12 research outputs found
Symmetry as a new element to control molecular switches
The isomerization properties of an azocarbazole macrocycle in solution were investigated utilizing NMR spectroscopy with in situ irradiation in combination with DFT calculations. It was demonstrated that the position of azo units in a rigid macrocyclic system influences the photoisomerization pathway even if the initial all-E isomer is highly symmetric. Furthermore, the effect of ring strain on lowering the rates of thermal isomerization was demonstrated and a mechanism via an inversion-rotation proposed. The herein presented results and methods give new insights into the general nature of the azobenzene unit. In particular we illustrate the effect of symmetry changes due to macrocyclic arrangement on the photochemical and thermal isomerization properties, which will stimulate future development towards multinary molecular switches
Bidentate Lewis Acid Catalyzed Domino Diels-Alder Reaction of Phthalazine for the Synthesis of Bridged Oligocyclic Tetrahydronaphthalenes
A domino process consisting of an inverse and a normal electron-demand Diels-Alder reaction is presented for the formation of bridged tri- and tetracyclic 1,2,3,4-tetrahydronaphthalenes catalyzed by a bidentate Lewis acid. The products were synthesized in a one-pot reaction from commercially available starting materials and contain up to six stereogenic centers. The tetrahydronaphthalenes were isolated as single diastereomers and are derivatives of phenylethylamine, which is well-known as a scaffold of amphetamine or dopamine
Symmetry as a new element to control molecular switches
The isomerization properties of an azocarbazole macrocycle in solution were investigated utilizing NMR spectroscopy with in situ irradiation in combination with DFT calculations. It was demonstrated that the position of azo units in a rigid macrocyclic system influences the photoisomerization pathway even if the initial all-E isomer is highly symmetric. Furthermore, the effect of ring strain on lowering the rates of thermal isomerization was demonstrated and a mechanism via an inversion-rotation proposed. The herein presented results and methods give new insights into the general nature of the azobenzene unit. In particular we illustrate the effect of symmetry changes due to macrocyclic arrangement on the photochemical and thermal isomerization properties, which will stimulate future development towards multinary molecular switches
Bidentate Lewis Acid Catalyzed Inverse-Electron-Demand Diels-Alder Reaction for the Selective Functionalization of Aldehydes
The inverse-electron-demand Diels-Alder (IEDDA) reaction catalyzed by a bidentate Lewis acid was applied to enamines generated in situ from aldehydes. In general, a high functional group tolerance has been observed. Side reactions during the enamine forming step can limit the yield of the desired naphthalene. For citronellal as substrate, the initial intermediate after the catalyzed IEDDA reaction was trapped by an intramolecular Diels-Alder reaction to furnish a tricyclic compound. This scaffold represents the framework of natural products such as valerianoids A-C or the patchouli alcohol
First example of a CLICK reaction of a coordinated 4'-azido-2,2':6',2"-terpyridine ligand
The first example of a copper-catalysed [2 + 3] cycloaddition reaction of a coordinated 4′-azido-2,2′:6′,2″-terpyridine ligand is reported and the solid state structures of the precursor and product are described
Ultrafast dynamics of highly constrained azobenzene macrocycles
The ultrafast photoisomerization of model azobenzene macrocycles was studied by transient absorption spectroscopy. Our results reveal a strong dependence of the dynamics and the overall molecular properties on the geometric constraints and the intramolecular strain
Ultrafast dynamics of highly constrained azobenzene macrocycles
The ultrafast photoisomerization of model azobenzene macrocycles was studied by transient absorption spectroscopy. Our results reveal a strong dependence of the dynamics and the overall molecular properties on the geometric constraints and the intramolecular strain
Ultrafast dynamics of highly constrained azobenzene macrocycles
The ultrafast photoisomerization of model azobenzene macrocycles was studied by transient absorption spectroscopy. Our results reveal a strong dependence of the dynamics and the overall molecular properties on the geometric constraints and the intramolecular strain
Direct Microscopic Analysis of Individual C<sub>60</sub> Dimerization Events: Kinetics and Mechanisms
Modern
transition state theory states that the statistical behavior
of a chemical reaction is the sum of individual chemical events that
occur randomly. Statistical analysis of each event for individual
molecules in a three-dimensional space however is practically impossible.
We report here that kinetics and mechanisms of chemical reactions
can be investigated by using a one-dimensional system where reaction
events can be observed in situ and counted one by one using variable-temperature
(VT) atomic-resolution transmission electron microscopy (TEM). We
thereby provide direct proof that the ensemble behavior of random
events conforms to the Rice–Ramsperger–Kassel–Marcus
theory, as illustrated for [2 + 2] cycloaddition of C<sub>60</sub> molecules in carbon nanotubes (CNTs). This method gives kinetic
and structural information for different types of reactions occurring
simultaneously in the microscopic view field, suggesting that the
VT-TEM opens a new dimension of chemical kinetics research on molecules
and their assemblies in their excited and ionized states. The study
carried out at 393–493 K showed that pristine CNT primarily
acts as a singlet sensitizer of the cycloaddition reaction that takes
place with an activation energy of 33.5 ± 6.8 kJ/mol. On the
other hand, CNT suffers electron damage of the conjugated system at
103–203 K and promotes a reactive radical cation path that
takes place with an activation energy of only 1.9 ± 0.7 kJ/mol.
The pre-exponential factor of the Arrhenius plot gave us further mechanistic
insights
Direct Microscopic Analysis of Individual C<sub>60</sub> Dimerization Events: Kinetics and Mechanisms
Modern
transition state theory states that the statistical behavior
of a chemical reaction is the sum of individual chemical events that
occur randomly. Statistical analysis of each event for individual
molecules in a three-dimensional space however is practically impossible.
We report here that kinetics and mechanisms of chemical reactions
can be investigated by using a one-dimensional system where reaction
events can be observed in situ and counted one by one using variable-temperature
(VT) atomic-resolution transmission electron microscopy (TEM). We
thereby provide direct proof that the ensemble behavior of random
events conforms to the Rice–Ramsperger–Kassel–Marcus
theory, as illustrated for [2 + 2] cycloaddition of C<sub>60</sub> molecules in carbon nanotubes (CNTs). This method gives kinetic
and structural information for different types of reactions occurring
simultaneously in the microscopic view field, suggesting that the
VT-TEM opens a new dimension of chemical kinetics research on molecules
and their assemblies in their excited and ionized states. The study
carried out at 393–493 K showed that pristine CNT primarily
acts as a singlet sensitizer of the cycloaddition reaction that takes
place with an activation energy of 33.5 ± 6.8 kJ/mol. On the
other hand, CNT suffers electron damage of the conjugated system at
103–203 K and promotes a reactive radical cation path that
takes place with an activation energy of only 1.9 ± 0.7 kJ/mol.
The pre-exponential factor of the Arrhenius plot gave us further mechanistic
insights