Deuterium label, allyl isomerization

More recently, a number of reports dealing with 1,3-sulfonyl shifts which proceed by other mechanisms have been published. For example, Baechler and coworkers suggested that the higher activation enthalpy observed for the isomerization of the deuterium labeled methallyl sulfone 72 in nitrobenzene at 150°C as compared to the corresponding sulfide, together with the positive entropy of activation may be taken as evidence for a homolytic dissociation mechanism (equation 44). A similar mechanism has also been suggested by Little and coworkers for the gas-phase thermal rearrangement of deuterium labelled allyl sec-butyl sulfone, which precedes its pyrolysis to alkene and sulfur dioxide. 

In benzene solution the entropy of activation is — 14eu. Isomerization of deuterium-labeled allyl diphenylphosphinate and a- and y-methylallyl dipheny 1-phosphinates showed that the reaction proceeds with complete inversion of allylic structure and is effectively irreversible. The activation parameters and stereospecificity of this reaction are consistent with a concerted, cyclic mechanism. 

In the experiments using El-dj- and 3,3-d2-allyl alcohol in the presence and absence of pyridine bases, the results (Table IV) show that reaction occurs on Bronsted acid sites (Scheme 8), which results in diallyl ether formation with scrambling of the deuterium label via carbonium ion formation, and also on oxidizing sites on which both 1-d- and 3,3-d2-acrolein form via the formation and interconversion of the two isomeric 1,1 -d2- and 3,3-d2-allyl molybdates (27). These mechanisms are supported by data (Table IV) that show the suppression of ether formation and reduction of deuterium scrambling in the presence of base, which has little effect on the acrolein yield. The oxidation of O-allyl alcohol to acrolein over MOO3, which... 

A deuterium label at the primary position could thus be used as a very sensitive probe for allyl isomerization. Formation of the primary a-allyl would be expected to scramble any initial configuration of the label to a random mixture of syn and anti deuterium. 

While several explanations may be given for these findings, we focus here on our contention that this result supports the importance of allyl isomerization in diene polymerization. We postulate initially that the loss of bond forming stereospecificity in the diene polymerization is the result of allyl isomerization. This is particularly well supported by the completely stereorandom bond formation found for trans polymer. This is satisfactorily accounted for by rapid isomerization of the label, via primary allyls, to a random(l l) mixture of syn and anti deuterium. Any other mechanism for randomization, for example... 

A unique mechanism is shown in Figure 1. This scheme starts with an allyl/diene complex with both the growing polymer chain and the deuterium label in an anti configuration initially. Rapid allyl isomerization (left hand branch) leads to trans backbone and stereorandom deuterium rapid bond formation (right hand... 

The reflux for 4 h of 1-hexene with Fe(CO)5 yields a mixture of all possible straight chain hexenes in approximately their thermodynamic equilibrium concentration. One possible mechanism is allylic hydrogen transfer analogous to the PdCl2 isomerization. Allyl alcohol on treatment with Fe(CO)s undergoes rearrangement to propionaldehyde, undoubtedly via a ir-allyl hydride as shown by deuterium labeling studies ... 

Desulfurization through a-cleavage was shown to be the major product for a series of compounds 34 illustrated below [30]. These compounds, both benzylic and allylic, consistently showed cleavage on the benzylic side but suffered E/Z isomerization in the enal products 36 and 37 when not constrained by substitution. Deuterium labeling was consistent with the internal hydrogen transfer proposed to reach the sulEne 35. 

The results of isomerization of 2M3BN to 3PN, studied by deuterium labeling and computational methods, are consistent with this mechanism. The formal electron pushing mechanisms of the reversible reactions—dehydrocyanation of 2M3BN and conversion of the allyl intermediate to 5.56—are shown by reactions 5.6.1.3 and 5.6.1.4. 

From a mechanistic point of view (see below) it is important to note that the chloropalladation of labeled 2,2-diphenyl-l-methylenecyclopropane-3,3-d2 gave only two isomeric complexes, with absence of the isomer in which both the phenyl and deuterium reside on the allylic moiety (equation 323). This allows the exclusion of a symmetrically bound >/4-trimethylenemethane (TMM) intermediate or rapidly equilibrating >/3-TMM species,... 

The Fe3(CO)i2-catalyzed isomerization of 3-ethyl-l-pentene-3di gives 3-ethyl-2-pentene in which the deuterim label is randomly scrambled among the three methyl groups without loss of deuterium . The recovered 1-olefin also shows rapid random scrambling. These experiments demonstrate the intermediacy of w-allyl iron carbonyl hydrides and indicate Aat multiple addition-eliminations proceed before decomplexation of the catalyst occurs as... 

Confirmation and extension of this mechanism was obtained by Adams and Jennings 97,109) using propylene labeled with deuterium in various positions. Ammonia was also added to the feed to produce acrylonitrile. After reasonable corrections for small effects of propylene isomerization and deuterium exchange, the results were in quantitative agreement with a model in which allylic hydrogen abstraction occurs to form an allylic intermediate followed by hydrogen abstraction from either end. The model is illustrated by the following scheme for l-propene-3d. 

Formation of the strained TCO was quite unexpected and arouse further interest in the reaction. Baldwin and Greeley [11] made a labelling experiment preparing l,5-cyclooctadiene-D-3, 4. It was found by mass spectrometry that the product TCO, obtained from 4 by CuBr sensitized irradiation in ether, lacked the statistical factor of 1/8 (of eight equivalent allylic positions in COD one is labelled) of its original deuterium content. From this observation the intermediacy of allyl radicals in the isomerization was deduced which finally abstract hydrogen from the solvent. 

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