Hydrogen-bonded heterocyclic carbenes

Bartolome C, Garcia-Cuadrado D, Ramiro Z, Espinet P. Synthesis and catalytic activity of gold chiral nitrogen acycHc carbenes and gold hydrogen bonded heterocyclic carbenes in cyclopropanation of vinyl arenes and in intramolecular hydroalkoxylation of allenes. Inorg Chem. 2010 49 9758-9764. 

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. 

Aqueous two-phase hydrogenations are dominated by platinum group metal catalysts containing water-soluble tertiary phosphine ligands. The extremely stable and versatile N-heterocyclic carbene complexes attracted only limited interest, despite the fact that such complexes were described in the literature [62-65]. Recently, it was reported that the water-soluble [RuXY(l-butyl-3-methylimi-dazol-2-ylidene) ( 76-p-cymene)]n+ (X=Ch, H20 Y = C1-, H20, pta) complexes preferentially hydrogenated cinnamaldehyde and benzylideneacetone at the C = C double bond (Scheme 38.5) with TOF values of 30 to 60 h 1 in water substrate biphasic mixtures (80 °C, lObar H2) [66]. 

Elimination of trimethylchlorosilane and nitrogen occurs when the (phos-phino)(silyl)diazomethane la is reacted with para-toluenesulfinyl chloride at low temperature. The formation of the four-membered heterocycle 92, obtained in 87% yield, can be rationalized by a multiple-step mechanism involving the formation of the (phosphino)(sulfinyl)carbene 2v. The insertion of the (phosphoryl)(sulfenyl)carbene 91, resulting from a 1,3-oxygen shift from sulfur to phosphorus in 2v, into a carbon-hydrogen bond of a diisopropylamino group readily accounts for the formation of 92.84... 

A-heterocyclic carbenes in organometallic catalysis 02AG(E)1290. Hydrogen-bonding-induced phenomena in bifunctional heteroazaaromatics 03ACR832. 

By using an N-heterocyclic carbene catalyst bearing a hydroxyl moiety, the Ma group reported an asymmetric cyclization of aryl 3-bromoenals and isatins. A series of chiral spirooxindole butenolides, including an alkenyl-substituted compound, were produced in excellent yields and enan-tioselectivity (up to 99% yield, 92% ee). The authors demonstrated that the hydrogen bonding between the free hydroxyl group of the catalyst and the isatin substrates would enhance the reactivity and selectivity. However, the Cs ion may also be involved as a Lewis acid to facilitate this transformation (Scheme 7.44). 

Espinet and coworkers examined gold complexes of ADC ligands containing a hydrogen-bonded backbone - denoted hydrogen-bond supported heterocyclic carbenes (HBHCs) - in cyclizations of 1,6- and 1,7-enynes and observed unusual selectivities [27a]. Catalysts 24a and 24b were found to fevor the endo cycliza-tion product in reactions of enyne 23, whereas previously known phosphite-(25) and NHC-based (26) catalysts favored the exo product (Scheme 16.9). Notably, the differing steric bulk of 24a and 24b did not appear to impact the exo endo ratio, suggesting that the observed selectivity depends primarily on the donor properties of the HBHCs. 

Figure 9.1 Complexes bearing NR,NR- (A), NR,NH- (B), and NH,NH-substituted (C) A-heterocyclic carbenes (NHC)s and hydrogen bonding between a coordinated NR,NH-NHC and DMPU (D).

Fig. 13 Stabilization of N-heterocyclic carbene complexes by intramolecular hydrogen bonds between the N-H group and chloride (A) or hydride (B) ligands reported by Esteruelas et al.

There has been a summary of computational and experimental studies of the use of palladium complexes with A -heterocyclic carbenes (NHCs) in the asymmetric coupling of -hybridized carbon-hydrogen bonds with aryl halides. It has been shown that the electronic and catalytic properties of NHCs fused to porphyrins may be modified by varying the inner metal in the porphyrin. A DPT study of the use of palladium-NHC complexes in the asymmetric intramolecular a-arylation of 2-bromoaryl amides to give 3,3-disubstituted oxindoles (101) has been reported. The likely pathway involves insertion of the palladium into the arene-bromine bond to form a palladacycle which deprotonates to give an (9-enolate. Conversion into the C-enolate followed by reductive elimination gives the product. The intramolecular reaction of 0 a cyclopropane carbon-hydrogen bond in a 2-bromoanilide derivative has been used to form cyclopropyloxindoles, (102), in a palladium-catalysed, silver-mediated reaction. 

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