Autocatalytic oxidative addition of PhBr to Pd(PtBu 3)2 via Pd(PtBu3)2(H)(Br)

Article abstract of DOI:10.1021/ja711159y

We report that oxidative addition of bromobenzene to Pd(P^tBu₃)₂ occurs by an unusual autocatalytic mechanism. Studies on the effect of various additives showed that the degree of rate acceleration followed the trend: (P^tBu₃)Pd(Ph)(Br) ≈ (HP^tBu₃)Br < [(P^tBu₃)Pd(μ-Br)]₂ < (P^tBu₃)₂Pd(H)(Br). Studies on the reactions of Pd(P^tBu₃)₂ in the presence of (P^tBu₃)₂Pd(H)(Br) showed that the concentration of (P^tBu₃)₂Pd(H)(Br) decreased only after the Pd(0) complex had been consumed. These data indicated that the catalyst in this process is (P^tBu₃)₂Pd(H)(Br). Thermal decomposition of the three-coordinate oxidative addition product (P^tBu₃)Pd(Ar)(Br) during the reaction of Pd(P^tBu₃)₂ and bromoarenes ultimately leads to formation of (P^tBu₃)₂Pd(H)(Br). Parallel reactions of bromobenzene with (P^tBu₃)₂Pd(H)(Br) and Pd(P^tBu₃)₂ showed that the bromoarenes reacted considerably faster with the Pd(II) species than with the Pd(0) species. We therefore propose a catalytic cycle for oxidative addition in which PBut3·HBr reacts with the Pd(0) species to form (P^tBu₃)₂Pd(H)(Br), and (P^tBu₃)₂Pd(H)(Br) reacts with the bromoarene, possibly though the anionic species [HP^tBu₃⁺][(P^tBu₃)Pd(Br)^-], to form [Pd(P^tBu₃)(Ar)(Br)]. Copyright

Full text of DOI:10.1021/ja711159y

Published on Web 04/11/2008
Autocatalytic Oxidative Addition of PhBr to Pd(PtBu3)2 via
Pd(PtBu3)2(H)(Br)
Fabiola Barrios-Landeros, Brad P. Carrow, and John F. Hartwig*
Department of Chemistry, UniVersity of Illinois, 600 South Mathews AVenue,
Urbana, Illinois 61801, and Department of Chemistry, Yale UniVersity, P.O. Box 208107,
New HaVen, Connecticut 06520-8107
Received December 17, 2007; E-mail: jhartwig@uiuc.edu
The oxidative addition of aryl halides or sulfonates to Pd(0)
complexes is the first step in classic and recently developed
cross-coupling reactions. Among the most active catalysts for
many coupling reactions are complexes of PtBu3.1 Recently,
we reported the isolation of unusual three-coordinate products
from oxidative addition of bromoarenes to Pd(0) complexes of
PtBu3.2 We have now studied the mechanism of the oxidative
addition of bromoarenes to Pd(PtBu3)2, and this mechanism is
equally unusual. The reaction in the absence of a strong base is
autocatalytic, and a side product, Pd(PtBu3)2(H)(Br), is capable
of acting as the catalyst. In contrast to the typically higher
reactivity of haloarenes with Pd(0) species than with Pd(II), the
reaction of bromobenzene with Pd(PtBu3)2(H)(Br) occurs faster
than with Pd(PtBu3)2 to form Pd(PtBu3)(Ph)(Br). Studies that
support these conclusions are reported here.
Figure 1. Decay of Pd(PtBu3)2 (1) during the oxidative addition of PhBr
in toluene at 70 °C in the presence of additives (L ) PtBu3).
The oxidative addition of PhBr to Pd(PtBu3)2 (1) produces
the three-coordinate (PtBu3)Pd(Ph)(Br) (2).2 The yields of 2 and
side products in polar and nonpolar solvents are shown in eq 1.
The reaction in polar solvents, such as methyl ethyl ketone
(MEK), formed 2 in high yield. Reactions in toluene were
slower, and this slower rate allowed decomposition of the prod-
uct (vide infra) to occur in parallel with the oxidative ad-
dition. Thus, a typical product distribution in toluene was 2
Figure 2. Decay of Pd(PtBu3) (1), during the oxidative addition of PhBr
in THF at 70 °C in the presence of 5 mol % of NEt3 ·HBr, 30 mol % of
phosphazene base tert-butyliminotrispyrrolidino phosphorane (BTPP), or
50 mol % of NBu4Br.
reaction of 1 with PhBr. Reactions in toluene in the presence
of 5 mol % added (PtBu3)2Pd(H)(Br) occurred in less than half
of the roughly 3.5 h required in the absence of additive.
Because several of the compounds that affected the oxidative
addition contained HBr, we sought to determine if the accelerat-
ing effect of these additives resulted from H+, Br-, or a
combination of the two. The oxidative addition of PhBr to 1 in
the presence of 5 mol % NBu4Br6 (0.020 M) occurred much
faster than in the absence of an additive and was complete in
about 30 min (Figure 2). The reaction in the presence of only
5 mol % NEt3 ·HBr occurred faster than in the absence of an
additive, in this case with an exponential decay possessing a
half-life of about 30 min. Oxidative addition of PhBr to 1 in
the presence of the strong, neutral phosphazene base tert-
butyliminotrispyrrolidino phosphorane (BTPP)7,8 also occurred
with an exponential decay, in this case with a much longer half-
life of 6.5 h. Reactions conducted with weaker bases, such as
NEt3, did not quench the autocatalysis.9 These data show that
both Br- and H+ contribute to the autocatalysis, and that a
strong base is needed to quench the autocatalysis.
4
(60%), (PtBu3)2Pd(H)(Br)3 (3) (10%), [(PtBu3)Pd(µ-Br)]2 (4)
5
(13%), and [Pd(PtBu2C(CH3)2CH2)(µ-Br)]2 (5) (16%). Proto-
nated phosphine [HPtBu3]+ (6) (12%) was also detected by 31
P
NMR spectroscopy; the anion in this salt lacks an NMR active
nucleus and is likely to be bromide. Each of the side products3–5
was identified by independent synthesis and comparison of 31
NMR chemical shifts to those of the reaction mixture.
P
The rate of reaction of the Pd(0) complex 1 with bromoben-
zene was measured by 31P NMR spectroscopy with [1] ) 0.040
M and [PhBr] ) 1.9 M in toluene, THF, and 2-butanone solvent
at 70 °C. The reaction of 1 in toluene in the absence of any
additive (open squares, Figure 1) was clearly not exponential
and was characteristic of an autocatalytic reaction. To determine
the origin of this kinetic behavior, the rate of the reaction of
PhBr with 1 was conducted with 5 mol % of (PtBu3)Pd(Ph)(Br)
(2) and 5 mol % of the individual, independently synthesized
side products. Each of these species accelerated the oxidative
addition process but to different extents. The degree of ac-
celeration by the different additives followed the trend
(PtBu3)Pd(Ph)(Br) ≈ (HPtBu3)Br < [(PtBu3)Pd(µ-Br)]2
<
Because HBr adds to Pd(PtBu3)2 to form hydridopalladium
bromide 3, and because complex 3 had the largest accelerating
(PtBu3)2Pd(H)(Br). Added cyclometalated 5 did not affect the
9
5842 J. AM. CHEM. SOC. 2008, 130, 5842–5843
10.1021/ja711159y CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
and PhBr to initiate the autocatalysis. To determine the origin
of these species, we studied the relative stabilities of the initial
products from reaction of PhBr with complex 1 at different
temperatures. These studies showed that thermal decomposition
of arylpalladium bromide 2 at 50 °C in toluene forms µ-Br
complex 415 and that complex 4 degrades at 80 °C in toluene
to form hydridopalladium bromide 3 (5%), cyclometalated 5
(42%), and the phosphonium salt (53%). Thus, the decomposi-
tion of the simple oxidative addition product 2 to µ-Br complex
t
4 and then Bu3P·HBr appears to initiate the autocatalysis.
In conclusion, we have shown that the oxidative addition to
Pd(PtBu3)2 occurs, at least under certain conditions, by an
unusual and complex autocatalytic mechanism, and these studies
led to the counterintuitive observation that bromobenzene reacts
faster with the palladium(II) complex L2Pd(H)(Br) than with
the related palladium(0) species L2Pd. The formation of L2Pd-
(H)(Br) from a cascade beginning with the oxidative addition
product, along with the faster reaction of the bromoarene with
L2Pd(H)(Br) than with L2Pd, can account for this autocatalysis.
The fact that these processes occur in the absence of base or in the
presence of bases of modest strength could make the autocatalytic
pathway relevant to the mechanisms of Stille cross-couplings,16
certain Suzuki couplings,17 and Heck reactions.18 In fact, complex
3 is the resting state of the Heck reaction catalyzed by Pd(0)
complexes of PtBu3 under some conditions.19
Figure 3. Relative decay of Pd(PtBu3)2 (1) and (PtBu3)2Pd(H)(Br) (3) during
the oxidative addition of PhBr in 2-butanone at 70 °C.
effect on the oxidative addition, we focused on how this complex
could affect the reactions of PhBr with Pd(PtBu3)2. Profiles of
the reaction of PhBr with a 1:1 mixture of Pd(0) complex 1
and the hydrido bromide 3 at 70 °C in toluene (see Supporting
Information for data) show that the concentration of 3 decreased
only after Pd(0) complex 1 had been consumed. Because the
oxidative addition to 1 was faster in the presence of 3, but the
concentration of 3 did not change during the consumption of 1,
complex 3 plays the unexpected role of a catalyst for this
addition of PhBr to 1.
To probe the origin of the accelerating affect of hydrido
bromide complex 3 on the reaction of PhBr with Pd(0) complex
1, we studied the reaction of 3 with PhBr and compared the
rates of this reaction to that of 1 with PhBr. The reaction of
PhBr with hydrido bromide complex 3 at 70 °C in toluene
occurred faster than that with Pd(0) complex 1. The yield of 2
was modest (32% at 77% conversion), just as it was for reaction
of 1 with PhBr in toluene (vide supra) because the decomposi-
tion products 5 and (HPtBu3)2[PdBr4] (7)10 increased at higher
conversion of 3.11
Acknowledgment. We thank the NIH (NIGMS, GM 58108)
for support of this work, and Johnson-Matthey for PdCl2.
Supporting Information Available: Experimental procedures
and characterization of reaction products. This material is available
free of charge via the Internet at http://pubs.acs.org.
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HPtBu3 (0.19).
Scheme 1
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t
and Bu3P·HBr (6). Phosphonium salt 6 then transfers HBr to
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