INTRAMOLECULAR CARBON-NITROGEN COUPLING FROM ISOLATED MONOHYDROCARBYL PALLADIUM(IV) COMPLEXES PREPARED USING H2O2 AS TERMINAL OXIDANT
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Carbon-nitrogen coupling is achieved traditionally by coupling of aryl halides and amines through a Pd(0)/Pd(II) catalytic cycle (Buchwald-Hartwig amination). A newer, more atom-economical approach to the synthesis of amines is based on oxidative C-H amination. Recent studies of C-H amination propose the involvement of Pd(II)/Pd(IV) catalytic cycle through a C-H activation step.
This work seeks to develop new stoichiometric and catalytic ways of forming C-N bonds through a Pd(II)/Pd(IV) cycle using H2O2 as terminal oxidant. In this effort, di-2-pyridylketone(dpk) ligated palladacycles were synthesized, oxidized with H2O2, and the reductive elimination of the high oxidation state Pd(IV, d6) containing species monitored. N-R-2-aminobiphenyl – derived substrates with electron donating groups (R = H, Me, Et) readily form carbazoles at room temperature without formation of appreciable amounts of intermediates. The use of electron withdrawing group (R = COCH3, COCF3, SO2CH3, SO2CF3) slows down the reaction for intermediates to be observed and isolated. Mechanistic studies of the first ever C(sp2)-N reductive elimination from an isolated Pd(IV, d6) intermediate was observed to be accelerated in the presence of acids. Reductive elimination is proposed to occur from a 6-coordinate Pd(IV, d6) center.
The dpk ligated Pd(IV, d6) palladacycles derived from 4-X-substituted N-SO2CF3-2-tert-butylaniline (X = H, Br, I), reductively eliminate the corresponding C(sp3)-N coupled products, 5-X-substituted indolines in high yield only in the presence of halohalic acids (HCl, HBr and HI). This confirms the importance of a proton source and a nucleophilic anion for this process to take place. Reductive elimination is proposed to occur through several competing pathways based on the fractional order of reaction with respect to [Br-] in solution.
Catalytic heterocyclization reaction was achieved using H2O2 with N-acetyl-2-aminobiphenyl to form N-acetylcarbazole with yields dependent on temperature and rate of addition of H2O2.