Pericyclic reaction

Summary

In organic chemistry, a pericyclic reaction is the type of organic reaction wherein the transition state of the molecule has a cyclic geometry, the reaction progresses in a concerted fashion, and the bond orbitals involved in the reaction overlap in a continuous cycle at the transition state. Pericyclic reactions stand in contrast to linear reactions, encompassing most organic transformations and proceeding through an acyclic transition state, on the one hand and coarctate reactions, which proceed through a doubly cyclic, concerted transition state on the other hand. Pericyclic reactions are usually rearrangement or addition reactions. The major classes of pericyclic reactions are given in the table below (the three most important classes are shown in bold). Ene reactions and cheletropic reactions are often classed as group transfer reactions and cycloadditions/cycloeliminations, respectively, while dyotropic reactions and group transfer reactions (if ene reactions are excluded) are

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The aim of the project was to investigate iron bis(oxazolinylphenyl)amino (bopa) pincer complexes as pre-catalysts in the enantioselective Kumada cross-coupling of non-activated alkyl halides with aryl Grignard reagents. From this preliminary cross-coupling studies we were able to isolate potential intermediate iron species that were then investigated for their catalytic activity. Further reactivity studies, kinetic studies, and DFT computations revealed the feasible catalytic cycles. Some intermediate species also proved to be relevant in the enantioselective hydrosilylation reaction of ketones. In the first chapter, a short overview is given of state-of-the-art iron pincer complexes and their catalytic application. The focus was put on symmetric pincer systems which are rigidified with aromatic rings in the ligand backbone. They proved themselves as highly active catalysts for the hydrogenation and hydrosilylation reaction of olefins, acetylenes, and carbonyls. There are also some examples given for trapping and releasing of hydrogen as potential applications for the storage of hydrogen on a molecular level. Examples for C-C bond formations are given to a lesser extent. In chapter two, the enantioselective Kumada cross-coupling of alkyl halides with aryl Grignard reagents was investigated. Iron(III) bopa pincer complexes are efficient pre-catalysts for the cross-coupling of non-activated primary and secondary alkyl halides with aryl Grignard reagents. The reactions proceed at room temperature in moderate to excellent yields. A variety of functional groups can be tolerated. The enantioselectivity of the coupling of secondary alkyl halides is low. The cross-coupling with secondary benzylic bromides solely yielded the homo-coupling products, 2,3-diphenylbutane and biphenyl. Modifications on the oxazoline, and diphenylamino moiety showed similar reactivity but did not improve the enantiomeric excess (ee). In chapter three, we isolated and characterised well-defined iron complexes and probed and supported their catalytic roles. Reactivity studies identified an Fe(II) "ate" complex, [Fe(Bopa-Ph)(Ph)2]-, as the active species for the oxidative addition of alkyl halide. Experiments using radical-probe substrates and DFT computations reveal a bimetallic and radical mechanism for the oxidative addition. The kinetics of the coupling of an alkyl iodide with PhMgCl indicates that formation of the "ate" complex, rather than oxidative addition, is the turnover determining step. In chapter four, the preliminary results on a possible mechanism in the enantioselective hydrosilylation of 4-acetylbiphenyl is presented. We could show that [(Fe(bopa)OAc)2], a 5-coordinated dimeric iron complex, is formed as the catalytic active species from the reaction of bopa ligands with Fe(OAc)2. The gathered results are consistent with an inner sphere reaction pathway, including the formulation of an iron-hydride species. For the reactions with zinc, no mechanism could yet be proposed.

EPFL2015

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beta-Lactams are very important structural motifs because of their broad biological activities as well as their propensity to engage in ring-opening reactions. Transitionmetal- catalyzed C-H functionalizations have emerged as strategy enabling yet uncommon highly efficient disconnections. In contrast to the significant progress of Pd-0-catalyzed C-H functionalization for aryl-aryl couplings, related reactions involving the formation of saturated C(sp(3))-C(sp(3)) bonds are elusive. Reported here is an asymmetric C-H functionalization approach to beta-lactams using readily accessible chloroacetamide substrates. Important aspects of this transformation are challenging C(sp(3))-C(sp(3)) and strain-building reductive eliminations to for the four-membered ring. In general, the blactams are formed in excellent yields and enantioselectivities using a bulky taddol phosphoramidite ligand in combination with adamantyl carboxylic acid as cocatalyst.

Wiley-Blackwell2014

The thesis describes a simple approach for N-formylation of amines with CO2 and hydrosilane reducing agents, the use of organic salts as N-formylation catalysts, their physical properties required for high catalytic activity and their role in the catalytic cycle. The investigation is then extended to the synthesis of quinazoline-2,4(1H,3H)-dione from 2-aminobenzonitrile and CO2 as well as other C-N bond forming reactions. Basicity, as measured by pKb, is identified as the key property required for high catalytic activity of organic salt catalysts. Minor variations in catalytic activity among salts of equal basicity are assigned to the ion pairing energy, hydrogen bonding ability and steric factors. The N-formylation reaction catalysed by organic salts proceeds by the activation of the hydrosilane reducing agent. Although, the catalytic activity was originally assigned to the nucleophilicity of the anion, a detailed investigation of the reaction mechanism and physical parameters of numerous salt catalysts revealed that they are more active as bases. Extensive substituent exchange around the hydrosilane silicon centre confirms its activation and supports the proposed reaction mechanism, where in-situ formed carbamate salt acts as the reaction nucleophile. Accounting for small detrimental effect of ion paring, a linear relationship between the pKa of the salt and catalytic activity is observed. The onset of the base catalysed reaction mechanism is substrate dependent with further variations for amines of high basicity. Nevertheless, highly basic salt catalysts, such as tetra-n-butylammonium fluoride ([TBA]F), promote rapid N-formylation of all types of amines under extremely mild conditions. A linear relationship between the basicity of salt catalysts and their catalytic activity was also observed in the synthesis of quinazoline-2,4(1H,3H)-dione. Similarly, the reaction proceeds by in-situ formation of a carbamate salt, which requires a base catalyst. However, here the reaction mechanism is limited by the acidity of the quinazoline-2,4(1H,3H)-dione product. Quinazoline-2,4(1H,3H)-dione is deprotonated by more basic catalysts (pKa of conjugate acid > 14.7 in DMSO) leading to the neutralization of the base catalyst and the formation of the quinazolide anion, which then acts as the reaction catalyst. Analysis of the reported literature reveals that the findings are directly applicable to the vast majority of C-N bond forming reactions of amines with CO2 and to almost all types of organocatalysts. The reactions proceed by carbamate salt formation in the form [BaseH][RR'NCOO]. The anion of the carbamate salt then acts as a nucleophile in hydrosilane reductions of CO2 towards formamides, N-methylamines and aminals or internal cyclization reactions to quinazoline-2,4-diones, oxazolidinones and oxazin-2-ons or after dehydration as an electrophile in the synthesis of urea derivatives. The role of organocatalysts in the reactions indicates that all bases of sufficient strength should be able to catalyse the reactions.

EPFL2019

EPFL2015

EPFL2019