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ABSTRACT: It is reported a weakly coordination-assisted alkynylation of aryl and heteroaryl carboxylic acids with iridium catalysis. The reaction is catalyzed by [{Cp*IrCl2}2] complex without cyclization, forming ortho?alkynylated aryl and heteroaryl carboxylic acids, and features high functional group tolerance and broad substrate scope under an air atmosphere. 2- (Hetero)aryl-substituted acetic acids were amenable to the alkynylation by forming an unusual six-membered ring cycloiridiated intermediate. Aromatic alkynes are versatile building blocks in synthetic chemistry and have been widely used in the preparation of common chemicals and pharmaceutical and material molecules.1 The development of efficient methods to the buildup of these fundamentally important motifs has attracted broad synthetic interest. Common approaches that were used to form aromatic alkynes traditionally involve the Sonogashira cross-coupling using reactive aromatic halides.2 Recently, introducing directing groups as auxiliaries for transition-metal catalysis has led to many breakthroughs in synthetic chemistry,3 remarkably revolutioniz?ing the manner in the creation of appealing molecules by the functionalization of unreactive chemical bonds.4 Carboxyls are synthetically valuable functional groups that are readily amenable to transform to the relative C?C, C?O, and C?N bonds by the corresponding decarboxylative cross-coupling reactions.5 How?ever, the weakly σ-coordination ability of carboxyl groups causes them difficulty in the direction of transition metals to approach and break one regiospecific chemical bond. On the other hand, the reactivity of the resulting cyclometalated intermediates toward further transformation may be lowered. Methods for the arylation,6 alkylation,7 alkenylation,8,9 amination,10 acylation,11 annulation,8,12 and deuteration13 have been developed using carboxyl directing groups (Scheme 1, eq 1). However, to our knowledge, there is no report of an alkynylation reaction that was directed by a weak coordination carboxyl functionality, although approaches mediated by a strong coordination directing group have been described.14?21 The obstacle may arise from the easy addition of the carboxyl across the alkyne via a cyclization, which causes the ortho-alkynylation of aromatic carboxylic acid suffering from a selectivity issue.8 Herein, we report the first weak coordination-assisted ortho?alkynylation of aryl and heteroaryl carboxylic acids with iridium catalysis.22Carboxyl groups in both aromatic and heteroaromatic carboxylic acids were able to direct iridium in the cleavage of aromatic C?H bonds and the formation of ortho-alkynylated aryl and heteroaryl carboxylic acids under an air atmosphere (Scheme 1, eq 2). The competitive cyclization by the addition of the carboxyl across the alkyne did not occur under catalytic conditions. While unusual six-membered ring cycloiridiated intermediates can be accessed in the realization of the ortho－alkynylation 2-(hetero)aryl-substituted acetic acids. In conclusion, we have developed a weakly coordination?assisted alkynylation of aromatic and heteroaromatic carboxylic acids with iridium catalysis. The reaction was directed by a synthetically useful carboxyl functionality in featuring high functional group tolerance and broad substrate scope. The relative cyclization by the addition of a carboxyl group across the alkyne did not occur under our catalytic conditions. This reaction provides an efficient methodology for the preparation of ortho?alkynylated aryl and heteroaryl carboxylic acids under an air atmosphere. 发自小木虫Android客户端 |
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2楼2019-11-22 19:24:21
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