Alkanes oxidative addition

Prior to 1982, Crabtree s report of the reaction of cyclopentane with a solvated IrH2(PPh3)2+ species to give a cyclopentadienyl-iridium product stood as the only well characterized example of a reaction of an alkane with a homogeneous transition metal, in contrast to the widespread reactivity of arenes [2]. Based upon the instability of the platinum methyl hydride complex Pt(PPh3)2(CH3)H, it was believed that alkane oxidative addition might not be a thermodynamically feasible process, and consequently few attempts were made to attempt such a reaction [3]. It was not until the discovery of the formation of stable alkane oxidative addition products in 1982 that it was realized that reactions of hydrocarbons were in fact feasible. 

The first reports were based upon reactions of the [Cp IrL] fragment where L=PMe3 or CO and Cp =r 5-C5Me5 by Bergman and Graham, respectively [4,5]. A less stable alkane oxidative addition product to the fragment [Cp Rh(PMe3)] was also reported by Jones [6]. In all of these cases, irradiation was used to either... 

One other example of alkane oxidative addition to a higher oxidation state late transition metal has been reported by Goldberg. Reaction of the trispyra-zolylborate complex K[r 2-Tp PtMe2] with B(C6F5)3 leads to the abstraction of a methyl anion and the formation of a transient species that adds to the C-H bonds of benzene, pentane, or cyclohexane (Eq. 15). This result provides the first example of the intermolecular addition of a C-H bond to a Ptn species to give a stable PtIV product [71]. Earlier work by Templeton had demonstrated that the trispyrazolylborateplatinumdialkylhydride product would be stable [72]. 

Graham has reported that irradiation of (r 6-C6Me6)Os(CO)2 in alkane solution leads to the formation of alkane oxidative addition complexes in competition with C6Me6 loss [97]. Perutz and Werner have also reported the photochemical reaction of (r 6-mesitylene)Os(CO)H2 in methane matrices leading to the formation of the methane activation product (ri6-mesitylene)Os(CO)(CH3)H [98]. 

Analogous studies of Cp Rh(CO)2 were used to compare reaction with various alkanes.103 Photolysis of Cp Rh(CO)2 in liquid Kr initially produces Cp Rh (CO)(Kr), with i co = 1947 cm-1. In addition, CO absorptions in the range of 2000-2008 cm-1 due to the formation of the products of C—H oxidative addition, Cp Rh(CO)(R)(H), are observed. Using the mechanism for alkane oxidative addition shown in Scheme 11.48 and the expression for kobs, monitoring the rate of formation of Cp Rh(CO)(R)(H) using time-resolved IR spectroscopy allowed extraction of ky and Keq. These data were acquired at temperatures between 80 and 110°C. 

Scheme 1 Deuterium labeling in organometallic complexes and alkane oxidative addition/reductive elimination reactions.

Recent topics regarding alkane oxidative addition include the regioselective reaction of a methyl group with the Pt(II) and W(II) centers (eqs. (4) and (5)) (7,8). 

The unusual feature of oxidative addition reactions of transition metals is the unusually wide range of addends A—B that can be involved, including such normally relatively unreactive molecules as silanes, H2, and even alkanes. Oxidative additions are a very diverse group of reactions in terms of mechanism, and we shall therefore consider each type separately. 

METALLATE or 1 6-electron complex + ALKYL HALIDE OR ALKANE (OXIDATIVE ADDITION)... 

The reactions typically proceed at 150°C with n-octane and di-pinacolboronate for 5-24 h, and the amount of catalyst typically is 1-5%. The yields of 1-octylboronate ester are good and the reaction is regioselective in the terminal alkyl position (second equation below, top of p. 418). The proposed mechanism involves oxidative addition of the B-B or B-H bond, followed by a-bond metathesis between the M-B and R-H bonds, which is driven towards formation of the B-R bond by the Lewis-acid property of boron. Note that 16e species (see Scheme p. 418) could also be involved in oxidative addition of the alkane to give an 18e intermediate M(Bpin)2(H)(R) or M(Bpin)(H)2(R) that would provide R-Bpin as well by reductive elimination. Calculations showed, however, that the o-bond metathesis path is preferred by about 10 kcal mol" over this alkane oxidative-addition path. 

An additional curious feature of alkylaromatic oxidation is that, under conditions where the initial attack involves electron transfer, the relative rate of attack on different alkyl groups attached to the same aromatic ring is quite different from that observed in alkane oxidation. For example, the oxidation of -cymene can lead to high yields of -isopropylbenzoic acid (2,205,297,298). 

This synthetic approach is known from the synthesis of L M(alkene)H compounds from LnM(CO)alkane precursors and can easily be applied to the analogous silyl complexes. The Si—H bond even shows an increased activity for oxidative addition reactions [42, 43]. 

The proposed catalytic cycle, which is based on experimental data, is shown in Scheme 6. Loss of 2 equiv. of N2 from 5 (or alternatively 1 equiv. of N2 or 1 equiv. of H2 from complexes shown in Scheme 3) affords the active species a. Olefin coordination giving b is considered to be preferred over oxidative addition of H2. Then, oxidative addition of H2 to b provides the olefin dihydride intermediate c. Olefin insertion giving d and subsequent alkane reductive elimination yields the saturated product and regenerates the catalytically active species a. 

Alkane oxidation via a hydroperoxide was suggested many years ago, and seems to be operative in Acinetobacter sp. strain M-1 that has, in addition, a rather unusual range of substrates that include both n-alkanes and -alkenes. The purified enzyme contains FAD and requires copper for activity (Maeng et al. 1996). 

Note that the main difference between zirconium hydride and tantalum hydride is that tantalum hydride is formally a d 8-electron Ta complex. On the one hand, a direct oxidative addition of the carbon-carbon bond of ethane or other alkanes could explain the products such a type of elementary step is rare and is usually a high energy process. On the other hand, formation of tantalum alkyl intermediates via C - H bond activation, a process already ob-... 

The most recent catalysts that operate under thermal conditions were then based on the premise that a Cp M fragment with ligands that dissociate under thermal conditions could be a catalyst for alkane borylation. After a brief study of Cp IrH4 and Cp Ir(ethylene)2, Dr. Chen studied related rhodium complexes. Ultimately, he proposed that the Cp Rh(ri" -C6Me6) complex would dissociate CeMce as an iimocent side product, and that Cp Rh(Bpin)2 from oxidative addition of pinBBpin (pin=pinacolate) would be the active catalyst. The overall catalytic... 

Two main mechanisms may be proposed for the first step of the alkane interaction with platinum(II) complexes (1) oxidative addition... 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

A mechanism has been proposed involving CO dissociation from the metal complex followed by oxidative addition of the diboron analog. This precedes the alkane functionalization process (Scheme 8). 

Iridium hydride complexes effectively catalyze addition of nitriles or 1,3-dicarbonyl compounds (pronucleophiles) to the C=N triple bonds of nitriles to afford enamines.42S,42Sa Highly chemoselective activation of both the a-C-H bonds and the C=N triple bonds of nitriles has been observed (Equation (72)). To activate simple alkane dinitriles, IrHs(P1Pr3)2 has proved to be more effective (Equation (73)). The reaction likely proceeds through oxidative addition of the a-C-H bonds of pronucleophiles to iridium followed by selective insertion of the CN triple bonds to the Ir-C bond. 

For the C-H activation sequence, the different possibilities to be considered are shown in Scheme 5 (a) direct oxidative addition to square-planar Pt(II) to form a six-coordinate Pt(IV) intermediate and (b, c) mechanisms involving a Pt(II) alkane complex intermediate. In (b) the alkane complex is deprotonated (which is referred to as the electrophilic mechanism) while in (c) oxidative addition occurs to form a five-coordinate Pt(IV) species which is subsequently deprotonated to form the Pt(II) alkyl product. 

For the oxidative addition pathway, however, it is not obvious why the C-H bond cleavage reaction should be more facile if the hydrocarbon first binds in the coordination sphere of the metal (Scheme 5, c). One argument could be that the equilibrium between the Pt(II) alkane complex and the five-coordinate Pt(IV) alkyl hydride has an intrinsically low activation barrier. Insight into this question together with detailed information about the mechanisms of these Pt(II) a-complex/Pt(IV) alkyl hydride interconversions has been gained via detailed studies of reductive elimination reactions from Pt(IV), as discussed below. 

The oxidative addition of alkane C-H bonds to Pt(II) has also been observed in these TpRa -based platinum systems. As shown in Scheme 19, methide abstraction from the anionic Pt(II) complex (K2-TpMe2)PtMe2 by the Lewis acid B(C6F5)3 resulted in C-H oxidative addition of the hydrocarbon solvent (88). When this was done in pentane solution, the pentyl(hydrido)platinum(IV) complex E (R = pentyl) was observed as a... 

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