Mechanisms intramolecular

Factor b above is discussed in Sections II, B, 1 II, B, 4 and II, C. A hydrogen-bonded structure such as 221 can account for the facile reaction of 5-bromouracil or for the unique, so-called hydrolyzability of carboxymethylthio-azines (237). The latter may also react via the intramolecular mechanism indicated in 136. The hydrogen-bonded transition state 238 seems a reasonable explanation of the fact that 3,4,6- and 3,4,5-trichloropyridazines react with glacial acetic acid selectively to give 3-pyridazinones while other nucleophiles (alkoxides, hydrazine, ammonia, or sulfanilamide anion) react at the 4- and 5-positions. In this connection, 4-amino-3,5-dichloro-pyridazine in liquid hydrazine gives (95°, 3hr, 60%yield)the isomer-... 

Fitting a rearranged system to the steric requirements shown above within a molecule is needed for an intramolecular mechanism. Even when the appropriate arrangement cannot be achieved within a single molecule, the required configuration, 29 or 30, can possibly be realized in a dimer or higher associate. For metal-centered migrants, an alternative intermolecu-lar mechanism of the dissociative SnI type is also possible. 

Quinoxaline A-oxides undergo rearrangement under a variety of conditions. Thus on treatment of 2-ethoxy- and 2-methoxy-quinoxa-line 4-oxide with hydrochloric acid, rearrangement and hydrolysis occurs to give quinoxaline-2,3-dione. A possible intramolecular mechanism of rearrangement is shown in Scheme 7. Reaction of 2,3-... 

The results of Jamieson and McNeill cannot be accounted for by the intramolecular mechanism proposed by Grassie and coworkers [136,137] for the thermal degradation behavior of VC/VAc copolymers (Eqs, [28] and [29]). They can be accounted for much more convincingly by the alternative approach proposed by Naqvi based on polar interactions within the PVC matrix. Just like in copolymers even in blends, the polar carbonyl group of PVAc intensifies the concentration of like-poles in the PVC matrix resulting in destabilization. 

The evidence presented so far excludes the formation of dissociated ions as the principal precursor to sulfone, since such a mechanism would yield a mixture of two isomeric sulfones. Similarly, in the case of optically active ester a racemic product should be formed. The observed data are consistent with either an ion-pair mechanism or a more concerted cyclic intramolecular mechanism involving little change between the polarity of the ground state and transition state. Support for the second alternative was found from measurements of the substituent and solvent effects on the rate of reaction. 

On the basis of the evidence presented above as well as some other pertinent data (e.g. negative entropies of activation), Darwish and Braverman have suggested that the rearrangement of allylic 2,6-dimethylbenzenesulfinates (6a-f) to corresponding sulfones (7a-f) proceeds by a cyclic intramolecular mechanism involving a five-membered transition state which may be represented by a resonance hybrid (8) of the following resonance structures. 

In order to account for the unusually facile thermal racemization of optically active allyl p-tolyl sulfoxide (15 R = p-Tol) whose rate of racemization is orders of magnitude faster than that of alkyl aryl or diaryl sulfoxides as a result of a comparably drastically reduced AH (22kcalmol- ), Mislow and coworkers44 suggested a cyclic (intramolecular) mechanism in which the chiral sulfoxide is in mobile equilibrium with the corresponding achiral sulfenate (equation 10). 

An intramolecular mechanism of the rearrangement has been shown in the special ESR study (refs. 21, 22), conducted on the model radicals, generated by abstraction of a bromine atom from T2 " the rate constant K2, equal to (5.0 + 0.3) X 104 sec- at 22°C, has been also determined. In addition, fragmentation of radical... 

Fig. 32 Proposed cooperative bimetallic intramolecular mechanism for the enantioselective Michael addition of a-cyanoesters 57 to vinylketones...

A method for the stereospecific synthesis of thiolane oxides involves the pyrolysis of derivatives of 5-t-butylsulfinylpentene (310), and is based on the thermal decomposition of dialkyl sulfoxides to alkenes and alkanesulfenic acids (equation 113). This reversible reaction proceeds by a concerted syn-intramolecular mechanism and thus facilitates the desired stereospecific synthesis The stereoelectronic requirements preclude the formation of the other possible isomer or the six-membered ring thiane oxide (equation 114). Bicyclic thiolane oxides can be prepared similarly from a cyclic alkene ... 

Thiocarbamate (tc, RHNCSO-) is a monodentate ambidentate ligand, and both oxygen- and sulfur-bonded forms are known for the simple pentaamminecobalt(III) complexes. These undergo redox reactions with chromium(II) ion in water via attack at the remote O or S atom of the S- and O-bound isomers respectively, with a structural trans effect suggested to direct the facile electron transfer in the former.1045 A cobalt-promoted synthesis utilizing the residual nucleophilicity of the coordinated hydroxide in [Co(NH3)5(OH)]2+ in reaction with MeNCS in (MeO)3PO solvent leads to the O-bonded monothiocarbamate, which isomerizes by an intramolecular mechanism to the S-bound isomer in water.1046... 

The intramolecular mechanism of this reaction is proved by cross experiments for the metallation and 1,3-rearrangement reaction. In the same manner it is possible to yield the benzoxasiloles 11 (Eq.(4)). The first step in the reaction of chloromethylsilylethers of o-bromophenol 10 with sodium in boiling toluene is the metallation, followed by 1,3-rearrangement and intramolecular cyclization [8]. 

Transition states for betaine isomerization to ylides via the intramolecular mechanism were not localized. We believe that these processes are intermolecular and involve donor solvent molecules or the second betaine molecule as a proton carrier. 

Moresco F, Meyer G, Rieder KH, Tang H, Gourdon A, Joachim C (2001) Recording intramolecular mechanics during the manipulation of a large molecule. Phys Rev Lett 87 088302... 

The photochemical decomposition of Mes3GeHgCl was studied by Castel and coworkers107. The main photoreaction is the ejection of Hg, and the formation of Mes3GeCl, presumably via an intramolecular mechanism. A number of other reactions are observed, however, all occurring via formation of Mes3Ge-. The photochemistry is summarized in Scheme 28, with the reactions being followed by E.S.R. and N.M.R. spectroscopies. 

Several authors have studied DEG formation, but the formation of dioxane in PET synthesis is rarely considered. Hovenkamp and Munting [60] investigated DEG formation in sealed tubes at 270 °C and found dioxane in amounts of up to 10% of the DEG value. They suggested an intramolecular mechanism forming dioxane from a terminal DEG group (Figure 2.10). 

---