140. Acid-Catalyzed -Sigmatropic Rearrangements of N-Propargylanilines

Article abstract of DOI:10.1002/hlca.19900730602

The acid-catalyzed rearrangement of N-(1',1'-dimethylprop-2'-ynyl)-, N-(1'-methylprop-2'-ynyl)-, and N-(1'-arylprop-2'-ynyl)-2,6-, 2,4,6-, and 2,3,5,6-, and 2,3,4,5,6-substituted anilines in mixtures of 1N aqueous H2SO4 and ROH such as EtOH, PrOH, BuOH etc., or in CDCl3 or CCl4 in the presence of 4 to 9 mol-equiv. trifluoroacetic acid (TFA) has been investigated (cf.Scheme 12-25 and Tables 6 and 7).The rearrangement of N-(3'-X-1',1'-dimethyl-prop-2'-ynyl)-2,6- and 2,4,6-trimethylanilines (X = Cl, Br, I) in CDCl3/TFA occurs already at 20 deg C with τ_1/2 of ca. 1 to 5 h to yield the corresponding 6-(1'-X-3'-methylbuta-1',2'-dienyl)-2,6-dimethyl- or 2,4,6-trimethylcyclohexa-2,4-dien-1-iminium ions (cf.Scheme 13 and Footnotes 26 and 34).When the 4 position is not substituted, a consecutive <3,3>-sigmatropic rearrangement takes place to yield 2,6-dimethyl-4-(3'-X-1'1'-dimethylprop-2'-ynyl)anilines (cf.Footnotes 26 and 34).A comparable behavior is exhibited by N-(3'chloro-1'-phenylprop-2'-ynyl)-2,6-dimethylaniline (45; cf.Table 7).The acid-catalyzed rearrangement of the anilines with a Cl substituent at C(3') in 1N aqueous H2SO4/ROH at 85-95 deg C, in addition, leads to the formation of 7-chlorotricyclo<3.2.1.0^2,7>oct-3-en-8-ones as the result of an intramolecular Diels-Alder reaction of the primarily formed iminium ions followed by hydrolysis of the iminium function (or vice versa; cf.Schemes 13, 23, and 25 as well as Table 7).When there is no X substituent at C(1') of the iminium-ion intermediate, a <1,2>-sigmatropic shift of the allenyl moity at C(6) occurs in competition to the <3,3>-sigmatropic rearrangement to yield the corresponding 3-allenyl-substituted anilines (cf.Schemes 12, 14-18, and 20 as well as Tables 6 and 7).The rearrangement of (-)-(S)-N-(1'phenylprop-2'-ynyl)-2,6-dimethylaniline ((-)-38; cf.Table 7) in a mixture of 1N H2SO4/PrOH at 86 deg C leads to the formation of (-)-(R)-3-(3'-phenylpropa-1',2'-dienyl)-2,6-dimethylaniline ((-)-91), (+)-(E)- and (-)-(Z)-6-benzylidene-1,5-dimethyltricyclo<3.2.1.0^2,7>oct-3-en-8-one ((+)-(E)- and (-)-(Z)-92, respectively), and (-)-(S)-2,6-dimethyl-4-(1'-phenylprop-2'-ynyl)aniline ((-)-93).Recovered starting material (10percent) showed a loss of 18percent of its original optical purity.On the other hand, (+)-(E)- and (-)-(Z)-92 showed the same optical purity as (-)-38, as expected for intramolecular concerted processes.The CD of (+)-(E)- and (-)-(Z)-92 clearly showed that their tricyclic skeletons possess enantiomorphic structures (cf.Fig. 1).Similar results were obtained from the acid-catalyzed rearrangement of (-)-(S)-N-(3'-chloro-1'-phenylprop-2'-ynyl)-2,6-dimethylaniline ((-)-45; cf.Table 7).The recovered starting material exhibited in this case a loss of 48percent of its original optical purity, showing that the Cl substituent favors the heterolytic cleavage of the N-C(1') bond in (-)-45.A still higher degree (78percent) of loss of optical activity of the starting aniline ...