Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Palladium/Norbornene-Catalyzed Ortho-Acylation and IpsoSelenation via C(O)−Se Bond Cleavage: Synthesis of α‑Carbonyl Selane Xiaozhong Fan and Zhenhua Gu* Department of Chemistry, Center for Excellence in Molecular Synthesis, and Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P. R. China S Supporting Information *
ABSTRACT: A palladium/norbornene-catalyzed synthetic method that combines selective ortho C−H bond activation with sequential reactions to form complex selenide compounds has been developed. The C(O)−Se bond in selenoates is successfully cleaved, and various kinds of α-carbonyl selanes were synthesized in decent to excellent yields. The reaction has good functional group tolerance and broad substrate scope. Scheme 1. Pd/NBE-Catalyzed Ortho-Functionalization
P
alladium/norbornene (NBE) catalysis has become a unique approach for efficient functionalization of the ortho position(s) of aryl halides and preparation of polysubstituted arenes. In 1997, the Catellani group showed that alkyl groups could be introduced to the ortho position of iodobenzenes, where norbornene works as an efficient shuttle for C−H palladation to form a critical palladacycle intermediate A (Scheme 1a).1 Four years later, the same research group described a homodimerization reaction of aryl iodides and, thus, successfully realized the ortho arylation reaction.2 In the one and a half decades since the discovery of this transformation, seminal works from the Catellani and Lautens groups demonstrated its vast utilities in the construction of densely functionalized arenes. The persuasive power in the functionalization of arenes made this transformation appealing to many synthetic chemists for versatile purposes, including the rapid total synthesis of natural products.3,4 Theoretically, different types of electrophiles could undergo oxidative addition with the key intermediate A under proper conditions.5 However, in 2013, Dong and co-workers developed an orthoamination reaction by the use of benzoyl hydroxylamine as electrophiles.6 In 2015, Liang, Dong, and our groups independently reported ortho acylation reaction by the use of (mixed)-anhydrides or acid chlorides.7 Subsequently, Dong and co-workers investigated different substituted mixed anhydrides and successfully established an α-alkoxylcarbonylation reaction by selective cleavage of the less steric hindered C(O)−O bonds in mixed carbonate anhydrides.8 Further experimental and computational investigation by us indicated that the cleavage of mixed carboxylic acid anhydrides was controlled by the electronic properties of two acid parts.9 In 2016, our group developed a palladium/norbornene/copper three-componentcatalyzed C(O)−S bond cleavage of thioesters for the synthesis of α-carbonyl diarylsulfane, where the catalyst CuI worked as activation reagent to assist C(O)−S bond cleavage.10 © XXXX American Chemical Society
It is widely recognized that selenium is one of the most important rare elements in human bodies. Selenides, the major form of selenium existing in our body, perform irreplaceable functions in life (Figure 1, B and C).11 On the other hand, in Received: January 10, 2018
A
DOI: 10.1021/acs.orglett.8b00112 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 2. Substrate Scopea
Figure 1. Selenide-containing compounds.
recent years, chiral selenophosphoramides and selenides emerged as novel catalysts or ligands in asymmetric catalysis (Figure 1, D and E).12 Notwithstanding the existence of efficient accesses for selenides,13 the development of new methods for the preparation of steric bulky selenides is challenging yet still desirable. Continuing our studies in this area, we reasoned that the C(O)−Se bond might be cleaved by palladium/norbornene catalysis with or without the assistance of copper species. Given the importance of organic selenide compounds, here we report a Pd/NBE-catalyzed orthoacylation/ipso-selenylation reaction for the synthesis of functionalized selenides with broad substrate scope. Our initial investigation began with 1-iodonaphthalene (1a) and Se-phenyl 2-methylpropaneselenoate (2a) in the presence of PdCl2, tris(2-furyl)phosphine (TFP), NBE, and K2CO3 in toluene. Pleasingly, the reaction did afford the desired product 3a, although the yield was only 39% (Table 1, entry 1). The Table 1. Optimization of Reaction Conditionsa
entry
base
solvent
1 2 3 4 5 6 7 8 9 10 11 12
K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 Cs2CO3 Na2CO3 NaOBut K3PO4 K2CO3 K2CO3 K2CO3
toluene dioxane MeCN DCE DCP DCP DCP DCP DCP DCP DCP DCP
a
Reaction conditions: the reactions were carried out with 1a (0.20 mmol), 2 (0.30 mmol, 1.5 equiv), PdCl2 (0.020 mmol, 10 mol %), TFP (0.050 mmol, 25 mol %), NBE (0.60 mmol), and K2CO3 (0.60 mmol) in DCP (4 mL) at 100 °C for 16 h. bDCP/DMI (20:1) was used as the solvent.
additive
yield (%)
CuI AgBF4 Au(PPh3)Cl
39 trace 26 47 83 25 trace trace 56 trace 10 17
branched carboxylic acids were suitable substrates for this crosscoupling reaction. For example, compound 3b was isolated in 73%, and the reaction with the bulky substrate, Se-phenyl 2,2dimethylpropaneselenoate, delivered the product 3c in 68% yield. Unfortunately, selenoates prepared from α-nonbranched carboxylic acid, such as acetic acid and propionic acid, were incompatible substrates, and the corresponding reactions either afforded meager yields or even no desired product. The easy enolization of these substrates might erode the efficiency of this transformation. It is foreseeable that aromatic acyl groups were introduced to the ortho position of 1-iodonaphthalene with good to excellent yields (3d−g). Notwithstanding the moderate yields (54−69%), this transformation was able to introduce αalkoxylcarbonyl groups (3h−k), which was only realized by Dong’s mixed carbonate anhydrides system recently.8 The structure of the alkoxycarbonylated compounds was established by the single crystal X-ray analysis of demethyl-3h. We next tested the generality with various substituted aryl iodides and selenoates. It was found that 1,3-dimethyl-2imidazolidone (DMI), initially reported by Liu and coworkers,3h,14 was beneficial for improving the yields in some cases (Scheme 3). The reaction of 1-iodo-2-alkylbenzene with Se-phenyl 2-methylbenzoselenoate proceeded smoothly to give the corresponding products in 60−88% (3l−p). Increasing the steric bulkiness by introducing substituents adjacent to the methyl group was beneficial for the yields (3m and 3n). The reaction of 1-iodo-2-methoxybenzene usually afforded a poor yield, although it was slightly improved by the use of bicyclo[2.2.1]hept-5-ene-2-carbaldehyde instead of norbornene (3q). The yields were advanced to 57−66% by either replacing methoxyl with an isopropoxyl group (3r) or by adding a
a
Reaction conditions: the reactions were carried out with 1a (0.20 mmol), 2 (0.30 mmol, 1.5 equiv), PdCl2 (0.020 mmol, 10 mol %), TFP (0.050 mmol, 25 mol %), and NBE (0.60 mmol) in the indicated solvent.
screening of the solvents, including dioxane, CH3CN, dichloroethane (DCE), or 1,3-dichloropropane (DCP), indicated that the reaction worked most efficiently in DCP to give 3a in 83% yield (entries 2−5). Surprisingly, the reaction with Cs2CO3, Na2CO3, or NaOBut as base performed with low efficiency or even no product formed, while the one with K3PO4 gave a decent yield of 3a (entries 6−9). In contrast to our previous observation for C(O)−S bond cleavage, group 11 metal complex additives, such as CuI, AgBF4, or Au(PPh3)Cl, gave negative effects (entry 10−12). Pleasingly, under the optimized conditions, a range of Sephenyl selenoates could be successfully coupled with 1iodonaphthalene (Scheme 2). Selenoates derived from αB
DOI: 10.1021/acs.orglett.8b00112 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 3. Substrate Scopea
Scheme 4. Control Experiments
previous Pd/norbornene-catalyzed α-acylation reactions, this reaction has a broad substrate scope: (1) both carbonyl and alkoxycarbonyl could be introduced to the ortho-position of aryl halides, and (2) both Se-aryl and Se-alkyl selenoates are compatible substrates.
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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.orglett.8b00112. Experimental procedure, spectroscopic data, and the 1H and 13C NMR spectra of the products (PDF)
a
Reaction conditions: all reactions were carried out with 1 (0.20 mmol), 2 (0.30 mmol, 1.5 equiv), PdCl2 (0.020 mmol, 10 mol %), TFP (0.050 mmol, 25 mol %), NBE (0.60 mmol), and K2CO3 (0.60 mmol) in DCP (4 mL) and DMI (0.2 mL) at 100 °C for 16 h. bThe compound was purified and characterized via the corresponding alcohol. cBicyclo[2.2.1]hept-5-ene-2-carbaldehyde instead of norbornene was used.
Accession Codes
CCDC 1815586 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.
substituent adjacent to methoxyl group (3s). The iodides bearing an ortho electron-withdrawing group trifluoromethyl or phenyl group are not good substrates; the yields of these reactions dramatically decreased (3t and 3u). Introducing substituents at the 4- or 7-position of 1-iodonaphthalene had a limited effect on the yields (3v and 3w). Furthermore, the Sealkyl or Se-benzyl selenoates are compatible substrates, and the synthetically useful yields could be achieved (3x−bb). Unfortunately, iodobenzene and 1-iodo-4-methoxybenzene were incompatible substrates, and the corresponding reactions were almost inert under the identical conditions. Mechanistically, in the presence of 2.0 equiv of TEMPO, the reaction proceeded uneventfully with a slight drop of the yield (see the Supporting Information). Under the standard reaction conditions, the reaction of 1a in the presence of Se-phenyl 2chlorobenzoselenoate (1.5 equiv) and Se-methyl 2-methylbenzoselenoate (1.5 equiv) gave 3g and 3x in 77% total yield, and no crossover product(s) were detected (Scheme 4a). In the presence of an additional 1.5 equiv of ethyl acrylate, significant amount of Heck product 4 has been obtained with a ratio of 3i/ 4 of 1.56:1 (Scheme 4b).15 In conclusion, we reported a palladium/norbornenecatalyzed ortho-carbonylation or alkoxycarbonylation and ipsoselenylation reaction via the cleavage of the C(O)−Se bond of selenoates. In sharp contrast to the cleavage of the C(O)−S bond of thioates, the addition of group 11 metal salts had adverse effects on the reactivity. In comparison with the
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhenhua Gu: 0000-0001-8168-2012 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partly supported by NSFC (21622206, 21472179), the ‘973’ project from the MOST of China (2015CB856600), and the Fundamental Research Funds for the Central Universities.
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(15) The corresponding reaction of thioesters in the presence of an additional 1.5 equiv of ethyl acrylate gave the thioether and Heck product with a ratio of 3.67:1. It indicated that the bonding of RSe− to the Pd atom is weaker than RS−. Thus, the addition of metal complex additives may give a negative effect by eradicating the bonding of RSe− to Pd atom.
D
DOI: 10.1021/acs.orglett.8b00112 Org. Lett. XXXX, XXX, XXX−XXX
