Intermolecular hydroarylation of unactivated olefins catalyzed by homogeneous platinum complexes

Article abstract of DOI:10.1002/anie.200800524

Designing catalysts: The five-coordinate platinum(IV) complex A and the platinum(II) trans complex B act as precatalysts for the hydroarylation of unactivated olefins. The catalytic cycle features an aryl-olefin insertion at PtII and a C-H bond activation of the arene solvent as key steps. The Pt ^II cis complex C has been observed in hydroarylation reactions of ethylene with benzene.

Full text of DOI:10.1002/anie.200800524

Communications
DOI: 10.1002/anie.200800524
Olefin Hydroarylation
Intermolecular Hydroarylation of Unactivated Olefins Catalyzed by
Homogeneous Platinum Complexes**
Avery T. Luedtke and Karen I. Goldberg*
À
The hydroarylation of olefins is a valuable C C bond forming
reaction used to produce alkyl arenes.[1] Olefin hydroarylation
can be catalyzed by Lewis acids,but such reactions proceed
through a Friedel–Crafts type mechanism involving an
intermediary carbocation. Thus,these reactions give predom-
inantly Markovnikov products,and ortho, meta,and para
selectivity is determined by the substituents on the aromatic
ring. In contrast,the use of transition-metal catalysts can
and olefins and present mechanistic evidence consistent with
À
a pathway involving aryl–olefin insertion and C H bond
oxidative addition at PtII. While Markovnikov products are
favored,anti-Markovnikov products are observed,and the
mechanistic insight gained is promising for rational design of
more selective and productive PtII catalysts for these reac-
tions.
We recently reported that thermolysis of the five-coor-
dinate PtIV complexes [(LX)PtMe3] {LX = dtbpp [3,5-di-tert-
butyl-2-(2-pyridyl)pyrrolide] (1a) or dppp [3,5-diphenyl-2-(2-
pyridyl)pyrrolide] (1b)} at 85–1008C in C6D6 in the presence
of C2H4 (9–60 equivalents) led to the release of ethane and
methane and formation of PtII complexes 2a or 2b,which
contain a cyclometalated substituted pyrrolide group and
C2H4 (Scheme 1).[9] This indicates that the (pyridyl)pyrrolide
afford different regioselectivities acting via a mechanism of
arene C H bond activation and olefin insertion. While in
the past transition-metal-catalyzed olefin hydroarylation
reactions were primarily limited to activated arenes wherein
[1]
À
a chelating functionality on the arene was available to assist
and direct the C H bond activation step, recently IrIII and
[1]
À
RuII catalysts have demonstrated hydroarylation with unac-
tivated arenes and olefins.[2,3] Mechanistic and computational
studies on these IrIII and RuII catalysts suggest that the
hydroarylation does not proceed through a Friedel–Crafts
type activation but through olefin insertion followed by
oxidative hydrogen migration. In addition,selectivity for anti-
Markovnikov over Markovnikov products (ca. 60:40) was
observed. However,significantly higher selectivities and
turnover numbers (TONs) are needed to make these pro-
cesses economical,so a broadly tunable system that can be
modified both sterically and electronically is likely needed.
One promising metal for olefin hydroarylation is plati-
À
num. There is considerable precedent for both arene C H
bond activation and olefin insertion at PtII;[4,5] however,
attempts at olefin hydroarylation with PtII have been dis-
appointing.[6] Selectivities consistent with an electrophilic
Friedel–Crafts type pathway were observed using a mixed
Ag–Pt catalyst system.[7] With a related PtII catalyst,the
hydroarylation of norbornene was reported,but other olefins
were found to be unreactive.[8] Finally,tridentate chelation of
a tris(pyrazolyl)borate ligand stabilized a potential PtIV
intermediate preventing catalytic turnover.[5] Herein,we
describe the rational development of an effective PtII system
for intermolecular hydroarylation with unactivated arenes
Scheme 1.
ligand on PtII effectively promoted two key reaction steps
needed for hydroarylation: C H bond activation and olefin
À
coordination. The stability of the cyclometalated complexes,
however,prevents further reaction. A complex containing a
ligand that does not form a stable cyclometalation product
may be expected to show different reactivity,and so
[(dmpp)PtMe3] [dmpp = 3,5-dimethyl-2-(2-pyridyl)pyrrolide
(1c)] was prepared. Deprotonation of dmpp-H (3)[10] with
KH in THF yielded green dmpp-K (4). Reaction of 4 with
[*] A. T. Luedtke, Prof. K. I. Goldberg
Department of Chemistry, Box 351700
University of Washington, Seattle, WA 98195-1700 (USA)
Fax: (+1)206-685-8665
À
[{PtMe3OTf}4] (OTf = SO3CF3 ) in Et2O produced a yellow
solution of 1c.[11] Although an X-ray crystal structure of 1c
reveals a non-centrosymmetric dimeric structure in the solid
state with a long bond [2.518(8) Š] between the C4-pyrrolide
carbon and a second Pt center (see Figure S16 in the
Supporting Information),the 1H NMR spectrum of 1c in
CD2Cl2 at room temperature is consistent with a fluxional
five-coordinate complex.[12]
E-mail: goldberg@chem.washington.edu
[**] We thank the National Science Foundation (CHE-0137394 and CHE-
0719372) for financial support, Dr. W. Kaminsky and Dr. J. Benedict
for performing the X-ray structural determinations and Prof. K.
McNeill (University of Minnesota) for initial samples of the ligand 3.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200800524.
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Remarkably,the change to methyl substituents on the
(pyridyl)pyrrolide ligand allowed for the observation of Pt-
catalyzed intermolecular hydroarylation of unactivated ole-
fins. Similar to the thermolyses of 1a and 1b,upon thermol-
ysis of 1c at 1008C for 5 h in C6D6 in the presence of C2H4
(70 equiv) ethane and methane (CH4 and CH3D) were
released. However,rather than a cyclometalated Pt II species,
the PtII product [(dmpp)Pt(CH2CH2C6D5)(C2H4)] (cis-[D5]5)
was observed in a 48% yield by 1H NMR spectroscopy
(Scheme 1). The configuration (cis) was assigned by
NOESY[11] and refers arbitrarily to the position of the
2-phenethyl ligand with respect to the pyrrolide group.
A variety of arene (C6H6, C6H5CH3, C6H5CF3) and olefin
(C2H4, C3H6,cyclohexene,norbornene) combinations were
examined with 1c or trans-6 as a precatalyst for hydro-
arylation. The product distributions were analyzed by GC-MS
and GC-FID,and the results are summarized in Table 1 (a
more complete listing can be found in Tables S1 and S2 in the
Supporting Information).[11] Hydroarylation of C2H4 (390–
410 mm initial concentration) in C6H6 to form ethylbenzene
gave TONs of 26 (1c) or 36 (trans-6); the higher TON for
trans-6 compared to 1c may be due to a more efficient
conversion to the active catalyst. Notably,when multiple
products are possible,hydroarylation product mixtures con-
tain similar ratios using 1c or trans-6 as a precatalyst. With
C3H6,both Markovnikov ( iPr-Ar) and anti-Markovnikov
(nPr-Ar) products are observed,with the former favored 5–
6:1. The isomeric distribution meta > para > ortho was
obtained for reactions with C3H6 in C6H5CH3 (o/m/p
9:66:25) and C2H4 in C6H5CF3 (o/m/p 6:62:32). Both the
fact that some anti-Markovnikov product is observed and that
a preference for meta and para over ortho functionalization is
Notably,the product of hydroarylation,C
6D5CH2CH2D
([D6]ethylbenzene),was also observed in the 1H NMR
spectrum,and the concentration of this organic product
continued to increase upon further heating for an additional
12 h.
However,complex cis-[D5]5 decomposed significantly
over this time to intractable products. The organic product,
C6D5CH2CH2D,was identified by 1H NMR spectroscopy and
GC-MS. When C6D12 was used as the solvent with C2H4 and
C6H6 (0.21m) added as reagents,no evidence of hydroaryla-
tion (or hydroalkylation) was observed in the 1H NMR
spectrum. Instead,decomposition of 1c to Pt0 was observed
with evidence of numerous unidentified compounds in the
1H NMR spectrum.
À
observed suggest that a C H bond activation pathway is
operative.
A small amount of styrenes (0.1–0.7 TON),presumably
formed by b-hydride elimination and also some dialkylation
products (0.5–1.6 TON) were observed.[11] Increasing the
amount of C2H4 from 220 to 390 mm caused a 1.4-fold
increase in TON in the hydroarylation of C6H6 using 1c as the
precatalyst. The relative amount of dialkylated products also
increased from 4% to 9% of the total TON under these
conditions.[11]
A related PtII catalyst precursor compound [(dmpp)Pt-
(SMe2)Ph] (trans-6) was prepared by the reaction of 3 with
[{Me2Pt(m-SMe2)}2] in C6H6.[13] The relative configuration,
[11]
phenyl trans to pyrrolide,was confirmed by NOESY.
Heating a C6D6 solution of trans-6 at 1008C caused partial
isomerization to cis-6 (greater than 2:1 cis/trans after
133 h).[11] Upon pressurization of a C6D6 solution of trans-6
with C2H4 and subsequent heating at 598C for 14 h,con-
version to cis-[D5]5 (59%) was observed in the 1H NMR
spectrum. Heating of either cis-[D5]5 or trans-6 under C2H4 in
C6D6 at 1008C produced the hydroarylation product
C6D5CH2CH2D. Similar attempts to synthesize [(dmpp)Pt-
{(C3H6)C6H5}(C3H6)] by heating trans-6 in C6D6 at 598C under
an atmosphere of C3H6 were unsuccessful. However, trans-6
can act as a precatalyst for the hydroarylation of C3H6 with
C6H6 or C6H5CH3 (Table 1).[14]
Reaction of C2H4 with a 1:1 solution of C6D6 and C6H6
using 1c as the precatalyst gave [D0]ethylbenzene through
[D6]ethylbenzene as observed by GC-MS,with the major
product being [D3]ethylbenzene (Supporting Information,
Figure S15). This isotopomer distribution suggests rapid
scrambling of H or D from the solvent (C6H6 or C6D6) into
ethylbenzene.[11]
Stoichiometric reactions of cis-5 or cis-[D5]5 in C6D6 with
no C2H4 present produced PhCH2CH3 or PhCH2CH2D,
respectively,as determined by 1H NMR spectroscopy.[11]
Thus,whether or not the C2 carbon of ethylbenzene bears
one D or only H is dependent upon whether the phenyl group
of the 2-phenethyl ligand of cis-5 was deuterated or not.
The mechanism shown in Scheme 2 is consistent with the
isotopic labeling results and the regioselectivity described
above. A PtII phenyl ethylene complex, A,[15] formed in situ
from 1c or trans-6,undergoes migratory insertion of olefin
Table 1: Products and TONs for the hydroarylation of olefins.[a]
Arene
C6H6
Olefin
Cat.
TON
o/m/p
iPr/nPr
C2H4
C3H6
1c
26
8
À
À
into the Pt Ph bond. Aryl C H bond cyclometalation of the
86:14
phenethyl group of B forms C. Insertion of an olefin into a
cyclohexene
norbornene
C2H4
8
10
4
II
À
Pt Ph bond followed by orthometalation of the Ph ring has
7:93[b]
2
recently been observed upon thermolysis of [TpMe PtPh-
C6H5CH3
C3H6
C2H4
2
2
10:63:27
6:62:32
85:15
(C2H4)] [TpMe = 3,5-dimethyl-tris(pyrazolyl)borate].
The
[5]
2
C6H5CF3
C6H6
observation of cis-5 and the results of the stoichiometric
reactions of cis-5 and cis-[D5]5 are consistent with a similar
reaction sequence in this system. However,here a five-
coordinate cyclometalated hydrido PtIV species C would be
C2H4
C3H6
C2H4
C3H6
trans-6
36
18
12
3
85:15
84:16
C6H5CH3
6:94
9:66:25
À
formed,and alkyl C H reductive elimination from five-
[a] Reaction conditions: 100–1108C, 1–3 mol% of 1c or trans-6, 17–
50 h.[11,17] [b] meta and para isomers could not be resolved and are listed
together.
coordinate PtIV structures is well precedented.[4] Coordination
of the solvent to the PtII product of C H reductive elimi-
À
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Communications
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Organo-
metallics 2004, 23,5007; d) J. Oxgaard,W. A. Goddard III, J.
Am. Chem. Soc. 2004, 126,442; e) M. Lail,B. N. Arrowood,T. B.
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tion and Functionalization of C-H Bonds (Eds.: K. I. Goldberg,
A. S. Goldman),ACS Symposium Series 885,Washington, 2004;
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[5] M. G. MacDonald,C. N. Kostelansky,P. S. White,J. L. Temple-
ton, Organometallics 2006, 25,4560.
[6] A recent success with Pt was reported as this Communication
was in press: B. A. McKeown,N. A. Foley,J. P. Lee,T. B.
Gunnoe, Organometallics, 2008, 27,4031.
Scheme 2. Proposed mechanism for the hydroarylation of C2H4 with
C6H6. The ligand L in B may be an open site, C2H4 (complex 5), or
SMe2.
nation would produce intermediate D,and a rapid equilibri-
um between arene complexes D and F would account for the
H,D exchange observed in the hydroarylation of C2H4 using a
1:1 mixture of C6H6 and C6D6. Scrambling of H,D between Pt-
bound Ph and arene groups has been previously reported.[16]
Replacement of the bound arene of F with an olefin releases
the product giving A. Displacement of the arene in D by the
olefin would lead to the formation of dialkylated arene
products,and b-hydride elimination from B would lead to
styrene formation.
[7] D. Karshtedt,A. T. Bell,T. D. Tilley, Organometallics 2004, 23,
4169.
[8] D. Karshtedt,J. L. McBee,A. T. Bell,T. D. Tilley,
Organo-
metallics 2006, 25,1801.
[9] A. T. Luedtke,K. I. Goldberg, Inorg. Chem. 2007, 46,8496.
[10] J. J. Klappa,A. E. Rich,K. McNeill, Org. Lett. 2002, 4,435.
[11] See the Supporting Information.
[12] At low temperature (À538C),the 1H NMR and NOESY spectra
of 1c in CD2Cl2 are consistent with the proposed dimeric
structure.
In summary,Pt-catalyzed intermolecular hydroarylation
of unactivated olefins has been rationally developed. Studies
of regioselectivity,relative reactivity of substrates,and
[13] For a similar reaction see: C. N. Iverson,C. A. G. Carter,R. T.
Baker,J. D. Scollard,J. A. Labinger,J. E. Bercaw, J. Am. Chem.
Soc. 2003, 125,12674.
[14] Heating a C6D6 solution of [{PtMe3OTf}4] or [{Me2Pt(m-SMe2)}2]
at 1008C under C2H4 does not produce [D6]ethylbenzene.
[15] Evidence has not yet been obtained to indicate the stereochem-
istry of the proposed intermediates.
[16] a) B. J. Wik,M. Lersch,A. Krivokapic,M. Tilset, J. Am. Chem.
Soc. 2006, 128,2682; b) C. M. Norris,S. Reinartz,P. S. White,
J. L. Templeton, Organometallics 2002, 21,5649.
[17] TONs were determined by GC-FID using the relative peak areas
versus an internal naphthalene standard. Relative response
factors versus the naphthalene standard were determined from
authentic samples that are commercially available: ethylben-
deuterium labeling are consistent with a mechanism of
II
À
olefin insertion into a Pt –aryl bond and aryl C H oxidative
addition at PtII center. Modification of both the sterics and the
electronics of the (pyridyl)pyrrolide ligand to promote
greater selectivity for anti-Markovnikov hydroarylation prod-
ucts and to increase the lifetime of the catalysts are currently
under investigation.
Received: February 1,2008
Revised: June 30,2008
Published online: August 25,2008
zene,2-phenylpropane,1-phenylpropane,
p-ethyltoluene, o-
ethyltoluene,phenylcyclohexane, m-cymene, p-cymene,and o-
cymene. The relative response factors of the other products were
assumed to be unity.
À
Keywords: arenes · C H activation · hydroarylation · olefins ·
platinum
.
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Angew. Chem. Int. Ed. 2008, 47, 7694 –7696