Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Amine-FUNC (Amine-Directed Diverse C(sp3)-H Functionalization)

Teaser

The development of new catalytic methods that enable the functionalization of aliphatic C–H bonds is an important challenge to the continued advance of chemical synthesis. Meanwhile, the synthesis of architecturally complex aliphatic amines remains an important challenge to...

Summary

The development of new catalytic methods that enable the functionalization of aliphatic C–H bonds is an important challenge to the continued advance of chemical synthesis. Meanwhile, the synthesis of architecturally complex aliphatic amines remains an important challenge to synthetic and medicinal chemists because their structural and functional properties are often fundamental to biological activity in many nitrogen containing molecules. Recently, palladium-catalyzed C–H functionalization of aliphatic amine derivatives has emerged as a potentially powerful tactic for the synthesis of complex variants of these important molecules. Central to the continued evolution of these synthetic strategies is the development of new activation modes and transformations on a range of aliphatic amine substrates. In fact, the direct use of the amine functionality as a directing group for cyclopalladation is less common, and most successful cases require protecting groups or auxiliaries to modulate the metal-binding properties of the nucleophilic nitrogen motif. As a result, catalytic strategies for the direct C-H activation of aliphatic amines remain underdeveloped.

In 2014, our group has been engaged in the development of C(sp3)–H activation reactions guided by the free (NH)-amine. Key to the success of this strategy has been the steric-induced destabilization of rapidly formed bisamine Pd(II) complexes, which leads to higher concentrations of the putative mono-amine Pd(II) complexes empirically required for C–H bond cleavage. As part of the evolution of this distinct C–H activation mode, this project aims to: (I) develop unprecedented diverse C(sp3)-H functionalization of amine technology, access diverse functionalized amine compounds, (II) develop novel Pd-catalyzed enantioselective C-H functionalization methods to make enantioenriched chiral molecules.

Conclusions of the action: over the last two years, we have developed a number of distinct transformations of aliphatic amine based on discrete 4- and 5-membered cyclopalladation pathways, such as C–H arylation and C–H alkenylation, which allows access to highly functionalized phenethyl amines and aliphatic N-heterocycles. By using chiral amino-acid-derived ligand, we have demonstrated the basis of a catalytic enantioselective arylation reaction. In addition, our studies also indicated that an amino-acid-derived ligand renders the C–H bond activation step reversible and promotes the traditionally difficult alkenylation process.

Work performed

1. C–H arylation project (Scheme 1)

First, stoichiometric reactions between a pre-formed amine cyclopalladation complex and various organoboron reagents were investigated. We found that the desired arylation reaction was observed with Ph-BPin in the presence of benzoquinone and Na2CO3. Based on these findings, we were able to render this transformation catalytic in Pd(OAc)2 by the addition of Ag2CO3 as terminal oxidant in combination with amino-acid-derived ligand, benzoquinone and Na2CO3. Then we found that a modest range readily available amines derivatives were suitable substrates for the C–H arylation. The reaction was also readily extended to a variety of arylboronic acid pinacol esters in good yields. Moreover, the products of the arylation can be advanced to complex polycyclic molecules by sequential C-H activation reactions. Finally, we investigated the potential for an asymmetric C–H arylation on the basis that the amino-acid-derived ligands can provide asymmetric induction. We screened a range of ligands and found that b-phenyl-phenylalanine combined with the bulky acyl side chain could give a 60% ee.

Overview of the results: a palladium-catalyzed C-H arylation of aliphatic amines with arylboronic esters is described, proceeding through a four-membered-ring cyclopalladation pathway. Crucial to the successful outcome of this reaction is the action of an amino-acid-derived ligand. A range of hindered secondary amines and arylboronic esters are compatible with this process and the products of the arylation can be advanced to complex polycyclic molecules by sequential C-H activation reactions.

2. C–H alkenylation project (Scheme 2)

The studies began with the investigation of a stoichiometric C–H alkenylation process using 3-ethyl-3,5,5-trimethylmorpholin-2-one. Its reaction with Pd(OAc)2 delivered a 3 : 1 mixture of palladacycles in favour of the 4-membered ring complex. When this mixture of palladacycles was treated with ethyl acrylate, we found that the bicyclic pyrrolidine product was formed; reaction of only the 5-membered ring palladacycle via sequential carbopalladation of the alkene, b-hydride elimination to a substituted acrylate and aza-Michael addition gave the pyrrolidine. Then we found that the use of ligand Ac-Gly-OH was reversing the cyclopalladation process, such that, in the presence of an alkene, the C–H alkenylation via the 5-membered ring palladacycle becomes a kinetic trap for the equilibrium, selectively forming pyrrolidine. So we were able to render this transformation catalytic in Pd(OAc)2 by the addition of AgOAc as terminal oxidant. Then we found that a range of functionalized a,a-disubstituted morpholinones underwent smooth ligand assisted C–H alkenylation. The reaction was also readily extended to a variety of acrylates in excellent yields. a,b-Unsaturated ketones, amides acrolein and even vinyl sulfone, vinyl phosphonate could also be incorporated into the pyrrolidine scaffold.

Overview of the results: the development of a ligand-assisted Pd-catalyzed C–H alkenylation of aliphatic amines is reported. Our studies indicated that an amino-acid-derived ligand renders the C–H bond activation step reversible and promotes the traditionally difficult alkenylation process. The C(sp3)–H alkenylation proceeds through a 5-membered-ring cyclopalladation pathway that allows access to complex aliphatic heterocycles that could be useful to practioners of synthetic and medicinal chemistry.

Final results

Aliphatic amines are central to the function of many biologically active molecules as evidenced by their prevalence in a large number of pharmaceutical agents. We believe that the developed catalytic C-H arylation and alkenylation methods provide facile and efficient processes that transform simple, readily available aliphatic amines into complex variants and will be valuable to the pharmaceutical, agrochemical and fine chemical industries. With these diverse novel amine compounds in hand, currently we were trying to screen them for biological and medicinal activities with the help of our collaborators, such as GSK, Pfizer, Syngenta, Hoffman-La-Roche, AstraZeneca, CRUK and Merck. We believe that this “cutting-edge” project would unlock the door to a range of molecules with untapped biological and medicinal potential and therefore, to establish further collaborations with the European chemical industry (crop science, pharmaceutical, medicinal chemistry etc.).

Meanwhile, the achievements and results of the project have already been disseminated through publication in high-impact scientific journals Angew. Chem. Int. Ed. and Chem. Sci., and also been presented as a talk in many international conferences. We believe that publications in these top quality journals, invited lectures will contribute to enhance EU scientific excellence and allow the beneficiary forge a successful future career.

Website & more info

More info: http://www-gaunt.ch.cam.ac.uk.