Xenon difluoride induced aryl iodide reductive elimination: A simple access to difluoropalladium(II) complexes

Article abstract of DOI:10.1021/ic701722f

Palladium(II) aryliodo complexes bearing chelating diphosphine ligands react with XeF₂, giving iodoarene and rare palladium(II) difluoro complexes. The reaction is general with regard to the aryl group, with even C₆F₅-I undergoing facile reductive elimination from a Pd center.

Full text of DOI:10.1021/ic701722f

Inorg. Chem. 2008, 47, 5−7
Xenon Difluoride Induced Aryl Iodide Reductive Elimination: a Simple
Access to Difluoropalladium(II) Complexes
Ariela W. Kaspi, Anette Yahav-Levi, Israel Goldberg, and Arkadi Vigalok*
School of Chemistry, The Raymond and BeVerly Sackler Faculty of Exact Sciences,
Tel AViV UniVersity, Tel AViV 69978, Israel
Received September 2, 2007
Scheme 1
Palladium(II) aryliodo complexes bearing chelating diphosphine
ligands react with XeF2, giving iodoarene and rare palladium(II)
difluoro complexes. The reaction is general with regard to the aryl
group, with even C6F5−I undergoing facile reductive elimination
from a Pd center.
Scheme 2
Late-transition-metal oxidative addition of aryl-halide
bonds is the key step in a variety of important catalytic
organic transformations.1 Today, there are dozens of metal
complexes that are capable of rapid insertion into even the
least active carbon-halogen bonds, and the requirements for
such reaction are generally well-understood.2 Precisely
against this background, remarkably little is known about
the reverse reaction: aryl halide reductive elimination.
Although a related C(sp2)-X reductive elimination reaction
was found to be an important step in the Monsanto methanol
carbonylation process,3 very few examples of Ar-X (X )
halide) were reported. In 1969, Ettorre showed that heating
(Et3P)2Pt2Ph2 in methanol gave iodobenzene and (Et3P)2-
PtPh.4 Many years later, Hartwig et al. demonstrated the
Ar-X (X ) Cl, Br, I) reductive elimination in a sterically
crowded PdII system.5 We recently reported evidence for an
Ar-Br and Ar-I reductive elimination reaction in a series
of PtIV complexes.6 Herein we present a highly efficient and
selective Ar-I reductive elimination reaction from a Pd
center triggered by reaction with xenon difluoride.
In the quest for synthetic routes toward diphosphine
difluoropalladium(II) complexes, we recently reported that
the reaction of the corresponding palladium(II) dimethyl
complex with XeF2 cleanly produces such products for
complexes bearing chelating alkylphosphine ligands (Scheme
1).7 The advantage of this reaction is that no laborious
workup is required because the byproducts remain in the gas
phase. In a continuation of our studies of XeF2 reactivity
with organometallic group 10 complexes, we found that upon
its reaction with the palladium(II) aryliodo complexes 3a-
c, clean formation of the difluoro complexes 2 and free
iodoarene was observed. Although Ar-X reductive elimina-
tion can generally be influenced by the chelate size,6 no such
effect was observed in this case, with both five- and six-
membered chelates undergoing clean Ar-I reductive elimi-
nation (Scheme 2). Only ligands bearing electron-donating
alkyl groups gave stable difluoro complexes. When the less
electron-rich rigid o-C6H4(Ph2P)2 ligand (dppbz) was used,
the resulting palladium difluoride could be observed at room
temperature immediately after the addition of two com-
pounds. It, however, decomposed within several minutes at
25 °C to form several P-F-bond-containing products.8 The
less rigid (dppp)PdF2 complex could only be observed at
* To whom correspondence should be addressed. E-mail:
avigal@post.tau.ac.il.
(1) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Sausalito, CA, 1987. (b) Tsuji, J. Palladium
in Organic Synthesis (Topics in Organometallic Chemistry);
Springer: Berlin, 2005.
(2) (a) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176.
(b) Bedford, R. B.; Cazin, C. S. J.; Holder, D. Coord. Chem. ReV.
2004, 248, 2283.
(3) Dekleva, T. W.; Forster, D. AdV. Catal. 1986, 34, 81.
(4) (a) Ettorre, R. Inorg. Nucl. Chem. Lett. 1969, 5, 45. (b) In our hands,
the reaction appeared to be more complex, giving large amounts of a
byproduct.
(7) Yahav, A.; Goldberg, I.; Vigalok, A. J. Am. Chem. Soc. 2003, 125,
13634.
(5) (a) Roy, A. H.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 13944.
(b) Roy, A. H.; Hartwig, J. F. Organometallics 2004, 23, 1533.
(6) Yahav-Levi, A.; Goldberg, I.; Vigalok, A. J. Am. Chem. Soc. 2006,
128, 8710.
(8) Fluoride migration to the coordinated phosphine ligand is the most
common decomposition pathway for late-transition-metal fluoro
complexes: (a) Grushin, V. V. Organometallics 2000, 19, 1888. (b)
Marshall, W. J.; Grushin, V. V. Organometallics 2003, 22, 555.
10.1021/ic701722f CCC: $40.75
Published on Web 12/04/2007
© 2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 1, 2008 5

COMMUNICATION
Figure 2. T1 measurement graphs for the 19F NMR signals of fluorine-
containing complexes and 4-fluoroiodobenzene.
CH2Cl2 (2.23 and 2.42 Å). The hydrogen bonding was
proposed to alleviate the strong π-donation properties of the
fluorides in their late-transition-metal complexes.11 The Pd-F
distances in 2a of 2.040(13) and 2.058(13) Å are only slightly
shorter than that in 2c [2.065(3) Å].7 There are also relatively
short distances between the fluoro ligands and the cyclohexyl
H atoms of the phosphine ligand. The P1-Pd-P2 angle is
expectedly smaller than that in 2c [87.37(2) vs 96.22(8)°].
Although the Pd-Ph complexes 4a and 4b easily undergo
the Ph-I reductive elimination with XeF2, using the 4-FC6H4
group instead of an unsubstituted phenyl group was found
to be helpful in a number of transformations at a metal center
due to the presence of a fluorine tag in the 19F NMR spectra.12
Surprisingly, the ratio between the 4-FC6H4I signal in the
19F NMR spectrum and the signal of 2 was found to be less
than 30% when 1 was reacted with XeF2 in a CH2Cl2
solution. Because no other product was detected in the
reaction mixture, we investigated the relaxation times of the
fluorine signal in 2 and aromatic aryl fluorides. We found
that the fluorine signal relaxation times in 2 were significantly
faster (several orders of magnitude) than those in 4-FC6H4I
(Figure 2). Increasing the delay time between the scans to
20 s gave the spectra with a correct integration ratio between
the products. Significant differences in the relaxation times
were also observed for the starting materials 3 and organic
Figure 1. Single-crystal ORTEP (thermal ellipsoids drawn at 50%
probability) structure of 2a. H atoms, with the exception of the atoms of
CH2Cl2, are omitted for clarity. Selected bond distances (Å) and angles
(deg): Pd-F1 2.0401(13), Pd-F2 2.058(13), Pd-P1 2.2061(6), Pd-P2
2.2177(6), F1-H28A 2.14, F1-H29A 2.23, F2-H27B 2.04, F2-H29B
2.42, F1-Pd-F2 90.62(5), P1-Pd-P2 87.37(2), F1-Pd-P2 92.87(4), F1-
Pd-P1 174.90(4).
low temperatures.7 The reaction appears to be quite general
with regard to the aryl group involved in the reductive
elimination. PdII complexes 4 and 5 also underwent smooth
Ar-I reductive elimination upon treatment with XeF2.
Importantly, even the strong C6F5-Pd bond was readily
cleaved under these conditions and C6F5-I was quantitively
formed together with 2. To our knowledge, the C6F5-X
reductive elimination is unprecedented.9
We were able to obtain transparent colorless prisms of
dcpePdF2 from a CH2Cl2/pentane solvent mixture at -30
°C.10 This crystal structure of 2a is only the second reported
for monomeric palladium(II) difluoro complexes.7 The X-ray
structure of 2a shows a square-planar arrangement at the
metal center, with two fluoro ligands involved in strong
hydrogen-bond interactions with three dichloromethane
molecules (Figure 1). No such interactions, however, were
observed in the crystal structure of the previously reported
2c, also crystallized from CH2Cl2. Coordination of three
CH2Cl2 molecules to fluoro ligands in a dimeric PdII system
has been described.11 The “terminal” solvent molecules show
relatively strong hydrogen-bond interactions with the fluoro
ligands (2.04 and 2.14 Å) compared with the bridging
1
fluorides, in both the 19F and H NMR spectra. Thus, the
NMR relaxation times of certain PdII complexes can be by
several orders of magnitude shorter than those of simple
aromatic molecules, and caution should be exercised when
assigning the product ratio based on the integration of the
NMR signals, even in the presence of an internal reference
compound.
It was proposed that XeF2 reacts with square-planar d8
metal complexes in an SN2-type mechanism, giving the trans
oxidative addition product.13 Organometallic palladium(IV)
phosphine complexes have so far eluded isolation; however,
(9) In fact, only a few Pd-catalyzed reactions involving the electron-
accepting C6F5 group are known: (a) Albeniz, A. C.; Espinet, P.;
Martin-Ruiz, B.; Milstein, D. J. Am. Chem. Soc. 2001, 123, 11504.
(b) Korenaga, T.; Kosaki, T.; Fukumura, R.; Ema, T.; Sakai, T. Org.
Lett. 2005, 7, 4915. (c) Frohn, H.-J.; Adonin, N. Y.; Bardin, V. V.;
Starichenko, V. F. Tetrahedron Lett. 2002, 43, 8111.
(10) X-ray structure data for 2a: C26H48F2P2Pd‚3CH2Cl2, M ) 821.76,
0.4 × 0.3 × 0.25 mm3, monoclinic, space group P21/c, a ) 12.34840-
(10) Å, b ) 14.20860(10) Å, c ) 21.0151(3) Å, â ) 103.3032(4)°, V
) 3588.23(6) Å3, Z ) 4, Nonius Kappa CCD, Mo KR radiation (λ )
0.710 73 Å), graphite monochromator, T ) 110(2) K, 8587 collected
reflections, 7142 unique reflections (Rint ) 0.0360). The structure was
determined by direct methods (SIR-97) and refined anisotropically by
least squares on F 2 data (SHELXL-97; 361 parameters with no
restraints). R1 ) 0.0356, wR2 ) 0.0466 for 11 587 data with I >
2σ(I) and R1 ) 0.0902, wR2 ) 0.0978 for all unique data.
(11) Grushin, V. V.; Marshall, W. J. Angew. Chem., Int. Ed. 2002, 41,
4476.
(12) (a) Yahav, A.; Goldberg, I.; Vigalok, A. Inorg. Chem. 2005, 44, 1547.
(b) Yahav, A.; Goldberg, I.; Vigalok, A. Organometallics 2005, 24,
5654.
(13) Cockman, R. W.; Ebsworth, E. A. V.; Holloway, J. H.; Murdoch, H.;
Robertson, N.; Watson, P. G. In Fluorine Chemistry (Toward the 21st
Century); Thrasher, J. S., Strauss, S. H., Eds.; American Chemical
Society: Washington, DC, 1994; Chapter 20, p 327.
6
Inorganic Chemistry, Vol. 47, No. 1, 2008

COMMUNICATION
formation of C6F5-I as the organic product was observed.
No 8b was observed in this reaction, and significant amounts
of P-F-bond-containing products were detected in the 19F
NMR spectra.
Figure 3.
We also studied the reactivity of the Pd(Ar)I complexes
with other reagents capable of influencing the Ar-I reductive
elimination. The reaction of 3 with Br2 resulted in the
simultaneous formation of the products of Ar-I (major) and
Ar-Br (minor) bond formation. Thus, the reaction of 3 with
XeF2 appears to be the most selective in terms of the
reductive elimination of iodoarene. To test the possibility
of Ar-I formation by oxidation, we reacted complexes 3
Scheme 3
their participation in various transformations as reactive
transients cannot be discounted.14 XeF2 reacts with related
platinum(II) phosphine aryl complexes, resulting in platinu-
m(IV) difluorides, although only cis positioning of the fluoro
ligands in the final product was observed.12a In the SN2-type
mechanism, formation of the cationic PdIV complex 6 can
be expected (Figure 3). Such a cationic intermediate can also
be formed with “electrophilic fluorination” reagents other
than XeF2. Interestingly, while no reaction was obtained upon
treatment of 3 with p-(difluoroiodo)toluene, the reaction
between 3 and 1-fluoro-2,4,6-trimethylpyridinium tetrafluo-
roborate (7) also resulted in the Ar-I reductive elimination.
With dcpp as the ligand, the cationic monofluoropalla-
dium(II) complex 8b (or its BF4- adduct) was observed and
remained stable for several hours in solution (Scheme 3).
The overall reaction was not as clean as the reaction with
XeF2. In addition to Ar-I, significant amounts of Ar-F (ca.
10%) were also observed.
While these observations may indicate the Ar-F reductive
elimination pathway from 6,15 we must also consider the
possibility that the electrophilic Pd-Ar bond cleavage with
the N-F reagent might be responsible for the formation of
the fluoroarene.16,17
The reaction of 3a with 7 also gave mixtures of Ar-I and
Ar-F; however, no stable palladium fluoro complexes were
isolated. When (dcpp)Pd(C6F5)I was reacted with 7, the
-
with Fc+ BF4 . No aryl iodide reductive elimination was
observed in this reaction. However, the reaction between 3
and cerium ammonium nitrate did produce some 4-FC6H4I
along with a number of unidentified products. These results
demonstrate that the reductive elimination can be, to a certain
extent, oxidatively induced. Yet, the difference in the
reactivity of XeF2 and 7 might indicate that the Ar-I
formation mechanism is more complex than the oxidative
addition-reductive elimination or oxidation-reductive elimi-
nation sequences. Although the reactions between late-
transition-metal complexes and XeF2 or its derivatives have
previously been studied,18 little is known about such chem-
istry involving complexes with alkyl- or aryl-metal bonds.
Attempts to observe the reductive elimination of Ar-Br from
the corresponding arylbromo complexes were, thus far,
unsuccessful. A complex mixture of products was observed
upon the reaction of palladium(II) arylbromo complexes and
XeF2.
In summary, the reaction of a series of chelating palla-
dium(II) aryl iodide complexes with XeF2 results in the
remarkably mild and selective reductive elimination of
iodoarene and the formation of the rare palladium(II)
difluorodiphosphine complexes. We are presently investigat-
ing the mechanism of this reaction in detail. Given that Xe
derivatives do not generate unwanted byproducts, their wider
use in synthetic organometallic chemistry should be consid-
ered.
(14) An organometallic PdIV compound was recently characterized at low
temperature: Canty, A. J.; Rodemann, T.; Skelton, B. W.; White, A.
H. Organometallics 2006, 25, 3996.
(15) We recently a proposed similar intermediate in Ar-I and Ar-Br
reductive elimination reactions from the chelating platinum com-
plexes.6
(16) Ar-F reductive elimination reactions remain a significant synthetic
challenge. A recently reported mechanism of Ar-F reductive elimina-
tion from a PdII center for the aryl ligand bearing a strong electron-
accepting nitro group [(a) Yandulov, D. V.; Tran, N. T. J. Am. Chem.
Soc. 2007, 129, 1342] has been contested: (b) Grushin, V. V.;
Marshall, W. J. Organometallics 2007, 26, 4997.
Acknowledgment. This work is dedicated to Prof. David
Milstein on the occasion of his 60th birthday. It was
supported by the German-Israel Science Foundation and Tel
Aviv University internal grant.
Supporting Information Available: Experimental details (PDF)
and X-ray data for complex 2a (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
(17) For the ligand-assisted synthesis of fluoroarenes using the electrophilic
N-F reagents, see: Hull, K. L.; Anani, W. Q.; Sanford, M. S. J. Am.
Chem. Soc. 2006, 128, 7134.
IC701722F
(18) Seppelt, K. Z. Anorg. Allg. Chem. 2003, 629, 2427.
Inorganic Chemistry, Vol. 47, No. 1, 2008
7