Intramolecular Diels-Alder Reactions Mediated byuthenium Vinylidene Complexes
Date Issued
2008
Date
2008
Author(s)
Chang, Shu-Hao
Abstract
The reactivity of a series of cationic ruthenium vinylidene complexes containing the cyclopentadienyl phosphine ligand are studied. Cationic metal vinylidene complexes [Ru]=C=CHCH(R)CH2CH=CH2 ([Ru] = CpRu(PPh3)2; 2a, R = para-C6H4-Ph; 2b, R = para-C6H4-F; 2c, R = naphthyl; 2d, R = ortho- C6H4-OCH2CH=CH2; 2e, R = ortho-C6H4-OCH2C6H5Ph; 2f, R = 9-Cl-10-anthracenyl) are prepared from the reaction of (η5-C5H5)(PPh3)2Ru-Cl with different 1,5-enyne. Complex 2a in chloroform transforms into [Ru]=C=CHCH2CH(R)CH=CH2 3a via a metathesis process giving the mixture of 2a and 3a in a ratio approximately 2:1. Electron withdrawing group on complex 2b makes metathesis process faster than that in 2a with an electron donating group. But the ratio of 2b and 3b didn’t changed. Complex 2a with only one phenyl group on Cγ has less steric effect than the vinylidene complex [Ru]=C=CHC(Ph)2CH2CH=CH2 having two phenyl groups on Cγ which could transforms into the metathesis product completely in our previously report. Therefore, the metathesis reaction was affected by the steric effect. Complex 2f (R = 9-Cl-10-anthracenyl) reacts with sodium methoxide to yield [Ru]-C≡C-CH(R)CH2CH=CH2 (4f). Complex 4f is treated with allyl iodide at room temperature giving the complex 6f via an intramolecular Diels-Alder process. This intramolecular Diels-Alder reaction provides a direct approach to complex polycyclic compounds under remarkably mild conditions. This reaction also gives very high diastereoselectivity. Only RS and SR form can be obtained. This intramolecular Diels-Alder reaction takes place only between the allyl group on Cß and the anthracenyl group. However, no intramolecular Diels-Alder reaction could be observed between the allyl group on Cγ and the anthracenyl group. It is interesting to explore how the intramolecular process is affected by the length of the spacer linker. With various length of the spacer linker, complexes 3fa and 3fd could be synthesized by the allenylidene complex with different Grignard agents. Reaction of 1-(anthracen-9-yl)prop-2-yn-1-ol with CpRu(PPh3)2Cl in the presence of KPF6 in CH2Cl2 afforded the cationic ruthenium allenylidene complex 2fa. Complex 2fa was treated with ethynylmagnesium bromide to afford the ruthenium acetylide complex 3fa, which has shorter distance between triple bond and the anthracenyl group than the distance between double bond on allyl group and the anthracenyl group in complex 4f. Complex 2fa was treated with but-3-enylmagnesium bromide to afford the ruthenium acetylide complex 3fd, which has longer distance between double bond on but-3-enyl and the anthracenyl group than the distance between double bond on allyl group and the anthracenyl group in complex 4f. However, there is no intramolecular Diels-Alder product observed in 3fa and 3fd. Furthermore, when complex 3fa is also treated with allyl iodide at room temperature, the intramolecular Diels-Alder product 5fa could be observed. It seems that the Diel-Alder reaction takes place only between the anthracenyl group and the allyl group on Cß.
Subjects
ruthenium
vinylidene
cyclization
rearrangement
Diels-Alder
Type
thesis
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