The Dimerization of Methylene

It is a short step from acetylene to ethylene, but instead of constructing a correlation diagram for addition of H2 across the acetylenic triple bond, let us consider the dimerization of two coplanar methylene molecules. For consistency with the axis convention of Figs. 4.1-4.4, the methylene molecules on the the left side of Fig. 4.7 are placed in in the zx plane and allowed to approach each other along their common C2 axis (x), leaving the Py orbitals free for tt bonding. The symmetry of the field exerted on the electrons by the nuclear frame is D2/1 no hypothetical external field need be postulated. 

The dashed half-arrows in Fig. 4.7 represent electrons present during the approach of two triplet methylenes. The full half-arrows (enclosed in a dashed circle to indicate their absence in the triplet dimerization) represent electrons present during the approach of two singlets. 

Omitting the occupied CH orbitals, because they retain their symmetry labels across the diagram, the ground-state configuration of ethylene is adequately characterized as 7r(62u) ]. Now, suppose that each of the methylenes is 

Scrutinizing Fig. 4.7 more carefully, we see that the cause of the forbiddenness is the refusal of to correlate with 7r(62ti)- How should the symmetry 

We note that the character of E and cr xy) is 1 in both B2u and whereas that of C2 z) and i is —1 in both irreps. The characters of all the other symmetry elements differ in the two representations when it is 1 in the first it is —1 in the second, and vice versa. It follows that if only the four elements E, C2 z), i, a.nda xy) are retained - that is, if the symmetry along the reaction path is reduced from D2/1 to in which the two recalcitrant orbitals have the same representation by) - the electron configuration of the reactant pair of singlet carbenes will correlate with that of the product ethylene, and the previously forbidden dimerization will have become allowed. 

---

These results can be easily rationalized in terms of a diabatic surface analysis. For illustrative purposes, we report here the diabatic surface analysis for the dimerization of methylenes and for the coupling reaction of methylene and silylene along the least-motion path the fragments here are in one case two CH2 and in the other case a CHj and a SiH2. The geometry of the CH2 fragment in both reactions is assumed to be equal to the geometry of ethylene (Rch=1-07A and = 115.9 ), while that of the SiHj... 

Fig. 7. Diabatic (SS and TT) and adiabatic (Ej) curves for the dimerization of methylenes along the least-motion path. TT denotes the curve associated with the triplet-triplet IFC. (From Bernardi and Robb Reproduced by permission of Taylor and Francis Ltd, Publishers.)... 

The simplest of all carbenes, methylene (CHj), initially proved difficult to observe in low temperature matrices, owing to its small size and high mobility. One of the first indications that methylene had been generated in matrices was the observation of the supposed chemiluminescence of ethene following photolysis of diazomethane in solid argon or nitrogen. The luminescence exhibited a distinct isotope effect when CDjNj was photolyzed and was presumed to arise from excited ethene molecules formed by the dimerization of methylene. Nevertheless, a reinvestigation of this reaction led to the conclusion that the excited ethene could have arisen either from the dimerization of CHj or from the reaction of CHj with the precursor CHjNj. 

At first, the dimeric nature of the base isolated from 3-ethyl-2-methyl-4-phenylthiazolium was postulated via a chemical route. Indeed the adduct of ICH, on a similar 2-ethylidene base is a 2-isopropylthiazolium salt in the case of methylene base it is an anilinovinyl compound identified by its absorption spectrum and chemical reactivity (45-47). This dimeric structure of the molecule has been definitively established by its NMR spectrum. It is very similar to the base issued from 2.3-dimethyl-benzo thiazolium (48). It corresponds to 2-(3 -ethyl-4 -phenyl-2 -methylenethiazolinilydene)2-methyl-3-ethyl-4-phenylthiazoline (13). There is only one methyl signal (62 = 2.59), and two series of signals (63= 1.36-3.90, 63= 1.12-3.78) correspond to ethyl groups. Three protons attributed to positions T,5,5 are shifted to a lower field 5.93, 6.58, and 8.36 ppm. The bulk of the ten phenyl protons is at 7.3 ppm (Scheme 22). 

DimeriZa.tlon. A special case of the [2 + 2] cyclo additions is the dimerization of ketenes. Of the six possible isomeric stmctures, only the 1,3-cyclobutanediones and the 2-oxetanones (P-lactones) are usually formed. Ketene itself gives predominandy (80—90%) the lactone dimer, 4-methylene-2-oxetanone (3), called diketene [674-82-8], approximately 5% is converted to the symmetrical dimer, 1,3-cyclobutanedione [15506-53-3] (4) which undergoes enol-acetylation to so-called triketene [38425-52-4] (5) (44). 

The reaction of benzoxazine in die presence of 2,6-xylenol does not occur until 135 C, presumably because die hydrogen-bonded intermediate depicted for the 2,4-xylenol reaction (Fig. 7.19) cannot occur. All three types of linkages are obtained in diis case. Para-para methylene-linked 2,6-xylenol dimers, obtained from the reaction of 2,6-xylenol with formaldehyde, formed in the decomposition of the benzoxazine (or with other by-products of that process) dominate. Possible side products from benzoxazine decomposition include formaldehyde and CH2=NH, either of which may provide the source of methylene linkages. Hie amount of ortho-para linkages formed by reaction of 2,6-xylenol with benzoxazine is low. Ortho-ortho methylene-linked products presumably form by a decomposition pathway from benzoxazine (as in Fig. 7.18). 

The results for the nitroso compounds are very similar to those for the methylene dimerization. The different paths for cis- and trans-approach were explored, and optimized reaction paths similar to those shown in Fig. 8 were obtained. No activation barrier was found for any of the reactions studied. Experimental values of a few kcal/mole have been reported for the dimerization of nitroso compounds. One interesting result is that the EHT... 

The suggested scheme for ethylene formation is dimerization of methylene-metal complexes ... 

The possibility of a biradical mechanism was suggested using the MNDO and AMI semiempirical methods, for the addition of protoanemonin (5-methylene-2(5Z/)-furanone) to butadiene105 and to several substituted dienes106. Experimental evidence for this kind of mechanism has recently been published133. A biradical mechanism has also been considered for the dimerization of butadiene96. For this reaction, CASSCF calculations... 

Cyanamide and its aromatic derivative, such as 4,4 -methylene bis(phenyl cyanamide), were reported to cure an epoxy resin at elevated temperatures.(4) It is also well known that the dimer of cyanamide (dicyandiamide) is the most important epoxy curing agent in one-package epoxy compounding.(5) Unfortunately, this dimer precipitates from the dispersion causing uneven mixing upon standing. 

Photolytic. Acher et al. (1981) studied the dye-sensitized photolysis of terbacil in aerated aqueous solutions over a wide pH range. After a 2-h exposure to sunlight, terbacil in aqueous solution (pH range 3.0-9.2) in the presence of methylene blue (3 ppm) or riboflavin (10 ppm) decomposed to 3-ter/-5-butyl-5-acetyl-5-hydroxyhydantoin. Deacylation was observed under alkaline conditions (pH 8.0 or 9.2) affording 3-/er/-5-hydroxyhydantoin. In neutral or acidic conditions (pH 6.8 or 3.0) containing riboflavin, a mono-Wdealkylated terbacil dimer and an unidentified water-soluble product formed. Product formation, the relative amounts of products formed, and the rate of photolysis were all dependent upon pH, sensitizer, temperature, and time (Acher et ah, 1981). 

The linear telomerization reaction of dienes was one of the very first processes catalyzed by water soluble phosphine complexes in aqueous media [7,8]. The reaction itself is the dimerization of a diene coupled with a simultaneous nucleophilic addition of HX (water, alcohols, amines, carboxylic acids, active methylene compounds, etc.) (Scheme 7.3). It is catalyzed by nickel- and palladium complexes of which palladium catalysts are substantially more active. In organic solutions [Pd(OAc)2] + PPhs gives the simplest catalyst combination and Ni/IPPTS and Pd/TPPTS were suggested for mnning the telomerizations in aqueous/organic biphasic systems [7]. An aqueous solvent would seem a straightforward choice for telomerization of dienes with water (the so-called hydrodimerization). In fact, the possibility of separation of the products and the catalyst without a need for distillation is a more important reason in this case, too. 

Other approaches to alkylidenecycloproparenes have been attempted without success. Aromatization of appropriate alkylidenecyclopropanes or their precursors could not be realized, and flash vacuum pyrolysis of methylene phthalide and 3-methylene-2-coumaranone afforded rearrangement products rather than alkylidenecycloproparenes via extrusion of 002. The photochemical or thermal decomposition of the sodium salt of benzocyclobutenone p-toluenesulfonyl hydrazone led to products derived from dimerization of the intermediate benzocy-clobutenylidene, or from its reaction with the solvent, but no ring-contracted products were observed. When the adduct of methylene-l,6-methano[10]annulene to dicyanoacetylene (249) was subjected to Alder-Rickert cleavage, phenylacety-lene (250) was formed, which derives reasonably from the parent 234. ... 

A disadvantage of diazo compounds is that they are quite polar, which endows them with a propensity to form dimers in the gas phase, even at concentrations as small as 1 5000. This feature becomes evident after their decomposition, which sometimes leads predominantly to carbene dimers (e.g., pentafulvalene in the case of diazocyclopentadiene" ), or adducts of the target carbene with its diazo precursor, as in the case of methylene, where the main products are CH2=NH and HCN." The problem disappears at very high guest/host ratios, but it is often impractical to achieve these. Fortunately, diazirines are less prone to dimerization in the gas phase. 

Vinylpyrroles and vinylindoles are extremely sensitive to acid-catalyzed dimerization and polymerization and it is significant that much of the early research was conducted on systems which were produced in situ. Even by this approach, the dimerization of, for example, 2-(3-indolyl)propene and l-(3-indolyl)-l-phenylethylene was difficult to prevent (see the formation of 110 and 120, Section 3.05.1.2.8). Similarly, although it is possible to isolate ethyl 2-(2- and 3-indolyl)propenoates, they appear to be extremely unstable at room temperature even in the absence of acid (81UP30500) to give [ 4 + 2] cycloadducts of the type (348) (cf. 77JCS(P1)1204>. For many years simple vinylpyrroles also eluded isolation, on account of their facile acid-catalyzed polymerization. Application of the Wittig reaction, however, permits the synthesis of vinyl-pyrroles and -indoles under relatively mild and neutral conditions (see Section 3.05.2.5). In contrast, heteroarylvinyl ketones, esters, nitriles and nitro compounds, obtained by condensation of the appropriate activated methylene compound with the heteroaryl aldehydes (see Section 3.05.2.5), are thermally stable and... 

Show that the dimerization of j/>2-hybridized CH2 is forbidden for approach with the geometry shown below but allowed if the n orbital of one methylene impinges on the unshared pair of the other. 

While diketene remains a very important synthetic precursor, there has been increasing interest in the chemistry of a-methylene-/3-lactones, 3-methylene-2-oxetanones. However, unlike diketene, which can be readily synthesized by the dimerization of aldehydic ketenes, there are few methods for the synthesis of a-methylene-/3-lactones in the literature. Recent strategies for the preparation of the compounds are discussed in Section 2.05.9.2. The kinetic resolution of racemates of alkyl-substituted a-methylene-/3-lactones has been carried out via a lipase-catalyzed transesterification reaction with benzyl alcohol (Equation 21) <1997TA833>. The most efficient lipase tested for this reaction was CAL-B (from Candida antarctica), which selectively transesterifies the (A)-lactone. At 51% conversion, the (R)-f3-lactone, (R)-74, and (A)-/3-hydroxy ester, (S)-75, were formed in very high enantio-selectivities (up to 99% ee). 

The salts of methylene derivatives of 2H- and 3//-pyrroles and -indoles are produced in the Ehrlich reaction (Scheme 32, Section 3.05.1.2.8) and they are also intermediates in the Vilsmeier-Haack reaction (Scheme 24, Section 3.05.2.1.6). Although 6-fV,fV-dimethyl-amino-l-azafulvene, i.e. 2-(Ar,Ar-dimethylaminomethylene)-2//-pyrrole, dimerizes spontaneously (see Section 3.05.2.5), the 6-aryl-6-Ar)Ar-dimethylamino-l-azafulvenes (514 R = aryl) can be isolated (71JCS(B)1405) but, curiously, they are more susceptible to reactions with nucleophiles at the 6-position than are the corresponding salts (B-77MI30508). The benzo[6 ]-1 -azafulvenes, obtained from the reaction of 2-formylindoles with dialkylamines, also dimerize spontaneously, but the isomeric benzo[c]-2-azaf ulvenes, derived from 3-formylindoles, are thermally more stable, although they are extremely moisture sensitive... 

From their scheme of dimerization of methylene radicals, Eidus and co-workers assigned exceptional importance to ethylene in the mechanism of hydrocarbon synthesis. Because both of its carbons can add new methylene radicals, its conversion is expected to occur at a high rate,... 

Lipatov798 recognized that the sorption of Methylene Blue on starch is a heterogenous neutralization reaction of neutralization in which starch acts as an acid. Adsorption isotherms of Methylene Blue on starch indicate that it is an equilibrium process and that swelling as well as other internal structural factors are responsible for the sigmoidal shape of the isotherm.799-800 It is also known that the Langmuir isotherm is affected by equilibrium between the monomer and dimer forms of Methylene Blue, both of which are capable of adsorption on starch (Table XLV).801... 

The polymerization of methylene on a catalyst surface would explain the formation of alkanes, whereas its dimerization on dinuclear clusters would explain the formation of ethylene with some particular homogeneous catalyst ... 

Diarylsilanes react with [Rh(/u.-H)(dippe)]2 to form [Rh(/x-SiAr2)(dippe)]2, but with dimethylsilane the product is (32), which is a catalyst precursor for the dimerization of diphenylsilane (see Silicon Inorganic Chemistry). The initial reaction of [Cp2Rh2(/u.-CH2)2(M-SH)]BPli4 with dimethylacetylene dicarboxylate is the insertion of one aUcyne into the S-H bond but the second equivalent of aUcyne couples with both methylene groups to form (33)." ... 

The polycyclic 1,2,4-trioxane 40 is formed when H2O2 is added to the hDA dimer of methylene cyclohexanone <03OBC2859>. 

The interaction of HCl and HBr with indene in methylene chloride and with acenaphthylene in pentane, methylene chloride and acetic acid as well as the reaction of HI with the latter monomer in methylene chloride only yielded the addition products, no polymerisation being detected. However, according to older reports the dimers of both these monomers can be obtained with HCI . Whatever the reason for this discrepancy, it is obvious that indene and acenaphthylene fall in the same category as styrene in that they do not offer the best conditions for the formation of active species in reacting with hydrogen halides. It would be interesting to try these interactions in more polar solvents. 

---