Carbene with carbon nucleophiles

Electrophilic vinylidene complexes, which can be easily generated by a number of different methods [128], can react with non-carbon nucleophiles to yield carbene complexes (Figure 2.9 for reactions with carbon nucleophiles, see Section 3.1). 

The related ferracyclic carbene complexes 13a has been shown to reaet with carbon nucleophiles such as lithium enolates and organocuprates to give 4-substituted-(3 )-diene triearbonyl iron complexes for example 13c (Scheme 12). Decomposition of 13a with formal loss of CO and MeOH can also occur to give 13b. 

The preparative value of this compound lies in the surprising fact that bis(l,3-diphenylimidazolidinylidenc-2) behaves in many reactions ie.g., with aromatic aldehydes,2,7 and with carbon acids 2 7-fJ) as if it dissociated to form a nucleophilic carbene. The hydrolytic cleavage of these derived imidazolidine derivatives makes possible the preparation of formyl compounds, so that the amino olefin can be considered as a potential carbonyla-tion reagent. In many reactions it is not necessary to isolate... 

The reaction of carbenes with alcohols can proceed by various pathways, which are most readily distinguished if the divalent carbon is conjugated to a tt system (Scheme 5). Both the ylide mechanism (a) and concerted O-H insertion (b) introduce the alkoxy group at the originally divalent site. On the other hand, carbene protonation (c) gives rise to allylic cations, which will accept nucleophiles at C-l and C-3 to give mixtures of isomeric ethers. In the case of R1 = R2, deuterated alcohols will afford mixtures of isotopomers. 

Cyclodditions to Carbon-Heteroatom Triple Bonds. Transient electrophilic carbenes are known to react with nitriles to give transient46 or even stable nitrile ylides 30.47 No reaction of transient nucleophilic carbenes with nitriles has been reported. 

Transition metal carbene complexes have broadly been classified into Fischer-type and Schrock-type carbene complexes. The former, typically low-valent, 18-electron complexes with strong 7t-acceptors at the metal, are electrophilic at the carbene carbon atom (C ). On the other hand, Schrock-type carbene complexes are usually high-valent complexes with fewer than 18 valence electrons, and without n-accepting ligands. Schrock-type carbene complexes generally behave as carbon nucleophiles (Figure 1.4). 

Carbonyl complexes also react with non-carbon nucleophiles. The resulting carbonic acid derivatives can serve as starting material for the preparation of bis-heteroatom-substituted carbene complexes [93]. Heterocyclic carbene complexes can be obtained from nucleophiles with a leaving group in -position (Table 2.2). 

Electrophilic vinylidene complexes, capable of reacting with non-carbon nucleophiles to yield Fischer-type carbene complexes, can be obtained by addition of electrophiles to alkynyl complexes (Figure 2.11, Table 2.7, Entries 11, 12) [134,144]. 

Our understanding of the chemistry of N-arylnitrenium ions is significantly more advanced than it was a decade ago. Nevertheless, this field of research is still considerably less developed than that of carbenium ions, carbenes, or nitrenes. For example, although singlet nitrenium ions behave as one might expect that their 4-imino-2,5-cyclohexadienyl resonance contributors would in their reactions with H2O, NJ, or Cl, their reactions with carbon, nitrogen, and sulfur nucleophiles, particularly d-G, are not so easily rationalized. Except for d-G, these reactions with soft nucleophiles have not been examined systematically and the regiochemistry exhibited by these nucleophiles is incompletely understood. 

Smaller-ring ketones, especially cydobutanones and more rigid cyclopentanones or cyclohexanones, give biradicals that follow the fourth of the pathways in which carbon monoxide is not tost. In this process a new oxygen-carbon bond is formed by attack of the oxygen of the acyl radical on the alkyl radical centre this generates a carbene which can subsequently react with a nucleophilic solvent such as methanol (4.28). 

Type III reactions proceed by attack of a nucleophile at the central sp carbon of the allenyl system of the complexes 5. Reactions of soft carbon nucleophiles derived from active methylene compounds, such as /i-kcto esters or malonates, and oxygen nucleophiles belong to this type. The attack of the nucleophile generates the intermediates 9, which are regarded as the palladium-carbene complexes 10. The intermediates 9 pick up a proton from the active methylene compound and n-allylpalladium complexes 11 are formed, which undergo further reaction with the nucleophile, as expected, and hence the alkenes 12 are formed by the introduction of two nucleophiles. 

Carbene complexes of transition metals [2,21,225-236] are typical representatives of compounds with a double metal-carbon bond. They are seen as derivatives of a two-covalent carbon in their singlet state [226,232,236]. As a rule, the carbene ligand is an effective a-donor and a comparatively weak n-acceptor. Formation of a cr-bond M — C takes place via transference of a nonbonding electronic pair with a nucleophilic a-orbital of the carbenic carbon to the metal atom. Simultaneously, it is also possible to form a 7t-bond as a result of the interaction of symmetrically appropriate metallic d-AO with a vacant electrophilic /7-orbital of the carbene [236,237], This situation is a key factor that determines the polarization of most of the carbene complexes according to type 145 (Fig. 2.6). 

Reaction of a stable carbene with iodine results in compound 46, which can be considered an isolated transition state which models the nucleophilic attack of the carbene on the iodine molecule (Scheme 25).55 The iodine-iodine bond js significantly lengthened and the carbon iodine bond distance of 2.104(3) A is slightly elongated when compared to that of iodoarenes. The authors report that protic solvents promote ionic dissociation to the 2-iodoimidazolium ion which is isoelectronic with the tellerium adduct 44. 

Hexafluoro-2-butyne and carbon disulflde react to give the tetrakis-(trifluoromethyl)tetrathiafulvalene quantitatively only in the presence of trifluoroacetic acid (70JA1412 73JA4379). The carbene initially formed is protonated the 1,3-dithiolium ion subsequently combines with the nucleophilic carbene to give the trifluoromethyl-substituted tetrathiafulvalene. 

The silver(l) carbene complex was used in the ROP of (L)-lactides [120]. Mechanistically, it is thought that the silver cation interacts with the carbonyl oxygen atom of the cyclic lactide. One of the carbene units then nucleophilicly attacks the carbonyl carbon atom resulting in C-O bond fissure and coordination of the oxygen atom on the silver cation. The role of the initial Ag-0=C interaction is to bring the lactide into position for nucleophilic attack by the carbene and to activate the carbonyl carbon atom for this attack (see Figure 4.40). 

Pentacarbonyl[methyl(methoxy)carbene]tungsten and other carbene complexes containing a hydrogen atom a to the carbene carbon atom react with butyllithium (but also with OMe" ) at low T to generate carbene anions. The moderate reactivity of these carbene anions toward carbon nucleophiles, including epoxides, aldehydes, a-bromoesters and a, -unsaturated carbonyl compounds can be used to prepare carbene complexes inaccessible via other synthetic routes (see refs. 7-9). The anion generated by treatment of (CO)5Cr[C(OMe)Me] with BuLi in THF at — 78°C is isolated as the air-stable bis(triphenylphosphane)iminium salt . 

Organylisothiocyanates (R = Me, Et, Ph) and -selenocyanates (R = Ph) also act as nucleophilic S and Se atom donors and react with carbene complexes 259 (M = Cr or W, R = H, Me, Br, OMe, CFj, or NMc2), to yield i/ -arylphenylthioketone and -selenoketone pentacarbonyl chromium and tungsten complexes (260) (170,171). Kinetic results suggest an associative stepwise mechanism with a nucleophilic attack of the sulfur at the carbene carbon in the first reaction step (172). It is also possible to isolate thio- and selenobenzaldehyde complexes 262 (M = Cr or W, E = S or Se,... 

Yet another kind of alkene addition is the reaction of a carbene with an alkene to 3field a cyclopropane. A carbene, R2C , is a neutral molecule containing a divalent carbon with only six electrons in its valence shell. It is therefore highly reactive and can be generated only as a reaction intermediate, rather than as an isolable molecule. Because a carbene has only six valence electrons on carbon, it is electron-deficient and behaves as an electrophile. Thus, carbenes react with nucleophilic C=C bonds much as other electrophiles do. The reaction occurs in a single step without intermediates. 

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