Asymmetric diimine ligands

In 1993, Jacobsen and Evans simultaneously reported that [7V-(p-tolylsulfonyl)imino]phenyliodinane (TsN=IPh, 195) is an efficient asymmetric nitrene transfer reagent to alkenes in the presence of a catalytic amount of a copper(i) salt and a chiral diimine ligand or a chiral bis(oxazoline) ligand (Equation (31)). Mechanistic study by Jacobsen and co-workers suggests that a discrete copper(iii) nitrene complex is an intermediate responsible to the reaction. ... 

Transition metal catalyzed carbene insertion into the Si—H bond provides a direct and efficient method for the synthesis of organosilicon compounds. When chiral spiro diimine ligand (/ )-24a was applied in Cu-catalyzed asymmetric insertion of a-diazo-a-arylacetates with silanes, the Si—H insertion products were obtained in high yields (85-97%) and excellent enantioselectivities (90-99% ee) (Scheme 51) [26a],... 

Scheme 6.14 Catalytic asymmetric aziridination using chiral diimine ligands ...

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

In a study published concurrently with the Evans bis(oxazoline) results, Jacobsen and co-workers (82) demonstrated that diimine complexes of Cu(I) are effective catalysts for the asymmetric aziridination of cis alkenes, Eq. 66. These authors found that salen-Cu [salen = bis(salicylidene)ethylenediamine] complexes such as 88b Cu are ineffective in the aziridination reaction, in spite of the success of these ligands in oxo-transfer reactions. Alkylation of the aryloxides provided catalysts that exhibit good selectivities but no turnover. The optimal catalyst was found to involve ligands that were capable only of bidentate coordination to copper. 

The impact of achiral ligands on the TOP of the catalyst in this system was then examined by following reaction conversions with various catalysts. In the absence of both Ph2-BINOL and diamine ligands, the reaction of diethylzinc with benzaldehyde is very slow, exhibiting about 1% conversion after 8h at 0°C. When Ph2-BINOL was employed in the asymmetric addition to this substrate, the catalyst TOP increased exhibiting 25% conversion after 2 h. To evaluate the TOP of zinc centers bearing diimine derived from ethylene diamine and 2,4,6-trimethylbenzaldehyde (Table 4, entry 5) or the meso diamine (Table 5, entry 7), these ligands were... 

Asymmetric transfer hydrogenation of ketones in the presence of soluble transition metal catalysts has been developed [8-10], enantioselectivities up to 99% ee being obtained using a ruthenium catalyst bearing mono-N-tosylated diphenyl-ethylenediamine as a ligand. Iridium complexes associated with fluorous chiral diimines 3a-3c or diamines 4a—4b have also been shown to be effective catalysts in hydrogen-transfer reduction of ketones [11,12]. 

In this study, the full catalytic cycle for the recently reported asymmetric Suzuki-Miyaura coupling between 1 -bromo-2-methylnaphthalene (2) and 1 -naphthal eneboronic acid (3) catalyzed by a [Pd(bis-hydrazone)] (1) complex [47], was theoretically investigated by means of DFT calculations. Importantly, the results derived from this study revealed that the transmetalation reaction does not occur in just one step, but occurs in three steps. This is owing to the relative lability of the bis-hydrazone ligand, which can easily dissociate one of the N atoms coordinated to the Pd catalyst. Very recently, this variant for the transmetalation mechanism has been reported for the Suzuki-Miyaura coupling catalyzed by a diimine chelated palladium complex [15]. 

By comparison of the a-diimine catalysts with the phosphine sulfonate-based palladium catalyst system, the change from a symmetric to an asymmetric ligand stmcture is apparent on first sight. Here (as well as in other ligand systems Section 3.24.4.1.4), it could be shown that this leads to major differences in reactivity. Despite the partially open catalyst stmcture, these complexes are of remarkable stability and activity in... 

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