PI-FACIAL SELECTIVITY IN NUCLEOPHILIC ADDITIONS TO 4,4-DISUBSTITUTED DIENONES - EXPERIMENTAL SUPPORT FOR ELECTROSTATIC CONTROL

The 4,4-disubstituted cyclohexadienones 5-10 and 32 were prepared by h ypervalent iodine oxidation of the corresponding phenols. Our study of the facial selectivity in nucleophilic carbonyl additions provided ex perimental evidence for dominant dipolar control in these substrates. Hyperconjugative orbital stabilization in the transition state and orb ital distortion effects appeared to be of secondary importance. Dienon es 5-10 showed predominant attack of methyl Grignard reagents anti to the oxygen substituent at C(4) from the alpha-face of the dienone. In contrast. fluorinated substrate 32 demonstrated inverse selectivity, i n accordance with an inversion of the molecular dipole moment mu(perpe ndicular to) The structure of all addition products was unequivocally established on the basis of NMR, X-ray, and chemical correlations. Dip ole moments were calculated with the AM1 MO method and compared to the experimentally observed facial selectivities. in a series of structur ally closely related dienones, an excellent linear correlation of the logarithm of the facial selectivities vs the calculated dipole moments was observed. Facial selectivities were strongly dependent on the nat ure of the nucleophile. Hydride ions and alkynyl groups added essentia lly nonselectively, whereas sp(2)- and sp(3)-hybridized C-nucleophiles led to alpha/beta ratios of 32:1 to 1:5.