C-X reductive elimination

Chelating ligands have a much lower propensity for dissociation. Yet detailed kinetic studies on similar systems with bidentate phosphine ligands, /ac-(L2)PtMe3X (L2 = dppe (bis(diphenylphosphino)ethane), dppbz (bis(diphenylphosphinobenzene) X = I, OAc, OPh) also showed that ligand dissociation was required prior to any C-C coupling (48-51). In this case, however, the X- group rather than the phosphine was lost to form a five-coordinate intermediate, as shown in Scheme 11. A competitive C-X reductive elimination also occurs from these complexes and involves the same five-coordinate cation (Section V. A). 

Although the C-X reductive elimination from Pd(IV) complexes has been established, it was recently suggested that Pd(III) dimers can also participate in the reductive elimination (Pig. 2) [57, 58]. Such, and relevant Pt(III) dimers, have been isolated and structurally characterized [59-63], however, it is not yet clear whether the mechanistic interpretation of the C-X elimination from these species should be treated differently from the established elimination fi om M(IV). As one... 

Consider now Fig. 4 showing two mechanisms of C-X reductive elimination from an octahedral metal species. According to the principle of microscopic reversibility, the mechanisms in Fig. 4 are the microscopic reverse of those in paths a and b in Fig. 3. 

Figure 9 shows a schematic representation of the potential energy profiles for a two-step C-X reductive elimination from a center with step a i being the rate limiting (solid line). A one-step C-X reductive elimination with an external nucleophile Z is also presented (dashed line). The two-step reaction path in Fig. 9 includes a five coordinate intermediate or a six-coordinate solvento-complex 4 and leads to a lower energy transition state TSs compared to the transition state TSse that corresponds to a direct nucleophilic attack of Z at the metal-bound carbon of the starting six coordinate compound. 

Fig. 9 A schematic representation of two possible potential energy profiles for an Sn2 reductive elimination reactions of an organic product R-Z from a center a two-step C-X bond elimination solid line) and one-step C-X reductive elimination dashed line). Sol designates a potential solvento ligand...

The first suggestion that ligand loss was important in reductive eliminatimi reactions from Pt(lV) complexes dates back more than 40 years. In 1969, Ettore observed that added iodide inhibited the reductive elimination of Phi from L2PtPh2l2 in methanol and suggested iodide loss to form a six-coordinate solvento intermediate [13]. Several years later, a five-coordinate intermediate was proposed by Puddephatt in studies of C-C reductive eliminations from Pt(IV) complexes (see Sect. 2.1.1) [15]. Since then, the involvement of five-coordinate intermediates has been supported so consistently in both experimental (e.g., [10,13-42]) and computational studies (e.g., [29, 30, 43 8]) of alkyl C-C, C-H, and C-X reductive elimination that it is now accepted as the norm in mechanistic schemes for reductive elimination from Pt(IV). 

An account of the redox chemistry of binuclear palladium complexes and the role of binuclear intermediates in Pd-catalysed oxidation reactions has been provided. Stoichiometric organometallic studies of the oxidation of binuclear Pd(II) complexes to binuclear Pd(III) complexes and subsequent C-X reductive elimination from the resulting binuclear Pd(III) complexes, which confirmed the viability of C-X bond-forming reactions mediated by binuclear Pd(III) complexes, has been described. The effect of ligand modification on the structure and reactivity of binuclear Pd(III) complexes has been presented to highlight the impact that ligand structure can exert on the structure and reactivity of binuclear Pd(III) complexes." ... 

Based on the discussed acylpalladium 7i-allylic complex (Scheme 5.22) and the reported X-ray structure of the (R)-MOP—Pd 7i-allylic complex [31], the acylpalladium (R)-MOP Ti-allylic complex C (Scheme 5.24) is proposed for the formation of the (R)-product. Complex D, which would give the (S)-product, suffers from steric compression between the MeO-naphthyl ring and the acyl group, while there is no such steric interaction in complex C. Thus, reductive elimination of Pd(0) from C would preferentially yield the... 

The Shilov reaction is an example of an Si 2 - t3q>e C(sp )-X reductive elimination reaction where several nucleophiles compete for the same electrophilic high-valent metal complex [18,19]. A detailed analysis of C(sp )-0 reductive elimination from symmetric (dpms)Pt Me(OH)2, 9, in aqueous solutions shows that this reaction also follows a complex mechanism characterized by the involvement of several competing nucleophilic Pt hydroxo and methoxo complexes (Fig. 10) [30]. 

The importance of 7t-electrophilic nature of aryl ligands was noted also in C-N, C-0 and C-S reductive ehmination from cw-Pd(C6H4-p-Y)(X)(diphosphine) complexes (X = NR2, OR, SR) [23,24], A common kinetic feature observed for these systems is the enhancement in the reaction rate hy a better jt-accepting Y and a better 0-donor ligand X. A kinetic study reported by Hartwig et al. for C-S reductive elimination is particularly detailed, leading to deep insights into the reaction mechanism (Scheme 9.6) [23a],... 

Diorganoplatinum(II) complexes are fairly stable towards reductive elimination. On the other hand, c 5-PtR(SiR3)L2 type complexes have been shown to be reactive towards C-Si reductive elimination [25]. For example, cis-Pt(vinyl)(SiPh3)(PMe2Ph)2 undergoes reductive elimination in toluene- g at 25 °C with the first-order rate constant 1.2 x 10 s ... 

Catalyst systems of the type [NiL X + AlEt Xj (where L = PR and X = halide) afford highly active catalysts for olefm dimerisation. However, when complex 11 (Scheme 13.8) is treated with AlEt Cl in the presence of 1-butene, in toluene at 20°C the only products observed were decomposition products, 12,13,14 no butene dimers were obtained [22], At low temperatures (-15°C) and using the complex with 1,3-diiso-propylimidazolin-2-ylidene as the NHC ligand, small amounts of butene dimers were observed. It is apparent from these results that Ni-NHC complexes are capable of olefin dimerisation, however, decomposition of the catalyst via reductive elimination predominates. 

The addition proceeds through (a) oxidative addition of the B-X bond to a low-va-lent metal (M=Pd, Pt) giving a ds-B-M-X complex (92), (b) migratory insertion of alkene or alkyne into the B-M bond (93 94), and finally (c) reductive elimination... 

This reaction typifies the two possibilities of reaction routes for M-catalyzed addition of an S-X (or Se-X) bond to alkyne (a) oxidative addition of the S-X bond to M(0) to form 94, (b) insertion of alkyne into either the M-S or M-X bond to provide 95 or 96 (c) C-X or C-S bond-forming reductive elimination to give 97 (Scheme 7-21). Comparable reaction sequences are also discussed when the Chalk-Harrod mechanism is compared with the modified Chalk-Harrod mechanism in hydrosily-lations [1,3]. The palladium-catalyzed thioboratiori, that is, addition of an S-B bond to an alkyne was reported by Miyaura and Suzuki et al. to furnish the cis-adducts 98 with the sulfur bound to the internal carbon and the boron center to the terminal carbon (Eq. 7.61) [62]. 

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