Ligand controlled orthogonal base-assisted direct C–H bond arylation in oxa(thia)zole-4-carboxylate series. New insights in nCMD mechanism
摘要:
Alkyl- and arylphosphines have been screened in competitive C2-H/C5-H direct phenylation of oxa(thia)zole-4-carboxylates using Cs2CO3 and Rb2CO3 carbonate bases. nCMD-based C2-H selective direct phenylation was highly kinetically reduced (or enhanced) in favor (or to the detriment) of CMD-based direct C5-H phenylation with bromo- and chlorobenzene, respectively, using highly electron-rich ligands. These results gave novel experimental proof in favor of the electrophilic substitution-type mechanism for nCMD process based upon a prior nitrogen-arylpalladium complex interaction that preludes the deprotonation step. (C) 2013 Elsevier Ltd. All rights reserved.
The Pd(0)-catalyzed regioselective C-2 (hetero)arylation of tert-butyl 4-thiazolecarboxylate with a broad (hetero)aryl halide is reported, including the direct coupling of pyridinyl halides. The process has allowed the preparation of valuable 2-pyridynyl-4-thiazolecarboxylates which are components of the complex heterocyclic core of thiopeptides antibiotics. As a first application, a synthesis of a tert-butyl sulfomycinamate thio-analogue from tert-butyl 4-thiazolecarboxylate is here described through a three-step direct pyridinylation, halogenation, and Stille cross-coupling sequence.