Aryl-halide versus aryl-aryl reductive elimination in Pt(IV)-phosphine complexes

Article abstract of DOI:10.1021/ja062166r

Upon the addition of Br₂ to complexes (P-P)Pt(Ar)₂, two different products were observed, depending on the bite angle of the bidentate phosphine ligand: a Pt(II) aryl bromide complex, the product of C-Br reductive elimination, and Pt(IV) oxidative addition complex. At high temperatures, the latter exclusively gave the product of the C-C reductive elimination. Copyright

Full text of DOI:10.1021/ja062166r

Published on Web 06/16/2006
Aryl-Halide versus Aryl-Aryl Reductive Elimination in Pt(IV)-Phosphine
Complexes
Anette Yahav-Levi, Israel Goldberg, and Arkadi Vigalok*
School of Chemistry, The Sackler Faculty of Exact Sciences, Tel AViV UniVersity, Tel AViV 69978, Israel
Received March 30, 2006; E-mail: avigal@post.tau.ac.il.
Scheme 1
Oxidative addition of C-X (X ) halide) bonds to late transition
metal centers is a well-studied process, yet its microscopic reverse,
carbon-halide reductive elimination, is extremely rare.1 Being
thermodynamically uphill, this reaction, nevertheless, often plays
an important role in catalysis, as illustrated in the Monsanto
methanol carbonylation process.2 Since the formation of a carbon-
halogen bond is usually suppressed when alternative pathways, such
as C-C reductive elimination, are available, very few examples
of the C-X reductive elimination from the high oxidation state
metal centers have been reported, with most of them restricted to
the sp3 C-X bond-forming reactions.3-5 In these reactions, the C-X
reductive elimination can be viewed as the reverse step of the SN2
Scheme 2
oxidative addition mechanism.3,6 With sp2 and sp-hybridized carbon
substituents, the C-X reductive elimination is generally not
observed, the C-C reductive elimination being by far the dominant
pathway.7 This paper reports the competitive aryl-halide and aryl-
aryl reductive elimination from a series of (P-P)Pt(IV) complexes
that show a remarkable selectivity dependence on the ligand’s bite
angle.
We recently reported that the addition of XeF2 to Pt(II) diaryl
complexes bearing the chelating phosphine ligands (dcpp, dppp,
or dppe) instantaneously produced the difluoro Pt(II) complexes
together with the product of C-C reductive elimination.8a As the
same reaction with the Pt(II) complexes bearing monodentate
phosphines gave stable Pt(IV) difluoro complexes,8b we considered
that, in the former case, the C-C reductive elimination might have
occurred from a cationic Pt(IV) intermediate, at a step prior to the
formation of the difluoro complexes. Since little information is
available with regard to the formation of (P-P)Pt(Ar)2X2 com-
plexes, we sought to study the mechanism of dihalogen oxidative
addition to square planar Pt(II) diaryl complexes.
Upon the addition of a solution of I2 to a series of the chelating
Pt(II) diaryl complexes 1a-d, facile formation of the aryl iodo Pt-
(II) complexes 2a-d was observed, and aryl iodide was obtained
as an organic product. The reaction proceeded smoothly, even at
very low temperatures, in different solvents, and no competition
from C-C reductive elimination was observed. These results
contrast with the exclusive C-C reductive elimination observed
for the vinyl-vinyl (sp2-sp2)9 and ethynyl-ethynyl (sp-sp)10 Pt-
(IV) systems, where no C-I reductive elimination was detected
and no such pathway was calculated in the DFT studies.9b-d It is
noteworthy that only when the small dmpe ligand was used was
the formation of a stable trans-Pt(IV) complex 3 observed at room
temperature.11 The 31P NMR spectrum showed a characteristic
signal at -24.37 ppm (about 40 ppm upfield from the starting
material) with a very low JPtP of 1200 Hz.12 At 50 °C in benzene
this compound underwent smooth conversion into the more stable
cis-product 4 (Scheme 1). These observations suggest that the trans
complex, such as 3, is the kinetic product of the X2 oxidative
addition to (P-P)Pt(Ar)2 and results from the SN2-type mechanism
involving cationic Pt(IV) intermediates.
Heating 4 at 90 °C for days gave the C-C reductive elimination
products.
Considering, however, the high kinetic lability and often
unpredictable behavior of iodides in organometallic chemistry,13
we decided to investigate the reactivity of complexes 1a-e with
Br2. Surprisingly, upon the addition of bromine to solutions of 1a
or 1b in different solvents (THF, toluene, acetone, or CH2Cl2),
instantaneous aryl-bromide reductive elimination was observed
together with the formation of dppePt(Ar)Br (5a) and dcpePt(Ar)-
Br (5b), respectively (Scheme 2). Small amounts of the Pt(IV)
oxidative addition products (both trans- and cis-isomers, 6a,b and
7a,b, respectively, in an approximately 1:2 ratio depending on the
conditions), as well as Pt(II) dibromide (8a,b, the product of the
reaction between 5a,b and Br2) were also observed. No biaryl
formation was detected in these reactions. On the other hand, the
addition of 1 equiv of Br2 to either 1c or 1d gave stable Pt(IV)
complexes (6c,d and 7c,d), which showed no Ar-Br reductive
elimination at room temperature or below (Scheme 2). The ratio
between the isomers varied slightly depending on the solvent, with
the polar solvents (THF vs CH2Cl2 or benzene) giving higher
percentage of the trans-product.14 When these complexes were
heated for several hours in benzene or THF, only aryl-aryl
reductive elimination was observed, and the corresponding Pt(II)
dibromides (8c,d) were obtained as the metal-containing species.
The reaction was significantly faster in the more polar solvent THF.
Addition of an excess of tetrabutylammonium bromide (TBA-Br)
to solutions of 6c,d and 7c,d in toluene or THF significantly slowed
the reductive elimination reaction (Figure 1), suggesting that initial
Pt-Br bond cleavage facilitates this reaction. Unlike the sp3
system,3b the direct anion attack at an sp2 carbon atom is not
possible, and aryl bromide formation is not aided by the presence
of TBA-Br. Similar to the reaction with I2, 1e reacted with Br2 to
9
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C O M M U N I C A T I O N S
biaryl and 8a,b. Thus, the added bromide shuts the regeneration of
I from 6, shifting the reaction from the C-Br to the C-C
elimination.18
Although the detailed mechanistic studies are still underway, it
is clear that the aryl-halide reductive elimination can compete
favorably with the more common C-C reductive elimination. The
reaction course appears to be extremely sensitive to even minor
changes in the ligand’s structure and reaction media.
Acknowledgment. This work was supported by the Germany-
Israel Science Foundation Young Investigator Award.
Supporting Information Available: Experimental details and
characterization of complexes 2-7. Crystallographic information for
complex 4 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 1. Rates of C-C reductive elimination from 7d at 80 °C in THF
(9), benzene (2), THF with 10 equiv TBA-Br (+), and benzene with 10
equiv TBA-Br (O).
References
Chart 1
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give a stable trans-Pt(IV) complex 6e, which upon prolonged
heating at 90 °C in THF converted to the cis-7e.
These results show that the bite angle plays a crucial role in
determining the outcome of the reductive elimination reactions in
Pt(IV) diaryl systems. Both steric bulk (with small dmpe as the
notable exception) and electronic properties of the substituents at
the phosphorus atoms appear to be of a lesser importance. Although
dppe and dcpe differ significantly in their steric and electronic
properties, they both favor the C-Br reductive elimination, while
dppp and dcpp favor C-C reductive elimination from isolated Pt-
(IV) complexes. The importance of the bite angle in organometallic
transformations is well-known, primarily with regard to its influence
on the reaction rates.15 In our case, a complete change in the reaction
pathways is obtained upon a slight change in the diphosphine
backbone.
To explain this phenomenon, we propose that the product
distribution in the reported reaction is primarily dependent on the
stability of the cationic intermediates produced during the oxidative
addition reaction (Chart 1). In the SN2-type mechanism, the square
pyramidal cation I is initially formed. This cation can either undergo
C-Br reductive elimination, react with the anion to give the trans
oxidative addition product or rearrange to the more stable square
pyramidal intermediate III. The last pathway is followed by the
reaction with the anion to give the cis oxidative addition product.
The rearrangement of I to III should be favored by the ligands
with the propane backbone (larger bite angles), because it is
accompanied by an increase in the P-Pt-P angle (cation II as a
putative intermediate).16
Interestingly, when a mixture of 5a with small amounts of 6a
and 7a (from the original reaction of 1a with Br2) was heated for
several hours at 70 °C, the Pt(IV) complexes gradually disappeared
to give more 5a and a small amount of the (dppe)PtBr2 (8a). The
equivalent amounts of the bromoarene and biaryl were also
observed, showing that, at high temperatures, both C-C and C-Br
reductive elimination reactions can proceed in the same system. In
an attempt to trap I prior to the C-Br reductive elimination step,
we reacted 1a,b with Br2 in the presence of 10 equiv of TBA-Br.
Under these conditions, 6a and 6b were obtained as the major
species (90 and 60%, respectively).17 Upon heating with TBA-Br,
these complexes underwent C-C reductive elimination to give the
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these complexes underwent rapid C-I reductive elimination.
(12) Yahav, A.; Goldberg, I.; Vigalok, A. Organometallics 2005, 24, 5654.
(13) Maitlis, P. M.; Haynes, A.; James, B. R.; Catellani, M.; Chiusoli, G. P.
Dalton Trans. 2004, 3409.
(14) See Supporting Information for full experimental details.
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(17) Although we observed no changes in the NMR spectra upon the addition
of TBA-Br to complexes 1a-e, the possibility of Br- precoordination to
the Pt(II) center before the reaction with Br2 cannot be excluded.
(18) In the presence of TBA-Br, the biaryl formation from 6a,b might occur
via the direct elimination or chelate ring-opening. Ion-pair rearrangement
mechanisms also cannot be ruled out at this stage.
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