Hydridorhodium complexes

Fig. 26 Regeneration of NADH by the [Cp Rh(bpy)(H20)]2+ complex. The corresponding hydridorhodium complex can be generated either electrochemically by cathodic reduction (method a) or chemically with formate (method b). (Cp = C5Me5 bpy = 2,2-bipyridine)...

The dinuclear hydridorhodium complex [ ( x-H)Rh P(0— Pr )3 2 2] is a catalyst for the stereoselective hydrogenation of dialkylalkynes and diarylalkynes to the corresponding rrans-alkenes. Although the hydrogenation rates are much lower than for terminal alkenes (approximately 1 turnover... 

Ru(bpy)3" ]. This reduced species is oxidized by Rh(bpysa).r, resulting in the formation of the reduced hydridorhodium complex, which provides further reduction of NAD" ". The NADH regenerated in this photochemical system was coupled with the enzymatic reduction of acetaldehyde to ethanol in the presence of alcohol dehydrogenase. 

As Scheme 5 illustrates, when a 1-alkene reacts with hydridorhodium complex with a chiral diphosphine ligand 18 two diastereomeric rr-olefin-Rh complexes,... 

The structures of the hydridorhodium complexes that are present in the catalytic system have been deduced by NMR spectroscopy. Brown showed that HRh(CO)2(PPh3)j exists as an 85 15 mixture of diequatoriakapical-equatorial isomers of HRh(CO)2(PPh3)2 (Scheme 17.11) that undergoes rapid equilibration at room temperature. This rapid equilibration of trigonal bipyramidal complexes could occur by either a Berry pseudorotation mechanism or a turnstile mechanism. In situ IR transmission spectroscopy on the catalytic system demonstrated that these two isomers are the resting state of the catalyst and were by far the predominant species present during hydroformylation of 1-octene (60-100 C, 5-20 atm, [Rh] = 1 mM, PPhj/Rh = 5). °... 

The solution structures of the trigonal bipyramidal hydridorhodium complexes containing diphosphite ligands, [HRh(L-L)(CO)2], which are the resting states in the hydroformylation reaction, have been analyzed in detail [13, 18-21] (see also Chapter 3). 

The key steps in the reaction are addition of hydridorhodium to the double bond of the alkene and migration of the alkyl group to the complexed carbon monoxide. Hydrogenolysis then leads to the aldehyde. 

Chiral diphosphites based on (2R,3R)-butane-2,3-diol, (2R,4R)-pentane-2,4-diol, (25, 5S)-hexane-2,5-diol, (lS -diphenylpropane-hS-diol, and tV-benzyltartarimide as chiral bridges have been used in the Rh-catalyzed asymmetric hydroformylation of styrene. Enantioselectivities up to 76%, at 50% conversion, have been obtained with stable hydridorhodium diphosphite catalysts. The solution structures of [RhH(L)(CO)2] complexes have been studied NMR and IR spectroscopic data revealed fluxional behavior. Depending on the structure of the bridge, the diphosphite adopts equatorial-equatorial or equatorial-axial coordination to the rhodium. The structure and the stability of the catalysts play a role in the asymmetric induction.218... 

To investigate whether a relationship exists between the solution structures of the hydridorhodium diphosphite species [RhH(CO)2(diphosphite)] [48] and catalytic performance, van Leeuwen and co-workers extensively studied the rhodium-diphosphite complexes formed under hydroformylation conditions by high-pressure NMR (HPNMR) techniques. It is well known that these complexes have a trigonal bipyramidal (TBP) structure. Two isomeric structures of these complexes, one containing the diphosphite coordinated in a bis-equatorial (ee) fashion and one containing the diphosphite in an equatorial-axial (ea) fashion, are possible (Fig. 4). 

The trans influence of a ligand arises principally from its inductive a-donor ability, transmitted to the trans-ligand by the appropriate metal pa-orbital. How far can this statement, made by Mason (18) and cited by Shustorovich (9), be applied to metalloporphyrins There are only a few examples of alkylmetalloporphyrins M(P)R with M = Fe, Co (69), or Rh (70) in the d6 configuration. In metal alkyl systems the strongest tram effects are to be expected hydridorhodium porphyrins are also known (71). Ogoshi (70) has prepared a variety of complexes Rh(OEP)LX ([22] ... 

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

Rhodium(Il) complexes with tertiary arsines were erroneously reported over 40 years ago. These complexes were, with the advent of NMR spectrometry, later proved to be hydridorhodium(III) complexes. Nevertheless, the only stable isolable monomeric rhodium(II) complexes are those containing tertiary phosphine and other similar group VB ligands. 

There are no tertiary arsine complexes of rhodium(II). Early reports of such complexes are erroneous.5,6 The complexes have been shown to be hydridorhodium(III) species. 

Rhodium(III) halides form [RhX2 Sb(o-C6H4Me)3 ] complexes when allowed to react with tri(o-tolyl)stibine. Despite their stoichiometry and diamagnetism there is no evidence for their being either o-metallated or hydridorhodium(III) complexes. It is believed they have the planar dimeric structure (32). Intramolecular interactions in such dimers could account for their diamagnetism and the non-equivalence of the methyl protons in their H NMR spectrum.278... 

By contrast the silyl complexes are important catalysts in a variety of hydrosilylation reactions.20 They can be prepared by the similar oxidative addition of hydrosilanes to rhodium(I) complexes, either directly or in solution (equation 189).935 However, in the presence of both additional base and triphenylphosphine a hydridorhodium(I) complex results (equation 190).936 Alternatively in the presence of a large excess of hydrosilane the monohydrido complex is transformed into a dihydrido complex.922... 

Table 79 Physical Properties of Hydridorhodium(III) Complexes Containing Bidentate Ligands...

Nitric acid converts hydridorhodium(I) complexes to the hfr(nitrato) complex (equation 52). Other examples... 

Extensive mechanistic studies have been performed on reactions catalyzed by rhodium and platinum complexes containing enantiopure C2-symmetric diphosphine ligands.As discussed above, (1) the formation of the Tr-olefin-Rh(H) complex 19, (2) stereospecific cis addition of the hydridorhodium to the coordinated olefin to form the alkyl-Rh complex 20 (and then 2, and (3) the migratory insertion of a carbonyl ligand giving the acyl-Rh complex 17 with retention of configuration, have been established in the hydroformylation of 1-alkenes or substituted ethenes. Thus, it is reasonable to assume that the enantioselectivity of the reaction giving a branched aldehyde is determined at the diastereomeric (1) TT-olefin-Rh complex 19 formation step, (2) alkyl-Rh complex 20 formation step, or (3) acyl-Rh complex 17 formation step. 

The potassium salt can be prepared by reducing [Rh2(PF3)s] with potassium amalgam (equation or by allowing the latter reagent to react with the hydridorhodium(I) complex (equation... 

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