Isomerism route-dependent

Figure 5.21 The route-dependent preparation of isomeric catenanes ...

The thermal equilibration of cis- and trans-[PtH2 2 [L = PMc3 or PEt3] has been examined. " The equilibrium positions are solvent dependent (as expected, polar solvents favor cis complexes) and ligand dependent. PMe3 leads to more of the cis isomer than PEt3. The isomerization route was not determined, but it was noted that the compounds catalyze scrambling between H2 and D2. An oxidative-addition/reductive-elimination route could accomplish this and could also be involved in the isomerization. 

There has been a consistent motivation for the work presented in this chapter the application to molecular synthesis in interstellar gas clouds (see, for example, Herbst,22 this volume). The species in these regions are detected spectroscopically and are thus automatically isomerically identified. The routes to the observed neutral species consistently involve ion-molecule reactions followed by dissociative electron-ion recombination.18 The first step in this process is to determine whether an isomeric ion can be formed which is likely to recombine to an observed neutral species. The foregoing discussion has shown that whether this occurs depends on the detailed nature of the potential surface. Certainly, this only occurs in some of the cases studied. Much more understanding will be required before the needs of this application are fulfilled. 

According to this sequence, formation of cis- and trani -stilbenes is preceded by formation of a magnetosensitive ion-radical by a singlet-triplet conversion. This means that spin polarization must be observed in cis- and trani -stilbene, and the isomerization rate must depend on the intensity of the magnetic held. These predictions were conhrmed experimentally (Lyoshina et al. 1980). Hence, the ion-radical route for trans —f cis conversion is the main one under photoirradiation conditions. Until now, the mechanisms assumed for such processes have involved energy transfer and did not take into account single-electron transfer. The electron transfer takes place in reality and makes the... 

Carbonylation of 2,3-homo-l//-azepines has been effected by means of their metal and carbonyl complexes and provides a useful route to a variety of isomeric azabicyclo-nonadienones. For example, the tricarbonyliron complex with carbon monoxide at 80 °C and 160 atm yields the 9-oxo-2-azabicyclo[3.3.l]nona-3,7-diene (167) (57%) or the 9-oxo-2-azabicyclo[3.2.2]nona-3,6-diene (168) (60%) depending on the exo or endo configuration of the tricarbonyliron complex. A third isomer, namely ethyl 7-oxo-9-azabicyclo[4.2.1]nona-2,4-diene-9-carboxylate (169), is formed on heating (125 °C) the azepine with carbon monoxide under pressure in the presence of the rhodium carbonyl complex [Rh(CO)Cl2] (78CB3927). 

The widely known Wilkinson catalyst is proposed to operate through this reaction mechanism. Computational evaluation of the full catalytic cycle showed that the rate-determining step implies the insertion and the subsequent isomerization process (27). Moreover, this catalyst has the particularity that the reaction mechanism depends on the hydrogen source since a monohydridic route has been proposed when 2-propanol is the hydrogen source (28). 

The hydrogen fluoride catalyzed fluorination of norbornene by xenon difluoride at room temperature leads to a mixture of at least seven components,39 but under milder conditions (— 78 to 26 C, 22 hours) the reaction affords a mixture of two main products 2-e,xo-5-cxo-difluoro-norbornane and 2-c-wfo-5- Yo-difluoronorbornane, ratio 2 1, in a total yield of 51-76%. If the same reaction is carried out in a limited temperature range between — 46 and — 39 C the yield of these products decreases, their ratio becomes equal, and the main product is 2-exo-l-ff //-difluoronorbornane (42 %).40 The structure dependence of the fluorination products of norbornene with xenon difluoride was studied. Solvent, temperature, reaction duration, catalyst (hydrogen fluoride, boron trifluoride, trifluoroacetic acid, pentafluorobenzenethiol) and the routes of product isomerization were analyzed.41-42... 

However, the most economical and octane effective route to reduce benzene is to eliminate benzene and benzene precursors from the naphtha reformer feed by pre-fractionation in an up-stream splitter and then send these precursors to an isomerization unit. With this approach, benzene will be saturated and isomerised to a mixture of higher octane saturated cyclics. Depending on the feed benzene content, various solutions can be applied to handle the heat load from the saturation of benzene. These heat management solutions entail saturation of the benzene in the iso-... 

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