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that the major diastereomer in the aliphatic case (e.g. 74e leading to 287e) results from a
transition state shown in Scheme 4.38 where the alkyl group is placed in a pseudoequatorial
position. In addition, there may be an interaction between the carbonyl oxygen of the methyl
ester and the oxophilic molybdenum center contributing to this transition state being favored.
The origin of reversed diastereoselectivity in the presence of an aromatic residue in the side
chain is subject to speculation. One potential explanation is based on coordination of the
aromatic ring in the side chain with the molybdenum center as shown on 74b (Scheme 4.38). It is
well known that Mo(CO)6 readily reacts with various arenes to form [Mo(η 6 -arene)(CO)3]
“piano-stool” complexes. 194 Since the Mo(CO)6-mediated cyclocarbonylation reaction takes
place in toluene, formation of the octahedral complex [Mo(η 6 -toluene)(CO)3] (291) has been
proposed as the initial step of the reaction. 195 A recent study has demonstrated that the arene
ligands in these molybdenum complexes can be readily exchanged even at rt. 196 Thus, it is
conceivable that temporary chelation of the phenyl ring in allenyne 74b with the molybdenum
center as shown in Scheme 4.38 may be responsible for the stereochemistry observed in the
major diastereomer 287b.
Scheme 4.38 Arene complexation rationale for the diastereoselectivity in the cyclocarbonylation
reaction.
Bz N O
MeO2C H
287e
H
Ph
O
N
MeO 2C
74e
H
H
H
H
ML n
Ph
O
N
MeO 2C
111
H MLn Bz N O
H
MeO2C H
H H
74b
Ph
287b
H
OC
Mo
CO
291
CO