Z-Selective Copper(I)-Catalyzed Alkyne Semihydrogenation with Tethered Cu-Alkoxide Complexes

Article abstract of DOI:10.1002/chem.201501739

A highly stereoselective alkyne semihydrogenation with copper(I) complexes is reported. Copper-N-heterocyclic carbene complex catalysts, bearing an intramolecular Cu-O bond, allow for the direct transfer of both hydrogen atoms from dihydrogen to the alkyn

Full text of DOI:10.1002/chem.201501739

DOI: 10.1002/chem.201501739
Communication
&
Homogeneous Catalysis
Z-Selective Copper(I)-Catalyzed Alkyne Semihydrogenation with
Tethered Cu–Alkoxide Complexes
Felix Pape, Niklas O. Thiel, and Johannes F. Teichert*[a]
Abstract: A highly stereoselective alkyne semihydrogena-
tion with copper(I) complexes is reported. Copper–N-het-
erocyclic carbene complex catalysts, bearing an intramo-
lecular CuÀO bond, allow for the direct transfer of both
hydrogen atoms from dihydrogen to the alkyne. The cor-
responding alkenes can be isolated with high Z selectivity
and negligible overreduction to the alkane.
Catalytic stereoselective semihydrogenation of alkynes to Z-al-
kenes[1] represents a synthetically useful transformation that re-
mains challenging to effect. Most reported catalysts,[2] such as
the widely employed heterogeneous Lindlar catalyst,[3] require
careful monitoring of hydrogen uptake due to partial overre-
duction and/or E/Z-isomerization processes.
Although several homogeneous catalysts for stereoselective
alkyne semihydrogenations aimed at overcoming these limita-
tions have been reported,[4–8] catalysts based on earth-abun-
dant transition metals are rare.[9,10]
Copper(I) hydrides[11–13] offer a viable alternative to the afore-
Scheme 1. Approaches to alkyne semihydrogenation with Cu complexes:
Use of separate hydride and proton sources vs. transfer of H2.
mentioned processes, as the syn-selective insertion of alkynes
into CuÀH bonds has been documented.[14,15] This reactivity
has found its application as a key step in reductive functionali-
zation[16] and formal semihydrogenation[14,17–19] of alkynes. The
first example of alkyne semihydrogenation relied upon stoi-
chiometric use of a copper hydride complex in the presence of
water (Scheme 1a).[14] Later, it was shown that in combination
with a stoichiometric hydride source such as a silane[17–19] or,
more recently, dihydrogen,[20] catalytic amounts of copper(I)
complexes were sufficient for the semihydrogenation reactions
to occur (Scheme 1b). However, as the hydride and the proton
are delivered by different reagents, these examples represent
formal semihydrogenations.[21]
Additionally, ease of product purification—one of the hall-
marks of hydrogenation processes in general—is hampered by
the use of alkoxide additives.
We therefore sought to develop a copper(I) catalyst for ste-
reoselective semihydrogenations of alkynes employing solely
dihydrogen as reducing agent (Scheme 1c). This approach
would circumvent the need for protic additives and/or alkoxide
activators, allowing for easier purification of the desired prod-
uct and effectively rendering this approach more atom-eco-
nomical.[23]
Catalytic CÀH bond-forming reactions with CuÀH complexes
can be performed with H2 as a terminal reducing agent.[24–33] In
these cases, the aforementioned additives serve to generate
a Cu–alkoxide 2 from a suitable precursor 1 (Scheme 2,
top).[20,24–33] Cu–alkoxides 2 in turn are thought to be preacti-
vated for the reaction with silanes or H2,[11–13,34] subsequently
generating a (putative) CuÀH complex 3 in situ.
Although the latter catalytic transformation (Scheme 1b) ex-
emplifies the utility of Cu complexes for alkyne semihydroge-
nation, the use of stoichiometric reducing agents (in the case
of silanes)[17–19] or relatively high amounts of alkoxide additives
(in the case of H2 as reducing agent)[20] could be viewed as not
optimal with regards to the atom efficiency[22] of the process.
We report herein a Cu-catalyzed alkyne semihydrogenation
with Z/E stereoselectivities of >99:1 and negligible overreduc-
tion to the corresponding alkanes, even at prolonged reaction
times (42 h). To replace protic solvents or additives, we rea-
soned that Cu complexes bearing an intramolecular chelating
CuÀO bond (5) should be suited for H2 activation, giving rise
to the analogous CuÀH complexes (6; Scheme 2, bottom).[35] In
[a] F. Pape,+ N. O. Thiel,+ Dr. J. F. Teichert
Institut für Chemie, Technische Universtität Berlin
Straße des 17. Juni 115, 10623 Berlin (Germany)
E-mail: johannes.teichert@chem.tu-berlin.de
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501739.
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Table 1. Optimization of alkyne semihydrogenation.[a]
Scheme 2. Copper–alkoxide complexes (L=NHC or phosphine) and alkox-
ide-tethered Cu–NHC complexes.
Entry Ligand PH
T
[8C]
t [h] Conversion of 7
8/9/10 [%]
2
[bar]
[%]
1
2
3
4
5
6
7[e]
8
9[f]
10[f]
4a
4a
4a
4a
4b
4c[d]
4a
–
100
100
100
100
100
100
100
100
1
40
40
40
30
40
40
40
40
RT
RT
7
81[b]
>99.9:0:0[b]
99.8:0.2:0[b]
99.7:0.3:0[b]
>99.9:0:0[b]
>99.9:0:0[b]
–:–:–
–:–:–
–:–:–
45.0:5.0:50.0[b]
0:0:>99.9[b,g]
18 99.5[b,c]
42 99.2[b]
21.5 87[b]
this manner, heterolytic H2 cleavage[34] would lead to a CuÀH
moiety and an alcohol in the same compound. The alcohol in
close proximity to Cu could facilitate an intramolecular proto-
decupration of the putative vinylcopper intermediate[15] giving
rise to the desired alkene. Hence, the tethered alkoxide serves
as an intramolecular catalytic proton source after heterolytic H2
cleavage, effecting the transfer of both H atoms from H2 to the
alkyne. Alkoxide-tethered Cu complexes 5 should be accessible
from previously reported alcohol-substituted N-heterocyclic
carbene (NHC) ligand precursors 4 (Figure 1).[36–38]
18 28[b]
18
18
18 irreproducible
1.5 100[b]
16 100[b,g]
–
–
–
–
1
[a] MesCu=mesitylcopper(I); [b] determined by GC; [c] isolated in 93%
yield; [d] 6 mol% 4c, 5 mol% MesCu was used; [e] no MesCu, 12 mol%
nBuLi; [f] 10 mol% Pd/CaCO3, poisoned with Pb (10 wt% Pd), 10 mol%
1
quinoline; [g] determined by H NMR spectroscopy.
sor 4c, bearing a tethered ether, led to no conversion, empha-
sizing the importance of the intramolecular CuÀO bond for re-
activity (Table 1, entry 6).[42] In the absence of a copper(I)
source, no reaction was observed; employing solely MesCu led
to irreproducible results, giving varying mixtures of 8, 9, and
10 (Table 1, entries 7 and 8).
Figure 1. Selected NHC precursors used in this study.[39]
The envisaged transformation was optimized using the semi-
hydrogenation of alkyne 7 as a test reaction (Table 1).[39] Pre-
liminary experiments showed that reproducible results were
obtained with mesitylcopper(I) (MesCu)[40,41] as source of cop-
per(I) and as base toward imidazol(in)ium salts 4 in THF. n-Bu-
tyllithium was employed as second equivalent of base with re-
gards to NHC precursors 4a and 4b to generate the proposed
complexes 5 in situ. NHC ligands with a tether length of 2
carbon atoms were identified as most active in early optimiza-
tion studies.[39] With mesityl-substituted imidazolinium hexa-
fluorophosphate 4a in THF at 408C, the semihydrogenation of
7 gave 81% conversion after seven hours. Full conversion was
reached after 18 h (Table 1, entries 1 and 2), with alkene 8 iso-
lated in 93% yield. In both cases, only Z isomer 8 was ob-
served with no E-alkene 9 or overreduced alkane 10 detected.
These ratios were found to be virtually unchanged even with
prolonged reaction time of 42 h (Table 1, entry 3). This is in
contrast to a related study reported diminishing Z/E selectivity
after increased reaction time.[20] Lowering the temperature to
308C led to a drop in conversion to 87% (Table 1, entry 4),
albeit with similar stereoselectivity.
Stryker’s reagent,[43,44] [Ph3PCuH]6, or the Lindlar catalyst did
not lead to chemoselective alkyne semihydrogenation of 7:
Under standard Lindlar conditions,[45] a mixture of the E/Z al-
kenes 8 and 9 in conjunction with the alkane 10 was observed
after 1.5 h (Table 1, entry 9). Alkane 10 was the sole product
when the reaction was run overnight (Table 1, entry 10). In
a similar vein, it should be noted that the quality of mesityl-
copper(I) is crucial for the observed chemoselectivity.[46] Em-
ploying commercially available or aged MesCu led to the for-
mation of alkane 10 (>10% GC yield), possibly due to the
presence of heterogeneous copper species.[41,47] Even though
the true nature of the catalytic species is difficult to exactly de-
termine as homo- or heterogeneous,[48] we put forward the hy-
pothesis that the formation of E-alkene 9 and alkane 10 can
be traced to heterogeneous Cu species.[49]
Subsequently, the substrate scope of the Cu-catalyzed
alkyne semihydrogenation was investigated at 408C and a H2
pressure of 100 bar (Table 2). In all cases, excellent Z selectivity
was maintained and no significant overreduction to the corre-
sponding alkanes was observed. Both electron-rich and -poor
phenylacetylene derivatives were semihydrogenated to the
corresponding alkenes in good to excellent yields, as demon-
Imidazolium-derived ligand precursor 4b gave only 28%
conversion at 408C (Table 1, entry 5). Importantly, NHC precur-
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as full conversion with 5 mol% of copper(I) catalyst was
achieved after 65 h (Table 2, entry 4). Alternatively, to ensure
brevity of reaction time, similar good results in terms of con-
version (100%) and stereoselectivity were achieved with
10 mol% catalyst within 18 h (Table 2, entry 3). With respect to
heterocyclic substituents, thiophene derivative 11 d gave only
17% conversion, whereas pyridinyl-substituted substrates
11 e–g gave the desired products in generally high yields
(Table 2, entries 5 and 6). The reactivity towards carbonyl
groups is also notable. Whereas ketone 11 h was converted
with a catalyst loading of 5 mol%, no reaction took place with
aldehyde 11 i (Table 2, entries 7 and 8). In the case of ketone
11 h and aldehyde 11 i, small amounts (1–3%) of a benzyl alco-
hol, presumably resulting from a carbonyl reduction, were ob-
served. This is noteworthy, as related CuÀH catalysts have suc-
cessfully been employed for the 1,2-reduction of carbonyl
groups,[11–13] which is almost fully suppressed in these cases.
The corresponding acetal-protected substrate 11 j, however, is
amenable to the semihydrogenation, giving 95% yield of Z-
alkene 12j (Table 2, entry 9).
Compounds bearing acidic protons, such as terminal alkyne
11 k and propargylic alcohol 11 l are not tolerated by the Cu
catalyst (Table 2, entries 10 and 11). This is probably due to the
basic Cu–alkoxide bond present in the in situ-formed com-
plexes of type 5 (Scheme 2).[50] Propargylic ether 11 m did not
lead to any conversion, whereas the dialkylacetylene 11 n gave
55% of the desired Z-alkene (Table 1, entries 12 and 13).
In the following, we investigated a selection of phenylacety-
lene derivatives (Table 3). The tert-butyl-dimethylsilyl (TBS) pro-
tecting group was tolerated by the semihydrogenation, as the
corresponding Z-alkene 14a was isolated in 92% yield (Table 3,
entry 1). Diphenylacetylene 13b was converted into Z-stilbene
14b with good yield (79%) and excellent stereoselectivity
(Table 3, entry 2). Similar results were obtained for nitrile-sub-
stituted 13c and the reaction of phenylpropyne 13d (Table 3,
entries 3 and 4). Finally, a,b-unsaturated ester 13e did not un-
dergo any conversion (Table 3, entry 5). The absence of any
conjugate reduction of the activated alkene in 13e appears
counterintuitive, as CuÀH complexes have found many applica-
tions in the conjugate reduction of these functional
groups.[11–13]
Table 2. Substrate scope of the Cu-catalyzed semihydrogenation of pen-
tynol derivatives.
Entry
1
Substrate
Yield of isolated product [%]
88[a]
2
3
75[b]
70[b]
4
5
11 c
100% conv.[a,c,d]
17% conv.[b,c,d]
2-pyridinyl (11 e): 71[e]
3-pyridinyl (11 f): 89
4-pyridinyl (11 g): 85
6
7
84[a,f]
[a,f]
8
–
9
95[b]
[b]
10
–
[b]
11
12
13
–
Besides 11 n (Table 2, entry 13), the present semihydrogena-
tion protocol is also applicable to dialkyl-substituted internal
alkynes: 6-Dodecyne 13 f and 3-hexyne 13g were succesfully
converted into the corresponding Z-alkenes (Table 3, entries 6
and 7). However, as with dialkylalkyne 11 n, lower yields were
observed.
[b]
–
55[b]
To underscore the hypothesis that both hydrogen atoms in
the semihydrogenated products stem from H2, labeling experi-
ments with D2 were conducted. When the reaction of phenyl-
[a] 5 mol% [Cu] was used; [b] 10 mol% [Cu] was used; [c] reaction was
run for 65 h; [d] determined by 1H NMR; [e] 95% conversion, 3% (E)-12e,
1
5% alkane, determined by H NMR spectroscopy; [f] 1–3% of benzyl alco-
hol detected.
acetylene
7 was carried out under standard conditions
(Table 1, entry 2) under D2 atmosphere, only 15% conversion
to [D2]8 was observed, indicating a significant kinetic isotope
effect. With longer reaction time (48 h) and higher catalyst
loading, full conversion of 7 could be achieved (Scheme 3).
The corresponding product [D2]8 showed 98% D-incorporation
in the b-position and 84% D-incorporation in the a-position.[51]
strated by benzoic ester 11 a (88%), anisole 11 b (75%), and
benzonitrile derivative 11 c (70%; Table 2, entries 1–3). With the
latter, one of the key features of this catalyst can be highlight-
ed: The catalyst remains active for a prolonged period of time,
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at prolonged reaction times. Due to this high stereo- and che-
moselectivity, the developed copper(I) catalyst offers a viable
alternative to semihydrogenation with heterogeneous catalysts
such as the Lindlar catalyst.
Table 3. Substrate scope of the Cu-catalyzed semihydrogenation contin-
ued.
Acknowledgements
J.F.T acknowledges support by the Fonds der Chemischen In-
dustrie through a Liebig-Stipendium. F.P. is supported by a pre-
doctoral scholarship of the Berlin International Graduate
School of Natural Sciences and Engineering (BIG-NSE). Prof. Dr.
Linda H. Doerrer (Boston University) is acknowledged for fruit-
ful discussions. Prof. Dr. Martin Oestreich (TU Berlin) is kindly
thanked for generous support.
Entry
1
Substrate
Yield of isolated product [%]
92[a]
2
3
4
79[a]
70[b,c]
Keywords: alkynes
· copper · homogeneous catalysis ·
hydrogenation · stereoselectivity
[1] W.-Y. Siau, Y. Zhang, Y. Zhao, Top. Curr. Chem. 2012, 327, 33–58.
100% conv.[b,d,e]
[2] a) C. Oger, L. Balas, T. Durand, J.-M. Galano, Chem. Rev. 2013, 113, 1313–
1350; b) M. Crespo-Quesada, F. Cµrdenas-Lizana, A.-L. Dessimoz, L. Kiwi-
Minsker, ACS Catal. 2012, 2, 1773–1786; c) A. Molnµr, A. Sarkany, M.
Varga, J. Mol. Catal. A 2001, 173, 185–221; d) E. N. Marvell, T. Li, Synthe-
sis 1973, 457–468.
[b]
5
6
7
–
[3] H. Lindlar, Helv. Chim. Acta 1952, 35, 446–450.
53
[4] For a review, see: A. M. Kluwer, C. J. Elsevier, in The Handbook of Homo-
geneous Hydrogenation (Eds.: J. G. de Vries, C. J. Elsevier), Wiley-VCH,
Weinheim, 2006, pp. 374–411.
100% conv.[f]
[5] Palladium: a) A. Bacchi, M. Carcelli, M. Costa, A. Leporati, E. Leporati, P.
Pelagatti, C. Pelizzi, G. Pelizzi, J. Organomet. Chem. 1997, 535, 107–120;
b) P. Pelagatti, A. Venturini, A. Leporati, M. Carcelli, M. Costa, A. Bacchi,
G. Pelizzi, C. Pelizzi, J. Chem. Soc. Dalton Trans. 1998, 2715–2721;
c) M. W. van Laren, C. J. Elsevier, Angew. Chem. Int. Ed. 1999, 38, 3715–
3717; Angew. Chem. 1999, 111, 3926–3929; d) M. Costa, P. Pelagatti, C.
Pelizzi, D. Rogolino, J. Mol. Catal. A 2002, 178, 21–26; e) M. W. van La-
ren, M. A. Duin, C. Klerk, M. Naglia, D. Rogolino, P. Pelagatti, A. Bacchi,
C. Pelizzi, C. J. Elsevier, Organometallics 2002, 21, 1546–1553; f) A. M.
Kluwer, T. S. Koblenz, T. Jonischkeit, K. Woelk, C. J. Elsevier, J. Am. Chem.
Soc. 2005, 127, 15470–15480; For a related Ru-catalyzed alkyne semihy-
drogenation, see: g) C. Belger, N. M. Neisius, B. Plietker, Chem. Eur. J.
2010, 16, 12214–12220.
[a] 5 mol% [Cu] was used; [b] 10 mol% [Cu] was used; [c] reaction was
run for 65 h; [d] determined by GC; [e] 5% alkane, detected by GC; [f] de-
tected by H NMR spectroscopy, product is volatile.
1
[6] Vanadium: H. S. La Pierre, J. Arnold, F. D. Toste, Angew. Chem. Int. Ed.
2011, 50, 3900–3903; Angew. Chem. 2011, 123, 3986–3989.
[7] Niobium: T. L. Gianetti, N. C. Tomson, J. Arnold, R. G. Bergman, J. Am.
Chem. Soc. 2011, 133, 14904–14907.
[8] Tantalum: D. R. Mulford, J. R. Clark, S. W. Schweiger, P. E. Fanwick, I. P.
Rothwell, Organometallics 1999, 18, 4448–4458.
[9] R. M. Bullock, Science 2013, 342, 1054–1055.
Scheme 3. Alkyne semihydrogenation with D2.
[10] For selected examples with Fe catalysts, see: a) C. Bianchini, A. Meli, M.
Peruzzini, F. Vizza, F. Zanobini, P. Frediani, Organometallics 1989, 8,
2080–2082; b) C. Bianchini, A. Meli, M. Peruzzini, P. Frediani, C. Bohan-
na, M. A. Esteruelas, L. A. Oro, Organometallics 1992, 11, 138–145.
[11] B. H. Lipshutz, in Modern Organocopper Chemistry (Ed.: N. Krause),
Wiley-VCH, Weinheim, 2002, pp. 167–187.
This result supports the notion that the present catalyst facili-
tates a genuine dihydrogenation of alkynes.
In summary, we have developed a novel Cu-catalyzed semi-
hydrogenation of alkynes in which both hydrogen atoms of di-
hydrogen are transferred. This atom-economic transformation
relies on Cu–NHC complexes bearing an intramolecular CuÀO
bond for H2 activation, effectively generating a catalytic source
of protons in situ, eliminating the need for additional activa-
tors or protic additives. Important features of the present semi-
hydrogenation protocol are high Z-stereoselectivity and the
absence of overreduction to the corresponding alkanes, even
[12] S. Rendler, M. Oestreich, Angew. Chem. Int. Ed. 2007, 46, 498–504;
Angew. Chem. 2007, 119, 504–510.
[13] C. Deutsch, N. Krause, B. H. Lipshutz, Chem. Rev. 2008, 108, 2916–2927.
[14] J. F. Daeuble, C. McGettigan, J. M. Stryker, Tetrahedron Lett. 1990, 31,
2397–2400.
[15] N. P. Mankad, D. S. Laitar, J. P. Sadighi, Organometallics 2004, 23, 3369–
3371.
[16] a) T. Fujihara, T. Xu, K. Semba, J. Terao, Y. Tsuji, Angew. Chem. Int. Ed.
2011, 50, 523–527; Angew. Chem. 2011, 123, 543–547; b) M. R. Uehling,
R. P. Rucker, G. Lalic, J. Am. Chem. Soc. 2014, 136, 8799–8803; c) S.-L.
Shi, S. L. Buchwald, Nat. Chem. 2014, 7, 38–44.
Chem. Eur. J. 2015, 21, 15934 – 15938
www.chemeurj.org
15937
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[17] K. Semba, T. Fujihara, T. Xu, J. Terao, Y. Tsuji, Adv. Synth. Catal. 2012, 354,
1542–1550.
[18] A. M. Whittaker, G. Lalic, Org. Lett. 2013, 15, 1112–1115.
[19] N. Cox, H. Dang, A. M. Whittaker, G. Lalic, Tetrahedron 2014, 70, 4219–
4231.
[20] During the course of this study, a related report appeared: K. Semba, R.
Kameyama, Y. Nakao, Synlett 2015, 26, 318–322.
[21] In these cases, it is suggested that a vinylcopper(I) intermediate, result-
ing from the reaction of a Cu catalyst, a silane and the alkyne, is
trapped by alcohols, which formally deliver the H+ equivalent for a sub-
sequent protodecupration reaction.
[22] a) R. A. Sheldon, Pure Appl. Chem. 2000, 72, 7; b) R. A. Sheldon, Chem.
Soc. Rev. 2012, 41, 1437–1451.
[23] a) B. M. Trost, Science 1991, 254, 1471–1477; b) B. M. Trost, Angew.
Chem. Int. Ed. Engl. 1995, 34, 259–281; Angew. Chem. 1995, 107, 285–
307.
[24] W. S. Mahoney, J. M. Stryker, J. Am. Chem. Soc. 1989, 111, 8818–8823.
[25] J. X. Chen, J. F. Daeuble, D. M. Brestensky, J. F. Stryker, Tetrahedron 2000,
56, 2153–2166.
[26] J. X. Chen, J. F. Daeuble, J. M. Stryker, Tetrahedron 2000, 56, 2789–2798.
[27] H. Shimizu, D. Igarashi, W. Kuriyama, Y. Yusa, N. Sayo, T. Saito, Org. Lett.
2007, 9, 1655–1657.
[38] For selected examples of Cu-catalyzed allylic substitutions with alcohol-
tethered NHC-ligands, see: a) A. O. Larsen, W. Leu, C. N. Oberhuber, J. E.
Campbell, A. H. Hoveyda, J. Am. Chem. Soc. 2004, 126, 11130–11131;
b) M. Magrez, Y. Le Guen, O. BaslØ, C. CrØvisy, M. Mauduit, Chem. Eur. J.
2013, 19, 1199–1203; c) F. Meng, K. P. McGrath, A. H. Hoveyda, Nature
2014, 513, 367–374.
[39] For further optimization studies, see the Supporting Information.
[40] T. Tsuda, T. Yazawa, K. Watanabe, T. Fujii, T. Saegusa, J. Org. Chem. 1981,
46, 192–194.
[41] M. Stollenz, F. Meyer, Organometallics 2012, 31, 7708–7727.
[42] Experiments with Cu complexes formed from Ligand 4c and sec-BuOLi
as external activator also showed alkyne semireduction with diminished
E/Z selectivity and significant overreduction. These experiments show-
case the advantage of Cu catalyst bearing an intramolecular alkoxide
tether. See the Supporting Information for details.
[43] The use of Stryker’s reagent ([Ph3PCuH]6), a reported catalyst for the hy-
drogenation of a,b-unsaturated carbonyl compounds (see refs. [24–26]),
led to full conversion of 7 under the standard conditions (2 mol%
[Ph3PCuH]6, THF, 408C, H2 (100 bar), 18 h), albeit with 6% (E)-9 and 2%
overreduction to 10. After 66 h, the ratio 8/9/10 was 65.0:6.0:29.0, as
determined by GC. In these hydrogenation experiments, formation of
Cu0 was observed.
[28] H. Shimizu, I. Nagasaki, K. Matsumura, N. Sayo, T. Saito, Acc. Chem. Res.
2007, 40, 1385–1393.
[29] H. Shimizu, T. Nagano, N. Sayo, T. Saito, T. Ohshima, K. Mashima, Synlett
2009, 19, 3143–3146.
[44] D.-w. Lee, J. Yun, Tetrahedron Lett. 2005, 46, 2037–2039.
[45] M. R. Kumar, K. Park, S. Lee, Adv. Synth. Catal. 2010, 352, 3255–3266.
[46] For experimental details on the preparation of mesitylcopper(I), see the
Supporting Information.
[30] H. Shimizu, N. Sayo, T. Saito, Synlett 2009, 8, 1295–1298.
[31] K. Junge, B. Wendt, D. Addis, S. Zhou, S. Das, S. Fleischer, M. Beller,
Chem. Eur. J. 2011, 17, 101–105.
[32] S. W. Krabbe, M. A. Hatcher, R. K. Bowman, M. B. Mitchell, M. S. McClure,
J. S. Johnson, Org. Lett. 2013, 15, 4560–4563.
[47] a) C. Barri›re, K. Piettre, V. Latour, O. Margeat, C.-O. Turrin, B. Chaudret,
P. Fau, J. Mater. Chem. 2012, 22, 2279–2285; b) M. Cokoja, B. R. Jagirdar,
H. Parala, A. Birkner, R. A. Fischer, Eur. J. Inorg. Chem. 2008, 21, 3330–
3339; c) C. Barri›re, G. Alcaraz, O. Margeat, P. Fau, J. B. Quoirin, C.
Anceau, B. Chaudret, J. Mater. Chem. 2008, 18, 3084–3086.
[48] a) R. H. Crabtree, Chem. Rev. 2012, 112, 1536–1554; b) J. A. Widegren,
R. G. Finke, J. Mol. Catal. A 2003, 198, 317–341.
[33] R. Watari, Y. Kayaki, S.-i. Hirano, N. Matsumoto, T. Ikariya, Adv. Synth.
Catal. 2015, 357, 1369–1373.
[34] G. V. Goeden, K. G. Caulton, J. Am. Chem. Soc. 1981, 103, 7354–7355.
[35] For preliminary investigations of the Cu–H complexes in solution, see
the Supporting Information.
[36] For a review, see: S. T. Liddle, I. S. Edworthy, P. L. Arnold, Chem. Soc. Rev.
2007, 36, 1732–1744.
[37] For the synthesis of alcohol-tethered NHC ligands, see: a) C. Jahier-
Diallo, M. S. T. Morin, P. Queval, M. Rouen, I. Artur, P. Querard, L. Toupet,
C. CrØvisy, O. BaslØ, M. Mauduit, Chem. Eur. J. 2015, 21, 993–997; b) H.
Clavier, L. Coutable, L. Toupet, J.-C. Guillemin, M. Mauduit, J. Organomet.
Chem. 2005, 690, 5237–5254; c) D. Martin, S. Kehrli, M. d’Augustin, H.
Clavier, M. Mauduit, A. Alexakis, J. Am. Chem. Soc. 2006, 128, 8416–
8417; d) S. Kehrli, D. Martin, D. Rix, M. Mauduit, A. Alexakis, Chem. Eur. J.
2010, 16, 9890–9904.
[49] For a discussion of the configurational stability of alkenylcopper(I) spe-
cies, see: G. M. Whitesides, C. P. Casey, J. K. Krieger, J. Am. Chem. Soc.
1971, 93, 1379–1389. For a study regarding the stability of tethered
CuI-hydrides, see reference 25.
[50] For a similar highly basic (NHC)CuÀOR bond (with R=H), see: G. Fort-
man, C. Slawin, M. Z. Alexandra, S. P. Nolan, Organometallics 2010, 29,
3966–3972.
[51] As judged by NMR spectroscopy.
Received: May 4, 2015
Revised: September 9, 2015
Published online on September 23, 2015
Chem. Eur. J. 2015, 21, 15934 – 15938
www.chemeurj.org
15938
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