Chiroptical studies. Part 92 The circular dichroism of strained, bridged-ring, and other ketones

Abstract

The systematic empirical re-evaluation of c.d. data (n→π*) for ketones, which in earlier papers has covered extended decalones, hexahydroindanones, and cyclopentanone derivatives, is now continued to include bridged-ring ketones and miscellaneous ketones with strained or conformationally mobile structures. Carbon–carbon bonds which are particularly strained by the molecular structure generally make c.d. contributions which are larger in the consignate (‘octant’) sense than those of similarly placed unstrained bonds. Data for a wide variety of ketones ranging from essentially unstrained to very highly strained structures have been used to derive an approximate empirical relationship between the strain energy in a bond and the contribution which the bond makes to the total value of Δε.

Approximate values of Δε for many ketones can be derived from the torsion angles ω_H and ω_C between the carbonyl bond and the C_α–H and C_α–C_β bonds, respectively, by using functions of the form k sin²ω. Six bonds need to be included in calculations, all ketones being treated as acetone derivatives. Coefficients k_H and k_C are used for C_α–H and C_α–C_β bonds, respectively. The numerical value of k_C is widely variable, depending upon the amount and distribution of bond strain in the molecule. The complete empirical equation for the total contributions of C_α–H and C_α–C_β bonds to Δε is (i). Application of this equation, and addition of the estimated contribution, δΔε=Σk_H sin²ω_H+Σk_C sin²ω_C(i), of any extended ‘primary zig-zag’ of C–C bonds, permits the derivation of approximate values of Δε for rigid ketones of diverse types, including strained bicyclo- and tricyclo-alkanones. Some non-rigid ketones (e.g. pregnan-20-ones and cycloheptanone analogues) can also be treated in this way if conformer populations are known.

Recommendations are offered to organic chemists wishing to use the data and methods in this and preceding papers in connection with problems involving chiroptical properties of ketones.