Transition states bridged

Other cycloadditions proceed in systems where the reaction partners are preoriented with a geometry ideally suited for the cycloaddition and the system is essentially forced towards the desired transition state. Bridged cyclic dimers or oligomers of norbornadiene and quadricyclane (6) serve as molecular frameworks. 

Dogonadze, R.R., Ulstrup,)., and Kharkats, Y.l. (1972) Theory of electrode reactions through bridge transition states-bridges with a discrete electronic spectrum. Journal of Electroanalytical Chemistry, 39, 47-61. 

Dogonadze R. R., Ulstrup J. and Kharkats Yu. I. (1972), Theory of electrode reactions through bridge transition states. Bridges with a discrete electron spectmm , J. Electronanal. Chem. 39,47-61. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

Although many overall rearrangements can be formulated as a series of 1,2-shifts, both isotopic tracer studies and con utational work have demonstrated foe involvement of other species. These are bridged ions in which hydride or alkyl groups are partially bound to two other carbons. Such structures can be transition states for hydride and alkyl-group shifts, but some evidence indicates that these structures can also be intermediates. 

These results indicate an energy profile for the 3-methyl-2-butyl cation to 2-methyl-2-butyl cation rearrangement in which the open secondary cations are transition states, rather than intermediates, with the secondary cations represented as methyl-bridged species (comer-protonated cyclopropanes) (Fig. 5.10). 

Fig. 5.11. Contrasting potential energy diagrams for stable and unstable bridged norbomyl cation. (A) Bridged ion is a transition state for rearrangement between classical structures. (B) Bridged ion is an intermediate in rearrangement of one classical structure to the other. (C) Bridged nonclassical ion is the only stable structure.

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

Another line of evidence that bridging is important in the transition state for solvolysis has to do with substituent effects for groups placed at C-4, C-5, C-6, and C-7 on the norbomyl system. The solvolysis rate is most strongly affected by C-6 substituents, and the exo isomer is more sensitive to these substituents than is the endo isomer. This implies that the transition state for solvolysis is especially sensitive to C-6 substituents, as would be ejqiected if the C(l)—C(6) bond participates in solvolysis. ... 

For alkyl-substituted alkynes, there is a difference in stereochemistry between mono-and disubstituted derivatives. The former give syn addition whereas the latter react by anti addition. The disubstituted (internal) compounds are considerably ( 100 times) more reactive than the monosubstituted (terminal) ones. This result suggests that the transition state of the rate-determining step is stabilized by both of the alkyl substituents and points to a bridged intermediate. This would be consistent with the overall stereochemistry of the reaction for internal alkynes. 

This rearrangement can be considered to occur via a transition state in which C(2)—C(4) bridging is accompanied by a 4 3 aryl migration ... 

The relation of rates of reduction with NaBH4 to variations in structure in a wide variety of monocyclic and bridged bicyclic compounds has also been discussed for example, a methyl a to a ketone slows the rate of reduction. Brown ° stated that reactions should not be discussed in terms of axial and equatorial attack, since the rates simply reflect differences in the energies of the possible transition states and not enough is known about the transition state to analyze it. He accepted th concepts of SAC and PDC, but preferred to call them steric strain contrpl and product stability control. ... 

To explain the stereospecificity of the reaction a transition state (6) is proposed rather than the bridged fluoronium ion (7),... 

The smooth intramolecular nucleophilic displacement of biphenyl carboxylic acids leading to benzocoumarins (See Section II.A.) inspired also investigation of the behavior of similar diphenyl ether, diphenyl sulfide and A-methyldiphenyl amine derivatives 458 under similar conditions. However, all these attempts to achieve cyclization to tricyclic compounds 459 were unsuccessful, probably due to the unfavorable stereochemistry for the formation of the required seven-mem-bered transition states and also to the presence of the deactivating bridge groups X (Eq. 42) [68JCS(C)1030]. 

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

When the carbonyl groups are present, the transition state for syn attack is sta-bihzed by interactions between the in-phase combination of the NN lone pairs and the antisymmetric n orbital of the CO-X-CO bridge (100). Although the secondary effect (SOI) operates only during syn approach and contributes added stabilization to this transition state, the primary orbital interaction (see 103) between the HOMO of the cyclohexadiene moiety of 100 and the n orbital of the dienophile (NN, Fig. 16) is differentiated with respect to the direction of attack, i.e., syn or anti, of triazolinedione (NN, Fig. 16). 

Because the pore dimensions in narrow pore zeolites such as ZSM-22 are of molecular order, hydrocarbon conversion on such zeolites is affected by the geometry of the pores and the hydrocarbons. Acid sites can be situated at different locations in the zeolite framework, each with their specific shape-selective effects. On ZSM-22 bridge, pore mouth and micropore acid sites occur (see Fig. 2). The shape-selective effects observed on ZSM-22 are mainly caused by conversion at the pore mouth sites. These effects are accounted for in the hydrocracking kinetics in the physisorption, protonation and transition state formation [12]. 

The transition states proposed for these exchange systems involve anion bridges For the system CrNCS -Cr, Ball and King suggest that either a nitrogen bridged transition state or a two stage process... 

Sykes et have proposed a transition state involving a hydroxide bridge for the processes defined by. ... 

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