Rhodium porphyrin alkylations with ammonium and quinolinium salts and cyclic ether formation via a palladium catalyzed dehydrogenative annulation
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The formation and study of metal–carbon σ-bonds can help unveil unique reactivities of organometallic complexes and provide support for further catalytic transformations. Rhodium porphyrins have shown exceptional reactivity through radical- type transformations, attracting significant attention towards understanding these metalloradical-mediated mechanisms. The stability and selectivity of rhodium porphyrins are promising for catalytic transformations, however, strong rhodium–carbon bonds frequently limit catalyst turnover. To gain a better understanding of Rh–C bonds in the porphyrin system, the synthesis of alkyl rhodium porphyrins through a C–N bond dealkylation of ammonium and quinolinium salts was conducted. The organometallic complexes were formed under air and with water, serving as a convenient method to prepare Rh–C bonds. Mechanistic studies support rhodium(I), rhodium(II), and rhodium(III) porphyrin intermediates operating in the alkylation, with a SN2-like reaction in the Rh–C bond forming step. A directed sp3 C–H bond functionalization strategy was also investigated to accomplish cyclic ether formation via an intramolecular alkoxylation reaction. An oxime vii directing group provided chemoselective activation at β-methyl positions, forming annulated products from the addition of tethered alcohol nucleophiles. Four- to seven- membered rings could be accessed through this dehydrogenative annulation pathway. Tethered primary, secondary, and tertiary free hydroxyl groups can all react to give the corresponding cyclized products. Protected silyl and benzyl alcohols were also compatible nucleophiles for the coupling. Preliminary mechanistic analysis supports an sp3 C–H activation/intramolecular SN2 pathway.