Synthesis and nuclear magnetic resonance spectroscopic studies of 1‐arylpyrroles

Abstract

A series of m‐ and p‐substituted 1‐phenyl, 1‐benzyl, 1‐benzoyl, and 1‐(2‐phenylethyl)pyrroles was prepared and their 1H and 13C nmr spectroscopic characteristics were examined. In general, good correlations were observed between the chemical shift values of the βH and the βC of pyrroles [except 1‐(2‐phenylethyl)pyrroles] and the Hammettt σ. The observation may be explained in terms of the electronic effects of the substituents which are transmitted through bonds and through space by interaction of the p orbitals between βCs of the pyrrole ring and m‐ and pCs of the phenyl ring. Substituent constants of 1‐pyrrolyl, 1‐pyrrolylmethyl, and 1‐pyrroloyl groups for the 1H and 13C chemical shifts of phenyl ring are also presented.

Computation of through-space NMR shielding effects in aromatic ring π-stacked complexes

“…When the mean chemical shifts of protons in the equilibrium structure dimer are compared to those in the monomer (Table 6), the shielding effect due to complex formation is apparent. Using published data on the chemical shift difference at two concentrations (0.01 and 0.10 M) [25] and assuming a linear relationship between chemical shift change and dimerization, it can be estimated that the 0.1 M solution consisted of at least 13% dimer. It is noteworthy that experimentally only a very slight change in the chemical shift Table 4 Differences (complex-isolated ring) between the Weinhold (npa) computed charges (in esu Â10 3 ) of the proximal proton of the diatomic hydrogen probe over an unsubstituted ring carbon at 2.5, 3.0, 3.5, 4.0 and 4.5 Å above the plane of each substituted aromatic ring in the p complex and the isolated substituted aromatic ring of the b-protons of phenylpyrrole was observed with changing concentration.…”

“…Lee et al have reported that N-phenypyrole exhibits a concentration-dependent NMR spectrum [25]. This phenomenon is not uncommon in heterocyclic aromatic compounds [26][27][28].…”

Rh(III)-Catalyzed Oxidative C–2 Coupling of N-Pyridinylindoles with Benzo[b]thiophene 1,1-Dioxides via C–H Bond Activation

“…All solvents were dried according to known methods and distilled prior to use . 1-(Pyridin-2-yl)-1 H -indole, 1-(pyrimidin-2-yl)-1 H -indole, 2-deuterio-1-(pyrimidin-2-yl)-1 H -indole, 1-(pyridin-2-yl)-1 H -benzo­[ d ]­imidazole, N , N -dimethyl-1 H -indole-1-carboxamide, 2-(1 H -pyrrol-1-yl)­pyridine, and benzo­[ b ]­thiophene 1,1-dioxide derivatives 2a – i and 2-deuteriobenzo­[ b ]­thiophene 1,1-dioxide 2a-D were synthesized by using H 2 O 2 /HCO 2 H (1:2) oxidation of the corresponding benzothiphenes according to literature procedures. ,,, The rhodium complex [Cp*RhCl 2 ] 2 was prepared according to the literature protocols . All other reagents were purchased from Sigma-Aldrich, Alfa Aesar, or Avra and used without further purification.…”

Five‐Membered Heterocycles: Pyrrole and Related Systems

“…Although some simple pyrroles are oils, many derivatives with higher molecular weights are solids. Pyrrole displays weakly acidic properties (pK a ¼ 17.25 in aqueous medium [13], 17.51 in aqueous hydroxide solution [14] and 23.05 in DMSO [15]). The dipole moment (m) of pyrrole is 1.74 AE 0.02 D, with the negative pole directed towards the ring carbon atoms [16].…”

“…The 13 C resonances of 1H-pyrroles lie in the aromatic region, and the shifts depend on the electronic effects transferred from the substituents. Alkyl- [9] or aryl [23] substituents at the nitrogen atom usually have only limited influence on the ring carbon chemical shifts regardless of their properties, whereas the presence of electron-withdrawing groups can cause relatively large downfield shifts of the resonances; the magnitude of this effect increases with the electron-withdrawing power of the N-substituent [24]. The tetrahedral carbon of 2H-pyrroles usually resonates at 78-98 ppm, in contrast to its counterpart in 3H-pyrroles, which appears in the range ppm.…”