Direct arylation of a cluster-bound alkyne ligand with benzene

Article abstract of DOI:10.1021/om100329r

Treatment of a triruthenium complex having μ₃-methylidyne and μ₃-propyne ligands, (Cp*Ru)₃(μ₃- CH)(μ₃-η²-HCCMe)(μ-H)₂ (6), with benzene results in arylation of the propyne ligand via the C-H bond activation of benzene to yield the μ₃-phenylmethylacetylene complex (Cp*Ru)₃(μ₃-CH)(μ₃- η²:η²(⊥)-PhCCMe) (7). An X-ray diffraction study of 7 clearly established that regiospecific arylation occurred at the terminal position of the propyne ligand.

Full text of DOI:10.1021/om100329r

4770 Organometallics 2010, 29, 4770–4773
DOI: 10.1021/om100329r
Direct Arylation of a Cluster-Bound Alkyne Ligand with Benzene†
Toshiro Takao, Makoto Moriya, Mana Kajigaya, and Hiroharu Suzuki*
Department of Applied Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
Received April 22, 2010
Scheme 1. Stoichiometric Arylation of an Alkyne Ligand on a
Triruthenium Clustera
Summary: Treatment of a triruthenium complex having μ3-
methylidyne and μ3-propyne ligands, (Cp*Ru)3(μ3-CH)(μ3-η2-
HCCMe)(μ-H)2 (6), with benzene results in arylation of the
propyne ligand via the C-H bond activation of benzene to yield
the μ3-phenylmethylacetylene complex (Cp*Ru)3(μ3-CH)(μ3-
η2:η2(^)-PhCCMe) (7). An X-ray diffraction study of 7
clearly established that regiospecific arylation occurred at
the terminal position of the propyne ligand.
Arylation of olefins and alkynes by way of C-H bond
activation of arenes has been one of the most intensely studied
areas of organometallic chemistry in years,1 emerging as an
economically and environmentally useful alternative to cross-
coupling reactions using aryl halides. Significant advances in
catalytic metal-mediated Ar-H bond activation have been
achievedinrecent years, as exemplified by selectiveC-C bond
formation at the ortho position by the directing effect of a
heteroatom tether2 and direct formation of styrene by Pd(II)-
catalyzed oxidative arylation of ethylene.3
Recently, two excellent catalytic systems for the direct
alkylation of benzene with linear preference were indepen-
dently reported by two groups led by Matsumoto and
Periana4 and by Gunnoe.5 Linear selectivity is determined
by the direction of olefin insertion into the metal-phenyl
bond formed by the C-H bond activation, whereas the
orientation of the insertion is controlled by the steric envir-
onment around the metal center. However, selectivity for
n-propylbenzene still remains around 60% when using
propene. In this context, an alternative approach for the
alkylation of benzene seems to be necessary.
a Legend: (a) 25 ꢀC, 30 min; (b) 180 ꢀC, 3 days; (c) 180 ꢀC, 24 h; (d) 180 ꢀC,
2 days; (e) 7 atm H2, 180 ꢀC, 3 days.
(^)-alkyne ligand (Scheme 1).6 This reaction was found to
proceed via the formation of μ3-benzyne complexes 3 and 4;
C-C bond formation took place between the μ3-benzyne
ligand and the C5 fragment separated by the Ru3 plane of 4.
Benzene was coordinated to the trimetallic site from the less
hindered face of the Ru3 plane of 2 and underwent C-H
bond activation. In this paper, we report another type of
arylation of an alkyne ligand using a trimetallic complex
having alkyne and μ3-methylidyne ligands on both faces of
the Ru3 plane. Occupation of both faces of the Ru3 plane
by hydrocarbyl groups prevents the approach of benzene
in the same way as in 2 to form a μ3-benzyne complex.
Nevertheless, arylation occurred at the alkyne ligand in a
manner different from that observed in 4.
We thus tried another route to access alkylbenzene using a
polyhydrido cluster and reported the stoichiometric alkyla-
tion of benzene using triruthenium complex 2 having a
† Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar
Seyferth in honor of his outstanding service as Editor-in-Chief of Organo-
metallics.
(1) (a) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res. 2001, 34,
633–639. (b) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826–834.
(c) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731–1770.
(2) Murai, S.; Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.;
Sonoda, M.; Chatani, N. Nature 1993, 366, 529–531.
(3) Jia, C.; Piao, D.; Oyamada, J.; Lu, W.; Kitamura, T.; Fujiwara, Y.
As described in our previous paper, the μ3-methylidyne-
μ3-η2( )-alkyne complex (Cp*Ru)3(μ3-CH)(μ3-η2( )-MeCCH)-
(μ-H)2 (6) was prepared by the reaction of 1 with butadiene at
80 ꢀC.7 Thermolysis of 6 in benzene at 180 ꢀC for 12 h resulted
in the exclusive formation of the (^)-phenylmethylacetylene
complex (Cp*Ru)3(μ3-CH)(μ3-η2:η2(^)-PhCCMe) (7) in 92%
Science 2000, 287, 1992–1995.
(4) (a) Matsumoto, T.; Taube, D. J.; Periana, R. A.; Taube, H.;
Yoshida, H. J. Am. Chem. Soc. 2000, 122, 7414–7415. (b) Matsumoto, T.;
Yoshida, H. Catal. Lett. 2001, 72, 107–109. (c) Periana, R. A.; Liu, X. Y.;
Bhalla, G. Chem. Commun. 2002, 3000–3001. (d) Matsumoto, T.; Periana,
R. A.; Taube, D. J.; Yoshida, H. J. Mol. Catal. A: Chem. 2002, 180, 1–18.
(e) Bhalla, G.; Oxgaard, J.; Goddard, W. A., III; Periana, R. A. Organome-
tallics 2005, 24, 3229–3232.
(5) (a) Lail, M.; Arrowood, B. N.; Gunnoe, T. B. J. Am. Chem. Soc.
2003, 125, 7506–7507. (b) Lail, M.; Bell, C. M.; Conner, D.; Cundari, T. R.;
Gunnoe, T. B.; Petersen, J. L. Organometallics 2004, 23, 5007–5020.
(c) Feng, Y.; Lail, M.; Barakat, K. A.; Cundari, T. R.; Gunnoe, T. B.; Petersen,
J. L. J. Am. Chem. Soc. 2005, 127, 14174–14175.
(6) Moriya, M.; Tahara, A.; Takao, T.; Suzuki, H. Eur. J. Inorg.
Chem. 2009, 3393–3397.
(7) Takemori, T.; Inagaki, A.; Suzuki, H. J. Am. Chem. Soc. 2001,
123, 1762–1763.
r
pubs.acs.org/Organometallics
Published on Web 05/18/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 21, 2010 4771
yield (eq 1).8 The rate of this reaction was significantly faster
than the transformation of 4 to 5, which required 2 days at
180 ꢀC. Reaction of 6 with benzene still proceeded at 120 ꢀC,
although it took more time (3 days).
It is noteworthy that the C-C bond between the methine
carbon of the alkyne ligand and benzene is formed in an
intermolecular fashion. Few examples of intramolecular
reactions between alkyne and arene moieties on a trimetallic
cluster are known,9 but to the best of our knowledge, this
type of direct arylation via the C-H bond activation of
unactivated benzene by a cluster compound is unprece-
dented. Although Friedel-Crafts alkylation of benzene
directly provides a C-C bond, it is a branched, not linear,
alkylbenzene. It should also be noted that C-C bond for-
mation occurred selectively at the terminal position of the
alkyne ligand; C-C bond formation at the 2-position of the
propyne ligand was completely suppressed on the Ru3 plane.
Perpendicular coordination of the phenylmethylacetylene
ligand in 7 was clearly established by an X-ray diffraction
study (Figure 1).10 The phenylmethylacetylene ligand is
coordinated to the Ru(1)-Ru(1#) edge in a perpendicular
mode and bisects the Ru3 triangle. On the basis of the
orientation of the alkyne ligand, two regioisomers for 7 are
possible. However, only one isomer, that with the phenyl
group positioned inside the Ru3 core, was obtained. This was
attributed to thermodynamic factors, where steric repulsion
between the Cp* groups and the phenyl group on the
acetylenic carbon are minimized.11
Figure 1. Molecular structure and labeling scheme of 7 with
thermal ellipsoids at the 30% probability level. Selected bond
lengths (A) and angles (deg): Ru(1)-Ru(1#), 2.6799(13); Ru(1)-
Ru(2), 2.7024(13); Ru(1)-C(1), 2.020(6); Ru(1)-C(2), 1.980(5);
Ru(1)-C(3), 2.213(6); Ru(2)-C(1), 1.960(7); Ru(2)-C(3),
2.064(8); C(2)-C(3), 1.466(10); C(2)-C(4), 1.514(11), C(3)-C(5),
1.480(10); Ru(2)-Ru(1)-Ru(1#), 60.28(2); Ru(1)-Ru(2)-
Ru(1#), 59.46(2); C(2)-C(3)-C(4), 125.2(6); C(2)-C(3)-C(5),
115.0(6).
˚
The size of the Ru3 triangle of 7 is significantly smaller
than that of the corresponding (^)-alkyne complex {Cp*Ru-
(μ-H)}3(μ3-η2:η2(^)-PhCCMe) (8), which was obtained from
the reaction of 1 and 1-phenyl-1-propyne.11a,12 The Ru(1)-
˚
Ru(1#) length of 2.6799(13) A, on which the phenylmethyl-
acetylene ligand is perpendicularly coordinated, is shorter
˚
than that of 8 by 0.16 A. The difference in the Ru-Ru bond
lengths between 7 and 8 likely arises from the presence of the
μ3-methylidyne ligand, which, in comparison to the μ-hy-
drido ligands, seems to bind the ruthenium atoms more
tightly.
(8) Benzene (10 mL) and 6 (50.6 mg, 0.066 mmol) were charged in a
glass autoclave. The reaction vessel was heated to 180 ꢀC for 12 h with
vigorous stirring. After the solvent was removed under reduced pressure,
the residual solid was washed with methanol (5 mL ꢀ 3). The residual
solid was then dried under reduced pressure, and 7 was obtained as a red
solid (51.2 mg, 0.061 mmol, 92% yield). 1H NMR (400 MHz, 23 ꢀC,
THF-d8): δ 1.41 (s, 15H, C5Me5), 1.72 (s, 30H, C5Me5), 2.28 (s, 3H,
˚
The C(2)-C(3) bond distance (1.466(10) A) is consider-
˚
ably longer than that of 8 (1.410(6) A), while the Ru(2)-C(3)
distance is significantly shorter: i.e. 2.064(8) A in 7 compared
˚
˚
to 2.139(5) A in 8. These structural features imply that back-
PhCCMe), 5.65 (dd, JH-H = 7.6, 1.0 Hz, 2H, o-Ph), 6.63 (tt, JH-H
=
donation from the metal centers to the π*(CC) orbital of the
alkyne moiety was significantly enhanced by the replacement
of the hydrido ligands with the μ3-methylidyne ligand.
Similar to the case for other (^)-alkyne complexes derived
from 1, the alkyne ligand of 7 exhibits dynamic behavior,
referred to as switchback motion.13 The alkyne moiety of 7
slips over the Ru3 core by alternating the position of the
inner- and outer-carbon atoms in each step, causing the
environments of all three Cp* groups to become equivalent
within the NMR time scale. While the 1H NMR spectrum of
7 revealed two well-separated signals assignable to the Cp*
groups at δ 1.41 (15H) and 1.72 (30H) at 25 ꢀC, these signals
became broaden with increasing temperature.
7.2, 1.0 Hz, 1H, p-Ph), 6.94 (dd, JH-H = 7.6, 7.2 Hz, 2H, m-Ph), 17.28 (s,
1H, μ3-CH). 13C{1H} NMR (100 MHz, 23 ꢀC, THF-d8): δ 11.6 (C5Me5),
27.5 (PhCCMe), 91.2 (C5Me5), 94.3 (C5Me5), 108.9 (PhCCMe), 121.3
(Ph), 124.6 (Ph), 127.3 (Ph), 147.3 (Ph), 223.6 (PhCCMe), 348.8 (μ3-CH).
Anal. Calcd for C40H54Ru3: C, 57.33; H, 6.49. Found: C, 57.62; H, 6.15.
(9) (a) Bruce, M. I.; Humphrey, P. A.; Skelton, B. W.; White, A. H.;
Costuas, K.; Halet, J.-F. Dalton 1999, 479–486. (b) Deng, M.; Leong,
W. K. Organometallics 2002, 21, 1221–1226.
(10) Crystal data for 7: C40H54Ru3, fw = 838.04, monoclinic, space
˚
˚
group C2/m (No. 12), a = 17.418(11) A, b = 18.059(7) A, c = 11.485(6)
A, β = 103.33(5)ꢀ, V = 3515(3) A , Z = 4, Dcalcd = 1.583 g/cm3,
temperature -120 ꢀC, μ(Mo KR) = 12.99 cm-1, R1 = 0.0420, wR2 =
0.1106 for 2685 reflections with I > 2σ(I).
3
˚
˚
(11) (a) Takao, T.; Takaya, Y.; Murotani, E.; Tenjimbayashi, R.;
Suzuki, H. Organometallics 2004, 23, 6094–6096. (b) Riehl, J.-F.; Koga,
N.; Morokuma, K. Organometallics 1994, 13, 4765–4780.
(12) Results of the X-ray diffraction studies of 8 and molecular
structure are given in the Supporting Information. Crystal data for 8:
C39H56Ru3, fw = 828.05, monoclinic, space group C2/m (No. 12), a =
The 13C signal of the inner carbon of a (^)-alkyne ligand
appears in the higher magnetic field region due to its five-
coordinated nature as well as the ring current shielding effect
caused by the three surrounding Cp* groups.11 In the 13C
˚
˚
˚
17.417(5) A, b = 18.432(5) A, c = 11.512(3) A, β = 104.445(10)ꢀ, V =
3
3578.9(17) A , Z = 4, Dcalcd = 1.537 g/cm3, temperature -50 ꢀC, μ(Mo
˚
KR) = 12.75 cm-1, R1 = 0.0379, wR2 = 0.0925 for 4036 reflections
with I > 2σ(I). Hydrogen atoms attached to the ruthenium atoms were
located by sequential difference Fourier synthesis and refined isotropi-
cally.
(13) Takao, T.; Kakuta,S.;Tenjimbayashi,R.;Takemori,T.;Murotani,
E.; Suzuki, H. Organometallics 2004, 23, 6090–6093.

4772 Organometallics, Vol. 29, No. 21, 2010
Takao et al.
NMR spectra of 7, a singlet assignable to the inner acetylenic
carbon was observed at δ 108.9, while the signal of the outer
carbon atom appeared in the lower magnetic field region
characteristic of a bridging alkylidene ligand (δ 223.6). Both
signals underwent a downfield shift of ca. 30-40 ppm in
comparison to those of 8 (Cin, δ 73.7: Cout, δ 181.1).11a
When the reaction was carried out at 100 ꢀC for 3 days,
slow formation of a trace amount of an intermediate,
(Cp*Ru)3(μ-H)2(μ3-CH)(μ3-η2( )-PhCCMe) (9), containing
a ( )-phenylmethylacetylene ligand was observed. Alterna-
tively, 9 was synthesized in 89% yield by treatment of 7 with
1 atm of H2 at 80 ꢀC (eq 2).14 Complex 9 was stable below
80 ꢀC; however, it was quantitatively converted to 7 upon
heating at 120 ꢀC. Although there have been a few examples
of reversible transformation between the two alkyne brid-
ging modes via the addition of CO,15 this is the first example
of the reversible transformation between them via the oxi-
dative addition of dihydrogen.
Figure 2. Molecular structure and labeling scheme of 9 with
thermal ellipsoids at the 30% probability level. Selected bond
˚
lengths (A) and angles (deg): Ru(1)-Ru(2), 2.7521(5); Ru(1)-
Ru(3), 2.7724(5); Ru(2)-Ru(3), 2.6765(5); Ru(1)-C(1), 1.951(4);
Ru(1)-C(2), 2.028(4); Ru(2)-C(1), 2.047(4); Ru(2)-C(2),
2.173(4); Ru(2)-C(3), 2.147(4); Ru(3)-C(1), 2.014(4); Ru(3)-
C(3), 2.104(4); C(2)-C(3), 1.395(5); C(2)-C(4), 1.507(6); C(3)-
C(5), 1.478(5); Ru(2)-Ru(1)-Ru(3), 57.955(3); Ru(1)-Ru(2)-
Ru(3), 61.402(12);Ru(1)-Ru(3)-Ru(2), 60.643(12);C(2)-C(3)-
C(4), 122.5(4); C(2)-C(3)-C(5), 126.5(4).
The parallel coordination mode of the phenylmethylace-
tylene ligand in 9 was unambiguously confirmed by an X-ray
diffraction study (Figure 2).16 Although a pair of enantio-
meric isomers having similar geometrical features was pre-
sent in the unit cell, only one enantiomer is depicted in
Figure 2. The alkyne moiety is σ-bonded to Ru(1) and Ru(3)
and π-bonded to Ru(2) in a μ3-η2( ) fashion. The C(2)-C(3)
of the hydrido ligands is located on the Ru(1)-Ru(3) edge,
while the other is on the Ru(2)-Ru(3) edge below the phenyl
group.
Since complex 6 is coordinatively saturated (48e config-
uration), elimination of dihydrogen is required to generate a
coordination site for benzene. Elimination of dihydrogen
from 6 yields the intermediate A, in which the alkyne ligand
adopts a perpendicular coordination mode due to the 46e
configuration nature of the complex18 (Scheme 2). The
(^)-alkyne ligand in A is then transformed into a μ3-prope-
nylidene ligand in B via a 1,2-shift of the methine proton. We
have reported facile interconversion between a perpendicu-
larly coordinated phenylacetylene complex and a μ3-styryli-
dene complex, where the μ3-styrylidene isomer was shown to
be more reactive than the (^)-alkyne complex.19 Thus, we
propose that benzene coordinates to the μ3-propenylidene
intermediate B and undergoes C-H bond scission to form
the phenyl intermediate D. For the formation of the phenyl-
methylacetylene ligand, a Z isomer is required to form for the
intermediate E. Therefore, it is reasonable to assume that
C-H bond activation occurs at the ruthenium center be-
neath the methyl group on the μ3-propenylidene ligand.
Subsequent reductive C-C bond coupling followed by
C-H bond cleavage at the μ-vinyl group in E affords
the parallel alkyne complex 9, and finally the (^)-alkyne
complex 7 was obtained after elimination of dihydrogen.
˚
distance of 1.395(5) A lies within reported values for ( )-
17
˚
alkyne ligands on triruthenium complexes (1.31-1.42 A).
The hydrido ligands are coordinated to the Ru3 core in an
unsymmetrical manner with respect to the alkyne ligand; one
(14) Heptane (20 mL) and 7 (222.4 mg, 0.265 mmol) were charged in a
glass autoclave. The reaction vessel was degassed and then charged with
1 atm of dihydrogen. The solution was heated at 80 ꢀC for 12 h with
vigorous stirring, after which the solvent was removed under reduced
pressure. The residual solid was dissolved in 10 mL of pentane and
purified by column chromatography on alumina (Merck, Art. No. 1097)
using toluene as eluent. The first red band, containing 9, was collected.
Drying under reduced pressure afforded 9 as an orange solid (196.5 mg,
0.235 mmol, 89% yield). 1H NMR (400 MHz, 23 ꢀC, THF-d8): δ -21.81
(s, 1H, RuH), -16.17 (s, 1H, RuH), 1.50 (s, 15H, C5Me5), 1.76 (s, 15H,
C5Me5), 1.89 (s, 15H, C5Me5), 2.17 (s, 3H, PhCCMe), 6.37 (br, 1H, o-
Ph), 6.83 (m, 1H, p-Ph), 7.04 (br, 2H, m-Ph), 7.10 (br, 1H, o-Ph), 14.39 (s,
1H, μ3-CH). 13C{1H} NMR (100 MHz, 23 ꢀC, THF-d8): δ 11.0 (C5Me5),
11.4 (C5Me5), 12.3 (C5Me5), 28.2 (PhCCMe), 94.7 (C5Me5), 95.1
(C5Me5), 95.3 (C5Me5), 123.0 (Ph), 127.2 (PhCCMe), 127.3 (Ph), 129.5
(Ph), 147.8 (Ph), 157.8 (PhCCMe), 312.9 (μ3-CH).
(15) (a) Rivomanana, S.; Lavigne, G.; Lugan, N.; Bonnet, J.-J. Inorg.
Chem. 1991, 30, 4110–4112. (b) Peng, J.-J.; Peng, S.-M.; Lee, G.-H.; Chi, Y.
Organometallics 1995, 14, 626–633. (c) Smith, A. K.; Harding, R. A. J.
Chem. Soc., Dalton Trans. 1996, 117–123. (d) Rivomanana, S.; Mongin, C.;
Lavigne, G. Organometallics 1996, 15, 1195–1207. (e) Mays, M. J.; Raithby,
P. R.; Sarveswaran, K.; Solan, G. A. Dalton Trans. 2002, 1671–1677.
(16) Crystal data for 9: C40H56Ru3, fw = 840.06, monoclinic, space
˚
˚
group P21/n (No. 14), a = 25.106(4) A, b = 11.6225(14) A, c = 25.482(4)
(18) (a) Schilling, B. E. R.; Hoffmann, R. J. Am. Chem. Soc. 1979,
101, 3456–3467. (b) Hoffman, D. M.; Hoffmann, R.; Fisel, C. R. J. Am.
Chem. Soc. 1982, 104, 3858–3875. (c) Granozzi, G.; Tondello, E.; Casarin,
M.; Aime, S.; Osella, D. Organometallics 1983, 2, 430–434. (d) Halet, J.-F.;
Saillard, J.-Y.; Lissillour, R.; McGlinchey, M. J.; Jaoen, G. Inorg. Chem.
1985, 24, 218–224. (e) Osella, D.; Pospisil, L.; Fieldler, J. Organometallics
1993, 12, 3140–3144. (f) Deabate, S.; Giordano, R.; Sappa, E. J. Cluster Sci.
1997, 8, 407–459.
A, β = 102.892(6)ꢀ, V = 7248.0(18) A , Z = 8, Dcalcd = 1.540 g/cm3,
temperature -160 ꢀC, μ(Mo KR) = 12.60 cm-1, R1 = 0.0498, wR2 =
0.1010 for 15 589 reflections with I > 2σ(I). Hydrogen atoms attached to
the ruthenium atoms were located by sequential difference Fourier
synthesis and refined isotropically.
3
˚
˚
(17) Structural data for 78 triruthenium complexes having a μ3-η2( )-
alkyne ligand were obtained from Cambridge Structural Database
System Version 5.31 (November 2009 þ 2 updates): Allen, F. H. Acta
Crystallogr. 2002, B52, 380–388.
(19) Tenjimbayashi, R.; Murotani, E.; Takemori, T.; Takao, T.;
Suzuki, H. J. Organomet. Chem. 2007, 692, 442–454.

Communication
Organometallics, Vol. 29, No. 21, 2010 4773
Scheme 2. Plausible Mechanism for the Arylation of the Propyne
Ligand in 6
In summary, direct arylation of the terminal alkyne ligand
in 6 by way of C-H bond activation of benzene was
achieved. This arylation performed on a trimetallic cluster
demonstrates the high regioselectivity for the C-C bond
formation at the terminal position of the alkyne ligand,
which likely arises from the formation of a μ3-vinylidene
intermediate. Facile transformations of a hydrocarbyl ligand
on the cluster appear to be crucial in demonstrating reactivi-
ties different from those observed in a monometallic com-
plex. In addition, this type of reaction has never been seen in
the common carbonyl cluster chemistry. This is probably due
to the nature of the Ru3 plane surrounded by the three Cp*
groups, which can stabilize the coordinatively unsaturated
species without deactivation by a coordinating ligand such as
CO. Since the reaction discussed here is stoichiometric, these
results imply the potential usefulness of the polyhydrido
cluster in synthetic reactions. We are now concentrating
our efforts on detailed mechanistic arylation studies using
substituted arene molecules.
Acknowledgment. This work was supported by Grant
No. 18105002 (Scientific Research (S)) and No. 19550058
(Scientific Research (C)) from the Japan Society of the
Promotion of Science.
Supporting Information Available: Text, tables, and figures
providing synthetic details for compounds 7 and 9 and CIF files
giving X-ray crystallographic data for 7-9. This material is
available free of charge via the Internet at http://pubs.acs.org.
Although more information is necessary for complete eluci-
dation, this mechanism is quite consistent with the initial
formation of 9 during the reaction carried out at 100 ꢀC.