Quantitative Analyses of Stereoisomeric 3,4-d₂-Cyclohexenes in the Presence of 3,6-d₂-Cyclohexenes

Abstract

The challenging analytical problem posed by mixtures of the four isomeric 3,4-d(2)-cyclohexenes and the three isomeric 3,6-d(2)-cyclohexenes has been solved through a novel one-dimensional NMR spectroscopic method dependent on recording (13)C resonances while broadband decoupling both proton and deuteron nuclei. Upfield deuterium perturbations of (13)C chemical shifts in chair conformationally locked cyclohexane derivatives readily secured from a mixture of the seven deuterium-labeled cyclohexenes allow quantitative analytical assessments of the four possible 3,4-d(2)-cyclohexenes in the mixture. This analytical capability is an essential prerequisite for uncovering the relative participations of the four possible stereochemical paths followed by the thermal structural isomerizations of vinylcyclobutane to cyclohexene. The unconventional NMR method validated in this work will likely prove invaluable in related stereochemical investigations.

Deuterium Quantification through Deuterium‐Induced Remote1H and13C NMR Shifts

“…The idea of employing deuterium‐induced 13 C NMR isotope shifts n Δ for deuterium quantification was published1 in 2009 as an “unconventional” and “novel” one‐dimensional NMR method. However, the authors1 used only 1 Δ and 2 Δ data as obtained with rather long relaxation delays (15 s), because they were not interested in more remote n Δ data of the types that are focused on in our present work.…”

“…The idea of employing deuterium‐induced 13 C NMR isotope shifts n Δ for deuterium quantification was published1 in 2009 as an “unconventional” and “novel” one‐dimensional NMR method. However, the authors1 used only 1 Δ and 2 Δ data as obtained with rather long relaxation delays (15 s), because they were not interested in more remote n Δ data of the types that are focused on in our present work. We have shown that the fractions of deuterated molecules (Table 1) may be quantified under conventional conditions of fast 13 C FT‐NMR spectroscopy, namely, continuous { 1 H} broadband decoupling with a very short relaxation delay and with the commonly used excitation of the total 13 C spectrum simultaneously, yet without {D} decoupling.…”

“…As a further advantage, many organic compounds exhibit more 13 C than 1 H NMR signals. A recommendable technique for potentially quantitative1 analysis by simple NMR integration is based on the phenomenon of deuterium‐induced isotope shifts,2, 3 n Δ= δ (deuterated)− δ (nondeuterated), which provide separate 1 H or 13 C NMR signals for the components of a mixture of “isotopologues” with the unlabeled parent compound through the interaction of deuterium atoms with 1 H or 13 C nuclei across n chemical bonds. Because the large majority of publications dealing with n Δ values do not use or mention such intensity measurements, we wish to draw attention to the merits of a reliable 13 C NMR integration procedure in its most convenient practical form, namely, under conventional conditions of fast accumulative recording with continuous broadband proton decoupling.…”

Degenerate thermal isomerizations of bicyclo[3.1.1]hept‐2‐ene: strategy and analysis

“…The procedure used here to estimate the 1 Δ 13 C deuterium shift at the site of substitution attributes the effect to the change in bond length of the CH bond when H is replaced by D. [16][17][18][19][20][21][22][23][24][25][26] In a recent paper [26] , it was shown for the specific relevant case of rigid hydrocarbons that the computed response of the 13 C chemical shift due to change in bond length is a good approximation to experiment. A more complete treatment includes contraction in the mean square amplitude of the stretching and both bending degrees of freedom when H is replaced by D. When all four contributions are included there is good agreement with experimental results for the longer range effects n Δ for n > 1.…”

“…The procedure used here to estimate the 1 Δ 13 C deuterium shift at the site of substitution attributes the effect to the change in bond length of the CH bond when H is replaced by D . In a recent paper, it was shown for the specific relevant case of rigid hydrocarbons that the computed response of the 13 C chemical shift due to change in bond length is a good approximation to experiment.…”