Scrambling thermodynamics

The thermodynamic stabilities of phenonium ions relative to the parent have been determined in the gas phase by measuring the position of the equilibrium between (46) and (47)7 The results followed a Yukawa-Tsuno relationship with a p value of -12.6 and an r+ value of 0.62, the general behaviour being more like benzenium ions than benzyl cations, with tt-delocalization less effective than in benzyl cations. A theoretical study of the elimination of molecular H2 from the benzenium ion C6H7+ shows that the barrier to this process appears to be very small.The gas-phase Friedel-Crafts alkylation reaction of CF3C6L6+ (L = H or D) with C2L4 is accompanied by isotopic scrambling, which has been used to elucidate the mechanism of this process. A theoretical calculation shows that the lifetime of triplet phenyl cation must be very short. ... 

The exchange in the alkoxyamine-based polymer occurs in a radical process that is tolerant of many functional groups. The exchange process is therefore applicable to polymers with various functional groups. TEMPO-based polyester 43 and polyurethane 44 were synthesized for studies of the scrambling of disparate polymers imder thermodynamic control (Fig. 8.11) [37], Two kinds of TEMPO-based polymers were mixed and heated in a closed system. After 24 hours when the crossover reaction achieved equilibrium, GPC and NMR analyses revealed that they were totally scrambled through bond recombination on the TEMPO units. 

Readout of the ligand information by a substrate is achieved at the rates with which L and S associate and dissociate it is thus determined by the complexation dynamics. In a mixture of ligands Li, L2. .. L and substrates Si, S2. . - S , information readout may assume a relaxation behaviour towards the thermodynamically most stable state of the system. At the absolute zero temperature this state would contain only complementary LiSi, L2S2. .. L S pairs at any higher temperature this optimum complementarity state (with zero readout errors) will be scrambled into an equilibrium Boltzmann distribution, containing the corresponding readout errors (LWS , n n ), by the noise due to thermal agitation. 

Dianions of the above types may not fall into the category of homoenolate in a strictly formal sense. Nevertheless the amide dianion does show a behavior typical of the homoenolate. Thus, the reaction of the isotopically labeled stannylpropionate results in scrambling of the label probably via a cyclopropane intermediate Eq. (47) [44]. As the result of such an equilibration, isomerization of a-methyl and a-phenyl substituted propionate homoenolates may occur to give the thermodynamically more favorable isomers, respectively. 

The Ce isomer distribution—2-methylpentane (28%), 3-methylpentane (14%), and w-hexane (32%)—is very far from thermodynamic equilibrium, and the presence of these isomers indicates that both isopropyl cation 31 [Eq. (5.62)] and n-propyl cation 34 [Eq. (5.63)] are involved as intermediates [as shown by 13C(2)—13C(1) scrambling in the stable ion147]. 

It is important that any attempted equilibrations are performed under identical conditions to those used for the preparation of the adducts under investigation. The only variation made to these attempted equilibration conditions from those used for the syntheses of 41 and 68 was the use of considerably greater excess of the unlabeled bisdiene 37 in order to drive the equilibrium towards the formation of 41. The hexadecadeuterio 2 1 adduct 68 was heated under reflux in toluene with 10 molar equivalents of the bisdiene 37 for ca. 48 hours (Scheme 18). By this time, indications of thermally-promoted degradation of the starting materials had become evident by TLC investigation. If thermodynamic equilibration had been occurring via retro-Diels-Alder reactions of 68, followed by scrambling of the labeled and unlabeled bisdienes, 66 and 37, respectively, prior to recombination... 

Scheme 18. If the repetitive Diels-Alder reaction adducts reported were being formed under thermodynamic control, heating the hexadecadeutero 2 1 adduct 68 under reflux in toluene in the presence of a 10 molar equivalent of the bisdiene 37 should give rise to a mixture of compounds comprised of 68, 69, and 41. A retro Diels-Alder reaction of 68, followed by scrambling of the labeled bisdiene 66 with the excess of the bisdiene 37 should favor preferential cycloaddition with 37 to afford 69. The excess of 37 should ultimately drive the equilibration process toward the unlabeled 2 1 adduct 41. No evidence for such an equilibration process has been found... 

One of the most striking differences between Winstein s and our work is the fact that the bicyclo[3.2.0] cations, reported in this chapter, are observable61. Although the [2.2.1] isomers have been observed in some cases, the thermodynamically more stable ions always turned out to have the [3.2.0] skeleton. The difference is nicely demonstrated in the five-carbon scrambling process. Winstein assumed a [3.2.0] cation 55 to be involved as an intermediate in the scrambling of the observed ion 37l3, (see also Scheme 11), whereas we assume that a [2.2.1 ] cation 56-d3 is an intermediate in the scrambling of the observed [3.2.0] carbon 51-d3 (Scheme 38). The position of the equilibrium is not dependent on the presence of the C=C double... 

One clear tendency is apparent from the equilibrium product distribution of Letvis acid-induced transfluorination and halogen scrambling if fluorine can migrate it vill al vays tend to concentrate at one carbon atom. Most preferred reaction products are trifluoromethyl derivatives, followed by geminal difluoromethy] derivatives. This thermodynamic product control is often referred to as the general fluorine effect (Scheme 2.19). 

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