Ketiminate ligands

Fig. 2.1.3.4 Central- and planar-chiral imine and ketimine ligands. Yields and selectivities were obtained using the conditions described in Scheme 2.1.3.2 [17].

The best results observed so far were with the central- and planar-chiral ketimine ligands, all of which were synthesized with the same chiral amine, (S)- or (R)-phenylethylamine, to result in the same central-chiral side-chain. The second generation of these ligands was synthesized with the various chiral amines depicted in Fig. 2.1.3.5 based on the AHPC 2 and BHPC 3. 

In summary, the configuration of the desired product is controlled by the planar-chiral imine and ketimine ligand backbone. The selectivity of the reaction depends on both the chiral center and the communication of the side-chain with the ligand backbone. We tuned the side-chain to increase the enantioselectivity up to 90% ee. In the case of the amino alcohol ligands, chiral cooperativity is also observed. However, the influence of the planar chirality is much lower, whereas central chirality is dominant in this instance. In most cases the enantioselectivity is lower than for the ketimines. 

In addition, we investigated a nonlinear-like effect (NLE), activity, temperature dependence, and kinetics of hydroxy[2.2]paracyclophane ketimine ligands with the 1,2-addition reaction of diethylzinc to cyclohexylcarbaldehyde. A linear correlation between the enantiomeric excess of AHPC ketimine ligands bearing a phenylethyl side group and the product was observed with 0.5 mol% of catalyst loading. When the catalyst loading of (Sp,S)/(Rp,R)-4a was increased to 4 mol%, a precipitate of the inactive heterochiral species was formed and resulted in a positive nonlinear like effect (Fig.2.1.3.6), while a linear behavior is observed with 5b (Fig. 2.1.3.7). The enantiomeric ratio was found to have linear temperature dependence. [16]... 

Taking this into consideration, we employed a further improved generation of ketimine ligands (Rp,S)-5b and (Sp,S)-5b, in which the cyclohexyl ring is more sterically demanding than the phenyl ring present in the ligands 5a and 6a (Scheme 2.1.3.4). 

The second generation of N,0-[2.2]paracyclophane ketimine ligands (Rp,S)-5b, 6b were investigated for their ability to catalyze the 1,2-addition of alkenylzinc... 

Figure 12 Metal complexes bearing tridentate ketiminate ligands...

FIGURE 4.15 Solvent-dependent oxygenation pathways for a mononuclear copper complex with a sterically hindered ketimine ligand.54... 

Bicyclic cyclopentenones. Cyclization of the Pauson-Khand type from enynes is achievable with Ni(cod)j (12 examples, 38-85%). A bulky bis-ketimine ligand and a CO equivalent are present in the reaction medium. For the latter (i-Pr)3SiCN is adequate. The silyl cyanide is in equilibrium with the isocyanide. The primary products are the cyclopentenone imines, which undergo hydrolysis on workup. 

In 2007, Lin and coworkers reported the synthesis of dimeric complexes [ [NNO Mg(OBn)]2 bearing tridentate [NNO] amino-ketiminate ligands (66) (Fig. 21) [81]. They acted as potent initiators for the ROP of l-LA, converting 50-200 equiv within 8 min and affording polymers with fully predictable molecular weights... 

Palladium-catalysed asymmetrie allylations of various carbonyl compounds have been studied by Hiroi et al. using various types of chiral sulfonamides derived from a-amino acids. In particular, the chiral bidentate phosphinyl sulfonamide derived from (5)-proline and depicted in Scheme 1.63 was employed in the presence of palladium to eatalyse the allylation of methyl aminoacetate diphenyl ketimine with allyl aeetate, leading to the eorresponding (7 )-product with a moderate enantioseleetivity of 62% ee. This ligand was also applied to the allylation of a series of other nueleophiles, as shown in Seheme 1.63, providing the eorresponding allylated produets in moderate enantioseleetivities. 

The first example of an asymmetric reduction of C=N bonds proceeding via DKR was reported in 2005 by Lassaletta et al. In this process, the transfer hydrogenation of 2-substituted bicyclic and monocyclic ketimines could be accomplished via DKR by using a HCO2H/TEA mixture as the hydrogen source and a chiral ruthenium complex including TsDPEN ligand,... 

Scheme 9.25 Ru-catalysed DKR-reductions of cyclic ketimines with TsDPEN ligand.

In 1994, the scope of this p-hydroxy sulfoximine ligand was extended to the borane reduction of ketimine derivatives by these workers. The corresponding chiral amines were formed with enantioselectivities of up to 72% ee, as shown in Scheme 10.57. It was found that the A -substituent of the ketimine had a major influence on the asymmetric induction, with a ketoxime thioether (SPh) being the most successful substrate. 

Scheme 10.57 Borane reductions of ketimine derivatives with P-hydroxy sulfoximine ligand.

Various phenyl-substituted ketimines and aldimines react with metallocenes 1 and 2, in a manner that depends on the substituents present [41]. In all cases, elimination of the al-kyne is observed. Complex 2b reacts with PhN=CMePh to give the r 2-complex 64, which is stabilized by an additional pyridine ligand [41a], In the reactions of 1 or 2a with the ketimine HN=CPh2, hydrogen transfer generates complexes 65. Two molecules of the aldimine PhN=CHPh are coupled by 2a to give the cyclic diamido complex 66 [41b]. 

Iridium-Catalyzed Asymmetric Hydrogenation of Olefins with Chiral N,P and C,N Ligands 71 Table 18 Asymmetric hydrogenation of acetophenone based A-aryl ketimines... 

These, in turn, can be condensed with amines to give imines 4 or ketimines 5 and 6, or reduced to give amino alcohols 7-9, respectively. The ligand structure is therefore vastly variable. Steric factors, such as flexibility of backbone and side-chains, as well as electronic factors (for example sp versus sp conflguration of the N-donors) can be easily modulated. The introduction of central chirality via chiral amine side-chains is also possible. The interaction of planar and central chirality, usually referred to as chiral cooperativity [11-13], can thus be studied in a ligand system which has both planar and central chiral elements. 

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