Positional interchange

Dyotropic rearrangements are uncatalyzed concerted dihydrogen exchange reactions, another class of orbital symmetry controlled processes, which involve the simultaneous migration of two cr-bonds. These conversions can be both thermal and photochemical. They can be subdivided into two types (1) reactions in which two migrating cr-bonds interchange their positions (equation 78), and (2) reactions without such positional interchange (equation 79)91,92. 

Note The pictorial term scrambling is used in mass spectrometry to describe rapid processes of (intramolecular) positional interchange of atoms. Scrambling may occur with hydrogens or may involve the complete carbon skeleton of an ion. Aryl radical ions and protonated aryl compounds are well known for their numerous scrambling processes. [54,55]... 

The formation of (C6H60)t from the molecular ion of 2-phenoxyethyl chloride has been shown to be accompanied by, in effect, a positional interchange of the phenoxy group and the chlorine atom [726, 832, 860]. The phenomenon is evocative of that encountered with (CH3)3CCH2OC6H5 (vide supra). From peak heights in the El mass... 

Thus, it is necessary for unhampered diffusion of cations in a zeolite that at least 1/3 of the octahedral and 1/4 of the cubic cation positions shall be unoccupied simultaneously, since every cubic cation position is adjacent to 3 octahedral positions, and every octahedral position is adjacent to 4 cubic cation positions—i.e., lo = 6/3 = 2 and Ic = 8/4 = 2. If lo a 2 and/or Ic < 2, cation diffusion by this mechanism is impossible. In that case, it proceeds by simultaneous position interchange between 2 cations. For Ca zeolites and Mn zeolites, the resulting maxi-mums are at Np = 0, 2, 3, and 5, or for Mg zeolites at Np = 0 and 2, while the minimums for Ca zeolites and Mn zeolites are at Np = 1, 3, and 6, or for Mg zeolites at Np = 1, 3, 4, 5, and 6, confirming these assumptions... 

To explain this D(y) relation, it is necessary to assume overlapping of 2 diffusion mechanisms a "pore—fissure diffusion and a diffusion by simultaneous position interchange between cations. In both cases, cation motion would be along the walls of cavities, since cations are strongly attracted by the negatively charged zeolite lattice. Rate of diffusion is smaller in the interchange mechanisms than in pore-fissure diffusion. 

A similar reaction was used for the synthesis of muscazone from ibotenic acid (Eq 17).40 Only the 2 and 3 positions interchange. 

Mass spectrometry by itself cannot usually tell what positional isomers are present in a mixture. In favorable cases, however, it is possible to differentiate between positional interchange and randomization by analyzing product ion distributions. The double hydrogen transfer in equation 3 represents a case in point. Photoionization of cyclooctyl 4-pyridyl ether at... 

As is well known, kinetic arguments can be used to disprove mechanistic hypotheses (as in this instance), but never to prove them. The validity of first-order kinetics has been experimentally verified for monoenergetic ions, but the general applicability to ions with a distribution of internal energies has been questioned.Conditions under which first-order kinetics represent a usable approximation have been discussed, and kinetic isotope eflfects (having positive definite values) have been extracted from experimental data for a number of unimolecular decompositions of ions produced by photoionization.Our interpretation of the dissociation in Scheme 6 in terms of positional interchange is treated in the next section. 

Without additional reagents Diaxial-diequatorial rearrangement with positional interchange of arylthio and sulfonyl groups... 

In Type II rearrangement, two migrating groups are transferred simultaneously to the new bonding sites without positional interchange, resulting in... 

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