Angewandte

Chemie

DOI: 10.1002/anie.201402562

C(sp3) H Activation Hot Paper

Palladium-Catalyzed C(sp3) H Activation: A Facile Method for the Synthesis of 3,4-Dihydroquinolinone Derivatives** Jia-Xuan Yan, Hu Li, Xiang-Wei Liu, Jiang-Ling Shi, Xin Wang, and Zhang-Jie Shi* Abstract: 3,4-Dihydroquinolinones were synthesized by the palladium-catalyzed, oxidative-addition-initiated activation and arylation of inert C(sp3) H bonds. Pd(OAc)2 and P(otol)3 were used as the catalyst and ligand, respectively, to improve the efficiency of the reaction. A further advantage of this reaction is that it could be performed in air. A relatively rare seven-membered palladacycle was proposed as a key intermediate of the catalytic cycle.

intramolecular[8] and intermolecular[9] activation reactions have been reported and have thus been confirmed to be efficient processes. Notably, for intramolecular C(sp3) H activation reactions, the formation of either four-membered[8a,c–e] or five-membered[8f–i] rings has been particularly successful, whereas the synthesis of six-membered or larger rings has still rarely been reported (Figure 1).[10] Obviously,

In bioactive molecules, pharmaceutics, natural products, and industrial materials, C H bonds are ubiquitous. For years, chemists have been considering the direct functionalization of C H bonds for the synthesis of important molecules to avoid tedious and sluggish synthetic procedures.[1] In recent years, C H bond activation reactions have experienced great developments, and among all of these progresses, palladium-catalyzed C H bond activation has shown its great advantages.[2] However, most of the current research mainly focuses on the activation of C(sp2) H bonds,[2e,h–j] as such transformations benefit substantially from the interactions between the catalyst and the p electrons,[2a,e] which enables catalyst–substrate binding and C(sp2) H bond cleavage through electrophilic metalation,[3] a concerted metalation– deprotonation (CMD) process,[4] or other pathways.[5] The aforementioned methods could also be applied to some C(sp3) H bond activation reactions. For example, several beautiful examples of the activation of benzylic C(sp3) H bonds and allylic C(sp3) H bonds have been described.[6] However, the activation of common C(sp3) H bonds has remained more challenging[7] because of a lack of p electrons and high steric hindrance. Fortunately, chemists have developed another strategy as an alternative method to the direct activation of C(sp3) H bonds that is initiated by the oxidative addition of organohalides to Pd0 precatalysts. In fact, both [*] J.-X. Yan, Dr. H. Li, Dr. X.-W. Liu, J.-L. Shi, X. Wang, Prof. Dr. Z.-J. Shi Beijing National Laboratory of Molecular Sciences (BNLMS) PKU Green Chemistry Centre and Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry Peking University, Beijing, 100871 (China) E-mail: [email protected] Prof. Dr. Z.-J. Shi State Key Laboratory of Organometallic Chemistry Chinese Academy of Sciences Shanghai 200032 (China) [**] Support from the NSFC (J1030413, 20925207, and 20821062) and the “973” Project from the MOST of China (2009CB825300) is gratefully acknowledged. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201402562.

the formation of seven-membered or even larger palladacycle intermediates during the catalytic process is more difficult than that of five- or six-membered palladacycles.[4f, 10] Herein, we reported a successful method to synthesize 3,3-disubstituted 3,4-dihydroquinolinone derivatives by an oxidativeaddition-initiated strategy that features seven-membered palladacycles as key intermediates through direct C(sp3) H bond activation. Inspired by the pioneering works from the groups of Cramer,[4f, 10b] Baudoin,[8d, e] Kndig,[8j, k] Martin,[11a, b] Su[11c,d] and others,[8a,b,h] we initially chose amide 1 a as a standard substrate to test the feasibility of intramolecular C(sp3) H bond activation to form six-membered rings. The blocked aposition of the carbonyl group, the nine chemically equal hydrogen atoms, and the gem-dimethyl structure of this molecule should support the desired C H bond activation and cyclopalladation process.[12, 13] Fortunately, the desired product was obtained in 30 % yield (as determined by 1 H NMR spectroscopy) in the presence of Pd(OAc)2 as the catalyst, PCy3 as the ligand, and PivOH as an additive

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Angew. Chem. Int. Ed. 2014, 53, 1 – 6

Figure 1. C(sp3) H activation reactions that are initiated by oxidative addition.

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. Angewandte Communications Table 1: Optimization of the intramolecular C(sp3) H activation of 1 a.

Entry[a]

Catalyst

Ligand

Additive

Solvent

Yield[b] [%]

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

Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OTFA)2 Pd(dba)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 Pd(OAc)2 –

PCy3 PtBu3·HBF4 P(o-tol)3 XPhos P(4-FC6H4)3 P(o-tol)3 P(o-tol)3 P(o-tol)3 P(o-tol)3 P(o-tol)3 P(o-tol)3 P(o-tol)3 P(o-tol)3 P(o-tol)3

PivOH PivOH PivOH PivOH PivOH PivOH PivOH PivOH PivOH PivOH PivOH PivOH – PivOH

DMF DMF DMF DMF DMF DMF DMF mesitylene toluene NMP DMAc NMP NMP NMP

30 25 43 40 21 27 28

Palladium-catalyzed C(sp(3))-H activation: a facile method for the synthesis of 3,4-dihydroquinolinone derivatives.

3,4-Dihydroquinolinones were synthesized by the palladium-catalyzed, oxidative-addition-initiated activation and arylation of inert C(sp(3) )H bonds...
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