Intramolecular interaction effects, carbonyl

An earlier study of the catalytic reduction of 13-oxolupanine (cf. Vol. 8, p. 69) was interpreted in terms of intramolecular interaction between a protonated nitrogen atom and the carbonyl group at C-13. Direct evidence for this proposal has now been provided by c.d. data for a number of 13-oxosparteine derivatives, showing, for example, that ketone Cotton effects disappear in acid solution.25... 

Abnormal (3 effects exist for substituents in the 3-position. This can be interpreted in terms of intramolecular interactions (electronic, steric and/or by hydrogen bridging) between the substituents and the neighbouring carbonyl group. Such interaction effects are discussed below. 

General discussion of intra- and intermolecular interactions 3 van der Waals interactions 3 Coulombic interactions 5 Medium effects on conformational equilibria 5 Quantum mechanical interpretations of intramolecular interactions 7 Methods of study 8 Introduction 8 Nmr and esr spectroscopy 8 Microwave spectroscopy (MW) 12 Gas-phase electron diffraction (ED) 12 X-ray crystallographic methods 13 Circular-dichroism spectroscopy and optical rotation 14 Infrared and Raman spectroscopy 18 Supersonic molecular jet technique 20 Ultrasonic relaxation 22 Dipole moments and Kerr constants 22 Molecular mechanic calculations 23 Quantum mechanical calculations 25 Conformations with respect to rotation about sp —sp bonds 27 Carbon-carbon and carbon-silicon bonds 28 Carbon-nitrogen and carbon-phosphorus bonds 42 Carbon-oxygen and carbon-sulphur bonds 48 Conformations with respect to rotation about sp —sp bonds Alkenes and carbonyl derivatives 53 Aromatic and heteroaromatic compounds 60 Amides, thioamides and analogues 75 Conclusions 83 References 84... 

Interaction effects vary from those that have a dramatic impact on the spectra to those that are barely detectable. The discussion of the interaction effect completes the survey of effects governing carbonyl stretching frequencies. They can be roughly divided into intramolecular and intermolecular types of interactions. 

Synclinal and antiperiplanar conformations of the TS are possible. The two TSs are believed to be close in energy and either may be involved in individual systems. An electronic tt interaction between the stannane HOMO and the carbonyl LUMO, as well as polar effects appear to favor the synclinal TS and can overcome the unfavorable steric effects.161bi 162 Generally the synclinal TS seems to be preferred for intramolecular reactions. The steric effects that favor the antiperiplanar TS are not present in intramolecular reactions, since the aldehyde and the stannane substituents are then part of the intramolecular linkage. 

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