Activation energies, for nucleophilic

In cases where comparable data are available, the activation energy for nucleophilic substitution by anions such as methoxide ion is... 

Activation Energies for the Nucleophilic Reaction OF NH3 WITH Pyridinium Salts 7a, 40, and 41"... 

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

In the case of the tricarbonylarene metals, enhancement of nucleophilic substitution relative to the free arene is reported 106), In contrast to earlier reports 106) Friedel-Crafts acylation of tricarbonylbenzene chromium occurs under mild conditions 18), Molecular-orbital calculations of the 7r-electron activation energies for these reactions 63) confirm enhanced nucleophilic reactivity and suggest electrophilic activity similar to that of the free arene. The nucleophilic displacement of halide by methoxide ion... 

Usually, the attack of the nucleophile on the bromonium ion is a fast process. On the other hand, kinetic investigations100 on the bromination shown in Scheme 19 indicate that bromonium ion formation (i.e. the ionization of the CT complex 45) cannot be the RDS. The apparent activation energy for the overall bromination (and the experimental reaction order in bromine, which changes by changing the temperature) confirms the neutralization of the bromonium ion to form the product (47) in a step limiting the observed rate of the overall process. 

Ring-opening reactions of phosphetanes (phosphetanium salts) are known to be promoted by nucleophiles. The reaction profile has been analyzed by theoretical calculations (HF/6-31+G(d)) <2001JOC915> and compared with those of a phosphirane and an acyclic phosphine as shown (Equations 2-4). The activation energy (/. /,) for the reaction of a four-membered ring is twice as high as that for a three-membered ring, whereas exothermicities (AE0) of both reactions are almost identical. 

The rate constants for addition of aliphatic alcohols and water to 19a decrease in the order MeOH > EtOH > H2O i-PrOH > t-BuOH in acetonitrile at 23 °C45, due to a combination of steric effects, nucleophilicity and acidity, all of which can be expected to affect the magnitudes of the individual rate constants involved in the mechanism for the reaction. The Arrhenius activation energy for addition of Z-BuOH to this silene, Ea = —1.7 0.4 kJmoP189, is closer to zero than that for MeOH addition, suggesting that the intracomplex product partitioning ratio is closer to 0.5 than is the case for the more reactive alcohol over the temperature range examined. It thus follows that the factor of ca 10 lower reactivity of t-BuOH compared to MeOH is mainly due to a reduction in the rate constant (kcj for initial complexation of the alcohol with the silene. 

The removal of electron density from the Pt(II) by multiple bonding to CN causes the side of the Pt(II) toward X to be electron deficient and more susceptible to nucleophilic attack by strong nucleophiles. Thus, the activation energy for forming the transition state is lower and the reaction proceeds faster. For a large number of ligands the apparent order of the trans effect produced is... 

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