Transition state bond order

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. 

This bond order deviation from an ideal transition state structure to an example of cyclopropene added to a benzo-fused heterocycle may now be applied. Before we exEunine bond order deviation from an ideal transition state, we can take a look at the sums of rings bond order in the transition state structures. To simplify this picture, we will focus only on exo transition state structures between cyclopropene and benzo[c]heterocycles. Previously, we mentioned that the Diels-Alder reaction with benzo[c]heterocycles as dienes is a HOMO controlled diene reaction, therefore, an electron rich (higher sum of bond order) transition state structure should be energetically preferred. If this is the case, the order of reactivity should be benzo[c]furan, benzo[c]thiophene, and then benzo[c]pyrrole, which is exactly the same as determined on the basis FMO energy change (Table 23). 

It is characterized by double-bifurcation reactions with three directly connected first-order transition states. These are two transition states 570 and transition state 562. The chemical behavior of cation 560 is determined by the stereocomposition of a cyclo-propylcarbynyl cation moiety in conjugation with two vinyl group. The 9-barbaralyl cation 560 is characterized by four weak C—C single bonds (bond orders = 0.54 and 0.84) and one strong nonbonded interaction that can be easily broken and closed. 

An intermediate situation between the A and D extremes is an interchange (/) mechanism. Here, the breaking of the E-Y bond and the formation of the E-Z bond occur simultaneously as a concerted process at no time is there an intermediate with both a fully-formed E-Y and an E-Z bond. The transition state is a complex which can be described as Y... EX ... Z. Like the A mechanism, the kinetics are second order, but the intermediate (or activated complex) has no real existence as a static entity in the reaction mixture unlike the intermediate in the A mechanism, there is no energy barrier to the breakdown of Y... EX ... Z, which appears as a maximum on the profile of energy... 

A concerted insertion mechanism with highly ordered transition state, close to three-centered, was corroborated by examination of the kinetic isotope effect, which was measured by competitive GeC insertion reactions into the C—Cl bonds of labeled 14CCl4 and 12CC14. The value obtained, ku/k 2 = 1.01 0.01, is very close to that calculated from the stretching frequencies and the ratio of the masses and moments of inertia of the isotopic molecules for a synchronous reaction (0.993) and differs significantly from the calculated value for a dissociative mechanism (0.900)52. 

As pointed out earlier, the aliphatic Claisen reairangement and its variants are powerful means to effect stereocontrolled C,C-bond formation. Highly ordered transition states effect a reliable transfer of stereochemistry from starting materials to products. Naturally, the geometry of the vinyl ether bond and the conformation of the transition state are crucial parameters in this process. The former issue is strongly dependent on the Claisen variant that is employed, whereas the transition state geometry is controlled by both steric and electronic features of the Claisen system. Additionally, the choice between... 

Initial efforts in this area involved the addition of BuaSnLi to fratw-crotonaldehyde and conversion of the racemic hydroxy stannane adduct to diastereomeric (-)-menthyloxy-methyl ethers by reaction with (-)-menthyloxymethyl chloride (Eq. 32) [52]. These dia-stereomers could be separated by careful chromatography. They formed diastereomeric anti, (Z) adducts with aldehydes upon heating to 130 °C. The results parallel those seen for the racemic OMOM allylic stannanes (Table 25). Formation of the (Z) double bond in these adducts is attributed to steric interactions between the allylic OR substituent and the adjacent stannane butyl groups in a chair-like transition state as pictured in Eq. (9). The excellent stereoselectivity of these additions is suggestive of a highly ordered transition state. 

The activation parameters (Table 3) are characterized by positive values of H koA negative values of and indicate that bond-formation plays an important role in forming the transition state. This is in agreement with previous work which showed that oxidative addition proceeds via an associative mechanism." For 2 in ethyl acetate, the oxidative addition rate constants could not be determined accurately, but /cr could be used for the calculation of the activation parameters. The values suggest less ordered transition states in which significant solvent interaction may occur, but it is clear that additional research is still required. 

The mechanism proposed on the basis of these data included a highly ordered transition state, responsible for the very negative values of the activation entropy. According to the observed value for the deuterium kinetic isotopic effect, the C-H bond breaking process must not be very important in this transition state. For the acid catalysed reaction a fast protonation equilibrium of species Via, followed by the rate-determining step was proposed. 

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