Facile and general electrochemical deuteration of unactivated alkyl halides

Herein, a facile and general electroreductive deuteration of unactivated alkyl halides (X = Cl, Br, I) or pseudo-halides (X = OMs) using D2O as the economical deuterium source was reported. In addition to primary and secondary alkyl halides, sterically hindered tertiary chlorides also work very well, affording the target deuterodehalogenated products with excellent efficiency and deuterium incorporation. More than 60 examples are provided, including late-stage dehalogenative deuteration of natural products, pharmaceuticals, and their derivatives, all with excellent deuterium incorporation (up to 99% D), demonstrating the potential utility of the developed method in organic synthesis. Furthermore, the method does not require external catalysts and tolerates high current, showing possible use in industrial applications.

Herein, we report our effort in developing this dehalogenative deuteration of unactivated alkyl halides, making it a general, economically and environmentally approach through electrochemistry (Fig. 1b). Notable features of this strategy include: (a) broad scope and excellent D-incorporation, including unsatisfactory tertiary chlorides in previous studies, (b) D 2 O as an economical deuterium source, (c) external catalysts-free and the use of electricity as an environmentally-friendly reductant, (d) catalytic amounts of electrolyte and no sacrificial anode, (e) tolerance with high current (up to 500 mA), making it potentially applicable in industrial production, (f) late-stage deuteration of complex natural products and drug derivatives.

Result
Optimization of electroreductive dehalogenation deuteration. Initially, we started our investigation using D 2 O as the economical deuterium source, unactivated alkyl bromide 1 as the substrate, and DMF as the solvent. After careful optimization, the corresponding product 2 was finally obtained in excellent yield and D-incorporation under 30 mA constant current with carbon felt (CF) and Pb as electrodes, in the presence of DIPEA as well as the catalytic amount of TBAI as the electrolyte in an undivided cell at room temperature for 10 hours (Table 1, entry 1). The control experiment revealed that the electricity was essential for the observed reactivity (entry 2). The reaction still worked well in the absence of DIPEA with 60% yield (entry 3), which showed that DMF could also be oxidized on anode. In addition, reducing the current to 20 mA led to lower yield and D-incorporation (entry 4). Surprisingly, the desired transformation proceeded smoothly with a higher current at 100 mA in 60 min (entry 5). It is notable that even under an extremely high current at 500 mA, the reaction still proceeded well to give the target product in only 15 min, without diminishing the yield and D-labeling level, showing potential usage in industrial amplification (entry 6). DMSO was proven to be a poor solvent for this transformation (entry 7), while MeCN gave a comparable yield but lower D-incorporation (entry 8). The reaction also worked well when n Bu 4 NBF 4 was used as the electrolyte, albeit in a slightly lower yield (entry 9). Finally, different electrodes were tested but none of the results surpassed entry 1 (entries 10-13).
Next, we turned our attention to a more challenging target, unactivated alkyl chlorides (Fig. 2C). To our delight, primary alkyl chlorides took part in the reaction with a broad range of functionalities and showed similar results to alkyl bromides, yielding the corresponding products in good efficiency and D-incorporation (33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43)(44)(45). Furthermore, other alkyl chlorides, especially tertiary substrates that did not work well in previous studies 24 , yielded the desired products unquestionably in satisfactory D-incorporation using this method (46)(47)(48)(49). To further demonstrate the generality, and environmentallyfriendly nature of the developed method, as well as its application in bioactive molecules and drug discovery, the late-stage deuteration was conducted using a series of natural products, pharmaceuticals, and their derivatives (Fig. 2D). The dehalogenative deuteration of pharmaceuticals including Estrone (50), Pregnenolone (52), DL-Menthol (53), Phytol (57), and Triclosan (58) was successfully achieved in 77-83% yield with 97-98% D-incorporation. Pharmaceutical intermediates ethynyl estradiol (51) and dihydro cuminyl alcohol (54) containing unsaturated bonds can also be used in this protocol. Ibuprofen (55) and naproxen (56), which are commonly used for the ease of pain, afforded the corresponding deuterated compounds in good yields (67%, 88%) with excellent deuterium incorporation (both 98% D). Furthermore, amino acid and glucose derivatives that widely exist in organisms can also be deuterated smoothly with high levels of D-incorporation (59−64, 97−99%). In addition, the tolerance of the reaction system to tertiary chlorides can be extended to complex substrates (65,66). Electrochemical dehalogenative deuteration of alkyl bromides with 500 mA current. As high current density has a key impact on the amplification of reaction scales, we further explored the practicability of some representative substrates at a current of 500 mA. To our delight, the reactions were complete during a much shorter reaction time (in only 15 minutes). Simple alkyl bromides with a variety of functional groups (2, 16, 23, and 26) as well as complex alkyl bromides derived from ibuprofen (55), L-phenylalanine (59), Diacetonefructose (61), and D-mannofuranose (63) were all amenable. level of D-incorporation has not changed at all (Fig. 3). Alkyl chloride was also well tolerated and the corresponding product 37 was obtained in 15 min. In addition, this electrochemical reduction protocol could be applied to the gram-scale preparation of deuterated product 8 under 500 mA constant current (Fig. 4a).
Mechanistic studies. Several mechanistic studies were conducted to gain insight into the reaction mechanism. Firstly, 2 equiv. of TEMPO was added to the reaction system and in the absence of D 2 O, the TEMPO adduct was obtained in 64% yield (67), together with hydrodehalogenated product 2-H in 28% yield (Fig. 4b, eq 1). Then, repeat the reaction with 2 equiv. of D 2 O resulted in the TEMPO adduct in 21% yield and deuterodehalogenated product 2 in 75% yield with 55% D-incorporation (Fig. 4b, eq 2). These results indicated that the alkyl radical might be involved in the electrochemical system. Then, we sought to gain further insight into the reaction mechanism through cyclic voltammetry (CV) experiments. A reduction peak of alkyl bromides 1 at −3.40 V (vs Ag/Ag + in CH 3 CN) was observed (Fig. 5a, the red line), and no obvious reduction peak of D 2 O was observed (Fig. 5a). At the same time, the oxidation potential of TBAI (Fig. 5b, pink line, 1.0 V) was very close to that of DIPEA (Fig. 5b, blue line, 1.1 V). Thus we inferred that both TBAI and DIPEA might act as the electron donors in this reaction. Notably, the anodic oxidation of alkyl bromide 1 was also indicated with higher potential than that of DIPEA (green line). This might explain the improved yield once external DIPEA was added by inhibiting the undesired oxidation or decomposition of the starting material (Table 1, entries 1 and  3). In addition, during the reaction system without TBAI, an increase in current can be clearly observed, indicating the electrolysis of DIPEA incorporated with formed bromo anion may further generate conductive substances (Fig. 5c).
Based on the mechanistic studies and previous literature 5, 48 , a plausible mechanism is proposed (Fig. 6). The reaction was initiated by the anodic oxidation of DIPEA and/or TBAI or DMF and the direct reduction of alkyl halides on the cathode to form the corresponding alkyl radical I and halide anions. The radical I continued to be reduced to an alkyl anion II at the cathode. Finally, intermediate II reacted with deuterated water to provide the expected product III. At the same time, iminium ion intermediate IV generated from DIPEA on the anode might combine with the released halogen anions to form ammonium salt, a possible conductive species.

Discussion
In conclusion, we have developed a general dehalogenative deuteration of unactivated alkyl (pseudo)halides (X = Cl, Br, I, OMs) driven by electrochemical force with high reactivity, selectivity, and excellent D-incorporation. D 2 O was used as an efficient and economical deuterium source. A wide range of unactivated alkyl halides containing diverse functionalities are well tolerated, as well as the extension to late stage deuteration of natural products, pharmaceuticals, and their derivatives. We envisage that the native features, including tolerance of high current and environmentally-friendly conditions, will bring further opportunity in its application to organic synthesis, drug discovery and modification, and even industrial production. Further electrochemical transformations of unactivated alkyl halides are currently ongoing in our laboratory.

Methods
General procedure of the electrochemical dehalogenative deuteration of alkyl bromide, iodide or pseudo-halides. The electrocatalysis was carried out in an undivided cell with a carbon felt anode (10 mm × 15 mm × 5 mm) and a lead cathode (10 mm × 15 mm × 0.3 mm). To a 15 mL pre-dried undivided electrochemical cell (15 mL) equipped with a magnetic bar were added alkyl bromide, iodide or pseudo-halides (0.5 mmol, 1 equiv), TBAI (36.9 mg, 0.1 mmol, 20 mol%), and DMF (5.0 mL). Then D 2 O (25 mmol, 50 equiv) and DIPEA (1.5 mmol, 3.0 equiv) were added via a syringe. The electrocatalysis was performed at room temperature with a constant current of 30 mA maintained for 10 h. The carbon felt anode was washed with EtOAc (3 × 5 mL) in an ultrasonic bath. H 2 O (20 mL) was added to the system, and the resulting mixture was extracted with EtOAc (3 ×20 mL). The combined organic phase was dried with anhydrous Na 2 SO 4 , filtered, and concentrated in vacuo. The crude product was purified by column chromatography to furnish the desired product.
General procedure of the electrochemical dehalogenative deuteration of alkyl chlorides. The electrocatalysis was carried out in an undivided cell with a graphite felt anode (10 mm × 15 mm × 5 mm) and a lead cathode (10 mm × 15 mm × 0.3 mm). To a 15 mL pre-dried undivided electrochemical cell (15 mL) equipped with a magnetic bar were added alkyl chloride (0.5 mmol, 1 equiv), TBAI (36.9 mg, 0.1 mmol, 20 mol%) and DMF (5.0 mL). Then D 2 O (25 mmol, 50 equiv) and DIPEA (1.5 mmol, 3.0 equiv) were added via a syringe. The electrocatalysis was performed at room temperature with a constant current of 50 mA maintained for 10 h. The carbon felt anode was washed with EtOAc (3 × 5 mL) in an ultrasonic bath. H 2 O (20 mL) was added to the system, and the resulting mixture was extracted with EtOAc (3 × 20 mL). The combined organic phase was dried with anhydrous Na 2 SO 4 , filtered, and concentrated in vacuo. The crude product was purified by column chromatography to furnish the desired product.
General procedure of the electrochemical dehalogenative deuteration of alkyl bromides and chloride with 500 mA current. The electrocatalysis was carried out in an undivided cell with a carbon felt anode (or graphite felt) (10 mm × 15 mm × 5 mm) and a lead cathode (10 mm × 15 mm × 0.3 mm). To a 15 mL pre-dried undivided electrochemical cell (15 mL) equipped with a magnetic bar were added alkyl bromide or chloride (0.5 mmol, 1 equiv), TBAI (36.9 mg, 0.1 mmol, 20 mol%) and DMF (5.0 mL). Then D 2 O (25 mmol, 50 equiv) and DIPEA (1.5 mmol, 3.0 equiv) were added via a syringe. The electrocatalysis was performed at room temperature with a constant current of 500 mA maintained for 15 min (reaction system exothermic). The carbon felt anode was washed with EtOAc (3 × 5 mL) in an ultrasonic bath. H 2 O (20 mL) was added to the system, and the resulting mixture was extracted with EtOAc (3 × 20 mL). The combined organic phase was dried with anhydrous Na 2 SO 4 , filtered, and concentrated in vacuo. The crude product was purified by column chromatography to furnish the desired product.
Gram-scale synthesis of 8. To an undivided reaction flask (diameter: 40 mm, length: 130 mm, volume: 200 mL) equipped with a teflon-coated magnetic stirring bar and teflon cap, a carbon felt anode (25 mm × 50 mm × 5 mm), and a lead cathode (25 mm × 50 mm × 0.3 mm) were added (4-bromobutoxy)benzene (3.44 g, 15 mmol), TBAI (1.11 g, 3.0 mmol), DMF (100 mL), D 2 O (15 mL, 0.75 mol) and DIPEA (5.80 g, 45 mmol). The electrocatalysis was performed at room temperature with a constant current of 500 mA maintained for 4 h (the reaction system was exothermic). The carbon felt anode was washed with EtOAc (3 × 20 mL) in an ultrasonic bath. H 2 O (200 mL) was added to the system, and the resulting mixture was extracted with EtOAc (3 × 200 mL). The combined organic phase was dried with anhydrous Na 2 SO 4 , filtered, and concentrated in vacuo. The crude product was purified by column chromatography to furnish the desired product.  Fig. 6 A plausible mechanism. Cathodic reactions converting organic halides to deuterated product. Anodic oxidation of DIPEA and/or TBAI or DMF was occurred on anode.
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