Trimethylenemethane Metal Complexes

Cobalt- and Silver-Promoted Methylenecyclopropane Rearrangements

The rate of the methylenecyclopropane rearrangement is enhanced by an alkyne-Co₂(CO)₆ complex bonded to the para position of a benzene ring. This is explained by a stabilizing effect on the transition state leading to the biradical intermediate. Computational studies indicate that the benzylic-type biradical intermediate is stabilized by a delocalization mechanism, where spin is delocalized onto the two cobalt atoms. Silver cation also enhances the rate of the methylenecyclopropane rearrangement. Computational studies suggest that silver cation can also stabilize a benzylic radical by spin delocalization involving silver. In the case of the silver-promoted reactions, the rate enhancements in a series of aryl-substituted methylenecyclopropanes correlate with σ⁺ values. The question remains open as to whether the silver-catalyzed methylenecyclopropane rearrangement proceeds via an argento-stabilized biradical or whether the reaction involves an argento-substituted allylic cation.

Planar Trimethylenemethane Dianion Chemistry of Lanthanide Metallocenes: Synthesis, Structure, Density Functional Theory Analysis, and Reactivity of [(C5Me5)2Ln]2[μ-η3:η3-C(CH2)3] Complexes

The unusual formation of planar trimethylenemethane (TMM) dianion complexes of lanthanide metallocenes, [(C5Me5)2Ln]2[mu-eta3:eta3-C(CH2)3] (Ln = Sm, 1; La, 2; Pr, 3; Nd, 4; Y, 5) has been examined by synthesizing examples with metals from La to Y to examine the effects of radial size on structure and to provide closed shell examples for direct comparison with density functional theory (DFT) calculations. Synthetic routes to 1-4 have been expanded from the [(C5Me5)2Ln][(mu-Ph)2BPh2]/neopentyl lithium reaction involving beta-methyl elimination to a [(C5Me5)2Ln][(mu-Ph)2BPh2]/isobutyl lithium route. The synthetic pathways are sensitive to metal radius. To obtain 5, the methylallyl complex, (C5Me5)2Y[CH2C(Me)CH2], 6, was synthesized and treated with [(C5Me5)2YH]x. In the Lu case, the neopentyl complex [(C5Me5)2LuCH2C(CH3)3]x, 7, was isolated instead of the TMM product. X-ray crystallography showed that the metrical parameters of the planar TMM dianions in each complex are similar. The structural data have been compared with DFT calculations on the closed-shell lanthanum and lutetium complexes that suggest only limited covalent interactions with the lanthanide ion. Benzophenone (2 equiv) reacts with 1 to expand the original four-carbon TMM skeleton to a dianionic bis(alkoxide) ligand containing a symmetrically substituted C=CH2 moiety in [(C5Me5)2Sm]2[mu-(OCPh2CH2)2C=CH2], 8. In this reaction, the TMM complex reacts as a bifunctional bisallylic reagent that generates an organic framework containing a central vinyl group.

Formation of a Bridging Planar Trimethylenemethane Dianion from a Neopentyl Precursor via Sequential β-Alkyl Elimination and C−H Activation

The reaction of neopentyllithium, Me3CCH2Li, with [(C5Me5)2Sm][(mu-Ph)2BPh2], 1, was investigated as a route to the unsolvated alkyl, [(C5Me5)2Sm(CH2CMe3)]x, and found to generate the first f element trimethylenemethane dianion complex, [(C5Me5)2Sm]2[mu-eta3:eta3-C(CH2)3], 2. Formation of the [C4H6]2- trimethylenemethane ligand from the [C5H11]1- neopentyl precursor can be explained by a combination of a beta-methyl elimination reaction to form isobutene and [(C5Me5)2SmMe]3, 3, with subsequent C-H activation reactions. This sequence has been modeled in several ways, including the synthesis of 2 from reactions of 3 with CH2=CMe2 and 3 with the 2-methylallyl complex, (C5Me5)2Sm[CH2C(Me)CH2], 4.

Mechanisms of the Cycloaddition Reaction of Methylenecyclopropane-Palladium and Oxa- and Azatrimethylenemethane-Palladium Complexes with Olefins

Mechanisms of palladium-catalyzed cycloaddition reactions of methylenecyclopropane with olefins and those of oxatrimethylenemethane (OTMM) and azatrimethylenemethane (ATMM) complexes with olefins have been studied by applying the ab initio molecular orbital method. Assuming an intramolecular coupling mechanism, we have examined some model complexes, Pd(eta(2)-methylenecyclopropane)(ethylene)(PH(3)) and Pd(eta(3)-OTMM or eta(3)-ATMM)(ethylene)(PH(3)), to determine the transition state structures. The coupling of methylenecyclopropane, OTMM, and ATMM with ethylene on the metal center takes place in two steps. In the reaction of methylenecyclopropane, the first step involves an opening of the cyclopropane ring promoted by an attack of ethylene. The methylenecyclopropane moiety has a pi-allylic form at the first transition state which leads to a metallacyclic intermediate. The first transition state of the reactions of OTMM and ATMM complexes looks very similar to that of the methylenecyclopropane complex, having pi-allylic coordinations. The two paths are separated in the second step. A [3 + 2] addition product is obtained by a reductive elimination from the intermediate metallacycle, whereas a prototropic shift followed by a reductive elimination affords a [2 + 1] addition product. It is the energetics of the second stage starting from the metallacycles that differentiates the reactions of the OTMM and ATMM complexes from the reaction of the methylenecyclopropane complex. The relative stabilities of various isomeric forms of these complexes have also been studied.