Ag+-catalyzed rearrangements. XIV. Influence of structural features of the course of bicyclo[1.1.0]butane rearrangements catalyzed by silver(I) ion

Abstract

Nine bicyclo[ 1.1.Ojbutanes, substituted to varying degrees with alkyl and aryl groups, have been rearranged with catalytic amounts of silver salts. In all cases, 1,3-dienes were produced but the positioning of substituents on this structural framework was found to vary with structure. When Ci and C3 are unsubstituted, the cleavage of diametrically opposed edge bonds occurs, a reaction which is kinetically accelerated with 2,2,4,4-tetramethyl substitution. In fact, this pathway also operates exclusively in the 1,2,2,4,4-pentamethyl derivative. Limited kinetic data allow for the possibility that this bond reorganization process is concerted when this level and type of substitution prevail. In those remaining cases in which Ci and/or C3 carry substituents, intermediate argento carbonium ion formation operates. 1,2-Hydride or vinyl shift (in that order) to the electron-deficient center and subsequent ejection of Ag+ leads to the diene products. Stereospecific considerations are also presented. Four general types of rearrangements have been observed in the Ag+-catalyzed rearrangement of substituted tricycloheptanes.1 234 One is the isomerization to 1,3-cycloheptadienes. This pathway, which proceeds by electrophilic attack of Ag+ at an edge bond with subsequent diametrically opposite edge bond rupture, operates exclusively in the case of the parent tricyclic hydrocarbon4•5 and is not affected by alkyl substitution at the "wing" positions.41 The second type, recognized to gain importance with substitution of the bridgehead carbons, is the conversion to alkylidenecyclohexenes. Kinetic deuterium isotope effect measurements and other relevant studies have revealed that this rearrangement proceeds by concurrent cleavage of the central bond of the strained ring and an edge bond with generation of intermediate argento carbonium ions.15-7 To the extent that tertiary argento carbonium ions can be formed, this isomerization is facilitated.

Ring-Strain-Enabled Reaction Discovery: New Heterocycles from Bicyclo[1.1.0]butanes

“…Their utility in the synthesis of natural and unnatural target molecules has been demonstrated, and a plethora of mechanistic information has been collected . The value of rhodium for valence isomerization reactions has also been recognized in the chemistry of bicyclo[1.1.0]­butanes. − Rhodium and nickel have played key roles in the determination of reaction mechanisms. − Silver­(I) ions can also efficiently catalyze the rearrangement of bicyclo[1.1.0]­butanes, albeit via a different mechanism. − …”

Sigmatropic Rearrangements and Related Processes Promoted by Silver

“…[22][23][24] Screening and kinetic investigations of these rearrangements suggested that the argento cationic intermediate formed on interaction of silver ion with one cyclopropane unit could further rearrange through ring opening of the second cyclopropane unit, but that the latter depended on the nature of the substituent. They can be considered as joined cyclopropanes and as such, readily react in the presence of catalytic amount of silver salts.…”

Synthesis of Stereodefined Polysubstituted Bicyclo[1.1.0]butanes

“…Furthermore, most of them necessitate electron-withdrawing groups (EWGs) on a bridgehead carbon center. − These limitations significantly restrict the diversity of accessible BCBs. Another approach to incorporate substituents is to metalate the bridgehead or the bridge position of simple BCBs followed by electrophile trapping. − The latter strategy is facilitated by the presence of EWGs. In this context, Anderson introduced an additional functionalization step involving the metalation of the bridge C–H bond using a lithium base followed by electrophile trapping/cross coupling, which results in the formation of bridge-substituted BCBs (Scheme b). , However, these approaches are limited in their ability to accommodate stereodefined quaternary centers.…”