Rhodaelectro-catalyzed access to chromones via formyl C–H activation towards peptide electro-labeling

Chromones represent a privileged scaffold in medicinal chemistry and are an omnipresent structural motif in natural products. Chemically encoded non-natural peptidomimetics feature improved stability towards enzymatic degradation, cell permeability and binding affinity, translating into a considerable impact on pharmaceutical industry. Herein, a strategy for the sustainable assembly of chromones via electro-formyl C–H activation is presented. The rational design of the rhodaelectro-catalysis is guided by detailed mechanistic insights and provides versatile access to tyrosine-based fluorogenic peptidomimetics.

In this work, we present an electro-formyl C-H activation via rhodaelectro catalysis for the assembly of substituted chromones to provide sustainable access to amino acid Fig. 1 Motivation, rationale, and development of rhodaelectro-catalyzed annulation of benzaldehydes. a Chromones as a privileged motive in pharmaceutical and bioactive compounds. b Strategy to access chromones via rhodaelectro catalysis and its use for electro-peptide labeling. c Synthesis of rhodium complex Rh-I and investigations of its redox properties by cyclic voltammetry in CH 2 Cl 2 with nBu 4 NPF 6 (0.2 M). d Reaction development, 0.25-0.50 mmol scale, 4.0-8.0 mL solvent, isolated yields. a With nBu 4 NPF 6 (0.1 M) for 4 h. Cp* pentamethylcyclopentadienyl, NaOPiv sodium pivalate, Fc ferrocene, GF graphite felt electrode, CCE constant current electrolysis, CPE constant potential electrolysis, tAmylOH 2-methyl-2-butanol. chromone hybrids and to label peptides 35,36 through metallaelectro catalysis.

Results and discussion
To put our hypothesis into practice, we designed intermediates that feature lower oxidation potentials than the sensitive aldehyde substrates themselves (Fig. 1b). To this end, a stoichiometric transformation of hydroxybenzaldehyde 1, alkyne 2 and [Cp*RhCl 2 ] 2 in the presence of base delivered rhodium(I) complex Rh-I (Fig. 1c), which was unambiguously characterized by X-ray diffraction analysis. With the proposed key intermediates in hand, we probed their redox properties towards an oxidation manifold under electrocatalytic conditions. Studies by cyclic voltammetry revealed that the complex Rh-I underwent irreversible oxidation to Rh(III) at E p = -0.11 V vs. Fc 0/+ and therefore exhibited a considerably lower oxidation potential than benzaldehyde 1 (E p = 1.68 V vs. Fc 0/+ ). With respect to an oxidatively induced reductive elimination from a rhodium(III/IV)species 37,38 , calculations by means of DFT at the PW6B95-D3(BJ)/ def2-QZVP+SMD(acetonitrile)//PBE0-D3(BJ)/def2-SVP level of theory at 298.15 K revealed an oxidation potential of E 1/2 = 0.48 V vs. Fc 0/+ of the corresponding seven-membered rhoda(III)-cycle, which is in proximity to the experimentally determined, oxidation potentials of related species at E p/2 = 0.68 V vs. Fc 0/+37, 38 . Further computational studies of the electro-formyl C-H activation were supportive of a kinetically, favorable oxidatively induced reductive elimination ( Supplementary Fig. 21).
The isolation and electroanalytical characterization of the key intermediate set the stage for studies on the electrocatalysis, initially with a constant potential of 1.0 V vs. Fc 0/+ , employing [Cp*RhCl 2 ] 2 as the catalyst and NaOPiv as the base to ensure an oxidatively induced reductive elimination. Hence, the desired chromone was obtained in 57% yield (Fig. 1d, entry 2), which is in line with our initial hypothesis for the rhodaelectro-catalyzed formyl activation. Further optimization demonstrated that the reaction furnished chromone 5 likewise under user-friendly galvanostatic conditions, with tAmOH/water (3:1) as the reaction medium avoiding additional electrolytes (Fig. 1d, entry 1). Control experiments revealed the crucial role of the rhodium precatalyst (entry 3). Importantly, the reaction was also viable with commercial equipment (Fig. 1d, entry 6). To test the role of electricity in the rhodaelectro-catalyzed formyl C-H activation, we performed an in-operando monitoring of the catalysis at different currents by in situ 1 H-NMR spectroscopy (Fig. 2a).
Indeed, the reaction rates are strongly dependent on the applied currents, indicating a turnover limiting electron transfer being operative. Additionally, an on/off experiment was conducted, clearly reflecting the key role of electricity for efficient catalyst turnover. To probe the catalysts mode of action, an intermolecular competition experiment between differently substituted salicylic aldehydes 6/7, revealed an inherent higher reactivity of the electron-rich substrate (Fig. 2b), being suggestive of a base-assisted internal electrophilic substitution-type (BIES) manifold 39 . A minor kinetic isotope effect was observed, again being in line with a rate limiting reoxidation (Fig. 2c).
To benchmark the presented electro-catalyzed formyl C-H activation, we compared its performance with challenging substrates 10, such as electron-deficient diphenylacetylenes and alkynes with aliphatic substituents. Thus, the efficacy towards the formation of products 12-15 was found to be uniquely effective under the electrocatalytic conditions, as compared to Cu(OAc) 2 as the oxidant, highlighting the superior performance of the  (Fig. 3a). Additionally, the scalability of the rhodaelectro-catalyzed transformation was highlighted with a multigram scale synthesis with reduced catalyst loading (Fig. 3b).
With the optimized conditions in hand, we explored the scope of salicylic aldehydes 16 (Fig. 4a). Overall, differently substituted aldehydes efficiently furnished the desired products 18-36. Especially redox sensitive groups, such as bromo-, iodo-, and thioether substituents were fully tolerated and delivered the corresponding products 27-29. The electron-deficient substrate with an ester substituent under water-free conditions, as well as salicylic aldehyde with an amine functionality were selectively converted to the corresponding chromones 30 and 31, respectively. Even bulky disubstituted aldehydes delivered the desired products 34 and 35 in very good yields. To our delight, also the estrone derivative was converted to the product 37.
Subsequently, a wealth of unsymmetrically substituted alkynes were efficiently converted to the desired products 46-57 (Fig. 4b). Thereby, terminal as well as keto-and amidosubstituted alkynes underwent the rhodaelectro catalysis with high efficacy (52-56). As to the regioselectivity of unsymmetrical alkynes, the alkyne substrates furnished chromones 49-55 with the aryl motif proximal to the oxygen-heteroatom. Under otherwise identical reaction conditions, alkynes with nitrile, ester or free acid groups provided less satisfactory results. It is noteworthy that hydroxyheptyne (10w) exclusively yielded the regioisomer 57. In order to gain insights into the origin of the regioselectivity for the synthesis of chromone 57, DFT calculations were carried out for the migratory insertion step at the PW6B95-D3(BJ)/def2-QZVP+SMD(methanol)//PBE0-D3(BJ)/ def2-SVP level of theory ( Supplementary Fig. 22). The transition state leading to regioisomer 57 with the hydroxyl distal to the carbonyl group of the substrate is favored by 2.4 kcal mol −1 , which can be attributed to favorable hydrogen bonding interactions with the hydroxyl group in the transition state structure (TS(1-2), Supplementary Fig. 22). Likewise, the regioselectivity of the electrocatalytic C-H activation with 4-methyl-4'-trifluoromethyltolane could be rationalized by DFT computation (Fig. 4c). The calculated regioselectivity was in good agreement with the experimental observations. Here, noncovalent interactions were found to be of minor relevance.
Chemically encoded peptidomimetics reduce enzymatic degradation and feature superior binding affinities, cell permeability, and pharmacokinetics 40,41 . However, the functionalization of structurally complex peptides mostly rely on terminal peptides, azide-based click chemistry or on the innate reactivity of cysteine [42][43][44] . Given the practical importance of late-stage peptide diversifications, we became attracted to tyrosine modifications  through rhodaelectro catalysis to access tyrosine-derived fluorescent amino acids. Indeed, tyrosines were chemo-selectively annulated with tolane and naphthalene derived alkynes furnishing the desired products 60-62. Next, we probed dipeptides to explore rhodaelectro-catalytic site-specific labeling (Fig. 5a). To our delight, a broad variety of dipeptides was efficiently converted to the corresponding products 63-68. Notably, even oxidation-sensitive serine (65) and methionine (66) containing peptides were transformed to the desired products. Furthermore, potentially coordinating dipeptides with unprotected tryptophan, or tyrosine regioselectively provided the desired products 67 and 68.
We explored an improvement of the photoelectronic properties of the thus-obtained labels by a late-stage annulation. The aryl moieties were selectively transformed into π-extended peptide labels in a photoelectrochemical process to obtain the desired products (Fig. 5b). Labels 69 and 70 demonstrated improved photoelectronic properties in comparison to the corresponding diaryl precursors 60 and 62 ( Fig. 5c and Supplementary Table 2).
Since compound 70 exhibited intense fluorescence at 458 nm, it bears considerable potential as a fluorogenic probe 45,46 .
Finally, we probed our strategy for the functionalization of structurally complex oligopeptide 71. Indeed, peptide 72 was obtained in excellent yield, highlighting the unique power of the rhodaelectro-catalytic labeling strategy (Fig. 5d).
In summary, we have reported on a rhodaelectro-catalyzed transformation of hydroxy-benzaldehydes by electrochemical formyl C-H activation, featuring scalability, high functional group tolerance, and improved efficiency in comparison to chemical oxidants. The strategy proved applicable to the functionalization of tyrosine derivatives, enabling difficult to perform, site-selective electro-labeling of amino acids and peptides by formyl C-H activation. A mediated photoelectrochemical oxidation allowed for an enhancement of fluorescence properties of the thus-obtained amino acids. The rhodaelectro catalysis was inspired by in-depth mechanistic insights, through the isolation and electroanalytical characterization of key intermediates, providing strong support for an oxidation-induced reductive elimination.

Data availability
The authors declare that the data supporting the findings of this study are available within the paper and its supplementary information files. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 2046225, 2046228-2046229, 2046502-2046508. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. All other requests for materials and information should be addressed to the corresponding authors. Source data are provided with this paper.