V-Heterocyclic carbene

V-Heterocyclic carbenes (NHCs) have become an incontrovertible class of mole-cnles for transition-metal- (TM) and organo-catalysis. Inception of the field dates back almost a centnry ago when Tschngajeff (Chugaev) and co-workers reacted... 

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

V-HETEROCYCLIC CARBENES IN TRANSITION METAL CATALYSIS AND ORGANOCATALYSIS... 

Nolan SP (2006) V-Heterocyclic carbenes in synthesis. Wiley, New York... 

Halogen abstraction from hypervalent sulfur halides has also been reported for the imidazol-2-ylidenes (IV) (Scheme 8.22). This reaction gives a nice example of the synthetic utihty of V-heterocyclic carbenes. Indeed, this adduct is the first structurally characterized derivative featuring the chlorosulfite ion (SO2CI ). ... 

Recently, spectacular achievements have been reported in the catalysis arena using stable cyclic diaminocarbenes III, IV, and VII, now called V-heterocyclic carbenes... 

Dixneuf used [RuCl2(/>-cymene)]2 as a catalyst for the reaction of enyne 72a in the presence of imidazolium salt and CS2CO3 and obtained the enyne metathesis product 73a in a high yield. The enyne silyl ether 72b is converted under similar reaction conditions into r/i7ra-compound 73b which after the Tamao oxidation gives diol 74 (Scheme 28). In this reaction, V-heterocyclic carbene should be generated to coordinate to the ruthenium metal, but the actual species for this reaction is not well documented. 

A wide range of a,p-unsaturated aldehydes, including 3-alkyl derivatives, undergo /V-heterocyclic carbene (NHC)-catalyzed annulations with A -sulfonyl ketimines under mild conditions to provide bicyclo[3.2.0]lactams 194 with outstanding dia-stereo- and enantioselectivity (Scheme 70) [101]. This concise route to p-lactams... 

V-Heterocyclic carbenes were demonstrated to be efficient catalysts for the Staudinger reaction of ketenes with /V-aryl-, /V-alkylcarbonyl imines [121]. Chiral /V-heterocyclic carbenes gave the corresponding < /.v-(3-lactams in good yields with good diastereoselectivities and excellent enantioselectivities (ee>99%). 

Scheme 113 Chiral /V-heterocyclic carbene-catalyzed kinetic resolution of cw-4-formyl-...

Li and coworkers have previously found that in the presence of an /V-hetero-cyclic carbene catalyst m-4-formyl-(3-lactams underwent the ring expansion reaction to afford succinimide derivatives [242]. More recently, they reported the kinetic resolution version of this transformation attempted with a chiral /V-heterocyclic carbene (Scheme 113), leading to m-4-formyl-(3-lactams with moderate ee of 64% [243]. 

Scheme 61, yielded thiazole 200 as the major product, along with minor amounts of carbinol 201 [152]. On the other hand, treatment of the imine formed from 199 and p-methoxyphenylamine with catalytic tetrabutylammonium cyanide, produced suc-cinimide derivative 202. In both cases, the process is initiated by nucleophilic attack to the carbaldehyde C=0 (or azomethine s C=N) group, which is followed up by an anionic rearrangement. A variation of the above process using as catalysts /V-heterocyclic carbenes (NHC) derived from base treatment of azolium, imidazo-lium, or triazolium salts, has also been developed to access gem-disubstituted succinimides [153, 154]. Unfortunately, an attempt of kinetic resolution of racemic 4-formyl (3-lactams by using chiral NHC resulted in moderate selectivities only [154]. 

The electronic structures of the l,3,2(A2)-diazasiloles 83-85 and 89 have been probed by He(i) and He(n) photoelectron spectroscopy. The experimental ionization potentials are summarized in Table 3. The molecular orbitals were assigned with the help of quantum mechanical computations for model compounds and in each case the HOMO was found to be a Jt-type MO of b, symmetry. In the case of the benzol] and pyiido[/4 fused compounds 85 and 89 also the HOMO-1 is a Jt-type MO. The next band was assigned to a lone pair-type MO of aj symmetry, which is mostly located at the dicoordinated silicon atom. Therefore, the HOMOs of the cyclic silylenes 83 and 84 differ in nature from those of the homologues V-heterocyclic carbenes which consist of an essentially aj lone pair-type MO at... 

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... 

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). 

Peris E, Crabtree RH (2004) Recent homogeneous catalytic apphcations of chelate and pincer V-heterocyclic carbenes. Coord Chem Rev 248 2239-2246... 

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