Oxidative addition/reductive elimination

The coordination chemistry of NO is often compared to that of CO but, whereas carbonyls are frequently prepared by reactions involving CO at high pressures and temperatures, this route is less viable for nitrosyls because of the thermodynamic instability of NO and its propensity to disproportionate or decompose under such conditions (p. 446). Nitrosyl complexes can sometimes be made by transformations involving pre-existing NO complexes, e.g. by ligand replacement, oxidative addition, reductive elimination or condensation reactions (reductive, thermal or photolytic). Typical examples are ... 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

This special feature arises from the combination of the transition metal behavior such as the coordination of a carbon-carbon multiple bond, oxidative addition, reductive elimination, P-hydride elimination, addition reactions and the behavior of classical c-carbanion towards electrophiles. 

Another reaction type to be mentioned in this section deals with oxidative addition/reductive elimination. Such reactions not only involve significant bond formation/bond breakage, but also a change in the oxidation state and coordination number of the metal complex. These effects cause significant volume changes such that large... 

Figure 12.3. Hydrogen as the source for hydride formation (4) and oxidative addition /reductive elimination related to hydride formation (5)...

Alkane metathesis was first reported in 1997 [84]. Acyclic alkanes, with the exception of methane, in contact with a silica supported tantalum hydride ](=SiO)2TaH] were transformed into their lower and higher homologues (for instance, ethane was transformed into methane and propane). Later, the reverse reaction was also reported [85]. Taking into accountthe high electrophilic character ofa tantalum(III) species, two mechanistic hypotheses were then envisaged (i) successive oxidative addition/reductive elimination steps and (ii) o-bond metathesis. Further work has shown that aLkyhdene hydrides are critical intermediates, and that carbon-carbon... 

Diyne cyclization/hydrosilylation catalyzed by 4 was proposed to occur via a mechanism analogous to that proposed for nickel-catalyzed diyne cyclization/hydrosilylation (Scheme 4). It was worth noting that experimental evidence pointed to a silane-promoted reductive elimination pathway. In particular, reaction of dimethyl dipropargylmalonate with HSiMc2Et (3 equiv.) catalyzed by 4 led to predominant formation of the disilylated uncyclized compound 5 in 51% yield, whereas slow addition of HSiMe2Et to a mixture of the diyne and 4 led to predominant formation of silylated 1,2-dialkylidene cyclopentane 6 (Scheme 5). This and related observations were consistent with a mechanism involving silane-promoted G-H reductive elimination from alkenylrhodium hydride species Id to form silylated uncyclized products in competition with intramolecular carbometallation of Id to form cyclization/hydrosilylation products (Scheme 4). Silane-promoted reductive elimination could occur either via an oxidative addition/reductive elimination sequence involving an Rh(v) intermediate, or via a cr-bond metathesis pathway. 

Organothorium complexes such as [Th(r 3-allyI )4] supported on dehydroxylated y-alumina have been shown to exhibit activities rivaling those of the most active platinum metal catalysts.123 Thorium maintains its original +4 oxidation states at all times that is, the mechanism does not follow the usual oxidative addition-reductive elimination pathway. Partially hydrogenated products cannot be detected... 

The results can be rationalized in terms of an oxidative addition-reductive elimination mechanism as illustrated with XXII. A similar mechanism has been proposed for the acid cleavage of Pt(II)-C sigma bond (57, 58). 

An oxidative addition-reductive elimination sequence is expected to result in gold-methyl bond cleavage, so the latter was dismissed as a possible mechanism. 

The regioselective and enantiospecific allylic substitution of alkyl-substituted allyl benzoates and carbamates with (Me2PhSi)2Zn and Cul has been shown to occur by an oxidative addition - reduction elimination mechanism rather than an SN2 mechanism.16... 

Synthetic routes to compounds containing M-C o bonds are fairly obvious. Substitution of, e.g., Cl by CH, can be effected by treatment with LiCH3 or CH3MgBr. A number of reaction types mentioned in Chapter 9 - oxidative addition, reductive elimination, insertion and cyclometallation (Sections 9.6 and 9.7) - have their uses in preparative routes to M-C bonds. The formation of organo-compounds of the lanthanides and actinides is an area of growing interest. Preparative methods are similar to those for other ER species where E is of relatively low electronegativity, e.g. ... 

The high formal oxidation states of metals in some of these adducts is noteworthy, e.g., Fe(IV) (entries 17 and 18), Ru(IV) (entries 21 and 22), and Pt(IV) (entries 55 and 56). Such adducts are important because they provide definite examples of species often postulated as intermediates in oxidative addition-reductive elimination processes (compare Section II,G,1) and in homogeneous catalysis (134,220a, 410a). In the case of germanium, a tris(germyl) adduct of Pt(IV) has been described (57), but no more than two silyl groups per metal atom are known to result from oxidative addition. 

An oxidative route to 1,3-thiazoles (39) and oxazoles, which bear the requisite functionality, such as amino groups and stereocenters, for incorporation into a variety of natural products was reported. Treatment of 1,3-thiazolines (36) with CuBr (l.leq), Cu(OAc)2 (l.leq) and t-butyl perbenzoate (1.5eq) under benzene reflux gave 1,3-thiazoles (39) in about 80% yield. A plausible mechanism included generation of a Cu (III) species (37) via oxidative addition, reductive elimination to the acyloxy thiazoline (38), and syn elimination on warming to produce the thiazole (39). [94TL6803]... 

A related topic that was already discussed in these first DFT/MM works is that of branching. The scheme shown in Fig. 1 would always produce always a linear polymer if ethylene was used as olefin. But a simple process of / C-H oxidative addition/reductive elimination, coupled with olefin rotation, can produce a branched polymer, as shown in Fig. 4. Calculations on the branching process for cationic diimine Ni(II) complexes [36, 37] indicated a small increase between 0.9 and 2.5 kcal/mol in the barrier for this process, associated with the introduction of the bulky substituents in the catalysts. 

The efficiency of /-elements in catalysis originates from unconventional electrophilic pathways. In contrast to rf-elements oxidative addition/reductive elimination sequences are not accessible. Instead, substrate adduct formation, ligand exchange and insertion reactions rule the mechanistic scenarios. Therefore, the main emphasis is put on the fine-tuning of the spectator ligand of the precatalyst. 

Each reaction type (oxidative addition, reductive elimination, etc.) was studied according to the electronic configuration (at this time only the even dn configurations have been considered), the coordination number, and the coordination geometry. The matrices that we have composed for the evaluation (Matrices 1, 2, 3, 4, 5, 6, and 7 see also Section I,C), show the structure d"-ML for which the reaction is allowed or forbidden. We must note that, in most of the cases, the rules that we present derivefrom theoretical studies found in the literature and that exceptions certainly exist. Another difficulty in this reaction evaluation is the importance of the coordination geometry (15b), related to the spin state (low or high), the choice of which is particularly difficult. 

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