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Table 4.2 Cyclocarbonylation reaction conditions for allenyne 73a.
BzN
MeO 2C
Bn
•
73a
H
conditions
Bz
MeO 2C
N
Bn
O
Bz N
O
MeO2C Bn
Bz
MeO 2C
270a 274a 111a
entry conditions a yield ratio 270a : 274a : 111a
1 Rh(CO)Cl(PPh 3) 2, AgOTf, CO, toluene, 50 o C 52% 4 : 1 : trace
2 Rh(CO)Cl(PPh 3) 2, AgOTf, CO, DCE, rt 65% 8.5 : 0 : 1.5
3 Rh(CO)Cl(PPh 3) 2, AgBF 4, CO, DCE, rt, 36h 76% 10 : 0 : 0
4 [Rh(CO) 2Cl] 2, PPh 3, AgBF 4, CO, DCE, rt, 1h 76% 10 : 0 : 0 b
a 10 mol % of catalyst and 1 atm of CO was used in all cases; b 10 mol % [Rh(CO)2Cl] 2, 30 mol % PPh 3, 22 mol % AgBF 4.
Based on observations that RhCl(PPh3)3 activated by AgBF4 on average gave longer reaction
times it was reasoned that reducing the amount of PPh3 would have an effect on increasing the
rate of the reaction. Therefore, an experiment was designed where the catalytic species was
prepared in situ from 10 mol % of [Rh(CO)2Cl]2, 30 mol % PPh3 and 22 mol % of AgBF4 (P : Rh
ratio = 1.5 : 1). o When allenyne was added under 1 atm CO, reaction occurred at rt in 1h to give
4-alkylidene cyclopentenone 270a in 76% yield (entry 4, Table 4.2). To our knowledge, this is
the first example of a Rh(I)-catalyzed cyclocarbonylation reaction occurring at rt (the initial
examples by Jeong and Narasaka required heating over 100 °C). 178
These conditions were then tested on a set of substrates to examine the scope. Variation
of the alkyne terminus with a TMS group (entries 2 and 6, Table 4.3) resulted in formation of α-
silyl-cyclopentenones 270c and 270h in high yield. This result is in contrast to Narasaka’s who
reported obtaining desilylated products in the reaction of trimethylsilyl-alkynes at higher
o PPh3 : Rh ratio of 1.5 : 1 proved necessary since increasing the amount of PPh3 (P : Rh = 2 : 1) led to slower
reaction (as in entry 3), whereas lowering the amount of PPh3 (P : Rh = 1 : 1) led to formation of triene 111a.
95
N
Bn