Reaction mechanisms and transition-state shifts

The currently accepted description of transition-state structural variations in terms of perturbations of free-energy surfaces leads to the expectation that changes in reactant structure which increase the rate of a reaction almost always make the transition state earlier in the sense of being more reactant-like (Hammond behaviour). 

The identity of r values for solvolysis reactivities and the gas-phase stabilities of the corresponding carbocations imphes the generality of the extended Brpnsted relationship or Hammond-Leffler rate-equihbrium relationship for benzylic solvolyses, i.e. (37a,b), 

The generally observed identity of the r value for solvolysis reactivity and gas-phase stability AAG(c+)h of the corresponding carbocation leads to an important prediction concerning the solvolysis transition state. In a typical (limiting) two-step SnI mechanism with a single dominant transition state, the r values of transition states for the various nucleophile-cation reactions should be essentially controlled by the intrinsic resonance demand of the intermediate cation the substituent effect should be described by a single scale of substituent constants (a) with an r value characteristic of this cation. In a recent laser flash-photolysis study (Das, 1993) on the recombination of stable trityl and benzhydryl cations with nucleophiles and solvents, McClelland et al. (1986, 1989) have treated the substituent effects on solvent-recombination processes by (2). 

The rate constants for reactions of highly stable triphenylmethyl and diphenylmethyl cations with various ionic and neutral nucleophiles have been measured (Gandler, 1985 McClelland etal., 1986) in aqueous acetonitrile and discussed from the view point of a reactivity-selectivity relationship. 

The rate constants for the solvent-recombination process of the carbocations [3C+(X,Y,Z)] were determined by the use of the azide clock method (Richard et al, 1984 Richard and Jencks, 1984a,b,c McClelland et al, 1991) and the rate constant kh of the forward reaction was derived using (38b) as kh = k Kn+ (McClelland et al, 1989,1991). While ordinary Hammett-type relationships were found to be inapplicable to the substituent effects on solvent recombination, there is a rate-equilibrium correlation for all available data on triarylmethyl cations, shown as the linear log kw vs. pXr+ plot, in Fig. 34 with a slope of 0.64. Such a relationship was earlier suggested by Arnett and Hofelich (1983) and Ritchie (1986). The correlation of kw with the scale was 

---

Structure-reactivity relationship in polyarylcarbocation systems 334 Conformations of carbocations 334 Reactivity-conformation relationship 337 Stabilities of carbocations in the gas phase 343 Structural effects 343 Tlie resonance demand parameter 355 Theoretically optimized structures of carbocations 362 Reaction mechanisms and transition-state shifts 365 Extended selectivity-stability relationships 365 Ground-state electrophilic reactivity of carbocations 366 Sn2 reactions of 1-arylethyl and benzyl precursors 372 Concluding remarks 378 Acknowledgements 379 References 379... 

The Claisen rearrangement is an electrocyclic reaction which converts an allyl vinyl ether into a y,8-unsaturated aldehyde or ketone, via a (3.3) sigmatropic shift. The rate of this reaction can be largely increased in polar solvents. Several works have addressed the study of the reaction mechanism and the electronic structure of the transition state (TS) by examining substituent and solvent effects on the rate of this reaction. 

Curvature in a Br nsted-type plot is sometimes attributed to a change in transition state structure. This is not a change in mechanism rather it is interpreted as a shift in extent of bond cleavage and bond formation within the same mechanistic pattern. Thus, Ba-Saif et al. ° found curvature in the Br nsted-type plot for the identity reactions in acetyl transfer between substituted phenolates this reaction was shown earlier. They concluded that a change in transition state structure occurs in the series. Jencks et al." caution against this type of conclusion solely on the evidence of curvature, because of the other possible causes. 

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

The correlation between selectivity and intracrystalline free space can be readily accounted for in terms of the mechanisms of the reactions involved. The acid-catalyzed xylene isomerization occurs via 1,2-methyl shifts in protonated xylenes (Figure 3). A mechanism via two transalkylation steps as proposed for synthetic faujasite (8) can be ruled out in view of the strictly consecutive nature of the isomerization sequence o m p and the low activity for disproportionation. Disproportionation involves a large diphenylmethane-type intermediate (Figure 4). It is suggested that this intermediate can form readily in the large intracrystalline cavity (diameter. 1.3 nm) of faujasite, but is sterically inhibited in the smaller pores of mordenite and ZSM-4 (d -0.8 nm) and especially of ZSM-5 (d -0.6 nm). Thus, transition state selectivity rather than shape selective diffusion are responsible for the high xylene isomerization selectivity of ZSM-5. 

---