The term organocatalysis describes the acceleration of chemical reactions through the addition of a substoichiometric quantity of an organic compound. The interest in this field has increased spectacularly in the last few years as result of both the novelty of the concept and, more importantly, the fact that the efficiency and selectivity of many organocatalytic reactions meet the standards of established organic reactions. Organocatalytic reactions are becoming powerful tools in the construction of complex molecular skeletons.[1,2]
 Special issue of Adv. Synth. Catal. 2004, 346, Nr. 9-10.
Increased substrate scope in phase-transfer-catalyzed asymmetric SNAr reactions was achieved by the use of arene chromium complexes as electrophiles. An efficient asymmetric synthesis of α,α-disubstituted α-amino acids containing various aromatic substituents is shown. PTC=phase-transfer catalyst.
A metal-free and base-free carbonylation has been developed which affords primary, secondary, and tertiary alkyl benzoates under irradiation with visible light in the presence of eosin Y as a photocatalyst. The mechanism has been studied by spectroscopic, theoretical, and preparative methods, and appears to involve intermediate aryl and aroyl radical species as well as a light-driven one-electron redox cycle without any sacrificial redox partner.
Useful skeletons: An electrophilic amination reaction catalyzed by chiral phosphoric acid is the basis for a highly efficient asymmetric dearomatization of naphthols. This protocol provides functionalized β-naphthalenone compounds with a chiral quaternary carbon center in excellent yields and enantioselectivity. These motifs are found in various biologically active natural products and therapeutic reagents.
Two series of dianhydrohexitol derivatives, the first preserving the original bis-fused THF backbones and the second originating from single THF ring-opening reactions, are used as asymmetric ligands in organometallic catalysis or as asymmetric organocatalysts. Their synthesis and their application in different asymmetric reactions for the formation of C–H, C–C, C–N, and C–S bonds are discussed.
Organocatalytic click! Recent advances in the metal-free enamine/enolate-mediated azide–carbonyl [3+2] cycloaddition reaction are discussed. These approaches require neither a metal catalyst nor alkyne substrates. Owing to the ready availability of carbonyl compounds, these methods thus offer excellent alternatives for the synthesis of 1,4-/1,5-disubstituted and 1,4,5-trisubstituted 1,2,3-triazoles.
Three in one to ease organocatalysis: A robust, tunable, and recyclable organocatalytic system has been prepared by anchoring N-heterocyclic carbenes on an all-aliphatic vinylic addition polynorbornene (VA-PNB) scaffold. These three features make this heterogeneous NHC-based organocatalyst most convenient.
A chiral disulfonimide catalyst is used to achieve the first catalytic enantioselective Abramov reaction. Several functionalized α-hydroxy phosphonates were synthesized in good yields and with excellent enantiomeric ratios of up to >99:1. The process was shown to be scalable and up to 1 g of starting material could be employed under mild reaction conditions.
Two roles for NFSI: The N-heterocyclic carbene (NHC) catalyzed asymmetric fluorination of readily available simple aliphatic aldehydes, α-chloro aldehydes, and even alcohols proceeds via azolium enolates and yields a wide range of α-fluoro esters, amides, and thioesters with excellent enantioselectivity. N-Fluorobenzenesulfonimide (NFSI) acts as both a fluorinating reagent and an oxidant in this transformation.
Cyclopeptoids are a novel class of efficient and tunable chiral macrocyclic phase-transfer catalyts. Screening of catalysts for the enantioselective alkylation of N-(diphenylmethylene)glycine tert-butyl ester shows the crucial role of N-arylmethyl residues.
From theory to practise: The mechanism of asymmetric organocatalysed Michael addition of β-ketoamides to enones was studied theoretically, demonstrating for the first time the unique role of the hydrogen atom of the amide in controlling both the reactivity and the enantioselectivity (see scheme). Additional reactions afforded original spiro hemiaminals, taking advantage of the reactivity of the Michael adducts with acrolein. Post-functionalisation led to valuable enantioenriched glutarimides and pyridones.
Two chiral carboxylic acid functionalized micro- and mesoporous metal–organic frameworks (MOFs) are constructed. The mesoporous MOF functions as a host for encapsulation of an enantiopure organic amine by acid–base interactions. The organocatalyst-loaded MOF is an efficient and recyclable heterogeneous catalyst for asymmetric direct aldol reactions, with significantly enhanced stereoselectivity relative to the homogeneous organocatalyst.
A crushing victory: The organocatalytic activity of α,β-dipeptides 1 a–c was evaluated in the asymmetric aldol reaction between ketones 2 and aldehydes 3. The best results in terms of yield and stereoselectivity were obtained under solvent-free conditions with ball-milling activation.
Quantity into quality: A very strong manifestation of a positive multivalency effect on the catalytic activity is observed in dendritic polymer-supported catalysts, based on N-alkylimidazoles, in the Baylis–Hillman reaction (see scheme). Tethering the catalytically active units to a dendritic spacer enhances the activity per unit dramatically, sometimes by an order of magnitude.
Fully functionalized 1,2,3-triazoles were synthesized by a metal-free clicking through an enolate-mediated organocatalytic azide–ketone [3+2]-cycloaddition (OrgAKC) reaction. Very simple and readily available aryl azides and enolizable arylacetones/deoxybenzoins were employed in this organocatalytic transformation (see scheme).
The synthesis of ferrocenophane–amines is developed. Compound L1 can be used as a ligand in the the copper-catalyzed oxidative coupling of 3-hydroxy-2-naphthoate to give the product in good yield with up to 92 % ee, and it also efficiently catalyzes the asymmetric Michael addition reaction as an organocatalyst.
Small ring, big ring: A novel strategy for the one-pot synthesis of substituted pyridines from N-arylmethyl 3-aziridinylpropiolate esters is described. The method employs a three-step procedure including the formation of allenyl imines, phosphine-catalyzed cyclization, and subsequent oxidation of the dihydropyridines. Depending on the reaction conditions of the final oxidation step, tri- and tetrasubstituted pyridines can be selectively produced.
An efficient process for the enantioselective synthesis of optically active 3,4-dihydrothiacarbazol-2(9H)-one derivatives has been developed The cascade Michael addition/thiolysis reactions of 9-methylindoline-2-thiones and N-alkenoylphthalimides were promoted by a chiral bifunctional squaramide catalyst to give the desired products in acceptable yields with 54–98 % ee.
A bifunctional squaramide-catalyzed sulfa-Michael/aldol cascade reaction between benzylidenechroman-4-ones and 1,4-dithiane-2,5-diol has been developed. This reaction enables facile asymmetric access to chiral spirocyclic tetrahydrothiophene chromanone derivatives with three contiguous stereocenters in high to excellent yields (up to 99 %) and with high enantioselectivities (up to 92 % ee).
Typecast: Reactions catalyzed by diphenylprolinol silyl ether can be categorized into three types (see figure): two involve an iminium ion intermediate, such as for a Michael-type reaction (type A) and a cycloaddition reaction (type B), and one proceeds via an enamine intermediate (type C). In type A, good enantioselectivity is realized if a catalyst with a bulky silyl moiety is employed. In types B and C, good enantioselectivity is obtained even when the silyl group is less bulky.
Lewis acid catalysis: A chiral disulfonimide (DSI) serves as a highly efficient precatalyst for vinylogous Mukaiyama–Mannich reactions of a readily available silyloxydiene with protected imines, delivering δ-amino-β-ketoesters. The synthetic utility of the reaction is illustrated by the preparation of valuable enantiomerically enriched building blocks and the formal synthesis of (−)-lasubin.
Mirror, mirror on the wall! HPLC monitoring of the aldol reaction between 4-nitrobenzaldehyde and acetone (see figure) in the absence of a chiral catalyst reveals that spontaneous mirror symmetry breaking takes place initially in about 50 % of the reactions. A novel coupled reaction network that involves indirect autocatalysis and does not require heterochiral inhibition in the aldol product can explain this behavior.
What we need is tolerance: A highly stereoselective aza-Henry reaction of α-aryl nitromethanes with aromatic N-Boc imines was established by using C1-symmetric chiral ammonium betaine as a bifunctional organic base catalyst. The reaction tolerated various substituted aryl groups for both imines and nitromethanes, and a series of precursors for the synthesis of unsymmetrical anti-1,2-diaryl ethylenediamines was provided.
The works: The discovery of a novel aerobic oxidative intermolecular azide–zwitterion reaction catalyzed by an organocatalyst is presented. It is demonstrated that the merger of the Lewis base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and electron-deficient olefins generates reactive zwitterion intermediates, which readily participate in cycloaddition reactions with an array of azides, thus providing facile entry to fully or highly substituted 1,2,3-triazole frameworks.
A strategy to combat the flu: The anti-influenza drugs zanamivir and laninamivir (see structures) were synthesized in only 13 steps from inexpensive D-araboascorbic acid by the use of an organocatalytic Michael addition and a metal-catalyzed anti-selective Henry reaction as key transformations. This cost-effective, straightforward, and efficient approach enabled the synthesis of more than 3.5 g of zanamivir.
Controlling tactic(s): The use of pyridine alone as organocatalyst of the ring-opening polymerization (ROP) of a cyclic O-carboxyanhydride resulted in atactic poly(mandelic acid) (PMA), however, a well-defined pyridine/mandelic acid adduct enabled excellent control over the ROP, providing highly isotactic chiral PMA with an enhanced heat resistance. Tg=glass-transition temperature.
A perfect combination: The combination of commercially available 4(5)-(hydroxymethyl)imidazole (HMI) and potassium iodide serves as a simple and efficient catalyst system for the coupling reaction of epoxides with carbon dioxide. Several epoxides are converted into cyclic carbonates in high yields (up to 99 %) under mild and solvent-free conditions within a short reaction time. This new catalyst system demonstrates higher activity than the previously reported potassium iodide/triethanolamine (TEA) system.
Less aromatic: The title reaction between indoles and quinone imine ketals was achieved by the two tandem approaches shown in the scheme. In both cases, enantiomerically pure indole derivatives bearing an all-carbon quaternary stereogenic center were generated in high yields and excellent stereoselectivities.
Going round in heterocycles: Benzoxazole and benzothiazole derivatives are synthesized by using mesoporous poly(melamine–formaldehyde) as a green and recoverable catalyst. This heterogeneous organocatalytic method can provide a novel and efficient strategy for the synthesis of other N-heterocycles.
Nucleophilic carbon: NHC catalysis is used for the functionalization of carboxylic anhydrides. The β carbon acts as a nucleophilic carbon and undergoes asymmetric reactions with electrophiles. Anhydrides with challenging β-alkyl substituents work effectively.
Hand-in-hand: Recent developments in Brønsted acid-assisted chiral phosphoric acid catalysis are discussed, which exhibit the superiority of the self-assembly and the specificity of substrate recognition.
A hybrid α1-Dawson polyanion bearing a lateral side chain with a 4-aminopyridine end group catalyzes the addition of indenyl allyl silanes to cinnamoyl fluorides. The chiral polyanionic framework influences the catalytic activity, the selectivity, and the stereoselectivity of the reaction (see figure).
Mirror mirror on the wall: We have developed an enantioselective, desymmetrizing, bromolactonization reaction of symmetric olefinic dicarboxylic acids that is promoted by a C3-symmetric trisimidazoline organocatalyst.
All in the family: A new family of chiral nucleophilic catalysts, biphenyl-derived phosphepines, and their use in the title reaction is reported. A range of one-carbon coupling partners can be employed to generate cyclopentenes bearing a fully substituted stereocenter. Initial mechanistic studies are described.
Hand in hand: The outlined diversity-oriented synthesis of a library of macrocycles is based on the orthogonal combination of multiple diversity-generating organocatalytic steps with alkene metathesis. In total, 51 macrocyclic structures bearing 48 unique scaffolds, drug-like chemophysical properties, and natural-product-like shape diversity were synthesized in only 2 to 4 steps without the need for protecting groups.
Procedures for the Brønsted acid catalyzed asymmetric synthesis of 9-substituted tetrahydroxanthenones and 3,9-disubstituted tetrahydroxanthenone derivatives have been developed. The procedures are based on the in situ generation of ortho-quinone methides and their subsequent reaction with 1,3-dicarbonyl compounds. The reaction provides products with a high level of asymmetric induction.
The organocatalytic intramolecular aza-Michael reaction (IMAMR) with conjugated esters, elusive to date, was achieved by using N-acyl pyrazoles as ester surrogates (see scheme; DIC=diisopropyl carbodiimide, CPME=cyclopentyl methyl ether). Squaramides were the catalyst of choice, and the reaction was performed under microwave irradiation at 90 °C. Furthermore, the process could be also performed in a tandem peptide-coupling IMAMR starting from the corresponding conjugated carboxylic acid derivatives.
Double bonanza: The title reaction in the presence of an imidazolidinone-based catalyst furnished N-bridgehead bicyclic alkaloids bearing [3.3.0], [3.4.0], [4.4.0], and [4.5.0] skeletons. By using this protocol, the total syntheses of (−)-epilupinine, (−)-tashiromine, and (−)-trachelanthamidine were achieved.
Ready to cyclize: Asymmetric synthesis of oxygenated 1,5- and 1,6-enynes is developed by means of an amine/Brønsted acid catalyzed direct cross-aldol reaction of unsaturated aldehydes. This reaction also works well with propargylic ketoesters yielding tertiary alcohols. Submission of the corresponding aldol adducts to standard O-silylation and subsequent Pauson–Khand conditions afforded the respective bicyclic products in good yields and stereoselectivities.
Selective catalysis by MOFs: Steric hindrance by a metal–organic framework (MOF) is shown to influence the outcome of a catalytic reaction by controlling the orientation of its intermediates, as demonstrated by using phosphine MOF LSK-3, which is evaluated with the aid of molecular modeling and NMR techniques and tested as a catalyst for coumarin synthesis, umpolung addition, Knoevenagel condensation, and [3+2] cycloaddition.
The methylation of amines with CO2 as C1 source and Ph2SiH2 as reducing agent was achieved with an N-heterocyclic carbene (NHC) as the catalyst. The catalyst is tolerant toward a variety of functional groups (including esters and ethers, nitro, nitrile, and carbonyl groups, and unsaturated C-C bonds); the reaction uses commercially available reagents and can be performed on a gram scale.
The catalytic asymmetric construction of a spirooxindole scaffold has been established by an organocatalytic three-component tandem reaction to provide structurally complex spirobenzodiazepine oxindoles with one quaternary stereogenic center (see scheme) in high yield with excellent enantioselectivity.
Light and simple: An organic molecule as simple as p-anisaldehyde can efficiently catalyze the intermolecular atom-transfer radical addition of a variety of haloalkanes onto olefins. The protocol requires irradiation from a household 23 W compact fluorescent light (CFL) bulb to proceed, and ambient temperature is sufficient to functionalize olefins in a synthetically useful fashion.
Organocatalysis: Proazaphosphatrane superbases prove to be highly active catalysts in the reductive functionalization of CO2, in the presence of hydroboranes. The new method makes possible the methylation of N-H bonds in a wide variety of amines, including secondary amines (see picture), with increased chemoselectivity.
Fully automated: A 3 D continuous-flow organocatalytic system is designed with the integration of the microreactor and multidimensional chromatography technologies. This enamine catalysis platform enables the production of chiral γ-nitroaldehydes with on-line monitoring of the reaction parameters. Boc=tert-Butoxycarbonyl, dr=diastereomeric ratio.
Sexually deceptive chiloglottones, antimicrobial dialkylresorcinols, and their many analogues are synthesized in very good yields in a sequential two-pot manner by using an “organocatalytic reductive coupling reaction” as the key step.
Carbohydrates, with their rigid backbones, large numbers of functional groups and high contents of chiral centres, are an appealing natural resource for the development of organocatalysts. They are becoming increasingly popular new tools for enantioselective synthesis, and this subject is reviewed for the period from 2009 to mid-2014.
The first asymmetric 1,4-addition of azlactones to α,β-unsaturated trichloromethyl ketones by using cinchona alkaloid derived bifunctional thiourea catalysts is described. A series of α,α-disubstituted α-amino acid derivatives bearing a quaternary stereocenter at the α-position are obtained in high yields with excellent diastereo- and enantioselectivities (up to >20:1 dr and 99 % ee).
Cinchona derivatives for the successful enantioselective addition of trichlorosilane to ketimines are developed. Excellent yields with good to high enantioselectivities (up to 91 %) are obtained in the reduction of differently substituted substrates. Remarkably high turnover frequencies for imine hydrosilylation are observed at a catalyst loading of only 1 mol-%.
The first catalytic Wittig reaction under microwave irradiation is reported. Readily available tributylphosphine oxide acts as the precatalyst. Under optimized conditions, a variety of aromatic, aliphatic, and heteroaromatic aldehydes could be converted efficiently. Moreover, one example of the first asymmetric version under these conditions is enclosed.
One thing led to another: An organocatalyzed diastereo- and enantioselective cascade aza-Michael/Michael addition of 2-tosylaminoenones to unsaturated pyrazolones has been developed to afford novel chiral spiropyrazolone tetrahydroquinolines containing three contiguous stereocenters (see scheme).
Unsaturated polyesters are obtained specifically from dimethacrylates by a new step-growth polymerization catalyzed by an N-heterocyclic carbene.
Phase transfer catalysis: Halogenated pentanidium salts are efficient phase-transfer catalysts for the enantioselective alkylation of sulfenate anions to sulfoxides with high enantioselectivity and high yield. Mechanistic studies indicate the ability of the catalysts to simultaneously activate/stabilize both nucleophile and electrophile through multiple noncovalent interactions (ion-pairing interaction, nonclassical hydrogen bonds, and halogen bonds).
A successful trio: Pyridinium cations carrying three electron-withdrawing substituents catalyze the alkylation of α-halo ethers with silyl ketene acetals through Coulombic anion binding. This C-C bond formation proceeds efficiently at low temperatures and at low catalyst loading.
Into the mix: N-Heterocyclic carbenes (NHCs) catalyze the generation of α,β-unsaturated acyl azoliums from α,β-unsaturated carboxylic acids via in situ generated mixed anhydrides for the enantioselective [3+2] and [3+3] cyclocondensation with sulfonylated α-amino ketones and alkyl(aryl)imines, respectively. The corresponding pyrrolidinones and dihydropyridinones were isolated in good yields with high to excellent enantioselectivities.
High enantioselectivities and excellent yields are obtained in the first example of a catalytic enantioselective addition to and nitronate protonation of trisubstituted nitroalkenes, generating a tetrasubstituted carbon atom. Thioacids reacted with both activated and unactivated nitroalkenes. The 1,2-nitrothioacetate products can be readily converted in two steps to biomedically relevant 1,2-aminosulfonic acids without loss of enantiopurity.
A rich seam: An enantioselective chiral phosphoric acid-catalyzed cyclization of unsaturated acetals has been utilized for the synthesis of functionalized chiral piperidines. The chiral enol ether products of these cyclizations undergo subsequent in situ enantioenrichment. A new computational method was utilized to elucidate the mechanism and stereoselectivity of this transformation. Cbz=benzyloxycarbonyl; S=resolution.
Lights on: Visible-light metal-free mediated hydroacylation of dialkyl azodicarboxylates was achieved by utilizing phenyl glyoxylic acid as the photocatalyst (see scheme).
Aromatics in 3D: Organocatalysis is now reaching beyond the control of stereogenic centers and opens new possibilities for the construction of complex polyaromatic structures with either helical or axial chirality.
A nonmetal catalytic oxidation of alcohols with preferential oxidation of secondary alcohols by diamine is developed. 1,2-Di(1-naphthyl)-1,2-ethanediamine (NEDA) is used as catalyst to selectively oxidize secondary alcohols in the presence of TBHP as terminal oxidant. The chiral NEDA is also studied and shows promising enantioselectivity in oxidative kinetic resolution of racemic secondary alcohols with good to excellent ee values.
Out with the old: The design and synthesis of new chiral phosphines, as well as their application in catalytic asymmetric reactions, have recently drawn a lot of attention. This review summarizes the advances in the field of enantioselective phosphine organocatalysis within the last couple of years.
(E/Z)-(Phenylsulfanyl)cycloalkanecarbaldehydes have been stereospecifically resolved through an enantioselective (S)-proline aldol reaction with ketones. (Z)-Aldehydes furnished the corresponding aldol adducts in good yields and excellent er (98:2 to >99:1). Further reduction of the aldol adducts and treatment with TsOH furnished enantiomerically enriched spirocyclic tetrahydrofuran derivatives.
A one-pot synthetic protocol for the regio- and stereoselective formation of highly substituted five- and six-membered carbacycles was developed. The two-step procedure includes an asymmetric allylic alkylation (AAA) of Morita–Baylis–Hillman (MBH) carbonates followed by a ring-closing alkene metathesis (RCM) reaction and affords the corresponding carbacycles in high yields with good enantioselectivity.
Flavin of the month: A new system for two-phase oxidations with diluted aqueous hydrogen peroxide that uses a flavinium phase-transfer catalyst (Fl+) is developed. The system uses both activation and the increasing lipophilicity of peroxide species by its covalent bonding to the amphiphilic flavinium salt.
This review provides an overview of recent advances in catalytic enantioselective protonation of preformed enol derivatives and catalytically generated enolates or equivalents through various cascade reaction sequences giving access to a large range of enantioenriched compounds containing tertiary stereocentres.
We like Mike: The Michael reaction of nitromethane to β,β-disubstituted α,β-unsaturated aldehydes catalyzed by diphenylprolinol silyl ether was developed to afford Michael adducts with all-carbon quaternary stereogenic centers and excellent enantioselectivity (see scheme). The E/Z isomerization of α,β-unsaturated aldehydes, the retro-Michael reaction, and the different reactivity between nitromethane and nitroethane are discussed.
A highly enantioselective, direct ε-regioselective bisvinylogous 1,6-addition reaction of β-allyl-2-cyclohexenone to β-substituted α,α-dicyanodienes is developed through trienamine catalysis of a bifunctional primary amine–thiourea compound. Excellent enantioselectivity is obtained, and more complex cyclic frameworks can be efficiently constructed in a highly stereoselectivity manner. OFBA = o-fluorobenzoic acid.
Pick your type: In the past several decades, highly useful epoxidation protocols have been developed with a variety of activation modes using a wide range of asymmetric organocatalysts. This review documents the rapid and expansive development in this area, thus providing a clear overview of the various catalyst types available for asymmetric organocatalytic epoxidations, as well as their mechanisms and applications.
The power of two: A highly enantio- and regioselective aminocatalytic and Lewis acid catalyzed α-hydroxyamination of β-keto esters and 1,3-diketones with N-hydroxycarbamates is realized in “one-pot” under aerobic conditions. The powerful dual catalysis strategy opens opportunities for developing new efficient organic transformations. Cbz=Benzyloxycarbonyl, Boc=tert-butoxycarbonyl.
Since initial reports of organocatalysis through hydrogen bonding interactions, a number of strategies have emerged to allow access to enhanced hydrogen-bond donor (HBD) organocatalysts. These strategies range from augmentation of existing HBDs to the design and synthesis of new HBD catalysts. The effects of recent dual HBD designs on catalyst performance are described.
Get straight to the point! The elusive and direct organocatalytic β-functionalization of saturated carbonyl compounds has been tackled by oxidative enamine catalysis, oxidative NHC catalysis and merging of photoredox catalysis with organocatalysis. This new activation mode expanded the horizons of chemical synthesis and offers new insight for organic transformations and complex molecule synthesis.
This review describes an approach to enantiopure products based on the enantioselective generation, through biocatalysis or organocatalysis, of chiral substrates for a multicomponent reaction (MCR). If the chiral substrates are able to control the newly formed stereogenic centers, this strategy allows fast and diversity-oriented entry to complex chiral substrates.
The Perfect Storm! Water facilitates catalyst turnover in enamine organocatalysis, and it is generated during enamine formation. A synergy between Lewis acids and organocatalysts allows for the development of new, selective, and innovative processes, but water-compatible Lewis acids must be employed that are capable of surviving and maintaining activity in the presence of water.
You say goodbye, I say halo: Derivatives of cinchona alkaloids are an important class of organocatalysts. Their applications in asymmetric halofunctionalization of alkenes and alkynes are summarized in this Focus Review. A wide range of nucleophiles, halogenation reagents, and substituted alkenes, alkynes, or enynes can participate in these reactions to afford diverse chiral building blocks.
BINSA, done that: The Brønsted acidity of catalysts is considered to be associated with their catalytic activity. Therefore, chiral 1,1'-binaphthyl-2,2-disulfonic acid (BINSA) has recently received much attention as a strong chiral Brønsted acid catalyst. This Focus Review summarizes the latest achievements in chiral BINSA chemistry from the perspective of their synthesis and their catalytic use in asymmetric organocatalysis.
Less is more: Metal-free systems, including frustrated Lewis pairs (FLPs), have been shown to bind CO2. By reducing the Lewis acidity and basicity of the ambiphilic system, it is possible to generate active catalysts for the deoxygenative hydroboration of carbon dioxide to methanol derivatives with conversion rates comparable to those of transition-metal-based catalysts (see scheme).
Esters—what else! A new strategy in NHC organocatalysis allows the α-, β- and γ-activation of saturated and unsaturated esters. The resulting acyl azolium intermediates efficiently participate in domino reactions with suitable substrates to generate synthetically valuable carbo- and heterocycles with very good diastereo- and excellent enantioselectivities.