Rhodium complexes, isomerization studies

The monosulfonated PPh derivative, Ph2P(m-C6H4S03K) (DPM) and its rhodium complex, HRh(CO)(DPM)3 have been synthesized and characterized by IR and NMR spectroscopic techniques. The data showed that the structure was similar to [HRh(CO)(PPh3)3]. The catalytic activity and selectivity of [HRh(CO)(DPM)3] in styrene hydroformylation were studied in biphasic catalytic systems.420 421 Rh1 complexes [Rh(acac)(CO)(PR3)] with tpa (131), cyep (132), (126), ompp (133), pmpp (134), tmpp (135), PPh2(pyl), PPh(pyl)2, and P(pyl)3 were characterized with NMR and IR spectra. Complexes with (131), (132), and (126) were catalysts for hydrogenation of C—C and C—O bonds, isomerization of alkenes, and hydroformylation of alkenes.422 Asymmetric hydroformylation of styrene was performed using as catalyst precursor [Rh(//-0 Me)(COD)]2 associated with sodium salts of m-sulfonated diarylphosphines.423... 

Murakami and colleagues132 studied the Diels-Alder reactions of vinylallenes with alkynes catalyzed by a rhodium complex. When a vinylallene lacking substituents at the vinylic terminus was reacted with a terminal alkyne, 1,3,5-trisubstituted benzenes were obtained, the reaction between vinylallene 197 and 1-hexyne (198) being a representative example (equation 55). The reaction was proposed to proceed via a rhodacycle which afforded the primary Diels-Alder adduct via reductive elimination. Aromatization via isomerization of the exocyclic double bond led to the isolation of 199. 

The spontaneous isomerization from the nitrito (—ONO) to the nitro (—N02) pentaammine-rhodium(III) complex has been well studied, and compared to the isomerizations of the analogous Co, Ir and Cr complexes. Isomerization was shown to occur in both the solid state and, about an order of magnitude more rapidly, in aqueous solution.517 The volume of activation (AF ) in aqueous solution is comparable for [M(NH3)5ONO]2+ (M = Co, Ir and Rh, AF = —6.7, —5.9 and — 7.4 + 4 cm3 mol-1 respectively), which is consistent with the anticipated intramolecular isomerization.584 The linkage isomerization is catalyzed by OH-,585 and by Hg2+, 5B6 In alkaline solution, the linkage isomerization of nitritopentaamminerhodium(III) obeys the rate expression rate =... 

The proposed mechanism, based on detailed studies [S.-I. Inoue, H. Takaya, K. Tani, S. Otsuka, T. Sato and R. Noyori, J. Am. Chem. Soc., 1990,112, 4897] is schematically represented in Figure 22. Dissociation of one of the BINAP ligands gives the solvated species 1 which coordinates an allylamine molecule affording 11. Complex 11 undergoes (3-elimination to form the transient imi-nium-rhodium hydrido n complex 111. This complex isomerizes to the aza-allyl... 

Although this rhodium complex has been studied in hydro-carbonylation and alkene isomerization, other rhodium catalysts give much higher yields and/or offer greater selectivity. 

A phosphite-modified calixarene with unsubstituted hydroxyl groups was used as a ligand in 1-hexene hydroformylation catalyzed by rhodium complexes [224], The reaction was carried out at a synthesis gas pressure of 6.0 MPa and 160 °C. Rh(acac)(CO)2 was a catalyst precursor. In 3 h, the conversion of the initial alkene virtually reached its theoretically predicted value the yield of aldehydes was 80-85%, and the normal-to-isomeric aldehyde ratio was approximately 1 1. Some similar phosphites 83 were also studied as components of catalytic systems for 1-octene hydroformylation [225]. It was shown that the nature and steric volume of substituent R have no essential effect on the main laws of the process. For example, the conversion was 80-90% at a selectivity with respect to nonanal of about 60% in all cases. The regioselectivity with respect to nonanal was considerably increased to 90-92% by using the chelate biphosphite 84 [220]. 

The preparation of two cyclo-octatetraene-gold complexes, (ct)AuCl and (cot)-AU2CI4, has been reported. The structures of biscyclo-octatetraenyl complexes of titanium, vanadium, thallium, and uranium, were deduced from their i.r. spectra. Protonation of (p-cyclo-octatetraene) (p-cyclopentadienyl) complexes has been studied. For the ruthenium and osmium complexes protonation occurs on the eight-membered ring to give CgH moiety co-ordinated to the metal atom via both an T -alkyl and an olefin-metal bond. For the cobalt and rhodium complexes a bicyclic cation (287) is produced which undergoes isomerization to the monocyclic (288). ... 

According to mechanistic studies, the hydrogenation of the aldehyde precedes the double-bond isomerization to the cyclic enolether, which is rapidly transformed by a Lewis acid-catalyzed acetalization. In this sequence, the cationic rhodium complexes obviously catalyze four steps. The three last steps all proceed in a domino manner and a rhodium-hydride complex is the presumed catalytically active species. 

There have been relatively few studies of conventional hydroformylation. Rhodium complexes of some of the chiral phosphines hitherto used in hydrogenation give asymmetric hydroformylation. " A comparison of cobalt and rhodium from the point of view of concurrent isomerization has been made/ " ... 

Despite the above, studies of isomerization processes at planar rhodium complexes are almost unknown, and this may reflect in part the fact that few isomer pairs are known for this element. The isolation of a series of iodo complexes cis- and tran5-[RhI(CO)(PR3)2] (R = aryl) may help this situation slightly, though the steric crowding due to the iodide makes these materials very prone to PR3 dissociation. " ... 

We have studied the reactions of SF4 with analogous complexes of rhodium. In general the results are similar, though the rhodium complexes are less stable and the initial reactions less simple, leading to the formation of more isomeric products. Fluxional processes are also similar, and are affected by ligand and by stereochemistry in the same sort of way. 

An especially important case is the enantioselective hydrogenation of a-amidoacrylic acids, which leads to a-aminoacids.29 A particularly detailed study has been carried out on the mechanism of reduction of methyl Z-a-acetamidocinnamate by a rhodium catalyst with a chiral diphosphine ligand DIPAMP.30 It has been concluded that the reactant can bind reversibly to the catalyst to give either of two complexes. Addition of hydrogen at rhodium then leads to a reactive rhodium hydride and eventually to product. Interestingly, the addition of hydrogen occurs most rapidly in the minor isomeric complex, and the enantioselectivity is due to this kinetic preference. 

Hayashi et al. proposed a catalytic cycle for the rhodium-catalyzed 1,4-addition of phenylboronic acid to 2-cyclo-hexenone (Scheme 28), which was confirmed by NMR spectroscopic studies.96 The reaction presumably involved three intermediates, phenylrhodium a, oxa-7r-allylrhodium b, and hydroxorhodium c complexes. Complex a reacted with 2-cyclohexenone to give b by insertion of the carbon-carbon double bond of enone into the phenyl-rhodium bond followed by isomerization into the thermodynamically more stable complex. Complex b was converted to c upon addition of water, liberating the phenylation product. Transmetallation of the phenyl group from phenylboronic acid to rhodium took place in the presence of triphenylphosphine to regenerate a. 

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). 

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