A boron-substituted analogue of the shvo hydrogenation catalyst: Catalytic hydroboration of aldehydes, imines, and ketones

Article abstract of DOI:10.1021/om801228m

The boron-substituted hydroxycycylopentadienyl ruthenium hydride [2,5-Ph₂-3,4-Tol₂(η⁵-C₄COBpin) Ru-(CO)₂H] (Bpin = 4,4,5,5-tetramethyl-l,3,2-dioxaborolane) 5 was synthesized by the addition of pinacol

Full text of DOI:10.1021/om801228m

Organometallics 2009, 28, 2085–2090
2085
A Boron-Substituted Analogue of the Shvo Hydrogenation Catalyst:
Catalytic Hydroboration of Aldehydes, Imines, and Ketones
Liza Koren-Selfridge,† Hannah N. Londino,† Jessica K. Vellucci,† Bryan J. Simmons,‡
Charles P. Casey,‡ and Timothy B. Clark*,†
Department of Chemistry, Western Washington UniVersity, Bellingham, Washington 98225-9150, and
Department of Chemistry, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706
ReceiVed December 31, 2008
The boron-substituted hydroxycycylopentadienyl ruthenium hydride [2,5-Ph2-3,4-Tol2(η5-C4COBpin)Ru-
(CO)2H] (Bpin ) 4,4,5,5-tetramethyl-1,3,2-dioxaborolane) 5 was synthesized by the addition of
pinacolborane to ruthenium dimer [2,5-Ph2-3,4-Tol2(η5-C4CO)Ru(CO)2]2 4. Complex 5 reacts with
aldehydes both stoichiometrically and catalytically, providing hydroboration products under mild reaction
conditions. A Hammett correlation plot of para-substituted benzaldehydes provided a F value of +0.91.
Catalytic hydroboration of aryl imines provided high yields of the corresponding amines. The hydroboration
of aryl ketones, however, required strongly electron-withdrawing substituents to induce hydroboration in
reasonable reaction times.
Introduction
Ligand-metal bifunctional catalysis has become a valuable
method for the hydrogenation of various organic substrates.1
The Shvo catalyst [2,3,4,5-Ph4(η5-C4COH)Ru(CO)2H] (RuHOH
1, Scheme 1) can be used in the hydrogenation of polarized
double (CdY) and triple bonds (CtY).2-5 Mechanistic studies
have revealed detailed insights into each elementary step of the
catalytic cycle.6-12 The key mechanistic step involves a unique
Figure 1. Boron-substituted analogues of the Shvo catalyst.
concerted, outer-sphere reduction in which the substrate does
Our group is interested in exploring the ability of the Shvo
catalyst to deliver reagents other than dihydrogen by ligand-metal
bifunctional catalysis.13 We have limited our attention to boron-
substituted analogues of the Shvo catalyst, [2,5-Ph2-3,4-Tol2(η5-
C4COH)Ru(CO)2B(OR)2] (RuB(OR)2OH, I, Figure 1) and [2,5-
Ph2-3,4-Tol2(η5-C4COB(OR)2)Ru(CO)2H] (RuHOB(OR)2, II),
due to the synthetic utility of the proposed boron-containing
organic products.14-17 Initial studies have focused on the
RuHOB(OR)2 complex (II) to determine the effectiveness of
boron as a surrogate for the acidic hydrogen of the Shvo catalyst.
We herein report the stoichiometric and catalytic reactivity of
[2,5-Ph2-3,4-Tol2(η5-C4COBpin)Ru(CO)2H](RuHOBpin, Bpin )
4,4,5,5-tetramethyl-1,3,2-dioxaborolane), a boron-substituted
analogue of the Shvo catalyst, in the hydroboration of aldehydes
and imines.
not coordinate to the metal prior to the addition of dihydrogen
(step a). The metal-mediated reaction of dihydrogen with a
carbonyl compound involves the addition of the metal-based
hydride hydrogen (red) to the carbonyl carbon and the addition
of the ligand-based acidic hydrogen (blue) to the carbonyl
oxygen simultaneously.
* To whom correspondence should be addressed. E-mail: clark@
chem.wwu.edu.
† Western Washington University.
‡ University of WisconsinsMadison.
(1) For reviews of ligand-metal bifunctional catalysis, see: (a) Noyori,
R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40–73. (b) Noyori, R.;
Kitamura, M.; Ohkuma, T. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5356–
5362. (c) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. ReV.
2004, 248, 2201–2237. (d) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol.
Chem. 2006, 4, 393–406.
(2) Blum, Y.; Czarkie, D.; Rahamim, Y.; Shvo, Y. Organometallics
1985, 4, 1459–1461.
(3) Shvo, Y.; Czarkie, D.; Rahamim, Y.; Chodosh, D. F. J. Am. Chem.
Soc. 1986, 108, 7400–7402.
(4) Menashe, N.; Shvo, Y. Organometallics 1991, 10, 3885–3891.
(5) Menashe, N.; Salant, E.; Shvo, Y. J. Organomet. Chem. 1996, 514,
97–102.
(6) Casey, C. P.; Singer, S. W.; Powell, D. R.; Hayashi, R. K.; Kavana,
M. J. Am. Chem. Soc. 2001, 123, 1090–1100.
The desired boron-substituted analogue of the Shvo catalyst
(complex II, Figure 1) requires a balance between a complex
that has sufficient Lewis acidity to promote hydroboration and
one that is stable toward hydrolysis or thermal decomposition.
Pinacolato-substituted boronate ester (Bpin) is known to be
(7) Casey, C. P.; Bikzhanova, G. A.; Cui, Q.; Guzei, I. A. J. Am. Chem.
Soc. 2005, 127, 14062–14071.
(8) Casey, C. P.; Johnson, J. B. J. Am. Chem. Soc. 2005, 127, 1883–
1894.
(13) For examples of additional ligand-metal bifunctional catalysts that
employ functional groups other than hydrogen, see: (a) Watanabe, M.;
Murata, K.; Ikariya, T. J. Am. Chem. Soc. 2003, 125, 7508–7509. (b)
Watanabe, M.; Ikagawa, A.; Wang, H.; Murata, K.; Ikariya, T. J. Am. Chem.
Soc. 2004, 126, 11148–11149. (c) Shibasaki, M.; Yoshikawa, N. Chem.
ReV. 2002, 102, 2187–2209. (d) Shibasaki, M.; Matsunaga, S. Chem. Soc.
ReV. 2006, 35, 269–279.
(9) Casey, C. P.; Johnson, J. B.; Singer, S. W.; Cui, Q. J. Am. Chem.
Soc. 2005, 127, 3100–3109.
´
(10) Samec, J. S. M.; Ell, A. H.; Åberg, J. B.; Privalov, T.; Eriksson,
L.; Ba¨ckvall, J.-E. J. Am. Chem. Soc. 2006, 128, 14293–14305.
(11) Casey, C. P.; Clark, T. B.; Guzei, I. A. J. Am. Chem. Soc. 2007,
129, 11821–11827.
(12) Casey, C. P.; Beetner, S. E.; Johnson, J. B. J. Am. Chem. Soc. 2008,
130, 2285–2295.
(14) Matteson, D. S. Chem. ReV. 1989, 89, 1535–1551.
(15) Matteson, D. S. Tetrahedron 1998, 54, 10555–10607.
(16) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457–2483.
(17) Takagi, J.; Takahashi, K.; Ishiyama, T.; Miyaura, N. J. Am. Chem.
Soc. 2002, 124, 8001–8006.
10.1021/om801228m CCC: $40.75
2009 American Chemical Society
Publication on Web 03/19/2009

2086 Organometallics, Vol. 28, No. 7, 2009
Scheme 1. Catalytic Cycle for Hydrogenation with Shvo Catalyst
Koren-Selfridge et al.
resistant to hydrolysis18 and was expected to have significant
reactivity as a Lewis acid. Several other common boron
substituents (9-BBN, catecholborane, and dimesitylborane) were
rejected due to the known uncatalyzed reactivity of H-BR2
toward carbonyl compounds.19,20
1H),6,21 a broad peak in the 11B NMR spectra at 21.8 ppm
(characteristic 11B NMR chemical shift for B(OR)3),22 and the
CdO stretching frequency in the IR spectrum at 2020 and 1959
cm-1. Analogous ruthenium hydride complexes display char-
1
acteristic chemical shifts in the H NMR that depend on the
molecularity of the complex. Monomeric ruthenium hydride
complexes have chemical shifts in the range of -9 to -11 ppm
{[2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)2H] 1′ (analogous to 1,
Scheme 1):6 δ ) -9.8 ppm; [2,5-Ph2-3,4-Tol2(η5-C4COH)Ru-
(CO)(PPh3)H]21 (7, Ru(PPh3)HOH): δ ) -10.4 ppm}; dimeric
ruthenium hydride complexes have chemical shifts in the range
of -17 to -19 ppm {complex 3 (bridging hydride, Scheme
1):6 δ ) -17.7 ppm}. The characteristic chemical shift in the
1H NMR for complex 5 confirms the identity of the regioisomer
formed and demonstrates that the complex is a monomer in
solution.
Results and Discussion
[2,5-Ph2-3,4-Tol2(η5-C4COBpin)Ru(CO)2H] (5) was syn-
thesized by oxidative addition of pinacolborane (H-Bpin) to
ruthenium dimer 4 (eq 1). A 74% isolated yield of 5 was
achieved by heating H-Bpin with ruthenium dimer 4 at 50 °C
for 2 h in toluene. Complex 5 was the sole regioisomer observed
in the addition of H-Bpin to complex 4. This regioisomer was
anticipated by analogy to the reported addition of triethylsilane
(Et3Si-H) to complex 4, providing ruthenium hydride 6 (eq 2).6
Reactivity of RuHOBpin 5 in the Hydroboration of
Aldehydes. The stoichiometric reactivity of RuHOBpin (5)
toward carbonyl compounds was examined to determine the
effect of the boron substituent on the reactivity of this complex
in reduction reactions.23 Prior to this work, the Casey group
showed that the acidic proton of [2,5-Ph2-3,4-Tol2(η5-
C4COH)Ru(CO)2H] 1′ is critical in the hydrogenation of
carbonyls and imines.6 The replacement of the acidic proton
with a triethylsilyl substituent (RuHOSiEt3, 6) was shown to
shut down the reactivity of these complexes toward aldehydes.
We postulated that the empty p-orbital on boron might allow
the resulting complex to react in a similar manner to the parent
complex 1′.
Stoichiometric hydroboration of benzaldehyde was examined
with RuHOBpin 5. Addition of 5 to benzaldehyde in the
presence of pyridine (trapping agent for coordinatively unsatur-
ated complex 2, Scheme 1) provided the hydroboration product
8 in 90% NMR yield after 1 h at 22 °C (eq 3). A control
experiment involving pinacolborane, pyridine, and benzaldehyde
resulted in <5% conversion to hydroboration product 8, ruling
out the possibility that the hydroboration could be occurring
The characteristic spectroscopic data of RuHOBpin 5 includes
the Ru-H resonance in the 1H NMR spectrum at -9.33 ppm (s,
(21) Casey, C. P.; Strotman, N. A.; Beetner, S. E.; Johnson, J. B.; Priebe,
D. C.; Vos, T. E.; Khodavandi, B.; Guzei, I. A. Organometallics 2006, 25,
1230–1235.
(22) No¨th, H.; Wrackmeyer, B. In 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.
(18) Tucker, C. E.; Davidson, J.; Knochel, P. J. Org. Chem. 1992, 57,
3482–3485.
(19) Ma¨nnig, D.; No¨th, H. Angew. Chem., Int. Ed. Engl. 1985, 24, 878–
879.
(20) Vogels, C. M.; Westcott, S. A. Curr. Org. Chem. 2005, 9, 687–
699.

B-Substituted Analogue of the ShVo Hydrogenation Catalyst
Organometallics, Vol. 28, No. 7, 2009 2087
Scheme 2. Proposed Catalytic Cycle of Ruthenium-Catalyzed Hydroboration
by an uncatalyzed process. The observed hydroboration of
benzaldehyde demonstrates that the boron substituent is Lewis
acidic enough to promote hydroboration through a ligand-metal
bifunctional catalyst system.
the Shvo hydrogenation catalyst 1 (Scheme 1). Complex 5
reduces benzaldehyde in stoichiometric reactions at higher
temperatures (22 °C) than the parent RuHOH 1 (-40 °C). The
decreased reactivity of RuHOBpin 5 in stoichiometric reductions
is consistent with a decreased Lewis acidity of the boronate
ester of 5 as compared to the acidic proton of 1.
Catalytically, complex 5 mediates reduction of aldehydes at
lower temperature (50 °C) and pressure (ambient, no H2 gas
required) than the Shvo catalyst (3). The Shvo catalyst typically
requires either elevated temperatures (>90 °C) or high pressure
of H2 (35 atm, 60 °C) for aldehyde reduction. The increased
catalytic reactivity of RuHOBpin 5 over complex 3 is consistent
with a mechanism for 5 that does not require the rate-limiting
dissociation of a bridging hydride (analogous to complex 3,
Scheme 1). RuHOBpin 5 exists in solution as a monomer due
to the steric bulk of the boron substituent (indicated by the
characteristic hydride chemical shift in the 1H NMR spectra, δ
) -9.33 ppm). The large pinacol substituent on boron prohibits
the formation of bridging hydride 10 (Scheme 2).21,24,25
Upon successful stoichiometric hydroboration of benzalde-
hyde, the complex was tested for catalytic competence. Using
2 mol % of 4 as a precatalyst and 1.5 equiv of pinacolborane,
the catalytic reduction of benzaldehyde provided quantitative
NMR yield of 8 in 21 h at 50 °C (eq 4). A control experiment
was conducted involving benzaldehyde, pinacolborane, and
benzene-d6 (in the absence of complex 4), resulting in less than
25% conversion to product 8 after 21 h at 50 °C. Column
chromatography of hydroboration product 8 resulted in a 78%
yield of benzyl alcohol, resulting from hydrolysis of 8 (eq 5).
The variety of aldehydes that were compatible with these
catalytic reaction conditions was explored. Both electron-rich
and electron-deficient aldehydes were tolerated (Table 1, entries
1-4), with qualitatively faster reactions for electron-deficient
aldehydes such as 4-nitrobenzaldehyde (entry 1) as shown by
shorter reaction times for comparable conversions. 4-Dimethyl-
aminobenzaldehyde (entry 5) reacted more rapidly than 4-meth-
oxybenzaldehyde (entry 4), which required higher temperature
to achieve significant conversion. This atypical reactivity trend
is attributed to coordination of the Lewis acidic boron substituent
of pinacolborane to the amine, attenuating the electron-donating
ability of the amine. Ortho-substituted benzaldehyde derivatives
were also tolerated (entries 6 and 7), providing high yields of
the desired alcohols. Hydrocinnamaldehyde was reactive under
these conditions (entry 8), providing 3-phenylpropanol in
moderate yield, demonstrating that an aromatic aldehyde is not
required to afford the hydroboration product.
Hammett Plot of Para-Substituted Benzaldehyde Deri-
vatives. The electronics of the transition state were further
investigated using a Hammett correlation plot. Competition
experiments between para-substituted benzaldehyde derivatives
and benzaldehyde were examined using 5 equiv of each
aldehyde, 1 equiv of pinB-H, and 4 mol % of precatalyst 4 at
The stoichiometric and catalytic hydroboration reactions
depicted in eqs 3-5 demonstrate the ability of the boronate ester
to activate aldehydes toward hydride addition. Unlike RuHO-
SiEt3 6 (eq 2),6 RuHOBpin 5 is a competent replacement for
1
50 °C for 2 h. The product ratios were determined using H
NMR spectroscopy with phenyltrimethylsilane as an internal

2088 Organometallics, Vol. 28, No. 7, 2009
Koren-Selfridge et al.
Table 2. Relative Rates of Hydroboration Reactions for
Para-Substituted Benzaldehydes (Eq 7)
a
substituent (X)
σ
kX/kH
log(kX/kH)
Table 1. Catalytic Hydroboration of Aldehydes (Eq 6)a
-OCH3
-CH3
-H
-0.28
-0.14
0
0.24
0.81
0.40
0.71
1.00
1.47
4.52
-0.40
-0.15
0
0.17
0.66
-Cl
b
-NO2
a
1
Product ratios determined by H NMR with phenyltrimethylsilane as
the internal standard and a 10 s relaxation delay. b Reaction run at 22 °C
for 2 h.
Figure 2. Hammett plot of log(kX/kH) vs σ for catalytic hydrobo-
ration of para-substituted benzaldehydes.
the Hammett correlation plot. To determine if the hydroboration
reaction is reversible, 4-methoxybenzaldehyde or 4-methylben-
zaldehyde was subjected to the optimized reaction conditions
(1.2 equiv of pinacolborane used, 1.0 equiv could not be used
due to low conversions) (Scheme 3). The reactions were run in
a Reactions conducted using Ru dimer 4 (2 mol %) and H-Bpin (1.5
equiv) and heated in a sealed reaction vessel. Workup involves passing
crude reaction mixture through a plug of silica gel. b Isolated yield of
alcohol.
1
benzene-d6 and monitored by H NMR spectroscopy. Upon
consumption of the aldehyde, 1 equiv of 4-nitrobenzaldehyde
was added to the reaction mixture. The reaction mixture was
heated to 50 °C for ∼2 h (conditions for competition experi-
ments) and examined by 1H NMR spectroscopy. In both cases,
there was no loss of the original hydroboration product, but
some hydroboration of the added 4-nitrobenzaldehyde was
observed. The small amounts of hydroboration of 4-nitroben-
zaldehyde that were observed is attributed to the reaction of
4-nitrobenzaldehdye with the slight excess of pinacolborane
required to consume the initial aldehyde (vide supra). When
these reactions were continued for extended times (∼24 h),
significant hydroboration of 4-nitrobenzaldehyde and regenera-
standard, providing >70% combined NMR yields in each
case.26,27 As was qualitatively observed by the reaction times,
electron-withdrawing substituents accelerate the hydroboration
reaction and electron-donating substituents decelerate the reac-
tion rate. Using the relative rates determined by each competition
experiment, a Hammett plot was constructed using log(kX/kH)
vs σ,28 where X is the para substituent of the benzaldehyde
analogue (Table 2, Figure 2). This plot gave a F value of +0.91
(R2 ) 0.98).
(23) For examples of ruthenium-catalyzed hydroboration reactions, see:
(a) Caballero, A.; Sabo-Etienne, S. Organometallics 2007, 26, 1191–1195.
(b) Montiel-Palma, V.; Lumbierres, M.; Donnadieu, B.; Sabo-Etienne, S.;
Chaudret, B. J. Am. Chem. Soc. 2002, 124, 5624–5625. (c) Burgess, K.;
Jaspars, M. Organometallics 1993, 12, 4197–4200.
(24) Casey, C. P.; Strotman, N. A.; Beetner, S. E.; Johnson, J. B.; Priebe,
D. C.; Guzei, I. A. Organometallics 2006, 25, 1236–1244.
(25) Casey, C. P.; Vos, T. E.; Singer, S. W.; Guzei, I. A. Organometallics
2002, 21, 5038–5046.
(26) The relative rates (kX/kH) were determined by using an excess of
each aldehyde (5 equivalents) and assuming that the concentration of each
aldehyde is approximately unchanged under the reaction conditions.
(27) See the Supporting Information for details.
(28) σ values were obtained from: Smith, M. B.; March, J. March’s
AdVanced Organic Chemistry: Reactions, Mechanisms, and Structure, 5th
ed; Wiley: New York, 2001; p 370.
The reversibility of the catalytic hydroboration reaction was
examined to determine the validity of the results obtained in

B-Substituted Analogue of the ShVo Hydrogenation Catalyst
Organometallics, Vol. 28, No. 7, 2009 2089
Scheme 3. Irreversibility of Hydroboration Reaction under Competition Experiment Conditions
lower than the F value for the stoichiometric hydrogenation of
substituted benzaldehydes with an analogue of the Shvo catalyst
[2,5-Ph2-3,4-Tol2(η5-C4COH)Ru(CO)(PPh3)H] 7 (F ) +1.77),24
which was shown to react through a concerted, outer-sphere
reduction mechanism. The decreased F value indicates a
decreased charge build-up in the transition state involving
complex 5 as compared to that of complex 7.
Table 3. Catalytic Hydroboration of Imines (Eq 8)
Catalytic Hydroboration of Imines. The scope of substrates
that are compatible with this catalyst was further investigated
by determining its reactivity with imines. N-Benzylideneaniline
was examined under the reaction conditions optimized for
aldehydes (50 °C, 2 mol % 4, toluene). Under these conditions,
the reaction was slow; complete consumption of the imine
required heating at 70 °C for over 5 days. By using 4 mol % of
precatalyst 4, the reaction rate was accelerated, providing amine
14 (after hydrolysis by silica gel chromatography) in 82% yield
after 5 days at 70 °C (entry 1). Electron-withdrawing groups
were also found to accelerate the reaction of imines. In the case
of imine 12 (entry 2), only 22 h at 70 °C was required to provide
amine 15 (after hydrolysis). Imine 13 was subjected to the
reaction conditions to determine if a significant rate effect would
be observed based on the nitrogen substituent (entry 3). No
qualitative difference in rates was observed with imine 13 over
imine 12.34
Catalytic Hydroboration of Ketones. The hydroboration of
ketones was desired due to the potential for a catalytic
asymmetric reduction reaction.1 Acetophenone was treated with
pinB-H and 4 mol % of precatalyst 4 at 70 °C, but low
conversion to product 17 was observed (eq 9). Because electron-
deficient aldehydes and imines displayed accelerated reaction
rates, hydroboration of 4′-nitroacetophenone was explored (eq
10). After 4.5 days at 70 °C, >95% conversion was observed
and alcohol 18 was isolated in 79% yield. Because the conditions
a Isolated yield of amines. b 4 mol % complex 4 used.
tion of tolualdehyde and anisaldehyde was observed, demon-
strating that the hydroboration of aldehydes is reversible over
extended reaction times. The reversibility of the hydroboration
reaction at extended reaction times is analogous to hydrogena-
tion reactions using the Shvo catalyst,29 which is reversible at
elevated temperatures. The competition experiments used in the
Hammett correlation plot were heated for 2 h at 50 °C,
conditions under which the reaction was shown to be irrevers-
ible. These control experiments verify that the competition
experiments were performed under kinetic control rather than
thermodynamic control.
While there is a clear dependence of the reaction rate on the
electron density of the aldehyde, the magnitude of the depen-
dence is much lower than other hydride addition reactions. The
F value for the hydroboration reaction is much smaller than that
for the reduction of substituted benzaldehydes by NaBH4 (F )
+3.8),30 substituted acetophenones by NaBH4 (F ) +3.06,
+2.02),31,32 and the reduction of substituted benzophenones by
LiAlH4 (F ) +1.95).33 More significantly, the F value of the
hydroboration reaction using RuHOBpin 5 was considerably
(29) For examples of dynamic kinetic asymmetric resolutions by transfer
hydrogenation with the Shvo catalyst, see:(a) Mart´ın-Matute, B.; Edin, M.;
Boga´r, K.; Kaynak, F. B.; Ba¨ckvall, J.-E. J. Am. Chem. Soc. 2005, 127,
8817–8825. (b) Fransson, A.-B. L.; Bore´n, L.; Pa`mies, O.; Ba¨ckvall, J.-E.
J. Org. Chem. 2005, 70, 2582–2587. (c) Mart´ın-Matute, B.; Ba¨ckvall, J.-E.
J. Org. Chem. 2004, 69, 9191–9195. (d) Pa`mies, O.; Ba¨ckvall, J.-E. J. Org.
Chem. 2002, 67, 1261–1265.
(30) Jacobs, J. W.; McFarland, J. T.; Wainer, I.; Jeanmaier, D.; Ham,
C.; Hamm, K.; Wnuk, M.; Lam, M. Biochemistry 1974, 13, 60–64.
(31) Bowden, K.; Hardy, M. Tetrahedron 1966, 22, 1169–1174.
(32) Mullins, R. J.; Vedernikov, A.; Viswanathan, R. J. Chem. Educ.
2004, 81, 1357–1361.
(33) Wiegers, K. E.; Smith, S. G. J. Am. Chem. Soc. 1977, 99, 1480–
1487.
(34) A competition experiment between the two imines showed a 1.2:1
ratio of hydroboration of imine 13 over imine 12.

2090 Organometallics, Vol. 28, No. 7, 2009
Koren-Selfridge et al.
131.9, 129.3, 129.1, 128.72, 128.70, 128.2, 104.2, 97.9, 84.3, 24.5,
21.3; 11B NMR (C6D6, 160 MHz) δ 21.7; IR (thin film) 3053, 2981,
2926, 2020, 1959, 1444, 1399 cm-1. Anal. Calcd for C39H37O5BRu:
C, 66.95; H, 5.62. Found: C, 66.57; H, 5.42.
for hydroboration of ketones proved to be too harsh, attempts
to determine the generality of the reaction and develop an
asymmetric method were not pursued.
General Procedure for the Catalytic Hydroboration of
Aldehydes, Imines, and 4′-Nitroacetophenone. Benzyl Alco-
hol.35 To a resealable glass tube containing {[2,5-Ph2-3,4-Tol2(η4-
C4CO)Ru(CO)2}2 (4) (0.068 g, 0.060 mmol), 15 mL of toluene,
and pinacolborane (0.700 mL, 4.50 mmol) was added benzaldehyde
(0.305 mL, 3.00 mmol). After 26 h at 50 °C, the reaction mixture
was stirred over silica gel for 3 h, filtered through a silica plug
with ethyl acetate (∼500 mL), and concentrated in vacuo to an
orange/brown oil. Purification by bulb-to-bulb distillation (70-110
°C, 5 mmHg) and column chromatography (2:98 to 3:97 ethyl
acetate/CH2Cl2) provided benzyl alcohol as a colorless oil (0.254
1
Conclusions
g, 78%): H NMR (CDCl3, 500 MHz) δ 7.37 (m, 4H), 7.32 (m,
1H), 4.69 (s, 2H), 1.98 (s, 1H); 13C NMR (CDCl3, 125 MHz) δ
RuHOBpin 5 was synthesized and shown to possess reactivity
in the hydroboration of carbonyls and imines. Catalytic condi-
tions for the hydroboration of aldehydes and imines were
developed requiring 50-70 °C and 2-4 mol % of ruthenium
dimer 4. The corresponding alcohols and imines were typically
isolated in high yields. A Hammett correlation plot quantified
the rate acceleration of electron-deficient aldehydes, providing
a F value of +0.91. Thus, boron is a suitable surrogate for the
acidic hydrogen of the Shvo catalyst.
141.1, 128.7, 127.8, 127.2, 65.3; IR (thin film) 3335, 3064, 3031,
2931, 2873, 1496, 1454, 1020, 734, 698 cm-1
.
Acknowledgment. Financial support to C.P.C was pro-
vided by the Department of Energy, Office of Basic Energy
Sciences and the National Science Foundation (CHE-
0209476). Financial support to T.B.C. was provided by the
National Institutes of Health in the form of a NRSA
fellowship (GM077962-01), the Petroleum Research Fund,
administered by the American Chemical Society (47598-
GB3), the M.J. Murdock Charitable Trust and Research
Corporation in the form of a department development grant,
and Western Washington University.
Experimental Section
[2,5-Ph2-3,4-Tol2(η4-C4OBpin)]Ru(CO)2H (5). A slurry of
{[2,5-Ph2-3,4-Tol2(η4-C4CO)Ru(CO)2}2 (4) (0.500 g, 0.439 mmol)
and pinacolborane (0.130 mL, 0.895 mmol) in 25 mL of toluene
was stirred for 2 h at 50 °C. The reaction mixture was removed
from heat and the solvent evaporated under high vacuum. The
resulting solid was suspended in 15 mL of pentane and filtered,
and the filtrate was rinsed with pentane (3 × 5 mL). The solid was
dried under high vacuum to give 5 as a brown/yellow solid (0.455
Supporting Information Available: General experimental
information, characterization data, details of the competition
experiments, and selected spectra of products. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM801228M
1
g, 74%): H NMR (C6D6, 500 MHz) δ 7.72 (d, J ) 7.5 Hz, 4H),
7.20 (d, J ) 8.0 Hz, 4H), 7.04 (t, J ) 7.5 Hz, 4H), 6.96 (m, 2H),
6.61 (d, J ) 10 Hz, 4H), 1.85 (s, 6H), 0.81 (s, 12H), -9.33 (s,
1H); 13C NMR (C6D6, 125 MHz) δ 202.5, 137.6, 133.6, 133.1,
(35) Shaikh, N. S.; Junge, K.; Beller, M. Org. Lett. 2007, 9, 5429–
5432.