Phosphido pincer complexes of palladium as new efficient catalysts for allylation of aldehydes

Article abstract of DOI:10.1021/om8008537

Palladium complexes supported by tridentate phosphido diphosphine ligands (P(o-C₆H₄PR₂)₂: 1, R = ⁱPr; 2, R = Ph) have been synthesized, characterized, and tested as catalysts for the electrophilic all

Full text of DOI:10.1021/om8008537

Organometallics 2008, 27, 5741–5743
5741
Phosphido Pincer Complexes of Palladium as New Efficient
Catalysts for Allylation of Aldehydes
Mina Mazzeo,*,† Marina Lamberti,† Antonio Massa,† Arrigo Scettri,† Claudio Pellecchia,†
and Jonas C. Peters‡
Dipartimento di Chimica, UniVersita` di Salerno, Via Ponte don Melillo, I-84084 Fisciano, Salerno, Italy,
and DiVision of Chemistry and Chemical Engineering, Arnold and Mabel Beckman Laboratories of
Chemical Synthesis, California Institute of Technology, Pasadena, California 91125
ReceiVed September 3, 2008
In 2003, Szabo`9 and co-workers achieved an interesting goal
demonstrating that pincer-based palladium complexes can be
highly active and robust catalysts for the catalytic allylation of
electrophiles with allylstannanes, showing several benefits
compared to generally used bis(allyl)palladium intermediates.
First of all, tridentate pincer ligands, occupying three of the
four coordination sites of a square-planar PdII complex, impose
an η1 coordination mode of the allyl fragment; therefore,
the catalytic activities are largely restricted to this single free
site on palladium. Moreover, the direct aryl-metal bonding,
providing an electron-rich palladium atom, assures a nucleophilic
character to the allyl group. The fact that this electron-supplying
part is forced to a trans position with respect to the allyl ligand
prevents Stille coupling between the pincer ligand and the allyl
moiety. Finally, a firm palladium-ligand bonding avoids
undesired dynamic processes and ligand exchange, ensuring a
high stability of the catalyst.10
Subsequent to the Szabo` finding, several pincer-based pal-
ladium complexes were tested in the catalytic allylation of
electrophiles with allylstannanes. In particular, while palladium
NCN-pincer complexes exhibited low catalytic activity, PCP-
pincer (with phosphine,10 phosphinite,10 and phosphite11 ancil-
lary ligands), SeCSe-pincer,12 and SCS-pincer13 complexes
proved to be much more efficient.
Although the electronic nature of the central ligand (trans to
the allyl moiety) plays a crucial role in the reactivity of these
(η1-allyl)palladium complexes, studies concerning the effects
of the presence of a different anionic central donor in the ligand
framework are definitively less numerous.13-15
Summary: Palladium complexes supported by tridentate phos-
phido diphosphine ligands (P(o-C6H4PR2)2: 1, R ) iPr; 2, R )
Ph) haVe been synthesized, characterized, and tested as catalysts
for the electrophilic allylation of aldehydes. The palladium
complex 2 resulted in an interesting catalyst for electrophilic
allylation in the presence of allyltributyltin, giVing good yields
under Very mild reaction conditions and eVen in the absence of
the solVent.
Palladium-catalyzed allylation is a reliable and widely used
method for the selective formation of new C-C, C-N, and
C-O bonds.1-4 In particular, it offers a versatile tool for the
introduction of an allylic moiety into both nucleophilic5 and
electrophilic6 substrates. Although allylic alkylation of nuclo-
philes is a very useful and well-established method, application
of electrophiles6,7 still receives considerable current attention.
Yamamoto and co-workers6a,8 first reported allylation of
aldehydes and imines with allyltributyltin catalyzed by a (η3-
allyl)(η1-allyl)palladium intermediate which serves as the nu-
cleophilic allyl transfer species. A considerable synthetic
limitation to the employment of this system is the difficult
control of the regioselectivity when the two allyl moieties bear
different substituents. A further problem is that the bis(allyl)pal-
ladium complexes may undergo allyl-allyl coupling prior to
the reaction with electrophiles.
* To whom correspondence should be addressed. E-mail: mmazzeo@
unisa.it.
† Universita` di Salerno.
‡ California Institute of Technology. Current address: Department of
Chemistry, Massachusetts Institute of Technology, Cambridge, Mas-
sachusetts 02139.
Recently the Peters group developed new diarylphosphido
phosphine (PPP) ligands16 that represent good candidates as
ligands for palladium-catalyzed electrophilic allylation of al-
dehydes in order to investigate the effects of the presence of a
different anionic donor atom on the catalytic behavior of the
metallic center. The incorporation of a strongly electron donating
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10.1021/om8008537 CCC: $40.75
2008 American Chemical Society
Publication on Web 10/22/2008

5742 Organometallics, Vol. 27, No. 22, 2008
Communications
Scheme 1. Synthesis of Complexes 1 and 2
and trans-labilizing donor group as the phosphido phosphorus
atom may promote the formation of coordinatively unsaturated
complexes that may exhibit different reactivity properties.
In this paper we report the synthesis and characterization of
two new PPP-based palladium complexes, bearing ligands with
different substituent groups on the side arms, and their use as
catalysts for the electrophilic allylation of aldehydes in the
presence of allyltributyltin.
Figure 1. ORTEP representation of the structure of complex 1.
Hydrogens have been omitted for clarity. Selected bond distances
(Å) and angles (deg): Pd(1)-P(1), 2.2533(9); Pd(1)-P(2),
2.2894(8); Pd(1)-P(3), 2.3146(8); Pd(1)-Cl(1), 2.4122(10);
P(1)-Pd(1)-P(2), 85.06(3); P(1)-Pd(1)-P(3), 84.17(3); P(2)-
Pd(1)-P(3), 164.429(19); P(1)-Pd(1)-Cl(1), 170.842(19);
P(2)-Pd(1)-Cl(1), 93.84(3); P(3)-Pd(1)-Cl(1), 98.55(3).
Synthesis and Characterization of Palladium Com-
plexes. The (R-PPP)H ligands (L1 and L2; see Scheme 1) were
synthesized according to a procedure previously reported in the
literature.16,17
Also for complex 2, the solution NMR data confirm the
square-planar geometry reminiscent of the analogous (Ph-
PCP)-21 and (Ph-PNP)-palladium22 complexes.
The solid-state structure of 1 was determined by an X-ray
diffraction study. The molecular structure of complex 1 and
relevant structural data are given in Figure 1.
The geometry of the palladium center is approximately square
planar, with the two neutral phosphorus atoms in a distorted
trans configuration with a P2-Pd-P3 angle of 164.4° (see
Figure 1). The environment about the phosphido phosphorus
atom is pyramidal with a stereochemically active lone pair. The
Pd-Cl distance (2.4122(9) Å) is slightly longer than that
observed in the (iPr-PNP)PdCl complex (2.3157(11) Å)23 and
similar to that observed in (tBu-PCP)PdCl (2.3969(6) Å),24
suggesting that the trans influence of the phosphido donor is
somewhat higher than that of the amido ligand and comparable
to that of an anionic aryl carbon.
The reactions between the ligands L1 and L2 and (COD)-
PdCl2 in THF in the presence of NEtiPr2 at 50 °C produce
orange solutions of desired complexes in high yield (see scheme
1). Solution NMR studies on the reaction performed in C6D6
indicate that the cyclooctadiene (COD) ligand is easily displaced
from (COD)PdCl2 by the ligands L1 and L2 at room temperature
1
upon mixing to form a solution of each complex. 31P and H
NMR spectra show that no intermediate species containing the
P-H functionality are initially formed, also in the absence of
the amine base, in agreement with the strong tendency of the
ligand to bind to the metal as an anionic (upon less of proton)
meridional PPP ligand. The NMR data are consistent with a
square-planar geometry for complex 1, where L1 is in a
meridional coordination mode, as evidenced by the multiplicity
of the resonances observed for the o-phenylene carbons in the
13C NMR spectrum.18
To test the activity of complexes 1 and 2, the allylation of a
number of representative aldehydes with allyltributyltin were
examined in DMF and in the absence of solvent under different
reaction conditions (Table 1, Scheme 2). Complex 2 showed
higher activity and gave higher yields in the presence of
benzaldehyde (entries 1-5). As expected, electron-poor alde-
hydes reacted more quickly with higher yields with both 1 and
2 under milder conditions at room temperature and with lower
catalyst loading (entries 7-9). In the case of o-NO2-benzalde-
hyde (entry 9), the presence of the nitro group in an ortho
position on the aromatic ring did not affect the reactivity of the
aldehyde. In contrast, the electronically deactivated anisaldehyde
was poorly reactive in the presence of 2 (entries 10 and 11)
and it was completely unreactive in the presence of 1 (entry
12). It is very interesting to note that, in the absence of solvent,
In the 13C NMR spectrum four resonances are observed for
four nonequivalent CH carbons of the isopropyl groups,
suggesting a low symmetry of the complex in solution and a
scarce flexibility of the chelate backbone.
In the 31P NMR spectrum, the two resonances corresponding
to the neutral and anionic phosphorus donors are strongly shifted
downfield from those of the corresponding ligand precursor.
The chemical shift (58.41 ppm) related to the neutral phosphorus
atoms is typical of square-planar Pd(II) complexes with trans-
phosphine; comparable values are reported for analogous (iPr-
PNP)-19 and (iPr-PCP)-palladium20 complexes.
Complex 1 is stable in solution; no decomposition was
detected by NMR after heating in C6D6 (80 °C, 24 h).
(17) Whited, M. T.; Rivard, E.; Peters, J. C. Chem. Commun. 2006,
1613.
(18) Crabtree, R. H. The Organometallic of The Transition Metals, 4th
ed.; Wiley-Interscience: Hoboken, NJ, 2005; p 276.
(19) Fan, L.; Yang, L.; Guo, C.; Foxman, B. M.; Ozerov, O. V.
Organometallics 2004, 23, 4778.
(21) Gorla, F.; Venanzi, L. M.; Albinati, A. Organometallics 1994, 13,
43.
(22) Huang, M.-H.; Liang, L.-C. Organometallics 2004, 23, 2813.
(23) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004, 23,
326.
(20) Frech, C. M.; Shimon, L. J. W.; Milstein, D. Angew. Chem., Int.
Ed. 2005, 44, 1709.
(24) Kimmich, B. F. M.; Marshall, W. J.; Fagan, P. J.; Hauptman, E.;
Bullock, R. M. Inorg. Chim. Acta 2002, 330, 52.

Communications
Organometallics, Vol. 27, No. 22, 2008 5743
Table 1. Allylation of Aldehydes Catalyzed by Complexes 1 and 2
Interestingly, under these mild reaction conditions, complex
2 was also able to catalyze the allylation of aliphatic aldehydes
in good yields (entries 13 and 14). However, in the case of
citronellal (entry 14), an aliphatic aldehyde with a chiral center
on the ꢀ-carbon, the diastereoselectivity was low. In all cases
complex 2 showed a higher catalytic activity than complex 1.
This could be attributable to the fact that the transmetalation
reactions with allylstannane are faster for less bulky and more
electron poor palladium systems.25
amt of
entry
no.
aldehyde
(R)
aldehyde
concn (M) cat.
cat.
T
yield
(%)a
(%) (°C)/t (h)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
p-NO2C6H4
p-NO2C6H4
o-NO2C6H4
p-OMeC6H4
p-OMeC6H4
p-OMeC6H4
PhCH2CH2
0.17
0.17
neat
neat
0.43
neat
0.43
0.43
0.43
neat
0.43
neat
neat
0.43
2
1
2
1
5
5
2.5
2.5
2.5
50/23
50/23
75
50
72
20
41
RTb/23
RT/23
RT/23
RT/23
RT/18
RT/18
RT/18
RT/23
50/23
2
no cat.
2
1
2
2
2
1
2
2
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
99
93
99
43
39
In summary, we have shown that the new (R-PPP)-Pd
chloride complexes 1 and 2 are efficient catalysts for the
allylation of aldehydes with allybutyltin. The activity of complex
2 is higher than those reported in the literature for palladium
pincer complexes that feature a chloride ligand. It is worth noting
that catalyst 2 requires very mild reaction conditions to achieve
good yields and, interestingly, it can be also used in the absence
of solvent.
RT/23
RT/23
no reacn
61
citronellal
RT/23 59(60/40)c
a All of the yields refer to isolated chromatographically pure
compounds. b RT: room temperature. c Values in parentheses refer to
syn/anti diastereoisomeric ratios that were determined by 1H NMR
analysis performed on the crude products.
Acknowledgment. We thank Matt T. Whited for as-
sistance with crystallographic studies.
Scheme 2. Catalytic Allylation Reaction of Aldehydes
Supporting Information Available: Text, figures, and tables
giving experimental procedures for the synthesis and characteriza-
tion data of compounds 1 and 2 and X-ray structural solution and
crystal data for 1; X-ray data for 1 are also given as a CIF file.
This material is available free of charge via the Internet at
http://pubs.acs.org.
2.5 mol % of 2 was able to achieve comparable yield with
benzaldehyde even at room temperature (entry 3), while the
control experiment without catalyst gave no product (entry 6).
The absence of solvent allowed us to omit the isolation
procedure, and the reaction mixture was directly purified by
column chromatography.
OM8008537
(25) Johansson, R.; Wendt, F. O. Dalton Trans. 2007, 488.