Organophotocatalytic Selective Deuteration of Metabolically Labile Heteroatom Adjacent C-H Bonds via H/D Exchange with D₂O

Abstract

We report a general approach for the efficient deuteration of the metabolically labile α-C-H bonds of widespread amides and amines. Temporary masking the secondary amine group as a carbamate allows for an unprecedented photoredox-HAT promoted α-carbamyl radical formation for efficient H/D exchange with D₂O. The mild protocol delivers structurally diverse α-deuterated secondary amines including ‘privileged’ piperidine and piperazine structures highly regioselectively with excellent levels of deuterium incorporation (up to 100%). Furthermore, we successfully implemented the strategy for α-deuteration of amides, lactams and ureas with high regioselectivity and D-incorporation. Finally, the observed efficient deuteration of secondary alcohol moieties in late-stage modification of complex amine containing pharmaceuticals allows for developing a viable method for the efficient α-deuteration of the important functionality.

Graphical Abstract

The deuterium labeling of organic molecules has broad applications across various domains. The deuterated compounds allows for the investigation of reaction mechanism,¹ and metabolic pathway,² and improving drug’s performance.³ Notable examples are the U.S. Food and Drug Administration (FDA) approval of deuterated drugs deutetrabenazine and deucravacitinib (Scheme 1A).^4,5 The improved drug performance underscores the importance of precision deuteration at the desired sites, particularly α-carbons of heteroatoms ‘N’ and ‘O’, which are prevalent in pharmaceuticals, but tend to be metabolically labile.^{3c, 6}

Scheme 1. Deuterated pharmaceuticals and photoredox-HAT mediated hydrogen–deuterium exchange reactions.

Direct deuteration of these sites via H/D exchange with α-C-H bond offers the one-step accessing deuterated structures without requiring prefunctionalization,^{1, 7} but is challenged by their structural complexity in which these heteroatoms are connected with α-carbons in diverse structural forms. For example, amides, which are prevalent in pharmaceuticals, peptides, proteins, and natural products,⁸ have two metabolic sites - α-carbon of carbonyl and amino groups. Significant efforts have been made on developing deuteration of the α-carbon of the carbonyl via deprotonation-enolation of the acidic α-C-H bond (Scheme 1B).⁹ However, deuteration of the amino site has limited success. As we are aware, Derdau and Kerr used iridium-catalyzed H/D exchange of amides with D₂ (Scheme 1B),¹⁰ but multiple steps are required to synthesize the metal complex. Furthermore, the primary, secondary and tertiary forms of amines have different chemical and physical properties, and thus confer different chemical reactivity, biological properties. Indeed, in α-deuteration of tertiary amines, MacMillan et al.¹¹ developed a photoredox mediated H/D exchange reaction by α-amino radical generated from the deprotonation of amine cation radical (Scheme 1C). In contrast, primary amines are more difficult to be oxidized for the generation of the corresponding amino radical cations because of higher oxidation potential (E_1/2^red = ca. + 1.4 V vs SCE) than tertiary amines (E_1/2^red = ca. + 0.8 V vs SCE).¹² Recently, Derdau¹³ and we¹⁴ achieved selective α-deuteration of primary amines by direct abstracting α-hydrogen by a HAT agent derived radical for H/D exchange (Scheme 1D). However, the α-deuteration of secondary amines remains elusive. Secondary amines have similar oxidation potential (E_1/2^red = ca. + 0.9 V vs SCE) to that of tertiary amines, but much lower than primary amines. In principle, the MacMillan’s strategy could be applied for the deuteration of the amines. Nonetheless, in our exploratory studies on pharmaceuticals troxipide and paroxetine, which contain piperidines, only 61 and 42% deuteration were obtained, respectively (Scheme S1). Despite extensive experimentation, we were unable to enhance the deuteration to a useful level (>90% D). These underscore the challenge and the need for a new strategy for the deuteration of this class of amines. Herein, we develop a general method that enables the deuteration of masked secondary amines, secondary amides and alcohols using D₂O as deuterium source (Scheme 1E).

Since the direct oxidation of secondary amines cannot deliver satisfied deuteration ratio, we propose to convert the amine into a carbamate as α-carbamyl radical precursor. Such modification raises the reduction potential significantly (for example, E_1/2^red = + 1.96 V vs SCE for N-Boc piperidine),¹² thus making it difficult to be directly oxidized. Although the α-carbamyl radical can be generated by an excited photocatalyst (PC) such as acridinium salt Mes-Acr with strong oxidant power (E_1/2^red = + 2.06 – 2.20 V vs SCE)¹⁵ via the deprotonation of the carbamyl cation radical,¹⁶ we proposed a distinct photoredox-HAT co-catalyzed formation of the radical, which could allow for the efficient H/D exchange (Scheme 2). Instead of direct oxidation of the carbamyl, the polarity match between the electrophilic thiyl radical 6 derived from HAT agent Ph₃SiSD(H) 7 (3) and hydridic α-C-H bond of the group enables selective abstraction of the H to give carbamyl radical 4.¹⁷ The resulting radical 4 undergoes D transfer (DAT) with 7, which is produced by a proton exchange with D₂O as a deuterium source, which was proved by H/D exchange experiment (Figure S4) to deliver the deuterated product 2. In this process, strong PC oxidant is not required since the mild photoexcited 4CzIPN* (E_1/2^red = + 1.35 V vs SCE)¹⁸ can efficiently oxidize the thiolate anion 5 (E_1/2^ox = + 0.5 V vs SCE) to give the thiyl radical 6 and regenerate 4CzIPN. The Stern–Volmer fluorescence quenching studies (Figure S5–S8) supported that the thiolate 5 quenched the excited state of the photocatalyst more effectively than the thiol 3 and Boc-carbamate 4.

Scheme 2. Proposed pathway for photoredox-HAT mediated deuteration of α-C-H bond of carbamyl.

To test the feasibility of the proposed process, our investigation began with N-Boc-piperidine as the model substrate. After a systematic exploration of the reaction conditions, an optimized reaction condition was established: 1 mol% 4CzIPN and 30 mol% Ph₃SiSH as HAT catalysts, along with 10 mol% K₃PO₄ as the base, in 4: 1 (v/v) EtOAc/D₂O at rt for 48 h (Table 1, entry 1). The desired product 2a was obtained in 88% yield with a high level of deuterium incorporation (99%). The process also proceeded highly regio- and chemo- selectively. Under the same conditions, i-Pr₃SiSH yielded lower deuterium incorporation (entry 2). This outcome may be attributed to the fact that i-Pr₃SiS^● is less electrophilic than Ph₃SiS^●.¹⁹ Other PCs, such as [Ir(dF(CF₃)ppy)₂(dtbpy)]PF₆ and 3DPA2FBN, were not suitable for this transformation (entries 3 and 4). A swap of D₂O to CD₃OD resulted in no desired product formation (entry 5). When the reaction was conducted in DMA or DMF, no deuterium was formed (entries 6 and 7). The removal of the base gave comparable D incorporation (entry 8). The base might play a role in promoting deprotonation process. Control experiments revealed that the reaction did not proceed in the absence of the photocatalyst, light, or thiol catalyst (entry 9).

Table 1.

Exploration and optimization.

Entry	Variation from the “Standard Conditions”a	D (%)b
1	none	99
2	i-Pr3SiSH as HAT reagent	86
3	[Ir(dF(CF3)ppy)2(dtbpy)]PF6 as PC	16
4	3DPA2FBN as PC	13
5	CD3OD as deuterium source	0
6	DMA as solvent	0
7	DMF as solvent	0
8	No base	97
9	No thiol, No PC or light	0

^aStandard conditions: Unless specified, a mixture of 1a (0.2 mmol), D₂O (0.5 mL), 4CzIPN (1 mol%), K₃PO₄ (10 mol%) and Ph₃SiSH (3, 30 mol%) in anhydrous EtOAc (2.0 mL) was irradiated with 40 W Kessil blue LEDs in a N₂ atmosphere at rt for 48 h.

^bDetermined by ¹H NMR.

With the optimal conditions in hand, a series of masked amines and amides were probed for the process. The investigation reveals that the process severs as a general approach for the deuteration of a broad range of amine derived structures including carbamates, amides and ureas with high level of deuterium incorporation and good regio-selectivity. The Boc protecting group can be removed by standard reaction conditions with HCl without erosion of incorporated deuterium (see SI). In addition to Boc, benzyloxycarbonyl (Cbz, 2b) is effective in this transformation with pyrrolidine (Scheme 3A). It is noted that piperidine is a ‘privileged’ structure, which exists in at least 72 FDA approved drugs.²⁰ The protocol allows for the direct, selective engineering deuterium into the piperidine ring in various pharmaceuticals including Boc protected flecainide (100%, 2c), troxipide (97%, 2d), paroxetine (84%, 2e) and anabasine (91%, 2f). The information of different incorporation percentage of ‘D’ atom at various positions may be useful for drug metabolism studies. It seems that steric effect plays a role in contributing to lower deuteration as is seen in anabasine. Interestingly, deuteration is also facially selective, observed in paroxetine (2e). Direct installation of deuterium into the piperidine ring in icaridin is achieved (2g). Encouraged by the results of the studies, we tuned our attention to piperazine, which is also often present in pharmaceuticals (59 drugs containing the moiety²⁰). Excellent, full deuteration (97%) was achieved for bis-Boc masked piperazine. We also probed the deuteration reaction on two “N” atoms bearing two discriminative groups. It was found that they exhibited different reactivities towards deuteration. Comparable level deuteration was observed with N-phenyl (2i). Further, when the reaction was carried out without base K₃PO₄, exclusive deuteration occurred at the N-phenyl α-carbon (100%, 2j). We believe two distinct reaction pathways are in operation. The direct deprotonation of the amine cation radical with the N-phenyl proceeds more efficiently than the thiyl radical mediated HAT with the N-Boc site. Not requiring a base allows the method to be employed in the presence of acidic functional groups with minimal threat of competing SET events. Furthermore, unexpectedly, when N-Boc 4-hydroxypiperidine was used as a substrate, excellent deuteration proceeded at the α-carbon of the secondary alcohol site (100%, 2k) compared with the primary alcohol (60%) in 2g. The method is also amenable to 5-, 7- and 4-membered (96%–100%, 2l-2n) ring and acyclic structures (100%, 2o) to deliver high to excellent deuterium incorporation.

Scheme 3. Carbamate, amide and urea substrate scope.

^aReaction conditions: unless otherwise noted, a mixture of 1 (0.2 mmol), D₂O (0.5 mL), 4CzIPN (1 mol%), K₃PO₄ (10 mol%) and triphenylsilanethiol (30 mol%) in anhydrous EtOAc (2.0 mL) was irradiated with 40 W Kessil blue LEDs in a N₂ atmosphere at rt for 48 h. ^bWithout base. ^cAnother 30 mol% of triphenylsilanethiol added after 24 h. ^d4 mL of EtOAc used.

The similar reactivity between carbamates and amides promoted us to explore the strategy for the deuteration of the α-amino carbon of tertiary amides. The realization of the process would provide an approach, which complementary to that reported by Atzrodt, Derdau, et al for secondary amides.^10a We demonstrated the protocol as a viable manifold for the deuteration of a variety of tertiary amides with high regioselectivity and excellent deuterium incorporation (Scheme 3B). For cyclic secondary amine derived amides, the variation of the acyl forms and ring sizes does not affect the deuteration efficiency (2p, 2q and 2r, 2s). Heterocyclic furan (2t) and thiophene (2u) performed successfully in this protocol (96–97% D%). Full deuteration of drug sunifiram was possible despite relatively low deuteration (76%, 2v). Our method could be applied for structurally diverse lactams, which are important structures in numerous bioactive molecules.²¹ High deuteration efficiencies are obtained for commonly seen five and six-membered lactams (90%–100%, 2w-2ac) including lenalidomide (93%, 2ad), a popular CRBN E3 ligase ligand in PROTAC molecules.²² Finally, we also validated acyclic amides including drug nikethamide (76%, 2af) and insect repellent DEET (93%, 2ag), ureas (96%–98%, 2ah-2ai) and methyl amides (Figure S3) as viable substrates for the process.

The observed excellent deuteration (100 D%) of α-hydroxy C-H bond in substrate 2k prompted us to explore alcohol substrates.²³ Under the same reaction condition, a series of alcohols were probed for the process. Acyclic secondary alcohols yielded their respective deuterated alcohols 3a-l with nearly uniformly excellent deuterium incorporation except 3a (93%) (Scheme 4). Functionalities such as aromatic C–F (3f), C–Cl (3c-3e), and C–Br (3g) at different positions were well-compatible with this transformation. Aromatic alkoxyl groups were well-tolerated to give their corresponding α-deuterated alcohols (3h, 3i) with excellent deuterium incorporation (100%) as well. CF₃ and CF₃O groups, often known to improve the performance of pharmaceutical leads in drug discovery,²⁴ were compatible with the protocol, exhibiting excellent deuterium incorporation (3j, 3k, 100%). In some cases (3a, 3d-3f, 3h and 3i), deuteration of the benzylic position is observed. It appears that the electronic effect of the substituents on the phenyl ring plays a role in deuteration chemo-selectivity for these substrates. Electron-withdrawing groups (3c-3f, 3j and 33k) tend to give less degree of deuteration than those donating (3h and 3i). It is believed that the donating groups can stabilize the resulting benzylic radicals. The polarity match between the electrophilic thiyl radical and electron rich benzylic C-H bonds also facilitates the radical formation.^1b,25 In addition to these functional groups, the deuteration reaction can tolerate other commonly used functional groups, such as ester (3l, 3r), sulfone (3n), and acetal (3p). Interestingly, significant deuteration occurred with the acetal group in compound 3p. Beside acyclic secondary alcohols, cyclic structures are also amenable giving high deuteration (76–100%) (3m-3s). The strained cyclobutane can be sustained under the mild reaction conditions (3r). We also probed primary alcohols. Under the standard reaction conditions, moderate level of deuteation was obtained (eg, 3t). Optimizing reaction conditions achieved 83% deuterium incorporation (Table S1).

Scheme 4. Alcohol substrates scope.

^aReaction conditions: Unless otherwise noted, a mixture of 1a (0.2 mmol), D₂O (0.5 mL), 4CzIPN (1 mol%), K₃PO₄ (10 mol%) and triphenylsilanethiol (30 mol%) in anhydrous EtOAc (2.0 mL) was irradiated with 40 W Kessil blue LEDs in a N₂ atmosphere at rt for 48 h. ^bAnother 30 mol% of thiol added after 24 h. ^c4 mL of EtOAc used.

Finally, we investigated the practical applicability of the methodology in the late-stage deuteration of natural products and pharmaceutics. The secondary alcohol in the steroid hormone (3u, 3v) underwent a smooth reaction, yielding the desired products with high deuterium ratio (97–100%). Pharmaceuticals including proxyphylline (3w, d₁), bucetin (3x, d_1.7), demonstrated compatibility with this reaction. Additionally, protected amino acids and saccharides such as Boc-Thr-OMe (3y) and diacetone-D-glucose (3z), proved to be appropriate substrates (94–96% D), providing access to the desired adducts in synthetically useful yields. It is worth noting that 3u, 3v, 3y will undergo racemization after the reaction, except for 3z.

Driven by the metabolically lability of the N-α-C-H bonds of widespread amides and amines in pharmaceuticals, we have successfully devised a general and mild photochemical approach for the selective deuteration of the hydridic C-H bonds. This process employs the cost-effective organophotocatalyst 4CzIPN with a thiol HAT as co-catalyst and D₂O as deuterium source, enabling highly regio-selective installation of deuterium to structurally diverse secondary amides, ureas and temporary masked secondary amines with an excellent level of deuterium incorporation (up to 100%). Furthermore, we successfully implemented the strategy for the α-deuteration of secondary amides including lactams and ureas and secondary alcohols with high regioselectivity and D-incorporation. It is expected that the method and the deuterated structures will provide a viable means for drug metabolism studies and drug discovery.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.