Synthesis of dienes by palladium-catalyzed couplings of tosylhydrazones with aryl and alkenyl halides

Article abstract of DOI:10.1002/adsc.201000700

Two different combinations of coupling partners can be employed for the synthesis of conjugated dienes by palladium-catalyzed cross-coupling with tosylhydrazones: α,β-unsaturated ketone and aryl halide or alkenyl halide and non-conjugated tosylhydrazone. Depending on the substrate, a vinylogous hydride elimination is responsible for the formation of the final dienes. Copyright

Full text of DOI:10.1002/adsc.201000700

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DOI: 10.1002/adsc.201000700
Synthesis of Dienes by Palladium-Catalyzed Couplings of
Tosylhydrazones with Aryl and Alkenyl Halides
Josꢀ Barluenga,a,* Marꢁa Tomꢂs-Gamasa,a Fernando Aznar,a and Carlos Valdꢀsa,*
a
Instituto Universitario de Quꢁmica Organometꢂlica “Enrique Moles”, Universidad de Oviedo, c/Juliꢂn Claverꢁa 8, 33006
Oviedo, Spain
Fax : (+34)-9-8510-3446; e-mail: barluenga@uniovi.es or acvg@uniovi.es
Received: September 9, 2010; Published online: December 5, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000700.
Abstract: Two different combinations of coupling
partners can be employed for the synthesis of con-
jugated dienes by palladium-catalyzed cross-cou-
pling with tosylhydrazones: a,b-unsaturated ketone
and aryl halide or alkenyl halide and non-conjugat-
ed tosylhydrazone. Depending on the substrate, a
Scheme 1. Pd-catalyzed cross-coupling reactions employing
carbonyls mediated by tosyl hydrazide.
vinylogous hydride elimination is responsible for
the formation of the final dienes.
hydrazones derived from a,b-unsatured ketones. We
now report that this catalytic process is an excellent
method to prepare dienic structures and, in particular,
homoanular cyclic dienes. Moreover, during the prog-
ress of this research, we have discovered a new reac-
tion pathway that proceeds through a formal d-hy-
dride elimination. Preliminary studies of this new
pathway show that it can be also employed in the
Keywords: cross-coupling; dienes; ketones; palladi-
um; tosylhydrazones
À
The discovery of new C C bond forming reactions preparation of conjugated dienes from tosylhydra-
that work efficiently and selectively, based on easily zones and haloalkenes.
available starting materials, is a field of constant inter-
Initially we selected the cross-coupling between the
est in organic chemistry. Palladium-catalyzed cross- hydrazone 1a derived from 4,4-dimethyl-2-cyclohex-
coupling reactions are among the most appealing en-1-one 4a and p-bromotoluene 2a. A screening of
methodologies for this purpose.[1] Their usefulness is reaction conditions with variation of bases, ligands,
reflected in a growing application for the synthesis of and reaction temperatures was carried out. The opti-
very complex organic molecules,[2] in drug discovery mal conditions found comprised [Pd
2ACHTUNGTRENNUNG(dba)3]
programs, and in industrial chemical processes.[3] Most (2.5 mol%) as the metal source, Xphos (2-dicyclohex-
À
of metal-catalyzed C C bond forming transformations ylphosphino-2’,4’,6’-triisopropylbiphenyl) as ligand,
consist of the combination of an organic halide with and LiO-t-Bu as base, in dioxane at 1108C. Under
an organometallic reagent. Nevertheless, great efforts these conditions, the diene 3a could be obtained in
have been devoted in the recent years towards the de- very good yield (Scheme 2). By decreasing either tem-
À
velopment of new processes for the creation of C C perature or catalyst loading, apart from an increment
linkages that do not employ stoichiometric organome- of the reaction time, formation of the Bamford–Ste-
tallics.[4–7]
vens by-product, that occurs through a competitive
In this context, we have recently uncovered a new uncatalyzed thermal process, took place.
Pd-catalyzed coupling reaction that employs N-tosyl-
We have previously demonstrated that the cross-
hydrazones as nucleophilic partner, suppressing the coupling reaction can be carried out in a one-pot fash-
stoichiometric organometallic reagent (Scheme 1).[8,9] ion directly from the carbonyl compound, by generat-
Since then, we have initiated a research program to ing in situ the hydrazone. Having established proper
evaluate the synthetic potential of this novel reaction. reaction conditions for the coupling process, we decid-
À
Continuing the study of the scope of this C C bond ed to investigate whether it might be possible to
forming reaction, we turned our attention to N-tosyl- apply the one-pot methodology to a,b-unsatured
Adv. Synth. Catal. 2010, 352, 3235 – 3240
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Table 1. Synthesis of dienes by cross-coupling of en-ones 4
with aryl halides 2.[a,b]
Scheme 2. Pd-catalyzed cross-coupling reaction with a,b-un-
saturated tosylhydrazone 1a: preliminary results.
ketone 4a. Indeed, the reaction starting directly from
4a and tosyl hydrazide provided the diene 3a in simi-
lar yield to that obtained from the preformed hydra-
zone 1a. No undesired products from side reactions
were detected (Scheme 2 and Table 1, entry 1).
These optimized conditions were applied to a set of
aryl halides 2, giving rise to the aryl-substituted ho-
moanular dienes 3 in very high yields in most of the
cases. In general, electronic effects played a minor
role and high conversions were achieved no matter if
the substituents were electron-withdrawing or elec-
tron-donating. The use of the electron-rich m-bromo-
toluene, p-bromothioanisole, or 2-bromo-6-methoxy-
naphthalene gave the desired dienes in good yields
(Table 1, entries 2, 4, and 6). Substituted bromoben-
zene derivatives, bearing electron-withdrawing groups
are also tolerated (Table 1, entry 7). Both aryl chlor-
ides as well as aryl bromides could be used (Table 1,
entries 1 and 2). The reaction with p-chlorobromo-
benzene leads exclusively to the 4-chloro-substituted
derivative (Table 1, entry 3). Moreover, a p-excessive
heterocycle participates efficiently in the coupling
(Table 1, entry 8), and 5-bromoindole was successfully
À
coupled without protection of the N H group
(Table 1, entry 9).
The coupling reaction was also successfully accom-
plished with 4,4-diphenyl-3-buten-2-one 4b, a b,b-dis-
usbtituted acyclic enone, leading to the expected acy-
clic dienes[10] (Table 1, entries 10–12). The best results
for this system were obtained by employing a one-
pot, two-steps sequence, stirring the hydrazide and
the ketone at 708C for 2 h in dioxane, before adding
the rest of the reagents (method B). Noteworthy,
these reactions proceeded with significantly lower cat-
alyst loading.
With respect to the cyclic en-one, the reaction also
takes place with other cyclic systems, like 2-cyclohex-
en-1-one 4c or 2-cyclohepten-1-one 4d. However, in
these cases, the linear-conjugated 1-aryldienes 5 were
obtained instead of the expected cross-conjugated sys-
tems (Table 2, entries 1–3). Furthermore, the coupling
[a]
Method A: ketone 4a, 1 mmol; TsNHNH2, 1.1 mmol;
ArX, 1 mmol; Pd2ACTHNURGTNEUNG(dba)3, 2.5 mol%; Xphos, 10 mol%;
LiO-t-Bu, 2.4 equiv.; dioxane, 1108C. Method B: ketone
4b, 1 mmol; TsNHNH2, 1.1 mmol; dioxane, 2 h, 708C;
then ArX, 1 mmol; Pd2ACHTNUTRGENNUG(dba)3, 2.5 mol%; Xphos,
10 mol%; LiO-t-Bu, 2.4 equiv., 1108C.
Xphos=2-dicyclohexylphosphino-2’,4’,6’-triisopropylbi-
phenyl.
Yield of isolated product.
[Pd2ACHTNUTRGENNG(U dba)3], 1.0–1.5 mol%, Xphos, 4–6 mol%.
[b]
[c]
[d]
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Synthesis of Dienes by Palladium-Catalyzed Couplings of Tosylhydrazones
Table 2. Synthesis of linear-conjugated dienes 5 by cross-
coupling of en-ones 4 with aryl halides 2.[a]
Table 3. Synthesis of chiral dienes by cross-coupling of R-
(+)-pulegone 6 with aryl halides.[a]
Entry
Ar
Ratio 7:8
Yield [%][b]
1
2
3
4
p-Tol
20:80
10:90
33:66
10:90
70
71
71
64
p-MeO-C6H4
p-Me2N-C6H4
p-biphenyl
[a]
Experimental
conditions:
pulegone
6,
1 mmol;
TsNHNH2, 1.1 mmol; ArX, 1 mmol; Pd2ACHTUNGTRENNUNG(dba)3,
2.5 mol%; Xphos, 10 mol%; LiO-t-Bu, 2.4 equiv.; diox-
ane, 1108C.
Isolated yield of the mixture of isomers.
[b]
[a]
Methods A and B: same conditions as in Table 1.
Yield of isolated product employing method A or B.
The yield for the reaction starting from the previously
[b]
[c]
isolated hydrazone is indicated in brackets.
reaction proceeded efficiently also with l-(À)-carvone
4e, affording a new cyclohexadiene bearing a stereo-
genic center (Table 2, entries 4–6). Noteworthy, these
examples led to the formation of a tetrasubstituted
alkene.[11] Again, the linear-conjugated dienes were
obtained for carvone derivatives. In the latter cases,
better results were obtained in a two-steps process,
starting from the preformed hydrazone.
Figure 1. Mechanisms proposed for the formation of cyclo-
hexadienes 3 and 5.
tion of the aryl halide to the Pd(0) catalyst, formation
In this context, we became interested in the struc- of Pd-carbene complex III by reaction of the diazo
ture of R-(+)-pulegone 6, a natural a,b-unsaturated compound II (generated from the tosylhydrazone)
ketone which features an exocyclic propenylidene with the arylpalladium complex I, and migratory in-
substituent. Interestingly, the coupling reaction gave sertion of the aryl group to the carbenic carbon to
rise to a mixture of two isomeric dienes, the expected form h1-allylpalladium intermediate IV. At this point,
diene 7 and the regioisomeric diene 8, which differ in b-hydride elimination on complex IV would release
the position of the double bonds. In fact, under the the olefin 3 and a Pd-hydride complex. This pathway
optimized reaction conditions, diene 8, which features operates in the case of hydrazones derived from 4,4-
linear conjugation with the aryl substituent is ob- dimethylcyclohexenone 4a (Table 1), but does not ex-
tained again as major product in all the examples ex- plain the formation of linear dienes from 4-unsubsti-
amined (Table 3).
tuted enones 4c, 4d and 4e (Table 2). For these sys-
The formation of the linear-conjugated dienes 5, tems, the formation of the linear diene 5 can be ra-
can be rationalized by considering the mechanism tionalized through a b-hydride elimination on com-
proposed for the cross-coupling reactions with tosyl- plex VI,[12] upon metallatropic rearrangement through
hydrazones (Figure 1), which involves: oxidative addi- the h3-allylpalladium complex V.[13] Moreover, a simi-
Adv. Synth. Catal. 2010, 352, 3235 – 3240
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Table 4. Synthesis of linear dienes 12 by cross-coupling of
aryl tosylhydrazones and bromoalkenes.[a,b]
Figure 2. Reaction of 1-bromoalkenes 10 and aromatic tosyl-
hydrazones 9
lar mechanism could be also proposed for the forma-
tion of linear dienes 8 from pulegone.
Taking into account this mechanistic model, we de-
cided to investigate other combinations of coupling
partners that could also participate through hydride
elimination involving a p-allylpalladium intermediate.
We selected the reaction of hydrazones 9 derived
from aryl aldehydes and alkenyl halides 10. As shown
in Figure 2, the only possible evolution of the allylpal-
ladium intermediate [11] would be the hydride elimi-
nation through the palladatropic rearrangement/b-hy-
dride elimination sequence to produce diene 12.
After some optimization work, the reactions pro-
ceeded as planned with moderate yields for 1-bromo-
2-methylpropene, (Table 4, entries 1–4), (bromome-
thylene)cyclohexane (Table 4, entry 5) and 1-bromo-
propene (Table 4, entry 6). Noteworthy, in all cases,
the trans isomer was obtained as a single stereoiso-
mer. Moreover, the reaction proceeded also with the
hydrazones derived from the non-enolizable ketones
benzophenone and fluorenone (Table 4, entries 7 and
8). For these reactions to take place, depending on
the particular substrates, it is necessary to employ the
proper ligand Xphos or Sphos, as well as the adequate
base NaO-t-Bu or LiO-t-Bu, leading again to the ex-
pected dienes through the vinylogous hydride elimi-
nation pathway.[14]
[a]
Experimental conditions: hydrazone 9, 1 mmol; bromoal-
kene 10, 2 mmol; Pd2ACTHNUTRGNE(UGN dba)3, 4 mol%; Sphos 16 mol%;
NaO-t-Bu, 2.4 equiv.; dioxane, 1108C.
Sphos=2-dicyclohexylphosphino-2’,6’-dimethoxylbiphen-
yl.
In summary, we have described a new methodology
for the preparation of aryl-substituted conjugated
dienes by the coupling reaction of tosylhydrazones
derived from a,b-unsaturated ketones. The reaction is
particularly effective for the preparation of homoanu-
lar dienes. Moreover, in the course of this research, a
variation of the reaction that involves a hydride elimi-
nation involving a p-allylpalladium intermediate has
been discovered. Preliminary studies indicate that this
new transformation may represent an efficient way to
[b]
[c]
[d]
Isolated yield.
The reaction is carried out in the presence of 5 equiva-
lents of water.
Experimental conditions: hydrazone 9, 1 mmol; bromoal-
[e]
kene 10, 2 mmol; Pd
2ACHTUNGRTEN(NUNG dba)3, 4 mol%; Xphos 16 mol%;
LiO-t-Bu, 2.4 equiv.; dioxane, 1108C.
carry out the synthesis of linear conjugated dienes Experimental Section
from readily available starting materials. Further ap-
plications of this methodology as well as detailed Reaction of Cyclic En-ones with Aryl Halides:
mechanistic studies are ongoing and will be reported Method A (Table 1, entries 1–9; Table 2, entries 1–3)
in due course.
A carousel reaction tube under a nitrogen atmosphere was
charged with 2-dicyclohexylphosphino-2’,4’,6’-triisopropylbi-
phenyl (Xphos) (23.8 mg, 0.05 mmol, 10 mol%), tris(diben-
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Synthesis of Dienes by Palladium-Catalyzed Couplings of Tosylhydrazones
zylideneacetone)dipalladium(0)
(11.4 mg,
0.013 mmol,
Acknowledgements
2.5 mol%), lithium tert-butoxide (96.0 mg, 2.4 equiv.), the
cyclic en-one 4 (0.55 mmol), p-toluenesulfonyl hydrazide
(0.64 mmol) and dioxane (2 mL). After 1 min, the aryl
halide 2 (0.5 mmol) was added. The system was heated at
1108C with stirring and reflux. The reaction was monitored
by GC-MS. When the reaction was completed, the crude re-
action mixture was allowed to reach room temperature,
taken up in dry pentane, hexanes or dichloromethane
(15 mL), and filtered through celite. The solvents were
evaporated under reduced pressure and the residue was pu-
rified by flash chromatography on silica gel or desactivated
silica gel.
This work was supported financially by DGI of Spain
(CTQ2007-61048/BQU) and Consejerꢀa de Educaciꢁn y
Ciencia of Principado de Asturias (IB08–088). An FPU pre-
doctoral fellowship to M. T.-G. is gratefully acknowledged.
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phenyl (Xphos) (38.1 mg, 0.08 mmol, 16 mol%) or dicyclo-
hexylphosphino-2’, 6’-dimethoxybiphenyl (Sphos) (32.9 mg,
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dioxane (2 mL). After 1 min, the alkenyl halide 10 (1 mmol)
was added, followed by the addition of 5 equiv. of water
(45 mL) when necessary. The system was heated at 1108C
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MS. When the reaction was completed, the crude reaction
mixture was allowed to reach room temperature, taken up
in dry pentane, hexanes or dichloromethane (15 mL), and
filtered through celite. The solvents were evaporated under
reduced pressure and the residue was purified by flash chro-
matography on silica gel.
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[10] The coupling reaction with hydrazones derived from
less substituted acyclic a,b-unsaturated ketones, such as
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Josꢀ Barluenga et al.
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benzylideneacetone, gives rise, under optimized reac-
tion conditions, to a 57: 43 mixture of the expected
diene and the pyrazole that is formed by decomposition
of the tosylhydrazone followed by electrocyclic ring
closure.
[12] DFT computations show that the TS of the b-hydride
elimination from VI, that leads to the linear dienes 5 is
13 kcalmolÀ1 (DG) more stable than the TS of the b-
hydride elimination from IV, that leads to the cross-
conjugated diene. A very large energy difference may
justify the exclusive formation of the linear dienes (see
Supporting Information for details).
[13] For some leading references regarding hydride elimina-
tions on p-allylpalladium complexes see: a) I. Shimizu,
in: Handbook of Organopalladium Chemistry for Or-
ganic Synthesis, (Ed.: E. Negishi), Wiley, Hoboken, NJ,
2002, p 1981; b) H. Matsushita, E. Negishi, J. Org.
Chem. 1982, 47, 4161; c) J. M. Takacs, E. C. Lawson, F.
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[14] The reaction with the hydrazone of 3-phenylpropanal
led to a 10: 1 mixture of the dienes derived from b-hy-
dride elimination and the vinylogous hydride elimina-
tion, respectively.
[11] The Pd-catalyzed synthesis of tetrasubstituted olefins
by the reaction of tosylhydrazones with aryl halides has
been recently published under slightly different reac-
tion conditions: E. Brachet, A. Hamze, J.-F. Peyrat, J.-
D. Brion, M. Alami, Org. Lett. 2010, 12, 4042. Never-
theless, we have observed that our protocol can be also
employed for the preparation of simple tetrasubstituted
alkenes.
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