Trimethylenemethane

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Trimethylenemethane
Chemdg trimethylenemethane 2rad.svg
Trimethylenemethane, average of three configurations. Formally, the radial bonds have valency ⁠4/3⁠. Each terminal carbon has ⁠2/3⁠ of an unfilled valence bond.
Names
Preferred IUPAC name
2-Methylidenepropane-1,3-diyl
Other names
Trimethylenemethane biradical; Trimethylenemethane diradical
Identifiers
3D model (JSmol)
PubChem CID
  • InChI=1S/C4H6/c1-4(2)3/h1-3H2
    Key: MOWBWPSGCTXGKR-UHFFFAOYSA-N
  • [CH2-]C(=C)[CH2+]
  • [CH2][C]([CH2])[CH2]
Properties
C4H6
Molar mass 54.092 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Trimethylenemethane (often abbreviated TMM) is a chemical compound with formula C
4H
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 C
4H
8 with two hydrogen atoms removed from the terminal methyl groups.

Contents

Structure

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 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:

Chemdg trimethylenemethane 2rad R0.svg Chemdg trimethylenemethane 2rad R1.svg Chemdg trimethylenemethane 2rad R2.svg

It has a triplet ground state (3A2′/3B2), and is therefore a diradical 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 three singlet 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 a D3h-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 a D3h-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 and D3h 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 −196 °C (77 K). [3]

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. [3]

Trimethylenemethane Trimethylenemethane.png
Trimethylenemethane

Trimethylenemethane has been obtained also by treating potassium with 2-iodomethyl-3-iodopropene and isobutylene diiodide (IH
2C)2C=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). [4]

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. [5] The latter case requires stabilization of a zwitterion, as with e.g. acetal 1: [6]

TMMMech2.png

Alternatively, palladium(0) or nickel(0) catalysts can stabilize the zwitterion: [7]

TMMMech3.png

Silylated allylic acetates, carbonates and other substituted allyl compounds may form TMM synthons under palladium catalysis. [8]

Organometallic chemistry

A number of organometallic complexes have been prepared, starting with Fe(C
4H
6)(CO)3, which was obtained by the ring-opening of methylenecyclopropane with diiron nonacarbonyl (Fe
2(CO)9). [3] The same complex was prepared by the salt metathesis reaction of disodium tetracarbonylferrate (Na
2Fe(CO)4) with 1,1-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 in organic 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]

Organic reactions

Unligated trimethylenemethanes are unstable, and rapidly close a ring to methylidenecyclopropanes. [5] 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. [6]

TMMGen.png

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. [6]

TMMScope2.png

TMM's singlet and triplet states exhibit different reactivity and selectivity profiles. [5] 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. [6]

TMMOrbs.png

Diazene-derived TMMs cyclize with an alkenic 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]

TMMMech1.png

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.

TMMMech4.png

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. [11]

TMMStereo1.png

In reactions of silyl-substituted allylic acetates, chiral sulfoxides can be used to enforce high diastereofacial selectivity. [12]

TMMStereo2.png

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. [13]

TMMScope4.png

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. [14]

TMMScope5.png

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. [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 to methylenecyclopropanes. [17]

TMMAlt.png

References

  1. 1 2 3 Slipchenko Lyudmila V., Krylov Anna I. (2003). "Electronic structure of the trimethylenemethane diradical in its ground and electronically excited states: Bonding, equilibrium geometries, and Vibrational frequencies". Journal of Chemical Physics. 118 (15): 6874–6883. Bibcode:2003JChPh.118.6874S. doi:10.1063/1.1561052. S2CID   4204676.
  2. ↑ C. A. Coulson (1948), Journal de Chimie Physique et de Physico-Chimie Biologique, volume 45, page 243. Cited by Slipchenko and Krylov (2003)
  3. 1 2 3 4 Paul Dowd (1972). "Trimethylenemethane". Accounts of Chemical Research. 5 (7): 242–248. doi:10.1021/ar50055a003.
  4. ↑ Skell Philip S., Doerr Robert G. (1967). "Trimethylenemethane". Journal of the American Chemical Society. 89 (18): 4688–4692. Bibcode:1967JAChS..89.4688S. doi:10.1021/ja00994a020.
  5. 1 2 3 4 Berson, J. A. Acc. Chem. Res.1978, 11, 446.
  6. 1 2 3 4 5 6 Nakamura, E.; Yamago, S.; Ejiri, S.; Dorigo, A. E.; Morokuma, K. J. Am. Chem. Soc.1991, 113, 3183.
  7. ↑ Binger, P.; Büch, H. M. Top. Curr. Chem.1987, 135, 77.
  8. 1 2 Trost, B. M. Angew. Chem. Int. Ed. Engl.1986, 25, 1.
  9. 1 2 J. S. Ward & R. Pettit (1970). "Trimethylenemethane complexes of iron, molybdenum, and chromium". Journal of the Chemical Society D (21): 1419–1420. doi:10.1039/C29700001419.
  10. ↑ Barry M. Trost (1979). "New conjunctive reagents. 2-Acetoxymethyl-3-allyltrimethylsilane for methylenecyclopentane annulations catalyzed by palladium(0)". Journal of the American Chemical Society. 101 (21): 6429–6432. Bibcode:1979JAChS.101.6429T. doi:10.1021/ja00515a046.
  11. ↑ Binger, P.; Schäfer, B. Tetrahedron Lett.1988, 29, 529.
  12. ↑ Chaigne, F.; Gotteland, J.-P.; Malacria, M. Tetrahedron Lett.1989, 30, 1803.
  13. ↑ Trost, B. M.; Sharma, S.; Schmidt, T. J. Am. Chem. Soc.1992, 114, 7903.
  14. ↑ Yamago, S.; Nakamura, E. J. Org. Chem.1990, 55, 5553.
  15. ↑ Boger, D. L.; Brotherton, C. E. J. Am. Chem. Soc.1986, 108, 6695.
  16. ↑ Crimmins, M. T.; Nantermet, P. G. J. Org. Chem.1990, 55, 4235.
  17. ↑ Curran, D. P.; Chen, M.-H. J. Am. Chem. Soc.1987, 109, 6558.
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