RhCl 2, oxidative addition

RhCl(PPh3)3 is a very active homogenous hydrogenation catalyst, because of its readiness to engage in oxidative addition reactions with molecules like H2, forming Rh—H bonds of moderate strength that can subsequently be broken to allow hydride transfer to the alkene substrate. A further factor is the lability of the bulky triphenylphosphines that creates coordinative unsaturation necessary to bind the substrate molecules [44]. 

The proposed reaction mechanism (Scheme 7-2) comprises (1) oxidative addition of ArSH to RhCl(PPh3)3 to give Rh(H)(Cl)(SPh)(PPli3)n, (2) coordination ofalkyne to the Rh complex, (3) ris-insertion of alkyne into the Rh-H bond with Rh positioned at terminal carbon and H at internal carbon, (4) reductive elimination of 16 from the Rh(III) complex to regenerate the Rh(I) complex. 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

The transformation of RhCl(PH3)2(HC=CH) to RhCl(PH3)2(C=CH2) has been calculated (MP2) to be exothermic by 7.8 kcal.mol"1. The intraligand 1,2-hydrogen shift mechanism found in the Ru11 system is not relevant to the present rhodium case. Starting from a T 2 C=C complex, both systems give a metal-( T]2 C-H) species in a subsequent step. In the case of the d6 Ru system this ri2 C-H complex is an intermediate. In contrast, the T 2 C-H coordinated state is a transition state in the d8 Rh1 system, the oxidative addition being a very facile process. 

The reactions on Rh/Ir usually proceed via oxidative addition to give hydrido (alkynyl) complexes, which then undergo 1,3-H shifts to form the vinylidene complexes. In general, a unimolecular mechanism has been considered to be operative. Recent studies of RhCl(PPr 2R)2 (R = C=NCBu =CHNMe) complexes have shown a remarkable acceleration of the isomerization, with the =C=CHBu complex being formed within seconds [32]. Suitable cross-over experiments showed that a bimolecular mechanism, earlier suggested by some experimental and computational results [33], did not operate. 

Another theoretical study also showed that the third pathway (bl +b3+b4), 1,3 hydrogen shift, through a hydrido-alkynyl intermediate could compete with the 1,2 hydrogen shift pathway (bl+b2) when the metal center is electron-rich enough [29, 30]. Indeed several hydrido-alkynyl intermediates have been detected or even isolated during the q -l-alkyne-to-vinylidene rearrangement on electron-rich metal centers, such as Co(I), Rh(I) and Ir(I) [73-78]. The ab initio M P2 calculations by Wakatsuki, Koga and their coworkers on the transformation of the model complex RhCl(PH3)2(HC=CH) to the vinylidene form RhCl(PH3)2(C=CH2) indicated that the transformation proceeded via the oxidative addition intermediate RhCl(PH3)2(H) (C CH) [30]. 

This sequence of events may be illustrated by the homogeneous hydrogenation of ethylene in (say) benzene solution by Wilkinson s catalyst, RhCl(PPh3)3 (Ph = phenyl, CeH5 omitted for clarity in cycle 18.10). In that square-planar complex, the central rhodium atom is stabilized in the oxidation state I by acceptance of excess electron density into the 3d orbitals of the triphenylphosphane ligands but is readily oxidized to rhodium (III), which is preferentially six coordinate. Thus, we have a typical candidate for a catalytic cycle of oxidative addition and subsequent reductive elimination ... 

Monohydrides play an important role in the following rhodium-complex-catalyzed hydrogenations in aqueous media. The catalyst precursor is [RhCl(PTA)3], which gives the catalytically active species (HRh(PTA)3] formed by dehydrochlorination of the primary product of H2 oxidative addition (88). The complex is an active catalyst for several reactants, e.g., olefinic and oxo adds, allyl alcohol, and sulfostyrene. 

Background. The Wilkinson Rh complex, RhCl[P(C6H5)3]3, catalyzes the hydrogenation of simple olefins in organic solvents under mild conditions. The mechanism which involves the oxidative addition of H2 to Rh(I) is shown in Scheme 2 (5-7). 

The reaction of [Rh2Cl2(CgHi4)4] with 2-aminopyridine led to a species believed to be a cationic solvated rhodium(I)-aminopyridine complex. This was more active than [RhCl(PPh3)3] or [RuH(Cl)(PPh3)3] for the hydrogenation of cyclohexene. The mechanism was thought to involve oxidative addition of hydrogen to rhodium(I) prior to alkene coordination.143... 

Oxidative addition of Cl2CNPh with the low-valent metal complexes Fe(CO)5, [Fe(CO)4]2-, Pt(PPh3)4, and RhCl(PPh3)3 produced Fe(CO)4 CNPh, PtCl2(CNPh)PPh3, and RhCl3(CNPh)(PPh3)2 (107, 108). 

There is general agreement on the mechanism for the stoichiometric decarbonylation of acid chlorides (9,14,15,16). The overall mechanism is shown by Equation set 2 where X = Cl. The stoichiometric decarbonylation reaction results from initial oxidative addition of the acid chloride to RhCl(PPh3)2 (Equation 2b, X = Cl). RhCl(PPh3)2 is a very reactive, low-concentration intermediate which is likely to be solvated (see Equation 2a) (17). 

At temperatures above ca. 200°C, the decarbonylation reaction can be driven catalytically (1,4,14, 20). Scheme I illustrates the proposed catalytic reaction scheme (15,16). This catalytic reaction is slow (activity for benzaldehyde decarbonylation at 178°C is 10 turnovers hr-1) presumably because the oxidative addition of RCOX to RhCl(CO)(PPh3)2 is difficult (7, 21, 22). Consistent with this, the rate is significantly greater when IrCl(CO)(PPh3)2 is used as the catalyst (benzaldehyde, 178°C, activity is 66 turnovers hr-1) (23). Oxidative addition to iridium complexes is well known to be more facile than with their rhodium analogues. 

The mono(diphosphine) complexes, [Rh(dppp)]BF4 and RhCl-(dppp), are less effective than [Rh(dppp)2] + but are still more active than RhCl(PPh3)3. The mono(diphosphine) catalysts also decompose slowly under the reaction conditions, which renders them less useful than the bis(diphosphine) catalysts. The slower rate of decarbonyla-tion observed with the mono(diphosphine) catalysts compared with the bis(diphosphine) catalysts presumably is due to the lower basicity of the former which retards the rate of oxidative addition (vide infra). Consistent with this is the observation that [Rh(COD)(dppp)]BF4 (COD = 1,5-cyclooctadiene) shows a higher rate for catalytic de-carbonylation of benzaldehyde than does [Rh(dppp)]BF4 (22). An additional observation is that the type of anion, Cl or BF4 , has no apparent effect on decarbonylation rates for the bis(diphosphine) catalysts however, for the mono(diphosphine) complexes the chloride salts show slightly lower rates than their tetrafluoroborate analogues. 

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