Isomerism intramolecular mechanisms

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. 

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... 

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. 

Intramolecular Folding The Excimer/Exciplex Mechanism and Dewar Isomerization (Butterfly Mechanism)... 

Light-catalyzed isomerization of azoxy derivatives to o-hydroxyazo-derivatives by exposure to sunlight, whereby the oxygen migrates from the nitrogen atom to which it is attached to the nucleus which is attached to the other nitrogen atom, seems to proceed via an intramolecular mechanism LXXVII I LXXX.12... 

Furthermore, the less pronounced effect of 2MN on the formation of HI suggests that a small part of isomerization of I into III may occur through an intramolecular mechanism. 

Fig.9.3 Mechanisms of xylene isomerization- a Intramolecular mechanism (1,2 methyl shifts) b Intermolecular mechanism via disproportionation and transalkylation steps...

Photolysis of six-coordinate [Fe(CN)s en]3 results in the loss of one cyanide to give287 the chelated complex [Fe(CN)4en]2 in high quantum yield. With ligands such as the cyanopyridines that have two donor sites [Fe(CN)5L]" complexes have been observed to exhibit coordination isomerization. Several such compounds have been studied288,289 and interestingly some of the isomerizations involve an intramolecular mechanism. 

What information does the difference spectrum provide concerning the intramolecular mechanism of the BR to K transition The frequency shifts observed (Fig 6.6-10) between 1300 and 1100 cm characterize an all-trans to 13-cis isomerization of retinal. The unusually strong out-of-plane vibrations at 960 cm and 814 cm indicate considerable distortion of the terminal part of the chromophore. In summary, the BR-K difference spectrum shows that isomerization forces the chromophore and the protein backbone into a strained conformation, generating tension in the protein, which in turn drives the ensuing reactions. 

Just as the equilibrium conformational properties of macromolecules, the theory of which has been developed in well-known classical works by Kuhn, Flory, Volken-stein and others the kinetic properties of polymer chains can be determined by two main mechanisms of intramolecular mobility. First, it is the discrete rotational isomeric (rotameric) mechanism of mobility caused by the jump of small-chain segments (kinetic units) from certain energically stable allowed conformers into others is4-i6S) gg ond it is the continuous mechanism of motion deter-... 

Cls-Pt(py)9CI9 also undergoes photoisomerization to the trans isomer in chloroform solutions (95). The Isomerization is the predominant process ( -0.04), but is accompanied by a dissociative photoreaction ( -0,025). Quantitative studies showed that most of the isomerization occurs via an intramolecular mechanism however, the details of the process are not yet certain. 

In polar media, initial protonation of a square complex will produce first a cationic 5-coordinate complex which may then isomerize by an intramolecular mechanism (page 140). Coordination of halide ion would finally give the oxidized product MXL3 + H+(solv) MHXL,... 

These results lead us to the conclusion that the rate of isomerization depends on the pressure of reagents. It provides another proof for an intermolecular isomerization pathway. Note that the fragmentation may occur also through an intramolecular mechanism. 

Aromatic sulphonic acids (X = SO3H) for example, are isomerized in this way. The change of position in the aromatic ring of the tertiary alkyl groups and bromine likewise cannot be confined as intramolecular mechanism with successive 1,2-shifts of the migrating group. Thus, the interconversions of ortho- and para-disubstituted benzenes with substituents of the above type proceed faster than their conversion into a thermodynamically more stable meta isomer... 

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