Transition stale

Transition stale search algorithms rather climb up the potential energy surface, unlike geometry optimi/.ation routines where an energy minimum is searched for. The characterization of even a simple reaction potential surface may result in location of more than one transition structure, and is likely to require many more individual calculations than are necessary to obtain et nilibrinm geometries for either reactant or product. 

With reactive aldehydes an early transition state is probably involved and therefore the steric demands of the aldehyde substituents are not highly influential. On the other hand, with less reactive ketones, the carbon-carbon bond formation is established further along the reaction coordinate, permitting the steric effects to play a greater role in the determination of the transition stale structure. 

The minimization of gauche pentane interactions in the transition stales is also an important consideration in the reactions of substituted allylboronates and a-heteroatom-substituted aldehydes4,52d 54,56. Transition states 8 and 11 have been identified by Roush and Hoffmann as the least sterically hindered ones accessible in reactions with (E)- and (Z)-allylboronates. 

For amide enolates (X = NR2), with Z geometry, model transition state D is intrinsically favored, but, again, large X substituents favor the formation of nt/-adducts via C. Factors that influence the diastereoselectivity include the solvent, the enolate counterion and the substituent pattern of enolate and enonc. In some cases either syn- or unh-products are obtained preferentially by varying the nature of the solvent, donor atom (enolate versus thioeno-late), or counterion. Most Michael additions listed in this section have not been examined systematically in terms of diastereoselectivity and coherent transition stale models are currently not available. Similar models to those shown in A-D can be used, however all the previously mentioned factors (among others) may be critical to the stereochemical outcome of the reaction. 

Transition stale structure, secondary deuterium isotope effects and, 31, 143 Transition states, structure in solution, cross-interaction constants and, 27, 57 Transition states, the stabilization of by cyclodextrins and other catalysts, 29, 1 Transition states, theory revisited, 28, 139... 

The physical organic chemistry of very high-spin polyradicals, 40, 153 Thermod5mamic stabilities of carbocations, 37, 57 Topochemical phenomena in solid-slate chemistry, 15, 63 Transition state analysis using multiple kinetic isotope effects, 37, 239 Transition state structure, crystallographic approaches to, 29, 87 Transition state structure, in solution, effective charge and, 27, 1 Transition stale structure, secondary deuterium isotope effects and, 31, 143 Transition states, structure in solution, cross-interaction constants and, 27, 57 Transition states, the stabilization of by cyclodextrins and other catalysts, 29, 1 Transition states, theory revisited, 28, 139... 

For discussion of transition stale geometries in this reaction, see Hoffmann Ditrich Froech Cremer Tetrahedron 1985, 41. 5517 Anh Thanh Nouv. J. Chim. 1986, 10. 681 Li Paddon-Row Houk J. Org. Chem. 1990, 55. 481 Denmark Henke J. Am. Chem. Soc. 1991, 113. 2177. 

There are inan common fragmentation processes which can help to explain and predict ion fragments found in El spectra. In general, these processes are promoted by the stability of the carbocation fragments produced and by six-membered transition stales in the rearrangements of ions. 

R1 R2 C-2—RC[) conformer - transition stale C-2—R conformer - transition state... 

This structure resembles the hydrogen bonded transition stale for the nucleophilic attack of hydroxide ion (Eq. 19.29) where the hydrogen bond promotes the attack on... 

In the transition state there is a p orbital at the carbon atom in the middle that shares one pair of electrons between the old and the new bonds. Both these pictures suggest that the transition stale for an S]sj2 reaction has a more or less planar carbon atom at the centre with the nucleophile and the leaving group arranged at 180° to each other. 

These reactions occur well without the enzyme (Chapter 36) but the enzyme accelerates this reaction by ah out a 106 increase in rate. There is no acid or base catalysis and we may suppose that the enzyme binds the transition state better than it binds the starting materials. We know this to be the case, because close structural analogues of the six-m embered ring transition state also bind to the enzyme and stop it working. An example is shown alongside—a compound that resembles the transition stale but can t react. 

The activation requires the interaction of a 0-CH bond with an acid center. A linear transition stale has been formulated by Hogeveen [43], but Olah concluded that the formation of a triangular cationic species (21) results from a front-side attack of a C-H bond. 

The transition stale represents the liighesl-encrgy structure involved in this step of the reaction. It is unstable and can f be isolcsterl, but we can nevertheless imagine it to be an activated complex of tlie two reactants in which both the C=C TT bond and H—Br bond are partially broken and the new C-H bond is partially formed (Figure 5.5). 

The first reaction variable to look at is the structure of the subslrafc. Because the S jZ transition stale involves partial bond formation between the incoming nucleophile and the alkyl halide carbon atom, it seems reasonable that a hindered, bulky substrate should prevent easy approach of the nucleophile, making bond formation difficult. In other words, the transition stale for reaction of a sterically hindered alkyl halide, tvhose carbon atom is "shielded" from approach of the incoming nucleophile, is higher in energy... 

The mechanism of the Diels-Alder cvcloaddition is different from that of other reactions we ve studied because it is neither polar nor radica.. Rather, the Dieis-Alder reaction is a pcricyciic process. Pericyclic reactions, which we ll discuss in more detail in Chapter 30, take place in a single step In- a cyclic redistribution of bonding electrons. The two reactants simply join together through a cvclic transition stale in wliich the two new carbon-carbon bonds form al the same time. 

Sharpless et a . have conhmied this mechanism in part by labeling experiments which demonstrated that the epoxide oxygen is derived exclusively from the peroxo ligands of the complex and not from the oxo oxygen. However, the reactivity of the molybdenum complex toward olefins closely parallels that of peracids, for which a three-membered cyclic transition stale is favored. ... 

We are indebted to Professor A. Streitwieser, Jr., for pointing out the details of the transition stale for the Ar + F" interaction, which are crucial to the understanding of the F addition ortho to E in the interaction with CsEFs+. 

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