Lewis Acid Catalyzed Z to E Isomerization of 1,2-Bis(diphenylphosphino)ethene

Article abstract of DOI:10.1515/znb-1998-1111

Z-1,2-Bis(diphenylphosphino)ethene, cis-Ph₂PCH=CHPPh₂, forms 1:2 adducts with GaBr₃ and GaI₃, the former of which has been identified in an X-ray diffraction study as the complex of the isomerized ligand, E-Ph₂PCH=CHPPh₂. The GaI₃ complex is believed to be analogous on the basis of analytical and spectroscopic data. InBr₃ affords a 1:1 complex with an ionic structure [(Ph₂PCH=CHPPh₂)₂InBr₂]⁺ [InBr₄]^- in which the cation contains the original cis-ligand. With InI₃ also a 1:1 adduct is obtained, where the metal triiodide unit is attached to only one phosphorus atom of the non-isomerized (cis) ligand in the solid state. There is rapid site exchange of the InI₃ unit in chloroform solution as followed by NMR spectroscopy. -The metal halide induced Z/E isomerization of Ph₂PCH=CHPPh₂ has been studied in various solvents and at variable temperature with stoichiometric and catalytic amounts of AlX₃ and GaX₃ Lewis-acids. InX₃ compounds proved ineffective (X = Cl, Br, I). Anhydrous AlBr₃ was found to be most efficient, giving a 90% Z/E conversion in 10 min at 100°C in toluene. A mechanism is proposed which is compatible with the experimental data.

Full text of DOI:10.1515/znb-1998-1111

Lewis Acid Catalyzed Z to E Isomerization of 1,2-Bis(diphenyl-
phosphino)ethene
Marcus Sigl, Annette Schier, Hubert Schmidbaur*
Anorganisch-chemisches Institut der Technischen Universität München,
Lichtenbergstrasse 4, D-85747 Garching, Germany
Z. Naturforsch. 53 b, 1301-1306 (1998); received August 13, 1998
Z/E Isomerization, Phosphino-Ethenes, Lewis Acid Catalysis, Gallium Halides, Aluminium
Halides, Indium Halides
Z-l,2-Bis(diphenylphosphino)ethene, cis-Ph2PCH=CHPPh2, forms 1:2 adducts with GaBn
and GaL, the former of which has been identified in an X-ray diffraction study as the complex
of the isomerized ligand, E-Ph2PCH=CHPPh2. The GaL complex is believed to be analogous
on the basis of analytical and spectroscopic data. InBr3 affords a 1:1 complex with an ionic
structure [(Ph2PCH=CHPPh2)2lnBr2]+ [InBr4]_ in which the cation contains the original cis-
ligand. With Inl^ also a 1:1 adduct is obtained, where the metal triiodide unit is attached to
only one phosphorus atom of the non-isomerized (cis) ligand in the solid state. There is rapid
site exchange of the Inl3 unit in chloroform solution as followed by NMR spectroscopy. -
The metal halide induced Z/E isomerization of Ph2PCH=CHPPh2 has been studied in various
solvents and at variable temperature with stoichiometric and catalytic amounts of AIX3 and
GaX3 Lewis-acids. InX3 compounds proved ineffective (X = Cl, Br, I). Anhydrous AlBr3 was
found to be most efficient, giving a 90% Z/E conversion in 10 min at 100°C in toluene. A
mechanism is proposed which is compatible with the experimental data.
Ph Ph
Introduction
PPh2
X3G a -V L
+
2 GaX3 --------
-
[I
( i )
The aluminium trihalides are important catalysts
for the rearrangement of hydrocarbons, cycloaddi-
tion reactions, Friedel-Crafts acylations, halogena-
tions, and many other reactions [1]. Their action is
based on the ability to coordinate as Lewis-acids to
7r-systems and heteroatoms [2]. Applications of the
corresponding gallium trihalides are limited to only
a few examples like the GaBr3-mediated methyl
transfer between alkylsilanes and -siloxanes [3]. In
the course of our studies of the reactivity of Group
III metal halides controled by various Lewis-bases
[4] we have now observed an olefin Z ^ E isomer-
ization induced by aluminium and gallium halides.
P
P
h 2
P -G aX 3
/ \
Ph Ph
J
X = B r: 1a
X = l : 1 b
crystalline products which are soluble in di- and
trichloromethane. The structure has been confirmed
by a crystal structure analysis of la. The same con-
clusion has been drawn from NMR spectroscopy:
After hydrolysis of the complexes the 31P NMR
spectra show only one resonance at -6 ppm which
is in good agreement with literature data for E-
dppee [5].
The ease of this isomerization is very astonishing
since C=C double bonds in general are expected to
be very robust. Although Z-dppee has been used as
a chelating ligand in a large number of complexes
and derivatives, only two isomerization reactions
have been observed. Heating of Z-dppee dioxide or
disulfide with PCI3 results in the corresponding E-
dppee derivatives [6], and the binuclear gold com-
plexes (Z-dppee)(AuX)2 were found to isomerize
upon photolysis to (E-dppee)(AuX)2 (X = Cl, Br, I,
p-S Q H 4CH3) [7],
Preparative Results
The reaction of two equivalents of anhydrous
GaX3 (X = Br, I) with Z-l,2-bis(diphenylphos-
phino)ethene, Z-dppee, results in the formation
of the dinuclear complexes of the Z —> E iso-
merized ligand (E-dppee)(GaX3)2 (1) (eq. (1)).
la and lb are air-stable, but moisture-sensitive
* Reprint requests to Prof. Dr. H. Schmidbaur.
0932-0776/98/1100-1301 $ 06.00 © 1998 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com
K
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1302
M. Sigl et al. ■l,2-Bis(diphenylphosphino)ethene
Table I. Isomerization of Z-l,2-Ph2PCH=CHPPh2 in
toluene.
Ph,P
[MX3]
(2 )
C
Catalyst
Temperature Time
PPh,
Yield (NMR)
The reaction of equimolar quantities of GaBr3 /
Gal3 and Z-dppee also results in the formation of 1
together with an equivalent of the non-coordinated
dppee in E-configuration. This result demonstrates
that the isomerization must be a catalytic process
(eq. (2)). Further investigations have shown that
GaCl3 and AlBr3 can also be applied, and that as
little as 2 mol% are sufficient for more than 90%
conversion in only 10 min (in toluene). Tempera-
tures higher than 80°C are required (Table I). As
a general rule catalytic activity increases with the
standard Lewis-acidity of the metal trihalide. The
best result was obtained with AlBr3, while indium
trihalides do not show any catalytic activity at all.
Control experiments in the dark established that the
Z —>E-isomerization is no photochemical process.
25 mol% GaBr^
25 mol% GaBrj
25 mol% GaBr3
5 mol% GaBn
5 mol% GaCb
2 mol% AlBrj
80°C
90°C
90°C
100°C
100°C
100°C 10 min
3 h
3 h
6 h
5 h
5 h
68 %
76 %
94 %
< 5 %
13 %
90%
Ph\ /Ph
pph2
,P — Ini,
+
Ini,
(3;
c
c
'p
Ph Ph
/ \
Scheme 1. Proposed mechanism for the Z to E Isomer-
ization of l,2-Bis(diphenylphosphino)ethene.
Anhydrous M 3 reacts with equimolar quanti-
ties of Z-dppee to give almost quantitative yields
of a colorless crystalline product (2, 94%, m.p.
249°C) which decomposes in water releasing free
cis-phosphane ligand (eq. (3)). Single crystal struc-
ture analysis has shown that the complex is an un-
symmetrical molecule with the indium atom con-
nected to only one of the two phosphorus atoms.
The structure is thus different from that of the analo-
gous l,2-bis(diphenylphosphino)benzene complex
[4J. At variance with the solid state structure, the
31P NMR spectrum shows only one resonance even
at -90°C. This indicates a fast exchange of the M 3
units between the two phosphorus atoms.
is associated with a halide redistribution between
two indium atoms to give an [InBr.*]- anion and a
bis-chelated cation [(Z-dppee)2lnBr2]+ with a hex-
acoordinated indium atom (eq. (4)).
Thus ligand Z —►E isomerization is observed
neither with InBr3 nor with M 3, and the process
appears to be restricted to the strong Lewis acids of
aluminium and gallium.
Proposed Mechanism
Formation of an ry1-dppee MX3 complex like in
2 is proposed as the initial step (Scheme 1). Penta-
or hexacoordinated intermediates as in 3, or in the
transition state for the site exchange of the M 3 unit
in 2 , are not likely to be relevant, as the chelat-
ing coordination forces the double bond into the Z-
configuration. By contrast, the coordination number
of gallium and aluminium in phosphane complexes
is largely limited to four [8].
Ph Ph
V
Ph
1
I
\ /Ph
I
Br
,PPh,
2 ll
+ 2 lnBr3
lnBr4e
c
( 4 )
)
c'P
'PPh,
/ \
Ph Ph
Ph Ph
Unlike the results obtained with Inl3, the 1:1 ad-
dition compound of Z-dppee with InBr3 has an ionic
structure (3, yield 92%, m.p. 202°C). The reaction
In a qualitative description of the isomerization,
the C=C double bond must be turned into a single
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1303
M. Sigl et al. • l,2-Bis(diphenylphosphino)ethene
Fig. 2. Molecular structure of 2 with atomic numbering
(ORTEP drawing, 50% probability ellipsoids, hydrogen
atoms omitted for clarity). Selected bond lengths [A] and
angles [°]: Inl-Pl 2.5871(12), Inl-P2 3.274(1), Inl-Il
2.7061 (5), In 1-12 2.7420(6), In 1-13 2.7003(5); In1-PI -C1
111.6(2), Inl-Pl-Cl 11 110.4(2), Ini-Pl-C 121 112.9(2).
Fig. 1. Molecular structure of la with atomic numbering
(ORTEP drawing, 50% probability ellipsoids, hydrogen
atoms of the phenyl groups omitted for clarity). Selected
bond lengths [A] and angles [°]: Ga-P 2.4002(8), Ga-Brl
2.3125(5), Ga-Br2 2.2936(6), Ga-Br3 2.3031 (5); Br1-Ga-
P 100.38(2), Br2-Ga-P 110.57(3), Br3-Ga-P 106.11(3),
Cl-P-Ga 102.85(9), Cl 11-P-Ga 112.80(10), C211-P-Ga
117.06(10).
bond to allow the rotation to occur. Ylide formation
induced by the formal positive charge of the co-
ordinated phosphorus atom appears to be the most
obvious model. The differences in catalytic activi-
ties are thus correlated with the acceptor properties
of the Group 13 trihalides.
It should be noted that reactions of Z-dppee
with BrNnsted acids like trifluoromethanesulfonic
or tetrafluoroboric acid also lead to a quatemisation
at the phosphorus center, but do not cause isomer-
ization of the C=C double bond upon heating. The
reaction may therefore require some direct action of
the MX3 unit at the double bond via 7r-complex for-
mation [9]. There are no experimental observations
at hand that could clarify these details.
Fig. 3. Molecular structure of one of the two indepen-
dent cations in 3 with atomic numbering (ORTEP draw-
ing, 50% probability ellipsoids, hydrogen atoms of the
phenyl groups omitted for clarity). Selected bond lengths
[A] andangles [°]: Inl-Brl 2.5985(11) [2.6280(11)], Inl-
Pl 2.696(3) [2.662(3)], Inl-P2 2.772(4) [2.770(3)]; Brl-
Inl-Brl' 180.0 [180.0], Pl-Inl-P2 77.83(9) [76.23(9)],
Brl-Inl-Pl 92.45(7) [87.59(7)], Brl-Inl-P2 90.92(7)
[82.84(7)]. The data in brackets refer to the second in-
dependent cation.
Structural Investigations
Compound 1 crystallizes in the triclinic space
group Pi with one formular unit in the unit cell.
The complex shows Q symmetry with the center of
inversion located in the middle of the C=C double
bond. The Ga-P distance [2.4008(2) Ä] is almost
identical with literature values for R3P-GaX3 com-
pounds [8, 10], The GaBr3 unit is tilted towards the
The crystals of compound 2 are monoclinic, space
C=C double bond as shown by the smaller angle group P2i/c with Z = 4 molecules in the unit cell.
Cl-P-Ga [102.8(5)°] as compared with C211-P-Ga Only one phosphorus atom (P1) is coordinated to the
[117.1(1)°] and Cl 11-P-Ga [112.8(1)°].
metal atom. Quatemisation of PI causes a formal
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1304
M. Sigl et al. • l,2-Bis(diphenylphosphino)ethene
rehybridisation from p3 to sp3. The increased s char- t, yPH = 20 Hz, 1H, PCH; 7.52 - 7.93, m, 10H, Ph. -
31P{1H} NMR (v. s.): 6 = -31.9, br s.
acter for PI leads to a shortening of the P-C dis-
tances (Pl-C average 1.802 Ä, P2-C average 1.823
A) and an opening of the C-P-C angles (C-Pl-C
average 107.2°, C-P2-C average 102.7°).
C26H22I6Ga2P2 (1297.28)
Calcd C 24.07 H 1.71%,
Found C 24.33 H 1.73%.
The product is the same for reactions with the molar ra-
tio of reactants 1:1, along with E-1,2-Ph2PCH=CHPPh2.
Hydrolysis of lb affords E-l,2-Ph2PCH=CHPPh2.
The compound with the net composition (Z-
dppee)InBr3 (3) was found to have the ionic struc-
ture [(Z-dppee)2lnBr2]+[InBr4]_ . The monoclinic
lattice (space group P2!/c, Z = 4) is built from
two crystallographically independent halves of two
cations, one anion and 1/2 CHCI3. The indium
atoms of the cations (Ini, In2) are residing on cen-
ters of inversion. The two cations are structurally
very similar and do not require a separate discus-
sion. The chelation of the phosphanes to the lin-
ear InBr2+ unit results in the formation of five-
membered C2P2In-heterocycles in envelope confor-
mation, with In 1and In2 as spiro-centers. The anion
has a standard tetrahedral structure.
[Z-1,2-Bis(diphenylphosphino)ethene[Ini3 (2)
Indium triiodide (409 mg, 0.825 mmol) is added to a
solution of Z -1,2-bis(diphenylphosphino)ethene (326 mg,
0.825 mmol) in 10 ml of dichloromethane. After stirring
for 1 h at room temperature the solution is layered with
pentane. Yield 688 mg (94%) of pale yellow crystals (m.p.
249°C).1H NMR (CDCI3,20°C): 6 = 7.21 - 7.59, m, PCH
and Ph. - 3IP{'H} NMR (v. s.): 6 = -32.7, br. - 13C{'H}
NMR (v. s.): 6 = 144.2, m, AXX', PCH; 129.0, 131.0,
129.2, 133.4, m, for ipso, ortho, meta, para of Ph.
C26H22InhP? (891.94)
Calcd C 35.01 H 2.49%,
Found C 34.82 H2.41%.
Experimental Part
General: All experiments were carried out routinely
in an atmosphere of purified dry nitrogen. Solvents were
dried and kept under nitrogen, and glassware was oven-
dried and filled with nitrogen. All starting materials were
commercially available. Gallium and indium trihalides
were stored and handled in a glove box.
Heating of the reaction mixture up to 100 °C for 1 h in
toluene leads to the same product.
[Z-1,2 -Bis(diphenylphosphino)ethene]2lnBr2+ InBr4 (3)
Z-l,2-bis(diphenylphosphino)ethene (127 mg, 0.32
mmol) and InBri (115 mg, 0.32 mmol) are reacted in
10 ml of toluene at room temperature for 2 h. The pre-
cipitate is recrystallized from chloroform / pentane. Yield
223 mg (92%) of colorless crystals (m.p. 202°C).1H NMR
(CDCI3,20°C): b = 7.26 - 7.71, m, PCH and Ph. - 31P {'H}
NMR (v. s.): 6 = -29.9, br. - 13C{'H} NMR (v. s.): 6 =
143.6, m, AXX', PCH; 127.5, 133.4, 129.2, 131.4, m, for
ipso, ortho, meta, para of Ph.
[E-l,2-Bis(diphenylphosphino)ethene](GaBrj)2 (la)
Gallium tribromide (156 mg, 0.50 mmol) and Z-1,2-
bis(diphenylphosphino)ethene (100 mg, 0.25 mmol) are
reacted in 10 ml of toluene at 80 °C for 1 h. The solvent is
distilled off in a vacuum and the residue is recrystallized
from chloroform / pentane. Besides traces of a yellow
oil, 203 mg (80%) of colorless crystals (m. p. 254°C) are
formed. 'H NMR (CDCI3, 20°C): 6 = 7.06, t, JPH = 20
Hz, 1H, PCH; 7.52 - 7.90, m, 10H, Ph. - 3,P{'H} NMR
(v. s.): 6 = -14.0, br s.
C52H44ln2Br6P4 (1501.88)
Calcd C 41.59 H 2.95%,
Found C 40.27 H 2.78%.
C26H22Br6Ga2P2 (1015.28)
Calcd C 30.76 H2.18%,
Found C 30.59 H2.18%.
Heating of the reaction mixture up to 100 °C for 1 h in
toluene leads to the same product.
Crystal structure determinations
The product is the same for reactions with the molar ra-
tio of reactants 1:1, along with E-l,2-Ph2PCH=CHPPh2.
Hydrolysis of la affords E-l,2-Ph2PCH=CHPPh2.
Specimens of suitable quality and size of compounds
la, 2, and 3 were mounted in glass capillaries and used
for measurements of precise cell constants and intensity
data collection on an Enraf Nonius CAD4 diffractome-
ter (Mo-A'q radiation, A(Mo-A'Q) = 0.71073 A). During
data collection, three standard reflections were measured
periodically as a general check of crystal and instrument
stability. No significant changes were observed for crys-
tals of compounds la and 2, wereas a decay of 53 %
[E-1,2-Bis(diphenylphosphino)ethene](Gal3)2(lb)
As described for la with gallium triiodide (248
mg, 0.55 mmol) and Z -1,2-bis(diphenylphosphino)ethene
(109 mg, 0.28 mmol). Yield 264 mg (74 %) of colorless
crystals (m. p. 242°C).1H NMR (CDCI3, 20°C): <5= 7.13,
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M. Sigl et al. ■l,2-Bis(diphenylphosphino)ethene
1305
Table II. Crystal data, data collection, and structure refinement.
la
2 • CHCI3
3 • 0.5 CHCI3
Crystal data
Formula
Mr
Crystal system
Space group
a (A)
b (A)
c (A)
<*(°)
C27H,3Chl3lnPo
1011.26
monoclinic
P 2\/c
21.650(3)
9.849(1)
15.883(2)
90
C2öH22Br6Ga2P2
1015.28
triclinic
C 5 2 .5 H 4 4 .5 B r6 C li.5 ln 2 P 4
1561.54
monoclinic
P 2\/c
21.517(1)
10.229(1)
27.509(1)
90
92.79(1)
90
6047.5(8)
1.715
P I
8.836(1)
9.463(1)
11.367(1)
67.43(1)
85.47(1)
67.99(1)
811.36(14)
2.078
101.03(1)
90
ß O
7 0 ,
V(A3)
3324.2(7)
2.021
Pcalc (g e m
)
Z
1
480
4
4
F(000)
1896
3012
ß(MoKa) (cm-1)
91.62
38.50
49.31
Data collection
+26
T (°C)
-80
-79
Scan mode
hkl Range
UJ
0—26,-12—0,-19— 19
-27—27, -13—0, 0—28
-11—»11,-12—>11,-14—4
0.624
5000
3518 [Rim= 0.0130]
3496
psi-scans
0.64
12357
12353 [/?int = 0.1384]
8738
DIFABS
0.62
6463
6463
6078
sin(#/A)max (A-1)
Measured reflections
Unique reflections
Refls. used for refinement
Absorption correction
DIFABS
Refinement
Refined parameters
167
325
596
Final R values [I >2cr(I)]
R \ W
0.0261
0.0332
0.0640
wR2w
(shift/error)max
ptin(max/min) (eA-3)
0.0570
0.1436
< 0.001
1.581/-0.852
0.0799
< 0.001
< 0.001
0.699/-0.453
1.243/-1.149
[al R = X(IIF0I- IFCII)/IIF0I; [b]wR2 = {[Iw (F02 - Fc2)2]/I[w(F02)2]}l/2; w = l/[a2(F02)+(apf+bp];p = (F02 + 2Fc2)/3;
a = 0.0246 (la), 0.0372 (2 • CHC13), 0.0896 (3 • 0.5 CHC13); b = 0.66 (la), 14.34 (2 • CHCI3 ), 0.000 (3 • 0.5 CHCI3 ).
was of obtained and corrected for crystals of compound
3, due to slow decomposition of the compound in the
X-ray beam. Lp correction was applied, and data of all
compounds were corrected for absorption. The structures
were solved by direct methods (SHELXS-8 6 ) and com-
pleted by full-matrix-least squares techniques against F2
(SHELXL-93). The thermal motion of all non-hydrogen
atoms was treated anisotropically, except for those of the
CHCI3 molecule in the crystal of compound 3, which
was refined isotropically. Due to its very large thermal
parameters, its occupancy was lowered to 50%. All hy-
drogen atoms were placed in idealized calculated posi-
tions and allowed to ride on their corresponding carbon
atoms with fixed isotropic contributions (UiS0(fix) = 1.5 x
Ueq of the attached C atom). Information on crystal data,
data collection and structure refinement are summarized
in Table II. Important interatomic distances and angles are
shown in the corresponding Figure Captions. Anisotropic
thermal parameters, tables of interatomic distances and
angles, and atomic coordinates have been deposited with
the Fachinformationszentrum Karlsruhe, Gesellschaft für
wissenschaftlich-technische Information mbH, D-76344
Eggenstein-Leopoldshafen. The data are available on re-
quest on quoting CSD No. 410070 (la), 410069 (2 •
CHCI3 ), and 410068 (3 • 0.5 CHC13).
Acknowledgements
This work was supported by VW-Stiftung and by Fonds
der Chemischen Industrie. The authors are grateful to Mr.
J. Riede for establishing the X-ray data sets.
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1306
M. Sigl et al. ■l,2-Bis(diphenylphosphino)ethene
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