Selectivity phenol hydrogenation

The assumed transition state of this reaction is shown in Scheme 5.3. Yb(OTf)3, (J )-(-h)-BINOL, and DBU form a complex with two hydrogen bonds, and the axial chirality of (J )-(-h)-BINOL is transferred via the hydrogen bonds to the amine parts. The additive would interact with the phenolic hydrogen of the imine, which is fixed by bidentate coordination to Yb(III). Because the top face of the imine is shielded by the amine, the dienophiles approach from the bottom face to achieve high levels of selectivity. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

Figure 5.2 Equilibrium conversion, yield and selectivity for phenol hydrogenation.

The catalyst treated in the presence of dodecane alone retains nearly all the activity of the freshly sulfided catalyst. The catalyst samples treated in the presence of water are both deactivated. They lost approximately two thirds of the initial activity. The evolution of the activity for the catalyst treated in the presence of water and hydrogen sulfide (H20-H2S(X)) indicates that 46 % of the activity are lost after 12 hours of treatment and that a stabilisation occurs between 12 and 36 hours. The selectivity for hydrogenation of the aromatic ring relative to direct elimination of phenolic OH group (hydrogenation/hydrogenolysis) does not vary significantly for any catalyst... 

Kvaerner Process Technology Cyclohexanone/cydohexanol Phenol/hydrogen Synthesis of KA oil with high selectivity to cyclohexanone 2 1998... 

Covering a supported metal catalyst with a thin liquid film that differs from the bulk solvent affects both the reaction rate and selectivityTreating a Pd/C catalyst with 2M KOH before using it in a phenol hydrogenation with a heptane solvent gave cyclohexanone in 97% yield. Apparently, the distribution of phenol and phenolate ions between the thin aqueous film around the catalyst and the bulk hydrocarbon solvent enhanced the adsorption of the phenolate ion on the catalyst and the facile transfer of the neutral cyclohexanone to the heptane after the selective hydrogenation was completed (Figure 17.1). 

Fig. 17.1 Depiction of a thin film of alkaline water on the metal catalyst and its effect on selectivity in phenol hydrogenation. ...

Hydrotalcite supported palladium catalysts were found to be highly active and selective for phenol hydrogenation. The activity of the catalysts depends on the method of preparation of the hdyrotalcite, interlayer anion and on the precursor of the palladium used. 

Achiral ytterbium Lewis acid was prepared from ytterbium triflate [Yb(OTf)3], ( )-(+)-l,l -bis(2-naphthol) (BINOL) and DBU (1), and subjected to aza Diels-Alder reactions of achiral imines (A-benzylidene-2-hydroxyanilines) and achiral dienophiles [79]. In this reaction the use of a chiral Lewis acid containing 1,3,5-trimethylpiperidine instead of 1 resulted in a lowering of the enantiomeric excess of adduct. Thus, the phenolic hydrogen of the imine interacts with DBU (1) in transition state, as shown in Figure 3.7, to increase the selectivity. 

Using the photo CIDNP method, resonances from particular amino acid side chains (tyrosine, histidine and tryptophan) can be selectively enhanced when these residues are situated at the surface of the protein. To this end a flavin dye, added to the sanq>le, is photo excited in the NMR probe by an argon laser. In this way triplet state flavin is generated, which in the case of tyrosine residues is able to abstract the phenolic hydrogen atom. Consequently a radical pair is formed (reaction 2 below) which reversibly yields flavin and tyrosine (reaction 3 below). 

The H2S concentration appears to be an important factor influencing the reactivity and the overall HDO mechanism of the biocrude components. " " Thus, during the HDO of GUA (7 MPa 473 and 543 K CoMo/AC), H2S had little effect on the overall conversion, but it inhibited the hydrogenolysis of the Car-0 bond. As a consequence, the (phenol+C6)/catechol ratio decreased. Similarly, H2S had little effect on the overall eonversion of ethyldecanoate but the selectivity to hydrogenated and deearboxylated products slightly decreased. The most adverse effect of H2S on the overall conversion was observed for 4-MA. In the presence of NH3 (from di-aminopropane) both conversion of GUA and deearboxylation of ethyldeeanoate were decreased. 

The derivative selected in any particular instance should be one which clearly singles out one compound from among all the possibilities and thus enables an unequivocal choice to be made. The melting points of the derivatives to be compared should differ by at least 5-10°. Whenever possible, a derivative should be selected which has a neutralisation equivalent as well as a melting point (e.g., an aryloxyacetic acid derivative of a phenol. Section IV,114,4, or a hydrogen S nitrophthalate of an alcohol. Section 111,25,5). 

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