Carbenes hydrogen complexes

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

The MCR toward 2//-2-imidazolines (65) has found apphcation in the construction of A(-heterocyclic carbene (NHC) complexes (74). Alkylation of the sp Af-atom with an alkyl halide followed by abstraction of the proton at C2 with a strong base (NaH, KOtBu) resulted in the formation of the free carbene species, which could be trapped and isolated as the corresponding metal complexes (Ir or Rh) [160]. The corresponding Ru-complexes were shown to be active and selective catalysts for the transfer hydrogenaticm of furfural to furfurol using iPrOH as hydrogen source [161]. 

In spite of the successful use of NHCs in a number of palladium-catalyzed reactions, no system for hydrogenation was reported until 2005. This can be easily explained as it had been observed that hydridopalladium-carbene species decompose due to attack of the hydride on the carbene, which results in its reductive elimination to yield the corresponding imidazolium salt [ 190]. However, Cavell and co-workers recently showed that the oxidative addition of imidazolium salts to bis-carbenic palladium complexes leads to isolable NHC-hydridopalladium complexes [191]. This elegant work evidenced the remarkable stabilizing effect of NHC ligands in otherwise reactive species and led to the development of the first NHC-palladium catalyst for hydrogenation. 

Careful treatment of 1,3-disubstituted imidazolium salts (e.g., 350) with sodium hydride (in some cases addition of a small amount of potassium /< r/-butoxide is also recommended) in tetrahydrofuran gives stable carbenes of type 351 (see also Section 2.4.4.2.3). They behave as rather strong bases (p , 24 in DMSO) and nucleophiles. Thus, carbene 351 and salt 350 form a crystalline bis(carbene)proton complex 352 which is a rare example of an asymmetrical hydrogen bridge between two carbon centers. By the action of BF3, B2H6 or AlH3NMe3 stable mesoionic adducts of type 353 are obtained. 

M. 1-Aza-t,3-butadiene Complexes via 1,4- and N-Atkenyl(amino)carbene Chromium Complexes via 1,5 Hydrogen Shifts... 

Hydrogen bonds with C-H-X interactions14 can be formed if the C—H bond is relatively polar as it is when C is bonded to electronegative groups as in HCC13. A b (carbene)-H+ complex has been shown to have the structure (2-IIA)15 with a linear C—H—C bond. 

As a consequence of the high reactivity of metal alkyls, there are few examples of olefin insertions in which the simple insertion product (with the metal still a bonded to the alkyl group) can be isolated. This lack of clear examples has led to alternative explanations involving formation of a carbene metal complex from the alkyl, which then forms a metallocyclobutane with the olefinA final hydrogen shift with ring opening yields the product ... 

Theoretical density functional calculations on the possibility of addition of imidazolium salts to electron-rich palladium centers predicted an exothermic enthalpy for such a process [36]. These results suggested that, under appropriate reaction conditions and with the use of a proper carbene precursor, this reaction should present a feasible synthetic path to carbene/palladium complexes. Only recently, the addition of the C(2)-H bond of an imidazolium salt, in the form of an ionic liquid, to a Pd(0)/NHC complex with the formation of a stable Pd-H bond has been reported [41]. These complexes bear three carbenes per metal center, the fourth coordination position being occupied by hydrogen. The isolation of these complexes has proven that the beneficial role of ionic liquids as solvent can lead to the formation of catalytically active palladium-carbene complexes (see Scheme 7). 

When Y in Scheme 7.21B is hydrogen, the reverse process yielding a metal carbene-hydride complex is c -hydrogen elimination process. It provides an important route leading to decomposition of metal alkyls beside the more often encountered -hydrogen elimination pathway giving metal hydride coordinated with an olefin. 

To clarify these points, we shall consider the carbene as an L-type ligand (4-36a). It therefore acts as a r donor, using its lone pair described by the tier orbital, which interacts with an empty orbital on the metal (e.g. z, 4-38a). In this model, the Tip orbital is empty, so the carbene acquires a r-acceptor character (single face) (4-38b). The interaction scheme is similar to that in the Dewar-Chatt-Duncanson model (Chapter 3, 3.4.1) used, for example, to describe ethylene complexes or molecular hydrogen complexes ( 4.1.4). 

In the molecular hydrogen complex [Os(NH3)4(CR2)(H2)] , a Fischer carbene and a dihydrogen molecule are in cis positions. We shall consider four limiting structures (0, 0), (0, 90), (90, 0), and (90, 90) that are characterized by the orientations of the carbene (first angle) and of the dihydrogen molecule (second angle). 

If ILs are to be used in metal-catalyzed reactions, imidazoHum-based salts may be critical due to the possible formation and involvement of heterocyclic imidazo-lylidene carbenes [Eqs. (2)-(4)]. The direct formation of carbene-metal complexes from imidazolium ILs has already been demonstrated for palladium-catalyzed C-C reactions [40, 41]. Different pathways for the formation of metal carbenes from imidazolium salts are possible either by direct oxidative addition of imidazolium to the metal center in a low oxidative state [Eq. (2)] or by deprotonation of the imidazolium cation in presence of a base [Eq. (3)]. It is worth mentioning here that deprotonation can also occur on the 4-position of the imidazolium [Eq. (4)]. The in-situ formation of a metal carbene can have a beneficial effect on catalytic performances in stabilizing the metal-catalyst complex (it can avoid formation of palladium black, for example). However, given the remarkable stability of this imidazolylidene-metal bond with respect to dissociation, the formation of such a complex may also lead to deactivation of the catalyst This is probably what happens in the telomerization of butadiene with methanol catalyzed by palladium-phosphine complexes in [BMIMj-based ILs [42]. The substitution of the acidic hydrogen in the 2-position of the imidazolium by a methyl group or the use of pyridinium-based salts makes it possible to overcome this problem. Phosphonium-based ILs can also bring advantages in this case. 

As stated in Chapter 3, carbene complexes can be divided into the five classes illustrated in Figure 13.2. One class of carbene complex encompasses the Fischer carbenes that were first prepared in the laboratory of E. O. Fischer. These complexes were the first transition metal carbene complexes prepared, and they contain a ir-donating group on the carbene carbon. Complexes of these carbenes are typically electrophilic at the carbene carbon. A second class of carbene complex was first prepared by Richard Schrock. These complexes contain alkyl groups or hydrogens on the carbene carbon and are called alkylidene complexes or often "Schrock carbenes." Complexes of these carbenes are typically electrophilic at the metal and nucleophilic at the carbene carbon. 

AAHeterocyclic carbene-borane complexes have been shown to be useful as hydrogen sources through radical mechanisms, as Chu et al [85] determined after their initial work with the reduction of xanthates mentioned earlier. Also, substituted imidazol-2-ylidene and triazol-3-ylidenes have been employed in the reduction of dodecyl iodide to dodecane without the addition of any radical initiator. Furthermore, even in the presence of usually borane-reactive species, such as ketones or alkenes, the reduction of the alkyl halide is the only reaction observed. Later, however, Ueng et al foimd that alkyl halides with nearby... 

To date there have been no reported complexes of NHCs to oxygen centers and only one to a sulfur center. This is certainly due to the reactivity of the atoms involved and the tendency to form lu-ea or thiourea derivatives. The NHC-suliur complex was generated from a frustrated Lewis pair reaction of elemental sulfur, giving an NHC-S-B(C6p5)3 complex the selenium analog was also prepared [265]. There have been structures of carbene-alcohol complexes reported however, the coordination occurs between a carbon and a hydrogen via a hydrogen bond [266]. A/]Af-Diamidocarbenes have been exploited in the homonuclear bond-activation of peroxides, disulfides, bromine (vide infra), and even some C—C bonds in diones and cyclopropenones as mentioned earlier [174]. Cyclo-propenyl-l-ylidene-stabilized S(II), Se(II), and Te(II) mono- and dications have also been prepared and characterized [267]. 

The reactions of organorhodium compounds or the catalytic reaction of a rhodium complex are insertions, formation reactions of carbene, hydrogenations, hydrometalations, decarbonylations, carbonylations, hydroformylations, cycliza-tions and cyclometalations, etc. 

The aim of this Chapter is to examine the application of well-defined N-hetero-cyclic carbene (NHC) complexes as well as the systems prepared in situ which involve free NHCs or the precursor salt for the reduction of imsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reductions contain electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types aeeording to reductants used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

Heckenroth M, Khlebnikov V, Neels A, Schurtenberger P, Albrecht M. Catalytic hydrogenation using abnormal N-heterocyclic carbene palladium complexes catalytic scope and mechanistic insights. ChemCatChem. 2011 3 167-173. 

Nucleophiles like lithium enolates and organocuprates can be added to the terminus of the allyl ligand of cationic cyclic allyl(carbene)iron complexes to give 4-substituted tricarbonyl[l,(3 )-diene]iron complexes. Subsequent to the nucleophilic addition, a ferra-Claisen rearrangement is supposed to be involved in the reaction mechanism. The free dienes can be released by treatment with ceric ammonium nitrate or alkaline hydrogen peroxide (Scheme 4-65). ... 

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