CO₂/Epoxide Coupling and the ROP of ε-Caprolactone: Mg and Al Complexes of γ-Phosphino-ketiminates as Dual-Purpose Catalysts

Abstract

γ-Phosphino-ketimines Ph 2 PC[C(Me)O][C(Me)NAr] (Ar = 2,6-diisopropylphenyl, L 1 H; Ar = 2,6-difluorophenyl, L 2 H) were synthesized by treating deprotonated ketimines with PPh 2 Cl. Their Mg complexes [(L 1 ) 2 (THF)Mg] (1) and [(L 2 ) 2 (THF) 2 Mg] (2) were obtained in excellent yields from a reaction between LH and di-nbutylmagnesium in THF. Addition of a slight excess of trimethylaluminum to LH in toluene yielded the Al complexes [L 1 AlMe 2 ] ( 3) and [L 2 AlMe 2 ] (4). Complexes 1 and 3 displayed high catalytic activity in the synthesis of cyclic carbonates from CO 2 and epoxides and also in the ring-opening polymerization (ROP) of ε-caprolactone. However, complexes 2 and 4, which contain fluoro substituents, showed poor activity in the synthesis of cyclic carbonates and could not initiate the ROP reaction. The pentacoordinated Mg complex 1 was found to be a better catalyst than the aluminum complex 3. It was also observed that the complexes 1 and 3 were more efficient than the unsubstituted ketiminate complexes reported in the literature.

N-Heterocyclic Carbene-Supported Aryl- and Alk- oxides of Beryllium and Magnesium

“…The second example, [Mg(L R )N(SiMe 3 ) 2 ] 2 (where L R = [OCMe 2 CH 2 (CNCH 2 CH 2 NR)]), features a magnesium atom supported by a bidentate alkoxide-NHC ligand [38]. It is noteworthy that magnesium complexes with Mg-OR units have been shown catalyze the formation of polylactide [14,38,[40][41][42][43][44], the ring opening polymerization of ε-caprolactone [45], as well as the hydroboration of aldehydes and ketones [46]. The chemistry of beryllium continues to be severely understudied relative to the heavier group 2 elements and NHC-beryllium oxides are hitherto unknown.…”

Homoleptic, bis‐ligated magnesium complexes for ring‐opening polymerization of lactide and lactones: Synthesis, structure, polymerization behavior and mechanism studies

“…However, previous reports showed that some homoleptic complexes were indeed active in ROP of cyclic esters in the presence of external alcohol (Scheme ) . The Ejfler group developed a kind of hetero‐ and homoleptic molecular zinc complexes with one or two equivalents of aminophenolate ligand in 2008 (Scheme a) .…”

“…The homoleptic zinc complex (Scheme e) based on salicylaldiminato ligands synthesized by the Wu group needed to be activated by raising the temperature to 130°C . Some homoleptic magnesium complexes (Scheme f–h) produced PCLs in 15–60 min, with MWs diverging from theoretical MWs or broad MWD . Though the homoleptic metal complexes were not traditionally regarded as effective initiators for ROP of cyclic esters, these elegant studies showed that the homoleptic, bis‐ligated organometallic complexes can be a potentially effective catalytic system in ROP of lactides and lactones, especially in the presence of external alcohol.…”

“…[41][42][43][44][45][46] However, previous reports showed that some homoleptic complexes were indeed active in ROP of cyclic esters in the presence of external alcohol (Scheme 1). [10,13,37,40,[47][48][49] The Ejfler group developed a kind of hetero-and homoleptic molecular zinc complexes with one or two equivalents of aminophenolate ligand in 2008 (Scheme 1a). [40] In 2014, Gerling et al reported bis-ligated zinc complexes, which could catalyze the ROP of L-lactide (L-LA) at high temperature with a more acidic phenol as an initiator (Scheme 1b).…”

“…[47] Some homoleptic magnesium complexes (Scheme 1f-h) produced PCLs in 15-60 min, with MWs diverging from theoretical MWs or broad MWD. [10,48,49] Though the homoleptic metal complexes were not traditionally regarded as effective initiators for ROP of cyclic esters, these elegant studies showed that the homoleptic, bis-ligated organometallic complexes can be a potentially effective catalytic system in ROP of lactides and lactones, especially in the presence of external alcohol. As aforementioned, previous reported monomers initiated by homoleptic complexes were mainly LA and ε-caprolactone (ε-CL).…”