New rhodacarborane-phosphoramidite catalyst system for enantioselective hydrogenation of functionalized olefins and molecular structure of the chiral catalyst precursor [3,3-{(S)-PipPhos}2-3-H-1,2-(o -xylylene)- closo -3,1,2-RhC2B<su

Article abstract of DOI:10.1021/om101201e

Formally 16-electron closo- and pseudocloso-(a³-cyclooctenyl) rhodacarboranes of the general formula [3-{(1-3-η³)-C ₈H₁₃}-1,2-R,R-3,1,2-RhC₂B₉H ₉] (1 (closo), R, R = μ-1′,2′-CH

Full text of DOI:10.1021/om101201e

ARTICLE
pubs.acs.org/Organometallics
New RhodacarboraneꢀPhosphoramidite Catalyst System for
Enantioselective Hydrogenation of Functionalized Olefins and
Molecular Structure of the Chiral Catalyst Precursor [3,3-{(S)-
PipPhos} -3-H-1,2-(o-xylylene)-closo-3,1,2-RhC B H ]
2
2 9 9
Leonid S. Alekseev, Sergey E. Lyubimov, Fedor M. Dolgushin, Valentin V. Novikov, Vadim A. Davankov, and
Igor T. Chizhevsky*
A. N. Nesmeyanov Institute of Organoelement Compounds of the RAS, 28 Vavilov Street, 119991 Moscow, Russian Federation
S Supporting Information
b
3
ABSTRACT: Formally 16-electron closo- and pseudocloso-(η -cyclooc-
3
tenyl)rhodacarboranes of the general formula [3-{(1ꢀ3-η )-C H }-
8
13
0 0 0 0
,2-R,R -3,1,2-RhC B H ] (1 (closo), R, R = μ-1 ,2 -CH C H CH ; 2
2 9 9 2 6 4 2
1
0
pseudocloso), R = R = PhCH ) coupled in situ with the chiral
2
(
phosphoramidite (S)-PipPhos (3) were found to catalyze an asym-
metric hydrogenation of functionalized olefins (enamides) with en-
antioselectivities as high as 97ꢀ99.7% and with 92ꢀ100% conversions. The key catalyst precursor [3,3-{(S)-PipPhos} -3-H-1,2-(o-
2
xylylene)-closo-3,1,2-RhC B H ] (16), independently prepaped by the stoichometric reaction of 1 with 3 in benzene, was found to
2
9 9
show of enantioselectivities and conversions upon the hydrogenation of prochiral enamides at the same levels as those observed for
the relevant precursor formed in situ from 1 and 3. The structure of 16 has been established on the basis of analytical and
multinuclear NMR data as well as a single-crystal X-ray diffraction study. In contrast to complex 1, complex 2 reacts with 3 to afford
the unstable hydridoꢀrhodium species [3,3-{(S)-PipPhos} -3-H-1,2-(PhCH ) -3,1,2-RhC B H ] (17), the formation of which
2
2 2
2
9
9
þ
ꢀ
and further conversion into the salt [(S)-(PipPhos) Rh] [7,8-(PhCH ) -nido-7,8-C B H ] (18) was detected by time-
dependent H NMR spectra. Some conclusions regarding the catalysis mechanistic pathway, which is consistent with that generally
4
2 2
2 9 10
1
accepted for the rhodacarborane-catalyzed alkene hydrogenation, are made.
9,10
’
INTRODUCTION
metallacarboranes of both exo-nido and closo structures, and
among these only the (R)-BINAP-mediated catalyst of the for-
Irida- and rhodacarboranes have been known since the mid-
2
1
mula [1,3-{μ-(3-η -(CH dCHCH CH ))}-3-H-3-PPh -closo-
2
2
2
3
1
970s, and many of them have found applications as efficient
2
3,1,2-RhC B H ] in ionic liquids has shown quite high en-
2 9 10
catalyst precursors for homogeneous catalysis. The first and
most extensively studied catalyst precursors of this family are
closo-bis(triphenylphosphine)hydridorhodacarborane,
antioselectivity (>99%) in the hydrogenation of aromatic ke-
tones to chiral alcohols under homogeneous catalysis by
9
metallacarborane. In the present work, a novel rhodium-based
[
H(PPh ) -closo-3,1,2-RhC B H ], and its 2,1,7- and 2,1,12-
3 2 2 9 11
3
catalytic system derived from the carbon-substituted closo- and
cage isomers. These icosahedral clusters usually display a unique
3
pseudocloso-(η -cyclooctenyl)rhodacarboranes of the general
tautomerism in solution between 18-electron Rh(III) closo and
3
0
formula [3-{(1ꢀ3-η )-C H }-1,2-R, R -3,1,2-RhC B H ]
8
0
13
2 9 9
1
6-electron Rh(I) exo-nido structures, the phenomenon firs4t
0
0
(1 (closo), R, R = μ-1 ,2 -CH C H CH ; 2 (pseudocloso),
R = R = PhCH ) are described, which in combination with the
2
6
4
2
discovered by Hawthorne and co-workers in the early 1980s.
0
2
The detailed studies of mechanistic aspects of metallacarborane
3d
chiral phosphoramidite (S)-PipPhos led to asymmetric hydro-
genation of enamides with both excellent conversions and
enantioselectivities.
catalysis have further revealed that the catalytically active
III
species of such systems contains a set of exo-BꢀRh ꢀH
functions formed in solution by the reversible oxidative addition
of Rh(I) to the terminal BꢀH bonds in the exo-nido precursor.
These and other exo-nido-metallacarboranes turned out to be
active and robust in a number of organic reactions such as the
’ RESULTS AND DISCUSSION
Using metalation reactions of the μ-chloride dimeric complexes
3
aꢀc,5
hydrogenation and isomerization of alkenes,
hydrogenoly-
cyclopropa-
2
2
þ
[
M(η :η -cyclodiene)Cl] (M = Rh, Ir) with K salts of the
2
3d,e
sis and hydrosilanolysis of alkenyl carboxylates,
nation of alkenes with ethyl diazoacetate, Kharasch addition to
alkenes, living polymerization of vinyl monomers,
reduction of aromatic ketones to alcohols. On the other hand,
there have been only very limited reports on either diastereo- and/or
enantiocontrolled catalytic reactions employing transition-metal
0
ꢀ
9 10
isomeric nido-carborane monoanions [nido-7,n-R,R -C B H ]
n = 8, 9; R, R = Alk, ArAlk), we have previously synthesized a
2
6
0
(
7
7a,8
and
series of closo- and pseudocloso-(η-carbocycle)metallacarborane
9
Received: December 27, 2010
Published: March 17, 2011
r 2011 American Chemical Society
1942
dx.doi.org/10.1021/om101201e
|Organometallics 2011, 30, 1942–1950

Organometallics
Scheme 1. Asymmetric Hydrogenation of Enamides 4ꢀ9 Catalyzed by (S)-PipPhos-Mediated Rhodacarborane Systems
ARTICLE
Table 1. Asymmetric Hydrogenation Results for Enamides
ꢀ9 Catalyzed by in Situ Generated Rhodacarborane/(S)-
values observed for many other rhodium catalysts based upon
15
4
BINOL-derived phosphoramidites. Very high to complete
conversion for each of the tested substrates is achieved within
a
PipPhos Systems
1
8ꢀ22 h. Clearly, the hydrogenation products thus obtained are
b
c
entry
cat.
substrate
t, h
conversn, %
ee, %
valuable amino acid derivatives and therefore, if required, can
easily be converted to the desired natural or unnatural amino
acids without loss of optical purity.
Hydrogenation reactions were carried out at ambient tem-
perature in a cylindrical stainless steel reactor (20 mL capacity)
charged with the catalyst precursor 1 or 2 and phosphoramidite
1
2
3
4
5
6
7
8
9
1/2L
2/2L
1/2L
2/2L
1/2L
2/2L
1/2L
2/2L
1/2L
2/2L
1/2L
2/2L
4
4
5
5
6
6
7
7
8
8
9
9
20
20
22
18
20
20
20
20
20
20
20
20
100
100
100
100
100
100
95
99.7 (R)
99.5 (R)
97 (R)
91 (R)
97 (R)
98 (R)
97 (R)
96 (R)
97 (R)
97 (R)
99.5 (R)
99 (R)
16
3
, taken in a 1/2 molar ratio, respectively, and the corresponding
enamides 4ꢀ9 (catalyst/substrate = 1 mol/100 mol) in acetone
solution (5 mL), followed by pressurization with argon (1.3 bar)
and hydrogen gas (10 bar).
100
100
96
Preparation and Characterization of Hydrogenation Cat-
alyst Precursors [3,3-{(S)-PipPhos} -3-H-1,2-μ-(o-xylylene)-
closo-3,1,2-RhC B H ] (16) and [3,3-{(S)-PipPhos} -3-H-
2
1
1
1
0
1
2
2
92
9
9
2
1
,2-(PhCH ) -closo-3,1,2-RhC B H ] (17) and the Catalytic
2 2 2 9 9
100
a
Activity of 16. In an attempt to elucidate the nature of catalyst
precursors used in situ for the hydrogenation of enamides
4ꢀ9, a stoichometric reaction of 1 with 2.2 equiv of (S)-PipPhos
in benzene was independently studied and found to produce the
All reactions were performed in a cylindrical stainless steel reactor
20 mL capacity). Reaction conditions: 0.6 mmol of substrate, 0.006
(
mmol of 1 or 2, 0.012 mmol of (S)-PipPhos, 5.0 mL of acetone at room
b
1
temperature, 18ꢀ22 h. Conversions were determined by H NMR
c
spectroscopy. Enantiomeric excesses were determined using HPLC
air-stable hydridoꢀrhodium complex [3,3-{(S)-PipPhos} -3-H-
2
(
see the Supporting Information).
1
,2-μ-(o-xylylene)-closo-3,1,2-RhC B H ] (16) in 92% isolated
2 9 9
yield. A single-crystal X-ray diffraction study (vide infra) com-
bined with analytical and multinuclear multiple-resonance
NMR data listed in the Experimental Section entirely confirmed
the closo structure of 16 both in the solid state and in solution.
complexes of rhodium and iridium with either saturated
1
1,12
11,13
1
8-electron
or formally unsaturated 16-electron
metal
1
31
1
centers. Among 16-electron complexes, there are several species
The H and P{ H} NMR characteristics are particularly
3
1
with the exo-polyhedral (1ꢀ3-η )-cyclooctenyl-type ligands in
convincing. Thus, the H NMR spectrum of 16 shows a some-
which the electron deficiency is relieved either by an agostic
what broadened metalꢀhydride resonance at ꢀ8.15 ppm
13aꢀc
1
7
CꢀH M bonding interaction
or, as in the case of
via donation to a metal atom of an
3
3 3
which is split into a triplet of doublets with the expected
31 1 103 1
1
3d,e
pseudocloso species,
J( P, H) = 22.5 Hz as well as J( Rh, H) coupling of less than 5
Hz. The 242.97 MHz P{ H} NMR spectrum of 16 contains a
31
1
additional electron density released from the polyhedral CꢀC
bond cleavage. Two rhodium complexes of both closo and
double set of AB quartets with the doublet components at 147.3
13c
13e
103
pseudocloso types, 1 and 2, are presented in this work,
and 146.4 ppm (P , J = 55.0, J( Rh,P) = 208.0 Hz) and 145.7
A
AB
103
and each of them coupled in situ with the chiral phosphoramidite
and 144.7 ppm (P , J = 55.1, J( Rh,P) = 220.8 Hz)
B AB
1
4
(
S)-PipPhos (3), exhibiting in acetone solution excellent
(Figure 1a). The correlation between doublets at 147.3 and
145.7 ppm as well as doublets at 146.4 and 144.7 ppm found in
catalytic properties with regard to enantioselectivities and con-
versions upon the hydrogenation of prochiral enamides
31
31
the 2D [ Pꢀ P]-COSY spectrum (Figure 1b) further supports
(Scheme 1). As summarized in Table 1, these unique phos-
the assignment made for P and P components of the ABX
A
B
103
31
1
phine-free catalytic systems give enantiomeric excesses of prod-
spin system (X =
Rh) observed in the P{ H} NMR
ucts 10ꢀ15 as high as 97ꢀ99.7%, which are among the highest
spectrum of 16.
1
943
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Organometallics
ARTICLE
3
1
1
Figure 1. (a, left) 242.97 MHz P{ H} NMR spectrum for 16 in CD Cl solution at 22 °C showing the assignment of the P and P resonances. (b,
2
2
A
B
31
1
31
1
right) 161.97 MHz [ P{ H}ꢀ P{ H}]-COSY spectrum for 16 showing the assignments of the A and B parts of the spectrum (CD
2 2
Cl , 22 °C).
Table 2. Asymmetric Hydrogenation of Enamides 4 and 5
Catalyzed by the HydridoꢀRhodium Complex 16 Indepen-
a
dently Synthesized by the Reaction of 1 with 3
entry
cat.
substrate
t, h
conversn, %
ee, %
1
2
16
16
4
5
18
18
100
100
99.4 (R)
98 (R)
a
Experimental conditions of catalytic reactions are the same as for the
processes catalyzed by in situ generated 1/(S)-PipPhos systems (see
Table 1).
with different phosphorus-containing ligands at the metal vertex
4
b,18
have been structurally characterized in numerous instances,
to the best of our knowledge, complex 16 is the first known closo-
hydridometallacarborane incorporating chiral phosphoramidite-
based ligands. Due to the presence of the bulky μ-(o-xylylene)
cage substituent, two phosphoramidite ligands and the rhodium-
bound hydride in 16 adopt the sterically most favorable orienta-
tion with respect to the nido-carborane open face: both (S)-
PipPhos groups lie as far as possible from the cage substituent,
while the terminal hydride, as the smallest ligand, is positioned
over and bisects the cage CꢀC bond. The observed conforma-
tion of 16 agrees well with those predicted on the basis of the
Figure 2. Molecular structure of [3,3-{(S)-PipPhos}
2
-3-H-1,2-(o-
xylylene)-closo-3,1,2-RhC B H ] (16). All hydrogen atoms except H-
2
9 9
(Rh) have been omitted for clarity. Selected interatomic distances (Å)
17
and angles (deg) (values for both independent molecules are quoted):
Rh(3)ꢀP(1) = 2.247(2), 2.242(3); Rh(3)ꢀP(2) = 2.259(2), 2.254(2);
C(1)ꢀC(2) = 1.631(11), 1.560(13); Rh(3)ꢀH(1) = 1.69, 1.50; Rh-
observed values of J(Rh,P) and J(Rh,H) coupling found from
1 31 1
the aforementioned H and P{ H} NMR spectra of 16. The o-
xylylene substituent in 16 is displaced from the C B plane of the
2
3
(
3)ꢀC(1) = 2.324(9), 2.281(10); Rh(3)ꢀC(2) = 2.298(8), 2.277(9);
cage ligand toward the metal atom in such a way that the angle
between the normals to the cage pentagonal plane and the least-
squares plane defined by the aromatic unit and bridging methy-
lene carbons is 35°. This can be compared with similar angles
found in [3,3-(Ph P) -3-H-1,2-μ-(o-xylylene)-closo-3,1,2-
Rh(3)ꢀB(4) = 2.254(9), 2.240(12); Rh(3)ꢀB(7) = 2.233(9), 2.235(10);
Rh(3)ꢀB(8) = 2.290(8), 2.324(12); P(1)ꢀRh(3)ꢀP(2) = 94.39(8),
9
7
5.37(9); P(1)ꢀRh(3)ꢀH(1) = 76.2, 73.6; P(2)ꢀRh(3)ꢀH(1) =
6.2, 59.3.
3
2
4a
RhC B H ] (38.7°) and [exo-6,10-{(Ph P)(Cy P)Rh}-6,10-
2
9
9
3
3
4
a
An X-ray diffraction study on a crystal of complex 16 deter-
μ-(H) -10-H-7,8-μ-(o-xylylene)-nido-7,8-C B H ] (81.0°);
2
2 9 7
mined the structure shown in Figure 2. Two independent
molecules have similar geometrical parameters (selected values
for both are given in Figure 2). It has become apparent from the
X-ray diffraction data that 16 in the crystalline phase represents a
bear in mind that one would expect the steric repulsion between
the o-xylylene group and the metal-containing moiety to be
present in the former but not in the latter complex.
To test the ability of 16 to serve as a true catalyst precursor in
the processes described above, asymmetric hydrogenation of
enamides 4 and 5 catalyzed by complex 16 alone has been carried
out. Remarkably, both reactions show the same levels of
closo species containing an (S)-(PipPhos) RhH vertex with two
2
phosphoramidites acting as monodentate ligands. Although
icosahedral carborane-containing hydridoꢀrhodium complexes
1
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Organometallics
ARTICLE
enantioselectivities and conversions as observed for the precur-
sor formed in situ from 1 and 3 under otherwise identical
reaction conditions (Table 2). It is noteworthy that the hydro-
genation process is strongly solvent dependent, and in chlori-
nated solvents such as CH Cl no conversion was observed
conversion into the new ionic species 18. This latter complex
could be prepared independently in 93% yield by treating 2 in
C H solution with a 4.5-fold molar excess of 3. Elemental
6
6
1
31
1
analysis for 18 as well as its H and P{ H} NMR spectra
(vide infra) are consistent with the formulation [(S)-(PipPhos) -
2
2
4
þ
ꢀ
2 9 10
11 11
1
either for the independently prepared precursor 16 or for in situ
Rh] [7,8-(PhCH ) -nido-7,8-C B H ] . The
B/ B{ H}
2
2
generated adducts of 1 or 2 with phosphoramidite 3.
NMR spectra of 18 were, to a large extent, identical with those
of the starting carborane monoanion from which both 2 and
18 were derived. It should also be noted that the ionic complex
itself was not active in the catalytic hydrogenation of enamides
4 and 5 (these were used as test examples).
Relatively fast transformation of 17 into the ionic complex 18
in the presence of 3 prevented us from obtaining the hydri-
Unlike complex 16, which was shown to represent the
observed closo isomer in all cases, i.e. both in solution (either
in acetone or CH Cl ) and in the solid state, the analogous
2
2
hydridoꢀrhodium species [3,3-{(S)-PipPhos} -3-H-1,2-(PhCH ) -
2
2 2
3
,1,2-RhC B H ] (17) (Chart 1) originating from 2 and 3
2
9 9
proved to be quite labile in solution due to a relatively fast
doꢀrhodium species 17 from a stoichometric reaction of 2 with
1
3
. Hence, for detection and characterization of 17 the H NMR
Chart 1. Schematic Representation of the Structurally
monitoring of the reaction of 2 in both CD Cl or d -acetone
2 2 6
Characterized (16) and Proposed (17) HydridoꢀRhodium
with either immediate addition of 4.2 equiv of 3/equiv of 2 or
with increasing amounts of 3 varying from 0.5 to 4.0 equiv/equiv
0
Complexes [3,3-{(S)-PipPhos} -3-H-1,2-R,R -3,1,2-
2
1
RhC B H ]
of 2 was used. The time-dependent H NMR experiment
2
9
9
(
Figure 3) clearly demonstrated that 17 is the primary inter-
mediate in the formation of ionic complex 18. In fact, complex 17
is observed in the H NMR spectrum with the high-field re-
1
2
31
1
1
103
1
sonance at ꢀ8.14 ppm (td, J( P, H) = 25.3 Hz, J( Rh, H) =
9
.6 Hz) even when the first 0.5 equiv of 3 had been added to 1.0
1
equiv of 2. The terminal metal hydride resonance in the H NMR
spectra of 17 first appeared at ꢀ8.14 ppm and then slowly
disappeared (all within ca. 45ꢀ50 min) to form instead a set of
broad resonances in the terminal BꢀH (ca. ꢀ0.25 to þ0.5 ppm)
and BꢀHꢀB bridging region (ꢀ2.46 ppm) that can be
1
1
11
Figure 3. Series of H NMR spectra from monitoring of the reaction of 2 with 4.2 equiv of 3 in CD
only the hydride region of the spectra is displayed).
2 2
Cl with the upper H{ B} NMR spectrum of 18
(
1
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Organometallics
ARTICLE
attributed to the anionic cage of complex 18; all these new
resonances show an increase in intensity with a decrease in
concentration of the hydridoꢀrhodium complex 17 (Figure 3)
following the addition of each new portion of 3. As expected,
interpretation. Nevertheless, this spectrum has been successfully
0
0
0
0
31
simulated as an AA MM X spin system, where A, A , M, M = P,
103
and X = Rh. As shown in Figure 4, experimental and simulated
spectra match very well, thus confirming the proposed formula of
compound 18.
1
the resonances derived from the starting complex 2 in the H
NMR spectrum disappear completely after addition of 4.0 equiv
The observed very large values of coupling constants between
2
2
of 3.
the nonequivalent phosphorus atoms ( J = J
18 are indicative of phosphorus nuclei lying trans to each other.
0
0
= 506 Hz) in
A M
19
AM
3
1
1
The P{ H} NMR spectrum of 18 consists of 42 lines
(Figure 4) and, therefore, is too complicated for an immediate
In contrast, the low values of other geminal couplings (JAA
0
= 45
3
1
1
0
0
0
Figure 4. Experimental (above) and simulated (below) P{ H} NMR spectra (MestreNova 6.0.2 software) for an AA MM X spin system with A, A ,
0
31
103
0
0
M, M = P, X = Rh, δ(A) = δ(A ) = 141.70, δ(M) = δ(M ) = 148.10 ppm, J
0
= 45 Hz, JAM = J
0
0
= 506 Hz, JAM
0
= J
0
= ꢀ54 Hz, J
0
= 80 Hz,
AA
A M
A M
MM
JAX = J
0
= 184.5 Hz, and JMX = J
0
= 190 Hz. Line widths for the phosphorus resonances are taken as 8 Hz. The resonance labeled with asterisks
M X
103 31
A X
exhibited J = 194.6 Hz, which could be assigned to the Rhꢀ P coupling of (S)-PipPhos-rhodium species having a symmetrical structure.
Chart 2. Schematic Representation of Two Possible Structures (A and B) for Complex 18
1
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Organometallics
ARTICLE
1
11
Figure 5. Variable-temperature H{ B} NMR spectra of complex 18. The BH resonances of 18 are labeled with asterisks. The resonances of the
1
piperidyl groups of 18 are labeled with circles. The upper nondecoupled H NMR spectrum is given for comparison.
Hz, JAM
for mutually cis P-atoms.
the square-planar geometry of the reported complex
Rh(MonoPhos) ]BF , which is the only structurally character-
0
= J
0
= ꢀ54 Hz, J
0
= 80 Hz) are typical of those
structures, however, could not be differentiated on the basis of
the NMR data alone.
A M
MM
19b
On the basis of these data and
Study of Dynamic Behavior of Ionic Complex 18 in Solu-
tion. There is one more feature in the NMR spectroscopic
properties of 18 that is worthy of note. On the basis of literature
[
4
4
ized rhodium species of this type with monodentate phosphor-
20
4a
31
1
amidite based on BINOL, we also suggest a square-planar (or
slightly distorted) structure for compound 18 (Chart 2). The
existence of two nonequivalent phosphorus groups in 18 can be
accounted for by two possible structures (A and B) with the
piperidine fragments located in both hemispheres; these two
data, as well as the overcomplicated P{ H} NMR spectrum of
18 described above, it seemed logical to assume that a weak
coordination between the carborane anion [7,8-(PhCH ) -nido-
2
2
ꢀ
9 10
7,8-C B H ] and the rhodium center of [{(S)-PipPhos}4-
2
þ
Rh] occurs through one or two BꢀH groups. Such a tight ion
1
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Organometallics
ARTICLE
pair might be fluxional in solution due to possible migration of
the rhodium-containing moiety over the surface of the anionic
carborane cage, by analogy with that observed in exo-nido
in the metallacarborane frameworks of 16 and 17, and complex
16 alone, as described above, is indeed highly active in the
asymmetric hydrogenation of enamides. Evidently, the hydro-
genation proceeds along a pathway that involves slow conversion
of Rh(III) complexes 16 and 17 into the respective exo-nido
Rh(I) tautomers in solution. However, no detectable amounts of
these exo-nido tautomers in equilibrium either with 16 or 17
could be observed by NMR spectroscopy, perhaps owing to their
high activity (and hence low concentration) with respect to
4
a
species.
1
Actually, the low-temperature (223 K) H NMR spectrum of
1
8 at higher field displays a set of three resonances, one of which
(
B
at ꢀ2.46 ppm, as assigned above) originates from the bridging
H
B hydrogen. Of the other two peaks at þ0.30
3
3 3 3 3 3
and ꢀ1.18 ppm, at least the more shielded one or perhaps even
both may possibly (in accord with the above assumption) be due
to the terminal BꢀH hydrogens involved in a weak interionic
BꢀH Rh interaction. Since, however, all these and other cage
oxidative addition reactions of [(R)-(PipPhos) Rh(I)] moiety
2
with BꢀH cage vertices.
On the other hand, the formation of the ionic species 18 from
the hydridoꢀrhodium species 17 provides some indirect evi-
dence for the intermediate formation of the relevant exo-nido
tautomer which, along with the competitive oxidative addition
processes mentioned above, displaced the rhodium-containing
3
3 3
1
BꢀH resonances in the boron-coupled H NMR spectrum of 18
were broad and those in the region from ꢀ0.25 to þ2.5
ppm partly overlapped, for their full assignment we have
1
examined the low-temperature (223 K) boron-decoupled H-
1
1
4a,5b
{
B} NMR spectrum (see Figure 5). Apart from a separate
moiety in the presence of a σ-donor (S)-PipPhos ligand
þ
BꢀHꢀB resonance at ꢀ2.46 ppm, this spectrum displays a set of
to form [(S)-(PipPhos) Rh]
stabilized by the [7,8-
4
ꢀ
three 2H and three 1H peaks in the range from þ0.25 to þ2.2
(PhCH ) -nido-7,8-C B H ] counterion.
2
2
2 9 10
1
ppm which, by comparison with those seen in the H NMR
spectrum, can correctly be assigned to the terminal BꢀH
1
11
’ CONCLUSIONS
protons. Moreover, the H{ B} NMR spectrum at 353 K
appears to indicate the presence of another set of well-defined
resonances integrated as having a total of 40 protons. These
resonances, including those at þ0.30 and ꢀ1.18 ppm, in the
The results of this study show that the use of a combination of
the easily accessible chiral phosphoramidite 3 and 16-electron
rhodacarboranes, either closo 1 or pseudocloso 2, provides a
potential route to new applications of such catalytically active
metallacarborane systems for other enantioselective organic
reactions as well as the opportunity to design new electronically
deficient metallacarboranes previously untried as catalyst pre-
cursors in asymmetric organic reactions.
1
11
H{ B} NMR spectrum at 230 K show no additional sharpening
1
as compared with the boron-coupled H NMR spectrum at the
same temperature and were therefore assigned to the piperidine
protons. A variable-temperature study showed it is precisely the
piperidine peaks that exhibited dynamic behavior, while all BꢀH
resonances in fact did not change their position in the spectra on
passing from 223 to 353 K. Finally, in order to elucidate possible
’
EXPERIMENTAL SECTION
þ
bonding interactions between [{(S)-PipPhos} Rh] and the
4
1
carborane anion cage in 18, an examination of its H DOSY
General Considerations. All reactions were carried out under an
(
diffusion ordered spectroscopy) spectrum was undertaken (see
argon atmosphere using standard Schlenk techniques. All solvents were
13c
the Supporting Information). The presence of two independent
sets of signals with different diffusion coefficients, corresponding
to [{(S)-PipPhos} Rh] and the carborane anion cage, respec-
tively, suggests the separate diffusion of these moieties in
distilled from appropriate drying agents prior to use. Compounds 1,
1
3e
14
1
2, and 3 were prepared according to literature procedures. The H,
þ
1
11
13
1
31
1
11 11
1
H{ B}, C{ H}, P{ H}, and B/ B{ H} NMR spectroscopic
4
1
1
measurements, including VT NMR and 2D [ Hꢀ H]-COSY and
1 13
[
Hꢀ C]-HSQC spectra, were performed with a Bruker Avance-600
solution.
instrument using TMS as an internal reference or 85% H PO and
We, therefore, concluded that (i) no specific interaction exists
3
4
þ
3 2
BF Et O as external reference. Elemental analyses for 16 and 18 were
between the cationic [{(S)-PipPhos} Rh] moiety and the
3
4
performed by the Analytical Laboratory of the Institute of Organoele-
ment Compounds of the Russian Academy of Sciences.
Preparation of [3,3-{(S)-PipPhos} -3-H-1,2-(o-xylylene)-
carborane cage anion of compound 18 in solution, and the
fluxional behavior of 18 revealed by the temperature-dependent
1
2
H NMR spectra can be attributed to the fluxionality of piperidyl
closo-3,1,2-RhC B H ] (16). Compounds 1 (30 mg, 0.067 mmol)
2
9 9
substituents in its (S)-PipPhos ligands and (ii) presumably four
(
6 6
and 3 (60 mg, 0.15 mmol) were dissolved in 1 mL of dry C H . After it
S)-PipPhos ligands in 18 produce a steric effect, which restricts
was stirred for 1 h at ambient temperature, the color of the resulting
mixture changed from red to a pale yellow; the reaction mixture was then
diluted with 10 mL of n-hexane, and the precipitate that formed was
separated by centrifugation. Washing with n-hexane (3 ꢁ 6 mL) and
drying in vacuo afforded analytically pure 16 (70 mg, 92%). Anal. Calcd
the fluxionality of piperidyl substituents at low temperature (at
least below 240 K), when they become “static”.
Some Conclusions Regarding the Hydrogenation Me-
chanistic Pathway. Assuming that the hydrogenation of enam-
ides catalyzed by complexes 16 and 17 proceeds by a mechanism
generally accepted for the rhodacarborane-catalyzed alkene
hydrogenation and related reactions, we attribute the catalytic
activity observed to the presence of a unique equilibrium
between 18 e Rh(III) closo and 16 e Rh(I) exo-nido rhodacar-
for C60
H
62
B
9
N
2
O
4
P
2
Rh: C, 63.36; H, 5.49; N, 2.46; B, 8.56. Found: C,
ꢀ1
6
3.32; H, 5.67; N, 2.42; B, 8.29. IR (KBr, cm ): 2541 (νBꢀH), 1942
3d
1
(
ν
RhꢀH). H NMR (600.22 MHz, CD
2
Cl
2
, 20 °C): δ 8.22ꢀ7.91 (total
6H, d-like each, CH
), 7.57ꢀ7.10 (total 18 H, m, CH
), 7.02
), 5.75
aromatic
aromatic
(t-like, 1H, C
6
H
4
), 6.95 (t-like, 1H, C
6
H
4
), 5.89 (d-like, 1H, C
6
H
4
4
a
borane tautomers wherein the key catalytically active species
(
t-like, 1H, C
6
H
4
), 5.04 (d-like, 1H, CHaromatic), 4.83 (d-like, 1H,
= 17.4, JP,H = 4.1 Hz),
III
involves the exo-nido BꢀRh ꢀH array. This conclusion was
CHaromatic), 4.24 (dd-like, 1H, CH H , J
A
B
H,H
based on the following observations. The intermediates formed
3.87 (br d-like, 1H, CH H , J = 17.0 Hz), 3.56 (dd-like, 1H, CH H ,
A B A B
in situ from the formally 16-electron clusters 1 and 2 and a chiral
JH,H = 18.2, JP,H = 5.3 Hz), 3.34 (br d-like, 4H, J = 29.0 Hz), 3.13 (br
d-like, 4H, J = 29.0 Hz), 2.97 (dd-like, 1H, CH H , J = 18.0, JP,H
phosphoramidite 3 are, apparently, closo (or pseudocloso)
=
A
B
H,H
III
rhodacarboranes containing a {(S)-(PipPhos) Rh H} vertex
8.6 Hz), 1.62 (m, 2H, N(CH ) ), 1.53 (m, 2H, N(CH ) ), 1.42 (m, 4H,
2
2 5
2 5
1
948
dx.doi.org/10.1021/om101201e |Organometallics 2011, 30, 1942–1950

Organometallics
ARTICLE
N(CH
2
)
5
), 1.28 (m, 4H, N(CH
2
)
5
), ꢀ8.15 (t-like, 1H, JP,H = 22.5 Hz).
’ ASSOCIATED CONTENT
Supporting Information. Figures giving HPLC chroma-
1
1
1
B{ H} NMR (128.38 MHz, CD Cl , 20 °C): 0.0 (3B), ꢀ6.9
2
2
1
3
1
S
(
2
6
2B), ꢀ9.0 (2B), ꢀ12.7 (2B). C{ H} NMR (150.93 MHz, CD
2
Cl
2
,
b
0 °C): 149.8ꢀ120.5 (sets of doublet and singlet resonances, Caromatic),
tograms of racemic samples and chiral hydrogenation products
1 1 13 1
2.2, 61.9 (br q, C ?), 45.1, 41.7 (CH ), 47.5, 47.4, 27.1, 26.6, 25.79,
10ꢀ15, [ Hꢀ H]-COSY and [ Cꢀ H]-HSQC NMR spectra
carb
2
1
25.05, 24.66 (sets of doublets (J
= 4.0ꢀ5.5 Hz) and singlets,
of complexes 16 and 17, DOSY H NMR spectrum of complex
C,P
31
1
31
1
31
1
N(CH
2
)
5
). P{ H} NMR (242.97 MHz, CD
2
Cl
2
, 20 °C): 146.75
17, and [ P{ H}ꢀ P{ H}]-COSY spectrum of complex 16
and a CIF file giving X-ray crystallographic data for complex 16.
This material is available free of charge via the Internet at http://
pubs.acs.org.
A B
(dd, P , JAB = 55.1, JP,Rh = 208.0 Hz), 145.32 (dd, P , JAB = 55.0, JP,Rh =
1
1
1
13
2
20.8 Hz). For the correlation 2D [ Hꢀ H]-COSY and [ Hꢀ C]-
HSQC spectra of 16 see the Supporting Information.
Preparation of the Ionic Complex [(S)-(PipPhos) Rh] -
þ
4
ꢀ
[
7,8-(PhCH ) -7,8-C B H ] (18). To a solution of 2 (60.0 mg,
2
2
2 9 10
0
.11 mmol) in 5 mL of C H was added 3 (200.0 mg, 0.50 mmol) as a
6 6
’
ACKNOWLEDGMENT
solid, and the resulting reaction mixture was stirred overnight at room
temperature. In the course of the reaction the color of the solution
changed from orange to yellow, affording a yellow oil. Then n-hexane
This work has been supported in part by Grants Nos. 09-
0
300211, 10-03-00837, and 10-0300505 from the Russian Foun-
dation for Basic Research. We thank Dr. M. Esernizkaya for
(
10 mL) was added and an additional yellow powder precipitated from
the mother liquor, while the oily deposits slowly solidified. The solids
were isolated by centrifugation and dissolved in ca. 4 mL of CH Cl , and
recording IR spectra and I. A. Godovikov for recording the
31
1
31
1
[
P{ H}ꢀ P{ H}]-COSY NMR spectrum of complex 16.
2
2
the resulting solution was transferred to a flask containing 15 mL of n-
hexane. The bulky precipitate that formed was again centrifuged, and the
mother liquor was decanted. This procedure was repeated two times,
and then the yellow powder was dried in vacuo to afford analytically pure
’
REFERENCES
(
1) See for instance: (a) Hoel, E. L.; Hawthorne, M. F. J. Am. Chem.
Soc. 1973, 95, 2712. (b) Hoel, E. L.; Hawthorne, M. F. J. Am. Chem. Soc.
975, 97, 6388. (c) Hardy, G. E.; Callahan, K. P.; Strouse, C. E.;
1
8 (212 mg, 92%). Anal. Calcd for C116H112B9N O P Rh: C, 69.17; H,
4 8 4
1
5.60; N, 2.78; B, 4.83. Found: C, 69.18; H, 5.81; N, 2.66; B, 4.86. IR
ꢀ
1
1
11
Hawthorne, M. F. Acta Crystallogr. 1976, B32, 264. (d) Jung, C. W.;
Hawthorne, M. F. Chem. Commun. 1976, 499. (e) Doi, J. A.; Teller,
R. G.; Hawthorne, M. F. Chem. Commun. 1980, 80.
(
8
2 2
KBr, cm ): 2519 (νBꢀH). H{ B} NMR (600.22 MHz, CD Cl ,
0 °C, J = J(H,H), Hz): 8.36ꢀ6.99 (m, 54 H, CHaromatic), 6.28 (d, 2H,
CH
PhCH
, J = 8.7), 5.73 (d, 2H, CH
, J = 8.7), 3.26 (d, 2H,
aromatic
aromatic
(
2) For the reviews on catalyst design and catalysis by
2
, JAB = 15.3), 3.16 (d, 2H, PhCH
2
, JAB = 15.3), 3.19, 3.09, 2.92,
metallacarboranes see: (a) Hawthorne, M. F. In Advances on Boron
and the Boranes; Liebman, J. F., Greenberg, R. E., Williams, R. E., Eds.;
VCH: New York, 1988; Chapter 10, p 225. (b) Teixidor, F.; Nu ~n ez, R.;
Flores, M. A.; Demonceau, A.; Vi n~ as, C. J. Organomet. Chem. 2000,
2
.40, 1.53, 1.28, 1.14, 0.82, 0.11 (set of 4H and 8H resonances (total 40
H), br m, N(CH 0, 2.10 (br s, 2H, BH), 1.95 (br s, 1H, BH), 1.48 (br s,
H, BH), 1.37 (br s, 2H, BH), 0.70 (br s, 1H, BH), 0.27 (br s, 1H,
2 5
)
2
11
1
11
BH), ꢀ2.32 (br s, 1H, BꢀHꢀB). B{ H}/ B NMR (128.38 MHz,
CD Cl
, room temperature, J = J(B,H), Hz): ꢀ9.4 (2B, J = 137), ꢀ10.7
1B, J = 168), ꢀ17.5 (3B, J = 125), ꢀ18.3 (1B, J = 127), ꢀ33.8 (1B,
6
14ꢀ615, 48. (c) Grimes, R. Coord. Chem. Rev. 2000, 200ꢀ202, 773. (d)
2
2
Teixidor, F.; Vi ~n as, C.; Demonceau, A.; Nu ~n ez, R. Pure Appl. Chem.
2003, 75, 1305. (e) Chizhevsky, I. T. Coord. Chem. Rev. 2007, 251, 1590.
(f) Grishin, I. D.; Grishin, D. F. Russ. Chem. Rev. 2008, 77, 633.
(3) (a) Paxson, T. E.; Hawthorne, M. F. J. Am. Chem. Soc. 1974,
96, 4674. (b) Behnken, P. E.; Belmont, J. A.; Busby, D. C.; Delaney,
M. S.; King, R. E., III; Kreimendahl, C. W.; Marder, T. B.; Wilczynski,
J. J.; Hawthorne, M. F. J. Am. Chem. Soc. 1984, 106, 3011. (c) Behnken,
P. E.; Busby, D. C.; Delaney, M. S.; King, R. E., III; Kreimendahl, C. W.;
Marder, T. B.; Wilczynski, J. J.; Hawthorne, M. F. J. Am. Chem. Soc. 1984,
(
13
1
J = 129), ꢀ36.5 (1B, J = 141). C{ H} NMR (150.93 MHz, CD
room temperature): 149.4ꢀ121.1 (sets of doublet and singlet reso-
nances, Caromatic), 61.8 (Ccarb.), 42.0 (CH Ph), 48.6, 26.9, 26.1, 24.9,
4.7 (sets of broad and sharp singlets, N(CH ) ). P{ H} NMR
2 2
Cl ,
2
31
1
2
2
5
(242.97 MHz, CD
2 2
Cl , room temperature, all resonances are given as
is): 150.2, 150.1, 149.9, 149.8, 149.7, 149.3, 149.2, 149.1*, 148.9, 148.8,
1
1
1
48.3*, 147.7, 147.6, 147.5, 147.4, 147.2, 146.9, 146.8, 146.7, 146.6,
46.3, 143.5, 143.2, 143.1, 143.0, 142.9, 142.6, 142.4, 142.3, 142.2, 142.1,
40.9, 140.8, 140.7, 140.6, 140.5, 140.1, 140.0, 139.9, 139.7, 139.6 (see
106, 7444. (d) Belmont, J. A.; Soto, J.; King, R. E., III; Donaldson, A. J.;
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Figure 4 caption for resonances labeled with asterisks).
1
990, 9, 2327.
4) (a) Long, J. A.; Marder, T. B.; Behnken, P. E.; Hawthorne, M. F.
X-ray Crystal Structure Analysis of Complex 16. Crystals of
(
1
6 (C60
H
62
B
9
N
2
O
4
P
2
Rh 2CH
2
Cl
2
0.25H
2
O, M
r
= 1311.61) are
3
3
J. Am. Chem. Soc. 1984, 106, 2979. (b) Knobler, C. B.; Marder, T. B.;
Mizusawa, E. A.; Teller, R. G.; Long, J. A.; Behnken, P. E.; Hawthorne,
M. F. J. Am. Chem. Soc. 1984, 106, 2990. (c) Long, J. A.; Marder, T. B.;
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Kreimendahl, C. W.; Marder, T. B.; Hawthorne, M. F. J. Am. Chem. Soc.
1984, 106, 5757.
monoclinic, space group P2
1
4
with a Bruker SMART APEX II diffractometer (λ(Mo KR) = 0.710 73 Å,
ω scans, 2θ < 56°) at 100 K. The structure was solved by direct methods
and refined by the full-matrix least-squares technique against F with
1
, at 100 K: a = 14.196(2) Å, b =
3
6.434(2) Å, c = 27.847(4) Å, β = 94.117(3)°, V = 6480(2) Å , Z =
3
ꢀ1
, dcalcd = 1.344 g/cm , μ = 0.526 mm . Data collection was carried out
2
(
5) (a) Teixidor, F.; Flores, M. A.; Vi n~ as, C.; Kivek €a s, R.; Sillanp €a €a ,
R. Angew. Chem., Int. Ed. Engl. 1996, 35, 2251. (b) Vi n~ as, C.; Flores,
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anisotropic displacement parameters for all non-hydrogen atoms
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1
998, 17, 2278. (c) Teixidor, F.; Flores, M. A.; Vi ~n as, C.; Sillanp €a €a , R.;
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the hydride ligands and carborane cages were located from the Fourier
synthesis; all other hydrogen atoms were placed geometrically and
included in the structure factor calculations in the riding motion
approximation. The refinement converged to wR2 = 0.1280 and
GOF = 1.058 for 22 637 reflections (R1 = 0.0548 was calculated against
F for 19 624 observed reflections with I > 2σ(I)). All calculations were
(
Hubert, A. J.; Chizhevsky, I. T.; Lobanova, I. A.; Bregadze, V. I.
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21
performed using SHELXTL PLUS 5.0.
1
949
dx.doi.org/10.1021/om101201e |Organometallics 2011, 30, 1942–1950

Organometallics
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(
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