Olefin isomerization, solvents

Solvent for Base-Catalyzed Reactions. The abihty of hydroxide or alkoxide ions to remove protons is enhanced by DMSO instead of water or alcohols (91). The equiUbrium change is also accompanied by a rate increase of 10 or more (92). Thus, reactions in which proton removal is rate-determining are favorably accompHshed in DMSO. These include olefin isomerizations, elimination reactions to produce olefins, racemizations, and H—D exchange reactions. 

The binuclear hydride salts are air sensitive, soluble in THF, acetone, CH3CN, MeOH, and diethyl ether, and insoluble in hydrocarbon solvents. They can be stored cold under an inert gas or in vacuo for several months. The salts tend to slowly decompose into W(CO)6 and [PPN] [FeH(CO)4], Carbon monoxide rapidly (within minutes) degrades the dimer into the same products.5 The salts react rapidly with CH3COOD or stronger deuterated acids to form the H-D exchanged products, [FeCrD(CO)9]. The hydrides also act as catalysts in olefin isomerization.10... 

The behavior of stilbene radical cations in the semiconductor catalysis is in keeping with the result of photoisomerization of other olefins like 6-methylstyrene sensitized by electron acceptors like chloranil in polar solvents (48). The semiconductor photocata-lyzed isomerization of strained cyclobutanes to strained dienes (isomerization of quadricyclene to norbomadiene and similar reactions of complex cage compounds (49)) is related to the olefin isomerization discussed above. 

This conclusion was challenged by Johnson (72), who followed the formation of isomeric aldehydes and isomeric olefins during the hydroformylation of 4-methyl-1-pentene. He found that 2-olefin was formed very rapidly under Oxo conditions and suggested that this did not affect the preferential formation of terminal aldehyde significantly, due to the fact that 2-olefin was so much less reactive to hydroformylation. These results have now been checked by Piacenti et al. (115), who found that 2-olefin was only rapidly formed if the rate of the reaction was sufficiently high that the solution was no longer saturated with carbon monoxide. By using a lower catalyst concentration in benzene solvent they were able to avoid this deficiency and found very little olefin isomerization. 

Structural Isomerization Reactions.—In earlier work Kropp and his co-workers19 suggested that the unsaturated and cyclopropane products formed by the irradiation of olefins in solvents of low nucleophilicity were produced by initial rearrangement to carbenes. In a later study Kropp and Fields20 have demonstrated that carbene intermediates are indeed formed in the irradiation of olefins... 

In order to eliminate the possibility for in situ carbene formation Raubenheimer et al. synthesized l-alkyl-2,3-dimethylimidazolium triflate ionic liquids and applied these as solvents in the rhodium catalyzed hydroformylation of l-hejEne and 1-dodecene [178]. Both, the classical Wilkinson type complex [RhCl(TPP)3] and the chiral, stereochemically pure complex (—)-(j7 -cycloocta-l,5-diene)-(2-menthyl-4,7-dimethylindenyl)rhodium(i) were applied. The Wilkinson catalyst showed low selectivity towards n-aldehydes whereas the chiral catalyst formed branched aldehydes predominantly. Hydrogenation was significant with up to 44% alkanes being formed and also a significant activity for olefin isomerization was observed. Additionally, hydroformylation was found to be slower in the ionic liquid than in toluene. Some of the findings were attributed by the authors to the lower gas solubility in the ionic liquid and the slower diffusion of the reactive gases H2 and CO into the ionic medium. 

This system nicely illustrates many of the principles outlined in the first section. Most of the work was done using Ni[P(OEr)3]4 or Ni[P(OMe)3]4 as catalysts in methanol solvent. In the presence of a strong acid cocatalyst, such as H2SO4 or HCl (but not without cocatalyst), butenes can be isom-erized within minutes at room temperature to an equilibrium mixture of the linear isomers. This is probably the most active olefin isomerization catalyst system known. 

Dimethyl sulfoxide is a favored solvent for displacement reactions in synthetic chemistry. The rates of reaction in DMSO are many times faster than in an alcohol or aqueous medium [6]. Dimethyl sulfoxide is the solvent of choice in reactions where proton (hydrogen atom) removal is the rate determining step. Reactions of this type include olefin isomerizations and reactions where an elimination process produces an olefin. Another application that uses DMSO is its use as an extraction solvent to separate olefins from saturated paraffins [7]. Several binary and ternary solvent systems containing DMSO and an amine (e.g., methylamine), sulfur trioxide, carbon disulfide/ amine, or sulfur trioxide/ammonia are used to dissolve cellulose, and act as spinning baths for the production of cellulose fibers [8,9]. Organic fungicides, insecticides, and herbicides are readily soluble in DMSO. Dimethyl sulfoxide is used to remove polymer residues from polymerization reactors. 

Olefin isomerization has been used as a probe for determination of the strength of various base-solvent systems. ... 

An example of carbon-carbon triple bond reduction in water is the reaction of disubstituted electron-deficient alkynes with water-soluble monosulfonated and trisulfonated triphenylphosphine [62] (Scheme 6.8). The water acts either as solvent or as reactant, and the amount of phosphine controls the cisitrans ratio of alkenes because it catalyzes the cis-trans olefin isomerization. 

The water-soluble pyrazolato complex (Rh(p-Pz)(CO)(TPPTS)]2 was used as precursor for olefin hydroformylation in an aqueous heptane solvent system 9]. Without additional ligand, olefin isomerization dominated hydroformylation at 7 bar CO/H2 (1 1) giving large amounts of 2-hexene. Isomerization was suppressed most effectively at 49.8 bar, leading to aldehyde chemoselectivities >90% and a 2.4-3.S njiso ratio. It was demonstrated that aerobic recycling of the aqueous catalyst phase by... 

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