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usually is DMSO. Heating to 100 ºC is generally required for reaction to proceed (Scheme 4.4).
The role of DMSO is believed to involve oxidation of one or more CO ligands to CO2, which
then leaves, opening a vacant site on the metal for coordination of the enyne. In this initial report,
it was shown that 1,3-enynes with diester, ether or nitrogen in the tether can participate in the
Mo(CO)6-promoted cyclization to afford the corresponding [5,5]-fused bicyclic systems (230 to
231, Scheme 4.4). Notably, attempts to prepare [6,5]-fused systems by extending the enyne
tether by one carbon were unsuccessful.
Scheme 4.4 Mo(CO)6-mediated cyclocarbonylation reaction.
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Mo(CO) 6, DMSO, toluene
100 o C, 8 h, 76%
230 231
There has been only one report on a catalytic version of the Mo-mediated cyclocarbonylation
reported to date, but it is limited to highly reactive substrates. 151 Recently, Carretero found that
Mo(CO)3(DMF)3 serves as a milder and more efficient promoter for the intramolecular
cyclization of enynes. 152 This air-sensitive complex can be synthesized from Mo(CO)6 itself and
allows the reaction of enynes to occur at rt with improved yields and without the necessity of
promoters (Scheme 4.5).
Scheme 4.5 Improved protocol for the Mo-mediated cyclocarbonylation reaction.
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Mo(CO) 3(DMF) 3, CH 2Cl 2
rt, 15 min, 83%
230 231
4.1.2 Late Transition Metal-Catalyzed Cyclocarbonylation Reaction.
Late transition metals such as Rh and Ru have been widely utilized in a variety of
catalytic transformations of 1,n-enynes including [4+2] and [5+2] cycloadditions, and ene-type
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