Selective Alkenylation and Hydroalkenylation of Enol Phosphates through Direct C-H Functionalization

Article abstract of DOI:10.1002/anie.201506437

An efficient and selective Rh-catalyzed direct C-H functionalization reaction of enol phosphates was developed. The method is applicable to a variety of coupling partners, including activated alkenes, alkynes, and allenes, and leads to the formation of various valuable alkenylated and hydroalkenylated enol phosphates through the action of the phosphate directing group. The versatility and utility of the coupling products were demonstrated through further transformations into synthetically useful building blocks. P points the way: A direct C-H functionalization of enol phosphates was developed. The method is applicable to a variety of coupling partners, including activated alkenes, alkynes, and allenes, and it leads to the formation of alkenylated and hydroalkenylated enol phosphates through the action of the phosphate directing group. The utility of the coupling products are demonstrated by further transformations into synthetically useful building blocks.

Full text of DOI:10.1002/anie.201506437

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201506437
German Edition: DOI: 10.1002/ange.201506437
Cross-Coupling
Selective Alkenylation and Hydroalkenylation of Enol Phosphates
À
through Direct C H Functionalization
Xu-Hong Hu, Xiao-Fei Yang, and Teck-Peng Loh*
À
Abstract: An efficient and selective Rh-catalyzed direct C H
functionalization reaction of enol phosphates was developed.
The method is applicable to a variety of coupling partners,
including activated alkenes, alkynes, and allenes, and leads to
the formation of various valuable alkenylated and hydro-
alkenylated enol phosphates through the action of the phos-
phate directing group. The versatility and utility of the coupling
products were demonstrated through further transformations
into synthetically useful building blocks.
partners (Scheme 1). This unprecedented catalytic method
provides a facile and atom-economical route to both alkeny-
lated and hydroalkenylated enol phosphates, which represent
P
hosphates are key structural motifs of many bioactive
natural products and pharmaceuticals. Of particular synthetic
relevance are enol phosphates, which have proven to be
versatile intermediates in metal-catalyzed cross-coupling
reactions. They serve as an attractive alternative to the
corresponding halide and triflate because of their stability and
low cost.[1] Given their widespread utility, the development of
convenient synthetic methods to prepare such phosphates
with structural diversity and functionality is thus of great
À
Scheme 1. Selective C H functionalization of enol phosphate.
importance.[2] As an atom-economically tool, direct C H
functionalization of simple enol phosphates is among the
À
À
most efficient strategies for forming C C bonds to furnish
significant versatile synthetic intermediates in organic and
medicinal chemistry.
a myriad of privileged phosphate scaffolds. As reported by the
groups of Kim,[3] Lee,[4] Miura,[5] and Glorius,[6] the phospho-
rus-containing groups have been identified as useful directing
groups analogous to their carboxylic counterparts.[7] Despite
these remarkable achievements, activation of the intrinsically
Our optimization study commenced with examining the
reaction of phenylvinyl phosphate 1a, which was easily
assembled from acetophenone, with n-butyl acrylate 2a.
Evaluation of various reaction parameters including oxidants
and solvents revealed that the cross-coupling product 1,3-
diene[10] 3a could be obtained in 83% yield and with a Z,E/
Z,Z ratio of 91:9 in the presence of [{Cp*RhCl2}2]
(2.5 mol%), AgSbF6 (10 mol%), and Cu(OAc)2·H2O
(1.1 equiv) in THF at 808C for 17 h (Conditions A).[11]
Control experiments indicated that all of the catalyst compo-
nents are indispensable in this reaction (see the Table S1 in
the Supporting Information).
With the optimal reaction conditions in hand, we next
investigated the effect of diverse substituents on the oxygen
atoms of both substrates (Table 1). A wide variety of
commercially available acrylates were initially utilized and
all were found to be viable coupling partners, allowing access
to the corresponding 2,4-dienoates with yields and selectiv-
ities that were generally little influenced by the substituent on
the oxygen atom. A gram-scale synthesis of 3aa demonstrated
the robustness of this methodology. After screening a series of
enol phosphate analogues bearing other directing groups, we
found that the electronic properties of the substituents
dominated the reaction efficiency (3ba–fa). Likewise, enol
phosphamide reacted readily to afford the desired product
3ga in 88% yield. A range of electrophilic alkenes were also
tested, and sulfone, styrene, and acrylonitrile proved to be
À
unreactive vinyl C H bond of enol phosphate still remains
undeveloped. Furthermore, auxiliary phosphorus-containing
directing groups, such as the monophosphonic acid and
phosphonate employed in the majority of these reported
methods, are often difficult to remove or further function-
alize,[8] thus largely limiting the synthetic applications of the
À
products. In line with our ongoing interest in the direct C H
functionalization of alkenes,[9] we herein disclose Rh-cata-
lyzed regio- and stereoselective cross-couplings of the vinyl
À
C H bond of enol phosphates with a variety of coupling
[*] Prof. Dr. T.-P. Loh
Department of Chemistry
University of Science and Technology of China
Hefei, Anhui 230026 (China)
Dr. X.-H. Hu,[+] Dr. X.-F. Yang,[+] Prof. Dr. T.-P. Loh
Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University
Singapore 637371 (Singapore)
E-mail: teckpeng@ntu.edu.sg
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506437.
Angew. Chem. Int. Ed. 2015, 54, 15535 –15539
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
15535

.
Angewandte
Communications
Table 1: Investigation of diverse enol phosphates and alkenes.[a]
Table 2: Scope of alkenylation with respect to enol phosphates.
[a] Reaction conditions A: 1 (0.15 mmol), 2 (2.0 equiv), [{Cp*RhCl2}2]
(2.5 mol%), AgSbF6 (10 mol%) and Cu(OAc)2·H2O (1.1 equiv) in
tetrahydrofuran (THF; 0.8 mL) at 808C for 17 h. Yields of isolated
product are given; Z,E/Z,Z ratios of the isomers given in parentheses
were determined by 1H NMR analysis. [b] 65% of 1 was recovered. [c] At
1008C. [d] 5 mol% Rh and 20 mol% AgSbF6 were used. NR=no
reaction.
[a] 1,2-dichloroethane as solvent. [b] At 1108C.
to coupling with 2b gave the desired product with the internal
double bond intact (3bb), thus suggesting that substitution at
the a-position was essential to facilitate the cleavage of vinyl
À
C H bond. Nevertheless, in the case of cyclic enol phos-
phates, only low yields of the coupling products were
observed (3bc,bd).
suitable coupling partners (3ag–ai). Ethyl crotonate, not
surprisingly, failed to give the coupling product.[12]
In light of these encouraging results, the scope and
limitation with respect to the enol phosphates was subse-
quently explored with methyl acrylate 2b as the coupling
partner (Table 2). The reaction is insensitive to the steric
effect of substitutions on phenyl vinyl phosphates, since
comparable results were observed when an ortho-, meta-, or
para-methyl group was present in the phenyl ring (3b,f,g,p).
Arenes bearing either an electron-withdrawing or electron-
donating substituent participated well under the current
conditions to afford the corresponding targets in 80–93%
yields (3c–q), where the former substitution had a beneficial
impact on the reactivity. Importantly, several valuable func-
tional groups such as iodo (3l), nitrile (3m), and nitro (3o)
groups are readily tolerated, delivering the alkenylation
products in excellent combined yields. The structure of
a major isomer 3o was unambiguously confirmed by an X-
ray crystallography. We were pleased to find that the scope
could be extended from the phenyl to naphthyl and phenan-
thryl systems (3r,s). In addition, heteroaryl enol phosphates
bearing furyl and thienyl moiety smoothly furnished the
corresponding products 3t and 3u with good selectivity, while
the pyridinyl group (3v) did not undergo the reaction,
presumably owing to strong coordination to the Rh catalyst.
Furthermore, the transformation is not restricted to aromatic
enol phosphates but is also applicable to enol phosphates with
aliphatic substituents (3w–z). Subjection of dienol phosphate
The unsuccessful result previously experienced with the
cross-coupling of 1a and ethyl crotonate prompted us to seek
À
a feasible C H activation pathway toward highly functional-
ized 1,3-dienoates by using the present catalyst system. To this
À
end, we conducted the conjugate addition of a vinyl C H
bond to electron-deficient alkynes as the Michael acceptors.
Gratifyingly, a series of multisubstituted 1,3-dienes (5a–e)
could be obtained with 34–83% yields and high selectivity
under mild reaction conditions (Table 3). It is worth noting
that this method offers a regioisomeric alternative to the
existing reductive couplings of alkenes with alkynes for the
synthesis of 1,3-dienes, which rely on coordination of the
alkene and alkyne to the metal catalyst.[13]
In view of that fact that enones are well-established as
superior electrophiles in catalytic conjugate addition reac-
tions,[14] we speculated that the direct addition of a Z-selective
À
vinyl C H bond to enone could be realized by using an
appropriate directing group, which will open new horizons for
the generation of g,d-unsaturated carbonyls in a complemen-
tary stereoselective manner by other mechanistically different
strategies.[15] In fact, compared to the impressive advance-
ments in the catalytic addition of aryl C(sp2)-M intermediates,
À
À
generated in situ upon C H activation, to polarized C X
(X = N, O, C) unsaturated bonds,[16] chelation-assisted vinyl
À
À
C H bond addition to C X bonds has received much less
attention and is still a challenging subject.[17] To our satisfac-
15536 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15535 –15539

Angewandte
Chemie
Table 3: Cross-couplings of enol phosphates with alkynes.
Scheme 2. Synthetic applicability of enol phosphate.
tion, the conjugate addition adduct 6a could be obtained by
employing methyl vinyl ketone as the coupling partner under
attributed to conjugate addition followed by an intermolec-
ular aldol condensation.[19] To further improve the synthetic
a
slightly modified catalytic system (Conditions B;
Table 4).[18] When altering other terminal enones, including
applicability of enol phosphates in this C H activation, we
À
performed the alkenylation reaction by using other structur-
ally related coupling partners containing a dienyl[20] or
allenyl[21] subunit under standard but unoptimized reaction
conditions, which resulted in the formation of trienes 7b and
7c in 66 and 30% yield, respectively. The unconsumed
starting material could be recovered completely.
Table 4: Scope of hydroalkenylation with respect to enol phosphates and
enones.[a]
As an additional dimension, the phosphate directing
group can be easily removed by using DIBAL-H or TBAF
in THF to give b,g-unsaturated ketone 8a and 1,5-diketone
8d, respectively (Scheme 3). Treatment of 3aa with DIBAL-
Scheme 3. Synthetic application of the coupling adducts. Dibal-H=dii-
sobutylaluminum hydride, TBAF=tetra-n-butylammonium fluoride,
COD=1,5-cyclooctadiene.
[a] Reaction conditions B: 1 (0.15 mmol), 2 (2.0 equiv), [{Cp*RhCl2}2]
(2.5 mol%), AgSbF6 (10 mol%) and Cu(OAc)2·H2O (60 mol%) in THF
(0.8 mL) at 808C for 17 h. Yields of isolated product are shown; numbers
in parentheses indicate the ratio between the Z isomer and the
corresponding diene product, as determined by 1H NMR analysis.
H in toluene cleanly provided the corresponding alcohol 8b in
86% yield without over-reduction of the phosphate moiety.
Under the Ni-catalyzed reaction system developed by Skryd-
strup,[1f–h] Suzuki–Miyaura cross-coupling with enol phos-
phate and phenyl boronic acid enables the installation of
increased diversity in the form of an dienoate to afford
product, 8c along with its 1,2-migration analogue.
Having demonstrated the application of the coupling
products, we finally pursued some preliminary mechanistic
studies of the process. Exposure of enol phosphate 1a to
conditions A in the presence or absence of 2b with the
addition of deuterated acetic acid led to an exclusively Z-
selective vinyl H/D exchange, thus suggesting reversible
cyclometalation modes (Scheme 4a). An intermolecular
kinetic isotope effect (KIE) for the alkenylation was deter-
both alkyl and aryl vinyl ketones, the reaction gave the
hydroalkenylation products in 62–89% yields (6b–i), along
with a trace amount of alkenylation products, which are
produced by the competitive b-hydride elimination. A
heteroaryl vinyl ketone embedded with a furyl moiety (6j)
was also compatible with this method, albeit with low yield.
Notably, when using acrolein as the Michael acceptor, the
expected adduct 6k could be obtained in reasonable yield.
It is interesting to note that the reaction of model
substrate 1a with a large excess of acrolein 2k delivered the
unexpected adduct 7a (Scheme 2). The result might be
À
mined with 1a and 1a-d2 to probe the relevance of C H
Angew. Chem. Int. Ed. 2015, 54, 15535 –15539
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15537

.
Angewandte
Communications
Hansen, J.-P. Ebran, M. Ahlquist, P. -O. Norrby, T. Skrydstrup,
Angew. Chem. Int. Ed. 2006, 45, 3349; Angew. Chem. 2006, 118,
3427; g) J.-P. Ebran, A. L. Hansen, T. M. Gøgsig, T. Skrydstrup, J.
Am. Chem. Soc. 2007, 129, 6931; h) A. L. Hansen, J.-P. Ebran,
T. M. Gøgsig, T. Skrydstrup, J. Org. Chem. 2007, 72, 6464; i) W.
You, Y. Li, M. K. Brown, Org. Lett. 2013, 15, 1610.
[2] a) G. Cahiez, V. Habiak, O. Gager, J. Org. Chem. 2008, 73, 6871;
b) P. H. Lee, S. Kim, A. Park, B. C. Chary, S. Kim, Angew. Chem.
Int. Ed. 2010, 49, 6806; Angew. Chem. 2010, 122, 6958; c) R. K.
Shiroodi, A. S. Dudnik, V. Gevorgyan, J. Am. Chem. Soc. 2012,
134, 6928.
Scheme 4. Isotopic labeling experiments for KIE studies.
[3] a) X. Meng, S. Kim, Org. Lett. 2013, 15, 1910; b) L. Y. Chan, L.
Cheong, S. Kim, Org. Lett. 2013, 15, 2186; c) B. C. Chary, S. Kim,
Y. Park, P. H. Lee, Org. Lett. 2013, 15, 2692; d) L. Y. Chan, S.
Kim, T. Ryu, P. H. Lee, Chem. Commun. 2013, 49, 4682; e) L. Y.
Chan, X. Meng, S. Kim, J. Org. Chem. 2013, 78, 8826; f) B. C.
Chary, S. Kim, Org. Biomol. Chem. 2013, 11, 6879.
[4] a) J. Seo, Y. Park, I. Jeon, T. Ryu, S. Park, P. H. Lee, Org. Lett.
2013, 15, 3358; b) T. Ryu, J. Kim, Y. Park, S. Kim, P. H. Lee, Org.
Lett. 2013, 15, 3986; c) S. Park, B. Seo, S. Shin, J.-Y. Son, P. H.
Lee, Chem. Commun. 2013, 49, 8671; d) Y. Park, J. Seo, S. Park,
E. J. Yoo, P. H. Lee, Chem. Eur. J. 2013, 19, 16461; e) C.-E. Kim,
J.-Y. Son, S. Shin, B. Seo, P. H. Lee, Org. Lett. 2015, 17, 908.
[5] a) Y. Unoh, Y. Hashimoto, D. Takeda, K. Hirano, T. Satoh, M.
Miura, Org. Lett. 2013, 15, 3258; b) M. Itoh, Y. Hashimoto, K.
Hirano, T. Satoh, M. Miura, J. Org. Chem. 2013, 78, 8098; c) Y.
Yokoyama, Y. Unoh, K. Hirano, T. Satoh, M. Miura, J. Org.
Chem. 2014, 79, 7649.
2
À
activation. A KIE value of 2.2 reveals that the C(sp ) H bond
cleavage might be involved in the rate-determining step
(Scheme 4b).[22]
In summary, we have accomplished the first Rh-catalyzed
cross-couplings of enol phosphates with a diverse array of
activated coupling partners. The directing ability of the
phosphate group enables the substrate to undergo syntheti-
cally tunable transformations through alkenylation with
acrylates and hydroalkenylation with enones. The latter
process involves a challenging conjugate addition of a vinyl-
À
rhodium species to enones through chelation-assisted C H
activation, which can be expanded to other Michael acceptors
such as electron-deficient alkynes. The versatility of the
catalyst system has been further demonstrated through
a series of cross-coupling reactions with other congeners,
which gave a facile access to highly functionalized conjugated
alkenes. Noteworthy features of this method include high
regio- and stereoselectivity, good functional-group compati-
bility, and versatile utility of the coupling products. Consid-
ering the practicability of the method for gram-scale synthesis
and the usefulness of such functionalized enol phosphates, we
anticipate their implementation as building blocks in complex
syntheses.
[6] D. Zhao, C. Nimphius, M. Lindale, F. Glorius, Org. Lett. 2013, 15,
4504.
À
[7] Reviews on carboxylate-assisted C H functionalization:
a) K. M. Engle, T.-S. Mei, M. Wasa, J.-Q. Yu, Acc. Chem. Res.
2012, 45, 788; b) L. Ackermann, Acc. Chem. Res. 2014, 47, 281.
[8] P. G. Steel, T. M. Woods, Synthesis 2009, 3897.
[9] a) Y.-H. Xu, J. Lu, T.-P. Loh, J. Am. Chem. Soc. 2009, 131, 1372;
b) S. Pankajakshan, Y.-H. Xu, J. K. Cheng, M. T. Low, T.-P. Loh,
Angew. Chem. Int. Ed. 2012, 51, 5701; Angew. Chem. 2012, 124,
5799; c) C. Feng, T.-P. Loh, Angew. Chem. Int. Ed. 2013, 52,
12414; Angew. Chem. 2013, 125, 12640; d) X.-H. Hu, J. Zhang,
X.-F. Yang, Y.-H. Xu, T.-P. Loh, J. Am. Chem. Soc. 2015, 137,
3169.
À
[10] C H functionalization of alkenes to synthesize 1,3-dienes: a) X.
Acknowledgements
Shang, Z.-Q. Liu, Chem. Soc. Rev. 2013, 42, 3253, and references
cited therein; b) S. Hu, D. Wang, J. Liu, X. Li, Org. Biomol.
Chem. 2013, 11, 2761; c) M. Boultadakis-Arapinis, M. N. Hop-
kinson, F. Glorius, Org. Lett. 2014, 16, 1630; d) R. Shang, L. Ilies,
S. Asako, E. Nakamura, J. Am. Chem. Soc. 2014, 136, 14349.
[11] Isomerization was not observed when the Z,E isomer was
subjected to conditions A, thus indicating that the Z,Z isomer
was formed during the catalytic cycle.
We gratefully acknowledge the National Natural Science
Foundation of China (21372210), the Singapore Ministry of
Education Academic Research Fund (Tier 2: MOE2014-T1-
1-001-102, MOE2012-T1-001-107), and National Environ-
ment Agency (NEA-ETRP-1002 111) for financial support of
this research. We also thank Dr. Yong-Xin Li for X-ray
analyses.
[12] Acrylamides, as exemplified by N,N-dimethylacrylamide, are not
applicable as coupling partners under this reaction system.
[13] a) A. T. Lindhardt, M. L. H. Mantel, T. Skrydstrup, Angew.
Chem. Int. Ed. 2008, 47, 2668; Angew. Chem. 2008, 120, 2708;
b) S. Mannathan, C.-H. Cheng, Chem. Commun. 2010, 46, 1923;
c) J. Zhang, A. Ugrinov, Y. Zhang, P. Zhao, Angew. Chem. Int.
Ed. 2014, 53, 8437; Angew. Chem. 2014, 126, 8577.
[14] Representative examples of catalytic conjugate additions to
enones: a) L. Yang, C. A. Correia, C.-J. Li, Org. Biomol. Chem.
2011, 9, 7176; b) L. Yang, B. Qian, H. Huang, Chem. Eur. J. 2012,
18, 9511; c) T. Yoshino, H. Ikemoto, S. Matsunaga, M. Kanai,
Angew. Chem. Int. Ed. 2013, 52, 2207; Angew. Chem. 2013, 125,
2263; d) Z. Shi, C. Grohmann, F. Glorius, Angew. Chem. Int. Ed.
2013, 52, 5393; Angew. Chem. 2013, 125, 5503; e) G. Rouquet, N.
Chatani, Chem. Sci. 2013, 4, 2201; f) B. Zhou, P. Ma, H. Chen, C.
Wang, Chem. Commun. 2014, 50, 14558; g) H. Jin, Z. Zhu, N. Jin,
À
Keywords: alkenylation · C H activation · enol phosphates ·
hydroalkenylation · rhodium
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15535–15539
Angew. Chem. 2015, 127, 15755–15759
[1] Selected reviews and examples of metal-catalyzed cross-cou-
plings of enol phosphates: a) H. Fuwa, Synlett 2011, 6; b) T.
Hayashi, T. Fujiwa, Y. Okamoto, Y. Katsuro, M. Kumada,
Synthesis 1981, 1001; c) K. C. Nicolaou, G.-Q. Shi, G. P. Gärtner,
Z. Yang, J. Am. Chem. Soc. 1997, 119, 5467; d) A. S. E.
Karlstrçm, K. Itami, J.-E. Bäckvall, J. Org. Chem. 1999, 64,
1745; e) J. Wu, Z. Yang, J. Org. Chem. 2001, 66, 7875; f) A. L.
15538 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15535 –15539

Angewandte
Chemie
J. Xie, Y. Cheng, C. Zhu, Org. Chem. Front. 2015, 2, 378; h) J. Li,
L. Ackermann, Org. Chem. Front. 2015, 2, 1035.
133, 10787; f) Y. Li, X.-S. Zhang, Q.-L. Zhu, Z.-J. Shi, Org. Lett.
2012, 14, 4498.
[15] a) A. Herath, B. B. Thompson, J. Montgomery, J. Am. Chem.
Soc. 2007, 129, 8712; b) H.-T. Chang, T. T. Jayanth, C.-C. Wang,
C.-H. Cheng, J. Am. Chem. Soc. 2007, 129, 12032; c) C.-Y. Ho, H.
Ohmiya, T. F. Jamison, Angew. Chem. Int. Ed. 2008, 47, 1893;
Angew. Chem. 2008, 120, 1919; d) S. Ogoshi, T. Haba, M. Ohashi,
J. Am. Chem. Soc. 2009, 131, 10350; e) A. T. Biju, M. Padma-
naban, N. E. Wurz, F. Glorius, Angew. Chem. Int. Ed. 2011, 50,
8412; Angew. Chem. 2011, 123, 8562.
[18] Although the exact mechanism of action of Cu(OAc)2·H2O is
unclear, a smaller amount of Cu(OAc)2·H2O led to partial
decomposition of the enol phosphate into the corresponding
ketone.
[19] X.-Y. Shi, C.-J. Li, Org. Lett. 2013, 15, 1476.
[20] a) C. E. Houlden, C. D. Bailey, J. G. Ford, M. R. GagnØ, G. C.
Lloyd-Jones, K. I. Brooker-Milburn, J. Am. Chem. Soc. 2008,
130, 10066; b) D. Zhao, F. Lied, F. Glorius, Chem. Sci. 2014, 5,
2869.
[21] a) H. Wang, B. Beiring, D.-G. Yu, K. D. Collins, F. Glorius,
Angew. Chem. Int. Ed. 2013, 52, 12430; Angew. Chem. 2013, 125,
12657; b) T.-J. Gong, W. Su, Z.-J. Liu, W.-M. Cheng, B. Xiao, Y.
Fu, Org. Lett. 2014, 16, 330.
[22] E. M. Simmons, J. F. Hartwig, Angew. Chem. Int. Ed. 2012, 51,
3066; Angew. Chem. 2012, 124, 3120.
À
[16] Reviews on transition-metal-catalyzed direct addition of a C H
À
bond to a C X unsaturated bond: a) D. A. Colby, A. S. Tsai,
R. G. Bergman, J. A. Ellman, Acc. Chem. Res. 2012, 45, 814;
b) G. Yan, X. Wu, M. Yang, Org. Biomol. Chem. 2013, 11, 5558;
c) X.-S. Zhang, K. Chen, Z.-J. Shi, Chem. Sci. 2014, 5, 2146; d) L.
Yang, H. Huang, Chem. Rev. 2015, 115, 3468.
[17] a) F. Kakiuchi, Y. Tanaka, T. Sato, N. Chatani, S. Murai, Chem.
Lett. 1995, 679; b) B. M. Trost, K. Imi, I. W. Davies, J. Am. Chem.
Soc. 1995, 117, 5371; c) D. A. Colby, R. G. Bergman, J. A.
Ellman, J. Am. Chem. Soc. 2006, 128, 5604; d) Y. Kuninobu, Y.
Fujii, T. Matsuki, Y. Nishina, K. Tanai, Org. Lett. 2009, 11, 2711;
e) C. Zhao, F. D. Toste, R. G. Bergman, J. Am. Chem. Soc. 2011,
Received: July 13, 2015
Published online: November 4, 2015
Angew. Chem. Int. Ed. 2015, 54, 15535 –15539
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15539