Generation of N-H Imines from α-Azidocarboxylic Acids through Ru-Catalyzed Decarboxylation.

A new method for the synthesis of N-H imines from α-azidocarboxylic acids was developed, which proceeds through decarboxylative C-C bond cleavage catalyzed by a commercial diruthenium complex ([CpRu(CO)2]2) under visible light irradiation at room temperature within several minutes. The reactive products undergo condensation, which forms cyclic trimers (2,4,6-trialkylhexahydro-1,3,5-triazines) or linear N,N'-bis(arylmethylidene)arylmethanediamines in quantitative yields. Alternatively, the N-H imines can be trapped with benzylamine and 2-(aminomethyl)aniline, providing stable N-benylimines and tetrahydroquinazolines, respectively. Subsequent oxidation of tetrahydroquinazolines produced quinazolines.

Concurrent Formation of N-H Imines and Carbonyl Compounds by Ruthenium-Catalyzed C-C Bond Cleavage of ß-Hydroxy Azides.

A commercial cyclopentadienylrutenium dicarbonyl dimer ([CpRu(CO)2]2) efficiently catalyzes the formation of N-H imines and carbonyl compounds simultaneously from ß-hydroxy azides via C-C bond cleavage under visible light. Density functional theory calculations for the cleavage reaction support the mechanism involving chelation of alkoxy azide species and liberation of nitrogen as the driving force. The synthetic utility of the reaction was demonstrated by a new amine synthesis promoted by chemoselective allylation of imine and synthesis of isoquinoline.

Base-Free Dynamic Kinetic Resolution of Secondary Alcohols with a Ruthenium-Lipase Couple.

We report the dynamic kinetic resolution (DKR) of various secondary alcohols by the combination of a ruthenium catalyst and an anionic surfactant-activated lipoprotein lipase. The DKR reactions performed under totally base-free conditions at room temperature provided the products of excellent enantiopurities (91-99% ee or greater) in high yields (92-99%). More importantly, the DKR of α-arylallyl alcohols was achieved for the first time with high yields (87-91%).

Ionic-surfactant-coated subtilisin: activity, enantioselectivity, and application to dynamic kinetic resolution of secondary alcohols.

In this work, we explored the utility of ionic-surfactant-coated Bacillus licheniformis subtilisin (ISCBLS) as the catalyst for the dynamic kinetic resolution of secondary alcohols. ISCBLS was prepared by freeze-drying Bacillus licheniformis subtilisin with both ionic surfactant 1 and dextrin. ISCBCL displayed 9300-fold enhanced activity relative to its native counterpart in the transesterificaion of N-acetyl phenylalanine ethyl ester with 1-propanol in hexane and 12 800-fold enhanced activity in the transesterification of trifluoroethyl butyrate with 1-phenylethanol in THF. The enantioselectivity of ISCBLS was examined with 50 secondary alcohols as the substrates for kinetic resolution in the presence of trifluoroethyl butyrate. ISCBLS displayed synthetically useful enantioselectivity for most of the secondary alcohols tested. The enantioselectivity of ISCBLS was in particular good to high for m- or p-substituted 1-phenylethanols. The DKRs of these secondary alcohols by the combination of ISCBLS and a ruthenium-based racemization catalyst provided the products of (S)-configuration with good results (80-94% yield, 90-99% ee). It is concluded that ISCBLS is of great use as the enantiocomplementary counterpart of (R)-selective lipase for the dynamic kinetic resolution of secondary alcohols.

Redox reaction between benzyl azides and aryl azides: concerted synthesis of aryl nitriles and anilines.

A unique and novel reaction between benzyl azides and aryl azides is described to synthesize aryl nitriles and anilines concurrently, which is catalyzed with a photoactivated diruthenium complex. N-Unsubstituted imines (N-H imines) are generated first from benzyl azides, followed by the hydrogen transfer reaction between N-H imines and aryl azides. A wide range of aryl nitriles and anilines were synthesized under neutral and mild reaction conditions.

Catalytic transformation of esters of 1,2-azido alcohols into α-amido ketones.

The esters of 1,2-azido alcohols were transformed into α-amido ketones without external oxidants through the Ru-catalyzed formation of N-H imines with the liberation of N2 followed by intramolecular migration of the acyl moiety. A wide range of α-amido ketones were obtained, and one-pot transformation into the corresponding oxazoles (or a thiazole) was demonstrated.

C-H activation guided by aromatic N-H ketimines: synthesis of functionalized isoquinolines using benzyl azides and alkynes.

Aromatic N-H ketimines were in situ generated from various benzylic azides by ruthenium catalysis for the subsequent Rh-catalyzed annulation reaction with alkynes to give the corresponding isoquinolines. In contrast to conventional synthetic methods for aromatic N-H ketimines, our protocol works under mild and neutral conditions, which enabled the synthesis of isoquinolines having various functionalities such as carbonyl, ester, alkenyl, and ether groups. In addition, the imidates generated from α-azido ethers were successfully used for the synthesis of 1-alkoxyisoquinolines.

Highly chemoselective aerobic oxidation of amino alcohols into amino carbonyl compounds.

The direct oxidation of unprotected amino alcohols to their corresponding amino carbonyl compounds has often posed serious challenges in organic synthesis and has constrained chemists to adopting an indirect route, such as a protection/deprotection strategy, to attain their goal. Described herein is a highly chemoselective aerobic oxidation of unprotected amino alcohols to their amino carbonyl compounds in which 2-azaadamantane N-oxyl (AZADO)/copper catalysis is used. The catalytic system developed leads to the alcohol-selective oxidation of various unprotected amino alcohols, carrying a primary, secondary, or tertiary amino group, in good to high yield at ambient temperature with exposure to air, thus offering flexibility in the synthesis of nitrogen-containing compounds.

Novel catalyst system for hydrostannation of alkynes.

A catalyst system was developed for the highly regio- and stereoselective hydrostannation of a range of alkynes with tributylstannane under mild conditions. The active catalytic species was generated from a stable diruthenium complex by illuminating household fluorescent light (30 W) at room temperature.

Postsynthetic functionalization of a hollow silica nanoreactor with manganese oxide-immobilized metal nanocrystals inside the cavity.

A postsynthetic protocol of functionalizing the preformed hollow nanoparticles with metal nanocrystals was developed based on galvanic replacement reaction on the Mn3O4 surface inside the cavity. The developed protocol produced hollow nanoreactor systems, in which a high density of ultrafine catalytic nanocrystals of a range of noble metals, such as Pd, Pt, Rh, and Ir and their alloys, are dispersively immobilized on an interior surface enclosed by a selectively permeable silica shell. The fabricated hollow nanoreactor exhibited highly enhanced activity, selectivity, and recyclability in catalyzing the oxidation of hydrosilanes, which are attributable to the synergistic combination of the porous silica nanoshell and the oxide-immobilized catalyst system.

Kinetic and dynamic kinetic resolution of secondary alcohols with ionic-surfactant-coated Burkholderia cepacia lipase: substrate scope and enantioselectivity.

Forty-four different secondary alcohols, which can be classified into several types (II-IX), were tested as the substrates of ionic surfactant-coated Burkholderia cepacia lipase (ISCBCL) to see its substrate scope and enantioselectivity in kinetic and dynamic kinetic resolution (KR and DKR). They include 6 boron-containing alcohols, 24 chiral propargyl alcohols, and 14 diarylmethanols. The results from the studies on KR indicate that ISCBCL accepted most of them with high enantioselectivity at ambient temperature and with useful to high enantioselectivity at elevated temperatures. In particular, ISCBCL displayed high enantioselectivity toward sterically demanding secondary alcohols (types VIII and IX) which have two bulky substituents at the hydroxymethine center. DKR reactions were performed by the combination of ISCBCL with a ruthenium-based racemization catalyst at 25-60 °C. Forty-one secondary alcohols were tested for DKR. About half of them were transformed into their acetates of high enantiopurity (>90% ee) with good yields (>80%). It is concluded that ISCBCL appears to be a superb enzyme for the KR and DKR of secondary alcohols.

A magnetically separable gold catalyst for chemoselective reduction of nitro compounds.

Magnetically separable gold-nanoparticle catalyst was prepared, and it showed excellent activity for chemoselective reduction of nitroarenes with hydrosilanes.

Photoactivated racemization catalyst for dynamic kinetic resolution of secondary alcohols.

Household fluorescent light activates a diruthenium complex to generate catalytic species highly active for the racemization of secondary alcohols under ambient conditions. This catalyst system is applicable for the chemoenzymatic dynamic kinetic resolution of racemic alcohols to give optically pure acetates under mild conditions.

Chemoenzymatic synthesis of rivastigmine via dynamic kinetic resolution as a key step.

A practical and efficient procedure for the synthesis of rivastigmine was developed. This procedure includes dynamic kinetic resolution using a polymer-bound ruthenium complex and a lipase in combination as a key step. Enantiopure (-)-rivastigmine was obtained from commercially available 3'-hydroxyacetophenone via five steps in overall 57% yield.

Synthesis of optically active amino acid derivatives via dynamic kinetic resolution.

The complete conversion of racemic amino acid amides to optically active amino acid derivatives was accomplished via lipase/palladium-catalyzed dynamic kinetic resolution (DKR). In the DKR, a lipase catalyzes the selective acylation of L-substrate in the presence of acyl donor while unreacted D-substrate is isomerized by a Pd nanocatalyst to L-substrate. The DKR reactions provided good yields (80-98%) and high enantiomeric excess (95-98% ee). Interestingly, the DKR reactions of phenylglycine amide in the presence of Z-Gly-OMe or Z-Gly-Gly-OMe yielded optically active di- and tripeptide .

One-pot synthesis of imines and secondary amines by Pd-catalyzed coupling of benzyl alcohols and primary amines.

Imines and secondary amines were synthesized selectively by controlling reaction conditions for the Pd-catalyzed one-pot reactions of benzyl alcohols with primary amines. The reactions did not require any additives and were effective for a wide range of alcohols and amines.

Asymmetric reductive acylation of aromatic ketoximes by enzyme-metal cocatalysis.

We have developed an efficient procedure for the asymmetric synthesis of chiral amides from ketoximes. This one-pot procedure employs two different types of catalysts, Pd nanocatalyst and lipase, for three consecutive transformations including hydrogenation, racemization, and acylation. Eight ketoximes have been efficiently transformed to the corresponding amides in good yields (83-92%) and high enantiomeric excesses (93-98%).

Highly enantioselective dynamic kinetic resolution of 1,2-diarylethanols by a lipase-ruthenium couple.

A practical procedure has been developed for the dynamic kinetic resolution of 1,2-diarylethanols. This procedure employs a highly enantioselective lipase from Pseudomonas stutzeri (trade name, lipase TL) as the resolution catalyst and a ruthenium complex as the racemization catalyst. Sixteen 1,2-diarylethanols have been efficiently resolved to provide their acetyl derivatives with good yields (95-97%) and high enantiomeric excesses (96-99%).