2-Phospha-4-silabicyclo[1.1.0]butane as a reactive intermediate

Article abstract of DOI:10.1002/anie.200454183

Thermally stable silylene Si{1,2(NCH₂tBu)₂C ₆H₄} reacts with 1H-phosphirenes to give the first isolated 2,3-dihydro-1,3-phosphasiletes, a unique type of zwitterion, and 1,2-dihydro-1,2-phosphasiletes. The novel 2-phospha-4-silabicyclo[1.1.0]butane (see picture) is a reactive intermediate in the reaction, whose course can be tuned by changing the substituents.

Full text of DOI:10.1002/anie.200454183

Communications
À
Phosphorus Heterocycles
diphosphetes of type C by isomerization or direct C P
insertion.[9]
Data on 2,4-disilabicyclo[1.1.0]butanes[10] is limited to two
disilabenzvalenes[11] and is absent for the corresponding
planar diyls. Only a single 2-silabenzvalene is known,
generated by photolysis of a silabenzene.[12] For the synthesis
of the 2,4-disila derivatives Ando et al. applied valence
2-Phospha-4-silabicyclo[1.1.0]butane as a
Reactive Intermediate**
J. Chris Slootweg, Frans J. J. de Kanter, Marius Schakel,
Andreas W. Ehlers, Barbara Gehrhus, Martin Lutz,
Allison M. Mills, Anthony L. Spek, and
Koop Lammertsma*
isomerization of bis(silirene) that involves an intramolecular
[11]
=
silylene addition to the C C bond of a silirene. The use of
stable silylenes might simplify this approach to a broader
array of hetero derivatives, which we decided to explore for
the unknown 2-phospha-4-silabicyclo[1.1.0]butanes 4.
Bicyclo[1.1.0]butane remains
a
fascinating compound.
À
Stretching of its central C C s-bond culminates in a planar
singlet diradical transition structure for inversion,[1] whereas
stretching of the peripheral bonds leads to valence isomer-
ization.[2] Bridging heteroatoms affect both processes. They
enable the coexistence of the puckered bicyclic and planar
diyl structures, and have thereby sparked intensive studies
dominated by recent discoveries of diphosphorus analogues.
Niecke et al. synthesized a crystalline 2,4-diphosphacyclobu-
tane-1,3-diyl A[3] that isomerizes photolytically to 2,4-diphos-
Reaction of silylene Si{1,2-(NCH2tBu)2C6H4} (Si(NN);
3)[13] with 1H-phosphirene 2a[14] in n-hexane at room temper-
ature results in red crystalline 2,3-dihydro-1,3-phosphasilete
5a instead of the desired 4 (Scheme 1).[15] Evidently, two
phabicyclo[1.1.0]butane B[4] and thermally to 1,2-dihydro-1,2-
diphosphete C.[5] A still more congested diyl A that does not
isomerize was reported by Yoshifuji and co-workers.[6] The
group of Bertrand obtained both stable planar diyl and
puckered forms for the isoelectronic 1,3-dibora-2,4-diphos-
phoniocyclobutanes.[7] 2,4-Diphosphabicyclo[1.1.0]butane is
puckered when the phosphorus atoms are endo,endo sub-
stituted as in dihydro diphosphabenzvalene 1, which Mathey
Scheme 1. Synthesis of 2,3-dihydro-1,3-phosphasiletes 5.
silylenes are involved, one in forming the novel ring structure
and the other as a substituent after transfer of a phenyl group.
No intermediate was detected by 31P NMR spectroscopy,
either at low temperatures or with different stoichiometries of
the reactants. The W(CO)5-complexed phosphaalkene part of
5a is characterized by the downfield absorptions at d(31P) =
278.3 ppm (1J(P,W) = 254.0 Hz) and d(13C) = 197.3 ppm
(1J(C,P) = 3.5 Hz). A single-crystal X-ray analysis confirmed
the structure of 5a (Figure 1).[16] Its four-membered ring
deviates from planarity owing to steric congestion, which is
and co-workers obtained by an intramolecular phosphinidene
[8]
=
addition to the C C bond of a 1H-phosphirene. Without
geometrical constraints, such additions invariably lead to 1,2-
[*] 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, 1081HV, Amsterdam (The Netherlands)
Fax: (+31)20-444-7488
À
À
À
reflected in the torsion angle of 11.6(2)8 for C1 P1 C2 Si1.
À
À
The P1 C1 and P1 C2 bond lengths (1.689(4) and 1.835(4) Š,
E-mail: lammert@chem.vu.nl
À
respectively) are normal, but the short Si1 C1 bond
Dr. B. Gehrhus
(1.841(5) Š) suggests some delocalization of charge.
Department of Chemistry
School of Life Science, University of Sussex
Brighton, BN19QJ (UK)
To lower the migratory aptitude of the phosphorus
substituent, we replaced the phenyl for a methyl group and
used 1H-phosphirene 2b[14] instead of 2a for the addition of
silylene 3. This had the desired effect, giving the crystalline
zwitterion 6b (68%), which only rearranged at elevated
temperatures (758C, 2 h) to 2,3-dihydro-1,3-phosphasilete 5b
(Scheme 1). The molecular structure of 6b (Figure 2,[16] the
crystal contains two independent molecules with similar
geometries) shows a unique fused tricyclic ring structure
which is retained in solution (d(31P) = À6.8 ppm (1J(P,W) =
Dr. M. Lutz, Dr. A. M. Mills, Prof. Dr. A. L. Spek
Bijvoet Center for Biomolecular Research
Crystal and Structural Chemistry, Utrecht University
Padualaan 8, 3584 CH, Utrecht (The Netherlands).
[**] The Netherlands Organization for Scientific Research (NWO/CW) is
acknowledged for partial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
3474
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200454183
Angew. Chem. Int. Ed. 2004, 43, 3474 –3477

Angewandte
Chemie
(d(29Si) = À33.4 ppm).[17] Furthermore, the cationic N2
[18]
À
À
lengthens the N2 Si and N2 C bonds.
We assume that the reaction starts by addition of the
=
silylene to the C C bond of the phosphirene to give 2-
phospha-4-silabicyclo[1.1.0]butane 4b as a transient inter-
mediate, followed by attack of a second silylene to the
bridgehead carbon atom in a manner analogous to the
reaction
of
nucleophiles with
bicyclo[1.1.0]butane
(Scheme 2).[19] The resulting 7b carries a silylium cation,
Scheme 2. Mechanism for the formation of 6b.
which requires stabilization[20] by coordination with the lone
pair of electrons of a nearby nitrogen atom.[21] This hypothesis
was tested with SCS-MP2/6-311 + G** energy calculations on
B3LYP6-31G* optimized model structures (labeled B),[22]
which are devoid of W(CO)5, carry H instead of Np
substituents, and have the nitrogen atoms linked by ethylene
instead of phenyl groups. This simplification is validated by
the similarity in optimized structures between 4B and that
containing the Np and W(CO)5 groups. Formation of 4B from
the reactants C2H4N2Si and C3H5P is exothermic by 17.5 kcal
molÀ1 with an additional 8.5 kcalmolÀ1 for the addition of a
second silylene to give 7B (Figure 3). Lewis base stabilization
Figure 1. Structure of 5a in the crystal (displacement ellipsoids drawn
at the 50% probability level; hydrogen atoms and the hexane solvent
molecule are omitted for clarity). Selected bond lengths [Š], angles [8],
À
À
À
and torsion angles [8]: W1 P1 2.4492(11), P1 C1 1.689(4), P1 C2
À
À
1.835(4), Si1 C1 1.841(5), Si1 C2 1.908(4); C1-P1-C2 92.9(2), P1-C2-
Si1 86.13(17), C1-Si1-C2 85.96(18), P1-C1-Si1 92.7(2); C1-P1-C2-Si1
11.6(2).
Figure 3. Relative SCS-MP2/6-311+G**//B3YLP/6-31G* energies (in kcal-
molÀ1) for the rearrangement of 6B to the thermodynamically more stable
5B with selected bond lengths [Š].
of the silylium cation gives the kinetically favored zwitterion
6B (DE = 21.7, DE° = 2.1 kcalmolÀ1), whereas a 1,3-methyl
shift from phosphorus to silicon results in the thermodynami-
cally favored phosphasilete 5B (DE = 49.6, DE° = 6.3 kcal
molÀ1). Such 1,3-shifts to an electron-deficient silicon are very
rare for both the methyl and phenyl groups.[23,24]
To reduce the attractiveness of the transient 2-phospha-4-
silabicyclo[1.1.0]butane for attack by a second silylene, a
methyl group was introduced at the bridgehead position by
treating 1H-phosphirene 2c[14] with 3. This had the desired
effect, but mainly valence isomerization occurred to the
isomeric 1,2-dihydro-1,2-phosphasiletes 8a and 8b,[25]
together with the formation of some 5c (5:2:2), because of
the elevated temperatures (808C, 13 h) required for the
reaction to take place (Scheme 3). After fractional crystal-
lization, 8a could be obtained as a yellow solid for which only
single resonances were observed at d(31P) = 15.2 ppm
Figure 2. Structure of 6b in the crystal, one of the two crystallographi-
cally independent molecules is shown (displacement ellipsoids drawn
at the 50% probability level; hydrogen atoms omitted for clarity).
À
Selected bond lengths [Š], angles [8], and torsion angles [8]: W1 P1
À
À
À
À
2.5363(12), P1 C1 1.793(4), P1 C2 1.863(4), Si1 N1 1.725(4), Si1 N2
À
À
À
À
1.923(4), Si1 C1 1.750(4), Si1 C2 1.906(4), Si2 N2 1.933(4), Si2 N3
1.710(4), Si2 N4 1.705(4), Si2 C2 1.828(4), N1 C10 1.409(6), N1
À
À
À
À
À
À
À
C16 1.471(6), N2 C11 1.471(5), N2 C21 1.510(5), C1 C4 1.455(6);
C1-P1-C2 90.79(19), P1-C2-Si1 85.70(18), C1-Si1-C2 90.70(19), P1-C1-
Si1 92.7(2); C2-P1-C1-Si1 À2.4(2), P1-C1-C4-C5 8.2(8).
240.0 Hz), d(29Si) = À11.2 (2J(Si,P) = 17.2 Hz, SiNN) and
À33.4 ppm (2J(Si,P) = 18.4 Hz, SiNN+). The negatively
charged C1 is in a planar environment and is surrounded by
À
À
À
shortened C1 P1 (1.793(4) Š), C1 C4 (1.455(6) Š), and C1
Si1 (1.750(4) Š) bonds that can be attributed to negative
hyperconjugation, which is supported by the shielding of Si1
Angew. Chem. Int. Ed. 2004, 43, 3474 –3477
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3475

Communications
phosphasiletes 5, zwitterion 6b, and 1,2-dihydro-1,2-phospha-
siletes 8 with the novel 2-phospha-4-silabicyclo[1.1.0]butanes
4 being a reactive intermediate in the reaction, the course of
which can be tuned by changing the substituents.
Experimental Section
5a: Complex 2a[14] (38.6 mg, 72.3 mmol) in n-hexane (5 mL) was
added slowly to a solution of 3[13] (39.7 mg, 144.6 mmol) in n-hexane
(2 mL) at À508C, warmed to ambient temperature, and stirred for
16 h. Filtration, concentration, and cooling at À208C affording red
crystals of 5a (56 mg, 66%): m.p. 2458C (decomp.); 29Si NMR
(79.5 MHz, C6D6): d = À5.1 (d, 2J(Si,P) = 6.9 Hz; Ph-SiNN),
Scheme 3. Isomerization of 4c.
(1J(P,W) = 205.0 Hz) and d(29Si) = À17.1 ppm (1J(Si,P) =
45.6 Hz) confirming the stoichiometry of the reaction. The
structure of 8a was established unequivocally by a single-
crystal X-ray structural analysis (Figure 4),[16] which shows
2
À13.5 ppm (d, J(Si,P) = 31.8 Hz; SiNN); 31P{1H} NMR (101.3 MHz,
CDCl3): d = 278.3 ppm (1J(P,W) = 254.0 Hz); HRMS (EI, 70 eV): m/z
(%): 1082 (2) [M+]; calcd for C51H63O5N4Si2PW: 1082.35840; found:
1082.35376.
5b: Complex 2b[14] (12.3 mg, 26.0 mmol) and
3 (14.3 mg,
52.1 mmol) were heated at 758C for 2 h in C6D6 (0.5 mL). Evaporation
to dryness, extraction into n-hexane, and subsequent filtration and
cooling at À208C gave red crystals of 5b (15 mg, 57%): m.p. 1508C
(decomp.); 29Si NMR (79.5 MHz, C6D6): d = 4.8 (d, 2J(Si,P) = 4.2 Hz;
Me-SiNN), À13.8 ppm (d, 2J(Si,P) = 33.2 Hz; SiNN); 31P{1H} NMR
(101.3 MHz, C6D6): d = 276.6 ppm (1J(P,W) = 246.1 Hz); HRMS (EI,
70 eV): m/z (%): 1020 (5) [M+]; calcd for C46H61O5N4Si2PW:
1020.34277; found: 1020.3391.
6b: Complex 2b (20.4 mg, 43.2 mmol) in n-hexane (5 mL) was
added to a solution of 3 (23.7 mg, 86.3 mmol) in n-hexane (2 mL) at
ambient temperature and stirred for 2 h. Concentration and cooling at
08C affording yellow crystals of 6b (30 mg, 68%): m.p. 1498C
(decomp.); 29Si NMR (79.5 MHz, C6D6): d = À33.4 (d, 2J(Si,P) =
18.4 Hz; SiNN+), À11.2 ppm (d, 2J(Si,P) = 17.2 Hz; SiNN);
31P{1H} NMR (101.3 MHz, C6D6): d = À6.8 ppm (1J(P,W) = 240.0 Hz).
8: Complex 2c[14] (44.9 mg, 81.9 mmol) and
3 (33.7 mg,
122.8 mmol) were heated at 808C for 13 h in C6D6 (0.5 mL).
31P{1H} NMR (101.3 MHz, C6D6): d = 12.0 (1J(P,W) = 198.5 Hz, 19%
yield of 8b by NMR spectroscopy), 15.2 (1J(P,W) = 205.6 Hz, 50%
yield of 8a by NMR spectroscopy), 308.1 ppm (br s, 19.5% yield of 5c
by NMR spectroscopy). Evaporation to dryness, extraction into n-
hexane, and subsequent cooling at À808C gave yellow crystals of 8a
(30 mg, 45%): m.p. 1578C (decomp.); 29Si NMR (79.5 MHz, C6D6):
d = À17.1 ppm (d, 1J(Si,P) = 45.6 Hz; SiNN); 31P{1H} NMR
(101.3 MHz, C6D6): d = 15.2 ppm (1J(P,W) = 205.0 Hz); HRMS (EI,
70 eV): m/z (%): 822 (80) [M+]; calcd for C36H39O5N2SiPW: 822.1876;
found: 822.1880. 5c: MS (EI, 70 eV): m/z (%): 1096 (30) [M+].
Figure 4. Structure of 8a in the crystal (displacement ellipsoids drawn
at the 50% probability level; hydrogen atoms are omitted for clarity).
À
Selected bond lengths [Š], angles [8], and torsion angles [8]: W1 P1
À
À
À
À
2.5269(6), P1 Si1 2.3074(8), P1 C1 1.854(2), P1 C10 1.841(2), Si1
À
À
À
N1 1.726(2), Si1 N2 1.7249(19), Si1 C2 1.863(2), C1 C2 1.360(3),
C1 C4 1.489(3), C2 C3 1.507(3); C1-P1-Si1 73.30(7), P1-Si1-C2
76.89(7); P1-C1-C2-Si1 7.46(17).
À
À
Received: March 4, 2004 [Z54183]
that the four-membered ring is nearly planar with a torsion
À
À
À
À
angle of 7.46(17)8 for P1 C1 C2 Si1 and that the P1 Si1
bond (2.3074(8) Š) is slightly elongated owing to steric
congestion.
Keywords: phosphorus heterocycles · reaction mechanisms ·
reactive intermediates · substituent effects · valence
isomerization
.
SCS-MP2/6-311 + G** calculations on B3LYP/6-31G*
model structures containing carbon substituents[22] show that
intermediate 2-phospha-4-silabicyclo[1.1.0]butane 4C can
isomerize to the thermodynamically preferred 1,2-dihydro-
1,2-phosphasiletes 8A and 8B with barriers of only 21.3 and
21.6 kcalmolÀ1, respectively. This similarity in DE° values is
reflected in the two experimentally observed isomers. The
valence isomerization of 4C to 8 is symmetry-allowed and can
be described as a [s2s+s2a] process.[26] Such a pathway is
unprecedented for the analogous isomerization of bicy-
clo[1.1.0]butane, for which s-trans-1,3-butadiene is the
favored product.
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phosphirenes 2 to give the first isolated 2,3-dihydro-1,3-
3476
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Angewandte
Chemie
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[16] Crystal structure determinations: X-ray intensities were meas-
ured on a Nonius KappaCCD diffractometer with rotating
anode (l = 0.71073 Š) at
a temperature of 150(2) K. The
structures were refined with SHELXL97[27] against F2 of all
reflections. Illustrations, structure calculations and checking was
performed with the PLATON package.[28] 5a (C51H63N4O5P-
Si2W·0.6C6H14): Mw = 1134.76, red block, 0.06 ” 0.18 ” 0.21 mm3,
monoclinic, P21 (No. 4), a = 10.5532(1), b = 16.0049(2), c =
17.2900(2) Š, b = 100.5233(5), V= 2871.21(6) Š3, Z = 2, 1 =
1.313 gcmÀ3; 51129 measured reflections, 13052 unique reflec-
tions (Rint = 0.050); structure solution with automated Patterson
methods;[29] 643 refined parameters, 46 restraints; R1/wR2 [I >
2s(I)]: 0.0338/0.0755, R1/wR2 (all reflections) = 0.0472/0.0794;
GoF = 1.072; Flack ” parameter: À0.024(4). 6b (C46H61N4O5P-
Si2W): Mw = 1020.99, yellow block, 0.18 ” 0.24 ” 0.30 mm3, tri-
[24] C. Eaborn, A. Kowalewska, W. Stanczyk, J. Organomet. Chem.
1998, 560, 41 – 46, and references therein.
[25] Only one other synthesis of 1,2-dihydro-1,2-phosphasiletes was
reported: a) S. Haber, R. Boese, M. Regitz, Angew. Chem. 1990,
102, 1523 – 1525; Angew. Chem. Int. Ed. Engl. 1990, 29, 1436 –
1438; b) S. Haber, M. Schmitz, U. Bergsträßer, J. Hoffmann, M.
Regitz, Chem. Eur. J. 1999, 5, 1581 – 1589.
[26] J. C. Slootweg, A. W. Ehlers, K. Lammertsma, Phosphorus
Sulfur Silicon Relat. Elem. 2004, 179, 803 – 807.
[27] G. M. Sheldrick, SHELXL-97, Program for crystal structure
refinement, University of Göttingen, Germany, 1997.
¯
clinic, P1 (No. 2), a = 10.6476(3), b = 21.2390(8), c =
22.6862(5) Š, a = 74.495(2), b = 89.5698(19), g = 82.814(3)8,
V= 4903.0(3) Š3, Z = 4, 1 = 1.383 gcmÀ3. The crystal appeared
to be non-merohedrally twinned with a 1808 rotation about
uvw = [100] as twin operation. The intensity data were evaluated
with EvalCCD,[30] taking this twin relation into account; 55937
measured reflections, 22336 unique reflections (Rint = 0.058);
sructure solution with direct methods;[31] 1064 refined parame-
ters, 388 restraints; bond lengths and angles of the two
independent molecules were restrained to be the same; R1/
wR2 [I > 2s(I)]: 0.0499/0.1070, R1/wR2 (all reflections) = 0.0767/
0.1216; GoF = 1.128; the refinement[24] of the twin fraction
resulted in 0.794(1):0.206. 8a (C36H39N2O5PSiW): Mw = 822.60,
yellow plate, 0.06 ” 0.36 ” 0.42 mm3, monoclinic, P21/c (No. 14),
[28] A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7 – 13.
[29] P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, S.
Garcia-Granda, R. O. Gould, J. M. M. Smits, C. Smykalla, The
DIRDIF99 program system, Technical Report of the Crystallog-
raphy Laboratory, University of Nijmegen, The Netherlands,
1999.
[30] A. J. M. Duisenberg, L. M. J. Kroon-Batenburg, A. M. M.
Schreurs, J. Appl. Crystallogr. 2003, 36, 220 – 229.
[31] G. M. Sheldrick, SHELXS-97, Program for crystal structure
solution, University of Göttingen, Germany, 1997.
a = 19.1665(1),
b = 10.6482(1),
c = 20.5181(1) Š,
b =
119.8343(4)8, V= 3632.53(5) Š3, Z = 4, 1 = 1.504 gcmÀ3; 58693
measured reflections, 8329 unique reflections (Rint = 0.046);
Angew. Chem. Int. Ed. 2004, 43, 3474 –3477
www.angewandte.org
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3477