Photochemically generated nitrilium phosphane-ylid tungsten complexes and their reactivity towards alkyne and nitrile derivatives

Article abstract of DOI:10.1039/a905752h

The photochemically generated nitrilium phosphane-ylid tungsten complex 2 reacts with different activated alkynes, dimethyl acetylenedicarboxylate (DMAD) (i) and ethyl acetylenecarboxylate (ii), and nitriles, ethyl cyanoformate (ECF) (iii) and 1-piperidin

Full text of DOI:10.1039/a905752h

Photochemically generated nitrilium phosphane-ylid tungsten complexes and
their reactivity towards alkyne and nitrile derivatives
Rainer Streubel,* Hendrik Wilkens and Peter G. Jones
Institut für Anorganische und Analytische Chemie der Technischen Universität Braunschweig, Postfach 3329,
D-38106 Braunschweig, Germany. E-mail: r.streubel@tu-bs.de
Received (in Basel, Switzerland) 14th July 1999, Accepted 6th September 1999
The photochemically generated nitrilium phosphane-ylid
tungsten complex 2 reacts with different activated alkynes,
dimethyl acetylenedicarboxylate (DMAD) (i) and ethyl
acetylenecarboxylate (ii), and nitriles, ethyl cyanoformate
(ECF) (iii) and 1-piperidinonitrile (iv), giving [3+2] cycload-
dition products such as the 2H-1,2-azaphosphole complexes
3 (i) and 4 (ii), the 2H-1,4,2-diazaphosphole complex 5 (iii)
and the 2H-1,3,2-diazaphosphole complex 7 in good to
excellent yields, the latter complexes, 4–7, being formed
regioselectively; the structure of the first ester-functionalized
2H-1,4,2-diazaphosphole complex 5 was established by X-
ray analysis.
regioisomer 7 provides support for the assumption of a
transylidation process, giving complex 6 and a subsequent
[3+2] cycloaddition of this reactive intermediate to benzonitrile.
It is also remarkable that satisfying results were obtained only if
pentane–benzonitrile mixtures were employed as solvent for
these low-temperature photochemical reactions (Scheme 2).
Nitrile ylides II, which are useful reactive intermediates in
synthetic chemistry, can be generated by thermally1 or photo-
chemically2 induced ring opening of 2H-azirenes I or by N-
terminal 1,1-addition of singlet carbenes III to nitriles3
(Scheme 1). So far, nitrilium phosphane-ylid complexes V have
been transiently formed only by thermal ring opening of 2H-
azaphosphirene complexes IV4 or by N-terminal 1,1-addition of
singlet type terminal phosphanediyl complexes VI to nitriles:5
we have termed the latter process C,N,P-1,3-dipole transylida-
tion. Although the yields are sometimes unsatisfactory, ther-
mally induced ring opening of 2H-azaphosphirene complexes
provides access to, e.g., 2H-1,2-azaphosphole,4 2H-1,4,2-diaza-
phosphole6 and 2H-1,3,2-diazaphosphole complexes.6 In order
to broaden the synthetic applicability of 2H-azaphosphirene
complexes in synthetic chemistry, we have now started to
investigate their photochemistry in the presence of activated
alkynes and nitriles as trapping reagents; the first results are
reported here.
Scheme 2 Reagents and conditions: (i), (ii) a solution of 0.14 mmol of 1, 80
mL n-pentane, 0.5 mL benzonitrile and 0.5 mmol alkyne was irradiated at
250 °C for 1.5 h (low-pressure Hg-lamp, Heraeus TQ150); evaporation of
the solvent yielded complexes 3 and 4 as orange to red oils, which, in the
case of complex 4, yielded 4 as an orange solid (mp 61 °C, decomp.) after
work-up by column chromatography at low temperature and crystallization
from pentane; (iii) a solution of 0.28 mmol of 1, 80 mL n-pentane, 0.5 mL
benzonitrile and 16 mmol ethyl cyanoformate was irradiated at 250 °C for
2 h, work-up by column chromatography at low temperature and
crystallization from pentane afforded 5 as red crystals (mp 67 °C, decomp.);
(iv) a solution of 0.14 mmol of 1, 80 mL n-pentane, 0.5 mL benzonitrile and
0.5 mmol 1-piperidinonitrile was irradiated at 250 °C for 1 h; evaporation
of the solvent yielded 7 as a yellow oil.
Scheme 1 Synthetic routes to nitrile ylides and nitrilium phosphane-ylid
complexes ([M] = metal complex fragment, R denotes ubiquitous organic
substituents).
Photochemical ring opening of the 2H-azaphosphirene
complex 17 in pentane–benzonitrile (50–100+1) at 250 °C in
the presence of 2 equiv. of dimethyl acetylenedicarboxylate
(DMAD) (i), ethyl acetylenecarboxylate (ii), ethyl cyano-
formate (ECF) (iii) and 1-piperidinonitrile (iv) furnished the
[3+2] cycloaddition products in good to excellent yields (80–95
%), which are significantly higher than those obtained by
thermal reactions. Reactions (i) and (ii) yielded 2H-1,2-aza-
phosphole complexes 34 and 4, and (iii) and (iv) the 2H-
1,4,2-diazaphosphole complex 5 and the 2H-1,3,2-diazaphos-
phole complex 7.6 Of note is the very high regioselectivity of
reactions (ii) and (iii) and also that of (iv); in the latter case the
Apart from the known complexes 3 and 7, the composition of
complex 4 and the ester-functionalized 2H-1,4,2-diazaphosp-
hole complex 5 were confirmed by elemental analyses and mass
spectrometry† and the structural formulation is based on the
characteristic NMR spectral data† in solution. Furthermore, the
ring constitution of complex 5 was confirmed by X-ray
structure analysis‡ (Fig. 1).
The phosphorus nuclei of 4 and 5 display resonances at d 94.4
and 122.4, which are in the expected range of such heterocycle
complexes, with phosphorus–tungsten coupling constants
Chem. Commun., 1999, 2127–2128
This journal is © The Royal Society of Chemistry 1999
2127

in 5 and 8 are both distorted tetrahedral, displaying identical
S°PPR values [310.9° (5) and 311° (8)], identical N1–P–C6
angles3 [89.05(8)° (5) and 90.2(3)° (8)6] and only slightly
different W–P–C8 angles [121.98(6)° (5) and 119.6(3)°
(8)6], but different phosphorus–tungsten distances of
2.5039(5) Å (5) and 2.532(2) Å (8).6 Together with the other
distances and angles, this shows a predominant electronic effect
of the ester group on these structural parameters.
We are currently investigating the reactivity of photo-
chemically generated nitrilium phosphane-ylid complexes to-
wards other p-systems.
This work was supported by the Fonds der Chemischen
Industrie and by the Deutsche Forschungsgemeinschaft; the
Figure was prepared by Mr F. Ruthe.
Notes and references
† Satisfactory elemental analyses were obtained for complexes 4 and 5.
NMR data were recorded in CDCl3 solution at 50.3 MHz (13C) and 81.0
(
31P), using SiMe4 and 85% H3PO4 as standard references; J/Hz. Selected
1
spectroscopic data: 4: 13C NMR: d 19.1 (d, JPC 4.4, PCH), 138.0 (d,
(2+3)
J
23.5, P–CNC), 162.4 (d, (2+3)JPC 9.0, P–CNC), 163.8 (d, 3JPC 17.2,
PC
Fig. 1 Molecular structure of complex 5 in the crystal. Radii are arbitrary.
Selected bond lengths (Å) and angles (°): P–W 2.5039(5), N1–P 1.7007(16),
P–C6 1.8678(19), C6–N2 1.284(2), N2–C7 1.438(2), C7–N1 1.287(2), P–
C8 1.8221(19); N1–P–C6 89.05(8), P–C6–N2 110.61(13), N2–C7–N1
119.60(16), C7–N1–P 110.08(13), W–P–C8 121.98(6).
CO2Et), 170.5 (d, (2+3)JPC 9.6, P–NNC), 196.3 (d, 2JPC 6.4, cis-CO), 198.4
(d, 2JPC 21.0, trans-CO); m/z (EI) 715 (M+, 10). 5: 13C NMR: d 21.9 (d, 1JPC
(2+3)
3
3.8, PCH), 167.2 (d,
J
7.0, P–NNC), 163.3 (d, JPC 28.2, CO2Et),
PC
195.6 (d, (1+4)JPC 29.8, P–CNN), 196.3 (d, 2JPC 6.2, cis-CO), 198.6 (d, 2JPC
23.1, trans-CO); m/z (EI) 716 (M+, 32).
‡ Crystal data for 5: C23H29N2O7PSi2W; M = 716.48, triclinic, space group
group P1, a = 10.5466(10), b = 10.7010(11), c = 13.8717(14) Å, a =
¯
|J(W,P)| of 227.2 and 233.6 Hz, respectively. The assignment of
the resonances to the carbon atoms of the heterocyclic system in
complexes 4 and 5 is unambiguous, if the carbon atoms are
bonded either to phosphorus or to hydrogen leading in the first
case to significantly greater magnitudes of |J(P,C)| (in general)
and/or to characteristic spectra if DEPT experiments were
performed. The carbon atom resonances of the heterocycle in
complex 4 appear at d 138.0 (C3), 162.4 (C4) and 170.5 (C5)
with phosphorus–carbon coupling constants |J(P,C)| of 23.5, 9.0
and 9.6 Hz, respectively. The 1H resonance at d 8.41 with a 32.7
Hz coupling to phosphorus is also structurally important; it is
significantly low-field shifted compared to 4-substituted con-
stitutional isomers, which have resonances at d ca. 7.5.8 As in
the 3,5-diphenyl-substituted 2H-1,4,2-diazaphosphole tungsten
complex 8,6 the ester-functionalized complex 5 displays a
typical phosphorus resonance (5+d 122.4, |J(W,P)| 233.6 Hz;
8+d 110.6, |J(W,P)| 227.8 Hz6). The ester group effectively
increases not only the coupling constant |J(W,P)|, but also
|J(P,C)|, which was observed in the 13C NMR spectra of
complex 5 for the C3- and C5-resonances (5+d 195.6, |J(P,C)|
29.8 Hz and 167.2, |J(P,C)| 7.0 Hz; 8: d 198.5, |J(P,C)| 22.3 Hz
and 169.5, |J(P,C)| 5.1 Hz6).
79.915(3), b = 82.215(3), g = 75.406(3)°, U = 1484.8(3) Å3, Z = 2, Dc
= 1.603 Mg m23, m = 4.065 mm21, F(000) = 708, 7327 independent
reflections to 2q max. 56°, T = 143 K, S = 1.019, R[F, > 4s(F)] = 0.0186,
wR(F2) = 0.0438, 25 restraints and 332 parameters, highest peak 1.142 and
deepest hole 20.539 e Å23
.
The X-ray data set was collected with monochromated Mo-Ka radiation
(l = 0.71073 Å) on a Bruker SMART 1000 CCD area detector. Absorption
correction based on multiple scans. The structure was solved by the
Patterson method and refined anisotropically by full-matrix least squares on
F2.10 H atoms were included using a riding model (except methyl groups:
refined as rigid groups). CCDC 182/1410. See http://www.rsc.org/
suppdata/cc/1999/2127/ for crystallographic files in .cif format.
1 P. K. Claus, in Houben Weyl, Methoden Org. Chem., 1990, vol.
E14b(1), p 1.
2 A. Padwa, Acc. Chem. Res., 1976, 9, 371.
3 A. Padwa and S. F. Hornbuckle, Chem. Rev., 1991, 91, 263.
4 R. Streubel, H. Wilkens, A. Ostrowski, C. Neumann, F. Ruthe and P. G.
Jones, Angew. Chem., Int. Ed. Engl., 1997, 36, 1492.
5 H. Wilkens, J. Jeske, P. G. Jones and R. Streubel, Chem. Commun.,
1998, 1529.
6 H. Wilkens, F. Ruthe, P. G. Jones and R. Streubel, Chem. Eur. J., 1998,
4, 1542.
7 R. Streubel, A. Ostrowski, S. Priemer, U. Rohde, J. Jeske and P. G.
Jones, Eur. J. Inorg. Chem., 1998, 257.
8 R. Streubel and H. Wilkens, unpublished work.
9 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen and
R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S1.
10 G. M. Sheldrick, SHELXL-97, program for crystal structure refinement,
University of Göttingen, 1997.
The five-membered ring system of the 2H-1,4,2-diazaphos-
phole complex 5 is almost planar and the phenyl group subtends
an interplanar angle to the five-membered ring of 4.2°, thus
enabling an efficient p–p electron interaction of the two rings.
The N–C distances, N1–C7 1.287(2) and N2–C6 1.284(2) Å,
are in the typical range of nitrogen–carbon double bonds;9 the
latter is somewhat shorter than the value of 1.302(9) Å in
complex 8.6 The coordination spheres of the phosphorus atoms
Communication 9/05752H
2128
Chem. Commun., 1999, 2127–2128