Role of the planar chirality of imine-phosphine hybrid ligands bearing chromium tricarbonyl in the palladium-catalyzed asymmetric allylic alkylation

Article abstract of DOI:10.1016/S0040-4039(00)00778-4

Optically active Cr-complexed imine-phosphine hybrid ligands having only a planar chirality showed excellent enantioselectivity for the palladium- catalyzed allylic alkylation with predictable absolute configuration. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00778-4

Tetrahedron Letters 41 (2000) 5083±5087
Role of the planar chirality of imine±phosphine hybrid
ligands bearing chromium tricarbonyl in the palladium-
catalyzed asymmetric allylic alkylation
Hye-Young Jang, Hwimin Seo, Jin Wook Han and Young Keun Chung*
School of Chemistry and Center for Molecular Catalysis, Seoul National University, Seoul 151-742, South Korea
Received 27 March 2000; revised 1 May 2000; accepted 12 May 2000
Abstract
Optically active Cr-complexed imine±phosphine hybrid ligands having only a planar chirality showed
excellent enantioselectivity for the palladium-catalyzed allylic alkylation with predictable absolute con®g-
uration. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: planar chirality; chromium tricarbonyl; allylic alkylation.
An asymmetric C±C bond formation reaction of allylic compounds catalyzed by palladium
complex has been investigated extensively.1 To achieve high chemo-, regio-, diastereo-, and
enantioselectivity, the P,N-ligands2 have been prepared and studied because they are easily
amenable by steric and electronic modi®cation. Most of the known P,N-chiral ligands possess
planar and central chirality. But ecient chiral ligands with only the planar chirality have not
been developed so much.3 Recently, planar chiral P,N-ligands bearing chromium tricarbonyl
have been used4 as chiral ligands in the catalytic asymmetric reaction. In the continuation of our
e€orts5 to explore new planar chiral P,N-ligands, we present here the preparation of planar chiral
imino±phosphine chromium ligands and their use in palladium-catalyzed asymmetric allylic
alkylation.
The planar chiral P,N-ligands 2a±e were synthesized from 1 (Eq. (1)).6,y
* Corresponding author. Tel: +82-2-880-6662; fax: +82-2-889-1568; e-mail: ykchung@plaza.snu.ac.kr
y
All the compounds have been fully characterized by 1H NMR, IR, and elemental analysis (See the Supporting
Information). Selected data for 2a: Mp 56ꢀC; IR nCO 1958, 1884 cm^1; 1H NMR (CDCl3): ꢀ 8.54 (d, 4.8 Hz, 1H), 7.43±
7.12 (m, 15H), 5.94 (d, 7.0 Hz, 1H), 5.45 (t, 6.5 Hz, 1H), 5.24 (t, 6.0 Hz, 1H), 4.65 (d, 6.5 Hz, 1H), 4.40 (q, 6.6 Hz, 1H),
1.20 (d, 6.6 Hz, 3H) ppm. Anal. calcd for C30H24CrNO3P: C, 68.05; H, 4.57; N, 2.65. Found: C, 67.68; H, 4.95; N, 2.86;
26
‰ꢁŠ ^84 (c 0.10, CH2Cl2).
D
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00778-4

5084
ꢁ1†
The palladium-catalyzed asymmetric allylic alkylation of rac-1,3-diphenyl prop-2-en-1-yl acetate
(A) with dimethyl malonate was carried out (Eq. (2)).
ꢁ2†
Reaction of A with sodium malonate in the presence of 2.4 mol% Pd(0) catalyst gave the allylic
alkylation product methyl-2-carbomethoxy-3,5-diphenylpent-4-enolate in high yields.
We have screened the reaction medium using chiral ligand 2a (entries 1±4 in Table 1).
Signi®cant solvent e€ects on the enantioselectivities were observed; the highest enantioselectivity
(82%) was achieved in DMSO at room temperature. Thus, DMSO was chosen a reaction medium
for our reactions.
Table 1
Pd-catalyzed allylic alkylation
We next investigated the ligand e€ect by employing 2b±e as a ligand (entries 5±8 in Table 1). All
of the ligands showed good to excellent enantioselectivities. It is noticeable that the change of the
central chirality in the imine (entry 4 versus 5 in Table 1) does not a€ect the absolute con®guration

5085
and enantioselectivity of the reaction. This observation was quite exceptional because it is widely
accepted6 that the absolute con®guration is governed mainly by the central chirality, but not by
the planar chirality. Interestingly, subjecting 2c having no central chirality to the same reaction
condition provided the S con®guration product with 79% ee. This observation suggests that the
central chirality may play no role in our reaction. Furthermore, for the chiral ligands 2d and e
having no central chirality, as the steric bulkiness increases, the ee values were promoted to 91
and >98%, respectively. Thus, it seems that the ee value is dependent upon the steric bulkiness
on the imino group, but not the central chirality.
To investigate the e€ect of planar chirality on the absolute con®guration of the product, an (S)-
planar chiral enantiomer 2f was synthesized (Eq. (3))7,{ and subjected to palladium-catalyzed
allylic substitution.
ꢁ3†
After the reaction, an R con®guration product was obtained with 95% ee. Thus, the absolute
con®guration is controlled only by the planar chirality.
Generally, the palladium-catalyzed allylic alkylation would proceed through the major
diastereomer at equilibrium.8 In the reaction, two diastereomeric allylpalladium complexes, the
exo±syn±syn and the endo±syn±syn isomer, can be considered as p-allyl intermediate. However,
for the CD2Cl2 solution of palladium complex derived from 2c only one isomer with the 31P
NMR chemical shift of 30.04 ppm was observed. The 1H NMR NOE study between allyl protons
and methyl protons of the tert-butyl imine veri®es that the major isomer is the exo±syn±syn one
(Fig. 1).
Figure 1. NOE connectivity between allyl and tert-butyl protons
Selected data for 2f: mp 148ꢀC. Anal. calcd for C35H26CrNO3P: C, 71.06; H, 4.43; N, 2.37. Found: C, 71.32; H, 4.65;
{
25
D
N, 2.42; ‰ꢁŠ +144 (c 0.10, CH2Cl2).

5086
To elucidate the stereochemical outcome, the X-ray crystal structure of palladium complex 3
prepared by the reaction of [Pd(Z3-PhCHCHCHPh)Cl]2 with 2c was determined. The crystal
structure shows the exo±syn±syn geometry with pseudo square-planar coordination around palla-
dium (Fig. 2),9,x which is consistent with the result of the NOE study. The 1,3-diphenylallyl ligand
is rotated in a clockwise manner along the Pd-allyl axis so that the terminal allyl carbon trans to
phosphorus is above the P-Pd-N coordination plane. The steric e€ect of the tert-butyl group would
enforce the allyl group away at one terminus and thereby the carbon atom trans to phosphorus
could be an enhanced reaction center for nucleophilic addition since it may carry more positive
charge character.10 The nucleophilic attack results in the S con®guration. Thus, the X-ray structure
explains the absolute con®guration of the product.
Figure 2. View of the structure of 3. The BF^4 ion is omitted for clarity
In conclusion, we have demonstrated that the imine±phosphine ligands with only the planar
chirality can produce an excellent asymmetric environment for palladium-catalyzed allylic alkyl-
ation. Furthermore, both R and S enantiomers were obtained in excellent enantioselectivities by
changing the enantiomer of the planar chiral ligand.
Selected data for 3: mp 180ꢀC (dec.); IR nCO 1968, 1916 cm^1; 1H NMR (CD2Cl2): ꢀ 8.14 (d, 4.2 Hz, 1H), 7.66±6.82
x
(m, 20H), 6.54 (dd, 9.8, 14.4 Hz, 1H), 6.41 (dd, 9.3, 14.3 Hz, 1H), 5.83 (t, 6.0 Hz, 1H), 5.63 (m, 1H), 5.42 (t, 6.4 Hz,
1H), 4.93 (t, 6.6 Hz, 1H), 4.19 (d, 9.2 Hz, 1H), 0.85 (s, 9H) ppm. Anal. calcd for C41H37BCrF4NO3PPd: C, 56.74; H,
4.30; N, 1.61. Found: C, 56.52; H, 4.67; N, 1.55. Crystal data for 3: C41H37BCrF4NO3PPd, M=867.90, orthorhombic,
P212121 (No. 19), a=11.146(2), b=18.170(3), c=19.107(4) A, U=3869.6(13) A3, Z=4, Dc=1.490 g cm^3, l(Mo±
Ka)=0.845 mm^1, Enraf±Nonius CAD4 di€ractometer, l=0.71073 A. 3833 re¯ections measured, 3806 unique re¯ec-
tions used in all calculations. The structure was solved by direct methods (SHELXS-86) and re®ned by full-matrix
least-squares (SHELXL-93) on F2. The ®nal R=0.1357 (obsd), wR(F2)=0.3411 (obsd), S=1.034 (obsd). Due to the
consistently poor quality of the crystals, re®nement of the structure has been unsatisfactory. A detailed discussion of
the structural parameters is therefore inappropriate, but it clearly shows that the molecular structure has the exo±syn±
syn geometry. Atomic coordinates, bond lengths and angles, and thermal parameters have been deposited at the
Cambridge Crystallographic Data Center. See: Notice Authors, Issue No. 1.

5087
Acknowledgements
We acknowledge the KOSEF (1999-1-122-001-5), the KOSEF through the Center for Molecular
Catalysis, and the Brain Korea 21 Program for ®nancial support.
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