Synthesis, structure, and reactivity of novel 3,4-diphosphacyclobutenes bearing silyl groups

Article abstract of DOI:10.1021/jo050195z

(Chemical Equation Presented) Copper-mediated homocoupling of sterically hindered 2-(2,4,6-tri-tert-butylphenyl)-1-trialkylsilyl-2-phosphaethenyllithiums afforded 1,2-bis(trialkylsilyl)-3,4-diphosphacyclobutenes (1,2- dihydrodiphosphetenes) through a form

Full text of DOI:10.1021/jo050195z

Synthesis, Structure, and Reactivity of Novel
3,4-Diphosphacyclobutenes Bearing Silyl Groups
Shigekazu Ito, Hiromichi Jin, Shigeo Kimura, and Masaaki Yoshifuji*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan
yoshifj@mail.tains.tohoku.ac.jp
Received January 31, 2005
Copper-mediated homocoupling of sterically hindered 2-(2,4,6-tri-tert-butylphenyl)-1-trialkylsilyl-
2-phosphaethenyllithiums afforded 1,2-bis(trialkylsilyl)-3,4-diphosphacyclobutenes (1,2-dihy-
drodiphosphetenes) through a formal electrocyclic [2+2] cyclization in the PdCsCdP skeleton as
well as 2-trimethylsilyl-1,4-diphosphabuta-1,3-diene. Reduction of 1,2-bis(trimethylsilyl)-3,4-diphos-
phacyclobutenes followed by quenching with electrophiles afforded ring-opened products, (E)-1,2-
bis(phosphino)-1,2-bis(trimethylsilyl)ethene and (Z)-2,3-bis(trimethylsilyl)-1,4-diphosphabut-1-ene.
The structures of the ring-opened products indicated E/Z isomerization around the CdC bond after
PsP bond cleavage of 5, and the isomerization of the PsCdC skeleton. Ring opening of 1,2-bis-
(trimethylsilyl)-3,4-diphosphacyclobutenes affording (E,E)- and (Z,Z)-1,4-diphosphabuta-1,3-dienes
was observed upon desilylation.
CHART 1
Introduction
The 1,4-diphosphabuta-1,3-diene skeleton (A) is one of
the simplest π-electron systems composed of phosphorus-
carbon multiple bonds. Basically, A is energetically
disfavored compared to the 3,4-diphosphacyclobutene
system (B), which has been suggested by theoretical
calculations1 as well as experimental investigations
(Chart 1).2 Although the strain energy in the unsaturated
four-membered-ring system is released, the stability of
the phosphorus-carbon double bond is inferior to that
of a carbon-carbon double bond, causing B to be more
stable than A.1a
SCHEME 1
Despite this, kinetic stabilization has been utilized to
synthesize various stable low-coordinated phosphorus
compounds bearing multiple bonds of phosphorus.3 We
have synthesized several kinetically stabilized 1,4-
diphosphabuta-1,3-dienes 1 by using copper-mediated
homocoupling of bulky phosphaethenyllithiums 2 bearing
the 2,4,6-tri-tert-butylphenyl group (hereafter abbrevi-
ated as Mes*) (Scheme 1).4 The stable 1,4-diphosphabuta-
1,3-dienes 1, which we have reported to date, have
displayed no cyclization upon irradiation or heating,
probably due to steric hindrance around the phosphorus
atoms.
Since we established preparation protocols for the 1,4-
diphosphabuta-1,3-dienes from the corresponding phos-
phaethenyllithiums, we have been interested in the
homocoupling of 1-phosphaethenyllithiums bearing silyl
groups such as 4. Contrary to our prediction, 3,4-
diphosphacyclobutenes 5, which are the formal result of
(1) (a) Bachrach, S. M.; Liu, M. J. Org. Chem. 1992, 57, 2040. (b)
Schoeller, W. W.; Tubbesing, U. Chem. Ber. 1996, 129, 419.
(2) Appel, R.; Barth, V. Tetrahedron Lett. 1980, 21, 1923.
(3) (a) Regitz, M.; Scherer, O. J. Multiple Bonds and Low Coordina-
tion in Phosphorus Chemistry; Thieme: Stuttgart, Germany, 1990. (b)
Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon Copy;
Wiley: Chichester, UK, 1998.
(4) (a) Ito, S.; Toyota, K.; Yoshifuji, M. Chem. Lett. 1995, 747. (b)
Ito, S.; Yoshifuji, M. Chem. Lett. 1998, 651. (c) Ito, S.; Yoshifuji, M.
Chem. Lett. 2000, 1390. (d) Ito, S.; Kimura, S.; Yoshifuji, M. Chem.
Lett. 2002, 708. (e) Ito, S.; Kimura, S.; Yoshifuji, M. Bull. Chem. Soc.
Jpn. 2003, 76, 405.
10.1021/jo050195z CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/26/2005
J. Org. Chem. 2005, 70, 3537-3541
3537

Ito et al.
SCHEME 2
CHART 2
FIGURE 1. An ORTEP drawing of the molecular structure
of 5a. One of the four independent molecules is displayed.
Hydrogen atoms are omitted for clarity. One of the p-tert-butyl
groups (right) is disordered and the atoms with the predomi-
nant occupancy factor (0.60) are displayed. Selected average
bond lengths (Å) and angles (deg): PsP 2.259, PsCMes* 1.871,
PsC 1.817, CdC 1.374, CsSi 1.882, PsPsC 74.8, PsCdC
102.2.
[2+2] cyclization, were obtained as the major product,
whereas the desired 2,3-bis(trialkylsilyl)-1,4-diphospha-
buta-1,3-diene (7) was not isolated. In this paper, we
report the isolation and structural determination of 5.
Moreover, we report here reduction with sodium naph-
thalenide to give the corresponding ring-opened products
through PsP bond cleavage. Although 3,4-diphospha-
cyclobutenes have been reported,5 the study of 5 revealed
novel findings concerning the properties of 3,4-diphos-
phacyclobutene.
displayed in the Supporting Information). Four indepen-
dent molecules of 5a have been identified and one of them
is displayed in Figure 1 together with the bond lengths
and angles. The four-membered ring is not planar and
the dihedral angle of the PsCdCsP is 23.3° (5a) or 21.4°
(5b). The bulky Mes* groups take a trans configuration
to avoid steric congestion. The average bond lengths of
the four-membered ring are comparable to the corre-
sponding parameters of the previously reported 3,4-
diphosphacyclobutenes [PsP 2.214-2.248 Å; PsC 1.814-
1.830 Å; CdC 1.356-1.358 Å].5b,9,10 The SisC(sp2) bond
length of 5a is close to that of 6a [1.897(2) Å].9
Theoretical calculations have suggested that 3,4-
diphosphacyclobutene is energetically favored over 1,4-
diphosphabuta-1,3-diene.1 However, the 1,4-diphospha-
buta-1,3-dienes bearing Mes*-P moieties (1) have not
shown [2+2] cyclization affording the corresponding 3,4-
diphosphacyclobutenes, because of the steric congestion
around the phosphorus atoms. Thus, the formation of 5
would involve a reaction mechanism other than the [2+2]
cyclization of the PdCsCdP skeleton. As an alternative
formation mechanism of 5 from 4, we propose an inter-
mediate containing a phosphinocarbene11,12 skeleton such
as 8, which affords the homocoupled product 5 in the
presence of copper(II) chloride (Chart 2). Indeed 8 is one
of the “stable” phosphinosilylcarbenes in which the
phosphorus donates a π-electron to the carbene center
whereas the silyl group behaves as a π-electron acceptor.12c
Efforts to prove the presence of intermediate 8 are in
progress.
Results and Discussion
Preparation and Structure of Diphosphacyclo-
butene. (Z)-2-Bromo-2-trialkylsilyl-1-phosphaethenes 36
were prepared from 2,2-dibromo-1-(2,4,6-tri-tert-butyl-
phenyl)-1-phosphaethene [Mes*PdCBr2].7 Compound 3a
was allowed to react with butyllithium to generate the
corresponding phosphaethenyllithium 4a6a followed by
the addition of copper(II) chloride. After the usual workup
procedures, 1,2-bis(trimethylsilyl)-3,4-diphosphacyclo-
butene 5a was obtained together with the protonated
adduct, 2-trimethylsilyl-1-phosphaethene [Mes*PdC(H)-
SiMe3]6 (Scheme 2).
The desired 2,3-bis(trimethylsilyl)-1,4-diphosphabuta-
1,3-diene (7; Chart 2) was not isolated in this reaction,8
whereas a trace amount of 2-trimethylsilyl-1,4-diphos-
phabuta-1,3-diene 6a9 was isolated after purification
indicating partial desilylation on the silica gel column.
Neither irradiation (λ > 300 nm) nor heating (110 °C) of
5a gave the ring-opened product 7a. Similarly, phospha-
ethenyllithium 4b, generated from 3b, afforded the
corresponding 3,4-diphosphacyclobutene 5b but in a low
yield. In the 31P NMR of 5, only one signal was observed,
indicating that 5 exists as a single diastereomer. Indeed,
compounds 5a,b were recrystallized from hexane and the
structure was confirmed by X-ray crystallography as
shown in Figure 1 (the molecular structure of 5b is
(5) (a) Tirla, C.; Me´zailles, N.; Ricard, L.; Mathey, F.; Le Floch, P.
Inorg. Chem. 2002, 41, 6032. (b) Charrier, C.; Guilhem, J.; Mathey, F.
J. Org. Chem. 1981, 46, 3. (c) Appel, R.; Barth, V.; Knoch, F. Chem.
Ber. 1983, 116, 938.
(6) (a) van der Sluis, M.; Wit, J. B. M.; Bickelhaupt, F. Organo-
metallics 1996, 15, 174. (b) van der Sluis, M.; Klootwijk, A.; Wit, J. B.
M.; Bickelhaupt, F.; Veldman, N.; Spek, A. L.; Jolly, P. W. J.
Organomet. Chem. 1997, 529, 107. (c) Appel, R.; Casser, C.; Immen-
keppel, M. Tetrahedron Lett. 1985, 26, 3551.
(7) Synthesis of Mes*PdCBr2: Sugiyama, S.; Ito, S.; Yoshifuji, M.
Chem. Eur. J. 2004, 10, 2700. See also: Goede, S. J.; Dam, M. A.;
Bickelhaupt, F. Recl. Trav. Chim. Pays-Bas 1994, 113, 278.
(8) A 31P NMR signal of δP 317, which seemed to be of 7a, was
observed in the reaction mixture.
(9) Niecke, E.; Fuchs, A.; Nieger, M. Angew. Chem., Int. Ed. 1999,
38, 3028.
(10) (a) Ce´nac, N.; Chrostowsk, A.; Sotirpoulos, J.-M.; Donnadieu,
B.; Igau, A.; Pfister-Guillouzo, G.; Majoral, J. P. Organometallics 1997,
16, 4551. (b) Schmidt, O.; Fuch, A.; Gudat, D.; Nieger, M.; Hoffbauer,
E.; Niecke, E.; Schoeller, W. W. Angew. Chem., Int. Ed. 1998, 38, 3028.
(c) Breen, T. L.; Stephan, D. W. Organometallics 1997, 16, 365. (d)
Heinemann, F. W.; Kummer, S.; Seiss-Brandl, U.; Zenneck, U. Orgno-
metallics 1999, 18, 2021. (e) Mackewitz, T. W.; Peters, C.; Bergstra¨sser,
U.; Leininger, S.; Regitz, M. J. Org. Chem. 1997, 62, 7605. (f) Jones,
C.; Richards, A. F. Organometallics 2002, 21, 438. (g) Mackewitz, T.
W.; Peters, C.; Bergstra¨sser, U.; Leininger, S.; Regitz, M. J. Org. Chem.
1997, 62, 7605.
(11) (a) Baceiredo, A.; Bertrand, G.; Sicard, G. J. Am. Chem. Soc.
1985, 107, 4781. (b) Igau, A.; Gru¨tzmacher, H.; Baceiredo, A.; Bertrand,
G. J. Am. Chem. Soc. 1988, 110, 6463. (c) Bourissou, D.; Guerret, O.;
Gabba¨i, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 58.
(12) van der Sluis, M. Dissertation, Vrije University, 1997.
3538 J. Org. Chem., Vol. 70, No. 9, 2005

3,4-Diphosphacyclobutenes Bearing Silyl Groups
FIGURE 2. An ORTEP drawing of the molecular structure of 9. Hydrogen atoms are omitted for clarity. The P2 atom disordered
and the atom with the predominant occupancy factor (0.89) is displayed. Typical bond lengths (Å) and angles (deg): P1sC1
1.871(3), P2sC2 1.868(3), P1sMe 1.867(3), P1sCMes* 1.870(2), P2sMe 1.867(3), P2sCMes* 1.886(3), C1sC2 1.375(3), C1sSi1
1.944(3), C2sSi2 1.949(3), P1sC1sSi1 121.6(1), P1sC1sC2 110.6(2), Si1sC1sC2 124.2(2), P2sC2sSi2 121.1(1), P2sC2sC1
109.8(2), Si2sC2sC1 124.8(2), Θ(P1sC1sC2sP2) 154.4(1), Θ(Si1sC1sC2sSi2) 161.0(1), Θ(P1sC1sC2sSi2) 2.3(3), Θ(Si1s
C1sC2sP2) 4.2(3).
SCHEME 3
CHART 3
are 0.325 and 0.334 Å, respectively). Accordingly, the C1
and C2 atoms slightly pyramidalize [sums of the bond
angles: ∑(C1) 356.4°, ∑(C2) 355.7°]. The P1sC1 and P2s
C2 distances are longer than the corresponding PsC
distances in (E)-1,2-bis(diphenylphosphino)ethane (11)
[1.807(3) and 1.808(3) Å].15 Moreover, the Si1sC1 and
Si2sC2 distances are longer than the corresponding
distances of tetrakis(trimethylsilyl)ethane (12) [1.909-
1.918 Å], which is one of the most crowded olefins.16 The
CdC distance of 9 is close to that of 12 [1.368(3) Å].16
These structural properties of 9 indicate that the steric
congestion is relieved mainly by elongation of the bond
lengths, whereas 12 displays a twisted conformation due
to the repulsion between the trimethylsilyl groups.16
When 5a was allowed to react with sodium naphtha-
lenide and ethanol, a diastereoisomer of 10 was observed
in the reaction mixture (δP ) 355.7, -35.9, JPP ) 146.1
Hz). The first observed diastereomer was completely
isomerized to afford 10 during silica gel column chroma-
tography. The isolated single diastereomer of 10 was
analyzed by X-ray crystallography as displayed in the
Supporting Information. The determined molecular struc-
ture shows an RS configuration at the P2 and C2 atoms.
The PdCsCsP skeleton twists with a torsion angle of
71.6(6)° probably as a result of steric interaction. Com-
pound 10 shows an isomerization involving a [1,3]
hydrogen shift, which is similar to the isomerization of
vinylphosphines to afford phosphapropenes.17
Reduction and Ring-Opening of 3,4-Diphosphacy-
clobutene. Reduction of 3,4-diphosphacyclobutenes with
alkaline metals has been established and used for the
synthesis of various heterocyclic compounds5,13 and thus
we are interested in the reactivity of 5a. Compound 5a
was allowed to react with sodium naphthalenide (ca. 3
equiv) to afford the corresponding anionic intermediate
as a deep-purple solution, which was stable at -78 °C
but afforded 5a at ca. -40 °C. The anionic intermediate
was quenched with iodomethane or ethanol at -78 °C to
give 1,2-diphosphinoethylene 9 or 1,4-diphosphabut-1-
ene 10 (Scheme 3).14 Similarly to the tetraphenyl-3,4-
diphosphacyclobutene reported by Mathey et al.,5,13 5a
revealed PsP bond cleavage upon reduction.
In contrast to the reduction of tetraphenyl-3,4-diphos-
phacyclobutene, 5a afforded ring-opened products indi-
cating E/Z isomerization during the formation of 9.
Additionally, 9 was obtained as a mixture of dl isomers.
Although the structure was not identified, a trace amount
of another diastereomer (δP ) 14.1) was observed in the
reaction mixture. Compound dl-9 was recrystallized from
hexane and the structure was determined by X-ray
crystallography (Figure 2). The C1 and C2 atoms locate
above the P1sSi1sP2sSi2 plane (the distances between
the C1 and C2 atoms from the P1sP2sSi1sSi2 plane
Ring-Opening of 3,4-Diphosphacyclobutene Af-
fording 1,4-Diphosphabuta-1,3-dienes. To understand
whether the trimethylsilyl groups in 5a can be removed
or replaced with other substituents, reaction of 5a with
tetrabutylammonium fluoride (TBAF) was carried out.
Desilylation took place when 5a was mixed with 2 equiv
(13) (a) Maigrot, N.; Ricard, L.; Charrier, C.; Mathey, F. Angew.
Chem., Int. Ed. Engl. 1988, 27, 950. (b) Maigrot, N.; Ricard, L.;
Charrier, C.; Mathey, F. Angew. Chem., Int. Ed. Engl. 1990, 29, 534.
(c) Charrier, C.; Maigrot, N.; Mathey, F.; Jeannin, Y. Organometallics
1986, 5, 623.
(14) Reaction of the anionic species from 5a with either iodine, bis-
(cyclopentadienyl)zirconium dichloride, or 1,2-dibromoethane gave 5a.
(15) Schmidbaur, H.; Reber, G.; Wagner, F. E.; Mu¨ller, G. Inorg.
Chim. Acta 1988, 147, 143.
(16) Sakurai, H.; Nakadaira, Y.; Tobita, H. J. Am. Chem. Soc. 1982,
104, 300.
J. Org. Chem, Vol. 70, No. 9, 2005 3539

Ito et al.
Colorless needles (EtOH), mp 72-74 °C; 31P{1H} NMR (162
MHz, CDCl3) δ 314.4; 1H NMR (400 MHz, CDCl3) δ 0.34 (d, J
) 0.8 Hz, 6H), 1.06 (s, 9H), 1.39 (s, 9H), 1.52 (s, 18H), 7.45 (d,
J ) 0.9 Hz, 2H); 13C{1H} NMR (101 MHz, CDCl3) δ -3.2 (d,
JCP ) 10.7 Hz), 19.0 (d, JCP ) 4.1 Hz), 27.8, 32.0, 33.4 (d, JCP
) 6.6 Hz), 35.6, 38.3, 122.5, 141.2 (d, JCP ) 65.7 Hz), 150.9,
153.0, 165.3 (d, JCP ) 79.4 Hz). Anal. Calcd for C25H44BrPSi:
C, 62.09; H, 9.17; Br, 16.52. Found: C, 62.37; H, 9.26; Br,
16.82.
SCHEME 4
of TBAF in wet THF. In contrast to our prediction, the
products were mainly 1,4-diphosphabuta-1,3-dienes 13
and no 3,4-diphosphacyclobutene derivative was observed
(Scheme 4). This result indicates that the 3,4-diphos-
phacyclobutene skeleton underwent thermal conrotatory
[π2s+σ2a] ring-opening, and thus, the (E,E) and (Z,Z)
isomers of 1318 were produced. It seems that silyl groups
sufficiently stabilize the sterically encumbered 3,4-
diphosphacyclobutene skeletons to avoid ring-openings.19
Preparation of 5a. To a solution of 3a (200 mg, 0.45 mmol)
in THF (15 mL) was added butyllithium (0.50 mmol) and the
mixture was stirred at -78 °C for 5 min. Copper(II) chloride
(0.50 mmol) was added to the reaction mixture and the solution
was then stirred for 1 h at -78 °C. The reaction mixture was
allowed to warm to room temperature, treated with ammonia
(10% NH3 in sat. NH4Claq) and extracted with ether (200 mL).
The organic layer was washed with water then dried over
anhydrous magnesium sulfate. The solvent was removed in
vacuo and silica gel column chromatography of the residue
(hexane) afforded crude 5a together with a trace amount of
6a. The crude 5a was recrystallized from hexane at -18 °C;
49 mg, 30%. Pale yellow prisms (hexane), mp 227-229 °C dec;
Conclusion
It has been established that copper-mediated coupling
of the 1-silyl-2-phosphaethenyllithiums (4) afforded 3,4-
diphosphacyclobutenes (5) through formal [2+2] cycliza-
tion of 1,4-diphosphabuta-1,3-diene. The structures of 5
showed a butterfly conformation. Reduction of 5a with
sodium naphthalenide gave the anionic intermediate that
was allowed to react with iodomethane and ethanol to
furnish 9 and 10, respectively, indicating PsP bond
cleavage. Particularly, the structure of 9 indicated E/Z
isomerization during the reaction, which had not been
known in the previous studies on 3,4-diphospha-
cyclobutenes. The isolated 9 and 10 are useful as novel
bulky P2-ligands.20 Furthermore, it has been clear that
silyl groups affect the molecular structures and reactivity
of phosphaethenyllithiums and 3,4-diphosphacyclobutenes.
1
31P{1H} NMR (162 MHz, CDCl3) δ 11.8; H NMR (400 MHz,
CDCl3) δ -0.30 (s, 18H), 1.33 (s, 18H), 1.60 (s, 36H), 7.39 (s,
4H); 13C{1H} NMR (101 MHz, CDCl3) δ 0.8, 31.6, 34.3 (pt,
(JCP+JCP)/2 ) 5.5 Hz), 35.3, 39.6, 122.0, 138.1 (pt, (JCP+JPC)/2
) 49.6 Hz), 151.7, 159.0 (pt, (JCP+JCP)/2 ) 6.6 Hz), 168.0 (pt,
(JCP+JCP)/2 ) 8.5 Hz); UV (hexanes) λmax (log ꢀ) 208 (3.68),
254 (3.51), 285 (3.43), 335 (sh, 2.85). Anal. Calcd for C44H76P2-
Si2: C, 73.06; H, 10.60. Found: C, 72.90; H 10.87.
Preparation of 5b. To a solution of 3b (850 mg, 1.8 mmol)
in THF (20 mL) was added butyllithium (1.8 mmol) and the
mixture was stirred at -78 °C for 5 min. Copper(II) chloride
(1.8 mmol) was added to the reaction mixture and the solution
was then stirred for 1 h at -78 °C. The reaction mixture was
allowed to warm to room temperature, treated with ammonia
(10% NH3 in sat. NH4Claq), and extracted with ether (300 mL).
The organic layer was washed with water then dried over
anhydrous magnesium sulfate. The solvent was removed in
vacuo and silica gel column chromatography followed by GPC
of the residue (hexane) afforded 12 mg of 5b (2% yield). Yellow
prisms (hexane), mp 186-188 °C; 31P{1H} NMR (162 MHz,
CDCl3) δ 3.7; 1H NMR (400 MHz, CDCl3) δ -0.94 (s, 6H), 0.13
(s, 6H), 0.86 (s, 18H), 1.33 (s, 18H), 1.50 (s, 18H), 1.67 (s, 18H),
7.28 (s, 2H) 7.37 (s, 2H); 13C{1H} NMR (101 MHz, CDCl3) δ
-4.6, -1.7, 20.1, 28.8, 31.5, 34.8, 35.2, 40.0, 123.3, 136.0 (pt,
(JCP+JCP)/2 ) 47.8 Hz), 151.4, 161.4 (pt, (JCP+JCP)/2 ) 17.9
Hz), 167.5 (pt, (JCP+JCP)/2 ) 11.9 Hz). Anal. Calcd for C50H88P2-
Si2: C, 74.38; H, 10.99. Found: C, 74.38; H, 11.05.
Reduction of 5a. To a solution of 5a (20 mg, 0.028 mmol)
in THF (1 mL) was added sodium naphthalenide (ca. 0.084
mmol) in THF (1 mL) at -78 °C and the mixture, colored deep
purple, was stirred for 5 min. An excess amount of io-
domethane or ethanol in THF was added to the reaction
mixture at -78 °C and the mixture, colored pale yellow, was
allowed to warm to room temperature. The solvent was
removed in vacuo and the residue was dissolved in hexane.
The insoluble materials were filtered off and the solution was
concentrated. Recrystallization from hexane afforded 9: 43%
yield, mp 166-168 °C; 31P{1H} NMR (162 MHz, CDCl3) δ 17.4;
1H NMR (400 MHz, CDCl3) δ -0.01 (br s, 18H), 1.30 (s, 18H),
1.37 (s, 18H), 1.45 (s, 18H), 1.60 (br s, 6H), 7.07 (br s, 1H),
7.17 (br s, 1H), 7.28 (br s, 2H); 13C{1H} NMR (101 MHz, CDCl3)
δ 6.7, 22.4 (pt, (JCP+JCP)/2 ) 8.7 Hz), 31.7, 34.2, 34.4, 35.2,
38.8, 40.5, 121.6, 123.3, 138.3 (dd, JCP ) 33.5 Hz, 26.5 Hz),
146.6, 154.2, 155.5, 180.6 (dd, JCP ) 15.6 Hz, 9.0 Hz). EA Calcd
for C46H82P2Si2‚H2O: C, 71.63; H, 10.97. Found: C, 71.77; H,
10.85. When the reaction mixture of 5a and sodium naphtha-
lenide in ethanol was concentrated in vacuo, a diastereoisomer
of 10 was observed in the 31P NMR spectrum (δP ) 355.7,
-35.9, JPP ) 146.1 Hz). Silica gel column chromatography
caused isomerization of the product to afford 10. 10: 82% yield,
Experimental Section
Preparation of 3b. To a solution of 2,2-dibromo-1-(2,4,6-
tri-tert-butylphenyl)-1-phosphaethene (Mes*PdCBr2; 1.00 g,
2.23 mmol)7 in THF (20 mL) was added butyllithium (2.3
mmol, 1.5 M solution in hexane) at -78 °C and the mixture
was stirred for 10 min. The solution was mixed with a THF (5
mL) solution of tert-butyldimethylsilyl chloride (2.45 mmol) at
-78 °C and the resulting mixture was stirred for 10 min. After
the reaction mixture was allowed to warm to room tempera-
ture the solvent was removed in vacuo. The crude residue was
purified by silica gel column chromatography (hexane) and
recrystallized from ethanol to give 0.85 g of 3b (78% yield).
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Guillemin, J. C.; Denis, J. M. J. Chem. Soc., Chem. Commun. 1994,
945. (e) Nguyen, M. T.; Landuyt, L.; Vanquickenborne, L. G. Chem.
Phys. Lett. 1993, 212, 543. (f) Yam, M.; Tsang, C.-W.; Gates, D. P. Inorg.
Chem. 2004, 43, 3719.
(18) Appel, R.; Hu¨nerbein, J.; Siabalis, N. Angew. Chem., Int. Ed.
Engl. 1987, 26, 779.
(19) One of the Reviewers pointed out that, as a result of theoretical
calculation, the substitution effect on energy difference between 1,2-
disilyl-3,4-diphosphacyclobutene and 2,3-disilyl-1,4-diphosphabutadi-
ene is large, indicating the silyl groups effectively stabilize the
diphosphacyclobutene structure. On the other hand, no ring-closure
of 1,4-diphosphabutadiene has been directly observed so far. It also
would be difficult to confirm that 5a is formed via ring-closure of 7a
even if the process is thermodynamically favorable. The substitution
effect on low-coordinated phosphorus compounds is described by:
Nyula´szi, L. J. Organomet. Chem. on line 26 Nov. 2004.
(20) Compound 11 was used for the preparation of several gold(I)
complexes as described in ref 15. See also: Brandys, M.-C.; Puddephatt,
R. J. J. Am. Chem. Soc. 2001, 123, 4839.
3540 J. Org. Chem., Vol. 70, No. 9, 2005

3,4-Diphosphacyclobutenes Bearing Silyl Groups
mp 155-158 °C; 31P{1H} NMR (162 MHz, CDCl3) δ -27.4,
347.2 (dd, JPP ) 110.8 Hz); 1H NMR (400 MHz, CDCl3) δ -0.25
(s, 9H), -0.23 (s, 9H), 1.31 (s, 9H), 1.35 (s, 9H), 1.54 (s, 9H),
1.57 (s, 9H), 1.60 (s, 9H), 1.68 (s, 9H), 2.93 (dd, J ) 36.0 Hz,
J ) 11.1 Hz, 1H), 5.33 (dd, J ) 241.0 Hz, J ) 10.8 Hz, 1H),
7.23-7.40 (br s, 4H); 13C{1H} NMR (101 MHz, CDCl3) δ 0.8,
1.8, 31.7, 33.8 (d, JCP ) 6.7 Hz), 34.1 (d, JCP ) 5.9 Hz), 34.3 (d,
JCP ) 10.3 Hz), 34.9, 35.3, 38.8, 39.1, 121.8, 122.1, 123.0, 133.4
(dd, JCP ) 56.1 Hz, 6.3 Hz), 139.8 (d, JCP ) 69.2 Hz), 148.3,
150.3, 154.0, 154.1, 155.1, 155.7, 194.6 (dd, JCP ) 66.0 Hz, 4.4
Hz); MS (ESI) m/z 725.5190 [M + H]; calcd for C44H79P2Si2
725.5195.
Reaction of 5a with TBAF. A mixture of 5a (60 mg, 0.083
mmol) and TBAF (tetrabutylammonium fluoride, 0.17 mmol)
in wet THF (5 mL) was stirred at room temperature for 1 h.
The solvent was removed in vacuo and the residue was
extracted with hexane. The hexane solution was concentrated
and purified by silica gel column chromatography (hexane) to
afford (E,E)- and (Z,Z)-1318 as a 1:1 mixture (14 mg, 30% yield).
X-ray Crystallography for 5a. A Rigaku MSC Mercury
CCD detector with graphite-monochromated Cu KR radiation
(λ ) 1.5418 Å) was used. The structure was solved by direct
methods (SIR92),21 expanded with Fourier techniques
(DIRDIF94),22 and then refined by full-matrix least squares.
Structuresolution, refinement, and graphical representation
were carried out with the teXsan package.23 C44H76P2Si2, M )
723.20, crystal dimensions 0.20 × 0.20 × 0.20 mm3, triclinic,
P1h (no. 2), a ) 13.963(1) Å, b ) 22.947(4) Å, c ) 31.616(5) Å,
R ) 70.01(2)°, â ) 87.85(2)°, γ ) 83.46(2)°, V ) 9458(3) Å3, Z
) 8, T ) 296 K, 2θmax ) 132.7°, Fcalc ) 1.016 g cm-3, µ(CuKR)
) 0.150 mm-1, 200 692 observed reflections, 26 573 unique
reflections (Rint ) 0.033), R1 ) 0.093 (I > 3σ(I)), RW ) 0.088
(all data) (CCDC-255489).
X-ray Crystallography for 9. A Rigaku RAXIS-IV imaging
plate detector with graphite-monochromated Mo KR radiation
(λ ) 0.71070 Å) was used. The structure was solved by direct
methods (SIR92),21 expanded using Fourier techniques
(DIRDIF94),22 and then refined by full-matrix least squares.
Structure solution, refinement, and graphical representation
were carried out with the teXsan package.23 C46H82P2Si2, M )
753.27, crystal dimensions 0.30 × 0.20 × 0.20 mm3, monoclinic,
P21/c (no. 14), a ) 10.650(1) Å, b ) 16.920(2) Å, c ) 27.253(1)
Å, â ) 100.309(3)°, V ) 4831.7(7) Å3, Z ) 4, T ) 130 K, 2θmax
) 55.0°, Fcalc ) 1.035 g cm-3, µ(MoKR) ) 0.167 mm-1, 36 384
observed reflections, 19 266 unique reflections (Rint ) 0.030),
R1 ) 0.065 (I > 3σ(I)), RW ) 0.088 (all data) (CCDC-255488).
Acknowledgment. This work was supported in part
by Grants-in-Aid for Scientific Research (Nos. 13304049
and 14044012) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. The authors
thank Dr. Chizuko Kabuto, Instrumental Analysis
Center of Chemistry, Graduate School of Science, To-
hoku University, for obtaining the X-ray data of 5a.
Supporting Information Available: ORTEP drawings
of molecular structures of 5b and 10, and X-ray crystal-
lographic data for 5a, 5b, 9 and 10 in CIF format. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(21) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994,
27, 435.
(22) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
de Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF94 program
system, Technical Report of the Crystallography Laboratory; Univer-
sity of Nijmegen, Nijmegen, The Netherlands, 1994.
JO050195Z
(23) Crystal Structure Analysis Package; Molecular Structure Cor-
poration, The Woodlands, TX, 1985 and 1999.
J. Org. Chem, Vol. 70, No. 9, 2005 3541