Alkylidenecarbenes, Alkylidenecarbenoids, and Competing Species:  Which Is Responsible for Vinylic Nucleophilic Substitution, [1 + 2] Cycloadditions, 1,5-CH Insertions, and the Fritsch−Buttenberg−Wiechell Rearrangement?

A Novel Synthetic Approach to Diaminoacetylenes: Structural Characterization and Reactivity of Aromatic and Aliphatic Ynediamines

“…[16,31] Apparently, polar solvents such as THF disfavor the FBW rearrangement by providing a long-lived vinylidene intermediate that undergoes intramolecular 1,5-CHinsertion, whereas the observed predominant formation of the diaminoalkyne 3 a in a nonpolar solvent such as toluene by 1,2-migration is in full agreement with recent high-level DFT calculations, showing "that the most facile FBW pathways occur in aggregated species." [32] Another interesting reactivity pattern associated with the presence of N-phenyl substituents in the diaminoacetylene 3 a was discovered upon studying its coordination chemistry towards transition metals.…”

“…This approach is based on the Fritsch-Buttenberg-Wiechell (FBW) rearrangement, which was originally developed for the preparation of diarylacetylenes (tolanes) from 2,2-diaryl-1-chloroalkenes [14,15] and involves the generation of halogen-metal-vinylidene species followed by 1,2-migration and concomitant metal halide elimination (Scheme 1b). [16] Notably, the FBW rearrangement has recently evolved into a valuable synthetic methodology for the preparation of polyyne structures, which can be accessed by treatment of 1,1-dibromo-2,2-dialkynyl-A C H T U N G T R E N N U N G ethenes with n-butyllithium (nBuLi). [17] Developing a similar protocol for the synthesis of diaminoalkynes required the preparation of the corresponding 2,2-dibromo-1,1-ethenedi-A C H T U N G T R E N N U N G amines of type 2 (Scheme 2), which, to the best of our knowledge, have not been described in the literature to date, although a limited number of dichloro derivatives have been reported that were obtained unexpectedly [18] or as byproducts.…”

“…[22] Mechanistically, the formation of 4 can be rationalized by an intramolecular insertion of the intermediate vinylidene species Int into an ortho-CÀH bond of one of the phenyl rings (Scheme 3), which resembles the formation of cyclopentene derivatives from thermally generated alkenylidenes by 1,5-CH insertion. [16,28] It should be noted, however, that insertion into alkyl C À H bonds is common, whereas examples of solution-state insertion into significantly less reactive aromatic CÀH bonds are rare and were only relatively recently reported.…”

Rhodium‐Catalyzed CH Alkynylation of Arenes at Room Temperature

“…[7a,d, 20, 22] Subsequently, intramolecular concerted 1,2-aryl migration [21] and heterolytic cleavage of the CÀRh bond allows generation of the intermediate F (Fritsch-ButtenbergWiechell-type rearrangement, Path 1). [22] Alternatively, D could follow stepwise migration and elimination via E to arrive at the same intermediate F (Path 2). Finally, protonation of F with another molecule of 2 b delivered the desired alkynylation product with concomitant regeneration of A.…”

Funktionalisierung von C‐H‐Bindungen: neue Synthesemethoden für Naturstoffe und Pharmazeutika

“…[146] Im Allgemeinen verläuft die Insertion von Alkylidencarbenen in primäre, sekundäre und benzylische sowie tertiäre C-H-Bindungen stereokontrolliert unter Bildung fünfgliedriger carbcyclischer (d. h. Cyclopenten) oder heterocyclischer Ringsysteme (d. h. Furan). Alkylidencarbene sind zwar auf vielen Arten zugänglich, [147] aber in der Synthese von Naturstoffen wurden drei spezielle Methoden häufig genutzt: 1) die a-Eliminierung aus terminalen 1-Halogenalkenen (Schema 58, Methode A) durch Umsetzung von Vinylhalogeniden mit starken Basen (KOtBu, KHMDS); [148] 2) die Bildung von Diazoalkenen aus Carbonylverbindungen (Methode B) durch Reaktion des Anions von (Trimethylsilyl)diazomethan (TMSCN 2 À ) mit Carbonylverbindungen; [149] 3) die Bildung von Alkinyliodoniumsalzen aus Alkinen (Methode C) durch Umsetzung eines Alkinylstannans mit PhI(CN)OTf und anschließende Zugabe einer geeigneten Base (z. B. Natrium-p-toluolsulfinat oder Natriumbenzolsulfinat).…”