Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Amide-Directed Cobalt(III)-Catalyzed C−H Amidation of Ferrocenes Dan-Ying Huang,† Qi-Jun Yao,§ Shuo Zhang,§ Xue-Tao Xu,*,†,‡ Kun Zhang,*,†,‡ and Bing-Feng Shi*,§ †
School of Biotechnology and Health Sciences, Wuyi University, Jiangmen, 529020, China International Healthcare Innovation Institute (Jiangmen), Jiangmen 529040, China § Department of Chemistry, Zhejiang University, Hangzhou 310027, China ‡
Org. Lett. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/25/19. For personal use only.
S Supporting Information *
ABSTRACT: The amide-directed cobalt(III)-catalyzed C−H amidation of ferrocene carboxamides using 1,4,2-dioxazol-5ones as robust and efficient amidating reagents has been developed. This reaction proceeds efficiently under mild reaction conditions with good functional group tolerance, providing expedient access to a broad range of ferrocenes containing a nitrogen group on the Cp ring.
F
Scheme 1. Transition Metal-Catalyzed C−H Amidation of Ferrocene Derivatives
errocene and its derivatives have attracted much attention due to their important applications in asymmetric synthesis,1 molecular switches,2 and pharmaceuticals.3 Therefore, tremendous efforts have been devoted to the synthesis of functionalized ferrocenes. Traditionally, functionalized ferrocenes were prepared via either Friedel−Crafts type electrophilic aromatic substitution or directed ortho-lithiation using stoichiometric alkyllithium reagents.4 However, the use of airsensitive, stoichiometric alkyllithium reagents imposes great limitations. Transition metal-catalyzed C−H functionalization has emerged as one of the most straightforward strategies to access these skeletons.5 However, these elegant methods have mainly focused on the formation of C−C bonds, such as alkylation,6 arylation,7 alkenylation,8 alkynylation,9 and acylation.10 To the best of our knowledge, C−N bond formation via C−H amination of ferrocene derivatives has not been realized until recently. You and co-workers reported the first Rh(III)catalyzed C−H amidation of ferrocenes using pyridyl as a directing group (Scheme 1a).11a Very recently, You et al.11b and Ackermann et al.11c elegantly demonstrated the Co(III)catalyzed C−H amidation of ferrocenes using pyridyl and thiocarbonyl as directing groups, respectively. Meanwhile, C− H functionalization of ferrocenes heavily rely on precious transition metal catalysts, prominently featuring iridium, rhodium and palladium complexes.5−10,11a In sharp contrast, C−H functionalization of ferrocenes with earth-abundant, inexpensive base metal catalysts continues to be scarce.11b,c,12,8i,13 Yu, Dai, and co-workers reported the first copper-mediated C−H thiolation of ferrocenes.13 Butenschön and co-workers reported a Co-catalyzed ortho-C−H alkenylation of ferrocenes.8i The incorporation of a nitrogen atom onto ferrocene via base-metal-catalyzed C−H amination using synthetically useful functional groups, such as amides, as directing groups would be highly desirable. In 2015, Chang and co-workers demonstrated a pioneering work on Rh(III)-catalyzed C−H amidation of arenes with 1,4,2-dioxazol-5-ones as robust and highly efficient amidating © XXXX American Chemical Society
reagents.14 Shortly after, the same group elegantly revealed that dioxazolones could also be utilized as a new type of amidating reagent in Co(III)-catalyzed C−H amidation reactions.15 Inspired by these pioneering work and our continuous interests in Co(III)-catalyzed C−H functionalization,16,17 herein we report a Co(III)-catalyzed C−H amidation of ferrocene carboxamides with 1,4,2-dioxazol-5-ones under mild conditions (Scheme 1b). We commenced our investigations by using 1-(pyrrolidine1-carbonyl) ferrocene (1a) as a model substrate to react with 3-phenyl-1,4,2-dioxazol-5-one (2a) for the optimization of Co(III)-catalyzed C−H amidation reaction conditions. Notably, the choice of 1,4,2-dioxazol-5-ones as amidating reagent has several advantages: (1) they can be easily prepared from the corresponding hydroxamic acids;18 (2) they can be used or stored without special precautions; (3) the reaction is oxidant Received: December 10, 2018
A
DOI: 10.1021/acs.orglett.8b03938 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 2. Scope of Various Dioxazolonesab
free and CO2 is released as the single byproduct. We were delighted to find that the desired amidation product 3a was obtained in 55% yield using [Cp*Co(CO)I2] as the catalyst in combination with AgNTf2 in DCE under nitrogen (Table 1, Table 1. Optimization of the Reaction Conditionsa
entry
solvent
additive (mol %)
T (°C)
silver salt
yield (%)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
DCE dioxane ethyl acetate toluene acetone acetone acetone acetone acetone acetone acetone acetone acetone acetone acetone
PivOH (20) PivOH (20) PivOH (20) PivOH (20) PivOH (20) PivOH (20) PivOH (20) PivOH (20) PhCO2H (20) 1-AdCO2H (20) AcOH (20) PivOH (40) none PivOH (20) PivOH (20)
60 60 60 60 60 60 60 60 60 60 60 60 60 40 80
AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgBF4 AgOTf AgSbF6 AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgNTf2 AgNTf2
55 62 64 54c 73c 60 42 65 46 65 47 41 14 70 64
a Reaction conditions: 1a (0.1 mmol), 2 (0.15 mmol), [Cp*Co(CO)I2] (0.01 mmol, 10 mol %), AgNTf2 (0.02 mmol), PivOH (0.02 mmol) in 1.0 mL acetone at 60 °C for 24 h. bIsolated yield.
a
Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), [Cp*Co(CO)I2] (0.01 mmol, 10 mol %), AgNTf2 (0.02 mmol), PivOH (0.02 mmol), solvent (1.0 mL), 60 °C, 24 h. b1H NMR yield using mesitylene as the internal standard. cIsolated yield. 1-AdCO2H = 1adamantanecarboxylic acid.
desired amidation product in relatively higher yields. Importantly, halides, such as chloride (3f and 3l), bromide (3g, 3n, and 3o), and iodide (3h), survived under the optimized conditions, giving the desired products in moderate to good yields. These groups could be further elaborated via conventional cross-coupling reactions. Alkyl substituent could also be tolerated, affording the desired product in moderate yield (3o, 43%). Dioxazolone bearing a heteroarene substituent at the 3-position also reacted smoothly with 1a to give the desired product in good yields (thiophene, 3p, 76%; furan, 3q, 82%). It is worth mentioning that ferrocene-derived dioxazolone 2r also acted as a good aminating reagent and the desired product (3r) was obtained in 69% yield. Of particular significance is the fact that [2.2]paracyclophane, an important scaffold widely applied in organic synthesis and materials science, is compatible with this protocol (3s, 53%). Encouraged by the results above, the scope of ferrocene derivatives was examined next. As shown in Scheme 3, ferrocene derivatives bearing various different amides could react smoothly with 2a to give the desired products in moderate to good yields (3aa−3ad). but the yield will not change much when the pyrrole ring is opened (3ad). Ferrocenes bearing a substituent on the other Cp ring were tested next. An isopropyl group was tolerated to give 3ae in 66% yield. 1-(Furan-2-yl)ethyl group was also compatible with this reaction, albeit in reduced yield (3af, 22%). The yield could be significantly increased when 1-phenylethyl was attached on the other Cp ring (3ag, 83%). Subsequently, the electronic effect of benzyl substituent was investigated, however, both electron-withdrawing and electron-donating groups led to reduced yields (3ah−3aj). When methyl prolinate was introduced to ferrocene carboxamide, it could act as a chiral auxiliary to give moderate diastereoselectivity
entry 1). Acetone turned out to be the optimal solvent (entry 5, 73% isolated yield), while protic solvents or strong coordinating solvents, such as DMF, DMSO, MeCN, and alcohols, totally inhibit the reaction (see the Supporting Information). After the screening of a variety of silver salts, AgNTf2 was found to be the most suitable additive for this transformation (entries 5−8). The judicious choice of carboxylic acids could significantly affect the efficiency and PivOH was proved to be the best (entries 9−13). We speculated that a bulky carboxylic acid, such as PivOH, could act as proper ligand in the C−H cleavage step via concerted metalation deprotonation (CMD) mechanism and suppress the aggregation of the cobalt by steric effect to maintain the activity. The reaction proceeded most efficiently at 60 °C, and increasing or lowering the temperature resulted in reduced yield (entries 14 and 15). The structure of 3a was unambiguously confirmed by X-ray crystallographic analysis (Scheme 2; also see the Supporting Information for details). With the optimal reaction conditions in hand, we set out to explore the generality of this amidation reaction. In general, the amidation reaction proceeded smoothly over a wide range of 3substituted-1,4,2-dioxazol-5-ones (Scheme 2). Both electrondonating groups (e.g., Me, MeO, t-Bu) and electron-withdrawing groups (e.g., F, Cl, Br, I, CF3) in substituted benzene are compatible with this reaction protocol (3b−3n). Dioxazolones bearing electron-withdrawing substituents on the aromatic ring are generally more efficient, giving the B
DOI: 10.1021/acs.orglett.8b03938 Org. Lett. XXXX, XXX, XXX−XXX
Organic Letters
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Scheme 3. Scope of Ferrocenesab
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03938. Experimental details and spectral data for all new compounds and X-ray crystallography/CIF for 3a (PDF) Accession Codes
CCDC 1871963 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (X.X.) *E-mail:
[email protected] (K.Z.) *E-mail:
[email protected] (B.-F.S.) ORCID
Bing-Feng Shi: 0000-0003-0375-955X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from Department of Education of Guangdong Province (No. 2017KTSCX185, 2017KSYS010, 2016KCXTD005) is gratefully acknowledged.
a Reaction conditions: 4 (0.1 mmol), 2a (0.15 mmol), [Cp*Co(CO)I2] (0.01 mmol, 10 mol %), AgNTf2 (0.02 mmol), PivOH (0.02 mmol) in 1.0 mL acetone at 60 °C for 24 h. bIsolated yield. cThe dr value was determined by 1H NMR.
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DOI: 10.1021/acs.orglett.8b03938 Org. Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.orglett.8b03938 Org. Lett. XXXX, XXX, XXX−XXX
