Search

Your search keyword '"Helaja, Juho"' showing total 223 results
Start Over Author "Helaja, Juho"
223 results on '"Helaja, Juho"'

6. A metal-free carbon catalyst for oxidative dehydrogenation of aryl cyclohexenes to produce biaryl compounds.

Author

Mingze Yang, Lenarda, Anna, Frindy, Sana, Yushuai Sang, Oksanen, Valtteri, Bolognani, Adriano, Hendrickx, Lisa, Helaja, Juho, and Yongdan Li

Subjects
OXIDATIVE dehydrogenation, X-ray photoelectron spectroscopy, CATALYTIC reforming, CARBON compounds, CATALYSTS, ACTIVATION energy
Abstract

A metal-free route based on a carbon catalyst to synthesize biphenyls through oxidative dehydrogenation (ODH) of phenyl cyclohexene has been investigated. Among the samples examined, an air-oxidized active carbon exhibits the best activity with a 9.1 × 10-2 h-1 rate constant, yielding 74% biphenyl in 28 h at 140 °C under five bar O2 in anisole. The apparent activation energy is measured as 54.5 kJmol-1. The extended reaction scope, consisting of 15 differently substituted phenyl cyclohexenes, shows the wide applicability of the proposed method. The catalyst's good recyclability over six runs suggests this ODH method as a promising route to access the biaryl compounds. In addition, the reaction mechanism is investigated with a combination of X-ray photoelectron spectroscopy, functional group blocking, and model compounds of carbon catalysts and is proposed to be based on the redox cycle of the quinoidic groups on the carbon surface. Additional experiments prove that the addition of the catalytic amount of acid (methanesulfonic acid) accelerates the reaction. In addition, Hammett plot examination suggests the formation of a carbonium intermediate, and its possible structure is outlined. [ABSTRACT FROM AUTHOR]

Published
2023
Full Text
View/download PDF

9. Carbon Materials as Catalytic Tools for Oxidative Dehydrogenations and Couplings in Liquid Phase.

Author

Lenarda, Anna, Wirtanen, Tom, and Helaja, Juho

Subjects
OXIDATIVE dehydrogenation, OXIDATIVE coupling, INHOMOGENEOUS materials, ALCOHOL oxidation, CARBON, ACTIVATED carbon
Abstract

Carbocatalysis is a heterogeneous metal-free catalytic technique of high potential for current and future synthetic processes. Carbocatalysts entail heterogeneous materials based on sp2 /sp3 interconnected carbons decorated with oxygen functional groups, defects, and other heteroatom dopants. In this short review, we cover a wide range of different carbocatalyzed oxidative dehydrogenation reactions, with particular emphasis on liquid-phase transformations that are relevant for synthetic organic chemists. 1 Introduction 2 Structures of Catalytic Carbon Materials 3 Oxidative Aromatizations with Activated Carbon 4 Oxidative Dehydrogenation CH–CH Couplings 5 Oxidative Dehydrogenation Coupling of Benzyl Amines 6 Oxidations of Alcohols 7 Other Oxidative Transformations by Heteroatom-Doped Carbon Materials 8 Asphaltene Oxide 9 Conclusions and Outlook [ABSTRACT FROM AUTHOR]

Published
2023
Full Text
View/download PDF

18. NH tautomerism in the natural chlorin derivatives

Author

Helaja, Juho, Stapelbroek-Mollmann, Maria, Kilpelainen, Ilkka, and Hynninen, Paavo H.

Subjects
Porphyrins -- Spectra, Chlorophyll -- Analysis, Chemical structure -- Analysis, Proton magnetic resonance -- Measurement, Organic compounds -- Synthesis, Biological sciences, Chemistry
Abstract

Results indicate that deformations in the chlorin skeleton are involved in the NH tautomeric exchange, which occurs in a stepwise proton-transfer manner via a hydrogen bridge as revealed by NMR spectroscopy and molecular modeling of the NH tautomerism of magnesium-free chlorophyll a and b derivatives.

Published
2000

19. NH tautomerism in the dimethyl ester of bonellin, a natural chlorin

Author

Helaja, Juho, Montforts, Franz-Peter, Kilpelainen, Ilkka, and Hynninen, Paavo H.

Subjects
Ethanes -- Research, Esters -- Research, Nuclear magnetic resonance spectroscopy -- Usage, Porphyrins -- Research, Biological sciences, Chemistry
Abstract

Nuclear magnetic resonance spectroscopy and molecular modeling were used to investigate the NH tautomerization process of bonellin diemthyl ester. Results suggested that the less aromatic cis NH tautomers are not significantly involved in the process. The overall tautomerization was found to proceed via the fully aromatic cis-tautomers and less aromatic trans-tautomer in a stepwise mechanism.

Published
1999

26. Locked conformations for praline pyrrolidine ring: Synthesis and conformational analysis of cis- and trans-4-tert-butylprolines

Author

Koskinen, M.P., Helaja, Juho, Kumpulainen, Esa T.T., Koivisto, Jari, Mansikkamaki, Heidi, and Rissanen, Kari

Subjects
Conformational analysis, Biological sciences, Chemistry
Abstract

A sterically demanding tert-butyl group at C-4 in trans- and cis-configurations is introduced. Both cis- and trans-4-tert-butyl groups strongly favor pseudoequatorial orientation, thereby causing opposite puckering effects for the pyrrolidine ring, cis-exo and trans-endo for (super L-)prolines, in contrast to the effects observed in the case of electronegative C-4 substituents.

Published
2005

35. Locked Conformations for Proline Pyrrolidine Ring: Synthesis and Conformational Analysis of cis- and trans-4-tert-4Butylprolines

Author

Koskinen, Ari M.P., Helaja, Juho, Kumpulainen, Esa T.T., Koivisto, Jari, Mansikkamäki, Heidi, Rissanen, Kari, Department of Chemistry, Aalto-yliopisto, and Aalto University

Subjects
cuprate reagent, tert-butylproline, conformational analysis
Abstract

The motional restrictions of the proline pyrrolidine ring allow this secondary amine amino acid to act as a turn inducer in many peptides and proteins. The pyrrolidine ring is known to exhibit two predominant pucker modes (i.e., C-4 (Cç) exo and endo envelope conformers whose ratio can be controlled by proper substituents in the ring). In nature, the exo puckered 4(R)-hydroxy-L-proline plays a crucial role as a building block in collagen and collagen-like structures. It has been previously concluded that the electronegativity of the 4-cis-substituent increases the endo puckering while the electronegativity of the 4-trans-substituent favors the exo puckering. Here, we have introduced a sterically demanding tert-butyl group at C-4 in trans- and cis-configurations. In the case of transsubstitution, the induced puckering effect on the pyrrolidine ring was studied with X-ray crystallography and 1H NMR spectral simulations. Both cis- and trans-4-tert-butyl groups strongly favor pseudoequatorial orientation, therebycausing opposite puckering effects for the pyrrolidine ring, cis-exo and trans-endo for L-prolines, in contrast to the effects observed in the case of electronegative C-4 substituents. The syntheses and structural analysis are presented for the conformationally constrained 4-tert-butylprolines. The prolines were synthesized from 4-hydroxy-L-proline, substitution with t-BuCuSPhLi being the key transformation. This reaction gave N-Boctrans-4-tert-butyl-L-proline tert-butyl ester in 94% ee and 57% de. Enantioselectivity was increased to 99.2% ee by crystallization of N-Boc-trans-4-tert-butyl-L-proline in the final step of the synthesis.

Published
2005

42. ChemInform Abstract: Cycloisomerization of 2-Alkynylanilines to Indoles Catalyzed by Carbon-Supported Gold Nanoparticles and Subsequent Homocoupling to 3,3′-Biindoles.

Author

Perea-Buceta, Jesus E., primary, Wirtanen, Tom, additional, Laukkanen, Otto-Ville, additional, Maekelae, Mikko K., additional, Nieger, Martin, additional, Melchionna, Michele, additional, Huittinen, Nina, additional, Lopez-Sanchez, Jose A., additional, and Helaja, Juho, additional

Published
2014
Full Text
View/download PDF

44. Klorofylliaggregaattien optisten ominaisuuksien parantaminen supramolekyläärisen suunnittelun avulla

Author

Helaja, Juho, Mikro- ja nanotekniikan laitos, Tittonen, Ilkka, Pale, Ville, Helaja, Juho, Mikro- ja nanotekniikan laitos, Tittonen, Ilkka, and Pale, Ville

Abstract

Klorofyllimolekyylit (lehtivihreä) ovat tehokkaimpia valoa kerääviä pigmenttejä, mitä luonnosta on löydetty. Niillä on myös uskomaton kyky siirtää viritysenergia toiseen klorofyllimolekyyliin jopa 90 % hyötysuhteella. Lisäksi niillä on erittäin hyvä fluoresenssin kvanttisaanto mahdollistaen niiden käytön uusissa optisissa materiaaleissa. Tämän evoluution muovaaman ilmiön hyödyntäminen voi johtaa uusiin mullistaviin fotoniikan materiaaleihin tai laitesovelluksiin, kuten energian tuottamiseen tai sensorisovelluksiin. Ennen tätä pigmentit täytyy kuitenkin pystyä integroimaan puolijohdemateriaaleihin. Tästä johtuen työssä on kehitetty menetelmä GaAs pinnan passivoimiseksi ja funktionalisoimiseksi. Työn toisena tavoitteena on ollut optimoida klorofyllien fluoresenssia supramolekyläärisen arkkitehtuurin avulla. Kirjallisuusosuus on jaettu kolmeen osaan. Ensimmäinen osa sisältää tietoa molekyylien viritysdynamiikasta. Toinen osa on omistettu klorofyllien optisten, kemiallisten ja aggregointi ominaisuuksien selittämiseen, jotka voidaan suoraan päätellä molekyylien rakenteen avulla. Viimeisessä osassa tutkitaan resonoivaa energiasiirtoa, joka on erittäin tärkeä molekyylien välinen vuorovaikutusmekanismi, joka on läsnä esimerkiksi kasveissa, yhteyttävissä bakteereissa ja johtavissa muoveissa. Kokeellinen osuus on jaettu kahteen osaan. Ensimmäisessä osassa GaAs pinnan passivointiin ja funktionalisointiin käytetään tioliyhdisteitä. Syntynyttä atomikerroksen paksuisen päällysteen ominaisuuksia karakterisoidaan taajuus ja aikatasossa fotoluminesenssin avulla. Toisessa osassa tutkitaan fluoresenssin tehokkuutta kahden polymeeri-pigmentti kompleksin ja klorofylli aggregaattin välillä. Näytteet valmistetaan lasisubstraateille "spin coating"-menetelmän avulla ja karakterisoidaan Uv-Vis absorption ja fluoresenssispektroskopian avulla., Chlorophyll molecules are the most efficient light-harvesting pigments found in nature. They also exhibit a remarkable ability to transfer the excitation energy to another chlorophyll molecule with 90 % efficiency. In addition, they possess a very good fluorescence quantum yield opening a possibility as new optical materials. To utilize this property could lead to new novel photonic materials or devices, e.g. in energy harvesting or sensor applications. However, for most applications this requires integration of the chromophores into the semiconductor materials. Hence, a method to passivate and functionalize GaAs surface has been developed. Also, another goal of this thesis has been to find the optimal supramolecular architecture to improve the fluorescence of the chlorophylls. The literature part is divided into three major parts. The first part contains information about the excitation dynamics of molecules. The second part is dedicated to explain the optical, chemical and aggregation properties of chlorophylls arising directly from the structural properties of these molecules. The third part investigates the resonance energy transfer, an important intermolecular interaction mechanisms present, e.g., in plants, photosynthetic bacteria and conductive polymers. The experimental part is divided into the two parts. First, a method to passivate and functionalize GaAs surface using thiol monolayer is investigated with continous wave and time-resolved photoluminescence spectroscopy. Second, the fluorescence between two different polymer-dye complexes and pure chlorophyll aggregates is studied. These samples are cast on a glass substrate using spin coating and the resulting films of are characterized by Uv-Vis absorption and fluorescence spectroscopy.

Published
2011

46. Light-Harvesting Nanorods Based on Pheophorbide-Appending Cellulose

Author

Sakakibara, Keita, primary, Granström, Mari, additional, Kilpeläinen, Ilkka, additional, Helaja, Juho, additional, Heinilehto, Santtu, additional, Inoue, Rintaro, additional, Kanaya, Toshiji, additional, Hill, Jonathan P., additional, Nakatsubo, Fumiaki, additional, Tsujii, Yoshinobu, additional, and Ariga, Katsuhiko, additional

Published
2013
Full Text
View/download PDF

50. Chemically controlled molecular hinges

Author

Helaja, Juho, Teknillinen korkeakoulu, Helsinki University of Technology, Kemian tekniikan osasto, Koskinen, Ari, Abe, Aki, Helaja, Juho, Teknillinen korkeakoulu, Helsinki University of Technology, Kemian tekniikan osasto, Koskinen, Ari, and Abe, Aki

Published
2005

Catalog

Books, media, physical & digital resources
See catalog results