Diels-Alder reactions isotope effect studies

A. Carbon-14 and Deuterium Isotope Effect Studies of the Diels-Alder Reaction... 

Seltzer /. Am. Chem. Soc. 1963, 85, 1360, 1965,87, 1534. For a review of isotope effect studies of Diels-Alder and other pcricyclic reactions, sec Gajewski Isot. Org. Chem. 1987, 7, 115-176. 

The elucidation of reaction mechanisms is a central topic in organic chemistry that led to many elegant studies emphasizing the interplay of theory and experiment as demonstrated, for example, by the seminal contributions of the Houk group to the understanding of the Diels-Alder and other pericyclic reactions.38 This reaction class is rather typical for the elucidation of reaction mechanisms. On the experimental side, the toolbox of solvent, substituent and isotope effect studies as well as stereochemical probes have been used extensively, while the reactants, products, intermediates and transition structures involved have been calculated at all feasible levels of theory. As a result, these reactions often serve as a success story in physical organic chemistry. 

A reverse Diels-Alder reaction, the decomposition of 2-methylfuran-maleic anhydride ejro-adduct (in iso-octane) was the object of an extensive study of isotope effects . Here rehybridisation is from ip to sp and therefore a ratio knlko > 1 is to be anticipated. This has, in fact, been found (/ h/ d)50 c = 108 per D atom, both for all possible deuterations on the maleic anhydride moiety (see below), either at X or at Y or at X and at Y, and for deuteration of the furan moiety at Z. 

An intramolecular secondary isotope effect has been measured by analysing the product of 1,2-cycloaddition of l,l-c(2-allene to acrylonitrile in benzene at 210-225°C. The result is (Ah/Ad)2s°c 1-18 per D atom it has the opposite direction of that found for a Diels-Alder reaction of M-r/j-allene, and is consistent with a two-step mechanism (see Section 4.1.5). A diradical intermediate seems probable. A similar result was obtained from the study of isotope effects for the dimerisation of allene. Since the reaction of / (-)-... 

Deuterium isotope effects have been studied for the Diels-Alder reactions between hexadeuterio-monosubstituted cyclo-octatetraenes (283) and several reactive dieno-philes leading to adducts (284). With dicyanomaleimide, isomerization of the cyclo-octatetraene to the bicyclo[4,2,0]octa-2,4,7-triene is the rate-determining step, but with the less reactive dienophiles (maleic anhydride and tetracyanoethylene) larger... 

The [4-1-2] cycloaddition of dienes to double bonds, the Diels-Alder reaction (see Pericycttc Reactions The Diels-Alder Reaction), is one of the most useful synthetic reactions in organic chemistry. The mechanism of the Diels-Alder reaction has been the subject of intensive study using a large number of different methods. This is at least in part due to the fact that, based on thermochemical estimates, there is a reasonable possibility for a stepwise mechanism. The energy of concert , i.e., the difference between the concerted and a diradicaloid mechanism, is small enough to be potentially overcome in some cases. The comparison of the results for the two different pathways suffers, however, from the difficulties of comparing closed-shell specie.s and diradicals within a single-determinant MO approach. Only recently, the use of multireference and DFT methods in combination with experimental kinetic isotope effect studies allow an unequivocal assessment of the reaction mechanism of the Diels-Alder reaction. ... 

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