Catalytic enantioselective synthesis of quaternary carbon stereocentres.

Quaternary carbon stereocentres-carbon atoms to which four distinct carbon substituents are attached-are common features of molecules found in nature. However, before recent advances in chemical catalysis, there were few methods of constructing single stereoisomers of this important structural motif. Here we discuss the many catalytic enantioselective reactions developed during the past decade for the synthesis of single stereoisomers of such organic molecules. This progress now makes it possible to incorporate quaternary stereocentres selectively in many organic molecules that are useful in medicine, agriculture and potentially other areas such as flavouring, fragrances and materials

N–Phenyl Pyrroloimidazolone Derivatives for Stereoselective Birch Reduction-Alkylation and Chiral Imidazolylidene-Iridium Complexation

This thesis describes the synthesis and use of an N– based proline–derived directing group towards the Birch reduction, diastereoselective alkylation, and the synthesis of NHC–iridium complexes that are precursors for the study of intramolecular aryl C–H activation. A pair of ortho–benzoate esters containing epimeric pyrroloimidazolone chiral auxiliaries underwent sequential Birch reduction and diastereoselective alkylation to provide products ranging from a 50:50 to 95:5 dr for the anti– epimer, and 88:12 to >95:5 diastereomeric ratio (dr) for the syn– epimer. Single crystal X–ray analysis of key anti–epimer–derived products, along with the comparison of the optical rotation measurements of enantiomers that were prepared from the syn–or anti– starting materials to its known enantiomer confirmed the stereoselectivity of the products. This work includes related Schultz stereoselective Birch reduction alkylation of anisole with a chiral benzamide except that the pyrroloimidazolone replaces the achiral methoxy group and serves as the stereodetermining element. In addition, the synthesis and evaluation of the N–phenyl iridium complex derived from the annulated aminal with syn–stereochemistry in the backbone was achieved. Exposure of the neutral Ir–complexes to anionic nucleophiles such as MeLi resulted in an increase of electron density at the Ir atom that initiated C–H bond activation. Lastly, a N–heterocyclic carbene ligand derived from the N–benzyl analogue of the auxiliary was also investigated. Attempts to design a monodentate ligand as well as a bidentate ligand bearing an alcohol side chain were both shown to be unsuccessful at this time

Chiral Modification of the Tetrakis(pentafluorophenyl)borate Anion with Myrtanyl Groups

The synthesis and characterization of chiral [B(C6F5)4]– derivatives bearing a myrtanyl group instead of a fluoro substituent in the para position are described. These new chiral borates were isolated as their bench‐stable lithium, sodium, and cesium salts. The corresponding trityl salts were prepared and tested as catalysts in representative counteranion‐directed Diels–Alder reactions and Mukaiyama aldol additions but no enantioselectivity was obtained. Preformation of a chalcone‐derived silylcarboxonium ion with the chiral borate as counteranion did not lead to any asymmetric induction in a reaction with cyclohexa‐1,3‐diene.TU Berlin, Open-Access-Mittel - 201

Catalytic addition of C–H bonds across C–C unsaturated systems promoted by iridium(i) and its group IX congeners

Transition metal-catalyzed hydrocarbonations of unsaturated substrates have emerged as powerful synthetic tools for increasing molecular complexity in an atom-economical manner. Although this field was traditionally dominated by low valent rhodium and ruthenium catalysts, in recent years, there have been many reports based on the use of iridium complexes. In many cases, these reactions have a different course from those of their rhodium homologs, and even allow performing otherwise inviable transformations. In this review we aim to provide an informative journey, from the early pioneering examples in the field, most of them based on other metals than iridium, to the most recent transformations catalyzed by designed Ir(I) complexes. The review is organized by the type of C–H bond that is activated (with C sp2, sp or sp3), as well as by the C–C unsaturated partner that is used as a hydrocarbonation partner (alkyne, allene or alkene). Importantly, we discuss the mechanistic foundations of the methods highlighting the differences from those previously proposed for processes catalyzed by related metals, particularly those of the same group (Co and Rh)This work received financial support from Spanish grants (SAF2016-76689-R, CTQ2016-77047-P, CTQ2017-84767-P and ORFEO-CINQA network CTQ2016-81797-REDC), the Consellería de Cultura, Educación e Ordenación Universitaria (2015-CP082, ED431C-2017/19 and Centro Singular de Investigación de Galicia accreditation 2019–2022, ED431G 2019/03), the European Union (European Regional Development Fund-ERDF corresponding to the multiannual financial framework 2014–2020), and the European Research Council (Advanced Grant No. 340055). DFF thanks Xunta de Galicia for his postdoctoral fellowship (ED481B-2019-005)S

Iridium-Catalyzed asymmetric hydrogenation : A: Studies on the synthesis of pyrazine-based P,N Ligands, B: Diastereoselective hydrogenation of chiral cyclohexenes

The synthesis of a new ligand class of pyrazine based ligands and their attempted complexation with iridium to form dimeric complexes is presented. Furthermore, the synthesis of chiral cyclohexenes by enantioselective Diels-Alder reactions is shown. These substrates were tested in the diastereoselective reduction by a chiral iridium catalyst. Last, some results of enantioselective palladium-catalyzed allylic substitution reactions and asymmetric iridium-catalyzed hydrogenation with different NeoPHOX ligands are presented

Iridium Complexes as Hydrosilylation Catalysts

Iridium complexes have been found to be active as catalysts for hydrosilylation reactions, especially for those involving 1,3-dienes and 1-alkynes. For ketones, iridium complexes show maximum activity if one molar equivalent (relative to iridium) of triphenylphosphine is added to the reaction mixture. Iridium complexes are also active catalysts for the hydrosilylation of a,ß-unsaturated ketones, although the regioselectivity differs from that obtained with rhodium complexes. Attempts at asymmetric hydrosilylation of keto compounds using iridium complexes resulted in extremely low enantiomeric excesses

A cobalt complex of a microbial arene oxidation product

We report the first synthesis of a cobalt Cp diene complex wherein the diene is derived by microbial dearomatising dihydroxylation of an aromatic ring. The complex has been characterised crystallographically and its structure is compared to that of an uncomplexed diene precursor

Selective Mono-Hydrogenation of Polyunsaturated Hydrocarbons: Traditional and Nanoscale Catalysis

Selective hydrogenation of olefins is an important process in both chemical and pharmaceutical industries. This chapter reviews intriguing catalytic studies accomplished by employing a variety of catalysts such as metal complexes, supported materials, supported metal complexes, and nanosized materials for polyene hydrogenation. In addition, new research area involving unsupported colloidal nanoparticle catalysts, which exhibit an excellent activity and selectivity toward the polyene hydrogenation is introduced. The high activity of colloidal metal nanoparticle catalysts often allows the reactions to be completed under mild conditions, at atmospheric pressure, and room temperature. These colloidal nanoparticle catalysts also offer an advantage of facile separation and multiple recycling without significant losses in activity and selectivity. This chapter provides important fundamental understandings on the influence of chemical environments (solvents, ligands, dopants, etc.) and compositions (metal complex, metals, alloys, etc.) toward the catalytic activity and selectivity of various catalysts in homogeneous, heterogeneous, and semi-heterogeneous conditions. The systematic evaluation discussed in this chapter would pave a way to further develop chemo-, regio-, and stereo-selective catalysts for polyene hydrogenation

Stereodivergent Access to Trisubstituted Alkenylboronate Esters through Alkene Isomerization

We report an efficient method for the preparation of synthetically valuable trisubstituted alkenylboronate esters through alkene isomerization of their readily available 1,1-disubstituted regioisomeric counterparts. Either stereoisomer of the target alkenylboronate motif can be obtained at will from the same starting material by employing different isomerization catalysts

Biomimetic asymmetric catalysis based on biological and synthetic macromolecular scaffolds

Gedurende de laatste decennia zijn er grote inspanningen verricht door chemici om replicas van enzymen te ontwerpen en fabriceren die zijn gebaseerd op verschillende macromoleculaire templates. Deze macromoleculen kunnen worden verdeeld in twee groepen: biologische platformen (eiwitten, peptiden en nucleotiden) en synthetische holtes met hydrofobe bindplaatsen. Kandidaten uit beide klasses kunnen een combinatie van interacties bieden (waterstof bruggen, π-π stapeling, electrostatische en hydrofobe interacties) aan het substraat, de overgangstoestand en het metaal complex. Daarom worden de macromoleculaire componenten rondom het metaal centrum gedefinierd als de tweede cöordinatie sfeer. Dit uitgebreide scala aan mogelijkheden heeft nieuwe reactiviteit en selectiviteit aan verschillende belangrijke organische reacties gebracht.Dit proefschrift betrekking op het ontwerp van nieuwe biomimetische katalytische systemen en deze toegepast asymmetrische katalysatoren voor verschillende chemische reacties