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with this metal under CO atmosphere. 148 In addition, a number of other transition metals have
been demonstrated as useful mediators or catalysts for the Pauson-Khand reaction, including Mo,
Ru, Rh, and Ti. 41a, 149 Currently, the name Pauson-Khand reaction is used to refer only to the
initial Co-mediated transformation developed by Pauson and Khand, whereas all other related
transformations are known as cyclocarbonylation reactions. The common mechanistic features of
nearly all cyclocarbonylations are presented in Scheme 4.3. The reaction is initiated by
coordination of the metal to the enyne, followed by the formation of a metallocycle 227 which
occurs via oxidative addition of the metal into the olefin and alkyne. Then, migration of one of
the metal-carbon bonds occurs into a CO ligand leading to an acyl-metallocycle 228, which
undergoes a reductive elimination to give the product 229.
Scheme 4.3 Common mechanistic features of transition metal-catalyzed cyclocarbonylation
reactions.
R 1
R2 229
O
reductive
elimination
R 1
R2 228
-ML n
O
ML n
migratory
insertion of CO
R2 225
R 1
R2 227
R 1
M(CO)L n
4.1.1 Mo(CO)6-Mediated Cyclocarbonylation Reaction.
ML n, CO
R 1
R2 226
M(CO)L n
oxidative
addition
Mo(CO)6 was reported to mediate the cyclocarbonylation reaction of 1,3-enynes by Jeong in
1993. 150 The reagent is used in a stoichiometric amount in conjunction with a promoter, which
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