Dirhodium species

Recent editions of Organic Reaction Mechanisms have highlighted a number of carbene and nitrene CH-insertion reactions. This field has now been reviewed with a focus on enantioselective reactions catalysed typically by dirhodium species.5 The use of C2-symmetric box ligands in asymmetric cyclopropanation reactions has been discussed in the context of a wider review of these ligands as a source of asymmetry.6... 

One of the attractions of dirhodium paddelwheel complexes is their ability to catalyse a wide variety of organic transformations such as C-H insertions, cyclopropanations and ylide formation. A review on the application of high symmetry chiral Rh2(II,II) paddlewheel compounds highlights their application as catalysts for asymmetric metal carbenoid and nitrenoid reactions, and as Lewis acids.59 Their impressive performance as catalysts in C-H functionalisation reactions has been exploited in the synthesis of complex natural products and pharmaceutical agents. A recent review on catalytic C-H functionalisation by metal carbenoid and nitrenoid insertion demonstrates the important role of dirhodium species in this field.60... 

UV Photoelectron Spectra. These in general have provided impressive support for both the qualitative ideas and the explicit quantitative results concerning the multiple M-M bonds (23, 28) and closely related ones such as the formal single bond (see Table 1) in dirhodium species (29). Such spectra have also strongly supported the accepted views on the electronic structures of many metal atom cluster compounds (30, 31). 

A more exciting application of PHIP for the mechanistic study of catalytic hydrogenations was the ability to observe and characterize intermediate species previously proposed but not directly detected. In 1994, Duckett et al. reported dihydride-olefm complexes of rhodium during the hydrogenation of those olefins using RhCl(PPh3)3 as catalyst under The expected mononuclear complexes were present, as were dirhodium species... 

The broad P resonances at 66 and 74 ppm correspond to the broadened hydride resonances at —6.2 and —8.5 ppm at 20°C. These are assigned to the hydride-containing bimetallic hydroformylation catalyst species. At —55°C these two hydride resonances start to resolve, with the resonance at —8.8 ppm forming what appears to be a pseudo-nonet, while the resonance at —6.3 ppm is only partially resolved. We assign this to the dirhodium species with one terminal and one bridging hydride, [Rh2(H)(p-H)(p-CO)(COMrac-et,ph-P4)] ", Hr, where x=l-3. 

Fig. 12 H and NMR spectra of [Rh2(nbd)2(rac-et,ph-P4)] in 30% water/de-acetone producing an exchanging mixture of the monocationic dirhodium species 14r and 15r...

Imidazolium ligands afforded the preparation of dirhodium species such as Rh2( -Bzim-A, C )2(CO)4 and chelate cationic Rh(i) dicarbonyl complexes [Rh((mim)2CH2)(GO)2]BPh4 19 [mim = Af-methylimidazol-2-yl] were also obtained. The related methylbenzimidazol complexes were also prepared. ... 

Reaction of the heterocubane cluster 14 (R = Ph) with (Cp Rhl2)2, a reagent that functions both as a Lewis acid (the metal center) and a Lewis base (the iodide ligands), generates the dirhodium complex 18. Complex 14 also adds to l,l-bis(diphenylphosphinoferrocene) to form the cyclic species 19.8... 

The reorientation takes place during the oxidation of the dirhodium complex [(triphos)Rh( -Cl)(/ -f/2,7/2-C2H2)(triphos)]Cl, the molecular structure of which is illustrated in Figure 22a, to the derivative [(triphos)Rh( -Cl)2( - /1, /1-C2H2)Rh(triphos)]2+ (the crystal structure of such dication could not be obtained, but that of the related species [(triphos)Rh(iu-Cl)(/i-OH)(/i- /1, 1-C2H2)Rh(triphos)]2+ is shown in Figure 22b).19... 

P-31 NMR was a powerful tool in studies correlating the structure of tertiary-phosphine-rhodium chloride complexes with their behavior as olefin hydrogenation catalysts. Triphenylphosphine-rhodium complex hydrogenation catalyst species (1) were studied by Tolman et al. at du Pont and Company (2). They found that tris(triphenylphosphine)rhodium(I) chloride (A) dissociates to tri-phenylphosphine and a highly reactive intermediate (B). The latter is dimerized to tetrakis(triphenylphosphine)dirhodium(I) dichloride (C). 

Two moles of aromatic aldehyde react with ethyl diazoacetate to form diastereomeric 1,3-dioxolanes.124 The reaction is catalysed by dirhodium(II) species, and proceeds via a carbonyl ylid. Stereo-control can be achieved using a bulky diazo substrate, and electronic effects of aromatic substituents are important. Different reactions show evidence of either a metal-stabilized ylid, a free ylid, or competition between the two. 

Rh2(DPB). In contrast to the dimers [Rh(P)]2, this compound reacted with dihydrogen only after intermediate addition of CO to yield a hydride (RhH)2(DPB) with two Rh-H bonds probably located inside the cavity, An insertion of CO into the Rh-H bonds, as with RhH(P) (see Sect. 4.2), was not observed. The dihydride was able to be dehydrogenated with molecular oxygen to reform Rh2(DPB) the formation of a dioxygen adduct (p-peroxo derivative) of the dirhodium(II) species which might be expected according to Scheme 3 (paths — q, t, u) was not observed. 

The lack of Rh" complexes generated from the reduction of Rh111 species was noted by Taube in 1968362 and in his recent review Felthouse notes that electrochemical studies of other metal-metal bonded systems have been a fertile area for study... although relatively few studies have been reported on dirhodium(II) compounds. 282 That is rapidly changing, as Rh2 electrochemistry has become a vigorously pursued research area. 

The use of chiral additives with a rhodium complex also leads to cyclopropanes enantioselectively. An important chiral rhodium species is Rh2(5-DOSP)4, which leads to cyclopropanes with excellent enantioselectivity in carbene cyclopro-panation reactions. Asymmetric, intramolecular cyclopropanation reactions have been reported. The copper catalyzed diazoester cyclopropanation was reported in an ionic liquid. ° It is noted that the reaction of a diazoester with a chiral dirhodium catalyst leads to p-lactones with modest enantioselectivity Phosphonate esters have been incorporated into the diazo compound... 

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