The Proton Resonance Spectra of Some Substituted Furans and Pyrroles

Abstract

The proton magnetic resonance spectra of some substituted fur'u~s ailcl pyrroles have been analyzed for spin coupling constants and chernical shifts.The relative insensitivity of the spin coupling constants to the nature of the substituent malres it possible to estimate their values in the parent rnoleculcs furan and pyrrole. Thc magnitude of the spin coupling constants is correlated with the angles made by the CH bonds with the C C bonds of the ring.The chemical shifts arc interpreted in terrns of ail effect due to the electronegativity of the substituent together with the effect due to conjugatiorl with the ring.The proton resonance spectra of the l~eterocyclic c o m p o~~n d s furan, pyrl-ole, ant1 thiophene and of their derivatives have been the subject of a number of investigations (1, 2, 3, 4). The present work is an attempt to determine more systematically the effects of substitution on the chemical shifts and spin coupling-constants of the ring protons in substituted furans and pyrroles.These compounds are well suited to a proton magnetic resonance investigation since their spectra are usually capable of a unique interpretation and assignment. The simplest spectra are those of the substituted furans. I-Iere, due to the large difference in the chemical shifts of the a ancl P protons of the ring, any inonos~~bstit~~tecl furan can be classed as an ABX-type spectrum for which a complete analysis is readily achievecl. In some cases, where fewer lines are observed than tl~eoretically expected, the spectrum obtained does not give explicit values of any of the coupling constants. This type of spectrum, which is obtained not only for some monosubstitutecl furans but also for the parent co~upounds furan and pyrrole, I n s been the subject of a previous publication ( 5 ) . The spectra of the substituted pyrroles are complicated by the additional interaction of the N-1-1 proton with the other ring protons. I-Iowever, a ~~ossible interpretation of the spectra of some monosubstitutecl pyrroles is presentecl. EXPERIMENTALThe compounds used were the co~ll~nercial samples except for the following. 2-Nitro furan was prepared by nitrating iuran with funling nitric acid and acetic anhydride following the procedure of fiilarquis (6). 5-Chlormetl~yl iurfural was prepared from sugar and concentrated I-ICI following the procedure of Aso (7). 3-Furoic acicl and itsderivatives were ltindly supplied by Prof. I-I. Gilman. The specinlen of 2-cyano pyrrole was lcindly supplied by Prof. I-I. J. Anderson.The spectra of the colnpou~lds denlonstrated their purity. Only in one case, 2-furoyltrifluoromethyl Itetone, was there any sign of any impurity.The spectrometer and the techniques for ineasuring the spectra have been described fully in a previous paper (1). The spectra are shown for 40

Studies of Organophosphorus Derivatives. Part I. Nuclear Magnetic Resonance Studies of the 2-Oxo-1,3,2-dioxaphosphorinane System

“…13A triplet X resonance of an ABX spin system implies equal AX and BX coupling constants only if the chemical shift difference between the A and B nuclei is significant (23).…”

Proton magnetic resonance of symmetrical molecules

“…All coupling constants are positive [15]. Downloaded by [McGill University Library] at 10:08 18 November 2014 coupling constants [23,24,[26][27][28][29][30]. The results are summarized in table 9.…”

Determination of the relative signs of proton spin coupling constants by double irradiation

“…The substance is 2-Furoic acid and it has been studied as a saturated solution in acetone on a Varian Associates 60 Mc/s high resolution spectrometer at about 24~ This substance has been investigated previously at 30 Mc/s [3], 40 Mc/s [4] and 60 Mc/s [5] as an ABX type molecule (the acid proton does not couple into the ring). At 60 Mc/s the spectrum is very nearly of the first order type (figure 1) but the ABX nomenclature has been retained and a normal ABX Table 1.…”