RHODIUM(I)-CATALYZED CYCLOISOMERIZATION OF NITROGEN TETHERED ENE-ALLENES: FORMATION OF TETRAHYDROAZEPINES

Abstract: A novel cycloisomerization process involving nitrogen-tethered 1,6-ene-allenes has been realized. Subjecting ene-allene 9 to rhodium biscarbonyl chloride dimer produces tetrahydroazepine 10 in moderate to high yields. Substituting the allene moiety with the bulky tert-butyl-group (R4 = t-Bu), while employing a trans-alkene tether, afforded the corresponding azepines in the highest yields. This formal Alder-ene transformation tolerated not only a variety of alkyl-substituents on both the alkene and allene portions but also silyl-[R2 = SiMe3, SiMe2Bn; R4 = Si(i-Pr)3] and phenyl-(R2 = Ph, R4 = Ph) substituents. A deuterium labeling study aided in the postulation of two possible mechanisms for this cycloisomerization. During the course of this investigation, a novel carbon monoxide insertion reaction was observed. Our findings to date are reported and discussed herein