Catalytic Activities of the Rh Complexes

Although pincer complexes are in general ubiquitous in catalysis [7,10], the reactions evaluated for Rh andir pincer complexes are still quite limited. Therefore, the few catalytic examples ofthose pincer complexes bearing two NHC moieties show the potential of these complexes rather than their limits, as the catalytic applications are highly diverse. 

The chemoselectivity was tested in a competition experiment containing dimethylphenylsilane, phenylacetylene, and styrene in a 1.1 1 1 ratio. The typical product distribution was found for the reaction with phenylacetylene, while styrene reacted slowly leading to 12% ethylbenzene. In comparison, the dinuclear Rh(I) NHC complex 37 led to a slight decrease in selectivity but comparable reactivities, and no ethylbenzene was found with this complex in the competition experiment. The results show that - in contrast to the findings by Peris - pincer Rh complexes are also be suitable catalyst precursors for the hydrogenation of C=C bonds. The results are consistent with the proposed mechanism for Rh catalysts by Ojima [37] that neutral Rh(I) complexes lead preferentially to Z-products while cationic Rh complexes lead to the -isomer [38]. 

In an exchange experiment, it was shown that the allyl ligand, bearing a terminal double bond, could be transferred from the Rh(lll) complex 28 to another Rh(I) complex 24 (bearing different N-substituents) in a fast equilibrium reaction. 

On the basis of these observations, complex 24 was tested as catalyst for the allylic alkylation of allyl carbonates. In general, Rh complexes led preferentially to the ipso alkylation products [41] while Ir complexes tended to produce the branched and Pd catalysts the linear as the major alkylation product [42]. The catalytic results of complex 24 showed a 6 1 ratio for the ipso product in case of 

In case of the linear carbonate, an Sj,j2 reaction would lead to the Rh(III) intermediate 52 bearing the allyl ligand with a terminal double bond. This complex will undergo allyl transfer much faster than the Rh(III) allyl intermediate 51 with the thermodynamically more stable internal double bond. As the oxidative addition is the rate-limiting step in this reaction, a certain amount of Rh(I) complex 24 is present in the reaction mixture to catalyze the isomerization by allyl transfer, leading to a lower ipso-selectivity with linear allyl carbonates. 

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Conjugate additions. Arylborates generated in situ from ArBr are effective as asymmetric addends to enones in the presence of a Rh-complex derived from BINAP and (acac)2Rh(C2H4)2. They give excellent chemical yields and ee (>91%, often 99%). In these processes (such as conjugate addition to alkenylphosphonates ), the use of arylboroxines (ArBO), instead of arylboronic acids is advantageous because catalytic activity of the Rh complex decimated by large amount of water can be avoided. 

Several dinuclear rhodium complexes such as the above-mentioned [Rh2(OAc)4] have been used as hydrogenation catalysts [22, 23]. Maitlis and coworkers have studied the chemistry and catalytic activity of the [Rh(C5Me5)Cl2]2 complex and related complexes. Kinetic studies suggested that cleavage into monomer occurs in the most active catalysts [90]. 

The thermoregulated phase-transfer function of nonionic phosphines has been proved by means of the aqueous-phase hydrogenation of sodium cinnamate in the presence of Rh/6 (N =32, R = n-CsHu) complex as the catalyst [16]. As outlined in Figure 2, an unusual inversely temperature-dependent catalytic behavior has been observed. Such an anti-Arrhenius kinetic behavior could only be attributed to the loss of catalytic activity of the rhodium complex when it precipitates from the aqueous phase on heating to its cloud point. Moreover, the reactivity of the catalyst could be restored since the phase separation process is reversible on cooling to a temperature lower than the cloud point. 

Further applications of TPSC have been attempted in the hydroformylation of cycloolefins, cyclohexene being a model substrate (Eq. 1). The experimental results indicate that the catalytic activity of PETPP/Rh complex is higher than that of the... 

The catalytic properties of chlorophosphine complexes of technetium such as lTcCl4(Ph3P)2]° for the hydrogenation of cyclohcxene and octcnc-1, dissolved in benzene, were studied in the presence of sodium amalgam and/or 0.1 MPa H2. The catalytic activity of the technetium complexes was shown to be evident but low as compared to the most active complexes of Ru, Rh or Ir [16],... 

Phosphorus-substituted carborane ligands can be mono-, bi-, or multidentate when they react with a metal center. Factors affecting this behavior include the structural features of the carborane cage (i.e., whether it is the ortho, meta, or para isomer, or has the closo or nido structure), the substitution pattern of the carborane cluster, the number and kind of donor atoms, and the electronic and steric properties of the ligand. All these factors have an influence on the catalytic activity of the metal complex. For example, the carborane cluster can act as an electron acceptor or electron donor at the phosphorus atom [22,29]. The donor atoms adjacent to phosphorus can act as labile ligands that block the coordination site at the metal until a substrate approaches. In nido cluster compounds, the metal ion can bind in two ways through the phosphorus substituent or by the decapped face of the cluster, whereby the former option is the most stable. In nido-carborme chemistry, B-H-Rh and... 

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

A series of studies deals with the catalytic activity of the dinuclear thiolate-bridged rhodium complex [ Rh( i-S Bu)(CO)(TPPTS) 2] in the hydroformylation ofpropene, 1-hexene and 1-octene (Scheme 4.4) [76-80]. Turnover frequencies up to 3100 h" were detected. 

None of the reactions gave rise to the formation of hexane, indicating that the complexes were under the chosen conditions inactive as hydrogenation catalysts. The catalytic activity of the most active complex 60 was considered better than that of the reference system [Rh(CO)2(acac)]/PPh3 (1 5) and roughly similar to that of complexes of monodentate 2.6-disubstituted phos-phabenzenes, but the inferior regioselectivity for n-aldehydes as compared to other catalysts makes further optimisation of the catalyst system mandatory [44]. 

Hirai et al.129 studied the hydrogenation of olefins catalyzed by poly(acrylic acid)-Rh(II) complexes in homogeneous solutions. The catalytic activity of the polymer-Rh complex was about 103 times that of the acetato-Rh complex. When olefins having another functional group, such as diallylether, allylaldehyde, and cyclohexene-1 -one, were used as the substrates, the olefinic bond was preferentially hydrogenized by the polymer-Rh complex. The polymer ligand was presumed to exercise a steric effect. 

Catalytic activity of synthesised Rh(I) siloxide complexes has been demonstrated in some reactions, i.e. in the hydrosilylation of alkenes [17] and allyl alkyl ethers [14, 18, 19] and in the silylative coupling of vinylsilanes with alkenes [20]. 

Pracejus was fascinated by the idea of functionalizing cellulose as the cheapest chiral material and to use it in this form as a carrier for monovalent rhodium for asymmetric hydrogenation. Rh was shown by Wilkinson to be useful as a catalyti-cally active central metal in phosphane complexes. However, the catalytic activities of the new cellulose immobilized complexes in the hydrogenation of unsaturated amino acid precursors were low and the enantioselectivities did not exceed 35%... 

No rationale is provided for the particular catalytic efficiency of the [Rh(CO)2Cl]2 for this transformation, or for the suppression of catalytic activity in the presence of ligands that improve the reactivity in closely related reactions [176], Nevertheless, this reaction is likely to share a common mechanism with other heterocyle arylation processes that include heterocycle coordination, C-H insertion and tautomerization to the rhodium(I) NHC complex, oxidative addition to the bromoarene, and reductive elimination of the arylated heterocycle (Scheme 20). 

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