Concerted reductive elimination mechanisms

For aromatic ether formation, electron-withdrawing groups on the arene accelerate the rate of reductive elimination. Fm-ther, the more basic alkoxide groups form ethers faster than phenoxides. These facts have led to the proposal that, in addition to the usual concerted reductive elimination mechanism, some substrates can undergo reductive elimination via a Meisenheimer type intermediate (equation 31). 

Intramolecular coupling reactions of group 4 imino-acyl complexes yielding the corresponding free imines are rather rare and mainly involve titanium complexes [35 — 38]. The mechanism of these reactions still remains unclear although a concerted reductive elimination step, that initially leads to an -irnine intermediate complex of the type II, has already been postulated [35] (Fig. 4). 

A similar pattern of reactivity has been reported for platinum(II) complexes [39d]. Ab initio calculations demonstrated that an oxidative addition-reductive elimination mechanism is more likely than the concerted metathesis [39e]. 

The key steps of a concerted three-center reductive elimination mechanism (Fig. 4, path b) are dissociation of the ligand L trans- to the hydrocarbyl R (step b-i), a concerted M-C bond cleavage and C-X bond formation (step h 2). and a displacement of the organic product R-Z by the ligand L (step b i). The reaction leads to the product of cis-elimination of R-Z with the retention of the configuration of the metal-bound carbon atom. 

The phenyl complex 12 of a similar structure is kinetically more resistant to C-O reductive elimination than the methyl analog 9 as different reductive elimination mechanisms are expected to be operational in these two cases, concerted three-center in the case of the aryl derivative and an Sn2 mechanism in the case of the alkyl complex. In particular, the symmetric phenyl complex (dpms)Pt Ph(OH)2, 12, in Fig. 6 is completely inert in acidic aqueous solutions at 100°C [25], whereas its methyl analog 9, (dpms)Pt Me(OH)2, eliminates methanol via an Sis/2 mechanism at room temperature [9]. 

On the basis of the analysis of the stereochemical outcome of oxidatively induced palladium-catalyzed C(sp )-0 forming reactions leading to substituted tetrahydrofurans, it was suggested that high valent alkyl palladium intermediates can react via a concerted three-center reductive elimination mechanism to form C (sp )-O bonds [31]. No characterization of these presumed high valent species or mechanistic studies of their reactions have been carried out. 

Combined experimental and computational studies have sought to probe the intimate mechanism for the formation of C2-methylated imidazolium salts from [Pd (Me)(NHC)]-type complexes. These studies illustrated that the reactions were unlikely to occur via a migratory process, and that a concerted reductive elimination was energetically far more favourable. " In further DFT (density functional theory) studies. 

Not all reductive eliminations are concerted. Many involve radicals. This makes good sense, because M-C bonds are weak. This means that warming above 150 °C will start to give significant amounts of M-C bond homolysis. Let s examine an interesting series of examples that shows a mixture of reductive elimination mechanisms. 

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.

The familiar standard de carbonyl at ion mechanism ( 3, 5) involving a concerted oxidative-addition of aldehyde, CO migration (with subsequent elimination), and reductive-elimination of product, would seem with metalloporphyrins to require coordination numbers higher than six, and in this case Ru(IV) intermediates. Although this is plausible, the data overall strongly suggest a radical mechanism and Ru(III) intermediates. 

The product-forming steps of dehalogenations by free radical pathways were discussed earlier (see Section 18.3.1.1). In non-radical mechanisms, the dehalo-genated products (RH) will be formed mostly by reductive elimination [193, 194] however, concerted processes lead directly from RX to RH (see Sections 18.3.1.2 and 18.3.1.3). 

A further possible mechanism is the concerted electrocyclization of the metalla-1,3,5-triene to yield an intermediate metallacyclohexadiene [333]. Reductive elimination of (C0)5M would also lead to the formation of cyclopentadienes. 

A cis-coordinating ligand is apparently required to bind and activate MeOH so that a methoxy group is transferred to the polyketone chain and a hydride remains on palladium. Two mechanisms are possible for this reaction (i) nucleophilic attack by the oxygen at the acyl carbonyl with concerted formation of Pd-H (ii) formation of a Pd(acyl) (methoxy) complex and H, followed by reductive elimination and subsequent proton attack on a Pd center. No experimental evidence favoring either mechanism in ethene/CO copolymerisation has been provided so far. 

Whether this condition can be fulfiUed depends on the electron count of the metal, and the stereochemistry of the elimination. For instance, in m-elimination from octahedral d , or square planar d , systems, metal ndipP -y ) acts as acceptor, and this should be a facile process ( e Fip. 1, 2). For /rans-elimination, on tiie other hand, the lowest empty orbital of correct symmetry is (n + l)p. Such elimination Kerns energetically less Ukely, unless a non-concerted pathway (such as successive anionic and cationic loss) is available. The same arguments apply, of course, to oxidative additions. It foUows that the many known cases of traits oxidative addition to square planar t/ systems are unlikely to take place by a concerted mechanism, and this conclusion is now generally accepted There are special complexities in reductive elimination from trigonal systems, and these are discussed furdier in Part III. 

The reverse reaction is reductive elimination. No mechanism is implied in reaction (13.3). The addition may be stepwise, radical, electrophilic, or nucleophilic or concerted. Oxidative additions of H—H [reaction (13.1)] or H—R [reaction (13.2)] tend to be concerted. 

We thus began to question the traditional belief that concerted cis elimination of R-R (simple intramolecular reductive elimination) was in fact the mechanism ... 

The reductive elimination eventually releases the newly formed organic product in a concerted mechanism. In the course of this process, the electron count is reduced by two. Iron has a great tendency for coordinative saturation, which in general does not favor processes such as ligand dissociation and reductive elimination. This aspect represents a potential limiting factor for catalytic reactions using iron. 

The gas-phase reactions of the cationic Irm complexes follow a previously unreported mechanism for their observed a-bond metathesis reactions. Previous discussions had considered a two-step mechanism involving intermolecular oxidative addition of either [Cp Ir(PMe3)(CH3)]+ or [CpIr(PMe3)(CH3)]+ to the C-H bond of an alkane or arene producing an Irv intermediate, followed by reductive elimination of methane, or a concerted a-bond metathesis reaction sim-... 

---