Helical Semiquinones: Proof of Chirality by ENDOR Spectroscopy

Abstract

4,5-Disubstituted phenanthrenequinones I1 are helical and therefore, in contrast to the unsubstituted species I, chiral. The twist is mainly caused by the repulsion between the substituents in 4,5 positions. The nonbonding oxygen-oxygen interaction adds a smaller contribution. ['] For the methyl substituted compounds the free activation enthalpy of racemization lies between 94 and 131 kJ mol-', that is, the enantiomers can be separated at room temperature by liquid chromatography (LC)."] The corresponding semiquinones are also expected to have a twisted ground state and a relatively high enantiomerization barrier. EPR and ENDOR studies of thallium complexes of these semiquinones show smaller values of the thallium coupling constants for the nonplanar species than for the planar ones.r21 An estimate of the height of the enantiomerization barrier was not possible by this method.The helical form of these semiquinones has now been proved by ENDOR spectroscopic studies of the diastereomeric radicals, which were prepared by reduction of the corresponding quinones with prochiral auxiliaries. The 2,2'-biphenyldiyl@-toly1)tin chloride B provides a suitable probe.r31 For comparison, triphenyltin chloride A was also used. A and B react with quinones, whereby in each case an aryl substituent is lost and semiquinone chelates of trigonalbipyramidal, pentacoordinate tin are formed, in which the bidentate ligand takes up axial-equatorial positions because of the ring strain.l31 Thus the chlorine atom in chelates with B must occupy an equatorial position, and tin is therefore a chiral center. When A is used, the chlorine atom, as an electronegative substituent, is bound axially, and the organometallic component is achiral. Studies of the corresponding benzoquinone complexes have that the permutation isomerization at room temperature is slow on the EPR time scale.Since the behavior of these paramagnetic complexes is determined by the central atom, a similar situation is anticipated with the phenanthrenesemiquinones. In order to distinguish between the spectral properties of the diastereomers and "nonchiral" effects, planar as well as helical phenan- threne quinone~'~' were treated with A and B (in 7 the positions 3 and 4 as well as 5 and 6 are linked by dioxymethylene bridges The EPR spectra are only partly resolved, and the large number of coupling nuclei prevents a complete interpretation (Fig. 1). In all cases they are symmetric and the g factors R Fig. 1. EPR spectrum of 5 B in toluene at room temperature. vary only slightly (e.g. g,, = 2.00339 and g,, = 2.00341). The 'H coupling constants were obtained from ENDOR spectra at various temperatures (Table 1).The assignment of the coupling constants in phenanthrene semiquinones is known". and can be applied to the tin chelates. This was confirmed by the determination of their relative signs. This means large values of the coupling constants for the positions 1, 3, 6, and 8, but small ones for the positions 2, 4, 5, and 7. However, the equivalence of the positions 1/8, 217, 3/6, ...