Aniline carbene complexes

As in carboxylic esters it is possible to substitute alkoxy groups of Fischer-type carbene complexes by non-carbon nucleophiles, such as other alcohols [73,214,218], enols [219], aliphatic amines [43,64,66,220-224], aniline [79], imines [225], or pyrroles [226]. Strong nucleophiles can also lead to a dealkylation of methoxy-substituted carbene complexes (5 2 at the methyl group, [227]), in the same way as methyl esters can be cleaved by nucleophiles such as iodide. Carbon... 

The first reports of N-H insertion reactions of electrophilic carbene complexes date back to 1952 [497], when it was found that aniline can be N-alkylated by diazoacetophenone upon treatment of both reactants with copper. A further early report is the attempt of Nicoud and Kagan [1178] to prepare enantiomerically pure a-amino acids by copper(I) cyanide-catalyzed decomposition of a-diazoesters in the presence of chiral benzylamines. Low enantiomeric excesses (< 26% ee) were obtained, however. 

In acetonitrile all reactions of pyrrolidine addition exhibited AV values between -15 and -17cm3mol-1.262 However, upon decreasing the solvent polarity, AV became markedly more negative and a good correlation with the solvent parameter qv (i.e. the pressure derivative of q, the polarisability of the solvent) could be demonstrated. It was concluded that the addition of pyrrolidine followed a two-step process with a polar transition state leading to a zwitterionic intermediate. Addition of / ara-substituted anilines to a similar Fischer carbene complex in acetonitrile was characterised by more negative values of AV (-21 to -27cm3mol-1) than those for addition of pyrrolidine. 263 Second-order rate constants for the addition reaction exhibited an excellent correlation with the basicity of the aniline derivatives used. The trend in activation volumes could be correlated with an early or late transition state for the fast and slow addition reactions, respectively Scheme 12. 

It was concluded that the addition of pyrrolidine follows a two-step process with a polar transition state leading to a zwitterionic intermediate. The addition of a series of p-substituted anilines to a Fischer carbene complex as shown in Scheme... 

Che has reported the tandem hydroamination/hydroarylation of aromatic amines wirh terminal alkynes to form dihydroquinolines in which 1 equiv of aniline combines with 2 equiv of alkyne [23]. For example, reaction of 3-methoxyanilme with pheny-lacetylene (5 equiv) and a catalytic 1 1 mixture of the gold(I) N-heterocydic carbene complex (IPr)AuCl (IPr= l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) and AgOTf at 150 °C under microwave irradiation led to isolation of dihydroquinoline 21 in 82 % yield (Eq. (11.15)). Alternatively, reaction of o-acetylaniline with pheny-lacetylene catalyzed by a mixture of (IPr)AuCl and AgOTf at 150 °C led to isolation of the quinoline derivative 22 in 93% yield via incorporation of a single equivalent of alkyne (Eq. (11.16)). Arcadi has reported the gold(IlI)-catalyzed hydroamination/ hydroarylation of 2-alkynylanilines with a,p-enones to form C3-alkyl indoles [24]. As an example of this transformation, treatment of 2-(phenylethynyl)aniline with 4-phenyl-3-buten-2-one and a catalytic amount of sodium tetrachloroaurate dihydrate in ethanol at 30°C formed 1,2,3-trisubstituted indole 23 in 88% yield (Eq. (11.17)). 

The ligand CNR, isoelectronic to CO, is also very easily attacked by nucleophiles, even neutral ones for instance, the attack by aniline leads to a carbenic complex ... 

In a series of papers (Vol. 5, p. 396) Belluco has shown that the formation of carbene complexes by addition of amines to isocyanide derivatives proceeds by a stepwise mechanism through an intermediate produced by amine attack on the isocyanide carbon atom and this undergoes proton transfer to give the final product. Recent kinetic studies of reaction (17) indicate that with ur/Ao-substituted anilines steric... 

Strain in the intermediate produces an overall decrease in the rate such that reversible attack by amine on the isocyanide carbon becomes comparable in rate with subsequent stages leading to carbene formation. Steric factors are also cited in explanation of the fact that with bis-isocyanide complexes c -[PdCl2(CNAr )2] secondary amines give only monocarbene derivatives c -[PdCl2 C(NHAr )NRAr CNAr ] whereas relatively unhindered primary anilines react further to give bis-carbene complexes c/s-[PdCl2 C(NHAr )NHAr Ij]. Reaction (18) (Ar = />-MeCeH 4) carried... 

Rourke and co-workers showed that the reaction of a simple A-aryl imi-dazolium ion with [Pd(OAc)2] in DMSO led to a Pd-carbene complex, but the presence of an aniline ligand derived from the initial imidazolium salt in the coordination sphere of the Pd implied the presence of a significant decomposition process. ... 

The Iwasawa group developed a platinum-catalyzed generation of unsaturated carbene complexes 325 by cyclization of methoxyalkyne-substituted aniline derivatives 321 and elimination of methoxide as depicted in 324 (Scheme 19.86) [159]. The carbene complexes 325 are useful for [3 + 2] cycloaddition with enol ethers 322, leading to efficient formation of cyclopentene-fused indoles 323. In this case, the indolylmetal species 326 would act as the nucleophile to promote the second cyclization with the oxonium moiety. 

A relay catalytic hydroamination/redox reaction for the synthesis of cyclic aminals 83 from tertiary amine-substituted 3-en-l-yne derivatives 81 and various aniline derivatives 82 was recently developed by Gong and coworkers (Scheme 12.40) [43]. The gold carbene complex [(IPrlAuNTfj] and TfOH were found to be optimal cocatalysts for this cascade reaction. In their preliminary studies, the authors found that this reaction could be carried out in a highly enantioselective manner, despite that 2 equiv. of an expensive chiral phosphoric acid are required. Interestingly, the diastereoselectivity for a particular substrate appears to be independent of the catalyst or ee. This suggests that the enantioselectivity may result from the ring-closure step, rather than the hydride shift step. 

Complexation of [Cp IrCl2]2 with iV-heterocyclic carbenes has led to complexes such as 25, developed by Peris and coworkers [107, 108], and 133, developed by Crabtree and coworkers [12]. Complex 24 is activated by the addition of silver triflate and is effective for the iV-alkylation of amines with alcohols and for the iV-alkylation of anilines with primary amines. Complex 25 has also been shown to couple benzyl alcohol 15 with a range of alcohols, including ethanol 134, to give ether products such as ether 135 (Scheme 31). Complex 133 was an active hydrogen transfer catalyst for the reduction of ketones and imines, using 2-propanol as the hydrogen source. It was also an effective catalyst for the iV-alkylation of amines... 

The photoreactions characteristic for the halogenated phenols and anilines have shown to occur in most members of the series of chlorophenoxy and halophenylurea derivative pesticides, and in some other related biocides as well. Photohydrolysis is a frequently encountered process, but carbene formation has been demonstrated in several cases too. The phototransformation mechanisms of these molecules, however, are frequently complex, owing to the greater complexity of the molecules in question, and homolytic cleavage steps have been found to contribute in several cases. 

Electron-rich and ortho-substituted aryl chlorides were converted to the respective anilines in excellent yields within 1 h on a 1 mmol scale, emphasizing the exceptional activation in combination with the electron-rich phosphine. Another class of strong a-donors are N-heterocychc carbenes. Their Pd(0) complexes can undergo facile oxidative addition of aryl halides. Nolan et al. [44] used the SIPr ligand (SIPr = (l,3-bis(2,6-diisopropyl)phenyl-4,5-dihydroimidazol-2-ylidene), 19) for the Pd-catalyzed arylation of stericaUy encumbered anilines with stericaUy demanding aryl chlorides at room temperature (Scheme 13.66). 

While the focus of this chapter is on hydroamination with amines, impressive recent advances in the hydroamidation and hydrocarbamation of allenes have been disclosed. Espinet showed that acyclic carbenes can be used as ligands for Au(I) to realize intramolecular hydrocarbamation [240], while Widenhoefer has used the commercially available l,3-bis[(2,6-diisopropylphenyl)imidazole-2-ylidine] (IPr) NHC in combination with cationic Au(I) to realize regioselective intermolecular hydroamination of 1,1-disubstituted allenes with benzyl carbamate to access allylamines with quaternary centers adjacent to N. This same catalyst can also accommodate 1,3-disubstituted allenes and even tetrasubstituted allenes (Table 15.19) [241]. Interestingly, these products are in contrast to the preferred products accessed with related Au(I)-phosphine complexes in combination with aniline substrates (Table 15.18) [239]. Hydroureation has been achieved intramolecularly [242, 243] and will be later discussed (Section 15.3.6). 

Af-Heterocydic carbene and phosphine systems were compared, and in some cases the bis-phosphine copper complex [Cu(NHPh)(dtbpe)] (dtbpe= l,2-bis(di-tert-butylphosphino)ethane) outperformed the NHC-based systems. Indeed, the transformation of aniline with acrylonitrile reached 95% conversion after 3h with dtbpe, whereas 12 h were required with [Cu(NHPh)(IPr)]. However, for the reaction of disubstituted cyclohexenone with aniline, [Cu(NHPh)(IPr)] outshone... 

J. Organomet. Chem. 254,333,1983 see so ref. 20.11 b) B. Crociani, P. Uguagliati, U. Belluco - Steric Role of Aromatic Ring Ortho-Substituents in the Mechanism of Carbene Formation from Palladium(II) Arylisocyanide Complexes and Anilines, J. Organomet. Chem. 117,189,1976. 

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