Oxygen Induced Cyclization of the Ruthenium Vinylidene Complex Prepared from 1,6-Enyne Containing Cyclohexyl group
Date Issued
2011
Date
2011
Author(s)
Liu, Sui-Yu
Abstract
A series of 1,6-enynes 1a-c containing pendant olefinic groups, which have either none or two methyl substituents, were prepared. Then, 1a-1c were treated with CpRu(PPh3)2Cl and CpRu(dppe)Cl to yield various metal complexes. Interestingly, these complexes display novel reactivity under different reaction conditions.
Treatment of the acetylide complexes 4a with HBF4 in MeOH at 0oC, caused surprising transformation. Protonation of 4a is followed by methoxide addition to generate the methoxycarbene complex 5a. The following mechanism is proposed: Protonation of the acetylide complex formed the vinylidene complex, and then MeOH eliminated. The presence of MeOH as solvent brings about addition of a methoxide group at Cα to give the α-methoxycarbene complex 5a.
Astonishingly, with the presence of two hydrogen at the terminal carbon, complex 2a underwent cyclization in air to give the vinylidene complex 6a, and the characteristic of 6a is distinct from that of the ruthenium vinylidene complex 2a, inspiring us to attempt more experiments. We characterized complex 6a by 31P, 1H, 13C, 1H-1H COSY, 1H-13C HSQC and 1H-13C HMBC NMR spectroscopy. Oxygen molecule reacts with 2a to yield the cyclized vinylidene complex 6a. For the formation of 6a, the ruthenium metal center may then serve to assist this [2+2+2] cyclization leading to B. PPh3 insertion into the dioxygen bond of the endoperoxide B gives the intermediate C. Addition of water at the phosphonium ion gives OPPh3 and the intermediate E, finally elimination of H2O results in the final product 6a’.
Moreover, we get the 1,3-migration product 7a and 7a’ by protonation of 6a, and then base induced reaction of complexes 7a and 7a’ leads to the neutral ruthenium acyl complex 8a. Finally, the organic compound 9a with an aldehyde group was obtained from the reaction of 8a with HCl.
Besides, the reaction of 1,6-enyne 1b also proceeds via a cyclization with both CpRu(PPh3)2Cl or CpRu(dppe)Cl by a 5-exo-dig pathway to give the carbene complexes 2b and 2b’, respectively.
Subjects
Rutheniun
vinylidene
methoxycarbene
cyclization
acyl
oxidative addition
Type
thesis
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