1. Oxidative Dearomatization of Pyridines and Limitations in Heteroatom Functionalization

1.1 Synthetic Value of Pyridine Dearomatization

Pyridines and piperidines are widely found in pharmaceuticals, natural products, and other biologically active molecules. Pyridine dearomatization enables stable, readily available aromatic heterocycles to be used directly to form new functional groups and stereocenters while reducing ring unsaturation, making it an important strategy for constructing functionalized piperidines.[1,2]

Classical pyridine dearomatization is based primarily on hydrogenation of the aromatic ring and nucleophilic addition to activated pyridinium species. In recent years, metal-catalyzed, photochemical, and asymmetric dearomatization methods have further expanded the types of available transformations, but many approaches still focus mainly on forming C-H or C-C bonds. By comparison, methods that directly form heteroatom bonds such as C-O and C-N while disrupting pyridine aromaticity remain relatively limited.[1,2]

As a result, the synthesis of polyfunctionalized piperidines often requires construction of the carbon framework first, followed by stepwise oxidation or transformation of nitrogen-containing functional groups. Direct conversion of pyridines into oxygenated dihydropyridines, followed by further functionalization of the remaining unsaturation, could substantially alter the synthetic sequence used to access these heterocycles.

1.2 The Conflict Between Aromaticity and Alkene Oxidation

Conventional alkenes can form C-O bonds through established methods such as osmium tetroxide (OsO₄) dihydroxylation and metal-catalyzed epoxidation. In pyridine, however, the π electrons are part of an aromatic conjugated system, and the corresponding bonds cannot simply be treated as isolated alkenes.

Dearomatization requires disruption of aromatic stabilization while avoiding nonselective oxidation of the substrate or product. Simply increasing oxidant reactivity may lead to pyridine N-oxidation, side-chain oxidation, and further reaction of the dearomatized products.

Biocatalytic studies have shown that certain nitrogen-containing aromatic heterocycles can undergo dioxygenation. For pyridine rings, however, hydroxypyridine products are usually observed experimentally, whereas the corresponding cis-dihydroxydihydropyridines are thought to undergo rapid elimination and rearomatization. Siddiqi and co-workers likewise identified the stability of these intermediates as one of the central challenges in establishing a method for the oxidative dearomatization of pyridines in 2024.[1,3,7]

This gives rise to two sequential questions:

How can an aromatic pyridine be converted into a structure that can undergo conventional alkene oxidation, and, after oxidation, how can the oxygenated dihydropyridine be released and used without rearomatization?

The study addressed both questions through temporary dearomatization mediated by N-methyl-1,2,4-triazoline-3,5-dione (MTAD).[1]

2. MTAD Photochemical para-Cycloaddition and Temporary Dearomatization

2.1 MTAD Forms a Bicyclic Alkene Amenable to Further Oxidation

MTAD is a highly reactive π electrophile of the triazolinedione class and can also serve as an arenophile in the photochemical dearomatization of aromatic systems.[4,5]

The arenophile-mediated dearomatization platform previously established by the Sarlah group consists of three consecutive steps:

Aromatic substrate

↓ Arenophile photocycloaddition

Nonaromatic bicyclic alkene

↓ Alkene functionalization

Functionalized bicyclic intermediate

↓ Cycloreversion

Functionalized nonaromatic product

The arenophile-mediated dihydroxylation of arenes reported in 2016 demonstrated that a visible-light-excited small-molecule arenophile can undergo para-cycloaddition with simple arenes, converting π bonds that were originally part of an aromatic system into localized C=C bonds amenable to alkene reactions.[4,5] A 2020 study further extended this pattern to the synthesis of arene oxides and oxepines.[6]

The 2024 pyridine study followed the same reaction design. Because the electronic properties of pyridines differ substantially from those of simple arenes, however, the initial MTAD cycloaddition became the key step determining whether the method would be viable.[1]

2.2 C2 Substitution Is an Important Structural Requirement for Effective Photocycloaddition

The researchers first examined the photochemical reactions of different pyridine substrates with MTAD. Unsubstituted pyridine did not effectively undergo the targeted para-cycloaddition; further screening showed that the presence of a substituent at the C2 position of pyridine was an important structural requirement for promoting the MTAD [4+2] photocycloaddition.[1]

The study then used 2-phenylpyridine as the model substrate. Under low-temperature visible-light conditions, 2-phenylpyridine and MTAD formed a nonaromatic bicyclic cycloadduct. During optimization, the reaction performed well when 2-phenylpyridine was used in a threefold amount relative to MTAD; reducing the amount of pyridine prolonged the reaction time and lowered the yield.[1]

This result defines the principal substrate class of the current method: the reaction accommodates a range of C2-substituted pyridines, whereas unsubstituted pyridine exhibits insufficient reactivity in the photocycloaddition.

2.3 Transient Cycloaddition Provides an Installable and Removable Covalent Activation Structure

The cycloaddition structure formed by MTAD is not part of the final product. This covalent structure can be removed by cycloreversion after subsequent functionalization is complete; this process does not mean that pyridine and MTAD establish a simple, rapid thermodynamic equilibrium under the reaction conditions.

The structural role of MTAD throughout the reaction can be divided into three stages:

This mode differs from conventional activation through N-alkylation or N-acylation of pyridinium species. The starting C2-substituted pyridines do not require prior conversion into pyridinium salts, although the reaction still proceeds through well-defined temporary covalent activation and dearomatization.[1]

3. cis-Dihydroxylation of the Bicyclic Alkene

3.1 One-Pot Photocycloaddition and Dihydroxylation

After MTAD photocycloaddition, the original aromatic π system of pyridine has been converted into a conformationally constrained, nonaromatic bicyclic alkene. The subsequent oxidation targets this localized C=C bond rather than directly oxidizing the aromatic pyridine.

The study designed the photocycloaddition and dihydroxylation as a two-step, one-pot sequence. The model system employed OsO₄-catalyzed dihydroxylation, with N-methylmorpholine N-oxide (NMO) as the co-oxidant and acetone/water as the reaction medium.[1]

Compared with the previous arenophile-mediated dihydroxylation of simple arenes, the pyridine-derived bicyclic intermediate required reoptimization of the oxidation conditions. Citric acid was an important variable: no target dihydroxylation product was detected in its absence, whereas two equivalents of citric acid gave a favorable result. Reaction concentration, temperature, and the amount of OsO₄ also affected the yield.[1]

Under the optimized conditions, dihydroxylation product 3a derived from 2-phenylpyridine was isolated in 58% yield as a single constitutional isomer and a single diastereomer.[1]

3.2 Bicyclic Structure and Oxidation Selectivity

The bicyclic alkene generated by MTAD cycloaddition has a constrained conformation, placing the two faces of the double bond in different steric environments. Across the substrate scope examined, the dihydroxylation products formed as single constitutional isomers and single diastereomers. This experimental result is consistent with control of facial selectivity by the bicyclic structure.[1]

The structure formed by MTAD therefore serves two functions:

① It temporarily interrupts the aromatic π system of pyridine, enabling oxidation of a localized alkene;

② It establishes a conformationally constrained bicyclic environment that provides pronounced regioselectivity and diastereoselectivity in the subsequent dihydroxylation.

3.3 Substrate Scope of the Dihydroxylation

The two-step, one-pot dihydroxylation is applicable to a variety of C2-substituted pyridines. The principal structural classes covered in the original study include the following:[1]

The study also included scale-up experiments, in which the corresponding substrates showed only a slight decrease in yield on a larger reaction scale.[1]

These results indicate that, within the reaction conditions and substrate range examined, C2 substitution is closely associated with effective photocycloaddition. A range of electron-withdrawing or halogen substituents on the aryl group, as well as certain alkyl, alkoxy, and ester functionalities, are compatible with the subsequent oxidation conditions.

4. Epoxidation and the Pyridine Oxide-1,4-Oxazepine System

4.1 Mn-Catalyzed Epoxidation Provides a Second Oxidation Pathway

In addition to cis-dihydroxylation, the MTAD-derived bicyclic alkene can undergo epoxidation.

The study employed a manganese-catalyzed system comprising manganese(II) perchlorate, picolinic acid, and peracetic acid to epoxidize the bicyclic alkene. The optimized system required a substantially higher manganese catalyst loading than that used in the previous epoxidation of simple arenes, with typical conditions employing approximately 25 mol% manganese(II) perchlorate.[1,6]

The authors proposed that the high catalyst loading may be associated with two factors:

Epoxidation of the pyridine-derived bicyclic alkene may itself be challenging;

Excess pyridine remaining after the photocycloaddition may competitively coordinate to the manganese center.

4.2 Epoxidation Substrate Scope and Chemoselectivity

The epoxidation route successfully provided more than a dozen pyridine oxides. Compatible substrates included:

① 2-Phenylpyridine and several 2-arylpyridines bearing chloro, nitro, or cyano substituents;

② 2,2′-Bipyridine and 2,3′-bipyridine;

③ Various 2-alkylpyridines;

④ 2-Methoxypyridine and its substituted derivatives.[1]

For bipyridine substrates, both the photocycloaddition and the subsequent epoxidation selectively occurred on the C2-substituted pyridine ring that met the structural requirements of the reaction.

The epoxidation products in the study were obtained as single constitutional isomers and single diastereomers with an anti relative configuration. The structure of the model bicyclic epoxide was also confirmed by X-ray crystallography. No N-oxidation of the pyridine nitrogen was observed throughout the process.[1]

2-Isopropylpyridine revealed a clear chemoselectivity limitation: photoexcited MTAD could undergo competitive benzylic C-H amination at its side chain. Substrates containing similarly reactive C-H sites must therefore be evaluated with consideration of the intrinsic photochemical reactivity of MTAD.[1]

The original study noted that, compared with previous five- to six-step routes to pyridine oxides, the arenophile strategy shortened the preparation and expanded the substrate range that could be examined.[1]

4.3 Pyridine Ring Oxides Are Not Pyridine N-Oxides

The pyridine ring oxides (pyridine oxides) obtained after cycloreversion are structurally distinct from conventional pyridine N-oxides.

4.4 Valence-Isomeric Equilibrium Between Pyridine Ring Oxides and 1,4-Oxazepines

Arene oxides and oxepines can interconvert through valence isomerization. In 2020, the Sarlah group used an arenophile strategy to establish a method for synthesizing arene oxides and oxepines.[6]

The pyridine system exhibits the corresponding relationship:

Pyridine ring oxide ⇌ 1,4-oxazepine

This process is a valence isomerization involving reorganization of the σ and π bonds between the epoxide structure and the seven-membered heterocycle; it is not an ordinary proton-transfer tautomerization.[1,6]

The two valence isomers have different reactive sites, allowing the same dearomatized product to enter different downstream reactions.

For example, 6-phenylpyridine 3,4-oxide can react with nitrosobenzene without prior purification to form a bicyclic hydroxylamine derivative. Subsequent reduction of the N-O bond and the aminal structure affords an aminohydroxylated piperidine. A different compound, 5-methoxy-7-phenyl-1,4-oxazepine, can be converted by hydrogenolysis into a partially reduced dihydro-1,4-oxazepine.[1]

5. MTAD Cycloreversion and the Construction of Polyfunctionalized Piperidines

5.1 cis-Diols Require Stabilization Before Removal of the MTAD-Derived Structure

After dihydroxylation, direct release of the free cis-dihydroxydihydropyridine can lead readily to elimination and rearomatization. The study therefore first converted the bicyclic cis-diol into its acetonide-protected form before carrying out a mild cycloreversion.[1]

The principal sequence is as follows:

MTAD-derived bicyclic cis-diol

↓ Acetonide protection

Protected bicyclic diol

↓ Partial hydrolysis of the urazole

↓ Nickel peroxide-mediated cycloreversion

Acetonide derivative of a cis-dihydroxydihydropyridine

This method was successfully applied to several representative diol substrates.

The dihydroxylation product derived from 6-methyl-2-methoxypyridine was particularly sensitive to purification. Rather than subjecting this product to conventional isolation, the study converted it directly into the acetonide-protected form before continuing to cycloreversion. This handling procedure prevented aromatization of the sensitive intermediate during isolation.[1]

The acetonide derivatives of cis-dihydroxydihydropyridines obtained after cycloreversion remain unsuitable for conventional workup, purification, or long-term storage, but they can immediately enter subsequent functionalization reactions. The synthetic value of these structures therefore lies mainly in their in situ generation and sequential use rather than in their long-term storage as stable intermediates.

5.2 Reduction to Dihydroxylated Piperidines

Several cis-dihydroxydihydropyridines can be reduced directly with sodium borohydride (NaBH₄) to give the corresponding dihydroxylated piperidines 6a, 6b, and 6f.[1]

The overall structural transformation can be represented as follows:

C2-substituted pyridine → MTAD-mediated temporary dearomatization → cis-dihydroxylation → cycloreversion → alkene reduction → dihydroxylated piperidine

The oxygen functionalities are installed in a single operation during dearomatization of the aromatic heterocycle rather than through separate C-H oxidations after formation of the saturated piperidine.

5.3 Different Reduction and Nucleophilic Addition Conditions Produce Distinct Functionalization Patterns

An ionic reduction system comprising trifluoroacetic acid (TFA) and triethylsilane converts a cis-dihydroxydihydropyridine into monohydroxylated imine product 7.[1]

cis-Dihydroxydihydropyridine 5a can also react with a Grignard reagent in the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf) to form double-addition product 8. The authors proposed that this transformation may proceed through sequential SN2′ substitution and imine addition.[1]

These reactions demonstrate that the dihydropyridines generated by oxidative dearomatization are not merely reduction intermediates. Their remaining double bonds and nitrogen-containing structures can undergo further ionic reduction or C-C bond formation, allowing a single dearomatization event to be translated into multiple stereocenters and functional groups.

5.4 Nitroso Cycloaddition Provides Additional C-N Bond Formation

The acetonide derivatives of cis-dihydroxydihydropyridines can also undergo highly regioselective and highly diastereoselective cycloaddition with nitrosobenzene.

The corresponding bicyclic products can be further converted into functionalized piperidines 9 and 10 through CuCl-mediated N-O bond cleavage or hydrogenolysis.[1]

This pathway first establishes C-O bonds through oxidative dearomatization and then uses the remaining unsaturation to form new nitrogen-containing functional groups, sequentially converting a simple pyridine framework into a saturated heterocycle containing multiple C-O and C-N bonds.

5.5 MTAD-Derived Structures Can Also Be Used Through Reductive Cleavage

The MTAD-derived structure does not necessarily have to be completely removed by cycloreversion.

The researchers established an alternative route in which acetonide-protected bicyclic intermediates 4a or 4h were treated with lithium aluminum hydride (LiAlH₄) and then Raney Ni to afford aminohydroxylated piperidines 11 and 12.[1]

MTAD-derived structures therefore have two distinct downstream processing modes:

6. Substrate Scope, Chemoselectivity, and Method Limitations

The method demonstrates that an arenophile strategy can provide direct access from pyridines to cis-diols, ring oxides, and polyfunctionalized piperidines, while the experimental results also define clear conditions for its applicability.

7. Future Research Directions Arising from Current Structural Limitations

(1) Reducing Dependence on C2 Substitution

The initial MTAD photocycloaddition is one of the main steps currently limiting expansion of the substrate scope. Altering the electronic properties, steric structure, or excited-state characteristics of the arenophile could be used to test the cycloaddition feasibility of unsubstituted pyridine and pyridines with different substitution patterns.

(2) Expanding the Types of Subsequent Alkene Functionalization

The current study has demonstrated cis-dihydroxylation and epoxidation. If other alkene functionalizations are compatible with the MTAD-derived bicyclic intermediate and permit subsequent cycloreversion, transformations such as halogenation, aminohydroxylation, diamination, and borylation merit further investigation.

The central requirements for this direction are whether the target reaction can selectively act on the MTAD-derived bicyclic intermediate and whether the installed functional groups can tolerate the subsequent cycloreversion.

(3) Establishing Enantioselective Control

The existing reactions exhibit high constitutional selectivity and diastereoselectivity, indicating that the bicyclic intermediate can effectively differentiate its two faces, but the products generally still lack enantioselective control.

Whether chiral arenophiles, chiral oxidation catalysts, or other modes of asymmetric induction can provide additional control over absolute configuration must be evaluated separately for the initial cycloaddition and the subsequent alkene oxidation.

(4) Shortening the Lifetime of Sensitive Intermediates

cis-Dihydroxydihydropyridines and their protected derivatives have limited stability. Directly coupling cycloreversion with reduction, nucleophilic addition, or cycloaddition could reduce the accumulated concentration of sensitive intermediates and decrease the number of isolation steps.

8. Representative Chemicals Related to MTAD-Mediated Oxidative Dearomatization of Pyridines: Substrates, Oxidation/Cycloreversion Systems, and Downstream Functionalization Reagents

Table 1. Pyridine Substrates and Structural Reactivity Studies

Table 2. Representative Auxiliary Reagents Related to Dihydroxylation, Epoxidation, and Cycloreversion

Table 3. Reagents for Reduction, C-C/C-N Bond Formation, and Derivatization of Dihydropyridines and Epoxidation Products

Note: The products listed above are representative Aladdin products relevant to scientific research. Their specific uses should be determined with reference to product specifications, batch COAs, and the intended reaction or evaluation system. Additional information on product specifications, grades, and COAs can be found by searching the Aladdin website by product name, CAS number, or catalog number.

References

[1] Siddiqi Z.; Bingham T. W.; Shimakawa T.; Hesp K. D.; Shavnya A.; Sarlah D. Oxidative Dearomatization of Pyridines. J. Am. Chem. Soc. 2024, 146, 2358–2363. DOI: 10.1021/jacs.3c13603.

[2] Escolano M.; Gaviña D.; Alzuet-Piña G.; Díaz-Oltra S.; Sánchez-Roselló M.; del Pozo C. Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines. Chem. Rev. 2024, 124, 1122–1246. DOI: 10.1021/acs.chemrev.3c00625.

[3] Boyd D. R.; Sharma N. D.; Modyanova L. V.; Carroll J. G.; Malone J. F.; Allen C. C. R.; Hamilton J. T. G.; Gibson D. T.; Parales R. E.; Dalton H. Dioxygenase-Catalyzed cis-Dihydroxylation of Pyridine-Ring Systems. Can. J. Chem. 2002, 80, 589–600. DOI: 10.1139/V02-062.

[4] Southgate E. H.; Pospech J.; Fu J.; Holycross D. R.; Sarlah D. Dearomative Dihydroxylation with Arenophiles. Nat. Chem. 2016, 8, 922–928. DOI: 10.1038/nchem.2594.

[5] Okumura M.; Sarlah D. Arenophile-Mediated Dearomative Functionalization Strategies. Synlett 2018, 29, 845–855. DOI: 10.1055/s-0036-1591940.

[6] Siddiqi Z.; Wertjes W. C.; Sarlah D. Chemical Equivalent of Arene Monooxygenases: Dearomative Synthesis of Arene Oxides and Oxepines. J. Am. Chem. Soc. 2020, 142, 10125–10131. DOI: 10.1021/jacs.0c02724.

[7] Garrett M. D.; Scott R.; Sheldrake G. N.; Dalton H.; Goode P. Biotransformation of Substituted Pyridines with Dioxygenase-Containing Microorganisms. Org. Biomol. Chem. 2006, 4, 2710–2715. DOI: 10.1039/B606113C.

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