Schiff-base catalysts

In the presence of primary alcohols the cobalt II) Schiff-base complex Co NMe-salpr) has been found to catalyze the oxidation of both internal and terminal olefins by 0 to the corresponding 2-ketone and 

The observed behavior and product pattern have been interpreted in terms of a mechanism in which the primary alcohol solvent is oxidized to the corresponding aldehyde and via intervention of free 

Olefin oxidation takes place via Markovnikov addition of Co 2 across the double bond  

The cobalt complex catalyzed decomposition of the hydroperoxide via a Haber-Weiss type mechanism affords the observed products  

The Haber-Weiss [182] type decomposition of the hydroperoxide ROOH may involve the sequence of steps  

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The Jacobsen group introduced the term Schiff base catalyst [122] to demonstrate that the structure of this novel catalyst class originates from Schiff base ligands and incorporates a Schiff base moiety notably, this term does not indicate that these catalysts operate as bases, but their high... 

Figure 6.13 Diaminocyclohexane-derived Schiff base catalyst 10 representing the optimized structure identified from parallel catalyst screening and its polymer-free (solution-phase) counterpart 11 prepared for application in asymmetric Strecker reactions.

On the basis of the observed stereoinduction trend, the addition of HCN took place over the diaminocyclohexane portion of the catalyst away from the amino acid and amide unit. The last hypothesis led to the prediction that a more sterically demanding amino acid or amide unit (Figure 6.14) could additionally favor the cyanide attack compared to the less bulky diaminocyclohexane unit and thus making the Schiff base catalyst more enantioselective in Strecker reactions of aldimines and ketimines. To evaluate this perspechve, the authors performed a model-(mechanism-) driven systematic structure optimizations by stepwise modification of the amide, the amino acid, and the (thio)urea unit of catalyst 42 and examined these derivatives of 42 (lmol% loading ) in the model Strecker reaction (toluene ... 

Systematic investigations of the catalyst structure-enantioselectivity profile in the Mannich reaction [72] led to significantly simplified thiourea catalyst 76 lacking both the Schiff base unit and the chiral diaminocyclohexane backbone (figure 6.14 Scheme 6.88). Yet, catalyst 76 displayed comparable catalytic activity (99% conv.) and enantioselectivity (94% ee) to the Schiff base catalyst 48 in the asymmetric Mannich reaction of N-Boc-protected aldimines (Schemes 6.49 and 6.88) [245]. This confirmed the enantioinductive function of the amino acid-thiourea side chain unit, which also appeared responsible for high enantioselectivities obtained with catalysts 72, 73, and 74, respectively, in the cyanosilylation of ketones (Schemes 6.84 and 6.85) [240, 242]. 

Based on the modular structure of Schiff base catalysts such as first-generation Strecker urea catalyst 42 [196, 198] (Figure 6.15 Schemes 6.41 and 6.45), Yoon... 

Figure 6.58 Schiff base catalyst 42 and glucosamine hydrochloride as starting material for the synthesis of saccharide-based catalyst 198.

Urea 198 was prepared from enantiomerically pure polyfunctLonal glucoseamine hydrochloride, which is readily accessible from chitin as a component of the natural chiral pool it appeared to be an alternative backbone structure supplanting the trans-l,2-diamocyclohexane of Schiff base catalyst 42 (Figure 6.58). 

In contrast to the results obtained by Jacobsen et al. when utilizing Schiff base catalyst 42, the decrease of reaction temperature to -40 °C reduced the yield as well as enantioselectivity of the resulting Mannich adduct (Scheme 6.175) [201]. Catalyst 198 found to be less effective in the Mannich reaction in terms of yield and enantiomeric induction due to reduced basicity of the N-acylamine and weaker hydrogen-bonding interactions compared to the more basic Strecker substrates (Scheme 6.174). 

New chiral Schiff base catalysts for the enantioselective addition of diethylzinc reagents to aldehydes have been developed.115,116 The reaction of aldehyde with diethylzinc in the presence of 1-2 mol% of the chiral Schiff base catalyst has provided the corresponding secondary alcohol with excellent enantiomeric excess (up to 96% ee). 

In addition, chiral Schiff base catalysts, which were developed previously for the Strecker reaction, were also found to be suitable catalysts for the Mannich reaction starting from imines and enolates [36, 37]. Very recently, further efficient organocatalysts for the Mannich reaction, such as chiral pyrrolidinyltetrazole and chiral binaphthyl phosphoric acids, have been reported [38]. 

Sigman and Jacobsen reported the first example of a metal-catalyzed enantioselective Strecker-type reaction using a chiral Alnl-salen complex (salen = N,N -bis(salicyhdene)-ethylenediamine dianion) [4]. A variety of N-allylimines 4 were evaluated in the reaction catalyzed by complex 5 to give products 6, which were isolated as trifluoroacetamides in good yields and moderate-to-excellent enantioselectivities (Scheme 3). Substituted arylimines 4 were the best substrates, while alkyl-substituted imines afforded products with considerably lower ee values. Jacobsen and co-workers also reported that non-metal Schiff base catalysts 8 and 9 proved to be effective in the Strecker reaction of imines 7 with hydrogen cyanide to afford trifluoroacetamides 10 after reaction with trifluoroacetic anhydride, since the free amines were not stable to chromatography (Scheme 4) [5]. 

Scheme 6.6 Schiff base catalysts for the Strecker reactions of aldimines.

Sigman MS, Jacobsen EN (1998a) Schiff base catalysts for the asymmetric Strecker reaction identified and optimized from parallel synthetic libraries. J Am Chem Soc 120 4901... 

One of the most powerful catalysts of the Mukaiyama aldol reaction is a chiral Ti(IV)-Schiff base complex 91 prepared from Ti(0 Pr)4 and enantiomerically pure salicylaldimine reported by Carreira [103-105]. This catalyst furnished aldol adducts in good yields and with excellent enantioselectivity. The Ti(IV)-Schiff base catalyst system is unique among the aldol catalysts yet reported in terms of operational simplicity, catalyst efficiency, chirality transfer, and substrate generality. Because the Ti(IV)-Schiff base complexes are remarkably efficient catalysts for the addition of ketene acetals to a wide variety of aldehydes, the polymeric version of catalyst 92 was prepared [106]. The activity and enantioselectivity of the polymer-supported chiral Ti(IV)-Schiff base complex were, however, much lower than were obtained from the low-molecular-weight catalyst (Eq. 28). 

A difficult challenge in developing ARO reactions with carbon nucleophiles is identifying a reagent that is sufficiently reactive to open epoxides but at the same time innocuous to chiral metal catalysts. A recent contribution by Crotti clearly illustrates this dehcate reactivity balance. The lithium enolate of acetophenone added in the presence of 20 mol % of the chiral Cr(salen) complex 1 to cyclohexene oxide in very low yield but in 84% ee (Scheme 10) [23]. That less than one turnover of the catalyst was observed strongly suggests that the lithium enolate and the Schiff base catalyst are not compatible under the reaction conditions. 

The cobalt(II) Schiff base catalyst forms a superoxocobalt(III) complex which converts the phenol into an aryloxy radical, itself being transformed into free or coordinated HO radical ... 

The Schiff base catalyst 4 was the first chiral catalyst for the intramolecular cyclopropanation of diazoketone, although low enantioselectivity (8% ee) and only moderate yield (64%) were obtained (125). A major breakthrough occurred when semicorrin 6 was employed. Although the yield remained moderate, high... 

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