The nature of the reformatsky reagent. Crystal structure of (BrZnCH2COO-t-Bu·THF)2

Article abstract of DOI:10.1021/om00087a015

The nature of the Reformatsky reagents derived from the ethyl and tert-butyl esters of bromoacetic acid has been studied by association measurements and NMR spectroscopy in various solvents. It appears that the reagents are dimeric in all but the most polar solvents. A dimeric structure containing both Zn-O and Zn-C single bonds is present in the crystalline state of (BrZnCH₂COO-t-Bu·THF)₂, the final X-ray analysis of which is reported. Crystals are monoclinic of space group P2₁/n with two dimers in a cell of dimensions a = 10.322 (3) A?, b = 12.358 (3) A?, c = 11.655 (2) A?, and β = 112.65 (2)°. The structure was refined to R = 0.055 on 881 observed reflections with I > 2.5σ(I). It is concluded that the dimeric structure found in the crystal persists in solution. In the very polar solvent Me₂SO, the reagents are monomeric C-metalated species. The consequences of these findings for the mechanism of the Reformatsky reaction in the commonly used solvents are discussed.