Issue 23, 2019

Alkali metal and stoichiometric effects in intermolecular hydroamination catalysed by lithium, sodium and potassium magnesiates

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)2K2Mg(CH2SiMe3)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(CH2SiMe3)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 of 7 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.

Graphical abstract: Alkali metal and stoichiometric effects in intermolecular hydroamination catalysed by lithium, sodium and potassium magnesiates

Supplementary files

Article information

Article type
Paper
Submitted
01 Mar 2019
Accepted
16 Apr 2019
First published
16 Apr 2019

Dalton Trans., 2019,48, 8122-8130

Alkali metal and stoichiometric effects in intermolecular hydroamination catalysed by lithium, sodium and potassium magnesiates

L. Davin, A. Hernán-Gómez, C. McLaughlin, A. R. Kennedy, R. McLellan and E. Hevia, Dalton Trans., 2019, 48, 8122 DOI: 10.1039/C9DT00923J

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