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Please login to access the full content or check if you have access via Method 8: Titanium(II)–Carbonyl Complexes as Catalysts in the Pauson–Khand Reaction

DOI: 10.1055/sos-SD-002-00576

Mikami, K.; Matsumoto, Y.; Shiono, T.Science of Synthesis, (20032516.

The transition-metal-promoted cyclocarbonylation, the so-called PausonKhand 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 PausonKhand 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.

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