Arduengo carbenes,

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],... 

Due to their isoelectronic relationship with Arduengo carbenes , stabilization of cyclic phosphenium ions by Ti-electron delocalization has been a matter of debate. 

Carbenes are defined as molecular species with formally divalent and two-coordinate carbon atoms bearing various substituents X and Y and a lone pair of electrons. While the simple representatives are of low stability (such as CH2) and may only appear as short-lived reaction intermediates or in adducts with electron donors, some cyclic systems can be readily isolated. This is particularly true for many of the A-heterocyclic carbenes (NHCs), which are now widely applied as ligands to metals ( Wanzlick-Arduengo carbenes ). Such carbenes based on imidazol and benzimidazol have become the working horses in this branch of organogold chemistry (Scheme 54). 

The most common ligands are those derived from imidazole and benzimidazol (Scheme 54), followed by the (benz)thiazols. The free Wanzlick-Arduengo carbenes can be isolated and employed for the synthesis of the complexes, but often it is more convenient to prepare the carbenes in situ from the dimers or the corresponding onium salts, or to use carbene-transfer reactions.256-259... 

No new calculations were specifically devoted to this heterocylic system since CHEC-II(1996). Redox properties of chalcogeno-ureas possessing this heterocyclic skeleton and resulting from the reaction of Arduengo carbenes such as 108 with sulfur or selenium was investigated through semi-empirical calculations <2000EJI1935>. 

Ardel polyarylate resins, 10 190 Ardeparin, 4 95t Arduengo carbenes, 26 847 Area, exponents of dimensions in absolute, gravitational, and engineering systems, Cross-sectional area Head- area meters Surface area Variable-area flowmeters Area detectors, 26 431 Area per surfactant molecule, 24 136 Arechloral hydrate, anesthetic properties of, 2 69... 

Further stabilization of 1,3,2-diazaphospholane cations can be achieved by incorporating the N-P(+)-N unit in a 6n system, resulting in an electronic structure analogous to the Arduengo carbene [30, 237-243], It has been shown that while the most important stabilizing effect is exerted by the amino groups on the phosphenium center, the 6it-delocalization indeed has a contribution [30] to the stability of these systems. It is of further interest that the diazaphospholes derived from the... 

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). 

An amino substituent on the phosphenium phosphorus makes cationic phosphenium complexes stable, but the ji-donation from an amino substituent is only a minor contribution. So what is the role of an amino substituent The role may be, as is proposed for Arduengo carbene, to protect the approach of a nucleophile to phosphenium phosphorus by high p7i lone pair density flanking the phosphenium center.32... 

Prior to the isolation of Arduengo carbenes. many closely related carbene complexes had also been prepared via the thermolytic cleavage of electron-rich alkenes in the presence of various transition metal reagents (Figure 5.6). 

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]. 

While silicocene is 7t-base stabilized by the pentamethylcyclopentadienyl substituents, the stabilization in the silicon analog of the Arduengo carbene is mainly caused by the C=C-7C system... 

The cationic titanium species [Cp2TiMe]+ can be stabilized by coordination of neutral Arduengo carbene ligands with strong bonds between the Ti and the carbon atoms. The observed conformational orientation of the carbene ligand indicates its strong cr-donor character, confirmed by the results of the theoretical calculations (Scheme 533).1408... 

This upheld chemical shift is in no way restricted to anionic ligands, as can be seen by the strong cr-donor dimesityl-imidazolydene, an Arduengo carbene (see Eig. 4.5). 

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... 

Recently, Heinekey et al. [68] reported the synthesis and structural characterization of a tetramethylfulvene complex of iridium bearing an Arduengo carbene ligand. 

A journalist recently asked me if I foresaw when my group prepared the first stable carbene that this topic would become a field in its own right. My answer was clear-cut certainly not Indeed, in 1988 and even for a few years after, I believed that these species were too fragile to be really useful, and that they would remain laboratory curiosities. I was not totally wrong as far as our group s first stable carbene is concerned, and it is obvious that Arduengo carbenes, the famous NHCs, have been responsible for the fantastic development of this field of chemistry. 

In the second paper, the non-hindered Arduengo carbene PF5 adduct (56) was prepared by thermolysis of 1,3-dimethyl-lff-imidazolium hexa-fluorophosphate (57) under reduced pressure. This observation... 

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