Reactions of Metal-Metal Quintuple Bonds with Alkynes: [2+2+2] and [2+2] Cycloadditions

Abstract

Benzene, C6H6, is one of the most important molecules in organic chemistry. The term aromaticity was developed from the study of benzene and has become an important and fundamental concept in contemporary chemistry since Kekul proposed a ringlike structure for benzene in 1865. Today, aromatic compounds are not only limited to carbocycles, but also extended to heterocycles containing maingroup elements. For example, one or two CH groups of benzene can be replaced by group 14 and 15 elements, such as silicon, nitrogen, and phosphorus, to give heterocyclic aromatics. Furthermore, an aromatic compound can be a transition-metal-containing organometallic species. Hoffmann et al. predicted that a metallabenzene could be possible by replacing one CH group with a transition-metal fragment. On the experimental side, since the recognition of the first metallabenzene, osmabenzene, in 1982, many metallabenzenes have been subsequently synthesized and structurally characterized. There are many ways to synthesize aromatic compounds. Among the methods, the [2+2+2] cycloaddition reaction of alkynes is a powerful tool to construct highly substituted homoand heterocyclic aromatic molecules. Such a reaction is thermally allowed, but it does not proceed at ambient temperature without transition-metal catalysts. It is however worth noting that Sekiguchi et al. have demonstrated a catalyst-free [2+2+2] cycloaddition reaction for the synthesis of 1,2-disilabenzene by reacting a kinetically stable disilyne RSi SiR (R= SiiPr[CH(SiMe3)2]2) and phenylacetylene at room temperature. The chemistry of transition-metal multiple bonding was recently reawakened by the recognition the first isolable quintuply bonded dimeric chromium complex Cr2Ar’2 (Ar’= C6H3-2,6-(C6H3-2,6-iPr2)2 by Power and co-workers. [10] Since then, several quintuply bonded dinuclear group VI metal complexes stabilized by nitrogen donors have been reported. Besides interesting bonding, the chromium derivatives have recently been shown to display novel reactions with AlMe3, [11f] N2O, RN3, [14] and P4. [15] Reactions of the chromium dimers with alkynes through a [2+2] cycloaddition process to give 1:1 adducts were recently explored as well. In light of the bonding analogy between the C C p components of alkynes and metal–metal d components in the quintuply bonded species (Figure 1), we became inter-