Nickel cyclopentenone synthesis

A recent synthesis of methylenomycin-B (74), based on these observations, features an allylation/car-bonylation (69) - (70) followed by a C-1—C-2 bond/nickel acyl insertion (70) -> (71) and a final meth-oxycarbonylation (71) -> (72). Thus 2-butyn-l-ol (1,2-dialkylacetylenes are inert) afforded in one synthetic operation a 1 4 mixture (78%) of regioisomeric cyclopentenones (72) and (73) which was converted to the antibiotic (74) (Scheme 16). ... 

Other C-C bond-forming reactions have been successfully developed using SCCO2 and liquid CO2 as reaction media. Examples include the synthesis of cyclopentenones via cobalt-catalyzed cocyclizations of alkynes with alkenes and carbon monoxide (Pauson-Khand reaction) (Scheme 30) , enantioselective nickel-catalyzed hydrovinylation of styrenes (Scheme 31) , and the palladium-catalyzed hydroarylation of acyclic jS-substimted-o , j8-enones with aryl iodides (formal conjugate addition) (Scheme 32). ... 

Allyl complexes have contributed significantly to the development of the organometallic chemistry of nickel and the applications of nickel complexes in organic synthesis, for example, nucleophilic attack on coordinated allyl ligands. In addition, allylnickel complexes have been identified as key intermediates in the oligomerization and cyclization of olefins and dienes. For example, the Ni(0)-catalyzed hydrocyanation of butadiene to adiponitrile, the main component of a major commercial process for the production of nylon, involves Ni (7r-allyl) intermediates. Moreover, the 77-rearrangements of allylnickel species have helped explain the facile isomerization of olefins in the presence of nickel complexes. The Ni-catalyzed homoallylation of carbonyl compounds with 1,3-dienes also involves Ni(7r-allyl) complexes this subject has been reviewed recently. New applications include the cleavage of G-G bonds in the deallylation of malonates, the preparation of cyclopentenones by carbonylative cycloaddi-... 

Electron-rich alkynes, including ethoxy(trimethylsilyl)-acetylene, reacted with chromium alkenylcarbene conplexes at — 10°C in acetonitrile in the presence of a stoichiometric amount of Ni(cod)2 with warming to 20 °C over 2 h to afford moderate yields (40-49%) of cyclopentenones (eq 10). This completely stereoselective nickel(0)-mediated [3 + 2] cyclization reaction of chromium alkenyl(methoxy)carbene complexes with both electron-withdrawing and electron-donating substituted alkynes provides a general synthesis of substituted 2-cyclo-pentenone derivatives, important synthons for the construction of more complex molecules. ... 

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