Alkyl Ethers as Traceless Hydride Donors in Brønsted Acid Catalyzed Intramolecular Hydrogen Atom Transfer

Abstract

A new protocol for the deoxygenation of alcohols and the hydrogenation of alkenes under Brønsted acid catalysis has been developed. The method is based on the use of either a benzyl or isopropyl ether as a traceless hydrogen-atom donor, and involves an intramolecular hydride transfer as a key step, which is achieved in a regio- and stereoselective manner.

Enantioselective Redox-Neutral Coupling of Aldehydes and Alkenes by an Iron-Catalyzed “Catch–Release” Tethering Approach

“…We assume that the aldehyde R substituent and the carbinol residue R 1 adopt pseudoequatorial orientations during the cyclization event, at which point the first stereogenic center of the product is formed. During the hydride transfer step, 11 the original chiral element of the substrate is deleted concomitantly with the setting of the second stereogenic center of the product. As hydride transfer must proceed suprafacially, this ensures perfect chirality transfer and rationalizes the excellent stereoselectivities observed even when acetaldehyde, otherwise a challenging aldehyde for enantioselective C–C bond forming reactions is employed ( 3r ).…”

Lewis Acid Catalyzed Reductive Cyclization of 2‐Aryloxybenzaldehydes and 2‐(Arylthio)benzaldehydes to Unsubstituted 9H‐Xanthenes and Thioxanthenes in Diisopropyl Ether

“…In recent literature, alkyl ethers, in the presence of suitable Lewis acids/Bronsted acids, have been utilized as traceless hydride transferring agents for highly effective intramolecular [20] as well as intermolecular [21] reduction of transient carbenium ions. For instance, Gagosz, Chiba and co‐workers reported an elegant TsOH catalyzed intramolecular hydrogenation of alkene using benzyl and isopropyl ether as the traceless hydride transferring groups under redox‐neutral conditions [20a] . Similarly, Xiong, Chen and co‐workers demonstrated that diarylmethyl carbenium ions, generated in situ from diarymethyl alcohols by treating with stoichiometric amount (1.2 equiv.)…”

Hydroarylation of Alkenes by Protonation/Friedel–Crafts Trapping: HFIP-Mediated Access to Per-aryl Quaternary Stereocenters

“…Pleasingly, use of γ-terpinene 24 as the hydride source and iodoindane 23 as alkenyl substrate led to the desired alkane 25 with the sensitive aryl iodide moiety still intact (Scheme c). Intramolecular 1,5-hydride transfer has been achieved by Chiba and Xiao using Brønsted acid catalysis, − but to the best of our knowledge, this is the first example of a Brønsted acid catalyzed intermolecular transfer hydrogenation. Whereas there are a plethora of reports of intermolecular hydride transfer to carbonyls and imines from Hantzsch esters, this HFIP-mediated reaction is notable because alkene 23 is a poor electrophile and skipped diene 24 is a poor hydride donor as judged by their respective Mayr coefficients …”