S. P. Nolan, S. Gaillard et al.
methane, the first half equivalent of acid affords complex 4
and if more acid is added, the formation of 2 is observed.
Nevertheless, when a biphasic system of water/dichlorome-
thane is used, only complex 4 is obtained and the excess
HBF rests in the aqueous layer.
4
Interestingly, the chemical shift of the two protons of the
IPr backbone can be correlated to the Lewis acidic charac-
ter of the gold(I) center and shows that the more Brønsted
acid is added to 1, the stronger the Lewis acidic character of
the resultant species. Indeed, if a Lewis acid character rank-
Scheme 2. Equilibrium of 1 in aqueous medium.
1
3
ing is established using C NMR chemical shift data for the
carbenic carbon (d=171.6 ppm in 1, d=162.6 ppm in 4, d=
relative to 1 (1.5 equivalents) led to the nearly exclusive for-
mation of 2, with only traces of 4. Cleary, a difference in re-
activity appeared between THF and dichloromethane solu-
1
59.8 ppm in ethereal species 2, and d=159.0 ppm in 2) the
[28,29]
following order is found: 2> 2·ether> 4> 1.
In the
1
H NMR spectra, the chemical shift of the two protons
found at d=7.20 ppm in 1, d=7.22 ppm in 4, d=7.38 ppm
in the ethereal species 2, and d=7.48 ppm in 2 in dry
CD Cl , shows that a more downfield shifted signal corre-
tions. Since HBF is more soluble in water than in organic
4
solvents, and as 4 appears to be a stable form of 2, we pro-
pose that, in the case of an immiscible mixture, complex 4 is
initially formed and that the excess acid migrates to the
aqueous layer. However, in the case of a miscible solvent
system, the acid remains available and 4 is in equilibrium
with species 2 [Eq. (5)].
2
2
lates to a stronger Lewis acid character of the gold(I)
center.
As the presence of water is important for the formation
of 4 and as water is used in several reactions involving cat-
ionic gold(I) catalysis, the behavior of the catalytic species
generated by acid activation of 1 was investigated in mix-
tures of water and water-miscible organic solvents. A 1:1
mixture of [D ]THF/D O was first selected to mimic the
8
2
previously developed reaction conditions for nitrile hydra-
[37] 1
tion.
H NMR spectra of well-defined 1 and 4 were re-
corded in a 1:1 mixture of [D ]THF/D O. In both cases, the
8
2
spectra showed the presence of another complex. Assuming
that the major product observed was the starting complex,
the H NMR spectrum of 1 revealed it to be in equilibrium
Importantly, this equilibrium is shifted towards 2 when
excess acid is present. A similar H NMR investigation was
1
1
with another species which exhibits signals similar to those
found for 4. A similar reaction using well-defined 4 reveals
features that can be attributed to the cationic species 2.
Based on computational results, Toste and co-workers previ-
ously proposed an equilibrium that exists in the case of tris-
undertaken using a 10:1 mixture of CD OD/D O. These
3
2
conditions are relevant to previous catalytic studies dealing
[41]
with the Meyer–Schuster rearrangement. The same equi-
librium phenomenon between complexes 1, 4, and species 2
was observed [Eq. (5)]. Noteworthy, when only one equiva-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[phosphinegold(I)]oxonium in the presence of an allenyne
lent of aqueous HBF is used, the equilibrium is already
4
[38]
as substrate in cycloisomerization processes. This gold(I)
oxonium species can be in equilibrium with a dinuclear gold
hydroxide and a gold acetylide complex with the allenenyne.
These species can also be in equilibrium with a gold hydrox-
ide complex and a cationic species coordinated to the
shifted to the almost exclusive formation of 2. Indeed, in a
10:1 CD OD/D O mixture, 1 when reacted with one equiva-
3
2
lent of acid led to less than 4% (versus 9% in THF/water
conditions) of 4.
In summary, formation of complex 4 can be viewed as oc-
curring in two consecutive steps. First, the addition of
+
alkyne of the acetylide complex. Moreover, as [LAu] is
considered isolobal with H
+
[39]
and as water is miscible with
0.5 equivalents of HBF to 1 leads to the formation of the
4
[
D ]THF, an analogy to the equilibrium involving the hy-
species 2 by simple protonolysis. In a second step, the bind-
ing of 2 to unreacted 1 affords complex 4 (Scheme 3).
8
droxonium with water could be envisaged with complex 1
and a bis[(IPr)Au(I)] hydroxide (Scheme 2).
ACHTUNGTRENNUNG
Noteworthy, when complex 1 was treated with 0.33 equiv-
Catalytic studies: As 1, 3, and 4 were isolated and can be
generated in situ under well determined conditions (vide
infra), the catalytic activity of these complexes (acid-activat-
ed or not) was evaluated in a number of important organic
transformations. A broad range of previously reported cat-
ionic gold(I)-catalyzed reactions were targeted, such as the
alents of HBF , no trigold(I) or tetragold(I) complexes were
4
observed like the phosphine gold(I) complexes reported by
[40]
Schimdbaur and co-workers.
The activation of 1 in a
[
D ]THF/D O mixture by acid addition was next attempted,
8
2
and when 0.5 equivalents of aqueous HBF were added to 1,
4
[37]
[42]
formation of 4 was observed with 2 as the minor product.
Then with a stoichiometric amount of acid, a 4:1 ratio of 2
and 4 was observed. Finally, increasing the amount of acid
hydration of nitriles
and alkynes,
the synthesis of
[43]
enones via the isomerization of propargylic acetates, skel-
[1o,m,10c,44]
[9a,44]
etal rearrangements,
and alkoxycyclization
of
13732
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 13729 – 13740
Gaillard, Sylvain
