Mechanism and scope of the cyanide-catalyzed cross silyl benzoin reaction

In this work, cross silyl benzoin addition reactions between acylsilanes (1) and aldehydes (2) catalyzed by metal cyanides are described. Unsymmetrical aryl-, heteroaryl-, and alkyl-substituted benzoin adducts can be generated in moderate to excellent yields with complete regiocontrol using potassium cyanide and a phase transfer catalyst. From a screen of transition metal cyanide complexes, lanthanum tricyanide was identified as an improved second-generation catalyst for the cross silyl benzoin reaction. A study of the influence of water on the KCN-catalyzed cross silyl benzoin addition revealed more practical reaction conditions using unpurified solvent under ambient conditions. A sequential silyl benzoin addition/cyanation/O-acylation reaction that resulted in two new C-C bonds was achieved in excellent yield. The mechanism of cross silyl benzoin addition is proposed in detail and is supported by crossover studies and a number of unambiguous experiments designed to ascertain the reversibility of key steps. No productive chemistry arises from cyanation of the more electrophilic aldehyde component. Formation of the carbon-carbon bond is shown to be the last irreversible step in the reaction.     

Cyclopropyl iminium activation: reactivity umpolung in enantioselective organocatalytic reaction design

Trick the iminium! The concept of cyclopropyl iminium activation for the organocatalytic desymmetrization of meso-cyclopropylcarbaldehydes is presented. A combination of nucleophilic and electrophilic chlorinating reagents leads to a formal addition of Cl(2) across one of the cyclopropyl bonds giving access to 1,3-dichlorides in an enantioselective, catalytic fashion.     

Chiral phosphine-squaramides as enantioselective catalysts for the intramolecular Morita-Baylis-Hillman reaction

Novel squaramides containing tertiary phosphine were developed as chiral bifunctional organic catalysts to promote the asymmetric intramolecular Morita-Baylis-Hillman reaction of ω-formyl-enones. The adducts were obtained in high yields with good-to-excellent enantioselectivity (up to 93% ee).     

Diarylprolinol silyl ether as catalyst of an exo-selective, enantioselective Diels-Alder reaction

In combined use with CF3CO2H, 2-[bis(3,5-bis-trifluoromethylphenyl) triethylsiloxymethyl]pyrrolidine was found to be an effective organocatalyst of an exo-selective, enantioselective Diels-Alder reaction of alpha,beta-unsaturated aldehydes.     

Novel catalysts for the enantioselective Henry reaction

Two novel catalytic systems based on the Cu^II complexes with N-(3,5-dibromo-2-hydroxybenzyl)- and N-(2-hydroxy-3-nitrobenzyl)-(S)-α,α-diphenylprolinols were developed. These systems catalyze the condensation of 4-nitrobenzaldehyde with nitromethane to produce S-nitroaldol with maximum enantiomeric excess of >90% (99% yield). The reactions of nitromethane with aliphatic aldehydes give the corresponding products in the yields above 80% and ee > 90%.     

Novel sulfonylpolystyrene-supported prolinamides as catalysts for enantioselective aldol reaction in water

Five novel prolinamides derived from 1,n-diamines supported on commercially available sulfonylpolystyrene have been synthesized and successfully applied in diastereo- and enantioselective aldol reaction in water or, in one case, neat conditions. Excellent dr and er were obtained by reaction of different cyclohexanone derivatives with aromatic and heteroaromatic aldehydes in the presence of the catalysts and AcOH as co-catalyst. The supported organocatalysts work in low loading (5 mol %) exhibiting similar performances than homogeneous prolinamides, and can be reused, after activation, without loss of activity.     

New (S)-proline derivatives as catalysts for the enantioselective aldol reaction

(2S)-N- 2-[(R)-Hydroxy(phenyl)methyl]phenylpyrrolidine-2-carboxamide, (2 S)-N-{2-[(S)-hydroxy(phenyl)methyl]phenyl}pyrrolidine-2-carboxamide, and (S)-N-{2-[hydroxy-(diphenyl)methyl]phenyl}pyrrolidine-2-carboxamide were obtained and tested as catalysts for an asymmetric aldol reaction of 4-nitrobenzaldehyde with acetone.     

Mechanism of enantioselective Ti-catalyzed Strecker reaction: peptide-based metal complexes as bifunctional catalysts.

Kinetic, structural, and stereochemical data regarding the mechanism of Ti-catalyzed addition of cyanide to imines in the presence of Schiff base peptide ligands are disclosed. The reaction is first order in the Ti-ligand complex; kinetic studies reveal DeltaS(dagger) = -45.6 +/- 4.1 cal K(-1) mol(-1), indicating a highly organized transition structure for the turnover-limiting step of the catalytic cycle. A mechanistic model consistent with the kinetic and stereochemical data is presented, where the Ti center is coordinated to the Schiff base unit of the ligand and the AA2 moiety of the peptidic segment of the chiral ligand associates and delivers HNC to the activated bound substrate. Thus, these studies illustrate that these non-C2-symmetric catalysts likely operate in a bifunctional fashion.     

Chiral bisphosphazides as dual basic enantioselective catalysts

Chiral bisphosphazides complexed with lithium salts efficiently catalyze the direct enantioselective 1,4-addition of dialkyl malonates to acyclic enones. Spectroscopic studies on the stoichiometry of the bisphosphazide and lithium salt have indicated the formation of a 1:1 species as the active enantioselective catalyst. It is suggested that the catalyst generates a complex of the protonated phosphazide and the chiral nucleophile as the key intermediate. The phosphazide moiety appears to be a promising dual basic functionality for stereo- and chemoselective catalytic transformations.     

Nitroxyl peptides as catalysts of enantioselective oxidations

The achiral, nitroxyl-containing alpha-amino acid TOAC (TOAC = 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid), in combination with the chiral alpha-amino acid C(alpha)-methyl valine [(alphaMe)Val], was used to prepare short peptides (from di- to hexa-) that induced the enantioselective oxidation of racemic 1-phenylethanol to acetophenone. The best catalyst was an N(alpha)-acylated dipeptide alkylamide with the -TOAC-(alphaMe)Val- sequence folded in a stable, intramolecularly hydrogen-bonded beta-turn conformation with large, lipophilic (hydrophobic) N- and C-terminal blocking groups. We rationalized our findings by proposing models for the diastereomeric intermediates between (R)-[and (S)]-1-phenylethanol and the catalyst Fmoc-TOAC-L-(alphaMe)Val-NHiPr, based on the X-ray diffraction structure of the latter.     

Histidine-based salt as an ionic tag for proline: application in enantioselective cross aldol reaction in ionic liquids

Abstract

A new ionic-liquid supported aminocatalyst was synthesized by the peptide coupling of proline on a ring-dialkylated histidine salt serving as ionic support. The catalytic activity and enantioselectivity of the obtained dipeptide catalyst were evaluated in the enantioselective cross aldol reaction of substituted benzaldehyde derivatives with acetone, in organic solvent and in ionic liquids, and both were found to be significantly higher in ionic liquids than in organic solvents. The [bmim][OTf] – catalyst system could be recycled for up to five cycles without any loss in activity.     

Imidazole based solid-supported catalysts for the benzoin condensation

New polymer-supported imidazolium salts have been synthesised, and their utility as pre-catalysts in the benzoin reaction has been demonstrated.     

Heterogeneous enantioselective catalysts: strategies for the immobilisation of homogeneous catalysts

Enantioselective formation of C-H, C-C, C-O and C-N bonds has been extensively studied using homogeneous asymmetric catalysts for many years. However, these catalysts have yet to make a significant impact in the industrial synthesis of fine chemicals. A central reason is that homogeneous asymmetric catalyst design requires relatively bulky ligands and catalyst re-use through recovery and recycle often causes problems. One mechanism to overcome this problem is to immobilise the asymmetric catalyst onto a support and the resulting heterogeneous asymmetric catalyst can, in principle, be readily re-used. This tutorial review covers the different methodologies for immobilisation, including: adsorption, encapsulation, tethering using a covalent bond and electrostatic interaction and is aimed at both researchers new to the field and those with a wider interest in the immobilisation of homogeneous catalysts. Most importantly, recent studies will be highlighted that demonstrate that immobilised catalysts can give higher enantioselection when compared with their non-immobilised counterparts and the question of how high enantioselection can be achieved is addressed.     

Discovery of exceptionally efficient catalysts for solvent-free enantioselective hetero-Diels-Alder reaction.

Combinatorial coordination chemistry strategy combined with high-throughput screening techniques has been successfully applied to engineering practical enantioselective catalysts for asymmetric hetero-Diels-Alder reaction. The reaction of Danishefsky's diene with a variety of aldehydes can be carried out with 0.1-0.005 mol % of H4-BINOL/Ti/H4-BINOL or H4-BINOL/Ti/H8-BINOL catalysts at room temperature under solvent- and MS-free conditions to afford dihydropyrone derivatives with up to quantitative yield and 99.8% ee.     

Cyanohydrins as substrates in benzoin condensation. : Regiocontrolled mixed benzoin condensation

O-Benzoylated cyanohydrins of aromatic aldehydes have been used as one of the substrates in the benzoin condensation. They were treated with aromatio aldehydes under phase-transfer conditions to result in benzoin benzoates. By this method aldehydes which fail to undergo condensation using traditional conditions could be converted into benzoins. By the Umpolung of reactivity of the pertinent aldehyde it was possible to prepare both isomeric unsymmetrical benzoins, including the thermodynamically less stable ones.     

Chiral aminophosphines as catalysts for enantioselective double-Michael indoline syntheses

The bisphosphine-catalyzed double-Michael addition of dinucleophiles to electron-deficient acetylenes is an efficient process for the synthesis of many nitrogen-containing heterocycles. Because the resulting heterocycles contain at least one stereogenic center, this double-Michael reaction would be even more useful if an asymmetric variant of the reaction were to be developed. Aminophosphines can also facilitate the double-Michael reaction and chiral amines are more readily available in Nature and synthetically; therefore, in this study we prepared several new chiral aminophosphines. When employed in the asymmetric double-Michael reaction between ortho-tosylamidophenyl malonate and 3-butyn-2-one, the chiral aminophosphines produced indolines in excellent yields with moderate asymmetric induction.     

Chiral hydroxythiols as catalysts for enantioselective borane ketone reductions

Several chiral 1,2- and 1,3-hydroxythiols derived from (R)-camphor, (1S)-(+)-10-camphorsulfonyl chloride, cysteine and cystine derivatives were prepared and evaluated as catalysts in borane ketone reductions. Under the best experimental conditions (10 mol% catalyst, THF, 35°C), a 95% yield of (R)-1-phenylethanol of 64% ee was obtained in the borane reduction of acetophenone.