Metal-Catalyzed Rearrangement of Cyclopropenes to Allenes

Abstract

A novel transition-metal-catalyzed rearrangement of silylated cyclopropenes to the corresponding allenes is described. The presence of both the trimethylsilyl group on the cyclopropene and the platinum catalyst are crucial for this rearrangement.

Theoretical Insight into PtCl2-Catalyzed Isomerization of Cyclopropenes to Allenes

“…According to the suggestion of Lee and coworkers, 6 pathway I proceeds via the cleavage of the C2−C3 bond in 2 to form open-chained zwitterionic intermediate 3, which then undertakes the 1,2-silyl shift, giving the final allene product P. Our calculations indicate that the C2−C3 bond cleavage of 2 takes place via ring-opening transition state TS3 with an energy requirement of 18.8 kcal/mol. The optimized geometry of 3 shows that the Pt−C2 bond distance is 1.858 Å, which is remarkably shorter than that in 2 (2.133 Å).…”

“…However, for the present system, we find that the direct dissociation process requires an energy of 35.7 kcal/mol (pathway (a) in Figure 4), which appears to be too high to explain the observed fact that the reaction was carried out under mild conditions. 6 So we want to know what the reasonable product formation mechanism is. Here, we propose two potential pathways (pathways (b) and (c) in Figure 4) based on the geometry of IM7 (Figure 2), where the PtCl 2 unit still keeps the Cl−Pt−Cl linear disposition like that in the Pt-π-coordinated complex 2.…”

“…Role of Trimethylsilyl Substituent. In Lee's experiment, 6 an intriguing observation is presented, that is, upon replacement of the trimethylsilyl substituent by a hydrogen atom, the cyclopropene does not provide any allene, which is in strong contrast to the formation of allene in excellent yield from the substrate with the trimethylsilyl substituent under the same conditions. So, we also performed detailed calculations for allene formation from the H-substituted cyclopropene, from which we expect to reveal the role of the trimethylsilyl substituent in the reaction.…”

“…Calculated free energy profiles in dichloromethane solution for the formation of the product precursor IM4 along the pathways proposed by Lee et al6 The relative free energies are given in kcal/mol.…”

[(NHC)NiIIH]‐Catalyzed Cross‐Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)NiII]‐Assisted C−C Rearrangement

“…Other than the use of traditional methods, [1] catalysts like Au, [2] Rh, [3] Cu, [4] Zn, [5] Pt, [6] and Pd [7] are particularly useful in ring-opening reactions of 1. By controlling the (NHC)Ni II Hr elative insertion reactivity with cyclopropene and alkyne,abroad scope of cyclopentadienes was obtained with highly selectively.The structural features of the new (NHC)Ni II catalyst were important for the success of the reaction.…”