Metal‐Free Coupling of 3‐Alkenyl Oxoindoles by Nucleophilic Vinylic Substitution of Nitroolefins

Abstract

The oxindole framework is a privileged heterocyclic motif that is present in the core of a large family of bioactive natural products and pharmaceutically active compounds, [1] such as Wasalexin A, [1a] Costinone B, [1f] and Sunitinib [1g] (Scheme 1). This versatile privileged structure and potential bioactvivity have aroused wide interest from chemists. Diverse transformations into related structures have been reported. [2] Nucleophilic vinylic substitution (S N V) [3] has emerged as a powerful method for the formation of the carboncarbon [4] or heteroatom-substituted-carbon-carbon double bonds, [5] in which a leaving group at the sp 2 -hybridized carbon center can be replaced by a nucleophile. This method is versatile for the synthesis of vinyl ethers, enamines, a variety of heterocyclic systems, [6] olefins, and intermediates for producing pharmaceutically important compounds. However, the existence of p-p conjugation between the leaving group and the olefin makes this transformation a challenging task. During the past three decades, many fascinating examples of exquisite S N V transformations have been described and mechanistic understanding of several processes, including biological processes, has been achieved. Notably, the main body of research on S N V reactions is still focused on those that use halogens, alkoxyl group, [7] and sulfonates [8] as nucleofuges. To our knowledge, there is still no report on the S N V replacement of nitro groups in nitroolefins.In general, the strong electron-withdrawing effect of a nitro group can orient nucleophilic attack at the b position of a nitroolefin, which results in 1,4-addition (Eq. (1), Scheme 2) or 1,4-addition/1,2-trans elimination [9] (Eq. (2), Scheme 2). The p-p conjugation between the nitro group and the CÀC double bond drastically increases the difficulty of the substitution. Therefore, S N V reactions of nitroolefins in which the nitro group is a nucleofuge are not only a great challenge, but are also significant transformations that have a high theoretical value (Eq. (3), Scheme 2).On the basis of our understanding [10] of the S N V mechanism in terms of transition state and perpendicular nucleophilic attack, we postulated that the electron-withdrawing groups R 1 and R 2 in the b position adjacent to the C a reaction site could increase nucleophilic reactivity in the presence of base, which might lead to nucleophilic vinylic substitution (Eq. (3), Scheme 2). Herein, we describe the successful execution of these ideas and present an S N V reaction of nitroolefins assisted by p-p conjugation between a nitro group and an olefin. This reaction not only successfully constructs carbon-heteroatom bonds, but also forms carbon-carbon bonds. It is a metal-free coupling reaction. We hope that this type of organic transformation can serve as a prodigious starting point for the discovery of new reaction modes for the S N V reaction. Moreover, this reaction could be a versatile method for the synthesis of biologically [a]