Palladium(II) allylic complexes by carbene transmetalation and migratory insertion reactions: Synthesis and side reactions

Article abstract of DOI:10.1016/j.jorganchem.2009.10.027

New 1,1-alkoxy, aryl substituted palladium η³-allyls [Pd(μ-Br){η³-C(C₆F₅)(OMe)CHR¹CHR²}]₂ can be synthesized from [W(CO)₅{C(OMe)CHR¹{double bond, long}CHR²}] and a palladium perfluoroaryl complex. The allyls are formed by transmetalation of the carbene fragment followed by migratory insertion of C₆F₅ to the putative and highly reactive Pd carbene complex. This reaction pathway predominates in all cases, but insertion of the double bond of the tungsten alkoxyvinylcarbenes into the Pd-C₆F₅ bond leads to secondary products, namely C₆F₅(OMe)C{double bond, long}CR¹CH(C₆F₅)R².

Full text of DOI:10.1016/j.jorganchem.2009.10.027

Journal of Organometallic Chemistry 695 (2010) 441–445
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Palladium(II) allylic complexes by carbene transmetalation and migratory
insertion reactions: Synthesis and side reactions
*
*
Ana C. Albéniz , Pablo Espinet , Alberto Pérez-Mateo
IU CINQUIMA/Química Inorgánica, Universidad de Valladolid, 47071 Valladolid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 August 2009
Received in revised form 21 October 2009
Accepted 22 October 2009
Available online 27 October 2009
New 1,1-alkoxy, aryl substituted palladium
g l-Br){g
3-allyls [Pd( 3-C(C6F5)(OMe)CHR1CHR2}]2 can be
synthesized from [W(CO)5{C(OMe)CHR1@CHR2}] and a palladium perfluoroaryl complex. The allyls are
formed by transmetalation of the carbene fragment followed by migratory insertion of C6F5 to the puta-
tive and highly reactive Pd carbene complex. This reaction pathway predominates in all cases, but inser-
tion of the double bond of the tungsten alkoxyvinylcarbenes into the Pd–C6F5 bond leads to secondary
products, namely C6F5(OMe)C@CR1CH(C6F5)R2.
Keywords:
Carbenes
Ó 2009 Elsevier B.V. All rights reserved.
Palladium
Transmetalation
Migratory insertion
Allylic complexes
Insertion
1. Introduction
bridging vinylic carbene and SnPh4 that leads to [Pd(g3
-
CPh2CHCH2)Cl(PPh3)] and presumably follows a migratory inser-
tion mechanism of the Ph group to a palladium carbene [7].
A second reaction center, the C–C double bond, is present in the
vinylic tungsten carbenes and should also be considered in these
reactions, since the formation of minor products through insertion
of the C@C bond into the Pd–C6F5 bond is also observed.
The migratory insertion reaction of an aryl group to a carbene in
the palladium coordination sphere is a process that readily occurs
for electrophilic carbene fragments, such as those bearing only one
heteroatom-containing substituent. Since few palladium carbenes
of this type are known, the migratory insertion reaction has been
rarely observed [1–3]. We have reported that transmetalation of
carbene fragments {CXR1} (X = NR2, OR; R1 = hydrocarbyl) from
tungsten to palladium can be used as a synthetic route for palla-
dium carbenes [1,4,5]. When the palladium complex contains a
Pd–aryl moiety, migratory insertion follows leading to a carbene–
aryl coupling product [1,4]. This is specially fast for methoxy car-
bene fragments and the [PdR2{C(OMe)R1}X]2 intermediate cannot
be detected. Using R1 = CH@CHPh and R2 = C6F5, we could isolate
the alkyl palladium migratory insertion product as a stable palla-
2. Results and discussion
The reaction of the tungsten vinylic carbenes 1a–d, with the
palladium complex 2 leads to a series of palladium allyls 3a–d
(Scheme 1). Complex 3a has been described by us before [1b].
Along with complexes 3, the vinylic ethers 4 were identified. The
reactions were monitored by NMR and Scheme 1 collects the molar
percentages of the products in the reaction mixtures, as deter-
mined by integration of 19F signals in the 19F NMR spectra. Aside
from 1a, the reactions with the other tungsten carbenes are not
very clean and some decomposition products coming from 2, such
as C6F5H and [Pd(C6F5)2(NCMe)2] were also formed, along with
other unidentified products. This lowers the yields of the allylic
palladium complexes 3b–c.
Complexes 3 are formed by transmetalation of the carbene frag-
ment and migratory insertion in a putative palladium carbene
(Scheme 2). The formation of this carbene has been supported by
isolation of the analogous diethylaminocarbenes, which undergo
a similar subsequent reactivity [4].
dium
g l-Br){g
3-allyl, [Pd( 3-C(C6F5)(OMe)CHCHPh}]2 [1b].
Following these results, we describe here the reactions of a pal-
ladium pentafluorophenyl complex with several tungsten vinylic
carbenes and the formation, by transmetalation and migratory
insertion, of the corresponding palladium allyls. Although
palladium allyls can be synthesized by a variety of available and
convenient ways [6], this is a novel route that leads to
g
3-allyls
with an unusual 1,1-substitution pattern. Besides this example,
Kurosawa et al. described the reaction of a Pd(I) complex with a
* Corresponding authors. Tel.: +34 983 184621; fax: +34 983 423013.
E-mail addresses: albeniz@qi.uva.es (A.C. Albéniz), espinet@qi.uva.es (P. Espinet).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.10.027

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A.C. Albéniz et al. / Journal of Organometallic Chemistry 695 (2010) 441–445
Scheme 1. Reaction of complexes 1 and 2.
Scheme 2. Reaction pathway for the formation of 3.
The second product, 4, contains two C6F5 groups and must come
from a different route. The possibility of an allyl–C6F5 coupling in
the course of the reaction, by pentafluorophenyl transmetalation
and subsequent reductive elimination, was discarded since decom-
position of 3b or 3c in the presence of 2 does not give 4 (Scheme 3,
a). The reductive elimination of an allyl and an aryl fragment is a
Scheme 3. Possible routes for the formation of 4.

A.C. Albéniz et al. / Journal of Organometallic Chemistry 695 (2010) 441–445
443
slow coupling favored in the presence of electron withdrawing li-
gands [8,9]. We have observed that the transmetalation of a car-
bene fragment from [W(CO)5(carbene)] to palladium occurs with
the concomitant transfer of CO, a potential electron withdrawing
ligand. However, CO does not promote the allyl–C6F5 coupling
and, again, the reaction of 3b and 2 in the presence of CO does
not give 4. A different route must then operate, and we propose
an insertion of the double bond into the Pd–C6F5 bond, followed
by intramolecular transmetalation of the carbene (Scheme 3, b).
This would give a metalacyclopropene (B) that could rearrange to
an alkenyl complex (C). The second C6F5 group would be intro-
duced by transmetalation from other palladium complex and
reductive elimination (Scheme 3, b). Insertion of C@C bonds into
Pd–C6F5 bonds is well documented [10], as well as fluoroaryl group
exchange between palladium centers [11]. Palladium metalacyclo-
propenes of type B (Scheme 3, b) have not been reported but this
type of metalacycles has been isolated for group 8 metals.
Compound 4 is a minor product of the reaction in all cases. Its
final ratio depends on the substitution of the carbene double bond,
the least substituted one (c, R1 = R2 = H) leading to a higher per-
centage of 4 (Scheme 1). This is consistent with the insertion of
the C@C bond into the Pd–C6F5 bond being the first step in the for-
mation of 4.
served for other g
3-allylic complexes, compounds 3 are a mixture
of isomers in solution due to the cis–trans arrangement of both al-
lyl groups in the dimer [1b,10b]. Four diastereoisomers are ex-
pected from the cis–trans arrangement of both central C2 and
both external C1 (or C3) in the allylic groups. The isomers intercon-
vert at a rate dependent on the concentration and temperature
[13]. Fig. 1 depicts the Fortho region in the 19F NMR spectra of 3b,
showing signals due to the four possible isomers at 223 K and fast
exchange of all of them at 293 K. The monomeric acac derivative
[Pd(g
3-CPf(OMe)CHCHPh)(acac)] (5a) does not show this isomer-
ism and just one species is observed in solution.
Besides the process just described, 3c shows a fast syn–anti
interconversion of the two H3 hydrogens at room temperature.
At 213 K the exchange is slow enough to detect both syn-H3 and
anti-H3 as separated and well resolved signals. Syn–anti exchange
is a common fluxional process for palladium allyls but it is notice-
ably faster for 3c than for other halogen bridged Pd allyls. This re-
flects the very different substitution pattern on both ends of the
allyl moiety of 3c that seems to favor the
r
-C3 coordination of
the allyl that triggers the exchange [6,14].
3. Conclusion
The relative importance of the transmetalation of the carbene
vs. the insertion of the C@C bond is difficult to quantify, as the
crude reaction mixture contains some compounds with a pentaflu-
orophenyl group bound to C that we could not identify [12], and
could derive from either route. Nonetheless, complexes 3 are the
major products of the reactions in all cases and substitution of
the double bond with a phenyl group (complexes a) makes inser-
tion of the double bond unimportant (76% of 3a obtained). This
points to transmetalation of the carbene as the main reaction path-
way for the reaction of vinylic carbenes.
New 1,1-alkoxy, aryl substituted palladium
g
3-allyls can be
synthesized from tungsten alkoxyvinylcarbenes and a palladium
perfluoroaryl complex. The allyls are formed by transmetalation
of the carbene fragment followed by migratory insertion of C6F5
to the new and highly reactive Pd carbene complex. This reaction
pathway predominates in all cases, but insertion of the double
bond of the tungsten alkoxyvinylcarbenes into the Pd–C6F5 bond
leads to secondary products. Although the insertion route is less
important than the transmetalation one, its incidence increases
for double bonds with no or small substituents.
Complexes 3 where characterized by NMR spectroscopy and
show the same stereochemistry as 3a, whose molecular structure
was previously determined by X-ray diffraction [1b]. The
g
3-allyls
4. Experimental
have an anti-C6F5 syn-OMe and a syn-R2 stereochemistry. The anti-
C6F5 arrangement is characterized by a 19F NMR downfield shift for
the Fortho signals (ꢀ127 and ꢀ136 ppm vs. ꢀ142 ppm for a typical
C–C6F5 group) and this congested position leads to hindered rota-
tion about the C–C6F5 bond in all the complexes. The arrangement
of the allylic substituents was confirmed by an NOE experiment on
complex 3b that shows the proximity of the central H with the syn-
Me (6% NOE) and the syn-OMe groups (11% NOE). As has been ob-
4.1. General methods
All manipulations were carried out using Schlenk techniques.
Solvents were dried and distilled under nitrogen prior to use. 1H,
13C and 19F NMR spectra were recorded on Bruker AC-300 and
ARX-300 spectrometers. Chemical shifts are referenced to TMS
for 1H and 13C and to CFCl3 for 19F. The spectral data were recorded
Fig. 1. Fortho signals in the 19F NMR spectra of 3b at different temperatures, showing the expected four isomers of a non-symmetrical allylic dimeric complex (b). Restricted
rotation leads to inequivalent Fortho atoms (a and b).

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A.C. Albéniz et al. / Journal of Organometallic Chemistry 695 (2010) 441–445
at 293 K unless otherwise stated. C, H and N elemental analyses
were performed on a Perkin–Elmer 240 microanalyzer. Com-
pounds 1a [15], 1b–d [16], 2 [10b], were prepared according to lit-
erature methods. Complex 3a has been described before [1b].
Percentages of products in the reactions mixtures were determined
by integration of 19F NMR signals.
CH3 acac), 1.76 ppm (s, 3H; C0H3 acac); 19F NMR (282 MHz, d,
CDCl3): ꢀ162.4 (ma, 2F; Fmeta), ꢀ152.8 (t, 1F; Fpara), ꢀ137.0 (ma,
1F; Fortho), ꢀ132.7 (ma, 1F; Fortho); 13C{1H} NMR (75.4 MHz, d,
CDCl3, 273 K): 188.2 (s, C–Me acac), 187.4 (s, C0-Me acac), 150.0–
125.0 (o-, m-, p-Pf), 137.5 (s, i-Ph), 128.9 (s, m-Ph), 127.7 (s, p-
2
Ph), 127.6 (s, o-Ph), 111.3 (t, JC–F = 18.9 Hz, i-Pf), 103.9 (s,
C1OMe(Pf)), 99.7 (s, CH acac), 87.8 (s, C2H), 65.1 (s, C3HPh), 56.3
(s, OCH3), 28.1 (s, CH3 acac), 28.0 ppm (s, C0H3 acac); Anal. Calc.
for C21H17F5O3Pd: C, 48.62; H, 3.30. Found: C, 48.94; H, 3.51%.
4.2. Synthesis of complexes 3
4.2.1. [Pd(l-Br){(g
3-C(C6F5)(OMe)CHCHMe}]2 (3b)
To a solution of 1b (0.250 g, 0.613 mmol) in CH2Cl2 (10 mL) was
added 2 (0.2668, 0.613 mmol) and the mixture was stirred for 4 h
at room temperature. The resulting dark suspension was filtered
through activated carbon and the filtrate was evaporated to dry-
ness. The residue was triturated with n-pentane and the yellow so-
lid was filtered, washed with n-pentane and vacuum-dried.
(0.116 g; yield: 42%). Two isomers of the dimeric complexes (3b
and 3b0) can be distinguished in some NMR spectra at room
temperature (see text). 1H NMR (300 MHz, d, CDCl3): 5.30 (d,
3JH–H = 11.4 Hz, 1H; H2), 3.81 (s, 3H; OCH3), 3.47 (m, 1H; H3),
1.45 ppm (d, 3JH–H = 6.2 Hz, 3H; Me); 19F NMR (282 MHz, d, CDCl3):
ꢀ163 (b, 2F; Fmeta), ꢀ152.0 (t, 1F; Fpara, 3b), ꢀ151.9 (t, 1F, Fpara, 3b0),
ꢀ136.0 (b, 1F, Fortho), ꢀ131.0 (b, 1F, Fortho); 13C{1H} NMR (75.4 MHz,
d, CDCl3, 263 K): 143.0–137.0 (o, m, p-Pf), 113.3 (s; C1OMe), 110.5
(i-Pf), 90.2 (s, C2H, 3b), 90.1 (s, C2H, 3b0), 71.0 (b, C3HMe), 57.2 (s,
OCH3), 18.3 ppm (s, CH3); Anal. Calc. for C22H16Br2F10O2Pd2: C,
30.20; H, 1.84. Found: C, 30.27; H, 2.14%.
4.4. Characterization of compounds 4
The equimolar reactions of 1b–d and 2 (0.1 mmol) in CH2Cl2
were monitored by 19F NMR. When complex 2 was consumed,
the mixture was filtered through activated carbon and the filtrate
evaporated to dryness. The residue was triturated with pentane
to obtain complexes 3 (see above) and the mother liquors were
evaporated. The residue was purified by column chromatography
through silica using a mixture of pentane/CH2Cl2 (1:1) as eluent.
Compounds 4 were spectroscopically characterized and 4b was
separated as a pale yellow oil.
4.4.1. (C6F5)(OMe)C@CH{CH(Me)(C6F5)} (4b)
1H NMR (300 MHz, d, CDCl3): 5.30 (dt, JH–H = 9.0 Hz,
3
3
5JH–F = 2.1 Hz, 1H, H2), 4.55 (q, JH–H = 9.0 Hz, 1H, H3), 3.39 (s, 3H,
3
OCH3), 1.45 (d, JH–H = 9.0 Hz, 3H, Me); 19F NMR (282 MHz, d,
CDCl3): ꢀ162.9 (m, 2F, F3meta), ꢀ161.8 (m, 2F, F1meta), ꢀ158.1 (t, 1F,
F3para), ꢀ153.0 (t, 1F, Fp1ara), ꢀ143.7 (m, 2F, F1ortho), ꢀ140.1 (m, 2F,
F3ortho); EI (70 eV): m/z (relative intensity), 418 (M+, 42), 403 (91),
211 (21), 195 (100), 181 (98), 167 (44).
Complexes 3c and 3d were prepared in a similar way. Complex
3c (28% yield) is mixed with a small amount of other unidentified
products that could not be separated by conventional methods.
Complex 3d (30% yield) was further purified by chromatography
in a silica column using Et2O as eluent.
4.4.2. (C6F5)(OMe)C@CH{CH2(C6F5)} (4c)
1H NMR (300 MHz, d, CDCl3): 5.00 (t, 3JH–H = 8.0 Hz, 1H, H2), 3.70
(d, 2H, CH2), 3.49 (s, 3H, OCH3); 19F NMR (282 MHz, d, CDCl3):
ꢀ163.0 (m, 2F, Fm3 eta), ꢀ161.8 (m, 2F, F1meta), ꢀ157.7 (t, 1F, F3para),
ꢀ152.9 (t, 1F, Fp1ara), ꢀ144.0 (m, 2F, Fo1rtho), ꢀ140.0 ppm (m, 2F,
F3ortho).
4.2.2. Data for 3c
Two isomers (3c and 3c0) can be distinguished in some NMR
spectra (see text). 1H NMR (300 MHz, d, CDCl3): 5.35 (t,
3JH–H = 9.5 Hz, 1H; H2), 3.80 ppm (s, 3H, OCH3); 1H NMR
3
(300 MHz, d, CDCl3/CD3CN, 213 K): 5.19 (dd, JH–H = 11, 8 Hz, 1H;
3
H2), 3.40 (s, 3H, OCH3), 3.35 (m, JH–H = 8 Hz, 1H; syn-H3),
4.4.3. (C6F5)(OMe)C@CMe{CH2(C6F5)} (4d)
3
2.20 ppm (da, JH–H = 11 Hz, 1H; anti-H3); 19F NMR (282 MHz, d,
19F NMR (282 MHz, d, CDCl3): ꢀ157.4 (t, 1F, Fp3ara), ꢀ152.7 (t, 1F,
CDCl3): ꢀ161.0 (ma, 2F, Fmeta, 3c/3c0), ꢀ151.1 (m, 1F, Fpara, 3c),
F1para), ꢀ141.8 (m, 2F, Fo1rtho), ꢀ139.1 (m, 2F, F3ortho).
ꢀ150.8 (m, 1F, Fpara
,
3c0), ꢀ136 (ma, 1F, Fortho
,
3c/3c0),
ꢀ130.0 ppm (ma, 1F, Fortho, 3c/3c0).
Acknowledgements
4.2.3. Data for 3d
Financial support is gratefully acknowledged from theSpanish
MEC (DGI, Grant CTQ2007-67411/BQU; Consolider Ingenio 2010,
Grant INTECAT, CSD2006-0003) and the Junta de Castilla y León
(Project GR169).
Two isomers (3d and 3d0, in a 1:2 ratio) can be distinguished in
some NMR spectra (see text). 1H NMR (300 MHz, d, CDCl3): 3.82 (b,
1H; syn-H3), 3.55 (s, 3H; OCH3), 2.41 (b, 1H; anti-H3), 2.33 ppm (s,
3H; Me); 19F NMR (282 MHz, d, CDCl3): ꢀ161.1 (b, 2F, Fmeta, 3d),
ꢀ160.6 (b, 1F, m-Pf, 3d0), ꢀ150.6 (t, 1F, Fpara, 3d), ꢀ150.2 (t, 1F,
Fpara, 3d0), ꢀ136.5 (b, 1F, Fortho, 3d/3d0), ꢀ128.5 (m, 1F, Fortho, 3d),
ꢀ127.7 ppm (m, 1F, Fortho, 3d0). Anal. Calc. for C22H16Br2F10O2Pd2:
C, 30.20; H, 1.84. Found: C, 30.55; H, 2.15%.
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