Structural and reactivity insights in Mg–Zn hybrid chemistry : Zn–I exchange and Pd-catalysed cross-coupling applications of aromatic substrates

Bluemke, Tobias D. and Clegg, William and García-Alvarez, Pablo and Kennedy, Alan R. and Koszinowski, Konrad and McCall, Matthew D. and Russo, Luca and Hevia, Eva (2014) Structural and reactivity insights in Mg–Zn hybrid chemistry : Zn–I exchange and Pd-catalysed cross-coupling applications of aromatic substrates. Chemical Science, 5 (9). pp. 3552-3562. ISSN 2041-6520

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    Abstract

    Expanding the synthetic potential of Mg-Zn hybrid organyl reagents (generated via transmetallation reactions), this study uncovers a versatile approach, involving a sequence of direct Zn-I exchange and Pd catalysed cross-coupling reactions which grants access to a wide range of asymmetric bis(aryls). By combining X-ray crystallography with ESI-MS and non-deuterium NMR spectroscopic studies, new light is shed on the heterobimetallic constitution of the intriguing organometallic species [(THF)4MgCl 2Zn(tBu)Cl] (1) and [{Mg2Cl3(THF) 6}+{ZntBu3}-] (2), formed through transmetallation of tBuMgCl with n equivalent amounts of ZnCl2 (n = 1 and 3 respectively). Operating by cooperative effects, alkyl-rich hybrid 2 can effectively promote direct Zn-I exchange reactions with aromatic halides in short periods of time at room temperature in THF solution. The structural elucidation of key organometallic intermediates involved in some of these Zn-I exchanges, provides new reactivity insights into how these bimetallic systems operate. Thus, while the reaction of 2 with 3 equivalents of 2-iodoanisole (3b) gives magnesium dizincate [{Mg(THF)6}2+{Zn(o-C 6H4-OMe)3}2 -] (4) which demonstrates the 3-fold activation of the tBu groups attached to Zn in 2, using 2-iodobenzonitrile (3i), only two tBu groups react with the substrate, affording [(THF)4MgCl(NC-o-C6H4)ZnI(o-C 6H4-CN)(THF)] (7). In 7 Mg and Zn are connected by an aryl bridge, suggesting that the formation of contacted ion-pair hybrids may have a deactivating effect on the outcome of the Zn-I exchange process. A wide range of homoleptic tris(aryl) zincate intermediates have been prepared in situ and used as precursors in Pd catalysed cross-coupling reactions, affording bis(aryls) 6a-s in excellent yields under mild reaction conditions without the need of any additive or polar cosolvent such as NMP or DMI.