Allenes: Versatile Building Blocks in Cobalt-Catalyzed C–H Activation

Abstract

The unique reactivity of allenes have led to their emergence as resourceful coupling partners in transition metal mediated C-H activation reactions. On the other hand, cobalt due to its wide abundance and higher Lewis acidity is garnering widespread interest as useful catalyst for C-H activation. Summarised herein are the cobalt-catalyzed C-H activations engaging allenes as coupling partners followed by our studies on Co(III)-catalyzed C-8 dienylation of quinoline N-oxides with allenes bearing leaving group at α-position for realizing a dienylation protocol.

Challenging Task of Ni-Catalyzed Highly Regio-/Enantioselective Semihydrogenation of Racemic Tetrasubstituted Allenes via a Kinetic Resolution Process

“…The chemistry of allenes with the intriguing structural architecture of the orthogonal cumulative π-system has been an area of considerable interest over the past few decades. − Allenes work as important building blocks with versatile reactivity and have always been regarded as the hybrids of alkenes and alkynes, which have been engaged in various chemical reactions to access complicated skeletons. A number of efficient transformations of mono- and disubstituted allenes are well developed in modern organic synthesis; however, tri- and tetrasubstituted allenes working as substrates have not always been an easy endeavor and are less developed (Scheme a). , Among them, the carbometalation of allenes with organometallic M–Nu intermediates always resulted in the chemo- and regioselective generation of two disparate carbometalated intermediates M-alkenyl INT (sp 2 -M bond formation) or M-π-allyl INT (sp 3 -M bond formation; Scheme b). − These highly reactive intermediates went through a variety of synthetic transformations and always required rare and expensive noble-metal catalysts, including protonation, annulation, β-hydride elimination, and reductive elimination, providing structurally important organic molecules. Comparatively, the transformations of thermodynamically disfavored M-alkenyl INT were challenging and reported scarcely.…”

Rh(iii)-catalyzed sp3/sp2–C–H heteroarylations via cascade C–H activation and cyclization

“… 15 A similar strategy via C(sp 3 )–H activation would be even more attractive and challenging. Building on our recent work on merging nucleophilic cyclization with hydrofunctionalization 16 ( Scheme 1c ) and transition-metal catalyzed C–H activations, 17 we envisioned that the metallacycle generated after C(sp 3 )–H or C(sp 2 )–H bond activation of 8-methylquinoline or quinoline- N -oxide could promote the nucleometallation of 2-alkynylaniline derivatives, providing access to a range of complex heterocyclic embedded quinolines ( Scheme 1d(i) and (ii) ). The key challenges ( Scheme 1e ) associated with the envisioned cascade are manifold.…”

Synthesis of structurally diverse pyrazolo[1,2-a]pyrazolones based on the selective C–H bond alkenylation–annulation of 1-arylpyrazolidinones with allenyl acetates

“…As allenes have two orthogonal CC bonds, they often demonstrate reaction patterns different from those of normal alkenes and alkynes. 7 To be specific, allenes can undergo coordination and migratory insertion reactions with the organometallic species generated via the C–H bond cleavage of substrates with diverse regioselectivity to afford protonation, cyclization or β-elimination products. 7,8 In this context, Cheng et al disclosed a formal [5 + 1] annulation of 2-vinylphenols with allenes to give 2 H -chromenes and a [3 + 3] annulation of anilides with benzylallenes to furnish 1,2-dihydroquinolines (Scheme 1a and b).…”

“…7 To be specific, allenes can undergo coordination and migratory insertion reactions with the organometallic species generated via the C–H bond cleavage of substrates with diverse regioselectivity to afford protonation, cyclization or β-elimination products. 7,8 In this context, Cheng et al disclosed a formal [5 + 1] annulation of 2-vinylphenols with allenes to give 2 H -chromenes and a [3 + 3] annulation of anilides with benzylallenes to furnish 1,2-dihydroquinolines (Scheme 1a and b). 8 a , b Volla et al described a cobalt-catalyzed [4 + 2] heteroannulation reaction of arylsulfonamides with allenes to give benzosultams (Scheme 1c).…”