A comparison between oxazoline-imidazolinylidene, -imidazolylidine, -benzimidazolylidene hydrogenation catalysts

Article abstract of DOI:10.1021/jo4013783

Imidazolinylidene, imidazolylidine, benzimidazolylidene complexes 1a-c were prepared and tested in asymmetric hydrogenations of a series of largely unfunctionalized alkenes. Similarities and differences in the catalytic performance of these complexes were rationalized in terms of the predicted mechanisms of these reactions, and their relative tendencies to generate protons under the hydrogenation conditions.

Full text of DOI:10.1021/jo4013783

Note
pubs.acs.org/joc
A Comparison between Oxazoline-imidazolinylidene,
-imidazolylidine, -benzimidazolylidene Hydrogenation Catalysts
Sakunchai Khumsubdee, Yubo Fan, and Kevin Burgess*
Department of Chemistry, Texas A&M University, Box 30012, College Station, Texas 77841, United States
S
* Supporting Information
ABSTRACT: Imidazolinylidene, imidazolylidine, benzimidazolylidene
complexes 1a−c were prepared and tested in asymmetric hydrogenations
of a series of largely unfunctionalized alkenes. Similarities and differences in
the catalytic performance of these complexes were rationalized in terms of
the predicted mechanisms of these reactions, and their relative tendencies
to generate protons under the hydrogenation conditions.
tereoelectronic variation is to be expected within the series
for another can have significant effects on catalytic performance.
Evidence for this assertion comes from the Grubbs’ metathesis
catalysts. Early work had shown that ring closing metatheses
mediated by the now widely appreciated Grubbs type II catalyst
Da tended to proceed with superior reaction rates and overall
conversions relative to Db.11 Delaude and co-workers
subsequently found that for favorable substrates the benzimi-
dazolylidene complex Dc could give faster initial rates for ring-
closing metathesis than Da.12
S
of N-heterocyclic carbene ligands a−c.1,2 With respect to
size, even members of this series with the same N-aryl
substituents may have various steric influences because they
have different flexibilities; for instance, the planar systems b and
c are less flexible than the saturated one a.1 Flexibility in the
ligands can also indirectly affect their electron donating
properties. Direct donation of π-electrons from the N-aryl
substituents to the metal in these systems can be significant,3,4
and it is bound to be orientation dependent. Consequently,
differences in the carbene core that perturb the preferred
orientations of the aryl groups also modulate the electron
densities they donate to the metal.5 Moreover, the saturated
imidazolinylidene a is the only member of this series that is not
aromatic, and this has ill-defined impacts on its electron
donating properties relative to the other two.6
In total, the considerations above indicate there is no reliable
way to extrapolate free ligand properties in the series a−c to the
catalytic performance of complexes, and that even small
stereoelectronic differences in the carbenes could have
significant effects. Consequently, we were curious to test the
series 1a−c that includes our carbene catalyst 1b. Complex 1b
and similar chiral analogs of Crabtree’s catalyst are suitable for
hydrogenation of trisubstituted alkenes without functional
groups that typically coordinate to the metal in these complexes
(CFGs).13−15 Consequently, this Note describes syntheses of
the new complexes 1a and c and use of these to hydrogenate
some largely unfunctionalized alkenes. Enantioselectivities were
the critical end-point parameter that we wished to measure
Several strategies have been devised to gauge stereoelectronic
differences in the series of ligands a−c. These approaches
consider dimerization of the parent carbene,7 IR and Raman
spectra8 of CO ligands in the same complex (sometimes
interpreted in terms of the Tolman electronic parameter),9
electrochemical studies,4 and theoretical approaches.10 Overall,
the most general conclusions from these studies are that all
three ligands are stronger σ-donors than any phosphine, but
electronic effects within the series tend to be small.1,10 For
instance, carbonyl complexes containing ligands a−c may have
IR absorbance within 3 cm−1 of each other, while others have
almost identical REDOX potentials.4
Even if electronic variability in the series a−c is small, and the
N-aryl groups are identical, substitution of one of these ligands
Received: June 25, 2013
© XXXX American Chemical Society
A
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Note
because the overriding application of 1b is in asymmetric
catalysts.
Table 1. Stereoselective Hydrogenations Using the Featured
Catalysts
Imidazoline E, required to make catalyst 1a, is known and
was prepared via the literature procedure.16 Benzimidazole F is
also a known compound,17 but we synthesized it using a
different method, i.e. reaction of the corresponding 1,2-
benzenediamine with trimethyl orthoformate under acidic
conditions as indicated in the Experimental Section. Scheme
1 shows how these two nucleophiles were reacted with the
appropriate iodo-oxazoline18 to give the corresponding carbene
precursors that were converted to the iridium complexes under
identical conditions. Complex 1a is yellow, whereas 1b and c
are orange. 13C NMR chemical shifts of the coordinated
carbene in this series 1a−c are very similar: 178.4, 179.5, and
187.2 ppm, respectively.
Scheme 1
a
b
c
Enantioselectivity unless it stated. Diastereoselectivity. Reactions
using 2 mol % of 1a−c.
Table 1 shows comparative data for a series of hydro-
genations using catalysts 1a−1c. The stilbene derivative in
entry 1 was included because it is the most widely studied
substrate for asymmetric hydrogenations using chiral Crabtree’s
catalyst analogs.13 The endocyclic alkene in entry 2 was chosen
because substrates of that kind are not reduced with high
stereoselectivities using catalyst 1b. The other substrates shown
in Table 1 are relevant to our studies on hydrogenations to
form fragments of polyketide-derived natural products and
other useful chirons.19,20
All the experiments summarized in Table 1 gave 100%
conversions under the conditions indicated. Stereoselectivities
in these reactions were almost identical in almost every case;
variation of the catalyst carbene feature had no significant effect.
We have previously reported evidence that the carbene-
containing catalyst 1b is significantly less prone to generate
protons in hydrogenation reactions than corresponding
complexes that have phosphorus-based ligands in place of the
carbene.21 Calculations in that work indicated that the carbene
complexes gave intermediates that are of the order of 107 times
less acidic than similar P-containing complexes.21 For the
current study, similar calculations were undertaken for the
series 1a−c, and the data obtained are outlined in Table 2.
Relative σ-donor and π-acceptor properties deduced from
similar calculations are also shown. In the event, calculated pKa
B
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Table 2. σ-Donor and π-Acceptor Potentials and pKa of
Complexes 1a−c
values for the complexes are within a 2.2 unit range. Similarly,
the calculated σ-donor and π-acceptor potentials for the free
carbenes were within relatively tight ranges: 0.2 and 0.7 eV,
respectively. These observations are consistent with Figure 1,
which shows the HOMOs from these calculations are all
relatively similar in shape.
Figure 2. Hydrogenation with catalyst 1: (a) Methyl red with acetic
acid in CH2Cl2 (control for acid); (b) methyl red with Et3N in CH2Cl2
(control for base); (c) 1a with methyl red; (d) 1b with methyl red;
and (e) 1c with methyl red.
Figure 1. HOMO of carbene electron from imidazoline (a), imidazole
(b), and benzimidazole (c).
Experimental validation of the calculated pKa trends
discussed above was obtained by performing the hydrogenation
shown in Figure 2 and then adding a pH indicator. Whereas, we
have shown previously that N,P-complexes tend to give red
solutions (similar to the control in Figure 2a), all three
complexes 1a−c gave light yellow solutions indicating the
reaction medium is not acidic.
Figure 3. Transformation of G to H is predicted to be rate-limiting in
the catalytic hydrogenation of trans-1,2-diphenylpropene mediated by
complex 1b.
to the transition state energy.22 Those studies also showed four
pathways should be considered. These are two for each
enantioface of the alkene and, for each enantioface, pathways
corresponding to two different π-complex rotamers related by
180° rotation around the metal to π-bond in the starting
intermediates G. These correspond to putting each of the two
inequivalent phenyl groups of the complexed alkene between
the smaller pocket between the bulky 2,6-di-isopropylphenyl
and adamantly substituents. The conclusion from this study is
that the pathway from intermediate G to H (based on the
relative energies of these two intermediates) for hydrogenation
of the alkene in Table 1, entry 1, to give the (S)-product was
5.91 kcal/mol more stable than any of the two pathways leading
to the (R)-enantiomer.
In the current study, calculations were preformed using
exactly the same procedures as before to assess the energies of
the same four types of pathways for the endocyclic substrate in
entry 2. Figure 4 shows two of the four key intermediates
(analogous to H) and their relative energy differences from the
starting π-complexes (analogous to G); all the values are
Our hypothesis regarding the mechanism of hydrogenation
with 1b, based on calculations, indicated22 a strong trans-effect
orients the alkene ligand opposite to the carbene in
intermediate G as indicated in Figure 3. These calculations
also indicate transformation of G into the transition state H was
predicted to be the rate-limiting step in the catalytic cycle. We
propose all the carbene ligands (abbreviated to C in Figure 3)
maintain that strong trans-effect in the catalysis, and there are
insufficient steric differences to perturb the enantioselectivities.
One of the referees for this paper suggested calculations
should be performed to explain the relatively poor performance
for catalyst 1b with endocyclic substrates, as in entry 2 of Table
1. Our calculations corresponding to use of catalyst 1b in entry
1, as reported previously, involved evaluating the energies of
three species. Specifically these were (i) the intermediate G in
Figure 3 before the turnover limiting step; (ii) the transition
state that relates G and H; and (iii) the energy of the
intermediate H after the turnover limiting step. These studies
proved that the energy of that intermediate H is closely related
C
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were obtained by passing the previously degassed solvents through
activated alumina columns. Reagents were purchased at a high
commercial quality (typically 97% or higher) and used without further
purification, unless otherwise stated. High field NMR spectra were
normalized relative to the most favorable pathway identified.
That most favorable pathway corresponds to production of the
(R)-enantiomer, but there is another calculated to be only 0.21
kcal/mol higher in energy that leads to the (S)-enantiomer.
Thus these calculations explain the poor enantioselectivities for
endocyclic alkenes like J: in both intermediates shown the
edge-fused ring system stacks above the (iPr)2C6H3 ligand aryl
substituent.
1
recorded at 300 MHz for H and 75 MHz for 13C. Chemical shifts of
1H and 13C spectra were referenced to the NMR solvents. Flash
chromatography was performed using silica gel (230−600 mesh). Thin
layer chromatography was performed using glass plates coated with
silica gel 60 F254. The following abbreviations were used to explain
the multiplicities: s = singlet, d = doublet, t = triplet, q = quartet, dd =
double doublet, dq = double quartet, m = multiplet, br = broad.
Calculations were performed as previously described.21,22
Preparation of 1-(2,6-Diisopropylphenyl)-4,5-dihydro-imi-
dazole E.
N-(2,6-Diisopropylphenyl)ethane-1,2-diamine (2.20 g, 10 mmol) was
dissolved in 10 mL of trimethyl orthoformate. A catalytic amount of p-
toluenesulfonic acid (0.10 g, 0.5 mmol) was added to the solution, and
then the mixture was refluxed using an air condenser to allow
methanol to escape until completion (18 h). Solvent was removed
under reduced pressure, and the residue was purified by recrystalliza-
tion (CH2Cl2/hexane) to obtain E (1.04 g, 4.5 mmol, 45%) as a yellow
1
solid. H NMR (300 MHz, CDCl3) δ 7.33−7.16 (3H, m), 6.82 (1H,
s), 4.06 (2H, t, J = 8.7 Hz), 3.58 (2H, t, J = 10.2 Hz), 3.11−3.04 (2H,
m), 1.23 (6H, d, J = 6.9 Hz), 1.18 (6H, d, J = 6.9 Hz); 13C NMR (75
MHz, CDCl3) δ163.0, 156.2, 155.9, 148.2, 134.1, 128.6, 124.1, 55.0,
51.6, 28.3, 28.0, 25.3, 24.5, 23.9, 23.5. IR (Thin film, cm−1) 3364,
3206, 3063, 3024, 2963, 2928, 2866, 1678, 1605, 1585, 1458, 1381,
1366, 1327, 1265, 1204, 1180, 1107, 1053, 961, 933, 887. HRMS (ESI,
TOF): Exact mass calcd for C15H22N2 i.e. [M+H]+ 231.1861. Found
231.1835.
Preparation of (S)-3-(2-(2-(Adamantan-1-yl)-4,5-dihydro-
oxazol-4-yl)ethyl)-1-(2,6-diisopropylphenyl)-4,5-dihydro-
imidazolium Iodide 2.
Figure 4. Intermediates in the two most preferred pathways for
hydrogenation of substrate J leading to different enantiomers have the
same energies (within error limits for these calculations).
(S)-2-(Adamantan-1-yl)-4-(2-iodoethyl)-4,5-dihydrooxazole (1.80 g, 5
mmol) and 1-(2,6-diisopropylphenyl)-4,5-dihydroimidazole E (1.15 g,
5 mmol) were dissolved in 10 mL of DMF under an Ar atmosphere.
The mixture was heated to 80 °C for 12 h. The solvent was removed
under reduced pressure, and the residue was washed with Et2O (5 × 5
mL) to yield compound 2 (2.89 g, 4.9 mmol, 98%) as a yellow solid.
1H NMR (300 MHz, CDCl3) δ 9.0 (1H, s), 8.05−8.01 (1H, m), 7.49−
There are two conclusions from this study. First, though we
cannot rule out the possibility that differences will emerge for
catalytic reactions mediated by complexes 1a−c, there are
negligible differences between them in the asymmetric
hydrogenation reactions shown in Table 1. Second, poor
enantioselectivities in the hydrogenations of endocyclic
substrates like J seem to arise because hydrogenation pathways
for the two enantiofaces of this substrate proceed through
energetically similar turnover limiting steps as illustrated in
Figure 4.
7.40 (2H, m), 7.31−7.20 (1H, m), 3.79 (2H, m), 3.57−3.46 (3H, m),
3.45−3.23 (1H, m), 2.92−2.70 (3H, m), 2.37 (1H, s), 1.91−1.54
(15H, m), 1.27−1.00 (14H, m); 13C NMR (75 MHz, CDCl3) δ. 183.6,
166.1, 162.8, 161.9, 147.0, 144.5, 143.9, 142.7, 142.3, 129.6, 124.4,
123.4, 122.5, 73.4, 50.7, 47.2, 20.3, 38.9, 38.6, 37.3, 36.4, 28.2, 28.0,
27.4, 25.1, 24.1, 23.5, 22.3. IR (Thin film, cm−1) 3066, 2978, 2922,
2844, 1610, 1602, 1458, 1352, 1268, 1161, 1131, 998, 931. HRMS
(ESI, TOF): Exact mass calcd for C30H44N3O+ i.e. [M]+ 462.3479.
Found 462.3487.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out under an inert
atmosphere (nitrogen, or argon where stated) with dry solvents under
anhydrous conditions. Glassware for anhydrous reactions was dried in
an oven at 140 °C for a minimum of 6 h prior to use. Dry solvents
Synthesis of (η4-1,5-Cyclooctadiene)(1-[(4S)-(2-(1-adaman-
tyl)-4-5-dihydrooxazolyl)ethyl]-3-(2,6-diisopropylphenyl)-4,5-
D
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dihydroimidazolin-2-ylidene)iridium(I) Tetrakis(3,5-bis(tri-
fluoromethyl)phenyl)borate 1a.
116.6, 115.8, 114.3, 79.6, 29.1. HRMS (ESI, TOF): Exact mass calcd
for C18H22N2O2 i.e. [M+H]+ 299.1760. Found 299.1733.
Preparation of N-(2,6-Diisopropylphenyl)benzene-1,2-dia-
mine J. 10 % Pd/C (0.02 g, 0.02 mmol) was added to a solution
of 2,6-diisopropyl-N-(2-nitrophenyl)aniline I (0.65 g, 2 mmol) in 15
mL of EtOH. The mixture was then placed into a steel pressure bomb
and then flushed with H2. The reaction was stirred under 20 bar of H2
for 12 h. The mixture was passed through Celite, and the solvent was
removed under reduced pressure to yield compound J (0.52 g, 1.96
mmol, 98%) as a yellow oil. 1H NMR (300 MHz, CDCl3) δ 7.36−7.29
(3H, m), 6.88−6.76 (3H, m), 6.29−6.26 (1H, m), 5.01 (1H, br), 3.60
(2H, br), 3.21−3.12 (2H, m), 1.37−1.23 (12H, m); 13C NMR (75
MHz, CDCl3) δ 145.6, 137.2, 136.5, 134.4, 126.2, 123.8, 120.4, 119.6,
116.5, 114.2, 28.0, 14.2. HRMS (ESI, TOF): Exact mass calcd for
C18H24N2 i.e. [M+2H]2+ 135.1043. Found 135.1054.
(S)-3-(2-(2-(Adamantan-1-yl)-4,5-dihydrooxazol-4-yl)ethyl)-1-(2,6-
diisopropylphenyl)-4,5-dihydroimidazolium iodide 2 (1 mmol) was
added to a round-bottom flask along with 1.5 equiv of lithium tert-
butoxide and 0.5 equiv of [Ir(COD)Cl]2 under Ar. THF was syringed
in to make the solution 0.03 M in imidazolinium salt. The mixture was
heated to 70 °C in an oil bath and stirred for 16 h. After cooling to
room temperature, the volatiles were removed under reduced pressure
and 1.5 equiv of NaBARF in 5 mL of CH2Cl2 was added. Water (5
mL) was added, and the mixture was stirred vigorously for 15 min.
The organic layer was removed, and the aqueous layer was washed
with an additional 5 mL of CH2Cl2. The organic layers were combined
and dried (Na2SO4), and the volatiles were removed in vacuo. The
residue was chromatographed using a short silica column and 10%
hexane/CH2Cl2 as the eluent to obtain compound 1a (55%). Mp
79.8−80.4 (decompose). 1H NMR (300 MHz, CDCl3) δ 7.74 (8H, s),
7.57 (4H, s), 7.41−7.39 (1H, m), 7.28−7.20 (2H, m), 4.73−4.68 (1H,
m), 4.41−4.29 (2H, m), 4.16−4.12 (1H, m), 3.97−3.58 (6H, m),
3.46−3.38 (1H, m), 3.25−3.21 (1H, m), 2.95−2.89 (1H, m), 2.68−
2.59 (1H, m), 2.17−1.59 (20H, m), 1.45−1.15 (16H, m), 0.95−0.84
(2H, m); 13C NMR (75 MHz, CDCl3) δ. 178.4, 177.5, 174.5, 163.6,
162.7, 162.0, 161.3, 160.7, 146.7, 136.3, 134,9, 134.8, 130.0, 129.2,
129.1, 128.7 (2 peaks), 128.6, 126.4, 124.8, 122.7, 117.5, 85.5, 62.1,
55.7, 39.3, 36.1, 29.1, 28.7, 27.4, 26.9, 25.4, 23.9, 22.9. IR (Thin film,
cm−1) 3067, 2967, 2916, 2851, 1609, 1606, 1458, 1354, 1277, 1161,
1126, 999, 929, 887. HRMS (ESI, TOF): Exact mass calcd for
Preparation of 1-(2,6-Diisopropylphenyl)-benzo[d]-imida-
zole F. N-(2,6-Diisopropylphenyl)benzene-1,2-diamine J (0.60 g, 2.3
mmol) was dissolved in 5 mL of trimethyl orthoformate. A catalytic
amount of p-toluenesulfonic acid (0.002 g, 0.01 mmol) was added to
the solution, and then the mixture was refluxed using an air condenser
to allow methanol to escape until completion (18 h). Solvent was
removed under reduced pressure, and the residue was purified by
recrystallization (CH2Cl2/hexane) to obtain F (0.19 g, 0.69 mmol,
1
30%) as a brown solid. H NMR (300 MHz, CDCl3) δ 7.93 (1H, s),
7.55−7.53 (1H, m), 7.38−7.24 (5H, m), 7.08−7.05 (1H, m), 2.34−
2.25 (2H, m), 1.13 (6H, d, J = 6.9 Hz), 1.06 (6H, d, J = 6.9 Hz); 13
C
NMR (75 MHz, CDCl3) δ147.5, 143.7, 143.1, 130.6, 130.2, 124.2,
123.6, 122.4, 120.3, 110.3, 28.3, 24.7, 23.9. IR (Thin film, cm−1) 3429,
3075, 2963, 2928, 2870, 1643, 1612, 1593, 1485, 1462, 1385, 1366,
1308, 1281, 1223, 1142, 1057, 1007, 976, 934, 887. HRMS (ESI,
TOF): Exact mass calcd for C19H22N2 i.e. [M+H]+ 279.1861. Found
279.1863.
Preparation of (S)-3-(2-(2-(Adamantan-1-yl)-4,5-dihydro-
oxazol-4-yl)ethyl)-1-(2,6-diisopropylphenyl)benzo[d]imida-
zolium Iodide 3.
−
C38H55IrN3O+ i.e. [M]+ 762.3969. Found 762.3954 and C32H12BF24
i.e. [M]− 863.0654. Found 863.0632.
(S)-2-(Adamantan-1-yl)-4-(2-iodoethyl)-4,5-dihydrooxazole (1.80 g, 5
mmol) and 1-(2,6-diisopropylphenyl)-benzo[d]imidazole F (1.39 g, 5
mmol) were dissolved in 10 mL of DMF under an Ar atmosphere. The
mixture was heated to 80 °C for 12 h. The solvent was removed under
reduced pressure, and the residue was washed with Et2O (5 × 5 mL)
1
to yield compound 3 (2.39 g, 3.8 mmol, 75%) as a brown solid. H
NMR (300 MHz, CDCl3) δ 8.06 (1H, s), 8.06−7.99 (1H, m), 7.57−
7.22 (5H, m), 7.06−7.03 (1H, m), 4.15−4.08 (1H, m), 2.97−2.90
(2H, m), 2.29−1.74 (14H, m), 1.37−1.21 (5H, m), 1.12 (6H, d, J =
6.9 Hz), 1.04 (6H, d, J = 6.9 Hz), 0.94−0.84 (2H, m); 13C NMR (75
MHz, CDCl3) δ182.9, 147.4, 146.7, 143.9, 135.6, 132.8, 130.4, 124.2,
123.9, 122.9, 120.1, 117.7, 110.5, 60.0, 51.8, 49.6, 49.1, 46.3, 40.5, 38.8,
36.5, 28.3, 27.9, 24.7, 23.9, 22.5, 22.4. IR (Thin film, cm−1) 3065,
2977, 2915, 2853, 1615, 1605, 1454, 1352, 1267, 1161, 998, 934, 889.
HRMS (ESI, TOF): Exact mass calcd for C34H44N3O+ i.e. [M]+
510.3479. Found 510.3499.
Preparation of 2,6-Diisopropyl-N-(2-nitrophenyl)aniline I. 2-
Fluoronitrobenzene (1.41 g, 10 mmol) was added into a solution of
K2CO3 (2.76 g, 20 mmol) and 2,6-diisopropylaniline (4.43 g, 20
mmol) in 30 mL of ethanol. The mixture was refluxed until
completion (36 h). The mixture was allowed to cool to ambient
temperature and then filtered through Celite. Solvent was removed
under reduced pressure, and the crude products were chromato-
graphed using 5% CH2Cl2/hexane to yield I (2.27 g, 7.6 mmol, 76%)
1
as a yellow oil. H NMR (300 MHz, CDCl3) δ 7.10 (2H, d, J = 7.8
Hz), 6.98 (1H, t, J = 7.5 Hz), 6.70−6.45 (4H, m), 3.65 (1H, br),
3.05−2.96 (2H, m), 1.34 (12H, d, J = 6.9 Hz); 13C NMR (75 MHz,
CDCl3) δ. 145.6, 143.0, 141.3, 126.3, 124.5, 123.8, 120.4, 118.6, 118.0,
Synthesis of (η4-1,5-Cyclooctadiene)(1-[(4S)-(2-(1-ada-
mantyl)-4-5-dihydrooxazolyl)-ethyl]-3-(2,6-diisopropyl-
E
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Note
phenyl)benzo[d]imidazol-2-ylidene)iridium(I) Tetrakis(3,5-bis-
(trifluoromethyl)phenyl)borate 1c.
ACKNOWLEDGMENTS
■
We thank The National Institutes of Health (GM087981) and
The Robert A. Welch Foundation (A-1121) for financial
support.
REFERENCES
■
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(S)-3-(2-(2-(Adamantan-1-yl)-4,5-dihydrooxazol-4-yl)ethyl)-1-(2,6-
diisopropylphenyl)-benzo[d]imidazol-3-ium iodide 3 (1 mmol) was
added to a round-bottom flask along with 1.5 equiv of lithium tert-
butoxide and 0.5 equiv of [Ir(COD)Cl]2 under Ar. THF was syringed
in to make the solution 0.03 M in imidazolinium salt. The mixture was
heated to 70 °C in an oil bath and stirred for 16 h. After cooling to
room temperature, the volatiles were removed under reduced pressure
and 1.5 equiv of NaBARF in 5 mL of CH2Cl2 was added. Water (5
mL) was added, and the mixture was stirred vigorously for 15 min.
The organic layer was removed, and the aqueous layer was washed
with an additional 5 mL of CH2Cl2. The organic layers were combined
and dried (Na2SO4), and the volatiles were removed in vacuo. The
residue was chromatographed using a short silica column and 10%
hexane/CH2Cl2 as the eluent to obtain 1c (43%). Mp 145.5−146.4
(decompose). 1H NMR (300 MHz, CDCl3) δ 7.72 (8H, s), 7.52 (4H,
s), 7.46−7.40 (3H, m), 7.28−7.26 (2H, m), 7.18−7.10 (1H, m), 6.92−
6.89 (1H, m), 4.65−4.50 (1H, m), 4.38 (2H, s), 3.78−3.71 (3H, m),
3.39−3.25 (2H, m), 2.87−2.70 (1H, m), 2.30−1.60 (7H, m), 1.56−
1.17 (18H, m), 1.17−1.11 (2H, m), 0.95 (6H, d, J = 6.9 Hz), 0.90
(6H, d, J = 6.9 Hz); 13C NMR (75 MHz, CDCl3) δ 187.2, 162.7,
162.0, 161.3, 160.7, 146.4, 142.5, 139.1, 134.8, 134.5, 131.7, 129.1,
129.0 (2 peaks), 128.6, 128.5, 128.3, 126.5, 126.3, 125.1, 124.8, 122.7,
117.9, 117.5, 117.4, 112.1, 70.5, 61.8, 36.2, 31.7, 29.7, 28.6, 24.3, 23.4,
22.7, 14.1. IR (Thin film, cm−1) 3067, 2967, 2916, 2851, 1609, 1458,
1354, 1277, 1161, 1126, 995, 949, 934, 887, 837. HRMS (ESI, TOF):
Exact mass calcd for C42H55IrN3O i.e. [M]+ 810.3974. Found 810.3998
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−
and C32H12BF24 ie [M]− 863.0654. Found 863.0634.
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Catalytic Hydrogenation Conditions. The corresponding
alkenes (0.25 mmol) and Ir-catalyst (1−2 mol %) were dissolved in
CH2Cl2 (0.5 M). The resulting mixture was degassed by three cycles of
freeze−pump−thaw and then transferred to a Parr Bomb. The bomb
was pressurized to 50 bar with hydrogen and the mixture was stirred at
300 rpm for 16 h. The bomb was then vented and solvent was
evaporated. The crude product was passed through a short silica plug
using 10 - 30% EtOAc/hexane as the eluent. The enantiomeric ratio
was then measured through chiral GC or HPLC analysis.
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ASSOCIATED CONTENT
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S
* Supporting Information
1H and 13C NMR spectra for compounds 1−3, E−F, I−J, GC
and HPLC traces for stereoselectivity determination after
hydrogenation, vCO IR absorbances for simple carbenes and
shape of carbenes electron for π-acceptor. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: burgess@tamu.edu.
Notes
The authors declare no competing financial interest.
F
dx.doi.org/10.1021/jo4013783 | J. Org. Chem. XXXX, XXX, XXX−XXX