Cycloaddition of phosphanylidene-σ4-phosphoranes ArP=PMe3 and quinones to yield 1,3,2-dioxophospholanes

Article abstract of DOI:10.1039/b309165a

The reaction of phosphanylidene-σ⁴-phosphoranes ArP=PMe₃ (Ar = 2,6-Mes₂C₆H₃ or 2,4,6-t-Bu₃C₆H₂) with select orthoquinones yields 1,3,2-dioxophospholanes, one of which

Full text of DOI:10.1039/b309165a

Cycloaddition of phosphanylidene-s4-phosphoranes ArPNPMe3 and
quinones to yield 1,3,2-dioxophospholanes†
Xufang Chen, Rhett C. Smith and John D. Protasiewicz*
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio, USA.
E-mail: jdp5@po.cwru.edu; Fax: +1 216-368-3006; Tel: +1 216-368-5060
Received (in Cambridge, UK) 4th August 2003, Accepted 27th October 2003
First published as an Advance Article on the web 21st November 2003
The reaction of phosphanylidene-s4-phosphoranes ArPNPMe3
(Ar = 2,6-Mes2C6H3 or 2,4,6-t-Bu3C6H2) with select ortho-
quinones yields 1,3,2-dioxophospholanes, one of which shows
interesting p-stacking of the aromatic groups in the solid
state.
attractive between rings, the phosphorus atom is actually distanced
from these two rings, as seen by the C(2)–C(1)–P(1) and C(6)–
C(1)–P(1) bond angles of 127.0(6) and 113.6(6)°, respectively. The
structural data for 3a can also be contrasted to that found in 3b,5
where longer P–O and P–C bond distances and a smaller O–P–O
bond angle indicate the greater steric presence of the Mes*
compared to the Dmp unit. An additional feature of interest for 3a
is the manner in which molecules aggregate in the solid state by
additional p-stacking between electron deficient and electron rich
aromatic rings (Fig. 2) along the c axis of the unit cell, with an
intermolecular distance of 3.70 Å between these rings. The
intramolecular p-stacking is not sufficient to strongly inhibit
rotation about the P–C bond in solution, as the mesityl rings are
equivalent by both 1H and 13C NMR spectroscopy.
The results of a single crystal X-ray diffraction study of 4a are
shown in Fig. 3. In addition to lacking the electronic disparity of the
two sets of aromatic rings in 3a, steric repulsions between tert-butyl
groups and the mesityl groups presumably also discourage the type
of intramolecular p-stacking observed in 3a. As in 3a, the C(2)–
C(1)–P(1) and C(6)–C(1)–P(1) bond angles of 128.8(4) and
111.9(4)°, respectively, are significantly different, and might
indicate Menshutkin-type interactions between the phosphorus
atom and the opposite mesityl ring, as have been invoked in the
related meta-terphenyls 2,6-Ar2–C6H3ECl2 (E = As, Bi or Sb).6
We have previously reported that reaction of the easily prepared
4
and stable phosphanylidene-s -phosphoranes ArPNPMe3 (Ar =
Dmp = 2,6-Mes2C6H3 (1) or Ar = Mes* = 2,4,6-t-Bu3C6H2 (2))
with aldehydes affords a rapid and convenient synthesis of
phosphaalkenes [eqn. (1)].1 During this study reagents 1 and 2 were
found to be unreactive towards ketones at ambient conditions.
Seeking to extend this variant of the phospha-Wittig reaction2 to
new systems, it was reasoned that reaction of these reagents with
more reactive CNO bonds of o-quinones might be more favorable.
In particular, reaction of o-quinones might lead to easy synthesis of
1,2-diphosphaalkenes [eqn. (2)], materials which are now drawing
attention as a new class of chelating ligands for transition metal
catalysts.3 Herein we report that the targeted reaction does not
proceed as initially intended, but instead gives products of
cycloaddition of the phosphanylidene center to o-quinones.
(1)
(2)
Reaction of 1 and 2 with either tetrachloro-o-benzoquinone or
3,5-di-tert-butyl-o-benzoquinone over a 1 hour time period in
toluene yielded pale yellow–green solutions. While the reaction
mixtures displayed 31P NMR resonances between 194.3 and 230.1
ppm, well within the range of values determined for phosphaalk-
enes,4 the resonance for PMe3, not the anticipated product ONPMe3,
was observed. Furthermore, the reaction stoichiometry was found
to be 1 : 1 for ArPNPMe3 : o-quinone (excess ArPNPMe3 was left
unreacted). The major products in these reactions are actually
1,3,2-dioxaphospholanes. From the reaction of 3,5-di-tert-butyl-o-
benzoquinone isolated yields of 1,3,2-dioxaphospholanes are quite
excellent (4a, 94.1%; 4b, 98.0%; Scheme 1).‡ Reactions of 1 and 2
with tetrachloro-o-benzoquinone gave somewhat lower yields of
the cycloadducts (40–45%). Best results for 3a were obtained for
reactions performed in toluene at reduced temperatures (235 °C).
Compound 3b has been previously isolated in 16% yield by
reaction of tetrachloro-o-benzoquinone with the diphosphene
Mes*PNPMes*.5
Scheme 1
An ORTEP diagram representing the results of crystallographic
determination for 3a is provided in Fig. 1.§ Immediately striking is
the disposition of the electron deficient ring above and parallel to
one of the two more electron rich mesityl rings on the Dmp unit.
This intramolecular p–p stacking is evinced by a distance between
rings of 3.23 Å. Although this interaction is presumed to be
Fig. 1 Structural diagram for 3a. Selected bond distances (Å) and angles (°):
P(1)–C(1), 1.824(8), P(1)–O(1) 1.675(6); P(1)–O(1) 1.674(6);
O(1)P(1)O(2), 93.5(3); C(1)P(1)O(1), 104.5(3); C(1)P(1)O(2), 104.7(3)
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b3/b309165a/
146
C h e m . C o m m u n . , 2 0 0 4 , 1 4 6 – 1 4 7
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

While the sum of angles at phosphorus (302.7°) for 3a is slightly
larger than that of compounds 4a (297.7°) and 3b (290.8°), each
phosphorus center is pyramidal.
electron-transfer pathways, thus explaining the lack of CNC or PNC
bond formation. Further work to increase the utility of ArPNPMe3
for ligand synthesis is currently underway. The authors thank the
National Science Foundation (CHE-0202040) for support.
The present method for preparing 1,3,2-dioxophospholanes adds
to the large number of reactions of ortho-quinones with phosphorus
compounds,7 but distinguishes itself in that reactions to produce
P(III) compounds (via low coordinate phosphorus compounds) are
not often as simple or high yielding. One can also compare the
current reactions to a limited set of reactions of these ortho-
quinones with non-carbonyl stabilized Wittig reagents RA2CNPR3
that yield 1,3-dioxoles.8 Such reactions may proceed by radical or
Notes and references
‡ Selected spectroscopic data for 3–4 (See ESI for full details): 3a mp
234–235 °C. 1H NMR (CDCl3): d 2.03 (s, 12H); 2.26 (s, 6H); 6.81 (s, 4H);
3
4
7.12 (dd, 2H, JHH = 7.6 Hz, JPH = 2.0 Hz); 7.66 (t, 1H, J = 7.6 Hz).
13C{1H} NMR (200 MHz, CDCl3): d 21.04 (s); 21.16 (d, J = 1.1 Hz);
127.96 (s); 130.05 (d, J = 1.8 Hz); 134.14 (s); 135.46 (d, J = 8.0); 136.15
(d, J = 2.2 Hz); 137.59 (s); 137.80 (s); 138.59 (s); 144.52 (s); 147.29 (s);
147.81 (s). 31P{1H} (CDCl3): d 230.1. 3b 1H NMR (CDCl3): d = 1.21 (s,
9H); 1.54 (d, 18H, J = 1.1 Hz); 7.13 (d, 2H, J = 1.1 Hz). 31P{1H} NMR
(CDCl3): d = 217.5. 4a mp 154–156 °C. 1H NMR (CDCl3): d = 1.23 (s,
9H); 1.25 (s, 9H); 1.76 (s, 6H); 2.14 (s, 6H); 2.36 (s, 6H); 5.87 (d, 1H, J =
2.0 Hz); 6.73 (d, 1H, J = 2.1 Hz); 6.78 (s, 2H); 6.94 (dd, 2H, 3JHH = 7.6
Hz, 4JPH = 1.8 Hz); 6.99 (s, 2H,); 7.47 (t, 1H, J = 7.6 Hz). 13C{1H} NMR
(200 MHz, CDCl3): d 20.81 (d, J = 5.3 Hz); 21.33 (s); 21.67 (s); 29.89 (s);
31.63 (s); 34.36 (s), 34.45 (s); 108.07 (d, J = 1.6 Hz); 116.34 (s); 127.86 (s);
128.28 (s); 130.07 (s); 131.56 (s); 134.01 (s); 135.94 (d, J = 2.0 Hz); 136.64
(s); 136.86 (d, J = 1.7 Hz); 137.32 (d, J = 4.6 Hz); 143.69 (s); 144.30 (s);
144.69 (s); 146.94 (s); 147.06 (s). 31P{1H} NMR (CDCl3): d 194.3. 4b mp
100–101 °C. 1H NMR (CDCl3): d 1.12 (s, 9H); 1.20 (s, 9H); 1.24 (s, 9H);
1.52 (s, 18H); 6.65 (d, 1H, J = 1.9 Hz); 6.75 (d, 1H, J = 2.2 Hz); 7.01 (d,
2H, J = 1.0 Hz). 13C{1H} NMR (300 MHz, CDCl3): d 30.23 (s); 31.02 (s);
31.57 (s); 34.14 (d, J = 9.2 Hz); 34.36 (s); 34.44 (s); 34.54 (s); 39.49 (d, J
= 2.7 Hz); 108.03 (s); 115.70 (s); 121.21 (s); 134.22 (s); 141.28 (s); 142.41
(s); 144.00 (s); 146.94 (d, J = 7.4 Hz); 149.54 (s); 156.62 (d, J = 6.8 Hz).
31P{1H} NMR (CDCl3): d 195.9.
§
Crystal data for 3a. C30H25Cl4O2P, M = 590.27, monoclinic, a =
8.451(3), b = 24.614(8), c = 14.068(4) Å, a = 90.00, b = 105.55(2), g =
90.00, U = 2819.1(14) Å3, T = 293 K, space group P2(1)/c, Z = 4, m(Mo–
Ka) = 0.503 mm21, 4432 reflections measured, 1851 unique (Rint
=
0.0618) which were used in all calculations. Final R1 = 0.0800, wR(F2) was
0.1900 (all data). CCDC 216923.
Crystal data for 4a. C38H45O2P, M = 564.71, monoclinic, a = 13.623(2),
b = 11.2320(17), c = 22.372(3) Å, a = 90.00, b = 98.531(12), g = 90.00,
U = 3385.4(9) Å3, T = 293 K, space group P2(1)/c, Z = 4, m(Mo–Ka) =
0.111 mm21, 5315 reflections measured, 2896 unique (Rint = 0.0342)
which were used in all calculations. Final R1 = 0.0930, wR(F2) was 0.2680
(all data). CCDC 216924. http://www.rsc.org/suppdata/cc/b3/b309165a/ for
crystallographic data in .cif format.
Fig. 2 Packing diagram for 3a illustrating p-stacking in the crystal.
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Fig. 3 Structural diagram for 4a. Selected bond distances (Å) and angles (°):
P(1)–C(1), 1.851(6), P(1)–O(1) 1.673(4); P(1)–O(1) 1.656(4);
O(1)P(1)O(2), 93.4(2); C(1)P(1)O(1), 100.6(2)
103.7(2)
;
C(1)P(1)O(2)
=
C h e m . C o m m u n . , 2 0 0 4 , 1 4 6 – 1 4 7
147