COMMUNICATION
Table 1. Reaction scope in symmetrically substituted substrates.[a]
the reaction mixtures. Substrates such as 4a–d, in which
both R1 and R2 were able to stabilize positive electron den-
sity, showed a clear (Z,Z) selectivity under [(IPr)Au (NTf2)]
catalysis (Table 2, entries 1, 3, 5, 6) whereas [(PPh3)Au-
AHCTUNGRTEGNUN(N NTf2)] favored the (1E,3Z) and (1Z,3E) selectivity
(Table 2, entries 2, 4, 7), analogously to the previous results
(Table 1). It is important to note that more sterically de-
manding substrate 4d dramatically improved the selectivity
towards 6d compared with the previous examples, thus re-
vealing the key role of sterics in the stereocontrol of these
transformations. In addition, substrate 4e, which features a
pull–push system, delivered the corresponding dienes 5e
and 6e as a 1:1 mixture, which clearly reflects the impor-
tance of electronic factors in the reaction stereocontrol. To
further study the effect of steric factors, we decided to pre-
pare substrates featuring similar electron-donating proper-
ties at C1 and substituents of different sizes at C4 (4 f–h). In-
dependent of the catalyst, we observed that (1Z,3E) isomers
6 f–h were always the major product of these reactions.
Gratifyingly, increasing the sterical bulkiness from methyl to
isopropyl (4g to 4 f) further enhanced the selectivity
(Table 2, entries 11–13). However, further increase in the
bulkiness (4h) was detrimental to the selectivity (Table 2,
Entry Substrate
Protocol Ratio
Product (Yield
[%])[c]
2:3[b]
1
2
3
4
5
1a (R=Ph)
1a
1b (R=3-OMeC6H4)
1b
1
2
1
2
1
10:1
1:9[d]
2a (86)
3a (80)
2b (87)
3b (83)
2c (92)
8:1
1:8[d]
1c (R=3,5-dimethoxy-
13:1
phenyl)
6
1c
2
1
2
1
2
1:10
8:1
1:7
–
3c (92)
2d (84)[e]
3d (80)[f]
1e
7
1d (R=cinnamyl)
8
1d
9
1e (R=4-CF3C6H4)
10
1e
–
1e
[a] Protocol 1: [(IPr)Au
ACHTNUGRTEN(NNUG NTf2)] (5%), RT, CH2Cl2; Protocol 2:
[(PPh3)Au(NTf2)] (2%), RT, CH2Cl2; see also reference [10]. [b] Ratio
ACHTUNGTRENNUNG
calculated by 1H NMR. [c] Isolated yield after column chromatography.
[d] The (E,E) isomer could be also detected. The effective ratios for en-
tries 2 and 4 are (1:18:1) and (1:16:1).[10] [e] Complete isomerization at
the cinnamyl moiety occurs to give (Z,Z,Z,Z). [f] The reaction was per-
formed in acetonitrile. 3d was isolated, and isomerized to 2d upon stand-
ing in solution.
Table 2. Reaction scope in nonsymmetrically substituted substrates.
[(PPh3)Au
ACHTUNGTRENNUNG
provide the 2,3-bisAHCTUNGTRENNUNG
1,3-dienes in excellent yields
(Table 1, entries 1, 2). Remark-
ably, no product of the poten-
tially competitive 1,3-acetate
migration could be detected. To
Entry
Substrate
Protocol[a]
Ratio 5:6:7[b]
Product (Yield [%])[c]
our delight,
a more careful
1
2
3
4
5
6
7
8
4a (R1 =Ph, R2 =3,5-dimethoxyphenyl)
1
2
1
2
1
1
2
1
2
1
2
1
2
1
2
>23:1:1
1:14:10
8:1:0
1:4.5:4
13:1
13:1:0
1:10:0
1:1:0
5a (73)
analysis of the reaction mix-
tures revealed that of the three
potential stereoisomers that the
reaction could afford, a high
stereoselectivity was observed,
depending on the catalyst:
4a
6a (53), 7a (40)
5b (85)
4b (R1 =Ph, R2 =3-OMeC6H4)
4b
6b+7b (80)[d]
5c (92)[e]
4c (R1 =cinnamyl, R2 =3,5-dimethoxyphenyl)
4d (R1 =4-OMeC6H4, R2 =Me, Ph)
5d (83)
6d (87)
4d
4e (R1 =4-OMeC6H4, R2 =4-CF3C6H4)
5e+6e (91)[d]
5e+6e (78)[d]
6 f (86)
9
4e
1:1:0
1:9:0
1:8:0
1:15:0
1:10: 0
1:1.5:0
1:1.5:0
[(IPr)AuACHTUNGTRENNUNG(NTf2)] selectively de-
10
11
12
4 f (R1 =4-OMeC6H4, R2 =Me)
livered the (1Z,3Z) isomers[11]
4 f
6 f (80)
6g (81)
4g (R1 =4-OMeC6H4, R2 =iPr)
(Table 1, entries 1, 3, 5, 7)
whereas the more cationic 13
14
4g
6g (69)
4h (R1 =4-OMeC6H4, R2 =tBu)
5h+6h (99)[d]
5h+6h (44)[d]
[(PPh3)Au
N
afforded
15
4h
(1Z,3E) isomer as the major
product of the reaction
(Table 1, entries 2, 4, 6, 8). The
importance of stabilizing the re-
action intermediates (VIII and
[a] See Table 1 for details of the protocols used. [b] Ratio calculated by 1H NMR. [c] Isolated yield of the
major isomer after column chromatography. [d] Isolated yield of the mixture after column chromatography.
[e] The cinnamyl moiety in 5c isomerizes to the Z isomer (5c’) upon standing in solution.
XI) was emphasized by the failure of substrate 1e to under-
go any type of rearrangement.
To examine the scope of this novel Au-catalyzed tandem
entries 14–15). In all cases and similarly to 4d, not having a
second stabilizing group clearly increased selectivity for
products of type 6 over 7.
1,2-/1,2-bisACHTUNGTRENNUNG(acetoxy) migration, we decided to apply both re-
action protocols to a set of unsymmetrically substituted sub-
strates. The results have been summarized in Table 2.
To evaluate the single effect of the substituent attached to
the acyloxy migrating group in the stereochemistry of the
We found that the 1,2-bis
(acetoxy) migration was favored
resulting vinyl acetate, we synthesized several bis
ACHTUNGTRENUN(NG acetates)
in all studied cases and no allene product was observed in
derived from commercially available 2-methyl-3-butyn-2-
Chem. Eur. J. 2009, 15, 5904 – 5908
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5905