Chapter 3
Alkane Complexes as Intermediates in C-H Bond Activation Reactions
Downloaded by UNIV OF MINNESOTA on September 19, 2013 | http://pubs.acs.org Publication Date: July 12, 2004 | doi: 10.1021/bk-2004-0885.ch003
William D. Jones, Andrew J. Vetter, Douglas D. Wick, and Todd O. Northcutt Department of Chemistry, University of Rochester, Rochester, NY 14627
The rearrangements of alkyl deuteride complexes are monitored and a kinetic model used to extract the rate constants for the fundamental processes involving the unseen alkane σ-complexes. Isotope effects for C-H and C-D bond formation and cleavage have been measured to permit these determinations. The relative rates of the processes available to an alkane σ-complex (C-H oxidative cleavage, C-D oxidative cleavage, dissociation, migration to an adjacent C-H bond) have been determined for methane, ethane, propane, butane, pentane, and hexane.
Introduction It has now been generally established that alkanes are activated by homogeneous transition metal complexes by way of initial complexation of the C - H bond to the metal, followed by oxidative cleavage of the C - H bond to produce an alkyl hydride product (eq 1) ( i ) . Evidence for this pathway has come from a variety of observations, including scrambling of a metal-deuteride
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© 2004 American Chemical Society
In Activation and Functionalization of C—H Bonds; Goldberg, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.
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Downloaded by UNIV OF MINNESOTA on September 19, 2013 | http://pubs.acs.org Publication Date: July 12, 2004 | doi: 10.1021/bk-2004-0885.ch003
into the α-position of the alkyl group (2-/0), temperature-dependent linewidth variations (//), and direct observation via N M R or IR spectroscopy (12,13). Despite these studies, little is known about the behavior of these alkane complexes due to their lability and the inability to determine the structure (i.e. binding site) in these species. In this article we present an overview of experiments that use deuterium scrambling to monitor the rearrangements of alkyl hydride complexes. Through rate measurements and kinetic modeling, it has proven possible to extract the relative rates of the various processes open to the alkane complex intermediates.
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-L
^ [LnM]
R-H ^e
