Platinum-based catalysts for the hydroamination of olefins with sulfonamides and weakly basic anilines

Article abstract of DOI:10.1021/ja052836d

Electrophilic Pt(II) complexes catalyze efficient hydroaminations of olefins by sulfonamides and weakly basic anilines. Catalysts include the structurally characterized complex (COD)Pt(OTf)₂ (1) and the known dimer [PtCl₂(C₂H₄)]₂, activated by AgBF₄. Experiments with substituted anilines establish an empirical pK_a cutoff (conjugate acid pK_a < 1) for the participation of nitrogen-containing substrates in this catalysis. Arylsulfonamides (conjugate acid pK_a ≈ = -6) with various para substituents hydroaminate olefins such as cyclohexene in yields greater than 95% at 90 °C. Hydroamination of propylene by p-toluenesulfonamide proceeds with Markovnikov selectivity, suggesting a mechanism that involves olefin activation at Pt. With norbornene and p-toluenesulfonamide as the substrates and 1 as the catalyst, intermediate [(COD)Pt-(norbornene)₂][OTf]₂ (3) was identified and characterized by ¹⁹F and ¹⁹⁵Pt NMR spectroscopies and mass spectrometry. Kinetic studies provide the empirical rate law, rate = k_obs[Pt][sulfonamide], and are consistent with a mechanism in which attack of a sulfonamide on the Pt-coordinated olefin is the rate-determining step.

Full text of DOI:10.1021/ja052836d

Published on Web 08/20/2005
Platinum-Based Catalysts for the Hydroamination of Olefins
with Sulfonamides and Weakly Basic Anilines
Dmitry Karshtedt,†,‡ Alexis T. Bell,*,‡,§ and T. Don Tilley*,†,‡
Contribution from the Departments of Chemistry and Chemical Engineering, UniVersity of
California, Berkeley, Berkeley, California 94720, and Chemical Sciences DiVision,
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720
Received May 2, 2005; E-mail: tdtilley@berkeley.edu
Abstract: Electrophilic Pt(II) complexes catalyze efficient hydroaminations of olefins by sulfonamides and
weakly basic anilines. Catalysts include the structurally characterized complex (COD)Pt(OTf)
known dimer [PtCl (C )] , activated by AgBF . Experiments with substituted anilines establish an empirical
pK cutoff (conjugate acid pK < 1) for the participation of nitrogen-containing substrates in this catalysis.
Arylsulfonamides (conjugate acid pK
2
(1) and the
2
H
2 4
2
4
a
a
a
≈ -6) with various para substituents hydroaminate olefins such as
cyclohexene in yields greater than 95% at 90 °C. Hydroamination of propylene by p-toluenesulfonamide
proceeds with Markovnikov selectivity, suggesting a mechanism that involves olefin activation at Pt. With
norbornene and p-toluenesulfonamide as the substrates and 1 as the catalyst, intermediate [(COD)Pt-
2 2
norbornene) ][OTf]
(3) was identified and characterized by 19F and Pt NMR spectroscopies and mass
195
(
spectrometry. Kinetic studies provide the empirical rate law, rate ) kobs[Pt][sulfonamide], and are consistent
with a mechanism in which attack of a sulfonamide on the Pt-coordinated olefin is the rate-determining
step.
Introduction
transition metal complexes, which tend to exhibit greater
stability and functional group tolerance, may represent more
versatile alternatives as hydroamination catalysts.
Hydroamination, the formal addition of an N-H bond across
an unsaturated organic fragment, is a convenient and atom-
economical method for the preparation of amine derivatives.
8
A number of studies have implicated electrophilic Pt com-
plexes as efficient catalysts for transformations that involve
1
Although several efficient catalysts for the hydroamination of
9
olefin activation at the metal center. For example, a recent study
alkynes,1 vinylarenes, dienes, and electron-deficient alkenes
b,2
3
4
5
details the addition of carboxamides to ethylene and propylene
have recently been discovered, general systems for intermo-
lecular hydroaminations of unactivated olefins remain elusive.
Lanthanide-based catalysts developed by Marks and co-workers
constitute a notable exception, as broad substrate scopes in both
the olefin and the amine have been realized.1e,7 However, late
9
c
catalyzed by a Pt-triphenylphosphine complex, while two
6
other reports describe PtBr2-catalyzed hydroamination of olefins
9
d,e
by aniline in an ionic solvent. Both of these systems appear
to feature attack of an amine on a coordinated olefin as a key
mechanistic step. Furthermore, related Pd-based amination
catalysts that participate in Wacker-type chemistry also proceed
†
Department of Chemistry, University of California, Berkeley.
Lawrence Berkeley National Laboratory.
Department of Chemical Engineering, University of California, Ber-
1
0
‡
§
by olefin activation. Many of the factors involved in the
hydroamination of olefins by electrophilic transition metal ions
keley.
(
1) (a) M u¨ ller, T. E.; Beller, M. Chem. ReV. 1998, 98, 675-704. (b) Bytschkov,
I.; Doye, S. Eur. J. Org. Chem. 2003, 6, 935-946. (c) Beller, M.; Seayad,
J.; Tillack, A.; Jiao, H. Angew. Chem., Int. Ed. 2004, 43, 3368-3398. (d)
Hartwig, J. F. Pure Appl. Chem. 2004, 76, 507-516. (e) Hong, S.; Marks,
T. J. Acc. Chem. Res. 2004, 37, 673-686.
(8) For early studies of late transition metal-catalyzed hydroamination of
norbornene, see: (a) Casalnuovo, A. L.; Calabrese, J. C.; Milstein, D. J.
Am. Chem. Soc. 1988, 110, 6738-6744. (b) Brunet, J.-J.; Commenges,
G.; Neibercker, D.; Philippot, K. J. Organomet. Chem. 1994, 469, 221-
228. (c) Dorta, R.; Egli, P.; Z u¨ rcher, F.; Togni, A. J. Am. Chem. Soc. 1997,
119, 10857-10858.
(9) (a) Hahn, C.; Cucciolito, M. E.; Vitagliano, A. J. Am. Chem. Soc. 2002,
124, 9038-9039. (b) Pastine, S. J.; Youn, S. W.; Sames, D. Org. Lett.
2003, 5, 1055-1058. (c) Wang, X.; Widenhoefer, R. A. Organometallics
2004, 23, 1649-1651. (d) Brunet, J.-J.; Cadena, M.; Chu, N. C.; Diallo,
O.; Jacob, K.; Mothes, E. Organometallics 2004, 23, 1264-1268. (e)
Brunet, J.-J.; Chu, N. C.; Diallo, O. Organometallics 2005, 24, 3104-
3110. (f) Liu, C.; Han, X.; Wang, X.; Widenhoefer, R. A. J. Am. Chem.
Soc. 2004, 126, 3700-3701. (g) Qian, H.; Han, X.; Widenhoefer, R. A. J.
Am. Chem. Soc. 2004, 126, 9536-9537. (h) Wang, X.; Widenhoefer, R.
A. Chem. Commun. 2004, 660-661.
(
(
2) Doye, S. Synlett 2004, 10, 1653-1672.
3) (a) Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc. 2000, 122, 9546-
9
547. (b) Utsunomiya, M.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125,
4286-14287. (c) Utsunomiya, M.; Kuwano, R.; Kawatsura, M.; Hartwig,
1
J. F. J. Am. Chem. Soc. 2003, 125, 5608-5609. (d) Utsunomiya, M.;
Hartwig, J. F. J. Am. Chem. Soc. 2004, 126, 2702-2703.
(
4) L o¨ ber, O.; Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123,
4
366-4367.
(
5) (a) Kawatsura, M.; Hartwig, J. F. Organometallics 2001, 20, 1960-1964.
(
b) Fadini, L.; Togni, A. Chem. Commun. 2003, 30-31. (c) Fadini, L.;
Togni, A. Chimia 2004, 58, 208-211.
(
6) For recent examples of intramolecular hydroaminations, see: (a) Ryu, J.-
S.; Marks, T. J.; McDonald, F. E. J. Org. Chem. 2004, 69, 1038-1052.
(10) (a) Fix, S. R.; Brice, J. L.; Stahl, S. S. Angew. Chem., Int. Ed. 2002, 41,
164-166. (b) Timokhin, V. I.; Anastasi, N. R.; Stahl, S. S. J. Am. Chem.
Soc. 2003, 125, 12996-12997. (c) Brice, J. L.; Harang, J. E.; Timokhin,
V. I.; Anastasi, N. R.; Stahl, S. S. J. Am. Chem. Soc. 2005, 127, 2868-
2869.
(
b) Bender, C. F.; Widenhoefer, R. A. J. Am. Chem. Soc. 2005, 127, 1070-
1
071.
7) Ryu, J.-S.; Li, G. Y.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 12584-
2605.
(
1
1
2640 J. AM. CHEM. SOC. 2005, 127, 12640-12646
9
10.1021/ja052836d CCC: $30.25 © 2005 American Chemical Society

Hydroamination of Olefins with Sulfonamides and Anilines
A R T I C L E S
Table 1. Summary of Crystallographic Data for
(COD)Pt(OTf)2‚0.5C6H6 (1‚0.5C6H6)
formula
2 6 6 13 15
PtS F O C H
MW
640.46
colorless, platelike
crystal color, habit
crystal dimensions
crystal system
3
0.05 × 0.15 × 0.15 mm
monoclinic
cell determination (2θ range)
3487 (5.00 < 2θ < 55.00°)
lattice parameters
a ) 24.017(2) Å
b ) 12.684(1) Å
c ) 13.349(1) Å
â ) 114.421(2)°
3
V ) 3702.7(7) Å
space group
Z value
Dcalcd
µ (Mo KR)
temperature
scan type
C2/c (no. 15)
8
3
2.298 g/cm
78.61 cm
-
1
-127 °C
ω (0.3° per frame)
no. of reflections measured
total: 11 764
unique: 4713 (Rint ) 0.040)
Lorentz-polarization absorption
corrections
Figure 1. ORTEP diagram of the X-ray crystal structure of (COD)Pt-
(
Tmax ) 1.00, Tmin ) 0.63)
(
OTf)2‚0.5C6H6 (1‚0.5C6H6). Hydrogen atoms and the C6H6 molecule are
omitted for clarity. Bond lengths (Å): Pt-O1 ) 2.085(3), Pt-O4 ) 2.087-
3), Pt-C1 ) 2.155(5), Pt-C2 ) 2.141(5), Pt-C5 ) 2.144(5), Pt-C6 )
.137(5), C1-C2 ) 1.393(7), C5-C6 ) 1.376(7).
structure solution
refinement
no. reflections observed
no. variables
direct methods (SIR97)
full-matrix least squares
3265 [I > 3.0σ(I)]
263
(
2
R; Rw; Rall
GOF
max peak in final diff. map
min peak in final diff. map
0.026; 0.029; 0.040
1.02
Table 2. Catalytic Hydroamination of Norbornene by ArNH2a
-
3
1.76 e /Å
Ar
pK
a
(conj. acid)
conditions
yield (%)
-
3
-0.73 e /Å
C6F5
p-NO2-C6H4
,5-(CF3)2C6H3
0.3
1.0
1.8
4.6
80 °C, 2 h
100
35
45
0
90 °C, 16 h
90 °C, 16 h
130 °C, 16 h
3
have been examined in recent theoretical studies.11 Still, while
hydroamination with late transition metal complexes is an
actively growing area, mechanistic aspects of this transformation
remain to be fully elucidated.
The current study was motivated by our recent finding that
certain electrophilic Pt complexes catalyze olefin hydroarylation,
which appears to involve nucleophilic attack of an arene onto
a platinum-bound olefin. This prompted an investigation of
analogous reactions involving the addition of N-H bonds to
olefins, since amines seemed to be potential candidates as
analogous nucleophiles in a related process. In this report, we
describe an efficient system for the hydroamination of olefins
with sulfonamides and weakly basic anilines.
C6H5
a
Catalyst: 10 mol % 1, solvent: 1,2-dichloroethane.
noted that 1 is highly water-sensitive, and attempted crystal-
lization of 1 from methylene chloride containing trace amounts
of water led instead to formation of the hydroxo-bridged
dinuclear complex [(COD)Pt(OTf)-µ-OH-(OTf)Pt(COD)]OTf
1
2
(
2), whose structure was established by X-ray crystallography.14
Thus, all manipulations involving 1 were performed under inert
atmosphere and with rigorously dry solvents.
Hydroamination with Weakly Basic Anilines. With 10 mol
%
of 1 as a potential catalyst, norbornene did not react with
either aniline or morpholine at temperatures up to 130 °C in
o-dichlorobenzene solvent. An examination of additional amine
substrates, however, revealed that 1 catalyzes the quantitative
hydroamination of norbornene with the weakly basic amine
pentafluoroaniline (conjugate acid pKa ) 0.3) at 80 °C over
Results and Discussion
Catalytst Characterization. Previous investigations of the
hydroarylation of olefins with electrophilic platinum complexes
demonstrated that (COD)Pt(OTf)2 (1) is a convenient catalyst
precursor, presumably because the triflate ligands are labile
enough to be readily displaced by the olefin substrate. To
obtain structural information on this complex, an X-ray dif-
fraction study was completed (Table 1). The molecular structure
1
5
the course of 2 h. Since hydroamination appeared to correlate
with amine basicity, the reactivity of anilines with conjugate
acid pKa values falling between those of pentafluoroaniline and
aniline (conjugate acid pKa ) 4.6) was examined. Both 3,5-
1
2
(bis-trifluoromethyl)aniline (conjugate acid pKa ) 1.8) and
1
of 1 (Figure 1) features η -coordination for both triflate anions.
p-nitroaniline (conjugate acid pKa ) 1.0) participated in the
catalyzed hydroamination of norbornene to form the corre-
sponding N-norbornyl aniline derivatives, but these reactions
required more forcing conditions and proceeded to lower yields
The Pt-O distances in this complex are fairly short, 2.085(3)
and 2.087(3) Å, suggesting strong Pt-triflate interactions as
compared to those in related phosphine complexes. For example,
Pt-O distances in (dppe)Pt(OTf)2 (dppe ) 1,2-bis-diphenyl-
phosphinoethane) are 2.120(4) and 2.138(4) Å.13 It should be
1
6
(
Table 2). Still, these transformations are highly selective for
hydroamination products, and unreacted olefin and aniline
(
11) (a) Senn, H. M.; Bl o¨ chl, P. E.; Togni, A. J. Am. Chem. Soc. 2000, 122,
4
098-4107. (b) Senn, H. M.; Deubel, D. V.; Bl o¨ chl, P. E.; Togni, A.;
(14) See Supporting Information for crystallographic data for 2.
(15) For this and all other products of norbornene hydroamination reported
herein, exo-substituted isomers were exclusively formed.
Frenking, G. THEOCHEM 2000, 506, 233-242.
(12) Karshtedt, D.; Bell, A. T.; Tilley, T. D. Organometallics 2004, 23, 4169-
4
171.
a
(16) For a compilation of pK values of conjugate acids of substituted anilines,
(13) Brunkan, N. M.; White, P. S.; Gagn e´ , M. R. Organometallics 2002, 21,
565-1575.
see: Gross, K. C.; Seybold, P. G.; Peralta-Inga, Z.; Murray, J. S.; Politzer,
P. J. Org. Chem. 2001, 66, 6919-6925.
1
J. AM. CHEM. SOC.
9
VOL. 127, NO. 36, 2005 12641

A R T I C L E S
Karshtedt et al.
Table 3. Catalytic Hydroamination of Olefins by
a
p-Toluenesulfonamide
Hydroamination of the inherently less reactive olefins cis-2-
butene and propylene was not catalyzed by 1 and required the
more active catalyst combination of Zeise’s dimer and AgBF4
(Table 3). A control experiment in which AgBF4 by itself was
used as a potential catalyst for a reaction between cis-2-butene
and p-toluenesulfonamide yielded no hydroamination products.
Notably, the reaction of p-toluenesulfonamide with propylene
led to exclusive formation of the Markovnikov product N-
isopropyl-p-toluenesulfonamide. The same selectivity was ob-
served by Widenhoefer and co-workers for propylene hydro-
amination by an alkylcarboxamide using a neutral Pt(II)-
triphenylphosphine catalyst, but under more forcing reaction
9
c
conditions (120 °C, 80 h, 100 psi). In a significant contrast
with the system described in this report, however, cyclohexene,
cyclopentene, and norbornene did not undergo Pt-catalyzed
20
hydroamination by carboxamides. Nevertheless, the results of
Widenhoefer’s study are consistent with the prediction that the
appropriate basicity of carboxamides (conjugate acid pKa of
2
1
benzamide ) -2.1) should enable their participation in the
Pt-catalyzed hydroamination. Indeed, norbornene was hy-
droaminated by benzamide at 110 °C over the course of 40 h
with 1 as the catalyst (1,2-dichloroethane solvent, >95%
a
Solvent: o-dichlorobenzene. b Catalyst 1 ) 10 mol % 1; catalyst 2 )
c
22
5
mol % [PtCl2(C2H4)]2 and 10 mol % AgBF4. 1 atm pressure.
yield).
Examination of additional olefin substrates revealed that
-hexene was rapidly isomerized to a mixture of internal hexenes
derivative account for the remaining components of the reaction
mixtures. Similar observations were made by Brunet and co-
workers, who reported that the number of turnovers for PtBr2-
catalyzed ethylene hydroamination increased with decreasing
basicity of the substituted aniline substrate.9d
1
that reacted with p-toluenesulfonamide at 85 °C over the course
of 12 h to give a mixture of two N-hexyl regioisomers, with an
overall yield of 60% (complex 1 was used as the catalyst).
Interestingly, although olefin isomerization is commonly pro-
moted by Pt(II) complexes, a recent report described PtBr2-
catalyzed Markovnikov hydroamination of 1-hexene by aniline
where no products derived from internal olefins were detected.9e
Furthermore, ethylene was unreactive with Zeise’s dimer and
Hydroamination with Sulfonamides. With the above ob-
servations, an empirical pKa cutoff for predicting the participa-
tion of amines in this catalytic system was established, leading
us to examine the reactivity of additional weak nitrogen bases
(conjugate acid pKa < 1) in olefin hydroaminations catalyzed
AgBF as the catalyst combination at temperatures up to 120
4
by 1. Sulfonamides were chosen for a more in-depth study
because of their appropriately low basicity (conjugate acid pKa
°C, while styrene was only oligomerized with either catalyst
system. In contrast, ethylene was reported to undergo Pt-
17
9c
of benzenesulfonamide ) -6.3) and their extensive use in
the pharmaceutical industry.18 The synthesis of N-alkylsulfona-
mides is normally achieved by the reaction of deprotonated
catalyzed hydroamination by various carboxamides at 120 °C.
This result further underscores the differences in substrate scope
between the two catalyst systems and suggests that, despite the
similarity of amide derivatives that were used, some mechanistic
aspects of the two transformations may be quite distinct.
Operative electronic factors for this catalysis were probed
by varying para-aryl ring substituents of the sulfonamides (eq
2). With cyclohexene as the olefin substrate, sulfonamides with
various para-aryl groups (-OMe, -Cl, -H, and -CF3) were
subjected to the standard reaction conditions (Table 4). All of
these derivatives participated in high-yielding hydroaminations
to form the corresponding N-cyclohexyl products. Note that, in
a related HOTf-catalyzed intramolecular hydroamination of
alkenes by tethered sulfonamides, substrates containing the
methoxybenzene functionality decomposed via acid-promoted
19a
sulfonamides with alkyl halides or by the condensation of
sulfonyl chlorides and amines,1 which generate stoichiometric
amounts of salt or acid byproducts, respectively. With p-
toluenesulfonamide as a test substrate and 1 as the catalyst, it
was found that norbornene, cyclohexene, and cyclopentene were
hydroaminated (eq 1) in yields in excess of 95% under the
experimental conditions given in Table 3. The equimolar (per
metal) combination of [PtCl2(C2H4)]2 (Zeise’s dimer) and AgBF4
catalyzed the hydroamination of cyclohexene by p-toluene-
sulfonamide under milder conditions than those for 1 (80 °C,
9b
2
h), but the active species presumably formed by chloride
abstraction from the Pt center could not be isolated.
2
3
ether cleavage. Thus, observation of clean hydroamination of
(
17) Virtanen, P. O. I.; Maikkula, M. Tetrahedron Lett. 1968, 47, 4855-4858.
18) Brackett, C. C.; Singh, H.; Block, J. H. Pharmacotherapy 2004, 24, 856-
(
(20) Widenhoefer, R. A., Duke University. Personal communication, 2005.
(21) Liler, M. Spectrochim. Acta, Part A 1967, 23, 139-147.
(22) For another example of norbornene hydroamination by benzamide, see:
Aufdenblatten, R.; Diezi, S.; Togni, A. Monatsh. Chem. 2000, 131, 1345-
1350.
8
70.
(19) (a) March, J. AdVanced Organic Chemistry; John Wiley & Sons: New
York, 1992; p 425. (b) March, J. AdVanced Organic Chemistry; John Wiley
&
Sons: New York, 1992; p 499.
12642 J. AM. CHEM. SOC.
9
VOL. 127, NO. 36, 2005

Hydroamination of Olefins with Sulfonamides and Anilines
A R T I C L E S
Table 4. Catalytic Hydroamination of Cyclohexene by RSO2NH2a
Scheme 1. Proposed Catalytic Cycle for the Hydroamination of
Olefins by Pt(II) Complexes
R
product
yield (%)
phenyl
p-OMe-phenyl
p-Cl-phenyl
p-CF3-phenyl
methyl
N-cyclohexylbenzenesulfonamide
>95
>95
>95
>95
>95
N-cyclohexyl-p-OMe-benzenesulfonamide
N-cyclohexyl-p-Cl-benzenesulfonamide
N-cyclohexyl-p-CF3-benzenesulfonamide
N-cyclohexylmethanesulfonamide
b
a
Conditions as in eq 2. b 110 °C, 30 h reaction time.
p-methoxybenzenesulfonamide by the Pt-based catalyst 1 re-
flects a degree of functional group tolerance.
Information). Unfortunately, attempts to isolate the complex
[(COD)Pt(norbornene)2][OTf]2 (3) by removing the solvent and
excess olefin under vacuum led to the formation of intractable
mixtures.
The electronic effects of aryl ring substituents in the sul-
fonamides were further accessed via competition experiments
involving reactions of excess norbornene with equimolar
mixtures of a para-substituted sulfonamide and the parent
benzenesulfonamide, and the yields of hydroamination products
were quantified by gas chromatography. No significant depen-
dence on the electronic nature of the substituents was observed,
as relative rates decreased in the order Me (2.0) > CF3 (1.5) >
OMe (1.4) > H (1.0) > Cl (0.9). This observation suggests
that the substituted arylsulfonamides that were examined may
have fairly similar basicities, and analogous results were reported
by Widenhoefer and co-workers for Pt-catalyzed ethylene
hydroamination with substituted arylcarboxamides.9c Interest-
ingly, the substrate methanesulfonamide, which contains a
relatively electron-donating methyl group, required rather forcing
conditions for cyclohexene hydroamination (Table 4). This result
is consistent with the general observation that higher nitrogen
basicity leads to lower reactivity in this catalysis.
In addition to its mass spectrometric identification, 3 was fully
1
19
195
characterized in solution by H, F, and
Pt NMR spec-
troscopies and was shown to be a persistent intermediate in the
catalytic hydroamination of nobornene by p-toluenesulfonamide.
Upon addition of excess (10 equiv) of norbornene to a solution
195
of 1 in 1,2-dichloroethane-d4 at 50 °C, the Pt NMR resonance
for the starting Pt complex (-3084 ppm) was replaced after 20
min by a new resonance at -3771 ppm, presumably corre-
sponding to complex 3. This signal persists after p-toluene-
sulfonamide (10 equiv) is added to the reaction mixture,
indicating that the initially formed olefin complex does not
undergo ligand substitution with the added sulfonamide under
these conditions. The signal at -3771 ppm persists as the
reaction mixture is further heated to 75 °C, and a new signal at
-3320 ppm, possibly due to a catalyst decomposition product,
appears only when most of the olefin (>90%) has been
1
consumed in the catalytic reaction. In addition, H NMR spectra
Identification of the Active Form of the Catalyst. Gener-
ally, two types of catalytic cycles are proposed for hydroami-
nations with late transition metal catalysts. The first is based
on initial N-H bond activation, followed by olefin insertion
and product-forming reductive C-H or C-N bond elimination,
while the second involves nucleophilic attack of an amine onto
of the reaction mixture taken during the course of catalysis show
that cyclooctadiene remains bound to Pt. In 1,2-dichloroethane-
d4, the olefinic resonance for the COD ligand of complex 3
appears at 5.72 ppm (JPt-H ) 80 Hz), as compared to 5.98 ppm
for 1 (JPt-H ) 77 Hz).
8a
1
9
1
Infrared and F{ H} NMR spectroscopies confirm the
-
a coordinated olefin and a product-forming proton-transfer step
displacement of OTf from the Pt center upon addition of
9
d
(
Scheme 1). The Markovnikov regioselectivity of the hy-
norbornene to 1. A solution IR spectrum of 3 contains a
-
1
droamination of propylene by p-toluenesulfonamide and the lack
of precedent for stoichiometric N-H bond activations by Pt(II)
seem to highly favor the latter mechanism for reactions catalyzed
by 1. Indeed, attack of nitrogen and oxygen nucleophiles on
Pt-coordinated olefin species has been demonstrated in a number
of cases for electrophilic Pt(II)-olefin complexes.2
characteristic band at 1242 cm that is indicative of ionic
triflate, while the IR spectrum of 1, featuring a band at 1344
-
1
cm , suggests the presence of covalently bound, monodentate
2
5
19
1
triflate. Furthermore, while the F{ H} NMR spectrum of
complex 1 contains a resonance at -78.0 ppm, a new resonance
at -79.1 ppm appears upon formation of 3 by the addition of
10 equiv of norbornene and heating to 50 °C in 1,2-dichloro-
4
To identify potential intermediates in the hydroamination
catalysis, reaction mixtures involving norbornene as the substrate
were examined by mass spectrometry and NMR spectroscopy.
Excess (10 equiv) of norbornene was added to complex 1
dissolved in 1,2-dichloroethane, and the resulting mixture was
analyzed by fast ion bombardment mass spectrometry (FAB-
1
9
1
ethane-d4. The F{ H} NMR chemical shift for 3 suggests the
presence of outer-sphere triflate, since the resonance of PPN-
2
6
[OTf], a salt that contains a free ionic triflate, is at -79.2
1
195
19
1
ppm. As with the H and Pt resonances, the F{ H} signal
for 3 persists after p-toluenesulfonamide is added and the
catalytic reaction proceeds. Thus, complex 3, which forms in
solution from 1 and excess norbornene, remains as the dominant
Pt-containing species during norbornene hydroamination with
p-toluenesulfonamide.
+
MS). A species at m/z ) 490 (M - 1) , resulting from the loss
2
+
of a proton from the dication [(COD)Pt(norbornene)2] , was
observed exclusively, and the actual and simulated isotope
patterns for this ion are virtually identical (see Supporting
(
23) Schlummer, B.; Hartwig, J. F. Org. Lett. 2002, 4, 1471-1474.
(25) Lawrance, G. A. Chem. ReV. 1986, 86, 17-33.
(26) PPN ) bis(triphenylphosphoranylidene)ammonium.
(
24) See: Hahn, C. Chem.-Eur. J. 2004, 10, 5888-5899 and references therein.
J. AM. CHEM. SOC.
9
VOL. 127, NO. 36, 2005 12643

A R T I C L E S
Karshtedt et al.
Reaction of 3 with Morpholine. The origin of the differences
in reactivity for various nitrogen bases in this system was further
probed by examining the interactions of 3 with morpholine.
While, as described above, 3 does not undergo a rapid reaction
with p-toluenesulfonamide, the addition of 10 equiv of mor-
pholine to 3 (generated from 1 and 10 equiv of norbornene in
acid additives have been used to promote the hydroamination
of electron-deficient olefins by aniline, also with a Pd-based
catalyst.
3
0
Further support for the mechanism depicted in Scheme 1 was
provided by a study of the kinetics of hydroamination of
norbornene by 4-n-butylbenzenesulfonamide, which was used
because of its high solubility in dichloromethane. Measurements
of the kinetics were conducted at 37 °C in CD2Cl2, and reaction
progress was monitored by quantifying the concentration of the
1
,2-dichloroethane-d4) at room temperature is accompanied by
the immediate replacement of the resonance for 3 with two new
195
resonances (-3708 and -3710 ppm) in the
Pt NMR
1
spectrum. While the components of this mixture could not be
identified further, the observation of a rapid reaction of
morpholine with the bis-norbornene complex 3 may provide a
justification for its lack of participation in catalytic hydroami-
nations.
N-norbornyl product by H NMR spectroscopy. Reaction orders
in the sulfonamide and catalyst 1 were determined by measuring
reaction rates under conditions of excess norbornene (10 equiv
of norbornene relative to the sulfonamide) and 5-20 equiv of
the sulfonamide relative to 1. The reaction order in norbornene
was determined by varying the concentration of norbornene (1-
On the basis of known reactivity modes for Pt-olefin
complexes with amines, there seem to be two reasonable
explanations for the fact that morpholine (and other amines for
which the pKa of the conjugate acid > 4) does not engage in
this hydroamination catalysis. First, the more basic amines may
bind too strongly to the Pt center, thus preventing the olefin
substrate from undergoing competitive activation via coordina-
tion. Second, it may be that these amines form intermediate
ammonium adducts (with the olefin substrate or with the COD
ligand) that are not acidic enough to undergo product-forming
proton transfer. Both ligand substitution and C-N bond
formation pathways have been observed in previous studies of
the stoichiometric reactions of amines with Pt-olefin com-
plexes. For example, a recent report indicates that various
1
0 equiv relative to the sulfonamide). These experiments
provided the empirical rate law, rate ) kobs[Pt][sulfonamide],
-
2
-1 -1
where kobs ) 1.28 × 10
M
s . This rate expression is
consistent with a pre-equilibrium involving the binding of
norbornene to form intermediate A, followed by a rate-
determining C-N bond-forming event that produces intermedi-
ate B, which transforms rapidly via proton transfer to form the
hydroamination product (Scheme 1). Another mechanistic
possibility that leads to the observed rate expression involves a
rapid equilibrium between intermediates A and B (with K ,
1
), followed by a rate-determining cleavage of the Pt-C bond
by the acidic proton. Thus, further studies are needed to
differentiate between the two pathways.
Attempts to establish the stereochemical outcome of the
hydroamination of norbornene by the reaction with N-deuterated
R-olefins are easily displaced from cis-(PPh3)(olefin)PtCl2 by
excess diethylamine at room temperature.27 In contrast, studies
31
p-toluenesulfonamide were complicated by deuterium scram-
bling into various positions of the N-norbornyl product. This
process, which may occur via a sequence of rapid C-H bond
by Vitagliano and co-workers demonstrate that dicationic
Pt(II)-olefin complexes react readily with aniline to form stable
zwitterionic Pt-aminoalkyl species.28 Given the spectroscopic
3
2
activation/C-D bond elimination steps, also hindered efforts
to establish the kinetic isotope effect by determining the rate
of norbornene hydroamination by N-deuterated 4-n-butylben-
results currently at hand, it is not possible to determine which
factor may be important in preventing morpholine from
participating in the catalysis. However, since poisoning of
potential hydroamination catalysts via alkylamine binding was
observed in a related system,29 we believe that ligand substitu-
tion of Pt-bound norbornene by morpholine is the more likely
pathway for inhibiting the catalytic reactivity for this substrate.
31
zenesulfonamide. The apparent propensity of hydroamination
products to undergo C-H bond activation has been noted by
Widenhoefer and co-workers, who established that the Pt-
catalyzed coupling of styrene and benzamide is reversible.33
Thus, deuterium labeling cannot serve as an accurate stereo-
chemical probe in this system.
Further Mechanistic Studies of Norbornene Hydroami-
nation. Weak nitrogen bases such as sulfonamides are not
expected to compete with olefins as ligands for Pt, and this is
confirmed by the persistence of 3 in the course of catalytic
hydroamination of norbornene by p-toluenesulfonamide. More-
over, intermediate B (Scheme 1) is predicted to be much more
acidic for sulfonamides than for alkylamines, allowing for a
facile proton transfer to cleave the Pt-C bond. The intermediacy
of a protonated sulfonamide species is consistent with the
observation that 1 equiv (relative to the Pt catalyst) of 2,6-di-
tert-butyl-4-methylpyridine, a sterically hindered base, com-
pletely shuts down the catalytic reactivity in this system.
Consistent with this observation, base additives have been used
in Pd-catalyzed oxidative amination chemistry to prevent the
Concluding Remarks
Catalytic hydroamination that proceeds via olefin activation
at the metal center appears to necessitate a nitrogen base that is
a weak donor, such that it does not compete with the olefin for
metal binding. However, the amine must be nucleophilic enough
to attack a carbon atom of the metal-olefin complex. These
conflicting requirements appear to limit the range of substrates
that may currently be used in hydroamination, as catalyzed by
electrophilic Pt complexes. Interestingly, many of the factors
relevant for late transition metal-catalyzed hydroamination (e.g.,
proton transfer and ligand substitution of olefin by amine) were
considered in a recently published theoretical article.1 That
1a
10b
formation of undesired olefin hydroamination products, while
(
30) Seligson, A. L.; Trogler, W. C. Organometallics 1993, 12, 744-751.
(
(
(
27) Pryadun, R.; Sukumaran, D.; Bogadi, R.; Atwood, J. D. J. Am. Chem. Soc.
004, 126, 12414-12420.
28) Hahn, C.; Morvillo, P.; Herdtweck, E.; Vitagliano, A. Organometallics 2002,
1, 1807-1818.
29) Schaffrath, H.; Keim, W. J. Mol. Catal. A: Chem. 2001, 168, 9-14.
(31) Uno, T.; Machida, K.; Hanai, K. Spectrochim. Acta, Part A 1968, 24, 1705-
2
1712.
(32) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118,
5961-5976.
(33) Qian, H.; Widenhoefer, R. A. Org. Lett. 2005, 7, 2635-2638.
2
1
2644 J. AM. CHEM. SOC. VOL. 127, NO. 36, 2005
9

Hydroamination of Olefins with Sulfonamides and Anilines
A R T I C L E S
study concluded that group 10 transition metal ions represent
promising potential catalysts for olefin hydroamination, and the
results described here, and in related investigations reported
an HP-5MS column. FAB-MS spectra were recorded on a ZAB2-EQ
spectrometer from Micromass.
General Procedure for Catalytic Runs. Reactions were conducted
in 5-mm Wilmad NMR tubes equipped with a J. Young Teflon valve
seal, which were heated in temperature-controlled oil baths. Samples
were prepared in the drybox by dissolving the catalyst, amine, and solid
or liquid olefin in the specified solvent. Gases were introduced on a
Schlenk line into J. Young NMR tubes containing the catalyst and amine
dissolved in the specified solvent. Reaction progress was monitored
9
c-e
elsewhere,
appear to bear out this prediction.
The catalytic system described in this report represents a
promising new approach for the hydroamination of olefins by
sulfonamides and weakly basic amines. While further studies
are needed to elucidate the details of this catalysis, the reaction
appears to involve olefin activation by coordination to Pt and a
subsequent attack on this species by an external nitrogen
nucleophile. Thus, hydroamination may proceed by a mechanism
similar to that for the Pt-catalyzed hydroarylation reported
1
by H NMR spectroscopy (a small amount of cyclohexane-d12 was
added to all samples to obtain a lock signal). Product yields were
quantified by GC, using a calculated response factor to account for the
difference in ionization between the integration standard (naphthalene)
and N-norbornyl-p-toluenesulfonamide.
Sample Procedure for the Isolation of Organic Products. In a
drybox, a flask equipped with a Teflon-sealable valve was charged with
1
2
earlier. This points to the potential generality of electrophilic
Pt complexes in olefin activation catalysis.
Although (COD)Pt(OTf)2 is a convenient and easily prepared
catalyst precursor, efforts to expand the scope of olefin
hydroamination to include basic anilines and alkylamines are
likely to involve complexes with more elaborate ligands on Pt.
In particular, electronic properties of the ligand may increase
the tendency of the metal center to bind olefins rather than
amines, while steric factors may be used to direct the hydroami-
nation toward anti-Markovnikov selectivity. In addition, Pt
complexes containing chiral ligands may catalyze an enantio-
selective variant of this transformation. Efforts directed toward
these goals are currently underway in our laboratories.
(
2
COD)Pt(OTf) (0.050 g, 0.083 mmol), norbornene (0.078 g, 0.830
mmol), and p-toluenesulfonamide (0.142 g, 0.830 mmol). These
reactants were dissolved in 5 mL of 1,2-dichloroethane, and the flask
was sealed and heated with stirring to 75 °C in a temperature-controlled
oil bath for 2 h. The reaction mixture was then cooled to room
temperature, diluted with 5 mL of diethyl ether, and filtered through a
plug of silica to remove metal-containing residues. The solvent was
then removed, yielding N-norbornyl-p-toluenesulfonamide as a crystal-
1
line colorless solid (0.202 g, 92%). The H NMR spectrum of the
product (CDCl ) agrees with the previously reported data.
3
3
6a
Competition Experiments. Reactions were conducted in sealed
NMR tubes containing 10 equiv of both benzenesulfonamide and a
substituted arylsulfonamide relative to catalyst 1 and 100 equiv of
norbornene relative to 1 in 1,2-dichloroethane solvent (a small amount
of cyclohexane-d12 was added to all samples to obtain a lock signal).
Product yields were quantified by GC.
Experimental Section
General. All experiments were conducted under a nitrogen atmo-
sphere using standard Schlenk techniques or in a Vacuum Atmospheres
drybox. Nondeuterated solvents were distilled under N
drying agents and stored in PTFE-valved flasks. Deuterated solvents
Cambridge Isotopes) were vacuum-transferred from appropriate drying
agents.
With the exception of 4-n-butylbenzenesulfonamide, which was
2
from appropriate
Measurements of Reaction Kinetics. Reactions were monitored
1
by H NMR spectroscopy with a Bruker AVB400 spectrometer, using
(
5
-mm Wilmad NMR tubes with a Teflon valve seal. Samples were
prepared in a drybox by dissolving the appropriate quantities of 1,
norbornene, and 4-n-butylbenzenesulfonamide in 1 mL of CD Cl
2
2
purchased from Maybridge, all olefins, anilines, and sulfonamides were
purchased from Aldrich. Norbornene was purified by sublimation.
Cyclopentene, cylcohexene, and 1-hexene were dried over sodium metal
and vacuum-transferred onto activated molecular sieves before use.
Ethylene, propylene, and 2-butene were used as received. Liquid anilines
were dried over molecular sieves, and solid anilines and sulfonamides
containing a known amount of bis-(4-fluorophenyl)methane standard.
The sample was then placed in an NMR probe preheated to 37 °C,
which was calibrated using a neat ethylene glycol sample and monitored
with a thermocouple. Single-scan spectra were acquired automatically
at preset time intervals, and the peaks were integrated relative to the
internal standard.
12
2
were dried by heating under vacuum. Catalyst precursors 1 and [PtCl -
X-ray Structure Determination of 1. X-ray quality crystals of
3
4
(
C
H )]
2 4 2
(Zeise’s dimer) were prepared as reported in the literature.
(
1
COD)Pt(OTf)
,2-dichloroethane solution containing a small amount of benzene to
assist the crystal growth. The compound crystallizes with eight
molecules of (COD)Pt(OTf) and four molecules of benzene in the unit
2
‚0.5C
6 6
H formed upon slow diffusion of pentane into a
1
19
1
Analytical Methods. H (400.1 MHz), F{ H} (376.5 MHz), and
Pt (86.0 MHz) NMR spectra were recorded using a Bruker AVB400
195
spectrometer equipped with a 5-mm BB probe or a Bruker AVQ400
spectrometer equipped with a 5-mm QNP probe. Spectra were recorded
at room temperature and referenced to the residual nondeuterated solvent
2
cell. The benzene is located on the crystallographic twofold axis and
is additionally disordered between the two possible orientations of a
twofold axis in the plane of a benzene molecule. The occupancy
parameter was refined with the total occupancy constrained to be unity.
All non-hydrogen atoms in the structure were refined anisotropically.
Positions of olefinic hydrogens were refined in the final least squares
cycles, and hydrogen atoms were calculated for the disordered benzene
of solvation. Other hydrogen atoms on the COD ligand were included
in the idealized calculated positions but not refined.
The X-ray analysis of compound 1 was carried out at the UC
Berkeley CHEXRAY crystallographic facility. Measurements were
made on a Bruker SMART CCD area detector with graphite mono-
chromated Mo KR radiation (λ ) 0.71069 Å). Data were integrated
by the program SAINT and analyzed for agreement using XPREP.
Empirical absorption corrections were made using SADABS. The
structure was solved by direct methods and expanded using Fourier
techniques. All calculations were performed using the teXsan crystal-
lographic software package.
1
19
195
for H, to external C
6 6 2 6
F for F, and to external K PtCl for Pt. FT-
infrared spectra were recorded in 1,2-dichloroethane solution using a
-
1
Mattson FTIR 3000 spectrometer at a resolution of 4 cm . Identities
1
35,36
of organic products were confirmed by H NMR spectroscopy
and
by GC-MS, using an Agilent Technologies 6890N GC system with
(
34) Chatt, J.; Searle, M. L. Inorg. Synth. 1957, 5, 210-215.
1
(
35) N-Norbornyl anilines were identified by comparison with the reported H
NMR data for N-norbornyl-3,5-(bis-trifluoromethyl)aniline and other
substituted anilines in: Ackermann, L.; Kaspar, L. T.; Gschrei, C. J. Org.
Lett. 2004, 6, 2515-2518.
(
36) (a) N-Norbornyl-p-toluenesulfonamide: Fleming, I.; Frackenpohl, J.; Ila,
H. J. Chem. Soc., Perkin Trans. 1 1998, 1229-1236. (b) N-s-Butyl-p-
toluenesulfonamide: White, E. H.; Lewis, C. P.; Ribi, M. A.; Ryan, T. J.
J. Org. Chem. 1981, 46, 552-558. (c) N-Isopropyl-p-toluenesulfonamide:
Hamura, S.; Oda, T.; Shimizu, Y.; Matsubara, K.; Nagashima, H. J. Chem.
Soc., Dalton Trans. 2002, 7, 1521-1527. (d) N-Cyclopentyl-p-toluene-
sulfonamide and N-cyclohexyl-p-toluenesulfonamide: Barluenga, J.;
Jim e´ nez, C.; N a´ jera, C.; Yus, M. J. Chem. Soc., Perkin Trans. 1 1984,
7
21-725.
J. AM. CHEM. SOC.
9
VOL. 127, NO. 36, 2005 12645

A R T I C L E S
Karshtedt et al.
Acknowledgment. Acknowledgment is made to Dr. Rita
Nichiporuk for carrying out mass spectrometry measurements,
to Dr. Herman van Halbeek for assistance with 1 Pt NMR
experiments, and to Dr. Fred Hollander and Dr. Allen Oliver
for assistance with the X-ray crystallography. We also thank
one of the reviewers for proposing an alternative mechanism
consistent with the experimental rate law for hydroamination.
This work was supported by the Director, Office of Energy
Research, Office of Basic Energy Sciences, Chemical Sciences
Division, of the U.S. Department of Energy under Contract No.
DE-AC03-76SF00098.
95
Supporting Information Available: Actual and simulated
isotope patterns for 3 (PDF), X-ray crystallographic data for 1
and 2 (PDF and CIF), and representative kinetic data (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA052836D
12646 J. AM. CHEM. SOC.
9
VOL. 127, NO. 36, 2005