Abstract
Conventional approaches to Pd-catalyzed alkene 1,2-carboamination rely upon the combination of a nucleophilic nitrogen-based component and an internal C-based or external oxidant. In this study, we outline an umpolung approach, which is triggered by oxidative initiation at an electrophilic N-based component and employs "standard" organometallic nucleophiles to introduce the new carbon-based fragment. Specifically, oxidative addition of a Pd(0)-catalyst into the N-O bond of O-pentafluorobenzoyl oxime esters generates imino-Pd(II) intermediates, which undergo 5-exo cyclization with sterically diverse alkenes. The resultant alkyl-Pd(II) intermediates are intercepted by organometallic nucleophiles or alcohols, under carbonylative or noncarbonylative conditions, to provide 1,2-carboamination products. This approach provides, for the first time, a unified strategy for achieving alkene 1,2-amino-acylation, -carboxylation, -arylation, -vinylation, and -alkynylation. For carbonylative processes, orchestrated protodecarboxylation of the pentafluorobenzoate leaving group underpins reaction efficiency. This process is likely a key feature in related Narasaka-Heck cyclizations and accounts for the efficacy of O-pentafluorobenzoyl oxime esters in aza-Heck reactions of this type.
Highlights
Palladium-catalyzed 1,2-carboaminations of alkenes are valuable but challenging processes.[1]
Oxidative processes were pioneered by the groups of Hegedus, Tamaru and Yoshida (Approach A).[2−4] Here, Pd(II)triggered aminopalladation generates intermediates 1 that undergo carbonylative trapping with alcohols to provide esters
In certain cases 1,1-disubstitution on the alkene is tolerated, but in general, this is limited to small alkyl substituents.7f organometallic reagents are not compatible with Approach A, and so, for associated amino-acylation protocols, C-based nucleophiles are constrained by Friedel−Crafts reactivity patterns.[5]
Summary
Palladium-catalyzed 1,2-carboaminations of alkenes are valuable but challenging processes.[1]. Because alkene coordination to Pd is intramolecular, relatively hindered 1,1-disubstituted olefinic partners should be compatible, which would complement existing carboamination approaches.[10] Second, as the site of oxidative addition is at the N-based fragment, a wide range of C−C bond formations should be accessible by trapping of common intermediate 1 with “standard” organometallic nucleophiles.[11] In this report, we show that this approach can be achieved by harnessing the oxidative addition of Pd(0)catalysts into N−O bond of O-pentafluorobenzoyl oxime esters, a process that is the basis of the Narasaka−Heck reaction (Scheme 1, gray box).[12−15] The versatility of this strategy is highlighted by prototypical methods for five distinct classes of alkene carboamination: 1,2-amino-acylation, -carboxylation,
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