Abstract
AbstractIn‐situ NMR spectroscopic analysis of homogeneous reactions is an essential tool for mechanistic analysis in organic and organometallic chemistry. However, rapid non‐equilibrium reactions, that are initiated by mixing, require specialized approaches. We report herein on a study of the factors that ensure quantitative results in a recently‐developed technique for stopped‐flow NMR spectroscopy. The influence of some of the key parameters on quantitation is studied by 19F NMR spectroscopic analysis of the kinetics and activation parameters for the base‐catalyzed protodeboronation of highly‐reactive polyfluorinated arylboronic acids, with half‐lives as low as 0.1 seconds. The effects of spin relaxation, pre‐magnetization, heat‐transfer versus reaction enthalpy, and mixing‐efficiency are analyzed in detail. We also compare and contrast choice of pulse angle, interscan delay, and use of pseudo real‐time by interleaving, as means to achieve an optimal balance between temporal resolution and sensitivity.
Highlights
The elucidation of chemical reaction mechanisms is essential for the informed optimization of known processes, and a powerful impetus in the design of new ones.[1]
Whilst the IR/quench techniques allowed us to determine a wide range of protodeboronation kinetics, n = 0 to 5, Scheme 2, they did not provide the key insights that NMR spectroscopy
Base-catalyzed protodeboronation of polyfluorophenyl boronic acids, with half-lives ranging from 7 months (3-fluorophenylboronate) to 2.6 milliseconds (2,3,4,5,6-pentafluorophenylboronate).[21]
Summary
The elucidation of chemical reaction mechanisms is essential for the informed optimization of known processes, and a powerful impetus in the design of new ones.[1]. SHARPER[14] allows activation parameters to be derived from temperature-dependent line-broadening and distortion effects, and the CPMG spin-echo pulse sequence reveals protein motions at microsecond to millisecond timescale.[15,16] monitoring rapid irreversible reactions by NMR spectroscopy poses a considerably different challenge, Scheme 1. Over the last decade we have investigated a wide range of anion-mediated processes involving organoboron[18,19,20,21] and organosilicon[22,23,24,25,26] reagents, including hydrolytic activation and degradation of boronic acids,[18,21] and their derivatives.[19,20] In the majority of examples we were able to employ standard 1H, 11B, 19F, and 29Si NMR spectroscopic techniques to analyze the reaction kinetics. Some reactions, for example the protodeboronation of polyfluorophenylboronic acids, 1, Scheme 2,[21] required fast in-situ spectroscopic techniques, such as SF-FT-IR, or very rapid ex-situ quench methods.[27,28,29]
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