Catalysis oxidative addition/reductive elimination

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. 

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. 

An alternative mechanism for catalysis of Si-Si bond formation by later transition metal complexes was originally proposed by Curtis and Epstein.33 This mechanism, illustrated in Scheme 3, also invokes a sequence of oxidative addition/reductive elimination steps but does not resort to a silylene intermediate. A serious weakness in this scheme is that, as far as we know, no example of the spontaneous reductive elimination of a disilane from a bis(silyl) complex has yet been observed. 

Homogeneous catalysis dissociation association oxidative addition reductive elimination... 

In the majority of catalytic reactions discussed in this chapter it has been possible to rationalize the reaction mechanism on the basis of the spectroscopic or structural identification of reaction intermediates, kinetic studies, and model reactions. Most of the reactions involve steps already discussed in Chapter 21, such as oxidative addition, reductive elimination, and insertion reactions. One may note, however, that it is sometimes difficult to be sure that a reaction is indeed homogeneous and not catalyzed heterogeneously by a decomposition product, such as a metal colloid, or by the surface of the reaction vessel. Some tests have been devised, for example the addition of mercury would poison any catalysis by metallic platinum particles but would not affect platinum complexes in solution, and unsaturated polymers are hydrogenated only by homogeneous catalysts. 

Evidence for alkane activation has also been seen by the observation of H/D exchange between two alkanes, an alkane and an arene, or an alkane and THE Using CpRe(PPh3)2H2 as the photo catalyst, thousands of turnovers have been observed. While the intermediate responsible for this catalysis was not identified, it does not appear to be [CpRe(PPh3)H2] undergoing Rem/Rev oxidative addition/reductive elimination, since no deuterium incorporation was observed in the dihydride catalyst [99]. Several other metal hydrides are known to catalyze H/D exchange between alkanes and deuterated benzene, such as Ir(PMe3)2H5 [100],CpMo(dmpe)H3 [101], and Re[P(c-hexyl)3]2H7 [102]. 

Various elementary processes such as oxidative addition, reductive elimination, olefin and CO insertion into the metal-to-carbon bond have found extensive applications in organic synthesis. Other processes such as attack of nucleophiles on metal-bound CO and olefins, unique reactions of metal carbene complexes, and a-bond metatheses are among the topics of special interest to organometalhc chemists as well as to synthetic organic chemists. Our aim is to provide the reader with detailed accounts of elementary processes with the hope that the information provided here is used for further development of molecular catalysis. 

Elementary steps in binuclear catalysis can differ significantly from those described for mononuclear complexes due to the proximity of a secrmd metal center. A brief description of binuclear oxidative addition, reductive elimination, ligand migration, and migratory insertion will be made in order to facilitate the understanding of the mechanisms discussed in this chapter. 

Organolanthanide complexes differ from late d-block transition metal complexes in several aspects. They are electrophilic, kinetically labile and lack conventional oxidative addition/reductive elimination pathways in their reactions. They have alternative mechanisms to perform catalytic transformations and are being increasingly used in homogeneous catalysis. The hydrophosphination reaction was proposed to proceed through the cycle depicted in... 

Homogeneous catalysis mediated by complexes of transition elements relies primarily on the ability of such complexes to undergo oxidative addition/reductive elimination reactions. ... 

Complexes bearing protic NHC ligands are accessible by various synthetic routes such as the deprotonation of azoles followed by reaction with a transition metal complex, the template-controlled cyclization of functionalized isocyanides, and the oxidative addition of different azoles to transition metal complexes. The complexes with simple monodentate NR,NH-NHCs often tend to tautomerize to give the N-bound azoles. This type of tautomerization is prevented in complexes with donor-functionalized NR,NH-NHCs. Recent smdies demonstrate that complexes with protic NHCs obtained from C2-H azoles are formed by an oxidative addition/reductive elimination reaction sequence. The N—H group in complexes with protic NR,NH-NHCs can serve as a hydrogen bond donor and thus as a molecular recognition unit and may enable various types of bifunctional catalysis. Recent smdies indicate that even biomolecules such as caffeine can be C8-metallated. It... 

Harrod has recently described theoretical work on a metal-silylene mechanism.In the former paper Tilley proposed that the mechanism for catalysis by the early transition metals differs from that of the late transition metals. With the early transition metals a sigma metathesis mechanism was proposed in which no formal change in the oxidation state of the metal occurs during the catalytic cycle. Tilley also suggested that oxidative-addition reductive-elimination mechanisms are the most consistent with results obtained from the late transition metals. This is an application of the general mechanisms... 

As mentioned at the very start of the chapter, +2 is the most common oxidation state for Group 10 metals due to the d electron configuration. For palladium, Pd(II) species have played a pivotal role in elementary reactions in palladium-catalysis, for example, oxidative addition/reductive elimination, and thus have received extensive research interest historically. In the following section, the advancements of dipalladium(II) compounds with Pd(II) - Pd(II) bonds in the last decade will be summarized. 

The phase-transfer catalysis method has also been utilized effectively for addition of dichlorocarbene to olefins,4 as well as for substitution and elimination reactions, oxidations, and reductions.18 The preceding procedure in this volume is another example.13... 

The acrylate complex 10 was suggested to be the major solution species during catalysis, since the equilibrium in Scheme 5-11, Eq. (2) lies to the right (fQq > 100)-Phosphine exchange at Pt was observed by NMR, but no evidence for four-coordinate PtL, was obtained. These observations help to explain why the excess of phosphine present (both products and starting materials) does not poison the catalyst. Pringle proposed a mechanism similar to that for formaldehyde and acrylonitrile hydrophosphination, involving P-H oxidative addition, insertion of olefin into the M-H bond, and P-C reductive elimination (as in Schemes 5-3 and 5-5) [11,12]. 

Key words ONIOM, hydrogenation, enantioselectivity, asymmetric catalysis, DFT, reaction mechanism, chiral phosphine, ab initio, valence bond, oxidative addition, migratory insertion, reductive elimination. 

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