Heteroscorpionate Rare‐Earth Metal Zwitterionic Complexes: Syntheses, Characterization, and Heteroselective Catalysis on the Ring‐Opening Polymerization of rac‐Lactide

Abstract

Novel neutral phosphine-modified heteroscorpionate ligand (3,5-Me(2)Pz)(2)CHPPh(2) (1) and its derivatives oxophosphine (2) and iminophosphine (3) heteroscorpionates were synthesized for the first time. These neutral heteroscorpionate ligands displayed unique chemistry towards rare-earth metal tris(alkyl)s [Ln(CH(2)SiMe(3))(3)(thf)(2)] (Ln=Y, Lu, Sc). The reaction between compound 1 and [Ln(CH(2)SiMe(3))(3)(thf)(2)] afforded heteroscorpionate rare-earth metal trialkyl adduct complexes 4a-c. Compounds 2 and 3 were treated with [Ln(CH(2)SiMe(3))(3)(thf)(2)] to give the unprecedented zwitterionic heteroscorpionate rare-earth metal dialkyls 5 and 6, respectively. In the process, the heteroscorpionates transferred to the carbanions by means of methine C-H bond cleavage that was attributed to the presence of the electron-withdrawing groups. In addition the ligand and central metal showed a concerted effect on both the catalytic activity and specific selectivity of complexes 4-6 for the ring-opening polymerization (ROP) of rac-lactide (rac-LA). All the adduct complexes 4 were nonselective and gave atactic polylactide (PLA), probably due to the dissociation of ligand 1 from the active metal center during the polymerization. Strikingly, zwitterionic complexes 5 catalyzed rapid ROP of rac-LA to produce PLAs with heterotacticity up to 0.87. However, the zwitterionic complexes 6 were less active and less selective than 5, which might be on account of the stronger coordination of the tetradentate ligand. Complexes 5 represent rare examples of the selective ROP of rac-LA mediated by rare-earth metal complexes supported by non-bisphenolate ligands.

Metal‐Size Influence in Iso‐Selective Lactide Polymerization

“…The two lutetium complexes 1 and 2 are approximately an order of magnitude slower than yttrium complexes A and B ( k obs =6.9×10 −4 , 7.9×10 −4 s −1 ). This finding is consistent with other examples of lutetium complexes 10b,f,h,j. 13 In contrast, lanthanum complex 3 was an extremely fast initiator, enabling almost complete conversion of 1000 equivalents of LA [versus initiator, in the presence of isopropyl alcohol (2 equivalents)] in less than 20 seconds (Table 1).…”

“…The use of different lanthanide centers enables an investigation of the influence of atomic size and coordination environment on polymer tacticity. Both metals have precedence in lactide ROP catalysis,10 although lutetium is rarely investigated 10b,e,f,h. Okuda and co‐workers reported heteroselective (syndiotactic) dithiaalkanediyl‐bridged bis(phenolato) yttrium and lutetium catalysts, but found that the stereocontrol decreased from Y to Lu 10b,f.…”

Synthesis, Structure, and Reactivity of Monoguanidinate Rare‐Earth Metal Aminobenzyl Enolate Complexes

“…In addition, the molecular weight of the polymers increases linearly with the ratio of [ rac ‐LA] 0 /[ 1a ] 0 (Table , entries 2, 9–12), which strongly suggests the controllable polymerization processes were initiated by complex 1a (SI Figure S20). These experimental results reveal that monoguanidinate rare‐earth metal aminobenzyl enolate complexes display efficient catalytic activity in the ROP of rac ‐lactide …”

Palladium-Catalyzed α-Arylation of Benzylic Phosphine Oxides

“…It is noteworthy that this compound has been demonstrated to be a potent inhibitor of the Kv1.5 potassium channel and a possible treatment for atrial fibrillation. 8a The prior synthesis provided only 2% yield of this biologically interesting compound, whereas the yield was significantly improved employing our method.…”

“…8 Surprisingly, very few methods have been reported for their synthesis. Classical methods involve the use of Michaelis-Arbuzov or Michaelis-Becker reactions (Scheme 1).…”