Diastereo- and enantioselective intramolecular C(sp3)-H arylation for the synthesis of fused cyclopentanes

Article abstract of DOI:10.1002/chem.201200018

All C-H bonds are not equal: The intramolecular arylation of unactivated C(sp³)-H bonds in the presence of a chiral Pd/binepine catalyst allows the synthesis of fused cyclopentanes efficiently and in an diastereo- and enantioselective manner (see scheme).

Full text of DOI:10.1002/chem.201200018

DOI: 10.1002/chem.201200018
3
À
Diastereo- and Enantioselective Intramolecular CACTHNUTRGNE(NUG sp ) H Arylation for the
Synthesis of Fused Cyclopentanes
Nicolas Martin, Cathleen Pierre, Michaꢀl Davi, Rodolphe Jazzar, and Olivier Baudoin*[a]
Transition-metal catalysis has recently emerged as a pow-
2
À
erful tool to functionalize otherwise unreactive C
N
3
[1–2]
À
and C
N
In this context, recent research
from our group[3] and others[4] has focused on the intramo-
3
À
lecular arylation of unactivated C
(sp ) H bonds to aryl hal-
ides under palladium(0) catalysis, giving rise to original pol-
ycylic molecules.[5] To date, very few examples of catalytic
enantioselective reactions involving the organometallic acti-
3
À
vation of unreactive
C
(sp ) H bonds have been de-
scribed.[6–9]
While our own work in this area was in progress, the syn-
thesis of (fused) indolines by palladium(0)-catalyzed enan-
3
À
tioselective intramolecular C
G
by the groups of Kꢀndig[8a] and Kagan,[8b] who reported
enantiomeric ratios (e.r. values) up to 97.5:2.5 by using
chiral N-heterocyclic carbenes (NHCs) and diphosphines as
the ligands, respectively (Scheme 1a). Herein, we report our
preliminary results on the synthesis of indanes by use of
a similar type of reaction (Scheme 1b). This substrate type
raises an additional stereochemical challenge, that is, the
presence of a quaternary benzylic carbon atom on the reac-
tion substrate (instead of a nitrogen atom) that bears both
Scheme 1. Palladium(0)-catalyzed enantioselective intramolecular
3
À
C
N
À
diastereotopic and enantiotopic C H bonds. In other words,
so far the major cis diastereoisomer 2a could not be formed
without significant proportions of 3a or 4a. For the asym-
metric version of this reaction, we tested as a priority chiral
ligands that gave good enantioselectivities in related pro-
cesses (Table 1). We initially employed screening conditions
that were previously optimized with achiral ligands, that is, 5
mol% PdACTHNURTGENNG(U OAc)2 as the precatalyst, 20 mol% of the phos-
phine ligand, and K2CO3 as the base, in DMF at 1408C for
14 h. We first checked for a possible background reaction
with PdACTHNGUETRNNU(G OAc)2 alone, which gave an 8% yield of 2a+3a
under the same conditions (Table 1, entry 1). As a conse-
quence, there should be little impact of this racemic back-
ground reaction on the enantioselectivity observed with
chiral ligands at this temperature. (R)-BINAP (L1) was
evaluated first, and gave indane 2a in moderate yield and
good diastereoselectivity, but almost no enantioselectivity
(Table 1, entry 2). (R,R)-Me-DUPHOS (L2), which gave
good enantioselectivities in the related synthesis of indoli-
nes,[8b] furnished racemic 2a in low yield (Table 1, entry 3).
Josiphos (L3, Cy=cyclohexyl),[10] with different phosphorus
substituents, gave similarly disappointing results (Table 1,
both high diastereo- and enantioselectivity must be achieved
to develop an efficient synthesis of the target carbocycles.
The reaction of aryl bromide 1a in the presence of chiral
ligands was studied extensively and Table 1 shows represen-
tative examples (see the Supporting Information for addi-
tional examples). As shown previously,[3b,e] compound 1a
may give rise to four different products by intramolecular
3
À
C
(sp ) H arylation, that is, both diastereoisomers of the de-
sired indane (2a and 3a), olefin 4a (also arising from
(sp ) H activation), and protodebromination product 5a.
It has already been shown that the structure of the (achiral)
ligand has a profound impact on the product selectivity, but
3
À
C
U
[a] Dr. N. Martin, C. Pierre, Dr. M. Davi, Dr. R. Jazzar,
Prof. Dr. O. Baudoin
Universitꢁ Claude Bernard Lyon 1, CNRS UMR 5246
Institut de Chimie et Biochimie Molꢁculaires et Supramolꢁculaires
CPE Lyon, 43 Boulevard du 11 Novembre 1918
69622 Villeurbanne (France)
entry 4), but its diACHTNUTRGNE(NUG PCy2) analogue L4 proved much more ef-
E-mail: olivier.baudoin@univ-lyon1.fr
ficient, providing an e.r. of 76:24 (Table 1, entry 5).[11] De-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200018.
creasing the temperature to 1108C with this ligand did not
4480
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4480 – 4484

COMMUNICATION
Table 1. Analysis of reaction parameters.
Pd precatalyst
([mol%])
L*
([mol%])
Base
Additive
(30 mol%)
Solvent
T
[8C]
2a/3a/4a/5a[a]
d.r.[a]
28:1
12:1
23:1
17:1
7:1
Yield of
e.r. of
G
E
G
2a+3a [%][b]
2a[c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
N
–
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
K2CO3
K2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
mesitylene
DMSO
DMSO
DMSO
DMSO
140
140
140
140
140
110
110
140
140
140
140
140
140
140
110
110
140
140
140
140
140
160
140
140
140
140
27.5:1:17:8
12:1:3:1
22.5:1:6.5:4
17:1:7:2
9.5:1.5:2:1
14:2:1:1
122:3:24:1
42:2:12:1
5.5:0:1:1
1.5:0:2.5:1
36:3.5:12:1
5:0:2:1
41:6:1:1
30:4:4:1
8 (18)
57
31
36
63
63
29 (47)
74
53
12 (86)
74
24 (64)
87
88
82
89
43
24
74
81
–
L1 (20)
L2 (20)
L3 (20)
L4 (20)
L4 (20)
–
52.5:47.5
50:50
52:48
76:24
75.5:24.5
–
50:50
50:50
n.d.
8:1
43:1
21:1
>98:2
>98:2
10:1
37:1
7:1
L5 (20)
L6 (20)
L7 (20)
L8 (20)
L9 (20)
L10 (20)
L11 (20)
L10 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L11 (20)
L13 (10)
L13 (10)
L11 (20)
–
53:47
n.d.
74:26
71.5:28.5
75.5:24.5
78.5:21.5
n.d.
8:1
8:1
8:1
51:6:1:1
84:11:3:1
30:1.5:31:1
19:1:46:1
12.5:1:3:0
159:28:3:1
90:3:15:1
19:2:16:1
46.5:2.5:1:1
1.5:0:1:0
T
–
–
–
20:1
19:1
12:1
6:1
29:1
11:1
20:1
n.d.
7:1
A
U
n.d.
Pd
Pd
Pd
N
61:39
78.5:21.5
66:34
69:31
79:21
–
PivOH
PivOH
56
39 (88)
84
2 (33)
75
88
[d]
A
U
tBuCO2Cs[d]
Pd
Pd
Pd
Pd
N
–
–
–
–
L12 (20)
L11 (10)
113:16:3:1
75.5:3:2:1
81:19
80:20
24:1
1
[a] Ratio of products estimated by GCMS analysis. The diastereomeric ratio (d.r.) of 2a/3a was almost identical (Æ5%) from GC and H NMR analysis.
[b] GC yield calculated by using tetradecane as the internal standard. The conversion was 100% in all cases unless otherwise provided (in parentheses).
[c] Determined by 19F NMR analysis of the Mosher amides prepared after reduction of 2a to the primary amine (see the Supporting Information). n.d.=
not determined. [d] With Cs2CO3 (1.5 equiv) and tBuCO2Cs (1 equiv).
seem to significantly affect the yield, d.r., or e.r. (Table 1,
entry 6).
However, we discovered that there was significantly more
of the background reaction at 1108C than at 1408C (Table 1,
entry 7), and thus this might affect the selectivities observed
with L4 at this temperature. In light of this observation, the
ligand screening was continued at 1408C to minimize the
possible contribution of the “ligand-free” reaction. We next
evaluated (S)-KenPhos (L5),[12] which has previously given
3
À
e.r. values of up to 75:25 in intermolecular C
G
tion reactions (Table 1, entry 8).[9] Disappointingly, L5 gave
a good yield and d.r., but no enantioselectivity. We next
Chem. Eur. J. 2012, 18, 4480 – 4484
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4481

O. Baudoin et al.
evaluated phosphoramidites L6–8,[13] which have furnished
high enantioselectivities in related Pd0-catalyzed intramolec-
2
[14]
À
ular CACHTUNGTRENNUNG(sp ) H arylation reactions (Table 1, entries 9–11).
However, these ligands did not give appreciable enantiose-
lectivities in this reaction.
We then turned our attention to the binepine family of li-
gands L9–12.[15–16] Indeed, these ligands should possess the
most similar electronic properties to trialkylphosphine li-
gands, which gave optimal results in the racemic reaction.[3e]
Gratifyingly, although binepine L9 was barely active
(Table 1, entry 12), ligands L10 and 11, bearing cyclohexyl
and tert-butyl P substituents, gave e.r. values above 70:30
(Table 1, entries 13 and 14), with a higher yield than Josi-
phos (L4; compare with Table 1, entry 5). The e.r. value
could be further improved by decreasing the temperature to
1108C (Table 1, entries 15 and 16), but we encountered
problems with reproducibility that we attributed to the
“ligand-free” background reaction (Table 1, entry 7).
As a consequence, more reliable conditions were sought
with ligand L11, by first varying the nature of the Pd preca-
talyst (Table 1, entries 17 and 18), then the base (Table 1,
entry 19), and finally by adding pivalic acid (PivOH;
30 mol%; Table 1, entry 20).[3c] Indeed, these last conditions
were found to provide indane 2a reproducibly in 81% yield,
with a d.r. of 6:1 and an e.r. of 78.5:21.5 at 1408C. At this
point, we were able to test NHC ligand L13, which gave an
e.r. of 97.5:2.5 in the indoline system[8a] under the same con-
ditions (Table 1, entry 21). For this system, this ligand gave
a moderate yield and an e.r. of 66:34. Applying the same re-
action conditions that were used for indolines[8a] failed to
furnish better results (Table 1, entry 22).
Scheme 2. Scope of the stereoselective synthesis of indanes. Reaction
conditions: PdACHTNUTRGNEG(UN OAc)2 (2.5 mol%), L11 (10 mol%), K2CO3 (2 equiv),
DMSO, 1408C. For each compound: yield of the isolated mixture of dia-
stereoisomers, d.r. determined by 1H NMR analysis, e.r. determined by
19F NMR analysis of the Mosher amides prepared after reduction of the
nitrile to the primary amine (2a–i) or by HPLC on a chiral phase (2j).
Absolute configurations of 2a–j were ascribed by analogy to 2k.
A final significant improvement of the reaction catalyzed
by Pd/L11 was found by using DMSO as the solvent, which
provided high diastereoselectivity (d.r.=20:1), as well as
a good yield (84%) and enantioselectivity (e.r.=79:21)
without the need for added pivalic acid (Table 1, entry 23).
In addition, no background reaction was found under these
conditions (Table 1, entry 24). At this point the more bulky
ligand L12, which was described by Fu and co-worker
during the course of our study,[17] was synthesized and tested
under these optimized conditions (Table 1, entry 25), but it
did not provide any significant improvement on L11. Finally,
the amount of catalyst could be decreased to a more reason-
yields and diastereoselectivities, and with e.r. values between
76:24 (2e) and 83:17 (2g). The absolute configuration of
indane 2d was determined to be (R,R) by derivatization to
form bromide 2k and X-ray diffraction analysis of a single
crystal (Figure 1).[20] The same absolute configuration was
ascribed to other indanes (2a–j) by analogy. In addition to
aryl bromides, the reaction was found to be applicable to
pyridine and thiophene bromides, giving rise to bicyclic
products 2i and j in good yields and stereoselectivities. In
particular, a better e.r. (9:1) was obtained for both 2i and j
than for 2a–h, indicating that reduced steric hindrance at
À
able loading (2.5 mol% Pd
(OAc)2/10 mol% L11), providing
the ortho position of the incipient CAr Pd bond has a posi-
tive influence on the enantioselectivity.
the optimized standard conditions for further studies
(Table 1, entry 26).[18]
This comparison of different chiral ligands, some of which
are known to provide good enantioselectivities in related
À
processes, shows that different asymmetric C H activation
reactions require different chiral ligands, even for closely re-
lated systems (Scheme 1). In this case, binepines appear to
be the most efficient and stereoselective ligands of the vari-
ous types tested.
Next, we studied the reaction scope with substituted aryl
bromides (Scheme 2).[19] Indanes 2a–h, bearing various
types of substituent on the benzene ring, were obtained
from the corresponding aryl bromides in consistently good
Figure 1. X-ray crystal structure of indane 2k, obtained by electrophilic
bromination of 2d (ellipsoids are shown at 30% probability).
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Chem. Eur. J. 2012, 18, 4480 – 4484

3
À
Diastereo- and Enantioselective Intramolecular CACTHNUTRGNE(NUG sp ) H Arylation
COMMUNICATION
5 min at room temperature and then for 14 h at 1408C. After cooling to
room temperature, the mixture was diluted with ethyl acetate and filtered
through Celite. The solvents were evaporated under reduced pressure
and the residue was purified by preparative TLC to provide the desired
indane.
As a first attempt at characterizing reaction intermediates
to gain an insight into the origin of the enantioselectivity ob-
served with binepine L11, we were able to isolate complex 6
from the oxidative addition of bromobenzene to a mixture
of [Pd
ACHTUNGTRENNUNG(cod)ACHTUNGTRENNUNG(CH2SiMe3)2] (COD=1,5-cyclooctadiene) and
L11 in hexane (Figure 2).[20] Complex 6, which, to the best
Acknowledgements
This work was financially supported by the ANR (programme blanc “Al-
CaCHA”), the Ministꢄre de lꢅEnseignement Supꢁrieur et de la Recher-
che, and the Institut Universitaire de France. We thank Johnson Matthey,
PLC for a loan of palladium acetate, Dr. A. Marinetti and A. Voituriez
(ICSN, Gif-sur-Yvette) for an initial gift of ligand L11, Prof. E. P. Kꢀndig
(University of Geneva) for a gift of ligand L13, Dr. B. Pugin (Solvias
AG, Basel) for a gift of ligand L4, and Dr. E. Jeanneau (University
Claude Bernard Lyon 1) for X-ray diffraction analysis.
Figure 2. X-ray crystal structure of [Pd(Ph)Br
ACHTUNGRTEN(NUNG L11)2] (6) obtained from
the oxidative addition of PhBr to [Pd(cod)(CH2SiMe3)2]/L11, with select-
ed H···C contacts (ellipsoids are shown at 30% probability, most H
atoms were omitted for clarity).
G
ACHTUNGTRENNUNG
À
À
Keywords: asymmetric catalysis · C C coupling · C H
activation · palladium
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of our knowledge, is the first crystallographically character-
ized oxidative-addition complex with Pd and a binepine
ligand, reveals a monomeric structure with two L11 ligands
bound to Pd. With the bulkier bromoarene 1a, the inter-
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À
mediate Pd complex undergoing C H activation should con-
tain only one phosphine ligand,[3e] and thus it is difficult to
extrapolate structural data from compound 6 to the corre-
sponding oxidative-addition complex of 1a (we have so far
been unable to isolate the latter). In complex 6, the Pd-
bound benzene ring adopts a perpendicular orientation to
the coordination plane, with its ortho hydrogen atoms in
proximity to a naphthalene ring (H···C=3.89 ꢃ) and the
tBu group (H···C=2.92 ꢃ) of each binepine ligand
(Figure 2). Such contacts, if preserved in the case of 1a,
would be key elements in the stereocontrol that could ex-
plain the stereochemical outcome of the reaction and could
help in the design of more enantioselective ligand analogues.
Future studies from our laboratory will be conducted along
these lines.
In conclusion, we have reported the first diastereo- and
enantioselective synthesis of indanes and heterocyclic ana-
3
[5] For recent reviews, see: a) R. Jazzar, J. Hitce, A. Renaudat, J.
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À
logues by intramolecular
C
G
a chiral Pd/binepine catalyst. The improvement in the enan-
tioselectivity and the origins of the observed stereoinduction
are currently being investigated.
À
[6] This definition excludes asymmetric C H insertion. For a general
À
review on stereoselective C H functionalization, see: R. Giri, B.-F.
Shi, K. M. Engle, N. Maugel, J.-Q. Yu, Chem. Soc. Rev. 2009, 38,
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3
[7] For asymmetric PdII-catalyzed C
(sp ) H alkylation and arylation,
À
Experimental Section
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2008, 120, 4960–4964; Angew. Chem. Int. Ed. 2008, 47, 4882–4886;
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3
Pd
N
Chem. Soc. 2011, 133, 19598–19601; for IrI-catalyzed C
N
À
K2CO3 (49 mg, 0.36 mmol) were added to a Schlenk tube containing
a magnetic stirring bar. The flask was closed with a rubber septum and
removed from the glovebox. A solution of aryl bromide (0.18 mmol) in
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4483

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[18] The Pd/ligand ratio of 1:4 was found to be optimal.
11483–11485.
3
À
[9] For a conceptually different enantioselective intermolecular C
N
H arylation, see: A. Renaudat, L. Jean-Gꢁrard, R. Jazzar, C. E. Ke-
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Top. Catal. 2002, 19, 3–16.
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commercially available precursors. The diisopropyl substitution at
the benzylic carbon was conserved for all examples because linear
and cyclic alkyl groups preferentially give rise to olefins such as 4a
(for more details, see reference [3b]). In addition, only a nitrile
group allowed the introduction of two isopropyl groups by dialkyla-
tion of the corresponding enolate. Compounds 2d–j have not been
reported before.
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[20] CCDC-854015 (2k) and CCDC-848401 (6) contain the supplementa-
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Received: January 3, 2012
Published online: March 12, 2012
4484
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