The nature and types of lone-pair effects on nuclear spin coupling constants are reviewed in the context of a localized bond description of molecular electronic structure. Emphasis is placed on the importance of residual delocalization involving the otherwise lone-pair orbital, in terms of which the effect of an X lone-pair, when compared with an isoelectronic Y-H or X-H+ group or with an X-R group, and its orientational dependence can be interpreted. One-, two-and three-bond coupling constants are considered and the importance of lone-pair effects for configurational and conformational information is stressed and illustrated. They also serve for a better understanding of substituent inductive effects on coupling constants.A large collection of illustrative examples are presented, with particular attention paid to couplings involving H, C, N, F and P nuclei, organized in a systematic manner into nine categories. The signs of the lone-pair effects on the reduced coupling constants are found to be independent of the actual nuclei under study in the same category. This is taken as an indication that the electron lone-pairs mainly affect the Fermi contact contribution to the coupling and, accordingly, an interpretation is given in terms of simple sum-over-states models. In addition, symmetry-based relationship are established involving the sign of lone-pair effects in coupling constants between nuclei which are a different number of bonds apart.KEY WORDS NMR Multinuclear spin coupling Lone-pair effects Substituent effects Stereochemistry MO Sum-over-states models Electron delocalization CONTENTS

Transition metal NMR chemical shifts are readily measured by modern 1D and 2D pulse techniques and serve as a probe into electronic and steric effects of ligands and substituents in metal complexes. Quantitative correlations of metal chemical shifts with reaction rates and catalytic activities, both experimental and as a result of quantum chemical calculations, give new mechanistic insights and permit reactivity predictions and a screening of homogeneous catalysts. A variety of such examples involving the spin-1/2 nuclei 57 Fe, 103 Rh, and 187 Os as well as the quadrupolar nuclei 51 V, 53 Cr, 55 Mn, 59 Co, and 91 Zr are discussed.

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SummaryThe 15N-NMR spectra of azoles, with natural isotope abundance, have been measured under different experimental conditions, and chemical shifts are reported for imidazoles, pyrazoles, oxazoles, isoxazoles, thiazoles, and isothiazoles. General trends of substituent effects in this heterocyclic series are discussed based on the data of 67 substituted azoles, dihydro-and tetrahydroazoles. "N, 'H spin-coupling constants have been determined from spectra obtained by ['HI 4 I5N polarizationtransfer experiments, i.e. an application of INEPT and DEPT pulse sequences. Two-bond and three-bond coupling constants are fully assigned and are discussed in terms of the specific pathways in azoles. The potential of structural applications of the new data is illustrated for isomeric nitro-imidazoles and highly-substituted pyrazoles, and in the case of ring-chain tautomerism of 2-substituted tetrahydrooxazoles.

The nitrogen nucleus is the third most important probe (after ' H and I3C) for structural investigations of organic and bioorganic molecules by NMR spectroscopy. For a long time, however, the insufficient sensitivity and low natural abundance of the I5N isotope hampered detection of the I5N nucleus, and the quadrupolar I4N nucleus proved unsuitable for the study of larger molecules with several nonequivalent nitrogen atoms. The advent of new techniques, such as pulse sequences and polarization transfer, in conjunction with the use of high-field magnets and large-sample probe heads largely solved the detection problem. As a result, the last few years have seen a dramatic development of "N-NMR spectroscopy as a versatile method for studying molecular structure, both in isotropic (liquid) and anisotropic (solid) phases. The scope of chemical applications extends from inorganic, organometallic, and organic chemistry to biochemistry and molecular biology, and includes the study of reactive intermediates, biopolymers, enzyme-inhibitor complexes, and nitrogen metabolism. Two-dimensional NMR techniques offer additional possibilities for detailed studies of biological systems. Pages 383-486Angew Chem. Inr. Ed. Engl. 25 (1986) 383-413 0 VCH Verlagsgesellschajl mbH. 0-6940 Weinheim. 1986 0S70-0833/86/05~15-0383 . $ 02.50/0 Anyen, Chem. Int. Ed. Engl. 25 (1986) 383-413

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Abstract-Vicinal C,H spin coupling ( 3 J~,~) in substituted alkenes has been investigated systematically. Emphasis is laid on the stereochemical significance (JLranr/Jcii;) and o n the various structural factors which influence 3J(!.H, such as r-bond order, torsional angle 4, bond angle 0, electronegativity of substituents and steric effects. A new type of y-effect is observed in 3J:,2s which appears to have the same origin as the y-shift effect. By comparison of 3JJc_rr,,,l and 3JH,11, it was found that the relation ' J c~,~ = 0.6 %J,,,H holds for both trans and cis coupling constants. Finally, it is concluded that 3 J~, f~ constitutes a valuable criterion to distinguish E-and Z-isomers, particularly in trisubstituted alkenes. Applications to natural products are presented.(1) INTRODUCTION THE STEREOCHEMICAL significance of vicinal H,H spin coupling in alkenes was recognised early and has ever since been utilised in numerous stereochemical assignments. This method, however, is not applicable to trisubstituted alkenes. In such cases, the configuration may be determined by proton NMR using differential chemical shifts,l allylic H,H spin coupling2 or nuclear Overhauser effect^,^ particularly when alkyl substituents are present. Allylic spin coupling though has proven much less reliable2 than vicinal spin coupling and certainly requires availability of both stereoisomers.With the advent of carbon magnetic resonance new criteria for stereochemical assignments became available. The increased sensitivity of the chemical shift parameter to structural and, in particular, to stereochemical changes, allows the assignment of cis-trans (or Z / E ) isomers by the differential shift m e t h~d .~ The two important criteria for assignment are the higher shielding of the olefinic carbons in the Z-isomers and the diamagnetic shift of cis-oriented substituents bearing hydrogen (y-effect5). This method also requires both stereoisomers for a reliable assignment, unless very close model compounds are available.It is the purpose of this paper to show that unambiguous stereochemical assignments in trisubstituted alkenes can be obtained from vicinal C,H spin coupling, i.e. from proton coupled 13C NMR spectra. It has previously been shown6 that in mono-and disubstituted ethylenes 3J6,H > 3J(1 U,H which, in principle, would offer an analogous criterion for cis-trans isomerism as in proton NMR. Applications to trisubstituted alkenes are as yet scarce. An example has been reported for the E-and 2-isomers of methylphosphoenol-u-ketobutyrate.7* Author to whom correspondence should be addressed. (2) ANALYSIS OF SPECTRA Chemical shift assignments are based upon offresonance decoupling experiments, a partial analysis of proton coupled spectra, selective 13C-{1H} decoupling experiments and spectral comparison. In the case of hydrocarbons with gem-dimethyl groups, the y-shift effect was also utilised. For the analysis of the proton coupled carbon spectra, a first order approach was justified in the majority of cases. The spectra of compounds 33, 44...

C-Fluorocurin[Z], C-Mavacurin [3] und C-Alkaloid Y [4] stellen in sudamerikanischen Strychnos-Arten und Calebassen-Curare vorkommende monoquartare Cz0-Alkaloide dar [5]. Fur die zwei erstgenannten Alkaloide sind im Jahre 1955 auf Grund ihrer vielgestaltigen chemischen Umwandlungen, von Ahbaureaktionen und biogenetischen Uberlegungen die Strukturformeln A und B vorgeschlagen worden [ 6 ] . OH OH -A B Diese Untersuchungen waren durch Materialmangel stark erschwert ; von den Abbauprodukten z. B. konnte nur Acetaldehyd in klassischer Weise identifiziert werden. In den Formeln A und B mussten vor allem Natur und Stellung der Hydroxylgruppe offen gelassen werden. C-Alkaloid Y ist das 2,7-Dihydroxymavacurin [7]. Alle drei Calehassen-Alkaloide sind chemisch miteinander korreliert worden. l) Teilweise vorgetragen : Oktober 1963 Gottingen, Februar 1964 Fribourg. z, Die Ziffern in eckigen Klammern verweisen auf das Literaturverzeichnis, Seite 910. Volumen 47, Fasciculus 3 (1964) -No. 99 879 Pleiocarpamin wurde zuerst 1961 aus den Wurzeln von Pleiocarpa mutica BEKTH. (Apocynaceae) [8] und zwei Jahre spater aus der Stammrinde von Hunteria ebumea PICHON (Apocylzaceae) [9] isoliert. Dem tertiaren Alkaloid kommt die massenspektrometrisch gesicherte Summenformel C,,H,,O,N, zu. Es besitzt ein Indolchromophor (Tab. 1) und zeigt im IR. in Tetrachlorkohlenstofflosung zwei Carbonylabsorptionen bei 1736 und 1.770 cm-1, in fester Phase (Nujol oder KBr) eine Bande bei 1727 cm-l;OH-und ind.-NH-Banden treten nicht auf. Das basische Stickstoffatom (pK& = 6,91) ist tertiarer Natur. Analytisch lassen sich die Anwesenheit einer 0-CH,-, einer C-CH,-Gruppe und die Abwesenheit einer N-CH,-Gruppe nachweisen.Pleiocarpamin (1) gab mit Lithiumaluminiumhydrid in Tetrahydrofuran eine kristallisierte Base (Pleiocarpaminol) (3) ([a],, = + 144" (MeOH)) der massenspektrometrisch gesicherten Summenformel C,,H,,ON,. Sie zeigt im UV. Indol-, im IR. (CHCl,) OH-Absorption bei 3584 cm-l; ind.-NH-und Carbonyl-Absorption tritt keine auf. Pleiocarpaminol hat man noch als kristallisiertes Hydrochlorid und als Methojodid 4 (C,,H,,ON,J) charakterisiert. Auf Grund dieser Befunde liegen die zwei Sauerstoffatome im Pleiocarpamin (1) in einer Carbomethoxy-Gruppierung3) vor ; Pleiocarpaminol (3) enthalt somit eine priniare Hydroxylgruppe. Pleiocarpaminol (3) und N(,,)-Methopleiocarpaminol (4) sind isomcr mit Normavacurin (7) bzw. mit C-Mavacurin (8). Aus der Beobachtung, dass Pleiocarpamin (1) und seine Derivate im 1R.-Spektrum keine Absorption fur indolisches NH erkennen lassen, kann man schliessen, dass diese Basen wie C-Mavacurin (8) am Indolstickstoff alkyliert sind. Pleiocarpaminol (3) und sein N,,,-Methoderivat (4) erwiesen sich aber im dunnschicht-bzw. papierchromatographischen Vergleich (siehe Tab. 2) als zwar recht ahnlich, aber doch deutlich verschieden von Normavacurin und bzw. C-Mavacurin. Pleiocarpaminol-methojodid (4) und C-Mavacurinjodid (8) sind auch in den 1R.-, NMR.-Spektren und den spezifischen Drehungen ([o(]D = + 142" bzw. + 246";wasseriges Aceton) von...