Stereo- and Regioselective Alkyne Hydrometallation with Gold(III) Hydrides

Article abstract of DOI:10.1002/anie.201607522

The hydroauration of internal and terminal alkynes by gold(III) hydride complexes [(C^N^C)AuH] was found to be mediated by radicals and proceeds by an unexpected binuclear outer-sphere mechanism to cleanly form trans-insertion products. Radical precursors such as azobisisobutyronitrile lead to a drastic rate enhancement. DFT calculations support the proposed radical mechanism, with very low activation barriers, and rule out mononuclear mechanistic alternatives. These alkyne hydroaurations are highly regio- and stereospecific for the formation of Z-vinyl isomers, with Z/E ratios of >99:1 in most cases.

Full text of DOI:10.1002/anie.201607522

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International Edition: DOI: 10.1002/anie.201607522
German Edition: DOI: 10.1002/ange.201607522
Reaction Mechanisms Very Important Paper
Stereo- and Regioselective Alkyne Hydrometallation with Gold(III)
Hydrides
Anna Pintus, Luca Rocchigiani, Julio Fernandez-Cestau, Peter H. M. Budzelaar,* and
Manfred Bochmann*
Dedicated to Professor Gerhard Erker on the occasion of his 70th birthday
Abstract: The hydroauration of internal and terminal alkynes
by gold(III) hydride complexes [(C^N^C)AuH] was found to
be mediated by radicals and proceeds by an unexpected
binuclear outer-sphere mechanism to cleanly form trans-
insertion
products.
Radical
precursors
such
as
azobisisobutyronitrile lead to a drastic rate enhancement.
DFT calculations support the proposed radical mechanism,
with very low activation barriers, and rule out mononuclear
mechanistic alternatives. These alkyne hydroaurations are
highly regio- and stereospecific for the formation of Z-vinyl
isomers, with Z/E ratios of > 99:1 in most cases.
G
old vinyl complexes have frequently been implicated as
intermediates in gold-catalyzed transformations of alkynes
and allenes,[1–3] most commonly by nucleophilic attack.[2]
Although vinyl complexes can be obtained by transalkyla-
tion,[4] in a number of cases they could also be isolated as
reaction intermediates,[5,6] including some rare examples of
structurally characterized gold(III) vinyl compounds.[7]
The formation of gold vinyl complexes by hydroauration,
Figure 1. Synthesis of gold(III) hydrides and crystal structure of 1d.
À
The Au H hydrogen atom was identified in the electron density map.
Selected bond distances [ꢀ] and angles [8] for 1d: Au1–H1 1.64(5),
Au1–N1 2.019(3), Au1–C9 2.060(3), Au1–C18 2.058(4); N1-Au1-C9
81.06(13), N1-Au1-C18 80.79(13), C9-Au1-H1 98.2(19), C18-Au1-H1
100.0(19), N1-Au1-H1 179.2(19), C9-Au1-C18 161.85(14). THF=tetra-
hydrofuran.
À
that is, addition of gold hydrides to C C multiple bonds, has
been reported in only two cases. Tsui et al. showed that the
ꢀ
gold(I) hydride [(IPr)AuH] reacts with internal alkynes, RC
CR, under trans insertion if R = COOMe, but there was no
reaction with R = Et or Ph [IPr= 1,3-bis(2,6-diisopropylphe-
nyl)imidazol-2-ylidene].[8] More recently, we isolated the
gold(III) hydride [(C^Npy^C)AuH] (1a, Figure 1), which
was found to insert allenes to give vinyl complexes, whereas
acetylenes failed to react.[9] Given the coordinative saturation
of square-planar gold(III) compounds and the reluctance of
these complexes to bind a fifth ligand, the mechanistic aspects
of these formal insertion reactions remained obscure. Such
fundamental reactions have a direct bearing on the role of
gold catalysts in organic transformations.[10–12]
Gold(III) forms square-planar structures typical of
d8 transition metals, a complex type where ligand exchange
kinetics are largely controlled by the trans effect.[13] We
therefore considered the possibility of enhancing the reac-
tivity of the AuIII H bond by introducing s- and p-donor
À
substituents at the 4-position of the pyridine ring, for example,
tBu (1b), OMe (1c), and NMe2 (1d; Figure 1).[14] In the
course of these studies we discovered that trace amounts of
radicals play an important role and greatly enhance gold
hydride reactivity.
[*] Dr. A. Pintus, Dr. L. Rocchigiani, Dr. J. Fernandez-Cestau,
Prof. Dr. M. Bochmann
School of Chemistry, University of East Anglia
Norwich Research Park, Norwich, NR4 7TJ (UK)
E-mail: m.bochmann@uea.ac.uk
Treatment of [(LR)AuCl] with a THF solution of LiAlH4
gave the hydrides 1a–d in high yields.[15] The pyrazine-based
complex [(C^Npz^C)AuCl][16] reacted similarly to give 1e.
Prof. Dr. P. H. M. Budzelaar
Department of Chemistry, University of Manitoba
Winnipeg, Manitoba R3T 2N2 (Canada)
E-mail: Peter.Budzelaar@umanitoba.ca
À
The Au H NMR chemical shifts show only a slight
dependence on the pyridine substituent: d = À6.51 (1a),[9a]
À6.41 (1b), À6.48 (1c), À6.26 (1d), and À6.50 (1e) (in
CD2Cl2). The nature of the compounds was confirmed by the
crystal structure of 1d, which shows the NMe2 substituent
coplanar with the pyridine ring. An analysis of the molecular
charge distributions by density functional theory (DFT)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201607522. CCDC 1495244, 1495245, and 1495246 con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
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Table 1: Alkyne hydroauration with 1c in the presence of AIBN.[a]
calculations showed only a marginal effect of the pyridine p-
À
Yield Z/E[b]
t
substituents on the Au H bond polarity (see the Supporting
Information), and indeed there was little difference in
reactivity between the complexes.
Substrate
Major product
[%][b]
[min]
For reasons of solubility we chose the methoxy derivative
1c to explore hydroauration reactivity. Solutions of freshly
prepared compounds were found to react with internal and
terminal alkynes over several days to weeks ([D8]toluene,
room temperature). Remarkably, the hydroaurations pro-
ceeded with essentially quantitative regio- and stereoselec-
tivity, to give exclusively the trans-insertion products.
Given that square-planar gold(III) compounds of type
1 stabilized by rigid pincer ligands have no coordination sites
available for substrate binding, the mechanism of this alkyne
hydroauration was not obvious. Earlier computational
attempts to search for the coordination of a phosphine to
the metal perpendicular to the molecular plane had failed to
find any evidence for an energy minimum.[17] Computations
with an alkyne as a possible ligand gave analogous results:
there was no evidence for a coordinative alkyne–gold
interaction. A [2+2] pathway by suitable alignment of the
2
3
4
5
6
7
8
9
90 >99:1
>95 >99:1
>95 >99:1
>95 >99:1
90 >99:1
85
5
35
5
110
35
60
60
40
25
40
>95 >99:1
85 >99:1
ꢀ
À
C C and Au H bonds was therefore ruled out. This finding
then led to the possibility of an outer-sphere mechanism,
conceivably bimolecular. Such an outer-sphere pathway
might be provided by the assistance of either a cation or
possibly a radical. Given that our alkyne reactions were
conducted in nonpolar solvents under mild reaction condi-
tions, and also given the known light sensitivity of many gold
compounds, the participation of trace amounts of radical
species seemed most plausible.
>95 >99:1
10 >95
82:18
11
75 >99:1
Indeed, a partially complete alkyne insertion reaction was
found to stop as soon as TEMPO was added as a radical
12 >95 >99:1
scavenger
(TEMPO = 2,2,6,6-tetramethylpiperidine
N-
13 >95
80:20 1020
oxide). Conversely, the addition of a radical source such as
azobisisobutyronitrile (AIBN) greatly accelerated the alkyne
insertions and reduced the reaction time from days to
minutes, without affecting the regio- and stereoselectivity of
the process. To accelerate the decomposition of AIBN, the
reactions were carried out at 508C [Eq. (1)].
[a] Reaction conditions: [D8]toluene, 508C; conversion of 1c was >95%
in all cases; [b] Determined by NMR spectroscopy.
range typical of Z couplings [9.0–10.8 Hz, the only exceptions
being of 5 and 6, for which a J(H,H) of 13.5 Hz is observed
3
owing to the b effect of silicon]. For the 1,2-disubstituted vinyl
1
2
1
=
derivatives [Au]-C(R ) CHR , H NMR NOE experiments
revealed the presence of dipolar interactions between the
1
vinyl H and both R and R2, and the absence of such
À
interactions between R1 and R2.
The hydroaurations proceed with high regio- and stereo-
specificity. For example, of the four possible isomers of the
ꢀ
A survey of alkynes with different substitution patterns
and functional groups resulted in the formation of the vinyl
complexes 2–13 (Table 1). In most cases only the Z-vinyl
products could be detected, the exceptions being 10 and 13
where minor amounts of the E isomer were also formed. The
structures were unequivocally assigned by NMR/NOE studies
(see the Supporting Information). In particular, vinyl 13C
resonances for a- and b-carbon atoms appear in the range of
d = 159.6–134.2 and d 130.8–123.9 ppm, respectively, and are
in agreement with the data for the previously reported vinyl
MeC CPh insertion, only a single product was formed, 3, in
which the gold center is bound in a-position to the methyl
group rather than the phenyl substituent. With silyl acety-
lenes, the gold is distal to the SiR3 group. This regioselectivity
is in contrast to that obtained in a number of alkyne
hydrometallation products, for example hydrostanna-
tions,[18,19] hydrosilylations,[20] hydroaluminations,[21] and
hydropalladations,[22] where the metal is predominantly
attached a to the phenyl substituent.
Gold(III) hydrides are stable to polar solvents, water, and
even acetic acid, and indeed a range of functional groups is
tolerated, including OH, NH, CHO, and COOH. Interest-
Py
[9a]
=
complex [(C^N ^C)AuC(Me) CMe2].
In the case of
3
terminal vinyl moieties, the J(H,H) values fall within the
2
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ingly, N-propargyl carboxamides, which are well-known for
that not too much importance should be attached to the C···H
their facile cycloisomerization to oxazoles in the presence of
gold catalysts,[2,23] form exclusively the gold vinyl 13 without
cyclization. Less reactive internal alkynes such as 3-hexyne
also gave clean hydroauration products.
distances in these optimized transition states. After including
thermal and dispersion corrections, the free-energy barrier for
hydride transfer from the incoming LAuH to the second
acetylene carbon atom is 2–6 kcalmolÀ1, thus leading to the
experimentally observed trans adduct. Structures for sta-
tionary points on the radical path towards the preferred
The stereochemical stability of the gold(III) vinyl prod-
ucts in our system contrasts with a recent report of extensive
cis–trans isomerization by AIBN in radical-initiated hydro-
stannations of propargylic ethers.[24] However, exposure of
our gold vinyls to light induced slow isomerization; for
example, irradiating a solution of 2 for 1 hour with UV light
changed the Z/E ratio from greater than 99:1 to about 1:1.
Scheme 1 depicts the proposed reaction sequence. Ther-
mal cleavage of AIBN provides radicals capable of H-
abstraction from the gold(III) hydride. Since the reactions
are conducted in the presence of excess alkyne, the gold(II)
radical thus formed is rapidly trapped. Reaction of the
resulting gold-vinyl radicals with a second molecule of
1 generates the gold-vinyl product. This scenario also effort-
lessly explains the observed trans hydroauration stereochem-
istry. The gold(II) radical is evidently sufficiently long-lived to
provide regiochemical control. For example, in the case of
ꢀ
adduct of MeC CPh are shown in the Supporting Informa-
tion (see Figure S50, see also Tables S3 and S4 for total and
relative energies for all species studied). The energy profiles
for this path and its non-observed regioisomeric alternative
are summarized in Figure 2.[28] The computational results
therefore support the proposed radical chain mechanism and
rule out alternative mononuclear mechanistic variations. An
alternative pathway assisted by the LAu+ cation instead of
a radical was also explored for comparison but appears less
favorable (see the Supporting Information).
The release of the gold vinyl was exemplified in the case of
3. Treating an NMR sample of 3 in [D6]benzene with a crystal
of iodine at room temperature generated the iodoalkene Z-
=
C(I)(Me) CHPh in quantitative yield, with retention of
stereochemistry. As in the case of alkyne hydroauration, the
mechanism of gold–carbon bond cleavage is likely not to be
straightforward. This aspect is currently under investigation.
In summary, the results provide the first demonstration of
the addition of unsaturated substrates to gold–hydrogen
bonds by a radical-initiated outer-sphere mechanism. This
intermolecular pathway overcomes the inability of gold(III)
pincer complexes to bind unsaturated substrates and to follow
intramolecular coordinative mechanisms of the type that are
commonplace for other transition metals. There is a growing
body of evidence that single-electron transfer steps may be
involved in certain gold reactions, such as the photochemi-
cally induced oxidative additions to gold(I).[29] The present
results support the notion that odd-electron gold(II) species
and outer-sphere processes may play an important role in gold
reaction pathways.
ꢀ
MeC CPh, a phenyl-stabilized vinyl radical will be preferred
over its methyl-substituted isomer, thus resulting in the
formation of 3, which is precisely what is observed. Further
support for the proposed intermediacy of (C^N^C)AuC is the
formation of trace amounts of the known[9,25] gold(II) dimer
[{(C^N^C)Au}2], which was detected as a by-product in some
reactions.[26] We have previously shown that (C^N^C)AuC is
capable of attacking [(C^N^C)AuH] to give a m-H inter-
mediate as part of the electrochemical reduction of 1a.[27]
The validity of this mechanistic proposal was probed by
DFT calculations for the addition of LAuH to the acetylenes
ꢀ
ꢀ
ꢀ
H CH, HC CPh, and MeC CPh [L = (C^N^C)]. All mono-
nuclear pathways tried had prohibitive free-energy barriers
(> 35 kcalmolÀ1), thus excluding their contribution under the
present reaction conditions. Acetylene coordination to LAuC
was found to have a small barrier (1–4 kcalmolÀ1) and is
modestly exergonic (2 to 12 kcalmolÀ1). The most stable
adducts have Au next to the H or Me substituent of the
acetylene. From there on, an incoming LAuH molecule
moves without much distortion towards the second acetylenic
C on an essentially flat potential-energy surface. Transition
states having one imaginary frequency with the correct
motion were located in all cases, but the surface is so flat
Figure 2. Free-energy profile (kcalmolÀ1) for LAuC-mediated trans addi-
py
ꢀ
tion of LAuH to MeC CPh (L=C^N ^C).
Acknowledgments
This work was supported by the European Research Council.
M.B. is an ERC Advanced Investigator Award holder (grant
no. 338944-GOCAT). We are grateful to the EPSRC National
Scheme 1. Proposed alkyne trans-hydroauration pathway.
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Ed. 2012, 51, 12935 – 12936; Angew. Chem. 2012, 124, 13109 –
13110.
[10] D.-A. Ros¸ca, J. A. Wright, M. Bochmann, Dalton Trans. 2015, 44,
20785 – 20807.
[11] H. Schmidbaur, H. G. Raubenheimer, L. Dobrzanska, Chem.
Soc. Rev. 2014, 43, 345 – 380.
Crystallography service, Southampton, UK, for collecting the
crystallographic data sets.
Keywords: alkynes · density functional calculations · gold ·
insertion · reaction mechanisms
[12] a) H. Schmidbaur, A. Schier, Organometallics 2010, 29, 2 – 23;
b) H. G. Raubenheimer, H. Schmidbaur, S. Afr. J. Sci. 2011, 107,
31 – 43; c) R. E. M. Brooner, R. A. Widenhoefer, Angew. Chem.
Int. Ed. 2013, 52, 11714 – 11724; Angew. Chem. 2013, 125, 11930 –
11941.
[13] a) M. Bochmann, Organometallics and Catalysis, an Introduc-
tion, Oxford University Press, Oxford, 2015, p. 120; b) J. F.
Hartwig, Organotransition Metal Chemistry, University Science
Books, Sausalito, 2010, p. 228.
[14] For details of ligand and complex synthesis and crystal structures
of the chloride complexes (LR)AuCl see the Supporting Infor-
mation.
[15] Reactions of (C^N^C)AuCl with commercial-grade solid
LiAlH4, on the other hand, gave products of variable purity
and lower reactivity, as did products of reduction with NaBH4.
[16] J. Fernandez-Cestau, B. Bertrand, M. Blaya, G. A. Jones, T. J.
Penfold, M. Bochmann, Chem. Commun. 2015, 51, 16629 –
16632.
[1] See for example: a) M. Joost, A. Amgoune, D. Bourissou,
Angew. Chem. Int. Ed. 2015, 54, 15022 – 15045; Angew. Chem.
2015, 127, 15234 – 15258; b) B. Ranieri, I. Escofet, A. M.
Echavarren, Org. Biomol. Chem. 2015, 13, 7103 – 7118; c) T.
Lauterbach, A. M. Asiri, A. S. K. Hashmi, Adv. Organomet.
Chem. 2014, 62, 261 – 297; d) W. Yang, A. S. K. Hashmi, Chem.
Soc. Rev. 2014, 43, 2941 – 2955; e) A. S. K. Hashmi, Top. Organo-
met. Chem. 2013, 44, 143 – 164; f) L.-P. Liu, G. B. Hammond,
Chem. Soc. Rev. 2012, 41, 3129 – 3139; g) H. Schmidbaur, A.
Schier, Arab. J. Sci. Eng. 2012, 37, 1187 – 1225; h) A. Fꢀrstner,
P. W. Davies, Angew. Chem. Int. Ed. 2007, 46, 3410 – 3449;
Angew. Chem. 2007, 119, 3478 – 3519; i) A. Fꢀrstner, Chem. Soc.
Rev. 2009, 38, 3208 – 3221.
[2] Examples: a) A. S. K. Hashmi, J. P. Weyrauch, W. Frey, J. W.
Bats, Org. Lett. 2004, 6, 4391 – 4394; b) A. Buzas, F. Gagosz, Org.
Lett. 2006, 8, 515 – 518; c) S. F. Kirsch, J. T. Binder, B. Crone, A.
Duschek, T. T. Hang, C. Liꢁbert, H. Menz, Angew. Chem. Int.
Ed. 2007, 46, 2310 – 2313; Angew. Chem. 2007, 119, 2360 – 2363;
d) M. Yu, G. Zhang, L. Zhang, Org. Lett. 2007, 9, 2147 – 2150;
e) Y. Yu, W. Yang, D. Pflꢂsterer, A. S. K. Hashmi, Angew. Chem.
Int. Ed. 2014, 53, 1144 – 1147; Angew. Chem. 2014, 126, 1162 –
1165; f) D. Malhotra, M. S. Mashuta, G. B. Hammond, B. Xu,
Angew. Chem. Int. Ed. 2014, 53, 4456 – 4459; Angew. Chem.
2014, 126, 4545 – 4548; g) W. Wang, G. B. Hammond, B. Xu, J.
Am. Chem. Soc. 2012, 134, 5697 – 5703; h) P. McGee, G.
Bellavance, I. Korobkov, A. Tarasewicz, L. Barriault, Chem.
Eur. J. 2015, 21, 9662 – 9665.
[3] a) I. D. Jurberg, Y. Odabachian, F. Gagosz, J. Am. Chem. Soc.
2010, 132, 3543 – 3552; b) P. Nçsel, T. Lauterbach, M. Rudolph,
F. Rominger, A. S. K. Hashmi, Chem. Eur. J. 2013, 19, 8634 –
8641; c) P. Nçsel, V. Mꢀller, S. Mader, S. Moggimi, M. Rudolph,
I. Braun, F. Rominger, A. S. K. Hashmi, Adv. Synth. Catal. 2015,
357, 500 – 506.
[4] a) A. N. Nesmeyanov, G. Perevalova, V. V. Krivykh, A. N.
Kosina, K. I. Grandberg, E. I. Smyslova, Izv. Akad. Nauk
SSSR Ser. Khim. 1972, 653 – 654; b) N. A. Barnes, A. K. Brisdon,
W. I. Cross, J. G. Fay, J. A. Greenall, R. G. Pritchard, J. Sher-
rington, J. Organomet. Chem. 2000, 616, 96 – 105; c) F. Mohr,
L. R. Falvello, M. Laguna, Eur. J. Inorg. Chem. 2006, 833 – 838;
d) A. S. K. Hashmi, T. D. Ramamurthi, F. Rominger, J. Organo-
met. Chem. 2009, 694, 592 – 597.
[5] a) A. S. K. Hashmi, Gold Bull. 2009, 42, 275 – 279; b) A. S. K.
Hashmi, A. M. Schuster, F. Rominger, Angew. Chem. Int. Ed.
2009, 48, 8247 – 8249; Angew. Chem. 2009, 121, 8396 – 8398;
c) M. Wieteck, M. Højer Larsen nꢁe Vilhelmsen, P. Nçsel, J.
Schulmeister, F. Rominger, M. Rudolph, M. Pernpointner,
A. S. K. Hashmi, Adv. Synth. Catal. 2016, 358, 1449 – 1462.
[6] T. J. Brown, D. Weber, M. R. Gagnꢁ, R. A. Widenhoefer, J. Am.
Chem. Soc. 2012, 134, 9134 – 9137.
[17] D.-A. Ros¸ca, J. A. Wright, D. L. Hughes, M. Bochmann, Nat.
Commun. 2013, 4, 2167.
[18] S. M. Rummelt, K. Radkowski, D. A. Ros¸ca, A. Fꢀrstner, J. Am.
Chem. Soc. 2015, 137, 5506 – 5519.
[19] T. J. Hadlington, M. Hermann, G. Frenking, C. Jones, Chem. Sci.
2015, 6, 7249 – 7257.
[20] S. Ding, L. J. Song, L. W. Chung, X. Zhang, J. Sun, Y. D. Wu, J.
Am. Chem. Soc. 2013, 135, 13835 – 13842.
[21] F. Gao, A. H. Hoveyda, J. Am. Chem. Soc. 2010, 132, 10961 –
10963.
[22] R. Shen, T. Chen, Y. Zhao, R. Qiu, Y. Zhou, S. Yin, Y. Wang, M.
Goto, L. B. Han, J. Am. Chem. Soc. 2011, 133, 17037 – 17044.
[23] J. P. Weyrauch, A. S. K. Hashmi, A. Schuster, T. Hengst, S.
Schetter, A. Littmann, M. Rudolph, M. Hamzic, J. Visus, F.
Rominger, W. Frey, J. W. Bats, Chem. Eur. J. 2010, 16, 956 – 963.
[24] D. P. Curran, T. A. McFadden, J. Am. Chem. Soc. 2016, 138,
7741 – 7752.
[25] D.-A. Ros¸ca, M. Bochmann, Organometallics 2016, 35, 27 – 31.
[26] In one case a small amount of crystals of the by-product were
obtained which were identified as [(LOMe)Au]2 by X-ray crys-
tallography: A. Pintus, J. Fernandez-Cestau, unpublished results.
[27] T. Dann, D.-A. Ros¸ca, J. A. Wright, G. G. Wildgoose, M.
Bochmann, Chem. Commun. 2013, 49, 10169 – 10171.
[28] Based on the energy profile of Figure 2 one would expect only
a slight regioselectivity for the observed product. This result
might be due to the limited basis set we were forced to use for
these calculations because of the size of the system.
[29] a) B. Sahoo, M. N. Hopkinson, F. Glorius, J. Am. Chem. Soc.
2013, 135, 5505 – 5508; b) A. Tlahuext-Aca, M. N. Hopkinson, B.
Sahoo, F. Glorius, Chem. Sci. 2016, 7, 89 – 93; c) M. S. Winston,
W. J. Wolf, F. D. Toste, J. Am. Chem. Soc. 2014, 136, 7777 – 7782;
d) X.-Z. Shu, M. Zhang, Y. He, H. Frei, F. D. Toste, J. Am. Chem.
Soc. 2014, 136, 5844 – 5847; e) S. Kim, J. Rojas-Martin, F. D.
Toste, Chem. Sci. 2016, 7, 85 – 88; f) D. V. Patil, H. Yun, S. Shin,
Adv. Synth. Catal. 2015, 357, 2622 – 2628; g) L. Huang, F.
Rominger, M. Rudolph, A. S. K. Hashmi, Chem. Commun.
2016, 52, 6435 – 6438; h) L. Huang, M. Rudolph, F. Rominger,
A. S. K. Hashmi, Angew. Chem. Int. Ed. 2016, 55, 4808 – 4813;
Angew. Chem. 2016, 128, 4888 – 4893.
[7] O. A. Egorova, H. Seo, Y. Kim, D. Moon, Y. M. Rhee, K. H.
Ahn, Angew. Chem. Int. Ed. 2011, 50, 11446 – 11450; Angew.
Chem. 2011, 123, 11648 – 11652.
[8] E. Y. Tsui, P. Mꢀller, J. P. Sadighi, Angew. Chem. Int. Ed. 2008,
47, 8937 – 8940; Angew. Chem. 2008, 120, 9069 – 9072.
[9] a) D.-A. Ros¸ca, D. A. Smith, D. L. Hughes, M. Bochmann,
Angew. Chem. Int. Ed. 2012, 51, 10643 – 10646; Angew. Chem.
2012, 124, 10795 – 10798; b) A. S. K. Hashmi, Angew. Chem. Int.
Received: August 4, 2016
Published online: && &&, &&&&
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Communications
Reaction Mechanisms
A. Pintus, L. Rocchigiani,
J. Fernandez-Cestau, P. H. M. Budzelaar,*
M. Bochmann*
&&&& — &&&&
Stereo- and Regioselective Alkyne
Hydrometallation with Gold(III) Hydrides
Gold(II) provides a helping hand: Gold-
(III) pincer complexes overcome their
lack of substrate binding sites by partic-
ipating in a radical-mediated bimolecular
pathway. Gold(III) hydrides react with
acetylenes in the presence of a radical
source in a kinetically controlled reaction
to give trans-hydroauration products with
high regio- and stereoselectivity.
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!