Diverse Reactivity of Diazatitanacyclohexenes: Coupling Reactions of 2<i>H</i>-Azirines Mediated by Titanium(II)

Abstract

2<i>H</i>-Azirines are versatile coupling partners for the synthesis of N-heterocycles. Herein, we present our studies on the reactivity of Cp₂Ti(BTMSA) (<b>1</b>; BTMSA = bis(trimethylsilyl)acetylene) with a variety of azirines. In all the cases examined, the initial organometallic products formed are diazatitanacyclohexenes, presumably formed via oxidative addition of Ti(II) into the C-N bond of the azirine to form an azatitanacyclobutene intermediate, followed by C═N insertion of a second equivalent of azirine into the Ti-C bond to form the observed products. Diazatitanacyclohexene <b>3</b>, bearing phenyl substituents and derived from 2,3-diphenyl-2<i>H</i>-azirine, fragments to form an azabutadiene and nitrile, which is shown to be catalytic in the presence of excess 2,3-diphenyl-2<i>H</i>-azirine. H-substituted complex <b>8</b>, derived from 3-phenyl-2<i>H</i>-azirine, decomposes via protonolysis of the Cp ligands. In contrast, the methyl-substituted diazatitanacyclohexene <b>10</b>, derived from 2-methyl-3-phenyl-2<i>H</i>-azirine, is thermally robust. Attempts to trap the putative azatitanacyclobutene intermediate with an alkyne were unsuccessful, resulting instead in the formation of titanacyclopentadiene (<b>12</b>) from coupling of alkyne with BTMSA. Initial reactivity studies found that <b>10</b> could be protonolyzed with AcOH to form mixtures of pyrrole and aziridine products, whereas reacting <b>10</b> with MeOH results solely in the formation of 2,4-dimethyl-3,5-diphenyl-1<i>H</i>-pyrrole.