Enantioselective synthesis of α-amino nitriles from N-benzhydryl imines and HCN with a chiral bicyclic guanidine as catalyst

Article abstract of DOI:10.1021/ol990623l

(Equation Presented) A novel catalytic enantioselective Strecker synthesis of chiral α-amino nitriles and α-amino acids is described and analyzed with regard to the possible mechanistic basis for stereoselectivity. Key features of the enantioselective process include (1) the use of the chiral bicyclic guanidine 1 as catalyst and (2) the use of the N-benzhydryl substituent on the imine substrate.

Full text of DOI:10.1021/ol990623l

ORGANIC
LETTERS
1
999
Vol. 1, No. 1
57-160
Enantioselective Synthesis of r-Amino
Nitriles from N-Benzhydryl Imines and
HCN with a Chiral Bicyclic Guanidine as
Catalyst
1
E. J. Corey* and Michael J. Grogan
Department of Chemistry and Chemical Biology, 12 Oxford Street,
HarVard UniVersity, Cambridge, Massachusetts 02138
corey@chemistry.harVard.edu
Received April 29, 1999
ABSTRACT
A novel catalytic enantioselective Strecker synthesis of chiral r-amino nitriles and r-amino acids is described and analyzed with regard to
the possible mechanistic basis for stereoselectivity. Key features of the enantioselective process include (1) the use of the chiral bicyclic
guanidine 1 as catalyst and (2) the use of the N-benzhydryl substituent on the imine substrate.
The classical Strecker synthesis of racemic R-amino acids
starting from aldehydes and ammonium cyanide (or equiva-
lent) is one of the simplest and most economical methods
drylimines (2), as outlined in Scheme 1 for the specific case
of the benzaldehyde imine.
1
of production. Consequently, enantioselective versions of
this process, which could lead to the practical production of
a wide range of (R)- or (S)-R-amino acids, have been of great
interest. Recently, several enantioselective catalysts for the
Strecker synthesis have been identified, including a chiral
Scheme 1
III
2a
2b
(
Salen)Al complex, a multifunctional salicylaldimine,
IV
2c
a Zr BINOL complex, and a guanidine-bearing diketo-
2
d
piperazine. In this paper we describe a novel catalytic
enantioselective Strecker reaction which utilizes the readily
2
available chiral C -symmetric guanidine 1 as a bifunctional
catalyst for the addition of HCN to achiral N-benzhy-
3
The reaction of imine 2a with HCN in the presence of
(
1) (a) Strecker, A. Ann. Chem. Pharm. 1850, 75, 27. (b) Duthaler, R.
O. Tetrahedron 1994, 50, 1539.
2) (a) Sigman M. S.; Jacobsen E. N. J. Am. Chem. Soc. 1998, 120, 5315.
b) Sigman, M. S.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, 4901. (c)
Ishitani, H.; Komiyama, S.; Kobayashi, S. Angew. Chem., Int. Ed. Engl.
998, 37, 3186. (d) Iyer, M. S.; Gigstad, K. M.; Namdev, N. D.; Lipton,
M. A. J. Am. Chem. Soc. 1996, 118, 4910.
4
1
0 mol % of 1 (synthesized as outlined in Scheme 3) in
(
(
(3) N-Benzhydrylimines were prepared by combining the aldehyde and
N-benzhydrylamine (5 mmol each) in benzene (5 mL) at 23 °C, removing
solvent in vacuo, filtering a benzene solution of the residue through activated
silica gel (0.2 g), and concentrating to the pure imines.
1
1
0.1021/ol990623l CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/24/1999

toluene at -40 °C over 20 h occurred cleanly to form the
R)-amino nitrile 3a in 96% yield and with 93:7 R/S
selectivity (86% ee). This reaction is quite general for
N-benzhydrylimines of aromatic aldehydes as shown by the
results which are collected in Table 1. The yields given in
poor (0-25%) enantiomeric purity. The N-(2,3:6,7-diben-
zocycloheptadienyl) analog of 2a afforded amino nitrile of
77% ee which is definitely below that with 2a itself.
Similarly, the placement of para substituents on the benz-
hydryl group of 2a resulted in products of somewhat reduced
(
3
enantiomeric excess (p-Me, OMe, Cl, and CF : 78, 70, 68,
and 59% ee, respectively, for the analogs of 3a).
The catalytic action of the bicyclic guanidine 1 can be
understood simply in terms of the mechanism for the
conversion of 2a to 3a which is outlined in Scheme 2. In
Table 1. Conversion of ArCHdNCH(C6H5)2 (2) to
ArCH(CN)NHCH(C6H5)2 (3) by Reaction with HCN in the Presence of
1
0 mol % of 1
2
a -j
Ar )
T( °C) t (h) % yield
% ee
3a -j
a
a
b
c
d
e
f
g
h
i
Ph
Ph
p-tolyl
3,5-xylyl
o-tolyl
4-t-Bu-Ph
4-TBSO-Ph
4-MeO-Ph
4-F-Ph
4-Cl-Ph
1-naphthyl
-40
-20
-40
-40
-20
-40
-40
-40
-40
-20
-20
20
8
96
99
96
96
88
80
98
99
97
88
90
86
82
80
79
50
85
88
84
86
81
76
(R)-3a
(R)-3a
(R)-3b
(R)-3c
3d
Scheme 2
20
16
12
72
38
28
23
20
12
3e
(R)-3f
(R)-3g
(R)-3h
(R)-3i
(R)-3j
j
5
Table 1 refer to isolated pure R-amino nitrile. The guanidine
catalyst 1 was easily separated from the crude reaction
mixture by extraction with oxalic acid and recovered for
5
reuse. The amino nitriles 3 upon heating at reflux with 6 N
HCl underwent benzhydryl cleavage and CN f COOH
conversion to form cleanly the corresponding (R)-aryl-
the first step of the cycle, HCN hydrogen bonds to the
catalyst 1, generating a guanidinium cyanide complex which
can serve as a hydrogen bond donor to the aldimine 2a
forming the pre-transition-state termolecular assembly shown
(4). Finally, attack by cyanide ion within the ion pair on the
hydrogen-bond-activated aldimine affords the Strecker prod-
uct (R)-3a. It is likely that the last step is rate-limiting since
hydrogen bond making and breaking are normally relatively
fast. It is also relevant that no kinetic H/D isotope effect on
the reaction rate was detected in experiments which com-
pared the velocity of the catalytic Strecker reaction with DCN
and HCN.
2d
glycines, the absolute configurations of which were deter-
mined by measurement of optical rotation. Enantiomeric
ratios were evaluated by HPLC analysis using chiral
5
columns. It is important to note that in the absence of
catalyst 1 there was no reaction between HCN and the
aldimines 2 in toluene at 10 °C or below and that N-methyl-1
is entirely inactive as a catalyst.
The N-benzhydryl subunit of the aldimine substrates 2 is
critical to the realization of good enantioselectivity. N-
Benzyl- or N-(9′-fluorenyl) aldimine analogs of 2 underwent
reaction with HCN in the presence of 10 mol % of 1 in
toluene at -10 to -20 °C to afford Strecker products of
The catalytic cycle which is summarized in Scheme 2
provides insights with regard to the origin of enantioselec-
tivity for the conversion of 2 to 3 in the presence of catalyst
(4) (3R,7R)-3,7-Diphenyl-1,4,6-triazabicyclo[3.3.0]oct-4-ene (1): color-
less solid, mp 159-160 °C; Rf 0.28 (1:10:90 NH4OH/MeOH/CH2Cl2);
1
and HCN. Modeling of the imine 2a hydrogen bonded to
23
[
R] D +23.8 (c 0.58, CHCl3); IR (thin film) 1452, 1492, 1674, 2829, 2926,
027, 3058, 3106, 3156, 3205 cm-1; H NMR (400 MHz, CDCl3) δ 7.37
1
the guanidinium in pre-transition-state assembly 4 indicates
the possibility of a three-dimensional arrangement depicted
in Figure 1 where the cyanide is positioned to attack the re
face of the imine carbon, a trajectory that leads to the
predominating enantiomer (R)-3a. A proximal phenyl group
of the catalyst can undergo π-stacking with one of the
benzhydryl phenyls, while the si face of the imine carbon is
blocked by the other phenyl of the benzhydryl group. At
the same time the aryl π-conjugated to the imine of 2a is
accommodated in a vacant quadrant of the guanidine face
and can experience van der Waals attractions with the
guanidine core and distal phenyl edge of catalyst 1. Rotation
of imine 2a by 180° about the H-N bond to expose the si
face to attack removes the van der Waals attractions and
3
(
d, 4 H, J ) 7.0 Hz), 7.32 (t, 4 H, J ) 7.1 Hz), 7.25 (t, 2 H, J ) 2.5 Hz),
6
.37 (bs, 1 H), 5.17 (t, 2 H, J ) 6.5 Hz), 3.50 (t, 2 H, J ) 7.8 Hz), 3.03
1
3
(
dd, 2 H, J ) 6.3, 7.8 Hz) ppm; C NMR (100 MHz, CDCl3) δ 169.3,
+
1
2
43.0, 128.5, 127.4, 126.4, 68.0, 57.2 ppm; CIMS 278(30) [M + NH4 ],
+
64(100) [M + H ].
(5) To a clear solution (0.2 M) of aldimine (0.18 mmol) and guanidine
1
(0.018 mmol) in toluene under N2 was added liquid HCN (0.36 mmol)
via a precooled 50-µL gastight syringe. Upon consumption of starting
material as indicated by TLC analysis, the reaction mixture was concentrated
in vacuo, acidified by addition of oxalic acid (0.018 mmol) in water (5
mL), and extracted with ether (3 × 15 mL). The combined extracts were
washed with brine, dried over MgSO4, and concentrated to give amino nitrile
3
that could be further purified by silica gel chromatography (1% ethyl
acetate-hexanes). Basification with 1 N NaOH of the oxalic acid wash
and extraction with ethyl acetate and concentration allows recovery of 1
(
80-90% yield). Enantioselectivity was analyzed by chiral HPLC with
Chiralpak AD, Chiralpak AS or Chiralcel OJ columns with 2-propanol-
hexanes as eluent.
158
Org. Lett., Vol. 1, No. 1, 1999

Figure 1. Pre-transition-state assemblies for the Strecker reactions of N-benzhydryl benzaldimine (4) and N-benzhydryl pivalaldimine (5).
introduces steric repulsion between the π-conjugated aldi-
mine aryl and the proximal phenyl of catalyst 1.
assembly 5 where the aldimine alkyl group contacts the
proximal phenyl of the catalyst and one benzhydryl phenyl
occupies a vacant quadrant of the guanidine face, as shown
in Figure 1. The inversion of product configuration from R
for aromatic imines to S for aliphatic imines indicates that
alkyl groups incur steric repulsions in the vacant quadrant
of guanidine 1 where an imine aryl or a benzhydryl phenyl
gains van der Waals attractions.
This working hypothesis receives support from studies of
the Strecker reaction between HCN, catalyst 1, and the
enantiomeric imines from benzaldehyde and (S)-1-phenethyl-
amine and (R)-1-phenethylamine. The former affords the
Strecker product (toluene, -20 °C, 12 h) with the (R)
configuration R to cyano with 93.5:6.5 R/S diastereoselec-
tivity, whereas the latter gives under the same conditions
the Strecker product with only 58:42 R/S diastereoselectivity.
Only in the pre-transition-state assembly derived from the
former can the two phenyl groups of the imine interact with
the catalyst in a manner similar to that shown in 4 in Figure
4
The bicyclic guanidine 1 was prepared from (R)-phen-
ylglycine by a route paralleling that previously utilized for
the synthesis of the bis-cyclohexyl (dodecahydro) analog
6
(
Scheme 3). Methyl (R)-phenylglycinate was converted to
the trityl-protected diamine 6 by amide formation in am-
monia-saturated methanol (23 °C, 24 h), tritylation (TrCl,
TEA), and lithium aluminum hydride reduction in ether at
reflux (79% yield for three steps). Coupling of 6 to (R)-N-
1. The deleterious effects of replacing benzhydryl in 2a by
N-benzyl or N-(9′-fluorenyl) (see above) are also consistent
with the three-dimensional model of the pre-transition-state
assembly shown in Figure 1. Finally, it should be noted that
(benzyloxylcarbonyl)phenylglycine (dicyclohexylcarbodiim-
6
the use of the bis-cyclohexyl (dodecahydro) analog of
ide, 1-hydroxybenzotriazole) yielded amide 7 (77% yield)
which was transformed to triamine 8 by carbamate hydro-
genolysis and amide reduction with sodium bis(2-methoxy-
ethoxy)aluminum dihydride (76% yield). Treatment of 8 with
thiophosgene and aqueous sodium carbonate effected ring
catalyst 1 instead of 1 leads to conversion of 2a to 3a with
only 56% enantiomeric excess.
The use of Schiff bases of benzhydrylamine with aliphatic
aldehydes in the Strecker process catalyzed by 1 also resulted
in efficient formation of R-amino nitriles. Thus with aldi-
mines from pivalaldehyde, cyclohexanecarboxaldehyde, and
n-heptanal, (S)-Strecker products were formed (toluene, -40
2
closure between NH and the proximal NH to form the
corresponding thiourea (95% yield) that was S-methylated
with iodomethane to generate the S-methylisothiouronium
iodide. When this compound was heated in DMF at 100 °C
guanidine 1 was formed (55% yield after basic workup and
silica gel chromatography). The three-dimensional structure
of the benzoate salt of 1 was determined by X-ray crystal-
°
C, 22 h) in ∼95% yield with ee values of 84, 76, and 63%,
respectively. In terms of a mechanistic model, the (S)
selectivity for aliphatic imines suggests a pre-transition-state
(6) Corey, E. J.; Ohtani, M. Tetrahedron. Lett. 1989, 30, 5227.
Org. Lett., Vol. 1, No. 1, 1999
159

1
Despite this distorted solid-state conformation, H NMR
spectra in CDCl at 23 °C are consistent with time averaged
-symmetry for bicycle 1.
In summary, bicyclic guanidine 1 is shown to be an
3
Scheme 3
C
2
effective catalyst for the asymmetric synthesis of R-amino
nitriles and R-amino acids, providing the first clear demon-
stration of a highly enantioselective catalyst for which the
guanidine functionality is alone sufficient for activity. The
mechanism proposed herein suggests a cause for the efficacy
of the Strecker catalyst described by Lipton et al.,2d
a
guanidine-bearing diketopiperazine, and provides a model
for developing new enantioselective methods with guanidine
catalysts. The mechanistic explanations of enantioselectivity
implied in Figure 1 depend on van der Waals attractive
interactions involving nonpolar groups as a source of three-
dimensional ordering in the transition state for reaction. Thus,
the examples described herein add to a small but growing
list of enantioselective processes thought to involve such
a
b
NH
3
, MeOH, 23 °C, 24 h, 86%. TrCl (1 equiv), TEA, CH
2
Cl
2
,
c
d
2
3 °C, 1 h, 97%. LiAlH
phenylglycine, DCC, HOBt, THF, 0 °C, 8 h, 77%. H ,10% Pd/C,
:1 THF/MeOH, 23 °C, 6 h. Red-Al (5 equiv), PhH, reflux, 2.5
h, 76%. Thiophosgene (1.05 equiv), Na
C, 15 min, 95%. h MeI (3 equiv), MeOH, 50 °C. DMF,100 °C,
h, 55% for two steps.
4 2
, Et O, reflux, 35 h, 95%. (R)-Cbz-
e
8
2
interactions.
f
1
g
2 3 2 2 2
CO , 1:1 CH Cl /H O, 0
Acknowledgment. This work was supported by a grant
from the National Institutes of Health and a predoctoral
fellowship for M.J.G. from Eli Lily and Co. We thank Dr.
Richard Staples for assistance with X-ray crystallography.
i
°
2
lographic analysis. The nitrogen at the 5/5 fusion (N-1) is
Supporting Information Available: Full experimental
procedures for synthesis and X-ray crystallographic analysis
of 1. This material is available free of charge via the Internet
at http://pubs.acs.org.
2
12-15° pyramidalized from planar sp , the N-C-N angle
involving the other two nitrogens is 131°, and the benzoate
7
ion is doubly hydrogen bonded with the guanidinium ion.
OL990623L
(
7) X-ray data for 1‚PhCOOH is available from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ,
U.K.: C24H24N3O2; orthorhombic; P2(1)2(1)2(1); a ) 8.7909(3) Å;
b ) 10.4417(4) Å; c ) 22.3969(3) Å; R ) â ) γ ) 90°; · ) 6; T ) 213
K; R1[I > 2σ(I)] ) 0.0416.
(8) See for example: (a) Corey, E. J.; Noe, M. C. J. Am. Chem. Soc.
1996, 118, 11038. (b) Corey, E. J.; Xu, F.; Noe, M. C. J. Am. Chem. Soc.
1997, 119, 12414.
160
Org. Lett., Vol. 1, No. 1, 1999