetallic rhodium complex. Thus, treatment of [bis(2-pyridyl) amino]diphenylphosphane (109) with [Rh(cod)Cl]2 led to the formation of a complex 108, which, according to X-ray crystal structure analysis, consisted of [Rh(cod) Cl2] anion and a rhodium cation stabilized by two P,N-ligands (Scheme 9.34) [73]. This bimetallic rhodium complex (108) allowed the direct arylation of benzene (87) with the aryl chloride 106 with a turnover number (TON) of 780 under comparably mild reaction conditions. 

A recent paper describing the use of these anchored complexes for aldehyde hydrogenation stated that cationic complexes were required for the formation of the anchored species and that this anchored catalyst was, essentially, an ion pair comprised of a rhodium cationic species and an oxygen... 

The major difference between the reactions of the isoelectronic carbon monoxide and tert-butylisocyanide with 2 comes, most probably, from the different stability of the hypothetical obtainable rhodium cations. The complex [Rh(CNBu )4]+, a stable and well-known cation, allows the formation of 14, while the relatively low stability... 

The functional group tolerance of this reaction is illustrated by olefins containing ester, amide, sulfone, and nitrile groups which can be applied to RhCl(PPh3)3-catalyzed orthoalkylation with remarkable efficiency (eq 68). These functionalized olefins are much more reactive than nonfunctionaUzed olefins. Nevertheless, when rhodium cationic species are employed as a catalyst, much higher yields of orthoalkylated products can be obtained under mild reaction conditions. 

There are only two controlled kinetic studies of asymmetric hydrogenation, one of which was carried out using neutral diop complexes before the significance of ionization in polar solvents was fully appreciated, and, hence, the pathway studied is not necessarily the most efficient one. In the second, Halpern and Chan demonstrate that the kinetic form of the hydrogenation of methyl z-a-acetamidocinnamate catalyzed by the 1,2- /5(diphenylphosphino)-ethane rhodium cation in methanol is ... 

Pamies O, Net G, Widhalm M et al (1999) Rhodium cationic complexes using macrocyclic diphosphines as chiral ligands application in asymmetric hydroformylation. J Organomet Chem 587 136-143... 

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