Claisen rearrangement kinetic isotope effect

Another series of publications from Ken s group compared kinetic isotope effects, computed for different possible transition structures for a variety of reactions, with the experimental values, either obtained from the literature or measured by Singleton s group at Texas A M. These comparisons established the most important features of the transition states for several classic organic reactions — Diels-Alder cycloadditions, Cope and Claisen rearrangements, peracid epoxidations, carbene and triazolinedione cycloadditions and, most recently, osmium tetroxide bis-hydroxylations. Due to Ken s research, the three-dimensional structures of many transition states have become nearly as well-understood as the structures of stable molecules. 

Al-Sader, B.H. and Al-Fekri, D.M., On the mechanism of flash vacuum pyrolysis of phenyl propargyl ether. Intramolecular deuterium kinetic isotope effect on Claisen rearrangement, J. Org. Chem., 43, 3626, 1978. 

Experimental Heavy atom, I60/I80, 12c/14c, (H/D2) kinetic isotope effects for the Claisen Rearrangement ... 

Supportive of the suggestion that ionization is not a major pathway in the Claisen rearrangement of the parent compound is the fact that the SDKIEs in aqueous solution are comparable to those in the gas phase and in m-xylene. Furthermore, attempts to solvolyze 1,1-dideuterioaUyl mesylate in aqueous methanol resulted in no ionization to an allyl cation instead, the direct displacement product was formed exclusively. Finally, determinations of a solvent kinetic isotope effect in deuterium oxide resulted in values around unity 10%. ° In the solvolysis reaction of tert-butyl chloride the value is 40% at room temperature. " It is possible to cause allylvinyl ethers to ionize by providing cation stabilizing substituents and Lewis acids or Lewis acidic solvents. ... 

The kinetic isotope effect and theoretical calculation of transition state model of the aromatic Claisen rearrangement were also reported. For example, Singleton et al. investigated the nature of the transition state of the Claisen rearrangement by a combined experimental and theoretical study [7]. It was shown that the experimental isotope effects coincide well with the predicted kinetic isotope effects. Detail of the theoretical studies is discussed in Chapter 11. 

Kupczyk-Subotkowska et al. studied heavy-atom kinetic isotope effects (KIE) of the thermal Claisen rearrangement of the parent [2- C]-, [4- C]-, [6- Cj- and [ 0]-allyl vinyl ether [38]. From the experimental KIEs they concluded, It is concerted a two-step process in which one bond is made (or broken) before the other is broken (or formed) is ruled out. From isotope effect calculations (BEBOVIB program) it was deduced that, "... the C4-O bond is 50-70% broken and the Cj-Cg bond 10-30% formed in the transition structure. ... 

Geometries, charge separation and kinetic isotope effects for the transition structure of the Claisen rearrangement of the parent aUyl vinyl ether were predicted on different levels of theory (RHF/6-31G, MCSCF/6-31G, CASSCF/6-31G ) by Yoo and Houk in 1994 [41]. The isotope effects calculated on the CASSCF/6-31G level were closest to the experimental values. The optimized MP2/6-31G structure of the transition structure was 1,4-diyl-like whereas the CASSCF/6-31G calculation predicted more oxallyl-allyl radical-pair character. 

The comparison of experimental and theoretically predicted kinetic isotope effects (KIEs) can be used to probe the accuracy of the computationally predicted transition structure provided that the experimental KIEs have been determined accurately. A comparison of predicted and experimental KIEs by Meyer et al. [55] led to the conclusion that, There is a firm disagreement in about half the cases between predicted and literature experimental heavy atom KIEs for both the aliphatic and aromatic Claisen rearrangements . Therefore, they reinvestigated the experimental KIEs for the Claisen rearrangement of aUyl phenyl and allyl vinyl ether and compared the determined values (solution) with the calculated data (gas phase). Eor the Claisen rearrangement of allyl vinyl ether, the transition structure was calculated on the MP4(SDQ)/6-31G level of theory and the predicted KIEs were in excellent agreement with the new experimental KIEs. The authors collected and compared previously reported data for the calculated distance between C- l/C-6 and O/C-4 and added their own predicted data (Scheme 11.40). 

A re-investigation of the isotope effects for the Claisen rearrangements of allyl vinyl ether and allyl phenyl ether has shown that there is disagreement between the predicted and literature experimental heavy-atom kinetic isotope effects for about half of these rearrangements studied. A new strategy for revealing the Claisen rearrangement of aUyl phenyl ether in the gas phase has been developed, and... 

---