Rhodium-catalyzed deallylation of allylmalonates and related compounds

Article abstract of DOI:10.1021/om0508583

Various allylmalonates and other related compounds were selectively deallylated in the presence of a catalytic amount of a rhodium complex and an excess of triethylaluminum. Comparison of several phosphane (PPh3) and BIAP (bis(imidazolonyl)pyridine) rhodium complexes showed that the latter are more active and general with respect to the structural diversity of the substrate then the former. It was shown that the role of triethylaluminum is not only to generate in situ a rhodium hydride, which is assumed to be the catalytically active species, but also to act as a Lewis acid to activate the carbonyl group of the substrate. Thus, the proposed mechanism of deallylation involves hydrorhodation of the double bond along with activation of the carbonyl group of the substrate by triethylaluminum followed by a sequence of bond formations and cleavages, furnishing an enolate and an alkene. The methodology provides an efficient and selective route to deallylation of allylmalonates under mild reaction conditions.

Full text of DOI:10.1021/om0508583

Organometallics 2006, 25, 901-907
901
Rhodium-Catalyzed Deallylation of Allylmalonates and Related
Compounds
Matya´sˇ Tursky´,† David Necˇas,† Pavel Drabina,‡ Milosˇ Sedla´k,*,‡ and Martin Kotora*,†,§
Department of Organic and Nuclear Chemistry and Centre for New AntiVirals and Antineoplastics, Faculty
of Science, Charles UniVersity, HlaVoVa 8, 128 43 Prague 2, Czech Republic, Department of Organic
Chemistry, Faculty of Chemical Technology, UniVersity of Pardubice, Na´m. Cˇ s. Legi´ı 565,
532 10 Pardubice, Czech Republic, and Institute of Organic Chemistry and Biochemistry, Czech Academy
of Sciences, FlemingoVo na´meˇst´ı 2, 166 10 Prague 6, Czech Republic
ReceiVed October 5, 2005
Various allylmalonates and other related compounds were selectively deallylated in the presence of a
catalytic amount of a rhodium complex and an excess of triethylaluminum. Comparison of several
phosphane (PPh3) and BIAP (bis(imidazolonyl)pyridine) rhodium complexes showed that the latter are
more active and general with respect to the structural diversity of the substrate then the former. It was
shown that the role of triethylaluminum is not only to generate in situ a rhodium hydride, which is
assumed to be the catalytically active species, but also to act as a Lewis acid to activate the carbonyl
group of the substrate. Thus, the proposed mechanism of deallylation involves hydrorhodation of the
double bond along with activation of the carbonyl group of the substrate by triethylaluminum followed
by a sequence of bond formations and cleavages, furnishing an enolate and an alkene. The methodology
provides an efficient and selective route to deallylation of allylmalonates under mild reaction conditions.
Rh catalysts are suitable also for the cleavage of acyclic C-C
bonds. Typical examples are the cleavage of the sp2-sp3 bond
of alkylbenzenes10 and the activation of the sp3-sp3 bond in
ketones11 and ketimines12 and of the sp-sp bond of alkynes.13
It is also worth mentioning that the C-C bond cleavage of
cyclopropane and cyclobutane rings is crucial for rhodium-
catalyzed higher order [5 + 2]14 and [6 + 2]15 cycloaddition
reactions.
Recently, we have described an iron phosphane complex
catalyzed alkylative cyclization of R,ω-2-chlorodienes with
trimethyl- and triethylaluminum, giving 1-methylidene-2-alkyl-
cyclopentanes. During the course of the study we observed that
the reaction of allyl(2-chloroallyl)malonate proceeds anoma-
Introduction
Rhodium belongs to the group of transition metals (Ru, Os,
Pd, etc.) that has caused a real revolution in synthetic organic
chemistry in the past 30 years. Although its utilization has been
overshadowed by the use of more successful (cheaper) metals,
e.g. Pd, there has been recently renewed focus on rhodium
catalysis within the context of the C-C bond forming reactions.
In this regard it has been used as a catalyst in 1,4-conjugate
additions, 1,2-additions to CdX bonds, cross-coupling reactions,
cycloisomerizations, cyclotrimerizations, carbometalations of
double and triple bonds, carbene chemistry, aldol condenzation,
etc.1,2 On the other hand, rhodium complexes are also effective
catalysts for the reverse processsC-C bond cleavage.3,4 They
proved to be excellent catalysts for the cleavage of strained
cyclic systems such as cyclopropane rings,5 cubane,6 the
cyclobutane ring of biphenylene,7 and rings of cyclobutanones8,9
and larger cycloalkanones.9 Similarly, it has been shown that
(8) Cleavage of activated cyclobutane rings: (a) Murakami, M.; Amii,
H.; Ito, Y. Nature 1994, 370, 540-541. (b) Murakami, M.; Amii, H.;
Shigeto, K.; Ito, Y. J. Am. Chem. Soc. 1996, 118, 8285-8290. (c) Murakami,
M.; Takahashi, K.; Amii, H.; Ito, Y. J. Am. Chem. Soc. 1997, 119, 9307-
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(h) Matsuda, T.; Fujimoto, A.; Ishibashi, M.; Murakami, M. Chem. Lett.
2004, 33, 876-877.
* To whom correspondence should be addressed. E-mail: kotora@
natur.cuni.cz (M.K.).
† Charles University.
‡ University of Pardubice.
§ Czech Academy of Sciences.
(9) Jun, C.-H.; Lee, H.; Lim, S. G. J. Am. Chem. Soc. 2001, 123, 751-
752.
(1) Modern Rhodium-Catalyzed Reactions; Evans, P. A., Ed.; Wiley-
VCH: Weinheim, Germany, 2005.
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1998, 687-688. (b) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed.
1999, 38, 870-883.
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2422-2428. (b) Jun, C.-H. Chem. Soc. ReV. 2004, 33, 610-618.
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Organometallics 1998, 17, 4532-4533. (c) Nishihara, Y.; Yoda, C.;
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2001, 3, 785-787. (e) Jun, C.-H.; Moon, C.-W.; Lee, H.; Lee, D.-Y. J.
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10.1021/om0508583 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 01/07/2006

902 Organometallics, Vol. 25, No. 4, 2006
Table 1. Deallylation of Allylmalonates by Rh(PPh3)4H (1)
Tursky´ et al.
a Usually 2 equiv. b GC or 1H NMR yield. In parentheses is given the amount of the unreacted starting material, if present (GC or 1H NMR analysis). n.r.
) no reaction. c At 60 °C isomerized product 30%, starting material 70%. d Butylpropylmalonate formed in 16% yield. e In the presence of 25 mol % of the
catalyst the deallylation was 100% complete. f Allylpropenylmalonate 6b formed in 55% yield.
Scheme 1. Dealyllation by Rh Complexes
lously: instead of alkylative cyclization, C-C bond cleavage
(deallylation) took place to furnish (2-chloroallyl)malonate.16
Additionally, we have reported that this reactionsdeallylations
can be catalyzed by a number of other transition-metal phos-
phane complexes (Ru, Co, Rh, Ni, and Pd) as well. Although
the deallylation products were obtained in all cases, a detailed
comparison of the Ni- and Ru-catalyzed reactions indicated a
distinct difference between the individual catalysts as far as the
selectivity for various substrates is concerned.17 The preliminary
results also showed that Rh(PPh3)3ClsWilkinson’s catalysts
could be the best choice of an catalyst for the allylbutylmalonate
deallylation in comparison with other complexes under the
standard conditions.
formed by the reaction of Ni(II) compounds with alkylalumi-
nums,18 we assumed that a cationic Ni(II) hydride was the
Further interest in Rh-catalyzed deallylation was based on
catalytically active species. On the other hand, the formation
conclusions made from the proposed reaction mechanism.17 It
of similar cationic species from a Rh(I) complex is unlikely
was reasonable to expect that the natures of the catalytically
under the same reaction conditions, because the reaction of
active species generated from Ni and Rh complexes should be
RhI-X with triethylaluminum should result in the formation
different. Since it has been known that cationic Ni species are
of a RhI-alkyl species that undergoes â-hydrogen elimination
to give a neutral RhI-hydride. This is supported by the reported
(14) (a) Wender, P. A.; Takahashi, M.; Witulski, B. J. Am. Chem. Soc.
formation of Rh-hydride complexes upon reaction with alkyl-
metals (Et2Zn,19 Et3Al,20 BuLi21), metal hydrides (Et3SiH,22
catecholborane,23 BH324), or hydrogen in the presence of a
base.25 The aforementioned applications, along with an interest
1995, 117, 4720-4721. (b) Wender, P. A.; Sperandio, D. J. Org. Chem.
1998, 63, 4164-4165. (c) Wender, P. A.; Rieck, H.; Fuji, M. J. Am. Chem.
Soc. 1998, 120, 10976-10977. (d) Wender, P. A.; Fuji, M.; Husfeld, C.
O.; Love, J. A. Org. Lett. 1999, 1, 137-140. (e) Wender, P. A.; Dyckman,
A. J. Org. Lett. 1999, 1, 2089-2092. (f) Wender, P. A.; Zhang, L. Org.
Lett. 2000, 2, 2323-2326. (g) Wender, P. A.; Bi, F. C.; Brodney, M. A.;
Gosselin, F. Org. Lett. 2001, 3, 2105-2108. (h) Wender, P. A.; Gamber,
G. G.; Scanio, M. J. C. Angew. Chem., Int. Ed. 2001, 40, 3895-3897. (i)
Wender, P. A.; Gamber, G. G.; Hubbard, R. D.; Zhang, L. J. Am. Chem.
Soc. 2002, 124, 2876-2877. (j) Wender, P. A.; Williams, T. J. Angew.
Chem., Int. Ed. 2002, 41, 4550-4553. (k) Wender, P. A.; Love, J. A.;
Williams, T. J. Synlett 2003, 1295-1298. (l) Wender, P. A.; Gamber, G.
G.; Hubbard, R. D.; Pham, S. M.; Zhang, L. J. Am. Chem. Soc. 2005, 127,
2836-2837.
(15) Wender, P. A.; Correa, A. G.; Sato, Y.; Sun, R. J. Am. Chem. Soc.
2000, 122, 7815-7816.
(16) Necˇas, D.; Kotora, M.; C´ısaˇrova´, I. Eur. J. Org. Chem. 2004, 1280-
1285.
(17) Necˇas, D.; Tursky´, M.; Kotora, M. J. Am. Chem. Soc. 2004, 126,
10222-10223.
(18) (a) Muzzio, F. J.; Loffler, D. G. Acta Chim. Hung. 1987, 124, 403-
406. (b) Skupinska, J. Chem. ReV. 1991, 91, 613-648.
(19) Sato, K.; Omote, M.; Ando, A.; Kumadaki, I. Org. Lett. 2004, 6,
4359-4361.
(20) Strauss, S. H.; Shriver, D. F. Inorg. Chem. 1978, 17, 3069-3074.
(21) Uma, R.; Davies, M. K.; Cre´visy, C.; Gre´e, R. Eur. J. Org. Chem.
2001, 3141-3146.
(22) Esteruelas, M. A.; Herrero, J.; Oliva´n, M. Organometallics 2004,
23, 3891-3897.
(23) Burgess, K.; Van der Donk, W. A.; Westcott, S. A.; Marder, T. B.;
Baker, R. T.; Calabrese, J. C. J. Am. Chem. Soc. 1992, 114, 9350-9359.
(24) Morrill, T. C.; D’Souza, C. A. Organometallics 2003, 22, 1626-
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(25) Joo´, F. Acc. Chem. Res. 2002, 35, 738-745.

Rh-Catalyzed Deallylation of Allylmalonates
Organometallics, Vol. 25, No. 4, 2006 903
Table 2. Deallylation with Complexes 2 and 3aa
a Reaction conditions: catalyst (5 mol %), Et3Al (2 equiv). b GC or 1H NMR yields. In parentheses is given the amount of the unreacted starting material,
1
if present (GC or H NMR analysis). n.r. ) no reaction. c Butylpropylmalonate 35%. d Cyclopentanone 8a was formed in 23% yield. e Isomerized product
6d was formed in 73% yield. f Reaction was not carried out. g Unidentified products 11% and 7%. h Butylpropylmalonate 7%. i Cyclopentanone 8b was
formed in 22% yield. j Unidentified products (19, 25, and 12%) could not be separated into individual substances. k Allylmalonate 5b in 23% yield.
l Diallylmalonate 4b 45%, allylmalonate 5b 26%. m Contains a mixture of unidentified compounds.

904 Organometallics, Vol. 25, No. 4, 2006
Tursky´ et al.
Scheme 2. Formation of Cyclopentanones 8 from 4
in revealing the scope of the reaction with respect to other
transition metals, provided a strong impetus to study the
suitability of rhodium complexes for the catalytic deallylation
reaction. Herein we to report a detailed study on Rh complex
(P or N ligated) catalyzed C-C bond cleavage (deallylation)
of allylmalonates.
Results and Discussion
Table 3. Deallylation of Substituted Allylbenzylmalonatesa
For our study we chose three typical representatives of
rhodium complexes: the stable hydride Rh(PPh3)4H (1), Rh-
(PPh3)3Cl (2; Wilkinson’s catalyst), and the new class of
nitrogen-ligated complexes Rh(BIAP)Cl3 (3; BIAP ) bis-
(imidazolonyl)pyridine).26 The catalytic activities of these
complexes were tested by themselves or in the presence of
triethylaluminum or other additives.
Reactions Catalyzed by Rh(PPh3)4H. The possibility of
catalytic deallylation with a neutral Rh(I) hydride was especially
attractive from a synthetic point of view. To carry out the
reaction under neutral conditions would mean broadening of
the scope of the reaction to substrates bearing functional groups
sensitive to alkylmetals. In view of the foregoing, the use of
the stable rhodium hydride (Rh(PPh3)4H) (1) as a catalyst
seemed to be inevitable.
The reaction with 1 was carried out under various conditions
in toluene (Table 1). Treatment of the allylbutylmalonate 4a
with a catalytic amount of 1 (5 mol %) at 20 or 60 °C for 48
h did not result in any visible reaction (entry 1). Similarly, the
deallylation did not proceed with a stoichiometric amount of 1
at 20 °C, but heating of the reaction mixture to 60 °C for 48 h
resulted in the double-bond shift and the formation of the
butylpropenylmalonate 6a in 93% yield (entry 2). The next
experiment was executed in the presence of NaH to promote
the formation of the enolate (at 20 and 60 °C); however, again
no reaction was observed (entry 3). Although these results were
disappointing, they gave us two hints: first, the hydrometalation
of the double bond proceeded (otherwise the isomerization
would not occur), and second, the function of triethylaluminum
was more than just to generate rhodium hydride or to trans-
metalate rhodium from the enolate.
Since triethylaluminum is a Lewis acid, it might coordinate
to the lone electron pair on the carbonyl oxygen and thus activate
bonds in the vicinity of the carbonyl group. We reasoned that
adding another Lewis acid into the reaction mixture might
induce the deallylation. Unfortunately, our expectations were
not met: the presence of B(C6F5)3, AlCl3, and ZnBr2 (entries
4-6) or Lewis acids with reductive properties such as ZnEt2
and BEt3 (entries 7 and 8) did not result in any deallylation.
However, in the last case a partial isomerization of the double
bond to 6a (24%) was observed (entry 8). The deallylation of
the allylbutylmalonate 4a to the butylmalonate 5a proceeded
smoothly in good yield (84%) only after addition of Et3Al into
the reaction mixture (entry 9). Along with 5a the butylpropyl-
malonate 7a (16%) was formed by the hydrogenation of the
double bond. However, the catalytic activity of 1 in the
deallylation of the diallylmalonate 4b to the allylmalonate 5b
(24%) was rather low and was accompanied by a double-bond
shift to the allylpropenylmalonate 6b in 55% yield (entry 10).
Reactions Catalyzed by Rh(PPh3)3Cl (2) and the Rh-
BIAP Complex 3a. Better deallylation results were obtained
with catalytic systems composed of Rh(PPh3)3Cl (2) and the
Rh-BIAP complex (3a) in the presence of Et3Al. Results are
product yield (%)b
entry
benzylmalonate
5
6
8
1
2
3
4q
4r
4s
42 (5q)
88 (5r)
66 (5s)
32 (6q)
25 (8c)
a Reaction conditions: catalyst (5 mol %), Et3Al (2 equiv). b 1H NMR
or GC yield.
summarized in Table 2. Deallylations by Rh(PPh3)3Cl (2) and
Et3Al proceeded in good yields with the alkyl(allyl)malonates
4a,c to give 5a,c in 65 and 77% yields, respectively (entries 1
and 2). In the former case a considerable amount (35%) of the
allylbutylmalonate 4a underwent hydrogenation to the butyl-
propylmalonate 7a, and in the latter, cyclopentanone 8a was
formed as a side product.
In the case of the allylphenylmalonate 4d the deallylation
did not proceed; only a migration of the double bond to give
the phenylpropenylmalonate 6d in 73% yield was observed
(entry 3). Reactions of malonates bearing a methyl substituent
on the double bond, 4e,f, resulted in low yields (28 and 6%) of
the deallylated products 5e,f (entries 4 and 5). The deallylation
of the diallylmalonate 4b and the unsymmetrically substituted
diallylmalonates 4i-k gave rise to 5b,i-k in 100, 82, 93, and
77% yields, respectively (entries 8-11). During deallylation of
4k the formation of cyclopentanone 8b as a side product in
22% yield was observed. The reaction of the allylcinnamylma-
lonate 4l resulted in the formation of an inseparable mixture of
unidentified compounds along with the unreacted starting
material (42%) (entry 12). Unlike the deallylation of the allyl-
(chloroallyl)malonate 4m catalyzed by Ru and Fe complexes,
the use of the rhodium catalytic system gave the (chloroallyl)-
malonate 5m in 41% yield and the allylmalonate 5b (23%) along
with other unidentified products (entry 13). The reaction of the
diallylcyanoacetate 4n and the diallylcoumaranone 4o afforded
complex reaction mixtures, which did not contain any traces of
deallylated products (entries 14 and 15). The butenyl(butyl)-
malonate 4p underwent deallylation in low yield after migration
of the double bond (entry 16).
The formation of cyclopentanones 8a,b during deallylation
of 4c,k indicated that the hydrometalation of the double bond
must have proceeded with the reverse regioselectivity, resulting
in the formation of an organoaluminum intermediate that
underwent intramolecular nucleophilic addition to the carbonyl
group (Scheme 2). Cyclization to the cyclopentanone derivatives
(26) Sedla´k, M.; Drabina, P.; C´ısarˇova´, I.; Ru˚zˇicˇka, A.; Hanusek, J.;
Macha´cˇek, V. Tetrahedron Lett. 2004, 45, 7723-7726.

Rh-Catalyzed Deallylation of Allylmalonates
Organometallics, Vol. 25, No. 4, 2006 905
Table 4. Deallylation of 4a,b with Rh(BIAP)Cl3 Complexes 3a-ea
a Reaction conditions: catalyst (5 mol %), Et3Al (2 equiv), 3 h. b 1H NMR or GC yield. c Unidentified products 54%. d Unidentified products 51%.
or any other reaction did not proceed in the absence of
Wilkinson’s catalyst. This assumption is supported by the report
of hydroalumination of terminal alkenes with triisobutylalumi-
num to n-alkylaluminums under catalysis of Pd, Co, and Rh
complexes.27 To shed light on the possible effect of the
neighboring benzylic group on the course of hydrometalation
of the double bond, substituted allylbenzylmalonates 4q-s were
prepared and subjected to deallylation conditions. Surprisingly,
in each case a different set of products was obtained (Table 3).
Thus, 4q was partially deallylated to 5q (42%) and partially
isomerized to the propenyl derivative 6q (35%). On the other
hand, 4r underwent deallylation to the monosubstituted malonate
5r in high yield (88%). In the case of 4s the deallylation to the
monosubstituted malonate 5s (66%) was again accompanied by
the formation of the cyclopentanone derivative 8c (25%)
(unreacted starting material 6%). The results obtained with
4c,q-s indicate that the aromatic ring of the benzyl group might
act as a pendant ligand that coordinates to the rhodium atom,
thus affecting its reactivity.28
and the butylcinnamylmalonate 4g (entries 4 and 6). The allyl-
(chloroallyl)malonate 4m was deallylated to the (chloroallyl)-
malonate 5m in 20% yield; the major part of the starting material
was reductively dehalogenated to the diallylmalonate 4b, which
was probably partially deallylated to 5b (entry 13). Of course,
the possible reductive dehalogenation of 5m to 5b cannot be
excluded. In the cases of the diallylcyanoacetate 4n and the
diallylcoumaranone 4o the deallylation did not proceed. The
butenyl derivative 4p was probably deallylated in two steps:
(i) the rhodium hydride catalyzed migration of the double bond
to give the propenyl derivative 4f and (ii) its deallylation to
give 5f in 30% yield (entry 16) along with the formation of
unidentified compounds. The deallylation of the propenyl
derivative 4f under these conditions proceeded in fair yield
(entry 5).
Reactions Catalyzed by Rh(BIAP)Cl3 (3). Since the Rh-
BIAP complex 3a proved to be a good catalyst for the
deallylation in the presence of triethylaluminum, we decided
to compare its catalytic activity with other four differently
substituted Rh-(BIAP) complexes, 3b-e. The catalytic activity
of these complexes was studied in the reaction with the
allylbutylmalonate 4a and the diallylmalonate 4b in the presence
of Et3Al and Et2Zn. The intention was to study the possible
effect of the imidazolone ring substituent on the course of the
reaction (Table 4). The catalytic activities of all five complexes
3a-e in the presence of Et3Al were the same. The deallylations
of the allylbutylmalonate 4a and the diallylmalonate 4b in all
of the cases were quantitative within 3 h. The use of Et2Zn as
the reductant and Lewis acid for the deallylation of 4a proved
to be totally ineffective; therefore, this combination was no
longer checked.
Much better selectivity for deallylation was obtained with a
catalytic system composed of the bis(imidazolonyl)pyridine
rhodium complex 3a in the presence of Et3Al. Its use enabled
us to deallylate different malonates quantitatively in most cases
(entries 1-3 and 7-12). Slightly lower efficiency was observed
in deallylation of the butylcrotylmalonate 4f (77%, entry 5),
and low yields were obtained with the methallylmalonate 4e
(27) Gagneur, S.; Makabe, H.; Negishi, E. Tetrahedron Lett. 2001, 42,
785-787.
(28) For typical examples of Rh-arene complexes, see: (a) Bowyer,
W. J.; Merkert, J. W.; Geiger, W. E.; Rheingold, A. L. Organometallics
1989, 8, 191-198. (b) Singewald, E. T.; Slone, C. S.; Stern, C. L.; Mirkin,
C. A.; Yap, G. P. A.; Liable-Sands, L. M.; Rheingold, A. L. J. Am. Chem.
Soc. 1997, 119, 3048-3056. (c) Huck, S.; Ginsberg, A.; Pritzkow, H.;
Siebert, W. J. Organomet. Chem. 1998, 571, 107-113. (d) Herberich, G.
E.; Eckenrath, H. J.; Englert, U. Organometallics 1998, 17, 519-523.
Although the deallylation of the diallylmalonate 4b was
quantitative within 3 h, stirring of the reaction mixture for 24
h resulted in further transformation of the deallylated product

906 Organometallics, Vol. 25, No. 4, 2006
Tursky´ et al.
Scheme 3. Hydrogenation of 5b under the Deallylation
Reaction Conditions
Scheme 5. Synthesis of Complexes 3
Scheme 4. Proposed Reaction Mechanisms for the
Deallylation Reaction
5b into other products. The major byproduct was the product
of hydrogenation of the double bond of the deallylated product
5bsthe propylmalonate 9. An independent experiment showed
that subjecting allylmalonate 5b to the aforementioned condi-
tions resulted in the formation of propylmalonate 9 in 63% yield
after 24 h at room temperature (Scheme 3).
Revised Reaction Mechanism. Although the reaction mech-
anism of the deallylation had been proposed earlier,17 new results
led to its revision in order to include information concerning
the role of triethylaluminum and to fit the obtained experimental
data. Thus, the revised reaction mechanism for the rhodium
complex catalyzed process can be summarized as follows
(Scheme 4). In the first step the Rh(I) complex is alkylated to
alkylrhodium species that undergo â-hydrogen elimination to
give the Rh-hydride 10. As for RhIII-BIAP complex catalyzed
deallylation, it is reasonable to assume that it is reduced to Rh-
(I) species prior to the hydrometalation of the double bond.
Then two scenarios can be envisioned (paths A and B). We
think that the most probable scenario (path A) proceeds via the
following steps. First, the hydride 10 hydrometalates the double
bond of an allylmalonate to form the sec-alkylrhodium species
11 with triethylaluminum (Lewis acid) coordinated to the
carbonyl group (Lewis base). Then follows a sequence of C-C,
C-O, C-Rh, and Rh-O bond formation and cleavage in 11
through a six-membered transition state promoted by coordina-
tion of Et3Al to the carbonyl group, resulting in the formation
of the rhodium enolate 12.29 During this process also the allyl
moiety is released as an alkene. Finally, the transmetalation of
the rhodium enolate 12 with Et3Al gives the aluminum enolate
13 and the ethylrhodium species, which after â-hydrogen
elimination goes back into the catalytic cycle as the hydride
10. Another possibility is that the shift of the ethyl group from
aluminum to rhodium accompanied by the concomitant C-C,
C-O, C-Rh, and Rh-O bond formation and cleavage pro-
cesses in 11 results in the direct formation of the aluminum
enolate 13, the alkylrhodium species, and an alkene. However,
in that case eight electrons would have to be moved, and that
is not probable.
An alternative reaction mechanism of deallylation could be
envisioned as well. It was reported that the allylic C-C bond
could oxidatively add to an electron-rich Pd(0) complex, forming
a π-allyl-PdII complex.30 Thus, the second scenario (path B)
is based on the oxidative addition of the allylic C-C bond to
the hydride 10, giving the allyl-RhIII complex 14.31 The ensuing
reductive elimination of propene would afford the rhodium
enolate 12. However, since the addition of rhodium hydrides
to the C-C double bond (hydrorhodation) is a well-established
reaction, we think that path A should be preferred over path B.
On the other hand, the possibility of the coexistence of both
reaction pathways cannot be excluded.
Preparation of Rh-BIAP Complexes. The synthesis of
rhodium-BIAP complexes followed the protocol recently
reported for the preparation of similar iron complexes.26 The
BIAP ligands were synthesized in three steps (Scheme 5): (a)
formation of the amide 17 by the reaction of racemic amide 15
and pyridine-2,6-dicarboxylic acid dichloride 16, (b) intramo-
lecular cyclization to 18, and (c) benzylation of 18 to ligands
(29) For leading references concerning the structure of rhodium-enolates,
see: (a) Slough, G. A.; Bergman, R. G.; Heathcock, C. K. J. Am. Chem.
Soc. 1989, 111, 938-949. (b) Slough, G. A.; Hayashi, R.; Ashbaugh, J.
R.; Shamblin, S. L.; Aukamp, A. M. Organometallics 1994, 13, 890-898.
(c) Hayashi, R.; Takahashi, M.; Takaya, Y.; Ogasawara, M. J. Am. Chem.
Soc. 2002, 124, 5052-5058.
(30) Nilsson, Y. I. M.; Andersson, P. G.; Ba¨ckvall, J.-E. J. Am. Chem.
Soc. 1993, 115, 6609-6613.
(31) For discussions of π-allyl- and σ-allyl-rhodium complexes, see:
(a) Evans, P. A.; Nelson, J. D. J. Am. Chem. Soc. 1998, 120, 5581-5582.
(b) Reference 1, pp 193-194.

Rh-Catalyzed Deallylation of Allylmalonates
Organometallics, Vol. 25, No. 4, 2006 907
19. Finally, the complexes 3 were prepared by refluxing of
ligands 19 with RhCl3‚nH2O in MeOH.
plexes, e.g. 3a, can be in many instances as high as that obtained
with the nickel-based systems. Furthermore, the results provide
reasonable evidence for a triple role of triethylaluminum: (a)
it generates the rhodium hydride, (b) it activates the carbonyl
group via the Lewis acid-Lewis base principle, which enhances
the formation of enolate and helps to promote the C-C bond
cleavage, and probably also (c) it transmetalates the rhodium
enolate to release rhodium into the catalytic cycle. As far as
the rhodium complexes are concerned, the ligation around the
central atom plays a crucial role in the selectivity of the
deallylation. In this regard the best results with respect to
selectivity and activity were obtained for the reactions of Rh-
(BIAP)Cl3 complexes with tridentate N ligands. Also worthy
of mention is the formation of cyclopentanone derivatives in
three cases of reactions catalyzed by Rh(PPh3)3Cl (2), which
probably proceeds by anti-Markovnikov hydrometalation of the
double bond.
Comparison of Rh-Catalyzed Deallylation with Other
Catalytic Systems. In a comparison of the deallylation results
under Rh catalysis with the results obtained with other transition-
metal complexes (e.g. Ni, Ru),17 it can be concluded that the
same principles should apply for them as well, at least in general
terms. The multiple role of Et3Al as a reducing agent, a Lewis
acid, and a transmetalating agent during deallylation seems to
be apparent. Furthermore, the effect of the ligation around the
central metal atom on the deallylation selectivity and the course
of the reaction is important (e.g. compare results obtained with
phosphane and BIAP Rh complexes in Table 2, entries 2 and
11). Last but not least, also the nature of the transition metal
considerably influences the course of the deallylation: Ni
complexes usually deallylate all kinds of allylic compounds,
whereas Ru complexes deallylate the unsubstituted ones only.
On the other hand, substrates bearing reactive functional groups
(CN, halides, etc.) seem to be sensitive to Rh-catalyzed reaction
conditions. Typical examples are given by comparisons of
deallylations of 4f and 5a (Rh-BIAP, 77% (Table 2, entry 5);
Ni-phosphane, 86%; Ru-phosphane, 0%17) and of 4n and 5n
(Rh-BIAP, 0% (Table 2, entry 14); Ni-phosphane, 60%; Ru-
phosphane, 59%17).
Acknowledgment. We thank Dr. Iva Tisˇlerova´ for NMR
measurements, Dr. Ivan Neˇmec for IR measurements, and
Michal Vala´sˇek for MS measurements. This project was
supported by Project No. 1M6138896301 to the Centre for New
Antivirals and Antineoplastics from the Ministry of Education
of the Czech Republic (M.K.) and Project No. 203/04/0646 by
the Grant Agency of the Czech Republic (M.S.).
Conclusion
Supporting Information Available: All experimental details
and conditions for all starting materials, deallylation reactions,
synthesis of ligands 18a-e, and complexes 3a-e. This material is
available free of charge via the Internet at http://pubs.acs.org.
We have found that the catalytic system composed of rhodium
complexes and triethylaluminum can be successfully used for
the deallylation of malonates via allyllic C-C bond cleavage.
In particular, deallylation selectivity of the Rh-BIAP com-
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