Sodium dialkyl-amidozincates: alkyl or amido bases? An experimental and theoretical case study

Alkali metal zincate reagents are attracting considerable attention at present in respect to their often special reactivity/selectivity in hydrogen-metal and halogen-metal interconversion reactions. Heteroleptic diorgano-amidozincates, typified by lithium di-tert-butyltetramethylpiperidinozincate, have proved to be especially useful reagents in such applications. In this paper the related sodium TMP-zincate, prepared as its TMEDA (N,N,N',N'-tetramethylethylenediamine) adduct, [TMEDA.Na(mu-tBu)(mu-TMP)Zn(tBu)], 1, is introduced. This new zincate was synthesized from a 1:1:1 mixture of tBu2Zn, NaTMP, and TMEDA in hexane solution, as a colorless crystalline solid in an isolated yield of 58%. It has been characterized in solution by 1H and 13C NMR spectroscopic studies. An X-ray crystallographic study reveals that 1 adopts a five-membered (NaNZnCC) ring system featuring a TMP bridge and an unusual, asymmetrical tBu bridge involving a Na...Me agostic contact. Probing the basicity of 1, reaction with benzene affords the new hetero(tri)leptic zincate [TMEDA.Na(mu-Ph)(mu-TMP)Zn(tBu)], 2, which has also been crystallographically characterized. Thus, in this hydrogen-metal exchange reaction 1 functions as an alkyl base, with the elimination of butane, as opposed to an amido base. Also reported are DFT calculations using B3LYP functionals and the 6-311G** basis set on model zincate systems, which intimate that the preference of 1 for tBu ligand transfer over TMP ligand transfer in the reaction toward benzene is due to favorable thermodynamic factors.