Stereochemistry of the insertion of disubstituted alkynes into the metal aminocarbyne bond in diiron complexes

Terminal alkynes (HCCR^′) (R^′=COOMe, CH₂OH) insert into the metal-carbyne bond of the diiron complexes Fe₂{μ-CN(Me)(R)}(μ-CO)(CO)(NCMe)(Cp)₂]SO₃CF₃] (R=Xyl, 1a; CH₂Ph, 1b; Me, 1c; Xyl=2,6-Me₂C₆H₃), affording the corresponding μ-vinyliminium complexes Fe₂{μ-σ:η³-C(R^′)CHCN(Me)(R)}(μ-CO)(CO)(Cp)₂]SO₃CF₃] (R=Xyl, R^′=COOMe, 2; R=CH₂Ph, R^′=COOMe, 3; R=Me, R^′=COOMe, 4; R=Xyl, R^′=CH₂OH, 5; R=Me, R^′=CH₂OH, 6). The insertion is regiospecific and C-C bond formation selectively occurs between the carbyne carbon and the CH moiety of the alkyne. Disubstituted alkynes (R^′CCR^′) also insert into the metal-carbyne bond leading to the formation of Fe₂{μ-σ:η³-C(R^′)C(R^′)CN(Me)(R)}(μ-CO)(CO)(Cp)₂]SO₃CF₃] (R^′=Me, R=Xyl, 8; R^′=Et, R=Xyl, 9; R^′=COOMe, R=Xyl, 10; R^′=COOMe, R=CH₂Ph, 11; R^′=COOMe, R=Me, 12). Complexes 2, 3, 5, 8, 9 and 11, in which the iminium nitrogen is unsymmetrically substituted, give rise to E and/or Z isomers. When iminium substituents are Me and Xyl, the NMR and structural investigations (X-ray structure analysis of 2 and 8) indicate that complexes obtained from terminal alkynes preferentially adopt the E configuration, whereas those derived from internal alkynes are exclusively Z. In complexes 8 and 9, trans and cis isomers have been observed, by NMR spectroscopy, and the structures of trans-8 and cis-8 have been determined by X-ray diffraction studies. Trans to cis isomerization occurs upon heating in THF at reflux temperature. In contrast to the case of HCCR^′, the insertion of 2-hexyne is not regiospecific: both Fe₂{μ-σ:η³-C(CH₂CH₂CH₃)C(Me)CN(Me)(R)}(μ-CO)(CO)(Cp)₂]SO₃CF₃] (R=Xyl, 13; R=Me, 15) and Fe₂{μ-σ:η³-C(Me)C(CH₂CH₂CH₃)CN(Me)(R)}(μ-CO)(CO)(Cp)₂]SO₃CF₃] (R=Xyl, 14, R=Me, 16) are obtained and these compounds are present in solution as a mixture of cis and trans isomers, with predominance of the former.