STRUCTURES OF SOME DIALKOXYPHENYLMETHYLIUM IONS - STERIC INHIBITION OF RESONANCE

In order to investigate the observation of decreased stability of phenyl vs methyl dialkoxy carbenium salts, the single crystal X-ray structures and properties of three aryl-substituted dialkoxy carbenium salts have been examined. Diethoxyphenylmethylium hexachloroantimonate, 3, crystallizes in the orthorhombic space group Pbn2(1), with a = 7.651(2) Angstrom, b = 13.543(3) Angstrom, and c = 17.792(3) Angstrom, V = 1843.5(7) Angstrom(3), and z = 4. Dimethoxy(p-methoxyphenyl)-methylium hexachloroantimonate, 4, crystallizes in a monoclinic crystal system, space group P2(1)/n, with a = 10.816(3) Angstrom, b = 14.119(4) Angstrom, c = 11.917(3) Angstrom, beta = 98.420(20)degrees, V = 1800.2(8) Angstrom(3), and z = 4. Diethoxy(2,4,6-trimethylphenyl)-methylium hexachloroantimonate, 5, crystallizes in the monoclinic space group P2(1)/n, with a = 10.872(2) Angstrom b = 14.804(3) Angstrom, c = 14.050(2) Angstrom, beta = 102.366(15)degrees, V = 2208.7(7) Angstrom(3), and z = 4. The X-ray studies revealed that in each case the aryl substituent is substantially twisted out of conjugation with the dialkoxy carbenium center (3, 38.9 degrees; 4, 30.5 degrees; 5, 64.6 degrees). This is consistent with the decrease in the stability of these compounds compared to their methyl analogues. The C-13 NMR shift of the para carbon of the aryl substituent was found not to be a good indication of the fraction of positive charge transferred to the aromatic ring.