Phosphines, steric property

Replacement of CO in MeCOMn(CO)5 with PPh3 seems to have little effect on the rate of the decarbonylation. As shown in Table IV, MeCO-Mn(CO)4PPh3 (an isomeric mixture) reacts only slightly faster than MeCOMn(CO)5 after provision is made for the difference in temperature 169). However, a recent kinetic study on the decarbonylation of CpMo-(CO)2L(COMe) (L = a tertiary phosphine) has shown that both inductive and steric properties of L are important 19a). Sterically demanding and weakly a-bonding phosphines increase the reaction rate. 

Based on the experience with tertiary phosphines, the importance of the steric properties of NHC hgands in determining chemical behaviour has been immediately recognised. The main practical problem, however, is that NHC hgands substantiaUy present a local symmetry axis, whereas phosphines present a local symmetry axis. This imphes that the well-accepted molecular descriptor used to quantify steric properties in phosphines, the Tohnan cone angle [78], cannot be applied to NHC ligands. 

One of the advantages of the as molecnlar descriptor to characterise the steric properties of ligands is its generality. This has allowed the placement of tertiary phosphines and NHCs on the same scale. The valnes reported in Fig. 1.18 for two of the most classical phosphines indicate that the less bnUcy PPhj compares with NHCs of intermediate bulkiness, snch as those presenting p-tolyl N-substitnents, while the bulkier PCyj compares with the bulky IPr and SlPr NHCs. Finally, farther refinement of the model was recently disclosed in the form of the dihedral angles ( )j and( )2 (Fig. 1.20). 

The origin and nature of the electronic and steric properties of NHCs, that render them phosphine surrogates, have aheady been discussed in depth in the general introduction of this book. Hence, we will focus our attention on the peculiarities that these properties impart to the metal complexes, and therefore the catalytic species. 

In a subsequent detailed study, other iridium phosphine systems prepared in situ were used. Depending on the steric properties of the phosphine employed and... 

Properties of the PPF-P 6u2 Ligand 183 Sterically Similar Phosphines Sterically Different Phosphines... 

Tert-butyl carbonates 229 can be used instead of free acids to obtain cyclic carbonates 230. The influence of the eledronic and steric properties of phosphine ligand in the catalytic system was studied and optimal behavior was observed for eledron-deficient phosphines such as P(C6F5)3 [48]. 

Abstract N-Heterocyclic carbene (NHC) has become a major ligand class and has proven to be more than just a phosphine mimic . Some important features like electronic and steric properties are discussed and typical examples of NHC are given herein. 

Studies of complexes with different phosphine and halide ligands led to the observation that there is no Unear correlation between the basicity of the phosphine and catalyst initiation (dissociation of the phosphine), since the steric properties of the phosphine also affect the activity of the catalyst. Higher activity was reported for chloride complexes than the corresponding bromide or iodide complexes [50]. 

Some aspects of the different electronic and steric properties of phosphines and car-benes have been summarized in short comparative overviews (Glorius 2007 Kantchev... 

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

The reaction is the basis of the Dimersol process for the production of octane enhancers.131 The activity of these catalysts is strongly dependent on the steric properties of the phosphine ligands, PEt3 << PPh3 < PCy3 < PPr Bu. 129... 

When tris-o-tolylphosphine was used as ligand the main product (Equation 13) was formed in ca. 84% yield by GC area analysis. No correlation of regioselectivity with electronic or steric properties was found for several different tertiary phosphines. The final step consisting of cis p-hydrogen elimination leading to the E product occurs stereoselectively because only one of the two possible conformations leading to elimination is favoured. 

The electronic and steric properties of the phosphine ligand(s) can have dramatic effects on the rate and selectivity of the rhodium catalysts. As mentioned above, electron-rich alkylated phosphines generally have a negative effect on the rate and regioselectivity, while more electron deficient phosphines such as PPhs and phosphites (see Electron Deficient Compound) generate more active and regioselective catalysts (Table 3). 

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