Towards Perfect Asymmetric Catalysis: Additives and Cocatalysts

Vogl, Erasmus M.; Gröger, Harald; Shibasaki, Masakatsu

doi:10.1002/(sici)1521-3773(19990601)38:11<1570::aid-anie1570>3.3.co;2-p

Cited by 78 publications

(102 citation statements)

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“…To improve the enantioselectivity further, several additives were screened 12. Although neither coordinating additives such as Ph 3 P(O), Et 3 N, Ph 3 P, or LiBr, nor Brönsted acids such as trifluoromethanesulfonic acid or trifluoroacetic acid produced positive effects, the addition of protic additives such as MeOH, EtOH, i PrOH, or t BuOH improved both the yield and enantioselectivity (Figure 2).…”

Section: Optimization Of the Reaction Conditions[a]mentioning

confidence: 99%

Multicenter Strategy for the Development of Catalytic Enantioselective Nucleophilic Alkylation of Ketones: Me₂Zn Addition to α‐Ketoesters

et al. 2003

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The catalytic construction of stereogenic tetrasubstituted carbon centers through the addition of carbon nucleophiles to ketones or ketoimines is very challenging, partly because of the lower reactivity of these substrates relative to aldehydes and aldoimines. [1] This task requires strong activation of the substrate and/or the nucleophile by an asymmetric catalyst. We developed Lewis acid-Lewis base two-center asymmetric catalysts (titanium and lanthanide complexes of 1) that promote the cyanosilylation of ketones and ketoimines with broad substrate generality.[2] The fundamental concept for the catalyst design was that the Lewis acid metal and the Lewis base (the phosphane oxide) activate both the substrate and the nucleophile (TMSCN) simultaneously at defined positions in the transition state. A logical extension of this concept is to target the diorganozinc addition to ketones, [3] because both Lewis acid activation of the substrate and Lewis base activation of the reagent are required to promote the reaction.[4] We report herein our initial investigations toward this goal: the catalytic enantioselective addition of Me 2 Zn to a-ketoesters. Our newly designed catalyst 2, which

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Section: Optimization Of the Reaction Conditions[a]mentioning

confidence: 99%

Multicenter Strategy for the Development of Catalytic Enantioselective Nucleophilic Alkylation of Ketones: Me₂Zn Addition to α‐Ketoesters

et al. 2003

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“…Shorter and longer linkers between the oxazoline rings were introduced, such as in oxalate‐ and phthalic acid‐derived ligands; however, these ligands did not lead to improved results compared to ligand 2 9. In addition, we also investigated the effect of additives upon the reaction 15. Various desiccants (e.g., molecular sieves, MgSO 4 ), as well as Lewis basic additives were tested, yet none of these resulted in enhanced selectivities 9…”

Section: Methodsmentioning

confidence: 99%

Catalytic Enantioselective Hosomi–Sakurai Conjugate Allylation of Cyclic Unsaturated Ketoesters

Shizuka

Snapper

2008

Angew Chem Int Ed

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Despite impressive advances over the years, [1] there are still important transformations that lack catalytic asymmetric variants. While Lewis acid catalyzed additions of allylsilanes to carbonyl compounds [2] and acetals [3] have been well studied using catalytic, [4] as well as auxiliary-based methods to control absolute configuration, [5] to the best of our knowledge, there are no effective methods for catalyzing the asymmetric 1,4-addition of allyltrimethylsilane to unsaturated carbonyl compounds. [6] In that regard, we report herein a catalytic enantioselective conjugate addition of allyltrimethylsilane to various activated cyclic enones with selectivities surpassing 98 % ee. The 1,4-addition of the air-and moisture-stable nucleophile to unsaturated carbonyl compounds proceeds to > 95 % conversion in the presence of Cu(OTf) 2 (10 mol %) with the commercially available di(tert-butyl)bis(oxazoline) (box) ligand (2). [7] We show how these products can be functionalized to a variety of useful enantiomerically enriched systems.Our initial studies into the development of a chiral Lewis acid catalyst indicated that simple cyclic and acyclic a,bunsaturated carbonyls (ketones and esters) did not react with a variety of metal-ligand combinations.[8] We therefore sought to activate the substrate by installation of a second electron-withdrawing/chelating group at the a-position of the enone (i.e., 1). In the presence of Cu(OTf) 2 (7 mol %) and bis(oxazoline) ligand 2 (8 mol %) in Cl(CH 2 ) 2 Cl, we obtained the desired 1,4-allyl-addition product 3 in > 95 % conversion (after 30 min at 0 8C) and 72 % ee as a mixture of keto-enol tautomers (Scheme 1). Alternative solvents (CH 2 Cl 2 , Et 2 O, toluene, EtOAc, etc.) and metal salts, including other copper salts, resulted in lower selectivities.[9] Other chiral ligands (e.g., peptide-based, [10] salen, [11] Trost ligand [12] ) led to high conversion (> 95 %), but with low selectivity (< 5 % ee).To identify a more effective catalyst, we prepared and screened approximately 40 mono-and bis(oxazoline) ligands.

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“…The use of additives is a common and convenient method for the enhancement of the efficiency of metal‐catalyzed reactions,9 as it allows the fine‐tuning of the catalyst system without redesigning the ligand 10. Recently, Jacobsen and co‐workers reported the beneficial effects of the presence of substoichiometric amounts of a carboxylic acid in epoxidation reactions with H 2 O 2 and the iron complex [Fe II (mep)(CH 3 CN) 2 ](SbF 6 ) 2 (mep= N , N ′‐dimethyl‐ N , N ′‐bis(2‐pyridylmethyl)‐ethylene 1,2‐diamine) 11.…”

Section: Methodsmentioning

confidence: 99%

Highly Enantioselective Iron‐Catalyzed Sulfide Oxidation with Aqueous Hydrogen Peroxide under Simple Reaction Conditions

2004

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Iron is one of the most abundant metals in the universe.[1] It is inexpensive, environmentally benign, and relatively nontoxic in comparison to other metals. For oxidation reactions the application of iron catalysts [2] in combination with aqueous hydrogen peroxide [3] would be most desirable. Asymmetric oxidations could then be achieved by the use of chiral iron complexes. In this context we recently reported a new enantioselective sulfide oxidation, [4][5][6] which provides optically active sulfoxides with up to 90 % ee.[7] The catalysis proceeds under very simple reaction conditions (at room temperature in a capped flask) and utilizes a readily available iron complex generated in situ from [Fe(acac) 3 ] and Schiff base 3.[8] Inexpensive and safe 30-35 % aqueous hydrogen peroxide serves as terminal oxidant (Scheme 1).Although the asymmetric induction in this iron-catalyzed sulfoxidation was remarkable, the practicability of the process was limited owing to the low conversion of 1. Thus, under the conditions required for high enantioselectivities, large amounts of unconverted sulfides remained, and the best yield of sulfoxide 2 was only 44 %. We now report on a significant improvement in this iron-catalyzed sulfide oxidation, leading to both higher enantioselectivities (up to 96 % ee) and better yields (up to 78 %).

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Towards Perfect Asymmetric Catalysis: Additives and Cocatalysts

Cited by 78 publications

References 0 publications

Multicenter Strategy for the Development of Catalytic Enantioselective Nucleophilic Alkylation of Ketones: Me₂Zn Addition to α‐Ketoesters

Multicenter Strategy for the Development of Catalytic Enantioselective Nucleophilic Alkylation of Ketones: Me₂Zn Addition to α‐Ketoesters

Catalytic Enantioselective Hosomi–Sakurai Conjugate Allylation of Cyclic Unsaturated Ketoesters

Highly Enantioselective Iron‐Catalyzed Sulfide Oxidation with Aqueous Hydrogen Peroxide under Simple Reaction Conditions

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Towards Perfect Asymmetric Catalysis: Additives and Cocatalysts

Cited by 78 publications

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Multicenter Strategy for the Development of Catalytic Enantioselective Nucleophilic Alkylation of Ketones: Me2Zn Addition to α‐Ketoesters

Multicenter Strategy for the Development of Catalytic Enantioselective Nucleophilic Alkylation of Ketones: Me2Zn Addition to α‐Ketoesters

Catalytic Enantioselective Hosomi–Sakurai Conjugate Allylation of Cyclic Unsaturated Ketoesters

Highly Enantioselective Iron‐Catalyzed Sulfide Oxidation with Aqueous Hydrogen Peroxide under Simple Reaction Conditions

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Multicenter Strategy for the Development of Catalytic Enantioselective Nucleophilic Alkylation of Ketones: Me₂Zn Addition to α‐Ketoesters

Multicenter Strategy for the Development of Catalytic Enantioselective Nucleophilic Alkylation of Ketones: Me₂Zn Addition to α‐Ketoesters