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affords a Co(I) catalyst species (A). Then, oxidative addition of a
propargyl acetate (1) takes place via C–O bond cleavage, giving
a Co(III) intermediate (B) (step a). Subsequent reduction of
propargyl Co(III) with manganese gives the propargyl cobalt(II)
species (C) (step b).3a,b,15 Then, the more nucleophilic3b Co(II)
species (C) reacts with CO2 to give the carboxylatocobalt inter-
mediate (D) (step c). Finally, the reduction of D with manganese
affords the corresponding manganese carboxylate and the Co(I)
catalytic species (A) regenerates (step d). Further studies on the
reaction mechanism are in progress.
We thank Professors Michinori Suginome and Toshimichi
Ohmura (Kyoto University) for measuring optical rotations. This
work was supported by a Grant-in-Aid for Scientific Research on
Innovative Areas (‘‘Organic synthesis based on reaction integration’’
and ‘‘Molecular activation directed toward straightforward
synthesis’’) from MEXT, Japan. K. N. is grateful to a Research
Fellowship of JSPS for Young Scientists.
Notes and references
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Scheme 1 Derivatization of carboxylated products.
introduced into the terminal alkyne moiety of 2u and 2v by the
Sonogashira coupling reaction14a (2w and 2x, Scheme 1d and e).
Moreover, Au-catalyzed intra-molecular cyclization14b of 2v pro-
vided unsaturated g-lactone 2y smoothly (Scheme 1f). Thus these
TMS moieties are very useful for the further derivatization.
When optically pure (S)-1a was employed as the substrate in the
present carboxylation, racemic 2a was obtained in 72% yield (eqn (1)).
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4 For another important C–C bond forming reaction employing CO2,
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J. Am. Chem. Soc., 2008, 130, 15254–15255; (g) J. Takaya, K. Sasano
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A plausible reaction mechanism is shown in Scheme 2.
Initially, the reduction of a Co(II) complex with manganese
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9 Recently, in the Ni-catalyzed carboxylation of benzylic chlorides with
CO2,3d C6H5CRCCH2Cl was used as a substrate and afforded
C6H5CRCCH2COOH in only 29% yield; other propargylic substrates
were not examined in the reaction.
Scheme 2 Plausible catalyst cycle.
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