Reductive elimination pathway

By one pathway, reductive elimination of R and the electrophile gives R-E with retention of stereochemistry at the metal-bound carbon (recall from Chapter 8 that concerted reductive elimination occurs with retention of configuration at this site). For example, the f/ireo-3,3-dimethylbutyl-l,2-(ij iron complex shown in Equation 12.16 reacts with mercuric chloride to form f/zreo-3,3-dimethylbutyl-l, 2-mercuric chloride. Similar results have been obtained for the reactions of t/u eo-PhCHDCHDFe(CO)2Cp and tra)is-(tlzreo-PhCHDCHD)... 

Catalytic mechanisms are proposed that invoke both aurophilic di-gold(I) intermediates and covalently bonded di-gold(II) entities. The proposed intermediates are semi-supported or unsupported, according to the definition of Figure 11.16. Geometries and energies of putative intermediates are calculated within density-functional theory the computations support a bimetallic pathway. Reductive elimination, induced by an arylboronic acid, proceeds with retention of stereochemistry at carbon. 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

The coupling of terminal alkynes with aryl or alkenyl halides catalysed by palladium and a copper co-catalyst in a basic medium is known as the Sonogashira reaction. A Cu(I)-acetylide complex is formed in situ and transmetallates to the Pd(II) complex obtained after oxidative addition of the halide. Through a reductive elimination pathway the reaction delivers substituted alkynes as products. 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

A comprehensive experimental and theoretical study was undertaken on the reaction, which was shown to be a concerted reductive elimination process kinetic studies were consistent with reductive elimination, and DFT calculations on complex 3 (Fig. 13.1) supported an associative reductive elimination pathway with a small... 

In studies involving the reaction of a cA-[PdCl(Me)(NHC)]j chloro-bridged dimer with CO it was demonstrated that reductive elimination is also extremely facile for NHC-Pd-acyl complexes, yielding 2-acylimidazolium salts and Pd(0) (Scheme 13.4) [22]. The product distribution was shown to depend on the stracture of the complexes from which reductive coupling took place (pathways A and B, Scheme 13.4). 

Pd, or Ni (Scheme 5-3). First, P-H oxidative addition of PH3 or hydroxymethyl-substituted derivatives gives a phosphido hydride complex. P-C bond formation was then suggested to occur in two possible pathways. In one, formaldehyde insertion into the M-H bond gives a hydroxymethyl complex, which undergoes P-C reductive elimination to give the product. Alternatively, nucleophilic attack of the phosphido group on formaldehyde gives a zwitterionic species, followed by proton transfer to form the O-H bond [7]. 

Fig. 5. Possible mechanisms for the MMO hydroxylation step. Pathway A insertion of the oxygen atom of Q into the C-H bond B concerted addition of the C-H bond to Q followed by reductive elimination C, D homolytic attack of Q on the C-H bond E reaction of the peroxo species with substrate.

Alternative paths for decomposition of the metal carboxylate can lead to ketones, acid anhydrides, esters, acid fluorides (1,11,22,68,77,78), and various coupling products (21,77,78), and aspects of these reactions have been reviewed (1,11). Competition from these routes is often substantial when thermal decomposition is carried out in the absence of a solvent (Section III,D), and their formation is attributable to homolytic pathways (11,21,77,78). Other alternative paths are reductive elimination rather than metal-carbon bond formation [Eq. (36)] (Section III,B) and formation of metal-oxygen rather than metal-carbon bonded compounds [e.g., Eqs. (107) (119) and (108) (120). Reactions (36) and (108) are reversible, and C02 activation (116) is involved in the reverse reactions (48,120). 

There has been little insight into potential decomposition pathways for the Ni(II) system due to sparse experimental evidence. Polymerization results with catalysts bearing different alkyl and fluorinated substituents have suggested that a C-H activation process analogous to that occuring with the Pd(II) catalysts is unlikely with Ni(TT) [28], Instead, side reactions between Ni and the aluminum coactivator, present as it is in such large excess, have been implicated. The formation of nickel dialkyl species and their subsequent reductive elimination to Ni(0) is one possible deactivation mechanism [68]. 

Fig. 8. Selected geometric parameters (A) of the optimized structures of the key species for reductive elimination via the most feasible stereochemical pathway for the competing routes affording VCH, ds-l,2-DVCB, and cis,cis-COD, respectively, for the generic catalyst along 2a -> 8a, 4a -> 9a, and 4a -> 10a. Free energies (AG, AG in kcal mol-1) are given relative to the favorable bis(r 3-yyn) stereoisomer of 4a.

Estimated Electronic and Steric Contributions to Intrinsic Activation Barriers and Reaction Energies for Reductive Elimination Affording c/ ,c/ -COD via the Most Feasible Pathway along 4a - 10aa... 

Oxidative coupling via la —> 2a and the reductive elimination routes, that commence from 4a as the precursor, involve different stereoisomers along the most feasible pathway. Accordingly, the conversions of the terminal allylic groups of the [Nin(octadienediyl)L] complex represent indispensable elementary processes. 

Some significant findings concerning the mechanistic details of well-established hydrogenation pathways (e.g., the insertion and reductive-elimination steps) have been published (see Section II), while at the other end of the scale in terms of characterized systems, the enzymatic hydro-genase system is beginning to draw attention (see Section IX). The well-established systems are reviewed first, as new systems are often compared to them in terms of reactivity and mechanism. 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

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