Arduengo-type carbene

Chiral diaminocarbene complexes of copper were used in asymmetric conjugate addition of diethylzinc to Michael acceptors. Achiral copper carbene complexes derived from imidazolium salts were synthesized and characterized for the first time by Arduengo in 1993 [43]. In 2001, Woodward reported the use of such Arduengo-type carbene in copper-catalyzed conjugate addition and showed their strong accelerating effect [44]. The same year, Alex-... 

Arduengo-type carbenes have also been successfully employed for complexation of group 13 trialkyls (Me3Ga- CNfPflCzMezNfPr1) 13091) and mixed hydrido/alkyls (Cp 2(H)Ga- CN(Me)C2Me2N(Me) 13192). Adduct 131... 

Figure 7 lnH3 adduct of an Arduengo-type carbene. 

In being considered as phosphine analogues, Arduengo-type carbenes have naturally attracted much attenhon for use as ancillary ligands in catalyhc applicahons. In 2002, Faller, Crabtree and coworkers published the details of an... 

Nickel catalysts are also efficient in promoting the coupling of halopyridines and amines. In the presence of nickel acetylacetonate and Arduengo type carbene precursors chloropyridines were coupled with a series of amines to give the aminopyridines in high yield. 3-Chloropyridine, for example, reacted with A -mcthylanilinc to yield the anilinopyridine in 93% (7.75.), Of the carbene precursors tested l,3-bis(2 ,6 -... 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

FIGURE 29. Arduengo-type carbenes with divalent atoms E = C, Si, Ge... 

Scheme 13. Reaction of 1 la with a stable Arduengo -type carbene...

A situation like that described can also be found if one compares the imidazolium cation to the imidazol-2-ylidene (the Arduengo-type carbene). ... 

Figure 5.7 Isolable (a) Bertrand- and (b) Arduengo-type free carbenes predated by (c) transition metal complexes...

Because of the high electron deficit at the two-coordinate phosphorus atom, phosphenium cations are only stable with electron-donating substituents R, predominantly with nitrogen donor atoms. This is an obvious parallel to the situation with stable carbenes of the Wanzlick-Arduengo type. Therefore most complexes in this series are derived from diazaphospholes (75a,... 

Organolanthanide carbene complexes have been elusive for a long time. With the use of Arduengo-type heterocyclic carbenes this interesting class of compounds has become readily available. The (imidazol-2-ylidene) samarium complexes shown in Scheme 105 were obtained from l,3,4,5-tetramethylimidazol-2-ylidene and Cp 2Sm(THF) in different molar ratios.519... 

Only limited number of neutral monocyclic 11,3,2 diazaphosphole representatives have been reported, which have mostly been prepared by [4+1] cyclocondensation of diaminomaleodinitrile (DAMN) with P(III) reagent and the alkylation of the initially formed 1,3,2-diazaphospholide [2, 4, 7], During recent times, 67t-aromatic [l,3,2]diazaphospholium ions of type 46 [45], more often represented as cyclic phosphenium cation 47 [46,47], have attracted more attention due to their isoelec-tronic nature with Arduengo carbenes . Nature of bonding and aromaticity of these cations have been the subject of several experimental and theoretical studies (Structure 2) [48-52],... 

Apart from the unique adduct 5150, in which two plumbylene moieties are bridged by a MgBr2(thf)4 molecule viaBr —> Pb interactions [Pb—Br 296.4(2) pm], and few cyclopen-tadienyl derivatives where chelating amines interact with the two-valent tetrel atom62, unsupported adducts of Lewis bases with tetrylenes have been restricted to carbene-type bases so far. However, only donor carbenes, such as Arduengo carbenes, isonitriles or ylids, form simple adducts with dative C —> E interactions (Table 8). Carbenes or analogous species which also exhibit a substantial acceptor ability form multiple bonds instead (see Section III). 

The first chiral NHCs of this type were developed by Herrmann and En-ders in 1996. Herrmann s group [6] synthesized a symmetric imidazolium salt 1 (as carbene precursor), starting from an enantiopure chiral amine which was readily converted to the heterocycle using a multi-component reaction previously developed by Arduengo [7]. After coordination to a rhodium(I) complex precursor (Scheme 1), this ligand was tested in the hydrosilylation of acetophenone. 

There are essentially three different types of transition metal carbene complexes featuring three different types of carbene ligands. They have all been named after their first discoverers Fischer carbenes [27-29], Schrock carbenes [30,31] and WanzUck-Arduengo carbenes (see Figure 1.1). The latter, also known as N-heterocycUc carbenes (NHC), should actually be named after three people Ofele [2] and Wanzlick [3], who independently synthesised their first transition metal complexes in 1968, and Arduengo [1] who reported the first free and stable NHC in 1991. Fischer carbene complexes have an electrophilic carbene carbon atom [32] that can be attacked by a Lewis base. The Schrock carbene complex has a reversed reactivity. The Schrock carbene complex is usually employed in olefin metathesis (Grubbs catalyst) or as an alternative to phosphorus ylides in the Wittig reaction [33]. 

Most carbenes are very short lived, although persistent carbenes are known. A persistent carbene (also known as a stable carbene or a Arduengo carbene) is a type of carbene demonstrating particular stability. The best-known examples are diaminocarbenes with the general formula (R2N)2C , where the R represents various functional groups. The groups can be bridged so that the carbon with unfilled orbitals is part of a heterocycle, such as imidazole or triazole. Persistent... 

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