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Real-Time High-Sensitivity Reaction Monitoring of Important Nitrogen-Cycle Synthons by 15N Hyperpolarized Nuclear Magnetic Resonance

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JournalJournal of the American Chemical Society
DateAccepted/In press - 19 Apr 2022
DateE-pub ahead of print - 4 May 2022
DatePublished (current) - 18 May 2022
Issue number19
Volume144
Number of pages14
Pages (from-to)8756–8769
Early online date4/05/22
Original languageEnglish

Abstract

Here, we show how signal amplification by reversible exchange hyperpolarization of a range of 15N-containing synthons can be used to enable studies of their reactivity by 15N nuclear magnetic resonance (NO 2 - (28% polarization), ND 3 (3%), PhCH 2NH 2 (5%), NaN 3 (3%), and NO 3 - (0.1%)). A range of iridium-based spin-polarization transfer catalysts are used, which for NO 2 - work optimally as an amino-derived carbene-containing complex with a DMAP- d 2 coligand. We harness long 15N spin-order lifetimes to probe in situ reactivity out to 3 × T 1. In the case of NO 2 - ( T 1 17.7 s at 9.4 T), we monitor PhNH 2 diazotization in acidic solution. The resulting diazonium salt ( 15N- T 1 38 s) forms within 30 s, and its subsequent reaction with NaN 3 leads to the detection of hyperpolarized PhN 3 ( T 1 192 s) in a second step via the formation of an identified cyclic pentazole intermediate. The role of PhN 3 and NaN 3 in copper-free click chemistry is exemplified for hyperpolarized triazole ( T 1 < 10 s) formation when they react with a strained alkyne. We also demonstrate simple routes to hyperpolarized N 2 in addition to showing how utilization of 15N-polarized PhCH 2NH 2 enables the probing of amidation, sulfonamidation, and imine formation. Hyperpolarized ND 3 is used to probe imine and ND 4 + ( T 1 33.6 s) formation. Furthermore, for NO 2 -, we also demonstrate how the 15N-magnetic resonance imaging monitoring of biphasic catalysis confirms the successful preparation of an aqueous bolus of hyperpolarized 15NO 2 - in seconds with 8% polarization. Hence, we create a versatile tool to probe organic transformations that has significant relevance for the synthesis of future hyperpolarized pharmaceuticals.

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