Catalytic Asymmetric Alkylations of Ketoimines. Enantioselective Synthesis of N-Substituted Quaternary Carbon Stereogenic Centers by Zr-Catalyzed Additions of Dialkylzinc Reagents to Aryl-, Alkyl-, and Trifluoroalkyl-Substituted Ketoimines

Asymmetric synthesis of trifluoromethylated amines via catalytic enantioselective isomerization of imines

A new approach toward the asymmetric synthesis of optically active trifluoromethylated amines was enabled by an unprecedented, highly enantioselective catalytic isomerization of trifluoromethyl imines with a new chiral organic catalyst. Not only aryl but also alkyl trifluoromethylated amines could be obtained in high enantioselectivities.

Organocatalytic asymmetric Strecker reaction of di- and trifluoromethyl ketoimines. Remarkable fluorine effect

A highly enantioselective Strecker reaction of difluoromethyl and trifluoromethyl ketoimines was developed. Remarkable fluorine effect on the reactivity and selectivity is observed and discussed.

Rhodium-catalyzed asymmetric addition of potassium organotrifluoroborates to N-sulfonyl ketimines

A rhodium-catalyzed asymmetric addition of readily available potassium organotrifluoroborates to both N-tosyl and N-nosyl ketimines has been developed. High enantioselectivity has been achieved by using a chiral diene ligand, and the nosyl group of the addition products can be easily removed while retaining the enantiomeric purity.

Highly enantioselective zinc/BINOL-catalyzed alkynylation of α-ketoimine ester: a new entry to optically active quaternary α-CF3 α-amino acids

An effective method for highly enantioselective alkynylation of ketoimine (α-trifluoromethyl ketoimine ester) has been developed via a zinc/BINOL catalyzed process. This protocol provides a useful and facile access to optically active quaternary α-trifluoromethyl α-amino acids and related derivatives of interest in life sciences.

Formation of C-C Bonds via Iridium-Catalyzed Hydrogenation and Transfer Hydrogenation

The formation of C-C bonds via catalytic hydrogenation and transfer hydrogenation enables carbonyl and imine addition in the absence of stoichiometric organometallic reagents. In this review, iridium-catalyzed C-C bond-forming hydrogenations and transfer hydrogenations are surveyed. These processes encompass selective, atom-economic methods for the vinylation and allylation of carbonyl compounds and imines. Notably, under transfer hydrogenation conditions, alcohol dehydrogenation drives reductive generation of organoiridium nucleophiles, enabling carbonyl addition from the aldehyde or alcohol oxidation level. In the latter case, hydrogen exchange between alcohols and π-unsaturated reactants generates electrophile-nucleophile pairs en route to products of hydro-hydroxyalkylation, representing a direct method for the functionalization of carbinol C-H bonds.

Catalytic reactions of titanium alkoxides with Grignard reagents and imines: a mechanistic study

The reactivity of Grignard reagents towards imines in the presence of catalytic and stoichiometric amounts of titanium alkoxides is reported. Alkylation, reduction, and coupling of imines take place. Whereas reductive coupling is the major reaction in stoichiometric reactions, alkylation is favored in catalytic reactions. Mechanistic studies clearly indicate that intermediates involved in the two reactions are different. Catalytic reactions involve a metal-alkyl complex. This has been confirmed by reactions of deuterium-labeled substrates and different alkylating agents. Under the stoichiometric conditions, however, titanium olefin complexes are formed through reductive elimination, probably through a multinuclear intermediate.

Asymmetric total synthesis of (+)-fumimycin via 1,2-addition to ketimines

The first asymmetric total synthesis of fumimycin was accomplished. As a key step, a 1,2-addition of methyl Grignard reagents to ketimines with quinine as additive was employed. The absolute configuration of (+)-fumimycin was determined by CD-spectroscopy combined with time-dependent density functional calculations.

Complete mapping of the stereochemical course of retentive deprotonation/alkylation of 1H-benzo[e][1,4]diazepin-2(3H)-ones

Enantiopure amino-acid derived 1H-benzo[e][1,4]diazepin-2(3H)-ones (BZDs) undergo highly retentive deprotonation/alkylation reactions. To confirm the role of stereolabile, axially chiral intermediates in these reactions and to determine the precise stereochemical course of deprotonation and alkylation, a new alanine-derived BZD (S)-1d was prepared. Because of slow diazepine ring inversion of the C3-alkylated derivatives of 1d, it proved possible to determine that electrophiles react at the concave face of the enolate derived from 1d. Furthermore, an enantiopure silyl enol ether derivative of (S)-1d was prepared and characterized by X-ray crystallography, confirming that deprotonation resulted in an (M)-configured axially chiral enolate. Activation parameters for diazepine ring inversion in the potassium enolate of 1d were determined experimentally and are well-matched by density functional calculations. Finally, the factors leading to concave-face alkylation of the enolate derived from 1d are analyzed based on calculated alkylation transition structures. Minimization of torsional effects at the BZD ring fusion and maximization of imine and amide resonance are proposed to favor concave-face alkylation.

Stereogenic-at-metal Zn- and Al-based N-heterocyclic carbene (NHC) complexes as bifunctional catalysts in Cu-free enantioselective allylic alkylations

Investigations detailed herein demonstrate the ability of chiral bidentate N-heterocyclic carbenes to promote directly-without the need for a Cu salt-site- and enantioselective C-C bond forming reactions. Within this context, catalytic allylic alkylations of various allylic phosphates with dialkylzinc and trialkylaluminum reagents, performed with chiral bidentate imidazolinium salts and in the absence of a Cu salt, are described. The Cu-free transformations deliver products bearing tertiary or (all-carbon) quaternary stereogenic centers with exceptional site- (>98:<2 S(N)2':S(N)2) and high enantioselectivity [up to 97.5:2.5 enantiomer ratio (er)]. A chiral Zn-based N-heterocyclic carbene (NHC) complex, which serves as the catalyst for the enantioselective allylic alkylation reactions, is isolated and fully characterized. Spectroscopic and X-ray crystallographic studies indicate that the sulfonate group within the stereogenic-at-Zn bidentate complexes coordinates syn to the proximal phenyl substituent of the NHC backbone (vs the initially expected anti). Investigations regarding a related NHC-Al complex reveal similar structural attributes. The studies outlined provide a plausible rationale regarding the mechanism of Cu-free allylic alkylation reactions that involve dialkylzinc or trialkylaluminum reagents as well as the previously reported alkylmagnesium halides. On the basis of the research disclosed, it is proposed that bidentate metal-carbene complexes can serve as effective bifunctional catalysts, a critical attribute that differentiates such NHC-based complexes from the corresponding monodentate variants.

Three component coupling of alpha-iminoesters via umpolung addition of organometals: synthesis of alpha,alpha-disubstituted alpha-amino acids

The synthesis of alpha,alpha-disubstituted alpha-amino acids by means of a three component coupling is reported. The coupling occurs through umpolung addition of organometallic reagents to the nitrogen of alpha-iminoesters. The resulting enolate intermediates subsequently react with electrophiles (aldehydes, imines, alpha,beta-unsaturated nitro, alkyl halides, acyl cyanides) to form a quaternary center. Tethering of the electrophile and nucleophile components provides cyclic alpha,alpha-disubstituted alpha-amino acid derivatives.

Ag-catalyzed diastereo- and enantioselective vinylogous Mannich reactions of alpha-ketoimine esters. Development of a method and investigation of its mechanism

An efficient diastereo- and enantioselective Ag-catalyzed method for additions of a commercially available siloxyfuran to alpha-ketoimine esters is disclosed. Catalytic transformations require an inexpensive metal salt (AgOAc) and an air stable chiral ligand that is prepared in three steps from commercially available materials in 42% overall yield. Aryl- as well as heterocyclic substituted ketoimines can be used effectively in the Ag-catalyzed process. Additionally, two examples regarding reactions of alkyl-substituted ketoimines are presented. An electronically modified N-aryl group is introduced that is responsible for high reaction efficiency (>98% conversion, 72-95% yields after purification) as well as diastereo- (up to >98:2 dr) and enantioselectivity (up to 97:3 er or 94% ee). The new N-aryl unit is crucial for conversion of the asymmetric vinylogous Mannich (AVM) products to the unprotected amines in high yields. Spectroscopic and X-ray data are among the physical evidence provided that shed light on the identity of the Ag-based chiral catalysts and some of the mechanistic subtleties of this class of enantioselective C-C bond forming processes.

Three-component Ag-catalyzed enantioselective vinylogous mannich and Aza-Diels-Alder reactions with alkyl-substituted aldehydes

Efficient protocols for three-component catalytic enantioselective vinylogous Mannich (VM) reactions of alkyl-substituted aldimines (including those bearing heteroatom-containing substituents) and readily available siloxyfurans are presented. High efficiency and stereoselectivity is achieved through the use of o-thiomethyl-p-methoxyaniline-derived aldimines. Reactions, performed under an atmosphere of air and in undistilled THF, can be promoted in the presence of as little as 1 mol % of easily accessible amino acid-based chiral ligands and commercially available AgOAc. The desired products are obtained in 44 to 92% yield, and in up to >98:<2 diastereomer and >99:<1 enantiomer ratio (>98% ee). Removal of the N-activating group is performed through a one-vessel oxidation/hydrolysis operation, which proceeds via a stable aza-quinone (characterized by X-ray crystallography). Evidence is presented indicating that reactions with chiral nonracemic aldehydes are subject to catalyst control: both substrate enantiomers react to afford the desired product diastereomers in high stereoselectivity. Aryl- and alkynyl-substituted o-thiomethyl-p-methoxyaniline-derived aldimines undergo Ag-catalyzed enantioselective VM reactions more efficiently and with higher selectivity than the corresponding o-anisidyl substrates. Additionally, Ag-catalyzed aza-Diels-Alder reactions of the alkyl-substituted aldimines bearing the structurally modified N-aryl unit afford enantiomerically enriched (up to 95% ee) products in up to 88% yield.