The chemistry of metallabenzynes has attracted considerable attention in the last two decades, since the first report of an osmabenzyne by our group in 2001. However, all the reported metallabenzynes contain one or more substituent(s) on the metallacycle. In this work, we have successfully synthesized the first example of parent osmabenzyne, namely, O_s(≡CCH=CHCH=CH)(PPh₃)₂Cl₂ (2-29), with no substituents on the osmacycle. Without the influence from substituents, we have studied the chemical reactivity of 2-29 with electrophiles and oxidants. The results showed that the regioselectivity of these reactions is electronically controlled by the compositions of the HOMO and LUMO of the osmabenzyne.

The chemical reactivity of polycyclic osmaarynes has rarely been reported previously. We have...[ Read more ]

The chemistry of metallabenzynes has attracted considerable attention in the last two decades, since the first report of an osmabenzyne by our group in 2001. However, all the reported metallabenzynes contain one or more substituent(s) on the metallacycle. In this work, we have successfully synthesized the first example of parent osmabenzyne, namely, O_s(≡CCH=CHCH=CH)(PPh₃)₂Cl₂ (2-29), with no substituents on the osmacycle. Without the influence from substituents, we have studied the chemical reactivity of 2-29 with electrophiles and oxidants. The results showed that the regioselectivity of these reactions is electronically controlled by the compositions of the HOMO and LUMO of the osmabenzyne.

The chemical reactivity of polycyclic osmaarynes has rarely been reported previously. We have now studied the reactivity of the β-osmanaphthalyne O_s(C₉H₅TMS)(PPh₃)₂Cl₂ (1-55). Its desilylation can be achieved by addition of acids or fluoride to produce an unsubstituted osmanaphthalyne, which is reactive towards water to generate an osmaindene species. Electrophilic substitution reactivity of 1-55 was also studied. Bromination of 1-55 regioselectively occurs at the C2- and C8-positions, which is controlled by its HOMO electron density distribution. Chlorination of 1-55 with one equivalent of chlorine also occurs at the C2-position; while further reaction with another equivalent of chlorine gives a dichlorinated α-osmanaphthalyne with the formal carbyne atom shifted to the C5 position. Nitrosylation of 1-55 gave an oxime-coordinated carbene complex, which was believed to be derived from a C2-nitrosyl-substituted osmanaphthalyne intermediate.

The reactivity of the tricyclic osmaanthracyne O_s(C₁₃H₇TMS)(PPh3)2Cl2 (1-57) was also studied. Its desilylation can also be accomplished by acids or fluoride, and the resulting unsubstituted osmaanthracyne is also reactive towards water. Electrophilic substitution reactions of 1-57 with chlorine and bromine gave C2- and C6-substituted species, among them a dichloro-substituted complex was characterized structurally and spectroscopically. Compared with the organic analog of anthracene, 1-57 is less reactive towards cycloaddition with dienophiles. By substituting its phosphine ligands with more σ-donating PBu₃ ligand, the resulting osmaanthracyne can react with EtO₂CC≡CCO₂Et to give the expected cycloaddition product.

Higher order polycyclic osmaarynes with more than three fused aromatic cycles have not been reported. We have attempted the syntheses of [2-(C≡CTMS)-3-(CH₂PPh₃)-anthracene]Br for the synthesis of osmatetracyne; and the quinone-based phosphonium salt [2-(C≡CTMS)-3-(CH₂PPh₃)-1,4-quinone]BPh₄ for the synthesis of polycyclic osmaarynes through Diels–Alder cycloaddition reactions.