Homogeneous catalysis steric effects

Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis. C. A. Tolman, Chem. Rev., 1977, 77, 313-348 (298). 

In square-planar, 16-electron complexes (i.e. coordinatively unsaturated) as found for many group 9 (Co, Rh, Ir) and 10 (Ni, Pd, Pt) metals, the associative process is most common. The trans effect and trans influence ligand series [19-23] are also useful measures in the study of homogeneous catalysis. Apart from very small ligands, such as CO, H2, and NO, steric repulsion between ligands, as well as complexes and incoming substrates, plays a dominant role in determining the kind of intermediates and complexes formed and their equilibria in solution. 

Although the reaction is successful with many representatives of the purine family, it relies on the protonation of the N -nitrogen atom [81] meaning that substituents at N and N cannot be chosen at will, thus limiting the scope for steric, electronic and chiral diversification of a purine derived carbene ligand for application in homogenous catalysis. Furthermore, the NH functionality may have adverse effects in catalytic reactions and the yields of C-bound complexes are generally low. 

Under the conditions of homogeneous catalysis, decomposition temperatures are normally significantly lower than with the heterogeneous catalysts mentioned above, and cyclopropane yields in general are higher. However, catalysts of type 2 must first be converted into the active form [presumably a copper(I) monochelate] by brief heating or by in situ reduction (see Table 10). Another soluble catalyst, copper(I) triflate, even decomposes diazoacetic esters and diazomalonic esters at temperatures below 0 °C and sterically more encumbered diazocarbonyl compounds (e.g. a-diazo-a-trialkylsilyl acetic esters " ) still at room temperature, and has shown its effectiveness in a number of cyclopropanation reactions. Since copper(I) triflate is... 

Libraries of a similar size were used for screening of homogenous catalysis under multiphase conditions and for the extraction of kinetic data for the asymmetric syntheses, thereby increasing the depth of information that was gathered. Steric, solubility, and electronic effects on reactivity were monitored for a substrate library for the isomerization of allyl alcohol derivatives conducted under liquid-liquid conditions. 

Hydroformylation is also a model reaction system in homogeneous catalysis as it contains so many aspects such as ligand effects (electronic, steric, bite angle), in situ studies, complicated kinetics, and effects of conditions and impurities. All this, combined with its practical value, makes it an ideal topic in education. 

---