Reaction of tungsten η₁-acetylide complexes [(η⁵-C₅H₅)(NO)(CO)W-C≡C-R]Li with iminium ions

The reaction of alkynyltungsten complexes [(CO)(NO)(Cp)W-C≡C-R]Li (R = H, C₆H₅, C(CH₃)₃) with differently substituted iminium ions is investigated. Due to the relative high acidity of the hydrogen atoms on the β-carbon atom of the η¹-vinylidene complex [(CO)(NO)-(Cp)W=C=CH₂)] (1), the parent η¹- acetylide complex [(CO)(NO)(Cp)W-C≡C-H)]^- is generated in situ simultaneously with the iminium ion by the reaction of η¹- vinylidene complex 1 with an enamine. The reaction of 1 with enamines 2a-e leads to vinylcarbene complexes 3a-e in good yield. The first step of this transformation is a Mannich reaction on the β-carbon atom of alkynyltungsten complexes, generating the expected β-aminoalkylated η¹-vinylidene complexes 6. This intermediate reacts further to 3a-e by migration of the hydrogen atom adjacent to the nitrogen atom to the α-carbon atom of the η¹-vinylidene moiety. The appearance of a η¹-vinylidene complex as an intermediate is supported by NMR experiments, and the postulated retro-imino-ene reaction is confirmed by the reaction of deuterated η¹-vinylidene complex 1-D with enamine 2b to the vinylcarbene complex 8. The scope of the reaction is demonstrated by the reaction of alkynyltungsten complexes 9, 16, and 17 with a series of differently substituted iminium ions to the corresponding vinylcarbene complexes. Activation parameters for the retro-imino-ene reaction of η¹- vinylidene complex 6a to vinylcarbene 3a in THF-ds were determined using ¹H NMR spectroscopy. On the basis of this experiment δH‡, δS‡, and δG‡ (at - 1°C) were found to be 20.5 ± 1.4 kcal/mol, -1.4 ± 0.6 cal/mol, and 20.9 ± 1.4 kcal/mol, respectively. The profile of the postulated retroimino-ene reaction is calculated on the model compound [(CO)(NO)(Cp)W=C=CH- CH₂-NH-CH₃] (6M), yielding [(CO)(NO)(Cp)W=CH-CH=CH ₂] using density functional theory at the B3LYP level. The calculation shows the process is more likely a single-step reaction where the hydrogen migration and carbon-nitrogen bond breaking are two consecutive reactions without formation of a true intermediate. Single-crystal X-ray diffraction data of 3a and 4 are reported.