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Chiral (1,2)‐Diphenylethylene‐Salen Complexes of Triel Metals: Coordination Patterns and Mechanistic Considerations in the Isoselective ROP of Lactide
Author(s) -
Maudoux Nicolas,
Roisnel Thierry,
Dorcet Vincent,
Carpentier JeanFrançois,
Sarazin Yann
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201304788
Subject(s) - bimetallic strip , chemistry , catalysis , ligand (biochemistry) , medicinal chemistry , lactide , gallium , alkyl , stereochemistry , polymerization , organic chemistry , receptor , biochemistry , polymer
The synthesis of enantiomerically pure aluminium, gallium and indium complexes supported by chiral ( R , R )‐( HH ONNO HH ) ( 1 ), ( R , R )‐( MeH ONNO HMe ) ( 2 ), ( R , R )‐( t Bu t Bu ONNO t Bu t Bu ) ( 3 ), ( R , R )‐( MeNO2 ONNO MeNO2 ) ( 4 ), ( R , R )‐( HOMe ONNO HOMe ) ( 5 ) and ( R , R )‐( ClCl ONNO ClCl ) ( 6 ) (1,2)‐diphenylethylene‐salen ligands is described. Several of these complexes have been crystallographically authenticated, which highlights a diversity of coordination patterns. Whereas all Ga complexes form [Ga 2 (CH 2 SiMe 3 ) 4 (ONNO)] bimetallic species (ONNO= 1 – 3 ), aluminium [AlR(ONNO)] (R=Me, CH 2 SiMe 3 ) and indium [In(CH 2 SiMe 3 )(ONNO)] derivatives are monometallic for ONNO= 1 , 2 and 4 – 6 , and only form the bimetallic complexes [Al 2 R 4 (ONNO)] and [In 2 (CH 2 SiMe 3 ) 4 (ONNO)] for the most sterically crowded ligand 3 . The [AlMe(ONNO)] complexes react with i PrOH to give [AlO i Pr(ONNO)] complexes that are robust towards further i PrOH. The [In(CH 2 SiMe 3 )(ONNO)] congeners are inert towards excess alcohol, whereas the Ga compounds decompose easily. All these alkyl complexes, as well as the [AlO i Pr(ONNO)] derivatives, catalyse the ring‐opening polymerisation (ROP) of racemic lactide ( rac ‐LA). The [AlMe(ONNO)] complexes require additional alcohol to afford controlled reactions, but [AlO i Pr(ONNO)] complexes are single‐component catalysts for the isoselective ROP of rac ‐LA, with values of P m in the range 0.80–0.90. Experimental evidence unexpectedly shows that chain‐end control leads to the isoselectivity of these aluminium catalysts; also, the more crowded the coordination sphere, the higher the isoselectivity. The bimetallic Ga complexes do not afford controlled reactions, but the binary [In(ONNO)(CH 2 SiMe 3 )/(PhCH 2 OH)] systems competently mediate non‐stereoselective ROP; evidence is given that an activated monomer mechanism is at work. Kinetic studies show that catalytic activity decreases when electronic density and steric congestion at the metal atom increase.