Formale anti‐Carbopalladierungen nicht‐aktivierter Alkine: Voraussetzungen, mechanistische Untersuchungen und Anwendungen

Abstract

Formale anti-Carbopalladierungsreaktionen von C-C-Dreifachbindungen sind ungewçhnliche,a ber äußerst nützlicheT ransformationen. Alkine,d ie gezielt anti-Carbopalladierungsprodukte ergeben, werden vorgestellt. Voraussetzung ist der Ausschluss anderer Reaktionswege,umeine cistrans-Isomerisierung der entstehenden Doppelbindung zu ermçglichen. Der entscheidende mechanistische Schritt wurde sowohle xperimentell als aucht heoretisch untersucht. Eine intramolekulare Version der Reaktion ermçglicht die einfache Herstellung sowohl von oligocyclischen Produkten als auch von Dibenzofuranen.

Intramolecular Pd‐Catalyzed Formal anti‐Carboalkoxylation of Alkynes: Access to Tetrasubstituted Enol Ethers

“…[12] Recently,w ed esignedaprocess that led to af ormal anti-dicarbofunctionalizationo fatriple bond. [13,14] The key step is the cis-trans-isomerization in the coordinations phere of a1 4VEP d species,w hich allows ac ontinuation of the cascade on the opposites ide of the emerging double bond. [13] It is important to use as ubstituent at the triple bond that does not allow b-hydride elimination, since this pathway is fastert han the internal isomerization.I na ll previous reports, the reactive anti-configured Pd species was intercepted by nucleophilic carbon moieties such as olefins, [15] or stannylateda renes.…”

“…[13,14] The key step is the cis-trans-isomerization in the coordinations phere of a1 4VEP d species,w hich allows ac ontinuation of the cascade on the opposites ide of the emerging double bond. [13] It is important to use as ubstituent at the triple bond that does not allow b-hydride elimination, since this pathway is fastert han the internal isomerization.I na ll previous reports, the reactive anti-configured Pd species was intercepted by nucleophilic carbon moieties such as olefins, [15] or stannylateda renes. [16] However,t he use of nucleophilic heteroatomic speciess uch as hydroxy units [17] would allow af acile generationo ft etrasubstituted enol ethers startingf rom internal alkynes.…”

“…For the transformation described here, our observations are supportive of a cis-trans-isomerization pathway in the coordination sphere of Pd (Scheme 4). [13] Oxidativea ddition of the Pd 0 speciesi nto the CÀBr bond of 1a forms intermediate A,…”

Palladium(0)‐Catalyzed Intermolecular Carbocyclization of (1,n)‐Diynes and Bromophenols: An Efficient Route to Tricyclic Scaffolds

“…[12][13][14][15][16][17] Thel ack of control is even more severe in [2+ +2+ +1] cyclizations, [16,17] generally leading to three inseparable regioisomers in apoor ratio.T oaddress this problem, we postulated that the utilization of carbon-or heteroatom-tethered (1,n)-diynes,w hich possess an interintramolecular feature,toreact with either phenol diazonium salts or b-naphthols would provide an effective solution by the regiospecific construction of am ore interesting tricyclic framework. [16,17] Intrigued by the recent advances on palladium(0)-catalyzed domino reactions of aryl halides with (1,n)-diynes for generating diversified polycycles, [18] we used the most abundant, commercially available bromophenols for promoting ap alladium(0)-catalyzed dearomative cyclization reaction with the tethered diynes.O ur task was to find suitable reaction conditions to enable the desired [2 + 2 + x] (x = 1o r2 )cyclization process,t hrough the dearomatization of phenols,b yp reventing the unwanted [2+ +2+ +2] aromatization of two reactants [19] and the self-consumption of bromophenols either through diarylether formation [20] or dehalogenation. [16,17] Intrigued by the recent advances on palladium(0)-catalyzed domino reactions of aryl halides with (1,n)-diynes for generating diversified polycycles, [18] we used the most abundant, commercially available bromophenols for promoting ap alladium(0)-catalyzed dearomative cyclization reaction with the tethered diynes.O ur task was to find suitable reaction conditions to enable the desired [2 + 2 + x] (x = 1o r2 )cyclization process,t hrough the dearomatization of phenols,b yp reventing the unwanted [2+ +2+ +2] aromatization of two reactants [19] and the self-consumption of bromophenols either through diarylether formation [20] or dehalogenation.…”

“…Theenvisioned reaction was first realized by heating amixture of 1a and 2a in THF at 110 8 8Cinthe presence of 5.0 mol %of Pd(OAc) 2 ,6.0 mol %PPh 3 ,and 2.0 equivalents of K 2 CO 3 .The desired tricyclic product 3a was obtained in 53 %y ield (entry 1), without generating any other undesirable side products. Finally,asolvent screening was carried out (entries [14][15][16][17][18], and the reaction was dramatically improved by switching to 1,4-dioxane as the solvent, thus providing 3a in 86 %y ield upon isolation (entry 18). By using the inexpensive ligand PPh 3 ,s everal other palladium precursors were then evaluated (entries 7-9), thus showing inferior performance in comparison with that of Pd(OAc) 2 .N ext, K 3 PO 4 ,KOAc, and Na 2 CO 3 were found to be effective for the reaction (entries 10-12), whereas Cs 2 CO 3 completely shut down the reaction (entry 13).…”