Structure of the transition state

Several structures of the transition state have been proposed (I. D. Williams, 1984 K. A. Jorgensen, 1987 E.J. Corey, 1990 C S. Takano, 1991). They are compatible with most data, such as the observed stereoselectivity, NMR measuiements (M.O. Finn, 1983), and X-ray structures of titanium complexes with tartaric acid derivatives (I.D. Williams, 1984). The models, e. g., Jorgensen s and Corey s, are, however, not compatible with each other. One may predict that there is no single dominant Sharpless transition state (as has been found in the similar case of the Wittig reaction see p. 29f.). 

The 8n2 mechanism is believed to describe most substitutions m which simple pri mary and secondary alkyl halides react with anionic nucleophiles All the examples cited in Table 8 1 proceed by the 8 2 mechanism (or a mechanism very much like 8 2— remember mechanisms can never be established with certainty but represent only our best present explanations of experimental observations) We 11 examine the 8 2 mecha nism particularly the structure of the transition state in more detail in 8ection 8 5 after hrst looking at some stereochemical studies carried out by Hughes and Ingold... 

What IS the structure of the transition state m an 8 2 reaction s In particular what is the spatial arrangement of the nucleophile m relation to the leaving group as reactants pass through the transition state on their way to products ... 

Wolfenden, R., 1972. Analogue approaches to die structure of the transition state in enzyme reactions. Accounts of Chemical Research 5 10-18. 

Obtain the energies of propene, dimethylborane, and 1-propyldimethyl borane, and calculate AH n for dimethylborane addition. Is this reaction exothermic or endothermic Use this result and the Hammond Postulate to predict whether the transition state will be more reactant like or more product like . Compare the geometry of the transition state to that of the reactants and products. Does the Hammond Postulate correctly anticipate the structure of the transition state Explain. 

Evidently, the structure of the transition state is such that cis as well as trans compounds can be formed with, more or less, a preference for the cis compounds. The reason for the latter may be that coordination of cis isomers is sterically favored. In agreement with this are the observations that cis compounds are more reactive than the corresponding trans compounds (49, 106). 

This reduction in activation energy will occur only when the structure of the transition state complex fits well in the zeoHte cavity. This is the case for the protonated toluene example in the zeoHte mordenite channel. The structure of the transition state complex in the cluster simulation and zeoHte can be observed to be very similar to the one in Figure 1.10. 

Figure 6.39. Structure of the transition states for ethylene hydrogenation, corresponding to Fig. 6.38 see text for details. [Adapted from M. Neurock, V. Pallassana and R.A. van Santen.J. Am. Chem. Soc. 122 (2000) 1150.]...

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

Some disagreement exists regarding the structure of the transition state (whether H or P3 is trans to the alkene) and whether or not solvent molecules occupy sites that are apparently vacant. In spite of some uncertainty regarding these details, the major issues regarding the catalyzed hydrogenation of alkenes using Wilkinson s catalyst are fairly well understood. 

A common theme of all the preceding mechanisms of catalysis is that the enzyme does something to assist the reaction in reaching the transition state. The structure of the transition state for a chemical reaction is slightly different from the structure of either the reactants or products. Some chemical bonds are at different angles and lengths, and charges are distributed differently. The enzyme can stabilize those features that occur... 

The STRUCTURE OF THE TRANSITION STATE is different from that of the substrate with respect to charge and shape. Because it looks different, the enzyme can recognize specific features of the transition state and stabilize them. This makes it easier to reach the transition state and makes the reaction faster. 

Fig. 8. Calculated structures of the transition states within different environmental models (159).

Figure 4.77 The optimized structure of the transition state II for the ethylene-insertion reaction II III (4.106), with forward activation energy A > = 6.90 kcalmol-1 relative to the metal-ethylene complex II.

The H-bonded H HOH product species was previously depicted in Fig. 5.16, while the structure and leading n— a interaction for the corresponding H2 OH-reactant species are shown in Fig. 5.33. Figure 5.34 similarly depicts the structure of the transition-state species and principal n—a interaction for the reactant-like Lewis structure that better describes the resonance hybrid (see below). 

Using kinetic isotope effects to determine the structure of the transition states of SN2 reactions, 41, 219... 

The Claisen rearrangement is an electrocyclic reaction which converts an allyl vinyl ether into a y,8-unsaturated aldehyde or ketone, via a (3.3) sigmatropic shift. The rate of this reaction can be largely increased in polar solvents. Several works have addressed the study of the reaction mechanism and the electronic structure of the transition state (TS) by examining substituent and solvent effects on the rate of this reaction. 

Pyramidalization is a common indication of increased reactivity at sp2 hybridized centres. These centres often become tetrahedral when they react, so the geometrical distortion is in the direction of the transition state for the reactions concerned. The way the rest of the structure responds to the distortion is therefore relevant to the structure of the transition state for such reactions. 

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