Structural origins of a dramatic variation in catalyst efficiency in enantioselective alkene aziridination: Implications for design of ligands based on chiral biaryldiamines

Full text of DOI:10.1021/ja994353d

7132
J. Am. Chem. Soc. 2000, 122, 7132-7133
Structural Origins of a Dramatic Variation in
Catalyst Efficiency in Enantioselective Alkene
Aziridination: Implications for Design of Ligands
Based on Chiral Biaryldiamines
Christopher J. Sanders,† Kevin M. Gillespie,† David Bell,‡ and
Peter Scott*,†
Department of Chemistry, UniVersity of Warwick
CoVentry, CV4 7AL, UK
SmithKline Beecham Pharmaceuticals
Harlow, Essex, CM19 5AW, UK
Figure 1. Molecular structure of 1 (hydrogen atoms and triflate
counterions omitted). The structure of 2 is closely related (see text).
ReceiVed December 13, 1999
C2-symmetric biaryl ligands, such as BINAP and its analogues,
have found application in many highly enantioselective metal-
catalyzed reactions, such as hydrogenation,1 Diels-Alder addi-
tion,2 and cyclopropanation.3 The success of these ligands is due
not least to the fact that the axial chirality of the ligand is very
well expressed in the steric environment of the active site and
also that the biaryl unit provides structural rigidity; substrate and
reagent are thus brought together at the metal center in highly
ordered circumstances. We were thus somewhat surprised at Itoh’s
observation4 that diiminobiaryl ligand L1 in conjunction with CuII
salts gave essentially no enantioselectivity in the catalytic
aziridination of alkenes: a reaction which has been moderated
by bis(oxazoline)5 and diiminocyclohexane6 complexes of the
same metal to ee >90%. In this paper we describe the structural
basis for this anomalous behavior and the subsequent design of
greatly improved catalysts.
plexes containing the ions [Cu2L2]2+, for example, [Cu2(L2)2]-
[OTf]2 1 and [Cu2(L3)2][OTf]2 2. X-ray crystal structures of both
complexes were determined,8 and the molecular structure of 1 is
shown in Figure 1. The two ligands L2 in each molecular unit
have the same relative configuration and form a double-helical
array about two linear 2-coordinate Cu+ centers. Despite this low
coordination number5c it is clear from space-filling models that
the approach of a further ligand would be severely sterically
hindered. One of the most striking features of the structure is the
presence of close edge-face interactions of the tert-butyl phenyl
rings, which make a contribution to the stability of the supramo-
lecular structure. The phenomenon of non-covalent interactions
between arene rings is well documented,9 and for edge-face
interactions the distances between the two ring centroids are found
in the range 4.5-7.0 Å,10 with a theoretical optimum distance of
∼5.2 Å.11 In 1 the centroid-centroid distances are .∼4.99 Å. The
structure of 2 is similar to that of 1 with the exception that in
this case the 2-naphthyl rings are aligned face-face with
centroid-centroid distances of ∼3.7-3.8 Å. Again, this appears
to be optimum.11 FAB mass spectra of dichloromethane solutions
of these complexes show strong m/z peaks for the species [Cu2L2]-
[OTf]+ and [Cu2L2]+, but interestingly no peaks appearing to arise
from a species [CuL]+. This suggests very strongly that the major
species in solution are the same as those in the solid state, that
is, the bimetallics.
A range of ligands L2-L7 were synthesized in a straightforward
manner by condensation of 2,2′-diamino-6,6′-dimethylbiphenyl7
and aromatic aldehydes. Reactions of racemic L2 and L3 with
To reduce the possibility of non-covalent arene-arene interac-
tions and thus promote formation of monomeric complexes,3a,5,6
proligands with 2,6-disubstituted iminoarene groups were inves-
tigated, for example, L4 and L.5 The reactions of these compounds
with [Cu(CH3CN)4]BF4 followed by recrystallization from dichlo-
romethane gave [Cu(L4)(CH3CN)2]BF4 3 and [Cu(L5)(CH3CN)2]-
BF4 4 respectively. X-ray crystal structures of both complexes
(e.g., 3, Figure 2) showed that they adopt monometallic C2-
symmetric structures in the solid state. FAB mass spectra of
dichloromethane solutions of these compounds gave no peaks of
higher mass than the monometallic molecular ion [CuL(CH3-
CN)2]+. Complexes of L4 and L5 derived from [{CuOTf}2(C6H6)],
that is, [CuL4(OTf)] and [CuL5(OTf)2], behave similarly.
[CuI(CH3CN)4]BF4 or with [{CuIOTf}2(C6H6)] followed by re-
crystallization from dichloromethane unexpectedly gave com-
* Author for correspondence: Telephone: +44 2476 523238. Fax: +44
2476 572710. E-mail: peter.scott@warwick.ac.uk.
† University of Warwick.
‡ SmithKline Beecham Pharmaceuticals.
(1) (a) Takaya, H.; Ohta, T.; Noyori, R. In Catalytic Asymmetric Synthesis;
Ojima, I., Ed.; VCH: Weinheim, 1993. (b) Noyori, R. Asymmetric Catalysis
in Organic Synthesis; John Wiley and Sons: New York, 1994.
(2) (a) Corey, E. J.; Imwikelreid, R.; Pikul, S.; Xiang Y. B. J. Am. Chem.
Soc. 1989, 111, 5493. (b) Chapuis, C.; Jurczak, HelV. Chim. Acta 1987, 70,
437. (c) Narasaka, K. Synthesis 1991, 1, 1. (d) Kagan, H. B.; Riant, O. Chem.
ReV. 1992, 92, 1007.
(3) (a) Gant, T. G.; Noe, M. C.; Corey, E. J. Tetrahedron Lett. 1995, 36,
8745. (b) Uozomi, Y.; Kyota, H.; Kishi, E.; Kitayama, K.; Hayashi, T.
Tetrahedron:Asymm. 1996, 8, 1603. (c) Suga. H.; Fudo, T.; Ibata, T. Synlett
1998, 933.
(4) Shi, M.; Itoh, N.; Masaki, Y. J. Chem. Res. M 1996, 1946.
(5) (a) Evans, D. A.; Woerpel, K. A.; Hinman, M. M.; Faul, M. M. J. Am.
Chem. Soc. 1991, 113, 726. (b) Evans, D. A.; Faul, M. M.; Bilodeau, M. T.
J. Org. Chem. 1991, 56, 6744. (c) Evans, D. A.; Woerpel, K. A.; Scott, M. J.
Angew. Chem., Int. Ed. Engl. 1992, 31, 430. (d) Evans, D. A.; Faul, M. M.;
Biodeau, M. T.; Anderson, B. J.; Barnes, D. M. J. Am. Chem. Soc 1993, 115,
5328.
(8) Crystal data for 1: C75H82Cl2Cu2F6N4O6S2, triclinic, P1, a )
11.4466(6) Å, b ) 18.1140(9) Å, c ) 19.5085(6) Å, R ) 69.3870(10)°, â )
78.2460(10)°, γ ) 82.4430(10)°, U ) 3698.7(3) Å3, Z ) 2, Dc ) 1.357 g
cm-3, T ) 180(2) K, λ(Μï ΚR) ) 0.71073 Å. Final R indices [for 16082
reflections with I > 2σ(I)]: R1 ) 0.0536, wR2 ) 0.1247. GOOF on F2
)
1.070. Crystal data for 3: C34H26BCl8CuF4N4, orthorhombic, Pbca, a )
13.2095(5) Å, b ) 23.9347(5) Å, c ) 25.5398(10) Å, U ) 8074(5) Å3, Z )
8, Dc ) 1.521 g cm-3, T ) 183(2) K, λ(Μï ΚR) ) 0.71073 Å. Final R indices
[for 5276 reflections with I > 2σ(I)]: R1 ) 0.0895, wR2 ) 0.2640. GOOF
on F2 ) 2.131. Data were collected on a Siemens SMART CCD. The structures
were solved by direct methods with additional light atoms found by Fourier
methods.
(6) (a) Li, Z.; Conser, K.; Jacobsen, E. N. J. Am. Chem. Soc. 1993, 115,
5326. (b) Li, Z.; Quan, R. W.; Jacobsen, E. N. J. Am. Chem. Soc. 1995, 117,
5889. (c) Quan, R. W.; Li, Z.; Jacobsen, E. N. J. Am. Chem. Soc. 1996, 118,
8156.
(9) Nishio, M.; Umezawa, Y.; Hirota, M.; Takeuchi, Y. Tetrahedron 1995,
51, 8665.
(10) Burley, S. K.; Petsko, G. A. Science 1985, 229, 23.
(11) Jorgenson, W.; Severance, D. L. J. Am. Chem. Soc. 1990, 112, 4768.
(7) Meisenheimer, J.; Horing, M. Berichte 1927, 60, 1425.
10.1021/ja994353d CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/11/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 7133
accordance with Itoh,4 very poor (entries 1, 2). The monometallic
precatalysts 3 and 4 were strikingly more efficient. Dissolution
of PhINTs was complete in minutes, isolated yields were good,
and the product ee was dramatically improved (entries 3, 4).14
The ligands L6 and L7 are expected on the basis of steric effects
to form monometallic and bimetallic complexes, respectively. The
corresponding high and low efficiencies were obtained in aziri-
dination (entries 5, 6).15
Essentially linear plots were obtained for eeligand versus eeproduct
for L2 and L,4 thus excluding the possibility that some extreme
non-linear effect might be responsible for the dramatic variation
in catalyst performance.16 We have also found that the enantio-
meric excess of the product obtained does not vary significantly
with % conversion. These results are consistent with the presence
of a catalyst which contains one ligand only and whose nature
does not change during the course of the reaction. In the case of
the predominantly bimetallic precatalysts, the low turnover
number is probably associated with the fact that the corresponding
active monometallic species [LCu]+ is in low concentration.17
Figure 2. Molecular structure of 3 (hydrogen atoms and triflate
counterion omitted). The structure of 4 is very similar.
It remains however that the catalysts giving low rates also give
low enantioselectivities. The 2,6-substituents in the most suc-
cessful ligands (L3 to L5) thus appear to have a dual role. First,
they discourage the formation of catalytically inactive L2M2
bimetallics; an observation that has implications for the design
of other ligand systems based on biaryldiamines. Second, they
are critically important in determining the steric and electronic
profile of the active site and thus the enantioselectivity, perhaps
via modulation of substrate-ligand arene-arene interactions.6c
We note in this context that the most successful ligands in
Jacobsen’s diiminocyclohexane6a and Suga and co-workers’
binaphthyldiimine catalysts for cyclopropanation3c incorporate 2,6-
disubstituted arene groups.
Scheme 1. Catalytic Enantioselective Synthesis of Aziridine 6
Table 1. Enantioselective Aziridination of Chromene 5
isolated
yield
time ee
entry ligand
Cu sourcea
temp of 6 (%) (min)b (%)
1
2
3
4
5
6
7
8
9
L2
L3
L4
L5
L6
L7
L4
L4
L4
[CuI(CH3CN)4]BF4
[CuI(CH3CN)4]BF4
[CuI(CH3CN)4]BF4
[CuI(CH3CN)4]BF4
[CuI(CH3CN)4]BF4
[CuI(CH3CN)4]BF4
rt
rt
rt
rt
rt
rt
53
75
83
79
56
0
85
80
87
900 13
900 16c
< 10 86
< 10 55
< 10 65
Acknowledgment. P.S. thanks EPSRC for a postdoctoral fellowship
(K.M.G.) and SmithKline Beecham for a CASE award (C.J.S.). We thank
EPSRC and Siemens Analytical Instruments for grants in support of the
diffractometer.
900
-
[CuII(CH3CN)4][BF4]2 rt
<10 85
300d 94
300d 99
[CuI(CH3CN)4]BF4
-40 °C
Supporting Information Available: Complete experimental proce-
dures and characterizing data for all ligands and complexes, chiral HPLC
traces for 6, plots of eeligand vs eeproduct for L2 and L4 and crystal data for
1-4 (PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
[CuII(CH3CN)4][BF4]2 -40 °C
a Use of [(CuI2(OTf)2].(C6H6) in place of [CuI(CH3CN)4]BF4 did not
affect the results significantly. b Time until all PhINTs had dissolved
c Opposite enantiomer to other ligand systems d The rate of the low
temperature reactions is probably limited by the rate of dissolution of
PhINTs at -40 °C.
JA994353D
(13) So¨dergren, M. J.; Alonso, D. A.; Bedekar, A. V.; Andersson, P. G.
Tettrahedron Lett. 1997, 38, 6897.
The complexes of chiral non-racemic L2-L5 (ee 99.8%) were
tested as catalysts for aziridination of the chromene 5 under
standard5d conditions (Scheme 1, Table 1).12 In the case of the
bimetallic precatalysts 1 and 2, dissolution of the sparingly soluble
trivalent iodinane nitrene source PhINTs13 was not complete after
900 min, and the enantiomeric excess of the product was, in
(14) Similar variations in efficiency were observed for other alkenes
including styrenes and cinnamate esters. For example, L4/Cu(I) catalyzed the
aziridination of trans-ethyl cinnamate to N-p-toluenesulphonyl-2-carboethoxy-
3-phenylaziridine in 75% ee and 63% yield (unoptimised) within minutes at
room temperature, while L3/Cu(I) gave 40% ee and 45% yield overnight.
(15) Rates and enantioselectivities in this reaction are not significantly
affected by substitution of the Cu(I) sources for Cu(II) e.g., [CuII(CH3CN)4]-
[BF4]2 (entry 7). See refs 5, 6. Lowering the temperature to -40 °C led to a
further improvement in enantioselectivity (entries 8, 9).
(12) Typical procedure: [Cu(CH3CN)4]BF4 (0.05 mmol) or [Cu2(C6H6)]-
[OTf]2 (0.025 mmol) were placed in a 25 mL round-bottom flask with
proligand (0.055 mmol) and dichloromethane (10 mL) was added under an
atmosphere of argon. The mixture was stirred for 10 min before chromene 5
(5.0 mmol) was added. The solution was stirred for a further 20 min before
PhINTs (1.0 mmol) was added in one batch. After the specified interval the
solution was filtered through a plug of silica gel, and the product was separated
from excess chromene using flash chromatography on silica gel using hexane:
ethyl acetate (6:1). Enantiomeric excess was readily determined using chiral
HPLC (Chiralcel OD).
(16) Kitamura, M.; Suga, S.; Oka, H.; Noyori, R. J. Am. Chem. Soc. 1998,
120, 9800.
(17) If a significant proportion of the turnover in the bimetallic systems
was being mediated by achiral catalysts arising directly from [CuI(CH3CN)4]-
BF4 or [{CuIOTf}2(C6H6)], we would expect, on the basis of control
experiments, the turnover rates to be very much lower. The recent synthesis
of soluble PhINTs analogues will allow us to make direct kinetic investigations
in this system: Macikenas, D.; Skrzypczak-Jankun, E.; Protasiewicz, J. D. J.
Am. Chem. Soc. 1999, 121, 7164.