Topochemical [2+2] Dimerization of Kinetically Stabilized 1-Phosphaallenes in the Solid State

Article abstract of DOI:10.1002/ejoc.200300471

The thermolysis of two bulky 1-phosphaallenes in the solid state afforded diphosphanylidenecyclobutane or 2,4-dimethylene-1,3-diphosphacyclobutane. The regioselectivity of the [2+2] dimerization depends largely on the crystal structure of the 1-phosphaallenes, and the reaction path seems to be affected by the presence of bulky 2,4,6-tri-tert-butylphenyl (Mes*) group. This topochemical reaction was also controlled by the substituents at the 3-position of the 1-phosphaallene skeleton. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejoc.200300471

FULL PAPER
Topochemical [2؉2] Dimerization of Kinetically Stabilized 1-Phosphaallenes in
the Solid State
Shigekazu Ito,[a] Satoshi Sekiguchi,[a] and Masaaki Yoshifuji*[a]
Keywords: Allenes / Dimerization / Phosphaalkenes / Solid-state reactions / Topochemistry
The thermolysis of two bulky 1-phosphaallenes in the solid
state afforded diphosphanylidenecyclobutane or 2,4-di-
tylphenyl (Mes*) group. This topochemical reaction was also
controlled by the substituents at the 3-position of the 1-phos-
methylene-1,3-diphosphacyclobutane. The regioselectivity phaallene skeleton.
of the [2+2] dimerization depends largely on the crystal struc-
ture of the 1-phosphaallenes, and the reaction path seems to ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
be affected by the presence of bulky 2,4,6-tri-tert-bu-
Germany, 2003)
Introduction
several 1-phosphaallenes afforded [2ϩ2] dimers in a head-
to-tail mode on heating in the solid state. The regioselectiv-
ity of the dimerization depended on the substituents at the
3-position, which was supported by X-ray crystallographic
analysis. PhosphorusϪcarbon multiple bonds are nearly
apolar, and contact between them is apparently difficult if
they are sterically protected by bulky substituents. Never-
theless, the crystal packing of 1-phosphaallene bearing the
bulky Mes* group was suitable for the regio- and stereose-
lective [2ϩ2] dimerization of the PϭCϭC moiety.
Chemical reactions in the solid state generally display
high regio- and stereoselectivity due to the topochemically
controlled conditions; each molecule is regularly disposed
and has low locomotive energy.[1] The [2ϩ2] cycloaddition
of alkenes in the solid state has been a well established[2]
reaction since the dimerization of (E)-cinnamic acid was re-
ported by Liebermann and Bergami.[3] The crystalline ar-
rangement of molecules is controlled by various non-coval-
ent interactions; however, it is difficult to design a molecule
that crystallizes at will.[2,4] Intermolecular interactions of
organic molecules through van der Waals forces are gener-
ally weak, and most nonpolar molecules tend to give effec-
tive crystal packing to minimize the intermolecular repul-
sions.[1] Most reactions performed in the solid state proceed
on irradiation, but some are known to occur on heating
or rubbing.[1,2]
We studied the synthesis and isolation of low-coordinated
phosphorus compounds[5] containing the 2,4,6-tri-tert-bu-
tylphenyl (ϭ Mes*) group as a sterically bulky protecting
group.[6] The inherent lability of the multiple bonds of
phosphorus is suppressed by the presence of the bulky sub-
stituent to permit their isolation at ambient temperature
(ca. 25°C) and even in air.[5] Recently, we reported the prep-
aration of several 1-phosphaallenes containing the PϭCϭC
skeletons[7] by using a lithium phosphanylidene carbenoid
[Mes*PϭC(Br)Li].[8] This synthetic protocol enabled us to
obtain 1-phosphaallenes in excellent yields and without any
tedious experimental procedures.
Results and Discussion
1-(2,4,6-Tri-tert-butylphenyl)-1-phosphaallene (1) was
synthesized as described in the literature[7a] and recrys-
tallized from ethanol (Scheme 1). A single crystal of 1 was
studied by X-ray crystallography, and Figure 1 displays the
structure obtained. The bond lengths and angles in the
ϪPϭCϭC moiety are nearly identical to those of 1-(2,4,6-
tri-tert-butylphenyl)-3,3-diphenyl-1-phosphaallene.[9] Two
molecules of 1 are paired in the crystalline state and the
hydrogen atom bonded to the C2 atom is in close proximity
to the aromatic ring of the other molecule. Indeed, the
CHϪπ interaction is assumed to be a noncovalent interac-
tion operating in the crystal structure.[10] As a consequence,
the C2···C1 intermolecular distance was found to be 3.83(1)
˚
A, as depicted in Figure 1, together with a C2···C2 distance
˚
of 3.66(2) A. Such an intermolecular distance is close to the
On the other hand, in the course of our research on kine-
tically stabilized 1-phosphaallene derivatives, we found that
[a]
Department of Chemistry, Graduate School of Science,
Tohoku University
Aoba, Sendai 980-8578, Japan
Fax: (internat.) ϩ 81-22-217-6562
E-mail: yoshifj@mail.tains.tohoku.ac.jp
Scheme 1
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DOI: 10.1002/ejoc.200300471
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Dimerization of Kinetically Stabilized 1-Phosphaallenes
FULL PAPER
˚
sum of the van der Waals radii (3.70 A) and lies in the
range of length within which two molecules can interact
with each other in the solid state.[1,2] Additionally, the mini-
˚
mal CH···aryl distance was observed to be 3.36 A.
Figure 2. The structure of 2 including a packing diagram. Hydro-
gen atoms are omitted for clarity. The tert-butyl group in the para-
position is disordered and the atoms with the predominant occu-
˚
pancy factor (0.58) are shown. Bond lengths (A) and angles: PϪC1
1.68(1), PϪCMes* 1.80(1), C1ϪC2 1.48(2), C1ϪC2* 1.53(2),
C1ϪPϪCMes* 94.8(6), PϪC1ϪC2 132(1), PϪC1ϪC2* 135(1),
C2ϪC1ϪC2* 91.5(10), C1ϪC2ϪC1* 88.5(10)
Figure 1. The structure of 1 including a packing diagram. Hydro-
gen atoms are omitted for clarity. The tert-butyl group in the para-
position is disordered and the atoms with the predominant occu-
˚
pancy factor (0.56) are shown. Bond lengths (A) and angles: PϪC1
1.637(7), PϪCMes* 1.894(5), C1ϪC2 1.34(1), C1ϪPϪCMes*
101.5(3), PϪC1ϪC2 174.3(6)
We then studied functionalized 1-phosphaallenes bearing
heteroatoms to form the 3-methoxy-1-phosphaallene 5. As
displayed in Scheme 2, the 1-bromo-2-phosphaethenylli-
thium 3 was allowed to react with chloromethyl methyl
The single crystals of 1 were heated at 120 °C for 1 day ether to afford the corresponding 2-bromo-3-methoxy-1-
during which time the color changed from colorless to yel- phosphapropene 4. Compound 4 was then treated with pot-
low. The 31P NMR spectrum of the reaction mixture (in assium tert-butoxide to afford the 3-methoxy-1-phosphaal-
solution) suggested that a new compound 2 was formed lene 5 in moderate yield. In the 31P NMR spectrum, the
(Scheme 1). The 31P chemical shift observed for 2 is within signal for the phosphorus atom in 5 showed a lower-field
the region of phosphaethenes (ϪPϭCϽ), indicating that δp shift than that in 1. In the 13C NMR spectrum of 5, the
the 1-phosphaallene cyclizes through the CϭC bonds. A signal corresponding to the sp carbon is observed at a
yellow crystal obtained after heating was analyzed by X-ray higher field than that in 1, whereas the signal for the ter-
crystallography, and we observed some contraction in the minal sp2 carbon is present at a lower chemical shift. These
crystal lattice of 2 compared with 1. As displayed in Fig- NMR spectral properties were similar to those found for
ure 2, 2 includes a planar 1,3-(diphosphanylidene)cyclobut- the 1-phenylallene derivatives.[11] Compound 5 was purified
ane skeleton formed by a head-to-tail [2ϩ2] dimerization of by column chromatography, because the attempted recrys-
the CϭC bonds. This regioselectivity can be explained by tallizations failed. On the other hand, heating 5 without
looking at the crystal structure of 1, as shown in Figure 1: solvent at 120 °C for 2 h in the solid phase afforded a di-
The crystal structures of both 1 and 2 are similar in shape. merized product 6. Figure 3 displays the X-ray structure of
The phosphorus atom of 2 deviates from the aromatic Mes* 6 revealing a planar 2,4-dimethylene-1,3-diphosphacyclo-
plane as indicated by the two dihedral angles butane skeleton with a trans conformation. Compound 6
PϪCipsoϪCorthoϪCtBu of 28(1)° and 29(1)°, not only due to was obtained as a single geometrical isomer. In contrast to
the release of steric congestion but also due to the approach 1, the PϭC bond of 5 dimerized in a head-to-tail manner.
of the PϭCϭC groups to each other. In comparison, the It is of interest to note that no [2ϩ2] dimerization of 5 oc-
PϪCipsoϪCorthoϪCtBu dihedral angles of 1, both 12(1)°, are curred in solution.
smaller than those of 2. It should be noted that this [2ϩ2]
[2ϩ2] Dimerization of 1,3,3-triphenyl-1-phosphaallene
dimerization of 1 upon heating did not occur in solution proceeded in a head-to-tail fashion with respect to the Pϭ
and thus can only occurs in the crystalline state. In order C bonds affording a 2,4-dimethylene-1,3-diphosphacyclo-
to obtain 2, the temperature of the reaction must be kept butane derivative.[12] Theoretical investigations indicated
just below the melting point. A probable cause of the in- that both the HOMO and the LUMO of 1-phosphaallene
completeness of the dimerization (see Exp. Sect.) is that the [HPϭCϭCH2] are dominated by the PϭC bond which have
supply of heat is not sufficient for the dimerization to pro- larger orbital coefficients.[13] The resonance structure
ceed to completion. Moreover, an amorphous solid of 1 did [HPϩCϪϭCH2] was also suggested although its contri-
not undergo the topochemical [2ϩ2] dimerization.
bution was found to be negligible.[9,13] These results hence
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S. Ito, S. Sekiguchi, M. Yoshifuji
FULL PAPER
ture (ca. 25°C). These dimerizations of phosphacumulenes
were performed in solution and biradical intermediates have
been predicted,[16] suggesting that the dimerizations of 1
and 5 may include the generation of a radical species.
This work provides the first example of a topochemically
controlled reaction of low-coordinated phosphorus com-
pounds. Indeed, the Mes* group protects the unsaturated
PϭC bond and enables its handling under ordinary con-
ditions, but it is also apparent that the Mes* group controls
the arrangement of the molecules in the crystalline state to
effect the regioselective [2ϩ2] cycloaddition of the 1-phos-
phaallenes. We are continuing our investigation of topoch-
emical reactions of 1, 5, and other low-coordinated phos-
phorus compounds on heating, irradiation, or rubbing.
Scheme 2
Experimental Section
Thermolysis of 1-(2,4,6-Tri-tert-butylphenyl)-1-phosphaallene (1):
Single crystals of 1 (50 mg, 1.7 mmol; crystallized from EtOH) were
heated at 120 °C in a 5-mm capillary tube for 24 h. The residual
yellow crystals were dissolved in [D]chloroform, and the peaks in
the 31P NMR spectrum corresponding to 1 and 2 were observed in
a 1:3 ratio. A yellow crystal was picked out for X-ray crystallogra-
phy. When single crystals of 1 were heated at 120 °C for 1 h, the
signals in the 31P NMR spectrum for 1 and 2 were observed in a
5:1 ratio. An amorphous solid of 1 did not undergo dimerization.
Moreover, after melting, the reaction did not proceed. 2: 31P NMR
(162 MHz, CDCl3): δ ϭ 214 (quin., 3JP,H ϭ 15 Hz) ppm. 1H NMR
(400 MHz, CDCl3): δ ϭ 7.38 (br. S, 4 H, m-Mes*), 2.81 (t, 3JP,H
ϭ
15 Hz, 4 H, CH2), 1.47 (s, 36 H, o-tBu), 1.34 (s, 18 H, p-tBu) ppm.
Elemental analysis of the crystals after heating. C20H31P·0.3H2O
(307.8): calcd. C 78.09, H 10.35; found C 78.01, H 10.16.
Preparation of the 2-Bromo-3-methoxy-1-phosphapropene 4: To a
solution of 2,2-dibromo-1-(2,4,6-tri-tert-butylphenyl)-1-phosphae-
thene[17] (300 mg, 0.67 mmol) in THF (15 mL) was added butyllith-
ium (0.67 mmol, 1.6  solution in hexane) at Ϫ78 °C and stirred
for 15 min. Chloromethyl methyl ether (0.67 mmol) was added to
the solution at Ϫ78 °C and stirred for 1 h. The reaction mixture
was warmed to room temperature (ca. 25°C) and the solvent was
removed in vacuo. Extraction with hexane and purification of the
resulting solution by silica gel column chromatography (hexane/
EtOAc, 20:1) gave 4 (212 mg, 77 % yield). 4: Pale yellow amorph-
ous solid. 31P{1H} NMR (162 MHz, CDCl3): δ ϭ 261 ppm.
Figure 3. The molecular structure of 6. Hydrogen atoms except
˚
H1 are omitted for clarity. Bond lengths (A) and angles (°): PϪC1
1.839(2), PϪC1* 1.832(2), PϪCMes* 1.883(2), C1ϪC2 1.338(3),
C2ϪO 1.335(3), OϪCMe 1.435(3), C1ϪPϪC1* 82.18(10),
PϪC1ϪP* 97.82(10), C1ϪPϪCMes* 101.37(10), C1*ϪPϪCMes*
123.9(1), PϪC1ϪC2 129.1(2), PϪC1*ϪC2* 129.4(2), C1ϪC2ϪO
121.7(2)
4
1H NMR (400 MHz, CDCl3): δ ϭ 7.45 (d, JP,H ϭ 1 Hz, 2 H,
3
m-Mes*), 4.43 (d, JP,H ϭ 19 Hz, 2 H, CH2), 3.47 (s, 3 H, OMe),
correspond to the reactivity of 5. In contrast, 1 cyclized
at positions corresponding to the terminal CϭC bond. As
indicated in Figure 1 and 2, the [2ϩ2] dimerization of 1 is
strongly affected by the arrangement of molecules in the
1.52 (s, 18 H, o-tBu), 1.37 (s, 9 H, p-tBu) ppm. C21H34BrOP
(413.4): calcd. C 61.01, H 8.29; found C 61.17, H 8.25.
Preparation of the 3-Methoxy-1-phosphaallene 5 and 6: To a solu-
crystalline state. In the case of 5, the methoxy group may tion of 4 (220 mg, 0.53 mmol) in THF (15 mL) was added a solu-
tion of potassium tert-butoxide (0.53 mmol) in 5 ml of THF at
0 °C during 30 min. The reaction was then warmed to room tem-
perature (ca. 25°C). The solvent was removed in vacuo, followed
by extraction with hexane. The hexane solution was purified by
neutral Al2O3 column chromatography (hexane) to afford 5
(114 mg, 64 % yield). 5: Pale yellow amorphous solid. 31P{1H}
NMR (162 MHz, CDCl3): δ ϭ 142 ppm. 1H NMR (400 MHz,
function to arrange the molecules under the topochemical
conditions.[14]
The mechanism of these topochemical dimerizations of
the 1-phosphaallenes has not yet been elucidated so far. It
is known that allenes dimerize through biradical intermedi-
ates, mostly to give a head-to-head dimer on irradiation.[15]
As for the low-coordinated phosphorus compounds, 1-
phosphacumulenes have displayed head-to-head dimeriz-
ation at the terminal CϭC bonds or head-to-tail dimeriz-
3
CDCl3): δ ϭ 7.40 (br. s, 2 H, m-Mes*), 7.06 (d, JP,H ϭ 19 Hz, 1
H, CH), 3.48 (s, 3 H, OMe), 1.61 (s, 18 H, o-tBu), 1.33 (s, 9 H,
p-tBu) ppm. 13C{1H} NMR (101 MHz, CDCl3): δ ϭ 230.9 (d,
2
ation with respect to the PϭC moieties at ambient tempera- 1JP,C ϭ 22 Hz, PϭC), 153.7 (d, JP,C ϭ 3 Hz, o-Mes*), 150.1 (s,
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Dimerization of Kinetically Stabilized 1-Phosphaallenes
FULL PAPER
2
1
p-Mes*), 139.4 (d, JP,C ϭ 7 Hz, CϭC), 136.7 (d, JP,C ϭ 70 Hz,
ipso-Mes*), 122.5 (s, m-Mes*), 57.9 (s, OMe), 38.7 (s, o-CMe3),
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was obtained. 6: Pale yellow crystals (hexane), m.p. 213Ϫ215 °C.
31P{1H} NMR (162 MHz, CDCl3): δ ϭ 25 ppm. 1H NMR
(400 MHz, CDCl3): δ ϭ 7.54 (4 H, brs, m-Mes*), 5.44 (dd, 3JP,H ϭ
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3
˚
116.620(5), γ ϭ 93.20(2)° V ϭ 1010.2(2) A , Z ϭ 2, ρcalcd. ϭ 1.093 g
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Acknowledgments
This work was supported in part by Grants-in-Aid for Scientific
Research (No. 13304049 and 14044012) from the Ministry of Edu-
cation, Culture, Sports, Science and Technology, Japan.
[20]
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