Unlocking a diazirine long-lived nuclear singlet state via photochemistry: NMR detection and lifetime of an unstabilized diazo-compound

TY - JOUR

T1 - Unlocking a diazirine long-lived nuclear singlet state via photochemistry

T2 - NMR detection and lifetime of an unstabilized diazo-compound

AU - Procacci, Barbara

AU - Roy, S. S.

AU - Norcott, Philip

AU - Turner, Norman

AU - Duckett, Simon

N1 - © 2018 American Chemical Society

PY - 2018/12/5

Y1 - 2018/12/5

N2 - Diazirines are important for photoaffinity labelling and their photoisomerization is relatively well known. This work shows how hyperpolarized NMR spectroscopy can be used to characterise an unstable diazo-compound formed via photoisomerization of a 15N2-labelled silyl-ether substituted diazirine. This diazirine is prepared in a nuclear spin singlet state via catalytic transfer of spin order from para-hydrogen. The active hyperpolarization catalyst is characterised to provide insight into the mechanism. The photochemical isomerisation of the diazirine into the diazo-analogue allows the NMR invisible nuclear singlet state of the parent compound to be probed. The identity of the diazo-species is confirmed by trapping with N-phenyl maleimide via a cycloaddition reaction to afford bicyclic pyrazolines that also show singlet state character. The presence of singlet states in the diazirine and the diazo-compound are validated by comparison of experimental nutation behaviour with theoretical simulation. The magnetic state lifetime of the diazo-compound is determined as 12 ± 1 s in CD3OD solution at room temperature whereas its chemical lifetime is measured as 100 ± 5 s by related hyperpolarized NMR studies. Indirect evidence for thegeneration of the photoproduct para-N2 is presented.

AB - Diazirines are important for photoaffinity labelling and their photoisomerization is relatively well known. This work shows how hyperpolarized NMR spectroscopy can be used to characterise an unstable diazo-compound formed via photoisomerization of a 15N2-labelled silyl-ether substituted diazirine. This diazirine is prepared in a nuclear spin singlet state via catalytic transfer of spin order from para-hydrogen. The active hyperpolarization catalyst is characterised to provide insight into the mechanism. The photochemical isomerisation of the diazirine into the diazo-analogue allows the NMR invisible nuclear singlet state of the parent compound to be probed. The identity of the diazo-species is confirmed by trapping with N-phenyl maleimide via a cycloaddition reaction to afford bicyclic pyrazolines that also show singlet state character. The presence of singlet states in the diazirine and the diazo-compound are validated by comparison of experimental nutation behaviour with theoretical simulation. The magnetic state lifetime of the diazo-compound is determined as 12 ± 1 s in CD3OD solution at room temperature whereas its chemical lifetime is measured as 100 ± 5 s by related hyperpolarized NMR studies. Indirect evidence for thegeneration of the photoproduct para-N2 is presented.

UR - http://www.scopus.com/inward/record.url?scp=85057544029&partnerID=8YFLogxK

U2 - 10.1021/jacs.8b10923

DO - 10.1021/jacs.8b10923

M3 - Article

SN - 0002-7863

VL - 140

SP - 16855

EP - 16864

JO - Journal of the American Chemical Society

JF - Journal of the American Chemical Society

IS - 48

ER -