Pd-Catalyzed Asymmetric Dearomative Heck/Tsuji-Trost Difunctionalization of Naphthalenes
Created on 07.24
1. English Official Research Introduction (Global Website Version)
Aromatic compounds are abundant fundamental feedstocks in organic synthesis and medicinal chemistry. Driven by the modern drug design concept of “escaping planar molecules”, dearomatization reactions have emerged as a core strategy to construct three-dimensional sp³-rich chiral molecular skeletons. Compared with electron-rich heteroarenes and activated aromatics such as indoles and naphthols, unactivated neutral aromatics including simple naphthalene and benzene possess higher aromatic stabilization energy, making their dearomative transformation a long-standing synthetic challenge.
In recent years, research teams led by Shu-Li You, Yi-Xia Jia, Xin-Jun Luan, Yamaguchi and other scholars have made breakthroughs and reported a series of dearomatization systems for naphthalene and benzene, covering 1,2-addition and 1,4-addition dearomative pathways. Very recently, the group of Professors Junliang Zhang and Zhan-Ming Zhang at Fudan University developed a palladium-catalyzed asymmetric dearomative tandem Heck/Tsuji-Trost 1,4-difunctionalization of naphthalenes.
The team designed a sterically bulky chiral sulfinamide phosphine Sadphos ligand M4, which precisely controls regioselectivity, diastereoselectivity and enantioselectivity via a steric shielding effect. This catalytic system tolerates both alkyl amines and aryl amines as nucleophiles, delivering a library of 4-amino spirocyclohexenyl oxindole derivatives bearing two remote chiral centers in moderate to excellent yields with up to 97% ee. The methodology features broad substrate compatibility and enables asymmetric derivatization of drug core scaffolds including coumarin, cytisine and paroxetine.
The reaction is scalable to gram scale, and the obtained products can undergo diverse orthogonal transformations at alkene, amide and amine sites to access complex functional molecules, demonstrating excellent synthetic practicability. Combined with kinetic studies, nonlinear effect analysis and control experiments, a plausible catalytic cycle is proposed, providing new synthetic tactics and mechanistic insights for asymmetric dearomative difunctionalization of unactivated arenes.
2. Core Advantages of Ming-Phos Ligand M4
- Powerful Stereocontrol Capability
Iterative structural optimization to build bulky M4 ligand with fused phenanthrene and tert-butyl groups generates prominent steric shielding, achieving a maximum enantioselectivity of 97% ee and enabling the precise construction of dual remote chiral centers from naphthalene dearomatization.
- Superior Chemo- and Regioselectivity
Effectively suppresses competitive side reactions including amine chelation, C2 nucleophilic attack and Buchwald-Hartwig coupling; exclusively directs nucleophilic addition at the C4 position and drastically minimizes undesired byproducts.
- Facile Structural Modification & Tunable Performance
The Ming-Phos scaffold is easy to derivatize; enantioselectivity can be gradually elevated by simply adjusting the steric bulk of aromatic substituents.
- Extensive Substrate Compatibility
Compatible with alkyl amines and various substituted anilines; enables asymmetric late-stage modification of pharmaceutical backbones such as coumarin and paroxetine, well-suited for pharmaceutical intermediate synthesis.
3. Catalytic System & Condition Screening
Standard Reaction Conditions
Pd Precursor: [7-PhC₃H₄PdCl₂] (2.5 mol%)Chiral Ligand: M4 (7 mol%)Additives: Ag₃PO₄, Na₂HPO₄·12H₂OSolvent: DCM, 80 °CKey screening conclusions:
- Pd(OAc)₂ delivers optimal yield and enantioselectivity among all palladium sources; Pd(TFA)₂, Pd₂(dba)₃ lead to obvious yield loss.
- Base screening: NaHCO₃, KF, CH₃COOLi exhibit good performance; DBU causes severe reaction inhibition.
- Solvent screening: DCM, MBTE, CH₃CN afford high yields and ee values; toluene, EA show inferior reactivity.
- Temperature optimization: 80–90 °C balances reaction efficiency and stereoselectivity; higher temperature (120 °C) erodes enantiocontrol.
4. Substrate Scope
A. Alkyl Amine Nucleophiles
Linear/branched alkyl amines, Boc-protected amines, benzyl amines, cycloalkyl amines are well tolerated; most substrates furnish 62%–95% yield with >90% ee.
B. Substituted Aniline Nucleophiles
Anilines bearing electron-donating (Me, OMe, SMe) and electron-withdrawing (F, Cl, CF₃, CN) substituents perform smoothly; single-crystal X-ray diffraction confirms the absolute configuration of representative product 3g.
C. Naphthalene Substrates
Naphthalene cores substituted with methyl, methoxy, chloro, bromo, ester and benzyl groups are compatible, delivering target spirocyclic products with good yields and high enantioselectivity.
5. Synthetic Utility & Late-Stage Drug Modification
1) Derivatization of Bioactive Molecules
The protocol achieves asymmetric modification of multiple marketed drug skeletons:
- Coumarin core derivative (5a, 88% yield, 95% ee)
- Cytisine derivative (5b, 90% yield, dr = 15:1)
- Norquetiapine derivative (5c, 82% yield, 94% ee)
- Paroxetine derivative (5d, 78% yield, dr >20:1)
2) Gram-Scale Synthesis & Downstream Transformations
- 1.22 g gram-scale reaction retains high yield and enantioselectivity without attenuation.
- Diversified derivatization routes of chiral spirocyclic products:
- Pd/C hydrogenation to reduce olefin double bonds;
- LiAlH₄ reduction of amide groups;
- Lawesson’s reagent-mediated thionation of carbonyls;
- Nucleophilic substitution via deprotonation of secondary amines to introduce acyl, alkyl and benzyl substituents;
- Reductive cyclization to construct fused polycyclic nitrogen-containing frameworks.
6. Mechanism Investigation
- Kinetic & Nonlinear Effect Experiments
Kinetic profiles and nonlinear effect curves clarify the reaction order of each substrate and the aggregation state of the chiral ligand-palladium complex in the catalytic cycle.
- Proposed Catalytic Cycle
- Oxidative addition of Pd(0) to naphthalene iodide substrate generates aryl-Pd(II) intermediate Int-1.
- Ligand M4 provides large steric hindrance to shield the C2 site, guiding migratory insertion toward the desired C4 position to form key intermediate Int-3 (suppressing undesired Int-2’).
- Amine nucleophile selectively attacks the C4 carbon center of Int-3 to form Int-4.
- Base-mediated reductive elimination releases the final chiral product and regenerates Pd(0) catalyst.
- Undesired pathways (nucleophilic attack at C2, amine chelation to Pd center) generate trace byproducts B/C/D, which are effectively suppressed by M4’s steric shielding effect.
7. Literature References for Ming-Phos (Selected Representative Works)
- Fang C., Ai J., Wang Q., Xu B., Zhang J., Zhang Z.-M. Nat. Commun.
- Li W., Zhang, J. Acc. Chem. Res.
- Wang J., Xu B., Wang Y., Xia G., Zhang Z.-M., Zhang J. J. Am. Chem. Soc.
- Zhou L., Li S., Xu B., Ji D., Wu L., Liu Y., Zhang Z.-M., Zhang J. Angew. Chem. Int. Ed.
- Zhang Z.-M., Xu B., Xu S., Wu H.-H., Zhang J. Angew. Chem. Int. Ed.
- Zhang Z.-M., Chen P., Li W., Niu Y., Zhao X.-L., Zhang J. Angew. Chem. Int. Ed.
8. Commercial Supply Notice (Global Official Website)
Kaitailai Platinum (Anhui) Co., Ltd. stocks more than 100 structurally diverse Ming-Phos chiral ligands including the sterically hindered M4 ligand used in this study. Custom synthesis services for novel Sadphos/Ming-Phos derivatives are available for academic laboratories and industrial R&D teams worldwide. Global cooperation and inquiry are warmly welcomed.