Phosphines cone angles

Figure 2.2 Steric properties of phosphines (cone angle)...

Very high phosphine concentrations increase the selectivity for the n-aldehyde product isomer, evidently by suppressing the formation of monophosphine species. The high selectivity arises since steric hindrance in the alkyl intermediate B is less for a primary alkyl than for a secondary one the selectivity is therefore promoted by the phosphine cone angle. At low phosphine concentrations the straight and branched chain aldehydes are formed in a -—3 1 ratio, as in the unmodified cobalt system. 

The enthalpy trend can be examined in terms of electronic and steric contributions to the enthalpy of reaction, the relative importance of which can be quantified in terms of the respective Ai /A2 ratios obtained in a treatment first proposed by Tolman, where enthalpies of reaction are correlated with steric (6, cone angle, see Tolman s Cone Angle) and electronic (v, carbonyl stretching frequency in Ni(CO)3L, L = tertiary phosphine) factors. A correlation factor of 0.95 is obtained when enthalpic data are fitted to equation (3) and a value of 2.32 is calculated for the A1/A2 and quantitatively denotes the overwhelming influence of steric factors. Since the steric factors have such a profound influence on the enthalpy of reaction, a direct relationship between the enthalpy of reaction and the phosphine cone angle can be established as shown in Figure 1. 

Figure 1 Enthalpy of reaction vs phosphine cone angle for a series of tertiary phosphine ligands in the Cp Ru(PR3)2Cl system (r/qpe =-1.94 R = 0.93)...

Figure 5 Enthalpies of reaction (kcal mol) for the CpRu(PR3)2Cl system vs phosphine cone angle (deg) slope = —1.85 R = 0.95)...

From a steric viewpoint, the considerably bulkier trialkylphosphine ligand should, and does, favor formation of linear products. While linear to branched ratios of only 2-3 1 are typically found for HCo(CO)4, higher regioselectivities of 7-8 1 occur for HCo(CO)3(PR3). There is a phosphine Cone Angle cutoff at about 132°, after which the steric effects of the phosphine ligand do not increase the product linear regioselectivity any further. 

Phosphine Cone angle (°) Rel. rate Lmear hranched ratio... 

Figure 3 Correlation between BDE and phosphine cone angle.

Fig. 2. Schematic representation of a phosphine cone angle 0 (based on Ref. 23)...

Reaction of [(NmCp)M(PMe3)(Me)(I)] (NmCp = neomenthylcyclopentadienyl M = Rh, Ir) with [AgSbFfi] resulted in intramolecular C-H activation affording " (7), one diastereomer (/ ,/ ) of which was structurally characterised for M = Ir. The syntheses and reactivities of a series of [IrCl(CO)(PR3)2(ti2-3,3-diphenylcyclopropene)] (R3 = MC3, Et3, Mc2Ph, MePh2) complexes, structurally characterised for R3 = Mc3, have been reported s. Lability of the aikene was found to increase as a function of phosphine cone angle. 

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