Kinetic isotope effects alkane activation

The existence of tr-complex intermediates in C-H activation chemistry has been suggested to explain inverse kinetic isotope effects in reductive elimination processes whereby alkanes are formed from alkyl metal hydrides (Scheme 3).9... 

This mechanism clearly implicated alkane complexes as precursors to C-H activation but the IR absorptions of [Cp Rh(CO)Kr] and [Cp Rh(CO)(C6Hi2)] were not resolved and were presumed to be coincident. The temperature dependent data gave values of AH = 18 (or 22) kj mol for the unimolecular C-H (or C-D) activation step representing a normal kinetic isotope effect, kn/fco 10- However, an inverse equilibrium isotope effect (K /Kq 0.1) was found for the slightly exothermic pre-equilibrium displacement of Kr by CoHn/C Dn implying that C6Dj2 binds more strongly to the rhodium center than does C Hn-... 

In the formation of any of these products, the first step of the reaction involves activation of the alkane molecule. Experimental results point to the fact that in general, breaking of the first C—H bond is a rate-limiting step. For example, on a V-P-O catalyst, a sizable deuterium kinetic isotope effect was... 

Yoshizawa, K.. (2002) Theoretical study on kinetic isotope effects in the C-H bond activation of alkanes by iron-oxo complexes, Coordination Chemistry Reviews 226, 251-259. 

Marks has examined the reactivity of thorium metallacycles with hydrocarbons, where ring strain is used to provide the thermodynamic driving force for alkane activation in a reaction with methane (Eq. 17). Reaction with CD4 shows a dramatic kinetic isotope effect, with kH/kD=6, which is typical of the four-centered electrophilic transition state hydrocarbon activations [76]. The metallacy-cle is formed by the elimination of neopentane from the bis-neopentyl derivative [77]. Reaction with cyclopropane and tetramethylsilane gave the bis-cyclopropyl product Cp 2Th(c-propyl)2 and the bis-TMS product Cp 2Th(CH2SiMe3)2, respectively [78]. 

In the literature [25, 26a,c-g], inverse kinetic isotope effects for the reductive elimination of alkanes from metal centers, which is the miaoscopic reverse of alkane activation by oxidative addition, have been explained by the presence of an a alkane intermediate. Recently, thermolysis of the diastereomerically pure complexes (R5),(5R)-[2,2-dimethylcyclopropyl) (Cp )-(PMe3)lrH] and (/ / ),(5 5)-[2,2-dimethylcyclopropyl)(Cp )(PMe3)IrH] (see Scheme VI.5) in CaDs has been shown [26h] to result in its interconversion to the other diastereomer. The analogous reaction of the deuterium-labeled complexes resulted additionally in scrambling of the deuterium from the a-position of the dimethylcyclopropyl ring to the metal hydride position. Diastereomer interconversion and isotopic scrambling occurred at similar rates and have been discussed in terms of a common intermediate mechanism involving a metal alkane complex (Scheme VI.5). 

Two recent articles describe details of the isotope effects in C-H activation. One separates the kinetic isotope effect on reductive coupling from that of oxidative cleavage by looking at the irreversible loss of propane from Tp Rh(CNneopenyl)(/ -propyl)D/H. The oxidative cleavage isotope effect was measured by irreversible activation of CH2D2. These separate measurements were combined to give an overall inverse equilibrium isotope effect on alkane loss. ... 

Similar substituent effects have been determined in the reaction of complexes 10 to form palladacycles 11 (Scheme 11.3) [31]. The opposite process-the intramolecular palladium-catalyzed arylation of alkanes to form dihydrobenzofuranes-has also been examined [32]. For this transformation, a mechanism based on a C—H bond-activation process by the aryl-Pd(II) involving a three-center transition state was found to be more consistent with the experimental kinetic isotope effect (3.6 at 115 °C), as well as with density functional theory (DFT) calculations. 

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