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A DFT Study of the Mechanism of Polymerization of ε‐Caprolactone Initiated by Organolanthanide Borohydride Complexes
Author(s) -
Barros Noémi,
Mountford Philip,
Guillaume Sophie M.,
Maron Laurent
Publication year - 2008
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.200800377
Subject(s) - chemistry , hydride , borohydride , polymerization , alkoxy group , medicinal chemistry , ring opening polymerization , aldehyde , monomer , alkoxide , polymer chemistry , stereochemistry , metal , catalysis , organic chemistry , polymer , alkyl
Abstract The mechanisms of polymerization of ε‐caprolactone (CL) initiated by either the rare‐earth hydride [Cp 2 Eu(H)] or the borohydrides [Cp 2 Eu(BH 4 )] or [(N 2 NN′)Eu(BH 4 )] were studied at the DFT level (Cp=η 5 ‐C 5 H 5 ; N 2 NN′=(2‐C 5 H 4 N)CH 2 (CH 2 ‐ CH 2 NMe) 2 ). For all compounds the reaction proceeds in two steps: a hydride transfer from the rare earth initiator to the carbonyl carbon of the lactone, followed by ring‐opening of the monomer. In the last step a difference is observed between the hydride and borohydride complexes, because for the latter the ring‐opening is induced by an additional BH bond cleavage leading to a terminal CH 2 OBH 2 group. This corresponds to the reduction by BH 3 of the carbonyl group of CL. Upon reaction of [Cp 2 Eu(H)] with CL, the alkoxy–aldehyde complex produced, [Cp 2 Eu{O(CH 2 ) 5 C(O)H}], is the first‐formed initiating species. In contrast, for the reaction of CL with the borohydride complexes [(L x )Eu(BH 4 )] (L x =Cp 2 or N 2 NN′), an aliphatic alkoxide with a terminal CH 2 OBH 2 group, [(L x )Eu{O(CH 2 ) 6 OBH 2 }] is formed and subsequently propagates the polymerization. The present DFT investigations are fully compatible with previously reported mechanistic studies of experimental systems.