期刊
CHEMISTRY-A EUROPEAN JOURNAL
卷 28, 期 35, 页码 -出版社
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.202200665
关键词
alkyl-alkyl coupling; beta-hydride elimination; cross-coupling; Negishi coupling; PEPPSI
Bulky Pd-N-heterocyclic carbene (NHC) catalysts have higher reactivity in cross-coupling reactions, and chlorinating the NHC core further increases the reactivity. However, a reversal of these trends is observed in the cross-coupling of primary alkyl electrophiles and nucleophiles.
Bulky Pd-N-heterocyclic carbene (NHC) catalysts (e. g., N-(di-2,6-(3-pentyl)phenyl), IPent) have been shown to have significantly higher reactivity in a wide variety of cross-coupling applications (i. e., C-C, C-S, C-N) than less hindered variants (e. g., N-(di-2,6-(isopropyl)phenyl), IPr). Further, chlorinating the backbone of the NHC ring sees an even greater increase in reactivity. In the cross-coupling of (hetero)aryl electrophiles to secondary alkyl nucleophiles, making the N-aryl groups larger reduces the amount of beta-hydride elimination leading to alkene byproducts and chlorinating the NHC core had an even greater effect, all but eliminating alkene formation. In the present study involving the cross-coupling of primary alkyl electrophiles and nucleophiles, a sharp and surprising reversal of all of the above trends was observed. Bulkier catalysts had generally slower rate of reaction and beta-hydride elimination worsened leading to extensive amounts of alkene byproducts.
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