Ligand effects, propene

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

Kumar D, de Visser SP, Sharma PK et al (2005) The intrinsic axial ligand effect on propene oxidation by horseradish peroxidase versus cytochrome P450 enzymes. J Biol Inorg Chem 10 181-189... 

Enantioselective imidation of alkyl aryl sulfides with A -alkoxycarbonyl azides as a nitrene precursor is effected by using (OC)Ru(salen) complex as catalyst. The steric and electronic nature of the Af-alkoxycarbonyl group strongly affect the enantioselectivity and the reaction rate. In a systematic and well-executed study of ligand effects on Lewis-acid-catalyzed Diels-Alder reaction, it has been shown that the attachment of aromatic a-amino acid ligands to copper(II) ions leads to an increase in the overall rate of the Diels-Alder reaction between 3-phenyl-l-(2-pyridyl)-2-propene-l-one (Din) and cyclopentadiene... 

The naphthyl derived ligand, (5)-1-mcthyl-2-[(l-naphthylamino)methyl]pyrrolidine (4) is especially effective in the stereoselective additions of (Z)-l-cthylthio-l-trimethylsilyloxy-l-propene to aldehydes. Thus, quantitative formation of. yyn-adducts is achieved, in addition to high reagent-induced stereoselectivity (>98% ee for the 3-hydroxy thioester products)23 32. 

The relative reactivity profile of the simple alkenes toward Wacker oxidation is quite shallow and in the order ethene > propene > 1-butene > Zi-2-butene > Z-2-butene.102 This order indicates that steric factors outweigh electronic effects and is consistent with substantial nucleophilic character in the rate-determining step. (Compare with oxymercuration see Part A, Section 5.8.) The addition step is believed to occur by an internal ligand transfer through a four-center mechanism, leading to syn addition. 

Considerable work has been conducted on a water-soluble catalyst using sulfonated phosphine-modified rhodium. Details of this chemistry will be described in Chapter 5. The general concept (Figure 2.3) is to make the catalyst water soluble, then after product formation, decant the product. In order for the water-soluble catalyst to be effective, the alkene must dissolve in the aqueous layer. This has been demonstrated on a commercial basis using propene. The low solubility of higher alkenes in the aqueous catalyst layer has proven problematic. The desirable characteristic of the ligand, water solubility, is needed in the separation step but is a disadvantage in the reaction step. 

We therefore evaluated how the bite angle affected regioselectivity, and studied the counterbalance of non-bonding and orbital effects. We choose two diphosphine ligands (benzoxantphos and homoxantphos) which among the series of xantphos ligands represent the extreme cases of natural bite angle, and used propene as a model for terminal aliphatic alkenes and styrene. 

Figure 11.6 Effect of varying the water soluble phosphine ligand on the hydroformylation of propene to butyraldehyde. P Rh ratio = TPPTS 80 BISBIS 7 NORBOS 14 BINAS 7...

So far we have only discussed catalysts based on diphosphine ligands with a focus on ethene. We should mention that the ligands that are most effective for this reaction are also capable of making terpolymers of ethene, propene and CO. This is important because the commercially interesting polymers must contain a few percent of propene in order to lower the melting point to around... 

These assembly ligands will be tested in suitable catalytic reactions that leave the assemblies intact. Salt-forming reactions are not attractive as the salts might interact with the assembly, nor is the use of catalytic metals that compete with the assembly metal for the salen type positions in the ditopic ligand ideally, all potential problems can be avoided if the same metal could be used. Rhodium-catalyzed hydroformylation of 1-octene is a suitable reaction, with the only disadvantage that high pressures are needed, but hydrogen or CO do not interfere with our assemblies. Metal salts do not interfere with the rhodium hydrides involved in the hydroformylation catalysis, as for instance the most effective industrial process today for propene hydroformylation... 

It was shown that room-temperature molten salts derived from the combination of 1,3-dialkylimidazolium chloride and A1C13 can be used as solvents in two-phase catalytic dimerization of propene to give hexenes catalyzed by Ni(II) compounds (125). The effects of phosphane ligands coordinated to nickel and operating variables were also investigated (126). The dimerization products separate as an organic layer above the molten salt. This reaction has been carried out with n-butenes as the reactant and cationic nickel complex catalysts dissolved in organochloroaluminate liquids (127). 

Some data are also available on the effect of the structure of the phosphine or diphosphine on the optical yield in the asymmetric hydrocarbalkoxylation of 2-phenyl-1-propene (24) (see Table III). The best optical yield was obtained using 2,2-dimethyl-4,5-bis(dibenzo-phosphol-5-ylmethyl)-l,3-dioxolane as the chiral ligand. Much lower optical yields were obtained using aminophosphines or monophosphines. 

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