Isomerization side reactions

The hydroformylation of trflns-3-octene at room temperature using the (non-encapsulated) rhodium catalyst based on tris(weta-pyridyl)phosphine afforded 2-ethylheptana] and 2-propylhexanal in exactly a 1 1 ratio. The encapsulated catalyst provided an unprecedented selectivity for 2-propylhexanal of 75% (Scheme 8.3). Again the selectivity is largely retained at 40 °C whereas at 80 °C the isomerization side reaction prohibits the selective formation of aldehydes. Similar regioselectivities were obtained in the hydroformylation of frflns-2-hexene, trans-2-nonene and trans-3-nonene at 25 °C. 

The parent system is prepared by oxidation of tetrahydrothiopyran with triphenylmethane and perchloric acid (66HCA2046) in high yield, and the method is applicable to the preparation of virtually all simple alkyl or aryl derivatives. In more complex cases, especially where the strong acid might cause elimination or isomerization side reactions, triphenylmethyl tetrafluoroborate may be used with the thiin precursor (equation 113) (78CL723). 

Increasing the CO partial pressure decreases the hydro-formylation reaction rate and the amount of alkene isomerization side reactions (see Hydrogenation Isomerization ofAlkenes), while increasing the aldehyde linear to branched product ratio. Pino proposed that the apparent marked difference between HCo(CO)4-catalyzed hydroformylation at low and high CO partial pressures was due to the existence of two active catalyst species, HCo(CO)4 and HCo(CO)3, formed from the CO association/dissociation equilibrium shown in... 

Hydrocarboxylation reactions generally do not have very high regioselectivities when carried out with C4 or higher alkenes, due to alkene isomerization side reactions catalyzed by both acid and metal. Thus, many of the reactions done industrially involve substrates such as acetylene and ethylene where isomerization side reactions will not present any problems. 

Grubbs reported tandem olefin metathesis/hydrogenation as a means to make satnrated cyclic prodncts and a one-pot RCM/transfer dehydrogenation/hydrogenation ronte to (R)-(-)-Mnscone utilizing (4a) (Scheme 19). Snapper and coworkers took advantage of the isomerization side reaction... 

Long-chain aliphatic olefins give only insufficient conversion to the acids due to low solubility and isomerization side reactions. In order to overcome these problems the effect of co-solvents and chemically modified /i-cyclodextrins as additives was investigated for the hydrocarboxylation of 1-decene [23], Without such a promoter, conversion and acid selectivity are low, 10% and 20% respectively. Addition of co-solvents significantly increases conversion, but does not reduce the isomerization. In contrast, the addition of dimethyl-/i-cyclodextrin increased conversion and induced 90% selectivity toward the acids. This effect is rationalized by a host/ guest complex of the cyclic carbohydrate and the olefin which prevents isomerization of the double bond. This pronounced chemoselectivity effect of cyclodextrins is also observed in the hydroformylation and the Wacker oxidation of water-insoluble olefins [24, 25]. More recent studies of the biphasic hydrocarboxylation include the reaction of vinyl aromatic compounds to the isomeric arylpropanoic acids [29, 30], and of small, sparingly water-soluble alkenes such as propene [31]. 

Catalytic alkylation of n-alkanes takes place only very slowly, particularly if conditions are not favorable for isomerization side reactions often predominate. 

A Br0nsted acid HX may be used as a co-catalyst. Long-chain alkenes give only insufficient conversion due to low solubility and isomerization side reactions. In order to overcome these problems the addition of co-solvents (such as y0-cyclo-dextrins cf. Section 2.2.3.3) was recommended. Their advantageous effect was rationalized by a host-guest complex of the cyclic carbohydrate and the alkene feed which prevents isomerization of the double bond [5]. 

ADMET polymerization is a versatile technique for the preparation of a wide variety of linear polymers (30,31). However, olefin isomerization side reactions may result in polymers with irregular repeating units (32). 

Fokou PA, Meier MAR. 2010. Studying and Suppressing Olefin Isomerization Side Reactions during ADMET Polymerizations. Macromol Rap Com 31 368-373. 

Organolithium compounds would be extremely desirable catalysts since the organometallic precursors such as alkyUithiums are readily available. It was found that intramolecular hydroamination of aminoalkenes can be mediated by a catalytic amount of -butyUithium [35, 106], The obvious drawback is the high basicity of lithium amides, which can result in double bond isomerization side reactions. Optimal conditions were found using a solvent mixture of THP and toluene at 1 lO C (3) although the reaction also proceeds at lower temperatures. 

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