Alkene Transfer Hydrogenation with Alkaline‐Earth Metal Catalysts

Abstract

The alkene transfer hydrogenation (TH) of avariety of alkenes has been achieved with simple AeN'' 2 catalysts [Ae = Ca, Sr,B a; N'' = N(SiMe 3 ) 2 ]u sing 1,4-cyclohexadiene (1,4-CHD) as aHsource.R eaction of 1,4-CHD with AeN'' 2 gave benzene,N''H, and the metal hydride species N''AeH (or aggregates thereof), whichi sacatalyst for alkene hydrogenation. BaN'' 2 is by far the most active catalyst. Hydrogenation of activated C=Cb onds (e.g. styrene) proceeded at room temperature without polymer formation. Unactivated (isolated) C=Cbonds (e.g. 1-hexene) needed ahigher temperature (120 8 8C) but proceeded without double-bond isomerization. The ligands fully control the course of the catalytic reaction, which can be:1 )alkene TH, 2) 1,4-CHD dehydrogenation, or 3) alkene polymerization. DFT calculations support formation of ametal hydride species by deprotonation of 1,4-CHD followed by Ht ransfer.C onvenient access to larger quantities of BaN'' 2 ,its high activity and selectivity,and the many advantages of TH make this as imple but attractive procedure for alkene hydrogenation. Scheme 1. Various concepts for transfer hydrogenation.

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

“…Such a transfer hydrogenation reaction would be orthogonal to known metal-catalyzed protocols. 42 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 5c). Intramolecular 1,5-hydride transfer has been achieved by Chiba and Xiao using Brønsted acid catalysis, 43−45 but to the best of our knowledge, this is the first example of a Brønsted acid catalyzed intermolecular transfer hydrogenation.…”