Carbene complexes coupling reactions

The coupling reaction of an a ,/3-unsaturated chromium carbene complex, e.g. 1, and an alkyne 2, through coordination to the chromium center, is called the Dotz reaction. The initial product is the chromium tricarbonyl complex of a hydroquinone derivative 3, which can easily be converted to a free hydroquinone or quinone. 

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

Sect. 2.1.1) and [3C+2S] cyclopentene derivatives. The product distribution can be controlled by choosing the appropriate reaction conditions [72]. Moreover, the cyclopentene derivatives are the exclusive products from the coupling of fi-pyrrolyl-substituted carbene complexes [72b,c] (Scheme 25). The crucial intermediate chromacyclobutane is formed in an initial step by a [2+2] cycloaddition. This chromacyclobutane rearranges to give the rf-complex when non-coordinating solvents are used. Finally, a reductive elimination leads to the formal [3C+2S] cyclopentene derivatives. 

Iwasawa et al. also developed a new reaction involving a three-component coupling process which affords five-membered heterocycles. This [2s+2sh-1c] cycloaddition reaction supposes the consecutive addition of an alkynyllithium derivative to a Fischer carbene complex followed by the addition of a third component which can be an aldehyde, an imine, an isocyanate, or C02 [119] (Scheme 74). 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

CM products from vinylhalides are highly desirable especially because of the possible use in metal catalysed coupling reactions. Johnson and co-workers, performed detailed studies of the possible deactivation pathways [161]. The Fischer-carbene complexes of the vinyl halides have an increased stabihty compared to their alkylidene counterparts and the Fischer carbenes may be deactivated either by migration of the phosphine or by elimination of HX leading to a carbide. 

Pyridine compounds 45 can also be produced by the NHC-Ni catalysed cycloaddition between nitriles 43 and diynes 44 (Scheme 5.13) [16]. The SIPr carbene was found to be the best ligand for the nickel complex in this reaction. The reaction required mild reaction conditions and low catalyst loadings, as in the case of cycloaddition of carbon dioxide. In addition to tethered aUcynes (i.e. diynes), pyridines were prepared from a 3-component coupling reaction with 43 and 3-hexyne 23 (Scheme 5.13). The reaction of diynes 44 and nitriles 43 was also catalysed by a combination of [Ni(COD)J, NHC salts and "BuLi, which generates the NHC-Ni catalyst in situ. The pyridines 45 were obtained with comparable... 

A -Heterocyclic Carbene Complexes in Cross-Coupling Reactions... 

Transition metal-catalysed reactions have emerged as powerful tools for carbon-carbon (C-C) bond formation [1], Cross-coupling reactions (Suzuki-Miyaura, Mizoroki-Heck, Stille, etc.) are recognised to be extremely reliable, robust and versatile. However, some other catalysed arylation reactions have been studied and have been reported to be very efficient [2]. In recent years, A -heterocyclic carbenes (NHC) have been extensively studied and their use as ligands for transition-metal catalysis has allowed for the significant improvement of many reactions [3]. This chapter highlights the use of NHC-bearing complexes in those arylation reactions. 

W-Heterocyclic Carbene Complexes in Arylation Reactions other than Cross-couplings 193... 

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