Chiral recognition of monomeric Me2MOR units resulting in the formation of homochiral dimeric species [Me2M(μ-OR)]2 (M = Ga, In), leads to heteroselective catalysts for the ring opening polymerization of rac-lactide (rac-LA).
Methods for the synthesis
of dialkylgalium compounds with alkoxide
or aryloxide ligands possessing N-heterocyclic carbene functionalities
have been established. As a result, the synthesis of a series of dialkylgallium
complexes Me2Ga(O,C) (1, 3–5), and Me2Ga(O,C)·Me3Ga (2, 6) is described, where (O,C) represents an
alkoxide or aryloxide monoanionic chelate ligand with an N-heterocyclic
carbene functionality. All complexes have been fully characterized
using spectroscopic and X-ray techniques. The presence of a strongly
basic NHC functionality in alkoxide or aryloxide ligands resulted
in the formation of monomeric Me2Ga(O,C) species. The reaction
of those complexes with the Lewis acid Me3Ga leads to Me2Ga(O,C)·Me3Ga adducts (2 and 6) with a strong Me3Ga–O dative bond. The
effect of (O,C) ligands with various steric and electronic properties
on the structure of obtained Me2Ga(O,C) and Me2Ga(O,C)·Me3Ga has been discussed on the basis of
spectroscopic data. Finally, the bond valence vector model has been
used to estimate the effect of a chelating (O,C) ligand on strains
in complexes 1–6 on the basis of
X-ray data.
Abstract:The investigation of the reactivity of six membered N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-1-ylidene (6-Mes) towards dialkylgallium and dialkylindium alkoxides/aryloxides has shown that both steric hindrances and donor properties of 6-Mes significantly influence the strength of M-C 6-Mes bond, as well as the formation, structure and reactivity of Me 2 MOR(6-Mes) (M = Ga, In) complexes. While the reactions of simple dimethylgallium alkoxides with 6-Mes lead to the formation of stable monomeric Me 2 Ga(OCH 2 CH 2 OMe)(6-Mes) (1) and Me 2 GaOMe(6-Mes) complexes, the analogous Me 2 InOR(6-Mes) are unstable and disproportionate to methylindium alkoxides and Me 3 In(6-Mes) (2). The use of bulky alkoxide ligand-OCPh 2 Me or aryloxide ligand-OC 6 H 4 OMe allowed for the synthesis of stable Me 2 M(OCPh 2 Me)(6-Mes) (M = Ga (3) and In (4)) as well as Me 2 M(OC 6 H 4 OMe)(6-Mes) (M = Ga (5) and In (6)). The structures of 1-6 have been determined using both spectroscopic methods in solution and X-ray diffraction studies, which confirmed the effect of both steric hindrances and donor properties of 6-Mes on their structure and catalytic properties in the ring-opening polymerization (ROP) of rac-lactide.
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