Imidazolin-2-ylidene

Based on these assumptions many different heteroatom-substituted carbenes have been synthesized. They are not limited to unsaturated cyclic di-aminocarbenes (imidazolin-2-ylidenes Scheme 3, A) [17-22] with stericbulk to avoid dimerization like 1 l,2,4-triazolin-5-ylidenes (Scheme 3, B), saturated... 

Figure 4.15 X-ray structure of l,3-bis(2,6-diisopropylphenyl) imidazolin-2-ylidene)-gold(l) fluoride [185].

Besides the addition of non-carbon nucleophiles to carbonyl and isonitrile complexes (Tables 2.2, 2.4), heteroatom-disubstituted carbene complexes can be prepared by direct addition of stable or latent carbenes to suitable complexes. The synthetic routes sketched in Figure 2.12 are those commonly used for preparing imidazoline-2-ylidene or imidazolidine-2-ylidene complexes. 

The most practical approach is the direct treatment of azolium salts with metal complexes under neutral or basic conditions [39,154-159]. Alternatively, the free carbenes can be generated in the presence of a suitable metal complex by reduction of a carbene precursor, e.g. a thiourea [160]. Stable, uncomplexed imidazoline-2-ylidenes, isolated for the first time in 1991 by Arduengo [161] (for further examples see [162-166]), are also convenient starting materials for the preparation of carbene complexes [167,168]. The corresponding diaminocarbene complexes can be obtained by treatment of the stable diaminocarbenes with transition metal complexes. Finally, at high temperatures many transition metal complexes catalyze the carbon-carbon bond scission of tetraaminoethylenes, forming carbene complexes [169-171]. Examples of such preparations are given in Table 2.8. 

Photolysis of cationic alkoxycarbene iron complexes [193] or alkoxycarbene manganese complexes [194] has been used to replace carbonyl groups by other ligands. The alkylidene ligand can also be transferred from one complex to another by photolysis [195], Transfer of alkylidene ligands occurs particularly easily from diaminocarbene complexes, and has become a powerful synthetic method for the preparation of imidazoline-2-ylidene complexes [155,196]. 

That was only 4 years after the preparation of the first Fischer-iyp carbene complex 6 years before the first Schrock-iyp carbene complex 4 was reported, and more than 20 years before the isolation of stable imidazolin-2-ylidenes by Arduengo in 1991 [Eq. (1)]. Once attached to a metal, these Wanzlick- or Arduengo-csabenes have shown a reaction pattern completely different from that... 

Later, imidazolidin-2-ylidenes such as 14, a saturated, more electron-rich and nonaromatic version of the imidazolin-2-ylidenes, were isolated. Isolation of a six-membered tetrahydropyrimid-2-ylidene 15 and of acyclic structures such as 1629,30... 

Precursors for NHCs with Unsaturated Backbone (Imidazolin-2-ylidenes... 

In many cases the synthesis of NHC complexes starts from iV,A/ -disubstituted azolium salts. Imidazolium salts as precursors for imidazolin-2-ylidenes are generally accessible by two ways complementing each other (i) nucleophilic substitution at the imidazole heterocycle or (ii) a multicomponent reaction building up the heterocycle with the appropriate substituents in a one-pot reaction. 

The broader subject of the interaction of stable carbenes with main-group compounds has recently been reviewed. Accordingly, the following discussion focuses on metallic elements of the s and p blocks. Dimeric NHC-alkali adducts have been characterized for lithium, sodium, and potassium. For imidazolin-2-ylidenes, alkoxy-bridged lithium dimer 20 and a lithium-cyclopentadienyl derivative 21 have been reported. For tetrahydropyrimid-2-ylidenes, amido-bridged dimers 22 have been characterized for lithium, sodium, and potassium. Since one of the synthetic approaches to stable NHCs involves the deprotonation of imidazolium cations with alkali metal bases, the interactions of alkali metal cations with NHCs are considered to be important for understanding the solution behavior of NHCs. 

The fact that NHCs form stable compounds with beryllium, one of the hardest Lewis acids known and without p-electrons to back donate, shows the nu-cleophilicity of these ligands. Reaction of l,3-dimethylimidazolin-2-ylidene with polymeric BeCl2 results in the formation of the neutral 2 1 adduct 23 or the cationic 3 1 adduct 24. The first NHC-alkaline earth metal complex to be isolated was the 1 1 adduct 25 with MgEt2- Whereas l,3-dimesitylimidazolin-2-ylidene results in the formation of a dimeric compound, the application of sterically more demanding l,3-(l-adamantyl)imidazolin-2-ylidene gives a monomeric adduct. ... 

The elimination of an alcohol from a neutral 2-alkoxy-1,2-dihydro-1//-imidazole leads to the formation of NHCs [Eq. (18)]. Upon heating, the elimination of alcohol forms the NHC, which in the case of imidazolin-2-ylidenes dimerizes to... 

Cyclic thiourea derivatives like l,3,4,5-tetramethylimidazole-2(3H)-thione— prepared by condensation of substituted thioureas with a-hydroxyketones—can be converted into the corresponding imidazolin-2-ylidene by desulfurization with sodium or potassium [Eq. (23)]. This method was used to prepare and isolate l,3-bis-neo-pentylbenzimidazolin-2-ylidene with Na/K. With LDA as the base it is also possible to generate free benzimidazolin-2-ylidenes in solution. ... 

Scheme 7. Reaction of chromocene with imidazolin-2-ylidenes and imidazolium salts.

In certain cases it is also possible to replace anionic ligands by neutral NHC ligands. Nickelocene and chromocene are capable of coordinating one imidazolin-2-ylidene ligand by shifting one of the cyclopentadienyl anions from to... 

In cases where the free NHC cannot be synthesized the complex formation has to be accomplished in situ from a ligand precursor, e.g., the imidazolium salt in the case of imidazolin-2-ylidenes. By this method, it is often possible to prepare complexes which do not have the maximum number of NHC ligands attached to the metal center. 

The electron density induced at the metal center increases in the order C(OR)R < C(NHR)R < C(NHR)2 imidazolidin-2-ylidene imidazolin-2-ylidene. According to these data the difference between diamino-substituted carbenes whether cyclic or acyclic, aromatic or nonaromatic, seems to be quite small. 

In C-NMR spectra, the signals for the carbene carbon are usually shifted upheld by about 20-30 ppm upon complexation of the free NHC to a transition metal. Cr-NMR data of [LCr(CO)s] complexes underline that NHC are a special case of carbene ligands because of their lack of tt-acceptor ability. Photoreactions of metal complexes containing NHCs by laser flash and continuous photolysis show that NHCs are quite inert ligands in photolysis reactions. He I and He II photoelectron spectra of platinum(O)- and palladium(O) bis(imidazolin-2-ylidene)... 

Palladium(ll) complexes of imidazolin-2-ylidenes bearing perfluorinated side chains 46 were prepared and suggested to be useful in flourous biphasic catalysis as well as in supercritical C02. ° However, catalytic applications have not been reported yet. 

Bioinorganic chemists have been attracted by the complex formations of NHC because the imidazolin-2-ylidene motif is encountered frequently in living organisms. The imidazole moiety is part of the purin bases in both DNA and RNA as well as the amino acid histidine which appears in proteins and enzymes and is in many cases considered to play a decisive role within the catalytically active center. The possible formation of NHC complexes under physiological conditions or in vivo has been addressed by investigation of A-confused caffeine 73 or purine 74 complexes. 

Although these two methods have found widespread application for the synthesis of free carbenes, they failed for selected saturated imidazolidin-2-ylidenes and especially in the preparation of triazolin-5-ylidenes. In these cases the free carbene species 7 can be obtained from 2-alkoxyimidazolidines 6 [44] or 5-aUcoxytriazoles [36] by thermally induced ot-elimination of an alcohol (Fig. 5). In addition to 2-aUcoxyimidazolidmes, 2-(pentafluorophenyl)imidazolidines [45, 46] have also been used for the generation of NHCs by cx-elimination. The adduct 8 eliminates acetonitrile upon heating [47] to yield the benzimidazolin-2-ylidene 9. In a more exotic procedure, imidazolium salts have been reduced electrochemically to give the free imidazolin-2-ylidenes [48]. 

Imidazolin-2-ylidenes of type 5 are normally obtained as colorless, diamagnetic, crystalline solids with surprisingly high melting points. In the absence of air and moisture, they exhibit a remarkable stability and even some air-stable derivatives... 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

Some bidentate bis(imidazolin-2-ylidene)s 18 have been used for the preparation of complexes with chelating dicarbene ligands [75-80]. The synthesis, properties, and coordination chemistry of tripodal tris(imidazolin-2-ylidene) ligands like 19... 

For example, complex 37 with an imidazolin-2-ylidene and a methyl ligand in cis-position to each other decomposes to yield the 1,2,3-trimethylimidazolium salt 38, Pd°, and cod (Fig. 13) [124], Additional examples for the reductive elimination of 2-alkyl and 2-aryl substituted azohum salts from palladium or nickel NHC complexes have been reported [125, 126]. Today, reductive elimination reactions have been established as one important reaction pathway for the deactivation of catalytically active metal NHC complexes [126, 127]. 

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