Steric Versus Electronic Effects

Significantly, the use of an ortho-aryl CF3 group in the cobalt systems can lead to substantial increases in activity (and catalyst Hfetimes) upon activation with MAO [39]. Notably, the cobalt-based catalyst bearing an ortho-CF group in combination with an ortho-F substituent (h) displays an activity lOOOOOgmmoT h bar , which is comparable in performance to the most active iron catalysts (e.g., le/MAO). 

The presence of cycloalkyl substituents in the 2,6-positions of the N-aryl groups (m) has been found to increase the temperature stability of the iron catalysts over their alkyl analogues (e.g., lb or Ic) [29]. On the basis of a quantum mechanical 

Study the extra stabihly is not associated with the M AO-induced activation reaction but rather the greater stability of the cycloaliphatic ontaining precatalyst 

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I. V. Glukhov, M. Y. Antipin, K. A. Lyssenko. C-C Bond variation in the 1-phenyl-o-carborane steric versus electronic effects. Eur. J. Inorg. Chem. 1379-1384(2004). 

Both steric and electronic effects can influence the regioselectivity of ene reactions. For example, a strong preference for hydrogen abstraction from the more substituted side of the double bond of the perepoxide intermediates, generated from trisubstituted alkenes, is observed. This effect has been called the cis effect and explained on the basis of orbital interactions1423 and activation entropy differences.1457 Scheme 6.267 shows the product distribution after photooxygenation of three alkenes (561 563).1461 There is an apparent steric effect of the methyl versus 2-propyl substituents in the first two cases. The cyclopropyl moiety in the last example remains unreacted, which rules out the formation of a biradical intermediate (see also Special Topic 6.10). 

In this equation, k/k is the relative rate of reaction of the substituted versus the unsubstituted (R = CH3) ester, and the subscripts B and A refer to basic and acidic conditions respectively. Equation (3.5) assumes that the reactivity in acid is independent of electronic effects, whereas in base the reactivity depends on both steric and electronic effects. The divisor of 2.48 is arbitrarily chosen to place (7 values on a scale similar to that of normal a values. Table 3.1 lists cr values of typical substituents. 

Several studies have been conducted aiming ai the separation of steric and electronic effects [71-73]. For a single step process such as an oxidative addition or one-electron change in electrochemical processes this may be useful, but for multi-step reactions as we are dealing with in catalysis, this technique will encounter many problems. There will be different effects on the distinct steps and linear free-energy relationships will be an exception rather than the rule. When for instance both an oxidative addition step and a reductive elimination step are involved volcano curves must be expected for reactivity versus a ligand property, as in a series of metal oxides when... 

Some unsaturated alcohols resist reaction with Mn02 due to steric reasons. Sometimes, epimeric unsaturated alcohols possess very different reactivities versus active Mn02, which points to the possible involvement of little-investigated stereo-electronic effects.35... 

The studies reviewed here are part of a continuing effort (4-10) to identify those properties of bimetallic systems which can be related to their superior catalytic properties. A pivotal question to be addressed of bimetallic systems (and of surface impurities in general) is the relative importance of ensemble (steric or local) versus electronic (nonlocal or extended) effects in the modification of catalytic properties. In gathering information to address this question it has been advantageous to simplify the problem by utilizing models of a bimetallic catalyst such as the deposition of metals on single- crystal substrates in the clean environment familiar to surface science. 

The effects of charge proximity on CD complexation behavior were evaluated (Fig. 7) by studying the complexation of two steroids by the sulfonate, sulfopropyl ether (SPE), and sulfobutyl ether (SEE) derivative.Electronic effects seem to be more of a factor than steric effects because even when only one sulfonate substituent is attached at the 6-position, (6-SAl-p-CD) the derivative loses its complexation capability. The binding constant for testosterone is only 64 for 6-SAl-p-CD versus 17,800 for the neutral p-CD. The attachment of a single negative charge close to the CD cavity appears to disrupt the thermodynamics driving the complexation. 

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%). 

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