Cone angle Tolman

Unfortunately, for all these reasons the conclusions cannot be applied quantitatively for description of the pH effects in the RCH-RP process. There are gross differences between the parameters of the measurements in [97] and those of the industrial process (temperature, partial pressure of H2, absence or presence of CO), furthermore the industrial catalyst is preformed from rhodium acetate rather than chloride. Although there is no big difference in the steric bulk of TPPTS and TPPMS [98], at least not on the basis of their respective Tolman cone angles, noticable differences in the thermodynamic stability of their complexes may still arise from the slight alterations in steric and electronic parameters of these two ligands being unequally sulfonated. Nevertheless, the laws of thermodynamics should be obeyed and equilibria like (4.2) should contribute to the pH-effects in the industrial process, too. 

The description of the steric properties of phosphanes using the Tolman cone angle [113] proved to be an excellent concept capable of explaining many phenomena in the coordination chemistry of phosphanes and their applications, especially in homogenous catalysis. That there is a steric influence connected with NHC was noticed very early, in fact it was thought that the steric hindrance introduced by the N-mesityl substituents was a contributing factor in the isolation of the first stable carbene in 1991 as opposed to dimerisation to the known tetraaminoethylenes [1],... 

It is comparatively easy to divide the N-substituents into bulky and nonbulky ones, but it is far more difficult to justify the decision in borderline cases and even more difficult to quantify the findings in a similar way to the Tolman cone angle, which gives a single value that can be calculated as the sum of the three contributing substituents on phosphorus. In short, the Tolman cone angle is valid for symmetrical and unsymmetrical (tertiary) phosphanes [113]. 

It is worth mentioning that Muller and Vogt have recently reintroduced the A, A concept for phosphinine ligands that have similar steric characteristics to NHC [119]. They call the two different angles the occupancy angles a and p, but the definitions are almost identical and they point out that the arithmetic average of a and p is very close to the Tolman cone angle 0 for tertiary phosphanes. 

Figure 2 Measurement of the Tolman cone angle, 0. A protractor is used to measure the angle between the straight edge and the block.

Figure 15 Schematic depiction of ligand cone angle for a generic phosphine ligand and representative Tolman cone angle values...

A series of monocarbonyl complexes were tested with phenyl groups on the diamine and various substituents on the phosphorus atoms (complexes 64-72). The electronic effects of the ligand can be quantified by use of the carbonyl stretching wavenumber of the complex while the steric effects can be expressed by the Tolman cone angle for groups around the phosphorus atoms. Figure 8 shows a plot of these two parameters versus the TOF for the ATH of acetophenone in isopropanol at 30 °C catalyzed by complexes 64-72 (0.02 mol%) which have been activated by KOtBu (0.16 mol%). 

Figure 8 The plot of catalyst activity for the ATH of acetophenone (TOF, h ) catalyzed by complexes 64-72 (0.02 mol% 0.2 mol% KOtBu) at 30 °C versus CO stretching frequency, as an electronic parameter, and Tolman cone angle, as a steric parameter. Reproduced from with permission from the American Chemical Society. (See insert for color/color representation of this figure)...

Figure 1 Illustration of the definition of the Tolman cone angle for a typical aryl phosphine ligand...

Figure 2 Calculation of the Tolman cone angle from X-ray crystallographic data...

From C- and H-NMR spectroscopy, we know that alkyl substitution on phenyl rings results in an upheld shift of the remaining C-H resonances. Phosphorus chemical shifts behave differently. Alkyl substituents have a - I effect on phosphorus. The phosphorus resonance is shifted downheld upon increasing alkyl subshtuhon (see Table 2.1). Phenyl substituents cause an upheld shift compared to respechve alkylated phosphorus compounds. PPhj has a chemical shift of < p=-6ppm, whereas PCyj has one of <5 p=9ppm. In addition, phosphorus chemical shifts are influenced by changes in the Tolman cone angle. 

Table 23.2 Tolman cone angles for selected phosphine and phosphite ligands.

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