V-Heterocyclic carbene ligands

Generally, octatriene formation is favored by higher temperatures, higher phosphine and/or butadiene concentrations and, importantly, by an increase in steric bulk of either the ligand or the nucleophile. Indeed, Harkal et al. showed a selectivity switch from telomerization products to 1,3,7-octatriene formation by altering the steric demand of the /V-heterocyclic carbene ligand in the reaction of butadiene with isopropanol under further identical reaction conditions [48]. For the more basic nucleophiles, such as the alcohols, the telomer products are stable under experimental conditions, i.e. product formation is irreversible, but for more acidic substrates such as phenol, product formation is reversible and more 1,3,7-octatriene will be formed after the substrate has been depleted. 

Cyclohexane is a saturated hydrocarbon in which no regioselectivity problems of C-H insertion can occur. The reaction of cyclohexane with ethyl diazoacetate was investigated in a thorough study by Perez et al.14 The /V-heterocyclic carbene ligand IPr (IPr= l,3-bis(diisopropylphenyl)imidazol-2-yliden) was used, all three compounds IPrMCl (M = Cu, Ag, Au) were inactive as catalysts in cyclohexane (Table 12.2, entry 1). Addition of the sodium BARF salt (NaB[3,5-(CF3)2C6H3]4) gave ethyl cyclohexyl acetate as the C-H insertion product of the carbene (Scheme 12.4). 

Extensive mechanistic studies have been conducted on the oxidative addition of aryl halides to Pd(0) complexes ligated by PPh3 in different media and with different additives175, 176. However, palladium complexes containing these ligands are not active catalysts for amination. Instead, one must consider the addition of aryl halides to palladium complexes bound by ligands relevant to amination. Studies of the mechanism of oxidative addition to palladium(O) complexes of P(tol-o)3, DPPF, BINAP, Q-phos, P(Bu-f)3 and an /V-heterocyclic carbene ligand have been reported. 

LMe,ff r2]y[ni(THF) is soluble in THF and toluene and sparingly soluble in Et20 and ra-pentane. The bound THF ligand may be readily removed by refluxing in toluene (see below) and may be displaced by other Lewis bases such as a /V-heterocyclic carbene ligand.1... 

ARYLATION OF KETONES, ARYL AMINATION AND SUZUKI-MIYAURA CROSS COUPLING USING A WELL-DEFINED PALLADIUM CATALYST BEARING AN /V-HETEROCYCLIC CARBENE LIGAND... 

Fryzuk and coworkers investigated the coordination chemistry of the tridentate diamido-V-heterocyclic carbene ligand 31 with tantalum (V) [72]. Aminoly-sis or alkane elimination reactions with the carbene ligand led to bidentate... 

Tejel C, Ciriano MA (2007) Catalysis and Organometallic Chemistry of Rhodium and Iridium in the Oxidation of Organic Substrates. 22 97-124 Tekavec TN, Louie J (2006) Transition Metal-Catalyzed Reactions Using N-Heterocyclic Carbene Ligands (Besides Pd- and Ru-Catalyzed Reactions). 21 159-192 Tesevic V, see Gladysz JA (2008) 23 67-89... 

The novel catalyst system based on palladium(O) /V-heterocyclic carbene complexes was developed by the group of Beller, in part prompted by the strong patent position of Dow on phosphine-based palladium catalysts [8]. The catalyst [37], either generated in situ from the corresponding imidazolium salt or the molecularly defined divinyldisiloxane complex [Pd(Imes)(dvds)] (Fig. 3), was used in the telomerization of 1,3-butadiene with methanol [38—40]. The /V-heterocyclic carbenes are in general better a-donor ligands and come with considerably different steric requirements than the phosphines. The [Pd(Imes)(dvds)] complex resembles the final telomer-palladium product complex and thus offers a facile and clean entry into the catalytic cycle. The metal carbene complex was shown to be... 

The Use of /V-Heterocyclic Carbenes as Ligands in Palladium-Mediated Catalysis... 

Robinson and coworkers have reported a remarkably general route to low-oxidation-state p-block element homodimers, exploiting V-heterocyclic carbenes (NHCs) as capping ligands (see Lintl, G. Schnockel, H. Coord. Chem. Rev. 2000,206, 285-319) ... 

Olefin metathesis reaction that reorganizes carbon-carbon double bonds provides fundamentally new strategies for natural product synthesis and polymer chemistry. Hilvert and coworkers built up an artificial metalloenzyme by covalently tethering a Grubbs-Hoveyda-type Ru complex to a protein scaffold [78]. An /V-heterocyclic carbene (NHC) ligand, which has been reported as a suitable ligand for a number of water-soluble ruthenium-based metathesis catalysts, was derivatized with an electrophilic bromoacetamide. The Ru carbene complex (27 in Figure 10.16) was then attached by site-selective alkylation of the cysteine... 

An exactly opposite trend was observed in olefin metathesis for which the mechanistic scheme for complexes 4 and 5 postulates the dissociation of a phosphine ligand from the metal centre as the key step in the dominant reaction pathway [23]. For instance, ruthenium benzylidene complexes 5 bearing only one /V-heterocyclic carbene entity were found to be significantly more active than those incorporating two of them (6) and the original Grubbs complex (4). This was clearly substantiated for the conversion of the suitable dienes into dihydropyrrole (Scheme 6, Table 2) [20d], and polyhydroxylated cyclohexene rings [24]. 

Oxidative addition is still one of the key processes observed in the literature. In fact, a better understanding of the steric and electronic properties of complexes that can promote C—H bond activation throughout this mechanism has been widely studied. The recent work of Conejero and coworkers [13] highlights this interest by the alteration of the environment of the V-heterocyclic carbene (NHC) ligands on a series of stable T-shaped [Pt(NHC )(NHC)][BAFj, where NHC represents the cyclometallated ligand, species (Fig. 25.6). 

Gabriele B, Salerno G, Costa M (2006) Oxidative Carbonylations. 18 239-272 Gade LH, Bellemin-Laponnaz S (2006) Chiral N-Heterocyclic Carbenes as Stereodirecting Ligands in Asymmetric Catalysis. 21 117-157 Gade LH, see Kassube JK (2006) 20 61-96 Gandon V, see Aubert C (2006) 19 259-294 Garcia JI, see Fraile JM (2005) 15 149-190... 

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