Regioselective proton abstraction and 1,3-migration of a phosphorus group in 1,3-dienes by iron coordination: A new method for the synthesis of α-phosphono-α,β-unsaturated ketones [17]

Full text of DOI:10.1021/ja016958r

J. Am. Chem. Soc. 2001, 123, 12117-12118
12117
Scheme 1
Regioselective Proton Abstraction and 1,3-Migration
of a Phosphorus Group in 1,3-Dienes by Iron
Coordination: A New Method for the Synthesis of
r-Phosphono-r,â-unsaturated Ketones
Tatsuo Okauchi,* Takao Teshima, Keishi Hayashi,
Nobuo Suetsugu, and Toru Minami*
Department of Applied Chemistry
Kyushu Institute of Technology
Sensui-cho, Tobata, Kitakyushu 804-8550, Japan
ReceiVed August 29, 2001
ReVised Manuscript ReceiVed October 23, 2001
Scheme 2
Vinylphosphonates containing various functional groups have
been widely studied due to their synthetic usefulness.1 We
previously reported that vinylphosphonates having an electron-
withdrawing group at the R-position underwent Lewis acid-
catalyzed cyclizations, for example, intramolecular ene reactions,2
[2 + 2] cycloadditions,3 and Nazarov cyclizations.4 Our interest
in vinylphosphonate chemistry led us to explore the synthesis of
cyclic vinylphosphonates bearing an electron-withdrawing group,
that is, R-phosphono-R,â-unsaturated cyclic ketones. Although a
few reports on the preparation of such kinds of cyclic vinylphos-
phonates have been found, the preparation procedures are
considered to be too lengthy.5 Thus, the need for a convenient
and general method for the preparation of the cyclic vinylphos-
phonate is manifest.
1,3-Migration of phosphorus from oxygen to carbon is con-
venient for preparing â-keto phosphonates (path a in Scheme 1),
since dienyl phosphates are easily obtained and rearranged
regioselectively to the C-1.6 This reaction involves regioselective
proton abstraction at the C-1 of dienyl phosphate 1 and subsequent
migration of phosphorus to give â-keto phosphonate. We envi-
sioned that the migration might be used for the preparation of
R-phosphono-R,â-unsaturated ketone if regioselective abstraction
of the proton at the C-3 in 1 could be performed.
phosphonoenone 4 after demetalation (path b in Scheme 1). We
now report the first example of the controlled regioselectivity of
the proton abstraction from 1,3-diene by iron coordination8 and
a convenient method for the synthesis of R-phosphono-R,â-
unsaturated cyclic ketones using iron-diene complexes.
Iron complex 2a was easily prepared from a dienyl phosphate
and (η4-benzylideneacetone)tricarbonyliron9 in 43% yield. Treat-
ment of 2a with 2.2 equiv of LDA at -78 °C selectively furnished
1,3-phosphorus migration product 3a in 78% yield (eq 1). This
demonstrates that coordination of Fe(CO)3 to the diene dramati-
cally altered the regioselectivity of the proton abstraction.
Removal of the iron moiety from 3a was accomplished by the
oxidation with Me3NO,10 giving the desired R-phosphono-R,â-
unsaturated cyclic enone 4a in 95% yield (eq 1).
Iron-1,3-diene complexes have a half-sandwich structure and
are described by the following resonance forms (Scheme 2).
Considering these forms, the complex would have dialkylmetal
characters to some extent, and C-1 and C-4 carbons of the 1,3-
diene part would have greater sp3 character than C-2 and C-3
carbons.7 It can, therefore, be presumed that when the complex
2 is treated with a strong base the proton abstraction would occur
at C-3 and phosphorus would successively migrate to C-3 to give
Various iron-diene complexes 2b-l having a phosphate group
were prepared from dienyl phosphates 1b-l and were subjected
to treatment with LDA. As summarized in Table 1, the phosphorus
migration to C-3 of cyclic and acyclic diene complexes proceeded
smoothly to give the corresponding products 3 in good yields
(except for 2d, entry 4). Interestingly, iron-diphenyl dienyl
phosphonate complex 2c was converted to iron-coordinated
R-phosphono-R,â-unsaturated ketone 3c (entry 3). In contrast to
this result, Wiemer reported that phosphorus migration to the
aromatic ring was observed when diphenyl vinyl phosphate was
treated with LDA.11 Therefore, we conclude that acidity of a
proton at C-3 of the iron-diene complex is higher than that of
an aromatic proton at the ortho position of the oxygen. For acyclic
complexes 2i-m, it is of interest to mention that the elimination
was suppressed by iron coordination and that the phosphorus
migration to the C-3 occurred exclusively to give iron-coordinated
R-phosphono-R,â-unsaturated ketones 3i-m, since treatment of
* Corresponding author. E-mail: okauchi@che.kyutech.ac.jp.
(1) For recent reviews, see: Minami, T.; Motoyoshiya, J. Synthesis 1992,
333; Minami, T.; Okauchi, T.; Kouno, R. Synthesis 2001, 349.
(2) Minami, T.; Utsunomiya, T.; Nakamura, S.; Okubo, M.; Kitamura, N.;
Okada, Y.; Ichikawa, J. J. Org. Chem. 1994, 59, 6717.
(3) Okauchi, T.; Kakiuchi, T.; Kitamura, N.; Utsunomiya, T. Ichikawa, J.;
Minami, T. J. Org. Chem. 1997, 62, 8419.
(4) Minami, T.; Nakamura, M.; Fujimoto, K. Matsuo, S. J. Chem. Soc.,
Chem. Commun. 1992, 190.
(5) Gil, J. M.; Hah, J.; Park, K. Y.; Oh, D. Y. Tetrahedron Lett. 1998, 39,
3205; Wada, E.; Kanemasa, S.; Tsuge, O. Bull. Chem. Soc. Jpn. 1989, 62,
860; Castagnino, E.; Corsano, S.; Strappaveccia, G. P. Tetrahedron Lett. 1985,
26, 93; Piers, E.; Abeysekera, B.; Scheffer, J. R. Tetrahedron Lett. 1979, 3279.
(6) Gloer, K. B.; Calogeropoulou, T.; Jackson, J. A.; Wiemer, D. F. J.
Org. Chem. 1990, 55, 2842 and the references therein.
(7) The right form in Scheme 2 is equivalent to the population of back-
bonding to the LUMO of butadiene. Back-bonding is not so extensive in metal
complexes with carbonyl ligands. However, C-3-C-4 and C-1-C-2 bonds
in diene-iron complexes are elongated compared with free butadiene, and
pyramidalization angles at C-1 and C-4 are greater than those of C-2 and
C-3. These are interpreted that back-bonding in diene-iron complexes is strong
enough to increase sp3 character of C-1 and C-4 higher than that of C-2 and
C-3. See, Deeming. A. J. Mononuclear Iron Compounds with η2-η6
Hydrocarbon Ligands. In ComprehensiVe Organometallic Chemistry; Wilkin-
son, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol.
4, p 377.
(8) The anion generation from iron-cyclobutadiene complexes is known.
However, this is a special case and distinct from normal diene-iron
complexes: see, Bunz, U. H. F. Organometallics 1993, 12, 3594.
(9) Howell, J. A.; Johnson, B. F. G.; Josty, P. L.; Lewis, J. J. Organomet.
Chem. 1972, 39, 329; Evans, G.; Johnson, B. F. G.; Lewis, J. J. Organomet.
Chem. 1975, 102, 507.
(10) Shuvo, Y.; Hazum, E. J. Chem. Soc., Chem. Commun. 1974, 336.
(11) Calogeropoulou, T.; Hammond, G. B.; Wiemer, D. F. J. Org. Chem.
1987, 52, 4185.
10.1021/ja016958r CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/08/2001

12118 J. Am. Chem. Soc., Vol. 123, No. 48, 2001
Table 1. 1,3-Phosphorus Migration of Iron Complexes 2a
Communications to the Editor
Table 2. 1,3-Migration of a Sulfonyl or Benzyl Group in Iron
Complex 5a
entry
5, Y
temp (°C)
time
yield of 6(%)b
1
2
3
5a, CF3(CF2)3SO2
5b, CF3SO2
5c, PhCO
-78
-78
0
5 min
5 min
0.5 h
80
83
96c
a Iron complex 5, LDA (1.1 equiv) were reacted in THF at -78 °C.
c
b Isolated yield. 7 was obtained.
shown in Table 2. The migration proceeded quite successfully to
give the expected iron complexes 6 or the corresponding
decomplexed enones 7. When the complex with a migrating group
having acidic protons, such as a methanesulfonyl or acetyl group,
was employed, no migration product was obtained; instead, iron-
dienol complex 5 (Y ) H) was formed quite rapidly even at low
temperature.
Next, we tried to trap the anion intermediate of the iron-diene
complex by an electrophile intermolecularly.13 Complex 2d was
selected as the starting substrate because its phosphonic amide
group did not migrate (Table 1, entry 4). 2d was treated with
LDA under the same conditions as shown in Table 1, and then
1
D2O was added into the reaction mixture at -78 °C (eq 3). H
NMR integration of recovered 2d (82%) showed 93% deuterium
incorporation at C-3. An attempt to trap the generated anion by
iodometane afforded the alkylated complex 8 in 76% yield. These
are the first examples of generation of the anion from an iron-
diene complex and of its reaction with an electrophile.
In conclusion, iron coordination to 1,3-diene with a phosphate
group allows proton abstraction at C-3 and subsequent 1,3-
migration or alkylation to occur. Removal of the iron moiety from
the migration product gives R-phosphono-R,â-unsaturated ketones.
This procedure provides a new approach to synthesize cyclic
vinylphosphonates bearing an electron-withdrawing group. Fur-
thermore, iron-diene complexes with a sulfonyl or an acyl goup
are also successfully employed in this reaction. These results
indicate that the acidity of protons at C-2 and C-3 of the iron-
1,3-diene complex may be higher than that of the C-1 and C-4
protons.
a Iron complex 2, LDA (2.2 equiv) were reacted in THF at -78 °C.
b Isolated yield. c Reaction run at -63 °C.
iron-free acyclic dienyl phosphates with LDA produces enynes
with an elimination of the diethyl phosphoric group.12
To determine whether this migration occurred inter- or in-
tramolecularly, a crossover experiment was conducted (eq 2).
Acknowledgment. We gratefully acknowledge financial support for
this research by a grant from the Ministry of Education, Science, Sports,
and Culture of Japan. We also thank the Center for Instrumental Analysis
KIT for the measurement of analytical data.
Supporting Information Available: Detailed experimental proce-
dures including spectroscopic and analytical data (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
Treatment of an equimolar mixture of 2c and 2e with LDA only
provided products 3c and 3e, indicating that the migration is an
intramolecular process.
A similar reaction of several iron-diene complexes bearing a
sulfonyloxy or benxyloxy group was investigated. The results are
JA016958R
(13) For the generation of a vinyl phosphate anion that could be trapped
with TMSCl: see, Koerwitz, F. L.; Hammond, G. B.; Wiemer, D. F. J. Org.
Chem. 1989, 54, 738.
(12) Negishi, E.; King, A. O.; Klima, W. L.; Patterson, W.; Silveira, A.,
Jr. J. Org. Chem. 1980, 45, 2526.