Direct deoxygenation of active allylic alcohols via metal-free catalysis. Issue 8 (8th February 2022)

• Record Type: Journal Article
• Authors: Liu, Qiaowen; Han, Feng; Zhuang, Hongfeng; Zhang, Tong; Ji, Na; Miao, Chengxia
• Is Part Of: Organic & biomolecular chemistry. Volume 20:Issue 8(2022)
• Page Start: 1680
• Page End: 1689
• Publication Date: 2022-02-08
• Subjects: Chemistry, Organic -- Periodicals; Bioorganic chemistry -- Periodicals; Chemistry, Physical organic -- Periodicals; 547
• DOI: 10.1039/d1ob02168k
• Languages: English
• ISSNs: 1477-0520

Abstract

Abstract : A simple and metal-free catalytic system for direct deoxygenation of highly active allylic alcohols was developed with TsOH·H2 O as the catalyst and p -methylbenzyl alcohol as the reductant. Abstract : Direct metal-free deoxygenation of highly active allylic alcohols catalyzed by a Brønsted acid was achieved, which avoids tedious reaction steps and eliminates metal contamination. By examining a series of Brønsted acids, alcohols, reaction temperatures and so on, up to 94% yield was obtained with 10 mol% TsOH·H2 O as the catalyst and 2 equiv. of p -methylbenzyl alcohol as the reductant at 80 °C for 2 h. The system was mainly suitable for aromatic allylic alcohols, and the yield was excellent as determined via gram-scale synthesis. The main product was double bond near the side of a more electron-rich aryl group when allylic alcohols featuring different substituents at the 1 and 3 positions were used as the substrates. Deuterium-labelled experiments clearly demonstrated that the hydrogen source was the methylene of p -methylbenzyl alcohol and other control experiments indicated the existence of two ether intermediates. Interestingly, in situ hydrogen transfer of allylic benzyl ether is a key process, but kinetic isotopic effect studies ( k H / k D = 1.28) showed that the C–H bond cleavage was not the rate-determining step. A possible mechanism involving carbocations, ether intermediates and hydrogen transfer is proposed.