Zn-Mediated Fragmentation of Tertiary Alkyl Oxalates Enabling

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Zn-Mediated Fragmentation of Tertiary Alkyl Oxalates Enabling Formation of Alkylated and Arylated Quaternary Carbon Centers Yang Ye,† Haifeng Chen,† Jonathan L. Sessler, and Hegui Gong* School of Materials Science and Engineering, Center for Supramolecular Chemistry and Catalysis and Department of Chemistry, Shanghai University, Shanghai, China 200444

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S Supporting Information *

IrIII and RuII photocatalysts in their excited states. For instance, reduction of 2 (E1/2red = −1.14 V vs SCE in CH3CN) with Ru(bpy)32+(E1/2II*/I = −1.33 V vs SCE in CH3CN) is thermodynamically viable due to the use of a photoexcited ruthenium complex as the electron donor and the NP moiety within the esters as the initial electron acceptor.8,10 In spite of these advances, the direct activation of the more readily accessible dialkyl oxalates, such as 3, remains a nearly unmet challenge.11 We thus set out to explore whether it would be possible to use simple dialkyl oxalates to generate C(sp3) radicals and under nominally less-forcing conditions than those provided by most current photocatalysis strategies (Scheme 1). We also sought to trap the putative tertiary C(sp3) carbon radicals thus broadening the range of accessible C−C bond coupled products (Scheme 1). To date, Michael acceptors have seen considerable use as productive traps for the tertiary alkyl radicals derived from Barton C−O bond scission. Overman has accomplished a number of natural product syntheses, e.g., cheloviolene A (Figure 1) using such an approach.12 Trapping an alkyl radical

ABSTRACT: Zn-mediated reduction of readily accessible dialkyl oxalates derived from tertiary alcohols provides an efficient approach to C−O bond fragmentation and alkyl radical formation. With MgCl2 as the indispensable additive and Ni as the promoter, trapping the radical with activated alkenes and aryl−Ni intermediates allows for the generation of alkylated and arylated all-carbon quaternary centers.

A

lcohols are ubiquitous in nature, and in the laboratory alkyl hydroxyls are among the most accessible of all organic functional groups. The development of effective coupling methods based on cleavage of unactivated C(sp3)− O bonds is synthetically and theoretically appealing. However, formidable challenges remain to be addressed.1 Carbon− oxygen bonds are strong (∼95 kcal/mol).2 Thus, forcing conditions are typically required to achieve useful conversions and even these are subject to limitations. Conventional Bartontype deoxygenation methods allow for the homolytic scission of C(sp3)−O bonds with good efficiency; however, they rely on the addition of toxic and/or less atom-efficient tin, silyl, and boryl radicals and require the use of specific thionyl and carbonyl esters.3 Recently, the generation of alkyl radicals through the photoredox catalyzed Barton fragmentation of C(sp3)−O bonds has been reported.4−8 Examples of this latter approach include Ir-catalyzed oxidative deoxygenations of hemialkyl oxalate salts (e.g., 1, Scheme 1)5,6 and Ru-catalyzed reductive deoxygenations of tertiary alkyl N-phthalimidoyl (NP) oxalyl esters (e.g., 2, Scheme 1).7−9 Here, advantage is taken of the strong driving force provided by the redox active

Figure 1. Structures of cheloviolene A, morphine, and (+)-bionectin A.

with an aryl−Ni(II) intermediate has been invoked to explain the Ni-catalyzed reductive or photoredox arylation of tertiary alkyl halides or boronated derivatives.13 This has allowed creation of arylated all-carbon quaternary stereogenic centers.14−18 Such centers are found in numerous naturally occurring compounds and active pharmaceutical ingredients (APIs),19 including morphine and (+)-bionectin A (Figure 1).20,21 Thus, being able to prepare all-carbon quaternary scaffolds directly using simple tertiary alcohol derivatives as the starting materials could prove useful. Here, we report one approach to meeting this synthetic challenge. Briefly, we have found that a Zn-mediated reduction of readily available dialkyl oxalates under Ni-promoted conditions leads to unprecedented C(sp3)−O bond fragmentation and generates tertiary

Scheme 1. Tertiary C(sp3)−O Bond Barton Fragmentation and C−C Bond Formation

Received: November 29, 2018 Published: December 20, 2018 © 2018 American Chemical Society

820

DOI: 10.1021/jacs.8b12801 J. Am. Chem. Soc. 2019, 141, 820−824

Communication

Journal of the American Chemical Society alkyl radicals. To our knowledge, the use of a chemical reductant to trigger a Barton C−O bond scission has not hitherto been reported.22 The radical intermediates can be trapped to generate a range of alkylated and arylated quaternary products as detailed below. We set out to test the reaction of a tertiary alkyl methyl oxalate (3a-Ox-Me) with benzyl acrylate. To our delight, the radical addition/hydrogen abstraction product 4a was obtained in an optimal 83% isolated yield when 1.2 equiv of 3a-Ox-Me was used along with a combination of NiCl2(Py)4/MgCl2/ PBI/Zn in DMA (method A, Table 1, entry 1). Control

Table 2. Scope of Tertiary Alkyl Methyl Oxalates and Alkenes

Table 1. Optimization of the Reaction of 3a-Ox-Me with Benzyl Acrylate

entry

variation from the standard conditions

yield %a

1 2 3 4 5 6 7 8 9 10 11 12 13 14

none w/o MgCl2 w/o PBI w/o Zn w/o Ni Mn in place of Zn Mg in place of Zn DMAP in place of PBI dtbbp in place of PBI iPr-Pybox in place of PBI NiCl2 NiBr2 NiI2 ZnCl2 instead of MgCl2

83b no reaction trace no reaction 32 47 no reaction 21 45 24 68 46 69 no reaction

The reaction was performed at 40 °C. bOxalate (1.5 equiv). cOxalate (1 equiv), benzyl acrylate (2.2 equiv). dThe reaction was run on a 10 g-scale using 5 mol % of Ni. eOxalate (2 equiv). a

a

quaternary products 4i−4l were obtained in excellent yields (Table 2). The construction of all C(sp3)-quaternary centers starting from hydroxyls present on 5-, 6-, and 7-membered rings also proved effective as exemplified by the preparation of 4m−4p in good to excellent yields. Likewise, menthone and estrone-derived tertiary oxalates allowed the generation of products 4q and 4r in good yields. A bicyclic scaffold derived from hematoxylin delivered a moderate yield of 4s. An attractive feature of the present approach is that it can be run at the 10-g scale using 3m-Ox-OMe and 5 mol % of the Ni salt to drive the chemistry; 4a was obtained in 74% yield with recovery of 70% of PBI during aqueous workup. We also examined the scope of the activated alkenes that could be used as coupling partners for 3a-Ox-Me. Acrylates decorated with different alkyl groups on the ester moieties and substituents at the internal vinyl carbon positions were effective as evidenced by the formation of 5 and 6 (Table 2). Conjugated vinyl sulfonates, phosphonates, and amides as well as 3-methylenedihydrofuran-2(3H)-one, cyclopent-2-en1-one, furan-2(5H)-one, and methyl cyclopent-1-ene-1-carboxylate could also be used; all generated the corresponding quaternary products 7−13 in good yields (Table 2). For the reaction of symmetrical oxalate 3a-Ox-3a with benzyl acrylate, a combination of NiF2, DMAP, and MgCl2 in

NMR yield using 2,5-dimethyl furan as the internal standard from a mixture containing other impurities after a quick flash column chromatography. bIsolated yield (average of 3 independent runs). Py = pyridine, PBI = 2-(pyridin-2-yl)-1H-benzo[d]imidazole, DMA = N,N-dimethylacetamide.

experiments revealed that MgCl2, PBI, and Zn were all crucial for the transformation; in their absence, no reaction occurs (entries 2−4). However, when the Ni salt was eliminated, 4a was still obtained in 32% yield (entry 5). This was taken as evidence that the key addition process is largely governed by Zn, MgCl2, and PBI. Use of other reductants, such as Mn and Mg, in lieu of Zn proved unsatisfactory (entries 6 and 7). Variation of other reaction parameters, including the use of different N-containing ligands/additives, Ni sources and ZnCl2 gave lower yields (entries 8−14 and Table S1).23 The C−O bond fragmentation method displayed excellent compatibility for a broad range of unsymmetrical oxalates. The tertiary alkyl oxalates containing geminal dimethyl groups delivered 4b−4h in good to excellent yields (Table 2). The more complex steroid-tethered tertiary alcohol can be converted to 4c in 86% yield. Dioxalate resulted in 4h in 61% yield, whereas the free hydroxyl substrate allowed formation of 4g in 70% yield. The more sterically hindered 821

DOI: 10.1021/jacs.8b12801 J. Am. Chem. Soc. 2019, 141, 820−824

Communication

Journal of the American Chemical Society

essentially to completion within 11 h under the conditions of Method A; this generated 4a (0.8 equiv), ∼0.3 equiv of the corresponding tertiary alcohol, and 0.1 equiv of unreacted oxalate (Figure S6). Here, the radical anion may abstract a hydrogen (e.g., from solvent), followed by decomposition to generate the tertiary alcohol (Scheme 2).27 The reaction of readily available 3a-Ox-Me with benzyl acrylate does not take place under the Ir- or Ru-catalyzed photoredox conditions (Scheme 1) used for oxalate cesium salts and NP−oxalates (eqs S1−S7).23 In contrast, our Zn/ MgCl2 conditions are unique for alkyl methyl oxalates to generate tertiary alkyl radicals via C−O bond fragmentation.22 In the field of cross-electrophile couplings,28−32 C−C bond forming processes often involve interception of an alkyl radical by a relatively stable organo-Ni(II) species generated from the oxidative addition of an electrophile to Ni(0).13,33 This prompted us to explore whether direct coupling of the tertiary alkyl oxalates with other electrophiles would prove possible. To our delight, using electron-deficient chloroarenes as the limiting reagent, the coupling of 3a-Ox-Me with methyl 4cholorbenzoate provided an inseparable mixture of 15a and its isomerization product (a ratio of 20:1 for the two products) in 54% yield (Table 3). Replacing the Me group in 3a-Ox-Me by a tBu moiety, a reaction with 3a-Ox-tBu as the limiting reagent gave 15a and its isomerization product (33:1 ratio) in 66%

DMA gave 4a in an optimal 77% yield (eq 1, method B and Table S2). The reaction was inhibited completely when TEMPO was added. Exposure of 3a-Ox-3a to TEMPO in the absence of benzyl acrylate produced 14 in 35% yield. Moreover, cyclization of 3t-Ox-Me afforded 4t in 76% yield (eq 2). Taken in concert, these findings provide support for the radical nature of the C−O bond scission event.

The role of MgCl2 and PBI was studied next. The reduction potential of 3a-Ox-Me was determined to be −1.29 V (vs SCE in CH3CN, Figure S7).23 In the absence of MgCl2, reduction of these diakyl oxalates using Zn (reduction potential −0.76 V vs SCE in water) appears nominally endoergonic, although a strict determination is not possible from the available data. In this event, control experiments indicated that Rieke Zn (with a more reactive surface than most commercial zinc powder) alone enabled the reaction of 3a-Ox-Me with benzyl acrylate to give 4a in 4% yield, which rose to 48% upon the addition of MgCl2 (Table S3).23 Under the present catalytic conditions using Zn powder, no reaction occurred in the absence of MgCl2 and PBI (Table 1). We propose that the role of MgCl2 and PBI is multifaceted. First, MgCl2 or the Mg(PBI)Cl2 complex activates Zn by removal of surface impurities.24 Second, the Lewis acidity of MgCl2 or Mg(PBI)Cl2 complex may enable its effective chelation with dialkyl oxalates (Figures S1−S5) and the radical anionic oxalate intermediates.23,25 This is expected to favor both the reaction kinetics and thermodynamics. The Ni−Py type complexes that are thought to result are expected to participate in and promote the reaction via a Ni(I) intermediate.26 The chemoselective cleavage of the C(sp3)−O bonds in unsymmetrical dialkyl oxalates is ascribed to reversible single-electron transfer from the Zn to a mixture of radical anions and vice versa. Irreversible formation of the more stable tertiary alkyl radical ultimately dictates the reaction pathway (Scheme 2). We also monitored the progress of the reaction of 3a-Ox-Me (1.2 equiv) with benzyl acrylate (1 equiv). The reaction went

Table 3. Arylation of Tertiary Alkyl Oxalate with Aryl Chloridesa,b

Scheme 2. Chemoselective Reduction of Dialkyl Oxalates with Zn/MgCl2 and Possible Pathway for Alcohol Formation

a

Without further notice, tRalkyl-Ox-tBu was used. bThe ratio in parentheses refers to that of quaternary product to its isomerization product after purification. c1.4 equiv of 3a-Ox-tBu, 1.0 equiv of fenofibrate, and 20 mol % of Ni were used at 40 °C. TMHD = 2,2,6,6tetramethylheptane-3,5-dionate.

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DOI: 10.1021/jacs.8b12801 J. Am. Chem. Soc. 2019, 141, 820−824

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yield (method C, Table 3). By contrast, methyl 4bromobenzoate was less satisfactory (Scheme S7).23 It was also noted that electron-neutral 1-chloro-4-methylbenzene was inert whereas its bromo analogue only delivered the coupling product in 25% yield (Scheme S7).23 Under these conditions, methyl 4-(tert-butyl)benzoate was obtained in 5% yield, reflecting excellent chemoselectivity even for di-tert-alkyl oxalates. In fact, a wide range of tRalkyl-Ox-tBu with different aryl chlorides proved viable as substrates (Table 3). The arylated quaternary products 15b−15f and 16−24 were obtained in yields comparable to those obtained using tertiary alkyl bromides.14 Both open-chain and cyclic tertiary oxalates were compatible. Side reactions arising from isomerization of the tertiary alkyl groups were generally negligible, except for more hindered 24.23 This stands in contrast to what is typically found using alkyl halides.14 Lastly, the utility of this arylation protocol is showcased via the late-stage installation of a tertiary alkyl group into a chlorine-containing commercial drug fenofibrate (a cholesterol-reducing agent used to treat cardiovascular disease). Here, coupling with 3a-Ox-tBu led to a quaternary product 25 in a reasonably good yield (modified method C, Table 3). In summary, we have demonstrated that dialkyl oxalates derived from tertiary alcohols undergo C−O bond fragmentation to afford tertiary alkyl radicals when treated with Zn and MgCl2 in the presence of Ni catalysts. The noninnocent roles of MgCl2 and pyridine-type additives in the process were attributed to activation of Zn and oxalates and stabilization of the radical anion intermediates. The generation of tertiary alkyl radicals was evidenced by trapping them with activated alkenes, TEMPO, radical cyclization and by creation of arylated quaternary carbon centers under Ni-catalyzed crosselectrophile coupling conditions. This work exploits earthabundant metal species. It thus provides a tool that can complement and extend the scope of current methodologies while offering new mechanistic insights into C−O bond fragmentation.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b12801. Detailed experimental procedures and characterization of new compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Jonathan L. Sessler: 0000-0002-9576-1325 Hegui Gong: 0000-0001-6534-5569 Author Contributions †

Y.Y. and H.C. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21871173 and 21572140). 823

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Journal of the American Chemical Society (18) Primer, D. N.; Molander, G. A. Enabling the Cross-Coupling of Tertiary Organoboron Nucleophiles through Radical-Mediated Alkyl Transfer. J. Am. Chem. Soc. 2017, 139, 9847−9850. (19) Quasdorf, K. W.; Overman, L. E. Catalytic Enantioselective Synthesis of Quaternary Carbon Stereocentres. Nature 2014, 516, 181−191. (20) Kim, J.; Movassaghi, M. Biogenetically-Inspired Total Synthesis of Pidithiodiketopiperazines and Related Alkaloids. Acc. Chem. Res. 2015, 48, 1159−1171. (21) Carlé, J. S.; Christophersen, C. Bromo-Substituted Physostigmine Alkaloids from a Marine Bryozoa Flustra Foliacea. J. Am. Chem. Soc. 1979, 101, 4012−4013. (22) In a recent report on Ni-catalyzed reductive coupling of primary benzyl methyl oxalates with alkyl halides, oxidative addition of a benzyl C−O bond to Ni(0) was considered plausible (see pages S37 to S40 in the Supporting Information for details): Yan, X.-B.; Li, C.-L.; Jin, W.-J.; Guo, P.; Shu, X.-Z. Reductive Coupling of Benzyl Oxalates with Highly Functionalized Alkyl Bromides by Nickel Catalysis. Chem. Sci. 2018, 9, 4529−4534. (23) See the Supporting Information for details. (24) Formation of Mg(DMAP)Cl2 and Mg(PBI)Cl2 complexes were confirmed by ICP-AES analysis (see page S28 in the Supporting Information). We reason that MgCl2 can remove the impurities on zinc surface, thus activating the zinc. Similar benefits have been inferred in the case of LiCl, see: Feng, C.; Cunningham, D. W.; Easter, Q. T.; Blum, S. A. Role of LiCl in Generating Soluble Organozinc Reagents. J. Am. Chem. Soc. 2016, 138, 11156−11159. (25) Metzger, A.; Bernhardt, S.; Manolikakes, G.; Knochel, P. MgCl2-Accelerated Addition of Functionalized Organozinc Reagents to Aldehydes, Ketones, and Carbon Dioxide. Angew. Chem., Int. Ed. 2010, 49, 4665−4668. (26) The redox potentials of 2,2′:6′,2″-terpyridine-Ni(I)-Me complex and 4,4′,4″-tritert-butyl-2,2′:6’,2″-terpyridine-Ni(I)-Me were determined to be −1.32 and −1.44 V (vs Ag/Ag+ in THF solution), respectively, Jones, G. D.; McFarland, C.; Anderson, T. J.; Vicic, D. A. Analysis of Key Steps in the Catalytic Cross-Coupling of Alkyl Electrophiles Under Negishi-like Conditions. Chem. Commun. 2005, 4211−4213. (27) Dolan, S. C.; MacMillan, J. A New Method for the Deoxygenation of Tertiary and Secondary Alcohols. J. Chem. Soc., Chem. Commun. 1985, 1588−1589. (28) Knappke, C. E. I.; Grupe, S.; Gärtner, D.; Corpet, M.; Gosmini, C.; Wangelin, A. J. Reductive Cross-Coupling Reactions between Two Electrophiles. Chem. - Eur. J. 2014, 20, 6828−6842. (29) Moragas, T.; Correa, A.; Martin, R. Metal-Catalyzed Reductive Coupling Reactions of Organic Halides with Carbonyl-Type Compounds. Chem. - Eur. J. 2014, 20, 8242−8252. (30) Weix, D. J. Methods and Mechanisms for Cross-Electrophile Coupling of Csp2 Halides with Alkyl Electrophiles. Acc. Chem. Res. 2015, 48, 1767−1775. (31) Wang, X.; Dai, Y.; Gong, H. Nickel-Catalyzed Reductive Couplings. Top. Curr. Chem. 2016, 374, 43−72. (32) Wang, X.; Ma, G.; Peng, Y.; Pitsch, C. E.; Moll, B. J.; Ly, T. D.; Wang, X.; Gong, H. Ni-Catalyzed Reductive Coupling of ElectronRich Aryl Iodides with Tertiary Alkyl Halides. J. Am. Chem. Soc. 2018, 140, 14490−14497. (33) Green, S. A.; Vásquez-Céspedes, S.; Shenvi, R. A. J. Am. Chem. Soc. 2018, 140, 11317.

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