Cyclopentanone, 2,5-dibromo cycloaddition reactions

Iron-stabilized oxallyl cations (generated in situ (eq 25) from a,o -dibromo ketones and Fe2(CO)9) react with alkenes. Noyori used this [3 + 2] cycloaddition reaction to produce cyclopentanone or cyclopentanone derivatives, as illustrated by a single-step synthesis of (i)-a-cuparenone (43) (eq 26). The reaction of a,a -dibromo ketones with enamines and Fe2(CO)9 yields substituted cyclopentenones in 50-100% yield (eq 27), as illustrated by the reaction with the a-morpholinostyrene (44). ... 

Reaction of styrene with the commercially available 2,4-dibromopentan-3-one (52) in the presence of a stoichiometric amount of Fc2(CO)9 gives a 65% yield of cyclopentanone (53) as a mixture of two dia-stereoisomers (equation 44). The cycloadduct is formed in a stepwise fashion via electrophilic attack of the incipient (oxyallyl)Fe complex (54) on the alkene to form the zwitterionic species (55). Ring closure of this intermediate affords the cyclopentanone product (equation 45). In light of this mechanism, it is not surprising tl.at the reaction is highly chemoselective, i.e. only electron-rich alkenes will participate in the cycloaddition. One is also limited to alkyl-substituted oxyallyl systems since both a,a -dibromo- and a,a,a, a -tetrabromo-acetone fail to yield any cycloadduct. Fe2(CO)9 is the most efficient reagent, although Fe(CO)s can also be used. The dibromo ketones are either commercially available or easily prepared by simple procedures. ... 

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