Oxidative Addition Pathways

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. 

In a study of the methane complex [(diimine)Pt(CH3)(CH4)]+ (diimine = HN=C(H)-C(H)=NH), relevant to the diimine system experimentally investigated by Tilset et al. (28), theoretical calculations indicate preference for the oxidative addition pathway (30). When one water molecule was included in these calculations, the preference for oxidative addition increased due to the stabilization of Pt(IV) by coordinated water (30). The same preference for oxidative addition was previously calculated for the ethylenediamine (en) system [(en)Pt(CH3)(CH4)]+ (151). This model is relevant for the experimentally investigated tmeda system [(tmeda)Pt(CH3)(solv)]+ discussed above (Scheme 7, B) (27,152). For the bis-formate complex Pt(02CH)2, a a-bond metathesis was assumed and the energies of intermediates and transition states were calculated... 

Unfortunately the potential alternative of an oxidative addition pathway was not investigated in this work. 

Given the discussion above detailing that in many systems C-H activation at Pt(II) occurs via an oxidative addition pathway, it is perhaps not surprising that Pt(II) species capable of C-H activation need not be very electrophilic. On the contrary, it might be intuitively expected that electron-rich Pt(II) centers are more suitable to oxidatively add a hydrocarbon molecule. Theoretical analyses, in contrast, seem to suggest rather that electron-poor Pt(II) centers add hydrocarbon molecules more favorably (see above, Section III.F). A recent experimental study... 

Oxidative addition consumes one equivalent of expensive Pd(OAc)2 in most cases. However, progress has been made towards the catalytic oxidative addition pathway. Knolker s group described one of the first oxidative cyclizations using catalytic Pd(OAc)2 in the synthesis of indoles [19]. They reoxidized Pd(0) to Pd(II) with cupric acetate similar to the Wacker reaction, making the reaction catalytic with respect to palladium [20]. 

Other possible mechanisms, corresponding to those discussed for the alkyne, have been considered. The oxidative addition pathway is excluded because all vinyl intermediates (see below) are found at high energy with respect to the n olefin complex. 

Even though the oxidative addition pathway of imidazolium salts has been shown to be possible at certain metal complexes under special circumstances, it is far from being generally applicable to the synthesis of NHC complexes [60-67]. 

We believe this definition captures the essence of numerous proposals in the organometallic literature [10], Indeed, the term C-H bond activation is used routinely to differentiate, for instance, between oxidative addition pathways and deprotonation [11],... 

Another instructive scenario may be found when considering the metalation of arenes. There are two distinct mechanisms for the metalation of aromatic C-H bonds - electrophilic substitution and concerted oxidative addition (Box2). The classical arene mercuration, known for more than a century, serves to illustrate the electrophilic pathway whereas the metal hydride-catalyzed deuterium labeling of arenes document the concerted oxidative addition mechanism [8, 17]. These two processes differ both in kinetic behavior and regioselectivity and thus we may appreciate the need to differentiate these two types of process. However, the choice of C-H bond activation to designate only one, the oxidative addition pathway, creates a similar linguistic paradox. Indeed, it is hard to argue that the C-H bond in the cationic cr-complex is not activated. 

The C-H activation step could, in principle, occur either by oxidative addition of the C-H bond - pathway (a) - or by electrophilic displacement - pathway (b). The oxidative addition pathway would proceed via the formation of a palla-dium(IV) species. Although such intermediates have been postulated in some coupling reactions catalyzed by palladacycles, as yet no conclusive experimental evidence has been presented [14], It is perhaps more likely that C-H activation results from electrophilic displacement of the ortho proton [15]. 

Especially for alkyl halides 6 the transfer of a single electron from the metal center is facile and occurs at the halide via transition state 6C, which stabilizes either by direct abstraction of the halide to a carbon-metal complex radical pair 6D or via a distinct radical anion-metal complex pair 6E. This process was noted early but not exploited until recently (review [45]). Alkyl tosylates or triflates are not easily reduced by SET, and thus Sn2 and/or oxidative addition pathways are common. The generation of cr-radicals from aryl and vinyl halides has been observed, but is rarer due to the energy requirement for their generation. Normally, two-electron oxidative addition prevails. 

Although oxidizing agents are not tolerated by most systems that activate C-H bonds through an oxidative addition pathway, they are compatible with boranes. In a series of elegant papers, Hartwig has demonstrated the selective formation of... 

Although formally considered Ru(II) species, an important class of organo-ruthenium complexes can be accessed through ring opening of cycloalkenes Such ruthenium carbene complexes have recently been synthesized through oxidative addition pathways. ... 

The preparation of allyltitanium compounds including those having functional groups is described by reaction of allylic halides or allylic alcohol derivatives with the system Ti(OPr1)4/MgXPr1 (X = C1, Br) (Scheme 7).24 Analogous allyltitanium complexes have also been reported by treatment of Ti(n) species with allylic alcohol derivatives, which proceeds via an oxidative addition pathway. Their reactions have been studied.25-27 These compounds are used to promote efficient syntheses of alkylidenecyclopropane and cycloalkane derivatives by regioselective reactions with carbonyl compounds,28,29 the stereoselective syntheses of optically active substituted piperidines and pyrrolidines... 

Polar Oxidative Addition Pathways Addition ofX2... 

Subsequent work with a Pt11 complex containing a labile triflate ligand showed evidence for the alkyl/aryl exchange labeled as type (c) in Fig. 1. While mechanistic studies were limited, the observation of little positional selectivity in the reaction with toluene argued in favor of the oxidative addition pathway, i.e., via a PtIV intermediate (Eq. 13) [60]. 

This isomerization process is thought to occur through the phosphine arm opening, but the Ar-H reductive-elimination-oxidative-addition pathway is not excluded [29]. 

Two independent pathways for the exchange mechanism, both of which accommodate experimental observations, have been proposed (57) and these are shown in Fig. 4. Pathway I may be called the oxidative addition pathway. It involves attack on 29 to give a 5-coordinate Pt(II) 7t-complex intermediate (or transition state), 30. Complex 30 then undergoes the oxidative addition (or metal insertion into an aromatic C—H bond) to give the Pt(IV) complex, 31. This step is... 

The group which brings the C—H bond with its two electrons into the vicinity of the metal centre has been called agostic by Brookhart and Green. These complexes can be considered as frozen structures on the pathway for the insertion of a metal into a C—H bond, in the same way that molecular hydrogen complexes were treated on the oxidative addition pathway that leads to a dihydride ( 4.1.4). 

In the activation of C-H bonds of hydrocarbons by either electrophilic or oxidative addition pathways, complexation of the alkane is believed to precede oxidative cleavage of the C-H bond. " In several cases, direct spectroscopic detection of these species (IR, NMR) has been possible, and provides further evidence for their presence beyond the early work with M(CO)s species in matrices " " and solution by photoacoustic spectroscopy. ... 

However, the mechanistic scheme for this catalytic cycle (Shilov system) is a topic of debate and there have been alternative proposals for of oxidative addition pathways that have been put forth, and experimental evidence provided to support the theory. Excellent review articles have focused on catalytic oxidation of alkanes, however herein we will discuss only those examples for the synthesis of bi(hetero)aryls involving an electrophilic aromatic substitution mechanism. 

Recent upsurge in the number of reports for the application of cyclome-talated complexes in C-C, C-F and C-H bond activation has therefore been a direct consequence of the pioneering work done by Chatt and others. Accordingly, in this section we will now discuss in more detail with recent examples, the impact of this oxidative addition pathway on C-H bond functionalization. 

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