NHC transfer reagent

Silver carbene complexes act as very efficient NHC transfer reagents for the synthesis of different metal-NHC complexes. On the other hand, as far as we know, there is only one example described to date where a silver-NHC complex was used for asymmetric catalysis. In 2006, Ferndndez and coworkers reported the first and only asymmetric catalysis using a chiral NHC-silver catalyst. Enantioselective diboration of styrenes was realized by using silver complex 111 as the catalyst (Scheme 3.70) [99], However, the diol was obtained in low yield and low enantioselectivity (less than 10% ee). 

Scheme 25.1.1 Applications of NHC-SiCU as an NHC-transfer Reagent Towards Transition Metal and Main Group Element Compounds.

As shown in Ref. (18), (C5HioN2)SiCl3H can be used as an NHC-transfer reagent. Furthermore, this adduct is very likely the key intermediate in the formation of NHC—SiCl2 via the addition of two equivalents of free NHC to HSiCl3 (19,20). 

Arnold and co-workers also reported the deprotonation of alkoxy imi-dazolium iodides with -butyl lithium to yield lithium alkoxide carbenes (Scheme 3).14 Single crystals of one of the complexes were grown from a diethyl ether solution, and revealed a dimer of LiL with lithium iodide incorporated to form a tetramer of lithium cations (7). The lithium-NHC bond distance of 2.131(6) A is similar to that of the lithium amide carbene 4. Also as in 4 there is distortion of the lithium-NCN bond which has an angle of 152.3°. The C2 carbon resonates at 200 ppm in the 13C NMR spectrum which is a relatively high-frequency, possibly as a result of the incorporated lithium iodide. The lithium salts were able to act as ligand transfer reagents and react with copper (II) chloride or triflate to afford mono- or bis-substituted copper(II) alkoxy carbene complexes. 

NHCs with an alkoxy-anchor coordinated to alkali metals serve as excellent ligand transfer reagents. The reagent synthesized by Arnold et al. also showed an extraordinary stability and was successfully used in the preparation of [(NHC)Ti(Oi-Pr)3] 19 through a salt metathesis reaction (Equation (6.6)). This transfer reagent was also employed for synthesizing a related [(NHQaUI] complex. 

Despite the considerable synthetic potential of this reaction, only two NHC-con-taining catalysts have been reported to date. The in situ generated complex from the reaction between [RhCl(COD)]2 and a NHC-silver ligand transfer reagent was active in the arylation of an A-phosphinoyl aldimine with phenylboronic acid, but... 

Complexes of the form [(NHC)CuX] are also useful carbene transfer reagents for the synthesis of a range of transition metal catalysts. The use of these reagents is attractive owing to the very low cost of copper. It has been shown... 

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. 

Despite the considerable synthetic potential of this reaction, only two NHC-containing catalysts have been reported to date. The in situ generated complex from the reaction between [RhCl(COD)]2 and the NHC-silver ligand transfer reagent 20 was found to be active in the arylation of an At-phosphi-noyl aldimine with phenylboronic acid but unfortunately only one catalytic test was performed [eqn (8.9)]. In a more systematic study, Suzuki and Sato screened various azolium salts, among which the system [RhCl(COD)]2/ lAd-HCl proved to be the most active catalyst for the arylation of a series of A -sulfonyl and A -phosphinoyl arylimines. ... 

Widely applicable is the last pathway shown in Scheme 4, the transmet-allation reaction . Although other reagents have been proposed [68], the use of Ag20 as introduced by Lin [69,70] is currently the most important pathway for the synthesis of NHC metal complexes. The imidazofiiun halide forms a silverhafide-NHC complex which then transfers the carbene ligand to the metal precursor. Scheme 4 shows a general representation of possible pathways for the synthesis of transition metal NHC complexes. 

---