Trimethylenemethane complexes are metal complexes of the organic compound trimethylenemethane. Several examples are known, and some have been employed in organic synthesis.[1]

History

The synthesis of cyclobutadieneiron tricarbonyl pointed to the possibility complexes of other organic ligands that are elusive in their free state. Trimethylenemethane (TMM) has a natural connection to cyclobutadiene, and, in 1966, Emerson and co-workers reported the first trimethylenemethane (TMM) transition metal complex, (CO)3FeC(CH2)3, which became the starting point for extensive studies.

Synthesis

Figure 1

Generally speaking, trimethylenemethane complexes are synthesized in the following four ways: (A) the dehalogenation of α, α'-dihalosubstituted precursors, (B) the thermal extrusion of XY (XY = HCl, Br2, and CH4,) from η3-methylallyl complexes, (C) the ring opening of alkylidenecyclopropanes, and (D) the elimination of Me3SiX [X = OAc, Cl, OS(O)2Me] from functionalized allylsilanes (Figure 1).

Dehalogenation of α, α'-dihalosubstituted precursors

Figure 2

η4-[C(CH2)3]Fe(CO)3, the first trimethylenemethane metal complex to be reported, was obtained from the reaction of 3-chloro-2-chloromethylprop-1-ene with Fe2(CO)9 or Na2[Fe(CO)4].[2] Followed by this result, a number of substituted trimethylenemethane iron complexes have been prepared.[3][4][5]

The thermal extrusion from η3-methylallyl complexes was reported by Emerson.The iron allyl complex, obtained from the reaction of 3-chloro-2-methylprop-1-ene with [Fe2(CO)9], decomposed on heating to afford the iron trimethylenemethane complex.[6]

Ring opening of alkylidenecyclopropanes

Stereochemistry of the ring opening of methylenecyclopropanes by Fe(0).[7]

In the presence of [Fe2(CO)9], the ring opening of 2-substituted methylenecyclopropanes leads to the formation of various η4-trimethylenemethane complexes containing different functional groups, such as (R1 = H, R2 = Ph), (R1 = Me, R2 = Ph), (R1 = R2 = Ph), and (R1 = H, R2 = CH=CH2).[8] The stereochemistry has been elucidated by deuterium-labeling experiments.

Elimination of Me3SiX [X = OAc, Cl, OS(O)2Me] from functionalized allylsilanes

[[Pd(PPh3)4]] is a precursor to highly reactive η3-trimethylenemethane complexes.[1] Allylsilanes oxidatively add to some low-valent d8 complexes resulting in the formation of an η1-allyl complexes, followed by the formation of an η3-allyl complex, and finally elimination of Me3SiX to yield the η4-trimethylenemethane complex. The isolation of the proposed intermidate further confirmed the mechanism.[9]

IrCl(CO)(PPh3)2 + CH2=C(CH2Cl)(CH2tms) → η4-[C(CH2)3]IrCl(PPh3)(CO) + tmsCl + PPh3 (Ph = C6H5)

Structure and bonding

Structure of η4-C(CH2)3]Fe(CO)3.

According to gas phase electron diffraction, η4-C(CH2)3]Fe(CO)3 adopts a staggered conformation about the iron center. The ligands, which include carbonyl and a trigonal-pyramidal trimethylenemethane, are arranged in the usual umbrella-type configuration. The central carbon of the trimethylenemethane ligand is closer to the iron center compared to the outer methylene carbons. This was confirmed by the Fe-C(central) distance measuring 1.94(1) Å, while the Fe-CH distances were measured at 2.12 Å.[10] Moreover, this result has also been confirmed by X-ray diffraction and vibrational spectrum.[11]

The primary bonding interaction occurs between the 2e set of the Fe(CO)3 fragment and e" on the trimethylenemethane ligand. However, if the metal-trimethylenemethane axis is rotated by 60° into an eclipsed geometry, the interaction between 2e and e" is minimized, which results in an increase in the energy of the HOMO in the complex, which is a significant factor that provides a barrier to rotation, as shown in Figure 6b.

Extended Huckel calculations give a barrier of 87 KJ mol−1 using a planar trimethylenemethane ligand.[12] Introducing a puckered conformation to the trimethylenemethane ligand, which resembles the experimental geometry, leads to an increase in the calculated barrier to 98.6 kJ mol−1. This puckering induces mixing of s character into e" orbitals, causing a more pronounced orientation toward the metal center. Consequently, the overlap between e" and 2e orbitals is enhanced. The degree of puckering, characterized by θ, falls within the range of 12°.[13] The mixing of s character into e" also results in the H-C-H plane being tipped away from the metal. The angle β, between C-1 and C-2 and the plane H-C-H, is typically about 15°.

Reactions

Trimethylenemethane complexes undergo a wide variety of reactions including those with electrophiles, nucleophiles as well as redox reactions.

η4-C(CH2)3]Fe(CO)3 adds hydrogen chloride to yield η3-CH3C(CH2)]Fe(CO)3. Substituted trimethylenemethane iron complexes, on the other hand, react with strong acids to produce cross-conjugated dienyl iron cations and η4-diene complexes.[14] η4-C(CH2)3]Mo(CO)2(C5H5)+ add nucleophiles to give charge-neutral η3-allyl complexes.[15]

[Fe{η4-C(CH2)3(L)3] (L = PMe or PMe2Ph) (complex 4) reacts with silver trifluoromethanesulfonate to give the 17-electron cation (Figure 7).[5]

References

  1. 1 2 Trost, Barry M.; Mata, Guillaume (2020). "Forging Odd-Membered Rings: Palladium-Catalyzed Asymmetric Cycloadditions of Trimethylenemethane". Accounts of Chemical Research. 53 (7): 1293–1305. doi:10.1021/acs.accounts.0c00152. PMID 32525684. S2CID 219606826.
  2. ↑ Emerson, G. F.; Ehrlich, K.; Giering, W. P.; Lauterbur, P. C. (July 1966). "Trimethylenemethaneiron Tricarbonyl". Journal of the American Chemical Society. 88 (13): 3172–3173. doi:10.1021/ja00965a077. ISSN 0002-7863.
  3. ↑ Ehrlich, Kenneth; Emerson, George F. (April 1972). "Trimethylenemethane iron tricarbonyl complexes". Journal of the American Chemical Society. 94 (7): 2464–2470. doi:10.1021/ja00762a045. ISSN 0002-7863.
  4. ↑ Bonazza, Benedict R.; Lillya, C. Peter; Magyar, Elaine S.; Scholes, Gary (July 1979). "(Cross-conjugated dienyl)tricarbonyliron cations. 2. 4-Methyl derivatives". Journal of the American Chemical Society. 101 (15): 4100–4106. doi:10.1021/ja00509a016. ISSN 0002-7863.
  5. 1 2 Grosselin, Jean Michel; Le Bozec, Hubert; Moinet, Claude; Toupet, Loic; Dixneuf, Pierre H. (May 1985). "Electron-rich, hydrocarbon-metal complexes: synthesis and reversible one-electron oxidation. X-ray structure of a 17-electron iron cation". Journal of the American Chemical Society. 107 (9): 2809–2811. doi:10.1021/ja00295a045. ISSN 0002-7863.
  6. ↑ Ehrlich, Kenneth; Emerson, George F. (April 1972). "Trimethylenemethane Iron Tricarbonyl Complexes". Journal of the American Chemical Society. 94 (7): 2464–2470. doi:10.1021/ja00762a045. ISSN 0002-7863.
  7. ↑ Pinhas, Allan R.; Carpenter, Barry K. (1980-01-01). "Frontier molecular orbital control of stereochemistry in organometallic electrocyclic reactions". Journal of the Chemical Society, Chemical Communications (1): 15–17. doi:10.1039/C39800000015. ISSN 0022-4936.
  8. ↑ Noyori, R.; Nishimura, T.; Takaya, H. (1969-01-01). "Reaction of methylenecyclopropanes with enneacarbonyldi-iron: a new route tricarbonyltrimethylenemethaneiron complexes". Journal of the Chemical Society D: Chemical Communications (3): 89. doi:10.1039/C29690000089. ISSN 0577-6171.
  9. ↑ Jones, Michael D.; Kemmitt, Raymond D. W.; Platt, Andrew W. G. (1986-01-01). "Trimethylenemethane metal complexes. Part 1. Synthesis of ruthenium, osmium, rhodium, and iridium complexes". Journal of the Chemical Society, Dalton Transactions (7): 1411–1418. doi:10.1039/DT9860001411. ISSN 1364-5447.
  10. ↑ Mousavi, Masoumeh; Frenking, Gernot (2013-12-15). "Bonding analysis of trimethylenemethane (TMM) complexes [(CO)3M–TMM] (M = Fe, Ru, Os, Rh+). Absence of expected bond paths". Journal of Organometallic Chemistry. Theory and Mechanistic Studies in Organometallic Chemistry. 748: 2–7. doi:10.1016/j.jorganchem.2013.03.047. ISSN 0022-328X.
  11. ↑ Churchill, Melvyn R.; Gold, Karen (1968-01-01). "The molecular configuration of (phenyltrimethylenemethane)tricarbonyliron". Chemical Communications (12): 693–694. doi:10.1039/C19680000693. ISSN 0009-241X. S2CID 95797623.
  12. ↑ Albright, Thomas A.; Hofmann, Peter; Hoffmann, Roald (November 1977). "Conformational preferences and rotational barriers in polyene-ML3 transition metal complexes". Journal of the American Chemical Society. 99 (23): 7546–7557. doi:10.1021/ja00465a025. ISSN 0002-7863.
  13. ↑ Yasuda, Norihiko; Kai, Yasushi; Yasuoka, Noritake; Kasai, Nobutami; Kakudo, Masao (1972-01-01). "X-Ray molecular structure of allene-trimer complexes of hexacarbonyldi-iron". Journal of the Chemical Society, Chemical Communications (3): 157–158. doi:10.1039/C39720000157. ISSN 0022-4936.
  14. ↑ Horn, Keith A.; Grossman, Robert B.; Whitenack, Anne A. (1987-10-06). "The palladium catalyzed synthesis of substituted phenylethynylpentamethyldisilanes and phenylethynylheptamethyltrisilanes". Journal of Organometallic Chemistry. 332 (3): 271–278. doi:10.1016/0022-328X(87)85094-5. ISSN 0022-328X.
  15. ↑ Allen, Stephen R.; Barnes, Stephen G.; Green, Michael; Moran, Grainne; Trollope, Lynda; Murrall, Nicholas W.; Welch, Alan J.; Sharaiha, Dima M. (1984-01-01). "Reactions of co-ordinated ligands. Part 30. The transformation of methylenecyclopropanes into cationic η4-trimethylenemethanemolybdenum complexes, reactions with nucleophilic reagents, and the molecular structure of [Mo{η4-C(CH2)3}(CO)2(η-C5Me5)][BF4]". Journal of the Chemical Society, Dalton Transactions (6): 1157–1169. doi:10.1039/DT9840001157. ISSN 1364-5447.
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