Rhodium achiral catalyst

Striking examples of this phenomenon are presented for allyl and homoallyl alcohols in Eqs. (5) to (7). The stereodirection in Eq. (5) is improved by a chiral (+)-binap catalyst and decreased by using the antipodal catalyst [60]. In contrast, in Eq. (6) both antipode catalysts induced almost the same stereodirection, indicating that the effect of catalyst-control is negligible when compared with the directivity exerted by the substrate [59]. In Eq. (7), the sense of asymmetric induction was in-versed by using the antipode catalysts, where the directivity by chiral catalyst overrides the directivity of substrate [52]. In the case of chiral dehydroamino acids, where both double bond and amide coordinate to the metal, the effect of the stereogenic center of the substrate is negligibly small and diastereoface discrimination is unsuccessful with an achiral rhodium catalyst (see Table 21.1, entries 9 and 10) [9]. 

Interactions between side chain and olefinic carbon atom of die substrate approaching the metal seem to be similar both in the aforementioned platinum(H) complexes and in the above transition states. In fact, in the deuterioformylation of racemic 3-methyl-1-pentene in the presence of an achiral rhodium catalyst it has been shown that the si-si face is preferentially attacked in the (S) antipode whereas,... 

An efficient enantioselective reductive amination of a-branched aldehydes (90) via d5namic kinetic resolution catalyzed by (89) has been described (Scheme 26). Reductive coupling of 1,3-enynes to heterocyclic aromatic aldehydes use an achiral rhodium-catalyst with a chiral Bronsted acid (89) as co-catalyst (Scheme 27). A highly efficient enantioselective aza-ene-type reaction of N-benzoylimines (91) with enecarbamates (92) has been achieved. The reaction can be performed at extremely low loading of the catalyst (93) without notable loss of enantioselectivity of P-aminoimines obtained (Scheme 28). ... 

After modification of the hydroxy group with an enantiopure ferrocenylphosphino-directing ester group, the resulting chiral dialkenyl-carbinols could be desymmetrized by using an achiral rhodium catalyst in almost perfect optical purity (Scheme 4.111) [22]. Removal of the directing auxiliary was... 

In a further attempt, the achiral rhodium catalyst was replaced by a chiral complex, based on (5,5)-Chiraphite as a ligand (Scheme 5.131) [20]. 

Hydroformylation of olefins has been established as an important industrial tool for the production of aldehydes. In recent years, novel asymmetric tandem reactions have included a rhodium-catalysed enantioselective hydroformylation. In this context, in 2007 Abillard and Breit ° and Chercheja and Eilbracht independently reported a novel domino hydroformylation-aldol reaction catalysed by an achiral rhodium catalyst and L-proline catalyst (Scheme 7.49). Possibly owing to the fact that proline is hard but the rhodium catalyst is soft, the proline can be compatible with the rhodium catalyst to allow this domino reaction to be achieved. By fine adjustment of the hydroformylation rate to that of the L-proline-catalysed aldol addition, the undesired homodimerisation of the aldehyde could be avoided. As a result, by in situ hydroformylation reaction, the donor aldehyde of a... 

The related chiral rhodium catalyst 4 has been used to effect kinetic resolution of these substrates.2 In this catalyst the achiral phosphine ligand of 1 is replaced by (R,R)-l,2-bis(o-anisylphenylphosphino)ethane (DIPAMP). Hydrogenation cat-... 

Che has reported that both achiral and chiral rhodium catalysts function competently for intramolecular aziridination reactions of alkyl- and arylsulfonamides (Scheme 17.29) [59, 97]. Cyclized products 87 are isolated in 90% yield using 2 mol% catalyst, PhI(OAc)2, and AI2O3. Notably, reactions of this type can be performed with catalyst loadings as low as 0.02 mol% and display turnover numbers in excess of 1300. In addition, a number of chiral dimeric rhodium systems have been examined for this process, with some encouraging results. To date, the best data are obtained using Doyle s Rh2(MEOX)4 complex. At 10 mol% catalyst and with a slight excess of Phl=0, the iso-... 

When the alkylation was performed with ethyl allyl carbonate as the precursor of the it-allyl intermediate, only 32% ee was obtained, indicative of a subtle proton-transfer process involved in the catalytic process such as in Scheme 8E.39. The chiral rhodium catalyst was shown to be the primary source of the asymmetric induction because the same reaction in the absence of the rhodium catalyst generated a racemic product in 91% yield. It is interesting that the use of only half an equivalent of the chiral ligand together with half an equivalent of achiral ligand (dppb) with respect to [Pd + Rh] was sufficient to give a high enantioselectivity (93% ee). 

Asymmetric hydrogenation of alkenes is efficiently catalysed by rhodium complexes with chiral diphosphite and diphosphoramidite ligands derived from BINOL or diphenylprolinol. Choice of a proper achiral backbone is crucial.341 Highly enantioselective hydrogenation of A-protected indoles was successfully achieved by use of the rhodium catalyst generated in situ from [Rh(nbd)2]SbF6 (nbd = norborna-2,5-diene)... 

The first step was development of a catalytic epoxidation cycle using stoichiometric amounts of achiral sulfides and rhodium acetate [212-214]. The nucleophilicity of the sulfide plays a key role. In addition, the absence of sulfides led to the formation of stilbenes, and homologated products were formed in the absence of rhodium acetate [214]. This emphasizes that the sulfide and the rhodium catalyst were required for the operation of the catalytic cycle shown in Scheme 6.87B [214], It was also found that the reaction proceeded to completion with catalytic amounts of the sulfide. A prerequisite is slow addition of the diazo compound over a longer period of time, e.g. 24 h, to avoid the assumed dimerization of the diazo compound as a competing reaction under those conditions [214, 215]. 

Chiral rhodium catalyst 118, pioneered by Nishiyama, has been put to use in the addition of allyltributylstannane to achiral aldehydes [91], This catalyst is relatively insensitive to water and can even be purified by silica gel chromatography. The optimized allylation conditions employ 1 equiv of the aldehyde, 1.5 equiv of allyltributylstannane, and 5 mol% of 118 (Scheme 10-53). The reactions with many different aldehydes can all be performed at room temperature to provide good yields of the desired homoallylic alcohols albeit in moderate to poor enan-tioselectivity. 

Enantioselective carbenoid cyclopropanation of achiral alkenes can be achieved with a chiral diazocarbonyl compound and/or chiral catalyst. In general, very low levels of asymmetric induction are obtained, when a combination of an achiral copper or rhodium catalyst and a chiral diazoacetic ester (e.g. menthyl or bornyl ester ) or a chiral diazoacetamide ° (see Section 1.2.1.2.4.2.6.3.3., Table 14, entry 3) is applied. A notable exception is provided by the cyclopropanation of styrene with [(3/ )-4,4-dimethyl-2-oxotetrahydro-3-furyl] ( )-2-diazo-4-phenylbut-3-enoate to give 5 with several rhodium(II) carboxylate catalysts, asymmetric induction gave de values of 69-97%. ° Ester residues derived from a-hydroxy esters other than ( —)-(7 )-pantolactone are not as equally well suited as chiral auxiliaries for example, catalysis by the corresponding rhodium(II) (S )-lactate provides (lS, 2S )-5 with a de value of 67%. 

Optically-active aldehydes are very important as precursors not only for biologically active compounds but also for new materials. Asymmetric hydroformylation is an attractive catalytic approach to the synthesis of a large number of chiral aldehydes. With the platinum precursor (Pt(PhCN)2Cl2), anhydrous tin(II) chloride was used as cocatalyst (SnCl2/Pt 1), which is essential for catalytic activity. In case of rhodium systems an excess amount (P/Rh = 4) of diphosphite ligand was always added to the catalyst precursor to exclude the formation of HRh(CO)4, which is an active achiral hydroformylation catalyst. 

The NP2 unit and the resultant achiral [Rh(NP2)(NBD)] moiety can also be attached easily at a specific site in a protein. The protein structure then provides the chirality required for enantioselective hydrogenation. Thus, hydrogenation of a-acetamidoacrylic acid to A/ -acetylalanine catalyzed by [Rh(NP2)(NBD)] bound to avidin at RT and 1.5 atm of H2 showed —40% S enantiomeric excess. Although these hydrogenation results with avidin are modest, it does demonstrate that asymmetric synthesis is accomplished by the -phosphine rhodium catalyst attached covalently to a protein. 

More recently, Cramer and coworkers reported an asymmetric version of this chemistry to access functionalized chiral dihydrobenzofurans that possess a quaternary stereocenter using chiral rhodium catalyst (Scheme 6.13) [23]. They found that O-tethered substrate 54 was deuterated more quickly than meta-methyl-substituted derivative 55. Furthermore, the reaction will take more time for both substrates when the chiral complex with a 1,2-disubstituted cyclopen-tadienyl ligand was replaced with achiral complex with a more hindered Cp ligand. These differences between substrate 54 and 55 emphasize the significance of the alkoxy substituent as a secondary directing group for rhodium-catalyzed reactions. 

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