First quinineꢀbased aryl phosphite
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 9, September, 2002
1749
6
cone angle (θ = 190°; the angle was determined by the
be expected to be manifested in other asymmetric reacꢀ
tions. In addition, this is presently the only example of
the involvement of chiral phosphites in catalytic enantioꢀ
selective rearrangements.
7
semiempirical AM1 quantumꢀchemical method ). This
angle is larger than the corresponding angles even in
such sterically hindered compounds as phosphite derivaꢀ
8
tives of calixarenes (θ = 160—176°). Hence, compound 1
is a promising ligand for asymmetric catalysis.
Experimental
In particular, ligand 1 was used in the enantioselective
rearrangement of Oꢀallylic thiocarbamate 2 into Sꢀalꢀ
lylic thiocarbamate 3 (Scheme 2). It should be noted
that the conversion of the substrate and the optical yield
were increased as the 1/Pd molar ratio was increased.
Thus, the conversion was 57% with ee 37% (S) and 100%
with ee 47% (S) at 1/Pd = 1 and 2, respectively.
The 1H, 13C, and 31P NMR spectra were recorded on a
Bruker AMXꢀ400 instrument (400.13 MHz for H, with respect
1
1
3
to Me Si; 100.61 MHz for C, with respect to CDCl , δ 76.91;
4
3
C
31
1
61.98 MHz for P, with respect to a 85% H PO solution in
3 4
D O). The assignment of the signals in the C NMR spectrum
13
2
of ligand 1 was made with the use of the DEPT procedure and
1
0
using the data published earlier. The optical rotation was
measured on a Perkin—Elmer 141 instrument. The mass specꢀ
tra (EI, 70 eV) were obtained on a Varian MATꢀ311 instruꢀ
Scheme 2
1
ment. The conversion of substrate 2 was monitored by H NMR
spectroscopy. The optical yield of compound 3 was determined
by GC with the use of an octakis(6ꢀOꢀmethylꢀ2,3ꢀOꢀdipentyl)ꢀ
9
γꢀcyclodextrin chiral column as described previously.
The starting substrate 2 was synthesized according to a
1
1
known procedure.
All reactions were carried out under an atmosphere of dry
argon with the use of anhydrous solvents.
Di(2,6ꢀdimethylphenyl) [(5ꢀvinylquinuclidinꢀ2ꢀyl)(6ꢀmethꢀ
oxyquinolinꢀ4ꢀyl)methyl] phosphite (1). A solution of di(2,6ꢀdiꢀ
1
2
methylphenyl) chlorophosphite (0.842 g, 2.7 mmol) in benꢀ
zene (8 mL) was added dropwise to a solution of quinine
(
(
0.876 g, 2.7 mmol) and Et N (0.4 mL, 2.7 mmol) in benzene
25 mL) with intense stirring and cooling to 0 °C. The resulting
3
solution was heated to boiling, cooled to ∼20 °C, and filtered
off. The filtrate was concentrated in vacuo (40 Torr) and hexꢀ
ane (25 mL) was added to the residue. The precipitate that
formed was filtered off. The filtrate was concentrated in vacuo
(
40 Torr) and then kept in vacuo (1 Torr) at 50 °C for 2 h.
Compound 1 was obtained in a yield of 1.40 g (87%) as a white
viscous substance, [α]D23 –212.2 (c 0.8, CHCl ). Found (%):
3
dba is dibenzylideneacetone, L* = 1 or 4
C, 72.21; H, 7.16; N, 4.82; P, 5.30. C36P41N2O4P. Calcuꢀ
lated (%): C, 72.46; H, 6.93; N, 4.69; P, 5.19.
1
3
The presence of the quinoline core in ligand 1 is a
necessary condition for the attainment of a noticeable
stereoinduction. Thus, quincoridineꢀbased aryl phosꢀ
phite 4 prepared by us earlier appeared to be completely
nonselective. In this reaction, we achieved only 2% ee
C NMR (C D ), δ: 158.10—101.69 (CAr); 142.19
6 6
2
(s, CH=); 114.14 (s, CH2=); 75.41 (d, CHOP, JC,P = 15.2 Hz);
3
6
4
2
1.20 (d, C(2), J = 6.8 Hz); 56.88 (s, C(6)); 54.98 (s, MeO);
C,P
2.53 (s, C(7)); 40.18 (s, C(5)); 28.09 (s, C(4)); 27.94 (s, C(8));
3.21 (s, C(3)); 18.15, 18.08, 17.91, and 17.85 (all s, Me(Ar)).
3
1
+
P NMR (C D ), δ: 147.73. MS, m/z (I (%)): 596 [M] (2),
6
6
rel
(
S) with the conversion of 60% (see Scheme 2).
+
+
4
1
75 [M – Me C H ] (23), 307 [M – (Me C H O) PO] (75),
2 6 3 2 6 3 2
Apparently, the low selectivity is attributable to the
+
22 [Me C H OH] (100).
2
6
3
fact that the quinoline fragment of phosphite 1 contains
Palladiumꢀcatalyzed rearrangement of Oꢀallylic thiocarbamate
2 into Sꢀallylic thiocarbamate 3. A solution of Pd (dba) •CHCl
2
the sp ꢀhybridized donor N atom capable of being inꢀ
2
3
3
3
–6
volved in additional coordination to the metal atom. It
should also be taken into account that the phosphorus
center in ligand 4 has a substantially smaller cone angle
θ = 157°).
It should be noted that the (S) enantiomer of comꢀ
pound 3 has been prepared recently9 with the nearly
quantitative optical purity. Nevertheless, the high degree
of enantioselectivity achieved in the reaction with the
use of ligand 1 demonstrates that this ligand has a high
potential as a stereoinducing reagent. This potential would
(0.009 g, 8.5•10 mol, 1.7 mol.%) and ligand 1 (0.012 g,
2•10 mol, 2 mol.% or 0.024 g, 4•10–5 mol, 4 mol.%) in
–5
CH Cl (5 mL) was stirred for 20 min. Then substrate 2 (0.171 g,
2
2
0
.001 mol) was added. The resulting solution was kept at 20 °C
(
for 48 h, a saturated NaCl solution (10 mL) was added, and the
reaction mixture was stirred for 1 h. The product was extracted
with CH Cl2 (3×10 mL). The combined organic phases were
2
dried with MgSO and filtered. The filtrate was concentrated in
4
vacuo (40 Torr). The residue (yellow oil) was chromatographed
on a column (2×30) with silica gel Kieselgel 60 (0.04—0.06 mm,
Merck) using a 4 : 1 pentane—EtOAc mixture as the eluent.
Bondarev
