Remarkable regioselectivity in the reaction between [(η5-C5Me4H)RhCl(μ-Cl)]2 and (C6F5)2PCH2CH2P(C6F5)2: Synthesis of a chiral-at-metal rhodium complex cation via carbon-fluorine and -hydrogen bond activation and carbon-carbon bond formation

Article abstract of DOI:10.1016/S0022-328X(98)00428-8

The reaction between [(η⁵-C₅Me₄H)RhCl(μ-Cl)]₂ and (C₆F₅)₂PCH₂CH₂P(C₆F₅)₂ proceeds in refluxing benzene via activation of two C-F and C-H bonds and formation of two C-C bonds to yield the chiral cation [{η⁵-C₅HMe₂-2,4-[CH₂C₆F₄P(C₆F₅)CH₂]₂-1,3}RhCl] ⁺, with 〉90% selectivity.

Full text of DOI:10.1016/S0022-328X(98)00428-8

Journal of Organometallic Chemistry 558 (1998) 209–212
Remarkable regioselectivity in the reaction between
[(p5-C5Me4H)RhCl(v-Cl)]2 and (C6F5)2PCH2CH2P(C6F5)2: synthesis of
a chiral-at-metal rhodium complex cation via carbon–fluorine and
–hydrogen bond activation and carbon–carbon bond formation1
Malcolm J. Atherton a, John H. Holloway b, Eric G. Hope b, Graham C. Saunders b,*
a F2 Chemicals Ltd., Springfields, Salwick, Preston, PR4 0XJ, UK
b Department of Chemistry, Uni6ersity of Leicester, Leicester, LE1 7RH, UK
Received 25 November 1997
Abstract
The reaction between [(p5-C5Me4H)RhCl(v-Cl)]2 and (C6F5)2PCH2CH2P(C6F5)2 proceeds in refluxing benzene via activation of
two C–F and C–H bonds and formation of two C–C bonds to yield the chiral cation [{p5-C5HMe2-2,4-[CH2C6F4P(C6F5)CH2]2-
1,3}RhCl]+, with \90% selectivity. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Regioselectivity; Chiral-at-metal rhodium complex; Bond activation
bonds to afford exclusively the complex [{p5-
The activation of carbon–fluorine bonds of
polyfluorinated compounds, which have formerly been
C5Me3[CH2C6F4P(C6F5)CH2]2-1,3}RhCl]+ as a mix-
ture of chloride (1a) and tetrafluoroborate (1b) salts in
quantitative yield under mild aerobic conditions (Eq.
(1)) [6,7].
considered as unreactive, is now being accomplished by
an increasing number of reagents under remarkably
mild conditions [1]. Recent advances have provided
catalytic methods for the hydrogenolysis of pe-
rfluoroarenes [2] and the defluorination of pe-
1/2[(p5−C5Me5)RhCl(v−Cl)]2
+(C6F5)2PCH2CH2P(C6F5)2“
rfluoroalkanes
to
perfluoroarenes
[3]
or
perfluoroalkenes [4]. A small number of reactions in-
volving the activation of carbon–fluorine bonds by
transition metal complexes lead to carbon–carbon
bond formation [5]. This type of reactivity could be of
immense synthetic value if high yields and regioselectiv-
ity can be obtained under mild conditions. The reaction
between [(p5-C5Me5)RhCl(v-Cl)]2 and (C6F5)2PCH2-
CH2P(C6F5)2 (dfppe) proceeds via the activation of two
C–F and C–H bonds and formation of two C–C
[{p5−C5Me3[CH2C6F4P(C6F5)CH2]2−1,3}RhCl]+
· X− +2HF (1a X− =Cl−, 1b X− =BF4−)
(1)
The reaction displays complete regiospecificity: only
one ortho C–F bond of each P(C6F5)2 moiety and C–H
bonds of methyl groups exclusively in a 1,3 disposition
are activated. Furthermore, of the two possible geomet-
ric isomers (Fig. 1) only isomer I is formed. This
regiospecificity is presumably imposed by the geometric
constraints of the reagents in the reaction. The re-
giospecificity, quantitative yield and mild conditions of
reaction (1) suggest that the synthetic potential of C–F
and C–H bond activation with concomitant C–C bond
formation can be realised.
* Corresponding author. Present address: School of Chemistry, The
Queen’s University of Belfast, David Keir Building, Belfast, BT9
5AG, UK; e-mail: g.saunders@qub.ac.uk
1 Dedicated to Prof. W.R. Roper FRS on the occasion of his 60th
birthday.
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00428-8

210
M.J. Atherton et al. / Journal of Organometallic Chemistry 558 (1998) 209–212
Scheme 1. C6H6, heat.
high yield (Scheme 1). The chloride salt, 2a2, was
As part of our programme to understand the mecha-
precipitated in 50% yield on treatment of dfppe with
[(p5-C5Me4H)RhCl(v-Cl)]2 in refluxing benzene, but
could not be obtained analytically pure due to contam-
ination by a small amount of tetrafluoroborate formed
nism and extend the scope of reaction (1) we have
found that the reaction between [(p5-C5Me4Et)RhCl(v-
Cl)]2 and dfppe, which yields products similar to 1a,
shows no discernible selectivity as to which C–H bonds
are activated [8]. Here we describe the remarkable
regioselectivity displayed by the reaction between [(p5-
C5Me4H)RhCl(v-Cl)]2 and dfppe to yield a chiral-at-
metal complex. Such complexes are of current interest
because of their possible applications in chiral synthesis
and catalysis [9].
The reaction between [(p5-C5Me4H)RhCl(v-Cl)]2 and
dfppe was expected to yield two isomers of the cation
of formulation [{p5-C5HMe2[CH2C6F4P(C6F5)CH2]2-
1,3}RhCl]+, dependent on the positions of the two
methyl groups. However, the reaction was found to
yield the asymmetric isomer with \90% selectivity in
Fig. 1. Diagrammatic representation of the geometric isomers of the
cation [RhCl{p5-C5Me3[CH2C6F4P(C6F5)CH2]2-1,3}]+ viewed along
the C5(centroid)-Rh axis.

M.J. Atherton et al. / Journal of Organometallic Chemistry 558 (1998) 209–212
211
of doublets at l 1.93, with couplings, 4JP–H, of 14.2 and
1.9 Hz, and a doublet at l 1.91 with JP–H 1.6 Hz,
by the reaction between HF and the borosilicate glass
reaction vessel. The tetrafluoroborate salt, 2b3, pre-
pared by anion metathesis of 2a with NH4BF4, was,
however, obtained pure and satisfactory elemental anal-
ysis was obtained. The positive ion FAB mass spectra
of 2a and 2b are identical and consistent with the
parent cation and [M–Cl–H]+. The 31P-{1H}-NMR
spectra exhibit two doublets of multiplets at ca. 70
ppm, with rhodium–phosphorus coupling constants,
1JRh–P, of ca. 140 Hz, consistent with the values of l
71.3 and 144 Hz for 1a [6]. The presence of two
resonances is indicative of non-equivalent phosphorus
4
whereas those of 2b occur as doublets at l 1.98 and
4
1.89 with JP–H 8.7 and 2.7 Hz respectively. Although
unequivocal assignment of these resonances cannot be
made, we tentatively assign the resonances with the
larger P–H couplings to the 4-methyl hydrogen atoms
(since this methyl group may be considered as trans to
one phosphorus atom) and the other resonances to the
2-methyl hydrogen atoms. Both spectra exhibit a dou-
3
blet at ca. l 5.5 with JP–H ca. 6.5 Hz which is assigned
to the hydrogen atom of the cyclopentadienyl ring. The
1
spectra are consistent with that of 1a, which exhibits a
atoms. The H-NMR spectrum of 2a possesses eight
4
doublet at l 2.04 with a coupling, JP–H, of ca. 6 Hz
resonances in the region l 3.5 to 5.0 indicating that all
the methylene hydrogen atoms, PCH2 and C5CH2C6F4,
are unique. Each C5CH2C6F4 methylene group shows
assigned to the equivalent 4- and 5-methyl groups.
(Originally this was erroneously described by us as two
singlets [6]). It is evident from the NMR data for 1a, 2a
and 2b that P–H coupling is small or negligible when
the P–Rh–CMe angle is not close to 180° and the
methyl and cyclopentadienyl hydrogen resonances show
couplings of \5 Hz to only one phosphorus atom. The
13C-{1H}-NMR and 1H–13C and 1H–31P correlation
spectra of 2a are also consistent with these observa-
tions. The 19F-NMR spectra of 2a and 2b are similar
and entirely consistent with a formulation in which
both C6F4 and both C6F5 groups are non-equivalent.
There are three chiral centres in the cations of 2a and
2b, the rhodium and both phosphorus atoms but, due
to the geometric constraints of the reaction, only one
pair of enantiomers can be formed. Presumably 2a is
formed as a racemic mixture.
2
two mutually coupled resonances with a coupling, JH–
H, of ca. 18 Hz, one of which is further coupled to one
4
phosphorus with a coupling, JP–H, of ca. 10 Hz, which
1
is confirmed by H-{31P}-NMR spectroscopy. In con-
trast, the respective C5CH2C6F4 resonances of 2b do
not show coupling to phosphorus. Thus, the anion has
a large effect on the spectroscopic properties of the
cation. The 1H-NMR spectra of 2a and 2b also differ in
their methyl resonances. Those of 2a occur as a doublet
2 Selected spectroscopic data for 2a: MS (FAB): m/z 977 (M+),
941 ([M–Cl–H]+). 1H-NMR (CDCl3, 400.13 MHz): l 5.45 (1H, d,
2
3JP%–H 6.9, C5H), 4.94 (1H, d, JH–H% 18.2, CHH%C6F4), 4.67 (1H, d,
2
2JH¦–H§ 17.7, CH¦H§C6F4), 4.44 (1H, m, PCH2), 4.18 (dd, JH¦–H§
17.7, 4JP%–H§ 9.8, CH¦H§C6F4), 4.08 (1H, dd, 2JH–H% 18.2, 4JP¦–H% 9.8,
CHH%C6F4), 3.88 (1H, m, PCH2), 3.58 (2H, m, P%CH2 and P¦CH2),
Multinuclear NMR spectroscopic and mass spectro-
metric investigations indicate that 2a is also the major
species (\50%) in the mother liquor. The data also
provide evidence for the formation of [(p5-
C5Me4H)RhCl(dfppe)]+ [m/z 1017 (M+)], and one or
more isomers of the singly C–F bond activated com-
plex [{p5-C5HMe3CH2C6F4P(C6F5)CH2CH2P(C6F5)2}-
4
4
4
1.93 (3H, dd, JP¦–H 14.2, JP%–H 1.6, 4-CH3), 1.91 (3H, d, JP–H 1.9,
2-CH3). 13C-{1H}-NMR (CDCl3, 100.62 MHz): l 86.6 (d, JP–C 7,
CH of C5 ring), 31.6 (dd, JP–C 34, JP–C 14, PCH2), 29.5 (d, JP–C
1
2
1
3
3
41, PCH2), 19.4 (d, JP¦–C 6, CH2C6F4), 18.4 (d, JP%–C 7, CH2C6F4],
12.6 (s, 2-CH3), 9.2 (d, JP¦–C 4, 4-CH3). 19F-NMR (CDCl3, 376.45
3
MHz): l −120.54 (2F, m, C6F4), −129.82 (2F, br s, Fo of C6F5),
−131.12 (2F, br s, Fo of C6F5), −135.30 (1F, m, C6F4), −135.61
(1F, m, C6F4), −143.52 (1F, ddd, JF–F ca. 20.8, ca. 20.8, 9.3, C6F4),
−143.68 (1F, ddd, JF–F ca. 20.8, ca. 20.8, 9.1, C6F4), −144.91 (1F,
m, Fp of C6F5), −145.06 (1F, m, Fp of C6F5), −153.08 (1F, dd,
JF–F ca. 21.7, ca. 21.7, C6F4), −153.38 (1F, dd, JF–F ca. 21.8, ca.
21.8, C6F4), −158.39 (4F, m, Fm of C6F5). 31P-{1H}-NMR (CDCl3,
1
RhCl]+ [m/z 997 (M+); lP 86.4 (dm, JRh–P 123 Hz),
58.3 (dm, 1JRh–P 141 Hz)], which are possible intermedi-
ates in the formation of 2a, together with a trace
amount of a complex with 31P-NMR spectral data [lP
1
1
161.99 MHz): l 78.8 (dm, JRh–P% 141, P%), 73.2 (dm, JRh–P¦ 141, P¦].
3 Selected spectroscopic data for 2b: 1H-NMR (CDCl3, 400.13
1
ca. 68.5 (d, JRh–P ca. 140 Hz)] similar to that of 1a,
3
2
which is tentatively assigned to the symmetric isomer of
2a. The ratio of asymmetric to symmetric isomers of
the doubly C–F bond activated product formed in the
reaction was determined to be \9:1 from the 31P-
NMR spectra. Thus, the reaction between [(p5-
C5Me4H)RhCl(v-Cl)]2 and dfppe not only displays the
regiospecificity of reaction (1), but also a remarkably
selective C–H bond activation.
MHz): l 5.58 (1H, d, JP–H 6.5, C5H), 4.09 (1H, d, JH–H 17.8,
2
CHH%C6F4), 4.07 (1H, d, JH–H 18.6, CH¦H§C6F4), 3.87 (1H, d,
2JH–H 17.8, CHH%C6F4), 3.58 (2H, m, PCH2), 3.33 (1H, d, JH–H
2
18.6, CH¦H§C6F4), 3.20 (1H, m, PCH2), 2.90 (1H, m, PCH2), 1.98
4
4
(3H, d, JP–H 8.7, 4-CH3), 1.89 (3H, d, JP–H 2.7, 2-CH3). 19F-NMR
(CDCl3, 376.45 MHz): l −120.50 (1F, m, C6F4), −120.68 (1F, m,
3
C6F4), −129.14 (2F, d, JF–F 19.6, Fo of C6F5), −131.06 (2F, d,
3JF–F 18.6, Fo of C6F5), −134.27 (1F, m, C6F4), −134.65 (1F, m,
C6F4), −143.67 (2F, m), −144.28 (2F, m), −152.87 (1F, dd, JF–F
ca. 20.6, ca. 20.6, C6F4), −153.07 (1F, dd, JF–F ca. 23.0, ca. 23.0,
C6F4), −153.70 and −153.76 (4F, 2s, 1:4, BF4−), −157.82 (2F, m,
Fm of C6F5), −158.00 (2F, m, Fm of C6F5). 31P-{1H}-NMR (CDCl3,
In conclusion, the reaction demonstrates the syn-
thetic potential that C–F and C–H bond activation
with concomitant C–C bond formation can provide by
1
1
161.99 MHz): l 75.9 [dm, JRh–P 140], 68.9 (dm, JRh–P 138).

212
M.J. Atherton et al. / Journal of Organometallic Chemistry 558 (1998) 209–212
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Acknowledgements
We are grateful to S.M. Martin for preliminary
work, Dr. G.A. Griffith for helpful discussion and
recording NMR spectra and Dr. G. Eaton for record-
ing the mass spectra. We thank F2 Chemicals Ltd.
(G.C.S.) and the Royal Society (E.G.H.) for support.
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