TMPZnN(SiMe3)2, [TMPZn(μ-OiPr)]2 and TMPZn[OCMe2C(O)OEt]. Their role in the ring-opening of rac-lactide and ε-caprolactone where TMP = 1,5,9-trimesityldipyrromethene

Abstract

From the reaction between Zn[N(SiMe3)2]2 and TMP-H (1 equiv.) in toluene at room temperature TMPZn[N(SiMe3)2] was prepared and subsequently reacted with PriOH (1 equiv.) and HOCMe2C(O)OEt (1 equiv.) to give [TMPZn(μ-OPri)]2 and TMPZn[OCMe2C(O)OEt], respectively. The three compounds were each crystallized and characterized by single crystal X-ray diffraction. The compound TMPZn[N(SiMe3)2] was a three coordinated N3Zn compound with an anagostic N(SiMe3)2 ligand where one SiMe3 was annexed to the Zn2+ ion. The compound [TMPZn(μ-OPri)]2 has a molecular C2 axis and is dimeric in toluene-d8 solution having a N2ZnO2 core for each Zn2+ ion. In TMPZn[OCMe2C(O)OEt] compound the Zn2 + ion is four-coordinated. This too is present in solution (IR) and also present in toluene-d8 and THF where there is evidence of fluxionality with Tc ∼15 °C and −85 °C, respectively. The compound [TMPZn(μ-OPri)]2 polymerizes both ε-caprolactone (ε-CL) and rac-lactide (rac-LA) to give polycaprolactone (PCL) and polylactic acid (PLA), respectively. The rate of the reactions are ε-CL > LA and are influenced by the nature of solvent, THF > CD2Cl2/CH2Cl2. The nature of PLA derived from rac-LA is heterotactic: Pr = 0.96 in THF and Pr = 0.75 in CD2Cl2/CH2Cl2. In the copolymerization of rac-LA: ε-CL by both [TMPZn(μ-OPri)]2 and TMPZn[OCMe2C(O)OEt] only PLA is produced, PCL is not formed.