Synthesis of ruthenium boryl analogues of the shvo metal-ligand bifunctional catalyst

Article abstract of DOI:10.1021/om1005808

Metal boryl complexes have received significant attention in the literature in recent years due to their role as key intermediates in a number of metal-catalyzed borylation reactions. The ligand scaffold is known to have a significant impact on the observed reactivity of these metal boryl complexes. A synthetic strategy to access ruthenium boryl analogues of the Shvo metal-ligand catalysts is described. Heating a precursor to Shvos catalyst (1) with bis(catecholato)diboron at 50 °C provided ruthenium boryl complex 3, [2,5-Ph₂-3,4-Tol₂(η⁵-C₄COBcat) Ru(CO)₂Bcat] (Bcat = catecholatoboryl). Addition of bis(catecholato)diboron to complex 1 in the presence of a phenol results in ruthenium boryl complex 5, [2,5-Ph₂-3,4-Tol₂(η ⁵-C₄COH)Ru(CO)₂Bcat], at 22 °C in 30% isolated yield. A single-crystal X-ray analysis of complex 5 confirmed the assigned structure. An improved synthesis of ruthenium boryl complex 5 was developed by the in situ formation of complex 3, [2,5-Ph₂-3,4- Tol₂(η⁵-C₄COBcat)Ru(CO)₂Bcat], followed by addition of the phenol, resulting in a 51% yield.

Full text of DOI:10.1021/om1005808

3896 Organometallics 2010, 29, 3896–3900
DOI: 10.1021/om1005808
Synthesis of Ruthenium Boryl Analogues of the Shvo
Metal-Ligand Bifunctional Catalyst
Liza Koren-Selfridge,† Ian P. Query,† Joel A. Hanson,† Nicholas A. Isley,† Ilia A. Guzei,‡
and Timothy B. Clark*,†
†Department of Chemistry, Western Washington University, Bellingham, Washington 98225-9150, and
‡Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706
Received June 12, 2010
Metal boryl complexes have received significant attention in the literature in recent years due to their
role as key intermediates in a number of metal-catalyzed borylation reactions. The ligand scaffold is
known to have a significant impact on the observed reactivity of these metal boryl complexes. A syn-
thetic strategy to access ruthenium boryl analogues of the Shvo metal-ligand catalysts is described.
Heating a precursor to Shvo’s catalyst (1) with bis(catecholato)diboron at 50 °C provided ruthenium
boryl complex 3, [2,5-Ph2-3,4-Tol2(η5-C4COBcat)Ru(CO)2Bcat] (Bcat = catecholatoboryl). Addi-
tion of bis(catecholato)diboron to complex 1 in the presence of a phenol results in ruthenium boryl
complex 5, [2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)2Bcat], at 22 °C in 30% isolated yield. A single-
crystal X-ray analysis of complex 5 confirmed the assigned structure. An improved synthesis of ruthe-
nium boryl complex 5 was developed by the in situ formation of complex 3, [2,5-Ph2-3,4-Tol2-
(η5-C4COBcat)Ru(CO)2Bcat], followed by addition of the phenol, resulting in a 51% yield.
Introduction
substituents into aryl or alkyl organic substrates.1,10-15 This
process allows for the conversion of inexpensive starting
materials into valuable fine chemicals based on the ability to
convert C-B bonds into C-O, C-N, and C-C bonds.16-21
The continued development of metal boryl complexes with
unique ligand frameworks has the potential to provide un-
explored reactivity. Metal-ligand bifunctional complexes
have led to highly active catalysts in hydrogenation and conju-
gate addition reactions in recent years.22,23 The Shvo and
Noyori catalysts have served as prototypes of these metal-
ligand bifunctional catalysts, which have been shown to have
Metal boryl complexes have received significant attention
over the past two decades due to their known intermediacy in
metal-catalyzed reactions that incorporate boron substitu-
ents into organic substrates.1-9 The design of metal boryl
complexes with unique ligand frameworks has led to com-
plexes with previously unknown reactivity. C-H borylation
reactions, for example, utilize iridium, rhodium, rhenium, and
ruthenium catalysts in the selective incorporation of boron
*To whom correspondence should be addressed. E-mail: clark@
chem.wwu.edu.
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r
pubs.acs.org/Organometallics
Published on Web 08/11/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 17, 2010 3897
mechanistically unique catalytic cycles.24-27 Our group is
interested in boron-substituted analogues of the Shvo com-
plex with the ultimate goal of designing metal boryl com-
plexes with unique catalytic activity based on metal-ligand
bifunctional catalysis.28-30 We herein report the synthesis of
two ruthenium boryl complexes, [2,5-Ph2-3,4-Tol2(η5-C4CO-
Bcat)Ru(CO)2Bcat] and [2,5-Ph2-3,4-Tol2(η5-C4COH)Ru-
(CO)2Bcat] (Bcat=catecholatoboronate), by the activation
of bis(catecholato)diboron by a coordinatively unsaturated
precursor to the Shvo catalyst.
Scheme 1. Synthesis of Ruthenium Boryl Complex 3
Results and Discussion
The synthesis of [2,5-Ph2-3,4-Tol2(η5-C4COBcat)Ru(CO)2-
Bcat] (3) was achieved by reaction of bis(catecholato)diboron
with ruthenium dimer 1. Previous studies on boron-substituted
analogues of Shvo complexes revealed that the activation
of pinacolborane could be achieved by ruthenium dimer 1 at
50 °C to provide ruthenium hydride 2 in 74% isolated yields
(Scheme 1).28 Direct replacement of pinacolborane with bis-
(catecholato)diboron resulted in the formation of 3 in 70%
NMR yield at 50 °C after 6 h. The structure of 3 was supported
by 11B NMR spectroscopy with a broad resonance at 47.8 ppm
(Ru-Bcat) and a sharp resonance at 22.3 ppm (Cp0O-
Bcat).15,28,31,32 Repeated attempts to isolate 3 were unsuc-
cessful due to the labile nature of the O-B bond. The greater
kinetic stability of the O-B bonds of 2 compared with 3 is
attributed to the known increased stability of pinacolato-
substituted boron substituents over those of catecholato-
substituted boron substituents.33 Analogous reactions of ruthe-
nium dimer 1 with bis(pinacolato)diboron (or other diboron
reagents), however, were unsuccessful in generating the desired
[2,5-Ph2-3,4-Tol2(η5-C4COBpin)Ru(CO)2Bpin] in appreciable
yields. These results are consistent with the known increased
reactivity of B2cat2 over B2pin2 in metal-mediated reactions.34
The oxidative addition of diboron reagents in the presence
of an alcohol was expected to provide [2,5-Ph2-3,4-Tol2(η5-
C4COH)Ru(CO)2B(OR)2] complexes directly. Detailed me-
chanistic and computational studies by Casey and Cui on the
activation of H2 have revealed a cyclic proton transfer
transition state (TS-A, Scheme 2) involving ethanol for this
oxidative addition.35 The activation of diboron reagents in
the presence of an alcohol was anticipated to proceed through an
analogous transition state (TS-B). Unlike the activation of
H2, where the exchange of a hydrogen atom is degenerate,
activation of a diboron reagent in the presence of an alcohol
was expected to result in the net addition of (RO)2B-H,
rather than (RO)2B-B(OR)2.
Addition of B2cat2 to ruthenium dimer 1 in the presence of
4-methoxyphenol resulted in the formation of [2,5-Ph2-3,4-
Tol2(η5-C4COH)Ru(CO)2Bcat] (5) in 74% NMR spectro-
scopic yield after 2 h at 22 °C (eq 1).36 The reaction required
that the alcohol additive did not possess an R-hydrogen. The
presence of an R-hydrogen resulted in dehydrogenation of
the alcohol to the carbonyl and formation of the correspond-
ing ruthenium hydride.24 Upon screening numerous phenols
(electron rich, electron poor, and 2,6-disubstituted) and tert-
butyl alcohol, 4-methoxyphenol was found to provide the
highest yield of 5. The formation of 5 is believed to proceed
via TS-B (Scheme 2), rather than by initial activation of
B2cat2 and protonolysis of the O-B bond by the phenol. This
conclusion is based on the reduced reaction time and tem-
perature in the presence of a phenol, as compared to the
addition of B2cat2 in the absence of the phenol (Scheme 1).
The increased reaction rate in the presence of the phenol is
consistent with a lower kinetic barrier to diboron activation
than the reaction in the absence of the phenol.35
(24) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.;
Kavana, M. J. Am. Chem. Soc. 2001, 123, 1090–1100.
(25) Casey, C. P.; Clark, T. B.; Guzei, I. A. J. Am. Chem. Soc. 2007,
129, 11821–11827.
(26) Abdur-Rashid, K.; Clapham, S. E.; Hadzovic, A.; Harvey, J. N.;
Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2002, 124, 15104–15118.
(27) Casey, C. P.; Johnson, J. B. J. Org. Chem. 2003, 68, 1998–2001.
(28) Koren-Selfridge, L.; Londino, H. N.; Vellucci, J. K.; Simmons,
B. J.; Casey, C. P.; Clark, T. B. Organometallics 2009, 28, 2085–2090.
(29) Conley, B. L.; Williams, T. J. J. Am. Chem. Soc. 2010, 132,
1764-1765.
In addition to the formation of complex 5, bridging hydride 6
(Figure 1) was identified by 1H NMR spectroscopic analysis
(10-25% NMR yield).24 The quantity of complex 6 was highly
dependent on both the identity and number of equivalents of
the phenol that was used. Initially, the source of complex 6
was unclear since phenols lack R-hydrogens and therefore
cannot act as a source of H2. The O-H borylation of the
phenol by the in situ generated complex 5, however, could
account for the generation of bridging hydride 6.37,38 To test
(30) Conley, B. L.; Williams, T. J. Chem. Commun. 2010, 4815–4817.
(31) 11B NMR spectra of borate esters typically have resonances in
€
the range 20-25 ppm; EtOBcat δ=23.0 ppm. Noth, H.; Wrackmeyer,
B. Nuclear Magnetic Resonance Spectroscopy of Boron Compounds;
Diehl, P.; Fluck, E.; Kosfeld, R., Eds.; NMR Basic Principles and Progress
14; Springer-Verlag: Berlin, 1978; pp 136-138.
(32) 11B NMR chemical shift of B2cat2 is 30.7 ppm.
(33) Tucker, C. E.; Davidson, J.; Knochel, P. J. Org. Chem. 1992, 57,
3482–3485.
1
(36) NMR spectroscopic yields were determined by H NMR spec-
troscopy relative to phenyltrimethylsilane as an internal standard and a
ten-second relaxation delay for integral integrity.
(37) Schlecht, S.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9435–
9443.
(34) Dang, L.; Zhao, H.; Lin, Z.; Marder, T. B. Organometallics 2008,
27, 1178–1186.
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(38) Marciniec, B.; Walkowiak, J. Chem. Commun. 2008, 2695–2697.

3898 Organometallics, Vol. 29, No. 17, 2010
Koren-Selfridge et al.
Scheme 2. Proposed Transition State for Diboron Activation Compared to Hydrogen Activation
this hypothesis, isolated complex 5 was treated with 4-methoxy-
phenol, resulting in 25% conversion into terminal hydride 4
after 4 h at 22 °C (eq 2). In the presence of ruthenium dimer 1,
terminal hydride 4 is known to readily convert into bridging
hydride 6.24,39 The formation of complex 6, therefore, is con-
sistent with competitive reaction rates of the phenol in diboron
activation (via TS-B, Scheme 2) and O-H borylation (eq 2).
Purification of ruthenium boryl complex 5 by trituration
with diethyl ether and pentane resulted in clean isolation of 5
in 30% yield. Bridging hydride 6 was found to be soluble in a
diethyl ether/pentane mixture, allowing its removal upon
successive triturations. The characterization data obtained
for complex 5 confirms the proposed ruthenium boryl struc-
ture. Notably, the 11B NMR spectrum revealed a broad reso-
nance at 47.0 ppm.15,28 The IR spectrum also showed charac-
teristic CO stretching frequencies at 2025 and 1968 cm-1
.
Although the spectroscopic data are consistent with the
assigned structure of 5, interactions between the metal-
bound boron substituent and the ligand-bound hydrogen
were considered. The presence of a σ-borane complex was
ruled out on the basis of the 1H NMR chemical shifts.
Known σ-borane complexes have broad resonances (due to
interactions with boron) of the boron-bound hydrogen and
have similar chemical shifts to typical metal hydrides (-5 to
-10 ppm),40,41 which is in stark contrast to the observed
sharp singlet at þ6.45 ppm. A transient σ-borane complex
was also ruled out on the basis of the known regiochemistry
of dialkoxyboranes [(RO)2B-H] with the coordinatively
Figure 2. X-ray crystal structure of 5.
unsaturated complex derived from dimer 1 (Scheme 1),
which provides the corresponding ruthenium hydride (such
as 2, Scheme 1).
The regiochemistry of boron addition and the nature of the
interactions between boron and hydrogen in complex 5 were
probed in the solid state using X-ray crystallography. X-ray
quality crystals were grown from CH2Cl2 and pentane at -30 °C.
The crystal structure confirms the identity of 5 and revealed
two unusual conformational features in the solid state arising
from hydrogen bonding of the Cp0OH to an oxygen atom on
the boryl group. First, the plane of the catecholatoboryl sub-
stituent is turned toward the hydroxycyclopentadienyl ligand
in spite of the steric congestion imposed by the tetraaryl-
substituted cyclopentadienyl ligand (19.9° torsion angle for
Cp0 centroid-Ru-B-O(2)).42-44 Second, the hydroxy sub-
stituent on the cyclopentadienyl ligand and catecholatoboryl
substituent are nearly eclipsed. X-ray crystal structures of the
Figure 1. Bridging hydride 6.
(42) Rankin, M. A.; MacLean, D. F.; McDonald, R.; Ferguson,
M. J.; Lumsden, M. D.; Stradiotto, M. Organometallics 2009, 28, 74–83.
(43) Lam, W. H.; Shimada, S.; Batsanov, A. S.; Lin, Z.; Marder,
T. B.; Cowan, J. A.; Howard, J. A. K.; Mason, S. A.; McIntyre, G. J.
Organometallics 2003, 22, 4557–4568.
(44) Hartwig, J. F.; Huber, S. J. Am. Chem. Soc. 1993, 115, 4908–
4909.
(39) Casey, C. P.; Beetner, S. E.; Johnson, J. B. J. Am. Chem. Soc.
2008, 130, 2285–2295.
(40) Hartwig, J. F.; Muhoro, C. N.; He, X. J. Am. Chem. Soc. 1996,
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Article
Organometallics, Vol. 29, No. 17, 2010 3899
Scheme 3. Independent Synthesis of Ruthenium Hydride 7
Shvo-type complexes were found to have the metal-hydride
bond rotated nearly anti to the hydroxy substituent of the
cyclopentadienyl ligand in the solid state.45-47
labile nature of the O-Bcat bond precluded the isolation of
complex 7 or 8.
Although the synthesis of complex 5 was achieved in the
presence of 4-methoxyphenol, the isolated yields were diffi-
cult to obtain reproducibly. This resulted in the evaluation
of other approaches to generate the ruthenium boryl com-
plex. The labile nature of the O-B bond in 3 prompted
attempts to selectively protonolyze the O-B bond in the
presence of the Ru-B bond. A solution to the irreproducible
yields was established with the initial formation of complex 3
(Scheme 3), followed by addition of 4-methoxyphenol,
providing 5 as the major product (eq 3). Again, the addition
of the phenol resulted in formation of bridging hydride 6,
but complex 5 was formed more cleanly using this protocol,
ultimately providing complex 5 in 51% isolated yield.
Notably, this optimized protocol resulted in readily repro-
ducible yields of 5.
Conclusions
The activation of bis(catecholato)diboron by ruthenium
dimer 1 was explored as a means to prepare ruthenium boryl
complexes possessing both acidic and Lewis acidic hydroxy-
cyclopentadienyl ligands. The direct activation of bis(cate-
cholato)diboron by 1 at 50 °C provided the corresponding
ruthenium boryl complex (3) with a second boron substitu-
ent bound to oxygen on the hydroxycyclopentadienyl ligand.
Conducting the same experiment in the presence of a phenol
resulted in rapid formation of ruthenium boryl complex 5 at
22 °C (with an O-H bond on the hydroxycyclopentadienyl
ligand). The concurrent formation of bridging hydride 6
decreased the isolated yield. Optimized yield of complex 5
was obtained by a two-step procedure in which ruthenium
boryl complex 3 was first generated, followed by selective
cleavage of the ligand O-B bond to provide 5 in 51% iso-
lated yield. An X-ray crystal structure of 5 was obtained to
confirm the identity of this ruthenium boryl complex.
Preliminary studies of the reactivity of 5 have been limi-
ted by the thermal instability of the complex. Heating 5 in
benzene-d6 to 80 °C results in decomposition to the corres-
ponding hydride, [2,5-Ph2-3,4-Tol2(η5-C4COBcat)Ru(CO)2H]
(7) (eq 4). In addition to complex 7, bridging hydride 8 was
observed (Scheme 3). Both of these complexes can be syn-
thesized by treatment of ruthenium dimer 1 with cate-
cholborane. The addition of 2 equiv of catecholborane to
ruthenium dimer 1 results in the predominant formation of
bridging hydride 8 in 30 min at 22 °C (∼10% terminal
hydride 7 observed by 1H NMR spectroscopy). The genera-
tion of terminal hydride 7 could also be facilitated by heat-
ing 8 to 50 °C with a slight excess of catB-H (3 equiv).
Unfortunately, only up to 75% conversion to 7 was possi-
ble under these conditions, and higher temperatures led to
decomposition of 7 and 8. The two complexes were readily
distinguishable by 1H NMR spectroscopy based on the
characteristic hydride resonances (7: δ = -9.3 ppm; 8: δ =
-18.3 ppm), which are consistent with the analogous com-
plexes from activation of dihydrogen (terminal hydride: δ=
-9.8 ppm; bridging hydride 6: δ=-18.2 ppm).24 Again, the
Experimental Section
General Experimental Information. All materials were mani-
pulated under dry nitrogen in an MBraun Inc. glovebox. NMR
spectra were collected on a UNITY-Inova spectrometer at 500 MHz
for H NMR, 125 MHz for 13C NMR, and 160 MHz for 11B
1
NMR. 1H NMR spectra are referenced to benzene-d6 at 7.16 ppm.
The 1H NMR data are reported as follows: chemical shift in ppm,
multiplicity (s = singlet, d = doublet, t = triplet, q = quartet,
qn = quintet, sep = septet, m = multiplet), coupling constants
(Hz), and integration. 13C NMR spectra are referenced to
benzene-d6 at 125.39 ppm. 11B NMR spectra were referenced
to an external BF3 Et2O sample in benzene-d6 (0.0 ppm).
3
Elemental analyses were performed by Desert Analytics in
Tucson, AZ. Elemental analysis data are reported as follows:
empirical formula; calculated percent carbon, calculated per-
cent hydrogen, calculated percent nitrogen; observed percent
carbon, observed percent hydrogen, observed percent nitrogen.
Deuterated solvents and pentane were dried, distilled, and
degassed prior to use. Toluene and diethyl ether were deoxy-
genated and dried in a solvent purification system (Innovative
Technology, Inc.) by passing through an activated alumina column
and an oxygen scavenging column under nitrogen, followed by
(45) Casey, C. P.; Strotman, N. A.; Beetner, S. E.; Johnson, J. B.;
Priebe, D. C.; Guzei, I. A. Organometallics 2006, 25, 1236–1244.
€
(46) Knolker, H.-J.; Baum, E.; Goesmann, H.; Klauss, R. Angew.
Chem., Int. Ed. 1999, 38, 2064–2066.
(47) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816–5817.

3900 Organometallics, Vol. 29, No. 17, 2010
Koren-Selfridge et al.
degassing by three freeze-pump-thaw cycles. Bis(catecholato)-
diboron and 4-methoxyphenol were purchased from Aldrich
and used without further purification. Catecholborane was pur-
chased from Aldrich and distilled under vacuum prior to use.
[2,5-Ph2-3,4-Tol2(η4-C4CO)]Ru(CO)2]2 (1) was synthesized accor-
ding to the literature procedure.24
Method B. A slurry of {[2,5-Ph2-3,4-Tol2(η4-C4CO)Ru-
(CO)2}2 (1) (0.500 g, 0.438 mmol) and bis(catecholato)diboron
(0.313 g, 1.32 mmol) in 38 mL of toluene was heated to 50 °C for
5 h. The reaction mixture was removed from heat, and 4-
methoxyphenol (0.114 g, 0.918 mmol) was added. After 20
min, the solvent was evaporated in vacuo to provide a yellow
solid. Et2O was added to the flask (20 mL), and the solution was
stirred in the glovebox for 2 h. Pentane (12 mL) was then added
to the solution, and the mixture was stirred for 10 min, during
which a yellow precipitate formed. The precipitate was filtered
and rinsed with additional pentane (∼40 mL) and dried in vacuo
to provide 5 as a pale yellow solid (0.308 g, 51%).
[2,5-Ph2-3,4-Tol2(η5-C4COBcat)Ru(CO)2Bcat] (3). To a resea-
lable NMR tube containing {[2,5-Ph2-3,4-Tol2(η4-C4CO)Ru-
(CO)2}2 (1) (0.016 g, 0.014 mmol) and 0.60 mL of benzene-d6
was added bis(catecholato)diboron (0.007 g, 0.029 mmol). The
reaction mixture was agitated to disperse the solids and was
1
heated to 50 °C for 6 h. A H NMR spectrum of the reaction
mixture showed a 70% NMR spectroscopic yield of 3 relative to
an internal PhSiMe3 standard. 1H NMR (benzene-d6, 300 MHz):
δ 7.72 (m, 4H), 7.25 (d, J=8.1, 4H), 6.92 (m, 6H), 6.72 (m, 2H),
6.60 (m, 8H), 6.45 (m, 2H), 1.85 (s, 6H). 11B NMR (benzene-d6,
160 MHz): δ 47.8, 22.3.
{[2,5-Ph2-3,4-Tol2(η5-C4CO)]2Bcat}Ru2(CO)4(μ-H) (8). To a
resealable NMR tube containing {[2,5-Ph2-3,4-Tol2(η4-C4CO)-
Ru(CO)2}2 (1) (0.033 g, 0.029 mmol) and 0.60 mL of benzene-d6
was added catecholborane (0.007 mL, 0.066 mmol). The reac-
tion mixture was agitated to disperse the solid. After 25 min a 1H
NMR of the reaction mixture showed complete conversion to 8
with about 10% 7. 1H NMR (benzene-d6, 300 MHz): δ 7.25 (d,
J=6.6 Hz, 6H), 7.13 (m, 6H), 6.99 (m, 18H), 6.71 (m, 2H), 6.49
(m, 8H), 1.80 (s, 12H), -18.36 (s, 1H). 11B NMR (benzene-d6,
160 MHz): δ 22.0.
[2,5-Ph2-3,4-Tol2(η4-C4OH)]Ru(CO)2Bcat (5). Method A.
A slurry of {[2,5-Ph2-3,4-Tol2(η4-C4CO)Ru(CO)2}2 (1) (0.300 g,
0.263 mmol), bis(catecholato)diboron (0.126 g, 0.530 mmol),
and 4-methoxyphenol (0.066 g, 0.532 mmol) in 11 mL of toluene
was stirred for 2 h at 22 °C. The solvent was evaporated in vacuo,
providing a yellow solid. Et2O was added to the flask (12 mL),
and the solution was stirred in the glovebox for 2 h. Pentane
(6 mL) was then added to the solution, and the mixture was stir-
red for 10 min, during which a yellow precipitate formed.
The precipitate was filtered and rinsed with additional pentane
(∼30 mL) and dried in vacuo to provide 5 as a pale yellow solid
(0.110 g, 30%). Crystals suitable for X-ray diffraction were
obtained by crystallization in a solution of CH2Cl2/pentane at
-30 °C. 1H NMR (benzene-d6, 500 MHz): δ 7.58 (d, J=6.5 Hz,
4H), 7.22 (d, J=8.0 Hz, 4H), 6.93 (m, 6H), 6.76 (dd, J=5.8, 3.2
Hz, 2H), 6.66 (d, J=8.0 Hz, 4H), 6.61 (dd, J=5.5, 3.5 Hz, 2H),
6.45 (s, 1H), 1.88 (s, 6H). 13C NMR (benzene-d6, 125 MHz): δ
201.7, 149.9, 138.0, 133.2, 132.6, 131.0, 129.4, 129.0, 128.9,
128.7, 122.5, 112.1, 107.1, 94.8, 21.3. 11B NMR (C6D6, 160
Acknowledgment. We thank Charles P. Casey for help-
ful discussions. Acknowledgement is made to the donors
of the American Chemical Society Petroleum Research
Fund for partial support of this research (47598-GB3),
the M.J. Murdock Charitable Trust and Research Corpo-
ration for Science Advancement in the form of a depart-
ment development grant, Western Washington Univer-
sity, and the National Institutes of Health in the form of a
NRSA fellowship (GM077962-01).
Supporting Information Available: 1H and 11B NMR spectral
data of 2, 3, 5, 7, and 8 and the X-ray crystallographic data
of 5 are provided free of charge via the Internet at http://
pubs.acs.org.
MHz): δ 47.0. IR (thin film): 3054, 2987, 2025, 1968, 1422 cm-1
.
Anal. Calcd for C39H29O5BRu: C, 67.74; H, 4.52. Found: C,
67.64; H, 4.38.