A new route to a 2-phosphanaphthalene

Article abstract of DOI:10.1021/ol302278p

The reaction between methylenechlorophosphine-pentacarbonyltungsten and an isobenzofuran affords a [4 + 2] adduct whose oxygen bridge is broken by BBr ₃, leading to a 2-phosphanaphthalene.

Full text of DOI:10.1021/ol302278p

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
A New Route to a 2‑Phosphanaphthalene
Yanli Mao, Kelvin Meng Hui Lim, Rakesh Ganguly, and Francois Mathey*
Division of Chemistry & Biological Chemistry, School of Physical & Mathematical
Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371
fmathey@ntu.edu.sg
Received August 16, 2012
ABSTRACT
The reaction between methylenechlorophosphine-pentacarbonyltungsten and an isobenzofuran affords a [4 þ 2] adduct whose oxygen bridge is
broken by BBr3, leading to a 2-phosphanaphthalene.
Despite their simplicity, only a very limited amount of
data are available in the literature on 2-phosphanaphtha-
lenes (isophosphinolines). The first synthesis of these species
was reported in 1973 by Bickelhaupt1 and relied on a lengthy
series of steps starting from acyclic precursors. The second,
shorter route relied on the ring expansion of phosphindoles
by acid chlorides.2 More recently, Regitz unveiled a third
route starting from phosphaalkynes and giving bifunctional
derivatives.3 The complexity of these various approaches
explains why the known chemistry of these derivatives is
essentially limited to a few cycloaddition reactions.3,4
Recently, we reported the conversion of furan into
2-hydroxyphosphinine5 by [4 þ 2] cycloaddition with
[CH2dPCl]W(CO)5.6 We were curious to see whether this
transformation could be applied to isobenzofurans and
give a simple access to 2-phosphanaphthalenes. Accord-
ingly, we allowed the methylenechlorophosphine tungsten
complex to react with isobenzofuran (1)7 and observed the
quantitative formation of the [4 þ 2] cycloadducts (2a) and
(2b) (Scheme 1). Both structures were checked by X-ray
analysis. As expected, the cycloadduct (2a) with the bulky
tungsten complexing group in the exo position is the major
isomer.
(1) de Graaf, H. G.; Dubbeldam, J.; Vermeer, H.; Bickelhaupt, F.
Tetrahedron Lett. 1973, 2397. de Graaf, H. G.; Bickelhaupt, F. Tetra-
hedron 1975, 31, 1097.
(2) Nief, F.; Charrier, C.; Mathey, F; Simalty, M. Nouv. J. Chim.
1981, 5, 187.
(3) Ruf, S. G.; Dietz, J.; Regitz, M. Tetrahedron 2000, 56, 6259.
(4) Klebach, T. C.; Turkenburg, L. A. M.; Bickelhaupt, F. Tetra-
hedron Lett. 1978, 1099.
(5) Mao, Y.; Mathey, F. Org. Lett. 2010, 12, 3384. Mao, Y.; Mathey,
F. Chem.;Eur. J. 2011, 17, 10745.
(6) Deschamps, B.; Mathey, F. J. Chem. Soc., Chem. Commun. 1985,
1010. Deschamps, B.; Mathey, F. J. Organomet. Chem. 1988, 354, 83.
(7) Kuninobu, Y.; Nishina, Y.; Nakagawa, C.; Takai, K. J. Am.
Chem. Soc. 2006, 128, 12376.
(8) 3: 31P NMR: (CDCl3): δ 183.8; 1H NMR (CDCl3): δ 7.46ꢀ7.56
(m, 12H, CH), 7.87ꢀ7.89 (m, 1H, CH), 7.96ꢀ7.98 (m, 1H, CH), 8.32
(d, 2JHP = 39.6 Hz, 1H, PdCH); 13C NMR (CDCl3): δ 125.55 (d, JCP
9.6 Hz, dCH), 126.80 (d, JCP = 5.7 Hz, dCH), 127.01 (d, JCP
=
=
3.9 Hz, dCH), 127.55 (2s, dCH), 128.22 (s, dCH), 128.33 (s, dCH),
129.16 (d, JCP = 4.5 Hz, dCH), 129.66 (s, dCH), 130.37 (d, JCP = 10.9
Hz, dCH), 133.19 (d, JCP = 10.1 Hz, dC), 135.97 (d, JCP =
11.0 Hz, dC), 142.64 (d, JCP = 24.8 Hz, dC), 142.93 (d, JCP = 3.6
1
Hz, dC), 143.41 (d, JCP = 53.8 Hz, PdCH), 147.44 (d, HRMS m/z
299.0986 (calcd for C21H16P: 299.0990). 5: 31P NMR: (CDCl3): δ 22.5;
1H NMR (CDCl3): δ 1.64 (s, 3H, CH3), 1.76 (s, 3H, CH3), 2.25ꢀ2.41 (m,
2H, CH2), 2.76ꢀ2.83 (m, 1H, CH2), 3.21 (pseudo t, 2JPH = 2JHH = 18.3
Hz, 1H, CH2), 6.32 (d, 2JPH = 15.6 Hz, 1H, dCH), 7.12ꢀ7.47 (m, 14H,
Ph); 13C NMR (CDCl3): δ 19.87 (s, CH3), 21.74 (d, JCP = 12.4 Hz, CH3),
30.60 (d, 1JCP = 69.6 Hz, P;CH2), 46.12 (s, CH2), 47.13 (d, 1JCP = 63.0
Hz, P;C), 118.20 (d, 1JCP = 90.6 Hz, P;CHd), 121.30 (d, 2JCP = 5.7
Scheme 1. Cycloaddition of Methylenechlorophosphine-Penta-
carbonyltungsten and Benzofuran
Hz, dC), 127.14 (s, dCH), 127.41 (s, dCH), 127.86 (d, JCP
=
9.5 Hz, dC), 128.19 (s, dCH), 128.32 (d, JCP = 5.7 Hz, dCH), 128.54
(s, dCH), 128.62 (s, dCH), 128.79 (s, dCH), 129.67 (s, dCH), 129.82 (d,
JCP = 3.8 Hz, dCH), 131.06 (s, dCH), 133.80 (d, JCP = 14.3 Hz, dC),
137.10 (d, JCP = 5.7 Hz, dC), 141.19 (d, JCP = 14.3 Hz, dC), 141.88 (d,
JCP = 5.7 Hz, dC), 155.34 (s, dC). HRMS m/z 395.1549 (calcd for
C27H24OP: 395.1565).
r
10.1021/ol302278p
XXXX American Chemical Society

Scheme 2. Conversion of the [4 þ 2] Cycloadducts into Phos-
Scheme 3. Reaction of 3 with Dimethylbutadiene
phanaphthalene
probably occurs during the second part of the reaction.
Phosphanaphthalene (3) was characterized by NMR and
HRMS.8 The 31P resonance is close to that observed for
other phosphanaphthalenes.1ꢀ3 In order to further confirm
its structure, it was converted into its Mo(CO)5 complex
(Mo(CO)6, toluene, 90 °C, 5 h) and the resulting PꢀMo-
(CO)5 complex (4) was characterized by X-ray crystal
structure analysis (Figure 1).
As expected the phosphanaphthalene nucleus is planar.
The planes of the two phenyl substituents make angles of
65.5° (C14) and 50.0° (C7) with this central plane. The two
PꢀC bonds are almost equal at 1.710(7) (P1ꢀC14) and
˚
1.734(8) A. This observation contrasts with the computed
data for the parent phosphanaphthalene (1.721 and 1.771
9
˚
A). The P1ꢀC14 is more reactive than the P1ꢀC6 bond as
a dienophile in [2 þ 4] cycloaddition reactions as shown by
the reaction of 3 with 2,3-dimethylbutadiene (Scheme 3).
The same regioselectivity was already observed by
Regitz.3 With this relatively simple access to phospha-
naphthalenes, we can consider that their chemistry is
now open for deeper investigation.
Figure 1. X-ray structure of 4.
Acknowledgment. The authors thank the Nanyang
Technological University in Singapore for the financial
support of this work.
Upon reaction with boron tribromide in the cold, the
oxygen bridge of 2a,b is cleaved, and upon further treatment
in situ with triethylamine and 1,2-bis(diphenylphosphino)-
ethane in boiling toluene overnight, the phosphanaphthalene
(3) is directly obtained (Scheme 2). It was purified by
chromatography on silica gel at ꢀ6 °C (to avoid P-oxidation)
with hexane/CH2Cl2 5:1 as the eluent and obtained as a
yellow solid in 46% yield. The loss of oxygen and tungsten
Supporting Information Available. Experimental sec-
tion and X-ray data for 2a, 2b, and 3. This material
is available free of charge via the Internet at http://pubs.
acs.org.
The authors declare no competing financial interest.
(9) Wang, L.; Wang, H. Int. J. Quantum Chem. 2007, 107, 1846.
B
Org. Lett., Vol. XX, No. XX, XXXX