Browsing by Author "Chang, Yung-Yu"

Now showing 1 - 3 of 3

Deformative Transition of the Menschutkin Reaction and Helical Atropisomers in a Congested Polyheterocyclic System
(2014-11-07)
Chang, Yung-Yu
;
Ho, Tse-Lok
;
Chung, Wen-Sheng
;
應用化學系
;
Department of Applied Chemistry
A 4,7-phenanthroline polycyclic 1A designed for probing the limits of the Menschutkin reaction was synthesized in a six-step sequence. The rotational barrier of the phenyl ring nearby the N-methyl group in rac-2A was estimated to be >= 18.1 kcal/mol from VT-NMR experiments, making them a new type of helical atropisomer. The methylation rate constants of 9 and 1A with MeI was found to be 2.22 x 10 (4) and 9.62 x 10 (6) s(1) mol(1) L, respectively; thus, the formation rate of (P/M)-2A is one of the slowest rates ever reported for a Menschutkin reaction. The N-methyl protons in (P/M)-2A exhibit a significant upfield shift (Delta delta 1.0 ppm) in its H-1 NMR, compared to those without a nearby phenyl, indicating a strong CH-pi interaction is involved. Conformational flexibility in dipyridylethene 9 is clearly shown by its complexation with BH3 to form helical atropisomers (P,P/M,M)-10. The pKa values of the conjugate acids of 1A and 9 in acetonitrile were determined to be 4.65 and 5.07, respectively, which are much smaller compared to that of pyridine 14a (pK(a) = 12.33), implying that the basicity, nucleophilicity, and amine alkylation rates of 1A and 9 are markedly decreased by the severe steric hindrance of the flanking phenyl rings in the polyheterocycles.
Different sensing modes of fluoride and acetate based on a calix[4]arene with 25,27-bistriazolylmethylpyrenylacetamides
(2014)
Hung, Hao-Chih
;
Chang, Yung-Yu
;
Luo, Liyang
;
Hung, Chen-Hsiung
;
Diau, Eric Wei-Guang
25,27-Bis{1'-N-(1-pyrenyl)-aminocarbonylmethyl-1H-[1',2',3']tri-azolyl-4'-methoxy}-26,28-dihydroxycalix-[4]arene, 4, is synthesized as a fluorescent chemosensor for the selective detection of both anions and ion pairs in MeCN. Sensor 4 uses bis-triazoles as ligands to bind a metal ion, bis-amides and bis-triazoles as the sites to recognize anions, and pyrenes as fluorophores. Among eight anions screened, chemosensor 4 showed a marked fluorescence change toward F-, H2PO4- and AcO-, but 4 responded to each anion in a distinct way. In the presence of F- at low concentrations, the dynamic excimer emission of compound 4 at lambda(max) 482 nm was quenched, but an emission at lambda(max) 472 nm appeared at large doses of F-. A control compound 6 showed very similar red shifts in the UV-vis and excitation spectra as 4 did, and its 472 nm emission band grew as the fluoride doses increased. Thus, the growth of the 472 nm emission of 4 and 6 in the presence of excess F- may be because strong H-bonding interactions of amido protons with F- favoured the formation of pyrene dimers in the ground state with charge transfer characteristics. The addition of H2PO4-, unlike F-, to a solution of 4 showed an enhanced monomer emission but a decreased excimer emission (lambda(max) 482 nm). Adding AcO- to 4 produced a systematic change from a dynamic excimer (lambda(max) 482 nm) to lambda(max) 472 nm but with very little change in the UV-vis spectrum. Time-resolved fluorescence measurements on compound 6 with F- and AcO- confirmed that the 472 nm emission band mainly came from static excimers for the former, but was partly from a dynamic excimer for the latter because it contained a growth component in the fluorescence decay traces. Without pre-treatment with an anion, chemosensor 4 showed recognition of only metal ions Cu2+, Hg2+ and Cr3+, but it became sensitive to Ag+ when it was pretreated with fluoride.
The Synthesis of Rigid Polycyclic Structures for the Study of Diatropic or Steric Effects of a Phenyl Ring on CF Bond
(2013-12-20)
Chang, Yung-Yu
;
Ho, I-Ting
;
Ho, Tse-Lok
;
Chung, Wen-Sheng
;
應用化學系
Polycyclic compounds 1a-c were synthesized to study the diatropic effects of a flanking phenyl ring on nearby CH and CF bonds. F-19 NMR spectra of 1b and 1c were strongly deshielded compared with those of the ring-opened compounds 3b, 7b, and 7c. DMol3 calculations on 1a-c provided quantitative bond lengths and torsional angles to support the conclusion that the downfield shifts in the F-19 NMR spectra are mainly due to steric interactions between the CF bonds and the pi clouds of the phenyl ring(s).