New chiral amino-phosphoramidite and bisphosphoramidite ligands derived from (R,R)-1,2-diaminocyclohexane: Application in Cu-catalyzed asymmetric conjugate addition of diethylzinc to 2-cyclohexenone

Article abstract of DOI:10.1016/S0957-4166(03)00447-6

New bidentate amino-phosphoroamidite and diphosphoroamidite ligands derived from inexpensive (R,R)-1,2-diaminocyclohexane have been synthesized and screened in the Cu-catalyzed asymmetric conjugate addition of Et₂Zn to 2-cyclohexenone. The high

Full text of DOI:10.1016/S0957-4166(03)00447-6

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 2127–2131
New chiral amino-phosphoramidite and bisphosphoramidite
ligands derived from (R,R)-1,2-diaminocyclohexane: application
in Cu-catalyzed asymmetric conjugate addition of diethylzinc to
2-cyclohexenone
Carmela G. Arena,* Vincenzo Casilli and Felice Faraone
Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universita` di Messina, Salita Sperone,
31 Villaggio S. Agata 98166, Messina, Italy
Received 4 April 2003; accepted 9 June 2003
Abstract—New bidentate amino-phosphoroamidite and diphosphoroamidite ligands derived from inexpensive (R,R)-1,2-
diaminocyclohexane have been synthesized and screened in the Cu-catalyzed asymmetric conjugate addition of Et2Zn to
2-cyclohexenone. The highest 74% ee value was reached with the N,N%-dimethyl substituted P,N-ligand and Cu(OAc)2·H2O.
© 2003 Elsevier Ltd. All rights reserved.
1. Introduction
asymmetric conjugate
2. Results and discussion
2.1. Synthesis of the ligands
Catalytic
addition
of
organometallic, Grignard and organolithium reagents
to a,b-unsaturated compounds is an attractive synthetic
method for carbonꢀcarbon formation.1 Remarkable
advances have been achieved in this area employing a
combination of an organozinc reagent, generally Et2Zn,
and a copper complex with a chiral ligand.2 Several
copper complexes of chiral phosphorus ligands3 have
been used in the catalytic conjugate addition of
dialkylzinc to enones with good to excellent enantiose-
lectivity. Even though, the use of chiral monodentate
phosphoroamidites in the Cu-catalyzed 1,4-addition of
R2Zn gave very high enantioselectivities,4 some biden-
tate P,P-5 and P,N-6 ligands have been described
affording excellent results.
Reaction of the disubstituted (R,R)-1,2-diaminocyclo-
hexanes8 1a–d (Scheme 1) with one equivalent of (3,3%-
bis-tert-butyl-5,5%-bis-methoxy-1,1%-biphenyl-2,2%-diyl)-
phosphorochloridite9 2 in toluene in the presence of
triethylamine, gave the desired amino-phosphoroamid-
ite ligand 3a only in the case of the N,N%-dimethyl
substituted amine 1a, whereas two equivalents of 2 were
necessary to obtain the N,N%-diethyl substituted ligand
3b. The presence of a larger group, such as benzyl or
isopropyl, in the N,N%-disubstituted diamines 1c–d did
not allow the synthesis of the corresponding P,N-chiral
ligand even upon exposure to a large excess of 2.
Moreover, treatment of N,N%-dimethyl amine 1a with 2
excess did not lead to the corresponding diphospho-
roamidite. The new chiral ligands 3a–b were obtained,
as white solid, in good yields (75–85%) after purifica-
tion by chromatography over silica gel. They are stable
to air in the solid state. The analytical data were in
accordance with the proposed structures, in particular
31P spectroscopy showed one singlet at chemical shift
values observed for similar compounds.10 Because the
rapid biphenyl moiety atropoisomerization on the
NMR time scale, diastereoisomers expected for the
ligands 3a–b was not detected by low temperature 31P
NMR. Finally, steric requirements of the N-sub-
stituents prevented us from introducing a binaphthol
fragment into N,N%-dialkyl diamines 1a–1d.
Following our interest in developing effective P,P- and
P,N-chiral ligands for enantioselective homogeneous
catalysis,7 we have designed a series of amino-phospho-
ramidite and bisphosphoramidite ligands derived from
the inexpensive and easily derivatizable chiral source
(R,R)-1,2-diaminocyclohexane. Herein we report the
synthesis of these ligands and their application in the
asymmetric conjugate addition of diethylzinc to 2-
cyclohexenone.
* Corresponding author. E-mail: c.arena@chem.unime.it
0957-4166/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0957-4166(03)00447-6

2128
C. G. Arena et al. / Tetrahedron: Asymmetry 14 (2003) 2127–2131
Scheme 1.
In view of these results, it appears that the N-sub-
stituents in (R,R)-1,2-diaminocyclohexane, 1, determine
the course of these P,N-ligand syntheses reaction.
Spilling et al.11 reported similar results on the synthesis
of a new series of amino-phosphine ligands derived
from (R,R)-1,2-cyclohexanediamine.
2-cyclohexenone 6 (Scheme 3). The catalytic system was
formed in situ by adding the corresponding ligand to a
suspension of Cu(OTf)2 or Cu(OAc)2·H2O. The results
are summarized in Table 1. Complete chemoselectivity
was found since no 1,2 product was detected by GC
analysis. Using the amino-phosphoroamidite ligands
3a, the highest enantioselectivity was achieved (74% ee,
entry 3) with Cu(OAc)2·H2O in toluene at −20°C. As
reported by Alexakis,4e the degree of enantioselectivity
increased using Cu(OAc)2·H2O (entry 3) compared to
Cu(OTf)2 (entry 1) while the catalytic activity is higher
with Cu(OTf)2. However, in contrast to Alexakis’s
findings, in this case the catalyst performance in ether is
very low (entry 6). Adding a one-fold excess of ligand
led to a decrease of the catalytic activity as well as the
enantioselectivity (entry 7 versus 3).
The chiral bisphosphoroamidite ligands 4–4b were pre-
pared in 80–90% isolated yields from the (R,R)-1,2-
diaminocyclohexane, 1, and either
2
or the
corresponding enantiomer of the (2,2%-binaph-
hol)phosphorochloridite9 2a–b in a 1:2 molar ratio, in
toluene in the presence of triethylamine (Scheme 2).
Ligand 4 was purified by chromatography over silica
gel and it is stable to air. In contrast to 4, ligands 4a–b
have to be stored under nitrogen. One singlet was
observed for 4–4b, in the 31P NMR spectrum at chemi-
cal shift values agree with those expected for phospho-
roamidites compounds.12
Surprisingly, the catalytic system Cu(OAc)2·H2O/3b in
toluene at −20°C gave a lower reaction rate and was
not selective (63% conv, 8% ee, entry 8). Probably, the
active catalyst was not stable under these reaction
conditions. Indeed, metallic copper was observed in the
reaction vessel after 3 h. Employing the system
Cu(OTf)2/3b the conversion reached 80% and (S)-3-
ethylcyclohexenone 6 was obtained in 57% ee (entry 9).
Comparing the results obtained with ligands 3a and 3b
(entry 1 and 9), it is worth noting that a slight increase
in the steric hindrance at nitrogen in N,N%-dialkyl
diamines 1a–b lead to a depletion of the conversion and
stereoselectivity. It is plain that amino-phosphoroamid-
ites derived from (R,R)-1,2-diaminocyclohexane show a
low tolerance to an increase of the sterically demanding
substituent at the nitrogen in the asymmetric copper-
catalyzed 1,4-addition to cyclohexenone.
2.2. Asymmetric conjugate addition of Et2Zn to 2-
cyclohexenone
The new chiral P,N-3a–3b and P,P-4–4b ligands were
tested in the Cu-catalyzed addition of diethylzinc to
Using the catalytic system formed by Cu(OTf)2 and
bidentate phosphoroamidites 4–4b, a marked decrease
Scheme 2.
Scheme 3.

C. G. Arena et al. / Tetrahedron: Asymmetry 14 (2003) 2127–2131
Table 1. Asymmetric conjugate addition of Et2Zn to 2-cyclohexenone
2129
Entrya
L
Copper salt
Solvent
T (°C)
Conv. (%)b
ee (%)b
1
3a
3a
3a
3a
3a
3a
3a
3b
3b
4
Cu(OTf)2
Cu(OTf)2
Toluene
Toluene
Toluene
Toluene
CH2Cl2
Et2O
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
−20
0
−20
0
−20
−20
−20
−20
−20
−20
−20
−20
95
>99
92
99
94
45
80
63
89
96
97
97
70 (R)
66 (R)
74 (R)
64 (R)
61 (R)
51 (R)
70 (R)
8 (R)
57 (R)
6 (R)
40 (S)
0
2c
3
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
4
5
6
7d
8
9
10
11
12
4a
4b
a Conditions, see Scheme 3 and Section 4.
b % Conversions and enantiomeric excesses determined by GC (Lipodex E column, 90°C).4e
c Determined after 1 h.
d Ligand:copper salt=2.2:1
4. Experimental
in the enantioselectivity was observed (0–40% entries
10–12) in comparison to P,N-ligands, while no signifi-
All syntheses were performed under purified nitrogen
using standard Schlenk techniques. Solvents were dried
by standard procedures. NMR experiments were car-
ried out using Bruker AMX R300 spectrometer. 1H and
13C NMR spectra were referenced to internal tetra-
methylsilane and 31P{1H} spectra to external 85%
H3PO4 (l=0 ppm).Unless otherwise indicated, all
materials were commercially available and were used
without further purification. (R,R)-1,2-diaminocyclo-
hexane was dried over NaOH pellets for several days
prior to use. For column chromatography, silica gel 60
(220–440 mesh) purchased from Fluka was used. Ele-
mental analyses were performed by Redox s.n.c.,
Cologno Monzese, Milano. Gas chromatographic
analyses were run on a Fisons GC 8000 instrument.
cant difference was observed in the reaction rates. Poor
results were obtained when Cu(OAc)2·H2O was used as
the copper salt. Substitution of the biphenol fragment
with the (S)-binaphthol groups afforded a relevant
improvement in the ee value (6%, entry 10 versus 40%,
entry 11) together an inversion in the absolute configu-
ration of the 3-ethylcyclohexanone 6 (R versus S). In
contrast to (SaSaRR) 4a, when the diastereomer
(RaRaRR) 4b, was employed the enantioselectivity
dropped to 0 (entry 12). Evidently the relative configu-
rations of binaphthol and cyclohexanediamine moieties
play an important role in determining the reaction
stereoselectivity. On the other hand, matching and mis-
matching combinations of chirality elements were
found for related bidentate phosphoroamidite ligands.12
4.1. N-[(3,3%-Bis-tert-butyl-5,5%-bis-methoxy-1,1%-
biphenyl-2,2%-diyl)phosphate]-(R,R)-N,N%-dimethyl-1,2-
diaminocyclohexane 3a
3. Conclusions
(R,R)-N,N%-Dimethyl-1,2-diaminocyclohexane8
(300
New bidentate amino-phosphoroamidite and bisphos-
phoroamidite ligands derived from inexpensive and eas-
ily derivatizable chiral source (R,R)-1,2-diaminocyclo-
hexane were synthesized and screened in the Cu-cata-
lyzed asymmetric conjugate addition of Et2Zn to 2-
cyclohexenone. The highest 74% ee value was reached
with N,N%-dimethyl substituted P,N-ligand 3a and
Cu(OAc)2·H2O. The asymmetric induction is strikingly
dependent on the steric hindrance of the nitrogen sub-
stituent. The bidentate phosphoroamidites 4–4b
afforded lower ee values. An enantiomeric excess of
40% was obtained with ligand 4a in which the chiral
elements (Sa)-binaphthol and (R,R)-1,2-diaminocyclo-
hexane proved to be the matched combination, while
the alternative RaRaRR combination in 4b gave no
stereoselectivity.
mg, 2.1 mmol) and Et3N (640 mg, 6.33 mmol) in
toluene (5 mL) were added slowly to a solution of
(3,3%-bis-tert-butyl-5,5%-bis-methoxy-1,1%-biphenyl-2,2%-
diyl)phosphorochloridite9 2 (889 mg, 2.1 mmol) in the
same solvent (10 mL) at 0°C. After the addition was
complete, the resulting solution was stirred overnight at
room temperature and then Et3N·HCl precipitate was
removed by filtration. The solvent was evaporated in
vacuum to afford a white foam which was purified by
column chromatography (SiO2, hexane:EtOAc=3:2).
Yield (85%, 1.78 mmol, 943 mg). [h]2D1=−168 (c 3.1,
hexane). Anal. calcd for C30H45N2O4P: C. 68.16; H.
1
8.58; N. 5.30. Found: C, 68.31; H, 8.60; N, 5.19. H
NMR (C6D6): l 0.99–1.56 (m, 7H); 1.66 (s, 9H), 1.72 (s,
9H), 1.90 (m, 2H), 2.15 (m, 1H), 2.30 (s, 1H), 2.41 (d,
JPH=3.0 Hz, 3H), 2.51 (s, 3H), 3.47 (s, 3H), 3.51 (s,
Further screening of these new chiral ligands with
respect to other enones and studies on the structure of
the actual catalyst complex are currently in progress.
3H), 6.85 (d, J=3.0 Hz, 1H), 6.91 (d, J=3.0 Hz, 1H),
7.30 (d, J=3.0 Hz, 1H), 7.32 (d, J=3.0 Hz, 1H). 31P
NMR (C6D6): l 148.6 (s). 13C NMR (C6D6): l 24.3,

2130
C. G. Arena et al. / Tetrahedron: Asymmetry 14 (2003) 2127–2131
26.6, 27.0, 30.4, 30.6, 31.0, 33.4, 35.2, 54.7, 58.7, 58.7,
112.6, 113.0, 114.4, 114.5, 135.1, 136.1, 141.4, 142.4,
144.0, 144.9, 152.2, 155.6.
4a in 90% yield (1.98 mmol, 1.47 g). Anal. calcd for
C46H36N2O4P: C, 74.39; H, 4.89; N, 3.77. Found: C,
1
74.50; H, 5.08; N, 3.65. H NMR (C6D6): l 1.03 (m,
2H); 1.3 (m, 4H), 1.87 (m, 2H), 2.67 (m, 2H), 3.10 (m
br, 2H), 6.91–7.70 (m, 24H). 31P NMR (C6D6): l 151.9
(s). 13C NMR (C6D6): l 24.6, 35.7, 56.3, 56.5, 121.0,
124.2, 126.0, 127.0, 128.3, 129.5, 130.3, 131.0, 133.0,
149.8.
4.2. N-[(3,3%-Bis-tert-butyl-5,5%-bis-methoxy-1,1%-
biphenyl-2,2%-diyl)phosphate]-(R,R)-N]-(R,R)-N,N%-
diethyl-1,2-diaminocyclohexane 3b
(R,R)-N,N%-Diethyl-1,2-diaminocyclohexane8 (255 mg,
1.5 mmol) and Et3N (607 mg, 6 mmol) in toluene (5
mL) were added dropwise to a solution of (3,3%-bis-tert-
butyl-5,5%-bis-methoxy-1,1%-biphenyl-2,2%-
4.6. N,N%-Bis[-(R)-1,1%-binaphthyl-2,2%-diyl)phosphite]-
(R,R)-1,2-diaminocyclohexane 4b
diyl)phosphorochloridite 2 (1268 mg, 3 mmol) in the
same solvent (10 mL) at 0°C. The reaction mixture was
allowed to warm to room temperature over 3 h and
then filtered under N2. Evaporation of the solvent
afforded a yellowish foam which was purified over SiO2
(hexane:EtOAc=3:2) to give a white foam (75%, 1.12
mmol, 626 mg). [h]2D1=−65.4 (c 1.3, hexane). Anal.
calcd for C32H49N2O4P: C, 69.04; H, 8.87; N, 5.03.
The crude product was obtained as a white solid which
was washed with hexane (3 mL) and dried to give 4b in
85% yield (1.87 mmol, 1.39 g). Anal. calcd for
C46H36N2O4P: C, 74.39; H, 4.89; N, 3.77. Found: C,
1
74.48; H, 5.05; N, 3.63. H NMR (C6D6): l 0.98 (m,
2H); 1.38 (m, 4H), 2.1 (m, 2H), 2.96 (m, 2H), 3.22 (m,
2H), 6.70–7.90 (m, 24H). 31P NMR (C6D6): l 154.2 (s).
13C NMR (C6D6): l 24.9, 35.9, 56.5, 56.8, 121.5, 124.6,
126.3, 127.0, 128.3, 130.0, 131.7, 134.0, 149.0.
1
Found: C, 69.19; H, 8.90; N, 4.98. H NMR (C6D6): l
0.99 (m, 2H), 1.20 (m, 5H), 1.31 (m, 5H), 1.64 (m, 2H),
1.69 (s, 9H), 1.74 (s, 9H), 2.00 (m, 2H), 2.63 (m, 2H),
2.96 (s, 3H), 3.23 (m, 1H), 3.47 (s, 3H), 3.48 (s, 3H),
6.86 (d, J=3.0 Hz, 1H), 7.33 (d, J=3.0 Hz, 2H). 31P
NMR (C6D6): l 151.4 (s). 13C NMR (C6D6): l 15.7,
17.2, 24.2, 26.0, 30.4, 30.8, 30.6, 31.0, 31.2, 32.1, 32.7,
35.3, 38.7, 40.7, 54.6, 54.7, 59.5, 59.6, 112.9, 114.4,
133.1, 134.1, 141.3, 142.2, 144.1, 145.0, 155.0, 155.6.
4.7. General procedure for asymmetric 1,4-conjugate
addition
0.017 mmol of copper salt and 0.019 mmol of ligand in
toluene (9 mL), were stirred, at room temperature, for
1 h. After cooling to (−20°C), 2-cyclohexen-1-one (3
mmol) and 3 mL of Et2Zn (1.1 M solution in toluene)
were added slowly. The resulting mixture was stirred at
−20°C and monitored by GC. The reaction was
quenched with HCl 1 M. The conversions and ee values
were determined by GC with a Lipodex E column.4e
4.3. General procedure for bisphosphoroamidite ligands
To
a
stirred solution of the appropriate
phosphorochloridite9 (4.4 mmol) in toluene (10 mL)
was added dropwise (R,R)-cyclohexane-1,2-diamine
(2.2 mmol) and Et3N (8.8 mmol) in the same solvent (5
mL) at 0°C. The reaction mixture was stirred overnight
at room temperature and the precipitate of Et3N·HCl
formed was removed by filtration. The toluene was
evaporated from filtrate to give the crude product.
Acknowledgements
We thank MURST for financial support.
References
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The crude product was obtained as a yellowish foam
which was purified by column chromatography (SiO2,
hexane:EtOAc=3:2) to give 4 in 80% yield (1.76 mmol,
1,56 g). Anal. calcd for C50H68N2O8P2: C. 67.70; H.
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was washed with cold hexane (3 mL) and dried to give

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