Synthesis of carborane-fused cyclobutenes and cyclobutanes

Article abstract of DOI:10.1007/s11426-014-5112-0

Transmetalation of carborane-fused zirconacycles to Cu(II) induces the C-C coupling reaction to form four-membered rings. This serves as a new efficient and general methodology for the generation of a series of carborane-fused cyclobutenes and cyclobutanes. A reaction mechanism involving transmetalation to Cu(II) and reductive elimination is proposed.

Full text of DOI:10.1007/s11426-014-5112-0

SCIENCE CHINA
Chemistry
August 2014 Vol.57 No.8: 1157–1163
doi: 10.1007/s11426-014-5112-0
• ARTICLES •
Synthesis of carborane-fused cyclobutenes and cyclobutanes
YUAN YingGen1,2, REN ShiKuo3, QIU ZaoZao1, WANG ShaoWu2* & XIE ZuoWei1,3*
1Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
2Laboratory of Functional Molecular Solids, Ministry of Education; Anhui Laboratory of Molecule-Based Materials; Institute of Organic
Chemistry, School of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China
3Department of Chemistry, The Chinese University of Hong Kong, Shatin, Shatin, New Territories, Hong Kong, China
Received February 27, 2014; accepted March 21, 2014; published online June 20, 2014
Transmetalation of carborane-fused zirconacycles to Cu(II) induces the CC coupling reaction to form four-membered rings.
This serves as a new efficient and general methodology for the generation of a series of carborane-fused cyclobutenes and cy-
clobutanes. A reaction mechanism involving transmetalation to Cu(II) and reductive elimination is proposed.
carborane, cyclobutene, cyclobutane, zirconacycle, copper
alkene and alkyne for the preparation of benzocarboranes
1 Introduction
[39], dihydrobenzocarboranes [40] and dihydrofulvenocar-
boranes [41], respectively. These reactions are co-mediated
by Zr and Ni, in which each metal plays a different role to
achieve the chemoselectivity. During the course of the above
studies, we learned that carborane-fused zirconacycles are
thermally very stable and chemically inert toward unsatu-
rated organic molecules, which is significantly different
from zirconacyclopentanes and zirconacyclopentenes with-
out carboranyl moiety [4244]. On the other hand, these
carborane-fused zirconacycles can readily undergo
transmetalation with metal halides to give reactive interme-
diates, thus opening up new opportunities for further func-
tionalization.
We previously communicated a carborane-fused cyclo-
butene, 1,2-[(Et)C=C(Et)]-1,2-C2B10H10, prepared in 46%
yield by treatment of 1,2-[Cp2ZrC(Et)=C(Et)]-1,2-C2B10H10
with CuCl2 in hot toluene [45]. It was also reported that
reaction of o-carboryne (1,2-dehydro-o-carborane) with
internal alkynes could afford the corresponding [2+2] cy-
cloaddition products, carborane-fused cyclobutenes in
< 20% yields [46, 47]. This series of highly constrained
molecules may serve as important monomers in ring-
opening metathesis polymerization/co-polymerization,
Transition metal-mediated C–C coupling reactions, as a
powerful strategy to construct a wide range of useful mole-
cules, have found many applications in organic synthesis,
mechanistic studies and the synthesis of functional materials
[1–7]. The majority of this research focuses on traditional
(tetravalent) carbon atoms. In view of the growing interest
in application of carborane derivatives in boron neutron
capture therapy [8–10], supramolecular design [11–16],
materials [17–22], and organometallics/coordination chem-
istry [23–28], we have extended transition metal-mediated
C–C coupling reactions to include carboranes, in which the
carbon is hypervalent [29]. Subsequently, several protocols
have been developed for cross-coupling reactions between
tetravalent carbon and hypervalent cage carbon of car-
boranes, leading to the synthesis of a variety of functional-
ized carboranes [30–38].
In 2012, we achieved three-component [2+2+2] and
[2+2+1] cross-cyclotrimerization of carboryne, alkyne/
Dedicated to Professor Qian Changtao on the occasion of his 80th birthday.
*Corresponding authors (email: swwang@mail.ahnu.edu.cn; zxie@cuhk.edu.hk)
© Science China Press and Springer-Verlag Berlin Heidelberg 2014
chem.scichina.com link.springer.com

1158
Yuan YG, et al. Sci China Chem August (2014) Vol.57 No.8
leading to a new class of polymeric materials incorporating
carborane units. In this connection, we investigated the
transmetalation of 1,2-[Cp2ZrC(R1)=C(R2)]-1,2-C2B10H10
and 1,2-[Cp2ZrCH(R)CH2]-1,2-C2B10H10 to copper as a
useful methodology for the synthesis of a series of car-
borane-fused cyclobutenes and cyclobutanes. These results
are reported in this article.
CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 144.7 (C=C),
75.0 (cage C), 29.3, 19.2, 13.9 (Pr). 11B{1H} NMR (96 MHz,
CDCl3): δ 8.0 (2B), 0.1 (2B), 11.0 (2B), 15.9 (4B).
+
HRMS: m/z calcd for C10H2411B810B2 : 252.2876. Found:
252.2864.
1,2-[(nBu)C=C(nBu)]-1,2-C2B10H10 (2c): Colorless oil;
1
yield, 75%. H NMR (400 MHz, CDCl3): δ 1.96 (t, J = 7.2
Hz, 4H, CH2), 1.39 (m, 8H, CH2), 0.92 (t, J = 7.2 Hz, 6H,
CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 144.6 (C=C),
75.0 (cage C), 29.7, 27.0, 22.4, 13.6 (Bu). 11B{1H} NMR
(96 MHz, CDCl3): δ 7.7 (2B), 0.1 (2B), 11.2 (2B), 16.1
2 Experimental
2.1 General procedures
+
(4B). HRMS: m/z calcd for C12H2811B810B2 : 280.3189.
All reactions were performed under an atmosphere of dry
nitrogen with the rigid exclusion of air and moisture in a
glove box. All organic solvents were freshly distilled from
sodium benzophenone ketyl immediately prior to use.
1H NMR spectra were recorded on either a Bruker DPX 300
spectrometer at 300 MHz or Bruker DPX 400 spectrometer
at 400 MHz (German). 13C{1H} NMR spectra were record-
ed on either a Bruker DPX 300 spectrometer at 75 MHz or a
Bruker DPX 400 spectrometer at 100 MHz. 11B{1H} NMR
spectra were recorded on either a Bruker DPX 300 spec-
trometer at 96 MHz or a Varian Inova 400 spectrometer at
128 MHz. All chemical shifts were reported in δ units with
reference to the residual protons and carbons of the deuter-
ated solvents for proton and carbon chemical shifts, and to
external BF3·OEt2 (0.00 ppm) for boron chemical shifts.
Mass spectra were obtained on a Thermo Finnigan MAT 95
XL spectrometer (German). Complexes 1,2-[Cp2ZrC(R1) =
C(R2)]-1,2-C2B10H10 (1) and Cp2Zr(μ-Cl)(μ-C2B10H10)-
Li(OEt2)2 were prepared according to literature methods [30,
36]. All other chemicals were purchased from either Sigma-
Aldrich or Acros Chemical Co. and used as received unless
otherwise specified.
Found: 280.3200.
1,2-[(Ph)C=C(Ph)]-1,2-C2B10H10 (2d): Colorless crystals;
1
yield, 81%. H NMR (300 MHz, CDCl3): δ 7.48 (m, 4H),
7.41 (m, 6H) (phenyl H). 13C{1H} NMR (75 MHz, CDCl3):
δ 138.9, 130.1, 129.7, 129.1, 126.0 (C=C and phenyl C),
73.9 (cage C). 11B{1H} NMR (96 MHz, CDCl3): δ 7.6 (2B),
1.1 (2B), 10.7 (2B), 14.6 (4B). HRMS: m/z calcd for
+
C16H2011B810B2 : 320.2563. Found: 320.2565.
1,2-[(Ph)C=C(Me)]-1,2-C2B10H10 (2e): Colorless crystals;
1
yield, 65%. H NMR (400 MHz, CDCl3): δ 7.40 (m, 3H),
7.19 (m, 2H) (phenyl H), 2.00 (s, 3H, CH3). 13C{1H} NMR
(100 MHz, CDCl3): δ 141.1, 138.7, 129.4, 129.1, 129.0,
125.5 (C=C and phenyl C), 75.2, 73.9 (cage C), 14.3 (CH3).
11B{1H} NMR (96 MHz, CDCl3): δ 8.1 (2B), 0.9 (2B),
10.6 (2B), 15.3 (4B). These data are identical to those
reported in the literature [47].
1,2-[(Ph)C=C(nBu)]-1,2-C2B10H10 (2f): Colorless crystals;
1
yield, 78%. H NMR (300 MHz, CDCl3): δ 7.43 (m, 3H),
7.21 (m, 2H) (phenyl H), 2.41 (t, J = 7.2 Hz, 2H), 1.61 (m,
2H), 1.49 (m, 2H) (CH2), 1.00 (t, J = 7.2 Hz, 3H, CH3).
13C{1H} NMR (75 MHz, CDCl3): δ 143.5, 140.3, 129.5,
129.1, 129.0, 125.6 (C=C and phenyl C), 74.6, 73.7 (cage
C), 28.8, 27.9, 22.7, 13.7 (nBu). 11B{1H} NMR (96 MHz,
CDCl3): δ 7.9 (2B), 0.8 (2B), 10.8 (2B), 15.1 (4B).
HRMS: m/z calcd for C14H2411B810B2: 300.2876. Found:
300.2877.
1,2-{(Ph)C=C[(CH2)3Cl]}-1,2-C2B10H10 (2g): White sol-
id; yield, 89%. 1H NMR (400 MHz, CDCl3): δ 7.40 (m, 3H),
7.22 (m, 2H) (phenyl H), 3.63 (t, J = 6.0 Hz, 2H), 2.59 (t, J
= 7.2 Hz, 2H), 2.06 (m, 2H) (CH2). 13C{1H} NMR (100
MHz, CDCl3): δ 142.0, 141.0, 129.5, 129.1, 125.8 (C=C and
phenyl C), 74.2, 73.5 (cage C), 43.8, 28.5, 26.1 (CH2).
11B{1H} NMR (128 MHz, CDCl3): δ 7.2 (2B), 0.3 (2B),
11.3 (2B), 15.7 (4B). HRMS: m/z calcd for
C13H2111B810B2Cl+: 320.2329. Found: 320.2326.
2.2 Preparation of 1,2-[(R1)C=C(R2)]-1,2-C2B10H10 (2)
A
representative procedure. To
a
suspension of
1,2-[Cp2ZrC(R1)=C(R2)]-1,2-C2B10H10 (1) (0.20 mmol) in
toluene was added Cu(OTf)2 (73 mg, 0.20 mmol). The
mixture was stirred at room temperature for 24 h. After re-
moval of the precipitate (Cu) by filtration, the resulting so-
lution was concentrated to dryness in vacuo. The residue
was subject to flash column chromatography on silica gel
(230400 mesh) using n-hexane as eluent to give 2.
1,2-[(Et)C=C(Et)]-1,2-C2B10H10 (2a): Colorless oil; yield,
1
55%. H NMR (300 MHz, CDCl3): δ 2.03 (q, J = 7.5 Hz,
4H, CH2), 1.03 (t, J = 7.5 Hz, 6H, CH3). 13C{1H} NMR (75
MHz, CDCl3): δ 144.9 (C=C), 74.6 (cage C), 20.7, 10.3 (Et).
11B{1H} NMR (96 MHz, CDCl3): δ 7.6 (2B), 0.4 (2B),
11.5 (2B), 16.3 (4B). HRMS: m/z calcd for
1,2-[(TMS)C=C(nBu)]-1,2-C2B10H10 (2h): Colorless
crystals; yield, 88%. 1H NMR (400 MHz, CDCl3): δ 2.06 (t,
J = 7.2 Hz, 2H), 1.43 (m, 4H) (CH2), 0.94 (t, J = 7.2 Hz, 3H,
CH3), 0.11 (s, 9H, TMS). 13C{1H} NMR (100 MHz, CDCl3):
δ 161.9, 150.1 (C=C), 78.5, 75.1 (cage C), 30.0, 28.0, 22.4,
13.7 (Bu), 2.2 (TMS). 11B{1H} NMR (96 MHz, CDCl3): δ
7.9 (2B), 1.4 (1B), 0.4 (1B), 10.4 (2B), 14.9 (4B).
+
C8H2011B810B2 : 224.2563. Found: 224.2561.
1,2-[(nPr)C=C(nPr)]-1,2-C2B10H10 (2b): Colorless oil;
1
yield, 43%. H NMR (400 MHz, CDCl3): δ 1.95 (t, J = 7.5
Hz, 4H, CH2), 1.48 (m, 4H, CH2), 0.94 (t, J = 7.5 Hz, 6H,

Yuan YG, et al. Sci China Chem August (2014) Vol.57 No.8
1159
HRMS: m/z calcd for C11H2811B810B2Si+: 296.2958. Found:
296.2943.
15.0 Hz, 1H, CHHPh), 2.96 (dd, J = 9.0 and 15.0 Hz, 1H,
CHHPh), 2.85 (dd, J = 9.0 and 12.0 Hz, 1H, CHHCH), 2.61
(dd, J = 6.0 and 12.0 Hz, 1H, CHHCH). 13C{1H} NMR (75
MHz, CDCl3): δ 136.7, 129.0, 128.6, 127.2 (phenyl C), 81.8,
74.5 (cage C), 48.9, 41.6, 37.9 (CH and CH2). 11B{1H}
NMR (96 MHz, CDCl3): δ 4.8 (1B), 5.4 (2B), 10.3 (2B),
11.1 (3B), 11.9 (2B). HRMS: m/z calcd for
1,2-[(TMS)C=C(Ph)]-1,2-C2B10H10 (2i): Colorless
crystals; yield, 71%; prolonged reaction time to 10 d was
required. 1H NMR (400 MHz, CDCl3): δ 7.40 (m, 3H), 7.22
(m, 2H) (phenyl H), 0.25 (s, 9H, TMS). 13C{1H} NMR (100
MHz, CDCl3): δ 148.3, 130.2, 129.0, 127.9, 126.0 (C=C and
phenyl C), 75.2 (cage C), 1.9 (TMS), another cage C was
not observed. 11B{1H} NMR (128 MHz, CDCl3): δ 6.5 (2B),
0.6 (1B), 1.0 (1B), 11.7 (2B), 15.5 (4B). HRMS: m/z
calcd for C13H2411B810B2Si+: 316.2645. Found: 316.2645.
1,2-[(Ph)C=C(CH2OCH3)]-1,2-C2B10H10 (2j): White
+
C11H2011B810B2 : 260.2568. Found: 260.2562.
1,2-[(Ph)CHCH2]-1,2-C2B10H10 (3d): Colorless crystal;
1
yield, 41%. H NMR (300 MHz, CDCl3): δ 7.41 (m, 3H),
7.12 (m, 2H) (phenyl H), 4.62 (t, J = 6.0 Hz, 1H, CH), 3.21
(d, J = 6.0 Hz, 2H, CH2). 13C{1H} NMR (75 MHz, CDCl3):
δ 136.7, 128.7, 127.6, 126.0 (phenyl C), 81.4, 74.4 (cage C),
50.4 (CH), 40.1 (CH2). 11B{1H} NMR (96 MHz, CDCl3): δ
4.5 (1B), 5.2 (2B), 10.8 (7B). HRMS: m/z calcd for
1
solid; yield, 24%. H NMR (400 MHz, CDCl3): δ 7.41 (m,
3H), 7.24 (m, 2H) (phenyl H), 4.10 (s, 2H, OCH2), 3.40 (s,
3H, OCH3). 13C{1H} NMR (100 MHz, CDCl3): δ 144.2,
138.4, 130.0, 129.1, 128.6, 126.5 (C=C and phenyl C), 73.8,
73.3 (cage C), 66.2 (OCH2), 59.2 (OCH3). 11B{1H} NMR
(96 MHz, CDCl3): δ 7.7 (2B), 0.8 (2B), 10.8 (2B), 15.2
(4B). HRMS: m/z calcd for C12H2011B810B2O+: 288.2512.
Found: 288.2510.
+
C10H1811B810B2 : 246.2406. Found: 246.2405.
1,2-[(4-Me-C6H4)CHCH2]-1,2-C2B10H10 (3e): Colorless
crystals; yield, 46%. 1H NMR (300 MHz, CDCl3): δ 7.18 (d,
J = 7.8 Hz, 2H), 6.98 (d, J = 7.8 Hz, 2H) (phenyl H), 4.56 (t,
J = 6.0 Hz, 1H, CH), 3.17 (d, J = 6.0 Hz, 2H, CH2), 2.36 (s,
3H, CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 137.4,
133.7, 129.3, 125.9 (phenyl C), 81.7, 74.5 (cage C), 50.2
(CH), 40.0 (CH2), 21.0 (CH3). 11B{1H} NMR (96 MHz,
CDCl3): δ 4.6 (1B), 5.3 (1B), 5.9 (1B), 9.2 (1B), 10.0
(1B), 11.0 (4B), 11.9 (1B). HRMS: m/z calcd for
2.3 Preparation of 1,2-[(R)CHCH2]-1,2-C2B10H10 (3)
A representative procedure. To a toluene solution (5 mL) of
Cp2Zr(μ-Cl)(μ-C2B10H10)Li(OEt2)2 (277 mg, 0.5 mmol) was
added alkene (1.0 mmol), and the mixture was heated to
reflux for 48 h. After cooling down to room temperature,
Cu(OTf)2 (272 mg, 0.75 mmol) was added. The resulting
mixture was stirred at room temperature for another 48 h.
Compound 3 was obtained using the same workup proce-
dures as reported for 2.
+
C11H2011B810B2 : 260.2568. Found: 260.2569.
1,2-[(4-Br-C6H4)CHCH2]-1,2-C2B10H10 (3f): Colorless
crystals; yield, 47%. 1H NMR (300 MHz, CDCl3): δ 7.51 (d,
J = 8.1 Hz, 2H), 6.98 (d, J = 8.1 Hz, 2H) (phenyl H), 4.53
(m, 1H, CH), 3.21 (dd, J = 7.2, 11.7 Hz, 1H, CHH), 3.13
(dd, J = 4.8, 11.7 Hz, 1H, CHH). 13C{1H} NMR (100 MHz,
CDCl3): δ 135.7, 131.9, 127.7, 121.8 (phenyl C), 80.8, 74.1
(cage C), 49.8 (CH), 40.2 (CH2). 11B{1H} NMR (96 MHz,
CDCl3): δ 4.4 (1B), 5.3 (1B), 6.0 (1B), 10.0 (2B),
11.0 (4B), 11.9 (1B). HRMS: m/z calcd for
C10H1711B810B2Br+: 325.1502. Found: 325.1508.
1,2-[(nBu)CHCH2]-1,2-C2B10H10 (3a): Colorless crystals;
1
yield, 44%. H NMR (400 MHz, CDCl3): δ 3.27 (m, 1H,
CH), 2.89 (dd, J = 8.4 and 11.6 Hz, 1H, CHH), 2.49 (dd, J =
4.4 and 11.6 Hz, 1H, CHH), 1.81 (m, 1H), 1.63 (m, 1H),
1.33 (m, 3H), 1.17 (m, 1H) (CH2), 0.91 (t, J = 6.9 Hz, 3H,
CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 82.3, 74.6 (cage
C), 48.7 (CH), 41.6, 31.4, 28.5, 22.2, 13.8 (CH2 and CH3).
11B{1H} NMR (96 MHz, CDCl3): δ 5.0 (3B), 10.5 (7B).
HRMS: m/z calcd for [M–2H]+ of [C8H2211B810B2]:
224.2563. Found: 224.2560.
1,2-[(2-Naphth)CHCH2]-1,2-C2B10H10 (3g): Colorless
crystals; yield, 35%. 1H NMR (300 MHz, CDCl3): δ 7.84 (m,
3H), 7.53 (m, 3H), 7.19 (d, J = 7.8 Hz, 1H) (naphthyl H),
4.76 (m, 1H, CH), 3.34 (dd, J = 4.8, 12.0 Hz, 1H, CHH),
3.13 (dd, J = 7.5, 12.0 Hz, 1H, CHH). 13C{1H} NMR (100
MHz, CDCl3): δ 134.1, 133.0, 132.5, 128.7, 127.9, 127.8,
126.8, 126.5, 124.9, 123.9 (naphthyl C), 81.4, 74.4 (cage C),
50.6 (CH), 40.1 (CH2). 11B{1H} NMR (96 MHz, CDCl3): δ
4.3 (1B), 5.1 (2B), 9.8 (1B), 10.9 (5B), 11.7 (1B).
1,2-[(nHex)CHCH2]-1,2-C2B10H10 (3b): Colorless oil;
1
yield, 58%. H NMR (300 MHz, CDCl3): δ 3.23 (m, 1H,
CH), 2.86 (dd, J = 6.9 and 11.7 Hz, 1H, CHH), 2.46 (dd, J =
4.2 and 11.7 Hz, 1H, CHH), 1.79 (m, 1H), 1.61 (m, 1H),
1.26 (m, 8H) (CH2), 0.88 (t, J = 6.3 Hz, 3H, CH3). 13C{1H}
NMR (100 MHz, CDCl3): δ 82.3, 74.6 (cage C), 48.7 (CH),
41.6, 31.8, 31.5, 28.8, 26.4, 22.5, 14.0 (CH2 and CH3).
11B{1H} NMR (96 MHz, CDCl3): δ 5.6 (3B), 10.4 (2B),
11.0 (3B), 11.9 (2B). HRMS: m/z calcd for
+
HRMS: m/z calcd for C14H2011B810B2 : 296.2570. Found:
296.2568.
2.4 X-ray structure determination
+
C10H2611B810B2 : 254.3037. Found: 254.3032.
Data were collected at 293 K on a Bruker SMART 1000
CCD diffractometer using Mo-Kα radiation. An empirical
absorption correction was applied using the SADABS pro-
gram [48]. The structures were solved by direct methods
1,2-[(Bn)CHCH2]-1,2-C2B10H10 (3c): White solid; yield,
33%. 1H NMR (300 MHz, CDCl3): δ 7.30 (m, 3H), 7.11 (m,
2H) (phenyl H), 3.58 (m, 1H, CH), 3.07 (dd, J = 9.0 and

1160
Yuan YG, et al. Sci China Chem August (2014) Vol.57 No.8
and subsequent Fourier difference techniques and refined
anisotropically for all non-hydrogen atoms by full-matrix
least-squares on F2 using the SHELXTL program package
[49]. All hydrogen atoms were geometrically fixed using the
riding model. Details of the crystal structures were deposit-
ed in the Cambridge Crystallographic Data Centre with
CCDC-988351 (2d) and CCDC-988352 (3d). Crystal data
and details of data collection and structure refinements were
given in Table 1.
A series of carborane-fused zirconacyclopentenes were
examined under the optimal reaction conditions (Table 2,
entry 6) and the results were summarized in Table 3. Phenyl
and TMS substituents offered much better yields of car-
borane-fused cyclobutenes than those of alkyls (Table 3,
entries 4–9 vs 1–3), suggesting that electron-withdrawing
groups favored such coupling reactions. For 1i, a prolonged
reaction time was necessary, probably due to steric reasons
(Table 3, entry 9). If a substituent bearing a donor atom
such as CH2OCH3 in 1j, the yield was significantly dropped
to 24%, which might be ascribed to the coordination of do-
3 Results and discussion
Table 2 Optimization of reaction conditions for preparation of 2a a)
3.1 Synthesis of carborane-fused cyclobutenes
Zirconocene-carboryne precursor Cp2Zr(μ-Cl)(μ-C2B10H10)-
Li(OEt2)2 reacted well with alkynes R1C≡CR2 in refluxing
toluene to generate the corresponding zirconacyclopentenes
1,2-[Cp2ZrC(R1)=C(R2)]-1,2-C2B10H10 (1). In the presence
of CuCl2, 1,2-[Cp2ZrC(Et)=C(Et)]-1,2-C2B10H10 (1a) gave a
CC coupling product 1,2-[(Et)C=C(Et)]-1,2-C2B10H10 (2a)
in 16% GC yield with the formation of copper mirror. The
yield was increased to 46% when the toluene solution was
Entry
[Cu]
CuCl
Solvent
toluene
toluene
toluene
toluene
toluene
toluene
THF
Yield (%) b)
1
2
0
trace
0
CuBr
o
heated at 80 C for 72 h [45]. We then examined the reac-
3
CuI
tion using different Cu salts and solvents to identify the op-
timal reaction conditions. The results were compiled in Ta-
ble 2. In general, Cu(II) salts were much more effective than
Cu(I), of which Cu(OTf)2 showed the highest activity. Sol-
vents were also crucial for this reaction. Toluene and Et2O
offered the best results of 78% and 77% yields, respectively
(Table 2, entries 6 and 9).
4
CuCl2
16
0
5
CuSO4
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
6
78
52
43
77
57
7
8
DME
9
Et2O
10
hexane
a) Reaction conditions: a mixture of 1a (8.9 mg, 0.02 mmol) and [Cu]
(0.02 mmol) in a solvent (0.6 mL) was stirred at room temperature for 24 h;
b) GC yield.
Table 1 Crystal data and structure refinements for 2d and 3d
2d
3d
Table 3 Cu(OTf)2 mediated formation of carborane-fused cyclobutenes a)
Formula
C16H20B10
0.40 × 0.30 × 0.20
320.4
C10H18B10
0.40 × 0.30 × 0.20
246.3
Crystal size (mm)
Fw
Crystal system
Space group
triclinic
monoclinic
P21/n
P(1)
a, Å
b, Å
c, Å
9.869(2)
10.739(2)
10.739(2)
75.10
63.66(1)
63.66(1)
911.5(3)
2
9.042(2)
14.042(3)
11.669(2)
90
98.04(1)
90
1466.9(5)
4
1.115
Entry
R1/R2
Et/Et
1
2
Yield (%) b)
, deg
, deg
 , deg
V, Å3
Z
1
2
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
55
43
nPr/nPr
3
nBu/nBu
75
Dcalcd, Mg/m3
1.168
4
Ph/Ph
81
5
Ph/Me
Ph/nBu
65
Mo Kα (0.71073)
4.1 to 50.0
0.058
Mo Kα (0.71073)
4.6 to 50.0
0.052
Radiation () Å
2 max, deg
, mm1
6
78
7
Ph/(CH2)3Cl
TMS/nBu
TMS/Ph
Ph/CH2OCH3
1g
1h
1i
2g
2h
2i
89
8
88
71 c)
F(000)
332
512
9
No. of obsd reflns
No. of params refnd
Goodness of fit
wR1
3043
2589
10
1j
2j
24
235
181
1.031
1.088
a) Reaction conditions: a mixture of 1 (0.2 mmol) and Cu(OTf)2 (0.2
mmol) in toluene was stirred at room temperature for 24 h; b) isolated
yields; c) prolonged reaction time of 10 d.
0.046
0.056
wR2
0.117
0.163

Yuan YG, et al. Sci China Chem August (2014) Vol.57 No.8
1161
nor atom to transition metal, disfavoring the transmetalation
(Table 3, entry 10). It was noted that the one-pot proce-
dure by the reaction of zirconocene-carboryne precursor
Cp2Zr(μ-Cl)(μ-C2B10H10)Li(OEt2)2 with 3-hexyne, followed
by treatment with Cu(OTf)2 also gave 2a in 53% isolated
yield, which was very comparable to that of 55% yield from
the reaction of 1a (Table 3, entry 1).
the yields of 3 were not obvious. The relatively lower yields
of 3 over those of 2 may be associated with the stability of
the resultant transmetalation products, carboranylcuprates as
1-[CH3CH(R)]-1,2-C2B10H11 or 1-[CH2(R)CH2]-1,2-C2B10H11
was always isolated in addition to 3. It was noted that the
known example of 1,2-(CH2)2-1,2-C2B10H10 was prepared
via several steps from o-carborane, with the last step being
the ring-closing of 1-TsOCH2CH2-1,2-C2B10H11 in the
presence of nBuLi in the best yield of 40% [50].
1
Compounds 2aj were fully characterized by H, 13C,
and 11B NMR spectra as well as high-resolution mass spec-
trometry. Both 1H and 13C NMR spectra indicated that 2ad
are symmetrical molecules as only one set of signals corre-
sponding to cyclobutene substituents was observed. The
cage carbons were observed in the range 73–78 ppm while
the olefinic carbons ranged from 130 to 162 ppm in the
13C NMR spectra. The 11B{1H} NMR spectra exhibited a
2:2:2:4 pattern spanning between 8 to 16 ppm though
some are unsymmetrical molecules of which one of the
peaks was split into two, leading to a 2:1:1:2:4 pattern for
2h and 2i.
1
Compounds 3ag were fully characterized by H, 13C,
and 11B NMR spectra as well as high-resolution mass spec-
1
trometry. The H NMR spectra of 3 showed unique two
doublets of doublets for methylene protons of four-
membered ring in the range 2.5 to 4.8 ppm. The four ring
carbons appeared in 13C NMR spectra at about 40 and 50
ppm corresponding to two sp3-C, and at about 75 and 82
ppm assignable to the two cage carbons. Their 11B NMR
spectra exhibited a similar pattern of 3:7 in the range 4 to
12 ppm.
As the above reactions worked only with Cu(II) rather
than Cu(I); and on the other hand, the Cu mirror was formed
during the reactions, the reaction mechanism may involve
the transmetalation of zirconacycle to Cu(II), giving an in-
termediate 1,2-[CuC(R1)=C(R2)]-1,2-C2B10H10 or 1,2-
[CuCH(R)CH2]-1,2-C2B10H10. The subsequent reductive
elimination readily afforded the cyclization product and Cu
mirror.
3.2 Synthesis of carborane-fused cyclobutanes
We have reported that terminal alkenes can react with
Cp2Zr(μ-Cl)(μ-C2B10H10)Li(OEt2)2 to give zirconacycles
1,2-[Cp2ZrCH(R)CH2]-1,2-C2B10H10 [37]. Inspired by the
aforementioned results, we applied the above method to the
synthesis of carborane-fused cyclobutanes. As expected,
treatment of 1,2-[Cp2ZrCH(R)CH2]-1,2-C2B10H10 with 1.5
equiv. of Cu(OTf)2 in toluene at room temperature for 48 h
gave 1,2-[CH(nBu)CH2]-1,2-C2B10H10 (3a) in 44% isolated
yield. One-pot reaction between Cp2Zr(μ-Cl)(μ-C2B10H10)
Li(OEt2)2 and 1-hexene in refluxing toluene, followed by
addition of 1.5 equiv. of Cu(OTf)2 at room temperature also
produced 3a in 44% yield. Both terminal alkyl and aryl al-
kenes were compatible with this reaction. The results were
summarized in Table 4. It seems that electronic effects on
3.3 Structure
The molecular structures of 2d and 3d were determined by
single-crystal X-ray analyses and are shown in Figures 1–2,
respectively. For 2d, two single bond distances of C(1)–
C(11)/C(2)–C(18) (1.544(2)/1.539(2) Å) and one double
bond length of C(11)–C(18) (1.365(2) Å) compare to those
Table 4 One-pot synthesis of carborane-fused cyclobutanes a)
Entry
R
nBu
3
Yield (%) b)
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3f
44
58
33
41
46
47
35
nHex
Bn
Ph
4-Me-C6H4
4-Br-C6H4
2-naphth
3g
Figure 1 Molecular structure of 1,2-[(Ph)C=C(Ph)]-1,2-C2B10H10 (2d).
Selected bond lengths (Å) and angles (deg): C(1)C(2) 1.612(2),
C(1)C(11) 1.544(2), C(2)C(18) 1.539(2), C(11)C(18) 1.365(2),
C(11)C(1)C(2) 85.3(1), C(1)C(11)C(18) 94.5(1), C(1)C(2)C(18)
85.5(1), C(2)C(18)C(11) 94.7(1).
a) Reaction conditions: a toluene solution of Cp2Zr(μ-Cl)(μ-C2B10H10)
Li(OEt2)2 (0.5 mmol) and alkene (1.0 mmol) was refluxed for 2 d, to which
was added Cu(OTf)2 (0.75 mmol) at room temperature. The reaction mixture
was then stirred for 2 d. b) Isolated yields.

1162
Yuan YG, et al. Sci China Chem August (2014) Vol.57 No.8
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WANG ShaoWu received his BSc and MS degrees from Anhui Normal University in 1985 and
1992, respectively. After obtaining a PhD from The Chinese University of Hong Kong in 1999, he
spent one year as a postdoctoral fellow at The Chinese University of Hong Kong. He then joined the
faculty of the Anhui Normal University as a full professor of chemistry. He serves as the
vice-president of the Anhui Normal University since June 2009, and has co-authored about 110 pub-
lications in peer-reviewed journals, 5 patents, and received two 2nd prize awards from the State
Commission of Education and Anhui Province. His research interests include organolanthanide
chemistry and organic synthesis.
XIE ZuoWei obtained a B.Sc. from Hangzhou University in 1983, a M.Sc. from Shanghai Institute
of Organic Chemistry (SIOC) in 1986, and his Ph.D. in 1990, working in a joint program between
SIOC and the Technische Universitӓt Berlin under the co-supervision of Profs. Changtao Qian,
Yaozeng Huang and Herbert Schumann. He is a Choh-Ming Li Professor of Chemistry in The Chi-
nese University of Hong Kong, a member of Shanghai-Hong Kong Joint Laboratory in Chemical
Synthesis and the 2008 recipient of the State Natural Science Prize.