In 1956, Longuet-Higgins and Orgel predicted the existence of transition-metal cyclobutadiene complexes, in which the degenerate eg orbital of cyclobutadiene has the correct symmetry for π interaction with the dxz and dyz orbitals of the proper metal. The compound was synthesized three years after the prediction and it serves as a beautiful case of theory before experiment. This successful attempt opens the door for the formation of novel compounds containing other organic ligands which in their free state are highly reactive molecules. Of all those reactive molecules, trimethylenemethane (TMM) has the most natural derivation from the cyclobutadiene complexes and in 1966, Emerson and co-workers reported the first trimethylenemethane (TMM) transition metal complex, (CO)3FeC(CH2)3, which became the starting point of the legends of trimethylenemethane complexes. Some good reviews on this aspect could be served as further resources for this topic.
Synthesis of trimethylenemethane complexes
Generally speaking, the trimethylenemethane complexes could be 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
Tris(carbonyl)-η4-trimethylenemethaneiron(0) was the first trimethylenemethane metal complex to be synthesized and was obtained from the reaction of 3-chloro-2-chloromethylprop-1-ene with [Fe2(CO)9,] or Na2[Fe(CO)4]. Followed by this result, A number of substituted trimethylenemethane iron complexes have been prepared by similar methods to those used to prepare tris(carbonyl)-η4-trimethylenemethaneiron(0).
Thermal extrusion from η3-methylallyl complexes
This reaction was first 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.
Ring opening of alkylidenecyclopropanes
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). The most interesting part of this strategy is the stereochemistry and it had been elucidated by stereospecific deuterium labeling experiments. (Figure 3)
Elimination of Me3SiX [X = OAc, Cl, OS(O)2Me] from functionalized allylsilanes
The use of [Pd(PPh3)4] to facilitate the elimination of Me3SiX (X=OAc) and generate a highly reactive η3-trimethylenemethane palladium was first reported by Trost and Chan. Subsequently, it was discovered that the reaction of allylsilanes with a d8 metal center proceeds via oxidative addition to the metal, resulting in the formation of an η1-allyl complex, followed by the formation of an η3-allyl complex, and finally elimination of Me3SiX to yield the η4-trimethylenemethane complex (Figure 4). The isolation of the proposed intermidate further confirmed the mechanism.
Structure and bonding
Structure conformation
In a study using gas phase electron diffraction, it was determined that tris(carbonyl)-η4-trimethylenemethaneiron 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. These findings are presented in Figure 5. Moreover, this result has also been confirmed by X-ray diffraction and vibrational spectrum.