Synthetic and reactivity studies of hetero-tri-anionic sodium zincates

Abstract

The synthesis and characterisation of several sodium zincate complexes are reported. The all-alkyl monomeric sodium zincate, (PMEDTA)·Na(μ-CH₂SiMe₃)Zn^tBu₂2, is prepared by combining equimolar quantities of ^tBu₂Zn, ⁿBuNa and PMDETA (N,N,N′,N′′,N′′-pentamethyldiethylenetriamine)]. A similar approach was used to prepare and isolate the unusual dimeric zincate [(PMEDTA)·Na(μ-ⁿBu)Zn^tBu₂]₂3. When an equimolar mixture of ⁿBuNa, ^tBu₂Zn and TMP(H) (2,2,6,6-tetramethylpiperidine) is combined in hexane, the hetero-tri-leptic TMP(H)-solvated zincate (TMPH)Na(μ-TMP)(μ-ⁿBu)Zn^tBu 4 results. Complex 4 can also be prepared using a rational approach [i.e., utilising two molar equivalents of TMP(H)]. When TMEDA is reacted with an equimolar mixture of ⁿBuNa, ^tBu₂Zn and TMP(H), the monomeric sodium zincate (TMEDA)Na(μ-TMP)(μ-ⁿBu)Zn^tBu 5 was obtained – this complex is structurally similar to the synthetically useful relation (TMEDA)·Na(μ-TMP)(μ-^tBu)Zn(^tBu) 1. By changing the sodium reagent used in the synthesis of 5, it was possible to prepare (TMEDA)Na(μ-TMP)(μ-Me₃SiCH₂)Zn^tBu 6. By reacting 5 with cis-DMP(H) (cis-2,6-dimethylpiperidine), the zincate could thermodynamically function as an amide base, to give the transamination product (TMEDA)Na(μ-cis-DMP)(μ-ⁿBu)Zn^tBu 7, although no crystals could be grown. However, when HMDS(H) (1,1,1,3,3,3-hexamethyldisilazane) or PEA(H) [(+)-bis[(R)-1-phenylethyl]amine] is reacted with 5, crystalline (TMEDA)Na(μ-HMDS)(μ-ⁿBu)Zn^tBu 8 or (TMEDA)Na(μ-PEA)(μ-ⁿBu)Zn^tBu 9 is isolated respectively. With PNA(H) (N-phenylnaphthalen-1-amine) the reaction took a different course and resulted in the formation of the dimeric sodium amide complex [(TMEDA)Na(PNA)]₂10. When reacted with benzene, it appears that a TMEDA-free variant of 5 functions thermodynamically as an ⁿBu base to yield the previously reported (TMEDA)Na(μ-TMP)(^tBu)Zn(μ-C₆H₄)Zn(^tBu)(μ-TMP)Na(TMEDA) 11. Finally when reacted with TEMPO (2,2,6,6-tetramethylpiperidinyloxy), 5 undergoes a single electron transfer reaction to form (TMEDA)Na(μ-TMP)(μ-TEMPO)ZnⁿBu 12.