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. 2022 Feb 7;28(8):e202103501.
doi: 10.1002/chem.202103501. Epub 2022 Jan 19.

Parahydrogen-Induced Polarization in Hydrogenation Reactions Mediated by a Metal-Free Catalyst

Danila O Zakharov  1 , Konstantin Chernichenko  2   3 , Kristina Sorochkina  1   2 , Shengjun Yang  1 , Ville-Veikko Telkki  1 , Timo Repo  2 , Vladimir V Zhivonitko  1
Affiliations

Parahydrogen-Induced Polarization in Hydrogenation Reactions Mediated by a Metal-Free Catalyst

Danila O Zakharov et al. Chemistry. .

Abstract

We report nuclear spin hyperpolarization of various alkenes achieved in alkyne hydrogenations with parahydrogen over a metal-free hydroborane catalyst (HCAT). Being an intramolecular frustrated Lewis pair aminoborane, HCAT utilizes a non-pairwise mechanism of H2 transfer to alkynes that normally prevents parahydrogen-induced polarization (PHIP) from being observed. Nevertheless, the specific spin dynamics in catalytic intermediates leads to the hyperpolarization of predominantly one hydrogen in alkene. PHIP enabled the detection of important HCAT-alkyne-H2 intermediates through substantial 1 H, 11 B and 15 N signal enhancement and allowed advanced characterization of the catalytic process.

Keywords: Lewis acids; Lewis bases; NMR hyperpolarization; organocatalysis; parahydrogen.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Scheme 1
Scheme 1
Steps of a substrate (Sub) hydrogenation over a catalyst (Cat).
Scheme 2
Scheme 2
Mechanism of alkyne hydrogenation with para‐H2 over HCAT.
Figure 1
Figure 1
1H NMR spectra of HCAT‐catalyzed hydrogenation of 1‐ethyl‐2‐phenylacetylene (1) with para‐H2 (R1). The upper (red) spectrum was acquired by using a π/4 pulse just after introducing para‐H2 into a solution containing ca. 0.1 M HCAT and 0.3 M of 1 in toluene‐d8 at 298 K. The enhanced antiphase signals correspond to NH and BH group hydrogens of the two regioisomeric HCAT‐alkyne‐H2 intermediates (a and b). The enhanced in‐phase negative signals correspond to one of the double‐bond hydrogens in (Z)‐1‐phenylbut‐1‐ene product and ortho‐H2. The other double‐bond hydrogen gains a weak antiphase polarization (see the inset). The lower (blue) trace shows the thermal signal after 4 min.
Figure 2
Figure 2
Selected fragments of 1H NMR spectra showing hyperpolarized species produced in the course of hydrogenation reactions R2–R5 over HCAT catalyst at 298 K in toluene‐d8. All spectra were measured immediately after para‐H2 was introduced into the corresponding substrate‐HCAT solutions. Left: the signals of NH and BH hydrogens of HCAT‐alkyne‐H2 intermediates; right: the signals of the hyperpolarized protons in cis‐alkenes. The signal of the double‐bond protons of (Z)‐1,2‐diphenylethylene‐d10 is marked with an asterisk. The spectral regions are scaled by different factors indicated in the figure.
Figure 3
Figure 3
a) 11B NMR spectra measured after adding para‐H2 to a solution of HCAT (0.1 M) and 1 (0.3 M) in toluene‐d8 at 298 K (red trace) and after para‐H2 was converted into normal H2 (blue trace). b) 15N NMR spectra acquired immediately after introducing a fresh portion of para‐H2 (upper trace) and 18 s later (lower trace) to 15N‐HCAT (0.1 M) and 1 (0.3 M) in toluene‐d8 at 298 K. The signals of the hyperpolarized species are marked using arrows in the spectra. See Figures 1 and 3 for their structures. HP=hyperpolarized, TH=thermal.
See this image and copyright information in PMC

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