Kinetic isotope effects hydrogen shifts

In contrast to the subsystem representation, the adiabatic basis depends on the environmental coordinates. As such, one obtains a physically intuitive description in terms of classical trajectories along Born-Oppenheimer surfaces. A variety of systems have been studied using QCL dynamics in this basis. These include the reaction rate and the kinetic isotope effect of proton transfer in a polar condensed phase solvent and a cluster [29-33], vibrational energy relaxation of a hydrogen bonded complex in a polar liquid [34], photodissociation of F2 [35], dynamical analysis of vibrational frequency shifts in a Xe fluid [36], and the spin-boson model [37,38], which is of particular importance as exact quantum results are available for comparison. 

Primary and secondary kinetic isotope effects are of general importance in the study of neighboring group participation. Isotopic substitution a to the incipient carbo-cation produces a secondary isotope effect whereas 0 and y substituents may give rise to both primary and secondary effects. For example, if the rate determining step of a solvolytic reaction involves a hydrogen shift or elimination, primary deuterium isotope effects are clearly implicated. 

In some instances, particularly in hydroxyla-tions meta to a halide substituent, the hydrogen on the hydroxylated carbon is quantitatively lost (i.e., there is no NIH shift), and a small deuterium kinetic isotope effect is observed These hydroxylations could result from direct oxygen insertion into the C-H bond, as in a true hydrox-ylation mechanism, but they are more likely to result from oxidation of the aromatic ring without the formation of a discrete epoxide intermediate. Isotope effect studies with deuterated benzenes bearing a variety of substituents have shed some light on this process A small, normal isotope effect is observed for weta-hydroxylation when deuterium is located meta- to the halogen in chlorobenzene = 1.1-1.3), but a small,... 

Okamura determined the primary deuterium kinetic isotope effect in a vitamin D-like model system as a function of temperature to obtain d - h = 1. 62 kcal/mol and Ah/Ad = 0.835 (Scheme 7.56). These results suggest a normal primary KIE without tunneling, not unlike those determined by Roth for the 1,5-hydrogen shift in cw-l,3-pentadiene (see Chapter 6, Section 2). 

The NMR from extensive pyrolysis of 7,8-dideuterio-l,3,5-cyclooctatriene revealed the presence of deuterium on C3, C4, C7, and C8. This is consistent with only 1,5-hydrogen shifts which would interconvert the two all cis-cyc ic trienes. Subsequently 5,8-dideuterio-l,3,6-cyclooctatriene was found to isomerize to the 1,3,5-isomer with an intramolecular kinetic isotope effect of 5.0 (Scheme 9.41). ... 

C which suggests a hyperconjugative isotope effect. Further, there is a kinetic isotope effect on both the homo- 1,5- and 1,7-hydrogen shifts of 4. 

To study the stereochemistry of the hydrogen shift, optically active 1-deuterio-l-methyl-3- r -butylindene was found to give l- r -butyl-3-methylindene = 5.5) with optical and deuterium labeling consistent with a suprafacial deuterium shift (Scheme 10.4). Moreover, the primary deuterium kinetic isotope effect was roughly 3. 

A double homodieny 1-1,5-hydrogen shift was proposed for the reaction with bicyclo[6.1.0]nona-2,5-diene being an intermediate. This was confirmed in the pyrolysis of 7,7-dideuterio tricyclic starting material, which exhibited a large kinetic isotope effect 1 /iP = 6.7), and led to 3,7-dideuterio-l,3,6-cyclononatriene. Interestingly, the second reaction must have involved a hydrogen and not deuterium shift, and this is reasonable considering the stereoelectronic requirements for such reactions (See Chapter 7, Section 4). 

FIGURE 3.8 [l,5]-Sigmatropic hydrogen shift in c -l,3-pentadiene. Temperature dependence of the kinetic isotope effect. 

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