Alkali metal complexes with carbenes

As mentioned in the Introduction, one of the synthetic approaches to stable nucleophilic carbenes involves C-deprotonation of imidazol-ium and related cations with alkali metal salts of strong bases such as NaH and KO Bu. Accordingly, the interactions with alkali metal cations with stable nucleophilic carbenes could prove to be important for understanding the solution behavior of the latter. Until recently, however, there were no examples of complexes of stable carbenes with... 

Another important issue has recently been raised by Alder. He proved that dia-minocarbenes, including aromatic carbenes, coordinate alkali metal ions (coming from the base), which is important with regard to the rate of dimerization." The metal ions might act as a Lewis acid catalyst for dimerization, as observed for protons, but alternatively, strong complexation might stabilize the carbene center and prevent its dimerization. This question is still open to debate. 

Complexation of Stable Carbenes with Alkali Metals... 

The interaction of [MN(SiMe3)2] (M = Li, Na, K) and LiTMP with carbenes (1) (R = i-Pr, R = Me), (3) (R = i-Pr) and (4) (R = Me) in solution was also reported. Evidence of complexation of these carbenes with the Li, Na, and K species in solution was given by the NMR shifts for the relevant carbene center. Recently, the reaction of carbene (1) with lithium l,2,4-tris(trimethylsilyl)cyclopentadienide was reported to result in monomeric carbene complexes with 1 1 stoichiometry (R = H R = f-Bu (6), adamantyl (7), or 2,4,6-trimethylphenyl(8)). The crystal structure of (6) showed that the cyclopentadienyl ring is coordinated in an -fashion to the lithium center (see Alkali Metals Organometallic Chemistry) and there is a single a-interaction present between the lithium and the carbene center (Li-C(carbene)... 

Carbamoyl complexes from metal carbonyls and amines 5.8.2.12.4 Carbanions reactions with alkene complexes 5.8.2.3,4 metal carbonyls 5.8.2.S.5 Carbene complexes by alkene metathesis 5.8.2.3.11 formation 5.8.2.8.5 Carbides alkali metal formation 5.10.2.1 bonding 5.10.2 formation 5.10.2 industrial uses 5.10.2 interstitial formation 5.10.2 Carbometallacycle formation 5.S.2.2.2 Carbometallacycles from n-allyl complexes 5.S.2.3.9 Carbon reaction with alkali metals 5.10.2.1 Carbon dioxide complexes formation 5.8.2.14.1 Carbon monoxide displacement by alkenes 5.8.2.3.1 Carbonyl complexes by ligand exchange 5.8.2.12.2 from carbon monoxide 5.8.2.12.1, 5.8.2.12.2... 

Catalysis by alkali metal ions has recently been reported as an alternative route. In an argon matrix, acetylene forms a n complex with the metal. On irradiation, it isomerizes to the vinylidene form, M C=CH2. When complexed with metals, vinylidene is much more stable, in the same way that metal carbenoids are generally much more stable than carbenes, and rearrangement of a tungsten alkyne complex to a tungsten vinylidene complex has been reported. ... 

We found [21] that diaminocarbenes form complexes with lithium (also discovered by others [22,23]), sodium, and even potassium ions. Since this has a substantial effect on the preparation and behaviour of the carbenes in solution, complexation with the alkali metals will be described in section 2.2. 

These results are interesting in themselves, but they are perhaps most significant in demonstrating that the generation of carbenes by deprotonation with strong anionic bases carrying alkali metal counterions will produce complexed species. Carbenes which dimerise like 8, 9, and 11 (R = Me, Et, /-Pr) can only be observed in solution, and it is an open question how much the observed properties are influenced by the complexation. This problem will be discussed further in sections 2.3 and 2.4, but we have been concerned for some time with... 

Barret and Hill have extensively studied the reactivity and catalytic activity of NHC-supported Group 2 amido complexes. Similar to several reports with the alkali metals, tricoordinate NHC-alkaline earth metal complexes 58 could be formed directly from the corresponding conjugate acid by addition of a metal amide salt (Scheme 5.8). Whereas carbene-Li complexes were found to be excellent carbene transfer reagents, 58 could function as a stable carbene equivalent. Indeed, in the presence of Lewis base donors such as triphenylphosphine oxide or protic substrates such as 2-methoxyethylamine, liberation of the free carbene was observed by and NMR. While the authors did not attempt to isolate this free carbene or investigate any additional reactivity, they claimed that the carbene was dissociated under catalyt-ically relevant conditions. 

The synthesis of sodium complexes 34 and 35, in which two different metals were added in situ to IPr, is essentially identical to a protocol recently reported by the Hill group. As was done with 34 and 35, Hill treated two equivalents of IPr with one equivalent each of an alkali and an alkaline earth metal salt and obtained mixed Group 1/Group 2 carbene complexes (50, Scheme 5.7). ... 

---