Regioselective alkylation of a versatile indazole: Electrophile scope and mechanistic insights from density functional theory calculations
Published in Beilstein Journal of Organic Chemistry, 2024, 20, 1940–1954 | DOI: 10.3762/bjoc.20.170
🔬 Key Takeaway: This study establishes highly regioselective N1– and N2-alkylations of methyl 5-bromo-1H-indazole-3-carboxylate (compound 6) using two complementary conditions. Over 30 alkylated products were isolated in >90% yield with excellent regiocontrol. DFT calculations revealed that Cs⁺ chelation drives N1-selectivity (Conditions A), while non-covalent interactions (NCIs) with the Mitsunobu intermediate drive N2-selectivity (Conditions B).
📄 Abstract
Herein, we report a pair of regioselective N1- and N2-alkylations of a versatile indazole, methyl 5-bromo-1H-indazole-3-carboxylate (6), and the use of density functional theory (DFT) to evaluate their mechanisms. Over thirty N1- and N2-alkylated products were isolated in over 90% yield regardless of the conditions. DFT calculations suggest a chelation mechanism produces the N1-substituted products when cesium is present and other non-covalent interactions (NCIs) drive the N2-product formation. Methyl 1H-indazole-7-carboxylate (18) and 1H-indazole-3-carbonitrile (21) were also subjected to the reaction conditions and their mechanisms were evaluated via natural bond orbital (NBO) analyses.
🧪 Two Complementary Alkylation Conditions
The key innovation: the same starting material (compound 6) can be selectively alkylated at either N1 or N2 simply by changing the reaction conditions.
| Condition | Target | Reagents | Solvent | Temp | Time | Yield | N1:N2 |
|---|---|---|---|---|---|---|---|
| A | N1-alkylation | ROTs (1.5 equiv), Cs₂CO₃ (2 equiv) | Dioxane | 90°C | 2 h | 90–96% | >7.5:1 |
| B | N2-alkylation | ROH (2 equiv), DEAD (2 equiv), PPh₃ (2 equiv) | THF | 50°C | 2 h | 90–97% | N2-selective |
🔬 Substrate Scope Highlights
| R Group | Cond A Yield (N1) | N1/N2 Ratio | Cond B Yield (N2) | N2/N1 Ratio |
|---|---|---|---|---|
| Methyl (CH₃) | 90% (15a) | 7.5:1 | 92% (16a) | 4.0:1 |
| Ethyl | 96% (15b) | 12.5:1 | 93% (16b) | 2.0:1 |
| Benzyl | 94% (15d) | 13.1:1 | 92% (16d) | 3.9:1 |
| Cyclopropylmethyl | 95% (15e) | 9.5:1 | 97% (16e) | 8.2:1 |
| Cyclohexyl | 96% (15f) | 13.8:1 | 93% (16f) | 5.1:1 |
| THP (tetrahydropyran) | 94% (15n) | >99:1 | 96% (16n) | 8.7:1 |
| Adamantyl-CH₂ | 95% (15p) | 15.8:1 | 95% (16p) | 4.9:1 |
| S-THF | 94% (15j) | 14.8:1 | 91% (16j) | 8.0:1 |
| R-THF | 96% (15k) | 12.4:1 | 97% (16k) | 3.9:1 |
⚛️ Mechanistic Insights
Condition A (Cs₂CO₃/Dioxane): DFT calculations (SMD(THF)-PBE0/def2-TZVP) revealed that Cs⁺ forms a chelate between the indazole N2 atom and the carbonyl oxygen of the C-3 ester. This five-membered chelate positions the electrophile for preferential attack at N1, with the N1-s-cis transition state being 2.1 kcal/mol lower in energy than the N2-pathway.
Condition B (Mitsunobu): Without Cs⁺ present, the reaction proceeds through non-covalent interactions (NCIs) between the MeOPPh₃⁺ intermediate and the indazole anion. The TS leading to the N2-product is 1.1 kcal/mol lower than the N1-analog, driven by NCIs between the carbonyl oxygen and the PPh₃ benzene ring, plus a hydrogen bond between the methyl electrophile and the ester oxygen.
Validation via probe substrates: Testing with methyl 1H-indazole-7-carboxylate (18) confirmed the chelation hypothesis — the regioselectivity reversed as predicted (N2-product under Cond A, N1-product under Cond B). Testing with 1H-indazole-3-carbonitrile (21), which cannot form the bidentate chelate, produced only the N1-product under both conditions, consistent with intrinsic nucleophilicity (Fukui index analysis).
💡 Significance
This is the first comprehensive DFT mechanistic study on regioselective indazole alkylation. The work demonstrates that methyl 5-bromo-1H-indazole-3-carboxylate can serve as a versatile common intermediate for accessing both N1- and N2-substituted indazoles in excellent yields from the same carbon sources through simple reagent control. This synthetic platform has direct applications in medicinal chemistry, particularly for the preparation of indazole-containing bioactive molecules such as danicopan, pazopanib, and CPI-637 analogs.
🔗 Related Products
- 5-Bromo-1H-indazole-3-carboxylic acid (CAS 1077-94-7) — the free acid form of compound 6. Inquire for pricing.
- 1H-Indazole-3-carboxylic acid (CAS 4498-67-3) — the parent core structure. Inquire for pricing.
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📖 Reference
Lu P, Juarez L, Wiget PA, Zhang W, Raman K, Kotian PL. Regioselective alkylation of a versatile indazole: Electrophile scope and mechanistic insights from density functional theory calculations. Beilstein J Org Chem. 2024 Aug 9;20:1940-1954. DOI: 10.3762/bjoc.20.170 | PMCID: PMC11318628
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