Hydrogen-first mechanism

Some of the most detaOed studies on this process were conducted by Gridnev and Imam-oto on reactions catalyzed by bidentate, P-chiral ligands containing alkyl substituents on phosphorus. ° Two pieces of data together imply that this catalyst reactsby the hydrogen-first mechanism in Scheme 15.8, instead of the alkene-first mechanism. First, the solvated bisphosphine dihydride complex [Rh(Lj)(S)2(H)J " is formed at -90 C, and this complex reacts with MAC at this temperature to form Thus, this complex is... 

Kharasch proposed that hydrogen bromide can add to alkenes by two different mechanisms both of which are regiospecific The first mechanism is electrophilic addi tion and follows Markovmkov s rule... 

Before we examine the hydrogenation of each type of unsaturation, let us first take a look at the basic mechanism assumed to be operating on metal catalytic surfaces. This mechanism is variously referred to as the classic mechanism, the Horiuti-Polanyi mechanism, or the half-hydrogenated state mechanism. It certainly fits the classic definition, since it was first proposed by Horiuti and Polanyi in 193412 and is still used today. Its important surface species is a half-hydrogenated state. This mechanism was shown in Chapter 1 (Scheme 1.2) as an example of how surface reactions are sometimes written. It is shown in slightly different form in Fig. 2.1. Basically, an unsaturated molecule is pictured as adsorbing with its Tt-bond parallel to the plane of the surface atoms of the catalyst. In the original Horiuti-Polanyi formulation, the 7t-bond ruptures... 

Basically, the cleavage of the C-O bond takes place through two mechanisms. The first mechanism (the hydrogenolytic cleavage) is a hydrogen-assisted bond-cleavage reaction, whereas the second mechanism is an ionic insertion of the metal into the C-O bond. The characteristic features of the two mechanisms are summarized in Table 4.1. 

The first mechanism appears to be the better basis for describing most of the results referred to by Cramer (56). It will, however, be noted that the addition-elimination mechanism requires that the metal catalyst be supplied as a metal hydride. Where the catalyst has not been supplied in this form, the reaction has usually been carried out in the presence of reagents known to convert transition metal compounds to hydrides (e.g. protonic acids, alcohols or hydrogen). These substances are known as co-catalysts and, where they have been used, induction periods have been encountered which are consistent with hydride formation as required in mechanism (a), but which would not be expected for (b). 

The fact that tetramethylethylene which contains no hydrogen on either of the double-bonded carbon atoms undergoes polymerization to yield dimer might be considered as a means of choosing between the carbonium ion mechanism and the hydrogen separation mechanism. However, regardless of which mechanism is used, it is necessary to assume that the olefin first undergoes isomerization to terf-butylethylene. 

Auration proposed in the first mechanism (Scheme 8.8) is possible in similar species, as Kharasch [68] and Fuchita [69] observed in stoichiometric reactions. In both pathways, the same intermediate was formed and no [3-hydrogen elimination was observed and not only furan but other electron-rich arenes could react [70]. Although AuCl3 was the precatalyst used, it was not possible to determine whether Au(III) or Au(I) were the real catalytic species (Scheme 8.9). 

Three different degradation mechanisms were proposed. In the first mechanism, the hydroxyl radical attacks atrazine by hydrogen abstraction from the secondary carbon of the ethylamino side chain, producing a free radical as shown in Equation (6.135). 

It is indirectly clear from Fig. 4.11 as to why it was necessary to search for a relatively long time for a mechanism pair which can explain the experimental evidence. The first mechanism pair, where x equals zero, does not correspond with an experimentally accessible situation, since it applies only within existing very high hydrogen peroxide concentrations. The second... 

The mechanism proposed by Ddtz involves the insertion of a carbon monoxide into the vinyl carbene complex intermediate with the formation of the vinyl ketene complex (255). Electrocyclic ring closure of (255) leads to the cyclohexadienone complex (252), which is related to the final tenzannulation product by a tautomerizadon when R is hydrogen. The mechanism proposed by Casey differs from that of Ddtz in that the order of the steps involving carbon monoxide insertion and cyclization to the aryl or alkenyl substiment is reversed. < Specifically, the vinyl carbene complex intermediate (248) first undergoes cyclization to the metallacyclohexadiene (249), followed by cartion monoxide insertion to give the intermediate (251), and finally reductive elimination to give cyclohexadienone intermediate (252). At this time the circumstantial evidence favors the intermediacy of vinyl ketene intermediates since they can be trapped from these reactions and isolated where the metal is dispaced from the vinyl ketene functionality however, there is not any evidence which can rule out the alternative mechanism. 

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