An official website of the United States government
The 1H- and 2H-1,2,3-triazoles are isomeric five-membered ring, aromatic heterocycles that may undergo chemical equilibration by virtue of intramolecular hydrogen migration (tautomerization). Using millimeter-wave spectroscopy in the 130–375 GHz frequency range, we measured the spectroscopic constants for thirteen 1H-1,2,3-triazole and sixteen 2H-1,2,3-triazole isotopologues. Herein, we provide highly accurate and highly precise semi-experimental equilibrium (reSE) structures for the two tautomers based on the spectroscopic constants of each set of isotopologues, together with vibration–rotation interaction and electron-mass distribution corrections calculated using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The resultant structures are compared with a “best theoretical estimate” (BTE), which has recently been shown to be in exceptional agreement with the semi-experimental equilibrium structures of other aromatic molecules. Bond distances of the 1H tautomer are determined to <0.0008 Å and bond angles to <0.2°. For the 2H tautomer, bond angles are also determined to <0.2°, but bond distances are less precise (2σ ≤ 0.0015). Agreement between BTE and reSE values is discussed.
more »
« less
Controlling Tautomerization in Pyridine‐Fused Phosphorus‐Nitrogen Heterocycles
Abstract Inclusion of a second nitrogen atom in the aromatic core of phosphorus‐nitrogen (PN) heterocycles results in unexpected tautomerization to a nonaromatic form. This tautomerization, initially observed in the solid state through X‐ray crystallography, is also explained by computational analysis. We prepared an electron deficient analogue (2 e) with a fluorine on the pyridine ring and showed that the weakly basic pyridine resisted tautomerization, providing key insights to why the transformation occurs. To study the difference in solution vs. solid‐state heterocycles, alkylated analogues that lock in the quinoidal tautomer were synthesized and their different1H NMR and UV/Vis spectra studied. Ultimately, we determined that all heterocycles are the aromatic tautomer in solution and all but2 eswitch to the quinoidal tautomer in the solid state. Better understanding of this transformation and under what circumstances it occurs suggest future use in a switchable on/off hydrogen‐bond‐directed receptor that can be tuned for complementary hydrogen bonding.
more »
« less
- Award ID(s):
- 2107425
- PAR ID:
- 10446274
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 28
- Issue:
- 22
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Direct and regioselective functionalization of pyridine is a topic of high scientific and technological importance. In spite of extensive efforts, the regioselective functionalization of pyridine still remains a significant challenge due to their low reactivity and presence of Lewis‐basic sp2nitrogen. Here, we studied the effect of hydrogen bonding interactions on the regiochemical outcome of Pd‐mediated C−H activation of pyridine by utilizing DFT calculations. We demonstrated that hydrogen bonding can act as a second independent factor to override the inherent regioselectivity of pyridine. This novel approach complements previously reported strategies, such as: (a) coordination of pyridine to transition metal center via its N‐center, (b) installation of directing group (DG) and then coordination of pyridine to the transition metal center via this DG (i. e. chelation assistant strategy), (c) protection of its nitrogen lone pair with N‐oxide or N‐imino groups or with Lewis acids, (d) the inherent positional reactivity of C−H bonds based on the electronic or steric properties of the substituents, and (e) by the identity of the oxidant used. We have also demonstrated that the oxidation state of the Pd catalyst has impact on the regiochemical outcome of the C−H activation step in pyridine. The implications of our study for regioselective C−H functionalization catalyst design of heteroarenes are twofold: It demonstrates (1) hydrogen bonding as a viable design principle, and (2) Pd(IV) as a catalyst for C−H functionalization.more » « less
-
Abstract 2,6‐Bis(pyrrol‐2‐yl)pyridines are important building blocks for supramolecular assemblies and photoluminescent main group and transition metal compounds. Sterically encumbered 2,6‐bis(5‐(2,4,6‐trimethylphenyl)‐3‐phenyl‐1H‐pyrrol‐2‐yl)pyridine, H2MesPDPPh, can adopt monomeric and dimeric structures in the solid state and in solution, controlled by competing inter‐ and intramolecular hydrogen bonds. Deprotonation in the presence of lithium cations provides Li2MesPDPPh, which can be isolated in monomeric and dimeric forms depending on solvent polarity. Protonation of H2MesPDPPhdisrupts intramolecular hydrogen bonding and provides the monomeric pyridinium salt [H3MesPDPPh]Cl. Independent of solvent polarity, all three derivatives exhibit intense fluorescence in solution. The absorption and emission spectra are highly sensitive to the level of protonation, which can be rationalized by the effects of (de)protonation on the HOMO and LUMO of the tricyclic π‐system.more » « less
-
Structural characteristics are reported for two thioether–ketones,DtdpeandMtdp[2-({2-[(2-oxo-2-phenylethyl)sulfanyl]ethyl}sulfanyl)-1-phenylethan-1-one, C18H18O2S2, and 2-[(2-oxo-2-phenylethyl)sulfanyl]-1-phenylethan-1-one, C16H14O2S], and for related derivatives, the bis(pyridylhydrazones)DhpkandPrpsb[2-((2E)-2-{(2Z)-2-phenyl-2-[2-(pyridin-2-yl)hydrazin-1-ylidene]ethylidene}hydrazin-1-yl)pyridine, C18H16N6, and 2-[(2Z,12Z)-3,12-diphenyl-14-(pyridin-2-yl)-5,10-dithia-1,2,13,14-tetraazatetradeca-2,12-dien-1-yl]pyridine, C30H32N6S2], as well as for the macrocyclic thiocarbohydrazide derivativeCtrsp[(3E,8Z)-3,9-dimethyl-1,11-dithia-4,5,7,8-tetraazacyclotetradeca-3,8-diene-6-thione, C10H18N4S3]. Three of the five compounds exhibit conformational enantiomerism in the solid state. The occurrence of intra- and intermolecular hydrogen bonding is commented upon through quantum mechanical (DFT) calculations. Weak C—H...S interactions are noted, while stronger N—H...N and N—H...S hydrogen bridges are delineated.more » « less
-
ABSTRACT The pursuit of sustainable organic synthesis has renewed interest in photochemistry, as sunlight‐driven reactions provide eco‐friendly alternative methods. Although the relationships among structure, properties, and reactivity are well established for ground‐state molecules, the understanding of excited states and reactive intermediates, such as triplet and singlet arylnitrenes, remains limited. Herein, we investigated the properties of triplet and singlet 4‐nitrenopyridine‐1‐pyridine oxide (1N), 3‐nitrenopyridine‐1‐pyridine oxide (2N), and phenylnitrene (PhN) using density functional theory (DFT), complete active space self‐consistent field (CASSCF(10,9)), and complete active space second‐order perturbation theory (CASPT2(10,9)) calculations. Bond length analysis demonstrated that31Nand11N, as well as12Nand1PhN, exhibit significant imine biradical character, whereas the structures of32Nand3PhNare better described as benzene‐like. Nucleus‐independent chemical shift (NICS(0), NICS(1.7)ZZ) and anisotropy of induced current density (ACID) calculations were performed to compare the induced magnetic currents in these molecules. These analyses demonstrated that31Nis weakly aromatic, whereas32Nand3PhNare best described as having Baird aromaticity. In contrast, singlet nitrenes11N,12N, and1PhNare nonaromatic. In addition, irradiation of1in argon matrices verified that31Nreacts photochemically to form corresponding ketenimine1K. Finally, the absorption difference spectrum of31Nin a frozen 2‐methyltetrahydrofuran (mTHF) matrix exhibited resolved vibrational structure, suggesting the vibrational coupling to another electronic state. These insights into the structure and aromaticity of heterocyclic nitrenes could provide new avenues for modulating the reactivity of triplet ground state and triplet excited molecules.more » « less
