Reversibility of [2+2] Cycloaddition in Ruthenium Vinylidene Complexes Containing a Tethering C=C Double Bond
Date Issued
2007
Date
2007
Author(s)
Kuo, Yi-Chun
DOI
en-US
Abstract
A series of ruthenium vinylidene complexes {[Ru]=C=CHC(Ar)2CH2CH=CH2} BF4 containing terminal vinyl group were prepared. In chloroform, the cationic complex 3a ([Ru] = CpRu(dppp), Ar = Ph) gradually transforms into 4a at room temperature via a metathesis process. The reaction takes place as soon as 3a is dissolved in solution. With the presence of a tethering terminal vinyl group bound to the vinylidene ligand, the [2+2] cycloaddition of two C=C double bonds readily occurred. It seems that intricate steric or electronic demand is required for an intramolecular metathesis to take place. Analogous complexes 3a” and 4a” ([Ru] = CpRu(PPh3)2, Ar = para-C6H4OMe) containing methoxy group at the para-position of gem-diphenyl group were similarly obtained from protonation of the corresponding acetylide complexes via formation of vinylidene intermediate. But only complex 3a’ ([Ru] = CpRu(PPh3)2, Ar2 = fluorenyl) is stable in solution, and decomposed to unidentified complex when heated to reflux in THF. Deprotonation of complex 4a in the presence of base gives neutral acetylide complex 6a. Addition of electrophiles at Cβ of σ-alkynyl complexes has been described as one of the most versatile entries into vinylidene derivatives. Thus, the alkylation of 6a with MeOTf causes formation of the cationic ruthenium vinylidene complex 7a. On the other hand, at room temperature in chloroform solution, complex 3b with an additional methyl group spontaneously undergoes a cyclization process to give complex 5, which contains coordinated cyclic allene ligand. Interestingly, in chloroform solution complex 7a follows the same route as that of 3b. Proposed pathways for the transformation from 7a to 5b are described. However, the 13C labeling experiments show that in complex 5b the two labeled carbon atoms are directly bonded. This result indicates that the transformation does not proceed via the pathway of simple cyclization followed by 1,3-hydrogen shift. We could not experimentally distinguish differences between these two pathways. In addition, protonation of the acetylide complex with a tethering terminal alkynyl group [Ru]C ¡CC(Ar)2CH2C≡CH (2c) generates the vinylidene complex {[Ru]=C=CHC(Ar)2CH2C≡CH}BF4 (3c) which again undergoes a transformation to give {[Ru]=C=CHCH2C(Ar)2C≡CH}BF4 (4c) possibly via a π-coordinated alkynyl complex followed by hydrogen and metal migration. No similar transformation is observed for the analogues 3c’ and 3c”. With an extra methylene group, complex {[Ru]=C=CHC(Ar)2CH2CH2CH=CH2}BF4 (3d, 3d’ and 3d”) are stable. The presence of a gem-diphenylmethylene moiety at the vinylidene ligand with the appropriate terminal vinyl or alkynyl group along with the proper steric environment implements such a novel reactivity in the ruthenium vinylidene complexes.
Subjects
釕金屬丙二烯錯合物
[2+2]環化加成反應
Ruthenium vinylidene
[2+2] cycloaddition
Type
thesis
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