Dehydrogenative and decarboxylative C–H alkynylation of heteroarenes catalyzed by Pd(II)–carbene complex

Recent Developments in Transition-Metal Catalyzed Direct C–H Alkenylation, Alkylation, and Alkynylation of Azoles

“…In the presence of a NiBr 2 •diglyme catalytic system with O 2 as oxidant, they coupled various arylacetylenes with benzoxazoles to afford the desired products in low to moderate yields. In the proposed reaction mechanism, an (alkynyl)nickel intermediate, generated with In 2015, Mannepalli and coworkers developed an air-and moisture-stable Pd(II) carbene complex for the alkynylation of azoles with terminal alkynes and aryl propiolic acids by crossdehydrogenative or decarboxylative coupling, respectively [100]. A wide range of azoles including benzoxazoles, benzothiazoles, imidazoles, and benzimizoles afforded the desired alkynylation products in moderate to high yields (Scheme 67).…”

Joy and Challenges of Alkynylation of Arenes and Heteroarenes through Double C−H Functionalizations

“…In the year 2013, Likhar, Mannepalli, and their co-workers described the direct oxidative alkynylation of heteroarenes with terminal alkynes using an anionic/amidocarbocyclic carbenePd (II) complex as pre-catalyst(Scheme 20). [27] They have investigated direct alkynylation of structurally diverse azoles with terminal alkynes found to provide the desired alkynylated product in good yields. In this protocol, Ag 2 O is used to oxidize Pd(0) to Pd(II), which was formed during the reductive Scheme 15.…”

Ligand-free Pd/Ag-mediated dehydrogenative alkynylation of imidazole derivatives

“…Among the synthetic procedures that allow the selective alkynylation of imidazoles and other ve-membered heteroarenes, 5,24 there is no doubt that the possibility of forming the new s carbon-carbon bond through the double activation of two distinct carbon-hydrogen bonds through a crossdehydrogenative alkynylation (CDA) represents the best synthetic approach in terms of atom economy and functional group tolerance (eqn (1), Scheme 1). [25][26][27][28][29][30][31][32][33][34] In fact, when compared with the transition metal-catalyzed direct C sp 2 -H alkynylation protocols involving 1-haloalkynes (the so-called "inverse Sonogashira coupling") (eqn (2), Scheme 1), [35][36][37][38][39][40][41][42][43][44][45] hypervalent iodine reagents (eqn (3), Scheme 1), [46][47][48] or a,b-ynoic acids (decarboxylative direct arylation) (eqn (4), Scheme 1), 26,49,50 the CDA methodology makes it possible to use terminal alkynes without the need for preliminary activation. 5,24 Despite several papers having been dedicated to the dehydrogenative alkynylation of pyrroles, 27 indoles, 27,31 oxazoles, 27,30,[32][33][34] benzoxazoles, 26,29,30,…”

“…[25][26][27][28][29][30][31][32][33][34] In fact, when compared with the transition metal-catalyzed direct C sp 2 -H alkynylation protocols involving 1-haloalkynes (the so-called "inverse Sonogashira coupling") (eqn (2), Scheme 1), [35][36][37][38][39][40][41][42][43][44][45] hypervalent iodine reagents (eqn (3), Scheme 1), [46][47][48] or a,b-ynoic acids (decarboxylative direct arylation) (eqn (4), Scheme 1), 26,49,50 the CDA methodology makes it possible to use terminal alkynes without the need for preliminary activation. 5,24 Despite several papers having been dedicated to the dehydrogenative alkynylation of pyrroles, 27 indoles, 27,31 oxazoles, 27,30,[32][33][34] benzoxazoles, 26,29,30,32,34 thiazoles, 27,30 benzothiazoles, 26,29,31,32,34 pyrazoles,…”

“…In this case Ag 2 O was used as the oxidant, Cs 2 CO 3 base, performing the coupling in DMF at 85 C under air. 26 Encouraged by these results but with the intention of developing a simplied procedure that would allow the use of a commercial Pd(II) pre-catalyst in the absence of any palladium ligands, we decided to review the conditions of reaction and, in this work, we are pleased to summarize the results obtained in the synthesis of 2-alkynylimidazole derivatives by dehydrogenative alkynylation with terminal alkynes (Scheme 2).…”