Phosphine steric parameter

Other examples correlating the reactivity to steric properties of ligands can be found, as in the formation of rhodium cort5)lexes containing norbornadiene-derived dimers, critically influenced by phosphine steric parameters. ... 

The nucleophilic carbcnes are phosphine-mimics but they are much more. They reside at the upper end of the Tolman electronic and steric parameter scales. Much remains to be explored with these ligands. With a rudimentary understanding of ligand stereoelectronic properties, we feel confident much exciting chemistry remains to be explored. 

Many attempts have been undertaken to define a reliable steric parameter complementary to the electronic parameter. Most often Tolman s parameter 0 (theta) is used. Tolman proposed to measure the steric bulk of a phosphine ligand from CPK models in the following way. From the metal centre, located at a distance of 2.28 A from the phosphorus atom in the appropriate direction, a cone is constructed which embraces all the atoms of the substituents on the phosphorus atom (see Figure 1.6). 

While many workers have used Tolman s steric parameter (the cone angle), not nearly as many have used his electronic parameter. The latter represents the net total of all electronic effects as reflected in the value of the vfCO) vibration (i.e. the contributions of a and n bonding are unresolved) of the LNi(CO)3 (L = phosphine) complex. Tolman further analyzed the contributions of various substituents to v by equation (55). 

Basic trialkyl phosphines are best suited as ligands for the palladium-catalyzed cyclotrimerization of 1 and CO2, and a strong influence of the ligand structure on the performance of the catalyst is observed. As noted earlier [4 c], the Tolman concept [10 a] of electronic (T ) and steric (0) parameters is obviously not sufficient to explain the observed ligand effects. The steric parameter Er, recently developed for phosphine ligands on the basis of molecular mechanics [11], also failed to show any correlation with the experimental results. The understanding of these... 

Carbenes in general are discussed in Chapter 11, but NHCs (4.15) are covered here because they have attained equal importance with phosphines as spectator ligands, particularly in catalysis Compared with phosphines, the range of accessible steric parameters is equally wide, but most NHCs are much stronger donors, best quantified in DFT calculations NHCs also seem to be modest tt acceptors ... 

The influence of steric factors on the position of tetrahedral - square-planar equilibria has been discussed in detail.38,39 jn general, two trends are apparent (i) the smaller the steric parameter, 0, of the phosphine ligand in a complex of the type [NiX2(PR3)2l, the greater the stabilization of the square-planar form, and in) the stabilization of the square-planar form is sensitive to the nature of the anionic ligand, X", and decreases in the order Cl > Br > P. 

Finally, we have constructed a QM/MM model of complex 3 whereby the phenyl phosphine groups are contained in the QM region, while the ferrocenyl and phenyl substituents on the pyrazole is accounted for on a steric basis only (model E). The agreement between the X-ray structure of 3 and model E is remarkable. For example, both the Pd-P distance and the twist of the coordination plane of the Pd center, 0, are virtually identical to those of the X-ray structure. In fact, the selected parameters displayed in Table 2 are generally better than those of the full QM calculation. The good agreement between the calculated and experimental structures is important for the detailed mechanistic study of the hydrosilylation that is presented in later sections of this chapter. 

The preparation of carbonyl-lr—NHC complexes (Scheme 3.1) and the study of their average CO-stretching frequencies [7], have provided some of the earliest experimental information on the electron-donor power of NHCs, quantified in terms of Tolman s electronic parameter [8]. The same method was later used to assess the electronic effects in a family of sterically demanding and rigid N-heterocyclic carbenes derived from bis-oxazolines [9]. The high electron-donor power of NHCs should favor oxidative addition involving the C—H bonds of their N-substituents, particularly because these substituents project towards the metal rather than away, as in phosphines. Indeed, NHCs have produced a number of unusual cyclometallation processes, some of which have led to electron-deficient... 

The dynamic behavior of the model intermediate rhodium-phosphine 99, for the asymmetric hydrogenation of dimethyl itaconate by cationic rhodium complexes, has been studied by variable temperature NMR LSA [167]. The line shape analysis provides rates of exchange and activation parameters in favor of an intermo-lecular process, in agreement with the mechanism already described for bis(pho-sphinite) chelates by Brown and coworkers [168], These authors describe a dynamic behavior where two diastereoisomeric enamide complexes exchange via olefin dissociation, subsequent rotation about the N-C(olefinic) bond and recoordination. These studies provide insight into the electronic and steric factors that affect the activity and stereoselectivity for the asymmetric hydrogenation of amino acid precursors. 

The most basic phosphine considered by Tolman was PBu this produced an A, frequency of 2056.1cm-1 and was made the arbitrary standard for the electronic parameter (Table 12). The electronic parameter, v, and the steric cone angle parameter, d, when plotted together produce a steric and electronic map (Figure 16). 

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