Conformational effects in β-diiminate ligated magnesium and zinc amides. Solution dynamics and lactide polymerization

The preparation of the compounds LMg(NiPr2)(THF) (1); and LZnNiPr2 (2), are reported for L=the bulky β-diiminate ligand, CH(CMeN-2-tBuC6H4)2. In solution compound 2 is shown to exist as a mixture of syn- and anti-rotamers that do not interconvert significantly. Compound 1 readily and reversibly dissociates THF in benzene-d6 or toluene-d8 from a site where THF is syn to the tBu group of L. Both 1 and 2 are catalyst precursors for the ring-opening polymerization of lactides and it is shown that the syn-conformer of 2 reacts much faster than the anti. Polymerization of rac-lactide employing 2 in benzene or CH2Cl2 or 1 in THF yield approximately 90% heterotactic PLA (isi+sis). These results are compared with related work by Coates [J. Am. Chem. Soc. 123 (2001) 3229]; and us [J. Chem. Soc., Dalton Trans. (2001) 222] and us employing the symmetric β-diiminate ligand CH(CMeN-2,6-iPr2C6H3)2.