Stereomutation mechanisms

The existence of this second AH4 minimum also differentiates the stereomutation mechanisms of silane (as well as those of its heavier homologues) from methane. CH4 can invert only via a single transition state (Cj) which is close in energy to complete dissociation into CH2 and H2 and higher in energy than CH3 and H (Fig. 3) [28]. 

It has been reported that in DjO, hydrolysis of 27, which is slower than epimerization, does not lead to deuterium incorporation at the C-3 position thus ruling out epimerization mechanisms which involve stereomutation at this carbon atom. However, deuterium exchange at the benzylic carbon of 27 does occur in the presence of Na0D-D20 and an alternative epimerization mechanism involving an ylide intermediate has been discussed Indeed, stcrcomutation of authentic ylide derived from 27 has been repotted and it has been suggested 5) that epimerization of 27 probably involves ylides and that interconversion of isomeric ylides may involve a square planar sp rf-hybridized phosphorus . Available data do not warrant further speculation regarding this unusual stereomutation mechanism. However, the base-catalyzed stereomutation of a phosphetanium ion incapable of ylide formation (e.g., II) would be of considerable interest. 

Studies on the opening of a spiro-OPA led to the proposal of a stereomutation mechanism of the MB4 type for the OPA decomposition. Several oxidation-Wittig and oxidation Wittig-Horner one-pot procedures have also been developed for the efficient synthesis of a variety of substituted stilbenes. ... 

As a result of recent extensive studies, especially that of Mislow (247) on the stereomutation of sulfoxides, it is recognized that racemization of optically active organosulfur compounds occurs under various conditions and according to different mechanisms. 

The molecular mechanisms for the ring openings of various cyclopropanone systems in the gas phase have been studied at the PM3 semiempirical level and shown to be disrotatory processes, while an experimental study of the stereomutation of 1,1-difluoro-2-ethyl-3-methylcyclopropane has confirmed the predicted preference for disrotatory ring opening and ring closure for this system. 

However, quantitative evaluation of the size of this preference depended on knowing the size of the secondary deuterium isotope effect on which C—C bond in 7b cleaves. With the seemingly reasonable assumption of a secondary isotope effect of 1.10 on bond cleavage, the experimental data led to the conclusion that double methylene rotation was favored over single methylene rotation by a factor of 50 in the stereomutation of 7b. Although the error limits on the measurements were large enough to allow the actual ratio to be much smaller, Berson wrote, There is no doubt that the double rotation mechanism predominates by a considerable factor. ... 

Sixteen years later Baldwin and co-workers published the results of even more elegant experiments in which the stereomutation of optically active 7-l- C-l,2,3-was studied. Because a deuterium atom is attached to each of the carbons in this compound, it was unnecessary for Baldwin to assume the size of a secondary deuterium isotope effect on which bond cleaves, in interpreting his kinetic data. The results of his experiments led him to conclude, the double rotation mechanism does not predominate by a substantial factor. ... 

Ramirez, Ugi and coworkers28,84 have put forward the turnstile rotation (TR) mechanism, which can explain the fast stereomutations of compounds such as 41,42 or 43 without contradicting the conclusions of Whitesides and coworkers. Referring to Figure 17, the process can be defined with reference to three types of movements of the ligands of a TBP structure ... 

One of the interesting points connected with this type of coordination springs from the fact that nucleophilic substitution on phosphoranes probably occurs via the formation of an octahedral complex242-245. Further, it should be recalled that the mechanism of irregular stereomutations of pentacoordinated structures in basic media may also involve a hexacoordinated intermediate. Figure 26 shows just a few of the numerous and diverse structures known (196-203) but, even then, some general observations can be made. 

All of the experimental and theoretical work on the stereomutations of cyclopropanes and vinylcyclopropanes covered above seems consistent with and understandable in terms of kinetically significant involvements of Cj(ts), Cs(ts) and EF(ts) structures and partitionings of EE trimethylene intermediates resulting in the formation of klt k2 and kl2 products at comparable rates. For trans-1,2-disubstituted cyclopropanes, neither the Smith mechanism (one-center stereomutations only) nor any two-center-only formulation can be correct, as demonstrated by Crawford and Lynch in 1968143 and reinforced by numerous subsequent studies (Figures 2 and 3). 

The experimental and theoretical work published by the early 1970s viewed the stereomutations of cyclopropanes as kinetically competitive one-center and two-center stereomutations some details, especially regarding relative rate constants for one-center epimerizations which defined relative rotational propensities, remained unclear, but all agreed that neither the Smith mechanism (one-center only) nor any two-center-only formulation for stereomutations could be sufficient. Thus when kinetic studies275 278 on the isomerizations shown by chiral samples of l-phenyl-2-d-cyclopropane and 1,2-d2-cyclo-propane purported to show that, actually, two-center stereomutations were kinetically dominant, many were stimulated to fresh speculations and accommodations. Theoretical work at times hinted that the parent hydrocarbon might be an exceptional case and might... 

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