Dissociative Solvolytic Cleavage of Methyl (ortho-Carboxymethyl)Aryl Phosphate Diesters Mediated by Yb3+in Methanol Gives a 1012-Fold Rate Acceleration Attributable to Leaving Group Assistance

2009 ◽  
Vol 131 (1) ◽  
pp. 368-377 ◽  
Author(s):  
David R. Edwards ◽  
Alexei A. Neverov ◽  
R. Stan Brown
Keyword(s):  
Rate Acceleration ◽  
Leaving Group ◽  
Phosphate Diesters

Bio-inspired approaches to accelerating metal ion-promoted reactions: enzyme-like rates for metal ion mediated phosphoryl and acyl transfer processes

2015 ◽  
Vol 87 (6) ◽  
pp. 601-614 ◽  
Author(s):  
Robert Stan Brown
Keyword(s):  
Metal Ion ◽  
Catalytic Efficiency ◽  
Acyl Transfer ◽  
Transfer Reactions ◽  
Medium Effect ◽  
Phosphoryl Transfer ◽  
Leaving Group ◽  
Bonding Properties ◽  
Catalyzed Reactions ◽  
Phosphate Diesters

Abstract Intense efforts by many research groups for more than 50 years have been directed at biomimetic approaches to understand how enzymes achieve their remarkable rate accelerations. Nevertheless, it was noted in 2003 that, despite numerous efforts to design models for catalyzing the cleavage of such species as phosphate diesters, “none of the several models so far described approaches the enormous catalytic efficiency of natural enzymes”. The same could be said for biomimetics of other enzymes promoting acyl or phosphoryl transfer reactions, particularly those mediated by metal ions such as Zn(II). Clearly other important factors were being overlooked or awaiting discovery. In this manuscript we describe two important effects that we have implemented to accelerate metal ion catayzed phosphoryl and acyl transfer reactions. The first of these relates to a medium effect where the polarity of the solution, as measured by dielectric constant, is reduced from that of water (ε = 78) to values of 31.5 and 24.3 when the solvent is changed to methanol or ethanol. Among organic solvents these light alcohols are closest to water in terms of structure and properties as well as retaining important H-bonding properties. The second important effect involves a known but difficult to demonstrate mode of catalysis where the leaving group (LG) in a solvolysis reaction is accelerated as it becomes progressively poorer. In the cases described herein, the LG’s propensity to depart from a substrate during the course of reaction is accelerated by coordination to a metal ion in a process known as leaving group assistance, or LGA. These two effects can each impart accelerations of 109–1017 for certain metal ion catalyzed reactions relative to the corresponding solvent, or base induced reactions.

Structure and reactivity of phosphate diesters. Dependence on the nonleaving group

2015 ◽  
Vol 93 (4) ◽  
pp. 422-427 ◽  
Author(s):  
Anthony J. Kirby ◽  
Bruno S. Souza ◽  
Faruk Nome
Keyword(s):  
Methyl Ester ◽  
Continuum Model ◽  
Water Molecules ◽  
Wide Range ◽  
Leaving Group ◽  
Leaving Groups ◽  
Lower Temperature ◽  
Phosphate Diesters

The hydrolytic reactivity of simple phosphate diesters with very good leaving groups is known to be practically independent of the nonleaving group at 100 °C. Calculations on the (too slow to measure) hydrolysis at 25 °C of a series of p-nitrophenyl diesters ROPO2––OpNP with a wide range of nonleaving group OR indicate a small but significant effect at the lower temperature, making the R = methyl ester the most reactive. This is in the opposite sense to the much larger effect observed for the reactions of triesters and consistent with a reaction driven primarily by leaving group departure. The calculations use a continuum model with up to four discrete water molecules: two or three waters give the best results.

scholarly journals Inter- and Intra-Molecular Organocatalysis of SN2 Fluorination by Crown Ether: Kinetics and Quantum Chemical Analysis

Molecules ◽  
2021 ◽  
Vol 26 (10) ◽  
pp. 2947
Author(s):  
Young-Ho Oh ◽  
Wonhyuck Yun ◽  
Chul-Hee Kim ◽  
Sung-Woo Jang ◽  
Sung-Sik Lee ◽  
...  
Keyword(s):  
Crown Ether ◽  
Quantum Chemical ◽  
Metal Salt ◽  
Ion Pair ◽  
Lewis Base ◽  
Free Energies ◽  
Quantum Chemical Analysis ◽  
Rate Acceleration ◽  
Leaving Group

We present the intra- and inter-molecular organocatalysis of SN2 fluorination using CsF by crown ether to estimate the efficacy of the promoter and to elucidate the reaction mechanism. The yields of intramolecular SN2 fluorination of the veratrole substrates are measured to be very small (<1% in 12 h) in the absence of crown ether promoters, whereas the SN2 fluorination of the substrate possessing a crown ether unit proceeds to near completion (~99%) in 12 h. We also studied the efficacy of intermolecular rate acceleration by an independent promoter 18-crown-6 for comparison. We find that the fluorinating yield of a veratrole substrate (leaving group = −OMs) in the presence of 18-crown-6 follows the almost identical kinetic course as that of intramolecular SN2 fluorination, indicating the mechanistic similarity of intra- and inter-molecular organocatalysis of the crown ether for SN2 fluorination. The calculated relative Gibbs free energies of activation for these reactions, in which the crown ether units act as Lewis base promoters for SN2 fluorination, are in excellent agreement with the experimentally measured yields of fluorination. The role of the metal salt CsF is briefly discussed in terms of whether it reacts as a contact ion pair or as a “free” nucleophile F−.

The Effect of Searching Versus Surfing on Cognitive and Emotional Responses to Online News

2009 ◽  
Vol 21 (2) ◽  
pp. 49-59 ◽  
Author(s):  
Kevin Wise ◽  
Hyo Jung Kim ◽  
Jeesum Kim
Keyword(s):  
Heart Rate ◽  
Skin Conductance ◽  
Skin Conductance Level ◽  
Emotional Responses ◽  
Online News ◽  
Self Report ◽  
Mixed Design ◽  
Information Need ◽  
News Stories ◽  
Rate Acceleration

A mixed-design experiment was conducted to explore differences between searching and surfing on cognitive and emotional responses to online news. Ninety-two participants read three unpleasant news stories from a website. Half of the participants acquired their stories by searching, meaning they had a previous information need in mind. The other half of the participants acquired their stories by surfing, with no previous information need in mind. Heart rate, skin conductance, and corrugator activation were collected as measures of resource allocation, motivational activation, and unpleasantness, respectively, while participants read each story. Self-report valence and recognition accuracy were also measured. Stories acquired by searching elicited greater heart rate acceleration, skin conductance level, and corrugator activation during reading. These stories were rated as more unpleasant, and their details were recognized more accurately than similar stories that were acquired by surfing. Implications of these results for understanding how people process online media are discussed.

General Cyclopropane Assembly via Enantioselective Redox-Active Carbene Transfer to Aliphatic Olefins

2018 ◽  
Author(s):  
Marc Montesinos-Magraner ◽  
Matteo Costantini ◽  
Rodrigo Ramirez-Contreras ◽  
Michael E. Muratore ◽  
Magnus J. Johansson ◽  
...  
Keyword(s):  
Total Synthesis ◽  
Asymmetric Synthesis ◽  
Chemical Space ◽  
Synthetic Approach ◽  
Asymmetric Cyclopropanation ◽  
Redox Active ◽  
Wide Range ◽  
Leaving Group ◽  
Stereoelectronic Properties ◽  
Carbene Transfer

Asymmetric cyclopropane synthesis currently requires bespoke strategies, methods, substrates and reagents, even when targeting similar compounds. This limits the speed and chemical space available for discovery campaigns. Here we introduce a practical and versatile diazocompound, and we demonstrate its performance in the first unified asymmetric synthesis of functionalized cyclopropanes. We found that the redox-active leaving group in this reagent enhances the reactivity and selectivity of geminal carbene transfer. This effect enabled the asymmetric cyclopropanation of a wide range of olefins including unactivated aliphatic alkenes, enabling the 3-step total synthesis of (–)-dictyopterene A. This unified synthetic approach delivers high enantioselectivities that are independent of the stereoelectronic properties of the functional groups transferred. Our results demonstrate that orthogonally-differentiated diazocompounds are viable and advantageous equivalents of single-carbon chirons<i>.</i>

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