Two alternative routes for the migratory insertion of a carbene ligand into a C-H bond

Article abstract of DOI:10.1021/om970784y

Reaction of [(η⁵-C₅H₅)Rh(=CPh₂)(PiPr ₃)] (2) with either PF₃ or HX (X = Cl, CF₃CO₂) leads to a migratory insertion of the carbene ligand into one of the C-H bonds of the cyclopentadienyl ring. The X-ray crystal structures of [(η⁵-C₅H₄CHPh₂)Rh(PF ₃)(PiPr₃)] (5) and [(η⁵-C₅H₄CHPh₂)RhCl ₂(PiPr₃)] (7a) have been determined.

Full text of DOI:10.1021/om970784y

1
0
Organometallics 1998, 17, 10-12
Tw o Alter n a tive Rou tes for th e Migr a tor y In ser tion of a
Ca r ben e Liga n d in to a C-H Bon d
Ulrich Herber, Elke Bleuel, Olaf Gevert, Matthias Laubender, and
Helmut Werner*
Institut f u¨ r Anorganische Chemie, Universit a¨ t W u¨ rzburg, Am Hubland,
D-97074 W u¨ rzburg, Germany
Received September 4, 1997X
5
Summary: Reaction of [(η -C5H5)Rh(dCPh2)(PiPr3)] (2)
with either PF3 or HX (X ) Cl, CF3CO2) leads to a
migratory insertion of the carbene ligand into one of the
C-H bonds of the cyclopentadienyl ring. The X-ray
Sch em e 1
5
crystal structures of [(η -C5H4CHPh2)Rh(PF3)(PiPr3)] (5)
and [(η -C5H4CHPh2)RhCl2(PiPr3)] (7a ) have been de-
5
termined.
It is well-known that transition-metal carbene com-
plexes readily undergo C-C coupling reactions not only
with alkenes (olefin metathesis) and alkynes (D o¨ tz
1
reaction) but also with CO and other C-nucleophiles.
During attempts to study the reactivity of the half-
5
sandwich-type compounds [(η -C5H5)Rh(dCPh2)(SbiPr3)]
(1) and the corresponding triisopropylphosphine deriva-
tive 2 (Scheme 1), which were prepared from the square-
2
planar precursors trans-[RhCl(dCPh2)L2] and NaC5H5,
we observed that the course of the reaction of 1 and 2
with CO and isocyanides depends critically on the ligand
L of the starting material. While the stibane complex
1
on treatment with CO or CNtBu yields the mixed
(
carbene)(carbonyl) and (carbene)(isocyanide)rhodium
5
compounds [(η -C5H5)Rh(dCPh2)(CX)] (3a , X ) O; 3b,
X ) NtBu), the phosphine derivative 2 affords the non-
carbene-containing products [(η -C5H5)Rh(PiPr3)(CX)]
5
(
4a ,b) together with diphenylketene or the correspond-
2
,3
ing ketenimine.
We have now found that PF3, despite
the similarity of its σ-donor/π-acceptor capabilities with
those of CO, reacts with 2 in a completely different way,
leading to an unprecedented migratory insertion of the
in the 31P NMR spectrum.5 Both 31P NMR signals show
strong Rh-P couplings of 446 and 221 Hz, respectively.
The X-ray crystal structure analysis of 5 (Figure 1)6
2
CPh2 unit into an sp C-H bond.
Treatment of compound 2 with PF3 in benzene at
room temperature leads to a smooth change of color
from blue to orange and finally to the isolation of the
orange air- and moisture-sensitive solid 5 in moderate
(4) The preparation of 5 is as follows. A slow stream of PF3 was
passed through a solution of 2 (100 mg, 0.20 mmol) in 20 mL of benzene
for 20 min at room temperature. After the solution was stirred for
another 20 min, the solvent was removed and the oily residue dissolved
in 3 mL of pentane. Column chromatography on Al O (neutral, activity
2 3
grade V) gave upon elution with pentane an orange fraction which was
brought to dryness in vacuo. The solid was recrystallized from pentane
4
yield. The most characteristic spectroscopic features
of 5 are the signals for the CHPh2 proton at δ 4.79 in
(
-78 °C) to afford orange crystals: yield 48 mg (41%).
1
the H NMR and the two resonances for the phosphorus
(5) Selected spectroscopic data for 5, 6a , 6b, and 7a (omitting the
1
13
H and C NMR data for isopropyl and phenyl groups) are as follows.
nuclei of the PF3 and PiPr3 ligands at δ 119.4 and 79.0
1
5
:
H NMR (200 MHz, C
6
D
6
) δ 5.1 (m, 4H, C
5
H
4
), 4.8 (m, 1H, CHPh
2
);
3
1
1
1
P NMR (81.0 MHz, C
6
D
6
) δ 119.4 (ddq, J (PF) ) 849.3, J (RhP) )
X
2
1
2
Abstract published in Advance ACS Abstracts, December 15, 1997.
446.3, J (PP) ) 77.6 Hz, PF ), 79.0 (dd, J (RhP) ) 221.0, J (PP) )
77.6 Hz, PiPr
3
1
3
(1) (a) D o¨ tz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert,
3
). 6a : H NMR (400 MHz, C
6
D
6
) δ 5.72 (d, J (RhH) )
), -12.21 (dd,
U.; Weiss, K. Transition Metal Carbene Complexes; Verlag Chemie:
Weinheim, Germany, 1983. (b) D o¨ tz, K. H. Angew. Chem. 1984, 96,
2.9 Hz, 1H, CHPh
2
), 5.1, 4.4 (both m, 2H each, C H
5 4
1
2
13
J (RhH) ) 35.1, J (PH) ) 13.8 Hz, 1H, RhH); C NMR (100.6 MHz,
D
H
1
2
5
73-594; Angew. Chem., Int. Ed. Engl. 1984, 23, 587-608. (c) Lappert,
C
C
6
6
2
) δ 124.5 (dd, J (RhC) ) 3.0, J (PC) ) 5.0 Hz, CCHPh ), 92.7 (s,
1 2 2
M. F. J . Organomet. Chem. 1988, 358, 185-214. (d) Erker, G. Angew.
Chem. 1989, 101, 411-426; Angew. Chem., Int. Ed. Engl. 1989, 28,
5
4
5 4
), 85.9 (dd, J (RhC) ) 3.6, J (PC) ) 8.9 Hz, C H ), 81.0 (d, J (PC)
2 31
) 5.8 Hz, C
NMR (162.0 MHz, C
(200 MHz, C ) δ 5.4 (s, br, 1H, CHPh
5
H
4
), 75.4 (d, J (PC) ) 6.4 Hz, C
5
H
4
), 48.9 (s, CHPh
2
);
) δ 81.7 (d, J (RhP) ) 145.0 Hz). 6b: H NMR
), 4.6 (m, 4H, C ), -11.04
(dd, J (RhH) ) 35.5, J (PH) ) 13.4 Hz, 1H, RhH); P NMR (81.0 MHz,
P
1
1
3
97-412. (e) Grubbs, R. H.; Tumas, W. Science 1989, 243, 907-915.
6 6
D
(f) Feldmann, J .; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1-74.
g) Hegedus, L. S. Organische Synthese mit U¨ bergangsmetallen; VCH
6
D
6
2
5 4
H
1
2
31
(
1
1
Verlag: Weinheim, Germany, 1995.
C
6
D
6
) δ 80.5 (d, J (RhP) ) 144.1 Hz). 7a : H NMR (400 MHz, C
6
D
6
) δ
), 4.84, 4.65 (both d, J (RhH) )
); C NMR (100.6 MHz, C ) δ 123.1 (s, br,
), 89.8, 79.7 (both s, br, C ), 47.7 (s, br, CHPh
) δ 59.4 (d, J (RhP) ) 135.1 Hz).
3
2
(
2) (a) Schwab, P.; Mahr, N.; Wolf, J .; Werner, H. Angew. Chem.
6.03 (d, J (RhH) ) 6.6 Hz, 1H, CHPh
2
13
1
1
993, 105, 1498-1500; Angew. Chem., Int. Ed. Engl. 1993, 32, 1480-
482. (b) Schwab, P. Dissertation, Universit a¨ t W u¨ rzburg, 1994.
2.0 Hz, 2H each, C
CCHPh
(162.0 MHz, C
5
H
4
6 6
D
3
1
2
5
H
4
2
); P NMR
1
(3) Review: Werner, H. J . Organomet. Chem. 1995, 500, 331-336.
6 6
D
S0276-7333(97)00784-X CCC: $15.00 © 1998 American Chemical Society
Publication on Web 01/05/1998

Communications
Organometallics, Vol. 17, No. 1, 1998 11
F igu r e 2. ORTEP diagram of compound 7a . Selected bond
distances (Å) and angles (deg): Rh-P, 2.328(1); Rh-Cl1,
2.382(1); Rh-Cl2, 2.400(1); Rh-C1, 2.155(4); Rh-C2,
2.167(4); Rh-C3, 2.135(4); Rh-C4, 2.237(4); Rh-C5, 2.226-
(4); P-Rh-Cl1, 89.79(4); P-Rh-Cl2, 95.72(4); Cl1-Rh-
Cl2, 94.30(4); C5-C6-C20, 111.1(4); C5-C6-C30, 110.2-
(4); C20-C6-C30, 115.9(4).
F igu r e 1. ORTEP diagram of compound 5. Selected bond
distances (Å) and angles (deg): Rh-P1, 2.275(3); Rh-P2,
2
.082(4); Rh-C1, 2.307(5); Rh-C2, 2.220(5); Rh-C3, 2.294-
(
6); Rh-C4, 2.258(5); Rh-C5, 2.304(6); C1-C6, 1.518(5);
P1-Rh-P2, 97.0(1); F1-P2-Rh, 117.6(2); F2-P2-Rh,
1
25.6(2); F3-P2-Rh, 125.1(2); C1-C6-C20, 112.3(3); C1-
C6-C30, 112.3(4); C20-C6-C30, 112.3(3).
of both 6a and 6b display a hydride resonance at high
field (δ -12.21 and -11.04), which due to Rh-H and
P-H coupling is split into a doublet of doublets.5 The
(di-p-tolylcarbene)rhodium compound 8 behaves simi-
larly to 2 and on treatment with HCl gives the corre-
sponding ring-substituted product 9 in good yield.
Attack of HX at the Rh-H bond of 6a and 6b leads
to the displacement of the hydride ligand and to the
formation of the dichloro and bis(trifluoracetato) com-
plexes 7a and 7b, respectively. In contrast to 6a and
6b, both substitution products are significantly more
stable and for a short period of time can even be handled
in air. The ORTEP plot (Figure 2)9 showing the
molecular structure of 7a illustrates the piano-stool
configuration of the molecule. While the Rh-C(ring)
distances in 7a are somewhat shorter than in 5, in
agreement with the oxidation state +III of the metal in
7a compared with +I in 5, the Rh-PiPr3 bond length in
7a is slightly longer than in 5. The P-Rh-Cl and Cl-
Rh-Cl bond angles in 7a are near to 90°, thus reflecting
the pseudooctahedral geometry.
reveals that the rhodium has a somewhat distorted
trigonal coordination sphere if the midpoint of the
cyclopentadienyl ring is taken as one coordination site.
The Rh-PF3 bond is significantly shorter than the Rh-
PiPr3 distance, which illustrates the distinct difference
in the π-acceptor strength of the two phosphine ligands.
Most unexpectedly, the migratory insertion of the
CPh2 moiety of 2 into a C-H bond of the five-membered
ring can also be initiated by HCl or CF3CO2H. If a
solution of 2 in acetone is treated with a solution of HCl
in benzene or with an equimolar amount of CF3CO2H
at room temperature, a spontaneous change of color
from blue to orange occurs and, upon addition of pen-
tane, orange crystals of 6a or 6b are isolated in virtually
quantitative yield.7 An alternative preparative proce-
dure for 6a consists of the reaction of the labile dimer
8
[
RhCl(PiPr3)2]2 with C5H5CHPh2, which also affords the
chloro hydrido complex almost quantitatively. In agree-
1
ment with the structural proposal, the H NMR spectra
(
6) Crystal data for 5: crystals from pentane (5 °C); crystal size 0.55
In order to elucidate the mechanism of the migratory
insertion process leading to 6a , a labeling experiment
was carried out. Treatment of 2-d5, which was prepared
from trans-[RhCl(dCPh2)(PiPr3)2] and TlC5D5 in THF,
with an equimolar amount of HCl in benzene affords
exclusively the compound 6a -d5 (Scheme 2). We assume
that in the initial step addition of HCl to the RhdCPh2
×
0.30 × 0.25 mm; monoclinic, space group C2/c (No. 15), Z ) 8; a )
3
4.48(9) Å, b ) 8.96(2) Å, c ) 17.67(3) Å, â ) 96.82(9)°, V ) 5427(3)
3 -3
Å , dcalcd ) 1.426 g cm ; 2θ(max) ) 48° (Mo KR, λ ) 0.709 30 Å,
graphite monochromator, ω/θ scan, Zr filter with factor 16.4, T ) 293-
(
2) K); 4548 reflections scanned, 4162 unique (R(int) ) 0.0136), 3736
observed (I > 2 σ(I)), Lorentz-polarization and empirical absorption
corrections (ψ scans, minimum transmission 80.0%); Patterson method
(
0
SHELXS-86), 408 parameters, reflex/parameter ratio 10.2; R1 )
-3
.0364, wR2 ) 0.0922; residual electron density 1.058/-0.425 eÅ
.
(
7) The preparation of 6a is as follows. A solution of 2 (119 mg, 0.24
mmol) in 5 mL of acetone was treated dropwise with 482 µL of a 0.5
M solution of HCl in benzene at room temperature. After the solution
was stirred for 5 min and then concentrated to ca. 0.5 mL in vacuo,
pentane was added, which led to the precipitation of an orange solid.
This was washed twice with 5 mL portions of pentane and dried: yield
(9) Crystal data for 7a : crystals from acetone (-20 °C); crystal size
0.40 × 0.25 × 0.08 mm; monoclinic, space group Cc (No. 9); a ) 8.865-
3
(2) Å, b ) 23.214(2) Å, c ) 13.172(3) Å, â ) 106.85(1)°, V ) 2594(1) Å ,
-3
d
calcd ) 1.447 g cm ; 2θ(max) ) 58° (Mo KR, λ ) 0.710 73 Å, graphite
monochromator, ω/θ scan, Zr filter with factor 16.4, T ) 173(2) K);
3624 reflections scanned, 3624 unique (R(int) ) 0.0000), 3488 observed
(I > 2σ(I)), Lorentz-polarization and empirical absorption corrections
(ψ scans, minimum transmission 89.48%); direct methods (SHELXS-
1
16 mg (91%). Compound 6b was prepared analogously, using 2 (54
mg, 0.11 mmol) and CF CO (8.35 µL, 0.11 mmol) as starting
materials: orange solid, yield 62 mg (93%).
8) Werner, H.; Wolf, J .; H o¨ hn, A. J . Organomet. Chem. 1985, 287,
95-407.
3
2
H
(
86), 290 parameters, reflex/parameter ratio 12.5; R1 ) 0.0284, wR2 )
0.0675; residual electron density +0.673/-0.813 e Å-
3
.
3

1
2
Organometallics, Vol. 17, No. 1, 1998
Communications
3
Sch em e 2
to the temporarily generated η -C5H5 unit, followed by
insertion into a C-H bond, could then afford the isolated
compound 5. It is conceivable that the repulsion
between the sterically demanding PiPr3 ligand, the
CPh2 unit, and the incoming PF3 donor in the postulated
1
:1 adduct facilitates the migration of the carbene ligand
to the five-membered ring.
Although various half-sandwich-type complexes [(η5-
C5H5)M(dCRR′)Ln], in particular those with n ) 2 and
1
,12
M ) Mn or Re, are known,
to the best of our
knowledge it has never been observed that they react
with either electrophilic or nucleophilic substrates by
migratory insertion of the carbene ligand into a C-H
bond of the ring. The closest analogy to the formation
of C5H4CHPh2 from C5H5 and CPh2 we are aware of goes
back to the work of Kirmse et al., which illustrates that
arylcarbenes, generated from the corresponding diazo
compounds, undergo an intramolecular insertion into
the ortho C-H bond of a connected phenyl ring.13
Currently we are trying to find out whether electro-
philes other than HCl or CF CO H and nucleophiles
bond takes place, similarly as in the reaction of 3a with
HCl. The intermediate A then reacts by migration of
the CHPh2 fragment to the C5D5 ligand to afford the
2
substituted cyclopentadienerhodium(I) species B.
A
subsequent 1,2-D-shift along the five-membered ring
generates the intermediate C, which by deuterium
3
2
3
transfer from the sp carbon atom of the C5 moiety to
other than PF can also initiate an insertion of a carbene
3
the metal would give the final product. An isotopomer
of C of composition [RhCl(η -C5H5CHPh2)(PiPr3)] is
ligand into a C-H bond, possibly not only into that of
4
C H but also into that of other π-bonded ring systems.
5
5
probably also involved in the reaction of [RhCl(PiPr3)2]2
with C5H5CHPh2 to yield 6a (see Scheme 1). With
regard to intermediate B, we note that quite recently
Hughes et al. reported the isolation of coordinatively
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support from the Deutsche Forschungsgemein-
schaft (Grant No. SFB 347), the Fonds der Chemischen
Industrie, and the BASF AG.
4
saturated (η -C5H5R)Rh complexes (R ) CF2CF2CF3,
5
CF(CF3)2), which were prepared from [(η -C5H5)Rh-
(
PMe3)2] and fluoroalkyl iodides.10
Su p p or tin g In for m a tion Ava ila ble: Fully labeled dia-
grams and tables of crystallographic data, data collection, and
solution and refinement details, positional and thermal param-
eters, and bond distances and angles for both 5 and 7a (12
pages). Ordering information is given on any current mast-
head page.
As far as the formation of 5 from 2 and PF3 is
concerned, a different mechanism probably operates.
Since it is known that ligand displacement reactions of
5
cyclopentadienylrhodium complexes [(η -C5H5)RhL2] with
1
1
Lewis bases L′ follow second-order kinetics, a reason-
able assumption is that from 2 and PF3 initially a labile
OM970784Y
1
:1 adduct is formed. A slippage of the carbene ligand
(
12) (a) Herrmann, W. A. Chem. Ber. 1975, 108, 486-499. (b) Casey,
(
10) Hughes, R. P.; Husebo, T. L.; Rheingold, A. L.; Liable-Sands,
L. M.; Yap, G. P. A. Organometallics 1997, 16, 5-7.
11) Basolo, F. Inorg. Chim. Acta 1985, 100, 33-39.
C. P.; Vosejpka, P. C.; Askham, F. R. J . Am. Chem. Soc. 1990, 112,
3713-3715.
(13) Guth, M.; Kirmse, W. Acta Chem. Scand. 1992, 46, 606-613.
(