The Racemization of trans-1,2-Divinylcyclopropane

Abstract

The synthesis of (-)-trans-l,2-divinylcyclopropane and the kinetics of its racemization and conversion to 1,4-cycloheptadiene, presumably through cis-l,2-divinylcyclopropane, are described. It is concluded that the ring opening is not an electrocyclic process.La synthtse du (-)-trans-divinyl-1,2 cyclopropane ainsi que la cinktique de sa rackmisation et transformation en cycloheptaditne-1,4, probablement via le cis-divinyl-1,2 cyclopropane, sont rapportkes. II en a ktk dtduit que l'ouverture du cycle n'est pas un processus blectrocyclique.

Concertedness in the Thermolysis of Some 1-Pyrazolines

“…In the case of 1 and 2 the product distribution is given in Table 5 along with that implied from studies on the implied diradical, 8, as produced during the racemization of (-)-6 (21). The recent work of Brown et al (22) makes it clear that any cis-1,2-divinylcyclopropane produced, under the conditions of thermolysis of 1 and 2, will be observed as 7 but it is not necessary that all of the 7 produced has been generated via cis-1,2-divinylcyclopropane since the diradical 8 may close directly to 7.…”

Recent applications of the divinylcyclopropane–cycloheptadiene rearrangement in organic synthesis

“…Nevertheless , trans -divinylcyclopropane 15 (see Scheme 3 ) can be used in the DVCPR, as it undergoes isomerization to the desired cis -isomer 9 at elevated temperature (≈200 °C [ 1 – 2 ], lowered for more conjugated systems). The isomerization pathways have been suggested to proceed either via the formation of intermediate diradical-species (pathway A, Scheme 3 ) [ 16 , 20 , 26 – 27 ] or through one-center epimerization (pathway B) [ 28 – 29 ]. Following pathway A, the C1–C2 bond of 15 is cleaved homolytically to give diradical 16.…”

Dynamic stereomutation of vinylcyclopropanes with metalloradicals

“…This orbital-symmetry-controlled stereo-retentive electrocyclic reaction has served as a key strategic transformation in total syntheses, but frequently requires the cis-isomer to be made in situ (owing to its high reactivity), which was achieved predominantly through direct construction of the cyclopropane ring. Construction of the more stable trans-divinylcyclopropane, followed by isomerization to cis requires high temperature (approximately 200 °C) and occurs under racemization [19][20][21][22] . Such thermally or photochemically induced homolytic scissions towards diradical intermediates 23 can also trigger the cis-to-trans isomerization of alternatively substituted cyclopropanes and, with appropriate metal complexation for specific substrates, also at lower temperature (80 °C) 24 , albeit under loss of enantiopurity towards the racemic, thermodynamic trans-isomer.…”

“…We subsequently examined whether additional substitution on the vinyl motif could also be tolerated and synthesized alkenyl cyclopropanes (19,(28)(29)(30)(31)(32)(33)(34)(35)(36) that contain internal alkenes in E or Z geometry. Although longer reaction times and elevated temperature were found to be necessary for these cases, high trans-yields were seen in all cases under full retention of the olefin (Z or E) geometry.…”

“…2c and the locked π-allyl configuration in Int2. Similarly, 1,1-disubstituted olefins (19,(35)(36) and higher substitution at the cyclopropane ring (18,19) were also tolerated and, in the absence of the vinyl group, the starting material was fully recovered (27).…”