Enantiomerically pure cyclopalladated diazaphospholidine

Article abstract of DOI:10.1016/j.tetasy.2007.09.005

Enantiomerically pure (S)-2-(anilinomethyl)pyrrolidine (S)-2 was obtained from (S)-proline using a modified four-step procedure in a total yield of 56%. Diamine (S)-2 was converted to diazaphospholidine (S)-1 using ^oTolP(NMe₂)_2<

Full text of DOI:10.1016/j.tetasy.2007.09.005

Tetrahedron: Asymmetry 18 (2007) 2011–2015
Enantiomerically pure cyclopalladated diazaphospholidine
Valery V. Dunina,a,* Olga N. Gorunova,a Valeriya A. Stepanova,a Pavel A. Zykov,a
Michail V. Livantsov,a Yuri K. Grishin,a Andrey V. Churakovb and
Lyudmila G. Kuz’minab
aDepartment of Chemistry, M.V. Lomonosov Moscow State University, 1 Leninskie Gory, 119991 Moscow, Russian Federation
bN.S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31 Leninsky Prosp.,
119991 Moscow, Russian Federation
Received 19 July 2007; accepted 5 September 2007
Abstract—Enantiomerically pure (S)-2-(anilinomethyl)pyrrolidine (S)-2 was obtained from (S)-proline using a modified four-step pro-
o
cedure in a total yield of 56%. Diamine (S)-2 was converted to diazaphospholidine (S)-1 using TolP(NMe2)2. The enantiomeric purity
of ligand (S)-1 and diamine (S)-2 was determined by 31P and 1H NMR spectroscopy, respectively, using a CN-palladacycle for their chi-
ral derivatization. Direct cyclopalladation of (S)-1, using Pd(OAc)2 in toluene under mild conditions regioselectively afforded the cyclo-
palladated complex with the (sp2)C–Pd bond. The aromatic C–H bond activation was confirmed by NMR spectral data and X-ray
diffraction study of the PPh3 derivative of the new P*,C*,N*-chiral phosphapalladacycle.
ꢀ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
The preparation and applications of enantiopure phospha-
palladacycles represent a new and promising field in the
chemistry of organometallic compounds. Only nine years
ago the preparation of the first optically active complex
of this kind was reported.1 Over the last decade, the first
representatives of CP- and PCP-complexes of several ste-
reochemical types have been described, including P*-chi-
ral,2 planar chiral,3 and axially chiral structures.4 These
complexes were prepared mainly from mono- or bidentate
phosphines. Examples of chiral cyclopalladated derivatives
of other classes of P-donor ligands remain rather scarce.
They include a couple of phosphite CP-palladacycles,5
and several pincer PCP-6a–c or PCN-complexes6d derived
from phosphites,6a,b phosphinites,6d or phosphorami-
dites,6c containing the {O3P:}, {C2OP:}, or {NO2P:} donor
groups, respectively. Herein we report our results on the
preparation of the first chiral CP-palladacycle containing
the {N2CP:} donor group in the polycyclic framework.7
Diazaphospholidine ligand (S)-1 was obtained8 from di-
amine (S)-2 by the method described for its P-ortho-anisyl
substituted analogue9 (Scheme 1).10,11
Ph
H
oTolP(NMe2)2
N
H
P
N
H
toluene, Δ, 2 h
N
NHPh
27%
Me
(S)-1
(S)-2
Scheme 1. Synthesis of diazaphospholidine (S)-1.
For the preparation of diamine (S)-2 from (S)-proline, a
modified version was developed combining advantages of
two known four-step protocols12a,b (Scheme 2). First, the
hydrogenation at the deprotection step was replaced by
the hydrolysis of anilide (S)-4 with CF3COOH12b,c by using
the tert-butyl carbamate (BOC) protection of (S)-proline13
instead of benzyloxycarbonyl. Secondly, in the amidation
step c the yield of anilide (S)-4 was increased from 77–
79%12a,d,e to 85% by using CH2Cl2 instead of EtOAc as
the solvent. Thirdly, in the reduction step f, the time
was shortened to 6 h and the yield of diamine (S)-2 was
*
Corresponding author. Fax: +7 495 932 8846; e-mail: dunina@org.
chem.msu.ru
0957-4166/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.09.005

2012
V. V. Dunina et al. / Tetrahedron: Asymmetry 18 (2007) 2011–2015
H
H
O
H
H
H
f
a
b,c
CO2H
d,e
NHPh
NHPh
N
H
N
H
N
N
N
CO2H
H
NHPh
O
BOC
BOC
(S)-2
(S)-5
(S)-3
(S)-4
Scheme 2. Synthesis of diamine (S)-2. Reagents and conditions: (a) (tBuOCO)2O, CH2Cl2, 0 ꢁC, 2.5 h, 95%; (b) ClCO2Et, N-methylmorpholine, 0 ꢁC,
10 min; (c) PhNH2, CH2Cl2, 0 ꢁC, 1 h; rt, 12 h, 85%; (d) TFA, CH2Cl2, 20 ꢁC, 4 h; (e) NaOHaq, 83%; (f) LiAlH4, THF, ꢂ15 ꢁC, 0.5 h; 0 ꢁC, 0.5 h; 65 ꢁC,
6 h, 83%.
increased from 63–66%12b,e to 83% by refluxing the reac-
tion mixture.
reported [a]D values vary from +15.312f to +19.712d (c 1.0–
1.1, EtOH). It is known that this method cannot be consid-
ered as being very reliable.17a Thus far, only one example of
the spectral (31P NMR) determination of diamine (S)-2
enantiopurity using covalent derivatization procedures
has been reported.14c However, the absence of any infor-
mation regarding diastereomeric signal dispersion, the use
of the reagent applicable only to C2 symmetric sub-
strates,17b along with general disadvantages of covalent
derivatization methodology, do not allow to consider this
method as a correct one.
It should be noted that our attempts to prepare diamine
(S)-2 from (S)-glutamic acid using a shorter two-step pro-
tocol14 provided only a racemic or scalemic diamine (<85%
ee) in low yields. The enantiomeric composition of the sam-
ples obtained was significantly dependent upon reaction
conditions.
The structures of diazaphospholidine (S)-1, diamine (S)-2,
and the synthetic intermediates (S)-3–5 were confirmed by
1H NMR spectra. Enantiomeric purity of ligand (S)-115
and diamine (S)-2 was determined by means of 31P and
1H NMR spectroscopy, respectively, using optically active
CN-dimer (S,S)-616 for their in situ coordinative chiral
derivatization (Scheme 3).
Diazaphospholidine ligand (S)-1 has two alternative sites
suitable for cyclopalladation: the (sp2)C–H bond of the
NPh-group (path A) and (sp3)C–H bond of the ortho-Me
group of the PTolo-substituent (path B); the latter route
was proposed in a retracted article.10,11 Although both
directions can result in the formation of optimal five-mem-
bered palladacycles, the first one has to be considered as
the preferred one by taking into account a higher reactivity
of aromatic C–H bond over aliphatic in cyclopalladation
(Scheme 4).18
The 31P NMR spectrum of the racemic diazaphospholidine
adduct with the cyclopalladated reagent revealed a signifi-
cant difference in the chemical shift values of the signals
belonging to the two diastereomers, (S,S)-7a and (R,S)-
7b (Dd 6.83 ppm). In the 1H NMR spectrum of the racemic
diamine adduct with the same reagent, (S,S)-8a/(R,S)-8b, a
good diastereomeric signal resolution (Dd 0.11 ppm) for the
anilide PhNH proton was observed; the signals of the a-
CH proton and the NMeax and NMeeq groups were also re-
solved, but at a lesser extent (Dd ꢀ 0.01 ppm). Therefore,
the presence of only one signal in the 31P NMR spectrum
of (S,S)-7a and one set of signals in the 1H NMR spectrum
of (S,S)-8a may be considered as evidence of the complete
Diazaphospholidine (S)-1 has showed a low reactivity to-
ward cyclopalladation. As a consequence, the conditions
of the C–H bond activation were optimized using racemic
ligand 1. Diverse metallation agents and conditions (vary-
ing the base and solvent nature, reaction temperature and
time) were tested. The best result was obtained in the reac-
tion of [PdCl2(NCPh)2] with ligand 1 in the presence of
AcONa in 1:1:1 ratio in a benzene–MeOH mixture under
mild conditions (rt, 2 h); the cyclopalladated dimer rac-9
was isolated in a yield of 27%.19 Both racemic dimer 9
and enantiopure (S,S)-9 were prepared in yields of 22–
25% using the more electrophilic reagent Pd(OAc)2 without
any base followed by anion metathesis (Scheme 5).20 Under
any conditions used, the reaction occurred regioselectively
to furnish dimer 9 as the only cyclopalladated complex;
enantiomeric purity (>98% ee) of the (S)-1 sample with
24
½aꢁD ¼ ꢂ375 (c 1, CHCl3) and the diamine (S)-2 sample
24
with ½aꢁD ¼ þ20:4 (c 1, EtOH).
To note, despite a wide application of diamine (S)-2 as a
chiral auxiliary, its enantiomeric composition was esti-
mated mainly by using polarimetry,12a,d–f although the
But
But
But
H
+
H
H
(S)-1
(S)-2
NMe2
2
NMe2
Cl
NMe2
Cl-
benzene, r.t., 2 h
31P NMR
CDCl3, r.t., 2 h
1H NMR
Pd
Pd
Pd
Ph
Cl
N
P
NHPh
N
Tolo
H
H
N
H
(S,S)-6
(S,S)-8a
(S,S)-7a
Scheme 3. Determination of the enantiomeric purity of diazaphospholidine (S)-1 and diamine (S)-2.

V. V. Dunina et al. / Tetrahedron: Asymmetry 18 (2007) 2011–2015
2013
H
H
H
B
Ph
A
N
N
N
N
N
(sp3)C-H bond
activation
N
P
(sp2)C-H bond
activation
P
Tolo
P
Pd
Pd
H
H
C
H2
(S)-1
Scheme 4. Alternative routes for the C–H bond activation of diazaphospholidine (S)-1.
H
i) Pd(OAc)2, toluene,
50 oC, 5 min
H
N
N
N
N
Tolo
P
ii) LiCl, Me2CO,
r.t., 30 min, 22%
P
Pd
Tolo
Cl
2
(S,S)-9
(S)-1
Scheme 5. Cyclopalladation of diazaphospholidine (S)-1.
intermediate coordination compounds and palladium(0)
were detected as side products.
To confirm the proposed regiochemistry of cyclopallada-
tion, dimer rac-9 was converted into its mononuclear phos-
phane derivative rac-10 (Scheme 6).21
H
H
N
N
N
PPh3, benzene
r.t., 1 h, 88%
N
Tolo
P
Tolo
P
Pd
Pd
Cl
Cl
2
Ph3P
Figure 1. Molecular structure and numbering scheme for complex rac-10
(benzene hemisolvate).
rac-9
rac-10
Scheme 6. Synthesis of mononuclear derivative rac-10.
to the sum of van-der-Vaals radii of these atoms of
The 31P NMR spectrum of the latter contains only two
doublets, which is the indicative of regioselective coordina-
tion of the auxiliary ligand with the CP-palladacycle. The
cis(P,P)-geometry of this complex is evident from a low
24
˚
3.1 A. Despite this proximity, the activation of the
aliphatic C–H bond was not observed in our experiments.
This may be considered as evidence of the greater reactivity
of the (sp2)C–H bonds compared to the (sp3)C–H ones.
2
1
value of the JPP constant (33.4 Hz).22 The H NMR spec-
trum of adduct 10 supports the aromatic C–H bond
activation: (i) it contains the [3H] singlet of the intact Me
group of the PTolo-substituent (d 2.70 ppm) and (ii) four
[1H] signals of the C6H4N group.
3. Conclusions
The first representative of chiral CP-palladacycles contain-
ing the {N2CP} donor group was prepared. Complete reg-
ioselectivity of cyclopalladation has been established in
favor of the aromatic C–H bond activation. It has been
shown that the new phosphapalladacycle is capable of
selective cis(P,P)-coordination of the auxiliary P-donor
ligand. The structure and stereochemistry of the CP-palla-
dacycle was unambiguously established by using NMR
spectroscopy and X-ray crystallography.
The most conclusive evidence of the proposed structure
and stereochemistry of the new phosphapalladacycle was
obtained from the X-ray diffraction study of complex
rac-10 (Fig. 1).23 It unambiguously confirms the formation
of the (sp2)C–Pd bond, relative ðSCSN1 SN3 SPÞꢃ-configura-
tion of the CP-palladacycle, and cis(P,P)-geometry of the
phosphane adduct. The CP-palladacycle has a slightly dis-
torted envelope conformation with an interplanar angle
{PdCCN1}/{PdP2N1} of 5.6ꢁ. A short contact of the metal
with the Me group of the ortho-tolyl substituent is to be
A modified protocol for the synthesis of the key synthetic
intermediate, (S)-2-(anilinomethyl)pyrrolidine, has been
˚
noted, with a Pdꢄ ꢄ ꢄH(27) distance of 2.74(4) A compared

2014
V. V. Dunina et al. / Tetrahedron: Asymmetry 18 (2007) 2011–2015
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1
diazaphospholidine ligand was determined by H or 31P
NMR spectroscopy, respectively, using the CN-pallada-
cycle for chiral derivatization.
13. Kamal, A.; Reddy, K. L.; Devaiah, V.; Shankaraiah, N.;
Reddy, Y. N. Tetrahedron Lett. 2004, 45, 7667.
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15. Determination of the enantiomeric purity of ligand (S)-1: A
solution of CN-dimer (S,S)-6 (0.018 mmol) and ligand (S)-1
(0.0362 mmol) in C6H6 (1 mL) was stirred at rt for 2 h, then
the solvent was evaporated. The crude diastereomer (S,S)-7a
was dissolved in CDCl3 and 31P NMR spectrum was
measured that contained one signal at d 108.85 ppm (s).
After using the same procedure for the racemic ligand 1, two
signals at d 108.84 and 102.01 ppm were detected.
16. Dunina, V. V.; Kazakova, M. Yu.; Grishin, Yu. K.;
Malyshev, O. R.; Kazakova, E. I. Russ. Chem. Bull. 1997,
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Yu. K.; Kuz’mina, L. G.; Kataeva, N. A.; Churakov, A. V.
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6. (a) Wallner, O. A.; Olsson, V. J.; Eriksson, L.; Szabo, K. J.
Inorg. Chim. Acta 2006, 359, 1767; (b) Baber, R. A.; Bedford,
R. B.; Betham, M.; Blake, M. E.; Coles, S. J.; Haddow, M.
F.; Hursthouse, M. B.; Orpen, A. G.; Pilarski, L. T.; Pringle,
P. G.; Wingad, R. L. Chem. Commun. 2006, 3880; (c) Benito-
17. (a) Parker, D. Chem. Rev. 1991, 91, 1441; (b) Brunel, J. M.;
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339.
´
Garagorri, D.; Bocokie, V.; Mereiter, K.; Kirchner, K.
19. Cyclopalladation of ligand rac-1:
A
solution of
Organometallics 2006, 25, 3817; (d) Motoyama, Yu.; Shi-
mozono, K.; Nishiyama, H. Inorg. Chim. Acta 2006, 359,
1725.
[PdCl2(PhCN)2] (0.35 mmol) and ligand rac-1 (0.35 mmol)
in benzene (5 mL) was stirred at rt for 40 min; then a solution
of AcONa (0.35 mmol) in MeOH (0.5 mL) was added and the
reaction mixture stirred at rt for 2 h. After chromatographic
purification on silica using a dry column (eluent C6H6) dimer
(R,S)-9 was isolated in a yield of 27%: mp (dec) 152–154 ꢁC,
Rf 0.79 (10:1 C6H6–Me2CO). 31P NMR: d 151.04 (s). Anal.
Calcd for C36H40N4P2Cl2Pd2: C, 49.44; H, 4.62; N, 6.41.
Found: C, 50.00; H, 4.49; N, 6.21.
7. (a) Preliminary data on this subject were reported recently:
Gorunova, O. N.; Livantsov, M. V.; Grishin, Yu. K.;
Churakov, A. V.; Kuz’mina, L. G.; Dunina, V. V. Book of
Abstracts of International Conference ‘Modern Trends in
Organoelement and Polymer Chemistry’, Moscow, May 30–
June 4; 2004; P91; (b) Gorunova, O. N.; Stepanova, V. A.;
Livantsov, M. V.; Grishin, Yu. K.; Dunina, V. V.; Churakov,
A. V.; Kuz’mina, L. G. Book of Abstracts of International
Conference Devoted to the Memory of Prof. Kost A. N.,
Moscow, October 17–21, 2005; C51.
20. Cyclopalladation of ligand (S)-1: A solution of Pd(OAc)2
(0.1857 mmol) and ligand (S)-1 (0.1856 mmol) in toluene
(3 mL) was stirred at 50 ꢁC for 5 min and then evaporated to
dryness. The residue was treated with LiCl (0.370 mmol) in
acetone (2 mL), the mixture was stirred at rt for 30 min and
then evaporated to dryness. After chromatographic purifica-
tion using dry column (eluents C6H6 and 20:1 C6H6–acetone),
dimer (S,S)-9 was isolated in a yield of 22% as a light-yellow
8. (2R,5S)-2-(2-Methylphenyl)-3-phenyl-1,3-diaza-2-phosphabi-
cyclo[3.3.0]octane, (S)-1.
A solution of diamine (S)-2
(0.01 mol, >98% ee) in toluene (25 mL) was treated with
oTolP(NMe2)2 (0.01 mol) under argon and refluxed while
stirring for 2 h. After twofold distillation and threefold
recrystallization from ethyl acetate ligand (S)-1 was obtained
25
amorphous powder. ½aꢁD ¼ ꢂ106:4 (c 0.235, CH2Cl2), 31P
24
NMR: d 151.04 (s).
as stable colorless crystals in the yield of 27%: ½aꢁD ¼ ꢂ345 (c
1.00, CHCl3), bp 112 ꢁC/1 mmHg. 31P NMR (d, ppm): 98.23
21. Phosphine adduct 10 preparation: A solution of dimer rac-9
(0.0172 mmol) and PPh3 (0.0378 mmol) in benzene (2 mL)
was stirred at rt for 1 h. It was evaporated in vacuo to
dryness, and the crude product was purified using dry column
chromatography (eluents hexane, C6H6, 20:1 and 10:1 and
C6H6–Me2CO) to afford adduct rac-10 in 88% yield as a
colorless powder. After recrystallization from a dichloro-
methane/hexane mixture: mp (dec) 192–195 ꢁC, Rf 0.56 (15:1
C6H6–Me2CO). Anal. Calcd for C36H35N2P2ClPd: C, 61.81;
H, 5.05; N, 4.01. Found: C, 62.43; H, 5.05; N, 4.14. 1H NMR:
1
o
(s). H NMR (d, ppm; J, Hz): 7.24–7.14 (m, 4H, Tol), 7.07
3
(m, 2H, m-H of NPh), 6.82 (d, 2H, JHH 7.9, o-H of NPh),
3
6.78 (t, 1H, JHH 7.4, p-H of NPh), 3.91 (m, 1H, 3JH5H4A 8:5,
3JH5H4B 2:5, C5H), 3.62 (ddd, 1H, 2JHH 8.8, 3JH4AH5 7:2, 3JHP
1.2, C4HA), 3.39 (m, 1H, C8HA), 3.23 (m, 1H, C8HB), 3.12
(ddd, 1H, 2JHH 8.8, 3JH4BH5 8:8, 3JHP 2.5, C4HB), 2.05 (m, 1H,
C6HA), 1.92 (m, 2H, C7H2), 1.83 (m, 1H, C6HB).
9. Breeden, S.; Wills, M. J. Org. Chem. 1999, 64, 9735.
10. The synthesis and cyclopalladation of ligand (S)-1 was
reported previously,11a but this publication was retracted by
the authors.11b
3
3
7.46 (dd, 6H, JHH 7.6, JHP 10.2, o-H of PPh3), 7.36 (t, 3H,
3JHH 7.7, p-H of PPh3), 7.26 (m, 6H, m-H00of PPh3); 7.19 (dd,
00
1H, 3Jav 7.5, H4 of Tolo), 6.80 (m, 1H, H3 of Tolo), 6.77 (dd,
11. (a) Brunel, J. M.; Hirlemann, M.-H.; Heumann, A.; Buone,
G. Chem. Commun. 2000, 1869; (b) Buone, G.; Brunel, J. M.;
Hirlemann, M.-H.; Heumann, A. Chem. Commun. 2002,
1318.
00
3
4
1H, Jav 7.4, H5 of Tolo);ꢀ 8.60 (dddd, 1H, JHP(PPh ) 7.2,
3
0
4JHP2 24.0, JHH 7.6, JHH 1.3, H6 of C6H4N), 7.05 (dd, 1H,
3
4
12. (a) Asami, M.; Ohno, H.; Kobayashi, S.; Mukayama, T. Bull.
Chem. Soc. Jpn. 1978, 51, 1869; (b) Lee, D.; Long, S. A.;
Murray, J. H.; Adams, J. L.; Nuttall, M. E.; Nadeu, D. P.;
00
ꢀ Signal of H6 proton of Tolo substituent is hidden under a solvent signal.

V. V. Dunina et al. / Tetrahedron: Asymmetry 18 (2007) 2011–2015
2015
0
0
3JH4
6
7:2, JH4
7:5, H4 of C6H4N), 6.82 (m, 1H, H50 of
by direct methods [SHELXS-97] and refined by full-matrix
least-squares on F2 [SHELXL-97] with anisotropic thermal
parameters for all non-hydrogen atoms. All H atoms (except
solvent benzene molecule) were found from difference Fourier
synthesis and refined isotropically. Hydrogen atoms of
solvent molecule were placed in calculated positions and
refined using a riding model. The final residuals were
R1 = 0.0253, wR2 = 0.0616 for 4083 reflections with
I > 2r(I) and 0.0648, 0.0682 for all data and 546 parameters.
3
0
0
H30
50
C H4HN), 6.48 (dd, 1H, JHP(PPh ) 3.9, JHH 7.5, H3 of
5
3
3
C6H4N); 3.82 (m, 1H, C8HA), 3.75 (m, 1H, C5H), 3.23–3.31
(m, 2H, C4H2), 3.16 (m, 1H, C8H), 2.70 (s, 3H, CH3 of Tolo),
2.00 (m, 1H, C6H), 1.76 (m, 2H, C7H2), 1.53 (m, 1H, C6H).
22. Dunina, V. V.; Gorunova, O. N. Russ. Chem. Rev. 2005, 74,
871.
23. C36H35Cl1N2P2PdÆ0.5C6H6, M = 738.50, monoclinic, a =
˚
9.630(2), b = 18.321(2), c = 19.566(2) A, b = 100.06(1)ꢁ,
3
V = 3399.0(9) A , space group P21/c, Z = 4, Dc = 1.443 g/
Goof = 1.020, maximum Dq = 0.326 e Aꢂ3. Crystallographic
˚
˚
cm3, F(000) = 1516, l(Mo Ka) = 0.750 mmꢂ1. The crystal of
approximate dimensions 0.4 · 0.3 · 0.2 was used for data
data (excluding structure factors) for the structure reported in
this paper have been deposited with the Cambridge Crystal-
lographic Data Centre as supplementary publication no.
CCDC-649695. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [fax: int. code +44(1223)336-033; e-mail:
deposit@ccdc.cam.ac.uk].
collection.
A total of 7865 reflections (5979 unique,
Rint = 0.0209) were measured on an Enraf-Nonius CAD4
diffractometer (graphite monochromatized Mo Ka radiation,
k = 0.71073 A) at room temperature. Data were collected in
2.11 < h < 24.98
21, ꢂ2 6 l6 23) using x scan mode. The structure was solved
˚
the
range
(ꢂ11 6 h 6 11, ꢂ2 6 k 6
24. Bondi, A. J. Phys. Chem. 1964, 68, 441.