First X-ray structure of a N-naphthaloyl-tethered chiral dirhodium(II) complex: Structural basis for tether substitution improving asymmetric control in olefin cyclopropanation

Article abstract of DOI:10.1002/chem.200903231

Good-ee! The first one-pot olefin cyclopropanation that gives high ee is reported. The optimized catalyst is the 4-Br-substituted chiral paddle-wheel tetracarboxylatodirhodium(II) complex based on N-naphthaloyl-(S)-tert-leucinate. X-ray structural analysis of the parent catalyst reveals a square chiral crown cavity shrouding the axial coordination site (see figure) Figure Presentation

Full text of DOI:10.1002/chem.200903231

COMMUNICATION
DOI: 10.1002/chem.200903231
First X-ray Structure of a N-Naphthaloyl-Tethered Chiral Dirhodium(II)
Complex: Structural Basis for Tether Substitution Improving Asymmetric
Control in Olefin Cyclopropanation
Ashraf Ghanem,*[a] Michael G. Gardiner,[b] Rachel M. Williamson,[c] and Paul Mꢀller[d]
Dedicated to Professor Volker Schurig on the occasion of his 70th birthday
The transition-metal-catalyzed cyclopropanation of olefins
with diazoesters occupies a prominent position in the field
of asymmetric catalysis. This stems not only from being the
reaction in which asymmetric catalysis by transition metals
was first demonstrated, but also because of its prevalence in
natural-product-based and synthetic drugs.[1] Once this prin-
ciple was demonstrated, highly selective catalysts were de-
veloped, mainly based on CuI and RhII associated with ap-
propriate chiral nonracemic ligands.[2] The intermediacy of
metallocarbenes in transition-metal-catalyzed decomposition
of diazo precursors is firmly established.[1,2] In the case of
the paddle-wheel dirhodium(II) catalysts based on amino
acid derived ligands, enantioselectivity arises from the con-
certed control of the carbene-transfer step (metal to olefin)
offered by the chiral ligand set flanking an axial coordina-
tion site of the dirhodium complex. A recent report dis-
cussed the role of the chiral crown cavity of the well-known
Hashimoto catalyst [Rh2{(S)-pttl}4] (pttl=N-phthaloyl-(S)-
tert-leucinate) formed by the four N-phthaloyl units in this
regard, Scheme 1.[3a]
Scheme 1. Carbene-transfer step during styrene cyclopropanation with a
carbene generated from a diazoester mediated by [Rh2{(S)-pttl)4}].
Initially, diazoacetate esters were the preferred reagents
although phenyl- and vinyl-substituted diazoacetates were
found to be equally suitable carbene precursors when appro-
priately modified catalysts were used.[4] Even though diazo-
AHCTUNGTREGaNNNU cetates are not explosive, the same cannot be said for all
diazo compounds and, in addition to their toxic and carcino-
genic properties,[5] alternative carbene precursors have an
advantage when striving for very general routes to a broad
range of cyclopropane syntheses such that industrial utiliza-
tion is not problematic owing to serious safety risks.[6] We
have, therefore, investigated the possibility of in situ genera-
tion of such intermediates. Herein we report the synthesis of
a set of dirhodium(II) carboxylate catalysts containing naph-
thoyl skeletons that contain the protected amino acids (S)-
N-naphthoyl-tert-leucine, (S)-N-naphthoyl-phenyl alanine
and their 3- or 4-substituted naphthoyl derivatives. These
catalysts were used for the one-pot cyclopropanation of ole-
fins with CH acidic reagents through intermediate phenyl-
[a] Dr. A. Ghanem
Australian Centre for Research on Separation Science
University of Tasmania
Private Bag 75, Hobart, Tasmania, 7001 (Australia)
Fax : (+61)3-62292858
E-mail: aghanem@utas.edu.au
[b] Dr. M. G. Gardiner
School of Chemistry, University of Tasmania
Private Bag 75, Hobart, Tasmania, 7001 (Australia)
[c] Dr. R. M. Williamson
Australian Synchrotron, 800 Blackburn Rd, Clayton
Victoria, 3168 (Australia)
[d] Prof. P. Mꢀller
Department of Organic Chemistry
University of Geneva, Quai Ernest Ansermet 30
Geneva 4, 1211 (Switzerland)
AHCTUNGTREGiNNNU odonium ylides to afford cyclopropane derivatives in up to
98% ee. We also report the first X-ray structure of a N-
naphthaloyl-based catalyst, [Rh2{(S)-nttl}4] (nttl=N-naph-
thoyl-tert-leucine) as a di(ethyl acetate) adduct, which has
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903231.
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3291

provided a structural explanation for its exceptional capabil-
ities in asymmetric cyclopropanations[7] and provides evi-
dence for the enhanced performance of analogues in this
study that are substituted in the 3- and 4- positions.
An attempt to avoid the isolation of the diazo precursors
for carbene transfer has been described by Aggarwal, who
used thermal decomposition of sodium salts of tosylhydra-
zones in the presence of catalytic amounts of Rh2ACTHNUTRGEN(UNG OAc)4
and sulfide for the in situ generation and decomposition of
diazo compounds.[6] Among the various approaches tried by
us, carbene generation through a elimination in the pres-
ence of appropriate catalysts was not successful.[8] Although
the decomposition of diphenylsulfinum ylides in the pres-
ence of Cu and Rh catalysts afforded products resulting
from enantioselective carbene transfer, the yields of the re-
action were disappointing and the approach was aban-
doned.[9] The decomposition of phenyliodonium ylides was
more promising and we could show, by conducing selectivity
studies, that the decomposition of preformed ylides with
Rh[10] or Cu catalysts[11] proceeds through the same reactive
intermediate as that of the corresponding diazo precursor,
although secondary reactions that probably bypass metallo-
carbene intermediates may occur, in particular in intramo-
lecular cyclopropanation reactions.[12]
The main advantage in using phenyliodonium ylides re-
lates to the possibility of carrying out carbene transfer in a
one-pot procedure, in which the phenyliodonium ylide is
generated and decomposed in situ in the presence of a tran-
sition-metal catalyst. The first one-pot procedure for asym-
metric carbene transfer with copper catalysts was described
by Dauban and co-workers although their main interest cen-
tered on nitrene transfer.[13] At the same time, Du Bois and
co-workers developed a one-pot procedure for RhII-cata-
lyzed nitrene transfer through in situ generated phenyliodi-
nanes.[14] A one-pot procedure for RhII-catalyzed carbene
transfer was described by Charette and Wutz,[15] and inde-
pendently by us.[16,17] For the one-pot asymmetric cyclopro-
panation of olefins with Meldrumꢂs acid (7) or dimethyl
malonate (10), our previously described [Rh2{(S)-nttl}4] cata-
lyst 4a proved to be particularly suitable, Scheme 2.[18] This
catalyst uses tert-leucinate, which is derived from 1,8-naph-
thanic anhydride as the ligand. We were surprised to find
that the introduction of substituents into the 4-position of
the naphthalene ring (catalyst 4d) produced a significant
enantioselectivity enhancement that reached 98 and 82% ee
for pentene and styrene, respectively, when using dimethyl
malonate as the carbene precursor.[17] This encouraged us to
examine the effect of 3-substituents and further ligand varia-
tions, such as the use of phenylalaninate, to replace tert-leu-
cinate (Scheme 2a, catalysts 4a–f and 5a–f), and the effect
of such variation on the enantioselectivity of the cyclopropa-
nation reactions by using either 7 or 10 as the carbene pre-
cursors (Scheme 2b).
Scheme 2. a) Synthesis of the chiral ligands and catalysts. b) Asymmetric
cyclopropanation of olefins by using in situ generated ylides (MS=mo-
lecular sieves).
The intermediate phenyliodonium ylide 8 was generated
with PhIACTHNUTRGNENG(U OAc)2 for the reactions with 7, whereas PhI=O
was used in the reactions with 10. The efficiency of the cata-
lysts was examined in the cyclopropanation of styrene 6a,
Table 1.
Table 1. Asymmetric cyclopropanation of styrene 6a with 7 or 10 by
using substituted [Rh2{(S)-nttl}4] catalysts 4a–f.[a]
Catalyst (X) 9a Yield [%] 9a ee [%][b] 12a Yield [%] 12a ee [%][c]
4a (H)
87
60
66
73
45
62
45
56
59
92
54
69
72
59
77
75
43
60
37
46
66
82
37
66
4b (3-Cl)
4c (4-Cl)
4d (4-Br)
4e (3-NO2)
4 f (4-NO2)
[a] In CH2Cl2, at 308C, tenfold excess of styrene. [b] and [c] Absolute
configuration R.
In all cases that were investigated with the derivatized
tert-leucinate ligand, and for both 7 and 10 as reagents, the
introduction of substituents lead to an enantioselectivity en-
hancement. Substitution in position 4 produced a more se-
lective catalyst than in position 3. The 4-Br substituent lead
to the most selective catalyst with 92% ee for the cyclopro-
panation of styrene 6a with 7 and 82% ee with 10. The ab-
solute configuration of the cyclopropanes 9a and 12a was R
with all catalysts, as previously determined.[16,17]
The optimization of the in situ generation and decomposi-
tion of phenyliodonium ylides has been previously report-
ed.[16,17] Reactions were carried out in CH2Cl2 with a tenfold
excess of olefin in the presence of 5 mol% of the catalyst.
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N-Naphthaloyl-Tethered Chiral Dirhodium(II) Complex
COMMUNICATION
Still higher enantioselectivities were achieved for catalyst
[Rh2{(S)-4-Br-nttl}4] 4d with other olefins, Table 2. With 7,
the highest ee value of 92% was observed with styrene 6a,
Table 2. Asymmetric cyclopropanation of olefins 6a–f with 7 or 10 by
using [Rh2{(S)-4-Br-nttl}4] 4d.[a]
Olefin (R)
9x Yield [%] 9x ee [%] 12x Yield [%] 12x ee [%]
6a (Ph)
6b (4-Me-C6H4) 67
6c (4-CF3-C6H4) 62
6d (4-Cl-C6H4)
6e (4-Br-C6H4)
6 f (C3H5)
73
92 (R)
72 (R)
55 (R)
43 (R)
52 (R)
53 (R)
75
71
62
65
63
56
82 (R)
65 (R)
87 (R)
90 (R)
87 (R)
98 (S)
45
54
56
[a] In CH2Cl2, at 308C, tenfold excess of olefin.
but substituted styrenes reacted with lower ee. With 10, 4-
chlorostyrene 6d produced a higher ee (90%) than the
parent styrene 6a with 82% ee, but the highest ee was re-
corded for 1-pentene 6 f (98%). For comparison, styrene
and substituted-styrene cyclopropanation enantioselectivities
with dimethyl diazomalonate reported in the literature were
50% ee at the most,[19] until recently when Charette et al. re-
ported an excess up to 96% ee for cyclopropanes containing
germinal dicarboxy groups prepared by using diazo reagents
and utilizing the trans-directing ability of an amide.[7] How-
ever, the use of diazo reagents in such a reaction was un-
avoidable.
The substituent effects noted here on catalyst enantiose-
lectivity performance is intriguing. The substituents are situ-
ated far away from the reactive center, so that a polar sub-
stituent effect appears unlikely. The data presently available
do not allow a convincing explanation of the phenomenon.
Similar spectacular effects on enantioselectivity have previ-
ously been reported by Hashimoto et al. for the N-phthalo-
yl-(S)-tert-leucinate-based catalyst [Rh2{(S)-pttl}4] and ring
substituted analogues, in which enantioselectivity for nitrene
insertion increased from 27% for the parent to 70% for the
tetrachloro-substituted N-phthaloyl system.[20] However, in
this latter case four Cl substituents per ligand were intro-
duced, whereas in the case of the N-naphthaloyl ligand in
this study, only one substituent is present.
Unfortunately we have been unable to grow crystals of
4b–f that are suitable for X-ray crystallographic studies,
which might have provided a structural basis for these varia-
tions in selectivity. However, we have succeeded with the
structure determination of the unsubstituted catalyst 4a as a
di(ethyl acetate) adduct, Figure 1a.[21] The complex exhibits
the so-called a,a,a,a conformation[22] in the solid state with
all N-naphthaloyl units arranged to form a nearly square-
shaped cavity (14.9ꢃ16.5 ꢄ wide). The N-phthaloyl ana-
logue (as a mono ethyl acetate adduct) was recently shown
to adopt a similar chiral crown structural motif, though in
that case a substantially narrower cavity dimension (11ꢃ
15 ꢄ) was discussed as being influential in the selective cata-
lytic performance.[3a]
Figure 1. Molecular structure of [4aACHTNUGRTENUNG(EtOAc)2]. Thermal ellipsoid dia-
gram (a) and space-filling representations viewed into the square chiral
À
crown cavity down the Rh Rh bond (b, onto Rh1), and a side view (c).
EtOAc and p-stacked endo-cavity-residing N-phthaloyl units of adjacent
molecules are shown in stick form in (b and c). Selected bond lengths
À
À
[ꢄ] and angles [8]: Rh1 Rh2=2.3816(5), Rh Oeq =1.999(3)–2.059(3),
À
À
Rh1 O17=2.360(3),
Rh2 O19=2.280(4).
N-Naphthaloyl
plane
(N1,N2,N3,N4)/Rh1 equatorial plane=39.69(5), 57.94(6), 47.25(7),
52.32(6).
The wider N-naphthaloyl units of 4a relative to [Rh2{(S)-
pttl}4] (namely, edge-fused aryl rings) may be responsible for
this relatively subtle cavity shape change, as indeed could
the four N-naphthaloyl units of adjacent molecules that are
p-stacked within the cavity as a solid-state feature (two are
present in the N-phthaloyl case, one each associated with
the wider cavity sides).[3a] N-Naphthaloyl incorporation
maintains the chiral nature of the crown cavity surrounding
the axial Rh coordination site through the clockwise twist of
these groups, Figure 1b. If 4d is structurally similar to 4a,
the 4-Br substituents would lie at the cavity rim and are
thus likely to exert a strong influence on enantiofacial dis-
crimination of the incoming alkene during carbene transfer.
In this regard, we note the improved performance of the 4-
Br substituted catalyst 4d over the 4-Cl analogue 4c, in
which the larger halide would clearly be expected to exert
more influence at the cavity rim in the case of 4d. Interest-
Chem. Eur. J. 2010, 16, 3291 – 3295
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3293

A. Ghanem et al.
ingly, desymmetrization of the N-naphthaloyl units through
3- and 4-substitution could give rise to diastereomeric ro-
tamers, some of which may be preferred for the chiral
cavity, others may be ruled out on steric grounds, or others
may even be responsible for drastic conformational changes
in the complex such as a,a,a,a to a,a,b,b flipping (the latter
is known for a related phenylalaninate complex).[23] Such
issues may underlie the selectivities observed here with a
range of ligand substitution patterns.
vealed an a,a,a,a conformation with a reasonably square
chiral crown cavity formed by the N-naphthoyl units, which
serves as a model that accounts for the 4-Br substituted ana-
logue having greatly improved enantioselectivity through
the cavity rim steric impedance. This structural study will, in
the wider field of chiral dirhodium(II) catalyst usage,[25] help
in understanding the mechanistic insights of the asymmetric
cyclopropanations of olefins through carbene-transfer reac-
tions.
On the other hand, the enantioselectivity of the RhII cata-
lysts that contain a 1,8-naphthoyl-protected (S)-phenylala-
nate ligand (5a–f) were very disappointing, Table 3. With 7,
Experimental Section
Cyclopropanation with Meldrumꢁs acid: Dichloromethane (10 mL) was
added through a syringe into a round bottom flask (50 mL) containing a
mixture of 7 (10 mmol, 1 equiv), PhIACHTNUTRGENNUG(OAc)2 (1.4 equiv), [Rh2ACHTUNGTRENNUNG(OAc)4] or
Table 3. Asymmetric cyclopropanation of styrene 6a with 7 or 10 by
using substituted [Rh2{(S)-ntpa}4] catalysts 5a–5 f.[a]
Catalyst (X) 9a Yield [%] 9a ee [%][b] 12a Yield [%] 12a ee [%][c]
chiral rhodium(II) catalyst, (5 mol%), Al2O3 (2.3 equiv) and molecular
sieves 4 ꢄ (250 mg), followed by the addition of the olefin 6 (10 equiv).
The reaction mixture was heated in a thermostatted oil bath to 308C and
stirred under argon. Samples (100 mL) were taken after several time in-
tervals. The samples were filtered by using a syringe filter holder (0.2 mm
pore size) and the organic layer was diluted with dichloromethane or
ethyl acetate (100 mL) before being analysed by GC. The reaction prog-
ress was monitored qualitatively and quantitatively by GC–MS by using
dodecane as an internal standard. When maximum conversion was
reached (2–4 h), the reaction was terminated by filtration through celite.
The residue on the celite was washed twice with dichloromethane. Evap-
oration of the combined filtrates under reduced pressure followed by
chromatography on a silica-gel column with pentane/ethyl acetate (2:1 v/
v) as the eluent afforded the desired cyclopropane derivatives 9a–f. Cy-
clopropanation with dimethyl malonate was as previously described.[16]
5a (H)
75
59
40
70
40
45
30
25
19
43
15
35
45
29
47
45
43
40
15
16
26
31
12
18
5b (3-Cl)
5c (4-Cl)
5d (4-Br)
5e (3-NO2)
5 f (4-NO2)
[a] In CH2Cl2, at 308C, tenfold excess of styrene. [b] and [c] Absolute
configuration R.
the highest ee was 43% for cyclopropanation of styrene 6a,
which was obtained with the 4-Br substituted catalyst 5d.
The same catalyst also afforded the highest enantioselectivi-
ty (31%) of styrene 6a with 10. Results with other olefins
were equally disappointing. Thus, pentene was cyclopropa-
nated in the presence of the 4-Br substituted catalyst 5d
with 22 and 28% ee with 7 and 10, respectively (data not
shown). At this stage we cannot be certain of the grounds
for the poor relative performance of the phenylalaninate-de-
rived catalysts 5a–f in comparison with the tert-leucinate
series 4a–f. We note, however, the contrasting solid-state
structures of the well-known N-phthaloyl analogues based
on these amino acids, with the (S)-phenylalanate complex
exhibiting an a,a,b,b conformation lacking the crown
cavity.[23] If 5a–f also adopt non-crown conformations, the
positioning of the 4-Br substituents would still exert an in-
fluence on the trajectory of the olefin during carbene trans-
fer (indeed 5d performs best in this series), but the influ-
ence of all four 4-Br groups shrouding the rim of the chiral
crown cavity would not be present in this case.
Acknowledgements
This work was supported by the Swiss National Science Foundation
(Projects No. 20–52581.97 and 2027–048156). The support and sponsor-
ship from COST Action D24 “Sustainable Chemical Processes: Stereose-
lective Transition Metal-Catalysed Reactions” are kindly acknowledged.
Thanks to the Australian Government for an Endeavor Award to A.G.
X-ray data were obtained on MX1 at the Australian Synchrotron, Victo-
ria, Australia.[24]
Keywords: amino acids · crystal growth · cyclopropanes ·
homogeneous catalysis · rhodium
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N-Naphthaloyl-Tethered Chiral Dirhodium(II) Complex
COMMUNICATION
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N
ACHTUNGTRENNU[GN 4aACHTUNRTEGG(NNUN EtOAc)ACTHUNGTRENNUNG
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R=0.035 for 11845 [I> 2s(I)] data and wR=0.095 for 12361 all
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[25] Note added in proof: during the preparation of the proofs, a paper
from Charette et al. discussing similar finding on the chiral dirhodiu-
m(II) complexꢂs conformation was published: V. N. G. Lindsay, W.
Lin, A. Charette, J. Am. Chem. Soc. 2009, 131, 16383–16385.
Received: November 26, 2009
Published online: February 19, 2010
Chem. Eur. J. 2010, 16, 3291 – 3295
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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