Hydrocarbyl complexes reductive elimination

Hydrocarbyl Complexes. Stable homoleptic and heteroleptic uranium hydrocarbyl complexes have been synthesized. Unlike the thorium analogues, uranium alkyl complexes are generally thermally unstable due to P-hydride elimination or reductive elimination processes. A rare example of a homoleptic uranium complex is U(CH(Si(CH2)3)2)3, the first stable U(I11) homoleptic complex to have been isolated. A stmctural study indicated a triganol... 

The conversion of the Ir(III) cyclohexyl hydride complex to an Ir/cyclohexane system involves a change in the formal oxidation state of Ir from + 3 to +1 (i.e., a formal two-electron reduction). As a result, this elementary reaction step is generally called a reductive coupling (Chart 11.4). From a metal hydrocarbyl hydride complex (i.e., M(R)(H)), the overall process of C H bond formation and dissociation of free hydrocarbon (or related functionalized molecule) is called reductive elimination (Chart 11.4). The reverse process, metal coordination of a C—H bond and insertion into the C—H bond, is called oxidative addition. Note Oxidative addition and reductive elimination reactions are not limited to reactions involving C and H.)... 

As a result of the combined effect of the factors mentioned above, Pd monoaUcyl complexes are expected to be the least stable among hydrocarbyls (M = Pd, Pt). Such complexes are currently unknown, whereas monoaUcyl ft " complexes were among the first compounds characterized in C(sp )-0 reductive elimination reactions [16-19]. Among monoaryl Pd " derivatives only few were isolated to date aU of them are stabilized by fluoride ligands [26, 27]. No monohydrocarbyl Pd complexes supported by 0-donor ligand have yet been reported and characterized. The most studied O-ligated organopalladium(lV) complexes are diaryl dicarbox-ylates 13 shown in Fig. 7 [11, 12]. 

An early theoretical study suggested that better 0 donors are eliminated more easily from cA-MR(R )L2 type complexes [8]. However, this prospect is now known to be applicable only to a dialkyl complex series [e.g., Me > Et > Pr > Bu]. Thus the rate of reductive elimination is rather sensitive to the other factors, especially to the orbital hybridization of hydrocarbyl ligands. In general, the reactivity decreases in the order [hydrido (s)alkenyl (sp ) > aryl (sp ) 3> alkynyl (sp) > alkyl (sp )]. While a Jt-allyl ligand ranks middle, its behavior in reductive elimination will be described separately in Section 9.4. 

N-heterocyclic carbenes (NHC) are considered extremely effective hgands for homogeneous catalysis (Figure 6.2). These specific carbenes often lead to high efficiencies in metal-catalyzed reactions compared to traditional phosphines [49, 50). NHC complexes are usually considered to be very stable, due to their electronic properties and the unusually high bond dissociation energies (BDE) associated with NHCs [51]. Previous work has shown that Ni-hydrocarbyl complexes of NHCs readily decompose by reductive elimination to yield the 2-substituted imidazohum salts [52, 53). Later studies have shown that the reverse reaction, oxidative addition of imidazohum salts to zerovalent Group 10 metals, is feasible [53]. 

The formation of a Th(C5H5)3 complex was suggested because the subsequent thermal reaction of the photogenerated thbrium monohydride with unreacted starting hydrocarbyl complexes, gives the binuclear reductive elimination of alkane ... 

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