Asymmetric neutral rhodium complex

Carbonyl groups are not reduced with classical Wilkinson catalysts. However, some cationic rhodium complexes show catalytic activity 52K There are only a few examples of asymmetric hydrogenation of ketones. Addition of base to a neutral rhodium complex is also a way to produce a catalyst for ketone reduction 44). Acetophenone... 

Without question, the most important developments in this field over the past 10 years have been in the area of enantioselective hydroborations. New chiral catalyst systems are typically tested in hydroborations of vinyl arenes, as reactions using HBcat and a cationic rhodium catalyst are well known to give selective formation of the unusual branched isomer. In related studies, enantiopure 2,2-disubstituted cyclopropyl boronates were easily prepared via the catalytic asymmetric hydroboration of 3,3-disubstituted cyclopropenes using a number of chiral neutral rhodium complexes (equation 13). Directing groups, such as esters and alkoxymethyl substituents, were necessary for achieving... 

Asymmetric Hydroboration. Rhodium complexes are known to catalyze hydroboration of alkenes with unreactive borane derivatives, e.g. catecholborane and oxaborolidine. This reaction proceeds enantioselectively by use of BINAP as a ligand for neutral " or cationic rhodium complexes. Reaction of styrene with catecholborane followed by oxidation affords (R)-1-phenylethanol in 96% ee in the presence of (R)-BINAP and [Rh(cod)2]Bp4 (eq 5). ... 

Asymmetric catalytic reactions under solvent-free conditions have also been reported. Contrary to the previous result, a neutral rhodium(I) complex provided comparable enantioselectivity with high chemical yield [24c], Eor certain cases, benzaldehyde gave improved enantioselectivity over cirmamaldehyde (Tab. 11.6), although the rationale behind choosing this particular CO source is not entirely clear. Additionally it should be noted that when the reaction was carried out using a stoichiometric amount of an aldehyde as the CO source in xylene, the reaction takes much longer and the enantioselectivity decreases substantially. 

Asymmetric catalytic version with Rh(i) under solvent-free conditions have also been reported by Shibata. Contrary to the previous results, a neutral rhodium(i) complex provided comparable enantioselectivities with high chemical yields. ... 

Crabtree and Chianese have extended the scope of Hoveyda s ligand by making the imidazolium salt 39 in two steps from l,l/-diamino-2,2/-binaphthyl (Fig. 10) [80]. They prepared neutral rhodium and iridium complexes with that ligand precursor and applied these complexes in the asymmetric hydrosilylation of acetophenone. Moderate enantioselectivities were obtained with the iridium derivative (up to 60% ee) whilst the rhodium catalysts only gave low enantioselectivities. 

The homogeneous catalyst has been prepared in alcoholic media and is a cation formed by loss of chloride. The procedure is described here for production of the neutral hydrido species HRh[( + )-diop]2, which is a slower catalyst than the in situ species for asymmetric hydrogenation but is equally effective in terms of optical yields. The method follows that of Levison and Robinson6 for synthesis of hydrido(triphenylphosphine)rhodium complexes. 

Achiwa et al. [46,47,48,49] further developed the approach to access the monobutyrolactone skeleton by creating a very efficient catalytic asymmetric method. Aryldensuccinic acid mono-methyl esters (23), obtained by Stobbe condensation of dimethyl succinate and the corresponding substituted aldehydes, were enantioselectively hydrogenated using a neutral rhodium (I) complex of (4[Pg.550]

There are only two controlled kinetic studies of asymmetric hydrogenation, one of which was carried out using neutral diop complexes before the significance of ionization in polar solvents was fully appreciated, and, hence, the pathway studied is not necessarily the most efficient one. In the second, Halpern and Chan demonstrate that the kinetic form of the hydrogenation of methyl z-a-acetamidocinnamate catalyzed by the 1,2- /5(diphenylphosphino)-ethane rhodium cation in methanol is ... 

Catalytic hydrogenation of a-keto-esters can be achieved in the presence of homogeneous neutral Rh complexes of the Wilkinson type. Asymmetric reduction occurs when chiral bis-phosphines are employed as ligands, and one of the best optical yields known for homogeneous a-keto-ester hydrogenation (76%) is observed with (20a) as a ligand and propyl pyruvate as substrate. Use of the ligand (20b) increases the lipophilicity of such rhodium catalysts, and hence their solubility in non-polar solvents. ... 

Rhodium complexes with the ferrocenyl ligand (i ,5)-Cy2PF-PPh2 (109) have been shown to catalyse asymmetric hydroalkynylation of norbornadienes with <99.9% ee A hydroxorhodium complex with (R)-Segphos (110) has been shown to catalyse the hydroarylation of 3-pyrrolines (111) with arylboroxines (112) under neutral conditions to give 3-arylpyrrolidines (113) (<96% ee) ... 

Importantly, asymmetric variants of the foregoing reactions have been developed by using the cationic or neutral rhodium(I)/axially chiral biaryl bisphosphine complexes as catalysts. Although the rhodium-based catalysts are expensive, these are highly stable and can be handled readily using conventional laboratory equipment. Therefore, I believe that rhodium-catalyzed [2 - - 2 - - 2] cycloaddition reactions wiU be employed continuously for the synthesis of complex aromatic compounds. 

It must be remembered that a change in the precatalyst structure or reaction conditions may bring about a change in the mechanism. Hydrogenation and asymmetric hydrogenation reactions, catalyzed by neutral and cationic rhodium complexes, respectively, clearly show this (see Section 5.1). Similarly, what happens to be the slowest step, i.e., the rate-determining step, under one set of reaction conditions need not necessarily be the rate-determining step under different conditions. 

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