Platinum‐Catalyzed Multistep Reactions of Indoles with Alkynyl Alcohols

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Abstract
PtCl2 effectively catalyzes the multistep reaction of N‐methyl indole (1 a) with pent‐3‐yn‐1‐ol (2 a) in THF at room temperature for 2 h to give indole derivative 3 a, which contains a five‐membered cyclic ether group at C3 in 93 % yield. Under similar reaction conditions, various substituted N‐methyl indoles 1 b–h and indole (1 i) reacted efficiently with 2 a to afford the corresponding indole derivatives 3 b–h and 3 i in 48–91 and 72 % yields. The results showed that N‐methyl indoles with electron‐donating substituents were more reactive affording higher product yields than those with electron‐withdrawing groups. Likewise, various substituted but‐3‐yn‐1‐ols 2 b–e and other longer chain alkynyl alcohols 2 f–i also underwent a cyclization–addition reaction with N‐methyl indole (1 a) to provide the corresponding cyclization–addition products 3 j–m and 3 a, 3 j, and 3 n–o in good to excellent yields. The present platinum‐catalyzed cyclization–addition reaction can be further extended into N‐methyl pyrrole. Mechanistically, the catalytic reaction proceeds by an intramolecular hydroalkoxylation of alkynyl alcohol to afford cyclic enol ether followed by the addition of the CH bond of indole to the unsaturated moiety of cyclic enol ether providing the final product. Experimental evidence to support this proposed mechanism is provided.

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