Cyclic transition complex

Calculations (10), similar to the RRR treatment of iinimolecnlar decomposition, based on a cyclic transition Complex, gave a value of 0.11 for the fraction of H % having translational energy m excess of 10 kcal. 

On the other hand, the conrotatory ring opening reaction of cyclobutene is thermaUy aUowed, and occurs via a Mdbius-Uke (43) cyclic transition complex according to Dewar (11) and Zimmermann... 

Step 1 is followed by the formation of a six-centered cyclic transition complex, in which a synchronized transfer of an electron pair takes place ... 

The assumption of a cyclic transition complex is equivalent to recognizing the diene synthesis as a c is-reaction consisting in the cis-addi-tion of the dienophile to the cisoid form of the diene. The most important conclusion from this is the necessary retention of the initial configurations of the diene and dienophile in the adduct formed. 

The relations 4- > 2-position in rate and 4- < 2-position in will apparently apply to reactions with anions, but the reverse relation is observed in piperidination, presumably due to 2-substitution being favored by hydrogen bonding in the zwitterionic transition state (cf. 47, 59, and 277) or by solvent-assisted proton removal from the intermediate complex (235). Substitutions of polychloroquino-lines (in which there is a combined effect of azine-nitrogen and unequal mutual activation of the chlorine substituents) also show 4- > 2-position in reactivity contrary statements are documented by these same references. Examples are cited below of the relation 2- > 4-position when a protonated substrate or a cyclic transition state is involved. 

The ethylenediamine derivative [31] possesses higher promoting activities than other diamines. This phenomenon may be ascribed to the copromoting effect of the two amino groups on the decomposition of persulfate through a CCT (contact charge transfer complex) formation. So we proposed the initiation mechanism via CCT as the intimate ion pair and deprotonation via CTS (cyclic transition state) as follows ... 

In the dialkylboron derivative of nitroacetic ester (102a) adopting the Z- con-hguration, the tetracoordinated boron atom is bonded with the carbonyl group (NMR spectroscopic data) (230). Also, a complex of this product with pyridine (103) has the E- conhguration (272). Apparently, the reaction of (102a) with pyridine also proceeds through the cyclic transition state A (Scheme 3.82). 

The propagation by both types of modified ester probably involves a 6-centred cyclic transition state. The same applies to a third type of /-cat propagation, in which a polymeric ester reacts with a D-A complex formed from M + I2. 

Following earlier studies of the oxidation of formic and oxalic acids by pyridinium fluoro-, chloro-, and bromo-chromates, Banerji and co-workers have smdied the kinetics of oxidation of these acids by 2, 2Tbipyridinium chlorochromate (BPCC) to C02. The formation constant of the initially formed BPCC-formic acid complex shows little dependence on the solvent, whilst a more variable rate constant for its decomposition to products correlates well with the cation-solvating power. This indicates the formation of an electron-deficient carbon centre in the transition state, possibly due to hydride transfer in an anhydride intermediate HCOO—Cr(=0)(0H)(Cl)—O—bpyH. A cyclic intermediate complex, in which oxalic acid acts as a bidentate ligand, is proposed to account for the unfavourable entropy term observed in the oxidation of this acid. 

Catalytic hydrogenation transfers the elements of molecular hydrogen through a series of complexes and intermediates. Diimide, HN=NH, an unstable hydrogen donor that can only be generated in situ, finds some specialized application in the reduction of carbon-carbon double bonds. Simple alkenes are reduced efficiently by diimide, but other easily reduced functional groups, such as nitro and cyano, are unaffected. The mechanism of the reaction is pictured as a transfer of hydrogen via a nonpolar cyclic transition state. 

Theoretical aspects of the effect of ring size on the acid-catalyzed reduction of cyclic sulfoxides by iodide ions have been studied by Tamagaki et who noticed that differences in reactivity are mainly dependent on the change in activation entropy, which is correlated to the rigidity of the transition complex 220, (Eq. 59). 

It is not necessary to incorporate the concept of macrosurfaces nor of olefinic coordination complexes of the metal in order to explain stereospecific polymerization. Simple 4 and 6 membered cyclic transition states account for steric control. 

From this mechanism the linear complex formed in reaction (47) may be visualized as the intermediate in the formation of the cyclic transition-state structure in step (48). Further implications of these quenching studies with respect to the mechanism of the primary bond-scission reaction (49) will be discussed in the next section. 

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