Catalysis Design for Ring-Opening Polymerization of Cyclic Esters: 1. Group 1 Metal and Thallium(I) Trispyrazolylborate Complexes with Hemilabile Ligands

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Abstract
The synthesis of 3-(2-methoxy-1,1-dimethylethyl)pyrazole, pz*H is described together with its reactions with the borohydrides MBH4, where M = Li, Na, and K, under melt conditions. At 180 °C, this procedure leads to a mixture of products for M = Li, and at higher temperatures, a derivative LiTp‘pz*H, 1, is isolated, wherein a B−H bond and a methyl group have been eliminated and a B−O bond has been formed. For M = Na, the reaction proceeds to give the tris-pyrazolylborate derivative NaTp*, 2, but at higher temperatures the tetra-pyrazolylborate complex NaB(pz*)4, 3, is obtained. The reactions involving KBH4 and pz*H yield the dinuclear complex K2(Tp*)2pz*H, 4. The reaction between NaTp* and TlOAc in CH2Cl2 at room temperature leads to the formation of TlTp*, 5, along with NaOAc. Thallium 5 reacts with methyllithium in diethylether to give LiTp*, 6, and thallium metal, and, similarly, 5 and KH react in tetrahydrofuran to give KTp*, 7 and Tl(0). 1−7 have been characterized by elemental analysis, NMR spectroscopy, and by single-crystal X-ray studies, the latter of which reveal the versatile modes of binding for this new ligand bearing hemilabile ether appendages.

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