Reaction of Tungsten η¹-Acetylide Complexes [(η⁵-C₅H₅)(NO)(CO)W−C⋮C−R]Li with Iminium Ions

Abstract

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-amino-alkylated 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-d8 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 retro-imino-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CH2] 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