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lack of double bond selectivity, since cyclocarbonylation was the predominating reaction
pathway. In an effort to inhibit the formation of the triene either sterically or electronically, PPh3
was added to the iridium catalyst. Formation of the triene 111f was avoided by using an in situ
prepared catalyst by addition of 20 mol % of PPh3 to 10 mol % [Ir(cod)Cl]2 (Ir : P ratio = 1 : 1).
However, these conditions also gave a mixture of cyclocarbonylation products in 60 % yield
with the α-alkylidene cyclopentenone 274f preferred (entry 2, Table 4.1). Both results indicate
that Ir-based catalysts may have a unique preference to promote the reaction with the proximal
double bond of the allene. This was confirmed in a report by Shibata in 2003 by converting
terminally disubstituted allenynes to α-alkylidene cyclopentenones using IrCl(CO)(PPh3)2. 172
Table 4.1 Cyclocarbonylation conditions for allenyne 73f.
CbzN
MeO 2C
•
73f
H
conditions
Cbz
MeO 2C
N
O
Cbz N
O
MeO2C Cbz
MeO 2C
270f 274f 111f
entry conditions a yield ratio 270f : 274f : 111f
1 [Ir(CO) 3Cl], CO, toluene, 90 °C 73% 2 : 6.5 : 1.5
2 [Ir(COD)Cl] 2, PPh 3, CO, toluene, 90 °C 60% 1.5 : 8.5 : 0
3 Rh(PPh 3) 3Cl, CO, toluene, 90°C, no reaction
4 Rh(CO)Cl(PPh 3) 2, CO, toluene, 90°C no reaction
5 Rh(CO)Cl(PPh 3) 2, AgOTf, b CO, toluene, 50°C 66% 7.5 : 2.5 : trace
a 10 mol % of catalyst and 1 atm of CO was used in all cases; b 10 mol %
Next, the effect of ligands on the Rh(I) catalyst was examined because it was reasoned that the
undesired β-hydride elimination pathway from metallocylopentene 271 could also be suppressed
in presence of phosphine ligands. It is well known that a vacant site on the Rh(III)-center is
required for β-hydride elimination to occur. 173, 174 Therefore, coordinatively saturated Rh(III)-
metallocycle is deemed more likely to undergo CO insertion than β-hydride elimination. With
this in mind, Rh(PPh3)3Cl (Wilkinson’s catalyst) 175 and trans-Rh(CO)Cl(PPh3)2 were tested. 176
93
N