Organic Syntheses, Vol. 78, pp. 1-13
Checked by Sarge Salman and Louis S. Hegedus.
1. Procedure
Caution! All reactions should be conducted in a well-ventilated hood.
2. Notes
3.
Dichloromethane
was purchased from Fisher Scientific, and distilled from calcium hydride
before use.
4. Silica gel 60 230-400 mesh ASTM was used.
5. Merck silica gel 60F-254 plates were used.
6. Specific rotation value of
2:
[α]D22 −143.5° (CHCl3,
c 1.24) [literature rotation for (S)-
2;
2
[α]D22 +142°
(CHCl3, c 1.035)];
1H
NMR δ: 7.27 (d, 2 H, J = 8.7), 7.41 (t, 2 H, J = 7.8),
7.58 (t, 2 H, J = 7.8), 7.62 (d, 2 H, J = 9.6), 8.00
(d, 2 H, J = 8.3), 8.13 (d, 2 H, J = 9.2).
If the material does not solidify, addition of equal amounts of
diethyl
ether and
hexane, followed by reevaporation, should
produce solid material.
8.
Palladium acetate
was purchased from Aldrich Chemical Company, Inc.,
and purified as follows:
Palladium acetate
is dissolved in hot
benzene
and filtered from insoluble material. After removal of the solvent, the residue is
triturated with a small amount of diethyl ether to give brown powder that is collected
by filtration, washed with diethyl ether and dried.
10.
Diisopropylethylamine
was purchased from Aldrich Chemical Company, Inc.,
and used without further purification.
11.
Dimethyl sulfoxide
was purchased from Aldrich Chemical Company, Inc.,
and distilled from calcium hydride before use.
12. The use of
5 mol% of
the catalyst [Pd(OAc)2-dppb] also gave a high yield of
3,
but the submitters recommend the use of 10 mol % of the catalyst to ensure high chemical
yield in the 12-hr reaction.
13. The physical properties of
3 are as follows:
[α]D20 +44.4° (CHCl3,
c 1.20),
[α]D22
+7.4° (CH2Cl2, c 1.40) [literature rotation
for (R)-
3;
3[α]D +6.29° (CH2Cl2,
c 1.00)]; IR (KBr) cm
−1
ν: 1410, 1205, 1140, and 945;
1H NMR δ: 6.9-8.1 (m, 22 H, aromatic);
31P NMR δ: 28.9 (s);
EIMS m/z 603 (M+1), 454, 201 (base peak).
Anal. Calcd for C
33H
22F
3O
4PS: C, 65.78;
H, 3.68. Found: C, 65.67; H, 3.89.
14. Assignment of all peaks in the
13C NMR is difficult
because of
13C-
31P coupling and the overlapping of peaks.
15.
Methanol and 1,4-dioxane
were purchased from Aldrich Chemical Company, Inc.,
and used without further purification.
16. An
analytically pure sample is isolated
by column chromatography on silica gel (see Note
4). The column is eluted with
50%
ethyl acetate-hexane and the
fractions are analyzed by TLC on silica gel (see Note
5) using the same eluent. Fractions containing the product are combined
and the solvent is evaporated on a
rotary evaporator to give
4 as a white powder. The physical properties of
4 are as follows (see
Note
14):
[α]D20
−105° (CHCl3, c 0.55),
[α]D20 −113° (CH2Cl2,
c 1.00) [literature rotation for (R)-
4;
3
[α]D −108.3° (CH2Cl2,
c 1.00)];
1H
NMR δ: 6.35-8.10 (m, 22 H), 9.01 (br s, 1 H);
31P NMR δ: 31.42 (s);
EIMS m/z 470 (M
+), 268 (base
peak); HRMS calcd for C
32H
23PO
2
470.1436, found 470. 1415. Anal. Calcd for C
32H
23O
2P:
C, 81.69; H, 4.93. Found: C, 81.66; H, 4.96.
18.
Celite 535 (45 gals/sq.ft/hour), purchased
from J.T. Baker, was used.
19. An analytically pure sample is isolated by column chromatography
on silica gel (see Note
4). The column is
eluted with
ethyl acetate and the fractions are analyzed by TLC
on silica gel (see Note
5) using the same
eluent. Fractions containing the product are combined and the solvent is evaporated
on a
rotary evaporator to give
5 as a white powder.
The physical properties of
5 are as follows (Note
14):
[α]D20 +121.5°
(CHCl3, c 1.30);
1H
NMR δ: 3.58 (s, 3 H), 6.75-8.01 (m, 22 H);
31P NMR δ: 28.88(s);EIMS m/z 484 (M
+), 453, 282 (base peak);
HRMS calcd for C
33H
25O
2P 484.1592, found 484.1574.
Anal. Calcd for C
33H
25O
2P: C, 81.80; H, 5.20. Found:
C, 81.77; H, 5.38.
20.
Triethylamine was
purchased from Aldrich Chemical Company, Inc., and used without
further purification.
21.
Toluene was purchased
from Aldrich Chemical Company, Inc., and distilled from calcium
hydride before use.
22.
Trichlorosilane
was purchased from Aldrich Chemical Company, Inc.,
and used without further purification. While it is difficult to measure the volume
of
trichlorosilane used accurately because of its volatility,
accurate measurement is not essential in the reduction of
5. The submitters
found that a large excess of
trichlorosilane does not interfere
with reduction of phosphine oxide in Part E, and recommend the use of trichlorosilane
(4 equiv or more to
5) to complete the reduction in an appropriate reaction
time.
23. Crystallization of the crude material from
dichloromethane-hexane
gave product
6 of 50% yield or lower. For efficiency of isolation, the submitters
recommend purifying
6 by column chromatography. The physical properties of
product
6 are as follows (see Note
14):
[α]D20 +95°
(CHCl3, c 0.27),
[α]D16
+75.7° (benzene, c 1.50) [literature rotation for (S)-
6;
4 [α]D16−59.3°
(benzene, c 1.0)];
mp 174-176°C
(recrystallization from CH
2Cl
2/n-hexane);
1H NMR δ: 3.34 (s, 3 H), 6.95-8.05 (m, 22 H);
31P NMR δ: −12.74
(s); EIMS m/z 468
(M
+), 437 (base peak); HRMS calcd for C
33H
25PO
2
468.1643, found 468.1672. Anal. Calcd for C
33H
25OP: C, 84.60;
H, 5.38. Found: C, 84.35; H, 5.44.
All toxic materials were disposed of in accordance with "Prudent Practices in the
Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
Most of the chiral phosphine ligands prepared so far and used for catalytic
asymmetric reactions are the bisphosphines,
5 which are expected to construct an effective
chiral environment by bidentate coordination to metal; they have been demonstrated
to be effective for several types of asymmetric reactions. On the other hand, there
exist transition metal-catalyzed reactions where the bisphosphine-metal complexes
cannot be used because of their low catalytic activity and/or low selectivity towards
a desired reaction pathway. Therefore chiral monodentate phosphine ligands
are required for the realization of new types of catalytic asymmetric reactions. Unfortunately,
only a limited number of monodentate chiral phosphine ligands have been reported,
6
which with few exceptions are not so useful as bisphosphine ligands. Recently, the
monodentate, optically active phosphine ligand,
2-diphenylphosphino-2'-methoxy-1,1'-binaphthyl
(MeO-MOP), and its analogs
7
have been demonstrated to provide high enantioselectivity in palladium-catalyzed hydrosilylation
of olefins
8 and
palladium-catalyzed reduction of allylic esters by
formic acid.
9 The
procedures described here allow the convenient preparation of MOP and have advantages
over previously published sequences.
4 MeO-MOP can be
prepared in five steps from binaphthol without racemization and the overall yield
is 72%. The key step in this process is the palladium-catalyzed monophosphinylation
of
2,2'-bis(trifluoromethanesulfonyloxy)-1,1'-binaphthyl,
which was originally reported by Morgans and co-workers.
3
Under the slightly modified conditions, ditriflate (R)-
2 was efficiently converted
into (R)-
3 (87% yield) without racemization. Hydrolysis of the remaining triflate
with aqueous
sodium hydroxide
in
1,4-dioxane and
methanol (2/1) gave (R)-
4. The
phenolic hydroxyl group of (R)-
4 was methylated by treatment with
methyl
iodide in the presence of
potassium
carbonate in acetone to give (R)-
5. Reduction of
phosphine oxide (R)-
5 was carried
out with trichlorosilane and triethylamine
10
and references cited therein. in toluene with heating to give the corresponding phosphine
(R)-
6 (86% yield over last 3 steps).
A variety of MOP derivatives bearing various alkoxy or siloxy groups were readily
prepared by changing the reagent used for the alkylation of
4.
7 Furthermore, the presence of the triflate group
11 in compound
3 allows one to prepare a wide range
of MOP derivatives functionalized at the 2'-position. Thus, the 2'-alkyl,
carboxyl, cyano, aminomethyl groups, etc. were introduced into the 2'-position
of MOP via transition metal-catalyzed cross-coupling, carbonylation, and cyanation
reactions.
7,12 Bis(substituted phenyl)phosphino groups were readily
introduced into the binaphthyl by the palladium-catalyzed reaction with the corresponding
diarylphosphine oxides. The same procedures used for the preparation of
3 were
followed with di(p-methoxyphenyl)phosphine oxide and
2. Subsequent hydrolysis,
alkylation, and reduction processes gave 2-di(p-methoxyphenyl)phosphino-MOP.
7 The flexibility of the synthetic route allows fine tuning of the
phosphine ligand by the introduction of several types of side chains and control of
the steric and electronic effects of the phosphino group. Needless to say, the synthetic
procedures shown here can be used for the preparation of MOPs having the (S)-absolute
configuration by using (S)-binaphthol as a starting material. In addition, a MOP analog
having the biphenanthryl skeleton (MOP-phen) was also prepared from optically active
4,4'-biphenanthrol through the same sequences mentioned above.
9b
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
(R)-2-Diphenylphosphino-2'-methoxy-1,1'-binaphthyl:
Phosphine, (2'-methoxy[1,1'- binaphthalen]-2-yl)diphenyl-, (R)-
(12);(145964-33-6)
(R)-2,2'-Bis(trifluoromethanesulfonyloxy)-1,1'-binaphthyl:
Methanesulfonic acid, trifluoro-, [1,1'-binaphthalene]-2,2'-diyl
ester, (R)- (12); (126613-06-7)
(R)-(+)-1,1'-Bi-2-naphthol: [1,1'-Binaphthalene]-2,2'-diol,
(R)-(+)- (8); [1,1'-Binaphthalene]-2,2'-diol, (R)-
(9); (18531-94-7)
Pyridine (8,9); (110-86-1)
Trifluoromethanesulfonic anhydride: Methanesulfonic
acid, trifluoro-, anhydride (8,9); (358-23-6)
(R)-(+)-2-Diphenylphosphinyl-2'-trifluoromethanesulfonyloxy-1,1'-binaphthyl:
Methanesulfonic acid, trifluoro-, 2'-(diphenylphosphinyl)[1,1'-binaphthalen]-2-yl
ester, (R)- (12); (132532-04-8)
Diphenylphosphine oxide: Phosphine oxide,
diphenyl- (8,9); (4559-70-0)
Palladium acetate: Acetic acid, palladium(2+)
salt(8,9); (3375-31-3)
1,4-Bis(diphenylphosphino)butane (dppb): Phosphine,
tetramethylenebis[diphenyl- (8); Phosphine, 1,4-butanediylbis[diphenyl-(9);
(7688-25-7)
N,N-Diisopropylethylamine: Triethylamine,
1,1'-dimethyl- (8); 2-Propanamine, N-ethyl-N-(1-methylethyl)-
(9); (7087-68-5)
Dimethyl sufoxide: Methyl sulfoxide
(8); Methane, sulfinyl bis- (9); (67-68-5)
(R)-(−)-2-Diphenylphosphinyl-2'-hydroxy-1,1'-binaphthyl:
[1,1'-Binaphthalene]-2-ol, 2'-(diphenylphosphinyl)-, (R)-
(12); (132548-91-5)
1,4-Dioxane: CANCER SUSPECT AGENT: p-Dioxane
(8); 1,4-Dioxane (9); (123-91-1)
(R)-(+)-2-Diphenylphosphinyl-2'-methoxy-1,1'-binaphthyl:
Phosphine oxide, (2'-methoxy[1,1'-binaphthalen]-2-y1)diphenyl-,
(R)- (13); (172897-73-3)
Methyl iodide: Methane, iodo-(8,9);
(74-88-4)
Triethylamine (8); Ethanamine, N,N-diethyl-(9);
(121-44-8)
Trichlorosilane: Silane, trichloro-
(8,9); (10025-78-2)
Diethyl phosphite: Phosphonic acid, diethyl
ester (8,9); (762-04-9)
Sodium (8,9); (7440-23-5)
Phenylmagnesium bromide: Magnesium, bromophenyl-
(8,9); (100-58-3)
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