Direct α-Functionalization of Saturated Cyclic Amines

Remote C(sp3)-H Oxygenation of Protonated Aliphatic Amines with Potassium Persulfate

This letter describes the development of a method for selective remote C(sp3)-H oxygenation of protonated aliphatic amines using aqueous potassium persulfate. Protonation serves to deactivate the proximal C(sp3)-H bonds of the amine substrates and also renders the amines soluble in the aqueous medium. These reactions proceed under relatively mild conditions (within 2 h at 80 °C with amine as limiting reagent) and do not require a transition metal catalyst. This method is applicable to a variety of types of C(sp3)-H bonds, including 3°, 2°, and benzylic C-H sites in primary, secondary, and tertiary amine substrates.

Iterative direct C(sp3)–H functionalization of amines: diastereoselective divergent syntheses of α,α′-disubstituted alicyclic amines

Facile access to the syn-α,α′-disubstituted N-heterocycles via iterative C–H oxygenation and C–C bond formation.

Decarboxylative Umpolung of conjugated enals to β-carbanions for intramolecular nucleophilic addition to an aldehyde

Decarboxylative Umpolung converts enals to β-carbanions for intramolecular nucleophilic addition to an aldehyde group with excellent regio- and diastereoselectivities.

Ruthenium catalyzed β-C(sp3)–H functionalization on the ‘privileged’ piperazine nucleus

Ruthenium catalysed β-C(sp³)–H functionalization of piperazines has been revealed.

Recent advances in the chemistry of metallated azetidines

The almost unexplored four-membered azetidines represent a particularly interesting class of molecules, among the family of saturated nitrogen heterocycles. This review reports recent developments in direct metal-based functionalization of the azetidine ring, focusing on the regio- and stereoselectivity of these reactions.

Diastereoselectively Complementary C-H Functionalization Enables Access to Structurally and Stereochemically Diverse 2,6-Substituted Piperidines

The preparation of 2,6-substituted piperidine derivatives through diastereoselective C-H functionalization of corresponding nitrogen heterocycles represents an appealing protocol and yet remains a formidable challenge. Here, we describe a stereochemically complementary oxidative C-H functionalization of N-carbamoyl tetrahydropyridines with a wide variety of building blocks, providing either the cis- or trans-2,6-substituted piperidines with diverse patterns of functionalities. The mild metal-free process exhibits excellent regio- and diastereoselectivities as well as functional group tolerance. The synthetic utilities in natural product and analogue syntheses are also described.

C-H functionalization of amines with aryl halides by nickel-photoredox catalysis

We describe the functionalization of α-amino C-H bonds with aryl halides using a combination of nickel and photoredox catalysis. This direct C-H, C-X coupling uses inexpensive and readily available starting materials to generate benzylic amines, an important class of bioactive molecules. Mechanistically, this method features the direct arylation of α-amino radicals mediated by a nickel catalyst. This reactivity is demonstrated for a range of aryl halides and N-aryl amines, with orthogonal scope to existing C-H activation and photoredox methodologies. We also report reactions with several complex aryl halides, demonstrating the potential utility of this approach in late-stage functionalization.

Redox-Neutral Aromatization of Cyclic Amines: Mechanistic Insights and Harnessing of Reactive Intermediates for Amine α- and β-C-H Functionalization

Cyclic amines such as pyrrolidine and piperidine are known to undergo condensations with aldehydes to furnish pyrrole and pyridine derivatives, respectively. A combined experimental and computational study provides detailed insights into the mechanism of pyrrole formation. A number of reactive intermediates (e.g., azomethine ylides, conjugated azomethine ylides, enamines) were intercepted, outlining strategies for circumventing aromatization as a valuable pathway for amine C-H functionalization.

Enantioselective amine α-functionalization via palladium-catalysed C-H arylation of thioamides

Saturated aza-heterocycles are highly privileged building blocks that are commonly encountered in bioactive compounds and approved therapeutic agents. These N-heterocycles are also incorporated as chiral auxiliaries and ligands in asymmetric synthesis. As such, development of methods to functionalize the α-methylene C–H bonds of these systems enantioselectively is of great importance, especially in drug discovery. Currently, enantioselective lithiation with (–)-sparteine followed by Pd(0) catalyzed cross coupling to prepare α-arylated amines is largely limited to pyrrolidines. Here we report a Pd(II)-catalyzed enantioselective α-C–H coupling of a wide range of amines, including ethyl amines, azetidines, pyrrolidines, piperidines, azepanes, indolines, and tetrahydroisoquinolines. Chiral phosphoric acids are demonstrated as effective anionic ligands for the enantioselective coupling of methylene C–H bonds with aryl boronic acids. This catalytic reaction not only affords high enantioselectivities, but also provides exclusive regioselectivity in the presence of two methylene groups in different steric environments.

Scandium-catalysed intermolecular hydroaminoalkylation of olefins with aliphatic tertiary amines

A homoleptic scandium trialkyl complex in combination with a borate compound served as an excellent catalyst for the C-H addition of aliphatic tertiary amines to olefins. This highly regiospecific, 100% atom efficient C-H bond alkylation reaction was applicable to a wide variety of tertiary amines and olefins, including functionalised styrenes and unactivated α-olefins. This work represents the first example of rare-earth catalysed olefin hydroaminoalkylation and also the first example of catalytic C-H addition of aliphatic tertiary amines to olefins with any catalyst.

Opportunities and challenges for direct C-H functionalization of piperazines

Piperazine ranks within the top three most utilized N-heterocyclic moieties in FDA-approved small-molecule pharmaceuticals. Herein we summarize the current synthetic methods available to perform C-H functionalization on piperazines in order to lend structural diversity to this privileged drug scaffold. Multiple approaches such as those involving α-lithiation trapping, transition-metal-catalyzed α-C-H functionalizations, and photoredox catalysis are discussed. We also highlight the difficulties experienced when successful methods for α-C-H functionalization of acyclic amines and saturated mono-nitrogen heterocyclic compounds (such as piperidines and pyrrolidines) were applied to piperazine substrates.

C-H functionalization of cyclic amines: redox-annulations with α,β-unsaturated carbonyl compounds

Cyclic amines such as pyrrolidine and 1,2,3,4-tetrahydroisoquinoline undergo redox-annulations with α,β-unsaturated aldehydes and ketones. Carboxylic acid promoted generation of a conjugated azomethine ylide is followed by 6π-electrocylization, and, in some cases, tautomerization. The resulting ring-fused pyrrolines are readily oxidized to the corresponding pyrroles or reduced to pyrrolidines.

Direct Acylation of C(sp(3))-H Bonds Enabled by Nickel and Photoredox Catalysis

Using nickel and photoredox catalysis, the direct functionalization of C(sp(3))-H bonds of N-aryl amines by acyl electrophiles is described. The method affords a diverse range of α-amino ketones at room temperature and is amenable to late-stage coupling of complex and biologically relevant groups. C(sp(3))-H activation occurs by photoredox-mediated oxidation to generate α-amino radicals which are intercepted by nickel in catalytic C(sp(3))-C coupling. The merger of these two modes of catalysis leverages nickel's unique properties in alkyl cross-coupling while avoiding limitations commonly associated with transition-metal-mediated C(sp(3))-H activation, including requirements for chelating directing groups and high reaction temperatures.

Redox-Annulation of Cyclic Amines and β-Ketoaldehydes

Benzo[a]quinolizine-2-one derivatives are readily assembled from 1,2,3,4-tetrahydroisoquinoline and β-ketoaldehydes by means of a new intramolecular redox-Mannich process. These reactions are promoted by simple acetic acid and are thought to involve azomethine ylides as reactive intermediates.

An Ugi Reaction Incorporating a Redox-Neutral Amine C-H Functionalization Step

Pyrrolidine and 1,2,3,4-tetrahydroisoquinoline (THIQ) undergo redox-neutral α-amidation with concurrent N-alkylation upon reaction with aromatic aldehydes and isocyanides. Reactions are promoted by acetic acid and represent a new variant of the Ugi reaction.

Palladium-catalysed transannular C-H functionalization of alicyclic amines

The discovery of pharmaceutical candidates is a resource-intensive enterprise that frequently requires the parallel synthesis of hundreds or even thousands of molecules. Carbon-hydrogen bonds are present in almost all pharmaceutical agents. As such, the development of selective, rapid, and efficient methods for converting carbon-hydrogen bonds into new chemical entities has the potential to dramatically streamline pharmaceutical development^1,2,3,4. Saturated nitrogen-containing heterocycles (alicyclic amines) feature prominently in pharmaceuticals, including treatments for depression (paroxetine, amitifadine), diabetes (gliclazide), leukemia (alvocidib), schizophrenia (risperidone, belaperidone), and nicotine addiction (cytisine and varenicline)⁵. However, existing methods for the C–H functionalization of saturated nitrogen heterocycles, particularly at sites remote to nitrogen, remain extremely limited ^6,7. Here we report a new approach to selectively manipulate the carbon–hydrogen bonds of alicyclic amines at sites remote to nitrogen. Our reaction leverages the boat conformation of the substrates to achieve the palladium-catalyzed amine-directed conversion of C–H bonds to C–C bonds on various alicyclic amine scaffolds. This approach is applied to the synthesis of novel derivatives of several bioactive molecules, including the top-selling smoking cessation drug varenicline (Chantix^®). We anticipate that this method should prove broadly useful in medicinal chemistry.

Stereocontrolled synthesis of vicinally functionalized piperidines by nucleophilic β-addition of alkyllithiums to α-aryl substituted piperidine enecarbamates

Substituted piperidines are emerging as important medicinally-active structural motifs. Here, we report highly stereoselective carbolithiation reactions of α-aryl piperidine enecarbamates that offer direct access to vicinally-substituted piperidine compounds. We have also demonstrated that the carbanion intermediates can be trapped with a carbon electrophile.

Mechanistic interrogation of the asymmetric lithiation-trapping of N-thiopivaloyl azetidine and pyrrolidine

A fundamental mechanistic study of the s-BuLi/chiral diamine-mediated lithiation-trapping of N-thiopivaloyl azetidine and pyrrolidine is reported. We show that lithiated thiopivalamides are configurationally unstable at -78 °C. Reaction then proceeds via a dynamic resolution of diastereomeric lithiated intermediates and this accounts for the variable sense and degree of asymmetric induction observed compared to N-Boc heterocycles.

Redox-Neutral α-Amino C-H Functionalization: When the Catalyst Is Also the Nucleophile

A redox-neutral functionalization of cyclic amines that leads to acyclic products is presented. The reaction hinges on generation of transient aryl radical intermediates by catalytic activation with a simple hydrazine. Those aryl radicals subsequently undergo translocation and further oxidation prior to trapping with the same hydrazine, thus resulting in an overall process where the catalyst unusually also acts as the nucleophile.

A highly efficient heterogeneous copper-catalyzed three-component coupling of tetrahydroisoquinolines, aldehydes and 1-alkynes

A highly efficient heterogeneous copper-catalyzed three-component coupling of tetrahydroisoquinolines, aldehydes and 1-alkynes has been described.

A short and modular approach towards 3,5-disubstituted indolizidine alkaloids

A short modular access to 5,9-trans-configurated 3,5-disubstituted indolizidines through double α-metalation of piperidine is presented.

Palladium-Catalyzed Directed C(sp3)-H Arylation of Saturated Heterocycles at C-3 Using a Concise Optimization Approach

Saturated heterocycles, such as THFs, pyrrolidines, piperidines and THPs, are essential components of many biologically active compounds. Examples of C-H functionalization on these important ring systems remain scarce, especially at unactivated positions. Here we report the development of conditions for the palladium-catalyzed stereoselective C(sp3)-H arylation at unactivated 3-positions of 5- and 6-membered N- and O-heterocycles with aminoquinoline directing groups. Subtle differences in substrate structures altered their reactivity significantly; and different conditions were required to achieve high yields in each case. Successful conditions were developed using a short empirical optimization approach to cover reaction space with a limited set of variables. Excellent cis-selectivity was achieved in all cases, except for the THP substrate where minor trans-products were formed through a different palladacyclic intermediate. Here, differences in reactivity and selectivity with other directing groups were examined.

Stereochemical Course of Deprotonation-Acylation of N-Boc- and N-Carbamoyl-2-cyano-6-methylpiperidines

The stereochemical course of electrophilic substitution of α-nitrile metallocarbanions generated by deprotonation from N-Boc- and N-carbamoyl-2-cyano-6-methylpiperidines was investigated. Deprotonation in the presence of an electrophile taking advantage of the high acidity of α-nitrile protons allowed examination of the effects of a chelating group on the nitrogen atom, a countercation, and the reactivity of an electrophile on the steric course. Analyses of reactions using aroyl chlorides and methyl iodide revealed the following: (1) the substitution reactions basically proceed with retention of configuration, (2) the extent of an inversion product increases with decreasing chelating ability of the N-substituent and with increasing leaving ability (ionic character) of a countercation (Li, Na, K) of the anionic species, and (3) the use of a more reactive electrophile results in an increase of the retention product.

α- and α'-Lithiation-Electrophile Trapping of N-Thiopivaloyl and N-tert-Butoxythiocarbonyl α-Substituted Azetidines: Rationalization of the Regiodivergence Using NMR and Computation

(1)H NMR and computational analyses provide insight into the regiodivergent (α- and α'-) lithiation-electrophile trapping of N-thiopivaloyl- and N-(tert-butoxythiocarbonyl)-α-alkylazetidines. The magnitudes of the rotation barriers in these azetidines indicate that rotamer interconversions do not occur at the temperature and on the time scale of the lithiations. The NMR and computational studies support the origin of regioselectivity as being thiocarbonyl-directed lithiation from the lowest energy amide-like rotameric forms (cis for N-thiopivaloyl and trans for N-tert-butoxythiocarbonyl).