Organocatalyzed enantioselective fluorocyclizations

An in situ directing group strategy for chiral anion phase-transfer fluorination of allylic alcohols

An enantioselective fluorination of allylic alcohols under chiral anion phase-transfer conditions is reported. The in situ generation of a directing group proved crucial for achieving effective enantiocontrol. In the presence of such a directing group, a range of acyclic substrates underwent fluorination to afford highly enantioenriched α-fluoro homoallylic alcohols. Mechanistic studies suggest that this transformation proceeds through a concerted enantiodetermining transition state involving both C-F bond formation and C-H bond cleavage.

Direct oxidative coupling of N-acetyl indoles and phenols for the synthesis of benzofuroindolines related to phalarine

Inspired by the biogenetic synthesis of benzofuroindoline-containing natural products, we designed an oxidative coupling between phenol and N-acetyl indoles. This straightforward and direct radical process, mediated by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and FeCl3 allowed the regioselective synthesis of benzofuro[3,2-b]indolines, whose structure is found in the natural product phalarine.

Catalytic asymmetric intermolecular bromoesterification of unfunctionalized olefins

An asymmetric intermolecular bromoesterification of unfunctionalized olefins catalyzed by (DHQD)(2)PHAL is described. Optically active bromoesters can be obtained with up to 92% ee.

An intramolecular cascade cyclization of 2-aryl indoles: efficient methods for the construction of 2,3-functionalized indolines and 3-indolinones

Efficient intramolecular N/O-nucleophilic cyclization of 2-aryl indoles has been developed to afford the corresponding 2-aza-3-oxaindolines and 3-indolinones in 80-95% yield. The methods provided convenient access to fused imidazo[1,2-c]oxazolidinone, oxazolidine, or tetrahydro-1,3-oxazine cores under mild conditions.

Catalytic enantioselective iodoaminocyclization of hydrazones

The first catalytic enantioselective iodoaminocyclization of β,γ-unsaturated hydrazones has been developed with the help of a trans-1,2-diaminocyclohexane-derived bifunctional thiourea catalyst and allows for the direct access to Δ(2)-pyrazolines containing a quaternary stereogenic center in high yield with good enantioselectivity (up to 95% yield and 95:5 er).

Straightforward Access to Hexahydropyrrolo[2,3-b]indole Core by a Regioselective C3-Azo Coupling Reaction of Arenediazonium Compounds with Tryptamines

A base-mediated regioselective electrophilic addition of arenediazonium salts at the C3-position of tryptamines followed by cyclization provides an efficient entry to C3-nitrogenated hexahydropyrrolo[2,3-b]indoles (HPIs) that can subsequently be transformed into 3-arylhexahydropyrrolo[2,3-b]indoles and other HPI derivatives. The reaction is the first example of a 1,2-diamination that utilizes easily accessible arenediazonium salts as nitrogenous electrophiles.

Asymmetric fluorination of α-branched cyclohexanones enabled by a combination of chiral anion phase-transfer catalysis and enamine catalysis using protected amino acids

We report a study involving the successful merger of two separate chiral catalytic cycles: a chiral anion phase-transfer catalysis cycle to activate Selectfluor and an enamine activation cycle, using a protected amino acid as organocatalyst. We have demonstrated the viability of this approach with the direct asymmetric fluorination of α-substituted cyclohexanones to generate quaternary fluorine-containing stereocenters. With these two chiral catalytic cycles operating together in a matched sense, high enantioselectivites can be achieved, and we envisage that this dual catalysis method has the potential to be more broadly applicable, given the breadth of enamine catalysis. It also represents a rare example of chiral enamine catalysis operating successfully on α-branched ketones, substrates commonly inert to this activation mode.

Asymmetric palladium-catalyzed directed intermolecular fluoroarylation of styrenes

A mild catalytic asymmetric direct fluoro-arylation of styrenes has been developed. The palladium-catalyzed three-component coupling of Selectfluor, a styrene and a boronic acid, provides chiral monofluorinated compounds in good yield and in high enantiomeric excess. A mechanism proceeding through a Pd(IV)-fluoride intermediate is proposed for the transformation and synthesis of an sp(3) C-F bond.

Highly diastereoselective and regioselective copper-catalyzed nitrosoformate dearomatization reaction under aerobic-oxidation conditions

An unprecedented copper-catalyzed acylnitroso dearomatization reaction, which expands the traditional acylnitroso ene reaction and acylnitroso Diels-Alder reaction to a new type of transformation, has been developed under aerobic oxidation. Intermolecular and intra-/intermolecular reaction modes demonstrate an entirely different N- or O-acylnitroso selectivity. Hence, we can utilize this reaction as a highly diastereoselective access to a series of new pyrroloindoline derivatives, which are important structural motifs for natural-product synthesis.

Stereoselectively fluorinated N-heterocycles: a brief survey

The stereoselective incorporation of fluorine atoms into N-heterocycles can lead to dramatic changes in the molecules’ physical and chemical properties. These changes can be rationally exploited for the benefit of diverse fields such as medicinal chemistry and organocatalysis. This brief review will examine some of the effects that fluorine substitution can have in N-heterocycles, including changes to the molecules’ stability, their conformational behaviour, their hydrogen bonding ability, and their basicity. Finally, some methods for the synthesis of stereoselectively fluorinated N-heterocycles will also be reviewed.

Kinetic resolution of unsaturated amides in a chlorocyclization reaction: concomitant enantiomer differentiation and face selective alkene chlorination by a single catalyst

The first example of a kinetic resolution via chlorofunctionalization of olefins is reported. The enantiomers of racemic unsaturated amides were found to have different hydrogen-bonding affinities for chiral Lewis bases in numerous solvents. This interaction was exploited in developing a kinetic resolution of racemic unsaturated amides via halocyclization. The same catalyst serves to both "sense chirality" in the substrate as well as mediate a highly face-selective chlorine delivery onto the olefin functionality, resulting in stereotriad products in up to 99:1 dr and up to 98.5:1.5 er. The selectivity factors were typically greater than 50 to allow for the simultaneous synthesis of both the products and unreacted substrates in highly enantioenriched form at yields approaching 50%. The reaction employs catalytic amounts (≤0.50 mol %) of a commercially available and recyclable organocatalyst.

Silver-catalyzed radical phosphonofluorination of unactivated alkenes

We report herein a mild and catalytic phosphonofluorination of unactivated alkenes. With catalysis by AgNO3, the condensation of various unactivated alkenes with diethyl phosphite and Selectfluor reagent in CH2Cl2/H2O/HOAc at 40 °C led to the efficient synthesis of β-fluorinated alkylphosphonates with good stereoselectivity and wide functional group compatibility. A mechanism involving silver-catalyzed oxidative generation of phosphonyl radicals and silver-assisted fluorine atom transfer is proposed.

Stereoselective halocyclization of alkenes with N-acyl hemiaminal nucleophiles

Halocyclization of alkenes was realized using N-acylhemiaminal nucleophiles. High diastereoselectivity could be achieved for the formation of three stereogenic centers in this halogen-mediated cyclization reaction. We also demonstrated that enantioselective bromocyclization of alkenes using N-acylhemiaminal nucleophiles was possible.

Iron-catalyzed asymmetric haloamination reactions

The first iron(III)/N,N'-dioxide-catalyzed asymmetric haloamination of 3-alkylidene- and 3-arylidene-indolin-2-ones was developed, affording the corresponding chiral oxindole derivatives bearing vicinal haloamine substituents with excellent results (up to 99% yield, 99% ee, >19 : 1 dr). This iron catalyst also exhibits perfect enantioselectivity for chalcone derivatives. The cooperative activation of the substrate and the reagent in concert guarantees the high stereoselectivity.

Introduction of fluorine and fluorine-containing functional groups

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.