NHC-TM complexes

Since the synthesis of NHC TM complexes was initially achieved by in situ formation of NHC from azolium salt in the presence of base followed by coordination to a metal center, a simpler procedure would be useful. In 1996, Arduengo reported the synthesis of Hg(IDM)2 (128) from Hg(OAc)2 and IDM HCl salt. The mechanism of this reaction is depicted in Scheme 21. Dissociation of one acetate ligand generates [Hg(OAc)]+ (126) and the mildly basic acetate anion drives the deprotonation of the imidazohum salt. Thus, the newly formed NHC coordinated promptly (126) and led to [(IDM)Hg(OAc)]+ (127). 

Although oxidative addition of C-Cl bond of thiazole, benzothiazole, and oxazole was reported as a feasible access to NHC-TM complexes by Stone in 1974, " this approach remained unexplored in the following 25 years. In 2001, Cavell and Yates demonstrated, with combined theoretical and experimental studies, that oxidative addition of the C-H bond... 

In 1968, Ofele reported that heating the adduct [IDMH][HCr(CO)5] at 120 °C yielded the NHC-Cr carbonyl complex [(IDM)Cr(CO)5]. Forty years later, reactions using metal hydrides to synthesize NHC-TM complexes are still scarcely nsed. Nonetheless, Sabo-Etienne and Chaudret synthesized a ntixture of stereoisomers of the Rn complex (152) from [RnH2(PCy3)2(H2)2] (150) and tetramethyltriazoilnm triflate (151) (equation 22). ... 

NHC-TM complexes can be involved in redox processes, especially reductive elimination, leading to the cleavage of the NHC-TM bond and so to the decomposition of the complex. This subject has been comprehensively reviewed by Cavell and McGuinness. ... 

While C H activation reactions have been observed for many NHC TM complexes, to the best of our knowledge, only one example of C-C bond activation has been reported to date. As shown in Scheme 45, complex (273) reported by Whittlesey and Macgregor was found capable of either C-C or C-H bond activation as a ftmction of reaction conditions. Reaction of three equivalents of free IMes with [Ru(PPh3)2(CO)(H)2] (272), at 80 °C, led to the formation of a mixture of monocarbene (273) and biscarbene complex (274). When more forcing conditions were used, product (276) was obtained in excellent yield resulting from the cleavage of... 

Another unusual behavior of NHC TM complex was recently reported by Nolan et al. and involves the insertion of an NHC into a platinum-olefin bond. In the course of preparation of new NHC-containing platinum complexes by reaction of equimolar amounts of [(l,5-hexadiene)PtCl2] and free NHC, the substitution product (286) in which one coordinated double bond was substituted by an NHC was isolated in good yield (Scheme 48). A by-product was also... 

The reductive elimination of imidazolium species represents a major catalyst deactivation pathway for NHC-TM complexes. This phenomenon (along with its reverse process, oxidative addition) was first reported by CavelF who, in collaboration with Yates, published a series of computational studies on this subject. The first of these considered loss of 1,2,3-trimethylimidaz-olium from [(IMe)Pd(Me)(PR3)2] (Scheme 4.1), with calculations indicating... 

The use of NHC complexes of silver is less developed in catalysis however, such species are widely employed as NHC transfer agents in the synthesis of NHC-TM complexes. Peris and Lledos showed that Ag20 readily depro-tonated [IMe-H] and that facile Agl transfer resulted in the formation of a [(IMe)Agl] complex. The presence of a second imidazolium strongly facilitated these processes by H-bonding through the C2-H group. The second imidazolium was first deprotonated by AgOH and then metallated. The overall... 

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