Arylation of α-chiral ketones by palladium-catalyzed cross-coupling reactions of tosylhydrazones with aryl halides

Article abstract of DOI:10.1002/anie.201003450

(Figure Presented) Papa was a rollin' ketone: Arylation of ketones with preservation of the chirality in configurationally unstable α-chiral ketones has been achieved by the palladium-catalyzed cross-coupling reaction between tosylhydrazones and aryl halides (see scheme; Boc=tert-butoxycarbonyl, Ts=4-toluenesulfonyl). The regioselectivity in the β-hydride elimination step is key for the retention of configuration.

Full text of DOI:10.1002/anie.201003450

Communications
DOI: 10.1002/anie.201003450
Cross-Coupling
Arylation of a-Chiral Ketones by Palladium-Catalyzed Cross-Coupling
Reactions of Tosylhydrazones with Aryl Halides**
Josꢀ Barluenga,* Marꢁa Escribano, Fernando Aznar, and Carlos Valdꢀs*
The carbonyl group is probably the most versatile functional
group in organic synthesis, owing to its rich and highly
developed chemistry. In particular, carbonyl compounds are
extraordinary sources of enantiomerically pure compounds,
that can be obtained from the chiral pool from terpenes,
carbohydrates, and amino acids, and also through asymmetric
catalysis[1] and organocatalysis.[2] However, carbonyl com-
pounds bearing chirality at the a carbon can be difficult to
manipulate owing to their configurational instability through
enolization. Moreover, the typical formation of alkenes by
nucleophilic-addition/elimination sequences usually affords
the more-substituted olefin,[3] with loss of the chiral informa-
tion (Figure 1a), and the reactions that proceed through the
(Figure 1c).[4,5] For these reasons, the development of meth-
odologies that allow the manipulation of the carbonyl
functionality with preservation of the a chirality are highly
desirable.
The palladium-catalyzed cross-coupling between tosylhy-
drazones and aryl halides, recently developed by our group,[6]
constitutes an efficient method to manipulate the carbonyl
functionality. The overall transformation is equivalent to a
nucleophilic addition/elimination sequence. However, as we
will show herein, in many cases the reaction gives the less-
substituted alkene, and importantly, with no erosion of the
chirality of the stereogenic center at the a position (Fig-
ure 1d); thus, we report the implementation of this new
methodology for the manipulation of a-chiral ketones.
The catalytic cycle proposed for the palladium-catalyzed
cross-coupling between tosylhydrazones and aryl halides is
presented in Figure 2. The characteristic steps are formation
Figure 1. Problems associated with the manipulation of a-chiral
ketones versus this work. Tf=trifluoromethanesulfonyl, Ts=4-toluene-
sulfonyl.
formation of enolates, such as cross-coupling reactions
through enol sulfonates, require tightly controlled kinetic
conditions to avoid the equilibration of the chiral center
Figure 2. Cross-coupling reactions of tosylhydrazones derived from
ketones with two enolizable positions and aryl halides: the regioselec-
tivity in the formation of the double bond is determined by the syn-b-
hydride-elimination step on alkylpalladium complex VIII. xphos=2-
dicyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl.
[*] Prof. J. Barluenga, M. Escribano, Prof. F. Aznar, Dr. C. Valdꢀs
Instituto Universitario de Quꢁmica Organometꢂlica “Enrique
Moles”, Universidad de Oviedo
c/Juliꢂn Claverꢁa 8, 33006 Oviedo (Spain)
Fax: (+34)985-103-446
E-mail: barluenga@uniovi.es
acvg@uniovi.es
of the palladium–carbene complex VII, migratory insertion of
the aryl to give the alkylpalladium complex VIII,[7,8] and syn-
b-hydride elimination, that releases the coupling product. For
tosylhydrazones derived from ketones with two enolizable
positions, such as I, two regioisomers II and III can be
obtained that differ in the position of the double bond.[6a,c,8]
[**] Financial support of this work from the DGI of Spain (CTQ2007-
61048/BQU), the Consejerꢁa de Educaciꢃn y Ciencia of Principado
de Asturias (IB08-088), and a FPU predoctoral fellowship from the
MCINN of Spain to M.E. is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003450.
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Thus, to achieve a coupling reaction with preservation of the
a chirality, a regioselective b-hydride-elimination step on
alkylpalladium complex VIII is needed, leading to the less-
substituted alkene III.[9]
In our previous work, we observed that the selectivity in
the b-hydride-elimination step depends on the type of
substrate. For instance, moderate selectivity had been
observed towards the more-substituted olefin 2 in methyl
n-alkyl hydrazone 1[6a] (Scheme 1). In contrast, in the reaction
Scheme 3. Synthesis of enantiomerically pure allyl ethers 10. Reaction
À
conditions: (À)-9 (0.55 mmol), Ar X, (0.5 mmol), [Pd2(dba)3]
(1 mol%), xphos (2 mol%), LiOtBu (1.4 mmol), dioxane (2 mL),
1108C, 4 h. [a] Determined by HPLC on a chiral stationary phase.
Encouraged by these results, we decided to examine other
types of a-chiral ketones. As the regioselectivity of the
b-hydride elimination is necessary to preserve the chiral
center, we decided to concentrate on methyl ketones. We
thought that the syn-b-hydride elimination of one of the
hydrogen atoms of the methyl group should be favored, thus
leading to the formation of the terminal disubstituted double
bond.
Methyl ketone 12, which was easily obtained in enantio-
merically pure form from l-proline,[12] was selected as model
to check this hypothesis. To avoid unnecessary steps, the
reactions were carried out directly from the ketone—without
isolation of the intermediate hydrazone—in a one-pot
process. As expected, in every case the disubstituted terminal
olefin 14 was the only coupling product observed. No
tetrasubstituted alkene was detected in the reaction mixture.
In most of the cases, the protected allylamines 14 were
obtained in 99% ee, as determined by HPLC. Again the
integrity of stereogenic center was preserved (Scheme 4).
However, in some examples, in the one-pot reaction, a slight
decrease in the enantiomeric excess was observed (14c and
Scheme 1. Preliminary studies on the regioselectivity of the reactions
of tosylhydrazones with aryl halides. [a] Yield of the isolated mixture of
isomers. Bn=benzyl, dba=trans,trans-dibenzylideneacetone, Tol=
tolyl.
of tosylhydrazone 4, formation of the trisubstituted olefin 6 is
now favored against formation of the tetrasubstituted alkene
5
[6a] (Scheme 1).
With these preliminary results in hand, we next turned our
attention to the reaction of the hydrazone 7 derived from
2-methylcyclohexanone. Interestingly, in this case, we
observed total regioselectivity, thus exclusively obtaining
the trisubstituted olefin 8 with high yield. A similar result was
obtained when hydrazone 9, derived from 2-methoxycyclo-
hexanone was employed, yielding the corresponding allylic
ether 10 (Scheme 2).
Scheme 2. Regioselectivity of the reactions of cyclic tosylhydrazones
with aryl halides.
These results suggested that this methodology could be
suitable for modifying cyclohexanones containing a chiral
center at the a position without erosion of the chirality. Thus,
we carried out coupling reactions on enantiomerically
enriched hydrazone (À)-9, obtained from (À)-2-methoxycy-
clohexanone 11 (98% ee).[10] To our delight, the resulting allyl
ethers 10 were all obtained with very high yield and 97% ee;
therefore, no erosion of the chirality had occurred
(Scheme 3). This important result represents a new method-
ology for the manipulation of chiral 2-substituted cyclo-
hexanones,[11] and offers numerous synthetic opportunities.
Scheme 4. Synthesis of enantiomerically pure disubstituted alkenes 14
from coupling reactions with proline derivatives 13. Reaction condi-
À
tions: 13 (0.55 mmol), Ar X, (0.5 mmol), [Pd2(dba)3] (2 mol%), xphos
(4 mol%), LiOtBu (1.4 mmol), dioxane (2 mL), 1108C, 12 h. [a] Deter-
mined by HPLC on a chiral stationary phase. [b] The ee value for the
one-pot process is indicated in brackets only for the cases in which
erosion of chirality was observed. Boc=tert-butoxycarbonyl.
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14d). Nevertheless, these allyl amines could be prepared in
99% ee from the previously isolated hydrazone 13 under the
standard conditions (Scheme 4).
The reaction was also attempted with ketone 15, derived
from l-alanine, as a representative of natural amino acid
derivatives. However, this transformation turned out to be
more challenging and required some additional experimental
work. In a first attempt, we performed the coupling reaction
under the standard reaction conditions from hydrazone 16,
but no coupling product was detected (Scheme 5). Surpris-
It is worth noting that the coupling reactions to produce
chiral alkenes discussed herein are very general with regard to
the aryl halide. In the synthesis of compounds 10, 14, and 17
we successfully employed electron-rich and electron-poor
benzene derivatives as well as heteroaromatic halides. More-
over, these reactions can be carried out in the presence of free
NH groups, as shown with tosylhydrazone 16.
The high regioselectivity observed in the reactions of
hydrazones derived from methyl ketones can be explained by
considering the syn arrangement required for the b-hydride
elimination on alkylpalladium complex VIII. As presented in
Figure 3, the conformation required for the formation of the
Scheme 5. Coupling reaction with alanine derivatives 15 and 16:
influence of the presence of H2O.
Figure 3. Different possibilities for the syn-b-hydride-elimination step
on systems derived from methyl ketones.
ingly, when the same reaction was conducted in a one-pot
procedure directly from ketone 15, with pre-formation of the
hydrazone in situ, the expected coupling product 17a was
isolated in 42% yield (Scheme 5).
tetrasubstituted double bond (VIII-a), is clearly disfavored,
because the bulky groups of both carbon atoms C1 and C2
have to adopt an eclipsed conformation. However, in the
conformation required for the formation of the disubstituted
double bond (VIII-b), the bulky groups attached at C1 are
eclipsed with hydrogen atoms of the methyl group, and
therefore, this situation is clearly favored.
Regarding the reactions with hydrazones derived from
2-substituted cyclohexanones (Scheme 2 and Scheme 3), the
higher regioselectivity observed when compared with similar
acyclic system 4 (Scheme 1) must be due to the restrictions
imposed by the cyclic structure. We propose that the
migratory insertion occurs on the less-hindered face of the
ring to give intermediate X (Figure 4), leaving the methyl
group and the palladium moiety in a cis arrangement. Then,
only syn-b-hydride elimination can occur to give the trisub-
stituted olefin (Figure 4). Formation of the tetrasubstituted
olefin, not observed, could only occur from the diastereo-
meric intermediate XI.[14]
The only difference between the two experiments was the
presence of 1 equivalent of water in the second reaction,
released in the formation of hydrazone 16 from tosylhydra-
zide and the ketone 15. For this reason, we carried out a study
of the influence of different amounts of water, which revealed
that the coupling reaction could be best accomplished in the
presence of 5 equivalents of water. Under these conditions,
the allyl amine 17a was obtained in an acceptable 65% yield
(Scheme 5).[13] Further experimentation revealed that the use
of Pd(OAc)2 instead of [Pd2(dba)3] provided higher yields.
Employing this methodology, a variety of chiral N-Boc-
protected allyl amines 17 were prepared from tosylhydrazone
16 (Scheme 6). In all cases, the reactions took place with total
retention of configuration of the stereogenic center.
Scheme 6. Synthesis of chiral N-Boc-protected allyl amines 17 from
the coupling reactions of tosylhydrazone 16 with aryl halides. Reaction
Figure 4. Rationale for the high regioselectivity observed in the cou-
pling reaction of tosylhydrazones derived from 2-substituted cyclo-
hexanones.
À
conditions: 16 (0.55 mmol), Ar Br, (0.5 mmol), Pd(OAc)2 (4 mol%),
xphos (8 mol%), LiOtBu (1.4 mmol), dioxane (2 mL), 1108C, 4 h.
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[7] For other examples of palladium-catalyzed processes involving a
In summary, we have shown that the palladium-catalyzed
cross-coupling between tosylhydrazones and aryl halides is a
powerful methodology for the manipulation of chiral ketones
with total preservation the stereochemistry of the a carbon.
As a proof of concept, we have synthesized aryl-substituted
chiral cyclohexenes from a-chiral cyclohexanones, and chiral
allylamines from methyl ketones that were easily obtained
from a-amino acids. Taking into consideration the wide
availability, but also the configurational instability, of a-chiral
ketones, this catalytic process may represent a very useful
transformation in organic synthesis. Our current work in this
area includes the extension of this methodology to other types
of chiral carbonyl compounds, the use of alkenyl halides in the
synthesis of dienes, and applications in natural products
synthesis.
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Received: June 7, 2010
Published online: August 3, 2010
Keywords: allyl amines · chiral ketones · cross-coupling ·
.
hydrazones · palladium
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