Olefin complexes steric effects

A particularly detailed description was obtained for complexes of Co- A zeolite with mono-olefins (24). Steric effects due to methyl groups adjacent to the double bond resulted in the ligand strength spectrochemical series... 

The hydrogenations become analogous to those involving HMn(CO)5 (see Section II,D), and to some catalyzed by HCo(CN)53 (see below). Use of bis(dimethylglyoximato)cobalt(II)-base complexes or cobaloximes(II) as catalysts (7, p. 193) has been more thoroughly studied (189, 190). Alkyl intermediates have been isolated with some activated olefinic substrates using the pyridine system, and electronic and steric effects on the catalytic hydrogenation rates have been reported (189). Mechanistic studies have appeared on the use of (pyridine)cobaloxime(II) with H2, and of (pyridine)chlorocobaloxime(III) and vitamin B12 with borohydride, for reduction of a,/3-unsaturated esters (190). Protonation of a carbanion... 

Electron spin resonance (ESR) signals, detected from phosphinated polystyrene-supported cationic rhodium catalysts both before and after use (for olefinic and ketonic substrates), have been attributed to the presence of rhodium(II) species (348). The extent of catalysis by such species generally is uncertain, although the activity of one system involving RhCls /phosphinated polystyrene has been attributed to rho-dium(II) (349). Rhodium(II) phosphine complexes have been stabilized by steric effects (350), which could pertain to the polymer alternatively (351), disproportionation of rhodium(I) could lead to rhodium(II) [Eq. (61)]. The accompanying isolated metal atoms in this case offer a potential source of ESR signals as well as the catalysis. 

Even in an excess of ligands capable of stabilizing low oxidation state transition metal ions in aqueous systems, one may often observe the reduction of the central ion of a catalyst complex to the metallic state. In many cases this leads to a loss of catalytic activity, however, in certain systems an active and selective catalyst mixture is formed. Such is the case when a solution of RhCU in water methanol = 1 1 is refluxed in the presence of three equivalents of TPPTS. Evaporation to dryness gives a brown solid which is an active catalyst for the hydrogenation of a wide range of olefins in aqueous solution or in two-phase reaction systems. This solid contains a mixture of Rh(I)-phosphine complexes, TPPTS oxide and colloidal rhodium. Patin and co-workers developed a preparative scale method for biphasic hydrogenation of olefins [61], some of the substrates and products are shown on Scheme 3.3. The reaction is strongly influenced by steric effects. 

Sterically Hindered Metalloporphyrins Capable of Direct Aerobic Oxygenation. The catalytic aerobic olefin epoxidation system of Quinn and Groves, (tetramesitylporphyrinato)Ru/02/olefin substrate, effects equations 3-6, that is, the direct oxygenation of substrate using O2 as the oxidant without consumption of reducing agent 14), The (tetramesitylporphyrinato)Ru complex sterically... 

Hirai et al.129 studied the hydrogenation of olefins catalyzed by poly(acrylic acid)-Rh(II) complexes in homogeneous solutions. The catalytic activity of the polymer-Rh complex was about 103 times that of the acetato-Rh complex. When olefins having another functional group, such as diallylether, allylaldehyde, and cyclohexene-1 -one, were used as the substrates, the olefinic bond was preferentially hydrogenized by the polymer-Rh complex. The polymer ligand was presumed to exercise a steric effect. 

By favoring the presence of species A, when olefin attacks, the steric effect of the two bulky triphenylphosphine ligands favors a high ratio of primary alkyl. If few triphenylphosphine ligands were present in the complex, a higher proportion of propylene would form secondary alkyl groups, giving more isoaldehyde. 

The dependence of the relative reaction rates on olefin geometry can be discussed with reference to Equation 14. As pointed out by Murray and co-workers (25), the complex formation occurs with cis-olefins rather than with their trans isomers for steric reasons hence, Kc (cis) > Kc (trans). However, during the formation of the primary ozonide by either path the olefinic carbon atoms change in hybridization, from sp2 to spz. The bond angles thus decrease from 120° to 109° in the cis isomers, this results in a compression of the substituents van der Waals radii. The repulsion between the substituents is increased, and so is the activation energy. Consequently, ki (trans) > ki (cis) and k2 (trans) > k2 (cis). In the final analysis, the geometry of the olefin has opposite effects (a) onKc and (b) on ki and k2. Present results seem to indicate that for large substituents the effect on Kc predominates since k (cis) > k (trans). 

There are factors other than olefin basicity and the steric effects of substituent groups on the olefin which can affect the stability of the silver ion complex. These include the energy required to displace solvate molecules from the coordination sphere of the metal ion and the degree of association between the cations and anions, especially in concentrated solutions or in solid salts. 

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