Pd-H elimination reactions in palladium(II) allylic complexes

Article abstract of DOI:10.1039/b706817d

The ease of H elimination from the 4- (β-) position in a series of allylic complexes [Pd(5-C₆F₅-1-3-η³- cyclohexenyl)XL]ⁿ⁺ (X, L = Br, N-, P-donor, C₆Cl ₂F₃; n = -1, 0, +1) was compared by analyzing their decomposition products at 50 °C. Pd-H elimination does not occur for cationic complexes, whereas it is the main decomposition pathway for neutral and anionic complexes. In addition to the charge of the complex, the ease of this Pd-H elimination is determined by the trans influence of the ligands (aryl > PMe₃ > Br, N-donor). The Royal Society of Chemistry.

Full text of DOI:10.1039/b706817d

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www.rsc.org/dalton | Dalton Transactions
Pd–H elimination reactions in palladium(II) allylic complexes†
Ana C. Albe´niz,* Pablo Espinet* and Blanca Mart´ın-Ruiz
Received 8th May 2007, Accepted 21st June 2007
First published as an Advance Article on the web 6th July 2007
DOI: 10.1039/b706817d
The ease of H elimination from the 4- (b-) position in a series of allylic complexes
3
[Pd(5-C6F5-1-3-g -cyclohexenyl)XL]n+ (X, L = Br, N-, P-donor, C6Cl2F3; n = −1, 0, +1) was compared
by analyzing their decomposition products at 50 ◦C. Pd–H elimination does not occur for cationic
complexes, whereas it is the main decomposition pathway for neutral and anionic complexes. In
addition to the charge of the complex, the ease of this Pd–H elimination is determined by the trans
influence of the ligands (aryl > PMe3 > Br, N-donor).
to evaluate these differences in more detail, including in the series
Introduction
some additional complexes. The results are reported here. It turns
out to be the case, according to the results discussed here, that
the hydrogen atoms in the 4-position of the Pd-allyls become b-H
atoms in a r-allyl intermediate, and from now on we will refer to
the corresponding H-elimination as b-H elimination.
b-H elimination is a fundamental process in organometallic
chemistry, often involved in many metal catalyzed reactions.1
The Pd-catalyzed synthesis of dienes from allylic acetates under
mild conditions is a direct consequence of this process.2 For
palladium alkyls, b-H elimination competes with other target
transformations such as C–C coupling reactions. The barrier for
b-H elimination in a palladium agostic alkyl complex has been
experimentally determined, and it is low (7.1 kcal mol−1).3 This
suggests that the higher resistance to b-H elimination found in
many palladium organometallic complexes arises from additional
barriers to attain the required four-centered transition state shown
in Fig. 1a, such as lack of an available coordination site, or
geometrical constraints in rigid systems.1 This is expectedly the
case for palladium g -allylic complexes, which are less prone to
H elimination from position 4 than Pd-alkyls from the equivalent
b-position. It is reasonable to assume that the g -allylic complexes
should isomerize to r-allyl complexes in order to reach the
four-coordinated, four-centered transition state needed for H
elimination (Fig. 1b). However, not much experimental evidence
is available to confirm this reasonable assumption.
Results
3
The palladium 5-pentafluorophenyl-1-3-g -cyclohexenyl com-
plexes studied are shown in Scheme 1 (N–P = o-Ph2PC6H4-
CH2NMe2). These allyl compounds differ in the two ligands
completing the coordination sphere of palladium and, depending
on them, in the net charge of the complex. Complexes 1, 3 and
5 have been described before.4,5 The rest have been prepared
as shown in Scheme 1. All these complexes were isolated as
solids except 2, which was synthesized in situ by addition of the
stoichiometric amount of NBu4Br. They were spectroscopically-
characterized and the crystal structures of complexes 3 and 6 were
determined.‡
3
3
Fig. 1 Four-centered transition state for b-H elimination in an alkyl
complex (a) and in an allylic compound (b).
In the course of a study of Stille coupling involving allylic
substrates,4 we noted a quite different stability towards Pd–H
3
elimination in a range of palladium g -allylic complexes containing
the same allyl moiety. The importance of this process prompted us
IU CINQUIMA/Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad de
Valladolid, Prado de la Magdalena s/n, 47005, Valladolid, Spain. E-mail:
albeniz@qi.uva.es, espinet@qi.uva.es; Fax: + 34 983 423 013; Tel: +34
982 184 621
3
Scheme 1 Synthesis of 5-C6F5-1-3-g -cyclohexenyl palladium complexes.
† Electronic supplementary information (ESI) available: Full tables
containing decomposition data for complexes 1–7. Figures with 19F
and 1H NMR spectra for some decomposition mixtures. See DOI:
10.1039/b706817d
‡ CCDC reference numbers 646249 and 646250. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b706817d
3710 | Dalton Trans., 2007, 3710–3714
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Table 1 Selected distances and angles for complex 3
Table 3 Decomposition data for complexes 1–7a
% complex (% 8 + 9 + 10)
Bond
Distance/A
Bonds
Angle/◦
˚
Entry
Complex
Net charge
2 h
1 day
Pd1–C1
Pd1–C2
Pd1–C3
Pd1–N1
Pd1–N2
C1–C2
C2–C3
C3–C4
C1–C6
2.128(5)
2.092(5)
2.133(5)
2.109(5)
2.111(5)
1.405(8)
1.403(8)
1.503(7)
1.515(7)
C3–C2–C1
N1–Pd1–N2
C1–Pd1–C3
N1–Pd1–C1
N2–Pd1–C3
114.8(5)
98.55(19)
67.5(2)
97.6(2)
96.4(2)
1
2
3
4
5
6
7
3
4
1
6
7
5
2
+1
+1
0
0
0
—
—
—
—
100 (0)
100 (0)
100 (0)
42 (31)b
95 (5)
—
—
−1
0 (92)c
—
−1
79 (3)d
a Decomposition reactions were carried out using CDCl3 solutions of com-
plexes at 50 ◦C. Percentages of products were determined by integration of
signals in the 19F spectra; they correspond to the percentage of C6F5 in the
compound, which is equivalent to the mol% except for the dimeric complex
1. b Complex 1 was also formed (18%); the percentage shown (31%)
corresponds to CPDs. 8 –10 and 12; several unidentified compounds were
also formed (9% total). c Several Pf-containing unidentified compounds
were also formed (8% total). d Compound 11 was also formed (9%) and
several unidentified compounds (9% total).
Table 2 Selected distances and angles for complex 6
Bond
Distance/A
Bonds
Angle/◦
˚
Pd1–C1
Pd1–C2
Pd1–C3
Pd1–Br1
Pd1–P1
C1–C2
C2–C3
C3–C4
C1–C6
2.220(10)
2.113(10)
2.134(10)
2.4924(15)
2.292(3)
1.422(15)
1.380(14)
1.512(13)
1.525(13)
C3–C2–C1
P1–Pd1–Br1
C1–Pd1–C3
Br1–Pd1 C2
P1–Pd1–C1
Br1–Pd1–C3
117.2(9)
94.22(8)
66.6(4)
129.7(4)
166.6(3)
162.5(3)
give the monoene 9 (Scheme 2). This type of H-transfer has been
studied by us before.8
Selected distances and angles for compounds 3 and 6 are
collected in Tables 1 and 2. The corresponding ORTEP repre-
sentations are shown in Fig. 2. Both complexes show a pseudo-
boat conformation, as was already found in the X-ray crystal
structure for complex 56 and by NMR analysis in the case of 1;5
this conformation leaves the bulky pentafluorophenyl substituent
in a less-hindered equatorial arrangement. C–C bond lengths in
the allylic fragment are similar to the distances found in other
palladium cyclohexenyl complexes.4,5,6,7 Pd–C distances reflect the
different trans influences of the ligands bound trans to the allylic
carbon (see the Discussion section) and this is clearly shown for
˚
complex 6 (Pd1–C1 trans to PMe3, 2.220 A vs Pd1–C3 trans to Br,
˚
2.134 A).
The decomposition of the allylic complexes was studied at 50 ◦C
in CDCl3 as solvent, and the results are summarized in Table 3. The
decomposition, initiated by b-H elimination, produces the diene
8 and a palladium hydrido complex that will either decompose
to Pd(0), or transfer the hydrido moiety to an allylic complex to
Scheme 2 Decomposition routes for complexes 1–7.
−
Fig. 2 ORTEP (40% probability level) of complexes 3 (BF4 is not shown) and 6.
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In the presence of Pd(0) 8 undergoes disproportionation to 10
and 9.9 Thus, the sum of 8–10 (Table 3) gives the prevalence of b-H
elimination in the decomposition of the Pd-allyls. For most cases,
this is the only decomposition route for the complexes studied.
It is still the major decomposition route for 2 and 6, where a
different route is additionally observed. Thus, for complex 2 the
formation of 11 is also observed (eqn (1) and Table 3), which
can arise by two plausible pathways that can compete with each
other: reductive elimination and nucleophilic attack of free Br−
on the allyl moiety.10 In the decomposition of 6, isomerization
of 8 to 1-pentafluorophenyl-1,3-cyclohexadiene (12) is observed,
and a mixture of 9, 10 and 12 is obtained. This is probably
due to the presence of phosphine which stabilizes the palladium
hydrido species formed by b-H elimination sufficiently to act as
an isomerization catalyst in the reaction medium.8 Moreover,
for 3 and 4 and no effect of the type of ancillary ligand can be
observed. In contrast, the anionic complexes 2 and 5, with more
electron-rich Pd centers, decompose under the same conditions
(entries 6 and 7). Moreover, a clear difference in decomposition
rate is observed, showing that the decomposition of complex 5
(with two strongly r-donating, high trans-influence C-ligands) is
much faster than that of 2, containing two electronegative, low
trans influence Br ligands. The effect of the ancillary ligands is
also seen in the series of neutral complexes (entries 3–5). All of
them are fairly stable, but the decomposition rate follows the trend
6 > 7 > 1, which is also the trend of trans influence of the neutral
ligand PMe3 > py > Br–Pd.17 This effect is also noticeable when the
cationic complex 4 with a coordinated P–N donor ligand, much
◦
more stable than the neutral analogue 6, decomposes at 100 C
slightly faster than the neutral 1 bearing the least trans-influencing
ligand.
◦
complex 6 produces 1 (17% after one day at 50 C, see Table 3).
This is probably associated with loss of PMe3 which is transferred
to the Pd(0) species formed in the decomposition (phosphine
oxidation is observed in only negligible percentage, while new
PMe3 complexed species are observed). Since 1 is stable in the
conditions and reaction times used, the b-H elimination products
can be unequivocally attributed to the decomposition of 6.
In summary, these results support the idea that H-elimination in
g -allyl palladium complexes occurs via b-H elimination on r-allyl
3
palladium intermediates. These are more accessible in electron-
rich palladium centers (that is, anionic is better than neutral is
better than cationic) and for ligands with higher trans influence.
It is synthetically fortunate that the conditions favoring allylic
alkylation disfavor H-elimination, which otherwise could be an
undesired complication.18
(1)
Experimental
Complexes 1 and 4, stable at 50 ◦C, were subjected to
stronger reaction conditions to check their robustness. Solutions
in tetracloethane-D2 were heated to 100 ◦C and monitored. Both
complexes remain unchanged after 3.5 h at this temperature.
Prolonged heating at 100 ◦C for 1 day leads to total decomposition
of 4 to a mixture of 8–10 and 12. In the same period of time 50%
of 1 decomposes to a mixture of 8–10 and 11.
General considerations
1H and 19F NMR spectra were recorded on Bruker AC-300 and
ARX-300 instruments. Chemical shifts are reported in d (ppm)
and referenced to Me4Si (1H) or CFCl3 (19F). All of the NMR
spectra were recorded at 293 K unless otherwise noted. 1H NMR
assignments are given according to the numbering shown in
Fig. 3. Organic products were analyzed using a HP-5890 gas
chromatograph connected to a HP-5988 mass spectrometer at an
ionizing voltage of 70 eV using a quadrupole analyzer. Elemental
CHN analyses were determined with a Perkin-Elmer 2400 CHN
microanalyzer. IR spectra were recorded with a Perkin-Elmer FT
1720 X and Perkin-Elmer IR 883 spectrophotometers.
Discussion
3
Only a few examples have been reported where a b-H of an g -
allylic complex interacts with the metal in an agostic fashion
allowing b-H elimination to occur directly. These are Ir(III)11
and Mn(I),12 hexacoordinated complexes where the agostic C–H
bond occupies the sixth coordination site of a highly unsaturated
metallic fragment. A direct H elimination via a pentacoordinated
Pd(II) complex in our system should be strongly disfavored,13,14
and severely hindered due to conformational restrictions in the
3
palladium cyclohexenyl system. An g - to r-allyl isomerization,
Fig. 3 Numbering scheme for Pd-cyclohexenyl complexes.
which creates a vacant site on palladium and simultaneously
enables the geometrical rearrangement to the agostic intermediate
shown in Fig. 1b, seems a most plausible mechanism.15 As a matter
Solvents were dried over CaH2 and distilled prior to use.
AgBF4, NBu4Br and PMe3 were obtained commercially and
were used without further purification. The palladium com-
plexes 1,5 34 and 54 were prepared according to the literature.
Compounds 8, 9, 10 and 11 have been described before.4 The
ligand Ph2PC6H4CH2NMe2 was prepared following a published
procedure.19
3
of fact, ligand or solvent induced g - to r-allyl isomerization is a
well documented fluxional process in allyl complexes.16 In such an
event one should expect that the factors favoring decoordination
3
of a double bond from a g -allylic complex should also favor the
b-H elimination process leading to decomposition. In effect, this
seems to be observed in Table 3.
Both cationic complexes 3 and 4, less prone to ligand dissocia-
tion, are stable towards b-H elimination and no decomposition is
observed after 1 day (entries 1 and 2). The comparatively short Pd–
C distances in 3 are also consistent with a more strongly-bonded
allylic fragment (Table 1). Thus, the factor of charge is dominant
(NBu4)[PdBr2(5-C6F5-1-3-g3-C6H8)] (2)
An NMR tube was charged with 1 (0.0200 g, 0.023 mmol), NBu4Br
(0.0149 g, 0.046 mmol) and CDCl3 (0.5 mL). The mixture was
stirred, and the product obtained was analyzed by NMR. 19F
3712 | Dalton Trans., 2007, 3710–3714
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NMR (282 MHz, CDCl3): d −163.11 (m, Fmeta), −157.59 (t, Fpara),
(300 MHz, CDCl3): d 5.86 (q, J = 6.7 Hz, JH-P = 6.7 Hz, 1H, H1)*,
5.53 (t, J = 6.5 Hz, 1H, H2), 4.83 (dt, J = 6.6, JH-P = 3.3 Hz, 1H,
H3), 2.81 (tt, J = 10.8, 5.3 Hz, 1H, H5), 2.45 (m, 2H, H6,H6ꢀ ), 2.30
(m, 1H, H4), 2.03 (m, 1H, H4ꢀ ), 1.54 (d, J = 10.0 Hz, 9H, Me);
1
−141.63 (m, Fortho); H NMR (300 MHz, CDCl3): d 5.47 (t, J =
6.0 Hz, 1H, H2), 5.29 (t, J = 6.0 Hz, 2H, H1, H3), 3.35 (m, 8H,
NCH2 Bu), 2.72 (tt, J = 10.8, 6.0 Hz, 1H, H5), 2.36 (dd, J = 10.8,
6.0 Hz, 2H, H4, H6), 2.05 (dt, J = 18.0, 10.8 Hz, 2H, H4ꢀ , H6ꢀ ), 1.68
(m, 8H, CH2 Bu), 1.46 (q, 8H, CH2 Bu), 0.98 (t, 12H, CH3 Bu).
1
31P{ H} NMR (121 MHz, CDCl3): d −14.84 (s).
*: H3, trans to Br; H1, trans to PMe3
[Pd(5-C6F5-1-3-g3-C6H8)(j2-P,N-Ph2PC6H4CH2NMe2)](BF4) (4)
[PdBr(5-C6F5-1-3-g3-C6H8)(py)] (7)
To a solution of AgBF4 (0.0068 g, 0.035 mmol) in CH3CN (3 mL)
was added 1 (0.0150 g, 0.017 mmol), and the mixture was stirred for
10 min protected from light. The off-white suspension was filtered
and the filtrate was added to Ph2PC6H4CH2NMe2 (0.0110 g,
0.034 mmol). n-Hexane (5 mL) was added to the resulting solution
and a white solid appeared which was filtered, washed with n-
hexane and vacuum dried (0.0188 g, 71% yield). Anal. calcd for
C33H30NBF9PPd: C, 52.16; H, 3.98; N, 1.84. Found: C, 52.07; H,
4.00; N, 2.09. 19F NMR (282 MHz, CDCl3): d −161.97 (m, Fmeta),
−155.98 (t, Fpara), −152.94 (m, BF4), −142.83 (m, Fortho); 1H NMR
(300 MHz, CDCl3): d 7.2–7.8 (m, 14H, aromatic), 6.27 (t, J =
7.3 Hz, 1H, H2), 6.14 (q, J = 7.3 Hz, JH-P = 7.3 Hz, 1H, H1)*, 4.31
(dt, J = 7.3, JH-P = 3.7 Hz, 1H, H3), 3.65 (m, 2H, CH2), 2.97 (s, 3H,
Me), 2.85 (s, 3H, Meꢀ), 2.75 (m, 2H, H5, H6), 2.21 (m, 1H, H6ꢀ ),
To a suspension of 1 (0.0736 g, 0.085 mmol) in CH2Cl2 (3 mL) was
added pyridine (0.017 mL, 0.212 mmol). The yellow suspension
turned to a colorless solution immediately. The mixture was stirred
for 10 min and n-hexane (3 mL) was added. The solvent was slowly
evaporated until a white solid appeared. It was filtered, washed
with n-hexane and air dried (0.0630 g, 72% yield). Anal. calcd
for C17H13BrF5NPd: C, 39.83; H, 2.56; N, 2.73. Found: C, 39.89;
H, 2.68; N, 2.67. 19F NMR (282 MHz, CDCl3): d −162.26 (m,
Fmeta), −156.59 (t, Fpara), −141.85 (m, Fortho); 1H NMR (300 MHz,
CDCl3): d 8.82 (m, 2H, o-py), 7.80 (t, 1H, J = 7.5 Hz, p-py), 7.40
(m, 2H, m-py), 5.71 (bt, 1H, H2), 5.30 (m, 2H, H1, H3), 2.81 (tt,
J = 12.3, 6.5 Hz, 1H, H5), 2.45 (m, 2H, H6,H6ꢀ ), 2.35, 2.23 (b, 4H,
H4, H6).
ꢀ
1
1.85 (m, 1H, H4), 1.57 (m, 1H, H4 ); 31P{ H} NMR (121 MHz,
Decomposition experiments
CDCl3): d 20.4 (s).
*: H3, trans to N; H1, trans to P.
An NMR tube was charged with 0.023 mmol of the allylic complex
in 0.5 mL of CDCl3. The mixture was introduced in a bath
at 50 ◦C. The decomposition processes were monitored by 19F
NMR. The products obtained 8, 9, 10 and 11 have been described
before.4 The amount of total decomposition products is collected
in Table 3 and separately in the ESI.†
[PdBr(5-C6F5-1-3-g3-C6H8)(PMe3)] (6)
To a◦suspension of 1 (0.0800 g, 0.092 mmol) in CH2Cl2 (2 mL)
at 0 C was added a 1 M solution of PMe3 in THF (0.1846 mL,
0.184 mmol). The mixture was stirred and a clear solution was
obtained. n-Hexane (4 mL) was added and the mixture was slowly
evaporated, whereupon a light yellow solid appeared. The solid
was filtered, washed with n-hexane and vacuum dried (0.0633 g,
67% yield). Anal. calcd for C15H17BrF5PPd: C, 35.40; H, 3.36.
Found: C, 35.29; H, 3.27. 19F NMR (282 MHz, CDCl3): d
−162.24 (m, Fmeta), −156.69 (t, Fpara), −142.17 (m, Fortho); 1H NMR
1-C6F5-1,3 cyclohexadiene (12): 19F NMR (282 MHz, CDCl3):
1
d −163.27 (m, Fmeta), −157.68 (t, Fpara), −141.90 (m, Fortho); H
NMR (300 MHz, d, CDCl3): 6.20–5.90 (m, 3H, H2, H3, H4), 2.50–
2.30 (m, 4H, H5, H5ꢀ , H6, H6ꢀ ); MS(EI) m/z (relative intensity):
246 (M+, 100), 205 (16), 181 (48), 169 (7), 123 (6), 99 (4), 77 (10),
51 (3).
Table 4 Crystallographic data for complexes 3 and 6‡
3
6
Empirical formula
Formula weight
Temperature
C16H14BF9N2Pd
522.50
C15H17BrF5PPd
509.57
298(2) K
298(2)
˚
˚
Wavelength
0.71073 A
0.71073 A
Crystal system
Space group
Unit cell dimensions
Triclinic
P-1
Monoclinic
P2(1)/c
˚
˚
˚
˚
˚
˚
a = 7.0298(7) A; b = 10.7158(11) A; c = 14.6871(15) A
a = 7.1018(18) A; b = 21.654(5) A; c = 12.220(3) A
a = 69.514(2)◦; b = 82.715(2)◦; c = 71.202(2)◦
a = c = 90◦; b = 102.445(5)◦
3
3
˚
˚
Volume
981.00(17) A
1835.1(8) A
Z
2
4
Density (calculated)
Absorption coefficient
No. of reflections measured
Scan range
1.769 g cm−3
1.844 g cm−3
1.032 mm−11
3.315 mm−1
4551
8553
1.48 < h < 23.25◦
1.88 > h > 23.29◦
No. of unique reflections
No. of reflections [I≥2r(I)]
No. of parameters
Final R indices [I > 2r(I)]
R indices (all data)
Dqmax/min
2799
2578
264
2643
1509
211
R1 = 0.0438, wR2 = 0.1249
R1 = 0.0463, wR2 = 0.1278
0.952, −0.573
R1 = 0.0576, wR2 = 0.1385
R1 = 0.1052, wR2 = 0.1612
1.432, −0.776
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X-Ray crystal structure determinations‡
Nicholas, Organometallics, 1993, 12, 3485–3494; (e) R. C. Larock, H.
Song, S. Kim and R. A. Jacobson, J. Chem. Soc., Chem. Commun.,
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8 A. C. Albe´niz, P. Espinet and Y.-S. Lin, Organometallics, 1997, 16,
4030–4032.
9 This heterogeneous Pd-catalyzed H-transfer has been known for a long
time: (a) N. D. Zelinsky and G. S. Pawlow, Ber. Dtsch. Chem. Ges.,
1933, 66, 1420–1423; (b) R. P. Linstead, E. A. Braude, P. W. D. Mitchell,
K. R. H. Wooldridge and L. M. Jackman, Nature, 1952, 169, 100–103;
(c) S. Carra, P. Beltrame and V. Ragaini, J. Catal., 1964, 3, 353–362.
10 H. Kurosawa, S. Ogoshi, Y. Kawasaki, S. Murai, M. Miyoshi and I.
Ikeda, J. Am. Chem.Soc., 1990, 112, 2813–2814.
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Soc., Chem. Commun., 1981, 506–507.
12 (a) M. Brookhart, W. Lamanna and M. B. Humphey, J. Am. Chem.
Soc., 1982, 104, 2117–2126; (b) F. Timmers and M. Brookhart,
Organometallics, 1985, 4, 1365–1371.
Crystals suitable for X-ray analysis were obtained by slow diffusion
of Et2O to a solution of 3 in CH2Cl2 at −20 ◦C and by cooling a so-
lution of 6 in a mixture of CH2Cl2–n-hexane at −20 ◦C. Crystals of
dimensions 0.10 × 0.20 × 0.25 mm (3) or 0.05 × 0.07 × 0.19 mm (6)
were mounted on the tip of glass fibers. X-Ray measurements were
made using a Bruker SMART CCD area-detector diffractometer.
Reflections were collected, intensities integrated, and the structure
was solved by direct methods procedure.20 Non-hydrogen atoms
were refined anisotropically and hydrogen atoms were constrained
to ideal geometries and refined with fixed isotropic displacement
parameters. Relevant crystallographic data are collected in Table 4.
13 (a) D. L. Thorn and R. Hoffman, J. Am. Chem. Soc., 1978, 100, 2079–
2090; (b) A. Dedieu, Chem. Rev., 2000, 100, 543–600.
Acknowledgements
14 A pentacoordinated intermediate has been proposed to explain the b-
H elimination in [PdEt2(PR3)2] to give ethane and ethene, where the
addition of phosphine has little effect on the reaction rate: F. Ozawa,
T. Ito and A. Yamamoto, J. Am. Chem. Soc., 1980, 102, 6457–6463.
15 Other less-common mechanisms have been reported for b-H elimina-
tion in palladium allyls, such as direct deprotonation of a b-H in the
presence of a base (see for example: (a) I. Schwarz and M. Braun,
Chem.–Eur. J., 1999, 5, 2300–2305; (b) J. M. Takacs, E. C. Lawson
and F. Clement, J. Am. Chem. Soc., 1997, 119, 5956–5957; (c) P. G.
Andersson and S. Schab, Organometallics, 1995, 14, 1–2; (d) J. M.
Takacs, F. Clement, J. Zhu, S. V. Chandramouli and X. Gong, J. Am.
Chem. Soc., 1997, 119, 5804–5817; (e) T. Hayashi, K. Kishi and Y.
Uozumi, Tetrahedron: Asymmetry, 1991, 2, 195–198) or, in the case of
a bis-allylic complex, a concerted 6-membered cyclic mechanism that
does not lead to a metal hydride: E. Keinan, S. Kumar, V. Dangur and
J. Vaya, J. Am. Chem. Soc., 1994, 116, 11151–11152. Only the first of
these two routes could have any relevance here, but the absence of the
bases that have been shown to effect the deprotonation (carboxylates,
mainly acetate), makes this mechanism unlikely.
Financial support from the Spanish MEC (DGI, Grant
CTQ2004–07667; Consolider Ingenio 2010, Grant CSD2006–
0003), and the Junta de Castilla y Leo´n (Project VA117A06) is
gratefully acknowledged.
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20 Data analysis and drawing were performed with the programs:
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3714 | Dalton Trans., 2007, 3710–3714
This journal is
The Royal Society of Chemistry 2007
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