Carbene anions

Finally, in some cases diradical negative ions can even be generated directly npon ionization of appropriate precnrsors. For example, nitrene and carbene anions can be formed by El of organic azides, diazo-compounds, and diazirines, whereas Branman and co-workers have reported the formation of oxyallyl anions by El of flnorinated acetyl componnds (Eq. 5.12). ... 

A peculiar reaction occurs when carbene anions are trapped with chlorosilanes the tetracarbonyl cobalt complex is recovered in high yield ... 

Benzene derivatives such as m-methylanisole (40) can be converted to distonic carbene ions. Reaction of 40 with O occurs with loss of H2, generating the conventional carbene anion 41 this anion reacts with molecular fluorine by dissociative ET, followed by nucleophilic attack of F on the methyl group, forming 42. In contrast to phenyhnethylene, 42 has a singlet ground state however, upon protonation it gives rise to the triplet state of m-hydroxyphenyl-methylene. This interesting reaction can be viewed as a spin-forbidden proton-transfer reaction. 

Ring closure to cyclobutanes and cyclopentane-s The synthesis of cyclo-propanones by elimination of lithium thiophenoxicMe with this base has been extended to similar syntheses of functionalized cyclob tanes and cyclopentanes, as shown in equations (I) and (II), When extended to synthesis of a cyclohexane derivative, this method resulted in a very low yield. One possible mechanism is formation of a dianion followed by loss of thiophenoxid e ion to give a carbene-anion, which cyclizes with loss of the second thiophenoxide ion. 

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 . 

The first Ce(IV) complex stabilized by an alkoxo functionalized Ai-heterocyclic carbene anion has recently been reported. The complex is made by oxidation of the corresponding trivalent cerium complex using benzoquinone (Figure 8.36). The cerium cation is coordinated by two bidentate ligands and two monodentate ligands, in which the NHC groups are unbound [123],... 

A special group of azomethine derivatives is diazoalkanes, RR C=N=N . They have been investigated in some detail electrochemically after it was shown that dissociative electron attachment to diazoalkanes was a convenient way of preparing carbene anion radicals in the gas phase. 

Reduction of diazoalkanes, such as 9-diazofluorene [100] or diazophenylmethane [101,102], in DMF or MeCN gives a high yield of the corresponding azine and sometimes the ketone and other compounds as minor products, as in eq. (31). The proposal that a carbene anion radical was a detectable intermediate in the reaction was later proved unlikely [103,104]. 

The diazocompounds, such as azibenzil [107], 2,3,4,5-tetraphenyldiazocyclopenta-diene [108], diazodimedone [109], and diethyl diazomalonate [106], whose anion radicals decompose to relatively stable carbene anion radicals, are all derived from compounds with relatively low pKa values (from 13 to 18 in DMSO). It thus seem essential for the lifetime of the carbene radical anion that the negative charge can be accommodated in the structure [110]. 

The reaction of the chromium and tungsten carbene anions [(CO)j-M=C(OMe)CH2 ] (M = W, Cr) with coordinated alkenes has been studied by Beck and co-workers (195,196), as well as by Geoffroy and coworkers (197). The reaction of the carbene anions with cationic complexes having a coordinated ethylene ligand leads to the formation of... 

A route to 2-cyclohexenones from manganese carbene anions and enones employs LDA to effect the cyclization. ... 

Cycloadditions of acylnitropyrrolidine derivatives [272] of 1.64 or 1.65 give interesting results, as do reactions of alkoxyimminium salts [295] or of 2-pyrrolidinobutadienes [296], The same pyrrolidine derivatives have been used in asymmetric Michael reactions of aminochromium carbene anions [297] and in nucleophilic additions to arenemanganese tricarbonyl complexes [298],... 

Carbon-Carbon Bond Formation via Metal Carbene Anions... 

The metal carbene anions represent C-nucleophiles which can be applied to aldol-type reactions. The first aldol reactions were performed with stoichiometric amounts of carbene complex anions [6a] later it turned out that higher yields are obtained when the reaction is run under equilibrating conditions (Scheme 4). [9] Clean aldol reactions generally require non-enolizable aldehydes. Nevertheless, an aldol condensation involving an enolizable aldehyde has been reported using catalytic amounts of base. [10]... 

Addition of Mel to the carbene anion [CpMo(CO)2 =C(NMe2)Ph results in formal insertion of the carbene unit into a C-H bond of Mel to form complex 64. A related process occurs in the reaction of BzCl with [CpM(CO)2 =C(ai5 e)] .M7 Addition of PMea to CpMo =C(OEt)Me)(CO)(NO) results in carbonyl substitution to form CpMo =C(OEt)Me)(PMe3)(NO). This reacts with SMe" " at the carbene to form the cation tCpMofPMesXNO) ti2-SMe-C(OEt)Me. 338... 

Alternatively, demetallation to give enol ethers 839 can be achieved by treatment with pyridine (Scheme 8.14). Under these conditions, as pyridine is a weak base, an equilibrium is established between the carbene 8.35 and its anion 837. The anion, however, can also reprotonate on chromium to give a chromium hydride 838. This is followed by reductive elimination. The enol ether 839 is obtained as its Z-isomer, a consequence of the carbene anion having S-geometry to keep the alkyl group away from the bulky Cr(CO)s moiety the chromium is converted into a pyridine complex. 

In spite of their great thermodynamic stability, carbene anions are reactive toward a variety of carbon electrophiles. Allylic and benzylic halides react with carbene anions at 0° to give alkylated carbene complexes (W. R. Bruns-void and C. P. Casey, unpublished results, 1975). Methyl fluorosulfonate acts as an efficient methylating agent of carbene anions (Casey ei al., 1972). 

Acid chlorides react readily with carbene anions to give acylated carbene complexes. When the initial acylated product contains an enolizable proton, the corresponding enol ester is isolated (Casey et a/., 1972). 

Aldehydes react with carbene anions to produce vinylcarbene complexes. Reaction of carbene anions with formaldehyde gives dimeric products that apparently arise by conjugate addition of a carbene anion to an initially... 

Carbene anions react with ethylene oxide to give 2-oxacyclopentylidene complexes (Anderson and Casey, 1974b). The reaction can be envisioned as nucleophilic attack of the carbene anion on the epoxide, followed by intramolecular displacement of methoxide from the intermediate adduct. As expected for a nucleophilic attack, propylene oxide is attacked by carbene anions at the least hindered carbon atom. 

Dialkylation is often an important side reaction in the alkylation of carbene anions. The dependence of the acidity and of the reactivity of carbene anions... 

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