Visible Light Promoted Direct Deuteration of Alkenes via Co(III)–H Mediated H/D Exchange
We report herein a visible light-mediated direct deuteration of alkenes with D2O or deuterated methanol (MeOD) using a cobaloxime as a hydrogen/deuterium (H/D) exchange catalyst. The synergistic photoredox/Co catalysis enabled facile deuterium (D)-incorporation of a variety of terminal and internal alkenes at either terminal or benzylic positions. We proposed that this process proceeded through a sequence of reversible addition-elimination reactions and fast proton exchange involving Co(III)–H, which was generated in situ by photoreduction.
Introduction
Direct hydrogen/deuterium (H/D) exchange of C–H bonds with readily available deuterium
sources such as D2O is arguably the most straightforward and economic strategy1–3 in D-labeling, which is of great importance in molecular tracing and drug discovery.4–6 Due to the thermostability of C–H bonds, catalytic systems capable of reversible
and facile C–H bond activations are generally required to facilitate direct H/D exchange.
Hence, the development of a new catalytic strategy is in high demand to enable selective
D-labeling of a distinctive chemical entity.7 Four-coordinated planar cobalt hydrides (Co(III)–H) are versatile catalytic intermediates
in chemical and energy-transforming processes such as hydrogen-gas evolution8–12 (Scheme 1a, Path A) and hydrofunctionalization of alkenes (Scheme 1a, Path B).13–25 Recently, planar Co(III)–H catalysis has been further advanced toward isomerization
of alkenes and dehydrogenative allylic alkylation under either photoredox or electrochemical
conditions.26–39 In these processes, we and others have observed that in-situ generated Co(III)–H
species would tend to undergo a fast and reversible addition-elimination process with
alkenes,29,33–35,39 a salient feature that remains unexploited. On the other hand, Co(III)–H is also
acidic enough to undergo a facile proton exchange, with pKa(Co–H) = 10–14 as a result of filled d8-electron configuration of Co(I) (Scheme 1a, Path C).40,41 On these bases, we envisioned that Co(III)–H might mediate selective H/D exchange
of alkene C–H bonds with deuterium sources such as D2O by combining its reversible alkene addition and acidity (Scheme 1b). Previously, transition metal catalysts such as Ir, Ru, and Pd have been reported
for direct deuterium (D)-incorporation of terminal styrenes and acrylic acid derivatives.42–52 Metal-free catalysis has also been explored in this regard but is limited to electron-deficient
alkenes or styrenes.53–55 Herein, we report a visible light photochemical protocol involving planer Co(III)–H
that enables selective H/D exchange of a wide range of alkenes including both terminal
and internal alkenes with readily available deuterium resources (Scheme 1b).56,57 Scheme 1 | Direct deuteration by catalytic H/D exchange strategy.
Results and Discussion
In our initial studies, the deuteration of 2-aryl-1-propene
![]() |
|||
---|---|---|---|
Entry | Variation from Above Conditions | Yield of
|
D (%) |
1 | None | 99 | 97 |
2 | Et3N instead of DIPEA | 87 | 97 |
3 | DBU or DABCO | 93/95 | 0 |
4 |
|
74 | 97 |
5 |
|
81 | 97 |
6 |
|
99 | 0 |
7 | MeOD as solvent | 75 | 95 |
8 | w/o
|
95/99/99 | 0 |
9 | In the dark | 95 | 0 |
![]() |
Under the optimized reaction conditions, the substrate scope was then examined. As
shown in Scheme 2, 2-aryl-1-propenes bearing para-substituents on the aryl ring such as alkyl (
Scheme 2 | Substrate scope. Reaction conditions: 1 (0.2 mmol), 3a (8 mol %), [Ir] (0.5 mol %), DIPEA (20–50 mol %), 0.6 mL of D2O and 1.2 mL MeCN (for condition A); 0.6 mL of MeOD and 2.0 mL DMF (for condition
B); 3d (1 mol %) instead of 3a and 0.5 mL MeOD (for condition C) as the solvent, deaerated and irradiated for 5–36 h
by 30 W blue LED under room temperature. Yield of the isolated product. Deuterium
incorporation percentages were detected by 1H NMR analysis. a 1.0 equiv of DIPEA added. b Reaction under condition C. MeOD, deuterated methanol; MeCN, methyl cyanide; DIPEA,
N,N-diisopropylethylamine; LED, light-emitting diode; 1H NMR, proton nuclear magnetic resonance.
Delightfully, alkyl propene could be applied to the H/D exchange reaction, and the
target deuterated 2-adamantyl propene
The current catalytic protocol could be extended to simple styrenes by using MeOD
in dimethylformamide (DMF) as the deuterium source (condition B, for the optimization
details, see the Supporting Information). The reaction afforded selectively terminal deuterated styrene
When 1-methylene-tetrahydronaphthalene
Scheme 3 | Scope of isomerization and deuteration. Reaction conditions: 4 (0.2 mmol), 3a (8 mol %), [Ir] (0.5 mol %), DIPEA (20∼50 mol %), 0.6 mL of D2O and 1.2 mL MeCN (for condition A); or 3d (1 mol %) instead of 3a and 1.0 mL MeOD (for condition C) as the solvent, deaerated and irradiated for 5–36 h
by 30 W blue LED under room temperature. Yield of the isolated product. Deuterium
incorporation percentages were detected by 1H NMR analysis. DIPEA, N,N-diisopropylethylamine; MeCN, methyl cyanide; MeOD, deuterated methanol; LED, light-emitting
diode; 1H NMR, proton nuclear magnetic resonance.
Next, we challenged the current strategy in the late-stage deuteration of structurally
complex substrates originating from natural products and pharmaceuticals. First, diacetone-D-glucose
derivative and menthol-containing 2-aryl propenes transformed into the corresponding
products with high levels of D-incorporation (Scheme 2, entries 24 and 25). Furthermore, an excellent yield was obtained for β-estradiol
derivative
A gram-scale deuteration of 1,1-diphenyl ethylene
Scheme 4 | Gram-scale reactions. Reactions were performed on a 10–20 mmol scale under corresponding
conditions (see Supporting Information for details). Yield of the isolated product. Deuterium incorporation percentages
were detected by 1H NMR analysis. a 3d as catalyst and MeOD as solvent. MeOD, deuterated
methanol.
Control experiments were conducted to elucidate this deuterium incorporation process.
First, treating (1-(2-phenylcyclopropyl)vinyl)benzene
Scheme 5 | Mechanistic insights. Reactions were performed on a 0.1 mmol scale under corresponding
conditions. Yield of the isolated product. Deuterium incorporation percentages were
detected by 1H NMR analysis. 1H NMR, proton nuclear magnetic resonance.
Based on the previous reports and our mechanism studies,26–32,39,61–63 a catalytic cycle was proposed as illustrated in Scheme 6. [Ir] combined with DIPEA provides an efficient photoreduction system to generate
the key Co(III)–H specie via a sequence electron and proton transfer process. The
Stern–Volmer fluorescence quenching experiments indicated that [Ir(III)]* mainly followed
the oxidative quenching process with cobaloxime
Scheme 6 | Proposed catalytic cycle.
Conclusion
We have developed a visible light-promoted direct deuteration of alkenes based on Co(III)–H medicated reversible H/D exchange strategy. This operationally simple and cost-effective protocol serves as a general and practical approach to various alkenes, including 2-aryl-propenes styrenes and alkyl alkenes, and could enable late-stage D-incorporation with structurally complex molecules. Furthermore, the current strategy is convenient and amenable for scale-up and will accelerate the construction of new deuterated compounds for drug discovery.
Footnote
a A reductive quenching pathway between [IrIII]* and DIPEA could not be excluded, for details see Supporting Information.
Supporting Information
Supporting Information is available and includes general information, substrates synthesis, optimization details, general experimental procedures, compound characterization, mechanistic studies, and details of NMR spectra.
Conflict of Interest
There is no conflict of interest to report.
Funding Information
The authors thank the Natural Science Foundation of China (grant nos. 91956000, 22031006, and 21861132003), Tsinghua University Initiative Scientific Research Program, and Haihe Laboratory of Sustainable Chemical Transformations for financial support.
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