Trimethylenemethane

{{Chembox |ImageFile = Chemdg trimethylenemethane 2rad.svg |ImageSize = 200px |ImageCaption = Trimethylenemethane, average of three configurations. Formally, the radial bonds have valency {{sfrac|4|3}}. Each terminal carbon has {{sfrac|2|3}} of an unfilled valence bond. |PIN = 2-Methylidenepropane-1,3-diyl |OtherNames = Trimethylenemethane biradical; Trimethylenemethane diradical |Section1 = {{Chembox Identifiers |CASNo = 13001-05-3 |PubChem = 11790068 |StdInChI=1S/C4H6/c1-4(2)3/h1-3H2 |StdInChIKey = MOWBWPSGCTXGKR-UHFFFAOYSA-N |SMILES = [CH2-]C(=C)[CH2+] |SMILES1 = [CH2][C]([CH2])[CH2] }} |Section2 = {{Chembox Properties |C=4|H=6 }} }}

'''Trimethylenemethane''' (often abbreviated '''TMM''') is a chemical compound with formula {{chem|C|4|H|6}}. It is a neutral free molecule with two unsatisfied valence bonds, and is therefore a highly reactive free radical. Formally, it can be viewed as an isobutylene molecule {{chem|C|4|H|8}} with two hydrogen atoms removed from the terminal methyl groups.

== Structure == The electronic structure of trimethylenemethane was discussed in 1948.<ref name="slip">{{cite journal |author=Slipchenko Lyudmila V., Krylov Anna I.|year=2003|title=Electronic structure of the trimethylenemethane diradical in its ground and electronically excited states: Bonding, equilibrium geometries, and Vibrational frequencies|journal=Journal of Chemical Physics|volume=118|issue=15|pages=6874–6883|doi=10.1063/1.1561052|bibcode=2003JChPh.118.6874S|s2cid=4204676}}</ref><ref name="couls2">C. A. Coulson (1948), Journal de Chimie Physique et de Physico-Chimie Biologique, volume 45, page 243. Cited by Slipchenko and Krylov (2003)</ref> It is a neutral four-carbon molecule containing four pi molecular orbitals. When trapped in a solid matrix at about {{cvt|90|K|C|0}}, the six hydrogen atoms of the molecule are equivalent. Thus, it can be described either as zwitterion, or as the simplest conjugated hydrocarbon that cannot be given a Kekulé structure. It can be described as the superposition of three states: {| |120x120px |120x120px |120x120px |} It has a triplet ground state (³''A''₂′/³''B''₂), and is therefore a diradical in the stricter sense of the term.<ref name="dowd2">{{cite journal |author=Paul Dowd |year=1972 |title=Trimethylenemethane |journal=Accounts of Chemical Research |volume=5 |issue=7 |pages=242–248 |doi=10.1021/ar50055a003}}</ref> Calculations predict a planar molecule with three-fold rotational symmetry, with approximate bond lengths 1.40&nbsp;Å (C–C) and 1.08&nbsp;Å (C–H). The H–C–H angle in each methylene is about 121°.<ref name="slip" />

Of the three singlet excited states, the first one, 1¹''A''₁ (1.17&nbsp;eV above ground), is a closed shell diradical with flat geometry and fully degenerate threefold (''D''_3h) symmetry. The second one, 1¹''B''₂ (also at 1.17&nbsp;eV), is an open-shell radical with a ''D''_3h-symmetric equilibrium between three equal geometries; each has a longer C–C bond (1.48&nbsp;Å) and two shorter ones (1.38&nbsp;Å), and is flat and bilaterally symmetric except that the longer methylene is twisted 79° out of the plane (''C''₂ symmetry). The third singlet state, 2¹''A''₁/¹''A''₁′ (3.88&nbsp;eV), is also a ''D''_3h-symmetric equilibrium of three geometries; each is planar with one shorter C–C bond and two longer ones (''C''_2ν symmetry).<ref name="slip" />

The next higher energy states are degenerate triplets, 1³''A''₁ and 2³''B''₂ (4.61&nbsp;eV), with one excited electron; and a quintet state, ⁵''B''₂ (7.17&nbsp;eV), with the p orbitals occupied by single electrons and ''D''_3h symmetry.

== Preparation == Trimethylenemethane was first obtained from photolysis of the diazo compound 4-methylene-Δ₁-pyrazoline with expulsion of nitrogen, in a frozen dilute glassy solution at {{cvt|-196|C|K|0}}.<ref name="dowd2"/>

It was also obtained from photolysis of 3-methylenecyclobutanone, both in cold solution and in the form of a single crystal, with expulsion of carbon monoxide. In both cases, trimethylenemethane was detected by electron spin resonance spectroscopy.<ref name="dowd2" /> center|400x400px|Trimethylenemethane Trimethylenemethane has been obtained also by treating potassium with 2-iodomethyl-3-iodopropene and isobutylene diiodide ({{chem|IH|2|C}})₂C={{chem|CH|2}} in the gas phase. However the product quickly dimerizes to yield 1,4-dimethylenecyclohexane, and also 2-methylpropene by abstracting two hydrogen atoms from other molecules (hydrocarbon or potassium hydride).<ref name="skell2">{{cite journal |author=Skell Philip S., Doerr Robert G. |year=1967 |title=Trimethylenemethane |journal=Journal of the American Chemical Society |volume=89 |issue=18 |pages=4688–4692 |bibcode=1967JAChS..89.4688S |doi=10.1021/ja00994a020}}</ref>

Three classes of compounds have been used to generate synthetically useful TMM-derivative reaction intermediates: diazenes, silyl-substituted allylic acetates and methylenecyclopropenes. In the first case, bridged diazenes are used to avoid competitive closure to MCPs and dimerization reactions.<ref name=Berson/> The latter case requires stabilization of a zwitterion, as with e.g. acetal '''1''':<ref name=NYEDM>Nakamura, E.; Yamago, S.; Ejiri, S.; Dorigo, A. E.; Morokuma, K. ''J. Am. Chem. Soc.'' '''1991''', ''113'', 3183.</ref> center Alternatively, palladium(0) or nickel(0) catalysts can stabilize the zwitterion:<ref>Binger, P.; Büch, H. M. ''Top. Curr. Chem.'' '''1987''', ''135'', 77.</ref> center Silylated allylic acetates, carbonates and other substituted allyl compounds may form TMM synthons under palladium catalysis.<ref name=TAng>Trost, B. M. ''Angew. Chem. Int. Ed. Engl.'' '''1986''', ''25'', 1.</ref>

== Organometallic chemistry == {{main|Trimethylenemethane complexes}} A number of organometallic complexes have been prepared, starting with Fe({{chem|C|4|H|6}})(CO)₃, which was obtained by the ring-opening of methylenecyclopropane with diiron nonacarbonyl ({{chem|Fe|2}}(CO)₉).<ref name="dowd2"/> The same complex was prepared by the salt metathesis reaction of disodium tetracarbonylferrate ({{chem|Na|2|Fe}}(CO)₄) with 1,1-bis(chloromethyl)ethylene (H₂C=C(CH₂Cl)₂).<ref name="ward2">{{cite journal |author=J. S. Ward |author2=R. Pettit |author2-link=Rowland Pettit |name-list-style=amp |year=1970 |title=Trimethylenemethane complexes of iron, molybdenum, and chromium |journal=Journal of the Chemical Society D |issue=21 |pages=1419–1420 |doi=10.1039/C29700001419}}</ref> Related reactions give M(TMM)(CO)₄ (M = Cr, Mo). The reaction leading to (TMM)Mo(CO)₄ also gives Mo({{chem|C|8|H|12}})(CO)₃ containing a dimerized TMM ligand.<ref name="ward2" />

TMM complexes have been examined for their potential in organic synthesis, specifically in the trimethylenemethane cycloaddition reaction (see {{slink||Cycloaddition}}) with only modest success. One example is a palladium-catalyzed [3+2<nowiki>]</nowiki>cycloaddition of trimethylenemethane.<ref>{{cite journal | doi = 10.1021/ja00515a046 | volume=101 | issue=21 | title=New conjunctive reagents. 2-Acetoxymethyl-3-allyltrimethylsilane for methylenecyclopentane annulations catalyzed by palladium(0) | year=1979 | journal=Journal of the American Chemical Society | pages=6429–6432 | author=Barry M. Trost | bibcode=1979JAChS.101.6429T }}</ref><ref name=Berson/> <gallery> File:PETCEWtopView.png|Structure of Ru(trimethylenemethane)(CO)₃, viewed down ''C''₃ axis.<ref name=NYEDM/> File:PETCEWside.png|Structure of Ru(trimethylenemethane)(CO)₃, viewed orthogonal to ''C''₃ axis. </gallery>

== Organic reactions == Unligated trimethylenemethanes are unstable, and rapidly close a ring to methylidenecyclopropanes.<ref name=Berson>Berson, J. A. ''Acc. Chem. Res.'' '''1978''', ''11'', 446.</ref> The problem is generally less severe for five-membered, cyclic TMMs due to ring strain in the corresponding methylidenecyclopropanes.

===Cycloaddition=== '''Trimethylenemethane cycloaddition''' is the formal (3+2) annulation of trimethylenemethane (TMM) derivatives to two-atom pi systems. Although TMM itself is too reactive and unstable to be stored, reagents which can generate TMM or TMM synthons ''in situ'' can be used to effect cycloaddition reactions with appropriate electron acceptors. Generally, electron-deficient pi bonds undergo cyclization with TMMs more easily than electron-rich pi bonds.<ref name=NYEDM/> center Usually, unless a cyclic pi system is involved TMM cycloadditions exhibit 2π periselectivity and do not react with larger pi systems. Polar MCPs, for example, react only with the 2,3 double bond of polyunsaturated esters.<ref name=NYEDM/> center TMM's singlet and triplet states exhibit different reactivity and selectivity profiles.<ref name=Berson/> A singlet (3+2) cycloaddition, when it is concerted, is believed to proceed under frontier orbital control. When electron-rich TMMs are involved, the ''A'' orbital serves as the HOMO (leading to fused products if the TMM is cyclic). When electron-poor (or unsubstituted) TMMs are involved, the ''S'' orbital serves as the HOMO (leading to bridged products if the TMM is cyclic). Cycloadditions involving the triplet state are stepwise, and usually result in configurational scrambling in the two-atom component.<ref name=NYEDM/> center Diazene-derived TMMs cyclize with an alkenic acceptor to either fused or bridged products.<ref name=NYEDM/> Fused products are generally favored, unless the methylene carbon bears electron-donating groups. The configuration of the alkene is maintained as long as the reaction is proceeding through a singlet TMM.<ref name=TAng/> center Unless catalyzed by transition metals, methylidenecyclopropane opening is also stereospecific with respect to alkene geometry, and exhibits high selectivity for ''endo'' products in most cases.

With catalysis, cyclization takes place in a stepwise fashion and does not exhibit stereospecificity. Rapid racemization of chiral π-allyl palladium complexes occurs, and only moderate diastereoselectivity is observed in reactions of chiral allylic acetates. Chiral non-racemic alkenes, however, may exhibit moderate to high diastereoselectivity. The reaction is highly regioselective, providing only the substitution pattern shown below regardless of the position of the R' group on the starting allylic acetate. center| Chiral auxiliaries on the alkene partner have been used for stereoselective transformations. In the reaction of camphorsultam-derived unsaturated amides, lower temperatures were needed to achieve high selectivities.<ref>Binger, P.; Schäfer, B. ''Tetrahedron Lett.'' '''1988''', ''29'', 529.</ref>

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In reactions of silyl-substituted allylic acetates, chiral sulfoxides can be used to enforce high diastereofacial selectivity.<ref>Chaigne, F.; Gotteland, J.-P.; Malacria, M. ''Tetrahedron Lett.'' '''1989''', ''30'', 1803.</ref>

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Carbonyl compounds may be used as the 2π component under the appropriate conditions. For instance, in the presence of an indium co-catalyst, the reactive 2π component of the cycloaddition below switches from the C-C to the C-O double bond.<ref>Trost, B. M.; Sharma, S.; Schmidt, T. ''J. Am. Chem. Soc.'' '''1992''', ''114'', 7903.</ref>

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Polarized trimethylenemethanes generated from polar MCPs are also useful substrates for (3+2) reactions with polar double bonds as the 2π component. Orthoester products are generally favored over ketene acetals.<ref>Yamago, S.; Nakamura, E. ''J. Org. Chem.'' '''1990''', ''55'', 5553.</ref>

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====Comparison with other methods==== Although 1,3-dipolar cycloaddition is a useful method for the generation of five-membered heterocyclic compounds, few methods exist to synthesize five-membered carbocyclic rings in a single step via annulation. Most of these, like TMM cycloaddition, rely on the generation of a suitable three-atom component for combination with a stable two-atom partner such as an alkene or alkyne. When heated, cyclopropene acetals rearrange to vinylcarbenes, which can serve as the three-atom component in cycloadditions with highly electron-deficient alkenes.<ref>Boger, D. L.; Brotherton, C. E. ''J. Am. Chem. Soc.'' '''1986''', ''108'', 6695.</ref> Zinc homoenolates can also serve as indirect three-atom components, and undergo cyclization to cyclopentenones in the presence of an unsaturated ester.<ref>Crimmins, M. T.; Nantermet, P. G. ''J. Org. Chem.'' '''1990''', ''55'', 4235.</ref> Tandem intermolecular-intramolecular cyclization of homopropargylic radicals leads to methylenecyclopropanes.<ref>Curran, D. P.; Chen, M.-H. ''J. Am. Chem. Soc.'' '''1987''', ''109'', 6558.</ref>

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==References== {{reflist}}

Category:Free radicals