Transient‐Ligand‐Enabled ortho‐Arylation of Five‐Membered Heterocycles: Facile Access to Mechanochromic Materials

Abstract

Reported herein is the first example of a direct arylation of heteroarenes by a transient-ligand-directed strategy without the need to construct and deconstruct the directing group. A wide range of heteroarenes undergoes the coupling with diverse aryl iodides to assemble a large library of highly selective and functionalized 3-arylthiophene-2-carbaldehydes. This route provides an opportunity to rapidly access new mechanofluorochromic materials. Moreover, a novel strategy for mechanochromic luminogens with chromism trends of red- and blue-shifts has been disclosed for the first time by facile functional-group modifications to a common structural core.

Transient Ligand‐Enabled Transition Metal‐Catalyzed C−H Functionalization

“…This strategy provided an opportunity to rapidly build up new mechanofluochromic materials. Through facile functional group modifications, we realized a novel synthetic strategy for mechanochromic luminogens with chromism trends of red‐ and blueshift based on TPA (triphenylamine)‐containing β‐aryl‐substituted five‐membered heterocyclic carbonyl compounds (Scheme a) . The proposed reaction mechanism is shown in Scheme b. Condensation of 1 with L19 forms the imine intermediate I , which coordinates with palladium species to afford the corresponding six‐membered cyclic palladium complex II .…”

“…[30b] In 2018, our group demonstrated the palladium-catalyzed direct arylation of heterocyclic aromatic ketones by at ransientligand-directed strategy.W ef ound that either 3-amino-3-methylbutanoic acid (L19)o rg lycine (L21)c ould be used as at ransient ligand to promote the direct C(sp 2 )ÀHf unctionalization (Scheme 14 c) . [19] Very recently,L in, Wei, and co-workersa lso demonstrated that ad irect arylation of aromatic ketonesc ould be carried out through Pd-catalyzed inert C(sp 2 )ÀHb ond functionalization with 2-amino-N-isopropyl acetamide (L28)a sanovel transient…”

“…Through facile functional group modifications, we realized an ovel synthetic strategy for mechanochromic luminogens with chromism trends of red-andb lueshift based on TPA( triphenylamine)-containing b-aryl-substituted five-membered heterocyclic carbonyl compounds (Scheme 6a). [19] The proposed reaction mechanism is shown in Scheme 6b.C ondensation of 1 with L19 forms the imine intermediate I,w hich coordinates with palladium speciest oa fford the corresponding six-membered cyclic palladium complex II.T hen cyclometalation of II forms the intermediate III by as ite-selective CÀHb ond activation process. Palladium(IV) complex IV is generatedb yo xidative addition of III with an aryl iodide.…”

Semicarbazide: A Transient Directing Group for C(sp3)−H Arylation of 2‐Methylbenzaldehydes

“…Substoichiometric TDG reagents first reversibly link to substrates forming temporary directing groups, facilitating substrates to undergo cyclometalations and C−H functionalizations, and then readily dissociate from the functionalized products without any extra prior and after operations . This TDG strategy has been applied to C( sp 2 )−H functionalization of benzaldehydes, diphenyls, ferrocene, aromatic ketones and five‐membered heterocycles . Recently, this strategy was also used to perform C( sp 3 )−H functionalizations of aldehydes, ketones and amines in moderate to good yields.…”

“…[2] This TDG strategy has been applied to C(sp 2 )À H functionalization of benzaldehydes, [3] diphenyls, [4] ferrocene, [5] aromatic ketones [6] and fivemembered heterocycles. [7] Recently, this strategy was also used to perform C(sp 3 )À H functionalizations of aldehydes, [8] ketones [9] and amines [10] in moderate to good yields.…”