Palladium alkoxides: Potential intermediacy in catalytic amination, reductive elimination of ethers, and catalytic etheration. Comments on alcohol elimination from Ir(III)

Full text of DOI:10.1021/ja963273w

J. Am. Chem. Soc. 1996, 118, 13109-13110
13109
Complex 1 decomposes in solution or as a solid over the
course of hours at room temperature. The dominant metal
products from this thermal chemistry are DPPF-ligated Pd(0)
complexes. The primary organic products are biphenyl, tert-
butylbenzene and 4,4′-di-tert-butylbiphenyl (Scheme 1).15 No
ether was observed. Even the heating of 1 in the presence of
PPh3, conditions known to dramatically increase yields of amine
reductive eliminations, produced no ether. Thus, electronically
neutral aromatic groups resist reductive elimination of ether in
this system.
However, this stability toward ether formation may be crucial
for effective catalytic amination of aryl halides by amines and
alkoxide base. The amination of aryl halides by amines in the
presence of NaO-t-Bu involves catalytic amounts of palladium
aryl halide intermediates in the presence of stoichiometric
quantities of NaO-t-Bu. Thus, it seems likely that aryl alkoxo
complexes such as 1 exist in the reaction solutions. Considering
recent N-H activation chemistry by dimeric hydroxo com-
plexes,16 the amination chemistry may occur by reaction of 1
with amine to form palladium amido aryl complexes.
Palladium Alkoxides: Potential Intermediacy in
Catalytic Amination, Reductive Elimination of
Ethers, and Catalytic Etheration. Comments on
Alcohol Elimination from Ir(III)
Grace Mann and John F. Hartwig*
Department of Chemistry
Yale UniVersity, P.O. Box 208107
New HaVen, Connecticut 06520-8107
ReceiVed September 17, 1996
Our group and those of Boncella and Hillhouse recently
observed carbon-nitrogen and -sulfur bond-forming reductive
eliminations of amines1-3 and sulfides4 from palladium and
nickel amides and thiolates. These observations led to improve-
ments in the palladium-catalyzed amination of aryl halides.5-9
In contrast, analogous chemistry to form ethers and alcohols
from palladium alkoxides has been elusive.3b,10-14 Further, the
potential intermediacy of palladium alkoxides in aryl halide
amination chemistry has not been evaluated. In this paper we
report (1) the isolation of a palladium aryl alkoxo complex, (2)
the formation of arylamines by reaction of this complex with
HNRR′, (3) the reductive elimination of aryl ethers from an
analog of this complex with an electron deficient aryl group,
(4) the catalytic formation of alkyl aryl ethers with DPPF-ligated
palladium, most likely by C-O bond-forming reductive elimi-
nation, and (5) the stability of an Ir(III) alkyl hydroxo complex
previously reported to eliminate alcohol thermally.
Motivated by our recent rapid reductive eliminations of
amines from DPPF-Pd(II) complexes and the potential for the
intermediacy of alkoxides in tin-free amination chemistry,6,9 we
prepared (DPPF)Pd(p-t-BuC6H4)(O-t-Bu) (1) (DPPF ) (1,1′-
diphenylphosphino)ferrocene) by addition of Na-O-t-Bu to
(DPPF)Pd(p-t-BuC6H4)(Br) (eq 1). Complex 1 was obtained
in pure form as yellow crystals by crystallization from pentane
at -30 °C.
To test this hypothesis, we studied the chemistry of 1 with
amines (Scheme 1). Reaction between ditolylamine and 1 at
room temperature gave the ditolylamido complex (DPPF)Pd-
(p-t-BuC6H4)((N(tol)2) (2) in 75% yield. Complex 2 was
independently prepared from (DPPF)Pd(p-t-BuC6H4)(Br) and
KN(tol)2; (DPPF)Pd(Ph)((N(tol)2) was prepared and fully
characterized previously.2 Ditolylamide 2 quantitatively formed
the triarylamine (4-tert-butylphenyl)bis(4-methylphenyl)amine
after less than 4.5 h at 75 °C in the presence of PPh3-d15.
Similarly, reaction between aniline and 1 formed HO-t-Bu and
(DPPF)Pd(C6H4-t-Bu)(NHPh) (3) (92% yield) that displayed two
new doublets in the 31P{1H} NMR spectrum, along with a new
1
t-Bu group in the H NMR spectrum, suggesting formation of
monomeric anilide complex 3. This assignment was confirmed
by observing 31P-15N coupling to both phosphorus atoms of
the DPPF (δ 26.1, dd, JNP ) 42 Hz, JPP ) 38 Hz; δ 5.3, dd,
JNP ) 5 Hz, JPP ) 38 Hz) in reactions conducted with [15N]-
aniline. Complex 3 formed (4-tert-butylphenyl)phenylamine in
75-85% yield after 1 h at room temperature. Though preclud-
ing isolation, these data further support its identity as the anilido
aryl complex. Addition of piperidine to 1 led to the formation
of N-(4-tert-butylphenyl)piperidine after 1.5 h at room temper-
ature in 57% yield. In this case, the presumed intermediate 4
was not observed, suggesting that the C-N bond-forming
reductive elimination is faster than N-H activation of the
alkylamine. These results demonstrate that the reaction chem-
istry of the alkoxo aryl complex with amines is faster than the
decomposition in the absence of amine. Further, they show
directly a pathway for formation of amido aryl species from
aryl halide complexes, amine, and alkoxide base.
Our studies on directly-observed reductive elimination of
amines and sulfides showed faster rates for electron deficient
aryl groups.4,17 Thus, we treated aryl halide complexes contain-
ing HC(O)-, PhC(O)-, F3C-, and NC- in the para-position
with NaO-t-Bu. The alkoxide products were too reactive to
isolate, but GC/MS of the reaction mixtures after 0.5 h at room
temperature showed formation of the alkyl aryl ethers, chemistry
that contrasted that of 1.
(1) Villanueva, L. A.; Abboud, K. A.; Boncella, J. M. Organometallics
1994, 13, 3921-3931.
(2) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1995, 117, 4708-
4709.
(3) For an example of oxidatively induced reductive elimination from
Ni(II) see: (a) Koo, K.; Hillhouse, G. L. Organometallics 1995, 14, 4421-
4423. (b) Koo, K.; Hillhouse, G. L.; Rheingold, A. L. Organometallics
1995, 14, 456.
(4) Baranano, D.; Hartwig, J. F. J. Am. Chem. Soc. 1995, 117, 2937-
2938.
(5) Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995, 36, 3609-3612.
(6) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7217-
7218.
(7) Guram, A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem., Int.
Ed. Engl. 1995, 34, 1348-1350.
(8) Wolfe, J. P.; Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc. 1996,
118, 7215-7216.
(9) Louie, J.; Driver, M. S.; Hamann, B. C.; Hartwig, J. F. Submitted
for publication.
We focused our attention on Pd(DPPF)(p-C6H4CHO)(O-t-
Bu) (5; Scheme 1), which showed the cleanest substitution
chemistry and which contained an aldehyde resonance in the
1H NMR spectrum to mark the aromatic group. Addition of
(10) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163-1188.
(11) Thompson, J. S.; Randall, S. L.; Atwood, J. D. Organometallics
1991, 10, 3906-3910.
(12) Matsunaga, P. T.; Hillhouse, G. L. J. Am. Chem. Soc. 1993, 115,
2075-2077.
(13) Matsunaga, P. T.; Mavropoulous, J. C.; Hillhouse, G. L. Polyhedron
1995, 14, 175-185.
(15) Organic products identified by 1H NMR spectroscopy and GC/MS.
(16) Driver, M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 4206-
4207.
(17) Hartwig, J. F.; Richards, S.; Baranano, D.; Paul, F. J. Am. Chem.
Soc. 1996, 118, 3626-3633.
(14) A palladium-catalyzed cyclization to form oxygen heterocycles has
now been reported: Palucki, M.; Wolfe, J. P.; Buchwald, S. L. J. Am. Chem.
Soc. 1996, 118, 10333.
S0002-7863(96)03273-8 CCC: $12.00 © 1996 American Chemical Society

13110 J. Am. Chem. Soc., Vol. 118, No. 51, 1996
Communications to the Editor
Table 1. Palladium-Catalyzed Etheration of Aryl Halidesa
Scheme 1
a 10% Pd(0) catalyst, 12 mol % DPPF, and 1.2 equiv of NaO-t-Bu
per ArBr in toluene at 100 °C for 8 h. b Isolated by chromatography
on silica gel, 30:1 pentane/ether. c Isolated by chromatography on
alumina, 30:1 pentane/ether. d 2,2,2-Trifluoroethanol and a catalytic
amount of triflic acid were added to the crude mixture and stirred for
10 min; phenol isolation by chromatography on silica gel, 10:1 pentane/
ether.
NaO-t-Bu to Pd(DPPF)(p-C6H4CHO)(Br) produced two new
31P{1H} signals (δ 30.0 (d), 8.7 (d), J ) 37 Hz), with similar
shifts and coupling constants to those of isolated 1 (δ 29.5 (d),
6.8 (d), J ) 34 Hz). In addition, the 1H NMR spectrum of the
reaction showed a new formyl resonance and a new t-Bu
resonance well resolved from NaO-t-Bu, HO-t-Bu, or the
product ether. These data strongly support the identity of 5 as
Pd(DPPF)(p-C6H4CHO)(O-t-Bu).
DPPE, BINAP, DPPP, and DPPBz were tested, but none proved
more effective than DPPF.18 The ratios of ether to arene
determined by GC were 0.6:1 (DPPE), 3.2:1 (DPPP), 5.1:1
(BINAP, 16% unreacted ArBr), 14:1 (DPPBz), and 23:1 (DPPF).
The good conversion and selectivity with DPPBz were surpris-
ing, considering its ineffectiveness in amination chemistry.
Further catalytic chemistry, ether reductive eliminations, and
general chemistry of palladium alkoxides will be reported in
due course.
Considering the restrictions on ether elimination from Pd(II),
we were puzzled by a previous report of methanol elimination
from an Ir(III) methyl hydroxo complex.11 At 0 °C, Ir(CO)-
(OH)(I)(Me)(P(p-tolyl)3)2 (6) was stated to first eliminate CH3-
OH and then add MeI to form Ir(CO)(I)2(Me)L2. Complex 6
was not isolated due to its reactivity below room temperature.
To investigate this chemistry for catalytic etheration, we added
a 3-fold excess of MeI to the PPh3 analog Ir(CO)(OH)(PPh3)2.19
In contrast to the earlier findings with 6, Ir(CO)(OH)(I)(Me)-
(PPh3)2 (7) was obtained in high yield and fully characterized
including X-ray diffraction and was stable at room temperature.20
Warming of this sample at 65 °C did not produce methanol,
but warming of 7 at 65 °C in the presence of MeI-d3 for several
hours gave CD3OH (eq 3). Thus, the formation of methanol in
Due to its room temperature reactivity, complex 5 could not
be purified from the ether and Pd(0) reaction products.
However, we separated 5 from unreacted NaO-t-Bu, in order
to rule out formation of ether by direct reaction of free NaO-
t-Bu with the metal-bound electron poor aromatic group. The
reaction between NaO-t-Bu and a small excess of the palladium
halide was stopped after ca. 60% of the halide was consumed.
A 3-fold volume of pentane was added, the mixture was filtered,
the solvent was removed, and the resulting solid was extracted
into a 5:1 pentane/benzene mixture. After removal of solvent,
1
a H NMR spectrum in C6D6 showed no NaO-t-Bu and only
alkoxide complex 5, p-t-BuOC6H4CHO, and (DPPF)2Pd. As
shown in Scheme 1, 5 converted to p-t-BuOC6H4CHO (85%
yield, after correcting for the initial ether) and Pd(0) in roughly
1 h at room temperature or 10 min at 85 °C. Although a number
of mechanisms are possible for this transformation, the formal
definition is reductive elimination of ether.
This result prompted us to test a variety of palladium
complexes in the catalytic formation of aryl ethers. Intermo-
lecular formation of ethers was observed in the presence of
Pd(0), DPPF, NaO-t-Bu, and electron deficient aryl bromides
(eq 2, Table 1). No significant formation of ether was observed
the previous report probably resulted from well-established
alkylation of the hydroxo group rather than from C-O bond-
forming reductive elimination. Thus, thermally-induced reduc-
tive elimination of ethers in high yields is currently unique to
the Pd(II) complexes in this report.
when electron neutral or electron rich aryl bromides were used
or when the metal or DPPF was omitted. Sodium alkoxides
containing â-hydrogens led to more arene than ether. Although
this chemistry is thus far limited to production of tert-butyl
ethers, the aryl alkyl ethers can be efficiently converted to
phenols by treatment of the final reaction mixture with 2,2,2-
trifluoroethanol and catalytic triflic acid, constituting an overall
one-step conversion of aryl bromides to phenols. The ligands
Acknowledgment. We gratefully acknowledge a National Science
Foundation Young Investigator Award, a Du Pont Young Professor
Award, a Union Carbide Innovative Recognition Award, and a Dreyfus
Foundation New Faculty Award, along with the Exxon Education
Foundation and Rohm and Haas for support for this work. J.F.H. is a
fellow of the Alfred P. Sloan Foundation. We would also like to thank
Johnson-Matthey Alpha/Aesar for donation of palladium chloride.
(18) DPPE ) 1,2-bis(diphenylphosphino)ethane, BINAP ) 2,2′-bis-
(diphenylphosphino)-1,1′-binaphthyl, DPPP ) 1,2-bis(diphenylphosphino)-
propane, DPPBz ) 1,2-bis(diphenylphosphino)benzene.
(19) Reed, C. A.; Roper, W. R. J. Chem. Soc., Chem. Commun. 1973,
1370.
(20) No unusual structural features were observed. Data are provided as
Supporting Information.
Supporting Information Available: Spectroscopic, analytical, and
X-ray data for 1, 2, and 7 (19 pages). See any current masthead page
for ordering and Internet access instructions.
JA963273W