Valence isomerization of 2-phosphabicyclo-[1.1.0]butanes

Article abstract of DOI:10.1002/anie.200502248

(Figure Presented) Remarkably stable 2-phosphabicyclo-[1.1.0]butanes (see picture; O red, P violet, W turquoise) were synthesized from the complexed phosphinidene Ph-P=W(CO)₅ and cyclopropenes. These compounds undergo valence isomerization to f

Full text of DOI:10.1002/anie.200502248

Angewandte
Chemie
Bicyclobutanes
DOI: 10.1002/anie.200502248
Valence Isomerization of 2-Phosphabicyclo-
[1.1.0]butanes**
J. Chris Slootweg, Steffen Krill, Frans J. J. de Kanter,
Marius Schakel, Andreas W. Ehlers, Martin Lutz,
Anthony L. Spek, and Koop Lammertsma*
The unique electronic properties of the strained bicyclo-
[1.1.0]butanes[1] are enhanced by the heteroatoms in the
molecular frame. Illustrative is the bond-stretch isomerization
of the P2C2 and P2B2 bicycles.[2] However, very few systems are
known with a single heteroatom,[3] probably because of their
high reactivity, which is only moderated when the heteroatom
occupies a bridgehead position as in the 1-aza derivatives.[4]
The increased reactivity of the hetero systems is due to the
valence isomerization to which the bicyclo[1.1.0]butanes are
prone. We now report on the first 2-phospha derivatives.
[5]
=
The carbene-like phosphinidene Ph-P W(CO)5 was
generated in situ by cheletropic elimination from 1 at 1108C
in toluene and then allowed to react with cyclopropene 2a[6]
(Scheme 1). This led to the desired W(CO)5-complexed 2-
phosphabicyclo[1.1.0]butanes exo-3a (d31P = ꢀ85.1 ppm) and
endo-3a (d31P = ꢀ36.7 ppm)[7] in a 10:9 ratio, which were
isolated in 69% yield as colorless solids. The remarkably
Scheme 1. Synthesis of the 2-phosphabicyclo[1.1.0]butanes 3.
M=W(CO)5.
[*] J. C. Slootweg, Dr. F. J. J. de Kanter, Dr. M. Schakel, Dr. A. W. Ehlers,
Prof. Dr. K. Lammertsma
Department of Organic and Inorganic Chemistry
Faculty of Sciences
Vrije Universiteit
De Boelelaan 1083, 1081 HV, Amsterdam (The Netherlands)
Fax: (+31)20-598-7488
E-mail: lammert@chem.vu.nl
Dr. S. Krill, Prof. Dr. K. Lammertsma
Formerly associated with the Department of Chemistry
University of Alabama at Birmingham
Birmingham, AL 35294 (USA)
Dr. M. Lutz, Prof. Dr. A. L. Spek
Bijvoet Center for Biomolecular Research
Crystal and Structural Chemistry
Utrecht University
Padualaan 8, 3584 CH, Utrecht (The Netherlands)
[**] This work was supported by the Council for Chemical Sciences of
the Netherlands Organization for Scientific Research (NWO/CW).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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stable products were characterized by single-crystal X-ray
analyses, which showed puckered geometries with P1-C1-C2-
C3 folding angles of ꢀ114.66(15)8 for exo-3a and
ꢀ
ꢀ120.65(14)8 for endo-3a, and transannular C1 C2 bonds
of 1.516(3) and 1.550(3) Š, respectively (Figure 1). The flatter
Scheme 2. Valence isomerization of 3 to form 5 and, for R=Me, 6.
M=W(CO)5.
The BP86/6-31G*(LANL2DZ) calculations[9] on models
(labeled with an apostrophe (’), no Csubstituents) did not
demonstrate a direct path from 3 to 5,[10] but instead the
involvement of 1-phosphabutadiene 4 was established (see
Scheme 2). Isomerization of exo-3’ to give trans-4’ proceeds
by a concerted, asynchronous conrotatory ring opening (DE =
ꢀ18.4, DEꢀ = 32.8 kcalmolꢀ1; Figure 2). With subsequent
conrotatory electrocyclic ring closure, via the gauche form
of 4’ (DE = 3.7, DEꢀ = 7.9 kcalmolꢀ1), the more stable
phosphacyclobutene 5’ is formed (DE = ꢀ4.4, DEꢀ =
19.2 kcalmolꢀ1). This route implies that 1-phosphabutadiene
4 is a reaction intermediate. The small energy difference
between 4’ and 3-phosphacyclobutene 5’ is reflected in
Tran Huy and Matheyꢀs[11] use of derivatives of 5 as masked
1-phosphabutadienes.[12] These results highlight that the
stability order 3’ ! 4’ < 5’ of the three valence isomers differs
from the established values for the hydrocarbons (3’’ ! 5’’ !
4’’).[13]
Figure 1. The displacement ellipsoid plot of exo-3a (top) and endo-3a
(bottom) with ellipsoids drawn at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Š], angles [8], and
torsion angles [8] for exo-3a: W1–P1 2.5013(6), P1–C1 1.808(2), P1–C2
1.811(2), P1–C6 1.820(2), C1–C2 1.516(3), C1–C3 1.515(3), C1–C4
1.500(3), C2–C3 1.512(3), C2–C5 1.505(3), C3–C12 1.488(3); C1-P1-C2
49.52(10), P1-C1-C2 65.35(12), P1-C1-C3 96.79(15),
C2-C1-C3 59.83(15), C2-C1-C4 139.6(2), P1-C2-C1
To gain more insight into the formation of phospholene 6,
we resorted to the more gentle CuCl-catalyzed reaction of
1
[5,14] with 2a (558C, 15 h; Scheme 3). This reaction gave, with
the exception of 6, the same products as the noncatalyzed
65.13(12), P1-C2-C3 96.78(15), C1-C2-C3 60.06(15),
C1-C2-C5 141.4(2), C1-C3-C2 60.11(15); P1-C1-C2-C3
ꢀ114.66(15). endo-3a: W1–P1 2.4851(5), P1–C1
1.801(2), P1–C2 1.792(2), P1–C6 1.823(2), C1–C2
1.550(3), C1–C3 1.511(3), C1–C4 1.502(3), C2–C3
1.521(3), C2–C5 1.500(3), C3–C12 1.495(3); C1-P1-C2
51.11(9), P1-C1-C2 64.13(11), P1-C1-C3 100.05(14),
C2-C1-C3 59.57(14), C2-C1-C4 135.80(19), P1-C2-C1
64.76(11), P1-C2-C3 100.08(13), C1-C2-C3 58.94(13),
C1-C2-C5 136.7(2), C1-C3-C2 61.49(13); P1-C1-C2-C3
ꢀ120.65(14).
ꢀ
endo structure with the longer C1 C2
bond is the least stable of the two and
decomposes at about 1308C. At this
temperature, the exo isomer under-
goes valence isomerization to give 3-
phosphacyclobutene complexes cis-5a
(d31P = 46.3 ppm)
and
trans-5a
Figure 2. The relative BP86/6-31G* (LANL2DZ for W) energies (ZPE corrected, in kcalmolꢀ1) for
the rearrangement of exo-3’ into 5’. Selected bond lengths [Š], angles [8], and torsion angles [8] of
exo-3’ (Cs): W1–P1 2.536, P1–C1 1.833, P1–C6 1.846, C1–C2 1.527, C1–C3 1.506; C1-P1-C2 49.2,
C1-C3-C2 60.9, P1-C1-C2-C3 120.2; TSexo-3’-trans-4’: W1–P1 2.541, P1–C1 2.709, P1–C2 1.801,
P1–C6 1.820, C1–C2 1.474, C1–C3 1.446, C2–C3 1.595; trans-4’ (Cs): W1–P1 2.515, P1–C2 1.703,
P1–C6 1.830, C1–C2 1.441, C1–C3 1.359; TStrans-4’-gauche-4’: P1-C2-C1-C3 95.2; gauche-4’: P1-
C2-C1-C3 31.0; TSgauche-4’-5’: W1–P1 2.568, P1–C2 1.774, P1–C3 2.558, C1–C2 1.391, C1–C3
1.419, P1-C2-C1-C3 26.4; 5’: W1–P1 2.539, P1–C2 1.838, P1–C3 1.915, P1–C6 1.848, C1–C2 1.355,
C1–C3 1.518.
(d31P = 55.2 ppm, 2J(H,P) = 9.5 Hz)[8]
in a 4:1 ratio (35%) besides the
phospholene complex 6 as the major
product
(41%,
d31P = 37.9 ppm;
Scheme 2). Formation of the phospho-
lene was confirmed by crystal structure
analysis.
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valence isomerization of the novel 2-phosphabicyclo-
[1.1.0]butanes is indeed directed by the bridgehead substitu-
ents (3b 508C; 3a 1308C). This behavior parallels that
observed for the all-carbon analogue 2,2-dimethyl-bicyclo-
[1.1.0]butane, where the 1,3-diphenyl derivative isomerizes at
1308Cand the 1,3-dimethyl derivative at temperatures above
2808C.[19]
In conclusion, W(CO)5-complexed 2-phosphabicyclo-
[1.1.0]butanes are remarkably stable compounds that
valence-isomerize to 3-phosphacyclobutenes via 1-phospha-
butadienes at elevated temperatures. In contrast to the
hydrocarbon analogues, the diene can be trapped as a
phospholene due to rearrangements that are specific for the
phosphorus compounds.
Scheme 3. The CuCl-catalyzed formation of 5a and 9. M=W(CO)5.
reaction, that is, exo- and endo-3a (4:1 ratio, 16%) and cis-
and trans-5a (7:8 ratio, 30%). Additionally, the vinyl-
phosphirane complexes anti-9 (d31P = ꢀ157.1 ppm) and syn-
9 (d31P = ꢀ150.2 ppm) were also produced in a 3:1 ratio
(6%). The vinyl-phosphiranes formed in this milder process
(558Cvs. 110 8C) corroborates the role of phosphabutadienes
as reaction intermediates. Heating of isolated anti-9 at 1108C
effected epimerization of the phosphorus center to give the
syn-9 isomer, which underwent the established [1,3]-sigma-
tropic shift[15] to phospholene complex 6. The presumed
reagent, the [PhP(Cl)W(CO)5]-Cu-alkene complex,[14] is more
Experimental Section
3a and 6: Compounds 1[5] (591 mg, 0.90 mmol) and 2a[6] (156 mg,
1.08 mmol) were heated in toluene (3 mL) at reflux for 17 h. Removal
of the solvents under vacuum and chromatography of the residue over
silica with pentane/toluene (9:1) as eluent gave endo-3a (155 mg,
30%) and exo-3a (200 mg, 39%) as colorless solids as well as 6
(20 mg, 4%) as a pale yellow solid together with minor amounts of
cis-5a (10 mg, 2%) and trans-5a (5 mg, 1%). exo-3a: m.p.: 115–
1168C; 31P{1H} NMR (101.3 MHz, CDCl3): d = ꢀ85.1 (1J(P,W) =
263.7 Hz). endo-3a: m.p.: 79–828C; 31P{1H} NMR (101.3 MHz,
CDCl3): d = ꢀ36.7 ppm (1J(P,W) = 260.3 Hz); HRMS (EI, 70 eV):
m/z (%): 576 (9) [M]+; calcd for C22H17O5PW: 576.0323; found:
576.0325. 6: m.p.: 1078C; 31P{1H} NMR (101.3 MHz, CDCl3): d =
37.9 ppm (1J(P,W) = 240.4 Hz); HRMS (EI, 70 eV): m/z (%): 576
(30) [M]+; calcd for C22H17O5PW: 576.0323; found: 576.0329.
=
sensitive than Ph-P W(CO)5 to steric congestion in the 1,2-
cycloaddition, which is reflected in the lower yield of the 2-
phosphabicyclobutanes 3a. The competitive reaction is, in our
opinion, the formation of zwitterion 7, for which there is
computational support on related systems.[16] Compound 7
can also rearrange to phosphabutadiene 4a in analogy to the
addition of dichlorocarbene to cyclopropenes.[17] Ring closure
then gives 5a,[8] while two known hydride shifts[18] lead, via the
secondary vinylphosphane complex 8, to vinyl-phosphirane 9
(Scheme 3). The 4’!9’ process is exothermic by ꢀ1.8 kcal-
molꢀ1 for the parent system (no phenyl substituent on the
diene).[9]
3b: Compounds 2b (122.6 mg, 0.46 mmol),
1
(598 mg,
0.91 mmol), and CuCl (10 mg, 0.1 mmol) in toluene (2 mL) were
heated at 558Cfor 30 min. The reaction was stopped at incomplete
conversion to ensure maximum yield of 3b. Removal of the solvents
under vacuum and chromatography of the residue over silica with
pentane/toluene (9:1) as eluent gave a 4:1 mixture of exo-3b and
endo-3b (107 mg, 33%) as a pale yellow solid. exo-3b: m.p.: 1248C
(decomp.); 31P{1H} NMR (101.3 MHz, CDCl3): d = ꢀ70.5 ppm (1J-
(P,W) = 262.0 Hz); HRMS (EI, 70 eV): m/z (%): 700 (22) [M]+; calcd
for C32H21O5PW: 700.0636; found: 700.0634. endo-3b: 31P{1H} NMR
(101.3 MHz, CDCl3): d = 3.8 ppm (1J(P,W) = 271.8 Hz).
Valence isomerization is sensitive to the nature of the
bridgehead substituents, and this also applies to 3!4!5.
Reaction of cyclopropene 2b, which has phenyl instead of
methyl substituents, with the phosphinidene precursor 1 at
1108Cgave as sole products the 3-phosphacyclobutene
complexes cis-5b (d31P = 43.8 ppm, 2J(H,P) = 6.2 Hz) and
trans-5b (d31P = 53.4 ppm, 2J(H,P) = 9.6 Hz)[8] (5:1 ratio,
95%); cis-5b was characterized by X-ray crystallography.
Apparently, 3b isomerizes faster than 3a. In this case,
phospholene 6 cannot be formed because the phenyl sub-
stituents render a hydride shift (to give 8) impossible.
3a, 5a, and 9: Compounds 1 (1.24 g, 1.90 mmol), 2a (410 mg,
2.84 mmol), and CuCl (10 mg, 0.1 mmol) in toluene (8 mL) were
heated at 558Cfor 15 h. Removal of the solvents under vacuum and
chromatography of the residue (an insoluble black residue remains)
over silica with pentane/toluene (9:1) as eluent gave endo-3a (40 mg,
4%), cis-5a (160 mg, 14%), exo-3a, and trans-5a in a 10:8 ratio
(310 mg, 28%), and syn- and anti-9 in a 3:1 ratio (65 mg, 6%) as
colorless oils. cis-5a: 31P{1H} NMR (101.3 MHz, CDCl3): d = 46.3 ppm
(1J(P,W) = 221.4 Hz). Crystallization of the mixture of exo-3a and
trans-5a from pentane at ꢀ808Cafforded the pale yellow crystals of
trans-5a: M.p. 1118C; 31P{1H} NMR (101.3 MHz, CDCl3): d =
55.2 ppm (1J(P,W) = 237.5 Hz); HRMS (EI, 70 eV): m/z (%): 576
(39) [M]+; calcd for C22H17O5PW: 576.0323; found: 576.0318. syn-9:
31P{1H} NMR (101.3 MHz, CDCl3): d = ꢀ157.1 ppm (1J(P,W) =
255.2 Hz); HRMS (EI, 70 eV): m/z (%): 576 (20) [M]+; calcd for
C22H17O5PW: 576.0323; found: 576.0318. anti-9: 31P{1H} NMR
(101.3 MHz, CDCl3): d = ꢀ150.2 ppm (1J(P,W) = 263.3 Hz).
2-Phosphabicyclo[1.1.0]butanes
exo-3b
(d31P =
ꢀ70.5 ppm) and endo-3b (d31P = 3.8 ppm)[7] could be
obtained (4:1 ratio, 33%) by the milder CuCl-catalyzed
reaction (2 equiv of 1, 558C, 0.5 h). The X-ray crystal
structure of the exo isomer revealed two different triclinic
polymorphs with Z = 2 and Z = 6, respectively, and very
similar molecular structures. Polymorph I shows a folding
5b: Compounds 1 (261 mg, 0.40 mmol), 2b (118 mg, 0.44 mmol),
and CuCl (10 mg, 0.1 mmol) in toluene (2 mL) were heated at 558C
for 38 h or, alternatively (without CuCl), at reflux for 16 h. Removal
of the solvents under vacuum and chromatography of the residue over
silica with pentane/toluene (4:1) as eluent gave cis-5b (215 mg, 77%)
as a pale yellow solid together with a trace of trans-5b ( ꢁ 2%).
angle (113.27(10)8) and
a transannular bond length
(1.510(2) Š) that are similar to those of exo-3a. Heating of
isolated 3b (exo + endo, 4:1) in toluene at 508Cfor 60 h gave
the favored phosphacyclobutene cis-5b, indicating that
Angew. Chem. Int. Ed. 2005, 44, 6579 –6582
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Removal of the solvents under vacuum and chromatography of the
[8] a) N. H. Tran Huy, L. Ricard, F. Mathey, Organometallics 1988,
7, 1791 – 1795; b) A. Marinetti, L. Ricard, F. Mathey, Organo-
metallics 1990, 9, 788 – 793.
residue over silica with pentane/toluene (4:1) as eluent gave a 5:1
mixture of cis-5b and trans-5b (300 mg, 95%) as a pale yellow oil.
Crystallization from pentane at ꢀ208Cafforded colourless crystals of
cis-5b: m.p.: 117–1188C; 31P{1H} NMR (101.3 MHz, CDCl3): d =
43.8 ppm (1J(P,W) = 230.9 Hz); HRMS (EI, 70 eV): m/z (%): 700
(23) [M]+; calcd for C32H21O5PW: 700.0636; found: 700.06486. trans-
5b: 31P{1H} NMR (101.3 MHz, CDCl3): d = 53.4 ppm (1J(P,W) =
247.0 Hz).
[9] Intrinsic reaction coordinate (IRC) calculations were performed
on the transition structures to ascertain the connection between
reactant and product: Gaussian 98 (RevisionA.11.4): M. J.
Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery,
Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam,
A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V.
Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q.
Cui, K. Morokuma, N. Rega, P. Salvador, J. J. Dannenberg, D. K.
Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J.
Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayak-
kara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W.
Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S.
Replogle, and J. A. Pople, Gaussian, Inc., Pittsburgh PA, 2002;
see also Supporting Information.
Crystal structure data (see Supporting Information): exo-3a
(C22H17O5PW): P21/c (no. 14); a = 6.8439(1), b = 21.3415(2), c =
14.4400(1) Š, b = 95.2844(3)8, V= 2100.13(4) Š3; Z = 4;
R [I >
2s(I)]: R1 = 0.0178, wR2 = 0.0406. endo-3a (C22H17O5PW): P21/c
(no. 14); a = 8.2108(1), b = 15.0945(1), c = 17.8628(2) Š, b =
99.0907(3)8, V= 2186.07(4) Š3; Z = 4; R [I > 2s(I)]: R1 = 0.0175,
wR2 = 0.0403. 6 (C22H17O5PW): C2/c (no. 15); a = 37.996(3), b =
8.9553(4), c = 26.4314(15) Š, b = 110.813(5)8, V= 8406.9(9) Š3; Z =
16; R [I > 2s(I)]: R1 = 0.0213, wR2 = 0.0462. cis-5b (C32H21O5PW):
P21/c (no. 14); a = 10.9428(1), b = 22.9359(2), c = 15.0824(1) Š, b =
133.2674(4)8, V= 2756.40(4) Š3; Z = 4; R [I > 2s(I)]: R1 = 0.0192,
¯
wR2 = 0.0474. exo-3b (polymorph I, C32H21O5PW): P1 (no. 2); a =
10.4005(8), b = 10.7789(8), c = 14.4436(12) Š, a = 83.618(6), b =
[10] a) J. C. Slootweg, A. W. Ehlers, K. Lammertsma, J. Mol. Model.
2005, 11, in press; b) J. C. Slootweg, F. J. J. de Kanter, M.
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Spek, K. Lammertsma, Angew. Chem. 2004, 116, 3556 – 3559;
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78.886(6), g = 61.670(5)8, V= 1398.2(2) Š3; Z = 2;
R [I > 2s(I)]:
¯
R1 = 0.0163, wR2 = 0.0332. exo-3b (polymorph II, C32H21O5PW): P1
(no. 2); a = 10.4176(8), b = 17.1058(17), c = 23.9196(19) Š, a =
96.436(6), b = 92.693(7), g = 97.143(8)8, V= 4194.7(6) Š3; Z = 6; R
[I > 2s(I)]: R1 = 0.0246, wR2 = 0.0444. CCDC 275002–275007 contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[11] a) N. H. Tran Huy, F. Mathey, Tetrahedron Lett. 1988, 29, 3077 –
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[12] a) S. Ito, H. Jin, S. Kimura, M. Yoshifuji, J. Org. Chem. 2005, 70,
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Kunze, Chem. Ber./Recueil 1984, 117, 3151 – 3159.
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Keywords: bicyclic compounds · heterocycles · phosphorus ·
substituent effects · valence isomerization
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