Rhodium complexes ester

Rhodium catalysts have also been used. Benzylic halides were converted to carboxylic esters with CO in the presence of a rhodium complex. In this case, the R could come from an ether R20, a borate ester B(OR )3, or an Al, Ti, or Zr alkoxide. Reaction with an a,co-diiodide, BU4NF and Mo(CO)e gave the corresponding lactone. ... 

Ester formation catalyzed by lipase (Mucor miehei) in conjunction with hydrogenation catalyzed by a rhodium complex Sol-gel immobilization of both catalysts... 

Ruthenium complexes of (129) and (130)336 were investigated for the asymmetric hydrogenation of prochiral 2-R-propenoic acids (Scheme 62a) rhodium complexes of these ligands were used for hydrogenation of acetoamido-cinnamic acid methyl ester (Scheme 62c) and hydrogenation of acetophenone-benzylamine (Scheme 62b). The results obtained with these... 

Rhodium compounds have also been used as catalysts since the late 1960s and mechanistic studies date from the 1970s.534,578-582 The binuclear rhodium complex [(Ph3P)4Rh2(//-OH)2] was found to be an effective catalyst for the reductive carbonylation of nitrobenzenes to carbamate esters. Electron-withdrawing groups at the para-position enhance the reactivity of the substrate.583... 

The hydrogenation of ketones with O or N functions in the a- or / -position is accomplished by several rhodium compounds [46 a, b, e, g, i, j, m, 56], Many of these examples have been applied in the synthesis of biologically active chiral products [59]. One of the first examples was the asymmetric synthesis of pantothenic acid, a member of the B complex vitamins and an important constituent of coenzyme A. Ojima et al. first described this synthesis in 1978, the most significant step being the enantioselective reduction of a cyclic a-keto ester, dihydro-4,4-dimethyl-2,3-furandione, to D-(-)-pantoyl lactone. A rhodium complex derived from [RhCl(COD)]2 and the chiral pyrrolidino diphosphine, (2S,4S)-N-tert-butoxy-carbonyl-4-diphenylphosphino-2-diphenylphosphinomethyl-pyrrolidine ((S, S) -... 

Scheme 6 Rhodium complexes of an ester-functionalised phosphine ligand...

The axial alignment of Rh2(5R-MEPY)4 leads to probable structures for the carbene intermediate as shown in Figure 17.15. Approach of styrene will occur with the phenyl group pointing away from the rhodium complex, and also in a trans (anti) fashion with respect to the ester group of the carbene moiety. The 2-phenylcyclopropane-l-carboxylic ester resulting from this is indeed the 1R,2R (1R-trans) diastereomer. 

The catalytic hydroboration of vinylarenes has also been well studied and, depending on the rhodium or iridium catalytic system used, the product distribution can be tuned. [lrCl2(T -C5Me5)]2 catalyzed the hydroboration of 4-vinylanisole in the presence of HBcat with the exclusive formation of the terminal hydroboration product, in contrast to the analogue rhodium complexes which mainly afford the branched alkylboronate ester (Scheme 7.13) [14]. 

Although enol esters have a similar structure to enamides, they have proven more difficult substrates for asymmetric hydrogenation, which is evident from the significantly fewer number of examples. One possible explanation is the weaker coordinating ability of the enol ester to the metal center, as compared to the corresponding enamide. Some rhodium complexes associated with chiral phosphorous ligands such as DIPAMP [100, 101] and DuPhos [102] are effective for asymmetric hydrogenation of a-(acyloxy)acrylates. 

An interesting asymmetric transformation is the asymmetric conjugate addition to a-acetamidoacryhc ester 30 giving phenylalanine derivative 31, which has been reported by Reetz (Scheme 3.10) [10]. The addition of phenylboronic acid 2m in the presence of a rhodium complex of l,T-binaphthol-based diphosphinite ligand 32 gave a quantitative yield of 31 with up to 11% enantiomeric excess. In this asymmetric reaction the stereochemical outcome is determined at the hydrolysis step of an oxa-7r-aUylrhodium intermediate, not at the insertion step (compare Scheme 3.7). 

It was apparent from the beginning (Scheme 16.7) that there were four potentially independent aspects of reactivity 1) the rate of bimolecular transfer of the diazo ester to the rhodium-complex [10a, 22] 2) the ratio [21] of C-H insertion to /9-H elimination [(34-1-35 -h 36 -h 37)/33] 3) the chemoselectivity [(34-i-35)/(36-i-37)] [4] and 4) the diastereoselectivity [9] of the insertion (34/35 or 36/37). As a prelude to the development of an effectively chiral catalyst, we felt that it was important to experimentally explore these aspects of reactivity. 

Transition metal catalysts and biocatalysts can be combined in tandem in very effective ways as shown by the following example (Scheme 2.21). An immobilized rhodium complex-catalyzed hydrogenahon of 46 was followed by enzymatic hydrolysis of the amide and ester groups of 47 to afford alanine (S)-9 in high conversion and enanhomeric excess. Removal of the hydrogenation catalyst by filtration prior to addition of enzyme led to improved yields when porcine kidney acylase 1 was used, although the acylase from Aspergillus melleus was unaffected by residual catalyst [23]. 

The same authors recently described the synthesis of similar rhodium-complexed dendrimers supported on a resin having both interior and exterior functional groups. These were tested as catalysts for the hydroformylation of aryl alkenes and vinyl esters (52). The results show that the reactions proceeded with high selectivity for the branched aldehydes, with excellent yields, even up to the tenth cycle. The hydroformylation experiments were carried out with first- and a second-generation rhodium-complexed dendrimers as catalysts, with a mixture of 34.5 bar of CO and 34.5 bar of H2 in dichloromethane at room temperature. Each catalyst was easily recovered by simple filtration and was reusable for at least six more cycles without... 

Other recent reports have also indicated that mixed-metal systems, particularly those containing combinations of ruthenium and rhodium complexes, can provide effective catalysts for the production of ethylene glycol or its carboxylic acid esters (5 9). However, the systems described in this paper are the first in which it has been demonstrated that composite ruthenium-rhodium catalysts, in which rhodium comprises only a minor proportion of the total metallic component, can match, in terms of both activity and selectivity, the previously documented behavior (J ) of mono-metallic rhodium catalysts containing significantly higher concentrations of rhodium. Some details of the chemistry of these bimetallic promoted catalysts are described here. 

Hashimoto and co-workers (55) reported that generation of ylide 152 from aryl ester 151 in the presence of a chiral rhodium complex Rh2(S-PTTL)4, a chiral phthalimide substimted carboxylate, followed by cycloaddition with DMAD, led to the formation of adduct 153 in good yield and in 74% enantiomeric excess (ee). 

A combination of rhodium complexes and phosphates promotes a highly regioselective allylic alkylation of unsym-metric allylic esters, where alkylation occurs at the more substituted allylic terminus of the esters (Equation (46)). As Evans and his co-workers reported, both the regio- and stereochemistry of the starting allylic esters are maintained in the allylic alkylated products (Equation (47)). Thus, the rhodium-catalyzed allylic alkylation takes place at the carbon substituted by a leaving group with net retention of configuration. A variety of carbon-centered... 

The main product is always the (R)-enantiomer of (28) 64). Employing other chiral catalysts, e.g. Schiff bases prepared from (S)-alaninemethyl ester or (S)-valinemethyl ester and 2-pyridinecarboxaldehyde in form of their rhodium complexes, in the same reaction, no or only very low asymmetric induction was observed. 

---