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3.3.1.1 Enantioselective Titanium-Catalyzed Pauson–Khand Reactions

DOI: 10.1055/sos-SD-203-00068

Shibata, T.Science of Synthesis: Stereoselective Synthesis, (20113125.

The low-valent titanium complex dicarbonylbis(η5-cyclopentadienyl)titanium(II) [Ti(Cp)2(CO)2] catalyzes the intramolecular PausonKhand reaction of various enynes under an almost atmospheric pressure of carbon monoxide, to provide bicyclopentenones in good to excellent yield.[‌5‌] This was the first PausonKhand reaction catalyzed by a non-cobalt transition-metal complex. The first catalytic and enantioselective PausonKhand reaction was also realized using the low-valent titanium complex 2 [(S,S)-Ti(EBTHI)(Me)2; EBTHI=1,1-ethylenebis(η5-4,5,6,7-tetrahydroindenyl)].[‌6‌] In the presence of the chiral titanium catalyst, where the metal center and cyclopentadienyl moiety are connected by a σ-bond, various carbon- and heteroatom-tethered 1,6-enynes 1 are transformed into the bicyclopentenones 3, which have a stereogenic center at the ring fusion.[‌7‌,‌8‌] The reaction tolerates an array of substituents at the alkyne terminus, including aryl groups with electron-donating or electron-withdrawing groups in addition to alkyl groups (Scheme 1). In the case of an enyne with an unsubstituted alkyne terminus, the yield is good but the enantioselectivity is poor. The reaction of an enyne with a 1,1-disubstituted alkene moiety affords a bicyclopentenone with an asymmetric quaternary carbon stereogenic center with modest enantiomeric excess. A practical limitation of the process is that the low-valent titanium species and chiral cyclopentadienyl ligands are unstable, and must be utilized under strictly oxygen- and moisture-free conditions.

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References


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