Bauer Heiko, Alonso Mercedes, Fischer Christian, Rösch Bastian, Elsen Holger, Harder Sjoerd
Chair of Inorganic and Organometallic Chemistry, Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058, Erlangen, Germany.
Algemene Chemie, Vrije Universiteit Brussel, Pleinlaan 2, 1050, Elsene, Belgium.
Angew Chem Int Ed Engl. 2018 Nov 12;57(46):15177-15182. doi: 10.1002/anie.201810026. Epub 2018 Oct 15.
Alkaline earth metal amides (AeN'' : Ae=Ca, Sr, Ba, N''=N(SiMe ) ) catalyze alkene hydrogenation (80-120 °C, 1-6 bar H , 1-10 mol % cat.), with the activity increasing with metal size. Various activated C=C bonds (styrene, p-MeO-styrene, α-Me-styrene, Ph C=CH , trans-stilbene, cyclohexadiene, 1-Ph-cyclohexene), semi-activated C=C bonds (Me SiCH=CH , 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'' 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.
碱土金属酰胺（AeN'' ：Ae = Ca、Sr、Ba，N'' = N(SiMe ) ）催化烯烃氢化反应（80 - 120 °C，1 - 6 bar氢气，1 - 10 mol %催化剂），活性随金属原子尺寸增大而增强。各种活化的C=C键（苯乙烯、对甲氧基苯乙烯、α-甲基苯乙烯、Ph C=CH 、反式二苯乙烯、环己二烯、1-苯基环己烯）、半活化的C=C键（Me SiCH=CH 、降冰片二烯）或未活化（孤立）的C=C键（降冰片烯、4-乙烯基环己烯、1-己烯）均可被还原。结果表明，中性的Ca或Ba催化剂在具有挑战性的孤立双键氢化反应中具有活性。对于活化烯烃（如苯乙烯），由于在催化剂引发过程中形成的N''H对高活性苄基中间体的快速质子化，聚合反应被完全抑制。以环己二烯作为氢源，首次报道了碱土金属催化的氢转移氢化反应。使用CaN''对苯乙烯氢化反应进行的密度泛函理论计算表明，苯乙烯齐聚反应与苯乙烯氢化反应相互竞争。计算还表明，苄基钙中间体与N''H的质子化是一条低能量的逃逸途径，从而避免了齐聚反应。
