Carbenes, generation stability

A number of close carbene analogues - silylenes and germylenes - have also been generated, stabilized in inert matrices and studied using the IR spectroscopic technique. 

The most common rearrangement reaction of alkyl carbenes is the shift of hydrogen, generating an alkene. This mode of stabilization predominates to the exclusion of most intermolecular reactions of aliphatic carbenes and often competes with intramolecular insertion reactions. For example, the carbene generated by decomposition of the tosylhydrazone of 2-methylcyclohexanone gives mainly 1- and 3-methylcyclohexene rather than the intramolecular insertion product. 

The stability of bis(2,4,6-trichlorophenyl)carbene " " generated by Zimmerman in 1964, was measured by using LFP. The transient absorption due to 106d, however, was found to disappear disappointingly quickly (within 100 ms). The decay was found to be second order, in accordance with the product analysis... 

Carbenes, generated by several methods, are reactive intermediates and used for further reactions without isolation. Carbenes can also be stabilized by coordination to some transition metals and can be isolated as carbene complexes which have formal metal-to-carbon double bonds. They are classified, based on the reactivity of the carbene, as electrophilic heteroatom-stabilized carbenes (Fischer type), and nucleophilic methylene or alkylidene carbenes (Schrock type). 

Other e. amples of [2+1] cycloaddition reactions with trifluoromethyl-substituted carbenes are known with carbenes bearing a halogen or a metho.xy substituent, which seems to result in a push-puir stabilization. Yields depend on the mode of generation of the carbenes best yields of eyclopropanes are obtained with carbenes generated from (polyfluorocthyl)mercury compounds. " ... 

Currently accepted mechanism of the Wittig reaction of aldehydes with non-stabilized ylides involves the formation of oxaphosphetanes through a [2-I-2]-cycloaddition-like reaction . The oxaphosphetanes are thermally unstable and collapse to alkene and phosphine oxide below room temperature. Under salt-free conditions there is no formation of betaine intermediates. The salt-free ylides can be prepared by the reaction of phosphines with carbenes generated in situ. Vedejs etal proposed a puckered 4-centre cyclic transition state I for sy -oxaphosphetane and planar structure J for anff-oxaphosphetane. In general, the flnfi-oxaphosphetane J is more stable than the syn-oxaphosphetane I, and under equilibrium conditions (when stabilized ylides are used) the E-alkene product is favoured (Scheme 4.24). However, kinetic control conditions, which appear to dominate when non-stabilized ylides are used, would lead to Z-alkene. 

The adducts 2 of (chloromethoxy)carbene (generated from chloromethyl dichloromethyl ether using alkyllithium as base) are of limited stability. They form stable methoxymethoxycyc-lopropanes 3 on reaction with sodium methoxide. These products do not react with mineral acids." ... 

Intramolecular insertion reactions are much more selective because the relative proximity of the various C-H bonds to the carbene center plays a major role in determining product distribution. For this reason, the reaction has synthetic utility and is especially valuable for generating derivatives of highly strained ring systems. Cyclopropanes formed by intramolecular insertion reactions are often major products from alkyl carbenes. A competing process for aliphatic carbenes is stabilization... 

CBrF) with (20), respectively, led to the formation of (25) (and (26)) via a spontaneous ring opening of intermediate gem-dihalocyclopropanes (21) (and (22)) to form cations (23) (and (24)). The mechanism has been further supported by DFT calculations highlighting the exceptional stability of cations (23) and (24). Besides, Cp2 carbene generated from trimethyl(trifluoromethyl)silane (i.e., the Ruppert-Prakash reagent) has been shown to add smoothly not only to C=C but also C C bonds, thus furnishing valuable gem-difluorocyclopropanes and. gem-difluorocyclopropenes, respectively. ... 

Many transition metals can form carbene complexes, often generated indirectly due to the instability of the corresponding carbene. While some of these complexes are reactive and unstable intermediates, many are stable and some are even commercially available. Carbene complexes are divided into two types Fischer and Schrock carbenes. Fischer carbenes, such as chromium complex 8.6 (Figure 8.1), contain metals from groups VI to VII, have rr-acceptor ligands, especially carbon monoxide, and are electrophilic. A donor atom on the carbene carbon stabilizes the carbene. Schrock carbenes, such as tantalum complex 8.7, involve early transition metals, do not have rr-acceptor ligands and are nucleophilic. 

The first attempt to use a carbene to stabilize a Group 15 element-centered radical was reported by Apeloig et al. in 2007. The phosphorus-centered radical -P(0)(Oi-Pr)2 was generated in situ in the presence of IMes to afford the NHC-diisopropylphosphoryl radical 211 (Figure 5.25). Despite the presence of the NHC, 211 could not be isolated, but its half-life (7.1 s) was sufficiently long to allow the authors to obtain an EPR spectrum. Spectral and computational data for 211 revealed considerable spin density on the NHC carbon (39%), with another 37% delocalized into the carbene backbone, and only 7.5% on the phosphorus. 

The area of intermolecular C-H activation has undergone explosive growth in recent years this can largely be attributed to the use of substituted carbenes in these processes [20]. A pivotal contribution by Davies in 1997 disclosed the first examples of intermolecular enantioselective C-H insertion processes onto simple hydrocarbons such as hexane and cyclopentane (Equation 31) [96], The stabilized carbene generated upon the interaction of Rh catalyst 78 with diazoester 179 underwent effective insertion into the C-H bond of cyclopentane to furnish the adduct 180 in 72 % yield and 96 % ee. 

The pentagon stabilization has been found in a biochemical phenomenon [80], The hydrogen on the thiazolium ring 9 (Scheme 7) is easily ionized to afford the corresponding carbene 10, a key catalyst in enzymatic reactions for which thiamine (vitamin B-1,11) pyrophosphate is the cofactor. The pentagon stability is expected to contribute to this unusual deprotonation. A lone pair generated on the carbon atom in 10 can similarly delocalize through the vicinal C-N and C-S a bonds in a cyclic manner. 

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

Nevertheless, a more traditional approach to the stabilization of carbenes and the investigation of their spectral properties deals with the direct generation of carbenes in low-temperature matrices, e.g. by the photolysis of diazo-compounds or ketenes. The method allows stabilization of carbenes in their ground electronic state, prevents intramolecular isomerization and also facilitates direct spectroscopic monitoring of their chemical transformations in low-temperature matrices. 

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