Correlation of the rates of solvolysis of neopentyl chloroformate-a recommended protecting agent

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Undergraduate Research, Data-Science Courses, and Volunteer Projects, Inform and Accelerate Wesley College's Retention Among First- and Second- Year Students

Wesley College is Delaware's minority-serving, private, undergraduate, liberal-arts institution¹. The College continually advances innovative programming to boost college readiness and to stem the dropping numbers of its first-time (freshmen) applicants. In order to help unprepared students for the rigors of its programs and to enhance a truly integrated STEM curriculum^2-8, Wesley strengthened diverse academic equity by implanting data-science as cornerstone undergraduate research projects^9-11. To further scale-up student-engagement and to nationally disseminate our high-impact STEM-education successes^12,13, the College developed an Undergraduate Research Center for Analytics, Talent, and Success (UR-CATS)¹⁴. To streamline procurement documents and hazardous-waste handling workflows, the College implemented the use of an online platform¹⁵ that resulted in major annual savings. To engage middle and high school students, Wesley STEM undergraduates established a Science Club to demonstrate hands-on science lessons, so STEM concepts come to life. To help explain the learning mechanisms by which integrated STEM experiences support outcomes, this paper presents interventions with particular attention to curriculum design for incoming STEM freshmen who place into pre-Calculus or a lower math course. To show that engagement with STEM college students as role models have a positive impact on high-school student interest, we provide examples where Science Club members pose challenges and show different perspectives on everyday items. In summary, Wesley project outcomes have allowed for coordinated interventions^3-9,12-14 in student well-being and student success.

LFER Studies Evaluating Solvent Effects on an α-Chloro-and two β,β,β-Trichloro-Ethyl Chloroformate Esters

To provide insight and to identify the occurrence of mechanistic changes in relation to variance in solvent-type, the solvent effects on the rates of solvolysis of three substrates, 2,2,2-trichloro-1,1-dimethylethyl chloroformate, 2,2,2-trichloroethyl chloroformate, and 1-chloroethyl chloroformate, are analyzed using linear free energy relationships (LFERs) such as the extended Grunwald-Winstein equation, and a similarity-based LFER model approach that is based on the solvolysis of phenyl chloroformate. At 25.0 °C, in four common solvents, the α-chloroethyl chloroformate was found to react considerably faster than the two β,β,β-trichloro-substituted analogs. This immense rate enhancement can be directly related to the proximity of the electron-withdrawing α-chlorine atom to the carbonyl carbon reaction center. In the thirteen solvents studied, 1-chloroethyl chloroformate was found to strictly follow a carbonyl addition process, with the addition-step being rate-determining. For the two β,β,β-trichloro-substrates, in aqueous mixtures that are very rich in a fluoroalcohol component, there is compelling evidence for the occurrence of side-by-side addition-elimination and ionization mechanisms, with the ionization pathway being predominant. The presence of the two methyl groups on the α-carbon of 2,2,2-trichloro-1,1-dimethylethyl chloroformate has additive steric and stereoelectronic implications, causing its rate of reaction to be significantly slower than that of 2,2,2-trichloroethyl chloroformate.

Kinetic studies that evaluate the solvolytic mechanisms of allyl and vinyl chloroformate esters

At 25.0 °C the specific rates of solvolysis for allyl and vinyl chloroformates have been determined in a wide mix of pure and aqueous organic mixtures. In all the solvents studied, vinyl chloroformate was found to react significantly faster than allyl chloroformate. Multiple correlation analyses of these rates are completed using the extended (two-term) Grunwald-Winstein equation with incorporation of literature values for solvent nucleophilicity (N_T) and solvent ionizing power (Y_Cl). Both substrates were found to solvolyze by similar dual bimolecular carbonyl-addition and unimolecular ionization channels, each heavily dependent upon the solvents nucleophilicity and ionizing ability.

Application of the Grunwald-Winstein Equations to Studies of Solvolytic Reactions of Chloroformate and Fluoroformate Esters

Chloroformates are important laboratory and industrial chemicals with almost one hundred listed in the catalogs of leading suppliers. They are, for example, of prime importance as protecting groups in peptide synthesis. In some instances, the more stable fluoroformate is preferred. In recent years, the specific rates of solvolysis (k) for chloroformates and fluoroformates in solvents of widely ranging nucleophilicity and ionizing power have been studied. Analysis of these rates using the extended (two-term) Grunwald-Winstein equation has led to important information concerning reaction mechanism. Also assisting in this effort have been studies of kinetic solvent isotope effects (KSIE), of leaving group effects (especially kF/kCl ratios), and of entropies of activation from studies of specific rate variations with temperature. For solvolyses of chloroformate esters, two mechanisms (addition-elimination and ionization) are commonly encountered. For solvolyses of fluoroformates, mainly because of a strong C-F bond, the ionization pathway is rare and the addition-elimination pathway is in most situations the one encountered.

Evaluation of Electronic Effects in the Solvolyses of p-Methylphenyl and p-Chlorophenyl Chlorothionoformate Esters

The solvolyses of p-tolyl chlorothionoformate and p-chlorophenyl chlorothionoformate are studied in a variety of organic mixtures of widely varying nucleophilicity and ionizing power values. This solvolytic data is accumulated at 25.0 °C using the titration method. An analysis of the rate data using the extended (two-term) Grunwald-Winstein equation, and the concept of similarity of substrates based on their l/m ratios, shows the occurrence of simultaneous side-by-side addition-elimination and unimolecular S_N1 mechanisms.

Correlation of the rates of solvolysis of tert-butyl chlorothioformate and observations concerning the reaction mechanism

The "parent" tertiary alkyl chloroformate, tert-butyl chloroformate, is unstable, but the tert-butyl chlorothioformate (1) is of increased stability and a kinetic investigation of the solvolyses is presented. Analyses in terms of the simple and extended Grunwald-Winstein equations are carried out. The original one-term equation satisfactorily correlates the data with a sensitivity towards changes in solvent ionizing power of 0.73 ±0.03. When the two-term equation is applied, the sensitivity towards changes in solvent nucleophilicity of 0.13 ± 0.09 is associated with a high (0.17) probability that the term that it governs is not statistically significant.

Use of Linear Free Energy Relationships (LFERs) to elucidate the mechanisms of reaction of a γ-methyl-β-alkynyl and an ortho-substituted aryl chloroformate ester

The specific rates of solvolysis of 2-butyn-1-yl-chloroformate (1) and 2-methoxyphenyl chloroformate (2) are studied at 25.0 °C in a series of binary aqueousorganic mixtures. The rates of reaction obtained are then analyzed using the extended Grunwald-Winstein (G-W) equation and the results are compared to previously published G-W analyses for phenyl chloroformate (3), propargyl chloroformate (4), p-methoxyphenyl choroformate (5), and p-nitrophenyl chloroformate (6). For 1, the results indicate that dual side-by-side addition-elimination and ionization pathways are occurring in some highly ionizing solvents due to the presence of the electron-donating γ-methyl group. For 2, the analyses indicate that the dominant mechanism is a bimolecular one where the formation of a tetrahedral intermediate is rate-determining.

Kinetic evaluation of the solvolysis of isobutyl chloro- and chlorothioformate esters

The specific rates of solvolysis of isobutyl chloroformate (1) are reported at 40.0 °C and those for isobutyl chlorothioformate (2) are reported at 25.0 °C, in a variety of pure and binary aqueous organic mixtures with wide ranging nucleophilicity and ionizing power. For 1, we also report the first-order rate constants determined at different temperatures in pure ethanol (EtOH), methanol (MeOH), 80% EtOH, and in both 97% and 70% 2,2,2-trifluoroethanol (TFE). The enthalpy (ΔH(≠)) and entropy (ΔS(≠)) of activation values obtained from Arrhenius plots for 1 in these five solvents are reported. The specific rates of solvolysis were analyzed using the extended Grunwald-Winstein equation. Results obtained from correlation analysis using this linear free energy relationship (LFER) reinforce our previous suggestion that side-by-side addition-elimination and ionization mechanisms operate, and the relative importance is dependent on the type of chloro- or chlorothioformate substrate and the solvent.