Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 0155 260

O

O

Chromatographic separation (50 60%ee) on triacetylcellulose and CD spectra of bicyclo[3.3.1]nonadiones 69 71 were described159. The enantiomer, (C)-(1S,5S)-69 ( ε302 C3.20), and monoketone 72 were reported earlier160.

An empirical estimation of the sign and magnitude of ε of ( )-quadrone (73) indicated that strain effects and octant-dissignate contributions of the pseudoaxial ˛-hydrogens dominate the CD spectrum of 73172.

A half-boat conformation of the cyclobutanone ring (in an octant projection) of 74 correctly explains the observed positive CD near 300 nm. A pseudo-axial chlorine substituent in 75 substantially enhances the positive Cotton effect. The cyclobutanone ring in 76 is nearly planar, thus the effect of exo and endo chloro substituents is effectively cancelled while the remainder of cholestane skeleton resides in a negative octant175.

O

(77)

− 0.68 (292) (C6 H12 ) −0.96 (289) (EtOH)178

The CD of fenchone (77), where the applicability of the octant rule is not obvious, and its sulfur and selenium analogues have been compared178.

The CD of (C)-camphor and ( )-carvone was measured in gas phase in the temperature interval 100 200 °C179. Gas-phase and vacuum UV CD measurements of the same ketones using synchrotron radiation were reported180. (C)-3-Methylcyclohexanone was the first example in synchrotron radiation CD measurement in the range of 130 205 nm181.

Crystallographic analysis of camphorquinone derivatives 79 and 80 showed that the ˛-diketone chromophore is planar186. The CD magnitude of model compounds similar to 79 varies with the polarizability of the vicinal substituent. An ‘octant rule’ with signs opposite to those for monoketones correctly predicts the long-wavelength CD sign in bicyclo[2.2.1]heptane-2,3-ones 81 84186. The CD of camphorquinone (78) radical-anion has been also reported188.

Schippers and Dekkers reported on the CD and circularly polarized fluorescence of 4,4- dideuterio-adamantan-2-one (85)193. The CD of 85 originates in transitions to a totally symmetric n ! Ł excited state with double minimum potential in the CDO out-of-plane bending mode.

The synthesis and CD of (1S)-2,4-adamantanedione-4-18O (86), whose chirality is solely due to isotopic substitution, have been described. The CD of a sample with 69%ee and 65% isotopic purity consists of three major positive bands at 320, 307 and 297 nm withε ³ 0.08194.

The chirality of (1S)-2-adamantanone-4-13C (87) and (1S)-2,4-adamantanedione-4-13C (88) is solely due to 13C substitution. Since the 13C ring carbon of 87 is located in a positive octant, from the negative Cotton effect of 87, it follows that 13C makes a smaller contribution than 12C. Diketone 88 exhibits three CD bonds with remarkably large amplitudes, which were attributed to different n ! Ł transitions195.

Meijer and Wynberg have reported preliminary results on the studies of compounds that are chiral solely in excited state196. This property can be found in meso compounds with two identical chromophores, one of them being selectively excited.

The optically active 1,2-dioxetane of 2,4-adamantanedione (89) was synthesized. Thermal activation of 89 yielded chemiluminescence ( max D 420 nm characteristic of ketone fluorescence), pointing to intermediate 90 which is chiral only in its excited state due to the out-of-plane geometry of one of the two carbonyl groups. However, circular polarization of chemiluminescence measurement of 90 has not detected optical activity at the moment of emission. The authors have concluded that fast, relative to the lifetime of ketone singlet excited state, intramolecular n, Ł energy transfer caused racemization of 90196.

Using a qualitative approach and implementation of empirical rules, a computer package program which combines computer graphics and molecular mechanics with rule-based correlations was developed to assist the prediction of CD properties from the threedimensional structure of a molecule197.

IV. UNSATURATED KETONES

When two carbonyl chromophores or a carbonyl and a carbon carbon double-bond chromophore are brought into close proximity, as in ˛,ˇ- and ˇ, -unsaturated ketones, an inherently dissymmetric extended chromophore is created if the two chromophores are dissymmetrically disposed19. The coupling is expressed through coulombic mixing of the local states of the separated chromophores and through interchromophoric charge transfer states. The relative importance of charge transfer diminishes with increasing distance between the chromophores. Consequently, whereas coulombic and charge transfer interactions are both important for coupling between CDO and CDC chromophores in ˛,ˇ-unsaturated ketones, charge transfer is generally of small importance in ˇ, , υ, etc. unsaturated ketones. For both theoretical and practical reasons, the octant rule does not apply to dissymmetric ˛,ˇ- and ˇ, -unsaturated ketones. Their n ! Ł Cotton effects have been explained on the basis of different chirality rules.

A. a,b-Unsaturation

Gawronski´ has published a comprehensive review of ˛,ˇ-unsaturated ketones198. ˛,ˇ- Unsaturated ketones are often non-coplanar and thus dissymmetric. For such cases, a helicity rule was proposed to correlate the sign of the Cˇ DC˛ CDO C C torsion angle with the n ! Ł Cotton effect. For cis-enones a negative torsion angle correlates with a negative Cotton effect; for trans-enones it correlates with a positive Cotton effect198. A few examples of applications of this helicity rule to the n ! Ł Cotton effects of trans and cis steroid ketones may be found in Table 6. In principle, a better correlation between CD and stereochemistry might be found for the ! Ł type transitions of ˛,ˇ-unsaturated ketones19; however, Gawronski´ analyzed three bands in the 185 260 nm ( ! Ł ) region and found that the Cotton effects are variously influenced by the presence of axial allylic substituents, ˛0 or ˇ0 axial alkyl groups as well as the enone dissymmetry198. For the special cases of planar ˛,ˇ-unsaturated ketones, Snatzke199 proposed a sector rule for the n ! Ł transition with a sign pattern opposite to that of the octant rule.

C8 H17

α

β

O cis

aData from Reference 198.

B.b,g -Unsaturation and the Extended Octant Rule

A chirality rule, called the extended octant rule, was proposed long ago to correlate the sign of the n ! Ł Cotton effect with absolute stereochemistry of ˇ, -enones19,200. Although not really an octant rule, this chirality rule states that when the intersection of the two planes containing the CDO and CDC chromophores has a dihedral angle ( , Figure 13) whose absolute value is greater than 90°, a negative Cotton effect is predicted for negative and a positive Cotton effect is predicted for positive 19,199. Here, the dihedral angle is defined as the C C C C torsion angle of ODC C˛ Cˇ DC . The rule was extended to other geometries of ˇ, -unsaturated ketones, including smaller and