Cationic rare-earth polyhydrido complexes: Synthesis, structure, and catalytic activity for the csi-1,4-selective polymerization of 1,3-cyclohexadiene

Article abstract of DOI:10.1002/anie.200603450

(Chemical Equation Presented) Rare activity from rare earths: Cationic rare-earth hydrides show regio- and stereoselectivity for the polymerization of 1,3-cyclohexadiene (CHD) to afford soluble crystalline cis-1,4-linked poly(CHD) that was previously unavailable. These cationic clusters are made by treating the corresponding neutral complexes with a borate activator (see scheme; Cp′: C₅Me₄SiMe³).

Full text of DOI:10.1002/anie.200603450

Communications
We recently reported the synthesis and hydrogenation
reactions of a new class of polynuclear rare-earth polyhydrido
complexes exemplified by [Y4(C5Me4SiMe3)4H8(thf)n] (1a:
n = 0, 1b: n = 1, 1c: n = 2).[2,3] In view of the unique reactivity
of these hydride clusters[2b–e,g] and the excellent olefin-
polymerization activity of the related cationic rare-earth–
alkyl complexes,[4,5] we became interested in the cationic
hydrido species generated from these rare-earth–hydride
clusters.
Herein, we report the synthesis, structural characteriza-
tion, and olefin-polymerization catalysis of the cationic
hydride complexes obtained from 1a–c (Scheme 1) and
related rare-earth–hydride clusters. These cationic poly-
hydrido complexes not only are the first cationic rare-earth–
hydride complexes but also show excellent regio- and
stereoselectivity for the polymerization of 1,3-cyclohexadiene
(CHD), which afforded, for the first time, soluble crystalline
cis-1,4-linked poly(CHD) (1,4 selectivity: 100%; cis selectiv-
ity: 99%). For comparison, the reaction of the neutral hydride
cluster, 1b, with CHD is also described. This reaction leads to
the formation of a structurally well-defined CHD insertion
product instead of the polymerization of CHD. Various metal
catalysts and initiators were reported previously for the
polymerization of CHD, but most yielded a mixture of 1,4-
and 1,2-poly(CHD)[6–10] or insoluble polymers,[10,11] and none
was reported to produce pure soluble crystalline cis-1,4-
linked poly(CHD).
Polymerization
DOI: 10.1002/anie.200603450
Cationic Rare-Earth Polyhydrido Complexes:
Synthesis, Structure, and Catalytic Activity
for the cis-1,4-Selective Polymerization of
1,3-Cyclohexadiene**
Xiaofang Li, Jens Baldamus, Masayoshi Nishiura,
Olivier Tardif, and Zhaomin Hou*
Reaction of the thf-free octahydrido yttrium cluster 1a[2b,f]
with one equivalent of [Ph3C][B(C6F5)4] in chlorobenzene or
toluene at 258C gave the cationic heptahydrido complex 2a
and Ph3CH (Scheme 1). In contrast to the neutral complex 1a,
which shows a good solubility in most organic solvents, the
cationic complex 2a was only slightly soluble in benzene and
toluene, and almost insoluble in hexane. Recrystallization of
2a from chlorobenzene/hexane afforded colorless single
Metal hydrides are fundamental components in a wide range
of stoichiometric and catalytic reactions. Their importance in
modern inorganic and organic chemistry cannot be over-
emphasized. Rare-earth (Group 3 and lanthanide) hydrides
are among the most active metal–hydride complexes.
Together with their alkyl analogues, metal–hydride com-
plexes of the rare-earth metals have occupied an especially
important position in the development of the organometallic
chemistry of the rare-earth elements. Generally, cationic
complexes differ in their structure and reactivity from their
neutral analogues. However, although a large number of
metal–hydride complexes of the rare-earth metals have been
synthesized and structurally characterized,[1] cationic com-
plexes of this type have not been reported previously.
[*] Dr. X. Li, J. Baldamus, Dr. M. Nishiura, Dr. O. Tardif, Prof. Dr. Z. Hou
Organometallic Chemistry Laboratory
RIKEN (The Institute of Physical and Chemical Research)
Hirosawa 2-1, Wako, Saitama 351-0198 (Japan)
and
PRESTO
Japan Science and Technology Agency (JST) (Japan)
Fax: (+81)48-462-4665
E-mail: houz@riken.jp
[**] This work was partly supported by Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science,
and Technology of Japan.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Synthesis of cationic yttrium–hydride clusters.
Cp’: C5Me4SiMe3.
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Angew. Chem. Int. Ed. 2006, 45, 8184 –8188

Angewandte
Chemie
crystals suitable for an X-ray diffraction study. X-ray analysis
revealed that 2a has a distorted tetrahedral Y4 frame, which is
bound by seven hydride ligands, one in m4, two in m3, and four
room temperature, which suggests that the unsymmetrical
{Y4H7(thf)} core in 2b is rigid. The four C6F5 groups in the
borate unit, however, showed one set of 19F NMR signals,
which indicates that the [B(C6F5)4]À ion of 2b does not have a
strong (or direct bonding) interaction with the cation.
Attempts to obtain a single crystal of the mono-thf complex
2b were not successful: Recrystallization of 2b from THF
yielded colorless single crystals of the bis-thf adduct 2c.
Alternatively, 2c could be made by reaction of the bis-thf
neutral complex 1c[2a,3b] with [Ph3C][B(C6F5)4] or by reaction
of 1b with [Ph3C][B(C6F5)4] in THF (Scheme 1).
À
in m2 bonding modes (Figure 1 and Table 1). The Y (m4-H)
An X-ray analysis established that 2c has a butterfly-like
{Y4} core bound by one m4-, two m3-, and four m2-hydride
ligands (Figure 2), which contrasts with the structure of the
neutral precursor 1c. This complex adopts a tetrahedral
{Y4H8} core structure with four m3- and four m2-hydride
ligands.[14] The interatomic distance between Y1 and Y2 in 2c
(5.2879(6) Š) is much longer than the other Y···Y interatomic
distances (3.4947(6)–3.5539(5) Š), and also much longer than
the Y···Y separations found in 1c
Figure 1. ORTEP drawing of 2a with 30% thermal ellipsoids. The
Me3Si and Me groups in C5Me4SiMe3 are omitted for clarity.
(3.3287(5)–3.9220(4) Š).[14]
In
Table 1: Summary of selected bond lengths (Š) of neutral and cationic yttrium–hydride clusters.[13]
Cp’: C5Me4SiMe3.
accordance with the butterfly
arrangement of the four Y atoms
in 2c, the Y1 H1 and Y2 H1 bonds
are much longer than the Y3 H1
1a[2f]
1b[2a,b]
1c[2a,3b]
2a
2c
3
À
À
À
À
Cp’ Yavg
2.599(3)
2.17(2)
2.39(2)
2.17(3)
–
2.624(4)
2.20(4)
2.27(3)
2.16(3)
2.403(3)
–
2.669(3)
–
2.24(3)
2.12(3)
2.407(2)
–
2.583(10)
2.36(6)
2.28(6)
2.08(6)
–
2.614(5)
2.45(3)
2.23(3)
2.07(2)
2.304(3)
–
2.626(4)
2.29(3)
2.26(3)
2.10(3)
–
À
Y (m4-H)avg
À
and Y4 H1 bonds (2.68(3) and
À
Y (m3-H)avg
Y (m2-H)avg
Y Oavg
À
Y1 F4
2.61(3) versus 2.31(3) Š and
À
À
2.21(3) Š), while the Y (m3-H)
À
À
and Y (m2-H) bond lengths in 2c
–
2.405(4)
–
are comparable with those in 1c
À
bonds in 2a with lengths from 2.23(6) Š (Y1 H1) to
À
2.43(6) Š (Y3 H1) and an average value of 2.36(6) Š are
significantly longer than those in 1a (average 2.17(2) Š),[2f] as
a result of more distortion of the Y4 frame in 2a than in 1a
À
from a normal tetrahedron. The lengths of the Y (m3-H) and
À
Y (m2-H) bonds in 2a are, however, similar to those in 1a
(Table 1).
A direct bonding interaction between the
[(C5Me4SiMe3)4Y4H7]+ ion and the [B(C6F5)4]À ion through
[12]
À
À
a Y F bond (Y1 F4: 2.405(4) Š) was also found in 2a.
1
The H NMR spectrum of 2a in C6D5Cl at room temper-
ature showed a broad singlet at 4.62 ppm for the seven
hydride ligands and one set of signals for the four C5Me4SiMe3
ligands (see the Supporting Information). Similarly, the four
C6F5 groups in the borate ion [B(C6F5)4]À also showed one set
of 19F NMR signals. These results suggest that 2a is highly
fluxional in solution, probably because of the rapid dissoci-
ation and coordination of [B(C6F5)4]À ions or solvent mole-
cules. This fluxionality was not fixed even at À408C in
Figure 2. ORTEP drawing of 2c with 30% thermal ellipsoids. The
Me3Si and Me groups in C5Me4SiMe3, hydrogen atoms in THF, and the
[B(C6F5)4]À ion are omitted for clarity.
(Table 1). Because of the greater electron deficiency of the
À
cationic metal centers in 2c, the Y Cp’ bonds in 2c are
À
significantly shorter than those in 1c, as are the Y O(thf)
1
C6D5Cl, which was shown by the H and 19F NMR spectra.
separations (Table 1). No direct bonding interaction between
the [(C5Me4SiMe3)4Y4H7(thf)2]+ ion and the [B(C6F5)4]À ion
was observed.
A similar reaction of the mono-thf complex, 1b,[2a,b] with
[Ph3C][B(C6F5)4] in chlorobenzene afforded analogously
complex 2b (Scheme 1). In contrast to the observation
made for 2a, the four C5Me4SiMe3 groups in 2b gave four
sets of signals and the seven hydride ligands showed a broad
singlet (1 m4-H) at 1.45 ppm, a pair of quartets (2 m3-H) at 3.24
(JY,H = 21.0 Hz) and 3.53 ppm (JY,H = 25.2 Hz), and a pair of
The {Y4H7(thf)2} core in 2c showed a similarly high
rigidity in solution as that in 2b. Signals corresponding to m4-
H, m3-H, and m2-H ligands could be distinguished at 2.83 ppm
(br s), 3.14 ppm (br m), and 5.41 ppm (dd,
JY,H = 39.6,
36.8 Hz), respectively, in the room-temperature NMR spec-
trum of 2b in C6D5Cl. The four C5Me4SiMe3 groups showed
two sets of 1H NMR signals, while the four C6F5 groups in the
triplets (4 m2-H) at 4.98 (JY,H = 37.5 Hz) and 5.40 ppm (JY,H
=
36.9 Hz) in the corresponding 1H NMR spectrum in C6D5Cl at
Angew. Chem. Int. Ed. 2006, 45, 8184 –8188
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8185

Communications
borate unit gave one set of 19F NMR signals. These results are
consistent with the crystal structure of 2c.
metals examined (Figure 3). In contrast, the polymerization
initiated by the trityl cation, [Ph3C][B(C6F5)4] (A), alone
showed very poor regio- and stereoselectivity and yielded
poly(CHD) with mixed 1,2- and 1,4-cis/trans microstructures
To the best of our knowledge, complexes 2a–c are the first
examples of cationic rare-earth–hydride complexes. The thf-
free cationic complex 2a and its mono-thf adduct 2b showed
very high activity for the polymerization of ethylene (ca.
103 kg of polyethylene per mol of Y, h, and atm) and
syndiospecific polymerization of styrene (ca. 10 kg of poly-
styrene per mol of Y and h, rrrr> 99%).[15] More remarkably,
these cationic complexes showed excellent regio- and stereo-
selectivity for the polymerization of CHD, which afforded
almost pure cis-1,4-poly(CHD) (1,4 selectivity: 100%; cis se-
lectivity: 99%; see the Supporting Information for details).
The bis-thf adduct 2c did not show polymerization activity
under the same conditions. Use of an isolated cationic hydrido
complex such as 2a or 2b is not required for the polymer-
ization of CHD. The cationic species generated in situ by the
reaction of 1a or 1b with one equivalent of [Ph3C][B(C6F5)4]
also showed similar or higher activity (Table 2, entry 3). As an
Figure 3. Plot of activity in the CHD polymerization versus the ionic
radius, R, of the cationic rare-earth polyhydrido complexes.
(Table 2, entry 1).[18] The activators
B and C alone showed no activity
for CHD polymerization under the
same conditions.
Table 2: Regio- and stereoselective polymerization of CHD by cationic rare-earth polyhydrido
complexes.[a]
For comparison, the reaction of
the neutral polyhydrido complex
1b with CHD was also examined.
It gave selectively the single CHD
insertion product, 3, in a quantita-
tive yield (Scheme 2). The overall
structure of the Y4 frame in 3 is
similar to that of 1b. The resultant
cyclohexenyl ligand adopts an
allylic form, which bridges two
Y atoms through the two terminal
carbon atoms of the allyl moiety,
each bonding to one metal center
in an h1 fashion (Figure 4). As with
those of 2a and 2c, the {Y4H7} core
in 3 also has one m4-H, two m3-H,
and four m2-H ligands. The
1H NMR signals for the m4-H and
the two m3-H ligands of 3 in C6D6 at
room temperature appear at
2.50 ppm (br s) and 3.91 ppm
(br m), respectively, while those
for the four m2-H ligands give two
triplets at 5.15 (t, JY,H = 35.7 Hz)
and 5.40 ppm (t, JY,H = 35.7 Hz),
[e]
[e]
[f]
Entry Ln
R[b]
[Š]
A[c]
T
[8C]
Polymer yield
1,4-Poly(CHD)
isomers[d]
Mn
Mw/Mn
Tm
[”103]
[8C]
[g]
[%]
trans
cis
tac
1[g]
2
3
–
Y
Y
Y
Y
–
A
–
25
25
25
25
25
0
25
50
25
25
25
25
25
0.80
–
100
–
43[h]
–
57
–
–
–
1.9
–
2.17
–
–
–
1.04
1.04
1.04
1.04
A
B
C
A
A
A
A
A
A
A
A
0.40
0.34
0.07
0.12
0.67
0.51
0.55
0.49
0.27
0.18
trace
50
42
9
1
3
99 73
97 75
6.1
6.3
4.2
3.6
3.5
2.0
4.5
7.8
6.8
2.9
–
2.23
2.18
2.00
1.78
1.84
1.93
2.10
1.85
1.89
1.72
–
253
254
236
249
247
248
251
247
250
226
–
4
5
5
95
–
6[i]
7
Gd 1.07
Gd 1.07
Gd 1.07
15
84
64
69
61
34
23
–
<1
>99 85
6
25
1
1
1
94 72
75
99 74
99 76
99 72
99 70
8
–
9
Dy
Ho 1.04
Er 1.03
Tm 1.02
Lu 1.00
1.05
10
11
12
13
1
–
–
–
[a] Conditions: [(C5Me4SiMe3)4Ln4H8(thf)]: 40 mmol, activator: 40 mmol, CHD: 10 mmol, V=2 mL
(toluene), t=15 h, unless otherwise noted. [b] Ionic radius of Ln3+ for CN=6; see reference [16].
[c] Activator: A=[Ph3C][B(C6F5)4], B=[PhMe2NH][B(C6F5)4], C=B(C6F5)3. [d] Determined by 1H and
13CNMR spectroscopy. Tacticity ( tac, either isotactic or syndiotactic) was determined by integrating the
ratio of the 13CNMR signals at 39.42 and 39.64 ppm for the cis-1,4-polymer, but was not unequivocally
identified. [e] Determined by gel permeation chromatography (GPC) in o-dichlorobenzene at 1408Cwith
polystyrene standard. Mn: number-average molecular weight of the polymer, Mw: weight-average
molecular weight. [f] Melting temperature of the polymer; measured by differential scanning calorimetry
(DSC). [g] t=1 min. [h] 1,4-Structure: 75%. [i] t=35 h.
activator, [PhMe2NH][B(C6F5)4] (B) was also effective,
whereas B(C6F5)3 (C) showed much lower activity under the
same conditions (Table 2, entries 4 and 5). In addition to Y,
analogous rare-earth metal clusters, such as those of Gd, Dy,
Ho, Er, Tm, and Lu,[17] were also effective catalysts for the cis-
1,4-selective polymerization of CHD under similar conditions
(Table 2). Moreover, a significant influence of the ionic radius
of the rare-earth metal centers on the catalytic activity was
observed, that is, an increase of the ionic radius led to an
almost linear increase in the catalytic activity among the
Scheme 2. Reaction of 1b with 1,3-cyclohexadiene.
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Chemie
D. W. Stephan, Coord. Chem. Rev. 2002, 233–234, 107 –129;
h) M. Ephritikhine, Chem. Rev. 1997, 97, 2193 –2242; i) H.
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[2] a) O. Tardif, M. Nishiura, Z. Hou, Organometallics 2003, 22,
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[3] For the formation and structure of 1c, see also: a) K. C.
Hultzsch, T. P. Spaniol, J. Okuda, Angew. Chem. 1999, 111,
163 –165; Angew. Chem. Int. Ed. 1999, 38, 227 –230; b) K. C.
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2003, 629, 1272 –1276.
Figure 4. ORTEP drawing of 3 with thermal ellipsoids set at 30%
probability. The C5Me4SiMe3 ligands and hydrogen atoms in the CHD
unit are omitted for clarity. Selected bond lengths [Š] and angles [8]:
À
À
À
À
Y1 C1 2.550(3), Y2 C3 2.568(3), C1 C2 1.357(5), C1 C6 1.476(6),
À
À
À
À
C2 C3 1.401(5), C3 C4 1.524(6), C4 C5 1.484(5), C5 C6 1.552(5);
C2-C1-C6 115.4(3), C1-C2-C3 128.4(4), C2-C3-C4 115.8(3), C2-C1-Y1
114.6(2), C2-C3-Y2 114.7(2).
[4] Selected reviews on cationic rare-earth–alkyl complexes:
a) P. M. Zeimentz, S. Arndt, B. R. Elvidge, J. Okuda, Chem.
Rev. 2006, 106, 2404 –2433; b) Z. Hou, Y. Luo, X. Li, J.
Organomet. Chem. 2006, 691, 3114 –3121; c) S. Arndt, J.
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J. F. Carpentier, A. Mortreux, Coord. Chem. Rev. 2004, 248, 397 –
410; e) W. E. Piers, D. J. H. Emslie, Coord. Chem. Rev. 2002,
233–234, 131 –135; f) Z. Hou, Y. Wakatsuki, Coord. Chem. Rev.
2002, 231, 1 –22.
[5] a) Y. Luo, J. Baldamus, Z. Hou, J. Am. Chem. Soc. 2004, 126,
13910 –13911; b) X. Li, Z. Hou, Macromolecules 2005, 38,
6767 –6769; c) X. Li, J. Baldamus, Z. Hou, Angew. Chem. 2005,
117, 984 –987; Angew. Chem. Int. Ed. 2005, 44, 962 –964; d) L.
Zhang, Y. Luo, Z. Hou, J. Am. Chem. Soc. 2005, 127, 14562 –
14563.
respectively. These results are consistent with the crystal
structure of 3 and show that the {Y4H7(C6H9)} core of 3 is
rigid.[19] No reaction was observed between 3 and CHD at
room temperature in [D8]toluene. However, on treatment
with one equivalent of [Ph3C][B(C6F5)4], 3 became active for
the cis-1,4-polymerization of CHD.[20]
In summary, by treating neutral rare-earth–hydride clus-
ters such as 1a–c with one equivalent of a borate compound
such as [Ph3C][B(C6F5)4], we have isolated and structurally
characterized the corresponding cationic polyhydrido com-
plexes such as 2a,c. In contrast to the neutral hydride cluster
À
1b, which yielded a single Y H addition product, 3, on
[6] For examples of CHD polymerization by radical initiators, see:
a) G. Lefebvre, F. Dawans, J. Polym. Sci. Part A 1964, 2, 3277 –
3295; b) H. Stücklen, H. Thayer, P. Willis, J. Am. Chem. Soc.
1940, 62, 1717 –1719.
treatment with CHD, the cationic hydride clusters (either
isolated or generated in situ) act as excellent catalysts for the
regio- and stereoselective cis-1,4-polymerization of CHD.
Studies on the reactions of cationic rare-earth–hydride
clusters with other unsaturated substrates, the activation of
small molecules, and the polymerization/copolymerization of
CHD by related cationic rare-earth–alkyl complexes are in
progress.
[7] For examples of CHD polymerization by anionic initiators, see:
a) H. Lussi, J. Braman, Helv. Chim. Acta 1967, 50, 1233 –1243;
b) L. A. Mango, R. W. Lenz, Polym. Prepr. Am. Chem. Soc.
Polym. Chem. Div. 1971, 12, 402 –409; c) Z. Sharaby, J. Jagur-
Grodzinski, M. Martan, D. Vofsi, J. Polym. Sci. Polym. Chem.
Ed. 1982, 20, 901 –915; d) B. Franc˛ois, X. F. Zhong, Makromol.
Chem. 1990, 191, 2743 –2753; e) P. E. Cassidy, C. S. Marvel, S.
Ray, J. Polym. Sci. Part A 1965, 3, 1553 –1565; f) I. Natori, S.
Inoue, Macromolecules 1998, 31, 4687 –4694; g) K. L. Hong,
J. W. Mays, Macromolecules 2001, 34, 782 –786; h) D. T. Wiliam-
son, J. F. Elman, P. H. Masison, A. J. Pasquale, T. E. Long,
Macromolecules 2001, 34, 2108 –2114; i) R. P. Quirk, F. You, C.
Wesdemiotis, M. A. Arnould, Macromolecules 2004, 37, 1234 –
1242.
[8] For examples of CHD polymerization by cationic initiators, see:
a) D. A. Frey, M. Hasegawa, C. S. Marvel, J. Polym. Sci. Part A
1963, 1, 2057 –2065; b) G. Lefebvre, F. Dawans, J. Polym. Sci.
Part A 1964, 2, 3277 –3295.
[9] For examples of CHD polymerization by Ziegler–Natta cata-
lysts, see: a) C. S. Marvel, G. E. Hartzell, J. Am. Chem. Soc. 1959,
81, 448 –452; b) A. H. K. Yousufzi, V. End, T. Otsu, J. Polym.
Sci. Polym. Chem. Ed. 1975, 13, 1601 –1605.
Received: August 23, 2006
Revised: September 25, 2006
Published online: November 14, 2006
Keywords: cations · cluster compounds · hydride ligands ·
.
polymerization · rare earths
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Noyori), Thieme, Stuttgart, 2002, pp. 849 –942; g) A. J. Hoskin,
[10] For examples of CHD polymerization by homogeneous titanium
catalysts, see: D. E. Heiser, J. Okuda, S. Gambarotta, R.
Mülhaupt, Macromol. Chem. Phys. 2005, 206, 195 –202.
Angew. Chem. Int. Ed. 2006, 45, 8184 –8188
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www.angewandte.org
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[11] For examples of CHD polymerization by nickel catalysts, see:
a) D. L. Gin, V. P. Conticello, R. H. Grubbs, J. Am. Chem. Soc.
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3004 –3009; d) P. Longo, C. Freda, O. R. Ballesteros, F. Grisi,
Macromol. Chem. Phys. 2001, 202, 409 –412.
[12] Interactions between rare-earth metals and fluorine were
observed previously in analogous base-free tetrakis(pentaflur-
ophenyl)borate complexes; see: a) M. W. Bouwkamp, P. H. M.
Budzelaar, J. Gercama, I. D. H. Morales, J. de Wolf, A. Meetsma,
S. I. Troyanov, J. H. Teuben, B. Hessen, J. Am. Chem. Soc. 2005,
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Kurazumi, A. C. Horiuchi, Y. Wakatsuki, Macromol. Rapid
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McDonald, J. Am. Chem. Soc. 2002, 124, 2132 –2133; d) X. Song,
M. Thornton-Pett, M. Bochmann, Organometallics 1998, 17,
1004 –1006.
[13] CCDC-617797 (1c), CCDC-617794 (2a), CCDC-617795 (2c),
and CCDC-617796 (3) contain the supplementary crystallo-
graphic 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.
[14] See the Supporting Information for details. See also referen-
ces [2a] and [3b].
[15] The reaction of the neutral hydride cluster 1b with styrene was
À
reported previously and gave a simple Y H insertion product:
reference [2b].
[16] R. D. Shannon, Acta Crystallogr. Sect. A 1976, 32, 751 –767.
[17] The Lu complex was reported previously; see reference [2a].
Other rare-earth–hydride clusters were prepared analogously to
1b. The Dy and Gd complexes were also structurally charac-
terized by X-ray analyses, which showed a tetranuclear structure
similar to that of 1b. More details will be reported elsewhere.
[18] Z. Sharaby, M. Martan, J. Jagur-Grodzinski, Macromolecules
1982, 15, 1167 –1173.
[19] The {Y4H7(C6H9)} core in 3 can be viewed as having a quasi
“mirror” symmetry, with Y1, Y3, H1, C2, and C5 being placed in
the mirror plane.
[20] In the absence of CHD, 2a was obtained when 3 was treated with
one equivalent of [Ph3C][B(C6F5)4].
8188
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