The existence of off-cycle catalyst cooperativity in amine/metal combined catalysis is disclosed. The experimental and density functional theory study of the amine/metal co-catalyzed enantioselective Michael/carbocyclization cascade reaction between allenes and α,β-unsaturated aldehydes reveals that the dual catalysts can perform off-cycle cooperativity that gives access to stable bicyclo[3.2.0]heptane species that limits the carbocycle product formation. Insight into this mode of co-catalyst cooperativity sheds new light on the chiral amine/metal co-catalyzed reactions of to date and gives deeper understanding for improved future design of this type of enantioselective reactions.
Palladium-catalyzed oxidative annulations between phenols and alkenylcarboxylic acids produced a library of benzofuran compounds. Depending on the nature of the substitution of the phenol precursor, either 2,3-dialkylbenzofurans or 2-alkyl-3-methylene-2,3-dihydrobenzofurans can be synthesized with excellent regioselectivity. Reactions between conjugated 5-phenylpenta-2,4-dienoic acids and phenol gave 3-alkylidenedihydrobenzofuran alkaloid motifs while biologically active 7-arylbenzofuran derivatives were prepared by starting from 2-phenylphenols. More interestingly, selective incorporation of deuterium from D2O has been discovered, which offers an attractive one-step method to access deuterated compounds.
The isomerisation of alkenols followed by reaction with aldehydes or N-tosylimines catalysed by rhodium complexes has been studied. The catalytically active rhodium complex is formed in situ from commercially available (cyclooctadiene)rhodium(l) chloride dimer [Rh(COD)Cl](2). The tandem process affords aldol and Mannich-type products in excellent yields. The key to the success of the coupling reaction is the activation of the catalysts by reaction with postassium tert-butoxide (t-BuOK), which promotes a catalytic cycle via alkoxides rather than rhodium hydrides. This mechanism minimises the formation of unwanted by-products. The mechanism has been studied by (1)H NMR spectroscopy and deuterium labelling experiments.
The total synthesis of capsaicin analogues was performed in one pot, starting from compounds that can be derived from lignin. Heterogeneous palladium nanoparticles were used to oxidise alcohols to aldehydes, which were further converted to amines by an enzyme cascade system, including an amine transaminase. It was shown that the palladium catalyst and the enzyme cascade system could be successfully combined in the same pot for conversion of alcohols to amines without any purification of intermediates. The intermediate vanillyl-amine, prepared with the enzyme cascade system, could be further converted to capsaicin analogues without any purification using either fatty acids and a lipase, or Schotten-Baumann conditions, in the same pot. An aldol compound (a simple lignin model) could also be used as starting material for the synthesis of capsaicin analogues. Using l-alanine as organocatalyst, vanillin could be obtained by a retro-aldol reaction. This could be combined with the enzyme cascade system to convert the aldol compound to vanillylamine in a one-step one-pot reaction.
Symmetrical and unsymmetrical diaryliodonium triflates have been synthesized from both electron-deficient and electron-rich arenes and aryl iodides with mCPBA and triflic acid. A thorough investigation of the optimization, scope and limitations has resulted in an improved one-pot protocol that is fast, high-yielding, and operationally simple. The reaction has been extended to the direct synthesis of symmetrical iodonium salts from iodine and arenes, conveniently circumventing the need for aryl iodides.
The highly enantioselective cascade reaction between N-protected a-cyanoglycine esters and a,beta-unsaturated aldehydes is disclosed. The reaction represents a one-step entry to polysubstituted 5-hydroxyproline derivatives having a quaternary a-stereocenter generally in high yields with up to >95:5 dr and 99:1 er. It is also a direct catalytic two-step entry to functionalized a-quaternary proline derivatives.
Aromatic aldehydes, along with aryl alkyl, heteroaryl alkyl, and dialkyl ketones were efficiently reduced to their corresponding primary and secondary alcohols, respectively, in high yields, using the commercially available and inexpensive polymeric silane, polymethylhydrosiloxane (PMHS), as reducing agent. The reaction is catalyzed by in situ generated iron complexes containing hydroxyethyl-functionalized NHC ligands. Turnover frequencies up to 600 h−1 were obtained
Two new highly modular carbohydrate-based, pseudodipeptide and thioamide ligand libraries have been synthesized for the rhodium- and ruthenium-catalyzed asymmetric transfer hydrogenation (ATH) of prochiral ketones. These series of ligands can be prepared efficiently from easily accessible D-xylose and D-glucose. The ligand libraries contain two main ligand structures (pseudodipeptide and thioamide) that have been designed by making systematic modifications to one of the most successful ligand families developed for the ATH. As well as studying the effect of these two ligand structures on the catalytic performance, we also evaluated the effect of modifying several of the ligand parameters. We found that the effectiveness of the ligands at transferring the chiral information in the product can be tuned by correctly choosing the ligand components (ligand structure and ligand parameters). Excellent enantioselectivities (ees up to 99%) were therefore obtained in both enantiomers of the alcohol products using a wide range of substrates.
A general strategy for the total asymmetric synthesis of valuable tropane alkaloids by catalytic stereoselective transformations is disclosed. The power of this approach is exemplified by the concise catalytic enantioselective total syntheses of (+)-methylecgonine, (-)-cocaine and (+)-cocaine as well as the first catalytic asymmetric total syntheses of a cocaine C-1 derivative and (+)-ferruginine starting from 5-oxo-protected-a,beta-unsaturated enals using only two and three column chromatographic purification steps, respectively.
A modular design for a novel heterogeneous synergistic catalytic system, which simultaneously activates the electrophile and nucleophile by the combined activation modes of a separate metal and non-metal catalyst, for asymmetric cascade transformations on a solid surface is disclosed. This modular catalysis strategy generates carbocycles (up to 97.5: 2.5 er) as well as spirocyclic oxindoles (97.5: 2.5 to > 99: 0.5 er), containing all-carbon quaternary centers, in a highly enantioselective fashion via a one-pot dynamic relay process.
The first example of a highly enantioselective organocatalytic aziridination of α-substituted α,β-unsaturated aldehydes is presented. The reaction is catalyzed by simple chiral amines and gives access to highly functional terminal azirdines containing an α-tertiary amine stereocenter in high yields and enantiomeric ratios (95.5:4.5–98:2).
A series of phenolic bio-oil components were selectively hydrodearomatized by palladium on carbon into the corresponding ketones or alcohols in excellent yields using polymethylhydrosiloxane and water as reducing agent. The selectivity of the reaction was governed by the water concentration where selectivity to alcohol was favoured at higher water concentrations. As phenolic bio-oil examples cardanol and beech wood tar creosote were studied as substrate to the developed reaction conditions. Cardanol was hydrodearomatized into 3-pentadecylcyclohexanone in excellent yield. From beech wood tar creosote, a mixture of cyclohexanols was produced. No hydrodeoxygenation occurred, suggesting the applicability of the reported method for the production of ketone-alcohol oil from biomass.
A range of ketones was efficiently reduced in the presence of catalytic amounts of lithium isopropoxide in 2-propanol under microwave heating, with alcohol products being formed in yields up to 99 %.
We report that transition metal-catalyzed nucleophilic activation can be combined with chiral amine-catalyzed iminium activation as exemplified by the unprecedented enantioselective conjugate addition of a dimethylsilanyl group to α,β-unsaturated aldehydes. These reactions proceed with excellent 1,4-selectivity to afford the corresponding β-silyl aldehyde products 3 in high yields and up to 97:3 er using inexpensive bench stable copper salts and simple chiral amine catalysts. The reaction canalso generate a quaternary stereocenter with goodenantioselectivity. Density functional calculations are performed to elucidate the reaction mechanism and the origin of enantioselectivity.
Cobalt salophen was encapsulated in a series of zeolites with a wide variation of the silicon-to-aluminium atomic ratio and with different cations. The zeolite-cobalt salophen catalysts were prepared using the “ship-in-a-bottle technique” where the complex was synthesized in the super cage of the zeolite and therefore locked into the pocket. The encapsulated catalysts were then tested in the aerobic oxidation of hydroquinone to p-benzoquinone; the best encapsulated catalyst was shown to be an efficient electron-transfer mediator in a palladium-catalyzed aerobic oxidative carbocyclization.
An efficient protocol for the synthesis of arylphosphonate diesters via a palladium-catalyzed cross-coupling of H-phosphonate diesters with aryl electrophiles, promoted by acetate ions, was developed. A significant shortening of the cross-coupling time in the presence of the added acetate ions was achieved for bidentate and monodentate supporting ligands, and for different aryl electrophiles (iodo, bromo and triflate derivatives). The reaction conditions were optimized in terms of amount of the catalyst, supporting ligands, and source of the acetate ion used. Various arylphosphonates, including those of potential biological significance, were synthesized using this newly developed protocol. Some mechanistic aspects of the investigated reactions are also discussed.
We have developed a novel method for the synthesis of allenylphosphonates and related compounds based on a palladium(0)-catalyzed reaction of propargylic derivatives with H-phosphonate,H-phosphonothioate, H-phosphonoselenoate, and H-phosphinateesters. The reaction is stereoselective and stereospecific, and provides a convenient entry to a vast array of allenylphosphonates and their analogues with diverse substitution patterns in the allenic moiety and at the phosphorus center. Some mechanistic aspects of this new reaction were also investigated.
A chemoenzymatic dynamic kinetic resolution (DKR) of N-heterocyclic amino alcohols is described. Various lipases were studied as biocatalysts for the kinetic resolution of N-heterocyclic 1,2-amino alcohols. The influence of the support of the enzymes on the enantioselectivity in the resolution of different substrates is highlighted. Various 3-acetoxypyrrolidines and -piperidines were obtained in high yield and high enantiomeric excess in efficient DKR reactions.
Our group has reported several one-pot protocols for the synthesis of diaryliodonium salts, which have been recognized as attractive multi-purpose reagents in areas ranging from organic synthesis to materials chemistry. Over the years, we have identified limitations in the published protocols concerning synthesis of mixed electron-rich and electron-poor, as well as highly electron-poor diaryliodonium salts, as the corresponding starting materials are either too reactive or too unreactive. In this update, we discuss the underlying limitations concerning the stability and reactivity of the involved reagents and provide strategies to overcome these challenges through updated synthetic protocols.
Highly enantioselective co-catalytic direct aldol reactions by a combination of simple hydrophobic acyclic amino acid and hydrogen-bond donating catalysts are presented. The corresponding aldol products are formed in high yields with high regio-, diastereo- (anti or syn) and enantioselectivity (up to 99.5:0.5 er). The catalyst loadings can be decreased to as little as 2 mol%.
A stereochemical divergent approach for the highly enantioselective synthesis of distinct bicyclic products with multiple stereocenters from a racemate using a single chiral catalyst is disclosed. It is based on switches of the overall reaction pathways in the chiral amine-catalyzed cascade reactions between racemic ?-nitro ketones and a,beta-unsaturated aldehydes using different achiral co-catalysts. The utility of the method is exemplified by the highly diasteroselective switch and stereoconvergent deracemization process by combination of chiral amine and achiral hydrogen-bond-donating catalysts.
A modular ligand library of -amino acid hydroxyamides and thioamides was prepared from 10 different N-tert-butyloxycarbonyl-protected -amino acids and three different amino alcohols derived from 2,3-O-isopropylidene--d-mannofuranoside. The ligand library was evaluated in the half-sandwich ruthenium- and rhodium-catalyzed asymmetric transfer hydrogenation of a wide array of ketone substrates, including simple as well as sterically demanding aryl alkyl ketones, aryl fluoroalkyl ketones, heteroaromatic alkyl ketones, aliphatic, conjugated and propargylic ketones. Under the optimized reaction conditions, secondary alcohols were obtained in high yields and in enantioselectivities up to >99%. The choice of ligand/catalyst allowed for the generation of both enantiomers of the secondary alcohols, where the ruthenium-hydroxyamide and the rhodium-thioamide catalysts act complementarily towards each other. The catalytic systems were also evaluated in the tandem isomerization/asymmetric transfer hydrogenation of racemic allylic alcohols to yield enantiomerically enriched saturated secondary alcohols in up to 98% ee. Furthermore, the catalytic tandem -alkylation/asymmetric transfer hydrogenation of acetophenones and 3-acetylpyridine with primary alcohols as alkylating and reducing agents was studied. Secondary alcohols containing an elongated alkyl chain were obtained in up to 92% ee.
Herein, we report a catalytic system based on the earth-abundant manganese for the ketone, amine, alkyne (KA2) reaction. The efficiency of manganese manifests at relatively high temperatures, combined with sustainable reaction conditions, and provides a tool for accessing propargylamines from structurally diverse starting materials, including synthetically relevant and bioactive molecules. Our efforts were also aimed at shedding light on the catalytic mode of action of manganese in this transformation, in order to explain its temperature-related behavior. The use of computational methods reveals mechanistic aspects of this reaction indicating important points regarding the reactivity of both manganese and ketones.
The nucleophilic addition of carbon nucleophiles to amides has traditionally been a difficult task, both due to reactivity and selectivity problems. When successful, these processes would represent straightforward routes towards carbonyl-type or amine compounds, depending on the fate of the generated tetrahedral intermediate. The direct addition of nucleophiles to amides for the preparation of ketones has been studied and applied to the syntheses of several natural products. On the other hand, the addition of nucleophiles to amides to obtain substituted amines represented a major challenge, and only scattered applications on particular substrates have appeared. Initial improvements were based on the activation of amides by introduction of particular substituents, such as in N-methoxy amides (Weinreb amides) or electron-withdrawing groups able to increase the carbon nucleophilicity. Although these strategies facilitate the introduction of nucleophiles, chemoselectivity issues arise when additional electrophilic moieties (i.e., carbonyls) are present, thus decreasing the versatility of the methods. In recent years, important advancements towards fully chemoselective methods have been realized. The capture of tetrahedral intermediates with acids generates highly electrophilic iminium species able to undergo chemoselective additions of various nucleophiles, thus accessing substituted amines. Alternatively, the in situ generation of an iminium triflate ion allows highly chemoselective additions of nucleophiles, yielding amines, ketones or ketimines. Also thioamides can be used as precursors of ketones or alpha-substituted amines. The success of the above methodologies is further showcased by the application in various syntheses of natural products or biologically active molecules.
The catalytic promiscuity of a ferulic acid decarboxylase from Enterobacter sp. (FDC_Es) and phenolic acid decarboxylases (PADs) for the asymmetric conjugate addition of water across the C=C bond of hydroxystyrenes was extended to the N-, C-and S-nucleophiles methoxyamine, cyanide and propanethiol to furnish the corresponding addition products in up to 91% ee. The products obtained from the biotransformation employing the most suitable enzyme/nucleophile pairs were isolated and characterized after optimizing the reaction conditions. Finally, a mechanistic rationale supported by quantum mechanical calculations for the highly (S)selective addition of cyanide is proposed.
Artificial metalloenzymes combine the excellent selective recognition/binding properties of enzymes with transition metal catalysts, and therefore many asymmetric transformations can benefit from these entities. The search for new successful strategies in the construction of metal-enzyme hybrid catalysts has therefore become a very active area of research. This review discusses all the developed strategies and the latest advances in the synthesis and application in asymmetric catalysis of artificial metalloenzymes with future directions for their design, synthesis and application (Sections 2-4). Finally, advice is presented (to the non-specialist) on how to prepare and use artificial metalloenzymes (Section 5).
An efficient method for the synthesis of alpha-iodoketones from allylic alcohols and elemental iodine is reported. We show in this paper that the isomerization of allylic alcohols catalyzed by iridium(III) complexes can be combined with an aerobic oxidative iodination protocol, resulting in a straightforward method for the synthesis of a wide range of alpha-iodoketones as single constitutional isomers and in high yields under mild reaction conditions.
Stereodefined cyclohexene and cyclopentene derivatives were prepared by the coupling of allylic alcohols and other allylic precursors with unsaturated aldehydes. These reactions are based on a multicatalytic one-pot approach involving palladium pincer complex-catalyzed boronation, allylation and ring-closing metathesis reactions. This reaction sequence can be performed in an operationally simple procedure affording the cycloalkene products in high overall yields and excellent regio- and stereoselectivities. The presented procedure has a broad synthetic scope and high functional group tolerance, which allows the synthesis of bicyclic lactone and spirane skeletons and various substitution patterns including hydroxy, silyl, vinyl, allyl, and sulfonyl groups. The studied catalytic one-pot reactions involve up to three individual processes performed by up to four acid- and transition metal-catalyzed events.
A Mo(CO)(6) (molybdenumhexacarbonyl) catalyzed reductive functionalization of amides to afford 5-amino substituted 4,5-dihydroisoxazoles is presented. The reduction of amides generates reactive enamines, which upon the addition of hydroximinoyl chlorides and base undergoes a 1,3-dipolar cycloaddition reaction that gives access to the desired heterocyclic compounds. The transformation of amides is highly chemoselective and tolerates functional groups such as nitro, nitriles, esters, and ketones. Furthermore, a versatile scope of 4,5-dihydroisoxazoles derived from a variety of hydroximinoyl chlorides and amides is demonstrated.
The operationally simple titanium(IV)- or zirconium(IV)-catalyzed direct amidation of non-activated carboxylic acids with ammonium carbamates generates primary, and tertiary N,N-dimethyl-substituted amides in good to excellent yields.
Porous organic polymers have prospects as functional substrates for catalysis, with quite different molecular properties from inorganic substrates. Here we disclose for the first time that porous palladium(II)-polyimines are excellent catalysts for cooperatively catalyzed and enantioselective cascade reactions. In synergy with a chiral amine co-catalyst, polysubstituted cyclopentenes and spirocyclic oxindoles, including the all-carbon quaternary stereocenter, were synthesized in high yields. High diastereo- and enantioselectivities were achieved for these dynamic kinetic asymmetric transformations (DYKAT) of enals with propargylic nucleophiles.
The discovery and investigation of solvent dependency in stereoselective intramolecular amidation of chiral 5-aminofunctionalized-2-fluoromalonate ester derivatives, which gives access to highly functionalized δ-lactams with a quaternary fluorine-containing stereocenter, is disclosed. Experimental work together with density functional theory calculations led to understanding of how to direct and switch the stereochemical outcome of the stereoselective δ-lactam formation. The merging of this solvent-dependent stereoselective switch with asymmetric catalysis and cascade reactions gives access to an unprecedented strategy for stereodivergent synthesis of all possible stereoisomers of fluorine-containing stereocenters adjacent to tertiary stereocenters of a wide range of heterocyclic compounds with 95->99% ee in one-pot. It is also useful for application in total synthesis of fluorine-containing pharmaceuticals.
The asymmetric synthesis of Maraviroc (UK-427,857), a chemochine receptor 5 (CCR-5) receptor antagonist, based on an expeditious organocatalytic enantioselective assembly of the chiral β-amino aldehyde key fragment is presented. The reactions were performed on a gram-scale and allow for the rapid construction of new Maraviroc analogues.
The highly enantioselective organo-co-catalytic aza-Morita–Baylis–Hillman (MBH)-type reaction between N-carbamate-protected imines and α,β-unsaturated aldehydes has been developed. The organic co-catalytic system of proline and 1,4-diazabicyclo[2.2.2]octane (DABCO) enables the asymmetric synthesis of the corresponding N-Boc- and N-Cbz-protected β-amino-α-alkylidene-aldehydes in good to high yields and up to 99% ee. In the case of aza-MBH-type addition of enals to phenylprop-2-ene-1-imines, the co-catalytic reaction exhibits excellent 1,2-selectivity. The organo-co-catalytic aza-MBH-type reaction can also be performed by the direct highly enantioselective addition of α,β-unsaturated aldehydes to bench-stableN-carbamate-protected α-amidosulfones to give the corresponding β-amino-α-alkylidene-aldehydes with up to 99% ee.The organo-co-catalytic aza-MBH-type reaction is also an expeditious entry to nearly enantiomerically pure β-amino-α-alkylidene-amino acids and β-amino-α-alkylidene-lactams (99% ee). The mechanism and stereochemistry of the chiral amine and DABCO co-catalyzed aza-MBH-type reaction arealso discussed.