A highly stable but highly reactive zinc catalyst for transesterification supported by a bis(imidazole) ligand

Abstract

We designed highly active and practical bis(imidazole)/zinc complexes for transesterification reactions. X-ray crystallographic analysis was used to confirm the structures of the zinc complexes and an equivalent of bis(imidazole) ligand was crucial for high catalytic activity. The octahedral zinc complex 8c was prepared in up to multigram scale by mixing Zn(OCOCF3)2·xH2O and meta-bis(imidazolylmethyl)benzene ligand 7j (two equivalents to zinc ion) and storable under air at room temperature for at least 9 months. The stable nature of the catalyst was amenable to recovery/reuse at least five times without a significant loss of reactivity. The transesterification reaction proceeded without strict reaction conditions, and expanded substrate generalities, including sterically demanding secondary and tertiary alcohols, were applicable. Remarkably, the present zinc catalyst proved highly effective for valuable monomer synthesis from readily available methyl acrylate derivatives. Chemoselective transesterification reactions of unprotected-amino alcohols were also achieved, using not only simple methyl ester but also unprecedented dimethyl carbonate. 5

Transesterification Reactions Catalyzed by a Recyclable Heterogeneous Zinc/Imidazole Catalyst

“…In the reverse direction, cyclic carbonate 12 was converted into 1,2‐diol 8k by changing the solvent to methanol. These results demonstrate that catalyst 6 could be used for the protection/deprotection of 1,2‐diols under almost neutral conditions without any acid or base …”

“…The use of four equivalents of imidazole to zinc ions under THF reflux conditions was optimal, and the zinc content of 6 was determined to be 0.399 mmol g −1 by ICP‐MS analysis. We also investigated the immobilization of a previously developed m ‐substituted bis(imidazole) unit on Merrifield resin in place of an imidazole unit, but the catalytic activity was much lower than that of zinc/imidazole catalyst 6 . In addition, a silica‐gel supported imidazole ligand gave only unsatisfactory results…”

“…We evaluated the stability of the present heterogeneous catalyst 6 by water‐addition control experiments (Scheme ) . In the presence of five equivalents of water, Zn 4 (OCOCF 3 ) 6 O ( 1 ) provided the product 9aa in moderate yield even after prolonged reaction time.…”

“…We previously reported that combined use of μ‐oxo‐tetranuclear zinc cluster Zn 4 (OCOCF 3 ) 6 O ( 1 ) (Figure ) with a catalytic amount of N ‐heteroaromatic compounds, such as DMAP, significantly facilitated the transesterification reaction rate through in situ generation of more active lower‐nuclear Zn species . Moreover, we recently developed the highly reactive octahedral zinc complex 2 supported by a designed bis(imidazole) ligand 3 . The high stability of zinc complex 2 enabled the practical use of 2 for transesterification reactions without strict concern regarding air and moisture.…”

Efficient synthesis of organic carbonates and poly(1,4‐butylene carbonate‐co‐terephthalate)s

“…Using this device, butane‐1,4‐diyl dimethyl dicarbonate was efficiently synthesized by feeding only 3 equiv DMC per BD. Various catalysts have been developed for the transesterification reaction, but we could convert all the OH groups in BD to methyl carbonate using a small amount of simple NaOH catalyst (0.5 mol % per BD) at 120 °C for 3 h. In the 1 H NMR spectra, C(O)OC H 2 and C H 3 OC(O) signals were observed at 4.16 and 3.76 ppm, respectively, with almost equal integration [Figure (a)]. This indicates that a mixture of MeOC(O)O(CH 2 ) 4 OC(O)OMe and MeO[C(O)O(CH 2 ) 4 O] 2 C(O)OMe was generated in 1:1 mole ratio.…”