Synthesis and catalytic application of magnesium complexes bearing pendant indolyl ligands

Abstract

Three novel indole-based ligand precursors [HIndPh(R), R = methoxy, HIndPh(OMe) (); thiomethoxy, HIndPh(SMe) (); and N,N'-dimethylamino, HIndPh(NMe2) ()] have been synthesized via Sonogashira and cyclization reactions with moderate to high yield. Reactions of these ligand precursors with 0.6 equivalent of Mg(n)Bu2 in THF afforded the magnesium bis-indolyl complexes , respectively. All the ligand precursors and related magnesium complexes have been characterized by NMR spectroscopy and elemental analyses. The molecular structures are reported for compounds and . Under optimized conditions, compound demonstrates efficient catalytic activities towards the ring opening polymerization of l-lactide and ε-caprolactone in the presence of BnOH.

Magnesium Pyrazolyl-Indolyl Complexes as Catalysts for Ring-Opening Polymerization of L-Lactide

“…The Mg-OTHF bond length of 1 (2.1516(8) Å) is longer than those found in cis-form (2.0661(10)-2.136(5) Å) [46][47][48]. The Mg-Nindolyl bond lengths (2.1067(9) Å) are close to those found in magnesium bis-indolyl complexes (2.0907(16) Å for trans-form; 2.126(2) and 2.138(2) Å for cis-form) [28]. The Mg-Npyrazole bond lengths (2.2156(9) Å) are within those found in magnesium complexes bearing pyrazole functionality (2.100(3)-2.21(1) Å) [35][36][37][38].…”

“…Since several magnesium complexes containing bis-indolyl ligands have demonstrated their catalytic activities towards the ROP of cyclic esters in the presence of alcohols [28], the homoleptic indolyl magnesium complexes reported here are expected to work as the catalysts for the ring opening polymerization. Representative results are collected in Table 3 for ROP of L-LA, respectively.…”

“…Previously, we reported the preparation of indole bearing pendant functionalities using the Sonogashira reaction followed by Zn-mediated cyclization [28]. In order to introduce pyrazolyl functionalities into indole molecules, copper-catalyzed N-arylation or the Bartoli indole synthesis are used to achieve this issue.…”

“…Therefore, development of novel routes for the preparation of indole derivatives has become an important theme in organic synthesis [30][31][32][33][34]. Recently, some magnesium complexes bearing pendant indolyl ligands have been reported by us [28]. They demonstrated efficient catalytic activities towards the ROP of cyclic esters.…”

“…Due to the promising catalytic activities, metal-based initiators/catalysts have been attractive interest over the past decades [6][7][8][9][10][11][12][13][14]. Among these studies, some metal complexes bearing anionic N-heterocyclic ligands, such as pyrrole [15][16][17][18][19][20][21][22][23][24][25][26], indole [27,28] or carbazole [29], have displayed good catalytic activities towards ROP (ring-opening polymerization) of cyclic esters. The indole ring system, one of the most important heterocycles in nature, keeps continuous attraction due to the potential biological activities demonstrated by various indole derivatives.…”