Dehalogenation of hexachlorocyclohexanes and simultaneous chlorination of triethylsilane catalyzed by rhodium and ruthenium complexes

Article abstract of DOI:10.1021/om049786q

Complexes RhCl(PPh₃)₃ and RhH₂Cl(P ⁱPr₃)₂ catalyze the dehalogenation of γ-hexachloro-cyclohexane to benzene and the simultaneous chlorination of HSiEt₃ to ClSiEt₃. In

Full text of DOI:10.1021/om049786q

Organometallics 2004, 23, 3891-3897
3891
Deh a logen a tion of Hexa ch lor ocycloh exa n es a n d
Sim u lta n eou s Ch lor in a tion of Tr ieth ylsila n e Ca ta lyzed
by Rh od iu m a n d Ru th en iu m Com p lexes
Miguel A. Esteruelas,*,† J uana Herrero,‡ and Montserrat Oliva´n†
Departamento de Quı´mica Inorga´nica, Instituto de Ciencia de Materiales de Arago´n,
Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain, and Departamento de Ingenier´ıa
Quı´mica y Qu´ımica Inorga´nica, Universidad de Cantabria, 39005 Santander, Spain
Received March 25, 2004
Complexes RhCl(PPh3)3 and RhH2Cl(PiPr3)2 catalyze the dehalogenation of γ-hexachloro-
cyclohexane to benzene and the simultaneous chlorination of HSiEt3 to ClSiEt3. In the
1
presence of RhCl(PPh3)3, the dehalogenation is inhibited by addition of PPh3. H and 31P-
{1H} NMR spectroscopy studies at variable temperature, on the behavior of the complex
RhCl(PPh3)3 in the presence of γ-hexachlorocyclohexane and/or HSiEt3, indicate that the
catalytic reaction involves the initial interaction of the catalyst with the silane and that
chloro-hydride intermediates related to RhH2Cl(PPh3)3 are the main species during the
catalysis. The complexes RuHCl(PPh3)3, RuH2Cl2(PiPr3)2, and RuHCl(η2-H2)(PiPr3)2 also
catalyze the dehalogenation of γ-hexachlorocyclohexane and the simultaneous chlorination
of HSiEt3 to ClSiEt3. In these cases, the dehalogenation products are 6/4 cyclohexene/
cyclohexane (RuHCl(PPh3)3) and 4/6 cyclohexene/benzene (RuH2Cl2(PiPr3)2 and RuHCl-
(η2-H2)(PiPr3)2) mixtures. The dehalogenations of R- and δ-hexachlorocyclohexane catalyzed
by RhCl(PPh3)3, RhH2Cl(PiPr3)2, and RuHCl(PPh3)3 have been also investigated. The initial
rates for the formation of ClSiEt3 decrease in the sequence γ- > R- > δ-hexachlorocyclohexane.
In tr od u ction
A limited number of methods for hydrogenolysis of
halogenated compounds employing homogeneous metal
catalysts have been reported. These methods involve the
use of molecular hydrogen,8 alcohols,9 cyclic amines,10
metal hydrides,11 metal alkoxides,12 sodium formate,13
sodium borohydrides,14 alkyl Grignard reagents,15 and
hydrosilanes.16 Among them, that based on the use of
silicon hydrides has the advantage of the formation of
chlorosilanes, which are valuable intermediates in the
silicon industry and in organic synthesis. Despite this
γ-Hexachlorocyclohexane is a noxious material.1 Its
degradation has been centered on microbiological meth-
ods2 and incineration.3 The latter generates highly toxic
fumes and hydrogen chloride.4 Chemical procedures are
scarce.5 They achieve dehalogenation by means of metal
particles or copper-activated metal particles (Mg, Fe, Al,
and Zn)6 and by palladium on alumina in hydrogen-
saturated water at room temperature and ambient
pressure.7 In all of the cases benzene is formed. As far
as we know, homogeneous catalysis has not been
employed as a tool for the dehalogenation of γ-hexachlo-
rocyclohexane and its isomers.
(8) (a) Love, C. J .; McQuillin, F. J . J . Chem. Soc., Perkin Trans. 1
1973, 2509. (b) Grushin, V. V.; Alper, H. Organometallics 1991, 10,
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Commun. 1992, 806. (d) Zhang, Y.; Liao, S.; Xu, Y. Tetrahedron Lett.
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* To whom correspondence should be addressed. E-mail: maester@
unizar.es.
(9) (a) Cucullu, M. E.; Nolan, S. P.; Belderrain, T. R.; Grubbs, R. H.
Organometallics 1999, 18, 1299. (b) Fujita, K.; Owaki, M.; Yamaguchi,
R. Chem. Commun. 2002, 2964.
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Lett. 1976, 855. (b) Imai, H.; Nishiguchi, T.; Tanaka, M.; Fukuzumi,
K. J . Org. Chem. 1977, 42, 2309.
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Catal. 1989, 54, L23. (b) Carfagna, C.; Musco, A.; Pontellini, R.;
Terzoni, G. J . Mol. Catal. 1989, 57, 23. (c) Qian, C.; Zhu, D.; Gu, Y. J .
Mol. Catal. 1990, 63, L1.
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M. S.; Grasa, G. A.; Nolan, S. P. Organometallics 2001, 20, 3607. (c)
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† Universidad de Zaragoza.
‡ Universidad de Cantabria.
(1) (a) Walker, K.; Vallero, D. A.; Lewis, R. G. Environ. Sci. Technol.
1999, 33, 4373. (b) Meharg, A. A.; Wright, J .; Leeks, G. J . L.; Wass, P.
D.; Osborn, D. Environ. Sci. Technol. 1999, 33, 2001. (c) Hazardous
Substances Databank (HSDB), a database of the National Library of
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Gamma- and Delta-Hexachlorocyclohexane; 205-93-0606; Research
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10.1021/om049786q CCC: $27.50 © 2004 American Chemical Society
Publication on Web 06/30/2004

3892 Organometallics, Vol. 23, No. 16, 2004
Esteruelas et al.
Ch a r t 1
of 1/15/108. They were followed by gas chromatography.
The formation of molecular hydrogen during the reac-
tion was confirmed by an additional experiment, in
which the gas formed was confined in a closed system.
Once the dehalogenation was finished, the closed system
was connected to a flask containing a solution of styrene
and catalytic amounts of OsHCl(CO)(PiPr3)2 in 2-pro-
panol.19 After 15 min at 60 °C the solution was analyzed
by GC-MS, showing a peak corresponding to ethylben-
zene. A control reaction in the absence of catalyst was
carried out. After 24 h at 70 °C, γ-hexachlorocyclohex-
ane and HSiEt3 were recovered from the solution. In
addition, to exclude a radical process, we have per-
formed the reaction in the absence of RhCl(PPh3)3 and
in the presence of catalytic amounts of benzoyl peroxide,
and again, γ-hexachlorocyclohexane and HSiEt3 were
recovered.
fact, the number of reports about the use of silicon
hydrides for the dehalogenation of organic substrates
is quite limited.17
In 1996, Chatgilialoglu and co-workers observed that
the heterogeneous system formed from PdCl2 and
HSiEt3 catalyzes the dehalogenation of a variety of
organic halides.16b Schubert and co-workers have re-
cently reported the dehalogenation of alkyl chlorides by
HSiMe2Ph in the presence of catalytic amounts of the
complex [Ph2PCH2CH2NMe2-κ2P,N]PtMeCl to give al-
kanes and ClSiMe2Ph.16d We have shown that the
homogeneous systems formed from RhCl(PPh3)3, [Rh-
(µ-Cl)(COE)2]2/PPh3 (COE ) cyclooctene), RhH2Cl-
(PiPr3)2, RuHCl(PPh3)3, and HSiEt3 are effective in the
dehalogenation of chlorobenzene and polychlorinated
benzenes.18
The complex RhCl(PPh3)3 is a more active catalyst
than RhH2Cl(PiPr3)2. In the presence of RhCl(PPh3)3,
γ-hexachlorocyclohexane is fully dehalogenated to ben-
zene after 31 min, while only a 29% yield of benzene is
obtained after 300 min in the presence of RhH2Cl-
(PiPr3)2.
We have now observed that rhodium and ruthenium
complexes are also effective homogeneous catalysts for
the dehalogenation of γ-hexachlorocyclohexane and its
R and δ isomers (Chart 1) and the simultaneous
chlorination of HSiEt3. In this paper we report a
mechanistic study on this catalysis, the influence of the
catalyst, and the reaction conditions on the dehaloge-
nation products and the influence of the stereochemistry
of the hexachlorocyclohexane isomers on the activity of
the catalysts.
The reactions were also studied by 1H NMR spectros-
copy, using toluene-d8 as solvent and a catalyst/γ-C6H6-
Cl6/HSiEt3 molar ratio of 1/25/150. At 70 °C, again,
benzene and ClSiEt3 were the products of the reactions.
In addition, a peak at 4.48 ppm was observed. It can be
assigned to the molecular hydrogen released in the
reaction. Partially dehalogenated cyclohexanes were not
detected. Under these conditions, complex RhCl(PPh3)3
is also a more active catalyst than RhH2Cl(PiPr3)2. Thus,
in the presence of RhCl(PPh3)3, γ-hexachlorocyclohexane
is fully dehalogenated to benzene after 1 h, while only
a 61% yield of benzene is obtained after 51 h in the
presence of RhH2Cl(PiPr3)2.
These results agree well with those found for the
dehalogenation of 1,2,4-trichlorobenzene to 1,2-dichlo-
robenzene with HSiEt3. While the complex RhH2Cl(Pi-
Pr3)2 requires 8 h to achieve an 80% yield of ClSiEt3,
complex RhCl(PPh3)3 reaches 90% yield after 65 min.18
Since the best catalytic results were obtained with the
complex RhCl(PPh3)3, we have performed spectroscopic
studies on the behavior of RhCl(PPh3)3 in the presence
of γ-hexachlorocyclohexane and/or HSiEt3, to obtain
information about the mechanism of the catalysis.
The 1H and 31P{1H} NMR spectra of the solution
resulting from heating at 70 °C for 15 min a toluene-d8
suspension of RhCl(PPh3)3 and 3 equiv of γ-hexachlo-
rocyclohexane show only resonances due to RhCl(PPh3)3
and γ-hexachlorocyclohexane. When the NMR tube is
heated for a longer time (30 min), an orange precipitate,
identified as [Rh(µ-Cl)(PPh3)2]2, is obtained. Since this
compound has been prepared on heating benzene or
toluene suspensions of RhCl(PPh3)3,20 this clearly indi-
cates that the catalytic dehalogenation does not involve
the initial reaction of complex RhCl(PPh3)3 with γ-hexa-
chlorocyclohexane.
Resu lts a n d Discu ssion
1. Deh a logen a tion of γ-Hexa ch lor ocycloh exa n e
Ca ta lyzed by Rh od iu m Com p lexes. The complexes
RhCl(PPh3)3 and RhH2Cl(PiPr3)2 catalyze the dehalo-
genation of γ-hexachlorocyclohexane to benzene and the
simultaneous chlorination of triethylsilane (eq 1).
C6H6Cl6 + 6HSiEt3 catalyst8 C6H6 + 6ClSiEt3 + 3H2
70 °C
(1)
The reactions were carried out under an argon
atmosphere in a round flask at 70 °C, using p-xylene
as solvent and a catalyst/γ-C6H6Cl6/HSiEt3 molar ratio
(16) (a) Pri-Bar, I.; Buchman, O. J . Org. Chem. 1986, 51, 734. (b)
Boukherroub, R.; Chatgilialoglu, C.; Manuel, G. Organometallics 1996,
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Organometallics 1998, 17, 570. (d) Sto¨hr, F.; Sturmayr, D.; Schubert,
U. Chem. Commun. 2002, 2222. (e) Rahaim, R. J ., J r.; Maleczka, R.
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Chem. Soc. 1947, 69, 2108. (b) Sommer, L. H.; Citron, J . D. J . Org.
Chem. 1967, 32, 2470. (c) Nagai, Y.; Matsumoto, H.; Yagihara, T.;
Moxishita, K. Kogyo Kagatu Zasshi 1968, 71, 1112. (d) Citron, J . D.;
Lyons, J .; Sommer, L. H. J . Org. Chem. 1969, 34, 638. (e) Doyle, M.
P.; McOsker, C. C.; West, C. T. J . Org. Chem. 1976, 41, 1393. (f)
Seyferth, D.; Prud’homme, C. C.; Wang, W.-L. J . Organomet. Chem.
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Y. Organometallics 1994, 13, 3233.
(19) OsHCl(CO)(PiPr3)2 is known to be a catalyst for hydrogenation
of unsaturated substrates. See, for example: (a) Andriollo, A.; Esteru-
elas, M. A.; Meyer, U.; Oro, L. A.; Sa´nchez-Delgado, R. A.; Sola, E.;
Valero, C.; Werner, H. J . Am. Chem. Soc. 1989, 111, 7431. (b)
Esteruelas, M. A.; Oro, L. A.; Valero, C. Organometallics 1992, 11,
3362.
(18) (a) Esteruelas, M. A.; Herrero, J .; Lo´pez, F. M.; Mart´ın, M.; Oro,
L. A. Organometallics 1999, 18, 1110. (b) D´ıaz, J .; Esteruelas, M. A.;
Herrero, J .; Moralejo, L.; Oliva´n, M. J . Catal. 2000, 195, 187.
(20) Osborn, J . A.; Wilkinson, G. Inorg. Synth. 1990, 28, 77.

Dehalogenation and Chlorination Catalyzed by Rh and Ru
Organometallics, Vol. 23, No. 16, 2004 3893
1
The H NMR spectrum at room temperature of the
orange solution resulting from heating at 70 °C for 15
min a toluene-d8 suspension of RhCl(PPh3)3 with 2.0
equiv of HSiEt3 shows a broad hydride resonance at
-7.90 ppm. At -80 °C, it is converted into a double
doublet of triplets by spin coupling with the rhodium
(J Rh-H ) 17 Hz) and the phosphorus (J P-H ) 104.7 and
17 Hz) of the phosphine ligands. At this temperature,
the 31P{1H} NMR spectrum contains a double doublet
at 42.8 ppm, with P-P and P-Rh coupling constants
of 24.7 and 171 Hz, and a double triplet at 37.2 ppm
with a P-Rh coupling constant of 144 Hz. These data
agree with those previously reported for the mono-
hydride complex RhH(PPh3)3,21 which is formed accord-
ing to eq 2. In agreement with this, the 1H NMR
F igu r e 1. Influence of the phosphine added on the yield
of the dehalogenation after 75 min of reaction. Condi-
tions: 0.02 mmol of RhCl(PPh3)3; 1 mmol of γ-hexachloro-
cyclohexane; 7 mmol of HSiEt3; 2 mL of p-xylene; 200 µL
of n-octane; 60 °C.
Sch em e 1
spectrum also shows peaks corresponding to ClSiEt3,
which was further characterized by GC-MS analysis.
The formation of ClSiR3 and metal-hydride complexes
by reaction of chloro transition-metal compounds and
silanes is a well-known elemental reaction.22
temperature, the 31P{1H} NMR spectrum contains a
doublet at 39.4 ppm, with a P-Rh coupling constant of
115 Hz and a broad resonance at about 20 ppm. At -40
°C, a double doublet at 39.5 ppm with P-P and P-Rh
coupling constants of 19.5 and 114.8 Hz and a double
triplet at 20.1 ppm with a P-Rh coupling constant of
90.8 Hz are observed. These data are similar to those
The reaction shown in eq 2 suggests that the first step
in the dehalogenation process involves the reaction of
RhCl(PPh3)3 with HSiEt3 to afford the monohydride
compound RhH(PPh3)3, which is an active intermediate
in the catalysis. Thus, we have observed that the 1H
and 31P{1H} NMR spectra of the resulting solution from
the addition at room temperature of 1.0 equiv of
γ-hexachlorocyclohexane to a benzene-d6 solution of
RhH(PPh3)3 contain the characteristic resonances of
RhCl(PPh3)3 and [Rh(µ-Cl)(PPh3)2]2. Furthermore, the
GC-MS spectrum of the solution indicates the presence
of C6H5Cl5. In addition to the resonances of RhCl(PPh3)3
and [Rh(µ-Cl)(PPh3)2]2, the 1H NMR spectrum shows
tiny broad resonances centered at -9.38 and -16.73
ppm, whereas the 31P{1H} NMR spectrum contains a
doublet at 39.4, with a P-Rh coupling constant of 115
Hz, and a broad resonance at about 20 ppm.
23
previously reported for the complex RhH2Cl(PPh3)3
and suggest that chloro-dihydride intermediates re-
lated to RhH2Cl(PPh3)3 are the main species during the
dehalogenation.
The aforementioned spectra also reveal that the
reaction of RhH(PPh3)3 with γ-hexachlorocyclohexane
leads, in addition to C6H5Cl5, RhCl(PPh3)3, and [Rh-
(µ-Cl)(PPh3)2]2, to RhH2Cl(PPh3)3. The formation of
these compounds can be rationalized according to
Scheme 1. Initially, complex RhH(PPh3)3 reacts with
γ-hexachlorocyclohexane to give C6H5Cl5, H2, and RhCl-
(PPh3)3. The oxidative addition of molecular hydrogen
to the latter affords RhH2Cl(PPh3)3, while the dissocia-
tion of triphenylphosphine from RhCl(PPh3)3 gives rise
to the dimeric species [Rh(µ-Cl)(PPh3)2]2.
To investigate the participation of highly unsaturated
species during the catalysis, which could be generated
by dissociation of phosphine, we have studied the
influence of the addition of triphenylphosphine to the
reaction. Figure 1 shows the yield of the dehalogenation
after 75 min as a function of the phosphine/catalyst
molar ratio. The extent of the reaction clearly depends
on the amount of PPh3 added. Excess phosphine has an
inhibitory effect on the dehalogenation activity of RhCl-
(PPh3)3. This indicates that, in fact, phosphine dissocia-
tion plays an important role during the dehalogenation.
Scheme 2 shows a reaction pathway which allows us
to rationalize the dehalogenation of γ-hexachlorocyclo-
1
The H NMR spectrum at room temperature of the
solution resulting from heating at 70 °C for 10 min a
suspension in toluene-d8 of RhCl(PPh3)3 with 3 equiv
of γ-hexachlorocyclohexane and 6 equiv of HSiEt3 shows,
in addition to the resonances corresponding to the
substrates and products of the dehalogenation, a broad
doublet centered at -9.38 ppm,with a J P-H value of 152
Hz, and a broad resonance at -16.73 ppm. At -40 °C,
the resonance at -9.38 ppm is converted into a complex
signal with H-H and H-Rh coupling constants of 7.5
and 18.3 Hz, respectively, and H-P coupling constants
of 155.4 and 12.3 Hz, while the resonance at -16.73
ppm is converted into a complex multiplet. At room
(21) Strauss, S. H.; Diamond, S. E.; Mares, F.; Shriver, D. F. Inorg.
Chem. 1978, 17, 3064.
(22) (a) Chalk, A. J . Chem. Commun. 1969, 1207. (b) Bennet, M.
A.; Charles, R.; Fraser, P. J . Aust. J . Chem. 1977, 30, 1201. (c)
Blackburn, S. N.; Haszeldine, R. N.; Parish, R. V.; Setchfield, J . H. J .
Chem. Res., Synop. 1980, 170. (d) Blackburn, S. N.; Haszeldine, R. N.;
Parish, R. V.; Setchfield, J . H. J . Organomet. Chem. 1980, 192, 329.
(e) Ferna´ndez, M. J .; Bailey, P. M.; Bentz, P. O.; Ricci, J . S.; Koetzle,
T. F.; Maitlis, P. M. J . Am. Chem. Soc. 1984, 106, 5458. (f) Campion,
B. K.; Heyn, R. H.; Tilley, T. D. J . Chem. Soc., Chem. Commun. 1992,
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1994, 411.
(23) (a) Young, J . F.; Osborn, J . A.; J ardine, F. H.; Wilkinson, G.
Chem. Commun. 1965, 131. (b) Osborn, J . A.; J ardine, F. H.; Young,
J . F.; Wilkinson, G. J . Chem. Soc. A 1966, 1711. (c) Tolman, C. A.;
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Soc. 1994, 116, 10548.

3894 Organometallics, Vol. 23, No. 16, 2004
Esteruelas et al.
Sch em e 2
Sch em e 3
hexane to benzene and the simultaneous chlorination
of HSiEt3, as well as the previously mentioned observa-
tions. The initial reaction of HSiEt3 with RhCl(PPh3)3
gives ClSiEt3 and a RhH(PPh3)x species, which by
oxidative addition of one of the C-Cl bonds of γ-hexachlo-
rocyclohexane should afford a chloro-hydride-alkyl
intermediate. The oxidative addition of C-Cl bonds to
unsaturated rhodium centers is a well-known process.24
The chloro-hydride-alkyl species could generate a
dihydride-alkyl intermediate upon reaction with HSi-
Et3 and subsequent elimination of ClSiEt3. Then, a
â-chlorine elimination reaction should lead to a short-
lived π-olefin RhH2Cl(η2-C6H6Cl4)(PPh3)x derivative,
which could evolve into RhCl(η2-C6H6Cl4)(PPh3)x by
reductive elimination of molecular hydrogen. In this
context, we note that chloride has been found to be a
better leaving group than hydrogen at the â-position of
palladium.25 Successive steps involving chlorination of
HSiEt3, C-Cl activation, chlorination of HSiEt3, C-Cl
activation, and hydrogen elimination should afford a
π-diolefin RhCl(η4-C6H6Cl2)(PPh3)x species via allyl
intermediates. Finally, a new sequence including chlo-
rination of HSiEt3, C-Cl activation, chlorination of
HSiEt3, and C-Cl activation should give benzene and
RhH2Cl(PPh3)x, which regenerates RhCl(PPh3)x by re-
ductive elimination of molecular hydrogen.
detectable amounts of partially chlorinated hydro-
carbons. This indicates that once the molecule of
γ-hexachlorocyclohexane enters in the coordination
sphere of the rhodium center, it remains bonded to the
metal until its complete dehalogenation. In addition, it
should be noted that RhH2Cl(η2-olefin)(PPh3)x and
RhH2Cl(η4-olefin)(PPh3)x species eliminate molecular
hydrogen faster than they hydrogenate the bound
olefins.
2. Deh a logen a tion of γ-Hexa ch lor ocycloh exa n e
Ca ta lyzed by Ru HCl(P P h 3)3 a n d Rela ted Ru th e-
n iu m Com p lexes. The complex RuHCl(PPh3)3 also
catalyzes the dehalogenation of γ-hexachlorocyclo-
hexane and the simultaneous chlorination of HSiEt3. In
this case, the reactions were carried out under an argon
atmosphere in a round flask at 70 °C, using toluene as
solvent and a catalyst/γ-C6H6Cl6/HSiEt3 molar ratio of
1/25/150. Under these conditions, after 71 h, ClSiEt3 was
formed in 80% yield. In addition, it must be pointed out
that, in contrast to RhCl(PPh3)3, cyclohexane and cy-
clohexene are obtained in a 4/6 molar ratio. As far as
we know, the dehalogenation of γ-hexachlorocyclohex-
ane to these products, which from an environmental
point of view are certainly friendlier than benzene, has
no precedent.
The participation of π-olefin, allyl, and π-diolefin
intermediates during the catalysis is consistent with the
selective formation of benzene and the absence of
(24) See, for example: Haarman, H. F.; Ernsting, J . M.; Kranenburg,
M.; Kooijman, H.; Veldman, N.; Spek, A. L.; van Leeuwen, P. W. N.
M.; Vrieze, K. Organometallics 1997, 16, 887.
(25) (a) Henry, P. M. Acc. Chem. Res. 1973, 6, 16. (b) Zhu, G.; Lu,
X. Organometallics 1995, 14, 4899.
Scheme 3 shows a sequence of reactions that allows
us to rationalize the dehalogenation of γ-hexachloro-

Dehalogenation and Chlorination Catalyzed by Rh and Ru
Organometallics, Vol. 23, No. 16, 2004 3895
Ta ble 1. Deh a logen a tion of
γ-Hexa ch lor ocycloh exa n e a n d Sim u lta n eou s
Ch lor in a tion of HSiEt3 Ca ta lyzed by Ru th en iu m
Com p lexesa
cyclohexane and the formation of cyclohexene. In a
manner similar to RhCl(PPh3)3, the initial reaction of
RuHCl(PPh3)3 with HSiEt3 should afford a RuH2(PPh3)x
dihydride species. Thus, the subsequent C-Cl activation
of γ-hexachlorocyclohexane followed by the reductive
elimination of molecular hydrogen could lead to a
chloro-alkyl-ruthenium(II) intermediate, which should
give RuH(C6H6Cl5)(PPh3)x by chlorination of HSiEt3.
The â-chlorine elimination on the alkyl ligand of this
intermediate could afford the π-olefin RuHCl(η2-C6H6-
Cl4)(PPh3)x species, which should evolve into a new
chloro-alkyl-ruthenium(II) intermediate by insertion
of the C-C double bond into the Ru-H bond. Three
successive groups of steps, each involving one chlorina-
tion of HSiEt3, â-chlorine elimination, and olefin inser-
tion into one of the Ru-H bonds, should yield RuCl-
(C6H10Cl)(PPh3)x. The reaction of this intermediate with
HSiEt3 could afford RuH(C6H10Cl)(PPh3)x, which should
give RuHCl(η2-C6H10)(PPh3)x, by a new chlorine â-elim-
ination reaction. The dissociation of cyclohexene from
this intermediate affords the olefin, regenerating the
active catalyst.
products of the
dehalogenation (%)
time
(h)
yield
(%)b
catalyst
C6H12
40
C6H10
C6H6
RuHCl(PPh3)3
71
94
175
80
77
81
60
40
36
RuH2Cl2(PiPr3)2
RuHCl(η2-H2)(PiPr3)2
60
64
a
Conditions: 70 °C; 4 × 10-2 mmol of catalyst; 1 mmol of
b
γ-C6H6Cl6; 6 mmol of HSiEt3; 4 mL of toluene. Percentage of
HSiEt3 converted into ClSiEt3.
RuHCl(η2-olefin)(PPh3)2 is favored with regard to the
other steps of the hydrogenation.
The elemental steps involved in the dehalogenation
of γ-hexachlorocyclohexane to cyclohexene and cyclo-
hexane are not significantly different from those in-
volved in the hydrogenation of olefins, and at first
glance, one should expect the same trend. Thus, the
difference in behavior between RhH2Cl(η2-olefin)(PPh3)2
and RuHCl(η2-olefin)(PPh3)2sthe first of them elimi-
nates H2 (Scheme 2), while the second one inserts the
olefin into the Ru-H bond (Scheme 3)scan explain why
the dehalogenation of γ-hexachlorocyclohexane in the
presence of RhCl(PPh3)3 yields benzene, while in the
presence of RuHCl(PPh3)3 cyclohexene and cyclohexane
are obtained.
Cyclohexane is the result of the hydrogenation of
cyclohexene in the presence of RuHCl(PPh3)3. In this
context, it should be noted that the complex RuHCl-
(PPh3)3 is an active catalyst for the hydrogenation of
olefins.26
The higher tendency of the ruthenium complex
RuHCl(PPh3)3 to yield dehalogenated saturated prod-
ucts, with regard to the rhodium derivative RhCl-
(PPh3)3, merits further comment. Both RhCl(PPh3)3 and
RuHCl(PPh3)3 have been shown to be active catalysts
for the hydrogenation of olefins. Kinetic studies have
proved that, for the hydrogenation of cyclohexene
catalyzed by the rhodium complex, the insertion of the
C-C double bond of the olefin into a Rh-H bond of a
RhH2Cl(η2-C6H10)(PPh3)x intermediate is the rate-
determining step of the hydrogenation.27 Theoretical
calculations have also shown that the intramolecular
migratory alkene insertion to give RhH(C2H5)Cl(PH3)2
is exothermic and is the rate-determining step for the
catalytic cycle.28 On the other hand, for the hydrogena-
tion of olefins catalyzed by RuHCl(PPh3)3, the oxidative
addition of molecular hydrogen to a Ru(alkyl)Cl(PPh3)2
intermediate is the rate-determining step of the cataly-
sis.26,29 The insertion of the C-C double bond of the
olefin into the Ru-H bond of RuHCl(η2-olefin)(PPh3)2
is fast. Even at high alkene/phosphine molar ratios, Ru-
(alkyl)Cl(PPh3)2 intermediates are the main metallic
species in the catalytic solutions.29b
The complexes RuH2Cl2(PiPr3)2 and RuHCl(η2-H2)-
(PiPr3)2 also catalyze the dehalogenation of γ-hexacho-
rocyclohexane and the simultaneous chlorination of
HSiEt3. However, under the same conditions, they are
less active than RuHCl(PPh3)3 (Table 1). Furthermore,
the behavior of these compounds is intermediate be-
tween that of RuHCl(PPh3)3 and that of RhCl(PPh3)3.
As for the rhodium complex, one of the dehalogenation
products is benzene and, as for RuHCl(PPh3)3, the other
product is cyclohexene. In agreement with the tendency
of these ruthenium systems to catalyze the hydrogena-
tion of olefins, when the reactions are carried out in a
sealed NMR tube, cyclohexane is obtained instead of
cyclohexene.
3. Deh a logen a tion of r- a n d δ-Hexa ch lor ocyclo-
h exa n e Ca ta lyzed by Rh Cl(P P h 3)3, Rh H2Cl(P iP r 3)2,
an d Ru HCl(P P h 3)3. The complexes RhCl(PPh3)3, RhH2-
Cl(PiPr3)2, and RuHCl(PPh3)3 catalyze not only the
dehalogenation of γ-hexachlorocyclohexane but also the
dehalogenation of its R and δ isomers. The catalysis was
also carried out under an argon atmosphere and in a
round flask. In this case the solvent employed was
p-xylene and the catalyst/C6H6Cl6/HSiEt3 molar ratio
used was 1/15/108. The reactions in the presence of
RhCl(PPh3)3 and RhH2Cl(PiPr3)2 lead, in both cases, to
benzene and ClSiEt3, while those catalyzed by RuHCl-
(PPh3)3 afford a 6/4 cyclohexene/cyclohexane mixture
and ClSiEt3.
The results from the kinetic studies on the mecha-
nisms of the hydrogenation of olefins catalyzed by RhCl-
(PPh3)3 and RuHCl(PPh3)3 indicate that, for the rhod-
ium complex, the insertion of the olefin into a Rh-H
bond of RhH2Cl(η2-olefin)(PPh3)2 is unfavorable with
regard to the other steps, while for RuHCl(PPh3)3, the
insertion of the olefin bond into the Ru-H bond of
Table 2 summarizes the reaction yields. As for the
dehalogenation of γ-hexachlorocyclohexane, RhCl(P-
Ph3)3 is the most active of the three catalysts studied.
Under the employed conditions, γ-hexachlorocyclo-
hexane is dehalogenated into benzene in 100% yield
after 31 min. The same yield for the dehalogenation of
the R isomer is obtained after 211 min, whereas with
the δ isomer as the starting material, benzene is formed
(26) Chaloner, P. A.; Esteruelas, M. A.; J oo´, F.; Oro, L. A. Homo-
geneous Hydrogenation, Kluwer: Dordrecht, The Netherlands, 1994.
(27) Halpern, J . Inorg. Chim. Acta 1981, 50, 11 and references
therein.
(28) Daniel, C.; Koga, N.; Han, J .; Fu, X. Y.; Morokuma, K. J . Am.
Chem. Soc. 1988, 110, 3773.
(29) (a) Hallman, P. S.; McGarvey, B. R.; Wilkinson, G. J . Chem.
Soc. A 1968, 3143. (b) J ames, B. R. Inorg. Chim. Acta Rev. 1970, 73.

3896 Organometallics, Vol. 23, No. 16, 2004
Esteruelas et al.
Ta ble 2. Deh a logen a tion of γ-, r-, a n d δ-Hexa ch lor ocycloh exa n e a n d Sim u lta n eou s Ch lor in a tion of HSiEt3
a
Ca ta lyzed by Rh Cl(P P h 3)3, Ru HCl(P P h 3)3, a n d Rh H2Cl(P iP r 3)2
a
Conditions: 70 °C, p-xylene (2 mL) as solvent, 1 mmol of C6H6Cl6; 7 mmol of HSiEt3; 7 × 10-2 mmol of catalyst; 200 µL of n-octane.
In addition, it should be mentioned that the rates and
extents of these reactions are unaffected by the presence
of hydroquinone, suggesting that in the dehalogenation
of the hexachlorocyclohexanes and in the simultaneous
chlorination of HSiEt3, the participation of radical-like
species as catalytic intermediates is not significant.
Con clu d in g Rem a r k s
This study reveals that rhodium and ruthenium
complexes, mainly RhCl(PPh3)3 and RuHCl(PPh3)3,
catalyze the dehalogenation of γ- (lindane), R-, and
δ-hexachlorocyclohexane and the simultaneous chlori-
nation of HSiEt3.
Spectroscopic studies on the behavior of the complex
F igu r e 2. Dehalogenation of hexachlorocyclohexanes cata-
RhCl(PPh3)3 in the presence of γ-hexachlorocyclohexane
lyzed by RhCl(PPh3)3 as a function of time: (O) γ isomer;
and/or HSiEt3 indicate that the catalytic reaction in-
(9) R isomer; (4) δ isomer.
volves the initial interaction of the catalyst with the
silane to give RhH(PPh3)3 and that chloro-hydride
in 74% yield after 371 min. Although the extent of the
intermediates related to RhH2Cl(PPh3)3 (RuHCl(PPh3)3
dehalogenation reactions is lower than in the presence
in the ruthenium case) are the main species during the
catalysis.
of RhCl(PPh3)3, a similar trend is observed when Ru-
HCl(PPh3)3 is used as catalyst. Thus, after 300 min, 40%
of the γ isomer was dehalogenated, while only 18 and
The products of the dehalogenation reactions show a
strong dependence on the metallic center of the cata-
lysts. Thus, while benzene is the dehalogenation product
when the complex RhCl(PPh3)3 is used as catalyst, a
mixture of cyclohexene and cyclohexane is obtained in
the presence of the ruthenium derivative RuHCl(PPh3)3.
9% of the R and δ isomers, respectively, were achieved.
When the catalyst used is RhH2Cl(PiPr3)2, the sequence
of the dehalogenation is the same, giving after 300 min
of reaction a conversion of the hexachlorocyclohexanes
to benzene of 29, 21, and 15%, respectively.
Figure 2 shows the course of the dehalogenation
reactions as a function of time when RhCl(PPh3)3 is used
as catalyst. In agreement with the results collected in
Table 2, the initial rates of the dehalogenation reaction
decrease in the sequence γ- > R- > δ-hexachlorocyclo-
hexane, as the number of chlorine atoms at axial
positions of the most favorable conformations of the
isomers decreases (γ, aaaeee; R, aaeeee; δ, aeeeee; a )
axial, e ) equatorial). This suggests that the chlorine
atoms at axial positions of the minimum energy con-
formation are more easily dehalogenated than those
located in equatorial positions.
The rate of the dehalogenation, which is inhibited by
addition of phosphine to the catalytic solutions, depends
on the metallic center of the catalyst and on the position
of the chlorine atoms in the hexachlorocyclohexane
isomers. Rhodium complexes are more active catalysts
than the ruthenium compounds. For both rhodium and
ruthenium catalysts, chlorine atoms at the axial posi-
tions of the minimum energy conformation of the
hexachlorocyclohexane isomer are more easily dehalo-
genated than those situated in equatorial positions.
Thus, the initial rate of the dehalogenation decreases
in the sequence γ- > R- > δ-hexachlorocyclohexane.

Dehalogenation and Chlorination Catalyzed by Rh and Ru
Organometallics, Vol. 23, No. 16, 2004 3897
NMR spectroscopy. The 1H NMR spectra show a triplet (δ 0.91,
J H-H ) 8 Hz, -CH3) and a quartet (δ 0.61, J H-H ) 8 Hz,
-CH2-), assigned to ClSiEt3 by comparison with a pure
sample. In addition, the 1H NMR spectra show peaks assigned
to benzene at 7.17 ppm and/or cyclohexane at 1.58 ppm.
Rea ction of Rh Cl(P P h 3)3 w ith HSiEt3: F or m a tion of
Rh H(P P h 3)3. A suspension in toluene of RhCl(PPh3)3 (100 mg,
0.108 mmol) was treated with HSiEt3 (43 µL, 0.27 mmol). The
resulting suspension was heated at 70 °C for 15 min, giving
an orange solution that was evaporated to dryness. Addition
of pentane afforded an orange yellowish solid that was washed
with pentane. The solid is oxygen sensitive both in solution
and in the solid state. The mass spectrum of the mother liquors
shows the presence of ClSiEt3. 1H NMR (300 MHz, toluene-
d8, 20 °C): δ 7.58-6.85 (m, 45H, PPh3), -7.90 (br, 1H,
Rh-H). 1H NMR (300 MHz, toluene-d8, -80 °C): δ 7.60-6.90
(m, 45H, PPh3), -7.90 (ddt, J Rh-H ) 17, J P(trans)-H ) 104.7,
J P(cis)-H ) 17, 1H, Rh-H). 31P{1H} NMR (121.4 MHz, toluene-
d8, 20 °C): δ 42.2 (d, J Rh-P ) 156). 31P{1H} NMR (121.4 MHz,
toluene-d8, -80 °C): δ 42.8 (dd, J P-P ) 24.7, J Rh-P ) 171), 37.2
(dt, J Rh-P ) 144, J P-P ) 24.7).
Exp er im en ta l Section
All manipulations were carried out with rigorous exclusion
of air using standard Schlenk techniques. Solvents were dried
by known procedures and distilled under argon prior to use.
Triethylsilane, n-octane, and the hexachlorocyclohexane iso-
mers were used without further purification. RhCl(PPh3)3,20
RhH2Cl(PiPr3)2,30 RuHCl(PPh3)3,31 and RuH2Cl2(PiPr3)232 were
prepared by published procedures. RuHCl(η2-H2)(PiPr3)2 was
prepared using a procedure similar to that described for
RuHCl(η2-H2)(PtBu2Me)2.33 The analysis of the products of the
reactions was carried out on a Hewlett-Packard 6890 series
gas chromatograph with a flame ionization detector, using a
100% cross-linked methyl silicone gum column (30 m × 0.25
mm, with 0.25 µm film thickness) and n-octane as the internal
standard. The oven conditions used are as follows: 35 °C (hold
6 min) to 280 °C at 25 °C/min (hold 4 min). The reaction
products were identified by comparison of their retention times
with those observed for pure samples. GC-MS experiments
were run on an Agilent 5973 mass selective detector interfaced
to an Agilent 6890 series gas chromatograph system. Samples
were injected into a 30 m × 250 µm HP-5MS 5% phenyl methyl
siloxane column with a film thickness of 0.25 µm (Agilent).
The GC oven temperature was programmed as follows: 35 °C
for 6 min, 35 °C to 280 °C at 25 °C/min, 280 °C for 4 min. The
R ea ct ion of R h H (P P h 3)3 w it h γ-H exa ch lor ocyclo-
h exa n e. In an NMR tube, the stoichiometric amount of
γ-hexachlorocyclohexane (6.5 mg, 2.2 × 10-2 mmol) was added
to a solution of RhH(PPh3)3 (20 mg, 2.2 × 10-2 mmol) in
carrier gas was helium at a flow of 1 mL/min. H and 31P{1H}
1
1
benzene-d6 (0.4 mL). After 10 min, the H and 31P{1H} NMR
spectra were recorded. The 31P{1H} NMR spectrum shows
peaks corresponding to RhCl(PPh3)3, [Rh(µ-Cl)(PPh3)2]2, and
RhH2Cl(PPh3)3. In addition, there is an orange precipitate
([Rh(µ-Cl)(PPh3)2]2) at the bottom of the NMR tube. GC-MS
analyses show the formation of C6H5Cl5.
NMR spectra were recorded either on a Varian Gemini 2000
or on a Bruker AXR 300 instrument. Chemical shifts are
referenced to residual solvent peaks (1H) or external H3PO4
(31P).
Ca ta lytic Deh a logen a tion in a Rou n d F la sk : Gen er a l
P r oced u r e. The dehalogenation reactions were carried out
in a two-necked flask fitted with a condenser and containing
a magnetic stirring bar. The second neck was capped with a
Suba seal to allow samples to be removed by syringe without
opening the system. Two reaction conditions were used.
(a) A 0.07 mmol portion of catalyst was dissolved in 2 mL
of a p-xylene solution contaning 1 mmol of hexachlorocyclo-
hexane, 7 mmol of triethylsilane, and 200 µL of n-octane. The
flask was then immersed in a bath at 70 °C, and the solution
was magnetically stirred. The reactions were periodically
checked by GC.
Rea ction of Rh Cl(P P h 3)3 w ith HSiEt3 a n d γ-Hexa ch lo-
r ocycloh exa n e: F or m a t ion of R h H2Cl(P P h 3)3. RhCl-
(PPh3)3 (10 mg, 1.1 × 10-2 mmol), γ-C6H6Cl6 (9.4 mg, 3.2 ×
10-2 mmol) and HSiEt3 (103 µL, 6.5 × 10-2 mmol) were mixed
in an NMR tube (0.4 mL, toluene-d8), and the resulting
suspension was heated at 70 °C for 10 min, giving an orange
yellowish solution. 1H and 31P{1H} NMR spectra were recorded
after this time. 1H NMR (300 MHz, toluene-d8, 20 °C): δ 8.00-
6.80 (m, 45H, PPh3), -9.38 (br d, J P-H ) 152, 1H, Rh-H),
1
-16.73 (br, 1H, Rh-H). H NMR (300 MHz, toluene-d8, -40
°C): δ 8.00-6.80 (m, 45H, PPh3), -9.38 (dddt, J P(trans)-H
)
(b) A 0.04 mmol portion of catalyst was dissolved in 4 mL
of a toluene solution contaning 1 mmol of hexachlorocyclo-
hexane, 6 mmol of triethylsilane, and 200 µL of n-octane. The
flask was then immersed in a bath at 70 °C, and the solution
was magnetically stirred. The reactions were periodically
checked by GC and/or GC-MS.
Ca ta lytic Deh a logen a tion in a n NMR Tu be: Gen er a l
P r oced u r e. The catalyst (4 × 10-3 mmol), γ-hexachlorocyclo-
hexane (0.1 mmol), and HSiEt3 (0.6 mmol) were dissolved in
toluene-d8 (0.4 mmol), and the tube was then immersed in a
bath at 70 °C. The samples were periodically checked by 1H
155.4, J P(cis)-H ) 12.3, J Rh-H ) 18.3, J H-H ) 7.5, 1H, Rh-H),
-16.67 (m, 1H, Rh-H). 31P{1H} NMR (121.4 MHz, toluene-
d8, 20 °C): δ 39.4 (d, J P-Rh ) 115), 20.0 (br). 31P{1H} NMR
(121.4 MHz, toluene-d8, -40 °C): δ 39.5 (dd, J P-P ) 19.5, J P-Rh
) 114.8), 20.1 (dt, J P-Rh ) 90.8, J P-P ) 19.5). On the GC-MS
of this sample, apart from HSiEt3 and γ-C6H6Cl6, peaks of C6H6
and ClSiEt3 are observed.
Ack n ow led gm en t. We acknowledge financial sup-
port from the Spanish “Ministerio de Ciencia y Tecno-
log´ıa” (Projects BQU2002-00606 and PPQ2000-0488-P4-
02). We thank Rafael de la Escalera, Mar´ıa J . Cueto,
and Patricia Domingo for technical assistance. M.O.
thanks the Spanish MCYT and the Universidad de
Zaragoza for funding through the “Ramo´n y Cajal”
Program.
(30) Werner, H.; Wolf, J .; Ho¨hn, A. J . Organomet. Chem. 1985, 287,
395.
(31) Schunn, R. A.; Wonchoba, E. R. Inorg. Synth. 1972, 13, 131.
(32) Gru¨nwald, C.; Gevert, O.; Wolf, J .; Gonza´lez-Herrero, P.;
Werner, H. Organometallics 1996, 15, 1960.
(33) Oliva´n, M.; Clot, E.; Eisenstein, O.; Caulton, K. G. Organo-
metallics 1998, 17, 3091.
OM049786Q