(S)-6-bromo-BINOL-based phosphoramidite ligand with C1 symmetry for enantioselective hydrogenation and allylic substitution

Article abstract of DOI:10.1002/chir.20845

(S)-6-Br-BINOL-derived phosphoramidite, a simple monodentate ligand with a stereogenic center at the phosphorus atom, was synthesized for the first time. This stereoselector generated a high level of enantioselectivity (80-95% ee) in the rhodium-catalyzed

Full text of DOI:10.1002/chir.20845

CHIRALITY 22:844–848 (2010)
(
S)-6-Bromo-BINOL-Based Phosphoramidite Ligand with C1
Symmetry for Enantioselective Hydrogenation and Allylic
Substitution
1
2
1
1
KONSTANTIN N. GAVRILOV, * EDUARD B. BENETSKY, VLADIMIR E. BOYKO, EUGENIE A. RASTORGUEV,
2
3
3
¨
¨
VADIM A. DAVANKOV, BENJAMIN SCHAFFNER, AND ARMIN BORNER
1
Department of Chemistry, Ryazan State University, Ryazan, Russian Federation
Laboratory of Stereochemistry of Sorption Processes, Institute of Organoelement Compounds, Russian Academy of Sciences,
Moscow, Russian Federation
2
3
Leibniz-Institut f u¨ r Katalyse an der Universit a¨ t Rostock e.V., Rostock, Germany
ABSTRACT
(S)-6-Br-BINOL-derived phosphoramidite, a simple monodentate ligand
with a stereogenic center at the phosphorus atom, was synthesized for the first time.
This stereoselector generated a high level of enantioselectivity (80–95% ee) in the rho-
dium-catalyzed hydrogenation of a-dehydrocarboxylic acid esters and was also success-
fully employed in the asymmetric palladium-catalyzed allylic substitution of (E)-1,3-
diphenylallyl acetate. The optical yield also showed significant dependence with reac-
tion type: up to 70% ee for allylic amination, up to 75% ee for allylic sulfonylation, and up
to 90% ee for allylic alkylation. Chirality 22:844–848, 2010. VV C 2010 Wiley-Liss, Inc.
KEY WORDS: asymmetric reactions; phosphorus ligands; hydrogenation; allylation
2
4
INTRODUCTION
on C -symmetric BINOL derivatives. In particular, phos-
1
phoramidites LA and LB (Fig. 1) showed excellent activ-
ities and enantioselectivities in Rh-catalyzed olefin hydro-
genation.
The preparation of new enantiopure ligands, which offer
chiral environment to coordinated metal atoms, is one of
the most straightforward challenges for organic chemists.
1
Herein, we describe an independent approach to the
Main families of successful phosphorus-containing stereo-
synthesis of new C -symmetric phosphoramidite from
1
selectors belong to C -symmetric chelates (usually chiral
2
readily available monosubstituted BINOL as a convenient
optically active auxiliary and its application in catalytic
asymmetric transformations.
diphosphine) or C2-symmetric monodentate ligands
2
–4
(
notably, chiral phosphites and phosphoramidites). How-
ever, to date, there has not been a substantial reason why a
C2-symmetric ligand should necessarily be superior to a
nonsymmetric counterpart. In contrast, good arguments
have recently surfaced suggesting that nonsymmetrical
MATERIALS AND METHODS
General
ligands provide a more effective enantiocontrol than C -
2
3
1
13
1
5
NMR spectra of P, C, and H were recorded with a
symmetric ligands for certain reactions. Also noteworthy,
3
1
Bruker AMX 400 instrument (162.0 MHz for P, 100.6
MHz for C, and 400.13 MHz for H). Complete assign-
ment of all the resonances in C NMR spectra was
achieved by the use of distortionless enhancement by
polarization transfer (DEPT) techniques. Chemical shifts
the idea that C -symmetric phosphite-type compounds with
1
1
3
1
*
P -stereocenters still remain as relatively rare class of chi-
1
3
6
–13
ral inductors.
This is rather surprising, as phosphites
and phosphoramidites are easy to prepare from readily avail-
able starting materials and are also less sensitive to air than
phosphines. Hence, this creates a challenge to develop a
new protocol for the whole process, including the ligand syn-
thesis, which does not necessitate the use of a glove box. In
addition, phosphites are characterized by pronounced p-
acidity and low-cost. As a whole, phosphites provide broad
opportunities for fine tuning of their donor-acceptor and
steric properties by incorporating oxygen and nitrogen into
the first coordination sphere of phosphorus and varying the
(
(
usually expressed in ppm) are given relative to Me Si
4
1
13
31
H and C) and 85% H PO ( P NMR). Mass spectra
3
4
electron ionization (EI) were recorded with a Varian MAT
11 spectrometer. Optical rotations were measured on a
3
Perkin-Elmer 341 polarimeter. Elemental analyses were
Contract grant sponsor: INTAS Open Call 2005-2006, Russian Science Sup-
port Foundation; Contract grant number: INTAS Ref. No. 05-1000008-8064;
Contract grant sponsor: RFBR; Contract grant number: 08-03-00416-a
1
4–23
O- and/or N-containing building blocks.
It is important to note that all nonsymmetrical phosphite-
type ligands are obtained on the basis of optically active eration. E-mail: k.gavrilov@rsu.edu.ru
*
Correspondence to: Konstantin N. Gavrilov, Department of chemistry,
Ryazan State University, 46 Svoboda street, 390000 Ryazan, Russian Fed-
Received for publication 17 March 2009; Accepted 6 January 2010
DOI: 10.1002/chir.20845
Published online 21 April 2010 in Wiley InterScience
amino alcohols, diamines or diols with C symmetry.
1
To the best of our knowledge, there are only few exam-
*
ples of P -chiral phosphites and phosphoramidites, based (www.interscience.wiley.com).
VV C 2010 Wiley-Liss, Inc.

ENANTIOSELECTIVE HYDROGENATION AND ALLYLIC SUBSTITUTION
845
stirred at 1108C for 45 min. Then the mixture was stirred
in vacuum (10 Torr, 908C) for 30 min to remove HNEt2
and cooled down to 208C. The residue was washed with
hexane (7 ml) and dried in vacuum (1 Torr) for 1 h to
yield 5 as yellowish amorphous solid in analytically pure
2
0
13
form (2.22 g, 95%). [a]D 5 2366.7 (c 1.0, CH Cl );
C
2
2
NMR (CDCl ): d 14.7 (d, J 5 7.3 Hz, 2x CH ), 38.3 (d, J 5
3
3
2
1.2 Hz, 2x CH ), 120.9 (Cq), 121.9 (CH), 122.9 (Cq),
2
1
23.3 (d, J 5 1.3 Hz, CH), 124.4 (d, J 5 5.1 Hz, Cq), 124.8
(
1
CH), 126.3 (CH), 126.7 (CH), 128.4 (CH), 128.7 (CH),
29.2 (CH), 130.1 (CH), 130.3 (CH), 130.5 (CH), 130.7
*
Fig. 1. Examples of BINOL-derived P -chiral monodentate phosphora-
midites.
(
(
Cq), 131.4 (Cq), 131.8 (Cq), 132.5 (Cq), 149.8 (Cq), 150.4
d, J 5 5.1 Hz, Cq); MS (EI), m/z (I%): 466 (52) [M] , 452
1
1
1
performed at the Laboratory of Microanalysis (Institute of (74) [M-Me1H] , 395 (100) [M-NEt
Organoelement Compounds, Moscow).
for C24 21BrNO P: C, 61.82; H, 4.54; N, 3.00; Found: C,
1H] ; Anal. Calcd
2
H
2
All reactions were carried out under a dry argon atmos- 61.98; H, 4.72; N, 2.95.
phere in freshly dried and distilled solvents; benzylamine,
pyrrolidine, and dipropylamine. Distillation was done suc-
cessively over KOH and then over a small amount of
0
0
RESULTS AND DISCUSSION
LiAlH4 before use. (S)-6-Bromo-2,2 -dihydroxy-1,1 -
binaphthyl [(S)-Br-BINOL 4] was synthesized using litera-
The synthetic route for (S)-6-Br-BINOL 4, the precursor
2
5
ture procedures, except for the modified technique for of structurally unusual phosphoramidite 5, is outlined in
2
6
the preparation of intermediate product 3. [Rh(cod) ]BF
Scheme 1 (as patterned from a modified literature
2
4
2
7
25
and [Pd(allyl)Cl]2 were prepared as described earlier. method ).
Rh-catalyzed hydrogenation of a-dehydrocarboxylic acid
According to this procedure, the commercial (S)-
esters 6a-d was performed in accordance to known proce- BINOL 1 was treated with pivaloyl chloride in the pres-
2
8,29
dures.
Furthermore, the Pd-catalyzed allylic substitu- ence of triethylamine in acetonitrile at 08C to give mono-
tion: amination of substrate 8 with benzylamine, dipropyl- ester 2 as the single product with a 94% yield. Compound
amine, and pyrrolidine; sulfonylation with sodium para- 2 was treated with 4 equivalent of bromine in 1,4-dioxane
toluene sulfinate and alkylation with dimethyl malonate at 208C for 2 h. After quenching with aqueous Na
were all carried out using the appropriate procedures.
SO
analytically pure substance 3 was obtained by simple
The starting substrates 6b and 8 were synthesized as extraction with CH Cl in quantitative yield without further
(S)-2,2 -Dihydroxy-1,1 -binaphthyl [(S)- purification. In contrast to the literature the monobromi-
BINOL 1], P(NEt ) , Pd(CF CO ) , dimethyl itaconate, nation step to obtain 3 was only successful with using of
, the
2
3
8,30–32
2
2
2
7,33
0
0
25
published.
2
3
3
2 2
methyl 2-acetamidoacrylate, ethyl 2-acetoxyacrylate, di- acetonitrile instead of 1,4-dioxane. Deprotection of the OH
methyl malonate, BSA [N,O-bis(trimethylsilyl) acetamide], group by saponification of 3 with potassium hydroxide in
and sodium para-toluene sulfinate were purchased from a mixture of THF/H
O at 258C gave the resulting 4 in
2
Aldrich and Acros Organics and used without further puri- quantitative yield. It is rather important, that this synthesis
fication.
of 4 is much easier than the preparation of precursors of
ligands LA and L , which involved more steps, including
B
2
4
ortho-metallation by butyllithium.
S)-6-Br-BINOL 4 was used as the starting material for
the nonsymmetrical phosphoramidite 5. Reaction of 4
A solution of bromine (1.96 ml, 38.3 mmol) in 1,4-diox- with hexaethylphosphorous triamide P(NEt (Scheme 1)
ane (20 ml) was slowly added to a solution of (S)-2- in solvent-free conditions followed by simple washing of
0
0
(
S)-6-Bromo-2-Hydroxy-2 -Pivaloyloxy-1,1 -Binaphthyl
3)
(
(
)
2 3
0
0
hydroxy-2 -pivaloyloxy-1,1 -binaphthyl 2 (3.55 g, 9.6 mmol) the residue with hexane gave analytically pure 5 in 95%
in 1,4-dioxane (170 ml) at 08C. The reaction mixture was yield without further chromatographic purification or
then stirred at 208C for 2 h and quenched with aqueous recrystallization. It should be noted that this convenient
Na SO . After addition of 200 ml of CH Cl , the organic method does not require additional base and that the vola-
2
3
2
2
phase was washed with saturated aqueous NaHCO and tile HNEt
is formed only as a by-product. The newly syn-
3
2
brine, and dried over MgSO . After removal of the solvent, thesized monodentate ligand is stable on prolonged stor-
4
3
(4.31 g, 100%) was obtained as a white solid. Optical age and can be obtained on a gram scale. Moreover, the
0
rotation and all spectroscopic data pertaining to compound monosubstitution at the 6-position of the 1,1 -binaphthyl
2
5
3
are consistent with published data.
skeleton not only reduced C to C symmetry, but also led
2 1
to the creation of a stereogenic center at the phosphorus
atom. The phosphorylation of 4 proceeded with complete
diastereoselectivity. Thus, the P NMR spectrum of 5 (in
0
0
0
(
S)-6 -Bromo-2-(Diethylamino)-Dinaphtho[2 ,1 -d:1@,2@-
31
f][1,3,2]Dioxaphosphepine (5)
CDCl ) shows a narrow singlet d 150.1. We did not estab-
3
0
0
*
A mixture of (S)-6-bromo-2,2 -dihydroxy-1,1 -binaphthyl lish configuration of the P -stereo center in the structure
(1.83 g, 5 mmol) and P(NEt ) (1.37 ml, 5 mmol) was of ligand 5. Nevertheless, on the basis of data presented
4
2
3
Chirality DOI 10.1002/chir

8
46
GAVRILOV ET AL
Scheme 1. Synthesis of Br-substituted phosphoramidite ligand.
2
4
in article, it is possible to assume, that we deal with
S ,S )-epimer of 5.
The reactions were performed in THF, propylene car-
bonate, or CH Cl at room temperature (with [Pd(al-
(
a
P
2
2
The products of catalytic hydrogenation of olefins sub- lyl)Cl] or Pd(CF CO ) , L/Pd 5 1 or 2). In the allylic ami-
2
3
2 2
stituted by both an amido and a carboxylic acid (or ester) nation of 8 with benzylamine, dipropylamine, and pyrroli-
group are important chiral building-blocks for the indus-
trial synthesis of peptides and for numerous biomedical
4
and medicinal applications. Accordingly, we describe
here results of the Rh-catalyzed hydrogenation of a-dehy-
drocarboxylic acid esters, as well-known benchmark sub-
strates, with ligand 5 (Table 1).
TABLE 1. Rh-catalysed hydrogenation of a-dehydrocarbox-
a
2
ylic acid esters (258C, 1 bar H )
The reactions were performed in propylene carbonate
or CH Cl at room temperature (with [Rh(cod) ]BF , L/
2
2
2
4
Rh 5 2). In the transformation of dimethyl itaconate 6a to
succinate 7a, phosphoramidite 5 has shown good enantio-
selectivity (80–81% ee), irrespective of the nature of the sol-
vent (Table 1, entries 1, 2). A very good optical yield (93%,
Table 1, entry 3) has been obtained in the hydrogenation
of substrate 6b, but only in propylene carbonate; in
CH Cl , product 7b was formed with a slightly smaller op-
2
2
tical purity (85%). With its high polarity propylene carbon-
ate offers great possibilities to recycle catalysts success-
c,d,e,f
Entry
Substrate
Solvent
Time (min)
ee (%)
3
4
fully.
In the Rh-catalyzed hydrogenation of esters 6c,d
b
excellent enantioselectivities were achieved in both sol-
vents (up to 93 and 95% ee, respectively, Table 1, entries
1
2
3
4
5
6
7
8
6a
6a
6b
6b
6c
6c
6d
6d
PC
200
60
80 (S)
CH
PC
CH
PC
CH
PC
2
CH Cl
2
Cl
2
2
2
2
81 (S)
93 (R)
85 (R)
93 (R)
90 (R)
95 (R)
95 (R)
1470
1440
540
500
120
90
5
–8). Interestingly, the Rh-catalyzed hydrogenation of sub-
2
Cl
strates 6a,d phosphoramidites L and L caused a consid-
A
B
erable dispersion of the asymmetric induction (0–97% ee
2
4
2
Cl
for 6a and 34–99% ee for 6d). Thus, being much easier
available, ligand 5 reliably provides enantioselectivities
that are rather close to the best results for L and L .
A
B
a
Metal-catalyzed enantioselective allylation led to the for-
mation of C ꢀꢀ H, ꢀꢀ C, ꢀꢀ O, ꢀꢀ N, ꢀꢀ S, and other bonds
with very high levels of asymmetric induction and toler-
ates a broad range of functional groups. This method,
100% conversion in all cases.
Propylene carbonate.
b
c
The conversion of substrate 6a and enantiomeric excess of 7a were
determined by GC (Lipodex E, 25 m 3 0.25 mm, 808C, 1 ml/min) or
HPLC (Daicel Chiralcel OD-H, C
6
H
14/i-PrOH 5 98/2, 0.8 ml/min, 220
therefore, has been applied successfully to the synthesis nm, t(R) 5 9.1 min, t(S) 5 16.1 min).
3
5
d
of many natural products and new chiral compounds. In
this connection, we turned our attention to the application
of 5 in the Pd-catalyzed allylic substitution of (E)-1,3-diphe-
nylallyl acetate 8 as a widely used model substrate with
N-, S- and C-containing nucleophiles (Table 2).
Chirality DOI 10.1002/chir
The conversion of the substrate 6b and enantiomeric excess of 7b were
determined by GC (Lipodex E, 25m 3 0.25 mm, 1458C, 1 ml/min).
e
The conversion of substrate 6c and enantiomeric excess of 7c were
determined by GC (50m Chiraldex ß-pm; Astec).
f
The conversion of substrate 6d and enantiomeric excess of 7d were deter-
mined by GC (XE-valin(tert-butylamide) 4 3 0.25 mm, 858C, 1 ml/min).

ENANTIOSELECTIVE HYDROGENATION AND ALLYLIC SUBSTITUTION
847
TABLE 2. Pd-catalysed allylic substitution of (E)-1,3-diphe- amidite 5 was achieved in the allylic alkylation of 8 with
nylallyl acetate 8 (208C)
dimethyl malonate as the C-nucleophile (Table 2, entries
3–18). Interestingly, the precatalyst Pd(CF CO ) pro-
1
3
2 2
vides a comparable conversion but considerably smaller
asymmetric induction in comparison with [Pd(allyl)Cl]2.
At the same time, with the participation of [Pd(allyl)Cl]2,
fairly good levels of conversion (85–100%) and enantiose-
lectivity (80–90%) were achieved, almost irrespective of
both the L/Pd molar ratio and the nature of the solvent.
CONCLUSION
In summary, we have prepared and characterized an
original (S)-6-Br-BINOL-derived C -symmetric phosphora-
1
Time Conv.
midite ligand that exposes a phosphorus stereocenter. It is
a very efficient ligand in the Rh-catalyzed hydrogenation
and Pd-catalyzed allylic alkylation. Moreover, the concept
of ligands design presented above looks very promising.
a
Entry
Precatalyst
L/Pd Solvent
(h)
(%)
ee (%)
60 (R)
b
Allylic amination with benzylamine
1
2
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
2
1/1
1/1
PC
CH Cl
12
12
91
99
60 (R) Indeed, there exist convenient approaches to the synthesis
2
2
c
Allylic amination with dipropylamine
of a whole set of 6-monofunctionalized 2,20-dihydroxy-1,10-
3
4
5
6
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
2
2
1/1
2/1
1/1
2/1
CH
CH
THF
dTHF
2
Cl
Cl
2
2
48
48
48
48
97
100
15
60 (2) binaphthyl compounds (including those with ionic frag-
3
6
2
65 (2)
60 (2)
64 (2)
ments), starting from (S)- or (R)-6-Br-BINOL. Accord-
ingly, the new route opens access to variable and highly
Pd(allyl)Cl]
2
19
*
prospective P -chiral phosphite-type stereoselectors for
Allylic amination with pyrrolidine
transition metal-catalyzed asymmetric reactions.
7
8
9
1
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
[Pd(allyl)Cl]
2
2
2
2
1/1
2/1
1/1
2/1
CH
CH
THF
THF
2
Cl
Cl
2
2
48
48
48
48
100
100
41
70 (S)
60 (S)
57 (S)
53 (S)
2
LITERATURE CITED
0
52
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7
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8
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e
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9
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1
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2
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2
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1
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1
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Chirality DOI 10.1002/chir