The pressure-dependence of the dimerization of 1,3-butadiene: Experimental and theoretical volumes of activation and reaction

Activation and Reaction Volumes in Solution. 3

“…A very important paper by Kla ¨rner 526 in 1994 decisively broke new ground in the long-debated question whether concerted pericyclic reactions are more strongly accelerated by pressure than stepwise analogues. He was able to mimick the activation and reaction volumes for both types of process by means of a Monte Carlo computer simulation based on a hard-sphere model, and the excellent agreement between observed and calculated volume changes strongly reinforces the notion that they are indicative of the basic mechanisms (entries [486][487][488].…”

“…The several ene reactions studied at high pressure (entries 518-524) are clearly of a concerted nature 536,537 except in the case of the azo compounds; in that case, the much reduced contraction in the transition state suggests that a diradical mechanism is operating. Sigmatropic shifts (entries [525][526][527][528][529][530][531][532][533][534][535] are subject to moderate pressure acceleration resulting from the assumption of a cyclic conformation; the bond-making and -breaking parts should largely cancel. 520 A very nice confirmation comes from a study of the 1,2-divinylcyclobutanes.…”

Reply to “Comment on ‘Phantom Activation Volumes'”

“…20 In the same vein, Klärner says "the effect on volumes caused by the change in bond lengths is rather small". 21 For this reason, the high-pressure approach does not tell us anything significant about (1) bond formation in the TS or (2) what the mechanism is. The Stewart-Klärner high-pressure papers cited by H&L indeed make an interesting case for duality of mechanism in chloroprene dimerization, 21,22 but when heavy atom effects are taken into account their data can also be reconciled to a single stepwise mechanism.…”

“…21 For this reason, the high-pressure approach does not tell us anything significant about (1) bond formation in the TS or (2) what the mechanism is. The Stewart-Klärner high-pressure papers cited by H&L indeed make an interesting case for duality of mechanism in chloroprene dimerization, 21,22 but when heavy atom effects are taken into account their data can also be reconciled to a single stepwise mechanism. 13 "S&F ... argued that the activation volume needs to be corrected because it is partly of a phantom nature, and that when this is done, the remaining and presumably real part is insufficient to support the claim of concertedness".…”