C O MMU N I C A T I O N S
Table 2. Ir-Catalyzed Reactions of Propargylic Esters with 5a
2). Ketene acetal 5d and ketene monothioacetal 5e retained the
preferential formation of 6k in reactions with 2k (entries 22 and
23 in Table 2). The ratio of 6k in the products increased with an
increase in the nucleophilicity of 5. On the other hand, steric
bulkiness at the nucleophilic site is advantageous for the formation
of 7. Because the contribution of the allenyl form in the cation
derived from 2k is estimated to be 10-30% on the basis of 13C
NMR,9 the selective formation of 7kc implies the participation of
thermodynamic factors. Regioselectivity aside, the present substitu-
tion of propargylic esters seems to be enhanced by the cation-
stabilizing effects of the substituent on the propargyl carbon. This
speculation is based on the reactivity order, 2k > 2a > 2b > 2h
) 2g > 2f ≈ 2e, which is roughly estimated by comparing the
reaction times. The intervention of certain cationic species derived
from 2 is supported by the finding that racemic 6ga was obtained
as the sole product in the respective reactions of (S)-2g (69% ee)
and racemic 2g with 5a.
propargylic ester
product (%)
entry
R1
R2
R3
enoxysilaneb
6
7
1
2
3
4
5
2b
2b
3b
2c
2d
2e
2e
2e
2e
2f
2f
2f
2f
2g
2g
2h
4i
nBu
nBu
nBu
Me3Si
H
Ph
Ph
Ph
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
5a (2)
5a (4)
5a (2)
5a (4)
5a (4)
5a (4)
5a (4)
5d (4)
5e (4)
5a (4)
5a (4)
5d (4)
5e (4)
5a (2)
5a (3)
5a (2)
5a (4)
5a (4)
5a (2)
5b (2)
5c (2)
5d (2)
5e (2)
5a (2)
ba
ba
ba
ca
da
ea
ea
ed
ee
fa
fa
fd
fe
ga
ga
ha
ia
84
94
84
86
81
66
84
83
88
33
72
38
77
93
95
86
72
68
34c
40c
0
0
0
0
5c
0
Ph
6
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
Me
Me
Me
Me
3c
4c
0
7d
8
9
0
0
10
11e
12
13
14f
15
16
17h
18i
19
20
21
22
23
24
-(CH2)5-
-(CH2)5-
-(CH2)5-
-(CH2)5-
0
4c
0
Ph
Ph
H
H
H
H
0
0
Naphg
Me
H
0
5c
4j
H
ja
1c
Acknowledgment. We gratefully acknowledge financial support
from the Ministry of Education, Science, Sports, and Culture, Japan.
2k
2k
2k
2k
2k
2l
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
ka
kb
kc
kd
ke
la
55
56
Supporting Information Available: Experimental procedures and
analytical data for new compounds (PDF). This material is available
86
49
54
8
39c
33c
77j
References
a Reactions were carried out on a 0.5 mmol scale at 25 °C in a CH2Cl2
solution containing 1 mol % of 1a activated by H2. They were stopped
when propargylic esters disappeared (4-24 h). b The number in parentheses
shows the molar equivalents of 5 used. c Estimated from the 1H NMR
spectrum of the sample isolated as a mixture of 6 and 7. d At 0 °C for 26
h. e At -20 °C for 24 h in the presence of 2 mol % of 1a. f At 50 °C for
16 h. g 1-Naphthyl. h At 50 °C for 5 h in the presence of 2 mol % of 1a.
i At 80 °C for 2 h in the presence of 2 mol % of 1a. j Allenyl product 7la
changed to dienone 9 during column chromatography on SiO2.
(1) (a) Hudrlik, P. F.; Hudrlik, A. M. In The Chemistry of the Carbon-Carbon
Triple Bond; Patai, S., Ed.; John Wiley & Sons: Chester, 1978; Chapter
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1994, 59, 2282. (b) Mahrwald, R.; Quint, S. Tetrahedron 2000, 56, 7463.
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In sharp contrast to the above results, reverse selectivity in the
regiochemistry of the substitution was observed in the reaction of
2k and 2l, which have two phenyl groups on the propargyl carbon
(entries 19-21 and 24 in Table 2). In particular, 7kc was isolated
as the sole product in the reaction of 2k with 5c (entry 21 in Table
JA0264089
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J. AM. CHEM. SOC. VOL. 124, NO. 31, 2002 9073