Stereoselective [2,3] sigmatropic rearrangement of acyclic allylic phosphinites

Article abstract of DOI:10.1039/b313979d

The [2,3] sigmatropic rearrangement of open-chain allylic phosphinites was found to proceed with high stereoselectivity, allowing the preparation of chiral (E) allylic phosphine oxides and aminophosphine oxides with 99% ee and a perfect transfer of the ch

Full text of DOI:10.1039/b313979d

Stereoselective [2,3] sigmatropic rearrangement of acyclic allylic
phosphinites
Frédéric Liron and Paul Knochel*
Ludwig-Maximilians-Universität München, Department Chemie, Butenandtstrasse 5-13, Haus F, D-81377
München, Germany. E-mail: Paul.Knochel@cup.uni-muenchen.de; Fax: (+)49 089 2180 77680; Tel: (+)49
089 2180 77681
Received (in Cambridge, UK) 4th November 2003, Accepted 4th December 2003
First published as an Advance Article on the web 5th January 2004
The [2,3] sigmatropic rearrangement of open-chain allylic
phosphinites was found to proceed with high stereoselectivity,
allowing the preparation of chiral (E) allylic phosphine oxides
and aminophosphine oxides with 99% ee and a perfect transfer
of the chiral information.
the preparation of chiral acyclic phosphine oxides (bearing an a-
quaternary center) as well as chiral aminophosphine oxides.
We first prepared a range of allylic alcohols of type 1 using
standard methods8 in order to study the influence of the substituents
on the stereoselectivity of the rearrangement. The treatment of the
alcohols 1 with Ph2PCl (1 equiv.) in the presence of DMAP (1
equiv.) in Et2O (rt, 0.5 h) provided quantitatively the corresponding
allylic phosphinites 2a–f. The heating of these phosphinites at 80
°C for 3–20 h provided the (E) and (Z) allylic phosphine oxides 3a–
f with good to excellent stereoselectivity (Table 1 and Scheme
1).
In contrast to cyclic allylic phosphinites, the acyclic phosphinites
2a–f can adopt two conformations suitable for the [2,3] sigmatropic
rearrangement (2A and 2B). In the case of 2A, there is a moderate
steric interaction between the allylic hydrogen atom and the
substituent R1, resulting in a weak 1,3-allylic strain.9 On the other
hand, the second conformation 2B that we can envision for the
sigmatropic rearrangement shows a strong interaction between the
Chiral phosphines and diphosphines are important ligands for the
performance of asymmetric metal catalyses.1 The stereoselective
synthesis of chiral phosphines has attracted much attention.2
Recently, we have shown that cyclic allylic phosphinites undergo
highly stereoselective [2,3] sigmatropic rearrangements,3 affording
chiral phosphines and diphosphines which are suitable ligands for
the performance of Rh(
)-catalyzed asymmetric hydrogenations4
I
and hydroborations.5 Although this rearrangement is known to be
highly enantioselective using chiral phosphorus centers,6 the
transfer of chirality in the carbon backbone of an open-chain system
has not been studied. Much work has also been reported on the
rearrangement of propargylic phosphinites,7 but the stereochemical
course of this rearrangement has not been addressed. Herein, we
wish to report applications of this sigmatropic rearrangement for
Table 1 Influence of the substituents on the stereoselectivity
Yield
(%)c
Entry
Alcohol R1
R2
R3
Producta E : Zb
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
Me
Bu
Me
2-pyr
H
H
H
H
H
Me
Me
H
H
3a
3b
97 : 3
96 : 4
95 : 5
85 : 15
> 99 : 1
> 99 : 1
75
60
50
50
50
50
Me 3c
H
H
H
3d
3e
3f
Bu
a R = CH2–CH2–(1-naphthyl). b Determined by 31P NMR. c Isolated yield
of analytically pure products.
Scheme 2 Reagents and conditions: (i) Bu3SnH (1.5 equiv.), PdCl2(PPh3)2
(0.05 equiv.), THF, rt, 0.5 h (70%); (ii) n-BuLi (2 equiv.), THF, 250 °C to
rt, 1 h, then ZnCl2 (2 equiv.), 250 °C to rt; (iii) 2-bromopyridine (3 equiv.),
Pd(PPh3)4 (0.15 equiv.), THF, 65 °C, 48 h (50%); (iv) 2-TfO-quinoline (3
equiv.), Pd(PPh3)4 (0.15 equiv.), THF, 65 °C, 48 h (35%); (v) I2 (1.1 equiv.),
CH2Cl2, rt, (80%), then 2-pyridylmethylzinc chloride (3 equiv.), THF, 65
°C, 48 h (25%); (vi) DMAP (1 equiv.), (2-furyl)2PCl, Et2O, rt, 0.5 h (95%);
(vii) DMAP (1 equiv.), Ph2PCl (1 equiv.), Et2O, rt, 0.5 h (95%).
Scheme 1 Reagents and conditions: (i) PPh2Cl (1 equiv.), DMAP (1 equiv.),
Et2O, rt, 0.5 h; (ii) toluene, 80 °C, 3 h.
304
C h e m . C o m m u n . , 2 0 0 4 , 3 0 4 – 3 0 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Notes and references
1 R. Noyori, in Asymmetric Catalysis in Organic Synthesis, Wiley-
Interscience, New York, 1994.
2 For recent reviews, see: F. Lagasse and H. B. Kagan, Chem. Pharm.
Bull., 2000, 48, 315; O. I. Kolodiazhnyi, Tetrahedron: Asymmetry,
1998, 9, 1279; M. Ohff, J. Holz, M. Quirmbach and A. Börner,
Synthesis, 1998, 1391; K. H. Pietrusiewicz and M. Zablocka, Chem.
Rev., 1994, 94, 1375; M. Baudler and K. Glinka, Chem. Rev., 1993, 93,
1623; J. Holz, M. Quirnbach and A. Börner, Synthesis, 1997, 983.
3 S. Demay, K. Harms and P. Knochel, Tetrahedron Lett., 1999, 40, 4981;
S. Demay, F. Volant and P. Knochel, Angew. Chem., Int. Ed., 2001, 40,
1235.
Scheme 3 Reagents and conditions: (i) PPh2Cl (1 equiv.), DMAP (1 equiv.),
Et2O, rt, 0.5 h; (ii) toluene, 110 °C, 12 h.
4 A. L. Casalnuovo, T. V. Rajanbabu, T. A. Ayers and T. H. Warren, J.
Am. Chem. Soc., 1994, 116, 9869.
substituents R and R1. The sigmatropic rearrangement via 2A leads
to the (E)-allylic phosphine oxides 3 whereas the thermal reaction
via 2B furnishes predominantly the (Z)-allylic phosphine oxides
3.
As can be seen from Table 1, the presence of a small substituent
R2 (R2 = H; entries 1–4) led to lower E : Z ratios (85 : 15 to 97 :
3), whereas the phosphinites bearing a methyl group at this position
(R2 = Me) gave only the (E) products (E)-3e and (E)-3f in 50%
yield (entries 5 and 6). The presence of a pyridine ring with a basic
nitrogen was also possible (entry 4; R1 = 2-pyr), however a lower
E : Z ratio was observed. Nevertheless, we have applied this method
to the preparation of three allylic alcohols 4–6 starting from the
propargylic alcohol 7 (Scheme 2).8
Chiral propargylic alcohol (S)-710 was used ( > 99% ee) for the
preparation of the allylic alcohol 5 with 99% ee. All three alcohols
were converted into the corresponding phosphinites 8–10 under
standard conditions, using either chlorodiphenylphosphine or
chlorodi(2-furyl)phosphine. The [2,3] sigmatropic shift was re-
alized in the case of the 2-furyl substituted phosphinites 8 and 10 at
110 °C (3 h) and afforded only the (E)-allylic phosphine oxides 11
and 13 respectively in 70 and 48% yield ( > 99% E), Scheme 2. In
the case of the chiral phosphinite 9, a smooth rearrangement
occurred at 80 °C (3 h) and provided the desired aminophosphine
oxide 12 in 90% yield (99% ee, > 99% E, Scheme 2).
Finally, we examined the rearrangement of the (E)-cinnamic
alcohol derivative 14 which was prepared in 99% ee.11 Its
conversion to the corresponding phosphinite by the reaction with
Ph2PCl (1 equiv.) in the presence of DMAP (1 equiv.) in Et2O was
complete at rt within 0.5 h. Heating in toluene at reflux for 12 h
furnished the desired allylic phosphine oxide 15 in 75% yield as a
single stereoisomer (99% ee, > 99% E), Scheme 3.
5 J. M. Brown, D. I. Hulmer and T. P. Langzell, J. Chem. Soc., Chem.
Commun., 1993, 1673; H. Doucet, E. Fernandez, T. P. Langzell and J.
M. Brown, Chem. Eur. J., 1999, 5, 1320; E. Fernandez, K. Maeda, M.
W. Hooper and J. M. Brown, Chem. Eur. J., 2000, 6, 1840; K. Burgess
and M. J. Ohlmeyer, Chem. Rev., 1991, 91, 1179; I. Beletskaya and A.
Pelter, Tetrahedron, 1997, 53, 4957.
6 A. W. Herriott and K. Mislow, Tetrahedron Lett., 1968, 3013.
7 T. Pollok and H. Schmidbaur, Tetrahedron Lett., 1987, 28, 1085 and
references cited therein.
8 1-Bromonaphthalene (1 equiv.) was reacted with n-BuLi (1 equiv.) at
250 °C for 1 h, transmetallated to the copper derivative (0.5 equiv.
CuCN·2LiCl) at 280 °C and reacted with acrolein (2 equiv.) and
TMSCl (4 equiv.) at 280 °C overnight yielding 3-(1-naphthyl)propanal
in 40% yield. This aldehyde was reacted with 1-propynyllithium13 (280
°C to rt), leading to the corresponding propargylic alcohol 7 in 80%
yield. This alcohol was oxidized to the ketone according to Swern14 in
80% yield. See ref. 10 for the asymmetric reduction to alcohol (S)-7.
9 R. W. Hoffmann, Chem. Rev., 1989, 89, 1841; R. W. Hoffmann, Angew.
Chem., Int. Ed., 2000, 39, 2055.
10 For the preparation of (S)-propargylic alcohol 7, (1-naphthyl)ethyl
propynyl ketone was reduced with Alpine-borane (0 °C to rt, overnight,
neat, 80%, > 99% ee). See H. B. Brown and G. G. Pai, J. Org. Chem.,
1985, 50, 1384.
11 Preparation of chiral alcohol 14: enantiomerically pure (S)-pentynol was
prepared following a published procedure from ethyl lactate.15 It was
regioselectively converted to the desired vinyl stannane via a stannylcu-
pration reaction.16 The corresponding vinylstannane was cross-coupled
with 2-bromoiodobenzene according to Scheme 3, leading to the allylic
alcohol 14 in 30% yield.
12 Typical procedure: An argon-flushed flask was charged with DMAP (1
mmol, 1 equiv.), an allylic alcohol (1 mmol, 1 equiv.) and Et2O (5 mL).
When a clear solution was obtained, neat chlorophosphine was added
dropwise (1 mmol, 1 equiv.). A white precipitate was formed. It was
stirred at rt for 30 min, then filtered through a short pad of dry silica gel.
The solvents were evaporated in vacuo and toluene (5 mL) was added.
The phosphinite was heated at the required temperature. The solvents
were evaporated in vacuo and the residue was chromatographed (Et2O–
CH2Cl2, 1 : 1), leading to the pure phosphine oxide.
The excellent transfer of the chirality observed in the preparation
of the chiral aminophosphine oxide 13 and the allylic phosphine
oxide 15 demonstrates the synthetic utility of this [2,3] sigmatropic
rearrangement for the elaboration of new ligands for metal
catalysis. Efforts in this direction are currently underway in our
laboratories.12
We thank the Fonds der Chemischen Industrie for financial
support. We also thank BASF AG (Ludwigshafen), Chemetall
GmbH (Frankfurt) and Bayer Chemicals (Leverkusen) for the
generous gift of chemicals.
13 J. Suffert and D. Toussaint, J. Org. Chem., 1995, 60, 3550.
14 A. J. Mancuso, S.-L. Huang and D. Swern, J. Org. Chem., 1978, 43,
2480.
15 J. A. Marshall and S. Xie, J. Org. Chem., 1995, 60, 7230.
16 J.-F. Betzer, F. Delaloge, B. Muller, A. Pancrazi and J. Prunet, J. Org.
Chem., 1997, 62, 7768.
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