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Simple Alkaline‐Earth Metal Catalysts for Effective Alkene Hydrogenation
Author(s) -
Bauer Heiko,
Alonso Mercedes,
Fischer Christian,
Rösch Bastian,
Elsen Holger,
Harder Sjoerd
Publication year - 2018
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.201810026
Subject(s) - styrene , alkene , chemistry , catalysis , norbornene , protonation , cyclohexene , double bond , metal , norbornadiene , polymerization , medicinal chemistry , polymer chemistry , photochemistry , copolymer , organic chemistry , ion , polymer
Alkaline earth metal amides (AeN′′ 2 : Ae=Ca, Sr, Ba, N′′=N(SiMe 3 ) 2 ) catalyze alkene hydrogenation (80–120 °C, 1–6 bar H 2 , 1–10 mol % cat.), with the activity increasing with metal size. Various activated C=C bonds (styrene, p ‐MeO‐styrene, α‐Me‐styrene, Ph 2 C=CH 2 , trans ‐stilbene, cyclohexadiene, 1‐Ph‐cyclohexene), semi‐activated C=C bonds (Me 3 SiCH=CH 2 , norbornadiene), or non‐activated (isolated) C=C bonds (norbornene, 4‐vinylcyclohexene, 1‐hexene) could be reduced. The results show that neutral Ca or Ba catalysts are active in the challenging hydrogenation of isolated double bonds. For activated alkenes (e.g. styrene), polymerization is fully suppressed due to fast protonation of the highly reactive benzyl intermediate by N′′H (formed in the catalyst initiation). Using cyclohexadiene as the H source, the first Ae metal catalyzed H‐transfer hydrogenation is reported. DFT calculations on styrene hydrogenation using CaN′′ 2 show that styrene oligomerization competes with styrene hydrogenation. Calculations also show that protonation of the benzylcalcium intermediate with N′′H is a low‐energy escape route, thus avoiding oligomerization.