Thermal [2 + 2] cycloadditions between diphosphene complexes and alkynes

Article abstract of DOI:10.1021/om961072v

According to the experimental conditions, the thermal decomposition of the 7-phenyl-7-phosphanorbornadiene-W(CO)₅ complex 1 affords the diphosphene complex 3 (60°C, CuCl as a catalyst), the triphosphirane complex 4 (120°C, 8 h), or the tetrapho

Full text of DOI:10.1021/om961072v

2506
Organometallics 1997, 16, 2506-2508
Articles
Th er m a l [2 + 2] Cycloa d d ition s betw een Dip h osp h en e
Com p lexes a n d Alk yn es
Ngoc Hoa Tran Huy, Yoichi Inubushi, Louis Ricard, and Franc¸ois Mathey*
Laboratoire “He´te´roe´le´ments et Coordination”, URA 1499 CNRS, DCPH, Ecole Polytechnique,
91128 Palaiseau Cedex, France
Received December 18, 1996X
According to the experimental conditions, the thermal decomposition of the 7-phenyl-7-
phosphanorbornadiene-W(CO)5 complex 1 affords the diphosphene complex 3 (60 °C, CuCl
as a catalyst), the triphosphirane complex 4 (120 °C, 8 h), or the tetraphosphetane complex
5 (120 °C, 15 h). The reaction of 3 with alkynes at 120 °C in toluene gives the corresponding
1,2-dihydro-1,2-diphosphete complexes 6-8 in modest yield (ca. 25%) via a [2 + 2]
cycloaddition. This result gives some insight into the reaction of cyclopolyphosphines with
alkynes.
A great deal of work has already been done on the
synthesis of 1,2-dihydro-1,2-diphosphetes by thermal
reaction of cyclopolyphosphines (RP)n with alkynes.1
Whereas the mechanism of this synthesis has never
been investigated, one possible explanation2 would be
a direct PdP + CtC [2 + 2] cycloaddition. We thus
decided to evaluate this possibility. In fact, the problem
is not trivial, since very bulky substituents are needed
to provide diphosphenes with a sufficient kinetic stabil-
ity. This may be the reason why only a few [2 + 2]
cycloadditions of diphosphenes have been described in
the literature, including the classical cyclodimeriza-
tion2 and the condensation of metallodiphosphenes
[Fe]sPdPsR with electron-poor alkenes.3 Alterna-
tively, it is possible to stabilize unhindered diphos-
phenes by complexation. The reactivity of the PdP bond
of such complexes has already been investigated in some
depth by Huttner.4 We decided to select this approach,
which parallels our work with phosphaalkenes.5
Optimization of the reaction conditions allowed the
synthesis of 3 in 46% yield (based on P) from 1. While
optimizing the synthesis of 3 via the CuCl-catalyzed
decomposition of 1, we also investigated the uncatalyzed
thermal splitting of the bicyclic skeleton. At 120 °C in
toluene, the only detectable organophosphorus product
was the mono complex of the triphosphirane 4 (eq 2).
Most of the tungsten was recovered as W(CO)6.
Resu lts a n d Discu ssion
All our work has been done with the diphosphene
complex 3, resulting from the dimerization of the
unstable terminal phosphinidene complex [PhPfW-
(CO)5] (2) as generated from the appropriate 7-phos-
phanorbornadiene precursor 1 (eq 1).6
Complex 4 displays a characteristic ABX 31P {1H}
NMR spectrum which implies a trans stereochemistry
for the PhP-PPh unit. The two uncomplexed phos-
phorus atoms resonate at δ -126.8 (A) and -133.2 (B).
The complexed phosphorus appears at δ -94.1 (X). As
expected, the 1J (P-P) coupling increases when the
tetracoordinated phosphorus is involved: 1J (AB) )
X Abstract published in Advance ACS Abstracts, May 15, 1997.
(1) (a) Mahler, W. J . Am. Chem. Soc. 1964, 86, 2306. (b) Ecker, A.;
Schmidt, U. Chem. Ber. 1973, 106, 1453. (c) Charrier C.; Guilhem, J .;
Mathey, F. J . Org. Chem. 1981, 46, 3. (d) Charrier, C.; Maigrot, N.;
Mathey, F.; Robert, F.; J eannin, Y. Organometallics 1986, 5, 623. (e)
Phillips, I. G.; Ball, R. G.; Cavell, R. G. Inorg. Chem. 1988, 27, 2269.
(2) For a review on the chemistry of diphosphenes, see: Weber, L.
Chem. Rev. 1992, 92, 1839.
(3) Weber, L.; Frebel, M.; Boese, R. Chem. Ber. 1990, 123, 733.
(4) Borm, J .; Huttner, G.; Orama, O.; Zsolnai, L. J . Organomet.
Chem. 1985, 282, 53. Huttner, G.; Borm, J .; Zsolnai, L. J . Organomet.
Chem. 1986, 304, 309.
1
1
146.5 Hz, J (AX) ) 220.3 Hz, and J (BX) ) 231.6 Hz.
Since the π-bonded [W(CO)5] group is known to be
labile in 3,4 we suspected that 4 was formed via the
reaction of the phosphinidene complex 2 with the
diphosphene complex. We verified this hypothesis by
a
31P NMR study of the reaction of the precursor 1 with
1 equiv of 3 (eq 3).
(5) Mathey, F. Acc. Chem. Res. 1992, 25, 90.
(6) Marinetti, A.; Charrier, C.; Mathey, F.; Fischer, J . Organome-
tallics 1985, 4, 2134.
S0276-7333(96)01072-2 CCC: $14.00 © 1997 American Chemical Society

Thermal [2 + 2] Cycloadditions
Organometallics, Vol. 16, No. 12, 1997 2507
Upon longer heating of 1 alone, we observed the
formation of 4 as expected, but the four-membered ring
5 was the other major product of the reaction (eq 4).
Complex 5 was also obtained by reaction of the precur-
sor 1 with the triphosphirane complex 4 under similar
experimental conditions. The 31P{1H} NMR spectrum
of 4 shows three types of phosphorus atoms at δ -70.6
(1P), -21.9 (2P) and +0.06 (1P), respectively. Its
formulation was established by an X-ray crystal struc-
ture analysis (Figure 1). The P-P bond lengths are
normal, between 2.226 and 2.241(1) Å. The phenyl
groups display an all-trans disposition. The four-
membered ring is folded around the P2-P4 axis by 51.24
+ 0.05°.
Having established the experimental conditions for
obtaining the various oligomers of [PhPfW(CO)5], we
came back to our initial problem. Heating complex 3
with various alkynes in refluxing toluene led to the
corresponding 1,2-dihydro-1,2-diphosphetes, as expected
(eq 5).
F igu r e 1. ORTEP drawing of one molecule of 5, as
determined by a single-crystal X-ray diffraction study.
Ellipsoids are scaled to enclose 50% of the electron density.
Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (deg): W-P(2) ) 2.520(1), P(1)-P(2)
) 2.229(1), P(1)-P(4) ) 2.226(1), P(1)-C(1) ) 1.832(4),
P(2)-P(3) ) 2.241(1), P(2)-C(7) ) 1.832(4), P(3)-P(4) )
2.227(1), P(3)-C(13) ) 1.816(4), P(4)-C(19) ) 1.846(4);
P(2)-P(1)-P(4) ) 85.58(5), P(2)-P(1)-C(1) ) 106.0(1),
P(4)-P(1)-C(1) ) 105.5(1), W-P(2)-P(1) ) 124.51(5),
W-P(2)-P(3) ) 125.08(5), W-P(2)-C(7) ) 113.7(1), P(1)-
P(2)-P(3) ) 82.79(5), P(1)-P(2)-C(7) ) 104.9(1), P(3)-
P(2)-C(7) ) 100.0(1), P(2)-P(3)-P(4) ) 85.26(5), P(2)-
P(3)-C(13) ) 107.3(1), P(4)-P(3)-C(13) ) 107.1(1), P(1)-
P(4)-P(3) ) 83.19(5), P(1)-P(4)-C(19) ) 101.1(1), P(3)-
P(4)-C(19) ) 100.1(1).
tuted or functional derivatives such as 7 and 8. Thus,
working with protected diphosphenes offers new syn-
thetic opportunities, since it is possible to recover free
dihydrodiphosphetes from their complexes.7
Exp er im en ta l Section
Complex 6 was identified by comparison of its 1H, 13C,
and 31P NMR spectra with those of an authentic sample
prepared by reaction of [W(CO)5(THF)] with trans-
1,2,3,4-tetraphenyl-1,2-dihydro-1,2-diphosphete. Par-
ticularly significant is the 13C resonance corresponding
to the ring carbons, which appears as a pseudotriplet
at δ 148.57 (CDCl3). Due to their lack of symmetry, the
identification of the original complexes 7 and 8 was
easier. Both complexes 7 and 8 give rise to character-
istic 31P AX spectra: 7, δ 27.73 and 46.64, ∑J (P-P) )
23.9 Hz; 8, δ 37.92 and 41.92, ∑J (P-P) ) 27.6 Hz. The
low P‚‚‚P couplings result from the superimposition of
All reactions were performed under nitrogen; the solvents
were purified, dried, and degassed by standard techniques. 1H,
13C, and 31P NMR spectra were recorded on a Bruker AC 200
SY spectrometer operating at 200.13, 50.32, and 81.01 MHz,
respectively. All chemical shifts are reported in ppm downfield
from internal TMS (1H and 13C) and external 85% H3PO4 (31P).
Mass spectra (EI) were obtained at 70 eV by the direct inlet
method.
[1,2,3-T r i p h e n y lt r i p h o s p h i r a n e ]p e n t a c a r b o n y l-
tu n gsten (4). A solution of complex 1 (1 g, 15 × 10-4 mol) in
toluene (10 mL) was refluxed at 120 °C for 8 h. After
evaporation, the residue was chromatographed on silica gel
with hexane. The triphosphirane complex 4 was thus obtained
as a light yellow powder in 45% yield (0.15 g). 31P NMR: see
text. Mass spectrum (184W): m/z 648 (M+, 2%), 620 (M+ - CO,
3
1J and J couplings of opposite signs.1d
Since we have checked that 3 is stable upon heating
in refluxing toluene for 4 h, it is reasonable to admit
that the first step of the reaction is a reversible
unmasking of the PdP double bond followed by a [2 +
2] PdP + CtC cycloaddition. Thus, this series of
experiments suggests that a similar mechanism is at
work in the synthesis of 1,2-dihydro-1,2-diphosphetes
from alkynes and cyclopolyphosphines. However, the
experimental conditions depicted in eq 4 are much
milder than those of the R1CtCR2 + (RP)n route. For
example, the reaction of PhCtCPh with (PhP)5 is
performed at 240 °C for 6 h and affords the correspond-
ing dihydrodiphosphete in only 15% yield.1c Besides,
this earlier route gives no access to partially unsubsti-
7%), 592 (M+ - 2CO, 7%), 564 (M+ - 3CO, 14%), 536 (M+
-
4CO, 9%), 508 (M+ - 5CO, 45%), 262 (64%), 183 (100%). Anal.
Calcd for C23H15O5P3W: C, 42.59; H, 2.31. Found: C, 42.23;
H, 2.18.
[1 ,2 ,3 ,4 -T e t r a p h e n y l t e t r a p h o s p h e t a n e ]p e n t a -
ca r bon yltu n gsten (5). A solution of complex 1 (3 g, 45 ×
10-4 mol) in toluene (20 mL) was refluxed at 120 °C for 15 h.
Chromatography of the residue, first with hexane as the
eluent, afforded 4 (0.4 g) and then with hexane/CH2Cl2 (10:1)
gave 5 as a yellow powder (0.4 g, 12%). 31P NMR (CDCl3): δ
1
-70.6 (1 PC), -21.9 (2 PB), +0.06 (1 PA, complexed), J AB
≈
155 Hz, J AC ≈ 75 Hz, J BC ≈ 100 Hz. Mass spectrum (184W):
1
1
(7) Maigrot, N.; Ricard, L.; Charrier, C.; Le Goff, P.; Mathey, F. Bull.
Soc. Chim. Fr. 1992, 129, 76.

2508 Organometallics, Vol. 16, No. 12, 1997
Tran Huy et al.
m/z 728 (M+ - CO, 4%), 700 (M+ - 2CO, 2%), 672 (M+ - 3CO,
12%), 644 (M+ - 4CO, 6%), 616 (M+ - 5CO, 25%), 262 (100%),
183 (95%). The base peak probably corresponds to Ph3P, which
could be formed by dismutation of the [PhP] units.
1952 (vs), 1716 (w) cm-1. Anal. Calcd for C27H16O12P2W2: C,
33.69; H, 1.66. Found: C, 33.65; H, 1.79.
X-r a y Str u ctu r e Deter m in a tion for 5. Crystals of 5,
C
29H20O5P4W, were grown from a dichloromethane/pentane
Syn th esis of 1,2-Dih yd r o-1,2-d ip h osp h ete Com p lexes
6-8. A solution of complex 3 (0.45 g, 3.8 × 10-4 mol) and
alkyne (3.8 × 10-4 mol) was refluxed in toluene (5 mL) at 120
°C for 4 h. After evaporation, the residue was chromato-
graphed on silica gel to give 6-8 as yellow powders.
solution of the compound. Data were collected at -150 ( 0.5
°C on an Enraf-Nonius CAD4 diffractometer using Mo KR
radiation (λ ) 0.710 73 Å) and a graphite monochromator. The
crystal structure was solved and refined using the Enraf-
Nonius MOLEN package. The compound crystallizes in space
group P21/n (No. 14), with a ) 10.315(1) Å, b ) 22.604(3) Å, c
) 12.892(1) Å, â ) 106.41(1)°, V ) 2883.5(1.0) Å3, Z ) 4, dcalc
) 1.742 g/cm3, µ ) 43.5 cm-1, and F(000) ) 1472. A total of
7560 unique reflections were recorded in the range 2° e 2θ e
56.1°, of which 2538 were considered as unobserved (F2 < 3.0σ-
(F2)), leaving 5022 for solution and refinement. Direct methods
yielded a solution for all atoms. The hydrogen atoms were
included as fixed contributions in the final stages of least-
squares refinement while using anisotropic temperature fac-
tors for all other atoms. A non-Poisson weighting scheme was
applied with a p factor equal to 0.03. The final agreement
factors were R ) 0.025, Rw ) 0.029, and GOF ) 1.02.
Complex 6: eluent hexane/CH2Cl2 9:1, yield 0.1 g (25%). 31P
NMR (CDCl3): δ 46.34, 1J (P-W) ) 231.9 Hz. Mass spectrum
(
184W): m/z 1042 (M+, 38%), 958 (M+ - 3CO, 52%), 874 (M+
-
6CO, 100%), 762 (M+ - 10CO, 79%). IR (CH2Cl2): ν(CO) 2069
(m), 1949 (vs) cm-1
.
Complex 7: eluent hexane/CH2Cl2 (7:3) yield 0.1 g (27%).
31P NMR (CDCl3): see text. 13C NMR (CDCl3): δ 153.79 (dd,
1J (P-C) ) 30.25 Hz, 2J (P-C) ) 11.5 Hz, dCPh). Mass
spectrum (184W): m/z 965 (M+, 42%), 881 (M+ - 3CO, 64%),
770 (M+ - 7CO, 75%), 685 (M+ - 10CO, 100%). IR (CH2Cl2):
ν(CO) 2069 (m), 1951 (vs) cm-1
.
Anal. Calcd for C30H16
-
O
10P2W2: C, 37.26; H, 1.65. Found: C, 37.88; H, 1.78.
Complex 8: eluent hexane/CH2Cl2 (1:1), yield 0.1 g (27%).
31P NMR (CDCl3): see text. 1H NMR (CDCl3): δ 2.72 (dd,
3J (P-H) ) 11.0 Hz, 4J (P-H) ) 1.6 Hz, 3H, CH3), 3.86 (s, CO2-
CH3). 13C NMR (CDCl3): δ 20.46 (t, 2J (C-P) ) 15.1 Hz, 3J (C-
Su p p or tin g In for m a tion Ava ila ble: Tables giving crys-
tal data and refinement details, positional and thermal
parameters, and bond distances and angles for 5 (8 pages).
Ordering information is given on any current masthead page.
2
P) ) 6.6 Hz, CH3), 53.11 (s, CO2CH3), 163.12 (dd, J (C-P) )
3
1
21.4 Hz, J (C-P) ) 12.4 Hz, CO), 168.51 (dd, J (C-P) ) 27.6
Hz, 2J (C-P) ) 12.4 Hz, dCMe). IR (CH2Cl2): ν(CO) 2070 (m),
OM961072V