Transition-state selectivity definition

The same models as for intermolccular processes are applied for intramolecular diastereoface differentiating double-bond additions. However, there are some advantages in the intramolecular version. Firstly, the entropy factor lowers the barrier of activation and allows reactions to proceed at lower temperatures, which increases the selectivity. Secondly, the cyclic transition states introduce the elements of ring strain and transannular interactions, which lead to enhanced differences between two diastereomorphous geometries. Both of these factors cooperate to increase the selectivity of the intramolecular reaction. For example, halolactonization, by definition, is an intramolecular process. 

Streitwieser40 pointed out that the correlation which exists between relative rates of reaction in deuterodeprotonation, nitration, and chlorination, and equilibrium constants for protonation in hydrofluoric acid amongst polynuclear hydrocarbons (cf. 6.2.3) constitutes a relationship of the Hammett type. The standard reaction is here the protonation equilibrium (for which p is unity by definition). For convenience he selected the 1 -position of naphthalene, rather than a position in benzene as the reference position (for which er is zero by definition), and by this means was able to evaluate p -values for the substitutions mentioned, and cr -values for positions in a number of hydrocarbons. The p -values (for protonation equilibria, x for deuterodeprotonation, 0-47 for nitration, 0-26 and for chlorination, 0-64) are taken to indicate how closely the transition states of these reactions resemble a er-complex. 

The condition 1) states that unequal amounts of stereoisomers are produced or destroyed by the reaction together conditions 2) and 3) assure that the increase in chiral genus is due to a stereoselectivity which is caused by chiral influences alone. Condition 1) is an essential part of the definition it is interrelated with condition 3), and together they restrict the type of chirality increasing stereoselective reactions that will be called an asymmetric synthesis34. In fact, a stereoselective reaction is never infinitely selective , because this would require an infinite free enthalpy difference of stereoisomers, stereoisomeric transition states respectively (with thermodynamic control, or with productive selectivity) in the case of an idealized destructive selectivity infinite selectivity would be reached at infinite reaction time, when none of the considered stereoisomers would be left over3S ... 

Phase space theory can be thought of as, in effect, considering a loose or, as it is sometimes called, orbiting [333] transition state regardless of the nature of the reaction. The need to select transition state properties for each individual reaction considered is avoided and it has been argued that a virtue of the theory is that it gives definite predictions [452]. 

From this state, ring strain facilitated predissociation to a "biradical-like" transition state [135] or vibrational relaxation (k ) to S may occur. It is also conceivable that transition state [135] could be produced directly from S °. Alternatively, molecules in the S ° state could intersystem cross (kST) to the triplet manifold (T ). For 2-alkylidenecyclobutanones, reactivity is manifested in isomerization about the exocyclic carbon-carbon double bond, while for the saturated cyclobutanone derivatives studied, definitive evidence for solution-phase reactivity is not available. If analogy is again made to the vapor-phase photochemistry of cyclobutanone [21], reactivity could conceivably result in decarbonylated products. Indeed, preliminary evidence has been obtained from sensitization experiments employing m-xylene as triplet sensitizer that decarbonylation of a saturated cyclobutanone is enhanced by selective population of its state (35). ... 

The transition states of the latter are therefore more sensitive to stereochemical and electronic influences, which also leads to a higher selectivity than in analogous electrochemical conversions. At some oxide electrodes, such as the Ni(OH)2 electrode [13], the oxidation occurs as inner sphere electron transfer by hydrogen atom transfer. Also at doped titanium anodes this seems to be partially the case [14]. It cannot be definitely excluded that also in some oxidations at platinum anodes, higher valency oxides at the surface act as inner sphere electron transfer agents. Electrochemical" inner sphere electron transfers are intentionally used in indirect electrochemical conversions where selective chemical oxidants or reductants are regenerated by electron transfer from the electrode [15]. They are also immobilized by attaching polymer-bound electrocatalysts as mediators to the electrode surface [16]. 

This category refers to enzymes that catalyze different reactions (and not just different substrates) than the one they evolved for. As is the case with substrate and coenzyme ambiguity, the enzyme s activity with these alternative substrates is purely accidental, and is under no selection, and is therefore promiscuous by definition. As suggested, these cases include chemical transformations where the bonds that are broken, or formed, are different than those in the native substrate and reaction, and/or transformations that proceed through a different transition state. As discussed later, the promiscuous chemical transformations can be performed by the same catalytic side chains, and by essentially the same mechanism, as the native enzymatic function (Section 8.03.6). But there are also cases in which the enzyme utilizes different subsets of active-site residues, and somewhat different mechanisms, for the native and promiscuous functions (Section 8.03.6.1.4). 

Now we would like to use a transition state ring bond order uniformity (n-molecular orbital delocalization) as a measure of its stability, and therefore the selectivity between two or more isometric transition state structures. A view that transition state structures can be classified as aromatic and antiaromatic is widely accepted in organic chemistry [54], A stabilized aromatic transition state will lead to a lower activation barrier. Also, it can be said that a more uniform bond order transition state will have lower activation barriers and will be allowed. An ideal uniform bond order transition state structure for a six-membered transition state structure is presented in Scheme 4. According to this definition, a six-electron transition state can be defined through a bond order distribution with an average bond order X. Less deviation from these ideally distributed bond orders is present in a transition state which is more stable. Therefore, it is energetically preferred over the other transition state structures. 

The ideal template for the selection of the best catalyst is the transition state, which is by definition unstable and therefore not useful. Therefore, a TSA should be designed that, on the one hand, is sufficiently precise to mimic the transition state but, on the other hand, is sufficiently stable to be used for the selection procedure. Recent progress in molecular modehng and the ever-increasing computer power facilitates the identification of the transition state and for the design of analogs thereof. 

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