Synthesis and chemistry of 2-phosphafurans

Article abstract of DOI:10.1021/om100778f

The reaction between (chlorophosphinidene)pentacarbonyltungsten and benzylideneacetophenone affords a [4 + 1] adduct whose dehydrochlorination by N-methylimidazole affords a 2-phosphafuran complex. Upon further reaction, the phosphafuran is displaced from

Full text of DOI:10.1021/om100778f

Organometallics 2010, 29, 5757–5758 5757
DOI: 10.1021/om100778f
Synthesis and Chemistry of 2-Phosphafurans
Matthew P. Duffy, Yuhan Lin, Feny Ho, and Francois Mathey*
Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences,
Nanyang Technological University, 21 Nanyang Link, Singapore 637371
Received August 11, 2010
2-Phosphafurans (1,2-oxaphospholes) are among the sim-
plest and least well-known members of the broad family of
aromatic heterophospholes.1 In fact, only one compound of
this class has ever been described in the literature, as an unstable
product formed via a complex route that was characterized
by NMR at low temperature.2 We wish to present here the
straightforward synthesis of a 2-phosphafuran as a reasonably
stable species and to give some preliminary results concern-
ing its chemistry. Our approach is based on the availability of
the transient chlorophosphinidene complex [Cl-P-W(CO)5].3
This species, as generated from a chlorophosphole complex
and dimethyl acetylenedicarboxylate, reacts at 80 °C with
benzylideneacetophenone in toluene to give the correspond-
ing [1 þ 4] cycloadduct 1 as a mixture of two isomers. This, in
turn, reacts with N-methylimidazole between -20 °C and
room temperature to give a red solution of the very reactive
phosphafuran complex 2 (Scheme 1). This species is char-
acterized by its 31P resonance at low field: δ(31P) 223.7 (1JPW
=
296 Hz). It can be kept without noticeable decomposition
for 1 h at room temperature and for 2 weeks at -25 °C. Its
identity was definitively established by a series of trapping
reactions (Scheme 2). The X-ray crystal structures of 3 and 4
are shown in Figures 1 and 2.
Figure 1. Molecular structure of 3.
Scheme 1
Selected NMR data for 3-5 are given in ref 4. Full experi-
mental data are available in the Supporting Information.
*To whom correspondence should be addressed. E-mail: fmathey@
ntu.edu.sg.
(1) Review: Schmidpeter, A., Heterophospholes. In Phosphorus-Carbon
Heterocyclic Chemistry: The Rise of a New Domain; Mathey, F., Ed.; Elsevier:
Oxford, U.K., 2001; pp 363-461.
€
(2) Mack, A.; Bergstraβer, U.; Reiss, G. J.; Regitz, M. Eur. J. Org.
Chem. 1999, 587.
(3) Duffy, M. P.; Mathey, F. J. Am. Chem. Soc. 2009, 131, 7534.
(4) Data for 3: 31P NMR (CD2Cl2) δ 159.6, 1JPW=281 Hz; 1H NMR
(CD2Cl2) δ 1.82 (br s, Me), 1.86 (s, Me), 5.63 (d, JHP=17.8 Hz, dCH);
13C NMR (CD2Cl2) δ 20.86 (d, JCP=2.5 Hz, CH3), 21.19 (d, JCP=2.5 Hz,
CH3), 43.37 (s, CH2), 45.12 (d, JCP=9.8 Hz, CH2), 61.09 (d, JCP=17.1 Hz,
P-C-Ph), 106.56 (d, JCP = 3.7 Hz, dCH), 157.36 (d, JCP = 6.1 Hz,
dC(Ph)O), 195.95 (d, JCP =7.4 Hz, cis CO), 199.75 (d, JCP =29.5 Hz,
Scheme 2
1
trans CO). Data for 4: 31P NMR (CD2Cl2) δ 117.8, JPW = 308 Hz;
1H NMR (CD2Cl2): δ 4.05 (dd, JHH=7.7 Hz, JHP=2.7 Hz, CH-CO),
4.38 (dd, JHH=7.7 Hz, JHP=3.2 Hz, CH-CO); 13C NMR (CD2Cl2) δ
51.93 (s, CH(CO)), 53.84 (d, JCP=7.3 Hz, P-CH(CO)), 98.23 (d, JCP
=
3.7 Hz, Ph-CO), 142.68 (s, dCH), 171.51 (s, NCO), 172.97 (s, NCO),
194.35 (d, JCP = 8.5 Hz, cis CO), 197.29 (d, JCP = 34.1 Hz, trans CO).
1
Data for 5a: 31P NMR (CD2Cl2) δ 171.9, JPW = 331 Hz; 1H NMR
(CD2Cl2) δ 3.42 (d, JHP=12.4 Hz, OMe), 4.86 (dd, JHH=2.3 Hz, JHP
=
14.6 Hz, CH-P), 6.01 (dd, JHH = 2.3 Hz, JHP = 18.3 Hz, dCH); 13C
NMR (CD2Cl2) δ 55.45 (d, JCP=3.9 Hz, OMe), 62.91 (d, JCP=18.2 Hz,
CH-P), 103.61 (s, dCH), 157.08 (d, JCP =6.7 Hz, PhC-O), 195.82 (d,
JCP=8.7 Hz, cis CO), 198.55 (d, JCP=34.6 Hz, trans CO). Data for 5b
31P NMR (CD2Cl2) δ 185.6, 1JPW =336 Hz; 1H NMR (CD2Cl2) δ 3.75
(d, JHP=11.4 Hz, OMe), 4.45 (dd, JHH=3.7 Hz, JHP=9.6 Hz, CH-P),
6.04 (dd, JHH = 3.7 Hz, JHP = 19.3 Hz, dCH); 13C NMR (CD2Cl2) δ
55.20 (d, JCP=8.7 Hz, OMe), 59.68 (d, JCP=13.5 Hz, CH-P), 103.99 (s,
dCH), 157.80 (d, JCP = 6.7 Hz, PhC-O), 195.32 (d, JCP = 9.6 Hz, cis
CO), 198.68 (d, JCP =35.5 Hz, trans CO).
2-Phosphafurans are computed to be reasonably aromatic with
.
an ASE (aromatic stabilization energy) of ca. 13.2 kcal mol-1 5
Thus, this explains why it is possible to synthesize these
species by dehydrochlorination of 1 with a mild base. On the
~
(5) Cyranski, M. K.; Krygowski, T. M.; Katritzky, A. R.; Schleyer,
P. v. R. J. Org. Chem. 2002, 67, 1333.
r
2010 American Chemical Society
Published on Web 10/14/2010
pubs.acs.org/Organometallics

5758 Organometallics, Vol. 29, No. 22, 2010
Duffy et al.
Scheme 3
Phosphafuran 6 can be kept at room temperature for 10 days
without noticeable decomposition. It reacts at room tem-
perature with an excess of 2,3-dimethylbutadiene to give the
[2 þ 4] cycloadduct (δ(31P) 144 ppm in toluene) whose sulfur-
ization gives 7, which was fully characterized (including a
X-ray crystal structure analysis).8 Clearly, the complexation
by tungsten provides some kinetic stability to the 2-phospha-
furan ring by steric protection as it does for 2H-phospholes9
but does not fundamentally alter the reactivity of the ring.
Acknowledgment. We thank Nanyang Technological
University and the Singapore Ministry of Education
Research Fund Tier 2 (MoE 2009-T2-2-065) for financial
support of this work and Dr. Li Yongxin (NTU) for the
X-ray crystal structure analyses.
Figure 2. Molecular structure of 4.
other hand, this aromaticity is not high enough to suppress
the reactivity of the PdC double bond. Indeed, it is striking
to see that 2 behaves as a nonaromatic 2H-phosphole in
terms of reactivity.6 Of course, these observations concern
the complexed phosphafurans and it must be recalled that
P-complexation is known to significantly reduce the aroma-
ticity of related species such as phosphinines.7 Thus, the free
species might have a different reactivity. We discovered that
it suffices to heat 2 at 60 °C for 2 h in the presence of an excess
of N-methylimidazole to get the free phosphafuran 6, which
displays a 31P resonance at 285.5 ppm in toluene with no
P-W coupling (Scheme 3). This reaction suggests that the
phosphafuran is a poor P-donor.
Supporting Information Available: Text and figures giving full
experimental details and characterization data and CIF files
giving X-ray data for 3, 4, and 7. This material is available free of
charge via the Internet at http://pubs.acs.org.
(8) Data for 7: 31P NMR (CD2Cl2) δ 123.5; 1H NMR (CD2Cl2) δ 1.74
(d, JHP =5.0 Hz, Me), 1.79 (s, Me), 5.74 (d, JHP =24.7 Hz, dCH); 13
C
NMR (CD2Cl2) δ 20.67 (d, JCP=6.7 Hz, CH3), 21.29 (d, JCP=2.9 Hz,
CH3), 41.66 (d, JCP=56.6 Hz, CH2), 42.53 (s, CH2), 55.01 (d, JCP=59.4 Hz,
P-C-Ph), 107.19 (d, JCP = 5.8 Hz, dCH), 156.97 (s, dC(Ph)O).
The structure has been checked by X-ray analysis (see the Supporting
Information).
(6) Mathey, F. Acc. Chem. Res. 2004, 37, 954.
(7) Mathey, F.; Alcaraz, J.-M. Tetrahedron Lett. 1984, 25, 207.
(9) Holand, S.; Charrier, C.; Mathey, F.; Fischer, J.; Mitschler, A.
J. Am. Chem. Soc. 1984, 106, 826.