Rhodium-catalyzed enantioselective conjugate addition of arylboronic acids to dihydronitronaphthalenes

Article abstract of DOI:10.1002/adsc.201200241

A highly enantioselective (up to 91% ee) rhodium-catalyzed asymmetric addition of arylboronic acids has been achieved leading to the challenging dihydro-3-nitronaphthalenes using one equivalent of phosphoramidite ligand to rhodium catalyst. A concise formal asymmetric synthesis of the dopamine D1 agonist, dihydrexidine was accomplished using the method. Copyright

Full text of DOI:10.1002/adsc.201200241

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DOI: 10.1002/adsc.201200241
Rhodium-Catalyzed Enantioselective Conjugate Addition of
Arylboronic Acids to Dihydronitronaphthalenes
Saumen Hajra,a,* Rajib Ghosh,b,c Sagar Chakrabarti,b,c Amit Ghosh,b,c
Swarup Dutta,b Tushar K. Dey,b,c Rajesh Malhotra,c Sonika Asijaa,c Subho Roy,b
Shantanu Dutta,b and Sourav Basub,*
a
Department of Chemistry, Indian Institute of Technology Khargapur, Kharagpur – 721302, India
Fax : (+91)-32-2225-5303; phone: (+91)-32-2228-3340; e-mail: shajra@chem.iitkgp.ernet.in
TCG Life Sciences Ltd, Saltlake, Kolkata – 700091, India
b
Fax: (91)-33-2367-3058; e-mail: sourav@chembiotek.com
Department of Chemistry, Guru Jambheshwar University of Science and Technology Hisar, Haryana 125001, India
c
Received: March 22, 2012; Revised: May 16, 2012; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200241.
Abstract: A highly enantioselective (up to 91% ee)
rhodium-catalyzed asymmetric addition of arylbor-
onic acids has been achieved leading to the chal-
lenging dihydro-3-nitronaphthalenes using one
equivalent of phosphoramidite ligand to rhodium
catalyst. A concise formal asymmetric synthesis of
the dopamine D1 agonist, dihydrexidine was ac-
complished using the method.
reduction of the nitro group and cyclization could
provide an efficient and concise protocol for the syn-
thesis of such compounds (Scheme 1). However, de-
spite its great synthetic importance the Rh-catalyzed
asymmetric addition of arylboronic acids to dihydro-
3-nitronaphthalenes 2 is still a challenge and known
to be unsuccessful.[7] Our interest[8] towards the effi-
cient asymmetric synthesis of biologically important
hexahydrobenzophenanthrenes and related com-
pounds led us to investigate the unprecedented
chemistry. Herein we report the Rh-catalyzed asym-
metric addition of arylboronic acids to dihydro-3-ni-
tronaphthalenes with high enantioselectivity (ee upto
91%) and its application towards the asymmetric syn-
thesis of the dopamine D1 agonist, (+)-dihydrexidine.
The use of 1.1 equivalents of monodentate phosphor-
Keywords: asymmetric catalysis; conjugate addi-
tion; dihydrexidine; dihydro-3-nitronaphthalenes;
rhodium
The rhodium-catalyzed asymmetric conjugate addi- amidite ligand to rhodium-catalyst is also effective for
tion of organoboronic acids to electron-deficient ole- the catalysis.
fins, pioneered by Miyaura and Hayashi, is one of the
Initial trials on the reaction of dihydronitronaph-
À
most efficient methods for enantioselective C C bond thalene 2a with PhB(OH)2 3a in the presence of
formation.[1] High levels of enantioselectivity have Rh
(acac)
R
been achieved using a variety of mono- and bidentate standard conditions failed. A number of bidentate
chiral ligands and the reaction is also applicable to phosphine ligands was tested under the same reaction
a wide range of substrates.[2] In contrast, studies on ni- conditions (see the Supporting Information); most of
troalkenes are still limited.[3,4] The first Rh-catalyzed them did not show any reaction. Only (+)-(S,S)-Ph-
asymmetric addition of organoboronic acids to cyclic
aliphatic nitroalkenes was reported by Hayashiꢀs
group with excellent enantioselectivity.[3] Recently im-
pressive results were achieved for nitrostyrenes
too.[4a,b]
Annulated arene heterocycles and carbocycles such
as hexahydrobenzophenanthrenes and their homo-
logues 1 constitute important structural subunits of
various natural products and pharmaceuticals.[5,6]
Enantioselective 1,4-conjugate addition of arylboronic
zophenanthrene by catalytic enantioselective conjugate ad-
acids to dihydro-3-nitronaphthalenes 2 followed by dition of ArB(OH)2 3 to dihydronitronaphthalenes 2.
Scheme 1. A straightforward construction of hexahydroben-
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Table 1. Initial trial using bidentate and achiral monodentate
ligands.
dronitonaphthalene 2a was directed towards the
asymmetric reaction with chiral monodentate phos-
phine ligands (using 1.1 equivalents to rhodium cata-
lyst)
(diphenyl)phosphine L5 and phosphoramidites[4c–e,10]
L6–L10 (Table 2). Rajphos L4 gave very poor con-
version (entry 1) whereas, the neomenthyl-
(diphenyl)phosphine L5 showed smooth addition of
like
rajphos
L4,
(S)-(+)-neomenthyl-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
PhB(OH)2 to dihydronitronaphthalene 2a with 100%
conversion and high yield, but no ee (entry 2). Among
the phophoramidite ligands, phosphoramidite L6
Table 2. Screening of chiral monodentate ligands for the
asymmetric conjugate addition to dihydronitronaphthalene
2a.
Entry
Ligand
Mol%
Conv. [%][a]
Yield [%][b]
1
2
3
4
5
6
7
8
L
L2
L3
Ar3P
Me3P
T
A
ACHTUNGTRENNUNG
22
22
22
22
22
22
22
11
NR
10
20
NR
–
–
15
–
–
–
75
72
NR
<10%
100%
100%
[a]
1
Determined by H NMR analysis of the crude reaction
mixture.
[b]
Isolated yields of the addition product 4aa. Ar=Ph, p-
Tol, o-Tol.
BPE L2 and (À)-(S,S)-i-Pr-DUPHOS L3 gave the de-
sired phenyl addition product 4aa with 10–20% con-
version (Table 1; entries 2 and 3), but there was no re-
action with (+)-(S,S)-Me-BPE and (À)-(S,S)-Me-
DUPHOS. This result prompted us to realize that the
approach of bidentate chelated cyclic aryl-rhodium
(Ar-Rh) to the structurally rigid dihydronitronaphtha-
lene 2a might be inhibited and the sterically hindered
Ph-BPE L2 and i-Pr-DUPHOS L3 might be acting as
monodentate ligands providing the reaction. So we
began our investigation on the above reaction with
monodentate achiral phosphine ligands (Table 1).
There was also no reaction with triarylphosphines
Ar3P (Ar=Ph, p-Tol, o-Tol) ligands (entry 4). In
Entry
Ligand
Conv. [%][a]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
L4
L5
L6
L7
L8
L9
L10
L9
NR
100
NR
NR
<5%
63
–
84
–
–
–
56
50
55
–
0
–
–
contrast,
Rh(acac)
we
were
pleased
to
find
that
G
N
2 ACHTUNGTRENNUNG(cy-Hex)3P catalyst showed very
good efficiency with 100% conversion (entry 7). Inter-
estingly, the reaction was found to be successful even
with only 1.1 equivalents of (cy-Hex)3P to
–
86
86
86
7
55
60
RhACHTUNGTRENNUNG(acac)ACHTUNGTRENNUNG(C2H4)2 (entry 8) and it was a cleaner reac-
8[d]
tion too.[9] Lowering the catalyst loading to 5 mol%
and 2 mol% in the reaction gave incomplete conver-
sion.
[a]
1
Determined by H NMR analysis of the crude reaction
mixture.
Isolated yield of the addition product 4aa.
Enantiomeric excess was determined by chiral HPLC.
Reaction with 2.1 equiv. of ligand L9 to rhodium catalyst.
[b]
[c]
[d]
Preliminary findings revealed that the phosphine li-
gands should be single point-binding and have to be
electron-rich. Now the PhB(OH)2 addition to dihy-
2
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Rhodium-Catalyzed Enantioselective Conjugate Addition of Arylboronic Acids to Dihydronitronaphthalenes
Table 3. RhACHTUNGTRENNUNG(acac)ACHTUNGTRENNUNG(C2H4)2/L9-catalyzed asymmetric conjugate addition to dihydronitronaphathalenes 2.
Entry
2
3
4
Conv. [%][a]
Yield [%][b]
ee of 4 [%][c]
1
2
3
2a
2a
2a
2a
2b
2b
2b
2b
2c
2c
3a
3b
3c
3d
3a
3b
3c
3d
3a
3b
4aa
4ab
4ac
4ad
4ba
4bb
4bc
4bd
4ca
4cb
63
56
42
55
55
52
40
45
35
20
56
52
31
42
50
48
28
32
30
16
86
87
81
80
85
91
78
80
61
75
4
5[d]
6
7
8
9[d]
10[d]
[a]
1
Determined by H NMR analysis of the crude reaction mixture; 30–70% of the corresponding dihydro-2-nitronaphtha-
lene 2 were recovered.
[b]
[c]
[d]
Isolated yields of the respective addition product 4.
Enantiomeric excess was determined by chiral HPLC.
Reactions were carried with (S)-phosphorimidite ent-L9 and produced the other enantiomers of compound 4.
having a free NH and sterically hindered ligands L7
This method could produce a number of non-race-
and L8 did not show any reaction (entries 3–5). Steri- mic trans-1-aryl-2-nitrotetralins 4, if a different combi-
cally less hindered simple (R)-binaphthyldimethyl- nation of dihydronitronaphthalenes 2 and arylbornic
ACHTUNGTRENNUNGamine phosphoramidite L9 was found to be efficient acids 3 are reacted and subjected to subsequent epi-
and provided 63% conversion with high enantioselec- merization[3] with NaHCO3 in EtOH. So this rhodi-
tivity (ee 86%) as a mixture of diastereomers (dr= um-catalyzed asymmetric method was generalized
1:1), which on heating with NaHCO3 in EtOH afford- with other arylboronic acids and dihydro-3-nitronaph-
ed exclusively trans-isomer 4aa without any loss of ee thalenes under the developed conditions using only
(entry 6). A phosphoramidite having the partially sa- 1.1 equiv. of ligand L9 to rhodium (Table 3). In partic-
turated binaphthyl unit L10 showed similar results ular, bromo-substituted dihydro-3-nitronaphthalenes
with slightly lower conversion (entry 7). These results were tested, where the bromo functionality might pro-
might further support the lesser steric demands of the vide a further avenue for structural elaboration by
arylboronic acid addition to dihydronitronaphthalene different kinds of coupling reactions and subsequent
as presumed. Interestingly, reactions of dihydronitro- reactions. Under the developed conditions, we were
naphthalene 2a and PhB(OH)2 3a in the presence of pleased to find that most of the reactions provided
rhodium/L9 using 2.1 equiv. (entry 8) and 1.1 equiv. clean addition products with high enantioselectivity of
(entry 6) ligand to rhodium afforded the same result. both diastereomers (dr varies from 1:1 to 2:1; see the
The active catalyst for the both cases might, therefore, Supporting Information). Reactions of four arylbor-
be the same. The addition of PhB(OH)2 3a with dihy- onic acids with unsubstituted dihydronitronaphtha-
dronitronaphthalene 2a was also tested with other lene 2a afforded all the desired products with high
rhodium catalysts such as [RhClACTHNUTRGNEG(UN C2H4)2]2/L9 in the enantioselectivity (ee up to 87%), but with incomplete
presence of additive either KOH or KHF2 or K3PO4 conversion (entries 1–4). 8-Bromodihydro-3-nitro-
and it provided similar results only in toluene/H2O, naphthalene 2b showed a similar reactivity as 2a with
but no reaction in dioxane/H2O. The catalytic asym- higher enantioselectivity (ee up to 91%; entries 5–8).
metric addition of arylboronic acids to dihydro-3-ni- In contrast, 6-bromodihydronitronaphthalene 2c was
tronaphthalenes with high enantioselectivity and use found to be less reactive and showed lower enantiose-
of one equivalent of ligand to rhodium represents lectivity (ee up to 75%; entries 9–12). The diastereo-
a very good achievement.
meric mixture of compound 4 on refluxing with
NaHCO3 in EtOH afforded exclusively the trans-
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Scheme 2. Catalytic asymmetric formal synthesis of the dopamine D1 agonist, (+)-dihydrexidine.
isomer 4 (dr>95:5) without any loss of enantioselec- Again (1R,2R)-trans-N-nosyl-1-amino-2-phenyltetralin
tivity.[3] This method is an unprecedented asymmetric 5 is a known[8c,d] advanced precursor for the synthesis
addition of arylboronic acids to dihydro-3-nitronaph- dopamine D1 agonist, dihydrexidine, thus completing
thanlenes with very good to excellent enantioselectivi- the formal synthesis.
ty, although with incomplete conversion.
In summary, we have developed the first rhodium-
Chiral trans-2-amino-1-aryltetralins are the key pre- phosphoramidite L9-catalyzed asymmetric conjugate
cursors to many biologically active natural products, addition of arylboronic acids to challenging dihydro-
and pharmaceuticals.[5,6,8,11] These trans-aminotetralins 3-nitronaphthalenes. The reaction is found to be ef-
could easily be obtained from 1-aryl-2-nitrotetralins 4. fective only with monodentate ligand. Use of one
Consequently, the synthetic utility of this methodolo- equivalent of ligand to rhodium provided the same
gy was further demonstrated by the transformation of result as two equivalents ligand. Under optimal condi-
dihydro-3-nitronaphthalene 2d to (+)-dihydrixdi- tions (using one equivalent ligand to rhodium), the
ne,[8c,d,11b–d] a dopamine D1 agonist (Scheme 2). To this addition reaction proceeds well with a range of sub-
end, 2d was reacted with PhB(OH)2 3a in the pres- strates and provided high enantioselectivity for both
ence of RhACHTUNGTRENNUNG(acac)ACHTUNGTRENNUNG(C2H4)2-L9 as a catalyst under opti- of the diastereomers that could easily be epimerized
mized conditions. It provided a diastereomeric mix- exclusively to the trans-isomer without any loss of ee.
ture (dr=1.5:1) of 2-nitro-1-phenyltetralin 4da in The usefulness of the method was demonstrated by
57% yield with 72% and 73% ee, respectively. Epime- a catalytic enantioselective formal synthesis of the
rization[3] of the cis-isomer to trans-one, by heating of dopamine D1 agonist, dihydrexidine. Further investi-
the diastereomeric mixture 4da with NaHCO3 in gation on efficiency, mechanism and applications are
EtOH, provided trans-2-nitro-1-phenyltetralin trans- underway in the laboratory.
4da in high diastereoselectivity (dr>95:05) without
any loss of enantioselectivity (ee 72%). Zn/HCl-medi-
ated reduction of the trans-4da produced trans-2-
amino-1-phenyltetralin and subsequent nosyl protec-
Experimental Section
tion with NsCl and Et3N in DCM provided N-nosyl-
ACHTUNGTRENNUNGaminotetralin 5 as a pale yellow solid with 61% yield
General Procedure for the Asymmetric Conjugate
Addition Reaction
over two steps. Recrystalization of the compound 5
from methanol at 08C crystallized out the minor
enantiomer and enhanced the ee of the mother liquor
to 98% with 72% recovery yield.[8c,d] The optical rota-
tion of 5 {[a]2D5: À60 (c 1.0, CHCl3)} matches very well
with literature[8c] data {[a]D25: À64.2 (c 1.0, CHCl3)},
thus confirming the absolute stereochemistry as
1R,2R. By analogy, the absolute stereochemistry of
To a mixture of RhACHTUNGTRENNUG(acac)ACHTUNGTRNE(NUGN C2H4)2 (0.001 g, 0.042 mmol,
10 mol%), ligand L9 (0.017 g, 0.047 mmol, 11 mol%) and 3-
methoxyphenylboronic acid 3b (0.325 g, 2.14 mmol,
5.0 equiv.) was added 1,4-dioxane (3 mL) and, the mixture
was stirred at room temperature for 3 min. 1,2 Dihydroni-
tronaphthalene 2a (0.075 g, 0.42 mmol, 1.0 equiv.) and water
(0.3 mL) were then added to it and the whole mixture was
stirred at 100–1058C for 20 h. The reaction mixture was
the trans-isomer of all compounds 4 was assumed. cooled to room temperature, followed by the addition of an
4
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Rhodium-Catalyzed Enantioselective Conjugate Addition of Arylboronic Acids to Dihydronitronaphthalenes
aqueous solution of sodium bicarbonate (5 mL). It was then
extracted with ethyl acetate (3ꢂ25 mL). The combined or-
ganic extract was washed with brine, dried over sodium sul-
phate and evaporated under vacuum. The residue was puri-
fied by flash column chromatography using ethyl acetate
and hexanes to obtain compound 4ab (yield: 0.06 g; 52%) as
a mixture of diastereomers (dr=2:1) along with the starting
1,2-dihydronitronaphthalene 2a (recovery: 0.03 g; 40%).
The mixture was then refluxed with sodium bicarbonate
(10 equiv.) and 4.0 mL of ethanol for 12 h. It was evaporated
to dryness, passed through a filter column using 100–200
silica gel and ethyl acetate-hexanes as an eluent and provid-
ed trans-1-aryl-2-nitrotralin 4ab as a single diastereomer;
yield: 100% (dr>95:05). (In some cases addition product 4
and unreacted dihydronaphthalene 3 were found to have the
same Rf. Preparative HPLC was adopted to purify the de-
sired addition product).
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Acknowledgements
Acknowledgements- We thank DBT (BIPP), New Delhi
(sanction no.: 102/IFD/SAN/1191/2009-2010) for providing
financial support. We are also grateful to the referees for their
valuable suggestions.
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6 Rhodium-Catalyzed Enantioselective Conjugate Addition of
Arylboronic Acids to Dihydronitronaphthalenes
Adv. Synth. Catal. 2012, 354, 1 – 6
Saumen Hajra,* Rajib Ghosh, Sagar Chakrabarti,
Amit Ghosh, Swarup Dutta, Tushar K. Dey,
Rajesh Malhotra, Sonika Asijaa, Subho Roy,
Shantanu Dutta, Sourav Basu*
6
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