Dimeric rhodium -complexes

Bicyclo[2.2.1]hepta-2,5-diene rhodium (I) chloride dimer (norbornadiene rhodium chloride complex dimer) [12257-42-0] M 462, m 240°(dec). Recrystd from hot CHCl3-pet ether as fine crystals soluble in CHCI3 and C H but almost insoluble in Et20 or pet ether. [7 Chem Soc 3178 1959.]... 

L = C3H3, C H ) and then [Rh(acac)(CO),] to yield the tetranuclear species 180 (85ICA(i00)L5), where the heterocyclic ligands are tridentate. The product reacts with the rhodium(I) dimer [Rh(CO)2Cl]3 to give the trinuclear complex 181. In the solid state, the molecules of this complex form the intermolecular stacks along the z-axis. 

Photolysis of the rhodium(III) complex of octaethylporphyrin gives a rhodium(II) dimer that readily undergoes addition reactions to afford rhodium(III) species (Figure 2.42). 

Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

The syntheses and spectroscopic and electrochemical characterization of the rhodium and iridium porphyrin complexes (Por)IVI(R) and (Por)M(R)(L) have been summarized in three review articles.The classical syntheses involve Rh(Por)X with RLi or RMgBr, and [Rh(Por) with RX. In addition, reactions of the rhodium and iridium dimers have led to a wide variety of rhodium a-bonded complexes. For example, Rh(OEP)]2 reacts with benzyl bromide to give benzyl rhodium complexes, and with monosubstituted alkenes and alkynes to give a-alkyl and fT-vinyl products, respectively. More recent synthetic methods are summarized below. Although the development of iridium porphyrin chemistry has lagged behind that of rhodium, there have been few surprises and reactions of [IrfPorih and lr(Por)H parallel those of the rhodium congeners quite closely.Selected structural data for rr-bonded rhodium and iridium porphyrin complexes are collected in Table VI, and several examples are shown in Fig. 7. ... 

Another reaction involving combining alkenes is the dimerization of ethene. That reaction is catalyzed by a rhodium complex, RhClL3 as is illustrated in Figure 22.12. 

The dithiophosphonic acid monoesters, RP(OR )(S)SH can be conveniently prepared by cleavage of dimeric, cyclic diphosphetane disulfides, [RP(S)S]2 with alcohols, silanols, or trialkylsilylalcohols180 and then can be converted into metal complexes M[SPR(OR )]2 without isolation.181 The substituted ferrocenyl anion, (N3C6H4CH20)(CpFeC5H4)PS2 has been prepared in two steps from P4Sio, ferrocene and hydroxymethylbenzotriazole (and its salt was used for the preparation of some nickel and rhodium complexes).182 Zwitter-ionic ferrocenylditiophosphonates,... 

As shown in the previous two sections, rhodium(n) dimers are superior catalysts for metal carbene C-H insertion reactions. For nitrene C-H insertion reactions, many catalysts found to be effective for carbene transfer are also effective for these reactions. Particularly, Rh2(OAc)4 has demonstrated great effectiveness in the inter- and intramolecular nitrene C-H insertions. The exploration of enantioselective C-H amination using chiral rhodium catalysts has been reported by several groups.225,244,253-255 Hashimoto s dirhodium tetrakis[A-tetrachlorophthaloyl-(A)-/ r/-leuci-nate], Rh2(derived rhodium complex, Rh2(i -BNP)4 48,244 afforded moderate enantiomeric excess for amidation of benzylic C-H bonds with NsN=IPh. 

Buchwald et al. have shown that 5-20 mol % Cp2Ti(CO)2 facilitates the PKR at 18 psi CO and 90 °C, giving yields in between 58 and 95% [38]. Moreover, Mitsudo et al. [39] and Murai et al. [40] reported independently on the employment of Ru3(CO)i2 as active catalyst. Cyclopentenones were isolated in moderate to excellent yields (41-95%). In addition, rhodium catalysts were successfully examined for use in the PKR. Narasaka et al. [41] carried out reactions at atmospheric CO pressure using the dimeric [RhCl(CO)2]2 complex. Also, in the presence of other rhodium complexes like Wilkinson catalyst RhCl(PPh3)3 and [RhCl(CO)(dppp)]2 [42] in combination with silver salts, cyclopentenones were obtained in yields in the range of 20-99%. Some representative examples of the catalytic PKR are shown in Eq. 2. 

Scheme 9.9 (a) Cleavage of the dimeric rhodium complex (b) exo-coordination of... 

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. 

In our initial studies on the [5+2] cycloaddition, several metal catalysts were screened. Rhodium(I) systems were found to provide the optimum yields and generality [26]. Since the introduction of this new reaction in 1995, our group and others have reported other catalyst systems that can effect the cycloaddition of tethered VCPs and systems. These new catalysts thus far include chlororhodium dicarbonyl dimer ( [RhCl(CO)2]2 ) [27], bidentate phosphine chlororhodium dimers such as [RhCl(dppb)]2 [28] and [RhCl(dppe)]2 [29], and arene-rhodium complexes [(arene)Rh(cod)] SbFs [30]. [Cp Ru(NCCH3)3] PFg has also been demonstrated to be effective in the case of tethered alkyne-VCPs [31], but has not yet been extended to intermolecular systems or other 2n -components. 

The reaction of VCP 79 illustrates the performance of the rhodium(I) dimer (Tab. 13.4). For reference, attempts to effect [5+2] cycloadditions with this substrate (79) and [RhCl(PPh3)3]/silver triflate resulted only in the formation of complex product mixtures. In remarkable contrast, when this same substrate was treated with 5 mol% [RhCl(CO)2]2 for 20 min in toluene at 110°C, the [5+2] cycloadduct 80 was obtained in 80% yield. Despite these significant advantages, tethered alkene-VCPs are not successfully converted with this catalyst. 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

Since the hydroformylation reaction for most substrates shows a first order dependence on the concentration of rhodium hydride, the reaction becomes slower when considerable amounts of rhodium are tied up in dimers. This will occur at low pressures of hydrogen and high rhodium concentrations. Dimer formation has mainly been reported for phosphine ligands [17, 42, 45], but similar dimeric rhodium complexes from monophosphites [47] and diphosphites [33, 39] have been reported. The orange side product obtained from HRh(15)(CO)2 was characterized as the carbonyl bridged, dimeric rhodium species Rh2(15)2(CO)2 [39]. 

Due to the heterogeneity of the recently advanced solid-support catalyst for the hydroformylation, direct structural information on catalyst surface has been collected by extended X-ray absorption fine structure (EXAFS). Iwasawa is the first to directly characterize the structure of dimeric rhodium complexes supported on... 

The dimerization of 19 also occurs in the presence of rhodium salts with stable crystalline rhodium complexes of 19 that can be isolated.26 it is interesting to note that the thermal dimerization of 19 gives only a 25% yield of the cyclodimer 20.27 The metal-catalyzed dimerization of cyclohepta-l,2,3-triene in the presence of tetrakis(triphenylphosphane)nickel(0) gives tricyclic 4-radialene 21.28 The structure of this cydobutane was confirmed by X-ray diffraction. 

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