Heck coupling with nonactivated alkenyl tosylates and phosphates: Examples of effective 1,2-migrations of the alkenyl palladium(II) intermediates

Article abstract of DOI:10.1002/anie.200600442

(Chemical Equation Presented) What the Heck! A catalytic system composed of a Pd complex with a basic, hindered alkyl phosphine ligand is capable of promoting Heck coupling of nonactivated vinyl tosylates and phosphates with electron-deficient olefins. An unexpected 1,2-migration of the alkenyl Pd ^II intermediates (see Scheme) leads to the isomerized Heck coupling product.

Full text of DOI:10.1002/anie.200600442

Angewandte
Chemie
Stille,[2] Suzuki,[2e,f,3] Negishi,[2b] Kumada,[2b,3e,4] Sonogashira,[2-
Buchwald–Hartwig,[4a,6] carbonyl enolate,[3d] and Heck
b,e,f, 5]
couplings,[7] as effective alternatives to the less stable and
typically more expensive alkenyl triflates and nonaflates.[8]
However, the majority of this work has focused on the use of
activated vinyl phosphates and tosylates, such as a,b-unsatu-
rated systems or a-heteroatom-substituted alkenes, for which
the oxidative-addition step is nonproblematic with palla-
dium(0) catalysts bearing aryl phosphine ligands. Less atten-
tion has been devoted to nonactivated counterparts, most
likely because of the greater difficulty in carrying out the first
step of the catalytic cycle, namely the oxidative addi-
tion.[3d,4,5,9]
We now report on catalyst systems composed of a
palladium complex with a basic, hindered alkyl phosphine
that can promote the Heck coupling of nonactivated vinyl
tosylates and phosphates with electron-deficient alkenes in
good yields, thereby increasing the scope of this important
cross-coupling reaction. Furthermore, during these studies we
observed an interesting 1,2-isomerization with certain alkenyl
tosylates and phosphates under reaction conditions that
provide coupling yields as high as 95%. A mechanistic
proposal supported by experimental results and DFT calcu-
lations is included.
Cross-Coupling
DOI: 10.1002/anie.200600442
To identify suitable reaction conditions for Heck cou-
plings with nonactivated alkenyl tosylates and phosphates, we
examined the use of palladium complexes with the ever
increasingly popular bulky electron-rich phosphines.[10] All
tosylates and phosphates were prepared from starting ketones
by base-promoted proton extraction and reaction with tosyl
anhydride (Ts2O) or (PhO)2POCl, as earlier reported.[4b,11,12]
Initial coupling attempts were performed between the
tosylate 1 and ethyl acrylate (Scheme 1). After examining a
Heck Coupling with Nonactivated Alkenyl
Tosylates and Phosphates: Examples of Effective
1,2-Migrations of the Alkenyl Palladium(II)
Intermediates**
Anders L. Hansen, Jean-Philippe Ebran,
Mårten Ahlquist, Per-Ola Norrby,* and
Troels Skrydstrup*
Considerable efforts have been undertaken by numerous
groups in academia and industry over the past decade to
expand the repertoire of coupling reagents in palla-
dium(0)-catalyzed cross-coupling reactions.[1] In particu-
lar, alkenyl phosphates and tosylates have proven their
worth in various cross-coupling reactions, such as the
Scheme 1. Heck coupling with a nonactivated vinyl tosylate.
[*] A. L. Hansen, Dr. J.-P. Ebran, Prof. Dr. T. Skrydstrup
The Center for Insoluble Protein Structures (inSPIN)
Department of Chemistry and the Interdisciplinary
Nanoscience Center
variety of reaction conditions, we noted that a combination of
[PdCl2(cod)](cod = 1,5-cyclooctadiene; 5 mol%) and the
P(tBu)3HBF4 salt[13] (10 mol%) in the presence of dicyclo-
hexylmethylamine (2 equiv) in dimethylformamide (DMF) at
858C could furnish the diene 2, albeit in low yield (approx-
imately 5%). The addition of one equivalent of LiCl, how-
ever, had a dramatic effect on the reaction outcome, thus
providing a 50% yield of the diene 2 after 24 hours.[14]
Increasing the reaction temperature to 1008C improved the
coupling yield to 66%. Other reactions conditions, including
change of solvent or examination of alternative catalyst
combinations, were incapable of promoting the cross-cou-
pling.
University of Aarhus
Langelandsgade 140, 8000 Aarhus (Denmark)
Fax: (+45)8619-6199
E-mail: ts@chem.au.dk
M. Ahlquist, Prof. Dr. P.-O. Norrby
Department of Chemistry, Building 201, Kemitorvet
Technical University of Denmark
DK-2800 Lyngby (Denmark)
[**] We thank the Danish National Science Research Council, the
Carlsberg Foundation, and the University of Aarhus for generous
financial support of this work.
To probe the scope of these reaction conditions, we
examined the Heck coupling of a variety of alkenyl tosylates
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 2: Heck coupling of alkenyl tosylates with isomerization.
with electron-deficient alkenes and styrene derivatives cata-
lyzed by the [PdCl2(cod)]/P(tBu)3 combination in the pres-
ence of LiCl (Table 1). Generally, all the reactions were
completed after 20 h and provided satisfactory coupling yields
Entry
1
R1
t [h]
Product
Yield [%][a]
79
Table 1: Heck coupling with nonactivated vinyl tosylates.
tBu
4
2
3
4
5
tBu
tBu
tBu
tBu
20
23
20
3
93
Entry
1
Vinyl tosylate
t [h]
Product
Yield [%][a]
64
95
23
72
80[b]
2
16
99
6
7
tBu
24
16
83[c]
33
3
4
5
17
17
17
88
96
92
SiMe3
[a] Yield of isolated product; unless noted all selectivities of isomerized
versus normal Heck products were >95:5. [b] Isomerized/normal
products=10:1. [c] cis/trans mixture=1:1.
result is in stark contrast to recent observations by Reissig and
co-workers, in which the Heck coupling between the corre-
sponding vinyl nonaflate and methyl acrylate under phos-
phane-free conditions was reported to give the expected
coupling product.[12c] Nevertheless, coupling of the same vinyl
tosylate with a variety of alkenes revealed this isomerization
to be quite effective with yields attaining 95% and with
exclusive trans selectivity in all but one case (Table 2,
entries 2–6). The use of acrylonitrile (Table 2, entry 6) led to
a 1:1 mixture of cis/trans-isomerized products undoubtedly
because of the small size of the nitrile group. One example
was attempted with a 1-trimethylsilylvinyl tosylate (Table 2,
entry 7). Although its coupling to styrene led to considerable
degradation, a 33% yield of the rearranged coupling product
could be secured, thus indicating the feasibility of such
isomerizations, even with silyl-substituted alkenes, although
further optimization is required.[16]
6
7
8
9
19
6
70
51
83
94
3
18
10
18
79
11
12
3
97
56
21
This isomerization–coupling step was also expanded to
alkenyl phosphates (Table 3). 1-tert-Butylvinyl O,O-diphenyl
phosphate behaved similarly as with the tosylate and coupled
effectively to styrene, p-phenylstyrene, and 4-vinylpyridine
(Table 3, entries 1, 2, and 4, respectively). Interestingly, these
reactions proved to be more sensitive to the solvent
employed. Whereas dioxane was the solvent of choice in
entries 1 and 2, DMF was more adequate in the coupling with
[a] Yield of the isolated product.
as high as 99%. Six- and seven-membered cyclic alkenyl
tosylates proved particularly effective for these coupling
reactions, although reactions with larger rings were unsuc-
cessful. The coupling of the readily available vinyl tosylate to
the acrylamide (Table 1, entry 12) demonstrates the potential
of this methodology for the simple introduction of a vinyl unit
to an a,b-unsaturated amide. Hence, the application of
alkenyl tosylates in Heck couplings provides a straightfor-
ward means for the preparation of functionalized butadienes
from simple starting materials.
Unexpectedly, the normal coupling product was not
isolated in the reaction between 1-tert-butylvinyl tosylate
and styrene. Rather the product from an apparent 1,2-
migration of the intermediate alkenyl palladium(II) species
was obtained in good yield (79%; Table 2, entry 1).[15] This
À
the vinyl pyridine. C C bond formation to the styrene
derivatives in DMF only provided coupling yields in the
order of 20%. Additional examples with two acrylamides and
one acrylate (Table 3, entries 3, 5, and 6) also revealed these
substrates to be effective for Heck couplings with isomer-
ization, whereas attempts with acrylonitrile were unsuccess-
ful. In all cases, the diene products were the result of an
apparent rearrangement, although in three cases an approx-
imate 9:1 ratio of the isomerized and the normal Heck
coupling products was detected (Table 3, entries 3, 5, and 6).
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Table 3: Heck coupling of alkenyl phosphates with isomerization.
product and recovered starting material after 24 h. In
contrast, the same Heck coupling performed in DMF led to
a 4:3 mixture of the isomerized and normal Heck products.
Recent work by Hartwig and co-workers has shown that
isomerization does not take place for Kumada couplings with
a-styryl tosylate in toluene.[4b] Hence, it is interesting to note
how subtle changes in both structure and solvent can have a
marked influence on the isomerization process in these Heck
couplings.
Entry
R
Solvent
t [h] Major product
Yield [%][b]
(isomerized/normal)[a]
1
2
tBu dioxane 24
tBu dioxane 24
65
75
A conceivable mechanism for this 1,2-migration involves
an initial b-hydride elimination of the palladium(II) species
after the oxidative-addition step to give an intermediate
alkyne-coordinated palladium hydride species.[17] To gain
further insight into the plausibility of this reaction mecha-
nism, a DFT study was conducted for the tert-butyl vinyl
substrates (Figure 1).[18] The starting point for the calculations
was a T-shaped complex 3 that coordinated P(tBu)3, a
chloride ion, and the tert-butyl vinyl group. Under the
experimental reaction conditions, chloride ions are present
in the reaction solution; thus, it is likely that a chloride ion
displaces the leaving-group anion (tosylate or phosphate).
Due to the large steric bulk of the tert-butyl alkene and
P(tBu)3, they are likely to be positioned trans to each other.
Palladium(II) complexes containing P(tBu)3 have previously
been shown to form tri-coordinate T-shaped complexes rather
than the more commonly observed tetra-coordinate square-
planar-type complexes.[19] A free coordination site is therefore
available to facilitate a potential hydride elimination from
any group containing a b hydrogen, as the need for de-
coordination of a ligand prior to the actual elimination is not
present.
3
4
tBu DMF
tBu DMF
24
24
71
64
(89:11)
(90:10)
5
6
tBu DMF
tBu DMF
24
24
77
68
(88:12)
7
8
Ph
Ph
dioxane 18
12
51
DMF
20
(4:3)
[a] Unless noted all selectivities of isomerized versus normal Heck
products were >95:5. [b] Yield of the isolated products.
From complex 3, there are then two possibilities, either
coordination and insertion of the olefin, or initial 1,2-
palladium migration with subsequent coordination and inser-
tion of the olefin. A transition state TS1 was characterized in
which the hydride is transferred to the palladium center, thus
yielding the tert-butyl acetylene hydride palladium(II) com-
Finally, a noteworthy solvent effect on the isomerization was
observed in the Heck coupling with a-styryl O,O-diphenyl-
phosphate (Table 3, entries 7 and 8). The reaction of acryl-
amide with this vinyl phosphate in dioxane was slow but
nevertheless afforded 12% yield of the nonisomerized
Figure 1. Schematic presentation of the relative energies of the structures 3–7 and TS1–TS4.
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plex 4. The transformation was calculated to proceed with a
Experimental Section
barrier of 62 kJmolÀ1, and the intermediate 4 was found to be
27 kJmolÀ1 higher in potential energy than 3. Subsequent
(E)-4-(2-(4-tert-Butylcyclohex-1-enyl)vinyl)pyridine: 4-Vinyl pyri-
dine (84 mL, 0.78 mmol), 4-tert-butylcyclohex-1-enyl tosylate
(60.0 mg, 0.19 mmol), Cy2NMe (83 mL, 0.38 mmol; Cy = cyclohexyl),
LiCl (8.2 mg, 0.19 mmol), and HBF4P(tBu)3 (5.6 mg, 0.019 mmol)
were dissolved in DMF (3 mL). [PdCl2(cod)](2.8 mg, 0.0097 mmol)
was then added, and the sample vial was fitted with a teflon-sealed
screwcap and removed from the glovebox. The reaction mixture was
heated for 16 h at 1008C. After completion, water was added to the
crude reaction mixture, followed by extraction with diethyl ether (3 ”
25 mL). The organic phases were washed with water (3 ” 10 mL) and
brine, dried over MgSO4, and concentrated in vacuo. The crude
product was purified by flash chromatography on silica gel using ethyl
acetate/pentane (1:2) as the eluent, thus affording 41.3 mg of the title
compound (88% yield) as a colorless solid. 1H NMR (400 MHz,
CD3CN): d = 8.45 (d, 2H, J = 6.4 Hz), 7.32 (d, 2H, J = 6.4 Hz), 7.06 (d,
1H, J = 16 Hz), 6.40 (d, 1H, J = 16 Hz), 6.05 (t, 1H, J = 2.8), 2.47–2.42
(m, 1H), 2.27–2.10 (m, 2H), 2.02–1.93 (m, 2H), 1.32 (ddt, 1H, J =
11.2, 4.8, 1.6 Hz), 1.20 (dq, 1H, J = 11.2, 5.2 Hz), 0.90 ppm (s, 9H);
13C NMR (100 MHz, CD3CN): d = 151.0, 146.3, 137.2, 136.5, 135.2,
123.31, 121.5, 45.1, 32.8, 28.7, 27.5, 26.5, 24.6 ppm; HRMS C17H23N
[M+H+]; calcd: 242.1909; found: 242.1902.
À
insertion of tert-butyl acetylene in the Pd H bond proceeds
via TS2 to yield the isomeric terminal tert-butyl vinyl
palladium(II) complex 5. The barrier from 4 was calculated
to be 21 kJmolÀ1, and the overall isomerization reaction from
3 to 5 is basically thermoneutral, with 5 having a potential
energy 1 kJmolÀ1 above that of 3. To conclude, the highest
barrier for isomerization between the two isomeric palla-
dium–vinyl intermediates was calculated to be 62 kJmolÀ1
(TS1). With such a low barrier, the reaction is likely to
proceed rapidly under the reaction conditions. Still, to
observe scrambling it is necessary that the barrier for
isomerization is lower than the carbopalladation of the
olefin. The difference in energy between the barriers for the
insertion step then determines the product outcome, thus
representing a typical Curtin–Hammett situation.
Further calculations are in support of this hypothesis.
Coordination of an olefin (here acrylamide) with 3 yields the
prereactive complex 6, and subsequent insertion takes place
via TS3 with an overall barrier of 87 kJmolÀ1 relative to 3. The
barrier for the b-hydride elimination described above was
found to be 25 kJmolÀ1 lower in potential energy and is thus
several orders of magnitude more rapid than the insertion of
acrylamide.[20] At the isomerized complex 5, coordination of
acrylamide results in complex 7. The insertion occurs via TS4
with an overall calculated barrier of 68 kJmolÀ1, 19 kJmolÀ1
lower than TS3, from which we conclude that the path that
leads to the isomerized product is strongly favored.
Finally, some experimental support for this migration was
obtained from a coupling experiment between p-phenyl-
styrene and the dideuterated vinyl tosylate 8 (Scheme 2).
Under identical coupling conditions with the nonlabeled
tosylate, a 72% yield of the diene 9 was isolated in which
migration of one deuterium atom was observed.[21]
Received: February 2, 2006
Published online: April 11, 2006
Keywords: 1,2-migration · alkenes · Heck reaction · phosphates ·
.
tosylates
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Reactions, 2nd ed., Wiley-VCH, Weinheim, 2004; b) J. Tsuji,
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dium Chemistry for Organic Synthesis (Ed.: E. Negishi), Wiley-
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[2]a) K. C. Nicolaou, G.-Q. Shi, J. L. Gunzner, P. Gärtner, Z. Yang,
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[7]a) J. W. Coe, Org. Lett. 2000, 2, 4205; b) X. Fu, S. Zhang, J. Yin,
T. L. McAllister, S. A. Jiang, T. Chou-Hong, K. Thiruvengadam,
F. Zhang, Tetrahedron Lett. 2002, 43, 573; c) A. L. Hansen, T.
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Skrydstrup, Org. Lett. 2005, 7, 5585.
Scheme 2. Isomerization studies with a dideuterated substrate 8.
In conclusion, we have demonstrated the potential for
nonactivated vinyl tosylates and phosphates to be worthy
substrates for Heck couplings with electron-poor alkenes and
styrene derivatives. Furthermore, effective 1,2-migrations of
the alkenyl palladium(II) intermediate has been observed,
thus providing a new variation of the Heck coupling. A
feasible path for this rearrangement has been located using
DFT methods. Further work is currently underway to identify
more effective conditions under which the catalyst loading
can be lowered and the selectivity to produce either the
normal or the isomerized Heck products can be controlled.
This work will be reported in due course.
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[8]For mechanistic studies on oxidative addition with aryl tosylates,
see reference [4a]and A. H. Roy, J. F. Hartwig, Organometallics
2004, 23, 194.
[9]For a few examples of cross-coupling reactions that employ Ni
catalysis, see: a) W. R. Baker, J. K. Pratt, Tetrahedron 1993, 49,
8739; b) Y. Nan, Z. Yang, Tetrahedron Lett. 1999, 40, 3321; c) J.
Wu, Z. Yang, J. Org. Chem. 2001, 66, 7875.
[10]For references on the development and application of bulky
electron-rich phosphines for cross-coupling reactions, notably
through the work of Fu, Buchwald, Hartwig, and Beller, see: A.
Zapf, M. Beller, Chem. Commun. 2005, 431.
121; c) M. Ahlquist, P. Fristrup, D. Tanner, P.-O. Norrby,
Organometallics 2006, DOI: 10.1021/om060126q.
[21]A cross-over experiment was also performed with the 1- tert-
butylvinyl tosylate by the addition of 1-hexyne representing a
less hindered alkyne. No hydropalladation and subsequent
coupling to an alkene was observed with this alkyne, thus
suggesting that the insertion step is probably faster than
dissociation of the tert-butylacetylene from the palladium
hydride intermediate.
[11]The synthesis of vinyl tosylates and phosphates was prepared
according to reference [4b]through proton abstraction of the
required ketones with lithium hexamethyldisilazide (LiHMDS)
in THF and subsequent treatment with tosyl anhydride or
diphenyl chlorophosphate.
[12]The lithium enolate required for the preparation of vinyl tosylate
from Table 1, entry 12 was prepared from cyclofragmentation
from lithiated THF; see: a) R. B. Bates, L. M. Kroposki, D. E.
Potter, J. Org. Chem. 1972, 37, 560; b) M. E. Jung, R. B. Blum,
Tetrahedron Lett. 1977, 18, 3791; c) I. M. Lyapkalo, M. Webel,
H.-U. Reissig, Eur. J. Org. Chem. 2001, 4189.
[13]M. R. Netherton, G. C. Fu, Org. Lett. 2001, 3, 4295.
[14]The rate acceleration effect of added halide ions to Pd-catalyzed
cross-coupling reactions has been proposed to be a result of a
medium effect and a more reactive halide-coordinated Pd0
species; for a recent discussion on the effect of added halide
ions with relevant references, see reference [8]. As our previous
results in Heck couplings with activated vinyl tosylates suggest
that the tosylate is not bound to the alkenyl palladium(II)
species after oxidative addition[7c,d] in solvents such as dioxane
and, undoubtedly also, DMF, we suggest that added LiCl assures
that the reaction passes through a neutral pathway in these Heck
couplings by trapping the PdII cationic intermediate.
[15]A couple of examples of this migration have been reported in
two other cross-coupling reactions, although in modest yield; see
reference [4b]and J. Takagi, K. Takahashi, T. Ishiyama, N.
Miyaura, J. Am. Chem. Soc. 2002, 124, 8001.
[16]For a recent review on 1,4-migrations, see: S. Ma, Z. Gu, Angew.
Chem. 2005, 117, 7680; Angew. Chem. Int. Ed. 2005, 44, 7512; .
[17]Along with a mechanism involving b-hydride elimination, an
alternative explanation was proposed by Hartwig and co-work-
ers in the 1,2-migration observed for the Kumada couplings with
vinyl tosylates.[4b] The presence of a strong base was suggested to
induce elimination of the tosylate, thus leading to an alkyne. We
do not believe this mechanism holds for these Heck couplings
because of the weak base used in these coupling reactions. N.
Frydman, R. Bixon, M. Sprecher, Y. Mazur, J. Chem. Soc. Chem.
Commun. 1969, 1044.
[18]DFT calculations were carried out with the Jaguar 4.2 program
package from Schrödinger Inc., Portland, Oregon: http://
www.schrodinger.com. All the calculations were performed at
the B3LYP/LACVP* level. The geometries were fully optimized
in solvent and simulated with the PB-SCRF model: B. Marten,
K. Kim, C. Cortis, R. A. Friesner, R. B. Murphy, M. N. Ring-
nalda, D. Sitkoff, B. Honig, J. Phys. Chem. 1996, 100, 11775. The
parameters were set to e = 38, probe radius = 2.47982 to simulate
DMF.
[19]J. P. Stambuli, C. D. Incarvito, M. Bꢀhl, J. F. Hartwig, J. Am.
Chem. Soc. 2004, 126, 1184.
[20]As the b-hydride elimination is a unimolecular process, while the
olefin insertion is bimolecular, the b-hydride is also strongly
favored entropically. Earlier studies have shown that a value of
approximately 40–60 kJmolÀ1 should be added to account for
the change in molecularity; see: a) M. S. Searle, M. S. Westwell,
D. H. Williams, J. Chem. Soc. Perkin Trans. 2 1995, 1, 141;
b) T. K. Woo, P. E. Blöchl, T. Ziegler, J. Phys. Chem. A 2000, 104,
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