Low Pressure Asymmetric Hydrogenation of Quinolines using an Annulated Planar Chiral N-Ferrocenyl NHC-Iridium Complex

Article abstract of DOI:10.1002/adsc.201500105

Annulated planar chiral N-ferrocenylimidazolones, obtained by acid-mediated cyclization of diphenylmethanol derivatives, may be reduced with diisobutylaluminium hydide (DIBAL-H) to afford a series of surprisingly stable and isolable hemiaminal ether aminals. Two of these derivatives can be oxidized with triphenylcarbenium tetrafluoroborate to imidazolinium salt precursors of N-heterocyclic carbenes (NHCs). Deprotonation of these salts in the presence of (cyclooctadiene)iridium chloride dimer {[Ir(COD)Cl]₂} provides chiral coordination complexes bearing N-ferrocenyl NHCs with unique rigid tetracyclic frameworks. Cationic analogues of these complexes catalyze the asymmetric hydrogenation of 2-substituted quinolines under very mild conditions (1 mol% complex, 1 mol% PPh₃, 1-5 atm H₂, toluene, 25 C) in appreciable enantioselectivity (up to 90:10 er). The sensitivity of the hydrogenation process to changes in the phosphine additive suggests that an outer-sphere reaction mechanism may be involved, as proposed for a related achiral NHC-Ir complex reported by Crabtree and co-workers.

Full text of DOI:10.1002/adsc.201500105

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DOI: 10.1002/adsc.201500105
Low Pressure Asymmetric Hydrogenation of Quinolines using an
Annulated Planar Chiral N-Ferrocenyl NHC-Iridium Complex
Joshni John,a Cody Wilson-Konderka,a and Costa Metallinosa,*
a
Department of Chemistry, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario, L2S 3A1, Canada
Fax: (+1)-905-641-8186; phone: (+1) 905-688-5550 ext. 3848; e-mail: cmetallinos@brocku.ca
Received: February 2, 2015; Revised: April 10, 2015; Published online: June 10, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500105.
Abstract: Annulated planar chiral N-ferrocenylimi- these complexes catalyze the asymmetric hydrogena-
dazolones, obtained by acid-mediated cyclization of tion of 2-substituted quinolines under very mild con-
diphenylmethanol derivatives, may be reduced with ditions (1 mol% complex, 1 mol% PPh3, 1–5 atm H2,
diisobutylaluminium hydide (DIBAL-H) to afford toluene, 258C) in appreciable enantioselectivity (up
a series of surprisingly stable and isolable hemiami- to 90:10 er). The sensitivity of the hydrogenation
nal ether aminals. Two of these derivatives can be process to changes in the phosphine additive suggests
oxidized with triphenylcarbenium tetrafluoroborate that an outer-sphere reaction mechanism may be in-
to imidazolinium salt precursors of N-heterocyclic volved, as proposed for a related achiral NHC-Ir
carbenes (NHCs). Deprotonation of these salts in complex reported by Crabtree and co-workers.
the presence of (cyclooctadiene)iridium chloride
dimer {[Ir(COD)Cl]2} provides chiral coordination Keywords: ferrocenes; hydrogenation; imidazolones;
complexes bearing N-ferrocenyl NHCs with unique iridium; lithiation; N-heterocyclic carbenes; quino-
rigid tetracyclic frameworks. Cationic analogues of lines
Introduction
P, N[10] ligands. Almost all of these catalyst systems re-
quire elevated hydrogen pressures or elevated tem-
Optically active tetrahydroquinolines display diverse peratures to achieve good coversions. A recent nota-
biological activity,[1] and have been investigated as ble exception has been reported by Crabtree and co-
chiral ligands for asymmetric synthesis.[2] Iridium-cata- workers[11] wherein Ir complex 1 (Scheme 1), which
lyzed asymmetric hydrogenations of 2-substituted qui- contains an achiral monodentate benzimidazolylidene
nolines offer convenient accesses to these com- ligand, has been found to catalyze the reduction of
pounds,[3] especially when the reactions are supported substituted quinolines under exceptionally mild condi-
by chiral diphosphine,[3,4] diphosphinite,[5] diphosphon- tions (1 mol% 1, 1 mol% PPh3, 1–5 atm H2, 358C).
ite,[6] phosphoramidite,[7] phosphite,[8] diamine,[9] and The atypical reactivity of 1 has been explained by in-
Scheme 1. Outer-sphere quinoline hydrogenation with 1.
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Joshni John et al.
Figure 1. Difference nOes from selective irradiation of hem-
ianinal ether methine hydrogens of anti- and syn-10.
tained by selective irradiation of the respective me-
thine hydrogens a to oxygen (Figure 1).[16] In the case
of syn-10, the a to oxygen methine signal at d=4.93
was observed to be a doublet (3J=7.5 Hz) as a result
of coupling to the neighbouring pyrrolidine methine
Scheme 2. Synthesis and acid-mediated annulation of 9.
voking an outer-sphere reaction mechanism[11] where- hydrogen. The magnitude of this coupling constant is
in an iridium dihydrogen complex protonates sub- similar to that of the starting material syn-8 (3J=
strate 2 and key intermediate 5, the latter preceding 6.3 Hz). Selective irradiation of the d=4.93 doublet
final hydride addition to prochiral iminium species of syn-10 produced a 4.4% nOe for the neighbouring
6.[12]
pyrrolidine methine at d=3.97, along with a 1.4%
As part of our ongoing studies of NHC-Ir com- nOe for a phenyl ring hydrogen at d=7.18. In the
plexes,[13,14] we were interested in investigating wheth- case of anti-10, the analogous a to oxygen methine
er an analogue of 1 bearing a chiral NHC ligand signal at d=5.46 is a singlet as a result of its near or-
would be capable of catalyzing the hydrogenation of thogonal dihedral angle with respect to the neigh-
quinolines in appreciable enantioselectivity, but also bouring pyrrolidine methine. This lack of observable
under similarly mild conditions. Described in this coupling has been seen in other derivatives with es-
paper are the syntheses of two unusual monodentate tablished anti-stereochemistry (e.g., anti-8). Irradia-
planar chiral N-ferrocenyl NHC ligands, prepared by tion of the d=5.46 singlet resulted in only 1.3% nOe
established stereoselective lithiation of syn- and anti-8 for the neighbouring pyrrolidine methine at d=3.90.
(Scheme 2 and Scheme 4).[15] The derived ferrocenyl Additional nOes were observed for the unsubstituted
NHC-Ir complexes were characterized crystallograph- Cp ring protons at d=3.72 (1.9%), the methylene hy-
ically and/or spectroscopically, and found to promote drogen at d=1.57 (2.2%), and for a phenyl ring
the asymmetric hydrogenation of 2-substituted quino- proton at d=7.76 (3.2%).
lines under mild conditions (1–5 atm H2, 258C) in up
to 90:10 er.
Reduction of anti-10 with four equivalents of
DIBAL-H afforded the unusual hemiaminal ether
aminal 11 (Scheme 3). This compound was surprising-
ly robust and indefinitely stable in a freezer (À158C)
under an inert atmosphere. Oxidation of 11 with tri-
phenylcarbenium tetrafluoroborate gave an imidazoli-
Results and Discussion
Having established that epimeric imidazolones syn- nium salt, which was not isolated due to its instabili-
and anti-8 induce opposite planar chirality in lithia- ty.[17] Instead, deprotonation of this NHC precursor
tion–substitution reactions,[15] attention turned to ap- with KO-t-Bu in the presence of 0.5 equivalents of
plications of products such as diphenylmethanol ad- [Ir(COD)Cl]2 afforded Ir(I) complex 12. Coordination
ducts 9 and 14 in ligand synthesis (Scheme 2 and of this atypical NHC ligand to iridium was deter-
Scheme 4). A potential entry into a new ligand motif mined initially by 13C NMR, where a characteristic yl-
was discovered when alcohol 9 underwent acid-medi- idene carbon signal was observed at d=206. Exposure
ated cyclization to give a 4:1 mixture of anti- and syn- of 12 to triphenylphosphine in dichloromethane gave
10, rather than elimination to the unsaturated imida- the cationic iridium complex 13 upon anion exchange
zolone.[15] Repetition of this experiment on a larger with potassium hexafluorophosphate.
scale (1.2 g) afforded exclusively anti-10, presumably
because the longer reaction time allowed for equili- tion studies on a single crystal grown from acetonitrile
bration of the stereoisomers. solution. The complex was found to crystallize in the
The structure of 12 was confirmed by X-ray diffrac-
The relative stereochemistries of anti- and syn-10 chiral triclinic space group P1 as two independent
were established by nOe difference spectroscopy, ob- molecules. The absolute stereochemistry was assigned
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Low Pressure Asymmetric Hydrogenation of Quinolines
Scheme 3. Synthesis of Ir(I) complexes 12 and 13.
Scheme 4. Synthesis and acid-mediated annulation of 14.
on the basis of a Flack value of À0.024(5), which cor-
roborates the known lithiation stereochemistry of
starting material syn-8.[15] The NMR-assigned anti ste-
reochemistry of methine hydrogens H-5a and H-6a
was also affirmed. Each independent molecule is
four-coordinate with square-planar geometry at iridi-
À
À
um and Cylidine Ir bond lengths of 1.999(7) Š [Ir(1A)
C(1A)] and 2.001(7) Š [Ir(1B) C(1B)], respectively.
À
Due to the broad similarity of the two complexes, the
remaining crystallographic metrics refer solely to
complex A, depicted in Figure 2. This complex is
characterized by ferrocenyl Cp rings that are virtually
eclipsed. Despite the rigid annulated structure of the
C1-symmetric NHC ligand, the torsion angle defined
À
À
À
by N-2a C-1a Ir-1a C-l1a [98.6(3)8] is closer to that
observed in related Ir(COD)Cl complexes bearing
non-annulated[18] chiral NHCs [96.9(3)8] rather than
those containing annulated chiral NHCs [104.0(2) and
112.89(18)8].[13a]
An analogous synthetic route was used for the syn-
thesis of an Ir(I) complex 18 (Scheme 5). Thus, diaste-
Scheme 5. Synthesis of Ir(I) complexes 17 and 18 from syn-
15.
reoselective lithiation of anti-8 followed by benzophe-
none quench gave carbinol 14 (Scheme 4). Acid-medi-
ated cyclization of this intermediate furnished a 1:1.4
mixture of ureas anti- and syn-15, which were separa-
ble by column chromatography.
As in the case of anti/syn-10, the stereochemistries
of anti- and syn-15 were assigned by selective irradia-
tion of the respective methine hydrogens a to oxygen,
3
and by J coupling constants. For syn-15, the methine
3
signal at d=5.58 was found to be a doublet with a J
coupling constant of 6.3 Hz, similar to syn-8 and syn-
10 (vide supra). Irradiation of the d=5.58 doublet in-
duced a 3.8% nOe for the neighbouring pyrrolidine
methine at d=4.21, a 2.9% nOe for a phenyl ring hy-
Figure 2. ORTEP depiction of 12 at 30% probability. Most
hydrogens have been omitted for clarity.[19]
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Joshni John et al.
Figure 3. Difference nOes from selective irradiation of hem-
ianinal ether methine hydrogens of syn- and anti-15.
drogen at d=7.75, and a 1.4% nOe for the Cp ring
hydrogens at d=3.77 (Figure 3). For anti-15, the me-
thine signal at d=4.60 was found to be a doublet with
a typically much smaller coupling constant (3J=
1.5 Hz). Difference nOe spectroscopy obtained by ir-
radiation of the d=4.60 signal revealed a much small-
er 0.9% nOe for the neighbouring pyrrolidine me-
thine at d=4.01, along with a 0.7% nOe for one of
the phenyl ring protons at d=7.21. In addition,
a 1.9% nOe was observed for one methylene hydro-
gen of the pyrrolidine ring (Figure 3).
Scheme 6. Optimization of the asymmetric reduction of 2-
methylquinoline (19a).
Reduction of ureas anti- and syn-15 with DIBAL-H Lowering the hydrogen pressure to 5 atm resulted in
gave the hemiaminal ether aminals anti-16 (60%) and no turnover for the 18-catalyzed reaction. These re-
syn-16 (78%), respectively. Of these products, only sults seem to indicate that the anti versus syn stereo-
aminal syn-16 was found to undergo oxidation with chemistry of the imidazoinylidene ligand framework
triphenylcarbenium tetrafluoroborate to give the re- is more important than the planar chirality in deter-
quired imidazolinium salt, which was cooridinated to mining the reactivity and sense of enantio-induction
Ir(I) without purification. Iridium complex 17 thus in asymmetric hydrogenations mediated by complexes
obtained was converted to cationic complex 18 by 13 or 18. Based on the preceding observations, the re-
treatment with PPh3, and isolated as the hexafluoro- maining optimization experiments were conducted
phospate salt upon anion exchange.
solely with complex 13 at 5 atm of hydrogen. Reac-
Based on the work by Crabtree and co-workers tion outcomes with 13 were found to be solvent de-
wherein benzimidazolylidenes were employed as an- pendent, with good yields of 20a being observed only
cillary ligands for the first time in the iridium-cata- in toluene (94%) and THF (86%) (Scheme 6). Metha-
lyzed hydrogenation of quinolines (e.g., complex nol was found to be poor a solvent for this reaction,
1),[11] chiral NHC-Ir complexes 13 and 18 were inves- both in terms of yield (47%), and enantioselectivity
tigated in the asymmetric hydrogenation of 2-substi- (55:45 er). Dichloromethane also led to some erosion
tuted quinolines. Initial experiments focused on iden- of yield (71%) and selectivity (69:31 er) in compari-
tifying optimum conditions for the enantioselective sion to the reactions conducted in toluene.
reduction of 2-methylquinoline (19a) with complex 13
The role of additives was also explored. Exclusion
(Scheme 6). Hydrogenation of 19a (1 mol% 13, of the 1 mol% of PPh3 from the reaction mixture was
1 mol% PPh3, PhMe, room temperature) at 45 atm of notably detrimental to the outcome, resulting in di-
hydrogen for 6 h afforded the desired tetrahydroqui- minished yield and selectivity (76% yield, 61:39 er)
noline 20a in 91% yield and 70:30 er favouring the S- and greater reaction time (24 h). The reaction also re-
enantiomer. Complex 13 retained significant activity sponded to the nature of the phosphine additive.
at 1 atm of hydrogen, as reported by Crabtree for 1, Whereas the use of 1 mol% of P(o-tol)3 gave similar
but provided 20a in lower yield (56%) and marginally results as PPh3 (90% yield, 67:33 er), the use of
better selectivity (73:27 er) after 16 h. The best reac- 1 mol% of P(2-furyl)3 afforded 20a as a racemate in
tion outcome was obtained at 5 atm of hydrogen pres- 53% yield. In contrast to some other catalysts that are
sure, where 20a was isolated in 94% yield and 79:21 expected to operate by an inner-sphere mecha-
er after 6 h.
nism,[20,26] the use of iodine as an additive in hydroge-
Unlike 13, complex 18 was found to catalyze hydro- nation with 13 led to complete inhibition of the reac-
genation of 19a only at 45 atm of hydrogen, requiring tion. The sensitivity of complex 13 to changes in phos-
longer reaction times (16 h) to give 20a in 84% yield phine additives or iodine, and its marked activity at
and 68:32 er (again favouring the S-enantiomer). low H2 pressures (1–5 atm) in the presence of PPh3, is
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Low Pressure Asymmetric Hydrogenation of Quinolines
chiral imidazolinium salts thus obtained may be coor-
dinated to [Ir(COD)Cl]2 in the presence of t-BuOK
to afford neutral complexes 12 and 17. Exposure of
12 and 17 to triphenylphosphine furnished cationic
complexes 13 and 18, which are chiral analogues of
complex 1. Of these two precatalysts, 13 was found to
mediate low pressure enantioselective hydrogenation
(1–5 atm H2) of prochiral 2-substituted quinolines in
up to 90:10 er (19c). Changes in reactivity that were
observed by altering the additive from PPh3 to P(o-
tol)3, P(2-furyl)3 or I2, and the similarity of reaction
conditions with complex 1, point to the likelihood
that this process operates under an outer-sphere
mechanism (Scheme 1). As such, this procedure rep-
resents a first attempt at exploiting low pressure
outer-sphere asymmetric hydrogenation of quinolines
Scheme 7. Asymmetric hydrogenation of 2-substituted qui-
nolines mediated by precatalyst 13.
consistent with an outer-sphere mechanism as pro- using an iridium complex bearing a chiral monoden-
posed by Crabtree for achiral complex 1. tate NHC ligand. A number of future directions may
Several additional 2-substituted quinolines were be considered for improvement of this process. In par-
subjected to hydrogenation with complex 13 ticular, we are investigating the synthesis of bidentate
(Scheme 7). Reduction of 6-methoxy-2-methylquino- NHC ligands based on related ferrocenyl ligand
line (19b) under optimized conditions (1 mol% 13, motifs as a means to enhance the observed enantiose-
1 mol% PPh3, 5 atm H2, PhMe, r.t.) gave 20b in 78% lectivity and yield of hydrogenation. Results in this
yield and 79:21 er within 24 h. Better results were ob- area of research will be reported in due course.
tained for 2-fluoro-2-methylquinoline (19c), which af-
forded 20c in 93% yield and 90:10 er within 10 h. No-
tably, 20c is a key intermediate in the synthesis of an-
tibacterial agent (S)-flumequine.[21] The higher enan-
tioselectivity observed for 19c seems to imply that en- Experimental Section
hanced electrophilicity of iminium species
6
(Scheme 1) may be important in differentiating enan- General
tiomers during the final hydride addition. This obser-
All reagents were purchased from commercial sources and
vation, in turn, may provide additional cursory sup-
port for an outer-sphere reaction mechanism (vide
supra). Sterically hindered substrates gave somewhat
poorer results. 2-Propylquinoline (19d) was reduced
used as received unless otherwise indicated. Tetrahydrofuran
(THF) was freshly dried and distilled over sodium/benzo-
phenone ketyl under an atmosphere of nitrogen. All alkyl-
lithium and lithium amide bases were titrated against N-
less selectively than 19a, b (69:31 er), while hydroge- benzylbenzamide[23] to a blue endpoint. All reactions were
performed under argon in flame- or oven-dried glassware
using syringe-septum cap techniques unless otherwise indi-
cated. The syntheses of metal complexes were conducted
under argon using dry, degassed solvents. 2-Propylquinoline
and 2-phenylquinoline were prepared according to reported
procedures.[24] Column chromatography was performed on
silica gel 60 (70–230 mesh). NMR spectra were obtained on
a Bruker Avance 300 or 600 MHz instrument and are refer-
enced to TMS or to the residual proton signal of the deuter-
nation of more sterically encumbered 2-pheylquino-
line 19e was sluggish, providing 20e in 25% yield and
75:25 er. Attempts to extend this method to hydroge-
nation of 2,3-dimethylquinoline resulted in a low
yield (18%) of racemic product (syn stereochemistry).
Additional heterocycles such as 1- and 3-methyliso-
quinoline, and 2-methyl-7,8-dihydroquinolin-5(6H)-
one, were inert to hydrogenation with complex 13.[22]
1
ated solvent for H spectra, and to the carbon multiplet of
the deuterated solvent for 13C spectra according to published
values. Enantiomeric ratios were determined on an Agilent
1100 series HPLC system at l=254 nm on a Chiralcel OD-
H column. FT-IR spectra were obtained on an ATI Mattson
Research Series spectrometer as KBr pellets for solids or on
KBr discs for liquids. Optical rotations were measured on
a Rudolph Research Autopol III automatic polarimeter.
Mass spectra were obtained on an MSI/Kratos Concept 1S
Mass Spectrometer. Combustion analyses were performed
by Atlantic Microlab Inc., Norcross, GA, USA. Melting
points were determined on a Kofler hot-stage apparatus and
Conclusions
Diphenylmethanol derivatives 9 and 14, prepared ste-
reoselectively by lithiation of syn- and anti-8, undergo
acid-mediated cyclization to give unusual annulated
imidazolones. These ureas may be reduced with
DIBAL-H to give surprisingly stable and isolable
hemiaminal ether aminals, two of which (11 and syn-
16) are amenable to oxidation with triphenylcarbeni-
um tetrafluoroborate. The rigid tetracyclic and planar are uncorrected.
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Synthesis of anti- and syn-10
(+)-(6aS,6bS)-5,5-Diphenyl-6a,6b,7,8,9,11-hexahydro-
5H-(Sp-ferroceno)[d]pyrrolo[1’,2’:3,4]imidazo[5,1-b]-
[1,3]oxazine (11)
A solution of 9[15a] (212 mg, 0.34 mmol) and p-toluenesulfon-
ic acid (129 mg, 0.68 mmol) in CHCl3 (4 mL) was stirred at
room temperature for 5 min. A distinct colour change from
orange to brown was observed. The solution was worked up
with saturated aqueous NaHCO3 solution (1 mL). The or-
ganic layer was washed with water and brine, dried over an-
hydrous Na2SO4 and concentrated under reduced pressure
to give a 4:1 mixture of anti/syn epimers; yield: 117 mg
(70%). Flash column chromatography (silica gel, 3:7
EtOAc/hexane) gave, sequentially, syn-10 (yield: 21 mg,
13%, Rf =0.43) and anti-10 (yield: 96 mg, 57%, Rf =0.23).
Data for syn-10 [(+)-(6aR,6bS)-5,5-diphenyl-6b,7,8,9-tet-
rahydro-5H-(Sp-ferroceno)[d]pyrrolo[1’,2’:3,4]imidazo[5,1-b]
[1,3]oxazin-11(6aH)-one]: Orange glassy solid; [a]2D0: +70.3
(c 0.85, CHCl3); CSP HPLC analysis (Chiralcel OD-H;
eluent: 80:20 hexanes/i-PrOH, 1.0 mLminÀ1): >99:1 er,
>98% ee [tR(minor)=5.59 min, tR(major)=10.77 min]; IR
A solution of anti-10 (610 mg, 1.24 mmol) in THF (60 mL)
at À788C under argon was treated with DIBAL-H (5 mL,
1M in hexanes, 4.98 mmol). The reaction mixture was al-
lowed to warm slowly to room temperature (ca. 5 h). After
16 h, the mixture was cooled to 08C, diluted with EtOAc,
and worked up with a solution of saturated aqueous potassi-
um sodium tartrate (2 mL). It was then stirred gently for
15 min until the organic and aqueous layers separated. The
organic layer was isolated, washed with water and brine,
dried over anhydrous Na2SO4, and concentrated under re-
duced pressure. Flash column chromatography (silica gel,
50:45:5 EtOAc/hexane/Et3N, Rf = 0.23) gave 11 as an orange
crystalline solid that was stable indefinitely in a freezer
under argon; yield: 470 mg (79%); mp 203–2048C; [a]2D0:
+312 (c 0.50, CHCl3); IR (KBr): nmax =3085, 3023, 2958,
(KBr): nmax =3085, 3058, 3026, 2954, 1714, 1504, 1402 cmÀ1
;
2937, 2861, 2848, 1490, 1469, 1360, 1136, 1105 cmÀ1
;
1H NMR (300 MHz, CDCl3): d=7.52–7.49 (d, 2H, J=
7.5 Hz), 7.43–7.37 (t, 2H, J=7.5 Hz), 7.32–7.30 (d, 1H, J=
6.9 Hz), 7.25–7.22 (m, 3H), 7.17–7.14 (m, 2H), 4.93 (d, 1H,
J=7.5 Hz), 4.69 (s, 1H), 4.16–4.07 (m, 2H), 3.87–3.74 (m,
7H), 3.19–3.11 (m, 1H), 2.36–2.28 (m, 1H), 2.18–2.12 (m,
2H), 1.98–1.93 (m, 1H); 13C NMR (150.9 MHz, CDCl3): d=
159.8, 146.3, 145.1, 128.0, 127.9, 127.8, 127.7, 126.7, 125.6,
94.1, 82.2, 80.5, 77.6, 69.8, 63.4, 62.7, 61.1, 56.0, 45.7, 25.4,
25.0; EI-MS: m/z (%)=490 (M+, 16), 105 (100), 84 (48), 77
(33), 49 (58); HR-MS (EI): m/z=490.1345, calcd. for
C29H26N2O256Fe: 490.1343.
1H NMR (300 MHz, CDCl3): d=7.75–7.74 (d, 2H, J=
7.2 Hz), 7.52–7.47 (t, 2H, J=7.2 Hz), 7.40–7.39 (t, 1H, J=
7.5 Hz), 7.12–7.09 (m, 3H), 6.96–6.93 (m, 2H), 4.89 (s, 1H),
4.45 (s, 1H), 3.99–3.97 (m, 1H), 3.91–3.88 (dd, 2H, J=12.3,
6.9 Hz), 3.84–3.80 (m, 2H), 3.68 (s, 5H), 3.22–3.21 (m, 1H),
2.88–2.87 (m, 1H), 2.25–2.24 (m, 1H), 1.86–1.77 (m, 3H);
13C NMR (75.5 MHz, CDCl3): d=147.8, 146.0, 127.7, 127.5,
127.4, 127.1, 126.9, 126.6, 100.2, 91.2, 81.7, 81.4, 75.2, 70.2,
70.0, 63.9, 63.3, 61.9, 56.0, 20.6, 26.7; EI-MS: m/z (%)=476
(M+, 44), 83 (100), 55 (12); HR-MS (EI): m/z=476.1554,
calcd. for C29H28N2O1Fe: 476.1551; anal. calcd. for
C29H26N2O256Fe: C 73.11, H 5.92; found: C 72.81, H 6.05.
Data for anti-10 [(+)-(6aS,6bS)-5,5-diphenyl-6b,7,8,9-tet-
rahydro-5H-(Sp-ferroceno)[d]pyrrolo[1’,2’:3,4]imidazo[5,1-b]
[1,3]oxazin-11(6aH)-one]: Orange solid; mp 251–2528C
(CH2Cl2/hexane); [a]D20: +168.0 (c 0.50, CHCl3); CSP HPLC
analysis (Chiralcel OD-H; eluent: 80:20 hexanes/i-PrOH,
1.0 mLminÀ1): >99:1 er, >98% ee [tR(minor)=8.64 min,
tR(major)=10.09 min]; IR (KBr): nmax =3085, 3058, 2968,
(À)-Chloro[h4-1,5-cyclooctadiene][(6aS,6bS)-5,5-
diphenyl-5,6a,6b,7,8,9-hexahydro(Sp-ferroceno)[d]-
pyrrolo[1’,2’:3,4]imidazo[5,1-b][1,3]oxazinylidene]-
iridium (12)
1
2896, 1712, 1502, 1392, 1028, 1001 cmÀ1; H NMR (600 MHz,
CDCl3): d=7.74–7.73 (d, 2H, J=7.8 Hz), 7.54–7.52 (t, 2H,
J=7.2 Hz), 7.44–7.42 (t, 1H, J=7.2 Hz), 7.15–7.10 (m, 3H),
6.88–6.87 (m, 2H), 5.46 (s, 1H), 5.17 (s, 1H), 4.12–4.11 (t,
1H, J=2.7 Hz), 3.90–3.85 (m, 2H), 3.70 (s, 5H), 3.68–3.63
(m, 1H), 3.18–3.14 (m, 1H), 2.23–2.18 (m, 1H), 2.11–2.07
(m, 1H), 1.99–1.94 (m, 1H), 1.58–1.52 (m, 1H); 13C NMR
(75.5 MHz, CDCl3): d=158.7, 146.6, 144.9, 127.9, 127.7,
127.6, 127.5, 127.3, 126.4, 92.5, 83.1, 82.8, 80.7, 69.9, 64.2,
64.1, 63.5, 60.5, 45.3, 28.4, 25.7; EI-MS: m/z (%)=490 (M+,
8), 86 (64), 84 (100), 49 (17), 47 (19); HR-MS (EI): m/z=
490.1336, calcd. for C29H26N2O2Fe: 490.1343; anal. calcd. for
C29H26N2O256Fe: C 71.03, H 5.34; found: C 70.51, H 5.14.
Scale-up: Repetition of the above procedure using a solu-
tion of 9 (1.2 g, 1.9 mol) and p-toluenesulfonic acid (716 mg,
3.76 mmol) in CHCl3 (20 mL), stirred at room temperature
for 20 min, gave only anti-10 after work-up. The crude mix-
ture was dissolved in a minimum amount of CH2Cl2, and
hexane was added to precipitate the product. Recrystalliza-
tion from CH2Cl2/hexane afforded anti-10 as an orange solid
(vide supra); yield: 645 mg (70%).
A solution of 11 (200 mg, 0.42 mmol) and triphenylcarbeni-
um tetrafluoroborate (139 mg, 0.42 mmol) in CH2Cl2 (4 mL)
was stirred in a Schlenk flask at room temperature under
argon, protected from light. After 5 h, the solvent was
evaporated under reduced pressure and the crude solid was
washed with dry diethyl ether (3”5 mL) under argon, then
dried under vacuum. In a glovebox, the crude imidazolinium
salt was treated with [Ir(COD)Cl]2 (141 mg, 0.21 mmol) and
degassed THF (14 mL). After removal from the glovebox,
the solution was cooled to À788C and KO-t-Bu (47 mg,
0.42 mmol) was added under an increased flow of argon.
After 1 h, the reaction mixture was warmed to room tem-
perature and the volatiles were removed under reduced
pressure. Flash column chromatography (silica gel, 2:8
EtOAc/hexane) gave a 4:1 mixture of coordination isomers.
The minor isomer was found to convert to the major isomer
in CH2Cl2 solution after 16 h at room temperature to afford
12 as a crystalline orange solid; yield: 194 mg (57%); Rf =
0.25 (2:8 EtOAc/hexane). Data for 12: mp 222–2238C
(CH3CN); [a]2D0: À245 (c 0.50, CHCl3); X-ray diffractometry
[CCDC 1030224] was performed on an orange crystal
(0.16”0.05”0.04 mm3):
810.19 gmolÀ1,
triclinic,
C37H38ClFeIrN2O:
P1, a=9.6609(9) Š,
M=
b=
11.9882(12) Š, c=14.7752(14) Š, V=1614.8(3) Š3, a=
2076
asc.wiley-vch.de
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2015, 357, 2071 – 2081

FULL PAPERS
Low Pressure Asymmetric Hydrogenation of Quinolines
74.139(2)8, ß=89.285(2)8, g=79.066(2)8, Z=2, 1cald
=
(À)-2-[(2Rp-(Diphenylhydroxymethyl)ferrocenyl]-1S-
triethylsilyloxy-7aS-hexahydropyrrolo[1,2-c]imidazol-
3-one (14)
1.666 gcmÀ3, F(000)=804, T=147(2) K; 26631 data were
collected. The structure was solved by Direct Methods
(SHELXTL) and refined by full-matrix least squares on F2
resulting in final R, Rw and GOF [for 13430 data with F>
2s(F)] of 0.0395, 0.0804 and 0.965, respectively, for solution
using the Sp enantiomer model [Flack parameter=
À0.024(5)]; IR (KBr): nmax =2956, 2922, 2873, 2828, 1699,
A solution of anti-8 (809 mg, 1.84 mmol) in THF (15 mL) at
À788C under argon was treated with t-BuLi (4.0 mL,
1.04M, 4.16 mmol). After stirring for 30 min, a distinct
colour change from orange to orange-red was observed. The
reaction mixture was quenched with a solution of Ph2CO
(837 mg, 4.59 mmol) in THF (5 mL) and stirred for 30 min
at that temperature. Work-up was conducted by addition of
water (0.5 mL) and, after warming to room temperature, the
reaction mixture was extracted with diethyl ether (2”
40 mL). The combined organic extract was washed with
water, brine, dried over anhydrous Na2SO4 and concentrated
under reduced pressure. Flash column chromatography
(silica gel, 2:8 EtOAc/hexane, Rf =0.43) gave 14 as a diaste-
reomerically enriched crystalline orange solid; yield: 1.06 g
(93%); mp 181–1828C (EtOH/H2O); [a]20: À67.0 (c 1.0,
CHCl3); IR (KBr): nmax =3221, 3086, 3054, D2954, 2909, 1686,
1
1518, 1413 cmÀ1; H NMR (300 MHz, CDCl3): d=7.71–7.68
(d, 2H, J=7.5 Hz), 7.55–7.50 (t, 2H, J=7.2 Hz), 7.45–7.40
(t, 1H, J=7.2 Hz), 7.16–7.11 (m, 3H), 6.88–6.84 (m, 2H),
6.28–6.27 (m, 1H), 5.74–5.73 (d, 1H, J=4.2 Hz), 4.85–4.80
(m, 1H), 4.69–4.63 (m, 1H), 4.48–4.39 (m, 1H), 4.20–4.19 (t,
1H, J=2.7 Hz), 4.05–3.98 (m, 1H), 3.93–3.91(m, 1H), 3.78
(s, 5H), 3.54–3.46 (m, 1H), 3.24–3.20 (m, 1H), 2.87–2.81(m,
1H), 2.36–2.15 (m, 12H); 13C NMR (75.5 MHz, CDCl3): d=
206.4, 146.4, 144.8, 127.8, 127.7, 127.6, 127.4, 126.1, 93.9,
89.4, 86.5, 86.3, 83.6, 81.4, 70.63, 68.5, 64.6, 64.0, 63.0, 54.7,
51.1, 46.3, 34.0, 32.8, 29.6, 28.9, 28.5, 25.5; FAB-MS: m/z
(%)=810 (M+, 2), 95 (50), 91 (36), 67 (70), 55 (100), 43
(99), 39 (54), 29 (73); HR-MS (FAB): m/z=810.1619, calcd.
1
1460 cmÀ1; H NMR (300 MHz, CDCl3): d=7.74 (d, 2H, J=
7.2 Hz), 7.46 (d, 2H, J=6.9 Hz), 7.31–7.29 (m, 1H), 7.23–
7.14 (m, 5H), 5.37 (s, 1H), 4.40 (s, 5H), 4.31- 4.29 (m, 1H),
4.02 (t, 1H, J=2.7 Hz), 3.45–3.43 (m, 1H), 3.38–3.29 (m,
1H), 3.23–3.18 (m, 1H), 2.96–2.90 (m, 1H), 1.82–1.81 (m,
1H), 1.74–1.66 (m, 2H), 0.97 (t, 10H, J=7.8 Hz), 0.67 (q,
6H, J=7.8 Hz); 13C NMR (75.5 MHz, CDCl3): d=161.5,
147.5, 145.9, 127.6, 127.3, 127.2, 127.1, 126.3, 126.2, 92.0,
90.8, 87.2, 76.1, 70.4, 70.1, 66.4, 66.0, 63.5, 45.3, 28.3, 24.9,
6.8, 5.2; EI-MS: m/z (%)=622 (M+, 3), 490 (56), 105 (58),
103 (100), 75 (79); HR-MS (EI): m/z=622.2285, calcd, for
C35H42N2O3Si56Fe: 622.2314; anal. calcd. for C35H42N2O3SiFe:
C 67.51, H 6.80; found: C 67.25, H 6.72.
for
C37H38N2OClFeIr:
810.1651;
anal.
calcd.
for
C37H38N2OCl56FeIr: C 54.85, H 4.73; found: C 54.57, H 4.75.
(À)-[h4-1,5-Cyclooctadiene][triphenylphosphine]
[(6aS,6bS)-5,5-diphenyl-5,6a,6b,7,8,9-hexahydro(Sp-
ferroceno)[d]pyrrolo[1’,2’:3,4]imidazo[5,1-b][1,3]-
oxazinylidene]iridium Hexafluorophosphate (13)
A solution of 12 (100 mg, 0.12 mmol) in CH2Cl2 (1 mL) was
treated with
a solution of triphenylphosphine (32 mg,
0.12 mmol) in CH2Cl2 (1 mL). The resulting red solution was
stirred at room temperature under argon for 3 h and then
concentrated under reduced pressure. The residue was dis-
solved in a minimum amount of CH3CN and a solution of
KPF6 (29 mg, 0.16 mmol) in a minimum amount of CH3CN
was added. The mixture was stirred at room temperature
under argon for 1 h, and then passed through Celite, rinsing
thoroughly with CH2Cl2. The volatiles were removed under
reduced pressure to give a red solid that was triturated with
pentane to afford 13 as a red crystalline solid; yield: 131 mg
(90%); mp 189–1908C; [a]2D0: À71.6 (c 0.50, CHCl3); IR
Synthesis of anti- and syn-15
A solution of 14 (1.10 g, 2.00 mmol) and p-toluenesulfonic
acid (761 mg, 4.00 mmol) in CHCl3 (20 mL) was stirred for
5 min at room temperature. A distinct colour change from
orange to brown was observed. The mixture was worked up
with saturated aqueous NaHCO3 solution. The organic layer
was washed with water and brine, dried over anhydrous
Na2SO4, and concentrated under reduced pressure to give
(KBr): nmax =2918, 2880, 2850, 1500, 1436, 838 cmÀ1
;
a
1.4:1 mixture of syn/anti epimers; yield: 650 mg
1H NMR (300 MHz, CDCl3): d=7.68–7.41 (m, 20H), 7.33–
7.32 (m, 2H), 7.16–7.14 (m, 3H), 6.92–6.90 (d, 2H, J=
6.3 Hz), 5.77 (s, 1H), 5.24 (s, 1H), 4.74 (m, 1H), 4.35 (s,
1H), 4.24 (m, 2H), 4.01 (m, 1H), 3.75 (m, 1H), 3.55–3.46
(m, 7H), 3.29–3.28 (m, 1H), 2.42–2.35 (m, 1H), 2.29–2.17
(m, 3H), 1.97–1.91 (m, 6H), 1.60–1.49 (m, 1H), 1.15–1.12
(m, 1H); 31P NMR (121.5 MHz, CDCl3): d=16.2, À144.2
(septet, J=713.2 Hz); 19F NMR (282.4 MHz, CDCl3): d=
À73.5 (d, J=711.6 Hz); 13C NMR (75.5 MHz, CDCl3): d=
202.9, 145.7–145.6 (d, JC,P =6.8 Hz), 133.7 (d, JC,P =19 Hz),
131.8, 129.2 (d, JC,P =9.8 Hz), 128.7, 128.4, 128.2, 127.9 (d,
(1.33 mmol, 66%). Flash column chromatography (silica gel,
3:7 EtOAc/hexanes, Rf (anti)=0.32, Rf (syn)=0.29) gave, se-
quentially anti-15 (yield: 270 mg, 0.55 mmol, 27%) and syn-
15 (yield: 380 mg, 39%).
Data for anti-15 [(À)-(6aS,6bS)-5,5-diphenyl-6b,7,8,9-tet-
rahydro-5H-(Rp-ferroceno)[d]pyrrolo[1’,2’:3,4]imidazo[5,1-
b][1,3]oxazin-11(6aH)-one]: Orange solid; mp 119–1208C;
[a]2D0: À230 (c 2.0, CHCl3); CSP HPLC analysis (Chiralcel
OD-H; eluent: 90:10 hexanes/i-PrOH, 1.0 mLminÀ1): >99:1
er, >98% ee [tR(major)=6.13 min, tR(minor)=9.88 min]; IR
(KBr): nmax =3085, 3058, 3006, 2970, 2920, 1714, 1489, 1392,
1
J
C,P =9.1 Hz), 127.6, 125.5, 94.0, 90.6–90.5 (d, JC,P =8.3 Hz),
1329 cmÀ1; H NMR (300 MHz, CDCl3): d=7.57 (d, 2H, J=
86.5, 85.1, 83.1, 81.4, 81.2, 80.7, 71.4, 70.2, 65.4, 64.1, 62.4,
45.2, 33.2, 32.2, 29.2, 29.0, 25.9, 24.8; FAB-MS: m/z(%)=
1037 (M+, 8), 83 (29), 71 (30), 69 (60), 67 (38), 55 (100), 43
(94); HR-MS (FAB): m/z=1037.2830, calcd. for
7.5 Hz), 7.40 (t, 2H, J=7.2 Hz), 7.32–7.28 (m, 1H), 7.23–
7.19 (m, 3H), 7.17–7.14 (m, 2H), 4.60 (d, 1H, J=1.5 Hz),
4.59–4.57 (m, 1H), 4.11 (t, 1H, J=8.4 Hz), 4.07 (t, 1H, J=
2.7 Hz), 3.97–3.92 (m, 1H), 3.84 (s, 5H), 3.66–3.58 (m, 1H),
3.25–3.16 (m, 1H), 2.05–1.86 (m, 3H), 1.26 (m, 1H);
13C NMR (75.5 MHz, CDCl3): d=160.5, 145.9, 145.1, 128.4,
127.9, 127.8, 127.7, 126.8, 125.7, 93.1, 82.5, 82.3, 81.5, 69.9,
C55H53N2OPFeIr:
1037.2874;
anal.
calcd.
for
C55H53N2OP56FeIr+PF6À: C 55.89, H 4.52; found: C 55.72, H
4.66.
Adv. Synth. Catal. 2015, 357, 2071 – 2081
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2077

FULL PAPERS
Joshni John et al.
63.7, 62.8, 57.2, 46.3, 45.4, 28.7, 25.6; EI-MS: m/z (%)=490
(M+, 83), 86 (65), 84 (96), 51 (44), 49 (100), 47 (20), 47 (18);
HR-MS (EI): m/z=490.1348, calcd. for C29H26N2O256Fe:
490.1343.
warm to room temperature (ca. 5 h). After 16 h, the solution
was cooled to 08C and worked up with aqueous potassium
sodium tartrate. It was then stirred for 15 min until separate
aqueous and organic layers were visible. The organic layer
was isolated, washed with water and brine, dried over anhy-
drous Na2SO4, and concentrated under reduced pressure.
Flash column chromatography (silica gel, 50:45:5 EtOAc/
hexanes/Et3N, Rf = 0.23) afforded syn-16 as an orange solid;
yield: 115 mg (0.24 mmol, 78%); mp 56–578C; [a]2D0: À318 (c
1.5, CHCl3); IR (KBr): nmax =3085, 3056, 2954, 2917, 2867,
Data for syn-15 [(À)-(6aR,6bS)-5,5-diphenyl-6b,7,8,9-tet-
rahydro-5H-(Rp-ferroceno)[d]pyrrolo[1’,2’:3,4]imidazo[5,1-
b][1,3]oxazin-11(6aH)-one]: Orange solid; mp 97–988C;
[a]2D0: À303 (c 1.0, CHCl3); CSP HPLC analysis (Chiralcel
OD-H; eluent: 60:40 hexanes/i-PrOH, 0.5 mLminÀ1): >99:1
er, >98% ee [tR(minor)=10.85 min, tR(major)=11.56 min];
IR (KBr): nmax =3086, 3057, 3026, 2924, 2853, 1713, 1504,
1
1508, 1490, 1473 cmÀ1; H NMR (300 MHz, CDCl3): d=7.66
1492, 1398 cmÀ1
;
1H NMR (300 MHz, CDCl3): d=7.72 (d,
(d, 2H, J=7.5 Hz), 7.48 (t, 2H, J=7.2 Hz), 7.40–7.37 (m,
1H), 7.14–7.12 (m, 3H), 6.99–6.96 (m, 2H), 5.07 (d, 1H, J=
4.5 Hz), 4.33 (s, 1H), 4.19 (d, 1H, J=4.8 Hz), 4.04–3.99 (m,
2H), 3.89–3.88 (m, 1H), 3.72 (s, 5H), 3.56 (d, 1H, J=
4.8 Hz), 3.05–3.00 (m, 1H), 2.58–2.54 (m, 1H), 2.30–2.26 (m,
1H), 1.87–1.76 (m, 3H); 13C NMR (75.5 MHz, CDCl3): d=
147.7, 146.1, 127.6, 127.5, 127.5, 127.0, 126.9, 126.4, 102.9,
86.3, 81.9, 81.7, 77.4, 74.7, 69.6, 67.0, 63.7, 63.0, 59.6, 55.2,
26.9, 24.5; EI-MS: m/z (%)=476 (M+, 78), 83 (100), 56 (63),
41 (34), 43 (31); HR-MS (EI): m/z=476.1547, calcd. for
C29H28N2O56Fe: 476.1551.
2H, J=7.2 Hz), 7.52 (t, 2H, J=7.8 Hz), 7.42 (t, 1H, J=
7.2 Hz), 7.14–7.08 (m, 3H), 6.87–6.84 (m, 2H), 5.58 (d, 1H,
J=6.6 Hz), 5.08–5.06 (m, 1H), 4.21–4.15 (m, 1H), 4.09 (t,
1H, J=2.7 Hz), 3.81–3.79 (m, 1H), 3.75 (s, 5H), 3.73–3.67
(m, 1H), 3.14–3.06 (m, 1H), 2.14–2.08 (m, 1H), 1.91–1.83
(m, 3H); 13C NMR (75.5 MHz, CDCl3): d=159.1, 146.9,
144.8, 127.7, 127.7, 127.6, 127.4, 127.3, 125.6, 93.1, 82.6, 80.9,
80.5, 70.0, 64.0, 60.4, 60.3, 46.2, 45.6, 26.5, 24.3; EI-MS: m/z
(%)=490 (M+, 4), 87 (12), 85 (59), 83 (100), 47 (18); HR-
MS (EI): m/z=490.1342, calcd. for C29H26N2O2Fe: 490.1343;
anal. calcd. for C29H26N2O2Fe: C 71.03, H 5.34; found: C
70.75, H 5.53.
(À)-Chloro[h4-1,5-cyclooctadiene][(6aR,6bS)-5,5-
diphenyl-5,6a,6b,7,8,9-hexahydro(Rp-ferroceno)[d]-
pyrrolo[1’,2’:3,4]imidazo[5,1-b][1,3]oxazinylidene]-
iridium (17)
(À)-(6aS,6bS)-5,5-Diphenyl-6a,6b,7,8,9,11-hexahydro-
5H-(Rp-ferroceno)[d]pyrrolo[1’,2’:3,4]imidazo[5,1-b]-
[1,3]oxazine (anti-16)
A solution of syn-16 (60 mg, 0.13 mmol) and triphenylcarbe-
nium tetrafluoroborate (42 mg, 0.13 mmol) in CH2Cl2
(2 mL) was stirred in a Schlenk flask at room temperature
under argon, protected from light. After 5 h, the solvent was
removed under reduced pressure, and the crude solid was
washed with diethyl ether (3”5 mL) under argon and dried
under vacuum. The crude material was transferred to a glo-
vebox, and treated with [Ir(COD)Cl]2 (42 mg, 0.06 mmol)
and degassed THF (4 mL). The mixture was removed from
the glovebox, cooled to À788C and, with an increased flow
of argon, KO-t-Bu (14 mg, 0.13 mmol) was added. The reac-
tion mixture was allowed to warm to room temperature.
After 16 h, the volatiles were removed under reduced pres-
sure. Flash column chromatography (silica gel, 3:7 EtOAc/
hexane, Rf =0.40) gave 17 as a crystalline orange solid;
yield: 26 mg (25%); mp 157–1588C; [a]2D0: À49.0 (c 0.5,
CHCl3); IR (KBr): nmax =3132, 2954, 2924, 2873, 1734, 1519,
To a solution of anti-15 (195 mg, 0.40 mmol) in THF
(20 mL) at À788C was added DIBAL-H (1.80 mL, 1.00M
solution in hexanes, 1.60 mmol). The reaction mixture was
allowed to warm to room temperature (ca. 5 h). After 16 h,
the solution was cooled to 08C and worked up with aqueous
potassium sodium tartrate. It was then stirred for 15 min
until separate aqueous and organic layers were visible. The
organic layer was isolated, washed with water and brine,
dried over anhydrous Na2SO4 and concentrated under re-
duced pressure. Flash column chromatography (silica gel,
50:45:5 EtOAc/hexanes/Et3N, Rf = 0.40) afforded aminal
anti-16 as an orange solid; yield: 114 mg (60%); mp 88–
908C; [a]2D0: À343 (c 1.0, CHCl3); IR (KBr): nmax =3084,
1
3026, 2954, 2919, 2865, 1491, 1473 cmÀ1; H NMR (300 MHz,
CDCl3): d=7.51 (d, 2H, J=7.5 Hz), 7.37 (t, 2H, J=7.2 Hz),
7.28–7.23 (m, 6H), 4.32 (d, 1H, J=7.5 Hz), 4.21 (d, 1H, J=
7.5 Hz), 4.12–4.11 (m, 1H), 4.06 (t, 1H, J=2.7 Hz), 3.98 (d,
1H, J=2.7 Hz), 3.87–3.82 (m, 1H), 3.80–3.79 (m, 1H), 3.77
(s, 5H), 3.20–3.13 (m, 1H), 2.72–2.64 (m, 1H), 2.15–2.07 (m,
1H), 1.79–1.68 (m, 2H), 1.61–1.55 (m, 1H); 13C NMR
(75.5 MHz, CDCl3): d=146.7, 145.5, 128.1, 127.8, 127.6,
127.3, 126.4, 125.6, 102.6, 88.7, 84.2, 81.0, 78.6, 70.2, 68.8,
63.0, 62.3, 54.9, 54.0, 29.4, 25.7; EI-MS: m/z (%)=476 (M+,
84), 83 (100), 56 (76), 57 (46), 41 (40); HR-MS (EI): m/z=
476.1544, calcd. for C29H28N2O56Fe: 476.1551.
1
1400 cmÀ1; H NMR (300 MHz, CDCl3): d=7.76 (d, 2H, J=
7.5 Hz), 7.53 (t, 2H, J=7.2 Hz), 7.45 (t, 1H, J=7.2 Hz),
7.12–7.10 (m, 3H), 6.95–6.92 (m, 2H), 6.13 (s, 1H), 5.69 (d,
1H, J=7.8 Hz), 4.80–4.75 (m, 1H), 4.68–4.61 (m, 1H), 4.57–
4.52 (m, 1H), 4.21–4.13 (m, 2H), 3.80–3.75 (s, 6H), 3.55–
3.51 (m, 1H), 3.28–3.26 (m, 1H), 3.08–3.06 (m, 1H), 2.45–
2.33 (m, 3H), 2.20–2.13 (m, 2H), 2.03–1.98 (m, 3H), 1.89–
1.81 (m, 2H), 1.74–1.68 (m, 2H); 13C NMR (75.5 MHz,
CDCl3): d=206.8, 146.6, 143.9, 128.2, 127.9, 127.7, 127.6,
127.5, 126.9, 94.4, 86.9, 86.6, 84.9, 83.1, 82.7, 70.2, 65.9, 65.0,
64.6, 61.7, 54.9, 49.6, 48.3, 34.7, 32.5, 30.4, 28.4, 25.9, 23.9;
FAB-MS: m/z (%)=810 (M+, 2), 95 (50), 91 (36), 67 (70),
55 (100), 43 (99), 39 (54), 29 (73); HR-MS (FAB): m/z=
810.1613, calcd. for C37H38N2OCl56FeIr: 810.1651.
(À)-(6aR,6bS)-5,5-Diphenyl-6a,6b,7,8,9,11-hexahydro-
5H-(Rp-ferroceno)[d]pyrrolo[1’,2’:3,4]imidazo[5,1-b]-
[1,3]oxazine (syn-16)
To a solution of syn-15 (150 mg, 0.31 mmol) in THF (15 mL)
at À788C was added DIBAL-H (1.40 mL, 1.00M solution in
hexanes, 1.60 mmol). The reaction mixture was allowed to
2078
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Low Pressure Asymmetric Hydrogenation of Quinolines
(À)-[h4--1,5-Cyclooctadiene][triphenylphosphine]
[(6aR,6bS)-5,5-diphenyl-5,6a,6b,7,8,9-hexahydro(Rp-
ferroceno)[d]pyrrolo[1’,2’:3,4]imidazo[5,1-b][1,3]-
oxazinylidene]iridium Hexafluorophosphate (18)
(À)-(S)-6-Methoxy-2-methyl-1,2,3,4-tetrahydro-
quinoline (20b)[25,26]
A vial containing a solution of 6-methoxy-2-methylquinoline
19b (87 mg, 0.50 mmol), triphenylphosphine (1.3 mg,
1 mol%) and iridium complex 13 (6 mg, 1 mol%) in toluene
(2 mL) was sealed in a Parr bomb. The vessel was evacuated
and back-filled with hydrogen three times, then pressurized
to 5 atm. The mixture was stirred at room temperature for
24 h, after which the pressure was carefully released. The re-
action mixture was concentrated under reduced pressure
and passed through a short column of silica gel, eluting with
10:86:4 EtOAc/hexane/Et3N. Evaporation of the solvent
under vacuum gave 20b as a yellow oil that solidified on
standing; yield: 69 mg (78%); [a]2D0: À59.7 (c 1.0, CHCl3)
{lit[27] (R)-20b: [a]D20: +80.3 (c 0.19, CHCl3, >99% ee)}; CSP
HPLC analysis (Chiralcel OD-H; eluent: 98:2 hexanes/i-
PrOH, 0.5 mLminÀ1): 79:21 er, 58% ee [tR(minor)=
19.58 min, tR(major)=22.85 min]; 1H NMR (300 MHz,
CDCl3): d=6.62–6.58 (m, 2H), 6.47–6.44 (m, 1H, J=
8.1 Hz), 3.73 (s, 3H), 3.33 (bs, 1H), 2.85–2.69 (m, 2H), 1.96–
1.88 (m, 1H), 1.65–1.51 (m, 1H), 1.22–1.20 (d, 3H, J=
6.3 Hz); 13C NMR (75.5 MHz, CDCl3): d=151.8, 138.8,
122.5, 115.5, 114.6, 112.8, 55.8, 47.4, 30.3, 26.9, 22.5.
A solution of 17 (13 mg, 0.02 mmol) in CH2Cl2 (0.5 mL) was
treated with
a solution of triphenylphosphine (4 mg,
0.02 mmol) in CH2Cl2 (0.5 mL) under argon. The resulting
red solution was stirred at room temperature for 3 h and
concentrated under reduced pressure. The residue was dis-
solved in a minimum amount of CH3CN, and a solution of
KPF6 (3.5 mg, 0.02 mmol) in a minimum amount of CH3CN
was added. After stirring at room temperature for 1 h, the
reaction mixture was passed through Celite, rinsing with
CH2Cl2. Removal of the solvent under reduced pressure
gave a red solid that was triturated with pentane to afford
18 as a red crystalline solid; yield: 17 mg (90%); mp 138–
1408C; [a]2D0: À319 (c 0.4, CHCl3); IR (KBr): nmax =3061,
1
2929, 2887, 1493, 1436, 1096, 847 cmÀ1; H NMR (300 MHz,
acetone-d6): d=7.68–7.58 (m, 2H), 7.56–7.49 (m, 3H), 7.46–
7.39 (m, 4H), 7.37–7.29 (m, 6H), 7.26–7.14 (m, 5H), 7.04 (t,
1H, J=7.2 Hz), 6.87 (t, 2H, J=7.8 Hz), 6.57 (d, 2H, J=
7.5 Hz), 6.16 (d, 1H, J=7.8 Hz), 5.85–5.84 (m, 1H), 4.87–
4.86 (m, 1H), 4.66–4.53 (m, 3H), 4.51–4.45 (m, 1H), 4.37–
4.36 (m, 1H), 3.80 (s, 5H), 3.59–3.47 (m, 1H), 3.40–3.36 (m,
1H), 3.00–2.93 (m, 2H), 2.80–2.78 (m, 3H), 2.62–2.50 (m,
4H), 1.95–1.75 (m, 3H), 1.70 (m, 1H), 1.58–1.51 (m, 1H);
31P NMR (242.9 MHz, CDCl3): d=18.9 (s), À144.3 (septet,
J=714.1 Hz); 19F NMR (564.7 MHz, CDCl3): d=À73.6 (d,
J=711.6 Hz);13C NMR (150.9 MHz, CDCl3): d=198.3 (d,
J=7.5 Hz), 146.9, 146.0, 134.0 (d, J=12.1 Hz), 131.5 (d, J=
15.1 Hz), 128.6 (t, J=10.6 Hz), 128.3, 127.9, 127.8, 127.7,
127.6, 95.2, 92.2, 85.6, 85.0, 84.1 (d, J=13.6 Hz), 83.6, 80.2,
79.8, 70.8, 67.2, 65.7, 63.8, 63.1, 45.0, 35.1, 34.7, 28.1, 27.2,
25.7, 21.2; FAB-MS: m/z (%)=1037 (M+, 17), 81 (42), 67
(41), 55 (100), 43 (93); HR-MS (FAB): m/z=1037.2682,
calcd. for C55H53N2OP56FeIr: 1037.2874.
(À)-(S)-6-Fluoro-2-methyl-1,2,3,4-tetrahydroquinoline
(20c)[25,26]
A vial containing a solution of 6-fluoro-2-methylquinoline
19c (81 mg, 0.51 mmol), triphenylphosphine (1.3 mg,
1 mol%) and iridium complex 13 (6 mg, 1 mol%) in toluene
(2 mL) under argon was sealed in a Parr bomb. The vessel
was evacuated and back-filled with hydrogen three times,
then pressurized to 5 atm. The mixture was stirred at room
temperature for 10 h, after which the pressure was carefully
released. The reaction mixture was concentrated under re-
duced pressure and passed through a short column of silica
gel, eluting with 2:94:4 EtOAc/hexane/Et3N. Evaporation of
the solvent under vacuum gave 20c as a colourless solid;
yield: 77 mg (93%); [a]2D0: À77.5 (c 0.65, CHCl3) {lit[27] (R)-
20c: [a]2D0: +80.3 (c 0.19, CHCl3, 98% ee)}; CSP HPLC anal-
ysis (Chiralcel OD-H; eluent: 98:2 hexanes/i-PrOH,
0.5 mLminÀ1): 90:10 er, 80% ee [tR(minor)=12.55 min,
tR(major)=16.66 min]; 1H NMR (300 MHz, CDCl3): d=
6.71–6.65 (m, 2H), 6.43–6.38 (m, 1H), 3.40–3.32 (m, 2H),
2.83–2.67 (m, 2H), 1.95–1.88 (m, 1H), 1.63–1.50 (m, 1H),
1.21 (d, 3H, J=6.3 Hz); 13C NMR (75.5 MHz, CDCl3): d=
157.0, 153.9, 140.9, 122.5, 122.4, 115.5, 115.2, 114.7, 114.6,
113.3, 113.0, 47.3, 29.8, 26.7, 22.4.
(À)-(S)-2-Methyl-l,2,3,4-tetrahydroquinoline
(20a)[25,26]
A vial containing a solution of 2-methylquinoline 19a
(0.07 mL, 0.51 mmol), triphenylphosphine (1.3 mg, 1 mol%)
and iridium complex 13 (6 mg, 1 mol%) in toluene (2 mL)
was sealed in a Parr bomb. The vessel was evacuated and
back-filled with hydrogen three times, then pressurized to
5 atm. The mixture was stirred at room temperature for 6 h,
after which the pressure was carefully released. The reaction
mixture was concentrated under reduced pressure and
passed through a short column of silica gel, eluting with
2:94:4 EtOAc/hexane/Et3N. Evaporation of the solvent
under vacuum gave 20a as a colourless oil; yield: 69 mg
(94%); [a]D20: À53.0 (c 3.0, CHCl3) {lit[27] (R)-20a: [a]D20:
+84.3 (c 0.20, CHCl3, 99% ee)}; CSP HPLC analysis (Chir-
alcel OD-H; eluent: 98:2 hexanes/i-PrOH, 0.5 mLminÀ1):
79:21 er, 58% ee [tR(minor)=14.16 min, tR(major)=
16.17 min]; 1H NMR (300 MHz, CDCl3): d=7.01–6.97 (m,
2H), 6.63 (t, 1H, J=7.6 Hz), 6.51–6.48 (m, 1H), 3.45–3.39
(m, 1H), 2.90–2.71 (m, 2H), 1.99–1.91 (m, 1H), 1.68–1.55
(m, 1H), 1.23 (d, 3H, J=6.3 Hz); 13C NMR (75.5 MHz,
CDCl3): d=144.7, 129.2, 126.6, 121.0, 116.9, 113.9, 47.1, 30.0,
26.5, 22.5.
(À)-(S)-2-Propyl-l,2,3,4-tetrahydroquinoline (20d)[25,26]
A vial containing a solution of 2-propylquinoline 19d
(0.08 mL, 0.50 mmol), triphenylphosphine (1.3 mg, 1 mol%)
and iridium complex 13 (6 mg, 1 mol%) in toluene (2 mL)
was sealed in a Parr bomb. The vessel was evacuated and
back-filled with hydrogen three times, then pressurized to
5 atm. The mixture was stirred at room temperature for
24 h, after which the pressure was carefully released. The re-
action mixture was concentrated under reduced pressure
and passed through a short column of silica gel, eluting with
2:94:4 EtOAc/hexane/Et3N. Evaporation of the solvent
under vacuum gave 20d as a colourless oil; yield: 68 mg
Adv. Synth. Catal. 2015, 357, 2071 – 2081
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2079

FULL PAPERS
Joshni John et al.
(78%); [a]D20: À48.0 (c 1.1, CHCl3) {lit[27] (R)-20d: [a]D20:
+80.3 (c 0.19, CHCl3, 99% ee)}; CSP HPLC analysis (Chir-
alcel OD-H; eluent: 98:2 hexanes/i-PrOH, 0.5 mLminÀ1):
69:31 er, 38% ee [tR(minor)=12.46 min, tR(major)=
Med. 1998, 64, 762; h) I. Jacquemond-Collet, S. Hanne-
douche, N. Fabre, I. FourastØ, C. Moulis, Phytochemis-
try 1999, 51, 1167.
[2] a) W.-B. Liu, H. He, L.-X. Dai, S.-L. You, Synthesis
2009, 2076; b) T. Pullmann, B. Engendahl, Z. Zhang,
M. Hçlscher, A. Zanotti-Gerosa, A. Dyke, G. Franciò,
W. Leitner, Chem. Eur. J. 2010, 16, 7517.
[3] a) Y.-G. Zhou, Acc. Chem. Res. 2007, 40, 1357; b) D.-S.
Wang, Q. A. Chen, S.-M. Lu, Y.-G. Zhou, Chem. Rev.
2012, 112, 2557; c) F. Glorius, Org. Biomol. Chem.
2005, 3, 4171; d) Y.-M. He, F.-T. Song, Q.-H. Fan, Top.
Curr. Chem. 2014, 343, 145.
[4] a) J. Wu, A. S. C. Chan, Acc. Chem. Res. 2006, 39, 711;
b) Z.-P. Chen, Z.-S. Ye, M.-W. Chen, Y.-G. Zhou, Syn-
thesis 2013, 3239; c) D.-Y. Zhang, C.-B. Yu, M. C.
Wang, K. Gao, Y. G. Zhou, Tetrahedron Lett. 2012, 53,
2556.
[5] a) W.-J. Tang, S.-F. L.-J. Xu, Q.-L. Zhou, Q.-H. Fan, H.-
F. Zhou, K. Lam, A. S. C. Chan, Chem. Commun. 2007,
613; b) K. H. Lam, L. Xu, L. Feng, Q.-H. Fan, F. L.
Lam, W.-H. Lo, A. S. C. Chan, Adv. Synth. Catal. 2005,
347, 1755.
1
14.84 min]; H NMR (300 MHz, CDCl3): d=6.98 (t, 2H, J=
6.6 Hz), 6.61 (t, 1H, J=7.8 Hz), 6.50 (d, 1H, J=8.1 Hz),
3.80 (bs, 1H) 3.29–3.25 (m, 1H), 2.84–2.75 (m, 2H), 2.01–
1.95 (m, 1H), 1.66–1.59 (m, 1H), 1.54–1.42 (m, 4H), 0.98 (t,
3H, J=6.9 Hz); 13C NMR (75.5 MHz, CDCl3): d=144.6,
129.2, 126.6, 121.4, 116.9, 114.0, 51.3, 38.8, 28.1, 26.4, 18.9,
14.2.
(+)-(R)-2-Phenyl-1,2,3,4-tetrahydroquinoline
(20e)[25,26,27,28]
A vial containing a solution of 2-phenylquinoline 19e
(103 mg, 0.50 mmol), triphenylphosphine (1.3 mg, 1 mol%)
and iridium complex 13 (6 mg, 1 mol%) in toluene (2 mL)
was sealed in a Parr bomb. The vessel was evacuated and
back-filled with hydrogen three times, then pressurized to
5 atm. The mixture was stirred at room temperature for
24 h, after which the pressure was carefully released. The re-
action mixture was concentrated under reduced pressure.
Purification by preparative TLC on silica gel, eluting with
5:95 EtOAc/hexane, gave 20e as a yellow solid; yield: 26 mg
(25%); [a]D20: +18.0 (c 0.4, CHCl3) {lit[29] (R)-20e: [a]D20:
+41.9 (c 1.0, CHCl3, 98% ee)}; CSP HPLC analysis (Chiral-
cel OD-H; eluent: 95:5 hexanes/i-PrOH, 0.6 mLminÀ1):
75:25 er, 50% ee [tR(minor)=17.68 min, tR(major)=
22.35 min]; 1H NMR (300 MHz, CDCl3): d=7.42–7.27 (m,
5H), 7.01 (t, 2H, J=6.9 Hz), 6.65 (t, 1H, J=7.5 Hz), 6.54
(d, 1H, J=8.1 Hz), 4.45 (dd, 1H, J=9.3, 3.3 Hz), 4.07 (bs,
1H), 2.99–2.90 (m, 1H), 2.74 (dt, 1H, J=16.5, 4.8 Hz), 2.17–
2.03 (m, 2H); 13C NMR (75.5 MHz, CDCl3): d=144.8, 144.7,
129.3, 128.5, 127.4, 126.9, 126.5, 120.8, 117.1, 113.9, 56.2,
30.9, 26.4.
[6] M. T. Reetz, X. Li, Chem. Commun. 2006, 2159.
ˇ ´
[7] a) N. Mrsic, L. Lefort, J. A. F. Boogers, A. J. Minnaard,
B. L. Feringa, J. G. de Vries, Adv. Synth. Catal. 2008,
350, 1081; b) M. Eggenstein, A. Thomas, J. Theuerkauf,
G. Franciò, W. Leitner, Adv. Synth. Catal. 2009, 351,
725.
[8] J. L. NfflÇez-Rico, H. C. Fernµndez-PØrez, J. Benet-
Buchholz, A. Vidal-Ferran, Organometallics 2010, 29,
6627.
[9] Z.-W. Li, T.-L. Wang, Y.-M. He, Z. J. Wang, Q.-H. Fan,
J. Pan, L.-J. Xu, Org. Lett. 2008, 10, 5265.
[10] a) A. Bruckmann, M. A. Pena, C. Bolm, Synlett 2008,
900; b) S.-M. Lu, X.-W. Han, Y.-G. Zhou, Adv. Synth.
Catal. 2004, 346, 909.
[11] G. E. Dobereiner, A. Nova, N. D. Schley, N. Hazari,
S. J. Miller, O. Eisenstein, R. H. Crabtree, J. Am.
Chem. Soc. 2011, 133, 7547.
Acknowledgements
[12] For a discussion of the classical inner-sphere reaction
mechanism for Ir-catalyzed quinoline hydrogenation,
see: D.-W. Wang, X.-B. Wang, D.-S. Wang, S.-M. Lu,
Y.-G. Zhou, Y.-X. Li, J. Org. Chem. 2009, 74, 2780.
[13] a) C. Metallinos, X. Du, Organometallics 2009, 28,
1233; b) C. Metallinos, J. Zaifman, L. Van Belle, L.
Dodge, L. M. Pilkington, Organometallics 2009, 28,
4534.
We thank NSERC Canada for support of our research pro-
gram and T. Jones and R. Simionescu for assistance with
spectroscopic data collection. We also thank Dr. Alan J.
Lough (University of Toronto) for X-ray crystallography of
complex 12.
[14] C. Metallinos, L. Van Belle, J. Organomet. Chem. 2011,
696, 141.
References
[15] a) C. Metallinos, J. John, J. Zaifman, K. Emberson,
Adv. Synth. Catal. 2012, 354, 602; b) C. Metallinos, J.
John, J. Nelson, T. Dudding, L. Belding, Adv. Synth.
Catal. 2013, 355, 1211.
[1] a) A. R. Katritzky, S. Rachwal, B. Rachwal, Tetrahe-
dron 1996, 52, 15031; b) V. Sridharan, P. A. Suryavan-
shi, J. C. MenØndez, Chem. Rev. 2011, 111, 7157; c) J.
Bµlint, G. Egri, E. Fogassy, Z. Bçcskei, K. Simon, A.
Gajµry, A. Friesz, Tetrahedron: Asymmetry 1999, 10,
1079; d) I. Jacquemond-Collet, F. Benoit-Vical, M. Val-
entin, A. Stanislas, E. MalliØ, M. FourastØ, Planta Med.
2002, 68, 68; e) P. -Y. Yang, Y.-G. Zhou, Tetrahedron:
Asymmetry 2004, 15, 1145; f) P. J. Houghton, T. Z. Wol-
demariam, Y. Watanabe, M. Yates, Planta Med. 1999,
65, 250; g) J. H. Rakotoson, N. Fabre, I. Jacquemond-
Collet, S. Hannedouche, I. FourastØ, C. Moulis, Planta
[16] For spectroscopic data of all new compounds (1 H,
13C NMR and nOe difference spectra) see the Support-
ing Information.
[17] For examples of achiral N-ferrocenyl NHCs, see: B.
Bildstein, J. Organomet. Chem. 2001, 617–618, 28, and
references cited therein. For a chiral C2-symmetric N-
ferrocenyl NHC, see: A. Bertogg, F. Camponovo, A.
Togni, Eur. J. Inorg. Chem. 2005, 347.
2080
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2015, 357, 2071 – 2081

FULL PAPERS
Low Pressure Asymmetric Hydrogenation of Quinolines
[18] H. Seo, B. Y. Kim, J. H. Lee, H.-J. Park, S. U. Son,
Y. H. Chung, Organometallics 2003, 22, 4783.
[23] A. F. Burchat, J. M. Chong, N. Nielsen, J. Organomet.
Chem. 1997, 542, 281.
[24] a) A.-H. Li, E. Ahmed, X. Chen, M. Cox, A. P. Crew,
H.-Q. Dong, M. Jin, L. Ma, B. Panicker, K. W. Siu,
A. G. Steinig, K. M. Stolz, P. A. R. Tavares, B. Volk, Q.
Weng, D. Werner, M. J. Mulvihill, Org. Biomol. Chem.
2007, 5, 61; b) V. Parekh, J. A. Ramsden, M. Wills, Tet-
rahedron: Asymmetry 2010, 21, 1549; c) R. V. Stevens,
J. W. Canary, J. Org. Chem. 1990, 55, 2237.
[25] C. Wang, C. Li, X. Wu, A. Pettman, J. Xiao, Angew.
Chem. 2009, 121, 6646; Angew. Chem. Int. Ed. 2009, 48,
6524.
[26] W.-B. Wang, S.-M. Lu, P. -Y. Yang, X.-W. Han, Y.-G.
Zhou, J. Am. Chem. Soc. 2003, 125, 10536.
[27] T. Wang, L.-G. Zhuo, Z. Li, F. Chen, Z. Ding, Y. He,
Q.-H. Fan, J. Xiang, Z.-X. Yu, A. S. C. Chan, J. Am.
Chem. Soc. 2011, 133, 9878.
[19] Crystallographic data (excluding structure factors) for
the structure reported in this paper have been deposit-
ed with the Cambridge Crystallographic Data Centre
as supplementary publication no. CCDC 1030224.
These data can be obtained free of charge from The
Cambridge
Crystallographic Data
Centre via
www.ccdc.cam.ac.uk/data_request/cif or on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.
[Fax: int. code +44-(1223)-336-033; E-mail: deposit@
ccdc.cam.ac.uk].
[20] L. Qiu, F. Y. Kwong, J. Wu, W. H. Lam, S. Chan, W.-Y.
Yu, Y.-M. Li, R. Guo, Z. Zhou, A. S. C. Chan, J. Am.
Chem. Soc. 2006, 128, 5955.
[21] J. Bµlint, G. Egri, E. Fogassy, Z. Bçcskei, K. Simon, A.
Gajµry, A. Friesz, Tetrahedron: Asymmetry 1999, 10,
1079.
[28] H. Lehner, K. Schlçgl, Monatsh. Chem. 1970, 101, 895.
[29] N. T. Patil, V. S. Raut, R. B. Tella, Chem. Commun.
2013, 49, 570.
[22] J. John, Ph.D. thesis, Brock University, 2014.
Adv. Synth. Catal. 2015, 357, 2071 – 2081
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2081