Reactions browning, catalyst, transition

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

The first kinetic study of the Friedel-Crafts alkylation reaction was repotted by Brown and Grayson in 1953. This involved the reactions of some substituted benzyl halides with aromatic compounds in the presence of AlClj-PhNOa catalyst. The transition state for the rate-determining step (attack of the aromatic component on a polar alkyl halide-AlCb addition compound) was depicted as a o -complex. 

The addition of HCN to olefins catalyzed by complexes of transition metals has been studied since about 1950. The first hydrocyanation by a homogeneous catalyst was reported by Arthur with cobalt carbonyl as catalyst. These reactions gave the branched nitrile as the predominant product. Nickel complexes of phosphites are more active catalysts for hydrocyanation, and these catalysts give the anti-Markovnikov product with terminal alkenes. The first nickel-catalyzed hydrocyanations were disclosed by Drinkard and by Brown and Rick. The development of this nickel-catalyzed chemistry into the commercially important addition to butadiene (Equation 16.3) was conducted at DuPont. Taylor and Swift referred to hydrocyanation of butadiene, and Drinkard exploited this chemistry for the synthesis of adiponitrile. The mechanism of ftiis process was pursued in depth by Tolman. As a result of this work, butadiene hydrocyanation was commercialized in 1971. The development of hydrocyanation is one of tfie early success stories in homogeneous catalysis. Significant improvements in catalysts have been made since that time, and many reviews have now been written on this subject. ... 

Vanadium complexes and in particular [VO(acac)2] are the most active catalysts for the oxidation of substituted anilines to nitro compounds. The effect of substituents upon reaction rate corresponds to a reaction involving an electron deficient transition state in that electron withdrawing groups decrease the rate and vice versa. The relative order of reactivity p-Me > m-Me > aniline > p-Cl > p-Br > m-Cl > m-Br is the same as observed in electrophilic aromatic substitution. Straight line correlations between the log of the relative rates and Hammett a or Brown constants were obtained with p values of— 1.42 and — 1.97, respectively, indicating an electron deficient transition state in the rate determining step. 

The use of anion-exchangers as the source of nucleophiles in nucleophilic substitution reactions has been referred to (Chapter 12). Brown and Jenkins (1976) and Regen (1975, 1976, 1977) have developed a triphase-catalysis method for carrying out such nucleophilic substitution reactions. The resin in these reactions acts as a polymer-bound phase-transfer catalyst. Quaternary ammonium groups that were introduced into the resin contained large alkyl groups to act as the linked phase-transit catalyst. 

---