Diamine ligand, chiral

In recent years, enantioselective variants of the above transannular C-H insertions have been extensively stiidied. The enantiodetermining step involves discrimination between the enantiotopic protons of a meso-epoxide by a homochiral base, typically an organolithium in combination with a chiral diamine ligand, to generate a chiral nonracemic lithiated epoxide (e.g., 26 Scheme 5.8). Hodgson... 

There is one report of a chiral reagent based on allylaluminum chemistry1 2 3 4 5 10. Bis(2-methyl-propyl)-2-propenylaluminum is treated with tin(II) triflate and chiral diamine ligand 4 to give a reagent, presumably a chiral allyltin species, that reacts with aldehydes at — 78 "C. Good enantioselectivity (80 -84% ee) is obtained with aromatic aldehydes, but with aliphatic aldehydes the selectivity is somewhat lower (53-64%)10. 

In 1991, Kobayashi el al. prepared novel chiral S/N ligands for the tin-mediated aldol reaction of silyl enol ethers with aldehydes. As an example, the reaction of benzaldehyde afforded the expected syn aldol product as the major product with a good yield and an enantioselectivity of up to 92% ee (Scheme 10.26). Moreover, other aldehydes such as substituted benzaldehydes or aliphatic unsaturated aldehydes were converted into their corresponding aldol products with enantioselectivities of more than 90% ee. It was checked that the corresponding diamine ligands provided less active complexes for the same reactions. 

Related catalysts include both a chiral BINAP-type phosphine and a chiral diamine ligand. A wide range of aryl ketones gave more than 95% enantioselectivity when substituted-l,l -binaphthyl and ethylene diamines were used.52... 

Scheme 6.7 shows some other examples of enantioselective catalysts. Entry 1 illustrates the use of a Co(III) complex, with the chirality derived from the diamine ligand. Entry 2 is a silver-catalyzed cycloaddition involving generation of an azome-thine ylide. The ferrocenylphosphine groups provide a chiral environment by coordination of the catalytic Ag+ ion. Entries 3 and 4 show typical Lewis acid catalysts in reactions in which nitrones are the electrophilic component. 

More recently, asymmetric Mannich-type reactions have been studied in aqueous conditions. Barbas and co-worker reported a direct amino acid catalyzed asymmetric aldol and Mannich-type reactions that can tolerate small amounts of water (<4 vol%).53 Kobayashi found that a diastereo- and enantioselective Mannich-type reaction of a hydrazono ester with silyl enol ethers in aqueous media has been successfully achieved with ZnF2, a chiral diamine ligand, and trifluoromethanesul-fonic acid (Eq. 11.31).54 The diastereoselective Mannich-type reaction... 

The 1,2-diol is liberated easily from cyclic osmate ester by either reductive or oxidative hydrolysis.213 Importantly, the ligand acceleration has been utilized extensively for the production of chiral 1,2-diols from (achiral) olefins using optically active amine bases (such as L = dihydroquinidine, dihydroquinine and various chiral diamine ligands).215... 

In 1996, Noyori and co-workers discovered that Ru(II) ri-arene complexes containing either a chiral 1,2-amino alcohol such as in 41 or a chiral N-monotosylated 1,2-diamine ligand, see 42, serve as excellent catalysts. It was a breakthrough for catalytic ATH reactions to ketones in terms of enantioselectivities, catalyst loading and... 

To improve the rate of reduction the amino alcohol ligand of the ruthenium complexes was exchanged for monotosylated 1,2-diamine ligands. For exploratory experiments AT-tosylethane-1,2-diamine was prepared hy monotosylation of ethane-1,2-diamine and attached to the primary face of P-CD yielding 80. With P-CD as the only chiral unit the ruthenium complex of 80 could reduce aromatic and aliphatic standard ketones 63 and 69 in 91% 5deld, 25% ee (S) and 68% 5deld, 58% ee, respectively, within only 4h under standard conditions (Fig. 24). 

Asymmetric C=0 hydrogenations in water were also reported by Lemaire et al. This catalytic system is based on Ir(cod)L complexes, where L is a hydrophilic chiral C2-symmetric diamine ligand such as p-substituted (IR 2R)-(-i-)-l,2-diphenylethylenediamine derivatives (29a-e Scheme 4.12). The use of such ligands allowed catalyst recovery without loss of activity and enantioselectivity in at least four acetophenone hydrogenation cycles [29]. The ee-values observed in the reduction of phenyl glyoxylate in the water phase were, however, lower than were found when running the tests in THF (Table 4.3), when the substituents were H and Me, and about the same with OH, OMe and 0-(C2H40)3Me. 

Ikariya and Noyori et al. also reported the synthesis of new chiral Cp Rh and Cp Ir complexes (13 and 14) bearing chiral diamine ligands [(R,R)-TsCYDN and (R,R)-TsDPEN] (Scheme 5.10) these are isoelectronic with the chiral Ru complex mentioned above, and may be used as effective catalysts in the asymmetric transfer hydrogenation of aromatic ketones [42], The Cp Ir hydride complex [Cp IrH(R,R)-Tscydn] (14c) and 5-coordinated amide complex (14d), both of which would have an important role as catalytic intermediates, were also successfully prepared. 

Analogous water-soluble Cp Rh and Cp lr complexes were prepared by Williams et al., and used in the asymmetric transfer hydrogenation of aromatic ketones under aqueous conditions [43]. These catalyst complexes contain water-soluble chiral diamine ligands (Scheme 5.11), and were prepared in situ by reacting [Cp MCl2]2 (M = Rh, Ir) with ligands 15a or 15b in the presence of a base, and used immediately. The results of the asymmetric transfer hydrogenation of... 

D. Reactions with the Chiral Diamine Ligand (—)-Sparteine... 928... 

Catalysts for ketone hydrogenation continue to be developed but one of the best systems is still the BINAP-DPEN catalyst first reported by Ohkuma et al. in 1995. " In this system ruthenium is combined with both a chiral diphosphine and a chiral diamine, forming an octahedral complex which gives a high degree of enantioselectivity. This stereoselectivity is considered to be a result of the synergistic effect of the chiral diphosphine and diamine ligands. 

A very simple yet elegant method for efficient epoxidation of aromatic and aliphatic alkenes was presented by Beller and coworkers [63, 64], FeCl3 hexahydrate in combination with 2,6-pyridinedicarboxylic add and various organic amines gave a highly reactive and selective catalyst system. An asymmetric variant (for epoxidations of trans-stilbene and related aromatic alkenes) was published recently [65] using N-monosulfonylated diamines as chiral ligands (Scheme 3.7). 

A water-soluble, recyclable ruthenium(II) complex including a chiral diamine ligand has been used for asymmetric transfer hydrogenation of cyclic imines and iminiums in water, with yields and ee up to 99%.49... 

The kinetic resolution of a variety of racemic epoxides has been performed using a chiral bicyclic diamine ligand (9).38 Using 5 mol % of catalyst, both epoxide and allylic alcohol can be obtained in up to 99% ee when the reaction is stopped shortly before or after 50% conversion is reached. 

As for the exodendrally functionalized dendrimer catalysts (Sect. 3.2), chiral diamine ligands have also been the objects of study in the investigation into the catalytic behaviour of core-functionalized dendrons. 

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