18-electron transition state

The intermediate coordination, trihapto of allyl type is also known but rare. It is important, because the shift of M-(r) -Cp) (18e) to M-(T] -Cp) (16e) temporarily liberates a coordination site on the metal, which can allow a catalytic function to operate or an associative ligand substitution to avoid a 20-electron transition state or... 

In certain cases the reaction may proceed by a concerted mechanism. With allyl ethers a concerted [2,3]-sigmatropic rearrangement via a five-membered six-electron transition state is possible " ... 

DFT has come to the fore in molecular calculations as providing a relatively cheap and effective method for including important correlation effects in the initial and final states. ADFT methods have been used, but by far the most popular approach is that based on Slater s half electron transition state theory [73] and its developments. Unlike Hartree-Fock theory, DFT has no Koopmans theorem that relates the orbital energies to an ionisation potential, instead it has been shown that the orbital energy (e,) is related to the gradient of the total energy E(N) of an N-electron system, with respect to the occupation number of the 2th orbital ( , ) [74],... 

In Slater s half electron transition state scheme the integral is replaced by a calculation of the orbital energy occupied by half an electron. In the generalised transition state method (GTS) [75] and its related method (UGTS) using unrestricted DFT, the integral is replaced by the expression shown in Equation (14). 

A common example of Cope rearrangement involving [3, 3] sigmatropic rearrangement in a 1, 5 diene is the pyrolysis of meso 3, 4 dimethyl hexa-1, 5 diene giving exclusively cis, trans isomer of 2, 6 octadiene. The process involves a six electron transition state which has a chair like configuration and substituents occupy equatorial sites. 

There are few examples of other allowed sigmatropic shifts involving six ten electron transition states, but these are not common reactions. A [3, 4] shift is observed in competition with a [1, 2] shift in cations derived from cyclohexane diols. This is a cationic equivalent of the Cope rearrangement,... 

Loss of sulfur from these species would give the carbodiimide, whereas addition of alkenes would give the cycloadducts. The authors infer that this intermediate cannot add conceitedly to alkenes in the supra-supra fashion because it would involve a four-electron transition state, but that a stepwise addition would hardly rationalize the stereospecificity observed this suggests that the thiaziridine (or the 1,3-dipole) participates in an antara facial reaction.63 However, according to recent discussions antarafacial addition is seldom observed68 and stereospecificity need not be lost in a stepwise process.69... 

A variety of sigmatropic rearrangements through six-electron transition states are known. In contrast to the rare [l,3]-migrations, [l,5]-shifts of hydrogen in dienes, suprafacially allowed, occur readily. The experiment outlined in Equation 12.88 confirms the predicted stereochemistry.139 The authors estimated the... 

It is necessary to distinguish between two different H-bridge exchange processes. Complexes with cyclic, symmetrical diene ligands like 37g, 37o, 38g, 38o. 39g, and 39o show an exchange between endo-H-5 and endo-H-6 or endo-H-7, respectively. By this process, the R and S configurations are interconverted via an achiral 16-electron transition state. The relative positions of the monodentate ligands and the diene unit are unaffected by this... 

Kinetic studies at different phosphine concentrations indicated that the substitution reactions occur totally by a dissociative mechanism, while the ring expansion reaction is by an associative mechanism. The associative reaction could proceed by an 18-electron transition state involving either a bent NO or an rj to rj ring slippage mechanism. The bent NO mechanism seems more likely, because the ring slippage mechanism is known to result in the formation of oxocyclobutenyl product with ring expansion and because the isoelec-tronic cobalt complex does not react by a parallel associative pathway. 

The two dominant characteristics for substitution reactions of 17-electron complexes are very rapid reactions and associative mechanisms. Each of these features is in contrast to reactions of 18-electron complexes. The reactivity has been attributed to the formation of a three-electron bond between the entering nucleophile and the 17-electron complex. Electron density analysis supports stabilization of the 19-electron transition state as the primary source for the labilization. ... 

The ene reaction, discovered 45 years ago by Alder, usually involves the thermal reaction of an al-kene containing an allylic hydrogen (ene) with an electron-deficient unsaturated compound (enophile) to form 1 1 adducts via a cyclic six-electron transition state (e.g. A -h B -> C -> D X = H Scheme 1). 

Fig. 12.4 shows two possible ways for this to happen the HOMO of the diene can combine with the LUMO of the dienophile or the LUMO of the diene can combine with the HOMO of the dienophile. The thermal reaction with this six-electron transition state is allowed, but the corresponding photochemical mechanism is forbidden. More generally, the Woodward-Hoffmann rule for concerted cycloaddition reactions can be stated If the number of electrons in the transition state equals An [An + 2], then thephotochemical [thermal] reaction will be allowed, but the thermal [photochemical] reaction will be forbidden. 

Our results clearly rule out a dissociative substitution pathway for the transient CpW (CO)3 radical. At present, however, we are unable to distinguish between a 19-electron transition state and a stable 19-electron intermediate in these reactions. 

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. 

When small alkanes come into contact with the strongest superacids such as HF-SbFs, even at temperatures below 0°C, rapid protimn exchange occurs between the acidic protons and the hydrocarbon. This is due to the fact that, in the strongest acid media, hydrocarbons behave as sigma basesl l which may accept a proton on the various C-H or C-C bonds, leading to 3 center-2 electron transition states (or carbonium ions), as shown in Scheme 1 for iso-pentane. 

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