Electron intermediates

Formally, the metal oxidation number x increases to x+2, while the coordination number n of ML, increases to n+2. If such oxidative addition reactions are intended to be the first step in a sequence of transformations, which eventually will lead to a functionalization reaction of C-X, then the oxidative addition product 2 should still be capable of coordinating further substrate molecules in order to initiate their insertion, subsequent reductive elimination, or the like [1], This is why 14 electron intermediates MLu (1) are of particular interest. In this case species 2 are 16 electron complexes themselves, and as such may still be reactive enough to bind another reaction partner. 

The most practicable and easy-to-use precursor complex for the desired 14 electron intermediate... 

The process is thought to be electrophilic in nature via a coordinatively unsaturated, 14-electron intermediate (Scheme 4). 

Until now, for most of the systems described here it has been accepted that alkane activation occurred through oxidative addition to the 14-electron intermediate complexes. Yet, Belli and Jensen [26] showed, for the first time, evidence for an alternative reaction path for the catalytic dehydrogenation of COA with complex [lrClH2(P Pr3)2] (22) which invoked an Ir(V) species. Catalytic and labeling experiments led these authors to propose an active mechanism (Scheme 13.12), on the basis of which they concluded that the dehydrogenation of COA by compound 22 did not involve an intermediate 14-electron complex [17-21], but rather the association of COA to an intermediate alkyl-hydride complex (Scheme 13.12). 

There is an alternative pathway to II, in which the phosphine dissociates before the alkene group coordinates pathway III. On the basis of electron accountancy alone, this should be viewed as unfavourable as it involves two 14-electron intermediates (26 and 27). However, it should be noted that the mechanism-derived rate equation for reaction via pathways I/III rather than I/II would be equally consistent with the empirical rate equation. 

In this dissociative pathway (which is assumed to be the major one today) first a phosphine is displaced from the metal center to form an active 14-electron-intermediate 42. After alkene coordination cis to the alkylidene fragment the 16-electron-olefine-complex 43 undergoes [2 + 2]-cycloaddition to give a metallacylobutane 44. Compound 44 breaks down in a symmetric fashion to form carbene complex 45. The ethylene is replaced in the conversion to complex 46. In the next steps (they are not further discribed above), another intramolecular [2 + 2]-cycloaddition joins up the eight-membered ring 11 regenerating the catalyst 42. Each step of the reaction is thermodynamically controlled making the whole RCM reversible. With additional excess of phosphine added to the reaction mixture an associative mechanism is achieved, in which both phosphines remain bound. 

Transition metal (TM) chemistry stands in contrast to this. Many compounds involve metal centres with partially filled d shells, and/or with one or several unpaired electrons. Therefore, it is not always straightforward to predict the orbital occupation pattern of a given stable compound. For intermediates on a reactive pathway, this is an even greater problem. This is also true for organometallic chemistry, despite the fact that many compounds obey the 18-electron rule and have closed-shell singlet ground states. Thus, there are many 16- or even 14-electron intermediates, odd-electron species [1], and polymetallic clusters and complexes for which the spin state is not readily predicted. 

The complexes are almost invariably planar, but with bulky ligands some distortion can occur. Cis and trans isomers of MX2L2 can interconvert, either by a dissociative pathway via a T-shaped 14-electron intermediate MX2L (as, for example, in the case of PtR R = alkyl or aryl, L = S-ligand)10 or by an associative pathway involving 5-coordinate intermediates, where the process is catalyzed by an excess of phosphine or a coordinating solvent (e.g., MeCN). 

Catalyst initiation involves the formation of a metathesis-active ruthenium species from the starting precatalyst and its entry into the catalytic cyde. For both Ru-2 and Ru-4, the initiation event consists of phosphine (PCys) dissodation to produce the 14-electron intermediate [(L)(Cl)2Rr CHR ], where L= PCys for Ru-2 and L = H2lMes for Ru-4) (Figure 6.4). Although this proposed spedes has not been observed in solution, it has been identified in the gas phase [7], and the ligand dissociation step has been studied by NMR magnetization transfer experiments. 

From the bis(ethylene) precursor complex the highly reactive unsaturated 14-electron intermediate Ir(PEt3)2Cl is generated as the actual catalyst by stepwise dissociation of ethylene. Related compounds M(Pr 3P)2Cl (M = Rh,... 

Dissociation of 47 to 48, a 14-electron intermediate (unless we count solvent coordination), followed by oxidative addition of H2 seems at first glance implausible... 

The mechanism of Ru-alkylidene-catalyzed reactions has been investigated. Note that Grubbs first- and second-generation catalysts are 16-electron species, so if the first step involves complexation of an alkene to the metal, this process could occur in an associative or dissociative manner. Evidence suggests (see Scheme 11.5) that this occurs in a dissociative manner, however, first forming a 14-electron intermediate 25 and then 26a or 26b after complexation of the alkene. Gas-phase mass spectral evidence supports the initial formation of 25. Complexes similar to 26a and b have been isolated from reaction mixtures under appropriate conditions, but ruthenacyclobutane 27 has not been directly observed until quite recently.37... 

At first fhe unprecedented activity of fhe second-generation catalysts was thought to be a result of faster phosphine dissociation [95]. This result indicated that fhe phosphine dissociation kinetics as well as fhe metathesis activity of the resulting 14-electron intermediate must be considered to rationalize the activity of these catalysts. Interestingly, returning to PPhs is valuable for fhe NHC complexes, in fhat fhe phosphine exchange rate for complex 11 is almost fhat of complex 6 [101]. 

Stable transition-metal complexes of tetramesityldisilene result upon irradiation with oxalato bis(tertiary phosphine) complexes of platinum87. The latter photofragment with loss of C02 to yield the 14-electron intermediate (R3P)2Pt, which then apparently adds to the disilene to yield 33 (R = Et or Ph) (equation 30). The two complexes were isolated in the form of air-sensitive orange-red oils and characterized by NMR and mass spectra. The structure of the triethylphosphine derivative was further secured by addition of methanol across the Pt-Si bond. 

More recent mechanistic studies have been able to distinguish between pathways (b) and (c), and all results indicate that (c) is operative [87]. The initial ligand dissociation and substitution steps have been studied using NMR magnetization transfer experiments, NMR and UV-vis kinetics, and mass spectrometry [87,88]. These investigations indicate that both phosphine/phosphine (Fig. 4.31a) and phosphine/olehn (Fig. 4.31b) substitution reactions in (L)(PR3)(X)2Ru= CHR complexes proceed by a dissociative mechanism involving a 14-electron intermediate (L)(X)2Ru=CHR (A). Although this proposed intermediate has not been observed in solution, presumably due to its low concentration, it has been identihed in the gas phase [88]. 

Despite steadily accumulating evidence over the years that some reactions of some platinum complexes proceed by ligand dissociation to three-coordinate T-shaped 14-electron intermediates, and notwithstanding the persuasive activation volume data reported above, the inability of kinetic data alone to rule out the intervention of solvent intermediates as alternative pathways has always left room for doubt. Thus a paper on the reversible loss of olefin from tran5-[PtCl2(ol)L]... 

---