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Rh(III)‐Catalyzed Cyclopropanation Initiated by C—H Activation: Ligand Development Enables a Diastereoselective [2 + 1] Annulation of N‐Enoxyphthalimides and Alkenes.

N-Enoxyphthalimides undergo a Rh(III)-catalyzed C−H activation initiated cyclopropanation of electron deficient alkenes. The reaction is proposed to proceed via a directed activation of the olefinic C−H bond followed by two migratory insertions, first across the electron-deficient alkene and then by cyclization back onto the enol moiety. A newly designed isopropylcyclopenta-dienyl ligand drastically improves yield and diastereose-lectivity. Rh(III) mediated C−H activation has proven to be a powerful approach for the synthesis of nitrogen containing heterocycles. 1 Building on a foundation of reactivity first described by Miura and Satoh 2 and Fagnou, 3 we 4 and others 5 reported the coupling of benzamides with alkynes to form isoquinolones via Rh(III) C−H activation. 6,7 Seminal work by Fagnou 8 and Glorius 9 demonstrated that an internal oxidant in the form of an N-pivaloxy substituent leads to efficient Rh(III)-catalyzed C−H activation giving dihydroisoquinolinones from benzamides and alkenes (eq 1, Scheme 1). A number of other internal oxidants 10 have been advanced including oximes, 11 hydra-zines, 12 and phenoxyamides (eq 2). 13 With prominent exceptions (such as eqs 3 and 4), most of these examples functionalize arene C−H bonds. Reasoning that the discovery of novel activation reactions must entail developing new directing groups, we sought to develop a protocol to activate vinyl C−H bonds and considered enoxyamides as potential precursors. The directing group would bear the ubiquitous amide carbonyl moiety required for activation but, similar to phenoxyamide directing groups, would result in release of the amide in the N−O cleavage/Rh reoxidation step. Herein, we report the development of the first Rh(III)-catalyzed cyclo-propanation via C−H activation (eq 5). Critical to the success of this reaction was the design of a monosubstituted isopropylcyclopentadienyl ligand. We selected phenyl-N-enoxyphthalimide 1a as a model substrate, readily accessible using Anderson's approach (eq 6). 14 With a suitable Rh(III)-catalyst, activation of the vinyl C− H bond could generate seven-membered rhodacycle I, allowing for further functionalization with a suitable electrophile. To our surprise, when alkene 2 was used as the coupling partner in initial screening, intermediate I acted as a one-carbon component in a [2 + 1] annulation giving the disubstituted trans-cyclopropane 3. Owing to their prevalence in biologically active natural and synthetic compounds, cyclopropane units are fundamental structures in organic chemistry. 15 As a result, the development of new strategies for their synthesis is of foremost interest, and stimulated our efforts to improve and generalize this transformation. Our discovery and initial development …