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Isotope Effects in C-H Bond Activation Reactions by Transition Metals WILLIAM D

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Isotope Effects in C-H Bond Activation Reactions by Transition Metals WILLIAM D Acc. Chem. Res. 2003, 36, 140-146 Isotope Effects in C-H Bond Activation Reactions by Transition Metals WILLIAM D. JONES FIGURE 1. Geometries of arene π-complexes and alkane σ-com- Department of Chemistry, University of Rochester, plexes. Rochester, New York 14627 Received July 1, 2002 complex (either dissociatively, as shown, or associatively) and the second involving the actual cleavage of the C-H ABSTRACT bond. This sequence can be used to describe a variety of The activation of alkane C-H bonds by oxidative addition and its hydrocarbon activations including aromatic, aliphatic, and reverse reaction, reductive elimination, are believed to occur via vinylic C-H bonds. For arenes, the initial complex has σ transient -alkane complexes. This Account summarizes how 2 isotope effects can be used to probe the nature of these intermedi- been shown to be an η -arene complex and can be actually ates and points out some pitfalls in interpreting kinetic data. observed and monitored as the second step occurs.3 For Comparisons are made with arene C-H activation and other alkanes, the initial complex has been described as a activation systems. ªσ-complexº,4 whose structure has not been experimen- tally determined but which is believed to involve a three- 1. Introduction center interaction of the metal, the carbon, and the hydrogen (η2-C,H) after the models provided by X-ray Over the past 30 years or so, a number of examples of structures of intramolecular agostic C-H interactions carbon-hydrogen bond activation by transition metals (Figure 1).5 While there have been several observations have appeared in the literature.1 This work took on greater of stable arene π-complexes,6 direct observations of σ-al- significance in 1982, when Bergman reported a reaction kane complexes are rare.7 in which a ªsimpleº oxidative addition of cyclohexane to The reaction steps in eq 1 are shown as equilibria, since a photochemically generated reactive IrI fragment had in many cases the formation of the alkyl or aryl hy- occurred.2 Since then, many other transition metal com- dride product is reversible. In fact, the intermediacy of plexes have been discovered that can activate alkane C-H an alkane σ-complex is oftentimes demonstrated by prep- bonds by a variety of mechanisms to give products with aration of an alkyl deuteride complex, which then revers- metal-carbon bonds. In this Account, the subject of ibly scrambles the deuterium into the R-hydrogens of the isotope effects in some of these reactions will be examined alkyl ligand, presumably via the unseen σ-complex. Dis- in detail, as these effects are oftentimes quoted as having sociation of the alkane from the σ-complex, followed by relevance to the mechanism(s) of C-H bond activation. irreversible trapping of the reactive fragment [L M] with A general sequence for activation of a hydrocarbon n an external ligand, is typically observed also as a slower C-H bond is shown in eq 1. In this process, two distinct process (eq 2).8 steps are typically proposed following generation of the reactive metal species [LnM], the first involving interaction of the hydrocarbon with the metal center to make a Although the reversible formation of the σ-alkane William D. Jones was born in Philadelphia, Pennsylvania, in 1953, and was inspired complex is detectable in these cases by virtue of the to work in inorganic chemistry as an undergraduate researcher with Mark S. exchange of the position of the labeled atom,9 there is one Wrighton at Massachusetts Institute of Technology (B.S., 1975). He obtained a Ph.D. degree in chemistry at California Institute of Technology (1979), working example in which dynamic NMR exchange has been used with Robert G. Bergman and completing his final year at Berkeley. He moved to to identify the reversible exchange.10 Several representative the University of Wisconsin as an NSF postdoctoral fellow with ACS President examples relying on isotopic exchange are shown in elect Chuck Casey, and in 1980 accepted a position as Assistant Professor at R the University of Rochester. He was promoted to Associate Professor in 1984 Scheme 1. In addition to the reversible -H/D exchange, and Professor in 1987, and is now the Charles F. Houghton Professor of Chemistry an effect of isotopic substitution on the rate of reductive and Department Chairman. Professor Jones has received several awards, elimination was also observed. These effects were mea- including an Alfred P. Sloan Research Fellowship (1984), a Camille & Henry sured by comparing the rate of alkane elimination in Dreyfus Foundation Teacher-Scholar Award (1985), a Royal Society Guest Research Fellowship (1988), a Fulbright-Hays Scholar (1988), a John Simon perdeuterio vs perprotio alkyl hydride complexes. In many Guggenheim Fellow (1988), and the ACS Award in Organometallic Chemistry cases examined, the observed kinetic isotope effect, (2003). He also serves as an Associate Editor for J. Am. Chem. Soc., beginning k /k , on alkane loss was inverse, as shown in Scheme in 2003. Professor Jones' research interests includes organometallic research H D 11,12 in strong C-X bond cleavage, catalysis, model studies, mechanisms, kinetics, 2. Hence, it became common to associate an inverse thermodynamics, and synthetic applications. isotope effect with the intermediacy of a σ-alkane com- 140 ACCOUNTS OF CHEMICAL RESEARCH / VOL. 36, NO. 2, 2003 10.1021/ar020148i CCC: $25.00 2003 American Chemical Society Published on Web 11/27/2002 Isotope Effects in C-H Bond Activation Jones Scheme 1 Scheme 2 plex. Furthermore, since virtually all of this isotope effect would likely be associated with the C-H/C-D bond- making/bond-breaking step, rather than the alkane dis- sociation step, the overall isotope effect for alkane loss was attributed to an inverse equilibrium isotope effect (i.e., H/D Keq < 1) separating the alkyl hydride complex from the σ-alkane complex. One explanation for the origin of the inverse equilib- rium isotope is that the σ-alkane complex contains an intact, strong C-HorC-D bond, whereas the alkyl hydride complex has a weaker M-HorM-D bond. Consideration of the zero-point energy differences asso- ciated with the stretching frequencies for these bonds leads to the expectation of an inverse equilibrium iso- tope effect.13 That is, with metal-hydrogen stretching frequencies being about two-thirds those of aliphatic C-H the transition state does not affect the equilibrium. Sec- bonds, a smaller difference in zero-point energies is ond, the observation of an inverse kinetic isotope effect expected in the alkyl hydride complex compared to the for the loss of alkane from an alkyl hydride/deuteride com- σ-alkane complex. There are two possibilities to be plex cannot, in itself, distinguish between these two cases. considered for the transition state separating these two Third, to determine which of the two cases pertains to complexes. In one case, shown in Scheme 3a, the zero- any given system, one must determine two of the three point energy difference in the transition state is assumed related isotope effects, since the equilibrium isotope effect to be intermediate between those of the reactant and is really just the ratio of the forward and reverse kinetic product. In the second case, shown in Scheme 3b, the isotope effects (eq 3). C-H/C-D stretching frequency is assumed to disappear k H/k D as it becomes the reaction coordinate to break the bond. K H/D ) RC RC (3) eq H D In the first case (a), one expects an inverse kinetic isotope kOC /kOC effect for the conversion of the alkyl hydride complex to With this somewhat lengthy introduction, we are now the σ-alkane complex (k H/k D), but a normal kinetic RC RC ready to describe a system that is the first to establish isotope effect for the reverse reaction (k H/k D). In the OC OC which of the two cases described above leads to the second case (b), one expects normal kinetic isotope effects inverse equilibrium isotope effect. We then offer com- in both directions, with the magnitude of the kinetic mentary on related systems described in the literature. isotope effect being larger for the conversion of the σ-alkane complex into the alkyl hydride complex. There are a few important points to be recognized 2. Kinetic Isotope Effect on Reductive immediately. First, both of these situations are consistent Coupling with an inverse equilibrium isotope effect separating the We use the term ªreductive couplingº to describe the alkyl hydride complex from the σ-alkane complex, since conversion of an alkyl hydride complex into a σ-alkane VOL. 36, NO. 2, 2003 / ACCOUNTS OF CHEMICAL RESEARCH 141 Isotope Effects in C-H Bond Activation Jones Scheme 3 complex without loss of alkane. We first set out to measure Scheme 4 the isotope effect on reductive coupling in a methyl hydride complex but found that, just as in the earlier examples of isotopic scrambling, the experiments de- scribed a combination of isotope effects rather than the isotope effect on a single, isolated step. We were fortunate to succeed by identifying a complex in which the reductive coupling step is irreversible, unlike the case with other alkyl hydride complexes. The work centers on the reactive metal fragment [Tp*Rh(CNR)] (Tp* ) tris(3,5-dimeth- ylpyrazolyl)borate, R ) neopentyl), which we have shown to activate a variety of alkane and arene C-H bonds via oxidative addition.14,15 The alkyl hydrides are stable for a few hours at room temperature before they undergo reductive elimination, allowing convenient study of their rearrangements. In benzene solvent, alkane loss is im- mediately followed by benzene activation to give the thermodynamically stable phenyl hydride complex, Tp*Rh- (CNR)(phenyl)H.
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