Catalytic deallylation of allyl- and diallylmalonates

Article abstract of DOI:10.1021/ja047320t

Substituted allylmalonates undergo the selective C-C bond cleavage in the presence of triethylaluminum and a catalytic amount of nickel and ruthenium phosphine complexes, resulting in the loss of the allyl moiety and formation of monosubstituted malonates. Comparison of reactivity of the nickel and ruthenium complexes showed that the use of the former is general with respect to the structure of the substituted allylmalonates, and the activity of the latter depended on the substitution pattern of the double bond of the allylic moiety. The smooth deallylation may encourage the use of the allyl group as a protective group for the acidic hydrogen in malonates. Copyright

Full text of DOI:10.1021/ja047320t

Published on Web 07/31/2004
Catalytic Deallylation of Allyl- and Diallylmalonates
†
†
,†,‡
David Ne cˇ as, Maty a´ sˇ Tursk y´ , and Martin Kotora*
Department of Organic and Nuclear Chemistry, Faculty of Science, Charles UniVersity, AlbertoV 2030,
1
28 43 Prague 2, Czech Republic, and Institute of Organic Chemistry and Biochemistry,
Czech Academy of Sciences, FlemingoVo n a´ m. 2., 166 10 Prague 6, Czech Republic
Received May 7, 2004; E-mail: kotora@natur.cuni.cz
A transition-metal complex-catalyzed carbon-carbon bond
cleavage under mild reaction conditions has been a long-pursued
process of considerable theoretical and practical interest. It can be
facilitated by the presence of an activating group and driven by
Scheme 1. Fe-Catalyzed Deallylation of 1a
1
decrease of steric strain. Nevertheless, the ultimate goal in this
area is the cleavage of unstrained and unactivated carbon-carbon
bonds under mild reaction conditions. Typical examples may serve
2
3
Scheme 2. Catalytic Deallylation of Allylbutylmalonate 1b
â-alkyl elimination in organometallics, skeletal rearrangements,
and cleavage of tertiary homoallyl alcohols to ketones and propene.4
Recently, we have reported that during attempts of Fe-catalyzed
alkylative cyclization of allyl(2-chloroallyl)malonate 1a, clean
deallylation to (2-chloroallyl)malonate 2a in 47% yield was
5
observed (Scheme 1). It is noteworthy that during the course of
the reaction the unactivated γ,δ-carbon-carbon bond was cleaved
and presumably the more reactive C-Cl bond remained intact.
Although deallylation of malonates was reported for titanium-
Table 1. Ru- and Ni-Catalyzed Deallylation of Monoallyl
Malonates
6
mediated process and was observed during some Pd-catalyzed
7
reactions, its scope and generality has not been studied in detail.
The lack of information on this reaction provided necessary impetus
to study this reaction, and herein we would like to report the first
catalytic deallylation of malonates and related compounds by
various transition-metal complexes.
We began our study exploring the scope of the deallylation with
respect to transition-metal complexes. Critical reaction parameters
previously identified, including the amount of the catalyst (5 mol
%), the amount of triethylaluminum, solvent (toluene), and tem-
perature, remained the same. The deallylation of the allylbutylma-
lonate 1b (Scheme 2) in the presence of several group VIII
transition-metal phosphine complexes was chosen as a model
reaction. As summarized in Scheme 2, all the complexes, with the
exception of the iron one, showed catalytic activity. The activity
of the complexes was decreasing in the following order Rh ≈ Ni
>
Ru > Co > Pd. It is worth mentioning that all deallylations
proceeded very cleanly and only deallylation products and unreacted
starting material were detected in the reaction mixtures. Except Et
Al, other organoaluminums were tested as well; however, the yields
of the deallylated product 2b were rather low (Me Al: 15%,
MAO: 15%, Et AlCl: 32%, i-Bu Al: 33%).
In the next step, we decided to compare the catalytic activity
and selectivity of RuCl (PPh and NiBr (PPh to better assess
3
-
3
a 1H NMR yields. b RuCl2(PPh3)3. c NiBr2(PPh3)2.
2
3
2
3
)
3
2
3
)
2
cinnamylbutylmalonate, 1g, did not react. Only partial success was
achieved in the deallylation of 1h.
the scope of the reaction. The first study focused on deallylation
of substituted monoallylmalonates bearing functional groups on the
Further attention focused on the deallylation of various substrates
1i-p to 2f-m (Table 2). As indicated, a considerable difference
in reactivity was again observed for both complexes. The nickel
complex was again more effective and afforded deallylated products
in high yields (60-99%). (Partial hydrogenation of 2f to the
propylmalonate in 22% yield was observed during the deallylation
of 1i.) A comparable activity of the Ru catalyst was observed for
the deallylation of the diallylmalonate 1i, the allylmethallylmalonate
1j, the allylcrotylmalonate 1k, and the diallylcyanoacetate 1p. The
8
double bond. As summarized in Table 1, the use of NiBr
2
(PPh
3 2
)
proved to be more effective than RuCl
2
(PPh
3
)
3
. The yields of the
deallylation were higher (73-99%), and it proceeded regardless
of the substitution on the double bond. The activity of the ruthenium
catalyst was comparable only in the deallylation of the phenyl, 1c,
and the benzylallylmalonate, 1d. The crotyl, 1e, methallyl, 1f, and
†
Charles University.
Czech Academy of Sciences.
‡
10222
9
J. AM. CHEM. SOC. 2004, 126, 10222-10223
10.1021/ja047320t CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Table 2. Ru- and Ni-Catalyzed Deallylation of Diallyl Malonates
alkylnickel species 4, (iii) the double bond shifts with the cleavage
of the C-C bond to the nickel enolate 5 and the alkene 6 (it further
isomerizes to the internal alkene 7), and (iv) transmetalation of 5
9
3
with Et Al to the enolate 8 and release of the nickel species back
into the catalytic cycle. Finally, hydrolysis of 8 will afford 2b.
Quenching of the reaction mixture with DCl resulted in the
2
formation of diethyl 2-[ H]-butylmalonate D-2b, which confirms
that the deallylation stops with the formation of the enolate 8.
In summary, we have developed the first and practical catalytic
method for smooth deallylation of 2-substituted-2-allylmalonates
to 2-substituted malonates via selective cleavage of the C-C bond
under mild reaction conditions. The reaction seems to be general
with respect to the transition-metal complexes; however, the
comparison of Ru and Ni catalysts indicates considerable differences
in their specific activity and selectivity. Last but not least, the
smooth deallylation offers an opportunity to use the allyl group as
an effective protective group for acidic hydrogen of malonic esters.
The scope of deallylation and mechanistic aspects of this reaction
with the respect to other substrates and transition-metal catalysts
are currently under investigation.
Acknowledgment. We would like to thank Dr. Iva Ti sˇ lerov a´
and Dr. Martin Sˇ t ´ı cha for measurement of NMR and MS. This work
was supported by a grant from the Fund for Development of Higher
Education No. 2800.
Supporting Information Available: Characteristics and spectral
data for all starting material and products as well as the reaction
condition details. This material is available free of charge via the
Internet at http://pubs.acs.org.
References
(
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Scheme 3. Proposed Reaction Mechanism of Catalytic Deallylation
9
1
1
1
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(
(
(
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deallylation of substrates bearing substituents exerting bigger steric
hindrance on the double bond such as the allylcinnamylmalonate
5) Ne cˇ as, D.; Kotora, M.; C ´ı sa ˇr ov a´ , I. Eur. J. Org. Chem. 2004, 1280-
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1
l and the allyl(3-methyl-2-buten-1-yl)malonate 1m proceeded in
6
moderate yields of 34 and 46%, respectively. Surprisingly, the Ru-
catalyzed deallylation of the allyl(2-(4′-bromobutyl))malonate 1n
and diallylcoumaranone 1o afforded 2k and 2l only in 35 and 16%
yields, respectively. In addition, the deallylation of 1i with the nickel
(
8) A typical experimental procedure was carried out as follows: Into a
solution of diethyl allylbutylmalonate 1b (128 mg, 0.5 mmol), NiBr
(PPh (19 mg, 0.025 mmol) in toluene (3 mL) was added a 2 M toluene
solution of Et
2
-
3 2
)
3
Al (0.5 mL, 1 mmol), and the reaction mixture was stirred
at 20 °C for 24h. Then the reaction mixture was quenched with 3 M HCl
1
2
catalyst proceeded also in the presence of Et Zn in 99% yield.
(2 mL) and analyzed by H NMR spectroscopy.
(
9) (E)-1-Propenylbenzene was found as the side product of the deallylation
of 1g and 1h.). An independent experiment showed that allylbenzene 6,
the side product of the deallylation of 1g, is quickly isomerized into (E)-
On the basis of the experimental results, we propose the following
reaction mechanism (Scheme 3): (i) alkylation of the Ni complex
1-propenylbenzene under the reaction conditions.
with Et
3
Al followed by â-hydrogen elimination to give the nickel-
hydride species 3, (ii) hydronickelation of the double bond to the
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J. AM. CHEM. SOC.
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