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Mild, Redox‐Neutral Formylation of Aryl Chlorides Through the Photocatalytic Generation of Chlorine Radicals

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Mild, Redox‐Neutral Formylation of Aryl Chlorides Through the Photocatalytic Generation of Chlorine Radicals Angewandte Zuschriften Chemie Deutsche Ausgabe:DOI:10.1002/ange.201702079 C HFunctionalization À Internationale Ausgabe:DOI:10.1002/anie.201702079 Mild, Redox-Neutral Formylation of Aryl Chlorides through the Photocatalytic GenerationofChlorine Radicals Matthew K. Nielsen+,Benjamin J. Shields+,Junyi Liu, Michael J. Williams,Michael J. Zacuto, and Abigail G. Doyle* Abstract: We report aredox-neutral formylation of aryl chlorides that proceeds through selective 2-functionalization of 1,3-dioxolane through nickel and photoredox catalysis.This scalable benchtop approach provides adistinct advantage over traditional reductive carbonylation in that no carbon mon- oxide,pressurized gas,orstoichiometric reductant is employed. The mild conditions give unprecedented scope from abundant and complex aryl chloride starting materials. Aromatic aldehydes are among the most versatile inter- mediates in the synthesis of pharmaceuticals,fragrances,fine chemicals,and natural products.[1] Indeed, the functional group can be rapidly elaborated through an ever growing host of C Cand C Xbond forming reactions.Despite the À À ubiquitous application of aryl aldehydes,synthetic methods for their preparation are limited. Classical approaches such as the Vilsmeier–Haack and Duff reactions proceed through electrophilic aromatic substitution;therefore their reactivity and regioselectivity are endogenous to the particular aryl substrate (Figure 1A).[2] As an alternative,traditional organ- Figure 1. A) Prior art in aryl formylation.B)Proposed redox-neutral formylation of aryl chlorides through C Hactivation of 1,3-dioxolane. ometallic methods,such as the addition of Grignard reagents À to DMF at cryogenic temperatures,impart regiocontrol at the expense of functional group compatibility.[3] To date,the most are incapable of employing aryl chlorides,[8] which are by far general method for the synthesis of aryl aldehydes is the the most abundant and diverse class of aryl halides.[9] palladium-catalyzed reductive carbonylation of aryl iodides Recently,our group[10] and others reported directing- and bromides,first reported by Heck in 1974 under 100 atm of group-free Csp3 Hcross-coupling platforms capable of À syngas (1:1 H /CO) at 1508C.[4] Although this procedure has carrying out C Harylations at room temperature.Incontrast 2 À been performed on amulti-ton scale,[5] the conditions are not to systems reported by MacMillan[11] and Molander,[12] which ideal for benchtop synthesis owing to the hazards associated primarily employ aryl bromides and iodides,our method with carbon monoxide and the specialized equipment utilizes aryl chlorides by design. Here,wedemonstrate that required for handling high-pressure syngas.Todevelop user- this manifold can be leveraged to enable redox-neutral friendly methods,several laboratories have reported alter- formylation through selective 2-arylation of the inexpensive native condensed-phase reductants[6] and CO surrogates[7] and abundant solvent 1,3-dioxolane with aryl chlorides such as the crystalline N-formylsaccharin.[7b] Nevertheless, followed by amild acidic workup (Figure 1B). Importantly reductive formylations are still limited to simple arenes owing this strategy overcomes the challenges of positional selectivity to the general requirements of high reaction temperatures and functional-group compatibility associated with classical and stoichiometric reducing agents.Moreover,most methods formylation reactions and obviates the need for the gaseous reagents,stoichiometric reductants,and high reaction temper- atures that are required for reductive carbonylation. [+] [+] [*] M. K. Nielsen, B. J. Shields, J. Liu, Prof. A. G. Doyle We envisioned that oxidative addition of Ni0 (Figure 2, 1) Department of Chemistry,Princeton University into an aryl chloride would produce NiII intermediate 2. 120 Washington Road, Princeton, NJ 08544 (USA) Simultaneously,visible-light irradiation of IrIII photocatalyst E-mail:[email protected] 3 would generate triplet excited *IrIII (4; t = 2.3 ms, *E = Dr.M.J.Williams, Dr.M.J.Zacuto 0 1/2 Drug Substance Development, Celgene Corporation 1.21 Vvs. SCE in MeCN) which could engage 2 (EP = 556 Morris Ave.,Summit, NJ 07901 (USA) 0.85 Vvs. SCE in THF) in photoinduced electron transfer III [10,13] [+]These authors contributed equally to this work. to give presumed Ni intermediate 5. According to our [10] II Supportinginformation and the ORCID identification number(s) for prior studies, photolysis of 5 produces Ni species 6 and the author(s) of this article can be found under: achlorine radical capable of abstracting ahydrogen atom https://doi.org/10.1002/anie.201702079. from 1,3-dioxolane.Aconcern in developing this reaction was Angew.Chem. 2017, 129,7297 –7300 2017 Wiley-VCH VerlagGmbH &Co. KGaA,Weinheim 7297 Angewandte Zuschriften Chemie that 1,3-dioxolane has two sets of chemically distinct a-oxy C Hbonds.Thermodynamic computations imply amoderate À driving force for 2-functionalization (DBDFE = 1.4 kcal 1 molÀ ), which we expected could be influenced by the Ni catalyst that accepts the resultant carbon-centered radical. Reductive elimination from NiIII species 7 would then give the aryldioxolane acetal, and hydrolytic workup would furnish the desired aryl aldehyde product. Finally,toclose both cycles,NiI species 8 (E [NiII/(0)] = 1.2 Vvs. SCE in DMF) 1/2 À can be reduced by IrII species 9 (E = 1.37 Vvs. SCE in 1/2 À MeCN).[13, 14] We began studying the proposed formylation reaction with emphasis on the development of auser-friendly and scalable method. We were pleased to find that the commercial bench-stable precatalyst systems Ir[dF(CF3)ppy]2- [15] (dtbbpy)PF6 and NiCl2·DME with 4,4’-di-t-butyl-2,2’-bipyr- idine ligand (dtbbpy), combined with two equivalents of potassium phosphate under irradiation with blue LEDs enabled the selective 2-functionalization of 1,3-dioxolane Figure 2. Proposed catalyticcycle. with numerous aryl chlorides.Asubsequent mild acidic workup gave the desired aryl (Figure 3, 10–18), heteroaryl Figure 3. Formylationofaryl chlorides:substrate scope. Yields are an average of two runs on 0.25 mmol scale. For 23–34,the name of the parent compound is listed. [a] Acid hydrolysis omitted. [b] Benchtop setup;single run. [c] Aldehyde does not form under hydrolysis conditions. [d] 2.3:1 d.r.(conserved from starting material). 7298 www.angewandte.de 2017Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Angew.Chem. 2017, 129,7297 –7300 Angewandte Zuschriften Chemie (19–22)and biologically relevant (23–34)aldehydes in good pleased to find that numerous pharmaceuticals (23, 24, 31–33) to excellent yields with minimal loss of material upon acetal and agrochemicals (25–28)were accommodated. Formyla- hydrolysis (76%yield 10 vs.74% yield 11). Interestingly,the tions that yielded amino ester 29,tripeptide 30,and glycoside average selectivity for the cross-coupling step favored meth- 34 are particularly noteworthy,demonstrating anew strategy ylene over ethylene C Hfunctionalization by 9:1. This for synthesizing biomolecular aldehydes,adesirable func- À observation is in qualitative agreement with computed C H tional handle in the field of bioconjugation.[16] More generally, À BDFEs,which predict aselectivity of 5:1purely based on the unparalleled scope of this transformation offers the thermodynamics and stoichiometry.However,the disparity possibility of late-stage formylation of typically inert aryl suggests that other factors such as polar effects in the rebound chlorides and thus the potential to disrupt current trends in of the radical to NiII may also be important. Based on these synthesis in which aryl aldehydes are employed early in observations,itisnoteworthy that the reaction of chlorme- asynthetic sequence.[1a,b] zanone afforded 23 in 89%yield, which is the maximum We recognized that in some instances,itmay be advanta- theoretical yield based on average selectivity.For most geous to employ aryl bromides or iodides in this trans- substrates,simple silica gel column chromatography was formation. 4-Bromobenzonitrile and 4-bromobiphenyl per- sufficient to obtain the desired product in high purity (see the formed modestly well in the coupling reaction, giving the Supporting Information for details). However,insome cases, corresponding aryl acetals (36-CN and 36-Ph) in 52%and amixture of aryl aldehyde and isomeric acetal resulting from 36%yield, respectively (Figure 4). Notably,aryl bromides are 4-functionalization of dioxolane was isolated. Notably,regioi- somerism can be avoided by employing 1,3,5-trioxane as aformyl source;for example,byusing 50 equivalents of 1,3,5- trioxane and benzene as asolvent, the trioxanyl acetal of 12 can be prepared in 57%yield under otherwise identical conditions.Control reactions demonstrated that in the absence of light, nickel, or photoredox catalyst, no product was formed. Reactions carried out on the benchtop afforded comparable yields to those set up in aglovebox;for example, 23 was obtained in 81%yield (vs.89% yield) when using Schlenk techniques.Inaddition, the reaction is amenable to Figure 4. Aryl halide selectivity studies. Yields determined by GC-FID operationally simple batch scale-up;fenofibrate gave 1.47 g using 1-fluoronaphthaleneasanexternal standard. See Tables S2 and of 24 in 83%yield on 5mmol scale.Moreover,benchtop S3 for additional experiments and reaction conditions. [a] With addi- scale-up reactions of 16 and 17 gave 64%yield and 70% tion of 1equiv TBACl. yield, respectively,onthe gram scale. Reaction scope investigations indicate that
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