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Rhodium(III)-Catalyzed Annulation between N-Sulfinyl Ketoimines and Activated Olefins: C-H Activation Assisted by an Oxidizing N-S Bond Qiang Wang, Yingzi Li, Zisong Qi, Fang Xie, Yu Lan, and Xingwei Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02297 • Publication Date (Web): 09 Feb 2016 Downloaded from http://pubs.acs.org on February 9, 2016
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ACS Catalysis
Rhodium(III)-Catalyzed Annulation between N-Sulfinyl Ketoimines and Activated Olefins: C-H Activation Assisted by an Oxidizing N-S Bond Qiang Wang,a Yingzi Li,b Zisong Qi,a Fang Xie,a Yu Lan,*,b Xingwei Li*,a a b
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, China
ABSTRACT: C-H activation under redox-neutral conditions, especially by Rh(III) catalysis, has offered attractive synthetic strategies. Previous work in redox-neutral C-H activation relied heavily on the cleavage of oxidizing N-O and N-N directing groups, and cleavable N-S bonds have been rarely used although they may offer complementary coupling patterns. In this work, N-sulfinyl ketoimines were designed as a novel substrate for the redox-neutral coupling with different activated olefins via a Rh-catalyzed C-H activation pathway. The coupling with acrylate esters afforded 1H-isoindoles with the formation of three chemical bonds around a quaternary carbon. Furthermore, the coupling with maleimides furnished pyrrolidone-fused isoquinolines. A broad scope of substrates has been established. The mechanism of the coupling with acrylates has been studied in details by a combination of experimental and computational methods. This coupling proceeds via imine-assisted C-H activation of the arene followed by ortho C-H olefination to afford a Rh(III) olefin hydride intermediate which, upon deprotonation, may exist in equilibrium with a Rh(I) olefin species. Cleavage of the N-S bond occurs only after C-H olefination to generate a Rh(III) imide species. DFT studies indicated that the imide group can undergo migratory insertion to produce a Rh(III) secondary alkyl which isomerizes under the assistance of acetic acid to a Rh(III) tertiary alkyl that is prone to insertion of the 2nd acrylate. KEYWORDS: C-H bond activation, Rh(III) catalyst, N-S bonds cleavage, N-sulfinyl imine, olefin, isoindole
Introduction In the past several years, Rh(III)-catalyzed C-H activation of arenes has been increasingly explored, which allowed the development of a large number of important synthetic methods with high reactivity, selectivity, functional group tolerance, and broad applicability.1 Building on the extensively studied Rh(III)-catalyzed C-H activation and coupling with various unsaturated partners under oxidative conditions using external oxidants,2 there has been increasing interest in taking advantage of an internal oxidant or an oxidizing directing group. Pioneering work by Fagnou3 and Glorius4 utilized oxidizing N-O and N-N bonds as directing groups, and these C-H activation reactions proceeded with redox-economy and high selectivity under relatively mild conditions.5 Inspired by these seminal works, the N-N, N-O, and even C-N bonds in oximes,6 hydrazines,7 N-phenoxyamides,8 N-oxides,9 N10 11 nitrosoanilines, N-hydroxyaniline and α-ammonium acetophenones12 have been designed and applied as viable oxidizing directing groups. Despite the progress, cleavage of a N-S directing group is highly rare.13 This is likely due to the lower oxidizing potential of a N-S bond. Thus development of coupling systems for arenes bearing a cleavable N-S bond is of significant interest and may complement existing internal oxidizing systems.
While the concept of oxidizing DG is appealing, it is important to note that the substrate should be readily available and C-H activation reactions using external oxidants or using alternative substrates are poorly accessible. We reasoned that readily accessible N-sulfinylimines may serve this purpose because the coordinating properties and polarity of the S=O bond may facilitate both C-H activation and N-S bond cleavage. Previously, C-H activation assisted by N-sulfonyl imine or other N-S functionalities have been extensively explored.14 However, only in one case was N-S cleavage involved (Scheme 1).13 On the other hand, although the N-O bond in oxime ethers/esters may well act as an internal oxidizing DG, their couplings with olefins only afforded sixmembered rings (Scheme 1).15 We now report complementary reactivity of N-sulfinylimines in C-H activation and coupling with different olefins, leading to efficient construction of 5and 6-membered heterocycles that are hardly accessible.
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Scheme 1. C-H Activation of Imines Bearing an Oxidizing DG Results and Discussions Optimization Studies. We initiated our studies with the exploration of the reactions parameters of the coupling between benzophenone-derived N-tert-butylsulfinyl ketoimine ((R)-1a) and ethyl acrylate (2a) catalyzed by [RhCp*Cl2]2/AgSbF6 in DCE at 100 oC (Table 1). The coupling proceeded with poor efficiency when no additive was employed (entry 1). Introduction of HOAc (1.2 equiv) as an additive improved the GC yield to 39% (entry 2), and the coupled product 3aa was fully characterized as a racemic 1Hisoindole, and the identity of one of its analogues has been unambiguously confirmed by X-ray crystallography (3ed, see Scheme 2).16 Thus, three bonds were constructed around a quaternary carbon in a single step.17 Other acid additives (TFA and PivOH) proved to be less efficient (entries 3, 4). Further improvement was realized when HOAc was used as a cosolvent, and the optimal volumetric ratio of DCE:HOAc was found to be 30:1, which corresponds to 8 equiv of AcOH (entries 7-9), under which conditions the product was isolated in 81% yield. In contrast to in situ activation of the rhodium catalyst via chlroide abstraction, coupling using a pre-formed cationic complex [RhCp*(MeCN)3](SbF6)2 only gave inferior results (entry 10). To our delight, high GC yield (92%) of 3aa was secured when Zn(OTf)2 (0.1 equiv) was further introduced as a co-additive (entry 11).9b Notably, the N-tert-butylsulfinyl group played an important role because switching this group to a N-tert-butylsulfonyl (1a’ ’ ) or to a N-para-tolylsulfinyl (1a’’ ’’) ’’ group all gave poor or no reaction (entries 13, 14). We also found that the reaction efficiency was soemwhat lower when AgOTf or Cu(OTf)2 was used as an additive (entries 15, 16). Table 1. Optimization Studiesa
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entry
additive (equiv)
solvent
yield (%)b
1
-
DCE
< 10
2
HOAc (1.2)
DCE
39
3
PivOH (1.2)
DCE
25
4
TFA (1.5)
DCE
