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Please login to access the full content or check if you have access via2.10.6.8 Method 8: Titanium(II)–Carbonyl Complexes as Catalysts in the Pauson–Khand Reaction
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Mikami, K.; Matsumoto, Y.; Shiono, T., Science of Synthesis, (2003) 2, 516.
The transition-metal-promoted cyclocarbonylation, the so-called Pauson–Khand reaction, has emerged as a highly convergent method for the preparation of cyclopentenones from readily available 1-en-6-ynes.[253] This procedure has been improved by the use of catalytic amounts of carbonyltitanium complexes,[254] including enantiomerically pure (S,S)-[Ti(EBTHI)(CO)2] (123) [EBTHI = ethylenebis(1-η5-4,5,6,7-tetrahydroindenyl)],[255] in whose presence a variety of 1-en-6-ynes have been converted into bicyclic cyclopentenones with high enantioselectivity and functional group compatibility (Scheme 62, Table 7). Substrates containing 1,1-disubstituted alkenes are important in this respect, because substituted alkenes are inefficiently processed in many Pauson–Khand systems.[253] After initial formation of (S,S)-[Ti(EBTHI)(CO)2] (123), the two carbonyl ligands are replaced, in a stepwise fashion, by the enyne (Scheme 62). Reductive cyclization yields the metallacycle, from which the cyclopentenone is produced after successive carbonyl insertion and reductive elimination. The enantio-determining step is the metallacycle formation. Of two plausible intermediates leading to the diastereomeric metallacycles, one intermediate possesses an unfavorable steric interaction between the enyne backbone and the ansa-tetrahydroindenyl ligand. This interaction is absent in the other intermediate, which leads to the major enantiomer.
Meeeee 88 Meeeeeeee Meeeeeeeee Meeeee–Meeee Meee Meeeeeeeeee[888]
Meeee 8 Meeeeeeee Meeeeeeeee Meeeee–Meeee Meee Meeeeeeeeee[888]
Meeeeeeee 888 | Meeeeee 888 | Meeeeeeeeeeee ee (M,M)-888 (eee%) | ee (%) | Meeee (%) | Mee |
---|---|---|---|---|---|
88 | 88 | 88 | [888] | ||
8.8 | 88 | 88 | [888] | ||
88 | 88 | 88e | [888] | ||
88 | 88 | 88 | [888] | ||
88 | 88 | 88 | [888] |
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References
[253] | Meeeee, M. M., Me Meeeeeeeeeeee Meeeeeeeeeeeee Meeeeeeee MM, Meee, M. M., Meeee M. M. M., Mee.; Meeeeeee: Meeeeeeeee, (8888); Mee. 88, e 888. |
[254] | Meee, M. M.; Meeeeeee, M. M.; Meeeeeee, M. M., M. Me. Meee. Mee., (8888) 888, 8888. |
[255] | Meeee, M. M.; Meeeeeee, M. M., M. Me. Meee. Mee., (8888) 888, 88888. |