This report describes a detailed investigation of acetate-assisted C–H activation at PdIV centers supported by the tris(2-pyridyl)methane (Py3CH) ligand. Mechanistic information about this ...transformation has been obtained through the following: (i) extensive one- and two-dimensional NMR analysis, (ii) reactivity studies of a series of substituted analogues, and (iii) isotope effect studies. These experiments all suggest that C–H activation at (Py3CH)PdIV(biphenyl)Cl2+ occurs via a multistep process involving chloride-to-acetate ligand exchange followed by conformational and configurational isomerization and then C–H cleavage. The data also suggest that C–H cleavage proceeds via an acetate-assisted mechanism with the carboxylate likely serving as an intramolecular base. The viability of acetate-assisted C–H activation at high valent palladium has important implications for the design and optimization of catalytic processes involving this transformation as a key step.
This paper demonstrates a catalytic cycle for Pd-catalyzed decarbonylative trifluoromethylation using trifluoroacetic esters as CF3 sources. The proposed cycle consists of four elementary steps: (1) ...oxidative addition of a trifluoroacetic ester to Pd0, (2) CO deinsertion from the resulting trifluoroacyl PdII complex, (3) transmetalation of a zinc aryl to PdII, and (4) aryl–CF3 bond-forming reductive elimination. The use of RuPhos as the supporting ligand enables each of these steps to proceed under mild conditions (<100 °C). These studies set the stage for the development of catalytic arene trifluoromethylation and perfluoroalkylation reactions using inexpensive trifluoroacetic acid derived CF3 sources.
Site-specific attachment of paramagnetic lanthanide ions to a protein generates pseudocontact shifts (PCS) in the nuclear magnetic resonance (NMR) spectra of the protein that are easily measured as ...changes in chemical shifts. By labeling the protein with lanthanide tags at four different sites, PCSs are observed for most amide protons and accurate information is obtained about their coordinates in three-dimensional space. The approach is demonstrated with the chaperone ERp29, for which large differences have been reported between X-ray and NMR structures of the C-terminal domain, ERp29-C. The results unambiguously show that the structure of rat ERp29-C in solution is similar to the crystal structure of human ERp29-C. PCSs of backbone amides were the only structural restraints required. Because these can be measured for more dilute protein solutions than other NMR restraints, the approach greatly widens the range of proteins amenable to structural studies in solution.
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•Four lanthanide tags allow determining atom positions in 3D space•NMR method for 3D fold determinations of proteins in solution•More sensitive than conventional NMR methods•Sufficient to measure restraints for backbone amides
Yagi et al. describe a method for protein structure determination in solution by NMR spectroscopy. The method uses only pseudocontact shifts of backbone amide protons generated by lanthanide ions attached at different sites. The method produced an improved 3D structure of the C-terminal domain of rat ERp29.
Cationic Pd(II) catalysts generated from chiral biphenyl diphosphine complexes or from COP-Cl promote enantioselective cyclization of E- and Z-configured allylic bis-trichloroacetimidates to highly ...enantioenriched 2-trichloromethyl-4-vinyloxazoline. This represents an exclusive example for olefin amination in high yield and enantioselectivity with trichloroacetimidate as the N-nucleophile by using a cationic palladium(II) complex as a catalyst providing an easy-to-deprotect enantioenriched vinylglycinol derivative.
Structural investigations of amyloid fibrils often rely on heterologous bacterial overexpression of the protein of interest. Due to their inherent hydrophobicity and tendency to aggregate as ...inclusion bodies, many amyloid proteins are challenging to express in bacterial systems. Cell-free protein expression is a promising alternative to classical bacterial expression to produce hydrophobic proteins and introduce NMR-active isotopes that can improve and speed up the NMR analysis. Here we implement the cell-free synthesis of the functional amyloid prion HET-s(218-289). We present an interesting case where HET-s(218-289) directly assembles into infectious fibril in the cell-free expression mixture without the requirement of denaturation procedures and purification. By introducing tailored
C and
N isotopes or CF
and
CH
F labels at strategic amino-acid positions, we demonstrate that cell-free synthesized amyloid fibrils are readily amenable to high-resolution magic-angle spinning NMR at sub-milligram quantity.
Synthesis of indoles labeled with 13C-1H and 13C-19F spin pairs is described. All syntheses utilize inexpensive carbon-13C dioxide as the 13C isotope source. Ruthenium-mediated ring-closing ...metathesis is the key step in construction of the 13C containing indole carbocycle. Fluorine is introduced via electrophilic fluorination at the 7-position and via palladium-mediated cross-coupling at the 4-position. Indole and fluoroindoles are viable tryptophan precursors for in vivo protein expression. We show that they are viable also in in vitro protein synthesis using standard E. coli S30 extracts. Incorporation of the synthesized 13C-1H and 13C-19F spin pair labeled tryptophans into proteins enables high-resolution and high-sensitivity nuclear magnetic resonance (NMR) spectroscopy.
This paper describes the synthesis of a series of PdIV complexes containing modular monoanionic tridentate facially coordinated NNN and NNC donor ligands. In all cases, these complexes are stable to ...reductive elimination for a minimum of several days in solution at room temperature. With appropriately designed tridentate ligands, the PdIV adducts participate in both ligand substitution and C–H activation reactions. Overall, this work shows that unsymmetrical fac-L2X type ligands can serve as versatile and tunable scaffolds for modulating the reactivity of octahedral PdIV complexes.
Global substitution of leucine for analogues containing CH
F instead of methyl groups delivers proteins with multiple sites for monitoring by
F nuclear magnetic resonance (NMR) spectroscopy. The 19 ...kDa
peptidyl-prolyl
isomerase B (PpiB) was prepared with uniform high-level substitution of leucine by (2
,4
)-5-fluoroleucine, (2
,4
)-5-fluoroleucine, or 5,5'-difluoroleucine. The stability of the samples toward thermal denaturation was little altered compared to the wild-type protein.
F nuclear magnetic resonance (NMR) spectra showed large chemical shift dispersions between 6 and 17 ppm. The
F chemical shifts correlate with the three-bond
H-
F couplings (
), providing the first experimental verification of the γ-gauche effect predicted by Feeney, J.
1996, 118, 8700-8706 and establishing the effect as the predominant determinant of the
F chemical shifts of CH
F groups. Individual CH
F groups can be confined to single rotameric states by the protein environment, but most CH
F groups exchange between different rotamers at a rate that is fast on the NMR chemical shift scale. Interactions between fluorine atoms in 5,5'-difluoroleucine bias the CH
F rotamers in agreement with results obtained previously for 1,3-difluoropropane. The sensitivity of the
F chemical shift to the rotameric state of the CH
F groups potentially renders them particularly sensitive for detecting allosteric effects.
Global substitution of leucine for analogues containing CH2F instead of methyl groups delivers proteins with multiple sites for monitoring by 19F nuclear magnetic resonance (NMR) spectroscopy. The 19 ...kDa Escherichia coli peptidyl–prolyl cis–trans isomerase B (PpiB) was prepared with uniform high-level substitution of leucine by (2S,4S)-5-fluoroleucine, (2S,4R)-5-fluoroleucine, or 5,5′-difluoroleucine. The stability of the samples toward thermal denaturation was little altered compared to the wild-type protein. 19F nuclear magnetic resonance (NMR) spectra showed large chemical shift dispersions between 6 and 17 ppm. The 19F chemical shifts correlate with the three-bond 1H–19F couplings (3 J HF), providing the first experimental verification of the γ-gauche effect predicted by Feeney, J. J. Am. Chem. Soc. 1996, 118, 8700–8706 and establishing the effect as the predominant determinant of the 19F chemical shifts of CH2F groups. Individual CH2F groups can be confined to single rotameric states by the protein environment, but most CH2F groups exchange between different rotamers at a rate that is fast on the NMR chemical shift scale. Interactions between fluorine atoms in 5,5′-difluoroleucine bias the CH2F rotamers in agreement with results obtained previously for 1,3-difluoropropane. The sensitivity of the 19F chemical shift to the rotameric state of the CH2F groups potentially renders them particularly sensitive for detecting allosteric effects.
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