Hans Dahn

17O NMR Spectra of Carbonyl Compounds ArCOX: Influence of Groups X on the Polarity of the Carbonyl Group

The 17O shift values of para‐substituted benzoyl compounds p‐YC6H4COX (X=H, CF3, COOEt, Br, Cl, F, SEt, OCOAr, OH, O‐Na+, NH2) were measured. The 17O shielding values δ(17O) of these and other aryl‐unsubstituted benzoyl compounds (Y=H) follow the electron‐donating power of X (resonance), whereas inductive electron attractors (CF3, COOEt) show only small effects. The sensitivity of the shift values to donor/attractor para substituents Y, measured as the Hammett–Taft ρ+=δ(17O)/σ+, follows both the resonance and the inductive effects; combined with the δ(17O) values, it is proposed as an empirical measure of the polarity (electrophilicity) of the carbonyl group (35 types of COX in Table 1).

The Origin of the Difference Between the 13C and 17O Shift Behaviour of Carbonyl Compounds RCOX: Ab initio Calculation of the Shielding Tensors

In carbonyl compounds RCOX (X=H, Me, SiR3, SR, Cl, F, OMe, OH, NH2, O‐; R=H, Me), the 17O shift values of the carbonyl group depend on the electron donor–acceptor properties of X, whereas the 13C shift values are determined by other factors too. By IGLO ab initio calculations, the difference between 13C and 17O has been traced to differences in the relative importance of the shielding tensor component in the direction of the C—O bond (σzz). The deshielding contribution of this component is mainly determined by the energy of the n–π* excitation; donor–acceptor‐type interactions influence the level of both orbitals. As the n orbital of this transition is essentially localized on the O atom, the excitation acts less on 13C and is thus mainly responsible for the difference in substituent sensitivity between 13C and 17O shifts. An analogous difference of substituent sensitivities between 13C and 17O shifts exists in aroyl compounds p‐YC6H4COX towards changes in the para substituent Y; it is explained on the same basis as the effect of X upon RCOX, without the necessity of referring to a ‘reverse’ substituent effect. Finally, the surprising absence of substituent effects upon the 17O shift in aroyl cations p‐YC6H4CO+ can be explained by the fact that, for symmetry reasons, the n–π*‐type excitation is absent in linear compounds.

(2+4)‐Cycloaddition with Singlet Oxygen. 17O‐lnvestigation of the Reactivity of Furfuryl Alcohol Endoperoxide¶

In earlier work, the use of furfuryl alcohol as a specific singlet oxygen acceptor was proposed because of the high ratio between the rate constants of chemical reaction and physical quenching. In contrast to furfuryl aldehyde, a number of products are formed by this type II photo‐oxidation of furfuryl alcohol. These products may be derived from the endoperoxide of furfuryl alcohol as a common intermediate. The present work focuses on the reactivity of this endoperoxide that was marked specifically by the use of 17O2 as a source for singlet oxygen. The analyses of the stable products, their yields and their labeling distribution reveal a strong solvent effect on the primary reaction pathways, and nucleophilic substitution reactions leading to hydroperoxide intermediates are dominant.

Concerning the chemical shift data (17O, 13C, 1H) of formaldehyde

The 17O NMR signal of formaldehyde was measured and the literature values of the 13C and 1H signals are discussed. They are compared with values obtained by other spectroscopic methods and by ab initio calculation.

NMR of terminal oxygen. Part 10. 17O NMR spectra of carbonyl phosphonium ylides and diazocarbonyl compounds: resonance stabilization, bond order and excitation energy

On the scale of carbonyl electrophilicities, measured by 17O NMR spectroscopy, carbonyl groups in aroyl (1), acetyl (2) and ethoxycarbonyl (3) phosphonium ylides are similar to those of amide groups, showing a considerable effect of resonance donation. The relative weight of the limiting formulae in the resonance in 1 and 3 is found to be close to earlier estimations which were based upon 1H and 13C NMR shift values. For comparison, a series of phenacyl compounds PhCOCH2X have been investigated; they show a very small effect of X on δo The δo values of aroyl (4) and ethoxycarbonyl (5) diazoalkanes are found to be less shielded than those of the corresponding ylides, demonstrating the importance of long-wave electronic transitions observable in the UV–VIS spectrum. However, the substituent sensitivity coefficient ρ+ of aroyldiazoalkanes 4 is similar to those of esters and amides.

Structure of the reaction product of 4-hydroxy-2,3- dioxo-4-phenylbutanoic acid 1,4-lactone with o-phenylenediamine

Abstract Examination of the structure of the yellow product ( 5a ), obtained by treating 4-hydroxy-2,3-dioxo-4-phenylbutanoic acid 1,4-lactone (4-phenyl-2,3-dioxobutyro-1,4-lactone) with two moles of o -phenylenediamine, by high-resolution 1 H-, 13 C-, and 15 N-n.m.r. spectroscopy, as well as by electron-impact mass spectrometry, confirmed without ambiguity the structure of the product as the quinoxaline amide 5a . When 4-hydroxy-2,3-dioxo-4-phenylbutanoic acid 1,4-lactone is treated with o -phenylenediamine, the Schiff base is first formed, which is then converted into a quinoxaline lactone ( 6a ). The excess of o -phenylenediamine then converted the quinoxaline lactone ( 6a ) into the yellow product ( 5a ).

NMR of terminal oxygen. Part 12. SO2 and isoelectronic compounds with a true π-bond: 17O NMR spectra of sulfinylamines R–NSO and sulfines RR′CSO. The conformation of ortho-substituted diaryl sulfines

The 17O NMR shift values δo of 20 sulfinylamines R–NSO 2 and of 14 sulfines RR′CSO 3(R and R′ mostly arene groups) were compared with those of the isoelectronic SO21. They show δo at much lower field than practically all other classes of S–O compounds; at the same time they exhibit the high substituent sensitivity which is typical for true πp bond systems, and absent in S+–O– compounds. The difference is discussed in terms of bond order and electronic excitation energy. Comparison of the E- and Z isomers of O-methyldiaryl sulfines yields the conformation of these compounds: one arene ring is (nearly) coplanar with the CSO group and the other (nearly) perpendicular to it. This demonstration for the molecules in solution is supported by an X-Ray structure determination.

NMR of terminal oxygen. Part 7. 17O NMR spectra of benzoyl derivatives of Ge, Se, Te and I: π-bond order and excitation energy in benzoyl compounds

The benzoyl derivatives of SeR, TeR and I show lower field 17O NMR signals than those of the lighter elements in the same group of the Periodic Table, whereas ArCOGeR3 is as low-field as ArCOSiR3. The δo values of 14 types of benzoyl compounds ArCOX are discussed in terms of π-bond order (obtained from arene substituent sensitivities using the ‘tool of increasing electron demand’) and of electronic excitation energy (obtained from UV spectra). It is concluded that, for 17O NMR, the terms ΔE–1 and ΣQ of the Karplus–Pople equation are, at least to a certain degree, independent of one another.

NMR of terminal oxygen. Part 11. 17O NMR spectra of disilyl ketone and related compounds: a case of strong deshielding by electronic excitation energy

The important deshielding effect on the 17O signal of the disilyl ketone 1(Δδ ca. 400 ppm, compared with normal ketones such as 3) and of the monosilylketones 2, is proportional to the λ values of the UV–VIS absorption spectra; in agreement with the r-term of the Karplus–Pople eqn. (1), it is four times larger than the analogous effect on the 13C signals of the carbonyl carbon.

NMR of Terminal Oxygen—17—Ab initio IGLO Study of the High Shielding of O (and C, N) in Linear Heteronuclear π‐Systems

Linear heteronuclear π‐systems of sp character, when compared with corresponding sp2 systems, show increased shift to higher field for all atoms concerned (O, C, N); acetylene compared with ethylene is a model case. In 17O NMR, the molecules NO+, PhCO+, RCNO, NξCξO−, RNξCξO and NO2+ were examined (in addition to known CO, CO2, COS and N2O). For these molecules, 13C and 15N shieldings were also analysed. As had been shown before for 13C and 15N, the increase in shielding of 17O in linear molecules is mainly attributable to the vanishing of the deshielding component along the linear axis of the molecule (manifest also in the increase in the anisotropy of shielding Δσ). Ab initio IGLO calculations were executed to evaluate Δσ ( = σ‖‐σ⊥). For each of the atoms concerned, the resulting calculated average values agreed well with the experimental values of isotropic shielding, σiso. As in the case of C and N, the values of σ‖ of 17O in linear compounds are close to that of the diamagnetic shielding of the isolated O atom, σd (atom). The particular case of organic isocyanates, RNCO, which, although not completely linear, show related behaviour, is discussed.