Alkali metal zincate chemistry : the importance of structure and stoichiometry on reactivity

• Main Author: Herd, Emma
• Published: University of Strathclyde 2014
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.632703

Focusing on the area of cooperative bimetallic reagents, this report aims to advance our understanding on the construction and reactivity of alkali metal zincates. Extending previous studies on the synthesis of dialkyl(amido)zincates, unsolvated lithium zincate [LiZn(HMDS)Me₂] 4a was prepared by a cocomplexation approach, and its interaction with a range of Lewis basic solvents of varying denticity was examined. The effect of these donors on constitution and stability was assessed, resulting in the determination of a novel dialkyl(amido)zincate, [(PMDETA)LiZn(HMDS)Me₂] 5, containing an unusual terminal amido ligand. The influence of the amido group in these mixed-metal systems was also studied. Using the amide [NHDipp]⁻, a new family of lithium zincates has been synthesised, leading to the characterisation of the first dialkyl(amido)zincates based on a primary amide - [(PMDETA)LiZn(NHDipp)Me₂] 10 and [(THF)₃LiZn(NHDipp)(Me)₂] 11. X-ray crystallographic studies evidenced that, depending on the donor solvent employed, novel zincate formulations can be accomplished - as demonstrated by the isolation of the unprecedented zinc-rich zincate [LiZn₂(NHDipp)₂(Me)₃(THF)₃] 12, containing an unusual 2:1 zinc to lithium ratio. By synthesising the homoaryl lithium zincates [LiZnPh₃] 15 and [Li₂ZnPh₄] 16, and investigating their reactivity towards several electron-deficient N-heterocycles, a new transition metal free method enabling the efficient, regioselective arylation of acridine has been developed. Isolation of key reaction intermediates such as [(THF)₃Li(NC₁₃H₉-9-Ph)] 17, has provided important clues as to how these bimetallic systems operate. The reactivity of trimetallic mixtures based on [Zn(OPiv)₂·2LiCl] 27 and ArMgCl towards N-heterocyclic molecules was also investigated. Although unreactive towards arylation, new light has been shed on the constitution, reactivity and stability of these multi-component reagents. Finally, the bulky bis(amido)silyl ligand [Ph₂Si(NDipp)²⁻] has been successfully incorporated into two novel zincates, [{Na(THF)₆}+{(Ph₂Si(NDipp)₂)ZnEt}⁻] 29 and [(Ph₂Si(NDipp)₂)Zn(TMP)Na(THF)] 30, which have been employed in the regioselective zincation of benzothiazole and pyrrole, yielding the novel compounds [Na(THF)₂Zn(Btz)₃]₂ 31 and [Na₂(THF)₄Zn(NC₄H₄)₄]∞ 32.