TY - JOUR
T1 - Multisite catalysis
T2 - A mechanistic study of β-lactone synthesis from epoxides and CO - Insights into a difficult case of homogeneous catalysis
AU - Molnar, Ferenc
AU - Luinstra, Gerrit A.
AU - Allmendinger, Markus
AU - Rieger, Bernhard
PY - 2003/3/17
Y1 - 2003/3/17
N2 - Carbonylation of epoxides with a combination of Lewis acids and cobalt carbonyls was studied by both theoretical and experimental methods. Only multisite catalysis opens a low-energy pathway for trans opening of oxirane rings. This ring-opening reaction is not easily achieved with a single-site metal catalyst due to structural and thermodynamic constraints. The overall reaction pathway includes epoxide ring opening, which requires both a Lewis acid and a tetracarbonylcobaltate nucleophile, yielding a cobalt alkyl - alkoxy-Lewis acid moiety. After CO insertion into the Co-Calkyl bond, lactone formation results from a nucleophilic attack of the alkoxy Lewis acid entity on the acylium carbon atom. A theoretical study indicates a marked influence of the Lewis acid on both ring-opening and lactone-formation steps, but not on carbonylation. Strong Lewis acids induce fast ring opening, but slow lactone formation, and visa versa: a good balance of Lewis acidity would give the fastest catalytic cycle as all steps have low barriers. Experimentally, carbonylation of propylene oxide to β-butyrolactone was monitored by online ATR-IR techniques with a mixture of tetracarbonylcobaltate and Lewis acids, namely BF3, Me3Al, Et2Al+ · diglyme, and a combination of Me3Al/dicobaltoctacarbonyl. We found that the last two mixtures are extremely active in lactone formation.
AB - Carbonylation of epoxides with a combination of Lewis acids and cobalt carbonyls was studied by both theoretical and experimental methods. Only multisite catalysis opens a low-energy pathway for trans opening of oxirane rings. This ring-opening reaction is not easily achieved with a single-site metal catalyst due to structural and thermodynamic constraints. The overall reaction pathway includes epoxide ring opening, which requires both a Lewis acid and a tetracarbonylcobaltate nucleophile, yielding a cobalt alkyl - alkoxy-Lewis acid moiety. After CO insertion into the Co-Calkyl bond, lactone formation results from a nucleophilic attack of the alkoxy Lewis acid entity on the acylium carbon atom. A theoretical study indicates a marked influence of the Lewis acid on both ring-opening and lactone-formation steps, but not on carbonylation. Strong Lewis acids induce fast ring opening, but slow lactone formation, and visa versa: a good balance of Lewis acidity would give the fastest catalytic cycle as all steps have low barriers. Experimentally, carbonylation of propylene oxide to β-butyrolactone was monitored by online ATR-IR techniques with a mixture of tetracarbonylcobaltate and Lewis acids, namely BF3, Me3Al, Et2Al+ · diglyme, and a combination of Me3Al/dicobaltoctacarbonyl. We found that the last two mixtures are extremely active in lactone formation.
KW - Carbonylation
KW - Homogeneous catalysis
KW - Multisite catalysis
KW - Polymerization
KW - Reaction mechanisms
UR - http://www.scopus.com/inward/record.url?scp=0037451476&partnerID=8YFLogxK
U2 - 10.1002/chem.200390144
DO - 10.1002/chem.200390144
M3 - Article
C2 - 12645016
AN - SCOPUS:0037451476
SN - 0947-6539
VL - 9
SP - 1273
EP - 1280
JO - Chemistry - A European Journal
JF - Chemistry - A European Journal
IS - 6
ER -