Reactivity Studies of Catalytically Relevant Palladium Model Complexes.

Recently high oxidation state palladium chemistry has emerged as efficient tool for direct C−H bond functionalization that does not require prefunctionalized starting materials. Investigation of transition metal model complexes is an efficient approach for optimization of known reactions and development of new methodologies. This dissertation describes reactivity studies of Pd(IV) model systems with an aim to gain better understanding of Pd(IV) mediated C−H bond functionalization reactions. In addition, investigation of Pd(II) model complexes to assess the feasibility of hypothetical Pd(0)/(II) catalyzed trifluoromethylation reaction with trifluoroacetic anhydride as a CF3 source is also described. In order to obtain detailed information about processes occurring at Pd(IV) during catalysis, relatively stable yet reactive model systems had to be identified. First, we designed and synthesized a number of Pd(IV) complexes supported by facial tridentate NNN and NCN ligands and investigated their reactivity. Next, we identified appropriate model system for detailed investigation of C−H bond cleavage at Pd(IV) centers. Mechanistic information about this transformation was 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 suggest that C−H activation at [(Py3CH)Pd(IV)(biphenyl)Cl2]+ model system 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. In a unrelated project we propose a catalytic cycle for nickel(0/II) or palladium(0/II) catalyzed decarbonylative trifluoromethylation using trifluoroacetic esters as CF3 sources. The catalytic cycle consists of four elementary steps: (1) oxidative addition of a trifluoroacetic ester to M(0) center, (2) CO deinsertion from the resulting trifluoroacyl M(II) complex, (3) transmetallation of a zinc-aryl to M(II), and (4) aryl–CF3 bond-forming reductive elimination. We demonstrated that the use of RuPhos as the supporting ligand for palladium enables each of these steps to proceed under mild conditions. These studies set the stage for the development of catalytic arene trifluoromethylation and perfluoroalkylation reactions using inexpensive trifluoroacetic acid-derived CF3 sources.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108903/1/maleckis_1.pd

Synthesis of 13C/19F/2H labeled indoles for use as tryptophan precursors for protein NMR spectroscopy

Synthesis of indoles labeled with 13C–1H and 13C–19F spin pairs is described. All syntheses utilize in- expensive 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 electrophi- lic 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-sensi- tivity nuclear magnetic resonance (NMR) spectroscopy.Financial support by the Australian Research Council for a Laureate Fellowship to G. O. (FL170100019), project funding (DP210100088) and through the Centre of Excellence for Innovations in Peptide & Protein Science (CE200100012) is gratefully acknowledged. A. M. is funded by the European Regional Development Fund (ERDF) PostDoc project nr. 1.1.1.2/VIAA/2/18/381

A method of assessing peripheral stent abrasiveness under cyclic deformations experienced during limb movement

Poor outcomes of peripheral arterial disease stenting are often attributed to the inability of stents to accommodate the complex biomechanics of the flexed lower limb. Abrasion damage caused by rubbing of the stent against the artery wall during limb movement plays a significant role in reconstruction failure but has not been characterized. Our goals were to develop a method of assessing the abrasiveness of peripheral nitinol stents and apply it to several commercial devices. Misago, AbsolutePro, Innova, Zilver, SmartControl, SmartFlex, and Supera stents were deployed inside electrospun nanofibrillar tubes with femoropopliteal artery-mimicking mechanical properties and subjected to cyclic axial compression (25%), bending (90°), and torsion (26°/cm) equivalent to five life-years of severe limb flexions. Abrasion was assessed using an abrasion damage score (ADS, range 1–7) for each deformation mode. Misago produced the least abrasion and no stent fractures (ADS 3). Innova caused small abrasion under compression and torsion but large damage under bending (ADS 7). Supera performed well under bending and compression but caused damage under torsion (ADS 8). AbsolutePro produced significant abrasion under bending and compression but less damage under torsion (ADS 12). Zilver fractured under all three deformations and severely abraded the tube under bending and compression (ADS 15). SmartControl and SmartFlex fractured under all three deformations and produced significant abrasion due to strut penetration (ADS 20 and 21). ADS strongly correlated with clinical 12- month primary patency and target lesion revascularization rates, and the described method of assessing peripheral stent abrasiveness can guide device selection and development

Trifluoromethylation of a well-defined square-planar Aryl-NiII complex involving NiIII/CF3 and NiIV−CF3 intermediate species

Ni-mediated trifluoromethylation of an aryl−Br bond in model macrocyclic ligands (Ln−Br) has been thoroughly studied, starting with an oxidative addition at Ni0 to obtain well-defined aryl-NiII-Br complexes ([Ln−NiII]Br). Abstraction of the halide with AgX (X=OTf− or ClO4−) thereafter provides [Ln−NiII](OTf). The nitrate analogue has been obtained through a direct C−H activation of an aryl−H bond using NiII salts, and this route has been studied by X-ray absorption spectroscopy (XAS). Crystallographic XRD and XAS characterization has shown a tight macrocyclic coordination in the aryl−NiII complex, which may hamper direct reaction with nucleophiles. On the contrary, enhanced reactivity is observed with oxidants, and the reaction of [Ln−NiII](OTf) with CF3+ sources afforded Ln−CF3 products in quantitative yield. A combined experimental and theoretical mechanistic study provides new insights into the operative mechanism for this transformation. Computational analysis indicates the occurrence of an initial single electron transfer (SET) to 5-(trifluoromethyl)dibenzothiophenium triflate (TDTT), producing a transient L1−NiIII/CF3. adduct, which rapidly recombines to form a [L1-NiIV-CF3](X)2 intermediate species. A final facile reductive elimination affords L1−CF3. The well-defined square-planar model system studied here permits to gain fundamental knowledge on the rich redox chemistry of nickel, which is sought to facilitate the development of new Ni-based trifluoromethylation methodologies

1.3 Å Crystal Structure of E. coli Peptidyl–Prolyl Isomerase B with Uniform Substitution of Valine by (2 S ,3 S )-4-Fluorovaline Reveals Structure Conservation and Multiple Staggered Rotamers of CH 2_2 F Groups

(2S,3S)-4-Fluorovaline (FVal) is an analogue of valine, where a single CH3 group is substituted by a CH2F group. In the absence of valine, E. coli valyl-tRNA synthetase uses FVal as a substitute, enabling the production of proteins uniformly labeled with FVal. Here, we describe the production and analysis of E. coli peptidyl–prolyl isomerase B where all 16 valine residues have been replaced by FVal synthesized with a 13C-labeled CH2F group. Although the melting temperature is lower by about 11 °C relative to the wild-type protein, the three-dimensional protein structure is almost completely conserved, as shown by X-ray crystallography. The CH2F groups invariably populate staggered rotamers. Most CH2F groups populate two different rotamers. The increased space requirement of fluorine versus hydrogen does not prohibit rotamers that position fluorine next to a backbone carbonyl carbon. 19F NMR spectra show a signal dispersion over 25 ppm. The most high-field shifted 19F resonances correlate with large 3JHF coupling constants, confirming the impact of the γ-gauche effect on the signal dispersion. The present work is the second experimental verification of the effect and extends its validity to fluorovaline. The abundance of valine in proteins and structural conservation with FVal renders this valine analogue attractive for probing proteins by 19F NMR spectroscopy

Non-Destructive Characterization of Peripheral Arteries using Intravascular Ultrasound

Peripheral Artery Disease (PAD) is the chronic obstruction of blood flow to the extremities caused by plaque buildup. Poor circulation results in exertional pain, numbness, and weakness, and in severe cases, can manifest critical conditions, including gangrene and limb loss. PAD affects approximately 8.5 million Americans and costs the United States $21 billion annually in direct medical expenses. High expenditures are attributed to operation and intervention failures resulting in frequent need for revascularization. Treatment of PAD typically involves lifestyle/diet adjustments, bypass surgery, or angioplasty/stenting. Unfortunately, repeated limb deformation during locomotion often results in adverse repair device-artery interactions, which hinder the long-term efficacy of endovascular therapies. Patient and lesion-specific device selection guided by computational modeling can help improve clinical outcomes, but these models rely heavily on accurately recorded three-dimensional arterial geometry and plaque composition. Intravascular ultrasound (IVUS) is a minimally invasive method of endovascular imaging that allows evaluation of the geometry and composition of the arterial wall, but its two-dimensional nature is often insufficient to capture complex three-dimensional plaques. We have developed a method of obtaining three-dimensional arterial geometry from two-dimensional IVUS images to build Computer-Aided Design models of calcified human femoropopliteal arteries. Our imaging method will allow for the characterization of calcium, necrotic core, fibrofatty, and fibrous tissue using IVUS. Correlation of IVUS images with conventional histology, micro-CT imaging, and clinical CTA data will help inform computational models.https://digitalcommons.unmc.edu/surp2021/1025/thumbnail.jp

Single‐ended transition state finding with the growing string method

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/110705/1/jcc23833.pd

Site-selective generation of lanthanoid binding sites on proteins using 4-fluoro-2,6-dicyanopyridine

The paramagnetism of a lanthanoid tag site-specifically installed on a protein provides a rich source of structural information accessible by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy. Here we report a lanthanoid tag for selective reaction with cysteine or selenocysteine with formation of a (seleno)thioether bond and a short tether between the lanthanoid ion and the protein backbone. The tag is assembled on the protein in three steps, comprising (i) reaction with 4-fluoro-2,6-dicyanopyridine (FDCP); (ii) reaction of the cyano groups with α-cysteine, penicillamine or β-cysteine to complete the lanthanoid chelating moiety; and (iii) titration with a lanthanoid ion. FDCP reacts much faster with selenocysteine than cysteine, opening a route for selective tagging in the presence of solvent-exposed cysteine residues. Loaded with Tb3+ and Tm3+ ions, pseudocontact shifts were observed in protein NMR spectra, confirming that the tag delivers good immobilisation of the lanthanoid ion relative to the protein, which was also manifested in residual dipolar couplings. Completion of the tag with different 1,2-aminothiol compounds resulted in different magnetic susceptibility tensors. In addition, the tag proved suitable for measuring distance distributions in double electron–electron resonance experiments after titration with Gd3+ ions.This research has been supported by the Australian Research Council (grant no. FL170100019 and DP210100088), the Australian Research Council Centre of Excellence for Innovations in Peptide and Protein Science (grant no. CE200100012) and the European Regional Development Fund (ERDF; PostDoc grant no. 1.1.1.2/VIAA/2/18/381)