The role of alkane coordination in C–H bond cleavage at a Pt(II) center

The Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology, 1200 East California Avenue, Mail Code 127-72, Pasadena, CA 91125

Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved February 15, 2007 (received for review December 11, 2006)

Abstract

The rates of CH bond activation for various alkanes by [(N–N)Pt(Me)(TFEd₃)]⁺ (N N = ArNC(Me)C(Me)NAr; Ar = 3,5-di-tert-butylphenyl; TFE-d₃ = CF₃CD₂OD) were studied. Both linear and cyclic alkanes give the corresponding alkene-hydride cation [(N–N)Pt(H)(alkene)]⁺ via (i) rate determining alkane coordination to form a CH σ complex, (ii) oxidative cleavage of the coordinated CH bond to give a platinum(IV) alkyl-methyl-hydride intermediate, (iii) reductive coupling to generate a methane σ complex, (iv) dissociation of methane, and (v) β-H elimination to form the observed product. Second-order rate constants for cycloalkane activation (C_nH_2n), are proportional to the size of the ring (k ∼ n). For cyclohexane, the deuterium kinetic isotope effect (k _H/k _D) of 1.28 (5) is consistent with the proposed rate determining alkane coordination to form a CH σ complex. Statistical scrambling of the five hydrogens of the Pt-methyl and the coordinated methylene unit, via rapid, reversible steps ii and iii, and interchange of geminal CH bonds of the methane and cyclohexane CH σ adducts, is observed before loss of methane.

Footnotes

^†To whom correspondence may be addressed. E-mail: jal{at}caltech.edu or bercaw{at}caltech.edu
Author contributions: G.S.C., J.A.L., and J.E.B. designed research; G.S.C. performed research; G.S.C. contributed new reagents/analytic tools; G.S.C., J.A.L., and J.E.B. analyzed data; and G.S.C., J.A.L., and J.E.B. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0610981104/DC1.
↵ ‡ We have not yet succeeded in isolating any of these products in pure form: they decompose on concentration, or more slowly on just standing in TFE-d ₃ solution.
↵ § It should be noted, however, that the reactions of propane (and ethane) with 2 do not give olefin-hydride products analogous to those found for higher linear alkanes; these reactions do appear to involve initial CH activation, but the final products exhibit more complex NMR spectra and have not yet been fully identified. Nonetheless, we believe that the inferences from these selective labeling experiments are probably valid.
↵ ¶ There may well be a connection between this behavior and the failure to observe stable olefin-hydride products from propane and ethane (see §), which can give only terminal olefin complexes.