Enantioselectivity rhodium reactions

Another approach in the use of chiral S/P ligands for the hydrosilylation reaction of ketones was proposed more recently by Evans et Thus, in 2003, these workers studied the application of new chiral thioether-phosphinite ligands to enantioselective rhodium-catalysed ketone hydrosilylation processes. For a wide variety of ketones, such as acyclic aryl alkyl and dialkyl ketones as well as cyclic aryl alkyl ketones and also cyclic keto esters, the reaction gave high levels of enantioselectivity of up to 99% ee (Scheme 10.44). 

Several chiral ligands have been developed for use with the rhodium catalysts, among them are pyrrolidinones and imidazolidinones.207 For example, the lactamate of pyroglutamic acid gives enantioselective cyclopropanation reactions. 

As for allylic ethers, there are relatively few reports of highly enantioselective rhodium-catalyzed isomerizations of allylic alcohols (for example, >80% ee). Relative to the other processes described above, reactions of allylic alcohols are more atom economical [6] since they obviate the need for a separate hydrolysis step (Eq. 5). 

I 70 Rhodium( )-Catahfzed Allylic Substitution Reactions and their Applications Tab. 10.11 Regio- and enantioselective rhodium-catalyzed allylic substitution. 

Following the success with cobalt and rhodium, Shibata reported Ir(i)-based enantioselective catalytic reaction. Right after their observation that the efficiency of [IrCl(COD)]2-catalyzed PKR substantially increased by addition of a phosphane co-ligand, they moved directly to use chiral phosphanes and examined the enantioselectivity. " TON and TOE of the reaction were low and the number of examples was limited. Typically, the reaction required a fair amount of Ir(i) catalyst [IrCl(COD)]2 (0.1-0.15 equiv.) and (reaction time. However, this has remained as the best in terms of enantioselectivity to date. Moreover, this catalytic system provided the first asymmetric intermolecular reaction as well. 

Mn(0Ac)3 2H20 <03EJ01410> <03S1977>. Muller, on the other hand, investigated the enantioselective rhodium-catalyzed reactions between 2-diazocyclohexane-l,3-diones and alkenes, which led to products with similar structures <03HCA3164>. Palladium-mediated coupling of alkenes and a 1,3-dicarbonyl derivative was utilized in the total synthesis of ( )-brevione B <03TL5235>. 

Other organometallic compounds, including aUylic stannanes, allylic samarium, allylic germanium, and allylic indium compounds add to aldi-mines in the same manner. Aryltrialkylstannanes also add the aryl group to Al-tosyl imines using a rhodium catalyst and sonication. Catalytic enantioselective addition reactions are well known, including reactions in an ionic liquid.Allylic... 

We wish to use the "molecular imprinting effect" to obtain highly enantioselective catalytic reaction The new catalyst preparation allows us to polymerise the chiral rhodium complex in presence of optically pure l-(s)-phenylethanol as a template. 

Ito and Sawamura showed that the use of rhodium and palladium in the presence of the TRAP-type ligand generates an effective catalyst combination for the reaction of an allyl carbonate with a cyanopropionamide [128]. The palladium-TRAP complex is proposed to generate a cationic Jt-allyl species. In addition, a rhodium-TRAP species complexes the cyano group of the nucleophile and induces formation of the enolate. Reaction of the enolate with the Tt-complex in assembly I generates the observed product. Scheme 45. The notion that enoliza-tion is caused by complexation to the cyano group is based on previous results in the enantioselective rhodium-catalyzed Michael addition. 

Recently, a rhodium(I)-catalyzed, highly enantioselective aldol reaction of the 2-cyanopropionate 17 has been achieved by the use of the frans-chelating... 

While copper and iron Lewis acids are the most prominent late transition metal Diels-Alder catalysts, there are reports on the use of other chiral complexes derived from ruthenium [97,98],rhodium [99],andzinc [100] in enantioselective cycloaddition reactions, with variable levels of success. As a comparison study, the reactions of a zinc(II)-bis(oxazoline) catalyst 41 and zinc(II)-pyridylbis(ox-azoline) catalyst 42 were evaluated side-by-side with their copper(II) counterparts (Scheme 34) [101]. The study concluded that zinc(II) Lewis acids catalyzed a few cycloadditions selectively, but, in contrast to the [Cu(f-Bubox)](SbFg)2 complex 31b (Sect. 3.2.1), enantioselectivity was not maintained over a range of temperatures or substitution patterns on the dienophile. An X-ray crystal structure of [Zn(Ph-box)] (01)2 revealed a tetrahedral metal center the absolute stereochemistry of the adduct was consistent with the reaction from that geometry and opposite that obtained with Cu(II) complex 31. 

The asymmetric diphosphine 16, used in rhodium-catalyzed enantioselective hydrogenation reactions [43], should exhibit surfactant properties due to the polyethylene glycol) functionality. The behavior of 16 in water (insoluble when n = 5 and water-soluble with n = 16) is typical for nonionic surfactant diphosphines and analogous to that of 13, which shows increasing water-solubility with increasing molecular mass [40 b]. 

The enantioselective hydrosilylation reaction has been extended from 1-phcnylethanone to other alkyl aryl and alkyl alkyl ketones. With rhodium catalysts containing phosphorus-based ligands results similar to those from 1-phenylethanone have been obtained3-3. 

This section has clearly demonstrated that enantioselective rhodium-carbenoid reactions only developed strongly in recent years, but it seems that today (1995), they are at the frontier of scientific activities on diazo-carbenoid chemistry. There, I see... 

Enantioselective rhodium-catalysed cyclopropanation reactions have enjoyed considerable success for intramolecular cases. However, by suitable choice of catalyst, intermolecular reactions have also been highly selective in some instances. 

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