Carben‐Umlagerungen, XX. trans ‐2‐(1,3‐Butadienyl)cyclopropyliden: Erzeugung und Umlagerungsverhalten

Abstract

20) and trans-l,2,4,6-heptatetraene (21) are formed from truns-2-(1,3-butadienyl)cyclopropylidene(oid) (l8), which is generated from the dibromocarbene adduct 17 with methyllithium at 0°C. The mechanisms of the rearrangements of 18 have been examined by '2C-labeling ('2C> 99.95%) in 17. 21 is formed from 18 by rupture of the distal bond B of the cyclopropylidene. Breakage of the lateral bond A in 18 induces a carbenecarbene rearrangement with 1,3-C migration. The rearranged carbene 28 stabilizes itself by three routes. The product ratio of the competing reactions in 18 is temperature dependent. At lower temperatures the carbene-carbene rearrangement 18 -+ 20 predominates, at higher temperatures increased formation of 21 takes place. -If 18 is generated by pyrolysis of the organotin compound 34, the high temperature (260 "C) necessary for the decomposition favors the allene formation and suppresses the carbene-carbene rearrangement. -21 is also formed when 18 is released from the N-nitrosourea 36b. 19 and 20 decompose under the reaction conditions. -A new method for the generation of carben(oid)s is described.

Quest for Even Higher Stabilized Foiled Carbenes

“…However, one pathway to bicyclo[3.1.0]hex-2-en-6-ylidene ( 2 ) still remains open and allows very easily a rearrangement with a calculated transition state of 3.4 kcal/mol. This pathway represents the first example of a retro-Skattebøl rearrangement, that is, the back reaction of the vinylcyclopylidene-cyclopentenylidene rearrangement . Cyclopropylidene 2 easily undergoes ring expansion to the energetically favored allene 1,2,4-cyclohexatriene ( 3 ) (Figure ).…”

“…Therefore, this second approach was chosen. Carbene 5 contains a vinylcyclopropylidene, which is expected to undergo the Skattebøl rearrangement to 4 . Additionally, this approach allows checking to see if the Skattebøl reaction in 5 takes place at all.…”

Three-Membered-Ring-Based Molecular Architectures

“…[360][361][362][363] 1,2-Dicyclopropylethenes have also been obtained by the addition of carbenes or carbenoids to 1,3,5-hexatrienes. [364][365][366][367][368] In many cases, the terminal double bonds are cyclopropanated selectively, and the second cyclopropanation (especially with dihalocarbenes) is much slower than the first one, which causes low yields of the bisadducts (Scheme 86). 367 However, in special cases, for example, the 2-fold cyclopropanation with dichlorocarbene of 533, endowed with particularly electron-rich terminal double bonds, the bisadduct 534 was formed in very good yield (Scheme 86).…”