Rh l Complexes

To explore the possibility of using tetrathiahelicene phosphanes as new phosphorous-based ligands for transition metals, with a view to applying them in asymmetric catalysis, helical-based phosphane 97 has been transformed into 

An interesting transformation of complexes 98—99 is observed after exposure to air for 24 h in a CDCI3 solution an oxidized new species is formed under these conditions whose structure, based on analytical and spectroscopic data, was determined to be a chelated phosphane—phosphine oxide Rh(I) complexes (2011EJO5649). 

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Scheme 1-12 Catalytic Cycle for Phosphine-Free Rh(l) Complexes...

Table 24.1. Enantioselective hydrogenation of dimethyl itaconate using Rh(l)-complexes ...

Scheme 10.2 Selection model of the Rh(l)-complex-catalyzed asymmetric hydrogenation to the (R)-amino acid derivative (according...

Almost all stable carbenes behave as 2-electron-cr-donating ligands with a few exceptions. In particular, in almost all cases corresponding Rh(l) complexes were targeted due to the easy synthetic method. An exception is the cyclopropenylidene carbene, with an extremely acute carbene angle. In this case, a second equivalent of carbene squeezes into the rhodium center, eliminating a chloride anion, giving the cationic dicarbenic rhodium(l) complex [51] (Scheme 5). 

Cyclic voltammetry measurements revealed that the 1,3-isomer [Rh (ri -CsPhs)) 1,3-cod)] (Ei/2 = -0.01 V vs. Fc/Fc ) is easier to oxidize than the 1,5-isomer [Rh ri -CsPhs)) 1,5-cod)] ( 1/2 = 0.09 V vs. Fc/Fc+), presumably because the Rh(l) complex of 1,3-isomer is less stable than the 1,5-isomer and the oxidized Rh(ll) complex profits from an additional agostic interaction between Rh(II) and an allylic hydrogen atom (a geometry not accesible for the 1,5-isomer). 

Several further experimental and theoretical studies of the oxidative addition of methyl iodide to Rh(l) complexes have been reported, in part because of its importance in the rhodium-catalyzed carbonylation of methanol (see Carbonyiation Processes by Homogeneous Catafysi to... 

A number of structural studies of iminophosphoranes have been reported. These include the product obtained from the reaction of phosphine (bisphosphine sulphide) (31) with p-tolyl azide which on the basis of its iH and iP n.m.r. exists in the C-ylide form (32) rather than as an iminophosphorane.16 Treatment of (32) with base gave the relatively stable iminophosphorane anion (33) which was isolated as a Rh(l) complex. The molecular structures of the iminophosphoranes (34),17 (35),1 and (36)19 have been determined by X-ray crystallography and their structural parameters compared with those determined for l,8-bis(dimethylamino)-... 

Compared to the rhodium-catalyzed asymmetric reactions, studies on the iridium-catalyzed ones have been limited until recently [8]. Hydrosilylation of acetophenone in the presence of a catalytic amount of lr(I) complex and the ligand 2 proceeded smoothly to give 1-phenylethanol in good yield, but with a much lower enantioselectivity (up to 23% ee) than the case of Rh(l) complex [6]. It is noteworthy that the alcohol of the opposite configuration was obtained when the metal is changed from Rh(I) to Ir(I) [6] as has been also observed in similar reactions using chiral oxazolinylferrocenylphosphines as ligands [8]. 

Corma, A., del Pino, M., Iglesias, M., Sanchez, F. (1992) Synthesis and characterization of new chiral Rh(l) complexes with A, A -ligands and A,P-ligands-a study of anchoring on the modified zeolites and catal54ic properties of heterogenized complexes, J. Organomet. Chem. 431, 233-246. 

Brunner, H., and Tracht, T. (1998) Asymmetric catalysis. 128. Diastereo-meric Rh(l) complexes in the enantioselective hydrogenation of ketopantolactone, Tetrahedron Asymm. 9,3773-3780. 

Some formally square four-coordinate structures are also fiuxional.Rearrangement of metal clusters and carbonyls which are bound to them have also been studied extensively.DNMR is not limited to solutions and can also be studied in solid materials. DNMR has also been observed with supramolecular complexes.Calixarene complexes based on Ir(l) and Rh(l) complexes also show dynamic motions associated with encapsulation of other molecules. " Although not an organometaiiic, a study of ring inversion in Fe(lll) porphyrins provides a particularly well-documented discussion of the methodologies that can be applied to DNMR. ... 

In essence, four types of Rh complexes have routinely been used as precursors for Rh-catalyzed hydrogenation reactions (see Fig. 3) neutral or cationic bis-diene (norbomadiene or 1,5-cyclooctadiene) Rh(l) complexes with CP or as anion, respectively, preformed diene-diphosphine complexes (see Fig. 1), mostly as anionic species and especially with air-sensitive ligands and/or P-chiral phosphines prone to racemization. Before the diene-diphosphine complexes become active catalysts, the diene has to be removed via hydrogenation. Depending on the diene and the ligand, this reaction can be rather slow, thereby leading to induction phases and/or decreased catalyst productivity for an overview, see Heller et al. [16]. The results presented show some clear trends as to which precursor type is most likely to be effective for a desired transformation ... 

Typical procedure for the reactions of Rh(l) complexes with aldehydes in THF-dg and... 

Ueno and coworkers showed that apo-Fr was able to coordinate Rh(l) complexes in the interior of the Fr cage as well [18]. Rhodium-bound apo-Fr was prepared by treating apo-Fr with an excess amoimt of [Rh(nbd)Cl]2- The purihed... 

Polbom and Severin studies have been reported that describe the use of MlPs in the reduction of ketones." This research was related closely to the work of Lemaire et al. on enantioselective hydride-transfer reactions using a catalytic Rh(l) complex" (see below), but, beneficially, an achiral catalyst and the catalytically more active Ru(ll) center was used. The MIP was prepared from a six-membered TSA for the reduction of benzophenone (Figure 2) and exhibited a sevenfold increase in hydrogenation rate relative to the control polymer. Impressive substrate selectivity was also observed with this imprinted catalyst because benzophenone was reduced preferentially with respect to acetophenone and in stark contrast use of the control polymer afforded the opposite selectivity. The 2-point connection of the catalyst to the polymer (Figure 2) was shown to improve the activity and selectivity over the analogous MIP with a single recognition site. 

Selke and coworkers have examined the application of carbohydrate-derived bisphosphinite-Rh(l) complexes from ligand 21 for the reduction of (Z)-acetam-idocinnamic acid derivatives. While a beneficial effect of sodium dodecyl sulfate (SDS) additive on enantioselectivity was observed in pure water, the Rh(l) complexes derived from these dihydroxy derivative of glucopyranoside had only limited solubility in water and they showed only moderate enantioselectivity. The polyhydroxyhc nature of carbohydrates was... 

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