Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Carboallylation of Electron-Deficient Alkenes with Organoboron Compounds and Allylic Carbonates by Cooperative Palladium/ Copper Catalysis Kazuhiko Semba,* Naoki Ohta, and Yoshiaki Nakao* Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
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ABSTRACT: The aryl- and alkylallylation of electrondeficient alkenes was achieved by cooperative palladium/ copper catalysis. The reaction affords various carbon skeletons from readily available alkenes, allylic carbonates, and organoboron compounds, whereby a variety of functional groups such as acetyl, alkoxycarbonyl, bromo, and cyano moieties are tolerated well.
T
that the allylarylation of 1a produced 4a in 78% yield in the presence of Pd(OAc)2 (1.0 mol %), P(n-Pr)3 (2.0 mol %), (IMes)CuCl (10 mol %), and NaOMe (10 mol %) at 80 °C for 24 h in 1,4-dioxane (entry 1), and the formation of 4methyl-1-allylbenzene (5a) was not observed. Other monodentate and bidentate phosphines afforded 4a in low yield (entries 2−5). Different N-heterocyclic carbenes (NHCs) on copper(I) chloride also affected the yield of 4a. For example, the sterically more demanding IPr and the saturated SIMes decelerated the reaction (entries 6 and 7). To prove the cooperativity of the catalysis, control experiments were conducted (entries 8−11), which revealed that all the catalyst components are indispensable for the present transformation. Under the optimized conditions, a variety of alkenes, allylic carbonates, and aryl boronates was screened (Scheme 1). Electron-rich and -deficient aryl groups were tolerated well under these conditions. Sterically demanding aryl boronate 2e afforded the corresponding product in moderate yield, while a 2-naphthyl group (2f) was successfully introduced in 1a. 3Thienyl boronate 2g can participate in the optimized conditions. Regarding the alkenes, electron-donating and -withdrawing substituents at the para-position on the benzene ring of 1a did not have an impact on the yield. 1-Cyano-1methoxycarbonyl-2-phenylethene (1d) and barbituric acid derivative 1e are viable alkenes under the optimized conditions. 2-Methylpropenyl methyl carbonate (3b), cinnamyl methyl carbonate (3c), and 1-cyclohex-2-enyl methyl carbonate (3d) afforded the corresponding product in moderate and high yield, respectively. It is worth mentioning that alkyl-9-BBN 2h, 2i, and 2j, which were prepared via the hydroboration of the corresponding alkenes with H-9-BBN,
he dicarbofunctionalization of alkenes is a powerful approach to construct carbon skeletons, especially considering the availability of a broad variety of alkenes.1 The dicarbofunctionalization of α,β-unsaturated alkenes with organometallic nucleophiles and organic electrophiles is a particularly well-established and powerful method to construct complex carbon structures, which has been applied to prepare biologically active molecules.2 However, typical reaction protocols require moisture- and/or O2-sensitive organometallic reagents and multiple steps, which renders the entire process complicated.3 A method based on bench-stable organometallics would therefore be highly desirable. Following a seminal report on the diallylation of electron-deficient alkenes by Pd catalysis,4a−c related Pd-catalyzed dicarbofunctionalization reactions of electron-deficient alkenes have been reported.4d−h Recently, we accomplished the carboallylation of electron-deficient alkenes by Pd/Cu catalysis using benchstable tetraorganosilicon reagents (HOMSi reagents) and allylic carbonates.5a Herein, we report the carboallylation of electron-deficient alkenes with aryl- or alkylboron reagents and allylic carbonates by synergistic Pd/Cu catalysis.5,6 The advantages of the present protocol relative to our previous method are (1) in contrast to the limited amount of aryl HOMSi reagents, a broader variety of arylboronic acids and their derivatives is commercially available; and (2) alkyl-9-borabicylo[3,3,1]nonane (alkyl-9-BBN) derivatives, which are easily prepared in situ from the uncatalyzed hydroboration of the corresponding alkenes with H-9-BBN, can be used as an alkylation agent. Mechanistic studies revealed that the reaction proceeds by cooperative Pd/Cu catalysis, not by a tandem catalysis as in the case of our recent example.5a Initially, we optimized the reaction conditions using benzalmalononitrile (1a), p-tolyl boronic acid neopentylglycol ester (2a), and allyl methyl carbonate (3a) as model substrates (Table 1). After screening a variety of parameters, we found © XXXX American Chemical Society
Received: March 14, 2019
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DOI: 10.1021/acs.orglett.9b00915 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Allylarylation of Benzalmalononitrile (1a) with pTolyl Boronic Acid Neopentylglycol Ester (2a) and Allyl Methyl Carbonate (3a)
entry
deviation from standard conditions
1 2 3 4 5 6 7
none PPh3 instead of P(n-Pr)3 PCy3 instead of P(n-Pr)3 P(t-Bu)3 instead of P(n-Pr)3 dppe instead of P(n-Pr)3 (IPr)CuCl instead of (IMes)CuCl (SIMes)CuCl instead of (IMes) CuCl w/o Pd(OAc)2 w/o Pd(OAc)2/P(n-Pr)3 w/o (IMes)CuCl w/o (IMes)CuCl/NaOMe
8 9 10 11
yield of 4a (%)a
yield of 5a (%)b
78 17 24
