Nucleophilicities and Nucleofugalities of Organic Carbonates

Abstract

Keywords: Kinetics / Linear free energy relationships / Solvolysis / Correlation analysis / Leaving groupsThe kinetics of the reactions of the methyl carbonate ion with benzhydrylium ions in acetonitrile have been studied by UV/ Vis spectrophotometry. Substitution of the resulting secondorder rate constants and the electrophilicity parameters E of the benzhydrylium ions into the linear free energy relationship log k = s(N + E) yielded the nucleophilicity parameters N 25 = 16.03 and s 25 = 0.64 for methyl carbonate in acetonitrile. The kinetics of the reverse reactions, i.e., of the solvolyses of ring-substituted benzhydryl alkyl carbonates in different aqueous solvents were followed by conductimetry. The

Photogeneration of carbocations: applications in physical organic chemistry and the design of suitable precursors

“…3), [113] and the more Lewis-basic tributylphosphine has been employed successfully in these cases. [45][46][47][48][49][50][51][52] For carbocations of very low Lewis acidity, even the formation of tributylphosphonium salts becomes reversible, as shown in Scheme 11 for the 2-phenyl-3,4-dihydroisoquinolinium ion. When the resulting equilibrium mixture is irradiated, the phospho- Figure 3.…”

“…[71,72] The excitation can thus be located entirely on the photonucleofuge (PLG), such as in the 355-nm photolysis of 3,4,5triamino-substituted pyridinium ions (Scheme 15), [71,72] or entirely on the photo-electrofuge (R), such as in the 266-nm photolyses of arylmethyl chlorides R-Cl [18][19][20][21][22][23][98][99][100][101][102] or tributylphosphonium ions R-P(nBu) 3 + . [45][46][47][48][49][50][51][52] In the 266-nm photolysis of benzhydryl triphenylphosphonium salts, [38] it is not clear where the excitation occurs.…”

“…Tertiary phosphines are the most versatile among the different classes of photo-leaving groups investigated: [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] Quaternary phosphonium salts are easy to synthesize, [109] stable in solution, even in solvents with very high ionizing power such as hexafluoroisopropanol/water mixtures, [43] and provide excellent yields of carbocations even in solvents of moderate polarity such as CH 2 Cl 2 . [37][38][39][40] Tris(p-chlorophenyl)phosphine is one of the most efficient photo-leaving groups presently known for the generation of highly reactive carbocations, [38][39][40] whereas tributylphosphine is the photo-leaving group of choice for the photogeneration and kinetic investigation of highly stabilized carbocations with low Lewis acidity.…”

“…Common precursors include alkyl halides R-Hal, [18][19][20][21][22][23][24] acetates R-OAc, [24][25][26][27][28][29][30][31][32][33] aryl ethers R-OAr, [24][25][26][27][28][29][30][31][32][33] ammonium salts R-NR 0 3 + , [18,34,37] and phosphonium salts R-PR 0 3 + . [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] For the sake of simplicity, neutral and charged precursors will both be denoted as "R-PLG", and the corresponding anionic (X À ) and neutral (X) photo-leaving groups will be denoted as "PLG À " in the following.…”

“…[24] In recent years, we have employed different precursors in laser flash photolysis experiments for the purpose of generating carbocations and studying their reactivities in bimolecular reactions on the 10 ns to 1 ms time scale. [21][22][23][30][31][32][33][34][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][68][69][70][71][72] Whereas the original papers mostly focus on the chemistry of the photogenerated carbocations, this review article summarizes those aspects of our work that are relevant for the rational design of precursors for the photogeneration of carbocations and offers some guidelines for the use of laser flash photolysis in kinetic experiments.…”