Involvement of intramolecular hydride transfer in the formation of alkanes from palladium alkyls

Article abstract of DOI:10.1021/om9703651

Intramolecular hydride transfer in a dimer between Pd atoms and reductive elimination of H and a benzyl moiety to give PhCH₂CH₂C₆F₅ (3) is the main decomposition pathway for the hydrido species generated from the η³-benzylpalladium derivative [Pd₂(μ-Br)₂(η³-CHPhCH₂C ₆F₅)₂] (1). The commonly accepted decomposition of the palladium hydride to give Pd(0) and HX, followed by acid attack on 1 to produce the alkane, is ruled out in this case.

Full text of DOI:10.1021/om9703651

4030
Organometallics 1997, 16, 4030-4032
In volvem en t of In tr a m olecu la r Hyd r id e Tr a n sfer in th e
F or m a tion of Alk a n es fr om P a lla d iu m Alk yls
Ana C. Albe´niz, Pablo Espinet,* and Yong-Shou Lin
Departamento de Quı´mica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid,
47005 Valladolid, Spain
Received April 30, 1997X
Summary: Intramolecular hydride transfer in a dimer
between Pd atoms and reductive elimination of H and a
benzyl moiety to give PhCH2CH2C6F5 (3) is the main
decomposition pathway for the hydrido species generated
from the η3-benzylpalladium derivative [Pd2(µ-Br)2(η3-
CHPhCH2C6F5)2] (1). The commonly accepted decom-
position of the palladium hydride to give Pd(0) and HX,
followed by acid attack on 1 to produce the alkane, is
ruled out in this case.
(82% yield). The η3-benzylic structure (Scheme 1) is
supported by the observation of a high-field aromatic
resonance (δ ) 6.23, H2) in its room temperature 1H
NMR spectrum in CDCl3. Due to the low solubility of
1 in CDCl3, its 13C NMR spectrum had to be recorded
in the presence of MeCN, which increases the solubility
but produces the coalescence of the signals for the two
ortho and meta C or H atoms (δ ) 106.3 for C2 and C6;
δ ) 6.79 for H2 and H6; δ ) 133.9 for C3 and C5; δ )
7.51 for H3 and H5).6 This is due to a rapid η3 T σ T η3
interconversion (1 T A, Scheme 1) exchanging both
ortho (and meta) phenyl sites, as reported for other η3-
benzyl complexes.7
Hydridopalladium complexes are intermediates in a
variety of catalytic processes. Important reactions, such
as the Heck functionalization of olefins, involve palla-
dium(II) hydrido derivatives in the final step of the
catalytic cycle.1 Elimination of HX and Pd(0) formation
is proposed to explain their decomposition. The acid
generated has been held responsible for the formation
of alkanes as byproducts in some of these reactions and
in the thermolysis of palladium monoalkyl complexes
(eqs 1 and 2),1c,2,3 as well as in the generation of
hydrogen in base-free olefin arylation reactions.4
In the presence of CDCl3 at room temperature, where
it is only sparingly soluble, 1 slowly gets in solution and
decomposes via Pd-(â-H) elimination, to E-PhCHdCHPf
(2) and an undetected hydrido-containing palladium
species. Since it is statistically unlikely that both
moieties of the dimer undergo simultaneous H-elimina-
tion, the hydrido-containing species is most probably the
mixed dimer depicted in Scheme 1.
[Pd(alkyl)XL2] f [PdHXL2] + alkene f
[PdL2] + HX + alkene (1)
Complete decomposition of a suspension of 1 in CDCl3
occurs slowly at room temperature, and it was followed
by 19F and 1H NMR. Organic compounds 2, 2-(pen-
tafluorophenyl)-1-phenylethane (3) and 1-bromo-2-(pen-
tafluorophenyl)-1-phenylethane (4) are formed in a ratio
of 2.3:1.5:1.8 A mixture of metallic Pd and PdBr2 is also
observed. The high amount of saturated compound 3
formed suggested that a mechanism might be operating
in this reaction different from that in eqs 1 and 2.3,4,9
When the reaction was carried out in CDCl3 solution
saturated with D2O, no deuterated compound 3 (neither
4) was detected. This discounts the involvement of HBr
formation in the reaction. Furthermore, the decomposi-
tion of 1 in the presence of a strong acid does not
increase the amount of saturated derivative 3 formed
and the presence of base does not eliminate its forma-
tion (Table 1).
[Pd(alkyl)XL2] + HX f [PdX2L2] + alkane (2)
Here, we study the decomposition of a benzylic
palladium complex which generates hydridopalladium
species by â-H elimination. The results show that
intramolecular hydrido transfer in a dimer followed by
reductive elimination to generate the alkane is the main
decomposition pathway, whereas acid attack is not
important in this case. This is a new pathway for the
decomposition of Pd-H species that could also be
important in other processes.
[Pd2(µ-Br)2(η3-CHPhCH2Pf)2] (1, Pf ) C6F5) was pre-
pared by reacting [PdPfBr(NCMe)2]5 and a stoichiomet-
ric amount of styrene for 5 min in CH2Cl2 at room
temperature. 1 is only slightly soluble in CH2Cl2 and
can be easily isolated by cooling and filtration at 0 °C
These results point to a direct H transfer from a
PdHBr fragment to an alkylpalladium moiety. Inter-
molecular hydride transfer has been proposed in the Pd-
X Abstract published in Advance ACS Abstracts, September 1, 1997.
(1) (a) Comprehensive Organometallic Chemistry II; Wilkinson, G.,
Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, 1995; Vol.
12. (b) Hegedus, L. S. In Organometallics in Synthesis; Schlosser, M.,
Ed.; Wiley: Chichester, 1994; Chapter 5. (c) De Meijere, A.; Meyer, F.
E. Angew. Chem., Int. Ed. Engl. 1994, 33, 2379. (d) Heck, R. F.
Palladium Reagents in Organic Syntheses; Academic Press: London,
1985. (e) Trost, B. M.; Verhoeven, T. R. In Comprehensive Organome-
tallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon Press: Oxford, 1982; Vol. 8, Chapter 57.
(2) Benhaddou, R.; Czernecki, S.; Ville, G. J . Org.Chem. 1992, 57,
4612.
(3) Kawataka, F.; Kayaki, Y.; Shimizu, I.; Yamamoto, A. Organo-
metallics 1994, 13, 3517.
(4) Portnoy, M.; Ben-David, Y.; Milstein, D. Organometallics 1993,
12, 4734.
(5) Albe´niz, A. C.; Espinet, P.; Foces-Foces, C.; Cano, F. H. Orga-
nometallics 1990, 9, 1079.
(6) Data for 1. Anal. Calcd for C28H16Br2F10Pd2: C, 36.75; H, 1.76.
Found: C, 36.64; H, 1.75. 1H NMR (300 MHz, δ, CDCl3): 7.6 (m, 1H,
H4), 7.5 (br, 2H, H3, H5), 7.35 (br, 1H, H6), 6.23 (br, 1H, H2), 3.66 (dd,
J ) 9.7, 5.6 Hz, 1H, HR), 3.11 (dd, J ) 14.6, 5.6 Hz, 1H, Hâ), 3.00 (dd,
J ) 14.6, 9.7 Hz, 1H, Hâ′). 19F NMR (282 MHz, δ, CDCl3): -162.3 (m,
Fmeta), -156.8 (t, Fpara), -142.7 (m, Fortho). Data for 1 + CH3CN (1:4).
1H NMR (300 MHz, δ, CDCl3): 7.61 (m, 1H, H4), 7.51 (m, 2H, H3, H5),
6.79 (br, 2H, H2, H6), 3.65 (dd, J ) 9.7, 5.9 Hz, 1H, HR), 3.10 (dd, J )
15.0, 5.9 Hz, 1H, Hâ), 2.98 (dd, J ) 15.0, 9.7 Hz, 1H, Hâ′). 19F NMR
(282 MHz, δ, CDCl3): -162.3 (m, Fmeta), -156.8 (t, Fpara), -142.7 (m,
1
Fortho). 13C NMR (75.4 MHz, δ, CDCl3): 133.9 (d, J C-H ) 164 Hz, C3,
1
1
C5), 128.8 (d, J C-H ) 164 Hz, C4), 114.2 (s, C1), 106.3 (d, J C-H ) 164
Hz, C2, C6), 58.3 (d, J C-H ) 157 Hz, CR), 23.2 (t, J C-H ) 134 Hz, Câ).
(7) Crascall, L. E.; Spencer, J . L. J . Chem. Soc., Dalton Trans. 1992,
3445 and references therein.
1
1
S0276-7333(97)00365-8 CCC: $14.00 © 1997 American Chemical Society

Communications
Organometallics, Vol. 16, No. 19, 1997 4031
Sch em e 1
Ta ble 1. Decom p osition P r od u cts fr om 1 in CDCl3
Solu tion a
mediated oxidation of allyl alcohol,10 but the dimeric
nature of 1 provides an easy way for intramolecular
hydride transfer to Pd-R.11 The different decomposi-
tion pathways and undetected intermediates involved
are summarized in Scheme 1.
entry
additive
2
3
4
efficiencyb
1
2
3
4
5
no additive
46
44
71
68
96
33
31
22
31
4
21
25
7
72
70
31
46
4
c
HBF4
proton sponged
An η3 f σ conversion gives A, which can produce 4
by reductive elimination of RBr (i, Scheme 1). Although
the reverse reaction, oxidative addition, is usually
observed, it is favored only in the presence of good
e
PPh3
PPh3
f
a
Room temperature; reaction time 2 days; percents based on
integration of the 19F NMR signals. Efficiency of H-transfer (%)
b
) ratio of 3/2. c HBF4/Et2O solution. Proton sponge:palladium )
d
(8) Decomposition experiments were carried out using a suspension
of 1 (15 mg, 0.016 mmol) in CDCl3 (0.6 mL) in an NMR tube at room
temperature and monitored by 1H and 19F NMR. Total decomposition
was observed after 7 days, although this time can change in the
presence of other additives (see Table 1). Data for 2. 19F NMR (282
MHz, δ, CDCl3): -163.5 (m, Fmeta), -157.0 (t, Fpara), -143.2 (m, Fortho).
1H NMR (300 MHz, δ, CDCl3): 7.35-7.55 (m, 5H, Ph), 7.44 (d, J )
16.8 Hz, 1H, PfCH), 6.98 (d, J ) 16.8 Hz, 1H, PhCH). MS, EI (70 ev):
m/z (relative intensity) 270 (M+, 98), 250 (100), 219 (45), 201 (49), 78
(58), 51 (83). Data for 3. 19F NMR (282 MHz, δ, CDCl3): -163.5 (m,
Fmeta), -158.0 (t, Fpara), -144.8 (m, Fortho). 1H NMR (300 MHz, δ,
CDCl3): 7.1-7.8 (m, 5H, Ph), 2.97 (m, 2H, PfCH2), 2.88 (m, 2H,
PhCH2). MS, EI (70 ev): m/z (relative intensity) 272 (M+, 10), 181 (6),
91 (100), 65 (13), 51 (4). Data for 4. 19F NMR (282 MHz, δ, CDCl3):
-162.4 (m, Fmeta), -155.6 (t, Fpara), -142.7 (m, Fortho). 1H NMR (300
MHz, δ, CDCl3): 7.3-7.55 (m, 5H, Ph), 5.17 (dd, J ) 8.8, 6.9 Hz, 1H,
PhCH, ABX system), 3.65 (ddb, J ) 14.3, 8.8 Hz, 1H, PfCHH′, ABX
system), 3.52 (ddb, J ) 14.3, 6.9 Hz, 1H, PfCHH′, ABX system). MS,
EI (70 ev): m/z (relative intensity) 352 (M+(Br81), 1), 350 (M+(Br79),
1), 272 (15), 271 (100), 250 (19), 251 (19), 201 (12), 171 (8), 169 (9),
103 (10), 89 (10), 78 (9), 77 (12), 63 (10), 51 (17).
2:1. e Pd:PPh3 ) 1:1.1, the reaction is complete in 10 min. f Pd:
PPh3 ) 1:2.2, the reaction is complete in 10 min.
donors, such as phosphines, that enhance the nucleo-
philicity of Pd(0) and are absent here. In fact, the
formation of 4 is suppressed in the presence of PPh3
(entries 4 and 5, Table 1).
â-H elimination in A gives intermediate B containing
a Pd-H moiety, from which 2 can be liberated (ii,
Scheme 1). This corresponds to eq 1 but in a dimer.
Hydrogen transfer between the Pd-atoms can occur
through an intermediate C containing a bridging hy-
drido ligand. C eventually undergoes reductive elimi-
nation of R-H (3), also giving 2, PdBr2, and Pd black
(iii, Scheme 1). The proposed H-transfer can be con-
sidered as a variation of the well-documented aryl or
alkyl exchange between transition metals. Mechanistic
studies on Pt-alkyl or Pd-aryl exchange reveal the
intermediacy of binuclear derivatives in such pro-
cesses.12 On the other hand, hydrido-bridged binuclear
(9) Main, L.; Nicholson, B. K. Adv. Met.-Org. Chem. 1994, 3, 27.
(10) Zaw, K.; Lautens, M.; Henry, P. M. Organometallics 1985, 4,
1286.
(11) Intramolecular refers here to a process occurring in a dimer.
The presence of bridging Br assists the formation of a bridging H
needed for a direct Pd to Pd hydride transfer (see main text).

4032 Organometallics, Vol. 16, No. 19, 1997
Communications
Sch em e 2
Pd(II) and Pt(II) complexes are well-known.13 According
to Scheme 1, the amount of 3 cannot exceed the amount
of 2, and the efficiency of iii versus ii can be calculated
from the ratio of 3:2. An efficiency of 72% is found for
the decomposition of 1 (Table 1). It does not change
when the decomposition occurs in the presence of a
strong acid (entry 2, Table 1). The ratio decreases to
31% in the presence of a strong base such as proton
sponge (entry 3, Table 1) because the base reacts with
the HBr formed in ii, favoring the decomposition from
B over the rearrangement to C. Finally, the addition
of PPh3 (entries 4 and 5, Table 1) produces a significant
decrease in the efficiency of H-transfer, the higher the
concentration of the ligand, the lower the efficiency. This
shows that coordinating species hinder the formation
of dimers, blocking the intramolecular H-transfer and
the formation of alkane, whereas alkene formation from
a monomer (as in eq 1) is not affected. In fact when 1
and PPh3 in a ratio of Pd:PPh3 ) 1:2 are mixed at 213
K, coordination of both phosphine molecules to pal-
ladium occurs.14 Decomposition of this complex is
observed at 273 K to form 2 and trans-[PdHBr(PPh3)2].15
The direct trapping of hydride by Pd-R was further
explored by the reaction of [Pd(C6F5)Br(NCMe)2] with
styrene in a 2:1 ratio in CDCl3 at room temperature.
Besides the products formed in the decomposition of 1
and unreacted pentafluorophenyl starting complex, a
new derivative, C6F5H, appears. It could be formed
from mixed dimers D as shown in Scheme 2. Again,
the addition of bases does not avoid the formation of 3
and C6F5H, and the presence of D2O does not produce
deuterium incorporation in 3 or C6F5D, thus ruling out
protonation as the mechanism for the formation of 3 and
C6F5H.
Although under catalytic conditions (usually with
large excesses of L) the formation of dimers is unlikely
and this intramolecular hydride transfer may be unim-
portant, whenever the formation of dimeric species is
possible (defect of L or L dissociation from a monomer),
the H-transfer to a Pd-alkyl or Pd-aryl is fast and
could be the main or a competing pathway for the
formation of saturated derivatives.
Ack n ow led gm en t. This work was supported by the
Direccio´n General de Investigacio´n Cient´ıfica y Te´cnica
(Spain) (Project No. PB96-0363), the Commission of the
European Communities (Network “Selective Processes
and Catalysis Involving Small Molecules”, Grant No.
CHRX-CT93-0147), and the J unta de Castilla y Leo´n
(Project No. VA 40-96). Y.-S.L. thanks fellowships from
the Spanish Ministerio de Educacio´n y Ciencia and the
AECI/ICD-Universidad de Valladolid.
OM9703651
(14) The species formed is most likely [Pd(η3-CHPhCH2Pf)(PPh3)2]-
Br according to its NMR spectra at 213 K: 31P{1H} NMR (121 MHz,
δ, CDCl3) 29.2 (d, 1P, J ) 52 Hz), 19.4 (d, 1P, J ) 52 Hz). 1H{31P}
NMR (300 MHz, δ, CDCl3) 7.6-6.8 (35H, aromatic), 3.86 (m, 1H, Hâ),
3.53 (d, 1H, J ) 9.8 Hz, HR), 2.95 (d, 1H, J ) 13 Hz, Hâ′). 19F NMR
(282 MHz, δ, CDCl3): -162.4 (m, Fmeta), -157.5 (t, Fpara), -142.8 (m,
Fortho).
(12) Scott, J . D.; Puddephatt, R. J . Organometallics 1983, 2, 1643.
(13) (a) Bronger, W.; a` Brassard, L. Angew. Chem., Int. Ed. Engl.
1995, 34, 898. (b) Terheijden, J .; Van Koten, G.; Grove, D. M.; Vrieze,
K.; Spek, A. L. J . Chem. Soc. Dalton Trans. 1987, 1359. (c) Paonessa,
R. S.; Trogler, W. C. J . Am. Chem. Soc. 1982, 104, 3529. (d) Bracher,
G.; Grove, D. M.; Pregosin, P. S.; Venanzi, L. M. Angew. Chem., Int.
Ed. Engl. 1979, 18, 155.
(15) trans-[PdHBr(PPh3)2]. 31P{1H} NMR (121 MHz, δ, CDCl3, 273
K): 29.18 (s). 1H NMR (300 MHz, δ, CDCl3, 273 K): 7.9-7.3 (m, 30H,
aromatic), -11.82 (t, 1H, J ) 9.5 Hz). These data agree with those
reported previously for the hydride complex, see: Heaton, B. T.; He´bert,
S. P. A.; Iggo, J . A.; Metz, F.; Whyman, R. J . Chem. Soc., Dalton Trans.
1993, 3081.