Transition state, suitable

Molecular mechanics calculations similar to those described in the previous sections allow us to evaluate energy differences between catalytic models (preinsertion intermediates and transition states) suitable for primary and secondary insertions. This energy difference, in the framework of the assumed mechanism, can give a rough estimate of the nonbonded energy contribution... 

Since molecular mechanics cannot be used to calculate the energy of transition states, suitable models were adopted. These models are extremely similar to the Jt-olefin complex with an orientation of the growing chain rather similar to that adopted when a a-agostic interaction is present. They were often called pre-insertion intermediates because the insertion transition state could be reached from these intermediates with a minimal displacement of the reacting atoms. 

The suitability of the model reaction chosen by Brown has been criticised. There are many side-chain reactions in which, during reaction, electron deficiencies arise at the site of reaction. The values of the substituent constants obtainable from these reactions would not agree with the values chosen for cr+. At worst, if the solvolysis of substituted benzyl chlorides in 50% aq. acetone had been chosen as the model reaction, crJ-Me would have been —0-82 instead of the adopted value of —0-28. It is difficult to see how the choice of reaction was defended, save by pointing out that the variation in the values of the substituent constants, derivable from different reactions, were not systematically related to the values of the reaction constants such a relationship would have been expected if the importance of the stabilization of the transition-state by direct resonance increased with increasing values of the reaction constant. 

Vinyl alcohol is a good system for discussing transition state optimizations. The (iptimizatior. to a transition state starting from the 9Sf form proceeds easily, making it a suitable introduction to the general topic. 

The applicability to alicyclic alcohols may be limited, since the mechanistic course via a cyclic transition state requires a suitably positioned hydrogen in order to afford the desired product. 

The reactions of allylboronates 1 (R = H or CH3) may proceed either by way of transition state 3, in which the a-substituent X adopts an axial position, or 4 in which X occupies an equatorial position. These two pathways are easily distinguished since 3 provides 7 with a Z-olefin, whereas 4 provides 8 with an E-olefinic linkage. There is also a second fundamental stereochemical difference between these two transition states 7 and 8 are heterochirally related from reactions in which 1 is not racemic. That is, 7 and 8 arc enantiomers once the stereochemistry-associated with the double bond is destroyed. Thus, the selectivity for reaction by way of 3 in preference to 4, or via 6 in preference to 5 in reactions of a-subsliluted (Z)-2-butenylboronate 2, is an important factor that determines the suitability of these reagents for applications in enantioselective or acyclic diastereoselective synthesis. 

Cyclization of an organolithium tethered to a suitably positioned carbon-carbon jt-bond is a thermodynamically favorable process that proceeds in a totally regioselective exo-fashion with a high degree of stereocontrol via a transition state in which the lithium atom is intramolecularly coordinated with the remote rc-bond.9 The stereochemical outcome of the cyclization of a substituted 5-hexenyllithium follows from the preference of the substituent to occupy a pseudoequatorial position in the chair-like transition state depicted below.7... 

On the basis of the examples given above, it is reasonable to suggest that the underlying principles for optimization of the overall reaction rate with respect to the choice of metal ion are similar. That is, there are basically three states along the reaction pathway which determine the most suitable choice of metal ion. These are (1) the reactant state with bound metal and substrate before the proton transfer step, (2) the intermediately created free OH nucleophile and, (3) the subsequent transition state associated with... 

The ratio F/Eq of width F and the mean energy of the transition Eo defines the precision necessary in nuclear y-absorption for tuning emission and absorption into resonance. Lifetimes of excited nuclear states suitable for Mossbauer spectroscopy range from 10 s to s. Lifetimes longer than 10 s produce too... 

The multistep radical elimination may involve the generation of discrete intermediates, which for instance could be formed by a cyclization process7) such as 15- 16- 17e). Alternatively, there may be no intermediate involved in the elimination sequence, but the actual transition states 19, 22 are substantially lower in energy due to the anchimeric assistance of suitable functional groups9,10) (4). 

The possible mechanisms for solvolysis of phosphoric monoesters show that the pathway followed depends upon a variety of factors, such as substituents, solvent, pH value, presence of nucleophiles, etc. The possible occurrence of monomeric metaphosphate ion cannot therefore be generalized and frequently cannot be predicted. It must be established in each individual case by a sum of kinetic and thermodynamic arguments since the product pattern frequently fails to provide unequivocal evidence for its intermediacy. The question of how free the PO ion actually exists in solution generally remains unanswered. There are no hard boundaries between solvation by solvent, complex formation with very weak nucleophiles such as dioxane or possibly acetonitrile, existence in a transition state of a reaction, such as in 129, or SN2(P) or oxyphosphorane mechanisms with suitable nucleophiles. 

Intramolecular cyclopropanation is a useful method for construction of [n.l.0]-bicyclic compounds.17-21 225 275 As a matter of course, alkenyl and diazo groups of the substrate are connected by a linker and the transition-state conformation of intramolecular cyclization is influenced by the length and the shape of the linker. Thus, the enantioselectivity of the reaction often depends upon the substrates used. Use of a catalyst suitably designed for each reaction is essential for achieving high enantioselectivity. 

Martin observed an extremely facile thermal decomposition of bisperester 53, which gave 54 (Scheme 7).74 The decomposition proceeds via a transition state 55, with the simultaneous participation of the cr (0 O)—np(S)—a (0-0) type.74 The transition state can be draw by the 5c 6e interaction. Alvalez discussed the structure and the stability of Br42, together with neutral Br4.lla The dianion could be analyzed by the 4c 6e model, if the four atoms align linearly with suitable distances. Farnham investigated the fluorine-bridged linear C-I- -F- I C interaction based on 5c-6e.llb... 

In the common parlance of physical organic chemists such phrases as product-like or reactant-like transition states are common. The degree of resemblance of transition states to either reactants or products is usually assessed for reaction series obeying the linear-free-energy principle on the basis of suitable reaction constants, such as Bronsted a- and (3-values, and Hammett reaction constants p. The question is inherently more complex for cyclisation reactions, since they are not expected to follow the linear-free-energy principle. 

Chevrier et al., 1983), solvent effects (Bensaude et al., 1979), and the effect of added salt on the rate of reaction (Bensaude et al., 1978) have been studied to provide information about this process. Molecular-orbital calculations confirm that a suitable transition state for the reaction is one involving bridging water molecules (Field et al., 1984). 

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