Palladium-Catalyzed trans-Hydroalkoxylation: Counterintuitive Use of an Aryl Iodide Additive to Promote C-H Bond Formation

We report an enantioselective palladium-catalyzed trans-hydroalkoxylation of propargylic amines with a trifluoroacetaldehydederived tether to build chiral oxazolidines. Diastereoselective hydrogenation using a heterogeneous palladium catalyst then gave access to protected benzylic amino alcohols in 45-87% yields and 84-94% ee values. Hydroalkoxylation of the alkynes required a catalytic amount of aryl iodide, highlighting the counterintuitive key role played by a putative Pd(II)/ArI oxidative addition complex to promote oxypalladation/protodemetalation.

Palladium Catalysed Intramolecular Carbo-oxygenation of Propargylic Amines to Access 1,2-Amino Alcohols and alpha-Amino Ketones via in situ Tether Formation

Phillip Gulliver Dominic Greenwood

a-Amino ketones and 1,2-amino alcohols are important structural motifs in organic chemistry, that can be observed in natural products, pharmaceutically and bioactive compounds. For these reasons, they constitute privileged targets for the development of new and effective synthetic methods. The dual-functionalisation of unsaturated carbon-carbon bonds is a powerful approach to access these crucial motifs. In this context, Pd catalysis has been comprehensively investigated and has consistently demonstrated its capability. Methods enabling the concomitant formation of a carbonâ heteroatom and a carbonâcarbon bond are highly valuable, but the challenges of intermolecular reactions have led to the development of tether systems that require additional steps for their introduction and removal. In situ tethering strategies have previously been explored within the Waser group to circumvent these limitations. This approach relies on a one-pot, three component, Pd catalysed carboetherification or amination reaction, leading to rapidly increase in complexity of simple substrates. So far, the in situ methodologies developed by the Waser group have focused on the functionalisation of alkenes from allylic amines and alcohols. In this regard, the goal of my PhD was to expand the in situ tethering methodologies established within the Waser group, to the synthesis of a-amino ketones and 1,2-amino alcohols. Specifically, my focus was on the formation a trifluoromethyl hemiaminal with a propargylic amine and using the resulting aminal to direct the oxyalkynylation and oxyarylation of the alkyne substrate via a Pd catalysed transformation. Upon optimisation, the reaction of a protected terminal propargylamine, trifluoroacetaldehyde ethyl hemiacetal and silylbromoacetylene using a Pd0 complex with DPEPhos as the catalytic system and Cs2CO3 as the base gave the desired oxazolidine alkenes in high yields and E:Z selectivity. Substrates with substitution on the terminal position and at the propargylic required a change of ligand and base. The transformation was broadly tolerant towards substitution on the nitrogen and the alkyne. An oxyarylative variant was also developed, using Ruphos as ligand, and worked well with a range of iodoarene partners. The obtained oxazolidines could be conveniently orthogonally deprotected to access the a-amino ketone and terminal alkyne; the hydrogenation of the unsaturated oxazolidines has allowed access to 1,2-amino alcohol scaffolds and the application of gold catalysis converted the enyne-oxazolidine system into trisubstituted silyl furans. The identification of a sideproduct of the oxyalkynylation process led to the development of a carboxy-alkynylation reaction using a "CO2" unit derived from the carbonate base. Optimisation established CsHCO3 to the best base/"CO2" source, using DPEPhos as ligand. This transformation gave convenient access to diverse library of cyclic carbamates. Work is ongoing to achieve an enantioselective oxyarylation using chiral phosphine ligands. The asymmetric synthesis of oxazolidines, followed by the diastereoselective alkene hydrogenation and subsequent tether removal would allow the stereodivergent access to interesting diarylalkyl-1,2- amino alcohols. Most recently, the product was synthesised in 66% yield, with 66%ee using commercially available Trost ligand.

EPFL2020

Suzuki-Miyaura cross-coupling of non-activated alkyl halides catalyzed by well-defined nickel pincer complexes

Thomas Clément Di Franco

Metal-catalyzed cross-coupling reactions are powerful and widespread methods for the carbon-carbon bond formation. Among the electrophile partners, non-activated alkyl halides remain challenging substrates because of their reluctance towards oxidative addition and their tendency to undergo the competitive ß-hydride elimination reaction. Due to the stability, availability and low toxicity of boron reagents, the Suzuki-Miyaura reaction is one of the most advantageous cross-coupling reaction. This dissertation is devoted to the development of Ni-catalyzed Suzuki-Miyaura reactions of non-activated alkyl halides. Chapter 2 describes the alkyl¿alkyl and alkyl¿aryl Suzuki¿Miyaura reactions of unactivated alkyl halides catalyzed by a well-defined pincer complex, Nickamine (1). The coupling of 9-alkyl-9-borabicyclo[3.3.1]nonane and 9-phenyl-9-borabicyclo[3.3.1]nonane (9-BBN) reagents with primary alkyl halides was achieved in modest to good yields. The reactions tolerated a variety of useful functional groups including ester, ether, furan, thioether and acetal. Reaction attempts with other boron reagents are also presented. In chapter 3, the first Ni-catalyzed Suzuki¿Miyaura coupling of alkyl halides with alkenyl-(9-BBN) reagents is reported. Both primary and secondary alkyl halides including alkyl chlorides can be coupled. The coupling method can be combined with hydroboration of terminal alkynes, allowing the expedited synthesis of functionalized alkyl alkenes from readily available alkynes with complete (E)-selectivity in one-pot. The method was successfully applied to the total synthesis of (±)-Recifeiolide, a natural macrolide. Replacement of a dimethyl amino group of the amidobis(amine) nickel(II) pincer complex (1), [(MeN2N)Ni¿Cl], by a pyrrolidino group resulted in a new nickel(II) pincer complex (2), [(PyrNMeNN)Ni¿Cl]. As shown in chapter 4, complex 2 is an efficient catalyst for Suzuki-Miyaura cross-coupling of non-activated secondary alkyl halides, while complex 1 is largely inactive. In contrast to the dimethylamino group in 1, the pyrrolidino group in 2 appears to be ¿hemilabile¿, which explains the significant activity difference towards the coupling of secondary alkyl halides. Synthesis and characterization of a new series of Ni-NNN pincer complexes are presented in chapter 5. No major structural difference was found between the new complexes except for the C-N-C angle of the various substituents of the side arm amino groups. The influence of these substituents on the catalytic performance of the Ni-complexes in alkyl-alkyl Kumada and Suzuki-Miyaura cross-coupling reactions was systematically investigated. No correlation was found between the catalytic activity and the key structural parameter (C-N-C angle), redox properties or Lewis acidity of the complexes. The cross-coupling of secondary alkyl halides is highly desirable since a stereogenic carbon center can be created during the process. Chapter 6 resumes the synthesis of new chiral pincer ligands in order to develop an enantioselective cross-coupling. Chiral centers were first introduced in the pyrrolidino group of the pincer ligand but the resulting Ni-complexes did not succeed in achieving a stereoconvergent reaction. The introduction of an oxazolino group in the pincer ligand structure resulted in more promising Ni-catalysts, efficient for both primary and secondary alkyl halides.

EPFL2016

Stereoselective synthesis of alkenes via base-metal-catalyzed addition reactions to alkynes and activation of hydrogen peroxide and carbon dioxide by iron complexes

Fedor Zhurkin

Alkene functionality can be found in the majority of natural products, drugs, catalysts and organic materials. Therefore, methods of C-C double bond formation constitute a cornerstone of organic synthesis. Selective formation of either (Z)- or (E)-isomer is especially desirable. 1,2-Addition to alkynes unites a large arsenal of methods of C-C double bond formation, among which the addition of carbon-centered species is the most interesting, since it offers a possibility of construction of the carbon skeleton. The first part of the present dissertation is devoted to the development of methods of selective addition of carbon reagents to alkynes. Chapter 1 reviews the existing methods of addition of carbon-centered species, generated from organometallic reagents, organic halides or carbonyl compounds to C-C triple bond with the accent on their stereoselectivity. Carbometalative mechanisms lay in the background of the majority of the reactions in this chemistry. The Z/E selectivity of 1,2-addition to triple bond can be controlled via predominance of either syn- or anti-addition or via Z/E isomerization of the addition intermediates. In Chapter 2 a novel method of disubstituted alkene synthesis via reductive addition of alkyl halides to terminal arylalkynes is described. The remarkable feature of this method is highly selective formation of (Z)-alkenes (generally Z/E > 10:1). Minor modifications of the reducing agent allowed the extension of the substrate scope to the primary alkyl iodides. Mechanistic studies revealed that this reaction proceeds as a formal anti-carbozincation. The resulting organozinc species form disubstituted (Z)-alkenes upon aqueous work-up. On the other hand, it would be interesting to utilize these species in cross-coupling reactions to allow the selective synthesis of trisubstituted alkenes. In Chapter 3 a catalytic system for highly regioselective copper-catalyzed allylation of these reagents is described. In Chapter 4 the results on stereoselective intermolecular addition of aldehydes and ketones to alkynes in the presence of zinc and trimethylchlorosilane are summarized. This reaction gives 1-chloro-1,3-dienes as products. The activation of small molecules has become an increasingly attractive area of research over the last years. In Chapter 5 an outline is given of the literature examples of small-molecule activation, namely activation of carbon dioxide and hydrogen peroxide, by iron complexes. In Chapter 6 the studies on small-molecule activation by iron-sulfur cubane clusters are presented. Electrochemical reduction of carbon dioxide and hydrogenation of alkenes were studied. As an additional result, synthesis and characterization of an unprecedented organic iron(I) ate-complex is reported. Structural modification of a non-heme iron complex with hydrogen-bond-donating substituents is described in Chapter 7. The deep impact of these modifications on the structure and reactivity of the resulting complex is discussed.

EPFL2017