scholarly journals Mechanistic Studies of Pd-Catalyzed Fluorination of Cyclic Vinyl Triflates: Evidence for in situ Ligand Modification

2021 ◽  
Author(s):  
Yuxuan Ye ◽  
Seoung-Tae Kim ◽  
Ryan P. King ◽  
Mu-Hyun Baik ◽  
Stephen L. Buchwald
Keyword(s):  
Trans Isomer ◽  
Computational Study ◽  
Reductive Elimination ◽  
Nucleophilic Attack ◽  
Ligand Modification ◽  
Low Efficiency ◽  
Vinyl Triflates ◽  
Nucleophilic Fluorination

<div><div><div><p>Pd-catalyzed nucleophilic fluorination reactions are important methods for the synthesis of fluoroarenes and fluoroalkenes. However, these reactions can generate a mixture of regioisomeric products that are often difficult to separate. While investigating the Pd- catalyzed fluorination of cyclic vinyl triflates, we observed that the addition of a substoichiometric quantity of TESCF3 significantly improves both the efficiency and the regioselectivity of the fluorination process. Herein, we report a combined experimental and computational study on the mechanism of this transformation focused on the role of TESCF3. We found that in the absence of additives such as TESCF3, the transmetalation step produces predominantly the thermodynamically more stable trans isomer of the key LPd(vinyl)F complex (L = biaryl monophosphine ligand). This intermediate, rather than undergoing reductive elimination, preferentially reacts through an intramolecular β-deprotonation to form a Pd-cyclohexyne intermediate. This undesired reactivity is responsible for the low efficiency (11% yield) and poor regioselectivity (1.8:1) of the catalytic reaction. When TESCF3 is added to the reaction mixture, the cis-LPd(vinyl)F complex is instead formed, through a pathway involving an unusual dearomatization of the ligand by nucleophilic attack from a trifluoromethyl anion (CF3–). In contrast to the trans isomer, this cis-LPd(vinyl)F complex readily undergoes reductive elimination to provide the vinyl fluoride product with desired regioselectivity, without the generation of Pd-cyclohexyne intermediates.</p></div></div></div>

scholarly journals Mechanistic Studies of Pd-Catalyzed Fluorination of Cyclic Vinyl Triflates: Evidence for in situ Ligand Modification

2021 ◽  
Author(s):  
Yuxuan Ye ◽  
Seoung-Tae Kim ◽  
Ryan P. King ◽  
Mu-Hyun Baik ◽  
Stephen L. Buchwald
Keyword(s):  
Trans Isomer ◽  
Computational Study ◽  
Reductive Elimination ◽  
Nucleophilic Attack ◽  
Ligand Modification ◽  
Low Efficiency ◽  
Vinyl Triflates ◽  
Nucleophilic Fluorination

<div><div><div><p>Pd-catalyzed nucleophilic fluorination reactions are important methods for the synthesis of fluoroarenes and fluoroalkenes. However, these reactions can generate a mixture of regioisomeric products that are often difficult to separate. While investigating the Pd- catalyzed fluorination of cyclic vinyl triflates, we observed that the addition of a substoichiometric quantity of TESCF3 significantly improves both the efficiency and the regioselectivity of the fluorination process. Herein, we report a combined experimental and computational study on the mechanism of this transformation focused on the role of TESCF3. We found that in the absence of additives such as TESCF3, the transmetalation step produces predominantly the thermodynamically more stable trans isomer of the key LPd(vinyl)F complex (L = biaryl monophosphine ligand). This intermediate, rather than undergoing reductive elimination, preferentially reacts through an intramolecular β-deprotonation to form a Pd-cyclohexyne intermediate. This undesired reactivity is responsible for the low efficiency (11% yield) and poor regioselectivity (1.8:1) of the catalytic reaction. When TESCF3 is added to the reaction mixture, the cis-LPd(vinyl)F complex is instead formed, through a pathway involving an unusual dearomatization of the ligand by nucleophilic attack from a trifluoromethyl anion (CF3–). In contrast to the trans isomer, this cis-LPd(vinyl)F complex readily undergoes reductive elimination to provide the vinyl fluoride product with desired regioselectivity, without the generation of Pd-cyclohexyne intermediates.</p></div></div></div>

On the Mechanism of the Nickel-Catalyzed Boron Insertion into the C−O Bond of Benzofuran

2019 ◽  
Author(s):  
Ana Mateo-Martínez ◽  
Hayate Saito ◽  
Hideki Yorimitsu ◽  
Carles Bo
Keyword(s):  
Reductive Elimination ◽  
Nucleophilic Attack ◽  
Dft Studies ◽  
Reaction Conditions ◽  
Elementary Steps

<p>The reaction of benzofurans with diboron reagents and Cs<sub>2</sub>CO<sub>3</sub>catalyzed by Ni(0) complexes results in the insertion of boron into the C-O bond. The reaction conditions mimic those reported by Martin and Hosoya for borylating aryl C-F bonds, but neither the role of the base nor the sequence of elementary steps is clear. Herein we report on the mechanism of such transformation on the basis of DFT studies, which suggest that the base activates the diboron reagent and generates a reactive boryl group, that Ni(II) is reduced back to Ni(0) during the boryl insertion into the Ni-O bond, and that the classical reductive elimination step is best viewed as a ring-contracting nucleophilic attack. </p>

On the Thermodynamic Control of Ring Opening of 4-Substituted 1,3,3-Tris-Carbethoxycyclobutene and the Role of the C-3 Substituent in Masking the Kinetic Torquoselectivity

2019 ◽  
Author(s):  
Veejendra Yadav ◽  
Dasari L V K Prasad ◽  
Arpita Yadav ◽  
Maddali L N Rao
Keyword(s):  
Ring Opening ◽  
Nucleophilic Attack ◽  
Thermodynamic Control ◽  
Heterolytic Cleavage ◽  
Methyl Phenyl ◽  
Halogen Acid

<p>The predominant transformations of 4-methyl- and 4-phenyl-1,3,3-<i>tris</i>-carbethoxycyclobutenes to s-<i>trans</i>-<i>trans</i>-1,1,3-<i>tris</i>-carbethoxy-4-methyl- and 4-phenyl-1,3-butadienes, respectively, proceed through a pathway entailing heterolytic cleavage of s<sub>C3C4</sub> bond rather than the usual four-electron conrotatory ring opening. The adventitious or in situ generated halogen acid catalyzes the reaction by either protonation of one of the two ester groups on C3 and, thus, weakening s<sub>C3C4</sub> bond to allow its heterolytic cleavage and formation of a stable cation or protonation followed by halide ion attack in S<sub>N</sub>2 manner on the methyl/phenyl-bearing carbon. Reorganization of the cation species formed in the former event and elimination of the elements of halogen acid from the halo-species formed in the latter event generate the observed product. The nucleophilic attack of DMSO to bring about heterolytic S<sub>N</sub>2 cleavage of s<sub>C3C4</sub> bond is also discussed.</p>

On the Thermodynamic Control of Ring Opening of 4-Substituted 1,3,3-Tris-Carbethoxycyclobutene and the Role of the C-3 Substituent in Masking the Kinetic Torquoselectivity

2020 ◽  
Author(s):  
Veejendra Yadav ◽  
Dasari L V K Prasad ◽  
Arpita Yadav ◽  
Maddali L N Rao
Keyword(s):  
Ring Opening ◽  
Nucleophilic Attack ◽  
Thermodynamic Control ◽  
Heterolytic Cleavage ◽  
Methyl Phenyl ◽  
Halogen Acid

<p>The predominant transformations of 4-methyl- and 4-phenyl-1,3,3-<i>tris</i>-carbethoxycyclobutenes to s-<i>trans</i>-<i>trans</i>-1,1,3-<i>tris</i>-carbethoxy-4-methyl- and 4-phenyl-1,3-butadienes, respectively, proceed through a pathway entailing heterolytic cleavage of s<sub>C3C4</sub> bond rather than the usual four-electron conrotatory ring opening. The adventitious or in situ generated halogen acid catalyzes the reaction by either protonation of one of the two ester groups on C3 and, thus, weakening s<sub>C3C4</sub> bond to allow its heterolytic cleavage and formation of a stable cation or protonation followed by halide ion attack in S<sub>N</sub>2 manner on the methyl/phenyl-bearing carbon. Reorganization of the cation species formed in the former event and elimination of the elements of halogen acid from the halo-species formed in the latter event generate the observed product. The nucleophilic attack of DMSO to bring about heterolytic S<sub>N</sub>2 cleavage of s<sub>C3C4</sub> bond is also discussed.</p>

scholarly journals Clicking Azides and Alkynes with Poly(pyrazolyl)borate-Copper(I) Catalysts: An Experimental and Computational Study

Catalysts ◽  
2019 ◽  
Vol 9 (8) ◽  
pp. 687 ◽  
Author(s):  
Lahoucine Bahsis ◽  
Hicham Ben El Ayouchia ◽  
Hafid Anane ◽  
Carmen Ramirez de Arellano ◽  
Abdeslem Bentama ◽  
...  
Keyword(s):  
Computational Study ◽  
Point Of View ◽  
Environmentally Benign ◽  
Form Complex ◽  
Benign Solvents ◽  
Catalytically Active ◽  
High Yields ◽  
Global And Local

The synthesis of 1,4-disubstituted-1,2,3-triazoles under a copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) regime was accomplished in high yields and a regioselective manner by using two homoscorpionate poly(pyrazolyl)borate anions: tris(pyrazolyl)hydroborate (HB(pz)3−) and bis(pyrazolyl)hydroborate (H2B(pz)2−), which stabilized in situ the catalytically active copper (I) center. The [3+2] cycloaddition (32CA) reactions took place under strict click conditions, including room temperature and a mixture of environmentally benign solvents such as water/ethanol in a 1:1 (v/v) ratio. These click chemistry conditions were applied to form complex 1,2,3-triazoles-containing sugar moieties, which are potentially relevant from a biological point of view. Computational modeling carried out by DFT methodologies at the B3LYP/6-31G(d) level showed that the coordination of poly(pyrazolyl)borate-copper(I) to alkyne groups produced relevant changes in terms of generating a high polar copper(I)-acetylide intermediates. The analysis of the global and local reactivity indices explains correctly the role of poly(pyrazolyl)borate ligands in the stabilization and activation of the copper(I) catalyst in the studied 32CA reactions.

On the Mechanism of the Nickel-Catalyzed Boron Insertion into the C−O Bond of Benzofuran

2019 ◽  
Author(s):  
Ana Mateo-Martínez ◽  
Hayate Saito ◽  
Hideki Yorimitsu ◽  
Carles Bo
Keyword(s):  
Reductive Elimination ◽  
Nucleophilic Attack ◽  
Dft Studies ◽  
Reaction Conditions ◽  
Elementary Steps

<p>The reaction of benzofurans with diboron reagents and Cs<sub>2</sub>CO<sub>3</sub>catalyzed by Ni(0) complexes results in the insertion of boron into the C-O bond. The reaction conditions mimic those reported by Martin and Hosoya for borylating aryl C-F bonds, but neither the role of the base nor the sequence of elementary steps is clear. Herein we report on the mechanism of such transformation on the basis of DFT studies, which suggest that the base activates the diboron reagent and generates a reactive boryl group, that Ni(II) is reduced back to Ni(0) during the boryl insertion into the Ni-O bond, and that the classical reductive elimination step is best viewed as a ring-contracting nucleophilic attack. </p>

THE ROLE OF SLOW LOW EFFICIENCY DIALYSIS (SLED) IN RENAL REPLACEMENT THERAPY

2019 ◽  
Vol 72 (11) ◽  
Author(s):  
Małgorzata Kościelska ◽  
Paweł Żebrowski ◽  
Jolanta Małyszko
Keyword(s):  
Renal Replacement Therapy ◽  
Replacement Therapy ◽  
Low Efficiency

{10-12} Twinning Mechanism During In Situ Micro-Tensile Loading of Pure Mg: Role of Basal Slip and Twin-Twin Boundary Interaction

2020 ◽  
Author(s):  
Nicolò Maria della Ventura ◽  
Szilvia Kalácska ◽  
Daniele Casari ◽  
Thomas Edward James Edwards ◽  
Johann Michler ◽  
...  
Keyword(s):  
Twin Boundary ◽  
Basal Slip ◽  
Tensile Loading ◽  
Boundary Interaction

The role of (bio)surfactant sorption in promoting the bioavailability of nutrients localized at the solid-water interface

1999 ◽  
Vol 39 (7) ◽  
pp. 91-98 ◽  
Author(s):  
Ryan N. Jordan ◽  
Eric P. Nichols ◽  
Alfred B. Cunningham
Keyword(s):  
Mass Transfer ◽  
Cell Membrane ◽  
Substrate Concentration ◽  
Water Interface ◽  
Industrial Systems ◽  
Solid Liquid Interface ◽  
Solid Liquid ◽  
Surfactant Sorption

Bioavailability is herein defined as the accessibility of a substrate by a microorganism. Further, bioavailability is governed by (1) the substrate concentration that the cell membrane “sees,” (i.e., the “directly bioavailable” pool) as well as (2) the rate of mass transfer from potentially bioavailable (e.g., nonaqueous) phases to the directly bioavailable (e.g., aqueous) phase. Mechanisms by which sorbed (bio)surfactants influence these two processes are discussed. We propose the hypothesis that the sorption of (bio)surfactants at the solid-liquid interface is partially responsible for the increased bioavailability of surface-bound nutrients, and offer this as a basis for suggesting the development of engineered in-situ bioremediation technologies that take advantage of low (bio)surfactant concentrations. In addition, other industrial systems where bioavailability phenomena should be considered are addressed.

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