The Effect of Bite Angle

Survey of Carbon-Carbon Bond-Forming Reductive Eiiminations 

Two examples of the many reductive eliminations to form C-C bonds from Pd(II) complexes are shown in Equations 8.45 and 8.46. These results reveal several of the trends noted in the above sections. Stille and Yamamoto showed that the reductive eliminations from LjPdMej complexes in Equation 8.45 occur after dissociation of ligand. Reaction from the complex containing the DEPHOS ligand in Equation 8.46 occurs more slowly because dissociation of the ligand is disfavored by chelation. 

Reductive eliminations to form C-C bonds from platinum complexes also include those containing both Pt(II) and Pt(IV) centers. Reductive elimination of biaryls and cyclopropanes from Pt(II) complexes were shown in Equations 8.39-8.41. Reductive eliminations from Pt(IV) were shown in Equations 8.35 and 8.37.  

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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... 

The effect of the bite angle has been reviewed.243,244 The 103Rh chemical shifts of a series of hydridorhodiumbis(carbonyl)diphosphine compounds containing chelating bidentate P-ligands have been obtained by inverse HMQC detection sequences.245... 

Phosphacyclic diphosphines (73a) and (73b) with wide natural bite angles were synthesized and the effect of the phosphacyclic moieties on the coordination chemistry in the [(diphosphine) Rh(CO)2H] complexes was studied. Both NMR and IR spectroscopy showed that the phosphacyclic xantphos ligands exhibit an enhanced preference for diequatorial chelation compared to the diphenylphosphino-substituted parent compound. In the hydroformylation of 1-octene the introduction of the phosphacyclic moieties leads to higher reaction rates. The dibenzophospholyl- and phenoxaphosphino-substituted xantphos ligands exhibit a high activity and selectivity in the hydroformylation of trans-2- and 4-octene to linear nonanal. CO dissociation rates from the... 

With the aim of rationalizing the effect of (wide) bite angle diphosphines in catalytic reactions a distinction can be made between two different effects, both related to the bite angle of diphosphine ligands [42] ... 

Supramolecular catalysis may also involve the combination of a host cavity and a metal active site as in the bis(diphenylphosphino)calix[4]arene nickel(II) complex 12.40 which acts as an efficient catalyst for ethylene and propylene polymerisation, and in tandem with zirconocene dichloride, for the formation of linear low-density polyethylene. In the latter case the complex gives very little branching - a significant advantage. The key to the effectiveness of the catalyst involves calixarene-induced changes in the bite angle at the Ni(II) centre, which is square planar in the active form of the catalyst.29... 

The effect of the ligand bite angle on the enantioselectivity in the copper(n) catalyzed Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazoUdin-2-one was studied using spiro bis(oxazolidine) based complexes 431a-d (Table The data... 

For further discussion of the effects of ligand bite angles on catalyst efficiency and selectivity, see P. Dierkes and P.W.N.M. van Leeuwen (1999) Journal of the Chemical Society, Dalton Transactions, p. 1519. 

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