Branched Phospha[7]triangulanes

Article abstract of DOI:10.1021/ja031648p

A highly strained, thermally stable (up to 150 °C) branched phospha[7]triangulane was synthesized from the second-generation bicyclopropylidene and transient phosphinidene [Ph?P=W(CO)₅], followed by demetalation in refluxing xylene. Bulkier transient CuCl?alkene-complexed phosphinidene gave 2-phosphabicyclo-[3.2.0]hept-1(5)-ene as an additional product. The "outer sphere" spirocyclopropanes provide a stabilizing factor for both of these novel compounds. Copyright

Full text of DOI:10.1021/ja031648p

Published on Web 02/24/2004
Branched Phospha[7]triangulanes
J. Chris Slootweg,† Marius Schakel,† Frans J. J. de Kanter,† Andreas W. Ehlers,†
Sergei I. Kozhushkov,‡ Armin de Meijere,‡ Martin Lutz,§ Anthony L. Spek,§ and Koop Lammertsma*,†
Department of Chemistry, Faculty of Sciences, Vrije UniVersiteit, De Boelelaan 1083,
1081 HV Amsterdam, The Netherlands, Institut fu¨r Organische und Biomolekulare Chemie der
Georg-August-UniVersita¨t Go¨ttingen, Tammannstrasse 2, D-37077 Go¨ttingen, Germany, and the BijVoet Center for
Biomolecular Research, Crystal and Structural Chemistry, Utrecht UniVersity, Padualaan 8,
3584 CH Utrecht, The Netherlands
Received December 11, 2003; E-mail: lammert@chem.vu.nl
Scheme 1. Synthesis of Phospha[7]triangulane Complex 3
Strain gives cyclopropane derivatives their unique electronic and
chemical properties.1 Spirofusion of three-membered rings augments
the strain by 8.5 kcal/mol,2 and yet many stable linear and branched
[n]triangulanes are known,3,4 but instead only few hetero[n]-
triangulanes are known, irrespective of whether it concerns spiro-
cyclopropanated aziridines, oxiranes, thiiranes, siliranes, or phos-
phiranes.3 The higher reactivity of the heterocyclic ring is believed
to be the underlying cause. In contrast, we now report on the
exceptional stability of a branched phospha[7]triangulane.
Reaction of carbene-like terminal phosphinidene complex [Ph-
PdW(CO)5],5 generated in situ by cheletropic elimination from 1
at 100 °C in toluene, with second-generation bicyclopropylidene
24 afforded W(CO)5-complexed phospha[7]triangulane 3 (mp 178-
179 °C, 88%) as the sole product (Scheme 1). Its 31P NMR
resonance at δ -119.6 is deshielded by 9.8 from the first-generation
triangulane6 and by 70.5 ppm from parent 4.7 This illustrates little
influence of the second spirocyclopropane sphere, but the larger
phospha[15]triangulane could not be synthesized from third-
generation bicyclopropylidene 54 because of its too congested
double bond.4b
Scheme 2. Decomplexation of 3
6′ (H for Ph; 224.2 kcal/mol) and alkene 2 (217.7 kcal/mol);
phosphirane C2PH5 has an SE of 20.8 kcal/mol. A contributing
factoristhereleaseofolefinstrain(OS)in2thatamountsto23.1kcal/
mol (see Supporting Information). The higher stability of 6 is also
related to a higher olefinic π-donor and π*-acceptor ability in 24
caused by spirocyclopropanation, which is reflected in the 9.3 kcal/
1
mol larger exothermicity for reaction of PH (A1) with 2 (82.9
kcal/mol) than for 1PH with ethylene (73.5 kcal/mol; G3(MP2)). This
behavior is in line with a higher HOMO (-8.45 vs -10.08 eV) and
a lower LUMO (4.12 vs 4.88 eV) for 2 when compared to ethylene.14
The higher reactivity of alkene 2 is also reflected in the CuCl-
catalyzed reaction with 1,15 but with surprises. Not only does the
reaction already take place at room temperature, instead of the usual
55-60 °C,15 but it also gives only a modest yield of 3 (42%) besides
The stabilizing W(CO)5 group was subsequently removed from
3. While oxidation with iodine at -30 °C8 afforded [W(CO)4I]+I--
complexed phospha[7]triangulane [δ(31P) -130.1], quenching with
N-methylimidazole8 only led to degradation. Instead direct ligand
exchange in refluxing xylene (150 °C!) with (Ph2PCH2)2 (dppe)9
was more successful, giving free phospha[7]triangulane 6 (mp 168-
169 °C, 82%) as the sole product (Scheme 2).10 Its δ(31P) at -164.0
shows the expected shielding on demetalation, and the increased
1J(P,C) coupling from 6.2 (3) to 37.3 Hz resembles that for the
parent 1-phenylphosphirane.7,11 The molecules of 6 are located on
an exact mirror plane in the crystal (Figure 1)12 and show slightly
elongated P-C and P-Ph bonds (by respectively 0.02 and 0.01
Å) due to the absence of the stabilizing W(CO)5 group.
The exceptional thermal stability of 3 and 6 is remarkable as
most phosphiranes eliminate or transfer [Ph-PdW(CO)5] at much
lower temperatures (e100-115 °C).10 That the six spirofused rings
indeed stabilize the CCP ring is evident from the mere 6.5 kcal/
mol difference in strain energies (SE), determined with homodes-
motic reactions at G3(MP2),13 between parent phospha[7]triangulane
Figure 1. Displacement ellipsoid plot (50%) of 6. Selected bond lengths
[Å] and angles [deg]: P1-C1 1.8430(11), C1-C1a 1.470(2), C1-C2
1.4889(14), C1-C5 1.4879(14), C2-C3 1.4862(15), C2-C4 1.4825(15),
C2-C5 1.4712(15), C3-C4 1.5305(17), C5-C6 1.4841(15), C5-C7
1.4869(16), C6-C7 1.5262(18); C1-P1-C1a 47.02(6), P1-C1-C1a
66.49(3). A: x, 0.5 - y, z.
† Vrije Universiteit.
‡ Georg-August-Universita¨t.
§ Utrecht University.
9
3050
J. AM. CHEM. SOC. 2004, 126, 3050-3051
10.1021/ja031648p CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
data for 3, 6, and 7 (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
References
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Figure 2. Displacement ellipsoid plot (50%) of 7 (molecule one of two).
Selected bond lengths [Å] and angles [deg]: P1-C1 1.805(6), P1-C12
1.877(5), C1-C8 1.339(7), C2-C5 1.524(7), C9-C12 1.539(7); C1-P1-
C12 89.2(3).
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Scheme 3. Formation of 3 and 7 under CuCl Catalysis
(8) Marinetti, A.; Mathey, F.; Fischer, J.; Mitschler, A. J. Chem. Soc., Chem.
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(12) 6: C20H21P, Fw ) 292.34, colorless needle, 0.46 × 0.36 × 0.09 mm3,
temperature ) 150(2) K, monoclinic, P21/m (No. 11), a ) 7.8462(1), b
) 12.9734(2), c ) 8.7058(1) Å, â ) 115.6968(6)°, V ) 798.537(18) Å3,
Z ) 2, Dx ) 1.216 g/cm3. No absorption correction (µ ) 0.164 mm-1).
15 383 measured reflections, 1913 reflections were unique [(sin θ/λ)max
) 0.65 Å-1]. 138 refined parameters. R-values [I > 2σ(I)]: R1 ) 0.0307,
wR2 ) 0.0784. R-values [all refl.]: R1 ) 0.0354, wR2 ) 0.0812. GOF
) 1.047. 7: C25H21O5PW, Fw ) 616.24, colorless plate, 0.09 × 0.09 ×
0.03 mm3, temperature ) 150(2) K, orthorhombic, Pca21 (no. 29), a )
18.9647(1), b ) 11.3278(1), c ) 21.7733(2) Å, V ) 4677.52(6) Å3, Z )
other products, including 7 (6%) (Scheme 3).16 The structure of 7,
the first 2-phosphabicyclo[3.2.0]hept-1(5)-ene derivative,17 spiro-
cyclopropanated at each carbon, was established by single-crystal
X-ray crystallography (Figure 2).12
8, Dx ) 1.750 g/cm3. Analytical absorption correction (µ ) 5.041 mm-1
,
0.58-0.89 transmission). 80 568 measured reflections, 10 736 reflections
were unique [(sin θ/λ)max ) 0.65 Å-1]. 577 refined parameters. Flack
parameter x ) -0.022(5). R-values [I > 2σ(I)]: R1) 0.0285, wR2 )
0.0378. R-values [all refl.]: R1 ) 0.0450, wR2 ) 0.0405. GOF ) 0.975.
(See Supporting Information for crystal structure determinations.)
(13) (a) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.; Rassolov, V.; Pople,
J. A. J. Chem. Phys. 1999, 110, 4703-4709. (b) Frisch, M. J.; Trucks, G.
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C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci,
B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.;
Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
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E. S.; Pople, J. A. Gaussian 98, revision A.11.3; Gaussian, Inc.: Pittsburgh,
PA, 2002. (c) See Supporting Information.
The formation of 7 is attributed to the influence of CuCl on the
cycloaddition process as 3 converts with added CuCl only slowly
in refluxing toluene to 7 and other products. This CuCl-catalyzed
cycloaddition behavior concurs with a recent analysis suggesting
that a CuCl-alkene complex facilitates the fragmentation of 1 to
give a reactive [PhP(Cl)W(CO)5]-Cu-alkene intermediate that
subsequently undergoes an SN2-type addition with alkenes.18 This
bulky Cu-containing reagent likely hampers the concerted [1+2]-
cycloaddition, thereby enabling the formation of zwitterion 8, which
can ring-close to 3 but also rearrange to 9 in analogy to the
cyclopropanation reaction of 2 with N2CHCO2Et in which both
products were obtained.4b However, contrasting its stable hydro-
carbon analogue, the more reactive PdC bond of 9 enables a
subsequent [1,3]-sigmatropic shift19 to give 7. This conversion is
19.9 kcal/mol exothermic at B3LYP/6-31G* for the parent system
(H for Ph, no W(CO)5).20
In conclusion, a highly strained, thermally stable (up to 150 °C)
branched phospha[7]triangulane was synthesized from second-
generation bicyclopropylidene 2 and phosphinidene [Ph-Pd
W(CO)5], followed by demetalation in refluxing xylene. Bulkier
transient CuCl-alkene-complexed phosphinidene gave also a
2-phosphabicyclo[3.2.0]hept-1(5)-ene. Spirocyclopropane-annela-
tion is stabilizing both of these novel compounds.
(14) Orbital energies at MP2/6-31G*.13b The lowest ionization energies (π-
IEv) determined by PE spectroscopy are 8.93 eV for bicyclopropylidene
(Gleiter, R.; Haider, R.; Conia, J.-M.; Barnier, J.-P.; de Meijere, A.; Weber,
W. J. Chem. Soc., Chem. Commun. 1979, 130-132. Hofland, A.; de Boer,
Th. J. Recl. TraV. Chim. Pays-Bas 1987, 106, 558-562. Gleiter, R.;
Spanget-Larson, J. In AdVances in Strain in Organic Chemistry; Halton,
B., Ed.; JAI Press: London, 1992; Vol. 2, p 143ff), 8.70 eV for
1-(cyclopropylidene)spiropentane, and 8.50 eV for 7-(cyclopropylidene)-
dispiro-[2.0.2.1]heptane) (Gleiter, R.; Kozhushkov, S. I.; de Meijere, A.,
unpublished results).
(15) Marinetti, A.; Mathey, F. Organometallics 1984, 3, 456-461.
(16) A byproduct in the crude reaction mixture [ca. 20%; δ(31P) ) 119.5, 1JP-W
) 254.4 Hz] did not survive workup.
(17) To our knowledge, only a 3-phosphabicyclo[3.2.0]heptene has been
reported earlier: Quin, L. D.; Middlemas, E. D. J. Am. Chem. Soc. 1977,
99, 8370-8371.
(18) Lammertsma, K.; Ehlers, A. W.; McKee, M. L. J. Am. Chem. Soc. 2003,
125, 14750-14759.
Acknowledgment. This work was supported by The Nether-
lands Foundation for Chemical Sciences (CW) with financial aid
from The Netherlands Organization for Scientific Research (NWO)
and the Fonds der Chemischen Industrie. We are grateful to the
companies BASF AG, Bayer AG, Chemetall GmbH, and Degussa
AG for generous gifts of chemicals.
(19) Bulo, R. E.; Ehlers, A. W.; Grimme, S.; Lammertsma, K. J. Am. Chem.
Soc. 2002, 124, 13903-13910. This rearrangement of 9 is a heteroanalogue
of the vinylcyclopropane-cyclopentene rearrangement: Baldwin, J. E. In
The Chemistry of the Cyclopropyl Group; Rappoport, Z., Ed.; Wiley:
Chichester, 1995; Vol. 2, pp 469-494. Baldwin, J. E. J. Comput. Chem.
1998, 19, 222-231. Baldwin, J. E. Chem. ReV. 2003, 103, 1197-1212.
(20) The geometries of 9′ and 7′ (PH instead of PhPW(CO)5) were optimized
at the B3LYP/6-31G* level (see Supporting Information).13b
Supporting Information Available: Crystallographic data (CIF)
of 6 and 7, experimental details and spectroscopic and computational
JA031648P
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J. AM. CHEM. SOC. VOL. 126, NO. 10, 2004 3051