Bite angle reductive elimination

The rate of the reaction increases with the electron donating power and steric bulk of the spectator ligands, L. If the latter are chelates, then reaction is favored by chelates with a larger bite angle. Reductive elimination is more favorable for 4d than for 5d metals, other factors being equal thus Pd(II) is more reactive than Pt(II) and reductive elimination is not rate-limiting in the systems described in the previous section. 

Reductive eliminations can be promoted by stabilisation of the low-valent state of the product. This means ligands that are good K-acceptors, bulky ligands, and ligands preferring bite angles more suited for tetrahedral than for square-planar complexes, when we deal with group 10 metals. 

Figure 11.5. Ligands with large bite angles promoting reductive elimination in nickel and...

In related Pd(Xantphos)-catalyzed cross-couplings of benzyl halides, reductive elimination was proposed as the slow step, since the overall rate increased with large bite angle ligands [98], P-C bond formation with isolated diastereomers of dinucleoside H-phosphonates was stereospecific, likely with retention of configuration at phosphorus, as above (Scheme 58). 

FIGURE 14.7 Reductive elimination of C—S bonds via Pd(ll) complexes with bidentate phosphines with varying bite angles. 

The reductive elimination and the /S-H elimination are competitive. The /S-H elimination takes place with trans dialkylpalladium complexes such as 36. The reductive elimination is favored by coordination of bidentate phosphine ligands which have larger bite angles (for the effect of bite angles, see van Leeuwen et al. [24]) to force other mutually cis ligands closer. Thus bidentate ligands of... 

Bisphosphine ligands having a certain bite angle, however, change the regioselectivity of allylsilane couphng from y- to a-carbon (Scheme 30) Retardation of a reductive elimination step by use of a dppe or dppp hgand is assumed, and this is considered to induce isomerization of the secondary y(cr)-aUyl in an allyl(aryl)Pd(ll) complex to a primary a-allyl complex. The a-selective reaction was applied to various aryl bromides or triflates (Scheme 31). 

The effects of chelating ligand structure on reductive elimination and /3-hydrogen elimination have not been studied extensively. However, one study did evaluate the ratios of products formed from the catalytic process as the ligand bite angle, the aryl group s electronic, and the aryl group s steric properties were systematically perturbed. ... 

Although reductive elimination from the four-coordinate species tends to be slower than from related three-coordiante species, the wide bite angle of dppf helps to accelerate the rate of reductive elimination. Thus, reductive elimination occurs from the sec-butylpalladium complex to form the sec-butyl organic product without competing rearrangements to form -butyl products or butene. 

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