Alkali metal and stoichiometric effects in intermolecular hydroamination catalysed by lithium, sodium and potassium magnesiates. Issue 23 (2nd May 2019) - Davin, Laia
Abstract : Cooperative catalytic hydroamination of diphenylacetylene and styrene is accomplished by magnesiates with efficiency dependent on the alkali metal and monoanionic or dianionic nature of the ate with dianionic [(PMDETA)2 K2 Mg(CH2 SiMe3 )4 ] performing best. Abstract : Main group bimetallic complexes, while being increasingly used in stoichiometric deprotonation and metal–halogen exchange reactions, have not yet made a significant impact in catalytic applications. This paper explores the ability of alkali metal magnesiates to catalyse the intermolecular hydroamination of alkynes and alkenes using sytrene and diphenylacetylene as principle setting model substrates. By systematically studying the role of the alkali–metal and the formulation of the heterobimetallic precatalyst, this study establishes higher order potassium magnesiate [(PMDETA)2 K2 Mg(CH2 SiMe3 )4 ] (7 ) as a highly effective system capable of catalysing hydroamination of styrene and diphenylacetylene with several amines while operating at room temperature. This high reactivity contrasts with the complete lack of catalytic ability of neutral Mg(CH2 SiMe3 )2, even when harsher reaction conditions are employed (24 h, 80 °C). A pronounced alkali metal effect is also uncovered proving that the alkali metal (Li, Na, or K) is not a mere spectating counterion. Through stoichiometric reactions, and structural and spectroscopic (DOSY NMR) investigations we shed some light on the potential reaction pathway as well as the constitution of key intermediates. This work suggests that the enhanced catalytic activity of7 can be rationalised in terms of the superior nucleophilic power of the formally dianionic magnesiate {Mg(NR2 )4 } 2− generated in situ during the hydroamination process, along with the ability of potassium to engage in π-interactions with the unsaturated organic substrate, enhancing its susceptibility towards a nucleophilic attack by the amide anion.