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-CtC-R]Li (R ) H, C 6 H 5 , C(CH 3 ) 3 ) with differently substituted iminium ions is investigated. Due to the relative high acidity of the hydrogen atoms on the -carbon atom of the η 1 -vinylidene complex [(CO)(NO)-(Cp)WdCdCH 2 )] (1), the parent η 1 -acetylide complex [(CO)(NO)(Cp)W-CtC-H)] -is generated in situ simultaneously with the iminium ion by the reaction of η 1 -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 -carbon atom of alkynyltungsten complexes, generating the expected -aminoalkylated η 1 -vinylidene complexes 6. This intermediate reacts further to 3a-e by migration of the hydrogen atom adjacent to the nitrogen atom to the R-carbon atom of the η 1 -vinylidene moiety. The appearance of a η 1 -vinylidene complex as an intermediate is supported by NMR experiments, and the postulated retro-imino-ene reaction is confirmed by the reaction of deuterated η 1 -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 η 1 -vinylidene complex 6a to vinylcarbene 3a in THF-d 8 were determined using 1 H NMR spectroscopy. On the basis of this experiment ∆H q , ∆S q , and ∆G q (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)WdCdCH-CH 2 -NH-CH 3 ] (6M), yielding [(CO)(NO)(Cp)WdCH-CHdCH 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.

Allene formation by gold catalyzed cross-coupling of masked carbenes and vinylidenes

“…Under our standard coupling conditions, addition of a catalytic amount of 3 to a solution of 6a did not yield allene 7a. This experiment also rules out the possibility of a metal-mediated retroiminoene fragmentation (40) from an in situ-generated 2 -propargyl amine gold complex, which can also be represented as an 1 -coordinated vinyl cation. Propargyl amine 6a also remained intact after adding a 5 mol% amount of a 10a/11a 1/1 mixture.…”

Rate Studies on the Release of Terminal Alkynes from Indenylruthenium(II) Vinylidene Complexes. Implications on the Mechanism of η1-Vinylidene into η2-Alkyne Isomerization

“…This apparent controversy prompted us to take into proper account the nature of the vinylidene moiety and to investigate further its behavior under the conditions of the kinetic experiments. In particular, we investigated the occurrence of acid−base exchange processes, which may arise from the acidic character of the β-H. , The addition of excess D 2 O to the complexes in acetonitrile- d 3 solutions caused the disappearance of the 1 H NMR signal of the RuCCHAr + group at 5.45 ( 2 ), 5.51 ( 4 ), 5.46 ( 5 ), 5.45 ( 6 ), and 5.48 ( 8 ) ppm, within the time of mixing at room temperature. Figure shows the 1 H NMR spectra of complex 4 in acetonitrile- d 3 (a) after the addition of an excess of D 2 O (b).…”

Transition metals in organic synthesis: Highlights for the year 2005

“…Chiral cationic iron carbene complexes participated in cis-and enantioselective cyclopropanations of unactivated alkenes [1609]. Tungsten acetylide complexes react with iminium ions to form alkenyl carbenes [1610]. Reaction of acid chlorides with tungsten alkylidenes furnished alkynes (Eq.…”