Electrophilic carbene atom

The synthesis and X-ray structural determination of a stable Ir111 hydride/alkylidene complex, (165), has been reported, in which the tridentate N3 ligand is TpMe2. 9 The complex undergoes reversible hydride migration onto the electrophilic carbene atom, as shown in reaction Scheme 20. 

The electrophilic carbene carbon atom of Fischer carbene complexes is usually stabilised through 7i-donation of an alkoxy or amino substituent. This type of electronic stabilisation renders carbene complexes thermostable nevertheless, they have to be stored and handled under inert gas in order to avoid oxidative decomposition. In a typical benzannulation protocol, the carbene complex is reacted with a 10% excess of the alkyne at a temperature between 45 and 60 °C in an ethereal solvent. On the other hand, the non-stabilised and highly electrophilic diphenylcarbene pentacarbonylchromium complex needs to be stored and handled at temperatures below -20 °C, which allows one to carry out benzannulation reactions at room temperature [34]. Recently, the first syntheses of tricyclic carbene complexes derived from diazo precursors have been performed and applied to benzannulation [35a,b]. The reaction of the non-planar dibenzocycloheptenylidene complex 28 with 1-hexyne afforded the Cr(CO)3-coordinated tetracyclic benzannulation product 29 in a completely regio- and diastereoselective way [35c] (Scheme 18). 

In view of the smooth addition of carbanions to the electrophilic P atom of phosphorins (see p. 78 and 66), Markl and Merz attempted the addition of carbenoids (carbenes ) by reacting dichloromethane, trichloromethane or dichloro-phenylmethane with potassium-tert-butoxide in the presence of 2.4.6-triphenyl-X -phosphorin 22 or 2.4.6-tri-tert-butyl-X -phosphorin 24. The desired bicyclocompound 7/5 or the X -phosphepin 116 were not obtained. Instead, benzene derivatives were formed by loss of a PCI fragment, the fate of which was not determined. Markl has proposed a mechanism in which intermediates 114,115 and 116 ZK invoked ... 

Although electrophilic carbenes react with the sulfur atom of CS double bonds, dimethoxycarbene reacts at the carbon atom.10 A review commentary covers the limited range of literature available covering 2,2,4,4-tetramethyl-l,3-cyclobutanedithione, diarylthiones, o-methyl thiobenzoate, methyl dithiobenzoate, and dimethyl xanthate. 

An interesting reaction between isothiazolone 103 and diazomalonoester was described. It is assumed that the electrophilic carbene formed from a diazo compound attacks the sulfur atom to give ylide 104. Its rearrangement affords product 105 [83JCS(CC)643] (Scheme 34). 

Cyclopropylcarbene complexes of the type L M=C(XR )R (X = O, S R = alkyl, aryl R = cyclopropyl) having a stabilizing heteroalkyl (XR ) group on the electrophilic carbene ligand (Scheme 3) have found widespread application in organic synthesis. These so-called Fischer carbene complexes are best known via their group 6 transition metal carbonyl complexes (CO)5M=(OR )R (M = Cr, Mo, W). Much less abundant are the Schrock-type cyclopropylcarbene complexes L M=CR R where no heteroatom is bound to the carbene carbon atom ... 

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]. 

For example, in attempts to realize benzannelation reactions, alkyloxy aryl carbene complexes of manganese failed to react with alkynes even in refluxing toluene, and the starting compounds could be recovered [4]. The documented low reactivity of the Mn as opposed to Cr and Mo carbene complexes may in part explain why the electrophilic carbene C-atom and the nucleophilic diazo C-atom tolerate each other in the same molecule. Besides, the bulky substituents at the silicon atom protect it fi"om being attacked by nucleophiles leading to desilylation as reported for trimethylsilyl substituted Cr carbene complexes [5]. 

Electrophilic carbenes and carbenoids can readily react with the nitrogen atom of a carbon-nitrogen multiple bond to effect immonium (cycloimmon-ium) or nitrile ylide formation. 1,3-Dipolar cycloaddition of these ylides... 

The reaction of carbenes with CO2 under the conditions of matrix isolation allows to qualitatively estimate the philicity of carbenes. [36] The initial step of this reaction is the nucleophilic attack of the carbene at the CO2 carbon atom. The primary adduct subsequently rearranges to give an a-lactone which is easily identified by IR spectroscopy. [36] Since carbenes act as a nucleophile in this reaction, the reactivity increases with increasing carbene nucleophilicity, and electrophilic carbenes like 2a (R = H), despite being of very high reactivity towards a variety of reactants (see below), can not be carboxylated. We thus... 

The chemical shifts of the carbene complex vary from 210 to 385 ppm in the iron series. These large differences have been interpreted by taking into account the more electrophilic nature of the carbene atom due to a possible stabilization of a positive charge by an alkyl group in some cases [102]. An increase of the chemical shift is also noted in going from the pentacoordi-nate complex to the hexacoordinate adduct. For example, the C chemical shift of the diphenyl carbene increases from 358.9 to 385.4 ppm with iron porphyrins [61]. 

Carbenes are uncharged reactive intermediates of general formula CR2, in which carbon atom has a sextet of electrons. Depending on the nature of substituents R, they can behave as electrophilic or nucleophilic species, able to enter a variety of synthetically useful reactions. Perhaps the most important reaction of electrophilic carbenes is addition to C=C double bond giving cyclopropanes. Many electrophilic carbenes can be generated via the base-induced a-elimination process—abstraction of a proton from a carbon atom, connected with a halogen, followed by departure of the halogen anion from the initially formed a-halocarbanion (eq. 101). 

Only a few examples have been obtained through the classical methodologies followed in group 6 metal chemistry. Most rf -Cs Fischer-type ruthenium and osmium carbenes arise from the nucleophilic additions of alcohol and amino groups at the electrophilic carbenic Ca-atom of both allenylidene and vinylidene complexes. The fate of the reaction depends on the electrophilicity as well as the steric hindrance around the Ca-atom, which can control its accessibility, especially for bulky nucleophiles. These features have been thoroughly discussed in a recent review. ... 

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