Double-Bond-Stabilizing Abilities of 1-Methyl-2-pyrrolyl, 9-Anthryl, and o-Tolyl Substituents

Article abstract of DOI:10.1021/jo00145a030

Equilibrium constants have been determined for the isomerisations of trans-PhCH=CHCH2X to trans-PhCH2CH=CHX, where X is 1-methyl-2-pyrrolyl, 9-anthryl, and o-tolyl.The values (estimated standard deviations) at 30 deg C in tert-butyl alcohol are 1.53 (0.04), 0.342 (0.022), and 0.407 (0.011), respectively.The equilibrium constant for the 9-anthryl case is only about two-thirds as large as a value reported previously.Steric factors are believed to be the most important reason 9-anthryl and o-tolyl groups are poorer double-bond-stabilizing substituents than phenyl groups.Resonance effects are probably decisive in making 1-methyl-2-pyrrolyl substituents better than phenyl but not as good as 2-furyl at stabilizing double bonds.The organolithium compounds formed in the reaction of butyllithium with either trans-1-(9-anthryl)-3-phenylpropene (7) or trans-3-(9-anthryl)-1-phenylpropene (8) react with water to give about 11percent 7, 27percent 8, and 58percent of what appears to be 9-(3-phenyl-2-propenylidene)-9,10-dihydroanthracene.Similar quenching of organolithium compounds in the o-tolyl case gave approximately the equilibrium mixture and in the 1-methyl-2-pyrrolyl case gave almost equal amounts of cis-1-(1-methyl-2-pyrrolyl)-3-phenylpropene (37percent), trans-1-(1-methyl-2-pyrrolyl)-3-phenylpropene (33percent), and trans-3-(1-methyl-2-pyrrolyl)-1-phenylpropene (30percent).The equilibrium constant for the trans-to-cis isomerization of PhCH2CH=CHX at 25 deg C is 0.164 (0.011) when X is 1-methyl-2-pyrrolyl, 0.801 (0.027) when X is 9-anthryl, and 0.039 (0.011) when X is o-tolyl. trans-1-(2-Pyrrolyl)-3-(dimethylamino)propene was prepared but could not be isomerized in the presence of potassium tert-butoxide, probably because of anion formation at the pyrrole NH position.In tert-butyl alcohol-dimethyl sulfoxide containing potassium tert-butoxide, trans-1-(1-methyl-2-pyrrolyl)-3-(dimethylamino)propene appeared to isomerize to trans-3-(1-methyl-2-pyrrolyl)-1-(dimethylamino)propene with an equilibrium constant of 21 (5), but the evidence for this is incomplete.