Pyridylic anions are soft nucleophiles in the palladium-catalyzed C(sp3)-H allylation of 4-alkylpyridines

Stability and reactivity of alkylidene dihydropyridines

Alkylidene dihydropyridines (ADHPs) are electron-rich nucleophilic intermediates that can be readily prepared by dearomatization of 4-alkylpyridines using chloroformate reagents and mild base. Their stability and reactivity can be tuned with the chloroformate reagent used as evidenced by NMR chemical shifts and oxidation potentials. ADHPs prepared with ethyl, allyl and trichloroethyl chloroformate undergo decomposition under an oxygen atmosphere at different rates (ethyl > allyl > trichloroethyl), predominantly to the corresponding 4-acylpyridine. The ADHPs derived from benzyl chloroformate are stable towards oxidation, and those derived from phenyl chloroformate hydrolyze readily.

Soft Fluorination of 4-Alkylpyridines

We report the mild and selective mono- and difluorination of 4-alkylpyridines. The process involves soft-dearomatization of pyridines to the corresponding alkylidene dihydropyridines and treatment with Selectfluor. The reaction tolerates a broad range of functional groups, including those bearing acidic and weak C-H bonds. In addition, selective fluorination of 4-alkylpyridines attached to 2-alkylpyridines and 2-alkylpyrimidines can be achieved in good yields, but a 4-alkylpyridine tethered to a 4-alkylpyrimidine is fluorinated at both heterobenzylic positions.

Palladium-catalyzed allylation of 2- and 4-alkylpyridines via N-allyl alkylidene dihydropyridine intermediates

A method to introduce allyl or cinnamyl groups to the picolyl positions of 2- or 4-alkylpyridines is described. Substituted N-allyl pyridinium salts are first treated with base (KOtBu) followed by catalytic [(η3-allyl)PdCl]2 and PPh3 to result in formal Pd-catalyzed transfer of N-allyl groups to the pyridine periphery. The reaction is believed to proceed through initial formation of nucleophilic alkylidene dihydropyridine intermediates that react with (π-allyl)Pd(II) electrophiles, thereby regenerating N-allyl pyridinium cations. Catalytic turnover and liberation of pyridine products is then achieved by oxidative addition of Pd(0) to these activated allyl groups.

Polar Heterobenzylic C(sp3)-H Chlorination Pathway Enabling Efficient Diversification of Aromatic Nitrogen Heterocycles

Site-selective radical reactions of benzylic C-H bonds are now highly effective methods for C(sp3-H) functionalization and cross-coupling. The existing methods, however, are often ineffective with heterobenzylic C-H bonds in alkyl-substituted pyridines and related aromatic heterocycles that are prominently featured in pharmaceuticals and agrochemicals. Here, we report new synthetic methods that leverage polar, rather than radical, reaction pathways to enable the selective heterobenzylic C-H chlorination of 2- and 4-alkyl-substituted pyridines and other heterocycles. Catalytic activation of the substrate with trifluoromethanesulfonyl chloride promotes the formation of enamine tautomers that react readily with electrophilic chlorination reagents. The resulting heterobenzyl chlorides can be used without isolation or purification in nucleophilic coupling reactions. This chlorination-diversification sequence provides an efficient strategy to achieve heterobenzylic C-H cross-coupling with aliphatic amines and a diverse collection of azoles, among other coupling partners.

Site-Selective Pyridylic C-H Functionalization by Photocatalytic Radical Cascades

An efficient pyridylic C(sp³ )-H functionalization has been developed through photocatalytic radical-mediated fluoroalkylation or cascade reactions. This method is enabled by the reversible formation of alkylidene dihydropyridine intermediates via the facile enolate formation of C4-alkyl N-amidopyridinium salts in the absence of an external base, thereby establishing the conditions necessary for subsequent intermolecular radical trapping. Rapid structural diversification of the pyridylic site can be achieved through photocatalytic multicomponent cascade reactions involving alkene trifluoromethylation, SO₂ -reincorporation, and sulfonyl radical addition. This operationally simple method features a broad substrate scope and high chemoselectivity and offers a unique approach for the rational modification of the heterobenzylic C-H bonds of pyridines and quinolines with uniform site-selective control. Furthermore, experimental and theoretical studies were performed to elucidate the reaction mechanism.

Iridium-Catalyzed Asymmetric Allylic Substitution of Methyl Azaarenes

Herein, an Ir-catalyzed asymmetric allylic substitution reaction of methyl azaarenes is described. Azaarenes such as (benzo)thiazole, oxazole, benzoimidazole, pyridine, and (iso)quinoline are all tolerated. The corresponding chiral azaarene derivatives are obtained in good yields with high enantioselectivity (up to 96 % yield and 99 % ee). The utilization of the Knochel reagent TMPZnBr⋅LiBr warrants the in situ formation of benzylic nucleophiles without additional activating reagents. ¹ H NMR studies suggested a two-fold function of the Knochel reagent in this reaction. The synthetic utility of this method has been showcased by a concise enantioselective synthesis of an allosteric protein kinase modulator.

Alkylidene Dihydropyridines Are Surrogates for Pyridylic Anions in the Conjugate Addition to α,β-Unsaturated Ketones

We show that alkylidene dihydropyridines, readily prepared from 4-alkylpyridines, behave as soft nucleophiles toward a range of α,β-unsaturated ketones under the influence of silyl Lewis acids to give the products of conjugate addition. In contrast to existing methods, which use strongly basic pyridylic anions, this reaction tolerates a wide array of functional groups, providing access to useful heterocyclic scaffolds.