H. Goossens

Cationic ring-opening polymerization of 2-propyl-2-oxazolines: Understanding structural effects on polymerization behavior based on molecular modeling

H. Goossens, S. Catak, M. Glassner, V. De La Rosa, B. Monnery, F. De Proft, V. Van Speybroeck, R. Hoogenboom
ACS Macro Letters
2, 651-654
2013
A1

Abstract 

The surprising difference in the cationic ring-opening polymerization rate of 2-cyclopropyl-2-oxazoline versus 2-n-propyl-2-oxazoline and 2-isopropyl-2-oxazoline was investigated both experimentally and theoretically. The polymerization kinetics of all three oxazolines were experimentally measured in acetonitrile at 140 °C, and the polymerization rate constant (kp) was found to decrease in the order c-PropOx > n-PropOx > i-PropOx. Theoretical free energy calculations confirmed the trend for kp, and a set of DFT-based reactivity descriptors, electrostatics, and frontier molecular orbitals were studied to detect the factors controlling this peculiar behavior. Our results show that the observed reactivity is dictated by electrostatic effects. More in particular, the charge on the nitrogen atom of the monomer, used to measure its nucleophilicity, was the most negative for c-PropOx. Furthermore, the electrophilicity of the cations does not change substantially, and thus, the nucleophilicity of the monomers is the driving factor for kp.

Synthesis of 2-Hydroxy-1,4-oxazin-3-ones through Ring Transformation of 3-Hydroxy-4-(1,2-dihydroxyethyl)--lactams and a Study of Their Reactivity

K. Mollet, H. Goossens, N. Piens, S. Catak, M. Waroquier, V. Van Speybroeck, M. D'Hooghe, N. De Kimpe
Chemistry - A European Journal
19 (10), 3383-3396
2013
A1

Abstract 

The reactivity of 3-hydroxy-4-(1,2-dihydroxyethyl)-β-lactams with regard to the oxidant sodium periodate was evaluated, unexpectedly resulting in the exclusive formation of new 2-hydroxy-1,4-oxazin-3-ones through a C3C4 bond cleavage of the intermediate 4-formyl-3-hydroxy-β-lactams followed by a ring expansion. This peculiar transformation stands in sharp contrast with the known NaIO4-mediated oxidation of 3-alkoxy- and 3-phenoxy-4-(1,2-dihydroxyethyl)-β-lactams, which exclusively leads to the corresponding 4-formyl-β-lactams without a subsequent ring enlargement. In addition, this new class of functionalized oxazin-3-ones was further evaluated for its potential use as building blocks in the synthesis of a variety of differently substituted oxazin-3-ones, morpholin-3-ones and pyrazinones. Furthermore, additional insights into the mechanism and the factors governing this new ring-expansion reaction were provided by means of density functional theory calculations.

Solvent-controlled selective transformation of 2-Bromomethyl-2-methylaziridines to functionalized aziridines and azetidines

S. Stankovic, H. Goossens, S. Catak, M. Tezcan, M. Waroquier, V. Van Speybroeck, M. D'Hooghe, N. De Kimpe
Journal of Organic Chemistry
77, 3181-3190
2012
A1

Abstract 

The reactivity of 2-bromomethyl-2-methylaziridines toward oxygen, sulfur, and carbon nucleophiles in different solvent systems was investigated. Remarkably, the choice of the solvent has a profound influence on the reaction outcome, enabling the selective formation of either functionalized aziridines in dimethylformamide (through direct bromide displacement) or azetidines in acetonitrile (through rearrangement via a bicyclic aziridinium intermediate). In addition, the experimentally observed solvent-dependent behavior of 2-bromomethyl-2-methylaziridines was further supported by means of DFT calculations.

Open Access version available at UGent repository

Synthesis of 3-Methoxyazetidines via an Aziridine to Azetidine Rearrangement and Theoretical Rationalization of the Reaction Mechanism

S. Stankovic, S. Catak, M. D'Hooghe, H. Goossens, K. Abbaspour Tehrani, P. Bogaert, M. Waroquier, V. Van Speybroeck, N. De Kimpe
Journal of Organic Chemistry
76 (7), 2157-2167
2011
A1

Abstract 

The synthetic utility of N-alkylidene-(2,3-dibromo-2-methylpropyl)amines and N-(2,3-dibromo-2-methylpropylidene)benzylamines was demonstrated by the unexpected synthesis of 3-methoxy-3-methylazetidines upon treatment with sodium borohydride in methanol under reflux through a rare aziridine to azetidine rearrangement. These findings stand in contrast to the known reactivity of the closely related N-alkylidene-(2,3-dibromopropyl)amines, which are easily converted into 2-(bromomethyl)aziridines under the same reaction conditions. A thorough insight into the reaction mechanism was provided by both experimental study and theoretical rationalization.

Reactivity of Activated versus Nonactivated 2-(Bromomethyl)aziridines with respect to Sodium Methoxide: a Combined Computational and Experimental Study

H. Goossens, K. Vervisch, S. Catak, S. Stankovic, M. D'Hooghe, F. De Proft, P. Geerlings, N. De Kimpe, M. Waroquier, V. Van Speybroeck
Journal of Organic Chemistry
76 (21), 8698-8709
2011
A1

Abstract 

The difference in reactivity between the activated 2-bromomethyl-1-tosylaziridine and the non-activated 1-benzyl-2-(bromomethyl)aziridine with respect to sodium methoxide was analyzed by means of DFT calculations within the supermolecule approach, taking into account explicit solvent molecules. In addition, the reactivity of epibromohydrin with regard to sodium methoxide was assessed as well. The barriers for direct displacement of bromide by methoxide in methanol are comparable for all three heterocyclic species under study. However, ring opening was found to be only feasible for the epoxide and the activated aziridine, and not for the non-activated aziridine. According to these computational analyses, the synthesis of chiral 2-substituted 1-tosylaziridines can take place with inversion (through ring opening/ring closure) or retention (through direct bromide displacement) of configuration upon treatment of the corresponding 2-(bromomethyl)aziridines with one equivalent of a nucleophile, whereas chiral 2-substituted 1-benzylaziridines are selectively obtained with retention of configuration (via direct bromide displacement). Furthermore, the computational results showed that explicit accounting for solvent molecules is required to describe the free energy profile correctly. To verify the computational findings experimentally, chiral 1-benzyl-2-(bromomethyl)aziridines and 2-bromomethyl-1-tosylaziridines were treated with sodium methoxide in methanol. The presented work concerning the reactivity of 2-bromomethyl-1-tosylaziridine stands in contrast to the behaviour of the corresponding 1-tosyl-2-(tosyloxymethyl)aziridine with respect to nucleophiles, which undergoes a clean ring-opening/ring-closure process with inversion of configuration at the asymmetric aziridine carbon atom.

Synthesis of 2-hydroxy-1,4-oxazin-3-ones through ring transformation of 3-hydroxy-4-(1,2-dihydroxyethyl)-β-lactams and study of their reactivity

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Talk

Conference / event / venue 

12th Chemistry Conference for Young Scientists
Blankenberge, Belgium
Thursday, 27 February, 2014 to Friday, 28 February, 2014

Synthesis of 2-hydroxy-1,4-oxazin-3-ones through ring transformation of 3-hydroxy-4-(1,2-dihydroxyethyl)-β-lactams and study of their reactivity

ISBN/ISSN:
Talk

Conference / event / venue 

17th Sigma-Aldrich Organic Synthesis Meeting
Blankenberge, Belgium
Thursday, 5 December, 2013 to Friday, 6 December, 2013

Cationic ring-opening polymerization of 2-propyl-2-oxazolines: understanding structural effects on polymerization behavior based on molecular modeling

ISBN/ISSN:
Talk

Conference / event / venue 

248th National Meeting of the American-Chemical-Society (ACS 2014 )
San Francisco, CA
Sunday, 10 August, 2014 to Thursday, 14 August, 2014

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