Fluxional olefins

AhHCl BulPCHzCHiCRCHiCH BuDlR = H(flux-ional) = CH3(not fluxional), olefinic and carbene complexes, and some new hydrides of iridium. 

Symmetrical and fluxional olefins give consistent products (table 8). 

Metathesis is a catalyzed reaction that converts two olefin molecules into two different olefins. It is an important reaction for which many mechanistic approaches have been proposed by scientists working in the fields of homogenous catalysis and polymerization. One approach is the formation of a fluxional five-membered metallocycle. The intermediate can give back the starting material or the metathetic products via a concerted mechanism ... 

The mechanisms of stereoselectivity which have been proposed for chain-end stereocontrolled polymerizations involving secondary monomer insertion also present a general pattern of similarity. In fact, molecular modeling studies suggest that for olefin polymerizations (both syndiospecific and isospecific, Section 4.1.2) as well as for styrene polymerization (syndiospecific, Section 4.2), the chirality of the growing chain would determine the chirality of a fluxional site, which in turn would discriminates between the two monomer enantiofaces. 

Both the 1 1 and the 1 2 bonded alkenes should show structural isomers, with, for the ethylene adduct, nonequivalence of the two protons bonded to the carbon. For substituted olefins, it is often possible to detect the presence of both isomeric forms, especially with the osmium derivatives. However, all these molecules are fluxional and the flux-... 

In a quest to increase the efficiency of olefin polymerization catalysts and their selectivity in the orientation of the polymerization, the highly effective Group IV metallocene catalysts, M(Cp)2(L)2, have been studied, since they all display high fluxionality. Following methide abstraction, the metallocene catalysts of general formula M(Cp-derivatives)2(CH3)2 (M= Ti, Zr, Hf), were turned into highly reactive M+-CH3 cationic species. The activation parameters for the methide abstraction, derived from variable temperature NMR experiments, establish a correlation between the enthalpies of methide abstraction, the chemical shift in the resulting cation, and the ethylene polymerization activities [149]. 

These complexes are effective catalysts in epoxidation reactions with H2O2 and alkyl hydroperoxides. Several detailed mechanistic studies have been carried out in particular, it has been shown that, when the alkyl chain contains a double bond, no autoepoxidation is observed both in the solid state and in solution. Nevertheless, if f-BuOOH is added, the epoxidation of the olefinic moiety immediately takes place. Therefore, it has been suggested that these complexes are not the active species in the oxygen transfer step to the substrate, but they behave as catalysts for the primary peroxidic oxidant. On the basis of kinetic, spectroscopic and theoretical studies, the authors provided a mechanism, whose key steps are sketched in Scheme 12. In this context a major role appears to be played by the fluxionality of the particular ligands used . ... 

Numerous investigations have been undertaken on the reactions of ruthenium carbonyls with olefins and acetylenes. Two complex types [55] and [56] result from the reaction of Ru3(CO)j2 with ethylene and other Simple olefins (127). The complexes [56], which belong to the products of reaction (1) (Chapter 2.5.) are also formed from Ru3(CO)i2 and diphenyl acetylene (183). [55] and [56] show interesting fluxional properties, and four different types of ligand scrambling are possible (163). 

The interesting fluxional behavior of complexes [56] (358) includes oscillation of the olefinic ligand as in [25a] [256]. Reactions of H2Os3(CO)j0 with acetylenes... 

In contrast to its ready reaction with acetylenes, Cp2Mo2-(CO)i+ does not react with simple olefins or dienes, e.g., C2Ht+, butadiene, C2H3CN, norbornadiene, etc. With TONE (C2(CN)iJ, 1 is oxidized and [CpMo(C0)il]+ [TONE]7 may be isolated (6). Allene does react, however, to form a complex, ll, in which each of the orthogonal C=C n-bonds donates two electrons to each molybdenum. This fluxional molecule has C2 symmetry in the solid state and has a Mo-Mo bond length of 3.117(1)A (32). 

The hydride iron carbonyl anion, [HFe(CO)4] , has a structure that is best described as a distorted tbp with the hydride ligand occupying an axial site. The anion is fluxional both in solution and in the solid state.113 The main use is found in organic synthesis and catalysis.114 A typical reaction is the reduction of olefins... 

Most recent quantum chemical calculations involving [n]-ladderanes suggest a defect in the ladderane framework, in the form of an olefin pair, can result in fluxional molecnles named sigmatropic shiflamers." " " The fluxional natnre wonld stem from the ability of the olefin pair to travel across the defective ladderane framework via a series of Cope rearrangements (Fignre 12). Calcnlations predict the flnxionahty of the molecnles depends on the size of the framework. Specifically, [n]-ladderanes with n > 7 are expected to be fluxional at and above room temperatnre. 

Irradiation of Re2(CO)io in the presence of ethylene gives (13) via a radical pathway which produces a [Re(CO)4(-n -olefin)]2 intermediate. At room temperature, (13) undergoes a rapid fluxional process in which the or- and ir-bonds rapidly interchange between the Re atoms. Re2(CO)io is reported to react with acyclic or cyclic olefins on photolysis. Thus with cyclohexene, (14) is... 

The NMR data for 11 displayed no evidence of any fluxional behavior for the complexes in solution. Structural features associated with the jLi-cr,fr-propenyl ligand of llh and llj are consistent with a strong metal-carbon cr bond, a typical sp -sp carbon-carbon single bond distance, and a significantly shorter olefinic carbon-carbon bond which is Tj -bonded to the second metal fragment (67,68). 

The titanium complexes bearing <9-phosphinophenoxo ligands shown in Scheme 380 are prepared by metallation of the corresponding phenol followed by reaction with CpTiCl3. The complexes were found to be fluxional by H NMR analysis at low temperature. The X-ray structure shows that in the solid state the molecule is chiral at the metal center. These complexes are effective catalysts for olefin hydrogenation and imine hydrosilylation.911... 

A platinum complex in which the ligand contains two independent olefinic bonds, PtCl2-C10H14O3, will be referred to in Sect. E.6. Cyclo-octatetraene (COT) may frequently be regarded as a conjugated ring system (e.g. in fluxional... 

This system is analogous to the HMo(T7 -C3H5)(diphos)2 system developed by Osborn et al. 17), which provided the first direct observation of the TT-allyl-hydride exchange mechanism proposed for the 1,3-hydride shifts found in many metal-catalyzed olefin reactions. Presumably, this hydride undergoes a fluxional rearrangement of the diphos ligands in addition to the metal(allyl)(hydride) <= metal (propene) interconversion. 

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