Showing papers by "Ramesh Giri published in 2014"

Site-Selective C(sp3)–H Functionalization of Di-, Tri-, and Tetrapeptides at the N-Terminus

Abstract: Although the syntheses of novel and diverse peptides rely mainly on traditional coupling using unnatural amino acids, postsynthetic modification of peptides could provide a complementary method for the preparation of nonproteinogenic peptides. Site selectivity of postsynthetic modification of peptides is usually achieved by targeting reactive moieties, such as the thiol group of cysteine or the C-2 position of tryptophan. Herein, we report the development of site-selective functionalizations of inert C(sp3)–H bonds of N-terminal amino acids in di-, tri-, and tetrapeptides without installing a directing group. The native amino acid moiety within the peptide is used as a ligand to accelerate the C–H activation reaction. In the long run, this newly uncovered reactivity could provide guidance for developing site-selective C(sp3)–H activation toward postsynthetic modification of a broader range of peptides.

Copper‐Catalyzed Coupling of Triaryl‐ and Trialkylindium Reagents with Aryl Iodides and Bromides through Consecutive Transmetalations

Abstract: An efficient copper(I)-catalyzed coupling of triaryl and trialkylindium reagents with aryl iodides and bromides is reported. The reaction proceeds at low catalyst loadings (2 mol %) and generally only requires 0.33 equivalents of the triorganoindium reagent with respect to the aryl halide as all three organic nucleophilic moieties of the reagent are transferred to the products through consecutive transmetalations. The reaction tolerates a variety of functional groups and sterically hindered substrates. Furthermore, preliminary mechanistic studies that entailed the synthesis and characterization of potential reaction intermediates offered a glimpse of the elementary steps that constitute the catalytic cycle.

Copper-Catalyzed Cross-Coupling of Aryl- and Heteroaryltriethoxysilanes with Aryl and Heteroaryl Iodides and Bromides

Abstract: Copper(I)-catalyzed coupling of aryl- and heteroaryltriethoxysilanes with aryl and heteroaryl iodides is described. The transformation also proceeds with activated aryl bromides, but in this case it requires a stoichiometric amount of the catalyst for best product yields. The current reaction requires a P,N-based bidentate ligand for aryl–aryl coupling while it proceeds without external ligands for aryl–heteroaryl coupling to afford good product yields. The reaction protocol can also be applied to achieve biarylation of diiodoarenes in reasonable yields.

Copper-Catalyzed Hiyama Coupling of (Hetero)aryltriethoxysilanes with (Hetero)aryl Iodides.

Abstract: A CuI-catalyzed Hiyama coupling was achieved, which proceeds in the absence of an ancillary ligand for aryl–heteroaryl and heteroaryl–heteroaryl couplings. A P,N-ligand is required to obtain the best product yields for aryl–aryl couplings. In addition to facilitating transmetalation, CsF is also found to function as a stabilizer of the [CuAr] species, potentially generated as an intermediate after transmetalation of aryltriethoxysilanes with CuI-catalysts in the absence of ancillary ligands.

Copper-Catalyzed Suzuki—Miyaura Coupling of Arylboronate Esters: Transmetalation with (PN)CuF and Identification of Intermediates.

Abstract: An efficient CuI-catalyzed Suzuki–Miyaura reaction was developed for the coupling of aryl- and heteroarylboronate esters with aryl and heteroaryl iodides at low catalyst loadings (2 mol %). The reaction proceeds under ligand-free conditions for aryl–heteroaryl and heteroaryl–heteroaryl couplings. We also conducted the first detailed mechanistic studies by synthesizing [(PN-2)CuI]2, [(PN-2)CuF]2, and (PN-2)CuPh (PN-2 = o-(di-tert-butylphosphino)-N,N-dimethylaniline) and demonstrated that [(PN-2)CuF]2 is the species that undergoes transmetalation with arylboronate esters.