Carbene anion radicals

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]. 

Electrical effects. These effects cause a variation in the electron density at the active site. They account for the ability of a substituent to stabilize or destabilize a cation, anion, radical, carbene or other chemical species. 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

Carbene type radical anions have been often postulated as reactive intermediates. An tetratrimethylsilyl substituted silylene (16) was reduced with several alkali metals in dimethoxyethane to yield a persistent silylene radical anion.169... 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

VIII. THE CYCLOPROPENYL CATION, ANION, RADICAL AND CARBENE. ... 

Many methodologies have been developed for the ring expansion of heterocycles involving different reaction intermediates, such as carbenes, nitrenes, radicals and a large number of anionic species these processes being promoted thermically, photo-chemically or by means of transition metals. 

Theoretical studies on the structure of the carbene cation and anion radical of the parent species (H2C and H2C , respectively) performed by MINDO/3, SCF/CI and ab initio (4-13G) calculations led to the conclusion that the structures H2C ( Ai) and H2C ( Bi) are similar to those of the neutral carbene in its lowest singlet ( Ai) and triplet ( Bi) state, respectively, if one compares CH bond lengths and HCH angles (see summary of Bethell and Parker, 1988 for neutral carbenes, see also Sect. 8.1 of this book). Removal of an electron from the neutral carbene is energetically much more expensive than addition of an electron. This theoretical result corresponds to conclusions that can be drawn from experimental experience concerning reaction products. They demonstrate that carbene cation radicals are very electrophilic. 

It should be noted that not only ordinary stable cationic, anionic, and molecular (neutral) Lewis acids and bases are included, but also highly reactive intermediates such as carbenes, free radicals, and hypothetical species. 

The single electron transfer (SET) reduction of Fischer alkynyl carbene complexes 95 with potassium graphite (CgK) results in the formation of cyclopentadienes 100 (Scheme 5.20) [31], The radical anion intermediates 96, generated by the SET reduction of Fischer alkynyl carbene complexes 95, undergo tail-to-tail dimerization to form bis-carbene anions 97. The intermediates 97 are protonated with a strong acid to produce the intermediates 98, which convert in situ into chromium cyclopentadienylcarbene 99. After the demetallation of the cyclopentadienylcarbene 99, cyclopentadienes 100 are obtained. 

Weak to moderate chemiluminescence has been reported from a large number of other Hquid-phase oxidation reactions (1,128,136). The Hst includes reactions of carbenes with oxygen (137), phenanthrene quinone with oxygen in alkaline ethanol (138), coumarin derivatives with hydrogen peroxide in acetic acid (139), nitriles with alkaline hydrogen peroxide (140), and reactions that produce electron-accepting radicals such as HO in the presence of carbonate ions (141). In the latter, exemplified by the reaction of h on(II) with H2O2 and KHCO, the carbonate radical anion is probably a key intermediate and may account for many observations of weak chemiluminescence in oxidation reactions. 

Three possible mechanisms may be envisioned for this reaction. The first two i.e. 1) Michael addition of R M to the acetylenic sulfone followed by a-elimination of LiOjSPh to yield a vinyl carbene which undergoes a 1,2 aryl shift and 2) carbometallation of the acetylenic sulfone by R M followed by a straightforward -elimination, where discarded by the authors. The third mechanism in which the organometallic reagent acts as an electron donor and the central intermediates is the radical anion ... 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

The SET mechanism is chiefly found where X = I or NO2 (see 10-104). A closely related mechanism, the SrnE takes place with aromatic substrates (Chapter 13). In that mechanism the initial attack is by an electron donor, rather than a nucleophile. The Srn 1 mechanism has also been invoked for reactions of enolate anions with 2-iodobicyclo[4.1.0]heptane. An example is the reaction of l-iodobicyclo[2.2.1]-heptane (15) with NaSnMe3 or LiPPh2, and some other nucleophiles, to give the substitution product. Another is the reaction of bromo 4-bromoacetophenone (16) with Bu4NBr in cumene. " The two mechanisms, Sn2 versus SET have been compared and contrasted. There are also reactions where it is reported that radical, carbanion, and carbene pathways occur simultaneously. ... 

This reaction can proceed by 1,1-proton abstraction to form a carbene radical anion, but can also occur by l,n-abstraction to form the negative ion of a diradical. Thus, reaction of O with methylene chloride results in the formation of CCI2 (Eq. S.Sa), reaction with ethylene gives vinylidene radical anion, H2CC (Eq. 5.8b), and the reaction with acetonitrile gives the radical anion of cyanomethylene, HCCN (Eq. 5.8c) Investigations of these ions have been used to determine the thermochemical properties of dichlorocarbene, CCI2, vinylidene, and cyanomethylene. ... 

An important synthetic application of this reaction is in dehalogenation of dichloro- and dibromocyclopropanes. The dihalocyclopropanes are accessible via carbene addition reactions (see Section 10.2.3). Reductive dehalogenation can also be used to introduce deuterium at a specific site. The mechanism of the reaction involves electron transfer to form a radical anion, which then fragments with loss of a halide ion. The resulting radical is reduced to a carbanion by a second electron transfer and subsequently protonated. 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

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