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Well‐Defined Cationic Alkyl– and Alkoxide–Aluminum Complexes and Their Reactivity with ε‐Caprolactone and Lactides
Author(s) -
Dagorne Samuel,
Le Bideau Franck,
Welter Richard,
BelleminLaponnaz Stéphane,
MaisseFrançois Aline
Publication year - 2007
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.200601112
Subject(s) - chemistry , adduct , alkoxide , cationic polymerization , reactivity (psychology) , ligand (biochemistry) , alkyl , denticity , medicinal chemistry , lewis acids and bases , stereochemistry , steric effects , electrophile , crystallography , polymer chemistry , organic chemistry , catalysis , crystal structure , biochemistry , alternative medicine , receptor , pathology , medicine
We describe the synthesis, structure, and reactivity of low‐coordinate Al–alkyl and –alkoxide cationic complexes incorporating the sterically bulky aminophenolate bidentate ligand 6‐(CH 2 NMe 2 )‐2‐CPh 3 ‐4‐Me‐C 6 H 2 O − ( N,O ). These complexes are derived from the ionization of neutral dialkyl Al complexes ( N,O )AlR 2 ( 1 a , R=Me; 1 b , R= i Bu), readily obtained by alkane elimination between AlR 3 and the corresponding aminophenol ligand, with the alkyl abstracting reagents B(C 6 F 5 ) 3 and [Ph 3 C][B(C 6 F 5 ) 4 ]. The reactions of 1 a , b with B(C 6 F 5 ) 3 yield complicated mixtures or decomposition products, however the ionization of the Al–diisobutyl derivative 1 b with [Ph 3 C][B(C 6 F 5 ) 4 ] affords a stable four‐coordinate AlPhBr cationic adduct [( N,O )Al( i Bu)(PhBr)] + ( 3 + ), as deduced from elemental analysis data. Complex 3 + readily coordinates Lewis bases such as THF to form the corresponding adduct [( N,O )Al( i Bu)(thf)] + ( 4 + ), and also rapidly chain‐transfers with 1‐hexene to yield the three‐coordinate Al–hexyl cation [( N,O )Alhexyl] + ( 5 + ). Both cations 3 + and 5 + slowly dimerize to form unprecedented organoaluminum dications [( N,O )AlR + ] 2 ( 3′ ++ , R= i Bu; 5′ ++ , R=hexyl) as deduced from X‐ray crystallographic analysis. Cation 3 + reacts quickly with i PrOH to form a stable Lewis acid/base adduct [( N,O )Al( i Bu)(HO i Pr)] + ( 6 + ), which constitutes the first X‐ray characterized adduct between an Al–alkyl complex and a simple ROH. The AlRO H proton in 6 + is readily abstracted by NMe 2 Ph to form the neutral isopropoxide Al complex [( N,O )Al( i Bu)(O i Pr)] ( 7 ). Upon reaction with THF, cation 6 + undergoes an intramolecular proton transfer to yield the ammonium AlTHF complex [(η 1 ‐H N,O )Al( i Bu)(O i Pr)(thf)] ( 8 b + ), in which the aminophenolate is η 1 ‐coordinated to the Al center. Cation 8 b + can then be converted to the desired Al–alkoxide derivative [( N,O )Al(O i Pr)(thf)] + ( 10 + ), by an intramolecular protonolysis reaction, as confirmed by X‐ray crystallography. The synthesized Al–alkyl cations form robust four‐coordinate adducts in the presence of cyclic esters such as ε‐caprolactone and ( d,l )‐lactide, but no insertion chemistry occurs, illustrating the poor ability of the AlR + moiety to ring‐open. In contrast, the Al–alkoxide cation 10 + polymerizes ε‐caprolactone in a controlled manner with excellent activity, but is inactive in the polymerization of ( d,l )‐lactide and L ‐lactide. Control experiments with L ‐lactide show that cation 10 + ring‐opens L ‐lactide to yield a robust five‐coordinated Allactate cation [( N,O )Al(η 2 ‐ L ‐lactate‐O i Pr)(thf)] + ( 13 + ), derived from a monoinsertion of L ‐lactide into the AlO i Pr bond of 10 + , that does not further react. Cation 13 + may be regarded as a structurally characterized close mimic of the initial intermediate in the ring opening polymerization (ROP), of lactides by [{LX}M(OR)(L)] (where LX − =bidentate monoanionic ligand and L=labile ligand) metal complex initiators.