Bronsted acid-catalyzed, highly enantioselective addition of enamides to in situ-generated ortho-quinone methides: A domino approach to complex acetamidotetrahydroxanthenes

Article abstract of DOI:10.1002/chem.201406044

The highly enantioselective conjugate addition of enamides and enecarbamates to in situ-generated ortho-quinone methides, upon subsequent N,O-acetalization, gives rise to acetamido-substituted tetrahydroxanthenes with generally excellent enantio- and dias

Full text of DOI:10.1002/chem.201406044

DOI: 10.1002/chem.201406044
Communication
&
Asymmetric Catalysis
Brønsted Acid-Catalyzed, Highly Enantioselective Addition of
Enamides to In Situ-Generated ortho-Quinone Methides: A
Domino Approach to Complex Acetamidotetrahydroxanthenes
Satyajit Saha and Christoph Schneider*[a]
that have attracted considerable attention over the years as
Abstract: The highly enantioselective conjugate addition
of enamides and enecarbamates to in situ-generated
ortho-quinone methides, upon subsequent N,O-acetaliza-
tion, gives rise to acetamido-substituted tetrahydroxan-
thenes with generally excellent enantio- and diastereose-
lectivities. A chiral BINOL-based phosphoric acid catalyst
controls the enantioselectivity of the carbon–carbon
bond-forming event. The products are readily converted
into other xanthene-based heterocycles.
a result of their wide range of biological properties, such as an-
algesic, antiviral, anti-inflammatory and antibacterial activi-
ties.[11]
We recently reported the phosphoric acid-catalyzed, highly
enantioselective conjugate addition of 1,3-dicarbonyl com-
pounds to in situ generated o-QMs and their application to the
high-yielding one-pot synthesis of 4-aryl-4H-chromenes and re-
lated heterocycles through a subsequent cyclodehydration re-
action (Scheme 1, pathway a).[12] As substrates for the in situ
Ortho-Quinone methides (o-QMs), featuring a unique assembly
of carbonyl and olefinic moieties in close proximity, have been
known for more than one century in organic chemistry and
exist in nature in a variety of medicinally important natural
products.[1] Because of their inherent highly electrophilic char-
acter as highly polarized 1-oxabutadienes, they react readily
with a broad range of nucleophiles and the thrust for aromati-
zation adds to the driving force for such conjugate additions.
Accordingly, they participate in many chemical, medicinal, and
biological processes, such as lignin biosynthesis, enzyme inhib-
ition, and DNA alkylation and cross-linking.[2]
Scheme 1. Brønsted acid-catalyzed reaction of o-QM with 1,3-diketones
(pathway a) and with enamides (pathway b).
Although the chemistry of o-QMs has been studied quite ex-
tensively, mainly in conjugate additions, [4+2]-cycloadditions,
and 6p-electrocyclization reactions,[3] reports on catalyst-con-
trolled, enantioselective reactions with o-QMs as substrates are
limited. Seminal contributions to this field have been disclosed
by the groups of Sigman,[4] Lectka,[5] Schaus,[6] Ye,[7] and
Scheidt,[8] who studied enantioselective palladium-, cinchona
alkaloid-, BINOL-, and NHC-catalyzed reactions of o-QMs, re-
spectively, with great success. Of particular relevance to the
present study were findings by the groups of Rueping[9] and
Bach,[10] which revealed the capacity of chiral phosphoric acids
to activate ortho-hydroxy benzyl alcohols for ensuing intramo-
lecular allylic alkylation reactions and indole alkylation reac-
tions, respectively.
preparation of o-QMs, we employed ortho-hydroxybenzhydryl
alcohols 1, which were converted into the reactive o-QMs
through acid-catalyzed dehydration. This methodology was,
however, restricted to reactive nucleophiles with sufficiently
high enol content and could not be extended to simple ke-
tones. Alternatively, enamides carrying an electron-withdraw-
ing group on the nitrogen atom constitute stable ketone enol
surrogates and have been employed as highly reactive nucleo-
philes in various carbon–carbon bond-forming reactions under
Lewis acid and Brønsted acid catalysis.[13] As an additional
design element, which was expected to support a highly or-
dered transition state in the present reaction, they carry an
acidic proton on the nitrogen atom, which offers the possibility
of forming an additional hydrogen bond between the chiral
catalyst and the nucleophile.
Xanthenes and in particular benzo-fused xanthenes consti-
tute an important class of compounds in organic chemistry
We report herein the first catalytic and highly enantioselec-
tive conjugate addition of enamides and enecarbamates to-
wards in situ-generated o-QMs under chiral Brønsted acid cat-
alysis (Scheme 1, pathway b). In a novel domino conjugate ad-
dition–N,O-acetalization process, a broadly applicable and
high-yielding synthesis of highly functionalized acetamido-sub-
stituted tetrahydroxanthenes has been developed, which rou-
[a] Dr. S. Saha, Prof. Dr. C. Schneider
Institut fꢀr Organische Chemie, Universitꢁt Leipzig
Johannisallee 29, 04103 Leipzig (Germany)
E-mail: schneider@chemie.uni-leipzig.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406044.
Chem. Eur. J. 2015, 21, 2348 – 2352
2348
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
tinely affords the products with excellent diastereo- and enan-
tioselectivities.
Table 2. Brønsted acid-catalyzed addition of enamides 2a and 2b to
ortho-hydroxy benzhydrols 1.[a–d]
We started our studies by examining the reaction of ortho-
hydroxybenzhydryl alcohol 1a (1 equiv) with N-(3,4-dihydro-
naphthalen-1-yl)acetamide 2a (1.2 equiv) in CH2Cl2 in the pres-
ence of various chiral phosphoric acids 3 (5 mol%; Table 1). Al-
though simple 3,3’-phenyl-substituted BINOL-phosphoric acid
3a gave rise to only poor enantioselectivity in this reaction
(Table 1, entry 1), the use of sterically more demanding 3,3’-aryl
substituents in the BINOL-backbone of the phosphoric acids
led to a significant increase in enantioselectivity. When for ex-
ample phosphoric acid 3b, with mesityl groups as 3,3’-sub-
stituents, was employed as Brønsted acid catalyst, the addition
product 4a was isolated in 77% yield as a single diastereomer
and with 97:3 e.r. after stirring for 12 h in CH2Cl2 at room tem-
perature (Table 1, entry 2).
Phosphoric acid 3d, with a larger para-substituent on the
3,3’-aryl groups, further increased the e.r. to 98:2 (Table 1,
entry 4), whereas catalyst 3e, with pentamethylated 3,3’-
phenyl groups, was found to be the optimal catalyst for this
reaction, giving the highest enantiomeric ratio of 99:1 in com-
bination with a complete diastereoselectivity (Table 1, entry 5).
Surprisingly, structurally similar catalysts 3c, 3g, and 3h were
found to be largely unreactive in this reaction, suggesting that
too much steric congestion in the catalyst backbone had shut
Table 1. Optimization studies.[a]
Entry
Cat.
Solvent
t [h]
Yield [%][b]
d.r.[c]
e.r.[d]
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3 f
3g
3h
3e
3e
CHCl3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
48
12
12
10
12
12
12
14
12
12
62
77
—
>98:2
>98:2
—
57:43
97:3
—
[f]
[g]
[g]
78
76
79
>98:2
>98:2
>98:2
98:2
99:1
97:3
[f]
[g]
[g]
—
—
—
—
—
—
[f]
[g]
[g]
[a] Reaction conditions: 1a–m (0.42 mmol, 1 equiv), cyclic enamide 2a/
2b (0.50 mmol, 1.2 equiv), 4 ꢁ MS (catalytic amount), catalyst 3e (13 mg,
5 mol%), CH2Cl2 (4 mL), RT, 12–84 h; [b] yield of the isolated major diaste-
reomer; [c] determined from 1H NMR spectrum of the crude product;
[d] e.r. determined through chiral HPLC analysis (see the Supporting Infor-
mation).
77
83
>98:2
>98:2
99:1
99:1
10[e]
CH2Cl2
[a] Reaction conditions: 1a (0.23 mmol, 1 equiv), 2a (0.27 mmol,
1.2 equiv), catalyst 3 (5 mol%) CH2Cl2 (2 mL), RT; [b] yield of the isolated
major diastereomer; [c] determined from 1H NMR spectrum of the crude
product; [d] e.r. determined through chiral HPL analysis (see the Support-
ing Information); [e] small amount of 4 ꢁ MS (5 mg per 0.23 mmol of 1a)
was used; [f] slow reaction; [g] not determined.
Chem. Eur. J. 2015, 21, 2348 – 2352
www.chemeurj.org
2349
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
down the reactivity in these cases (Table 1, entries 3,
7, and 8). A brief solvent screening revealed that di-
chloromethane was superior to the other solvents
that were investigated (see the Supporting Informa-
tion, Table S1). Gratifyingly, we found that the addi-
tion of activated powdered molecular sieves (4 ꢁ) fur-
ther improved the chemical yield of the reaction
without compromising the enantioselectivity (Table 1,
entry 10). We assume that the molecular sieves trap
the equimolar amount of water that is generated in
the reaction, preventing the undesired decomposi-
tion of the nucleophile.
Table 3. Brønsted acid-catalyzed addition of enamides 2c-g to ortho-hydroxy benzhy-
drols 1.[a–d]
To evaluate the scope of this process, various
ortho-hydroxybenzhydrols 1a–m were subsequently
subjected to the above-optimized reaction conditions
in the presence of enamides 2a and 2b (Table 2). In
all cases studied, the reaction proceeded smoothly,
typically within 1–2 days, and furnished pure or
almost pure diastereomers in typically good yields
with excellent enantioselectivity. A range of function-
al groups were readily tolerated at various positions
both within the quinone methide fragment and
within the b-aryl substituent, delivering the products
with excellent results. A crystal structure of xanthene
4l, obtained from MeOH, revealed both the relative
and absolute configuration of the products, which
was assigned to all other products as well
(Figure 1).[14]
To further broaden the scope of this reaction with
respect to the enamide component, we employed
some more carbocyclic and heterocyclic enamides in
this process, all of which produced the desired prod-
ucts with excellent selectivity (Table 3).
Thus, using the chromene- and thiochromene-
based enamides 2c and 2d as nucleophiles, the cor-
responding acetamido-substituted chromeno[4,3-
b]chromenes 6a and 6b and thiochromeno[4,3-
b]chromenes 7a and 7b, respectively, were obtained
in good yields and exceptional diastereo- and enan-
tioselectivities under the described conditions
(Table 3).
[a] All reactions were carried out on 0.2 mmol scale; [b] yield of the isolated major dia-
stereomer; [c] determined from 1H NMR spectrum of the crude product; [d] e.r. deter-
mined through chiral HPLC analysis (see the Supporting Information).
Interestingly, the regioisomeric enamide 2e delivered the
acetamido-substituted benzo[a]xanthenes 8a and 8b in good
yields, albeit with decreased selectivity. Enamide 2 f lacking the
annelated aromatic ring also conferred the addition products
9a and 9b in good yields, excellent diastereoselectivity and
e.r. up to 92:8. Ring-enlarged, benzofused cyclohepta[1,2-
b]chromenes 10a and 10b were easily accessed as well start-
ing from the seven-membered ring-based enamide 2g in
good to moderate enantiomeric ratios (Table 3).
In addition, the influence of the N-protecting group in the
enamide component was briefly investigated in this study. Ene-
carbamate 2h performed extremely well both in terms of yield
and enantioselectivity (Scheme 2). Upon subsequent elimina-
To further reveal the synthetic potential of this new process,
several of the products were subsequently converted into ver-
satile dihydrobenzo[c]xanthenes and related heterocycles 11 a–
f through an acid-catalyzed elimination of acetamide (Table 4).
The reactions generally occurred with excellent yields
(>90%) and the products completely retained their optical
purity.
Scheme 2. Brønsted acid-catalyzed addition of enecarbamate 2h to ortho-
hydroxybenzhydrol 1a.
Chem. Eur. J. 2015, 21, 2348 – 2352
www.chemeurj.org
2350
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
To further expand this protocol, the Brønsted acid-catalyzed
addition of 1,3-cyclohexanedione-derived enamide 2i to the o-
QM derived from 1a was carried out, affording the desired
product 13 in very good yield and enantiomeric ratio and as
a single diastereomer (Scheme 3). These observations substan-
tially enhance the scope and applicability of this methodology
and allow for elaboration of these scaffolds in library synthesis.
Scheme 3. Brønsted acid-catalyzed addition of enamide 2i to ortho-hydroxy
benzhydrol 1a.
On the basis of the crystal structure that we obtained for
xanthene 4l, we propose a transition structure, as shown in
Figure 2. We assume that the chiral phosphoric acid works as
a bifunctional catalyst that is hydrogen-bonded to the ortho-
quinone methide and at the same time attached to the enam-
ide NÀH moiety through an additional hydrogen bridge from
Figure 1. Crystal structure of xanthene 4l. Thermal ellipsoids are set to 50%
probability.
Table 4. Acid-promoted elimination of acetamide.[a–c]
Figure 2. Proposed transition state assembly.
the basic phosphoryl oxygen atom. In this highly ordered tran-
sition state assembly, the top face of the o-QM is effectively
shielded by the neighboring Ar2 group in the catalyst’s back-
bone and the conjugate addition preferentially occurs from
the bottom side. This transition state assembly would also ra-
tionalize the observed trans diastereoselectivity of the reaction.
In summary, we have developed a highly efficient, Brønsted
acid-catalyzed, conjugate addition of enamides to in situ-gen-
erated ortho-quinone methides followed by a N,O-acetalization
reaction, both of which proceeded with exceptional stereocon-
trol. A broad range of highly functionalized acetamido-substi-
tuted tetrahydroxanthenes and related heterocycles were ob-
tained with typically good to very good yields and excellent
enantioselectivities. Several of the products were subsequently
converted into dihydroxanthenes through an acid-catalyzed
[a] All reactions were carried out on 0.2 mmol scale; [b] yield of the isolat-
ed product; [c] e.r. determined through chiral HPLC analysis (see the Sup-
porting Information).
tion with trifluoroacetic acid (TFA), dihydrobenzo[c]xanthene
11 a was obtained in 72% overall yield and with 98:2 e.r.
Chem. Eur. J. 2015, 21, 2348 – 2352
www.chemeurj.org
2351
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
e) R. Jana, T. P. Pathak, K. H. Jensen, M. S. Sigman, Org. Lett. 2012, 14,
4074–4077.
[5] E. Alden-Danforth, M. T. Scerba, T. Lectka, Org. Lett. 2008, 10, 4951–
4953.
[6] Y. Luan, S. E. Schaus, J. Am. Chem. Soc. 2012, 134, 19965–19968.
[7] a) H. Lv, L. You, S. Ye, Adv. Synth. Catal. 2009, 351, 2822–2826; b) H. Lv,
W. Q. Jia, L. H. Sun, S. Ye, Angew. Chem. Int. Ed. 2013, 52, 8607–8610;
Angew. Chem. 2013, 125, 8769–8772.
elimination reaction with full retention of absolute configura-
tion. This study further extends the concept of phosphoric
acid-catalyzed, enantioselective reactions of ortho-quinone me-
thides and suggests that a broad range of additional nucleo-
philes may be employed accordingly.
[8] a) A. E. Mattson, K. A. Scheidt, J. Am. Chem. Soc. 2007, 129, 4508–4509;
b) J. Izquierdo, A. Orue, K. A. Scheidt, J. Am. Chem. Soc. 2013, 135,
10634–10637; c) A. Lee, A. Younai, C. K. Price, J. Izquierdo, R. K. Mishra,
K. A. Scheidt, J. Am. Chem. Soc. 2014, 136, 10589–10592.
[9] a) M. Rueping, U. Uria, M. Y. Lin, I. Atodiresei, J. Am. Chem. Soc. 2011,
133, 3732–3735.
[10] a) D. Wilcke, E. Herdtweck, T. Bach, Synlett 2011, 1235–1238; b) see
also: W. Zhao, Z. Wang, B. Chu, J. Sun, Angew. Chem. Int. Ed. 2014, DOI:
10.1002/anie.201405252; Angew. Chem. 2014, DOI: 10.1002/
ange.201405252.
Acknowledgements
We are grateful to Dr. P. Lçnnecke (University of Leipzig) for
the crystal structure analysis. We gratefully acknowledge the
donation of chemicals from Evonik and BASF.
Keywords: asymmetric catalysis · Brønsted acids · domino
reactions · organocatalysis · xanthenes
[11] a) H. N. Hafez, M. I. Hegab, I. S. Ahmed-Farag, A. B. A. El-Gazzar, Bioorg.
Med. Chem. Lett. 2008, 18, 4538–4543; b) R. W. Lambert, J. A. Martin,
J. H. Merrett, K. E. B. Parkes, G. J. Thomas, CT Int. Appl. WO9706178,
1997; c) T. Hideo, J. Teruomi, (Sankyo Co.), Jpn. Patent, 56005480, 1981;
d) J. P. Poupelin, G. Saint-Ruf, O. Foussard-Blanpin, G. Narcisse, G.
Uchida-Ernouf, R. Lacroix, Eur. J. Med. Chem. 1978, 13, 67–71.
[12] a) O. El-Sepelgy, S. Haseloff, S. K. Alamsetti, C. Schneider, Angew. Chem.
Int. Ed. 2014, 53, 7923–7927; Angew. Chem. 2014, 126, 8057–8061;
b) after publication of our manuscript a related communication ap-
peared: C. C. Hsiao, H. H. Liao, M. Rueping, Angew. Chem. Int. Ed. 2014,
53, 13258–13263; Angew. Chem. 2014, 126, 13474–13479.
[13] a) T. Majima, C. Pac, H. Sakurai, J. Chem. Soc. Perkin Trans. 1 1980, 2705–
2708; b) S. Nakanishi, Y. Otsuji, K. Itoh, N. Hayashi, Bull. Chem. Soc. Jpn.
1990, 63, 3595–3600; c) A. Couture, E. Deniau, P. Grandclaudon, S.
Lebrun, Tetrahedron Lett. 1996, 37, 7749–7752; d) T. Naito, Y. Yuumoto,
T. Kiguchi, I. Ninomiya, J. Chem. Soc. Perkin Trans. 1 1996, 281–288; e) T.
Bach, Angew. Chem. Int. Ed. Engl. 1996, 35, 884–886; Angew. Chem.
1996, 108, 976–977; f) S. S. Kinderman, J. H. van Maarseveen, H. E.
Schoemaker, H. Hiemstra, F. P. J. T. Rutjes, Org. Lett. 2001, 3, 2045–2048;
g) W. A. L. van Otterlo, E. L. Ngidi, C. B. de-Koning, Tetrahedron Lett.
2003, 44, 6483–6486; h) R. Matsubara, Y. Nakamura, S. Kobayashi,
Angew. Chem. Int. Ed. 2004, 43, 1679–1681; Angew. Chem. 2004, 116,
1711–1713; i) R. Matsubara, S. Kobayashi, Angew. Chem. Int. Ed. 2006,
45, 7993–7995; Angew. Chem. 2006, 118, 8161–8163; j) H. Kiyohara, R.
Matsubara, S. Kobayashi, Org. Lett. 2006, 8, 5333–5335; k) F. Berthiol, R.
Matsubara, N. Kawai, S. Kobayashi, Angew. Chem. Int. Ed. 2007, 46,
7803–7805; Angew. Chem. 2007, 119, 7949–7951; l) M. Terada, K. Ma-
chioka, K. Sorimachi, J. Am. Chem. Soc. 2007, 129, 10336–10337; m) M.
Terada, K. Sorimachi, J. Am. Chem. Soc. 2007, 129, 292–293; n) R. Matsu-
bara, S. Kobayashi, Acc. Chem. Res. 2008, 41, 292–301; o) D. R. Carbery,
Org. Biomol. Chem. 2008, 6, 3455–3460; p) M. Terada, K. Soga, N. Mo-
miyama, Angew. Chem. Int. Ed. 2008, 47, 4122–4125; Angew. Chem.
2008, 120, 4190–4193; q) K. Gopalaiah, H. B. Kagan, Chem. Rev. 2011,
111, 4599–4657.
[1] a) A. B. Turner, Q. Rev. Chem. Soc. 1964, 18, 347–360; b) M. A. Lewis,
D. G. Yoerg, J. L. Bolton, J. A. Thompson, Chem. Res. Toxicol. 1996, 9,
1368–1374; c) P. G. McCracken, J. L. Bolton, G. R. J. Thatcher, J. Org.
Chem. 1997, 62, 1820–1825; d) S. E. Rokita, J. Yang, P. Pande, W. A.
Greenberg, J. Org. Chem. 1997, 62, 3010–3012; e) P. Pande, J. Shearer, J.
Yang, W. A. Greenberg, S. E. Rokita, J. Am. Chem. Soc. 1999, 121, 6773–
6779; f) J. P. Richard, M. M. Toteva, J. Crugeiras, J. Am. Chem. Soc. 2000,
122, 1664–1674; g) Y. Chiang, A. J. Kresge, Y. Zhu, J. Am. Chem. Soc.
2000, 122, 9854–9855; h) M. M. Toteva, J. P. Richard, J. Am. Chem. Soc.
2000, 122, 11073–11083; i) E. Modica, R. Zanaketti, M. Freccero, M.
Mella, J. Org. Chem. 2001, 66, 41–52; j) T. Itoh, Prog. Polym. Sci. 2001,
26, 1019–1059; k) H. Amouri, J. Le Bras, Acc. Chem. Res. 2002, 35, 501–
510; l) T. Rosenau, A. Potthast, T. Elder, P. Kosma, Org. Lett. 2002, 4,
4285–4288; m) R. Rodriguez, R. M. Adlington, J. E. Moses, A. Cowley,
J. E. Baldwin, Org. Lett. 2004, 6, 3617–3619; n) S. B. Ferreira, F. d. C. da
Silva, A. C. Pinto, D. T. G. Gonzaga, V. F. Ferreira, J. Heterocycl. Chem.
2009, 46, 1080–1097; o) F. Dufrasne, M. Gelbcke, J. Neve, R. Kiss, J.-L.
Kraus, Curr. Med. Chem. 2011, 18, 3995–4011; p) M. M. Toteva, J. P. Ri-
chard, Adv. Phys. Org. Chem. 2011, 45, 39–91.
[2] a) K. I. Takao, T. Sasaki, T. Kozaki, Y. Yanagisawa, K.-i. Tadano, A. Kawashi-
ma, H. Shinonaga, Org. Lett. 2001, 3, 4291–4294; b) B. E. Barragꢂn-
Huerta, J. Peralta-Cruz, R. F. Gonzlezlaredo, J. Karchesy, Phytochemistry
2004, 65, 925–928; c) F. Doria, S. N. Richter, M. Nadai, S. Colloredo-Mels,
M. Mella, M. Palumbo, M. Freccero, J. Med. Chem. 2007, 50, 6570–6579;
d) M. Di Antonio, F. Doria, S. N. Richter, C. Bertipaglia, M. Mella, C. Sissi,
M. Palumbo, M. Freccero, J. Am. Chem. Soc. 2009, 131, 13132–13141;
e) H. Wang, S. E. Rokita, Angew. Chem. Int. Ed. 2010, 49, 5957–5960;
Angew. Chem. 2010, 122, 6093–6096; f) H. J. Martin, T. Magauer, J.
Mulzer, Angew. Chem. Int. Ed. 2010, 49, 5614–5626; Angew. Chem. 2010,
122, 5746–5758; g) R. Jansen, K. Gerth, H. Steinmetz, S. Reinecke, W.
Kessler, A. Kirschning, R. Muller, Chem. Eur. J. 2011, 17, 7739–7744.
[3] Reviews: a) N. J. Willis, C. D. Bray, Chem. Eur. J. 2012, 18, 9160–9173;
b) T. P. Pathak, M. S. Sigman, J. Org. Chem. 2011, 76, 9210–9215; c) R. W.
van de Water, T. R. R. Pettus, Tetrahedron 2002, 58, 5367–5405; d) Y.
Chiang, A. J. Kreesge, Y. Zhu, Pure Appl. Chem. 2000, 72, 2299–2308.
[4] a) Y. Zhang, M. S. Sigman, J. Am. Chem. Soc. 2007, 129, 3076–3079;
b) K. H. Jensen, T. P. Pathak, Y. Zhang, M. S. Sigman, J. Am. Chem. Soc.
2009, 131, 17074–17075; c) T. P. Pathak, K. M. Gligorich, B. E. Welm,
M. S. Sigman, J. Am. Chem. Soc. 2010, 132, 7870–7801; d) K. H. Jensen,
J. D. Webb, M. S. Sigman, J. Am. Chem. Soc. 2010, 132, 17471–17482;
[14] CCDC 1018807 (4l) contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. For details concerning the crystal structure of 4l, also see
the Supporting Information.
Received: August 13, 2014
Published online on December 8, 2014
Chem. Eur. J. 2015, 21, 2348 – 2352
www.chemeurj.org
2352
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim