Phosphine‐Catalyzed [4+1] Cycloadditions of Allenes with Methyl Ketimines, Enamines, and a Primary Amine

• Published in: Angewandte Chemie 2020-01, Vol.132 (5), p.1900-1906
• Main Authors: Cao, Ze‐Hun, Wang, Yu‐Hao, Kalita, Subarna Jyoti, Schneider, Uwe, Huang, Yi‐Yong
• Format: Article
• Language: English
• Subjects: [4+1]-Cycloaddition; Allene; Chemistry; Enaminone; Imides; Imines; Intermediates; Ketene; NMR; Nuclear magnetic resonance; Nukleophile Katalyse; Organic chemistry; Phosphine; Phosphines; Pyrazole; Substrates
• ISSN: 0044-8249; 1521-3757
• DOI: 10.1002/ange.201912263

Unprecedented phosphine‐catalyzed [4+1] cycloadditions of allenyl imides have been discovered using various N‐based substrates including methyl ketimines, enamines, and a primary amine. These transformations provide a one‐pot access to cyclopentenoyl enamines and imines, or (chiral) γ‐lactams through two geminal C−C bond or two C−N bond formations, respectively. Several P‐based key intermediates including a 1,4‐(bis)electrophilic α,β‐unsaturated ketenyl phosphonium species have been detected by 31P NMR and HRMS analyses, which shed light on the postulated catalytic cycle. The synthetic utility of this new chemistry has been demonstrated through a gram‐scaling up of the catalytic reaction as well as regioselective hydrogenation and double condensation to form cyclopentanoyl enamines and fused pyrazole building blocks, respectively. Gib mir fünf! Phosphin‐katalysierte [4+1]‐Cycloadditionen eines Allens mit Methylketiminen und ‐enaminen verlaufen über ein 1,4‐(bis)elektrophiles α,β‐ungesättigtes Ketenylphosphonium‐Intermediat und liefern Cyclopentenoylenamine bzw. ‐imine.