Access to Air-Stable 1,3-Diphosphacyclobutane-2,4-diyls by an Arylation Reaction with Arynes

Article abstract of DOI:10.1002/anie.201601907

Tuning of the physicochemical properties of the 1,3-diphosphacyclobutane-2,4-diyl unit is attractive in view of materials applications. The use of arynes is shown to be effective for installing relatively electron rich aryl substituents into the open-shell singlet P-heterocyclic system. Treatment of the sterically encumbered 1,3-diphosphacyclobuten-4-yl anion with ortho-silylated aryl triflates in the presence of fluoride under appropriate conditions afforded the corresponding 1-aryl 1,3-diphosphacyclobutane-2,4-diyls. The air-stable open-shell singlet P-heterocycles exhibit considerable electron-donating character, and the aromatic substituent influences the open-shell character, which is thought to be related to the property of p-type semiconductivity. The P-arylated 1,3-diphosphacyclobutane-2,4-diyl systems can be further utilized as detectors of hydrogen fluoride (HF), which causes a remarkable change in their photoabsorption properties.

Full text of DOI:10.1002/anie.201601907

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International Edition: DOI: 10.1002/anie.201601907
German Edition: DOI: 10.1002/ange.201601907
Phosphorus Heterocycles
Access to Air-Stable 1,3-Diphosphacyclobutane-2,4-diyls by an
Arylation Reaction with Arynes
Yasuhiro Ueta, Koichi Mikami, and Shigekazu Ito*
Abstract: Tuning of the physicochemical properties of the 1,3-
diphosphacyclobutane-2,4-diyl unit is attractive in view of
materials applications. The use of arynes is shown to be
effective for installing relatively electron rich aryl substituents
into the open-shell singlet P-heterocyclic system. Treatment of
the sterically encumbered 1,3-diphosphacyclobuten-4-yl anion
with ortho-silylated aryl triflates in the presence of fluoride
under appropriate conditions afforded the corresponding 1-
aryl 1,3-diphosphacyclobutane-2,4-diyls. The air-stable open-
shell singlet P-heterocycles exhibit considerable electron-
donating character, and the aromatic substituent influences
the open-shell character, which is thought to be related to the
property of p-type semiconductivity. The P-arylated 1,3-
diphosphacyclobutane-2,4-diyl systems can be further utilized
as detectors of hydrogen fluoride (HF), which causes a re-
markable change in their photoabsorption properties.
co-workers used the addition of phosphines to arynes in
a
multicomponent [3+2] cycloaddition with aldehydes
[Scheme 1, Eq. (3)].[5] Strongly electrophilic arynes accept
even phosphines readily, and can provide functionalized
phosphorus compounds with aryl substituents.
We previously reported that nucleophilic aromatic sub-
stitution (SNAr) of a sterically encumbered 1,3-diphospha-
cyclobuten-4-yl anion 1 can undergo arylation at the non-tert-
butyl-substituted skeletal phosphorus atom to provide air-
stable 1-aryl 1,3-diphosphacyclobutane-2,4-diyls (Scheme 2).
A
rynes are normally quite unstable and hard to isolate;
however, various methods for the generation of arynes have
been established and widely employed in organic synthesis.[1]
Arynes have also played key roles in organophosphorus
chemistry (Scheme 1). Wittig and co-workers reported the
reaction of arynes (dehydroaromatic compounds), generated
from fluoroarenes and butyllithium, with triarylphosphines to
afford the corresponding zwitterionic betaines [Scheme 1,
Eq. (1)].[2,3] This reaction procedure was utilized for the
synthesis of a variety of organophosphorus compounds. JugØ
and co-workers reported the synthesis of optically active
phosphine–borane derivatives [Scheme 1, Eq. (2)].[4] Biju and
Scheme 2. SNAr reaction of 1 with an electron-deficient N-heteroaryl
reagent. Mes*=2,4,6-tBu3C6H2.
The direct arylation is suitable for tuning the electronic
properties of the open-shell singlet P-heterocyclic moiety, and
is promising for the development of materials, such as
organoelectronics,[6] and sensing/operating processes involv-
ing hydrogen fluoride (HF).[7] However, the SNAr process was
only successful when electron-deficient N-heterocyclic aryl
halides were employed as the electrophiles, because the sp2
phosphorus atom of 1 is positive in nature and scarcely
contributes to the HOMO of 1.[7] The development of novel
methods for the direct arylation of 1,3-diphosphacyclobuten-
4-yl anions with relatively electron rich aromatic substituents
is quite attractive, as the experimentally characterized strong
electron-donating nature of the resulting air-stable 1,3-
diphosphacyclobutane-2,4-diyls makes these compounds suit-
able as p-type semiconductors and for the practical sensing of
HF. Furthermore, installation of the electron-rich aromatic
substituent on the phosphorus atom induces high stability
owing to the enlarged HOMO–LUMO gap.
Scheme 1. Addition of phosphines to arynes.
We speculated that highly electrophilic arynes should
react with 1 and afford stable and electron-donating open-
shell singlet P-heterocycles. Herein we demonstrate that
arynes, generated from the corresponding o-silyl triflates and
fluoride, react with 1 under appropriate conditions to provide
air-stable open-shell singlet P-heterocycles 2. The physico-
chemical properties of 2, including their semiconductivity,
were investigated. We also disclose the detection/release of
hydrogen fluoride by the P-arylated 1,3-diphosphacyclobu-
[*] Y. Ueta, Prof. Dr. K. Mikami, Prof. Dr. S. Ito
School of Materials and Chemical Technology
Department of Chemical Science and Engineering
Tokyo Institute of Technology
2-12-1 Ookayama, Meguro, Tokyo 152-8552 (Japan)
E-mail: ito.s.ao@m.titech.ac.jp
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201601907.
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tane-2,4-diyls, with an accompanying remarkable change in
their photoabsorption properties.
employed as a precursor of benzyne (Grignard reagent),[12]
the 1,3-diphosphacyclobutane-2,4-diyl bearing a 2-iodophenyl
substituent (6) was isolated in 64% yield (Scheme 3),
presumably via the anionic intermediate 2a’ and isopropyl
iodide.
On the basis of the results summarized in Table 1, we
examined the arylation of 1 with arynes to afford aryl-
substituted 1,3-diphosphacyclobutane-2,4-diyls that were dif-
ficult to synthesize by the SNAr process (Table 2). A 2-
naphthyl group was successfully installed by the use of 4b as
the precursor, and 2b was isolated in 43% yield (Table 2,
entry 1). A benzo[d][1,3]dioxole moiety and 2,3-dimethyl-
1-(2,4,6-Tri-tert-butylphenyl)-2-phosphaethyne
was
treated with tert-butyllithium (0.5 equiv) in THF to generate
the corresponding cyclic anion 1.[8] Next, conditions for the
generation of benzyne were examined (Table 1). At first,
a combination of o-bromophenyl triflate and an alkyl
lithium[9] was used in THF at À788C, but the reaction gave
an unidentified mixture (Table 1, entry 1). When the boronic
Table 1: Arylation of 1 with benzyne.
Entry
1
Conditions
generation of benzyne
Yield [%][a]
arylation of 1
À788C, 0.5 h
0
nBuLi, À788C
Scheme 3. Reaction of 1 with benzyne generated with a Grignard
reagent.
2
3
RT, 2 h
14
tBuLi, À78!À208C
Table 2: Arylation of 1 with arynes.
08C!RT, 2.5 h
0[b]
TBAF, 08C
[c]
4
5
4a, TBAF, À208C
À208C, 3 h
À408C, 3 h
–
4a, TBAF, À408C
59
Entry
1
Aryne precursor
Product(s)
Yield [%][a]
43
[a] Yield of 2a (isolated product). [b] The hydrofluorinated product was
observed. [c] Product 2a and the hydrofluorinated product were
observed in a ratio of 7:1. TBAF=tetrabutylammonium fluoride,
pin=pinacolato, Tf =trifluoromethanesulfonyl.
2
3
64
74
acid derivative 3 was used as a precursor of benzyne and
treated with tert-butyllithium,[10] the 31P NMR spectrum of the
reaction mixture showed the formation of 2a as a single
product; however, 2a was isolated in very low yield owing to
difficulties in its separation from the reaction mixture
(Table 1, entry 2). To improve the yield of 2a, we examined
the reaction of the benzyne precursor 2-(trimethylsilyl)phenyl
triflate (4a)[11] with 1 and TBAF at room temperature;
however, the hydrofluorinated products of 2a (see below),
1l5,3l5-diphosphetes,[7] were observed in the reaction mixture
(Table 1, entry 3). On the other hand, the equivalent reaction
at À208C afforded 3a together with the hydrofluorinated
products, thus indicating that the subsequent hydrofluorina-
tion was minimized at lower temperature (Table 1, entry 4).
When the reaction was performed at À408C, the hydro-
fluorination was completely avoided, and the arylated prod-
uct 2a was isolated in 59% yield (Table 1, entry 5).
4
5
57
32[b]
6
33[c]
The addition of 1 to benzyne should generate the
corresponding anionic intermediate, which would be proton-
ated promptly. However, when o-iodoaryl sulfonate 5 was
[a] Yield of the isolated product. [b] The reaction was carried out at
À408C for 3 h. [c] The reaction time was 2 h.
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phenyl group were successfully combined with the open-shell
P-heterocyclic unit, and 2c and 2d were isolated in moderate
yields (Table 2, entries 2 and 3). Compound 2e was selectively
produced in 57% yield (entry 4). This regioselectivity for 2e
could be explained by the inductive effect of the methoxy
group to promote the observed meta selectivity.[13,14] No
regioisomer of 2e was observed in the reaction mixture. On
the other hand, the naphthalene derivative 4 f gave a mixture
of 2b and 2 f with 2b as the major product (Table 2, entry 5).
Next, we examined reaction of 1 in the presence of 4g, which
normally reacts with predominant meta selectivity.[15]
Although 2g was obtained as the major product, a small
amount of 2h was also formed, presumably because of
coordination of the nitrogen atom to the countercation (Li+)
of 1.[16]
Figure 1. a) HOMOÀ1 (#207) and b) LUMO+1 (#210) of 2b [M06-
2X/6-31G(d)]. Values in parenthesis indicate the estimated amount of
electrons [CASSCF(6,6)/6-31G(d)].
The physicochemical and structural properties of 2 and 6
were next investigated. The isolated products 2 all showed
similar visible-absorption wavelengths (ca. 600 nm) and
oxidation potentials (ca. + 0.30 V; Table 3). As indicated
open-shell character and thus p-type semiconductivity,
although the mobility and on/off ratio of a drop-cast
fabricated field-effect transistor (FET) were low (mh = 1.29 ”
10À8 cm2 VÀ1 sÀ1, On/Off = 6, Vth = À4 V; see the Supporting
Information). Thus, the synthetic procedure based on the use
of arynes would be beneficial for the development of stable
congeners of cyclobutane-1,3-diyl that are significantly per-
turbed by (p-extended) aromatic substituents.
Table 3: Physicochemical properties of 2 and 6.
ox
Compound
lmax [nm][a]
E1/2 (V vs. Ag/AgCl)[b]
In an attempt to utilize 2 for a chemical transformation,
we examined the detection of HF by 2. In our previous study,
1,3-diphosphacyclobutane-2,4-diyls bearing relatively elec-
tron donating substituents trapped HF to afford 1l5,3l5-
2a
2b
2c
2d
2e
6
600
603
597
595
595
559
+0.29
+0.30
+0.28
+0.26
+0.30
+0.28
À
À
diphosphetes containing P H and P F bonds, and the
trapped HF could be removed upon treatment with sodium
hydride to recover the P-heterocyclic biradical system.[7] In
our study of HF trapping by the open-shell P-heterocyclic
system, the electron-rich 1,3-diphosphacyclobutane-2,4-diyl
system showed a remarkable blueshift of photoabsorption
assigned as the transition from the HOMO to the LUMO,
which enabled the identification of the presence of HF by
visual observation. Compound 2d showed a weak visible
absorption (595 nm) due to this HOMO–LUMO transition,
whereas 7 (Scheme 4), with a higher-lying LUMO as com-
pared to that of 2d, showed remarkably different photo-
absorption characteristics (Figure 2). Thus, 2d can be used as
a chemical probe for the detection of HF by the naked eye
(see Figure S3).
[a] In dichloromethane. [b] Conditions: 1 mm in dichloromethane, 0.1m
TBAP; working electrode: glassy carbon; counter electrode: Pt; scan
rate: 50 mVsecÀ1; ferrocene/ferrocenium: +0.54 V. TBAP=tetrabutyl-
ammonium perchlorate.
previously,[6] the HOMO has a significant amplitude at the
skeletal sp2 carbon atoms and the Mes* aromatic rings.
Nevertheless, the aryl substituent on the phosphorus atom
affects the physicochemical properties.[6,7] The effect of the
aromatic substituent on the 4-membered molecular system
was almost identical for all compounds 2. On the other hand,
6 showed a remarkable blueshift of photoabsorption as
compared to 2a, whereas the oxidation potential of 6 was
almost identical to that of 2a. The HOMO–LUMO energetic
difference (4.91 eV) of 6, optimized at the M06-2X/6-31G(d)/
SDD level, was larger than that of 2a (4.68 eV; M06-2X/6-
31G(d)).
The molecular structure of 2b was determined by X-ray
crystallography and revealed comparable metric parameters
to those of the previously reported air-stable 1,3-diphospha-
cyclobutane-2,4-diyls (see Figure S1 in the Supporting Infor-
mation). DFTand CASSCF(6,6) calculations for 2b identified
remarkable contributions of the 2-naphthyl substituent to the
molecular orbitals and the open-shell character. Figure 1
displays drawings of the orbitals HOMOÀ1 (#207) and
LUMO + 1 (#210), which show considerable contributions
by the naphthyl substituent (see also Figure S2). As indicated
by the estimated amount of electrons, the fused aromatic
substituent is decisive in establishing the extremely stable
In conclusion, arynes enabled the synthesis of P-arylated
air-stable 1,3-diphosphacycyclobutane-2,4-diyls that were dif-
ficult to prepare by the previously established SNAr process.
The installation of relatively electron rich and p-extended
aryl substituents on a skeletal phosphorus atom of the open-
shell singlet framework could tune the molecular orbitals
Scheme 4. Hydrofluorination of 2d and removal of HF: a) C5H5N
(2.0 equiv), Et2NH (4.1 equiv), THF, room temperature, 24 h; b) NaH
(2.0 equiv), 15-crown-5 (2.0 equiv), THF, room temperature, 20 h.
Ar=3,4-Me2C6H3.
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afford 7 (291 mg, 64% yield) as a yellow solid: M.p.: 199–2028C
(dec.); 1H NMR (300 MHz, [D8]THF): d = 0.41 (d, 3J(P,H) = 16.2 Hz,
9H, tBuP), 1.22 (s, 18H, p-tBu), 1.46 (s, 18H, o-tBu), 1.53 (s, 18H, o-
tBu), 2.29 (s, 3H, Me), 2.32 (s, 3H, Me), 7.03 (s, 2H, m-Mes*), 7.21–
7.26 (m, 3H, m-Mes* + Ph), 7.80 (dd, J = 13.7 Hz, J = 7.7 Hz, 1H, Ph),
7.91 (d, J = 13.5 Hz, 1H, Ph), 8.28 ppm (ddd, 1J(P,H) = 463.1 Hz,
4J(F,H) = 52.2 Hz, 3J(P,H) = 41.7 Hz, PH); 31P{1H} NMR (121 MHz,
[D8]THF): d = 7.3 (d, 2J(P,P) = 131.2 Hz, ArP), 75.0 ppm (dd, 1J-
(P,F) = 1106.9 Hz, 2J(P,P) = 131.2 Hz, tBuP); 19F NMR (282 MHz,
[D8]THF) d = À64.1 ppm (dd, 1J(P,F) = 1107.0 Hz, 4J(H,F) =
52.5 Hz); 13C NMR (75 MHz, [D8]THF): d = 19.6 (s, Me), 19.7 (s,
Me), 26.5 (s, PCMe3), 31.1 (ddd, 1J(P,C) = 103.4 Hz, 1J(P,C) = 92.1 Hz,
2J(F,C) = 23.4 Hz, CP2), 31.6 (s, p-CMe3), 34.0 (d, 5J(P,C) = 2.3 Hz, o-
CMe3), 34.6 (s, p-CMe3), 35.0 (d, 5J(P,C) = 3.0 Hz, o-CMe3), 37.1 (s, o-
CMe3), 38.8 (ddd, 1J(P,C) = 66.4 Hz, 2J(F,C) = 31.7 Hz, 3J(P,C) =
14.3 Hz, PCMe3), 39.5 (s, o-CMe3), 119.4 (s, m-Mes*), 124.5 (d,
4J(P,C) = 3.0 Hz, m-Mes*), 128.1 (dd, 1J(P,C) = 61.5 Hz, 3J(P,C) =
7.2 Hz, ipso-Ph), 129.2 (d, 2J(P,C) = 2.3 Hz, ipso-Mes*), 130.1 (d,
2J(P,C) = 12.1 Hz, Ph), 132.9 (d, 3J(P,C) = 12.1 Hz, Ph), 136.5 (d,
2J(P,C) = 12.1 Hz, Ph), 137.2 (d, 3J(P,C) = 12.1 Hz, Ph), 141.8 (d,
4J(P,C) = 3.0 Hz, p-Ph), 143.9 (pt, {5J(P,C) + 5J(P,C)}/2 = 2.6 Hz, p-
Figure 2. UV/Vis spectra of 2d (solid line) and 7 (dashed line).
effectively for the development of highly electron donating
molecular chromophores and the detection of hydrogen
fluoride. To develop novel functional open-shell singlet
molecules, we are attempting to synthesize P congeners of
cyclobutane-1,3-diyls with p-extended and/or highly function-
alized aromatic substituents by using various 1,3-diphospha-
cyclobuten-4-yl anions.
3
3
Mes*), 150.8 (d, J(P,C) = 6.8 Hz, o-Mes*), 151.2 ppm (dd, J(P,C) =
13.2 Hz, 3J(P,C) = 8.7 Hz, o-Mes*); MS(APCI): m/z calcd for
C50H77F1P2 [M + H]+: 759.5586; found: 759.5563.
Removal of HF from 7: 15-Crown-5 (19.8 mL, 0.10 mmol) was
added to a solution of 7 (38 mg, 0.050 mmol) and 60% NaH (4.0 mg,
0.10 mmol) in THF (1 mL) at room temperature under an argon
atmosphere. The mixture was stirred for 20 h at that temperature, and
then the solvent was removed in vacuo, and the resulting residue was
dissolved in hexane. The hexane solution was washed with MeCN,
and the solvent was removed in vacuo. The residual solid was washed
with MeOH to afford 2a (20.1 mg, 54% yield) as a blue solid.
Experimental Section
2b: tert-Butyllithium (0.520 mmol) was added slowly to a solution of
2-(2,4,6-tri-tert-butylphenyl)-1-phosphaethyne (300 mg, 1.04 mmol)
in THF (24 mL) at À788C under an argon atmosphere. The resulting
mixture was stirred for 15 min and then allowed to warm to room
temperature. The reaction mixture was cooled back down to À308C,
and 3-(trimethylsilyl)naphthalen-2-yl trifluoromethanesulfonate
(0.288 mL, 1.04 mmol) was added. TBAF (0.936 mmol) was then
added dropwise, and the mixture was stirred for 1.5 h at À308C.
Hexane and MeCN were added at this temperature, and the phases
were separated. The hexane layer was washed three times with
MeCN, and the solvent was removed in vacuo. The residual solid was
washed with MeOH to afford 2b (170 mg, 43% yield) as a blue-green
Acknowledgements
This research was supported in part by Grants-in-Aid for
Scientific Research (nos. 23655173, 25109518, 25288033, and
15H00923) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan, the Collaborative Research
Program of the Institute for Chemical Research, Kyoto
University (grant nos. 2014-09 and 2015-23), and Nissan
Chemicals Co. Ltd. We thank Prof. Dr. Toshiro Takao and
Prof. Dr. Masataka Oishi of Tokyo Institute of Technology for
support with X-ray crystallographic analysis. Prof. Dr. Jun-
ichi Nishida of the University of Hyogo supported the FET
study.
1
solid. M.p.: 129–1328C (dec.); H NMR (300 MHz, CDCl3): d = 0.85
(d, 3J(P,H) = 14.1 Hz, 9H, tBuP), 1.35 (s, 18H, p-tBu), 1.49 (s, 18H, o-
tBu), 1.72 (s, 18H, o-tBu), 7.28–7.40 (m, 5H), 7.46–7.67 (m, 4H), 7.68–
7.71 ppm (m, 2H); 31P{1H} NMR (121 MHz, CDCl3): d = À10.2 (d,
2J(P,P) = 347.5 Hz, ArP), 56.9 ppm (d, 2J(P,P) = 347.5 Hz, tBuP);
13C NMR (75 MHz, CDCl3): d = 29.6 (dd, 2J(P,C), 4J(P,C) = 3.8,
5
5.3 Hz, PCMe3), 31.7 (s, p-CMe3), 33.8 (d, J(P,C) = 7.5 Hz, o-CMe3),
Keywords: arynes · fluorine · heterocycles · phosphorus ·
radicals
34.8 (s, p-CMe3), 35.3 (s, o-CMe3), 38.0 (s, o-CMe3), 38.9 (s, o-CMe3),
47.6 (dd, 1J(P,C) = 54.3 Hz, 3J(P,C) = 13.6 Hz, PCMe3), 104.8 (br s,
CP2), 121.1 (s, m-Mes*), 123.2 (s, m-Mes*), 126.3 (s, Nap), 126.3 (s,
3
2
How to cite: Angew. Chem. Int. Ed. 2016, 55, 7525–7529
Angew. Chem. 2016, 128, 7651–7655
Nap), 126.7 (d, J(P,C) = 5.3 Hz, m-Nap), 127.0 (d, J(P,C) = 12.1 Hz,
2
o-Nap), 127.7 (s, Nap), 128.0 (s, Nap), 129.5 (dd, J(P,C) = 10.6 Hz,
2J(P,C) = 3.8 Hz, o-Nap), 132.6–132.7 (m, 2C), 133.3 (s, ipso-Mes*),
134.3 (dd, 1J(P,C) = 56.6 Hz, 3J(P,C) = 24.2 Hz, ipso-Nap), 145.4 (s, p-
Mes*), 148.5 (d, J(P,C) = 9.8 Hz, o-Mes*), 150.5 ppm (pt, {3J(P,C) +
3
3J(P,C)}/2 = 8.3 Hz, o-Mes*); MS(ESI): m/z calcd for C52H74P2 [M]+C:
760.5265; found: 760.5302.
[1] a) P. M. Tadross, B. M. Stolz, Chem. Rev. 2012, 112, 3550;
b) M. S. Singh, Reactive Intermediates in Organic Synthesis—
Structure, Mechanism, and Reactions, Wiley-VCH, Weinheim,
2014.
[2] a) G. Wittig, H. Matzura, Liebigs Ann. Chem. 1970, 732, 97; b) G.
Wittig, H. Braun, Liebigs Ann. Chem. 1971, 751, 27; c) G. Wittig,
E. Benz, Chem. Ber. 1959, 92, 1999.
7: NHEt2 (253 mL, 2.46 mmol) was added to a solution of 2d
(443 mg, 0.60 mmol) and pentafluoropyridine (125 mL, 1.2 mmol) in
THF (4 mL) at room temperature under an argon atmosphere. The
mixture was stirred for 24 h at that temperature, and then the solvent
was removed in vacuo, and the resulting residue was dissolved in
hexane. The hexane solution was washed with MeCN, and the solvent
was removed in vacuo. The residual solid was washed with EtOH to
[3] R. A. Dhokale, S. B. Mhaske, Org. Lett. 2013, 15, 2218.
7528
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[4] J. Bayardon, H. Laureano, V. Diemer, M. Dutarte, U. Das, Y.
[12] I. Sapountzis, W. Lin, M. Fischer, P. Knochel, Angew. Chem. Int.
Rousselin, J.-C. Henry, F. Colobert, F. R. Leroux, S. JugØ, J. Org.
Chem. 2012, 77, 5759.
[5] A. Bhunia, T. Kaicharla, D. Porwal, R. G. Gonnade, A. T. Biju,
Chem. Commun. 2014, 50, 11389.
[6] S. Ito, Y. Ueta, T. T. T. Ngo, M. Kobayashi, D. Hashizume, J.-i.
Nishida, Y. Yamashita, K. Mikami, J. Am. Chem. Soc. 2013, 136,
17610.
[7] Y. Ueta, K. Mikami, S. Ito, Inorg. Chem. 2015, 54, 8778.
[8] a) H. Sugiyama, S. Ito, M. Yoshifuji, Angew. Chem. Int. Ed. 2003,
42, 3802; Angew. Chem. 2003, 115, 3932; b) H. Sugiyama, S. Ito,
M. Yoshifuji, Chem. Eur. J. 2004, 10, 2700.
Ed. 2004, 43, 4364; Angew. Chem. 2004, 116, 4464.
[13] G. B. R. de Graaff, H. J. Hertog, W. C. Melger, Tetrahedron Lett.
1965, 6, 963.
[14] P. P. Wickham, K. H. Hazen, H. Guo, G. Jones, K. H. Reuter,
W. J. Scott, J. Org. Chem. 1991, 56, 2045.
[15] a) P. H.-Y. Cheong, R. S. Paton, S. M. Bronner, G.-Y. Im, N. K.
Garg, K. N. Houk, J. Am. Chem. Soc. 2010, 132, 1267; b) G.-Y.
Im, S. M. Bronner, A. E. Goetz, R. S. Paton, P. H.-Y. Cheong,
K. N. Houk, N. K. Garg, J. Am. Chem. Soc. 2010, 132, 17933.
[16] P. D. Pansegrau, W. F. Rieker, A. I. Meyers, J. Am. Chem. Soc.
1988, 110, 7178.
[9] T. Matsumoto, T. Hosoya, M. Katsuki, K. Suzuki, Tetrahedron
Lett. 1991, 32, 6735.
[10] Y. Sumida, T. Kato, T. Hosoya, Org. Lett. 2013, 15, 2806.
[11] Y. Himeshima, T. Sonoda, H. Kobayashi, Chem. Lett. 1983, 1211.
Received: February 23, 2016
Published online: May 2, 2016
Angew. Chem. Int. Ed. 2016, 55, 7525 –7529
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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