Free carbene

Co-condensation reaction of the vapors of l,3-di-rcrt-butylimidazol-2-ylidene and nickel, palladium, or platinum gives the coordinatively unsaturated 14-electron sandwiches [L M] (M=Ni, Pd, Pt) of the carbene type (990M3228). Palladium(O) carbene complexes can also be prepared by the direct interaction of l,3-R2-imidazol-2-ylidenes (R=/-Pr, r-Bu, Cy, Mes) (L) with the palladium(O) compound [Pd(P(o-Tol)3)2] (OOJOM(595)186), and the product at the first stage is [(L)PdP(o-Tol)3l, and then in excess free carbene [PdL ]. 

Dimethylbenzimidazolium iodide serves as a starting material for free carbenes (99CEJ1931, 99JA10626, 00AGE541) and carbene complexes (99JOM(585)241). Thus, with palladium(II) acetate it gives [L Pdl ] where L is... 

Free carbenes based on 1,2,4-triazole are not as numerous as those based on imidazole (70ZN(B)1421, 95AGE1021, 97JA6668, 98JA9100). The carbene complex 169 (Ar = Ph, p-Tol) is prepared by the [3 + 2] cycloaddition route from [W(CO)j(C+=NC-HCOOEt)]- and aryldiazonium (930M3241). Oxidative decomplexation causes tautomerization of the 1,2,4-triazole ligand, the products being 170 (Ar= Ph, i-Tol). 

By application of the Simmons-Smith reaction it is possible to synthesize a cyclopropane from an alkene by formal addition of carbene to the carbon-carbon double bond, without a free carbene being present in the reaction mixture the... 

Because most carbenes are so reactive, it is often difficult to prove that they are actually present in a given reaction. The lifetime of formylcarbene was measured by transient absorption and transient grating spectroscopy to be 0.15-0.73 ns in dichloromethane. In many instances where a carbene is apparently produced by an a elimination or by disintegration of a double-bond compound, there is evidence that no free carbene is actually involved. The neutral term carbenoid is used where it is known that a free carbene is not present or in cases where there is doubt. a-Halo organometallic compounds (R2CXM) are often called carbenoids because they readily give a elimination reactions (e.g., see 12-37). ° ... 

The reaction in which aldehydes are converted to methyl ketones, RCHO -f CH2N2 —> RCOCH3, while apparently similar, does not involve a free carbene intermediate. It is considered in Chapter 18 (18-9). 

Carbenes and substituted carbenes add to double bonds to give cyclopropane derivatives ([1 -f 2]-cycloaddition). Many derivatives of carbene (e.g., PhCH, ROCH) ° and Me2C=C, and C(CN)2, have been added to double bonds, but the reaction is most often performed with CH2 itself, with halo and dihalocarbenes, " and with carbalkoxycarbenes (generated from diazoacetic esters). Alkylcarbenes (HCR) have been added to alkenes, but more often these rearrange to give alkenes (p. 252). The carbene can be generated in any of the ways normally used (p. 249). However, most reactions in which a cyclopropane is formed by treatment of an alkene with a carbene precursor do not actually involve free carbene... 

Carbene itself is extremely reactive and gives many side reactions, especially insertion reactions (12-19), which greatly reduce yields. When it is desired to add CH2 for preparative purposes, free carbene is not used, but the Simmons-Smith procedure (p. 1088) or some other method that does not involve free carbenes is employed instead. Halocarbenes are less active than carbenes, and this reaction proceeds quite well, since insertion reactions do not interfere.The absolute rate constant for addition of selected alkoxychlorocarbene to butenes has been measured to range from 330 to 1 x 10 A few of the many ways in... 

Free carbenes can also be avoided by using transition metal-carbene complexes L M—CRR (L = a ligand, M = a metal),which add the group CRR to double bonds.An example is ... 

The actual product of the reaction is thus the ketene, which then reacts with water (15-3), an alcohol (15-5), or ammonia or an amine (15-8). Particularly stable ketenes (e.g., Ph2C=C=0) have been isolated and others have been trapped in other ways (e.g., as P-lactams, 16-64). The purpose of the catalyst is not well understood, though many suggestions have been made. This mechanism is strictly analogous to that of the Curtius rearrangement (18-14). Although the mechanism as shown above involves a free carbene and there is much evidence to support this, it is also possible that at least in some cases the two steps are concerted and a free carbene is absent. 

This group showed that isolable silver(I) diaminocarbene complexes can be use in situ instead of free carbenes, to generate the copper carbene complex. The silver salts that precipitates during the formation of the copper complex have not any negative effect on the conversion. This method is advantageous since most of the silver complexes are isolable, air-stable and easily obtained by treatment of the corresponding imidazohnium salt by 0.5 equiv of silver oxide (Scheme 53). The solid structure of 78 was analyzed by X-ray diffraction. 

It has been possible to record the IR and UV spectra of several derivatives of the carbene [75] - tetrachlorocyclopentadienylidene [80], indenylidene [81] and fluorenylidene [82] (Bell and Dunkin, 1985). These carbenes were formed by UV photolysis of the corresponding diazo precursors frozen in inert matrices and have a triplet ground state. The carbenes [80]-[82] react with CO in inert matrices at 30 K, but exhibit a lower reactivity than the carbene [75]. Furthermore, they were stabilized in a pure CO matrix at 12 K, whereas the free carbene [75] could not be detected under these conditions. The different reactivity towards CO between [75] and [80]-[82] may be associated with the different steric shielding of the carbene centres and with the different triplet-singlet gap as well. 

Transition metal-catalyzed carbenoid transfer reactions, such as alkene cyclopro-panation, C-H insertion, X-H insertion (X = heteroatom), ylide formation, and cycloaddition, are powerful methods for the construction of C-C and C-heteroatom bonds [1-6]. In contrast to a free carbene, metallocarbene-mediated reactions often proceed stereo- and regioselectively under mild conditions with tolerance to a wide range of functionalities. The reactivity and selectivity of metallocarbenes can be... 

A free carbene B base-containing complex C transmetallation D oxidative addition E C=C activation F C-H bond activation... 

The most convenient tool for the characterisation of NHCs is NMR spectroscopy, in particular C H NMR. As a case study, the carbenes IPr and SIPr, and their corresponding salts IPr HCl and SIPr HCl were chosen. As described above (Scheme 1.2), free carbenes are often obtained by deprotonation of the corresponding salt. The best diagnostic tool to observe the salt deprotonation, and thus indirectly monitor the carbene formation, is H NMR spectroscopy, by means of the disappearance of the characteristic acidic proton resonance. The signal corresponding to the latter (ff) is largely shifted downfield (typically 8-12 ppm) and disappears upon deprotonation (Fig. 1.5). 

The carbene formation can be monitored by C H NMR, as the carbene carbon atom of free NHCs has a signal significantly shifted downfield. Typically, the signal for the O atom is found between 200 and 250 ppm for the free carbene and between 130 and 160 ppm for the corresponding salt... 

Based on this work, it has been proposed that a specifically solvated carbene (Scheme 4.6, Reaction 2) nndergoes bimolecular reactions at slower rates than a free carbene (Scheme 4.6, Reaction 1). Other alternatives that mnst be considered are participation of rapid and reversible ylide formation with the ylide acting as a... 

Since the most direct evidence for specihc solvation of a carbene would be a spectroscopic signature distinct from that of the free carbene and also from that of a fully formed ylide, TRIR spectroscopy has been used to search for such car-bene-solvent interactions. Chlorophenylcarbene (32) and fluorophenylcarbene (33) were recently examined by TRIR spectroscopy in the absence and presence of tetrahydrofuran (THF) or benzene. These carbenes possess IR bands near 1225 cm that largely involve stretching of the partial double bond between the carbene carbon and the aromatic ring. It was anticipated that electron pair donation from a coordinating solvent such as THF or benzene into the empty carbene p-orbital might reduce the partial double bond character to the carbene center, shifting this vibrational frequency to a lower value. However, such shifts were not observed, perhaps because these halophenylcarbenes are so well stabilized that interactions with solvent are too weak to be observed. The bimolecular rate constant for the reaction of carbenes 32 and 33 with tetramethylethylene (TME) was also unaffected by THF or benzene, consistent with the lack of solvent coordination in these cases. °... 

In either neat dioxane or THF, carbene-ether ylides are observed as a broad IR absorption band between 1560 and 1610 cm , distinct from the IR bands of the free carbenes. With discrete spectroscopic signatures for the free carbene and its corresponding ether ylides, TRIR spectroscopy was used to confirm that the effects described above with dilute ether in Freon-113 were due to specific solvation of the carbene (Scheme 4.6, Reaction 2) rather than a pre-equilibration with the coordinating solvent (Scheme 4.6, Reaction 3) or reactivity of the ylide itself (Scheme 6, Reaction 4). In Freon-113 containing 0.095M THF simultaneous TRIR observation of both the free carbene (x = ca. 500 ns) and the carbene-THF ylide (x = ca. 5ps) was possible7 The observation that lifetimes of these species were observed to be so different conclusively demonstrates that the free carbene and the carbene-THF ylide are not in rapid equilibrium and that Reaction 3 of Scheme 4.6 is not operative. By examining the kinetics of the carbene 34 at 1635 cm directly in Freon-113 with small amounts of added dioxane, it was observed that the rate of reaction with TME was reduced, consistent with Reaction 2 (and not Reaction 4) of Scheme 4.6. 

There are a number of ways of generating carbenes that will be discussed shortly. In some cases, the reactions involve complexes or precursors of carbenes rather than the carbene per se. For example, carbenes can be generated by a-elimination reactions. Under some circumstances the question arises as to whether the carbene has a finite lifetime, and in some cases a completely free carbene structure is never attained. 

When a reaction appears to involve a species that reacts as expected for a carbene but must still be at least partially bound to other atoms, the term carbenoid is used. Some carbenelike processes involve transition metal ions. In many of these reactions, the divalent carbene is bound to the metal. Some compounds of this type are stable, whereas others exist only as transient intermediates. In most cases, the reaction involves the metal-bound carbene, rather than a free carbene. 

These reactions have very low activation energies when the intermediate is a free carbene. Intermolecular insertion reactions are inherently nonselective. The course of intramolecular reactions is frequently controlled by the proximity of the reacting groups.113 Carbene intermediates can also be involved in rearrangement reactions. In the sections that follow we also consider a number of rearrangement reactions that probably do not involve carbene intermediates, but lead to transformations that correspond to those of carbenes. 

The reactive intermediates under some conditions may be the carbenoid a-haloalkyllithium compounds or carbene-lithium halide complexes.158 In the case of the trichloromethyllithium to dichlorocarbene conversion, the equilibrium lies heavily to the side of trichloromethyllithium at — 100°C.159 The addition reaction with alkenes seems to involve dichlorocarbene, however, since the pattern of reactivity toward different alkenes is identical to that observed for the free carbene in the gas phase.160... 

Reactions involving free carbenes are very exothermic since two new theoretical treatment of the addition of singlet methylene to ethylene suggests that there is no activation barrier.168 The addition of carbenes to alkenes is an important method for synthesis of many types of cyclopropanes and several of the methods for carbene generation listed in Scheme 10.8 have been adapted for use in synthesis. Scheme 10.9, at the end of this section, gives a number of specific examples. 

A Mechanism for Alkylidene Formation. There is no unambiguous example of free-carbene capture by a metal substrate, and the mild reaction conditions used in the generation of these carbene complexes from diazoalkanes suggests that such a mechanism is highly unlikely here. Transition metal diazoalkane complexes, then, are almost certainly implicated as intermediates in these reactions. 

Synthesis via the reaction of AgX salt with free carbene ligand 204... 

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