Curtin-Hammett

Basic Mechanistic Concepts Kinetic versus Thermodynamic Control, Hammond s Postulate, the Curtin-Hammett Principle... 

SECTION 4.4. BASIC MECHANISTIC CONCEPTS KINETIC VERSUS THERMODYNAMIC CONTROL, HAMMOND S POSTULATE. THE CURTIN-HAMMETT PRINCIPLE... 

In a simple statement of the Curtin-Hammett principle, we know that the rate will depend not only on the separate reactivities of the two pathways, but also on the relative populations of the catalyst species. In the following table we highlight the parameters referring to reactivity and to the population. 

Does the Curtin-Hammett principle apply to photoelectrocyclic reactions when more than one conformation of reactant is present in solution Why ... 

Fig. 17 Curtin-Hammett dynamic binding in the interior of a zeolite.

The mechanism of this interconversion is known as apparent allyl rotation and results in a switching of the allylic termini in addition to the movement of the central allyl carbon from one side of the coordination plane to the other although, for situations with symmetrically substituted allyl units, only this latter change is noticeable As the name suggests, the process is non-trivial and, in reality, the mechanism is dependent on the specific conditions.1161 Nevertheless, in most cases, the interchange is rapid compared with the rate of nucleophilic attack and, therefore, the product outcome does not necessarily depend on the intermediate isomer ratio (Curtin-Hammett principle). 

The theory was tested in simple tetrahydropyranyl acetal systems, which exist in the axial conformation [92], but can be constrained to adopt the equatorial conformation [93] by building in appropriate structural features. It was found, for example, that the equatorial isomers of the oxadecalin acetals [94] are actually more reactive than the axial compounds [95], although only the latter have a lone pair on the donor ring oxygen antiperiplanar to the C-OAr bond (Kirby, 1984, 1987). This and other similar observations do not disprove the theory of stereoelectronic control because any stereoelectronic barrier can be got round if there is sufficient conformational flexibility. In other words, these are Curtin-Hammett systems, with free energy barriers between conformations much smaller than... 

It is well known that we usually deal with a mixture of rotamers in chemical reactions. If they are interconverted rapidly at a given temperature (eq. [6]), then the Curtin-Hammett relation (171) (eq. [7]) will explain the product dis-... 

Case B is very common and can also be worked out easily. It is seen that the barriers for both the forward and backward reaction of (1) are much lower than the barrier for (2). We are dealing with a fast pre-equilibrium and a ratedetermining reaction (2) ki, k i k2 (concentrations omitted). It is also referred to as Curtin-Hammett conditions in U S. literature it refers to the kinetics of a system of a number of rapidly equilibrating species or conformations, each one of which might undeigo a different conversion, but all that counts is the global, lowest barrier, as that is the direction the system takes. 

The major intermediate observed in solution is the alkene complex, but it interchanges rapidly with the aldehyde complex. The product formed according to this scheme is allyl alcohol, because the overall barrier 2 is lower than barrier 1 (above we named this Curtin-Hammett conditions). Barrier 2 is also the ratedetermining step in this sequence. 

Quantitative data for the difference in complexation of ethene and CO to hydrocarbylpalladium(dppp)+ were reported by Brookhart and co-workers [15,33], The equilibrium between CO and ethene coordination amounts to about 104 at 25 °C. Multiplied by the concentrations of the two gases and the two individual rate constants for the insertion they calculated that the ratio of CO insertion versus ethene insertion is about 105 in an alkyl-palladium intermediate under Curtin-Hammett conditions, that is to say fast exchange of coordinated CO and ethene ligands compared to insertion reactions. Figure 12.9 summarises this. 

Figure 12.9. Curtin-Hammett kinetics forethene/CO insertion...

Figure 2b shows the other extreme, whereby the rate of epimerization is fast relative to the rate of substitution. In this case, Curtin-Hammett kinetics apply, and the product ratio is determined by AAG. In the specific case of organolithium enantiomers that are rendered diastereomeric by virtue of an external chiral ligand, such a process may be termed a dynamic kinetic resolution. Both of these processes are also known by the more general term asymmetric transformation One should be careful to restrict the term resolution to a separation (either physical or dynamic) of enantiomers. An asymmetric transformation may also afford dynamic separation of equilibrating diastereomers, but such a process is not a resolution. "... 

If it is assumed that the Curtin-Hammett principle applies, one need only to compare the energies of the minima on the solid and dashed curves to be able to predict the structure of the major product. These curves also allow a direct comparison of Cram s, Cornforth s, Karabatsos s and Felkin s model for 1,2 asymmetric induction. Both Figures show the Felkin transition states lying close to the minima. The Corn-forth transition states (Fig. 3) are more than 4 kcal/mol higher and should contribute little to the formation of the final products assuming a Boltzmann distribution for the transition states, less than one molecule, out of a thousand, goes through them. Similarly, Fig. 4 shows the Cram and Karabatsos transition states to lie more than 2.7 kcal/mol above the Felkin transition states, which means that they account for less than 1% of the total yield. 

Cumyl propyl peroxide, determination, 708 Curing processes, 692, 707 Curtin-Hammett principle, electron-poor alkene photooxygenation, 877 Cyanine dye, TBARS assay, 667 Cyanohydrins, bis(trimethylsilyl) peroxide reactions, 797... 

In both of these situations, the reaction actually observed does not occur from the lowest-energy conformation of the reactants. That this need not be the case is a direct consequence the Curtin-Hammett principled This recognizes that some higher-energy reactive conformation , will be in rapid equilibrium with the global minimum and, assuming that any barriers which separate these conformations are much smaller than the barrier to reaction, will be replenished throughout the reaction. 

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