Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 129, Issue 19, Pages 6328-6335Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja070424u
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Funding
- NIGMS NIH HHS [GM67173] Funding Source: Medline
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A number of different Pd-II catalyst systems have been reported recently for the Wacker-type aerobic oxidative cyclization of alkenes bearing tethered nitrogen nucleophiles. This study examines the stereochemistry of the aminopalladation step with five different catalyst systems: Pd(OAc)(2)/DMSO (A), PdX2/pyridine [X = OAc (B), O2CCF3 (C)], Pd(IMes)(O2CCF3)(2)(OH2) (D), and Pd(O2CCF3)(2)/(-)-sparteine (E). Use of a stereospecifically deuterated cyclopentene substrate reveals that four of the five catalyst systems (A, B, C, and E) promote exclusive cis-aminopalladation of the alkene, whereas both cis- and trans-aminopalladation occur with the N-heterocyclic-carbene (NHC) catalyst system. If stoichiometric Bronsted base (NaOAc, Na2CO3) is added to the latter reaction conditions, however, only cis-aminopalladation is observed. The identity of the nitrogen nucleophile also affects the aminopalladation pathway, with results ranging from exclusively cis- to exclusively trans-aminopalladation. These results have important implications for ongoing efforts to develop enantioselective methods for Pd-catalyzed oxidative amination of alkenes.
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