Communication Cite This: Organometallics XXXX, XXX, XXX-XXX
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Cp*RhIII-Catalyzed C−H Amidation of Ferrocenes Shao-Bo Wang,‡ Qing Gu,† and Shu-Li You*,†,‡ †
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 LinglingLu, Shanghai 200032, People’s Republic of China ‡ School of Physical Science and Technology, ShanghaiTech University, 100 Haike Road, Shanghai 201210, People’s Republic of China S Supporting Information *
ABSTRACT: Amidation of N-containing heteroaryl ferrocenes with 1,4,2-dioxazol-5-ones as the amidated reagents was achieved via a Rh-catalyzed direct C−H functionalization reaction. Under mild reaction conditions, a wide range of Nferrocenyl amides were obtained in up to 99% yield. Transformations of the amide group were also feasible.
Optimization of the Reaction Conditions. We initially chose 2-ferrocenylpyridine (1a) and 3-phenyl-1,4,2-dioxazol-5one7a (2a) as model substrates to test our hypothesis. As shown in Table 1, in the presence of 2.5 mol % of [Cp*RhCl2]2, 0.15
Ferrocene and its derivatives have been widely applied in organic synthesis, materials science, and medicinal chemistry.1 Their application in asymmetric catalysis is also notable (Figure 1).2 The latter includes the application of (R,Sp)-Xyliphos as a
Table 1. Optimization of the Reaction Conditionsa
Figure 1. Selected ferrocene-based ligands.
highly efficient ligand for the industrial production of (S)metolachlor.3 Of particular note, ferrocenyl ligands mostly contain heteroatoms, such as P, N, S, etc., directly attached to the ferrocene backbone (Figure 1). To date, the most common strategy employed for the introduction of heteroatoms on the ferrocene backbone relies on lithiation of ferrocene and subsequent quenching by a proper electrophile.1d,e Transition-metal-catalyzed direct C−H bond functionalization has developed rapidly over the past decades because of its high step and atom economy.4 Therefore, it is highly attractive to achieve diverse ferrocene derivatives by employing this strategy. However, to our knowledge, the development of transition-metal-catalyzed direct C−H bond functionalization of the ferrocene backbone has mainly been limited to C−C bond formation.5 C−N bond formation by attaching an N atom on the ferrocene backbone, via a C−H functionalization reaction, has not yey been reported.6,7 Inspired by the pioneering work from the Chang group on Rh-catalyzed direct amidation reaction of arenes with 1,4,2-dioxazol-5-ones as new types of amidating reagents,7 we recently realized a C−N formation reaction on ferrocene by Rh catalysis. Herein, we report such a Rh-catalyzed C−H amidation reaction of ferrocenes with 1,4,2-dioxazol-5-ones. © XXXX American Chemical Society
entry
variation from standard conditions
yield of 3aa (%)b
1 2 3 4 5 6 7
none under air AgSbF6 instead of AgNTf2 [Cp*Rh(MeCN)3][SbF6]2 as catalyst DCM instead of DCE EtOAc instead of DCE MeOH instead of DCE
>95 89 67 >95 >95 19 −
a
Conditions: 1a (0.1 mmol), 2a (1.2 equiv), [Cp*RhCl2]2 (2.5 mol %), AgNTf2 (0.15 equiv), NaOAc (0.2 equiv), DCE (0.2 M), 40 °C, 2 h, under Ar. bDetected by 1H NMR with CH2Br2 as an internal standard.
equiv of AgNTf2, and 0.2 equiv of NaOAc, the reaction of 1a (0.1 mmol) with 2a (1.2 equiv) in DCE at 40 °C afforded the ortho amidation product (3aa) in 95% NMR yield (Table 1, entry 1). No diamidated product was observed even when 2.4 equiv of 2a was used. When the reaction was performed under air or with AgSbF6 instead of AgNTf2, the yield of 3aa dropped (Table 1, entries 2 and 3). Notably, [Cp*Rh(MeCN)3][SbF6]2 also proved to be an efficient catalyst (Table 1, entry 4). After the solvents were screened, DCE was found to be optimal and DCM was also suitable (Table 1, entries 5−7). Received: September 9, 2017
A
DOI: 10.1021/acs.organomet.7b00691 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
Table 3. Substrate Scope of Substituted Dioxazolonea
Substrate Scope. With the optimal conditions in hand (Table 1, entry 1), we then explored the substrate scope of the amidation reaction. As summarized in Table 2, substrates 1b−e Table 2. Substrate Scope of Substituted Ferrocenesa
a
Conditions: 1a (0.3 mmol), 2 (1.2 equiv), [Cp*RhCl2]2 (2.5 mol %), AgNTf2 (0.15 equiv), NaOAc (0.2 equiv), DCE (0.2 M), 40 °C. Yields of isolated products are reported.
substituted dioxazolones (2l,m) could react smoothly, affording amidation products in excellent yields (93−99%). Asymmetric Reaction. On the basis of works reported by the Cramer group4c,8 and our group9 on chiral CpRh-catalyzed asymmetric C−H functionalizations, a preliminary investigation of the enantioselective amidation of ferrocenylpyridine 1a with dioxazolone 2a was conducted under modified reaction conditions using chiral complexes Rh1 and Rh2 as catalysts, respectively (Scheme 1). The desired product 3aa was obtained a
Conditions unless noted otherwise: 1 (0.3 mmol), 2a (1.2 equiv), [Cp*RhCl2]2 (2.5 mol %), AgNTf2 (0.15 equiv), NaOAc (0.2 equiv), DCE (0.2 M), 40 °C. Yields of isolated products are reported. b [Cp*RhCl2]2 (5 mol %), AgNTf2 (0.30 equiv). cAt 70 °C.
Scheme 1. Enantioselective Synthesis of 3aa
bearing a methyl group on different positions of the pyridine ring could react smoothly with 2a, giving 3ba−ea in good yields (79−85%). An OiPr substituent was well compatible, as the reaction of 1f led to amidation product 3fa in 89% yield. Isoquinoline and pyrimidine could also act as directing groups, affording their desired products 3ga,ha in 89% and 52% yields, respectively, under slightly modified conditions. The structure of 3ha was confirmed by X-ray crystallographic analysis (see the Supporting Information for details). Next, substrates bearing a functional group on the other Cp ring were examined. Substrate 1i bearing a MOM group was found to be tolerated to give 3ia in 70% yield. However, for substrates 1j,k bearing Bz and pyridinyl groups, respectively, the desired products 3ja,ka were obtained with diminished yields (24% and 19%), probably due to the coordination of the Bz and pyridinyl groups with rhodium. Of particular note, for substrate 1k bearing two pyridinyl groups, no diamidation product was observed. Next, we tested different dioxazolones (2) with ferrocene 1a. As shown in Table 3, various substituents on the benzene ring of dioxazolone 2 gave their desired products 3ab−ai in good to excellent yields (74−99%), regardless of their electronic properties. Dioxazolones 2j (R = 2-thienyl) and 2k (R = 2furyl) were also proved to be suitable substrates (93−97% yields), while pyridinyl-substituted dioxazolone (R = 2pyridinyl) was found to yield no product due to the decomposition of the substrate. Notably, simple alkyl-
in 26% yield and 40% ee catalyzed by chiral Rh1, while a slightly higher yield (37%) and lower enantioselectivity (20% ee) were obtained using chiral Rh2. Synthetic Application. The amide group of 3aa could be transformed smoothly to thioamide by treatment of Lawesson’s reagent (1.2 equiv) to give 4aa in 89% yield. Moreover, reduction of the amide group by 5 equiv of DIBAL-H afforded benzylamine 4ab in 85% yield. Compound 4ab might be a precursor for an N,N-bidentate ligand (Scheme 2). Scheme 2. Transformations of 3aa
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DOI: 10.1021/acs.organomet.7b00691 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Notes
Proposed Catalytic Cycle. On the basis of previous reports,7a,b,g,h a plausible catalytic cycle was proposed by using 1a and 2a as substrates (Scheme 3). The cationic Cp*Rh,
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Key R&D Program of China (2016YFA0202900), the National Basic Research Program of China (973 Program 2015CB856600), the NSFC (21332009, 21421091, 21572250), and the Strategic Priority Research Program of CAS (XDB20000000, QYZDYSSWSLH012) for generous financial support.
Scheme 3. Proposed Catalytic Cycle
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formed by exposing [Cp*RhCl2]2 to AgNTf2, proceeds via C− H bond activation assisted by pyridine chelation, leading to rhodacycle I. Upon coordination with 2a, the formed intermediate II then undergoes decarboxylation and amidation, furnishing intermediate III. The desired product 3aa is obtained after protodemetalation, regenerating the cationic Cp*Rh species. In conclusion, we have realized a Rh-catalyzed C−H direct amidation of ferrocenylpyridines and their analogues with dioxazolones under mild reaction conditions, affording ferrocene derivatives bearing an nitrogen group attached directly on the Cp ring in good to excellent yields (up to 99%). The facile transformation of the benzoyl group further enhances the potential utility of this method.
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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/acs.organomet.7b00691. Experimental procedures, analysis data for all new compounds, and details of X-ray crystallographic analysis for 3ha (PDF) Accession Codes
CCDC 1573556 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 data_
[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 Author
*E-mail for S.-L.Y.:
[email protected]. ORCID
Qing Gu: 0000-0003-4963-2271 Shu-Li You: 0000-0003-4586-8359 C
DOI: 10.1021/acs.organomet.7b00691 Organometallics XXXX, XXX, XXX−XXX
Communication
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DOI: 10.1021/acs.organomet.7b00691 Organometallics XXXX, XXX, XXX−XXX
