Papers by jonathan buchspies
Context. The solar corona is a template to understand stellar activity. The Sun is a moderately a... more Context. The solar corona is a template to understand stellar activity. The Sun is a moderately active star, and its corona differs from that of active stars: for instance, active stellar coronae have a double-peaked emission measure distribution EM(T) with the hot peak at 8 − 20 MK, while the non flaring solar corona has one peak at 1 − 2 MK and, typically, much cooler plasma. Aims. We study the average contribution of flares to the solar emission measure distribution to investigate indirectly the hypothesis that the hot peak of the EM(T) of active stellar coronae is due to a large number of unresolved solar-like flares, and to infer properties on the flare distribution from nano-to macro-flares. Methods. We measure the disk-integrated time-averaged emission measure, EM F (T), of an unbiased sample of solar flares analyzing uninterrupted GOES/XRS light curves over time intervals of one month. We obtain the EM Q (T) of quiescent corona for the same time intervals from the Yohkoh/SXT data. To investigate how EM F (T) and EM Q (T) vary with the solar cycle, we evaluate them at different phases of the cycle between December 1991 and April 1998. Results. Irrespective of the solar cycle phase, EM F (T) appears like a peak of the distribution significantly larger than the values of EM Q (T) for T ∼ 5 − 10 MK. As a result the time-averaged EM(T) of the whole solar corona is double-peaked, with the hot peak, due to time-averaged flares, located at temperature similar of that of active stars, but less enhanced. The EM F (T) shape supports the hypothesis that the hot EM(T) peak of active coronae is due to unresolved solar-like flares. If this is the case, quiescent and flare components should follow different scaling laws for increasing stellar activity. In the assumption that the heating of the corona is entirely due to flares, from nano-to macro-flares, then either the flare distribution or the confined plasma response to flares, or both, are bimodal.
NJIT Academic Integrity Code: All Students should be aware that the Department of Chemistry & Env... more NJIT Academic Integrity Code: All Students should be aware that the Department of Chemistry & Environmental Science (CES) takes the University Code on Academic Integrity at NJIT very seriously and enforces it strictly. This means that there must not be any forms of plagiarism, i.e., copying of homework, class projects, or lab assignments, or any form of cheating in quizzes and exams. Under the University Code on Academic Integrity, students are obligated to report any such activities to the Instructor.
Catalysts, 2020
In this Special Issue on N-Heterocyclic Carbenes and Their Complexes in Catalysis, we report the ... more In this Special Issue on N-Heterocyclic Carbenes and Their Complexes in Catalysis, we report the first example of Suzuki–Miyaura cross-coupling of amides catalyzed by well-defined, air- and moisture-stable nickel/NHC (NHC = N-heterocyclic carbene) complexes. The selective amide bond N–C(O) activation is achieved by half-sandwich, cyclopentadienyl [CpNi(NHC)Cl] complexes. The following order of reactivity of NHC ligands has been found: IPr > IMes > IPaul ≈ IPr*. Both the neutral and the cationic complexes are efficient catalysts for the Suzuki–Miyaura cross-coupling of amides. Kinetic studies demonstrate that the reactions are complete in < 1 h at 80 °C. Complete selectivity for the cleavage of exocyclic N-acyl bond has been observed under the experimental conditions. Given the utility of nickel catalysis in activating unreactive bonds, we believe that well-defined and bench-stable [CpNi(NHC)Cl] complexes will find broad application in amide bond and related cross-couplings ...
Catalysts, 2019
Acyl Suzuki cross-coupling involves the coupling of an organoboron reagent with an acyl electroph... more Acyl Suzuki cross-coupling involves the coupling of an organoboron reagent with an acyl electrophile (acyl halide, anhydride, ester, amide). This review provides a timely overview of the very important advances that have recently taken place in the acylative Suzuki cross-coupling. Particular emphasis is directed toward the type of acyl electrophiles, catalyst systems and new cross-coupling partners. This review will be of value to synthetic chemists involved in this rapidly developing field of Suzuki cross-coupling as well as those interested in using acylative Suzuki cross-coupling for the synthesis of ketones as a catalytic alternative to stoichiometric nucleophilic additions or Friedel-Crafts reactions.
Catalysts, 2019
We report a general, highly selective method for Suzuki–Miyaura cross-coupling of N-acylphthalimi... more We report a general, highly selective method for Suzuki–Miyaura cross-coupling of N-acylphthalimides via N–C(O) acyl cleavage catalyzed by Pd–PEPPSI-type precatalysts. Of broad synthetic interest, the method introduces N-acylphthalimides as new, bench-stable, highly reactive, twist-controlled, amide-based precursors to acyl-metal intermediates. The reaction delivers functionalized biaryl ketones by acylative Suzuki–Miyaura cross-coupling with readily available boronic acids. Studies demonstrate that cheap, easily prepared, and broadly applicable Pd–PEPPSI-type precatalysts supported by a sterically demanding IPr (1,3-Bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) ancillary ligand provide high yields in this reaction. Preliminary selectivity studies and the effect of Pd–N-heterocyclic carbenes (NHC) complexes with allyl-type throw-away ligands are described. We expect that N-acylphthalimides will find significant use as amide-based acyl coupling reagents and cross-coupling precursors...
Molecules, 2018
Although the palladium-catalyzed Suzuki-Miyaura cross-coupling of aryl esters has received signif... more Although the palladium-catalyzed Suzuki-Miyaura cross-coupling of aryl esters has received significant attention, there is a lack of methods that utilize cheap and readily accessible Pd-phosphane catalysts, and can be routinely carried out with high cross-coupling selectivity. Herein, we report the first general method for the cross-coupling of pentafluorophenyl esters (pentafluorophenyl = pfp) by selective C–O acyl cleavage. The reaction proceeds efficiently using Pd(0)/phosphane catalyst systems. The unique characteristics of pentafluorophenyl esters are reflected in the fully selective cross-coupling vs. phenolic esters. Of broad synthetic interest, this report establishes pentafluorophenyl esters as new, highly reactive, bench-stable, economical, ester-based, electrophilic acylative reagents via acyl-metal intermediates. Mechanistic studies strongly support a unified reactivity scale of acyl electrophiles by C(O)–X (X = N, O) activation. The reactivity of pfp esters can be corre...
Carbon-Based Metal-Free Catalysts, 2018
Molecules
The formation of amide bonds represents one of the most fundamental processes in organic synthesi... more The formation of amide bonds represents one of the most fundamental processes in organic synthesis. Transition-metal-catalyzed activation of acyclic twisted amides has emerged as an increasingly powerful platform in synthesis. Herein, we report the transamidation of N-activated twisted amides by selective N–C(O) cleavage mediated by air- and moisture-stable half-sandwich Ni(II)–NHC (NHC = N-heterocyclic carbenes) complexes. We demonstrate that the readily available cyclopentadienyl complex, [CpNi(IPr)Cl] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), promotes highly selective transamidation of the N–C(O) bond in twisted N-Boc amides with non-nucleophilic anilines. The reaction provides access to secondary anilides via the non-conventional amide bond-forming pathway. Furthermore, the amidation of activated phenolic and unactivated methyl esters mediated by [CpNi(IPr)Cl] is reported. This study sets the stage for the broad utilization of well-defined, air- and moisture-stab...
Uploads
Papers by jonathan buchspies