Download (3249Kb) - D-Scholarship@Pitt - University of Pittsburgh

cycloisomerization pathway involves β-hydride elimination from the adjacent methyl group to
give Rh(III)-H species 272, followed by reductive elimination. Alternatively, migratory insertion
of CO in the vinyl-Rh(III) bond leads to acyl-metallocyclohexene 273, which is transformed to
cyclopentenone 270f via a reductive elimination.
Scheme 4.19 Mechanism for the cycloisomerization and cyclocarbonylation reaction of 73f.
Cbz
MeO 2C
N
O
CbzN
MeO 2C
Cbz
MeO 2C
N
73f
•
H
CO
Rh
Ln
Cbz
MeO 2C
271
270f
H
111f
Rh(I)L n
[Rh(CO) 2Cl] 2
B A C
D
Cbz
MeO2C N
O
Rh
Ln Cbz
MeO2C N
CO
Rh
Ln
H
Rh(I)(CO)Ln 273
A: CO insertion; B: reductive elimination; C: β-hydride elimination; D: reductive elimination.
The reaction of 73f in presence of [Rh(CO)2Cl]2 clearly indicated that the cycloisomerization
pathway leading to 111f is greatly favored with this catalyst. To circumvent this issue, other
catalytic conditions were examined.
In 2000, Shibata reported the use of Ir(I) catalysts for the intramolecular
cyclocarbonylation reaction of enynes. 171 Borrowing from this report, [Ir(CO)3Cl] was used in an
attempt to effect selective formation of 270f. Heating allenyne 73f to 90 °C under 1 atm of CO,
resulted in a mixture of all three possible products in 73% yield (entry 1, Table 4.1).
Interestingly, the α-alkylidene cyclopentenone 274f (48%) resulting from reaction with the
proximal double bond of the allene was formed preferentially over 270f (15%), along with a
small amount of undesired triene 111f (10%). This result was encouraging despite the complete
92
272
N