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Glycerol as an efficient medium for the petasis borono-mannich reaction.

Rosholm T, Gois PM, Franzen R, Candeias NR - ChemistryOpen (2014)

Bottom Line: Glycerol was compared with other solvents, and in some cases, it provided the reaction product in higher yield.Crude glycerol, as generated by the biodiesel industry, was evaluated and found to be a suitable solvent for the PBM reaction, successfully expanding the potential use of this industry by-product.Based on density functional theory (DFT) calculations and the obtained experimental results, the involvement of glycerol-derived boronic esters in the reaction mechanism is suggested to be competitive with the free boronic acid pathway.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Bioengineering, Tampere University of Technology, Korkeakoulunkatu 8 33101 Tampere (Finland).

ABSTRACT
The multicomponent Petasis borono-Mannich (PBM) reaction is a useful tool for the preparation of complex molecules in a single step from boronic acids, aldehydes/ketones, and amines. Here, we describe the use of glycerol in the PBM reaction of salicylaldehydes or 2-pyridinecarbaldehyde with several boronic acids and secondary amines. From these readily available starting materials, alkylaminophenols, 2-substituted pyridines, and 2H-chromenes were prepared in reasonable to good yields. Glycerol was compared with other solvents, and in some cases, it provided the reaction product in higher yield. Crude glycerol, as generated by the biodiesel industry, was evaluated and found to be a suitable solvent for the PBM reaction, successfully expanding the potential use of this industry by-product. Based on density functional theory (DFT) calculations and the obtained experimental results, the involvement of glycerol-derived boronic esters in the reaction mechanism is suggested to be competitive with the free boronic acid pathway. Similar Gibbs free energies for the aryl migration from the boronate species to the iminium were determined for both mechanisms.

No MeSH data available.


Petasis borono–Mannich (PBM) reaction with glycerol-derived boronic esters. Relative energies given in parenthesis (in kcal mol−1) refer to the optimised, zwitterionic 2-[dimethyliminio)methyl]phenolate (Im) and glycerol 1,2-phenylboronate (gbe1) set of reagents, and include the thermal correction to the Gibbs free energy in 1,2-ethanediol.
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sch4: Petasis borono–Mannich (PBM) reaction with glycerol-derived boronic esters. Relative energies given in parenthesis (in kcal mol−1) refer to the optimised, zwitterionic 2-[dimethyliminio)methyl]phenolate (Im) and glycerol 1,2-phenylboronate (gbe1) set of reagents, and include the thermal correction to the Gibbs free energy in 1,2-ethanediol.

Mentions: Considering gbe1 and gbe2 as the reacting boron species, the PBM mechanism was elucidated. The free-energy profiles obtained are represented in Figure 1, path 1 (solid lines) for the mechanism derived from gbe1 and path 2 (dashed lines) for gbe2. A general working model mechanism is represented in Scheme 4. The study reported here considers the starting material to be a zwitterionic iminium formed after condensation of dimethylamine with salicylaldehyde. It was previously reported that coordination of the boron species to the phenoxide was energetically more favoured than coordination with the phenol moiety.[15], [33] Hence, the activation of the glycerol boronic ester by the phenoxy group to form the “ate complex” was also considered presumed? to be the earliest step of this reaction. A decrease in both electronic and free energies for the ate complex formation was determined, demonstrating the stabilisation of the species by interaction of the zwitterionic iminium and the boron species through B−O bond formation, and corroborated by the 0.5–0.6 Wiberg indices (WI) determined for the B−O bond in ATE1 and ATE2. The stabilisation of the six-membered ring ate complex ATE1 is higher than the stabilisation of ATE2 by 4 kcal mol−1 when compared with the initial set of reagents. Inspection of the conformations of the ate complexes reveals the tetrahedral character of the boron atom, forcing the five-membered cyclic boronate ester moiety to adopt a puckered conformation in ATE2 and a chair conformation in ATE1.


Glycerol as an efficient medium for the petasis borono-mannich reaction.

Rosholm T, Gois PM, Franzen R, Candeias NR - ChemistryOpen (2014)

Petasis borono–Mannich (PBM) reaction with glycerol-derived boronic esters. Relative energies given in parenthesis (in kcal mol−1) refer to the optimised, zwitterionic 2-[dimethyliminio)methyl]phenolate (Im) and glycerol 1,2-phenylboronate (gbe1) set of reagents, and include the thermal correction to the Gibbs free energy in 1,2-ethanediol.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4380952&req=5

sch4: Petasis borono–Mannich (PBM) reaction with glycerol-derived boronic esters. Relative energies given in parenthesis (in kcal mol−1) refer to the optimised, zwitterionic 2-[dimethyliminio)methyl]phenolate (Im) and glycerol 1,2-phenylboronate (gbe1) set of reagents, and include the thermal correction to the Gibbs free energy in 1,2-ethanediol.
Mentions: Considering gbe1 and gbe2 as the reacting boron species, the PBM mechanism was elucidated. The free-energy profiles obtained are represented in Figure 1, path 1 (solid lines) for the mechanism derived from gbe1 and path 2 (dashed lines) for gbe2. A general working model mechanism is represented in Scheme 4. The study reported here considers the starting material to be a zwitterionic iminium formed after condensation of dimethylamine with salicylaldehyde. It was previously reported that coordination of the boron species to the phenoxide was energetically more favoured than coordination with the phenol moiety.[15], [33] Hence, the activation of the glycerol boronic ester by the phenoxy group to form the “ate complex” was also considered presumed? to be the earliest step of this reaction. A decrease in both electronic and free energies for the ate complex formation was determined, demonstrating the stabilisation of the species by interaction of the zwitterionic iminium and the boron species through B−O bond formation, and corroborated by the 0.5–0.6 Wiberg indices (WI) determined for the B−O bond in ATE1 and ATE2. The stabilisation of the six-membered ring ate complex ATE1 is higher than the stabilisation of ATE2 by 4 kcal mol−1 when compared with the initial set of reagents. Inspection of the conformations of the ate complexes reveals the tetrahedral character of the boron atom, forcing the five-membered cyclic boronate ester moiety to adopt a puckered conformation in ATE2 and a chair conformation in ATE1.

Bottom Line: Glycerol was compared with other solvents, and in some cases, it provided the reaction product in higher yield.Crude glycerol, as generated by the biodiesel industry, was evaluated and found to be a suitable solvent for the PBM reaction, successfully expanding the potential use of this industry by-product.Based on density functional theory (DFT) calculations and the obtained experimental results, the involvement of glycerol-derived boronic esters in the reaction mechanism is suggested to be competitive with the free boronic acid pathway.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry and Bioengineering, Tampere University of Technology, Korkeakoulunkatu 8 33101 Tampere (Finland).

ABSTRACT
The multicomponent Petasis borono-Mannich (PBM) reaction is a useful tool for the preparation of complex molecules in a single step from boronic acids, aldehydes/ketones, and amines. Here, we describe the use of glycerol in the PBM reaction of salicylaldehydes or 2-pyridinecarbaldehyde with several boronic acids and secondary amines. From these readily available starting materials, alkylaminophenols, 2-substituted pyridines, and 2H-chromenes were prepared in reasonable to good yields. Glycerol was compared with other solvents, and in some cases, it provided the reaction product in higher yield. Crude glycerol, as generated by the biodiesel industry, was evaluated and found to be a suitable solvent for the PBM reaction, successfully expanding the potential use of this industry by-product. Based on density functional theory (DFT) calculations and the obtained experimental results, the involvement of glycerol-derived boronic esters in the reaction mechanism is suggested to be competitive with the free boronic acid pathway. Similar Gibbs free energies for the aryl migration from the boronate species to the iminium were determined for both mechanisms.

No MeSH data available.