Tridentate chiral ligands, enantioselective

On the other hand, the enantioselective 1,4-addition of carbanions such as enolates to linear enones is an interesting challenge, since relatively few efficient methods exist for these transformations. The Michael reaction of p-dicarbonyl compounds with a,p-unsaturated ketones can be catalysed by a number of transition-metal compounds. The asymmetric version of this reaction has been performed using chiral diol, diamine, and diphosphine ligands. In the past few years, bidentate and polydentate thioethers have begun to be considered as chiral ligands for this reaction. As an example, Christoffers et al. have developed the synthesis of several S/O-bidentate and S/O/S-tridentate thioether... 

The advances that have taken place over the past five years in catalytic, enantioselective aldol addition reactions is evident in a number of important respects. The types of transition metals and their complexes that function competently as catalysts have been expanded considerably. Thus, in addition to B(III), Ag(I), Au(I), Sn(II), and La(III), chiral catalysts prepared from Cu(II), Ti(IV), Ln(III), Si(IV), Pt(II) and Pd(II) have been introduced. The expansion in the use of transition metals has taken place hand-in-hand with the design and synthesis of new bidentate and tridentate organic ligands based on nitrogen, oxygen, and phosphorus donors. Additionally, whereas the older methods primarily relied on the use of Lewis-acids for the activation of... 

R,R)-4,6-Dibenzofiirandiyl-2,2 -bis(4-phenyloxazoline), DBFOX/Ph (5), is a novel tridentate bisoxazoline ligand developed by Kanemasa and coworkers that has been successfully used as a chiral Lewis acid in enantioselective Diels-Alder-reactions, nitrone cycloadditions and conjugate additions of radicals and thiols to 3-(2-alkenoyl)-2-oxazolidinones. Representative examples for cycloadditions using the Ni(C104)2-6H20 derived complex are shown below. 

Koga has pioneered the use of chiral amine ligands in the enantioselective reactions of lithium enolates with achiral electrophiles [51 ]. In the initial work, a full equivalent of the chiral ligand 10 was used for the benzylation of cyclohexanone and l-tetralone,Eq. (15) [52].Rationale for the enantioselectivity was coordination of the lithium ion by the tridentate ligand which provides a chiral environment around the enolate, leading to an enantioselective reaction. 

Carreira s recent work is an extension of earlier studies [5] in which a titanium(IV) complex, prepared in situ from tridentate ligand ( )-l and Ti(Ot Pr)4, was found to catalyze Mukaiyama aldol reactions with high enantioselectivities. The chiral ligand used in both the ene and aldol chemistry is prepared from 3-bromo-5-ferf-butylsalicylaldehyde and 2-amino-2 -hydroxy-l,T-binaphthol. This... 

The first report of an external chiral ligand-mediated enantioselective addition of an organolithium reagent to an imine appeared in 1990 (Tomioka, in this volume) [27]. Tomioka and coworkers found that 1,2-addition of MeLi, n-BuLi and vinyllithium to the 4-methoxyphenylimine of benzaldehyde 15 gave, in the presence of tridentate aminoether 16, the corresponding tertiary amines 17 in excellent yields and good ees (71-77%) (Scheme 6). 

The polymer-supported tridentate ligand 234b was employed in the enantioselective triethylaluminium addition to aldehydes. To obtain the same level of enantioselectivity as precedent system, 28 equivalents of Ti(0/Pr)4 relative to the chiral ligand was necessary. With most aldehydes used, the enantioselectivities were slightly lower than those obtained with the heterogeneous 234a/ri(0/Pr)4 system. 

Asymmetric hydroamination that proceeds via a Michael-type addition can be promoted enantioselectively [70, 246, 294]. More recently, Togni and Fadini have shown that chiral ferrocenyl tridentate phosphine ligand (Pigiphos ligands) can... 

Pd(OAc)2 was reacted with an imidazolium salt with two ferrocenyl phosphine substituents to give a complex 122 with a chiral tridentate PGP ligand. Treatment with [Et30]PF6 gave a dicationic complex, [Pd(PGP)(NGMe)](PF6)2, which was an effective catalyst for the enantioselective addition of morpholine to... 

Asymmetric sulfoxidation with tridentate ligand Ti complexes and 30% H2O2 is quite efficient in terms of catalytic activity with yields of up to 96%, high selectivity to sulfoxide (90%), minimal over-oxidation to sulfone and rather low catalyst loading, usually <1% mol. Nevertheless, the enantioselectivity was not very high with a maximum ee of 60% after optimization of the chiral ligand. ... 

Although a majority of the catalytic complexes employed in the aldol reaction are bidentate, Carreira and coworkers published the synthesis of a new chiral tridentate chelating ligand for the efficient asymmetric induction of stereochemistry in aldol adducts. The Ti(IV) complex 68, an analog of the BINOL catalyst previously mentioned, was further stabilized by 3,5-di-tert-butylsalicyclic acid as a counterion to increase the yields, selectivity, and efficiency of the asymmetric reaction. This new catalyst is particularly effective in the addition of either O-trimethylsilyl, or O-ethyl, or O-methyl ketene to both aliphatic and aromatic aldehydes enantioselectively to obtain the respective aldol adduct. For example, the reaction of the silylketene acetal 90 with the aromatic aldehyde 89 in the presence of 68 obtains the aldol adduct 91 in high yield (91%) and excellent enantioselectivity (97% ee). 

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