Dyotropic type rearrangement

A similar rearrangement is observed between tetrasilyldisilenes 34 and 35 [Eq. (56)].14 The activation free energy for the rearrangement from 35 to (Z)-34 (or ( )-34) is 17.4kcalmol-1 at 283 K, which is ca. 1.7 kcal mol-1 larger than that for the /i,Z-isomerization and 7.7 kcal mol-1 smaller than those for the dyotropic type rearrangement of tetraaryldisilenes.96... 

Dyotropic type rearrangements were also found in stable disilenes (equation 5)31,32. The rates of these rearrangements were strongly dependent on the substituents. Thus, for the rearrangement in l,l-dimesityl(2,2-dixylyl)disilenes, 70 days were needed at 298 K to attain equilibrium, while the rearrangement in tetrasilyldisilenes was completed within 10 days at 273 K. 

Hetero-l,l-dimetallo-l-alkenes and hetero-1,1-dihalo-1-alkenes could be obtained in a stereoselective manner from 2-lithio-2,3-dihydrofuran via a dyotropic-type rearrangement of the derived cuprate. An example is depicted below <03SL955> <03S2530>. 

A dyotropic rearrangement is an uncatalyzed process in which two a bonds simultaneously migrate intramolecularly. There are two types. The above is an example of type 1, which consists of reactions in which the two a bonds interchange positions. In type 2, the two a bonds do not interchange positions. An example is... 

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. 

Furthermore, a brief review of dyotropic rearrangements starting with the hypothetical transformations of 1,2-disubstituted cyclobutenes was published98 in which two types of these processes were described and a general theory covering such rearrangements was outlined. Quantum chemical calculations of the reaction barrier for the dihydrogen... 

Type II dyotropic rearrangements involve double migration but no positional exchange. One such possibility is the squaric acid system in which thermally induced migration of the two ester functions toward the two carbonyl oxygen atoms is an orbital symmetry-allowed [ 7g -1- ... 

A special term ( dyotropic reaction see also Ref 222.374) proposed to denote the noncatalyzed processes in the course of which two o-bonds migrate simultaneously and intramolecularly, and a general theory of such reactions was developed In accordance with the definitions given in Ref. the rearrangement of ion (77) by the concerted route should be considered as a dyotropic reaction of type I. 

Good examples of Type I dyotropic rearrangements involving C—C bond as the stationary surface are provided by the interconversion of vicinal dibromides in cyclohexane and cyclohexanone ring systems (Scheme 6.7). 

Some examples of Type II dyotropic rearrangements by double C—H transfer are given in Scheme 6.9. The driving force for the reaction is provided by the proximity of the interacting orbitals and the rigid carbon framework. Furthermore, the reaction is accelerated by the release of the ring strain at the receptor 7t-bond and aromatization of the diene ring. 

The complex reaction sequence shown in equation 34 might provide some rationalization. The formation of the silylcarbene 141 is suggested, based on experimental results from related reactions , but there is no evidence for the formation of 141 nor for a silylene intermediate. Thus, the transformation 137 142 might proceed via a dyotropic rearrangement as well. The facile 1,3-methyl shift in 2-trimethylsilylsilenes which interconverts 142 139 is well known from Wiberg -type silenes . 139 (R = i-Bu) is stable in solution at room temperature over days and isomerizes only slowly to 140 (R = t-Bu) which rapidly dimerizes giving a 1,3-disilacyclobutane . 

Purely geometrical considerations dictate that the bromine atoms must migrate along their initial sides of the ir-system. The antarafacial interaction must therefore involve the utilization of a p-type orbital by one of the bromine atoms, but the inversion at this monovalent atom, of course, is not detectable in the rearranged product. The generality of dyotropic processes remains to be demonstrated. 

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