John Hudec

Spectroscopic properties of axially and equatorially substituted β-trimethylstannyl ketones and compounds with related chromophores

The synthesis of two optically active cyclohexanones, with a β-trimethylstannyl substituent, are described and their c.d. spectra are reported. Whereas the axial SnMe3 group confers a dissignate (anti-octant) contribution on the n→π* transition relative to the parent unsubstituted ketone, the equatorial SnMe3 group contributes strongly in a consignate (octant) sense. This contrasting behaviour is rationalized in terms of a conformationally dependent through-bond coupling between the lone pair on the carbonyl oxygen atom and the electron-donating C–Sn bond which gives rise to a higher energy transition in the equatorially substituted compound. This band is assigned to the second component of the split n→π* transition. The spectral properties of some βγ-unsaturated ketones, in which the p orbital on the β-carbon atom is in the same orientation with respect to the carbonyl group as the equatorial C–Sn bond, are identical. Thus the high energy band, formerly assigned to a charge transfer transition, should be reassigned as a second n→π* transition.

Relationship between the magnitude of Jgem and the spatial orientation of α-substituents

An empirical relationship between ΔJgem and a relative orientation of a methylene group attached to oxygen and sulphur, and two groups, sulphonyl and sulphinyl, has been experimentally determined. The empirical curve for oxygen shows marked similarity to a theoretically derived curve relating the magnitude of Jgem in the rotamers of methanol about the C–O bond. All four curves bear out the predictions of the theory of geminal coupling.

Orbital interaction in amino-ketones

A study of the orbitals of α-aminoacetone and α-aminoacetaldehyde as a function of the two important torsions in these molecules is reported. The individual level shifts are much greater than the variation of the total energy. We identify four significant interactions affecting the level ordering. Each interaction has its distinctive conformational dependence. In order of decreasing spectroscopic importance the interactions are as follows. (1) Through bond coupling of the nitrogen and oxygen lone-pairs. (2) Through space interaction of these lone-pairs. (3) Hyperconjugation between the C–N σ* level and the carbonyl π-system. (4) Direct overlap of the nitrogen lone-pair with the carbonyl π-system.

On the formation of biacetyl trimers in acidic media

Abstract Evidence is presented to show that the tricyclic trimer of biacetyl ( 1 ) is formed in acidic media via “biacetyl hydrates” rather than the dimeric aldol ( 2 ). A new trimer ( 3 ) has been isolated as well as a crystalline biacetyl trimer dihydrate.

Correlation of circular dichroism and conformation in γδ- and δε-unsaturated ketones

C.d. spectra of a number of unsaturated ketones (several γδ- and one δIµ-) are discussed. The orientation of the CC bond with respect to the CO group in terms of the pathway via the intervening C–C bonds is important in determining the sign of the Cotton effect of the n→π* transition, not the position of the double bond in a particular octant. Octant contribution can be expected only in those orientations of bonds which permit effective π back-donation from CC via C–C bonds to the carbonyl group. Anomalous solvent effects on steroidal Δ6-3-ketones are discussed.

Rationalisation of ‘dissignate’ and ‘front octant’ effects of chiral ketones by a quadrant rule

Theoretical study of a number of 2-alkanones reveals that both the normal and the abnormal (front octant and dissignate effect of substituents in rear octants) chiroptical behaviour of substituents in certain relative orientations with respect to the carbonyl group can be rationalised by a quadrant rule supplemented by a definition of the through-bond path that connects the substituent and the chromophore; the new rules are rationalised in terms of the change in the mixing of the x and y components of the total n MO orbital induced by the substituent.

Origin of stereofacial selectivity in electrophilic additions to methylenecyclohexanes and methylenedioxanes. A theoretical and experimental study

Addition reaction studies and ab initio calculations on methylenecyclohexane and 5-methylene-1,3-dioxane systems suggest that two electronic factors contribute to the stereoselectivity of epoxidation and diimide reduction. These are respectively the spatial anisotropy of the HOMO with respect to the two faces of the double bond, common to both molecules, which is likely to be responsible for the overall axial stereofacial selectivity exhibited, and a similar anisotropy in the electrostatic potential field of the methylenedioxane caused by the oxygens; which also favours attack from an axial direction by polarisable electrophilic species. The anisotropy of the HOMO arises from the important topological difference between the contributions made to the HOMO by the periplanar β C–H σ bonds and opposing β C–O or C–C σ bonds. Catalytic reduction proceeds with equatorial face selectivity for both the cyclohexane and the dioxane systems and appears to be governed largely by steric effects.

Determination of the structure of a trimer of biacetyl by carbon-13 autocorrelation nuclear magnetic resonance spectroscopy

The structure of one of the trimers of biacetyl (butane-2,3-dione) has been determined unambiguously using a novel n.m.r. technique : anew molecular structure (5) is proposed and discussed in terms of its latent symmetry properties.

The circular dichroism of 3-alkylthio- and 3-arylthio-cyclohexanones: an analysis of through bonds and dynamic coupling contributions to the optical activity

The c.d. spectra of a series of 3-alkylthio- and 3-arylthio-cyclohexanones were recorded and interpreted by means of MO calculations and dynamic coupling approximations. From the analysis, the absolute configurations of the compounds were deduced.

Indirect calculations of relative rotatory strengths of substituted ketones

The twists θ[θ(n)–θ(π*)] of the orbitals of the carbonyl group of a series of model substituted ketones reproduce accurately the variation in the c.d. of the n→π* transition of chiral substituted ketones even when the substituents are heteroatoms (F, Cl, SiH3).