A significant effect of anion binding ureas on the product ratio in the palladium(II)-catalyzed hydrocarbonylation of alkenes

Article abstract of DOI:10.1039/a806722h

Hydrogen bonding of urea derivatives to the anionic ligands X of (dppp)PdX₂ catalysts significantly increases the hydroacylation of cyclopentene relative to the hydroformylation, most probably due to a decreased coordination strength of the anionic ligands.

Full text of DOI:10.1039/a806722h

A significant effect of anion binding ureas on the product ratio in the
palladium(II)-catalyzed hydrocarbonylation of alkenes
Jan Scheele, Peter Timmerman and David N. Reinhoudt*
Laboratory of Supramolecular Chemistry and Technology, University of Twente, PO Box 217, 7500 AE Enschede,
The Netherlands. E-mail: smct@ct.utwente.nl
Received (in Cambridge, UK) 27th August 1998, Accepted 21st October 1998
Hydrogen bonding of urea derivatives to the anionic ligands
X of (dppp)PdX catalysts significantly increases the hydro-
acylation of cyclopentene relative to the hydroformylation,
most probably due to a decreased coordination strength of
the anionic ligands.
monitored by the turnover number of CO (TONCO), i.e. the
number of CO insertions per Pd center. Intermediate 10 can
react further in two different ways, either giving cyclopentane-
carbaldehyde 11 or yielding one of the ketones 13–15 after
insertion of a second molecule of cyclopentene. In all cases 8 is
regenerated either by b-elimination or by oxidative addition of
2
0
Transition metal complexes are important homogeneous cata-
HX to the Pd complex formed.
1
lysts for alkene polymerization and alkene/CO copolymeriza-
The selectivity for ketones increases from 14 to 98%
reflecting the decrease in coordination strength of the anionic
2
tion reactions. In the L
2
PdX
2
-catalyzed hydrocarbonylation of
) either aldehydes (hydro-
alkenes with synthesis gas (CO/H
2
ligands X in the series of (dppp)PdX
2
(X = TFA, OMs, OTs,
2
formylation), ketones (hydroacylation), or polyketones (copoly-
merization) are formed. The type of reaction is determined
mainly by the coordination strength of the anionic ligands X in
OTf, entries 1–4 in Table 1) with 6.0 3 10 < TONCO < 9.2 3
2
21 21
10 mol (mol Pd)
2
h . With (dppp)Pd(OAc) (entry 5) the
2
21 21
TONCO is reduced to 0.2 3 10 mol (mol Pd)
h
because of
3
II
L
(
2
PdX
e.g. X = TFA) show sufficient activity in such hydro-
carbonylation reactions. These catalysts are prepared by anion
metathesis reactions of L PdX with the corresponding silver
2
. Only Pd catalysts with weakly coordinating anions
the stronger coordinating acetate, and the selectivity for ketones
is 24%. Weaker coordinating anions may enhance the (intrinsic)
II
electrophilicity of the Pd center and, of course, these anions are
2
2
more easily displaced from the (fourth) coordination site which
4
5
salt (X = Cl) or Brønsted acid (X = OAc). Alternatively,
strong Lewis acids like methylalumoxane (MAO) or SnCl
facilitates the formation of intermediates 10 (increased TONCO
)
6
7
2
and 12 (increased selectivity for ketones).
are added.
We found that in the presence of 0.6 mol% [7.5 equiv. with
Our group has developed a variety of anion receptors based
2
respect to the (dppp)Pd(TFA) catalyst] of N,NA-diphenylurea 1
8
on multiple hydrogen bonding to (sulfon)amides or (thio)ur-
the selectivity for ketones increases from 14 to 25% (entry 6 in
9
10
2
eas, or coordination to a Lewis acidic uranyl center. These
anion receptors have been applied for anion-selective sensors
Table 1), whereas the TONCO increases from 6.0 3 10 to 7.8
2
21
h21.‡ With 0.6 mol% of the urea
3 10 mol (mol Pd)
(
CHEMFETs)11 and in membrane transport studies. Recently,
derivatives 2 and 3, containing either one or two electron-
we have described the catalytic activity of anion receptors in
withdrawing substituents at the phenyl rings that will increase
the anion affinity of the urea moiety, the selectivity for
acyl transfer reactions.12
17
Here we show that N,NA-disubstituted urea derivatives 1–5
ketones shows a sharp increase from 14 to 49 and 61%,
respectively (entries 7 and 8). In both cases the TONCO is
similar to that in the presence of urea 1. Both N-butyl-NA-
phenylurea 4 and N,NA-dibutylurea 5 do not significantly change
the selectivity for ketones (entries 9 and 10), which is in
accordance with the much lower acidity and anion binding
strength of (di)alkyl ureas compared to diaryl ureas.
The altered selectivities of the catalyst upon addition of
diarylureas 1 or 3 were also observed for the catalysts
O
O
R1
R2
Ph
Ph
N
N
N
N
H
H
1
= R2 = Ph
18
1
2
3
4
5
R
R
R
R
R
6
1
1
1
1
2
= p-F3CC6H4, R = Ph
= R2 = p-F3CC6H4
n
2
= Bu , R = Ph
= R2 = Bun
O
P
H2
Pd
significantly influence the performance of the (dppp)PdX
2
O
P
X
1
0
catalyst [dppp = 1,3-bis(diphenylphosphino)propane] in hy-
drocarbonylation reactions. The effect is attributed to the
interaction13 of the acidic urea protons with the counterions (X
H
CO
11
=
OAc, TFA, OTs) leading to a decrease in their coordination
P
P
P
P
X
X
P
P
H
X
P
P
strength. There are reports of hydrogen bonding to the anionic
ligands of transition metal complexes in the solid state.14 To the
best of our knowledge this is the first report in which hydrogen
bonding to the anionic ligands of a homogeneous catalyst alters
the product ratio of the reaction.15
+H2, –HX
Pd
Pd
Pd
Pd
X
X
O
2
1
7
8
9
O
H2
O
II
As a model reaction we used the Pd -catalyzed hydro-
carbonylation of cyclopentene with synthesis gas in anisole.†
The mechanism for this reaction is depicted in Scheme 1.
1
3
3
O
Hydride 8 is formed by reaction of precatalyst (dppp)PdX
2
with
H
2
. The rate of the subsequent exchange reaction of cyclo-
1
pentene for X depends strongly on the coordination strength of
the counterion to the Pd center.4,16 Migratory insertion gives the
s-alkyl–Pd complex 9 and consecutive CO insertion yields the
acyl–Pd intermediate 10. The formation of 10 from 8 is
14
8
Scheme 1 Catalytic cycle for the PdII-catalyzed hydrocarbonylation of
cyclopentene.
Chem. Commun., 1998, 2613–2614
2613

Table 1 Selectivity for ketones and turnover number for the hydro-
‡ The amount of added N,NA-diphenylurea 1 correlates well with the
carbonylation of cyclopentene with CO and H
2
in the presence of urea
selectivity for ketones formed in the reaction and the TONCO. With varying
a
derivatives 1–6
amounts (2–18 equiv.) of 1 as cocatalyst in the (dppp)Pd(TFA)
2
catalyzed
reaction both the selectivity for ketones and the TONCO are increased. A
2
2
21 21
Selectivity
(%)c
TONCO/10 mol
maximum of 37% and 8.9 3 10 mol (mol Pd)
h
was reached with 1.5
Entry
Anion
Receptorb
(mol Pd)
21
h
21 d
mol% (18 equiv.) of 1 (limited by the solubility of 1 in the reaction
medium).
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
OTf
—
—
—
—
—
1
2
3
4
5
1
3
1
3
6
6
6
6
98
54
41
14
24
25
49
61
16
10
45
80
82
95
14
12
25
51
8.7
8.2
9.2
6.0
0.2
7.8
7.8
8.3
5.9
5.1
0.4
0.4
7.0
10
§ Additional evidence for hydrogen bond formation of 1–5 to the anionic
1
OTs
OMs
TFA
OAc
TFA
TFA
TFA
TFA
TFA
OAc
OAc
OTs
OTs
TFA
TFA
OAc
OTs
ligands X of (dppe)PdX
2
(X = Cl, TFA, OTs) was obtained by IR, H and
3
1P NMR spectroscopic studies in CDCl
3
at room temperature (ref. 19).
solution of N,NA-diphenylurea
1 (free N–H vibration at 3422 cm ) gave rise to an additional N–H stretch
Addition of 2 equiv. (dppe)PdCl to an 1 m
2
M
2
1
2
1
1
frequency at 3330 cm in the FT–IR spectrum. The H NMR spectra of
ureas 1–5 show in all cases downfield shifts (0.40 > Dd > 0.15 ppm) for
2
the urea proton signals upon addition of 1 equiv. of (dppp)PdX , which is
indicative for hydrogen bond formation. Furthermore the 31P NMR
1
1
1
1
1
1
1
1
1
a
2
resonances of the (dppe)PdX complexes shift over 1 ppm downfield upon
addition of 2 equiv. of 1. Similar downfield shifting of the 31P NMR
resonances is also observed upon weakening of the coordination strength of
2
the anions of (dppe)PdX (X = TFA: d 63.1; X = OTs: d 69.9). In contrast
to this the addition of 1,3-dimethyl-1,3-diphenylurea 6 to the Pd complexes
did not induce any significant shift of the 31P NMR resonances.
e
6.4
5.8
0.3
9.6
f
¶ The TONs based on conversion of cyclopentene (TON
=
).can easily be
calculated from Table 1 according to TON
selectivity).
=
= TONCO 3 (1 +
Cyclopentene (5 ml), anisole (10 ml), (dppp)PdX
2
(0.08 mol%), 110
°
C, 80 bar (CO:H = 1:1), analysis by GC FID, integrals were not corrected
2
b
1
2
3
H. H. Brintzinger, D. Fisher, R. Mülhaupt, B. Rieger and R. M.
Waymouth, Angew. Chem., Int. Ed. Engl., 1995, 34, 1143; P. Margl, L.
Deng and T. Ziegler, Organometallics, 1998, 17, 933.
E. Drent, Eur. Pat. Appl. 121,965 A2, 1984; E. Drent, J. A. M. van
Broekhoven and M. J. Doyle, J. Organomet. Chem., 1991, 417, 235; A.
Sen, Adv. Polym. Sci., 1986, 73/74, 125.
E. Drent and P. H. M. Budzelaar, Chem. Rev., 1996, 96, 663; C. Pisano
and G. Consiglio, Gazz. Chim. Ital., 1994, 124, 393.
c
for sensitivities. 7.5 equiv. cocatalyst compared to Pd catalyst. Percen-
tage of hydroacylation products (13–15) of the total amount of products
formed, accuracy ±2%. Turnover number of CO determined as the sum of
TONs of all products 11, 13, 14 and 15; accuracy ±5% (see note ¶). 10
d
e
f
equiv. cocatalyst 5. 13 equiv. cocatalyst 6.
(
dppp)Pd(OAc)
2
(entries 11 and 12) and (dppp)Pd(OTs)
2
4 A. Yamamoto, J. Organomet. Chem., 1995, 500, 337.
5 E. Drent, Pure Appl. Chem., 1990, 62, 661.
(
entries 13 and 14). In both cases the stronger anion binding
6
7
8
L. K. Johnson, C. M. Killian and M. Brookhart, J. Am. Chem. Soc.,
995, 117, 6415.
F. Agbossou, J. F. Carpentier and A. Mortreux, Chem. Rev., 1995, 95,
485.
urea 3 causes the largest change in the selectivity for ketones,
i.e. from 24 to 80% for (dppp)Pd(OAc) and from 54 to 95% for
dppp)Pd(OTs) . The TONCO is enhanced from 0.2 3 10 to 0.4
1
2
2
(
3
2
2
2
2
2
21 21
10 and from 8.2 3 10 to 10 3 10 mol (mol Pd)
h ,
S. Valiyaveettil, J. F. J. Engbersen, W. Verboom and D. N. Reinhoudt,
respectively. These results suggest that the observed increase in
ketone formation is the result of complexation of the anionic
ligands by the urea derivatives 1–3 via hydrogen bonding which
decreases the coordination strength of the counterions to the Pd
center.
Experiments carried out in the presence of a large excess of
tetrasubstituted urea 6, which is unable to bind anions via
hydrogen bonding, show that neither the selectivity for ketones
nor the TONCO is affected to a significant extent (entries 15–18
in Table 1). This excludes the possibility that the observed effect
is due to coordination of the urea carbonyl to the Pd center or to
a change in the polarity of the reaction medium.§
Our results show that hydrogen bond formation to the anionic
ligands X of (dppp)Pd catalysts can significantly change the
selectivity of the catalyst in the hydrocarbonylation of cyclo-
pentene with synthesis gas. Addition of N,NA-diarylureas 1–3
strongly favours hydroacylation with respect to hydroformyla-
tion. The maximum effect is observed with the stronger anion
binding urea 3.
We thank the Shell Research & Technology Centre, Am-
sterdam, for financial support and Professor Dr E. Drent and Dr
W. P. Mul for helpful discussions.
Angew. Chem., Int. Ed. Engl., 1993, 32, 900.
9 J. Scheerder, J. P. M. van Duynhoven, J. F. J. Engbersen and D. N.
Reinhoudt, Angew. Chem., Int. Ed. Engl., 1996, 35, 1090.
10 D. M. Rudkevich, Z. Brzozka, M. J. Palys, W. P. R. V. Stauthamer, G. J.
van Hummel, S. M. Franken, S. Harkema, J. F. J. Engbersen and D. N.
Reinhoudt, J. Am. Chem. Soc., 1994, 116, 4341.
1 M. M. G. Antonisse, B. H. M. Snellink-Ruël, J. F. J. Engbersen and
D. N. Reinhoudt, J. Chem Soc., Perkin Trans. 2, 1998, 773.
1
1
2 V. van Axel Castelli, R. Cacciapaglia, G. Chiosis, F. C. J. M. van
Veggel, L. Mandolini and D. N. Reinhoudt, Inorg. Chim. Acta, 1996,
2
46, 1.
13 Ureas are known to bind to a variety of (delocalized) anions via
hydrogen bonding. E. Fan, S. A. Van Arman, S. Kincaid and A. D.
Hamilton, J. Am. Chem. Soc., 1993, 115, 369.
4 van Koten, Angew. Chem., Int. Ed. Engl., 1996, 35, 1959;K. Biradha and
G. R. Desiraju, Organometallics, 1996, 15, 1284; E. S. Shubina, N. V.
Belkova and L. M. Epstein, J. Organomet. Chem. 1997, 536–537, 17; F.
Grepioni, G. Cojazzi, S. M. Draper, N. Scully and D. Braga,
Organometallics, 1998, 17, 296.
5 The influence of thiourea on the palladium catalyzed reaction of
terminal alkynes has been reported. However, this effect is attributed to
coordination of the thiocarbonyl moiety to the Pd center and not to
hydrogen bonding. B. Gabriele, G. Salerno, M. Costa and G. P.
Chiusoli, J. Organomet. Chem. 1995, 503, 21.
1
1
1
6 G. P. C. M. Dekker, C. J. Elsevier, K. Vrieze, P. W. N. M. van Leeuwen
and C. F. Roobeek, J. Organomet. Chem., 1992, 430, 357.
Notes and references
17 C. S. Wilcox, E. Kim, D. Romano, L. H. Kuo, A. L. Burt and D. P.
Curran, Tetrahedron, 1995, 51, 621.
†
Experimental procedure: Hydrocarbonylation experiments were per-
formed in a 100 ml autoclave at 110 °C. 10 ml anisole, 5 ml cyclopentene,
.08 mol% of (dppp)PdX catalyst, and urea cocatalyst were brought under
a H atmosphere whereafter the autoclave was pressurized with 40 bar CO
and 40 bar H . After a reaction time of 20 h the autoclave was cooled down
1
1
8 S. Nishizawa, P. Bühlmann, K. P. Xiao and Y. Umezawa, Anal. Chim.
Acta, 1998, 358, 35.
9 Chloroform has approximately the same solvation strength as anisole.
Y. Marcus, Ion Solvation, Wiley, New York, 1985.
0
2
2
2
and the gas (pressure drop < 15 bar) was vented off. The products were
analysed by GC FID (CPSIL-5, 50 m).
Communication 8/06722H
2614
Chem. Commun., 1998, 2613–2614