 Trimethylenemethane, average of three configurations. Formally, the radial bonds have valency4/3. Each terminal carbon has2/3 of an unfilled valence bond. | |
| Names | |
|---|---|
| Preferred IUPAC name 2-Methylidenepropane-1,3-diyl | |
| Other names Trimethylenemethane biradical; Trimethylenemethane diradical | |
| Identifiers | |
3D model (JSmol) | |
| |
| |
| Properties | |
| C4H6 | |
| Molar mass | 54.092 g·mol−1 |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Trimethylenemethane (often abbreviatedTMM) is achemical compound withformulaC
4H
6. It is aneutral free molecule with two unsatisfiedvalence bonds, and is therefore a highly reactivefree radical. Formally, it can be viewed as anisobutylene moleculeC
4H
8 with twohydrogen atoms removed from the terminalmethyl groups.
The electronic structure of trimethylenemethane was discussed in 1948.[1][2] It is a neutral four-carbon molecule containing four pi molecular orbitals. When trapped in a solid matrix at about 90 K (−183 °C), the six hydrogen atoms of the molecule are equivalent. Thus, it can be described either aszwitterion, or as the simplestconjugated hydrocarbon thatcannot be given a Kekulé structure. It can be described as the superposition of three states:
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It has atripletground state (3A2′/3B2), and is therefore adiradical in the stricter sense of the term.[3] Calculations predict a planar molecule with three-fold rotational symmetry, with approximate bond lengths 1.40 Å (C–C) and 1.08 Å (C–H). The H–C–H angle in each methylene is about 121°.[1]
Of the threesinglet excited states, the first one, 11A1 (1.17 eV above ground), is a closed shell diradical with flat geometry and fully degenerate threefold (D3h) symmetry. The second one, 11B2 (also at 1.17 eV), is an open-shell radical with aD3h-symmetric equilibrium between three equal geometries; each has a longer C–C bond (1.48 Å) and two shorter ones (1.38 Å), and is flat and bilaterally symmetric except that the longer methylene is twisted 79° out of the plane (C2 symmetry). The third singlet state, 21A1/1A1′ (3.88 eV), is also aD3h-symmetric equilibrium of three geometries; each is planar with one shorter C–C bond and two longer ones (C2ν symmetry).[1]
The next higher energy states are degenerate triplets, 13A1 and 23B2 (4.61 eV), with one excited electron; and a quintet state,5B2 (7.17 eV), with the p orbitals occupied by single electrons andD3h symmetry.
Trimethylenemethane was first obtained fromphotolysis of thediazo compound4-methylene-Δ1-pyrazoline with expulsion of nitrogen, in a frozen dilute glassy solution at −196 °C (77 K).[3]
It was also obtained from photolysis of3-methylenecyclobutanone, both in cold solution and in the form of a single crystal, with expulsion of carbon monoxide. In both cases, trimethylenemethane was detected byelectron spin resonance spectroscopy.[3]
Trimethylenemethane has been obtained also by treatingpotassium with 2-iodomethyl-3-iodopropene and isobutylene diiodide (IH
2C)2C=CH
2 in the gas phase. However the product quicklydimerizes to yield 1,4-dimethylenecyclohexane, and also2-methylpropene by abstracting two hydrogen atoms from other molecules (hydrocarbon orpotassium hydride).[4]
Three classes of compounds have been used to generate synthetically useful TMM-derivative reaction intermediates:diazenes, silyl-substitutedallylicacetates and methylenecyclopropenes. In the first case, bridged diazenes are used to avoid competitive closure to MCPs and dimerization reactions.[5] The latter case requires stabilization of a zwitterion, as with e.g. acetal1:[6]
Alternatively,palladium(0) ornickel(0) catalysts can stabilize the zwitterion:[7]
Silylated allylic acetates,carbonates and other substituted allyl compounds may form TMM synthons under palladium catalysis.[8]
A number oforganometallic complexes have been prepared, starting with Fe(C
4H
6)(CO)3, which was obtained by the ring-opening of methylenecyclopropane withdiiron nonacarbonyl (Fe
2(CO)9).[3] The same complex was prepared by thesalt metathesis reaction ofdisodium tetracarbonylferrate (Na
2Fe(CO)4) with1,1-bis(chloromethyl)ethylene (H2C=C(CH2Cl)2).[9] Related reactions give M(TMM)(CO)4 (M = Cr, Mo). The reaction leading to (TMM)Mo(CO)4 also gives Mo(C
8H
12)(CO)3 containing a dimerized TMM ligand.[9]
TMM complexes have been examined for their potential inorganic synthesis, specifically in the trimethylenemethane cycloaddition reaction (see§ Cycloaddition) with only modest success. One example is a palladium-catalyzed [3+2]cycloaddition of trimethylenemethane.[10][5]
![Structure of Ru(trimethylenemethane)(CO)3, viewed down C3 axis.[6]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aPETCEWtopView.png)
Unligated trimethylenemethanes are unstable, and rapidly close a ring tomethylidenecyclopropanes.[5] The problem is generally less severe for five-membered, cyclic TMMs due toring strain in the corresponding methylidenecyclopropanes.
Trimethylenemethane cycloaddition is the formal (3+2)annulation of trimethylenemethane (TMM) derivatives to two-atompi systems. Although TMM itself is too reactive and unstable to be stored,reagents which can generate TMM or TMMsynthonsin situ can be used to effectcycloaddition reactions with appropriateelectron acceptors. Generally, electron-deficientpi bonds undergocyclization with TMMs more easily than electron-rich pi bonds.[6]
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 polyunsaturatedesters.[6]
TMM'ssinglet andtriplet states exhibit differentreactivity andselectivity profiles.[5] A singlet (3+2) cycloaddition, when it is concerted, is believed to proceed underfrontier orbital control. When electron-rich TMMs are involved, theA orbital serves as the HOMO (leading to fused products if the TMM is cyclic). When electron-poor (or unsubstituted) TMMs are involved, theS orbital serves as the HOMO (leading tobridged products if the TMM is cyclic). Cycloadditions involving the triplet state arestepwise, and usually result in configurational scrambling in the two-atom component.[6]
Diazene-derived TMMs cyclize with analkenic acceptor to either fused or bridged products.[6] 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.[8]
Unless catalyzed by transition metals, methylidenecyclopropane opening is alsostereospecific with respect to alkene geometry, and exhibits high selectivity forendo products in most cases.
With catalysis, cyclization takes place in a stepwise fashion and does not exhibit stereospecificity. Rapidracemization ofchiral π-allyl palladium complexes occurs, and only moderatediastereoselectivity is observed in reactions of chiral allylic acetates. Chiral non-racemic alkenes, however, may exhibit moderate to high diastereoselectivity. The reaction is highlyregioselective, providing only the substitution pattern shown below regardless of the position of the R' group on the starting allylic acetate.
Chiral auxiliaries on the alkene partner have been used for stereoselective transformations. In the reaction ofcamphorsultam-derived unsaturatedamides, lower temperatures were needed to achieve high selectivities.[11]
In reactions of silyl-substituted allylic acetates, chiralsulfoxides can be used to enforce high diastereofacial selectivity.[12]
Carbonyl compounds may be used as the 2π component under the appropriate conditions. For instance, in the presence of anindium co-catalyst, the reactive 2π component of the cycloaddition below switches from the C-C to the C-O double bond.[13]
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 overketene acetals.[14]
Although1,3-dipolar cycloaddition is a useful method for the generation of five-memberedheterocyclic compounds, few methods exist to synthesize five-memberedcarbocyclic 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 oralkyne. When heated, cyclopropene acetals rearrange to vinylcarbenes, which can serve as the three-atom component in cycloadditions with highly electron-deficient alkenes.[15]Zinc homoenolates can also serve as indirect three-atom components, and undergo cyclization to cyclopentenones in the presence of an unsaturated ester.[16] Tandem intermolecular-intramolecular cyclization of homopropargylic radicals leads tomethylenecyclopropanes.[17]
