BDI∗MgX(L) where X = nBu and OtBu and L = THF, py and DMAP. The rates of kinetic exchange of L where BDI∗ = CH{C(tBu)N-2,6-iPr2C6H3}2

BDI∗MgnBu(THF) and rac-LA to produce heterotactic PLA have been shown to increase from Pr = 0.5–0.65 from DCM to neat THF. The increase in concentration of THF from MTHF = 0 in toluene to neat THF is shown to occur by THF dissociation as deduced by 1H NMR studies. In related systems where X = nBu and OtBu and with ligands L = THF, py and DMAP similar processes occur by which external exchange of L occurs with added free ligand. For X = nBu and L = THF, the activation parameters for the dissociation process are ΔH╪ = 7.0 ± 0.5 kcal mol−1 and ΔS╪ = −11.6 ± 0.4 cal mol−1 and for X = OtBu and L = THF are ΔH╪ = 11.1 ± 1.0 kcal mol−1 and ΔS╪ = −3.3 ± 3.9 cal mol−1. The BDI∗(R = tBu) is notably larger than BDI(R = Me) and this limits the use of THF to coordinate during catalysis. The three coordinate complex BDI∗MgnBu has been structurally characterized by single crystal X-ray crystallography: where Mg–N = 2.010(4) and 2.026(4) Å and Mg–C = 2.117(5) Å, with N–Mg–N = 94.2(2)° and N–Mg–C = 130.0(2)° and 135.8(2)°. The related complexes BDI∗MgnBu(L), where L = py and DMAP have similar structural features.

BDI∗MgX(L), where X = nBu or OtBu and L = THF, 2-MeTHF, py and DMAP, undergo the reversible reaction shown above by a dissociative process involving the loss of the ligand, L. This process is involved in the rate of polymerization of rac-LA to give heterotactic PLA.Figure optionsDownload as PowerPoint slide