Isomerization | EPFL Graph Search

In chemistry, isomerization or isomerisation is the process in which a molecule, polyatomic ion or molecular fragment is transformed into an isomer with a different chemical structure. Enolization is an example of isomerization, as is tautomerization. When the isomerization occurs intramolecularly it may be called a rearrangement reaction. When the activation energy for the isomerization reaction is sufficiently small, both isomers will exist in a temperature-dependent equilibrium with each other. Many values of the standard free energy difference, \Delta G^\circ, have been calculated, with good agreement between observed and calculated data. Examples and applications Alkanes Skeletal isomerization occurs in the cracking process, used in the petrochemical industry. As well as reducing the average chain length, straight-chain hydrocarbons are converted to branched isomers in the process, as illustrated the following reaction of n-butane to i-butane.

Mécanismes d'isomérisation et d'échange de ligand de complexes octaédriques labiles de cobalt (II)

Fabrice Riblet

There are only few structural thermodynamic and kinetic studies on labile octahedral complexes available today. Octahedral paramagnetic compounds of cobalt(II) have an optimum electronic relaxation leading to NMR spectra with good resolution. Therefore, they are excellent candidates to follow substitution and isomerisation processes by multinuclear NMR at variable pressure and temperature. This work groups the study of nine labile complexes (three families of compounds with three examples each) which can exist in different isomeric forms. The first part of the work concerns octahedral complexes with bis-chelating ligands of the general formula [Co(LA)2Rpy2] with LA being a bidentate acetylacetonate ligand and Rpy a monodentate ligand of the γ-substituted pyridine type. The second part of the work concerns octahedral complexes with tris-chelating ligands of the general [Co(LA)2(LB)] with LA being a bidentate acetylacetonate ligand and LB a bidentate ligand of the γ-substituted bipyridine type. The third part of the work concerns trinuclear heterometallic complexes with the general formula [Fe2M(μ3-O)(μ2-Ac)6(Rpy)3] with M = CoII, NiII and Rpy a monodentate ligand of the γ-substituted pyridine type. Each type of the metal centres has an octahedral geometry. For each example chosen, the different species present in equilibrium in solution have been identified and the thermodynamic and kinetic parameters measured are discussed. Mechanisms for isomerisation and ligand exchange are proposed on the basis of variable temperature and pressure kinetics data. It has been shown that the bis-chelate compounds isomerise via a dissociative mechanism (with an intermediate of bipyramid geometry). The monodentate ligand participates more or less depending on its donor character. In the case of complexes with γ-methylpyridine, the mechanism consists essentially of an opening of one arm of the bidentate ligand (with little exchange of the pyridine). However, in the case of complexes with γ-trifluoromethylpyridine, a more frequent exchange of the monodentate ligand is observed competing with the opening of the arm of the bidentate ligand. In the case of tris-chelate compounds the isomerisation mechanism proposed on the bases of the exchange matrix used for spectral simulations is a twist with an intermediate of prismatic geometry. This observed rates (faster) and the obtained activation parameters (smaller) are very different of those found for complexes with bis-chelating ligands. The trinuclear heterometallic complexes maintain their structure in dichloromethane solution. Variable pressure experiments allowed assigning a dissociative mechanism (D) for the exchange of pyridine on both the divalent and the trivalent centres. Experiments at variable temperature indicated an increase of lability for the trivalent metals and a decrease in lability for the divalent metals compared to the homoleptic octahedral mononuclear complexes. The activation parameters for the trinuclear heterometallic complexes are bigger than those found for the bis- and tris-chelate compounds.

EPFL2007

Mechanistic Studies Related to Nickel-Catalyzed Alkyl-Alkyl Cross Coupling Reactions

Jan Breitenfeld

Cross-coupling reactions of non-activated alkyl halides are potentially useful chemical transformations. At the same time, however, they are challenging due to a series of unproductive side reactions. Recently, significant progress has been made to overcome these difficulties. With judicious choice of metal, ligands, and reaction conditions, the normally problematic β- H elimination can be suppressed. As a consequence, a number of examples of the successful coupling of non-activated alkyl halides have been demonstrated. The mechanistic understanding of such reactions is, in most cases, still primitive. In the majority of cases, the active catalysts are generated in situ and remain unidentified and unknown. This dissertation is devoted to mechanistic studies of alkyl-alkyl cross-coupling catalyzed by a well-defined nickel pincer complex [(MeNN2)Ni-Cl] (1; Nickamine). This complex showed excellent activity for the cross-coupling of unactivated alkyl halides. Moreover, several nickel alkyl complexes bearing β-hydrogen atoms have been isolated. These features offered a unique platform to carry out mechanistic studies to gain a better understanding of nickel-catalyzed crosscoupling reactions. Chapter one provides examples of palladium-, iron-, cobalt-, and nickel-catalyzed alkyl-alkyl cross-coupling reactions. In each case mechanistic details are discussed. At the end of the chapter cross-coupling reactions using Nickamine are presented. In chapter two the propensity of isolated nickel alkyl complexes to undergo -hydride elimination is explored. Isomerization and olefin exchange experiments show that -hydride elimination is kinetically viable but thermodynamically unfavorable in [(MeNN2)Ni-alkyl] complexes. The intermediacy of an [(MeNN2)Ni-H] (6) species was corroborated by trapping experiments. The alkyl complex [(MeNN2)Ni-propyl] catalyzes olefin isomerization reactions. In chapter three the aforementioned nickel(II) hydride complex 2 was synthesized by reaction of [(MeNN2)Ni-OMe] (10) with Ph2SiH2, and was characterized by NMR and IR spectroscopy, as well as X-ray crystallography. Complex 6 was unstable in solution, and decomposed via two reaction pathways. The first pathway was through intramolecular N-H reductive elimination to III give MeNN2H and Ni particles. The second pathway was intermolecular, giving H2, Ni particles, and a five-coordinate Ni(II) complex [(MeNN2)2Ni] (12) as the products. Compound 6 reacted with ethylene and acetone, forming [(MeNN2)Ni-Et] (2) and [(MeNN2)Ni-OiPr] (13), respectively. Complex 6 also reacted with alkyl halides, yielding nickel(II) halide complexes and alkanes. The reduction of alkyl halides was rendered catalytic, using 1 as catalyst, NaOiPr or NaOMe as base, and Ph2SiH2 or Me(EtO)2SiH as the hydride source. The catalysis appeared to operate via a radical mechanism. In chapter four a detailed mechanistic study of alkyl-alkyl Kumada coupling catalyzed by the preformed nickel(II) pincer complex 1 is reported. The coupling proceeds through a radical process, involving two nickel centers for the oxidative addition of alkyl halide. The catalysis is 2nd order in Grignard reagent, 1st order in catalyst, and 0th order in alkyl halide. A transient species, (MeNN2)Ni-Alkyl2, is identified as the key intermediate responsible for the activation of alkyl halide, the formation of which is the turnover-determining step of the catalysis. Finally, a catalytic cycle for Nickamine-catalyzed cross-coupling reactions was established. This dissertation elaborates the properties of nickel-alkyl and nickel-hydride complexes of the Nickamine system. It elucidates many mechanistic features of the alkyl-alkyl Kumada coupling reactions catalyzed by Nickamine, and establishes, for the first time, a bimetallic oxidative addition mechanism for nickel-catalyzed coupling reactions. The work significantly enhances the current understanding of cross-coupling reactions of alkyl halides.

EPFL2013

Polysulfones as new organic catalysts

Dean Markovic

Even unsubstituted butadiene adds to sulfur dioxide in the hetero-Diels-Alder mode more rapidly than in the chelotropic mode. The sultine can be observed in equilibrium with the diene and the sulfur dioxide only at low temperature and in the presence of CF3COOH. Crystals of the 4,5-dialkyl-sultine resulting from the SO2 addition to 1,2-dimethylidenecyclohexane have been obtained at -60 °C and analyzed by X-ray diffraction. Quantum chemistry calculations have shown that hyperconjugative interactions within the sulfinyl moiety are responsible for the anomeric effects observed in sultines that prefer pseudo-chair conformations with pseudo-axial S=O bonds. Mixing of sulfur dioxide and methylidenecyclopentane leads to the formation of alternating copolymer poly(methydilenecyclopentane sulfone) and isomerization of methylidenecyclopentane to 1-methylcyclopentene. Polysulfones are known to equilibrate with radicals upon heating (ceiling temperature of polysulfone formation). These sulfonyl radicals can abstract a hydrogen atom from methylidenecyclopentane (and from other 2-alkylalk-1-enes) to give an allyl radical type intermediate and sulfinic acid. The intermediate allyl radical abstracts an hydrogen atom from sulfinic acid and gives isomerized product (1-methylcyclopentene). Diphenyldisulfone or solid polysulfones are mild and efficient organic catalysts for selective cleavage of methylprenyl (2,3-dimethylbut-2-en-1-yl), prenyl (3-methylbut-2-en-1-yl), and methallyl (2-methylallyl) ethers. Our reaction conditions are compatible with the presence of other protecting groups such as acetals, acetates, and allyl, benzyl, and TBDMS ethers. Exposure of 2,3-dimethylbut-2-en-1-yl and 3-methylbut-2-en1-yl ethers to diphenyldisulfone or solid polysulfones led to the formation of 2,3-dimethylbuta-1,3-diene and isoprene, respectively. 2-Methylallyl ethers undergo isomerization to 2-methylpropenyl ethers, which are easily hydrolyzed into the corresponding free alcohols and isobutyraldehyde. We discovered that (2-methoxyprop-2-en-1-yl) silyl sulfinates are very effective agents for the silylation of alcohols and oximes. (2-methoxyprop-2-en-1-yl) silyl sulfinates allow easy, efficient and fast silylation of primary, secondy and tertiary alcohols. The diols are monosilylated at low temperature with high regioselectivity and high yields.

EPFL2005