Isocyanides have a broad range of applications in multicomponent reactions such as Passerini and Ugi processes. Recent advances in metal catalysis have tremendously increased the versatility of isocyanides in organic chemistry. Suitable metal catalysts could selectively activate various C-H bonds to allow direct functionalization under mild conditions, which represents a chemical process with broad synthetic potential. The synergy from the combination of isocyanide insertion and C-H bond activation offers an efficient and powerful tool to establish complicated reactions and to construct useful substances, from which the high potential of such strategy has been convincingly demonstrated in drug discovery, organic synthesis, and materials science. The present review highlights the most recent advances of isocyanide chemistry in metal-catalyzed C-H bond functionalization.
A bimetallic palladium/copper catalyst system allows the highly modular synthesis of azomethines via the decarboxylative coupling of aryl halides and a-iminocarboxylates, generated in situ from potassium a-oxocarboxylates and primary amines. The reaction proceeds already at 100 8C, a new record for redox-neutral decarboxylative crosscoupling reactions.
Decarboxylative cross-coupling reactions have recently emerged as a powerful methodology for the regioselective construction of CÀC and CÀheteroatom bonds. [1] Their key advantage over traditional cross-coupling reactions is that they draw on stable and readily available carboxylate salts as sources of carbon nucleophiles rather than expensive and sensitive organometallic reagents. In the last decade, a rapidly growing number of decarboxylative reactions have been discovered including decarboxylative Heck reactions, [2] allylations, [3] redox-neutral cross-coupling reactions, [4] oxidative coupling reactions, [5] CÀH arylations, [6] homocouplings, [7] and Chan-Lam-type reactions. [8] Redox-neutral decarboxylative cross-couplings mediated by Cu/Pd or Ag/Pd bimetallic catalyst systems proved to have a particularly broad scope with regards to both carboxylates and carbon electrophiles. In this variant, the decarboxylation step is mediated by a Cu I [9] or Ag I [10] catalyst, while a Pd complex catalyzes the coupling with the carbon electrophile. Whereas aryl bromides and iodides can be converted with very simple ligand systems, [11] the activation of aryl chlorides, [12] triflates, [13] and tosylates [14] requires the use of sophisticated catalyst systems containing electron-rich, bulky phosphine ligands. However, all attempts to develop decarboxylative couplings of the notoriously hard-to-activate methanesulfonates (mesylates) have failed so far (Scheme 1).Aryl and alkenyl mesylates are particularly attractive carbon electrophiles for preparative-and industrial-scale syntheses, since they have the lowest molecular weight of all sulfonate leaving groups. They are easily accessible by Angewandte Chemie 2955
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