A analysis group led by Ji-Min Yang from the School of Chemical Sciences, College of Chinese language Academy of Sciences, reported a novel class of monophosphine ligands primarily based on the norbornene skeleton. This work introduces the design idea of “three-dimensional (3D) isosteric benzene rings” from medicinal chemistry into phosphine ligand growth, utilizing inflexible norbornene to simulate the benzene ring core in conventional biarylphosphines. Whereas sustaining key geometric options, this strategy endows the ligand with superior stereo and digital properties. The catalytic system primarily based on this ligand achieves extremely environment friendly C–N coupling with extraordinarily low palladium loadings (all the way down to 2 ppm), reaching a most conversion quantity (TON) of 490,000, the very best reported so far for one of these response. The article was revealed as an open entry Analysis Article in CCS Chemistry, the flagship journal of the Chinese language Chemical Society.
Background data:
The Buchwald–Hartwig coupling response is a crucial technique for developing C–N bonds and has huge purposes in prescribed drugs, pesticides, and practical supplies. Over the previous thirty years, monodentate biarylphosphine ligands, represented by Buchwald-type biphenyldialkylphosphine, have change into one of the profitable ligand techniques for this response on account of their average steric hindrance and good electron-donating capability.
In the meantime, the “escape the airplane” technique (i.e., changing the planar fragrant ring with a saturated three-dimensional framework) has been broadly utilized in medicinal chemistry and supplies science. Impressed by this, the group proposed the next query: Can this idea be utilized to phosphine ligand design? That’s, to make use of a inflexible three-dimensional saturated framework to simulate the construction and performance of the benzene ring in biarylphosphine, thereby additional enhancing the spatial regulation capability and digital properties of the ligand whereas retaining its advantageous geometric options.
Highlights of this text:
1. Modular synthesis, versatile and adjustable construction. The analysis group used brominated or iodobornene as beginning supplies to effectively assemble a sequence of norbornene skeleton monophosphine ligands (L1 – L8) in two to a few steps. This artificial route is very modular: by altering the substituents (Cy, Ph, iPr) on the phosphine atom and the aryl substituents on the norbornene skeleton, the digital and steric properties of the ligands might be simply managed, facilitating subsequent efficiency optimization.
2. Structural verification: Key geometric options of biarylphosphine had been efficiently simulated. Utilizing single-crystal X-ray diffraction and DFT theoretical calculations, the authors systematically in contrast the structural parameters of the norcamphene ligand and the classical ligand Cy-JohnPhos. The outcomes present that though the norcamphene skeleton elongates the interatomic spacing of the skeleton carbon atoms, the dihedral angle impact permits the aryl substituents to be spatially compressed, efficiently simulating the “P-aromatic ring” spatial relationship in biarylphosphine. Extra importantly, the buried quantity proportion (%Vbur = 61.7%) of the norcamphene ligand L1 is considerably higher than that of Cy-JohnPhos (56.4%), whereas L7, containing a tert-butyl group, reaches 67.3%, exhibiting a stronger spatial shielding impact. Moreover, detailed DFT conformational evaluation revealed vital variations within the conformational conduct of the norcamphene skeleton ligand and the biaryl ligand throughout catalytic biking, offering a theoretical foundation for understanding their superior efficiency (see full article for particulars).
3. Catalytic efficiency: Document-breaking TON of 490,000. The ligand L7 (containing a tert-butyl substituted fragrant ring on the 3,5-position) displays distinctive efficiency in palladium-catalyzed C–N coupling to triarylamines, reaching a TON of 490,000, the very best reported worth for this transformation so far. This ligand system will also be used for the environment friendly synthesis of assorted commercially out there triarylamine OLED molecules, sustaining extraordinarily low palladium loading even in 10-gram scale-up experiments, demonstrating promising prospects for sensible purposes.
4. Extremely environment friendly coupling underneath room temperature and delicate situations. This catalytic system not solely performs excellently underneath high-temperature reflux situations but in addition demonstrates outstanding efficiency underneath milder situations. When the temperature drops to 70°C, solely 0.01 mol% Pd is required to quantitatively convert diphenylamine and bromobenzene to triphenylamine inside 2 hours; even at room temperature, the identical conversion might be achieved inside 8 hours utilizing 2.5 mol %Pd , and it’s equally efficient for chlorobenzene. Primarily based on this, varied substrates (akin to N-methylaniline, indole, sterically hindered diarylamines, and the commercially out there OLED molecule p-TPD) can all yield the goal merchandise in excessive yields at room temperature.
5. Broad substrate scope, suitable with complicated drug molecules. Catalytic techniques primarily based on one of these ligand have extraordinarily broad substrate versatility, relevant not solely to secondary amines, sterically hindered anilines, aryl chlorides and heteroaryl substrates, but in addition efficiently utilized to the late-stage modification of assorted drug molecules akin to paroxetine, clopidogrel, amoxapine, and duloxetine, demonstrating glorious practical group compatibility and sensible software potential.
Abstract and Outlook:
In abstract, this work, primarily based on a “three-dimensional benzene ring simulation” technique, launched the norbornene skeleton into phosphine ligand design, modularly developing a novel class of single phosphine ligands. These ligands successfully simulate the important thing geometric options of biarylphosphines structurally, whereas offering stronger and extra tunable steric hindrance and superior electron-donating capability. In palladium-catalyzed C–N coupling reactions, this ligand system achieved glorious catalytic effectivity, reaching 490,000 TON within the synthesis of triarylamines. The system maintained glorious catalytic exercise even underneath varied difficult substrates and room temperature response situations.
The design idea of norcamphene skeleton is anticipated to be prolonged to different inflexible three-dimensional skeletons. The totally different bond lengths and bond angles supplied by totally different skeletons will open up extra potentialities for the design of recent ligands and catalysts sooner or later.
This work was revealed as a Analysis Article in CCS Chemistry, with Affiliate Professor Ji-Min Yang from the College of Chinese language Academy of Sciences because the corresponding creator and postdoctoral researcher Yi-Xiong Dong as the primary creator. This work was supported by the Nationwide Pure Science Basis of China.
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In regards to the journal: CCS Chemistry is the Chinese language Chemical Society’s flagship publication, established to function the preeminent worldwide chemistry journal revealed in China. It’s an English language journal that covers all areas of chemistry and the chemical sciences, together with groundbreaking ideas, mechanisms, strategies, supplies, reactions, and purposes. All articles are diamond open entry, with no charges for authors or readers. Extra data might be discovered at https://www.chinesechemsoc.org/journal/ccschem .
In regards to the Chinese language Chemical Society: The Chinese language Chemical Society (CCS) is a tutorial group fashioned by Chinese language chemists of their very own accord with the aim of uniting Chinese language chemists at house and overseas to advertise the event of chemistry in China. The CCS was based throughout a gathering of preeminent chemists in Nanjing on August 4, 1932. It presently has greater than 120,000 particular person members and 184 organizational members. There are 7 Divisions overlaying the key areas of chemistry: bodily, inorganic, natural, polymer, analytical, utilized and chemical training, in addition to 31 Commissions, together with catalysis, computational chemistry, photochemistry, electrochemistry, natural strong chemistry, environmental chemistry, and lots of different sub-fields of the chemical sciences. The CCS additionally has 10 committees, together with the Lady’s Chemists Committee and Younger Chemists Committee. Extra data might be discovered at https://www.chinesechemsoc.org/ .
