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Allylic Alkylation Catalyzed by Molybdenum Complexes Containing Polydentate
Phosphinoalkyl-Silyl Ligands†
MAKOTO MINATO*, NAOTO WADA, TAKEFUMI KATSUREN, and HIROAKI SHU
Department of Materials Chemistry, Graduate School of Engineering,
Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan
Abstract: The reaction of allyl alcohol derivatives with soft carbon nucleophiles, such as
dimethyl sodiomalonate, in the presence of catalytic quantities of silyl-molybdenum hydrido
complexes ([MoH3{Si(R)[Ph2PCH2CH2P(Ph)C6H4-o]2}] or [MoH3([Ph2PCH2CH2P(Ph)C6H4o]R2Si-P,P,Si)]) is probed. The regioselectivity of the reaction and the catalyst activity were
found to be highly dependent on the solvents used. With trisubstituted double bond olefins,
the products alkylated at the unsubstituted allylic terminus were obtained in 95-99%
selectivity. This regioselectivity is complementary to the Ir-catalyzed reactions, in whichhighly regioselective alkylation at the substituted allylic terminus was achieved. Directconversion of allylic alcohol was also examined. The reaction of ethylbenzenesulfonylacetate
or diethyl malonate proceeds under neutral conditions without addition of bases from outside.
Keywords: high regioselectivity; Tsuji-Trost reaction; trans-influence.
†

Dedicated to the memory of Rastko D. Vukicevic, a friend and a fine colleague whose loss

has been felt very deeply by so many people.
RUNNING TITLE: ALLYLIC ALKYLATION CATALYZED BY Mo-COMPLEXESINTRODUCTION
In

previous

papers,wereportedtheunexpected

formationofthecomplexes[MoH3{[Ph2PCH2CH2P(Ph)C6H4-o]2(R)Si-P,P,P,P,Si}] (2), which were obtained by thethermal reaction of [MoH4(dppe)2] (1, dppe = Ph2PCH2CH2PPh2) with primary silanes RSiH3(R = Ph, n-C6H13).1 The resulting complexes possess an unusual quadruply chelated ligand
consisting of a P2SiP2 framework (Scheme 1). When secondary R2SiH2 (R = Ph, C4H3S) was1

employed in a similar reaction with 2, a trihydrido complex 3 with a tridentate ligand[MoH3([Ph2PCH2CH2P(Ph)C6H4-o]R2Si-P,P,Si)] consisting of a P2Si framework was isolated.2*Corresponding author. E-mail: minato@ynu.ac.jp
The formation of these silyl-molybdenum complexes is itself very novel, because activation of
not only the Si-H bonds but also the ortho C-H bonds of dppe ligands takes place in the same
system.
R

Ph

Ph

H

H

Ph

Ph
CH2
Mo
H2C
P
P CH2
Ph
Ph
H Ph
Ph H
H2C P

1

P

Ph
RSiH3
110 °C −H2

H2C P

H Si

2a (R = Ph) 2b (R = n-C6H13)

R2SiH2R
110 °C −H2

Ph
CH2
CH
2
P
Ph
Ph
P

Mo
H2C
P
Ph
Ph H H

Ph
H2C P

H Si
Mo

H2C
P
Ph
Ph H H

R

Ph
Ph
CH2
CH
2
P
Ph
Ph
P

3a (R = Ph) 3b (R =S)

Scheme 1. Reactions of 1 with RSiH3 and R2SiH2.
In addition to some interesting reactivities, these silyl-molybdenum complexes have exhibited
unique characteristics for catalytic CO2 fixation,3 dehydro-polymerization of arylsilanes,4 andsynthesis of polysiloxanes.5 We think that the catalytic activity of the complexes is due to thestrong trans-influence of the Si fragment since the parent complex 1 did not catalyse the above
processes. The electron-releasing properties and strong trans-influence of silyl ligands mayalter or enhance the catalytic performance of transition-metal catalysts.6 Thus, we have an
ongoing interest in the application of the complexes as catalysts for organic transformation. In
this report, we explored the catalytic activity of complexes 2 and 3 towards the allylic
alkylations. The palladium-catalyzed substitution reaction of allylic compounds with various
nucleophiles is a well-established method (Tsuji-Trost reaction) and used extensively in organic
synthesis.7 One recent and exciting breakthrough associated with research on the Tsuji-Trost
reaction has been the development of a general methods of enantioselectively creating
quaternary centers.8
EXPERIMENTAL
General procedures
All manipulations were performed under an inert atmosphere of nitrogen or argon using
standard Schlenk techniques. The following instrumentation was used for accumulating spectra
data: Perkin-Elmer 1600FT spectrometer (infrared), JEOL JNMEX-270, or JEOL JNMGX-500
spectrometers (nuclear magnetic resonance), and JEOL JMS-600 spectrometer (mass). GC

analyses were performed on a Shimadzu GC-14A using 0.33 mm × 50 m glass column packed
with CBP1-S50-050.
Materials
Commercially available reagent grade chemicals (Wako Chemical) were used as such without
any further purification. Solvents were purified according to standard procedures. Complexes
2 and 3 were prepared by literature procedures.1, 2 Allylic esters were prepared by the reactionof the corresponding alcohols with acetyl chloride or trifluoroacetic anhydride.
Allylic alkylation of 4a (Entry 1)
A Schelenk flask was charged with complex 2a (0.050 g, 0.050 mmol), allyl acetate (4a, 0.54
ml, 5 mmol), and toluene (10 ml) under argon. In a separate flask, dimethyl malonate (0.69 ml,
6.0 mmol) was added to a slurry of hexane-washed sodium hydride (0.24 g, 6 mmol) in toluene(17.5 ml) under argon. To the resulting suspension of sodium dimethyl malonate was added theformer via a cannula. After heating at 110 °C for 4h, usual work-up provided dimethyl
allylmalonate.
Allylic alkylation of 4b (Entry 5)
A Schelenk flask was charged with complex 2a (0.050 g, 0.050 mmol), cinnamyl
trifluoroacetate (4b, 0.30 g, 1.3 mmol), and DMI (10 ml) under argon. In a separate flask,
dimethyl malonate (0.30 ml, 2.61 mmol) was added to a slurry of hexane-washed sodium
hydride (0.12 g, 3.0 mmol) in DMI (20 ml) under argon. To the resulting suspension of sodium
dimethyl malonate was added the former via a cannula. Then the mixture was stirred for 48 h
at 110 °C. The usual workup and purification by column chromatography on SiO2 withhexane/ethyl acetate (8/1) gave a linear compound A (76%). GLC analysis showed the branched
product B in 1%. Methyl (E)-2-methoxycarbonyl-5-phenyl-4-pentenoate: 1H NMR d 2. 81 (td,J = 7.25, 1.32 Hz, 2H), 3.54 (t, J = 7.25 Hz, 1H), 3.74 (s, 6H), 6.10 (dt, J = 15.83, 7.25 Hz, 1H),
6.48 (d, J =15.83 Hz, 1H), 7.18-7.35 (m, 5H). MS: 248 (M+).
Allylic alkylation of 4c (Entry 7)
A Schelenk flask was charged with complex 2a (0.10 g, 0.10 mmol), geranyl trifluoroacetate(4c, 1.18 ml, 5.0 mmol), and toluene (10 ml) under argon. In a separate flask, dimethyl malonate(0.57 ml, 5.0 mmol) was added to a slurry of hexane-washed sodium hydride (0.18 g, 5.0 mmol)in toluene (10 ml) under argon. To the resulting suspension of sodium dimethyl malonate wasadded the former via a cannula. Then the mixture was stirred for 12 h at 110 °C. The usual
workup and purification by column chromatography on SiO2 with hexane/ethyl acetate (10/1)
gave a linear compound A (39%), which was identical with an authentic sample.11 GLC analysis

showed the branched product B in 1%. By the same procedure, neryl trifluoroacetate wasreacted with sodium dimethyl malonate to give a 95:5 mixture of A and B isomer in 35% yield.
Allylic alkylation of allylic alcohol with ethyl benzenesulfonylacetate (Entry 3)
A Schelenk flask was charged with complex 2b (0.050 g, 0.050 mmol), allylic alcohol (0.34 ml,
5.0 mmol), Tetrabutylammonium Fluoride THF solution (0.25 ml, 0.25 mmol), and toluene (10ml) under argon. The mixture was stirred for 30 min. To the resulting solution was added ethyl
benzenesulfonylacetate (0.0.57 g, 2.5 mmol) and n-tetradecane (0.20 ml, 2.0 mmol, internal
standard). Then the mixture was stirred for 6 h at 110 °C. The usual workup and purificationby column chromatography on SiO2 with hexane/ethyl acetate (7/1) gave ethyl 2benzensulfonyl-4-pentenoate (40%): 1H NMR d 1.14 (t, J = 7.25 Hz, 3H), 2.63~2.84 (m, 2H),
4.01 (dd, J = 10.55, 4.62 Hz, 1H), 4.09 (qur, J = 7.25 Hz, 2H), 5.09 (dd, J = 10.55, 1.32 Hz,
1H), 5.16 (dd, J = 15.50, 1.32 Hz, 1H), 5.68 (ddt, J = 15.50, 10.55, 6.59 Hz, 1H), 7.55~7.91 (m,5H). MS: 269 (M++1).
Allylic alkylation of allylic alcohol with diethyl malonate (Entry 4)
A Schelenk flask was charged with complex 3a (0.050 g, 0.046 mmol), allylic alcohol (0.34 ml,
5.0 mmol), Tetrabutylammonium Fluoride THF solution (0.25 ml, 0.25 mmol), and toluene (10ml) under argon. The mixture was stirred for 30 min. To the resulting solution was added diethyl
malonate (0.38 ml, 2.5 mmol) and n-tetradecane (0.20 ml, 2.0 mmol, internal standard). Thenthe mixture was stirred for 12 h at 110 °C. GLC analysis showed allyl diethyl malonate (42%),which was identical with an authentic sample prepared by the conventional palladium process.
RESULTS AND DISCUSSION
We carried out the allylation of dimethyl malonate (Scheme 2). The reaction conditions and theresults are summarized in TABLE I.

Scheme 2. Mo-catalyzed allylic alkylation

TABLE I. Scope of the allylic alkylation
Entry (cat.)
1 (2a)2 (1)
3 (2a)d4 (2a)e,g
5 (2a)f,g
6 (2a)f,i
7 (3a)d,h8 (3a)d,h

Allyl (X)
4a (AcO)
4a (AcO)
4b (CF3CO2))4b (CF3CO2))4b (CF3CO2))4b (CF3CO2))4c (CF3CO2))4d (CF3CO2))

NuH
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2
CH2(CO2Me)2

A:Ba
44:56
98:2
99:1
99:1
99:1
95:5

Yield (%) j
94b
22a
12c
38c
76c
54c
39c
35c

a

Ratios determined by GLC and 1H NMR; bDetermined by internal standard method with GLC; cIsolated yield;
The reaction was carried out in refluxing toluene for 12 h; eDMF was the solvent. fDMI was the solvent; gThereaction was performed with 4 mol% catalyst at 110 °C for 48 h; hThe reaction was performed with 2 mol%catalyst; iThe reaction was performed with 4 mol% catalyst at room temperature for 24 h; jThe products wereidentified by comparison of their 1H NMR spectra with those reported in the literature for these compounds (seeref. 9 and 11).
d

The initial reaction employed allyl acetate with dimethyl sodiomalonate in toluene at reflux.
The allylation catalyzed by complex 2a proceeded smoothly (Entry 1). The yield of the reaction
was much higher than that obtained for the system with complex 1 (Entry 2), thus the silyl
ligand in complex 2 proved to be effective for the transformation. Interest in transition-metalcatalyzed reactions of allyl substrates with nucleophiles stemmed from the issue ofregioselectivity. The palladium process tends to occur at the less substituted allylic terminus.
However, there are some notable exceptions and the regiocontrol appears somewhat confusing
as described below. On the other hand, with molybdenum catalysts, the regioselectivity is often
complementary to the Pd-catalyzed reactions. For example, with aryl-substituted allyl systems,
molybdenum catalysts favor attack at the more substituted terminus.9 Accordingly, next
experiments focused on the use of cinnamyl, geranyl, and neryl alcohol derivatives as thestarting materials (Entries 3~8).
Complex 2a was found to be ineffective for the conversion of cinnamyl acetate.10
However, 2a catalyzed the allylation of the more reactive cinnamyl trifluoroacetate. The
regioselectivity of the reaction and the catalyst activity were found to be highly dependent on
the solvents used. In toluene, the allylation generated only small amounts of products (12%,Entry 3). The regioselectivity was also considerably lower (44:56). On the other hand, using
Dimethylformamide (DMF) facilitated the catalysis (38%, Entry 4). In addition, thenucleophilic substitution takes place regioselectively at unsubstituted side of allylic system(98:2). This observation is in sharp contrast with that of molybdenum carbonyl complexes
system in which the molybdenum catalysts tend to lead to preferential alkylation at thesubstituted allylic terminus. 1,3-Dimethyl-2-imidazolidinone (DMI) proved to show moreadvantageous performance, affording the product in high yield (76%, Entry 5) with highregioselectivity (99:1). It is also interesting to note that the allylation in DMI is occurring
appreciably at room temperature (Entry 6). Equally good selectivity for attack at the lesssubstituted terminus occurred for both geranyl and neryl trifluoroacetates (Entries 7 and 8).
These reactions, however, required using complex 3a as a catalyst and heating at reflux intoluene. In striking contrast to the result of cinnamyl trifluoroacetate, with DMI, the reactions
failed due to an unknown reason. Previously Trost reported that Pd-catalyzed alkylation of
geranyl acetate with dimethyl sodiomalonate produced a mixture of regioisomers (A:B = 90:10).
Furthermore, in the case of neryl acetate, the major product of alkylation with dimethyl
sodiomalonate arose from attack at the tertiary carbon atom (A:B = 35:65) and thus theregioselectivity was opposite that of geranyl acetate.11 These results indicate that the silylmolybdenum complexes are efficient catalysts for regiospecific alkylation at the unsubstituted
allylic terminus.
From the viewpoint of the atom economy, direct conversion of allylic alcohols is far more
beneficial. In Pd(0) catalyzed reactions, allylic alcohols are less reactive allylating agents.
Several attempts have been reported in this connection. For example, allylation of malonateswith allyl alcohols was carried out by using a sp2-hybridized bidentate phosphine-Pd catalyst.12
Generally, a poor leaving group hydroxide can be used in the presence of some activators, such
as As2O3, B2O3, and Ti(O-i-Pr)4.13 The hydroxy group is activated by coordination of these
Lewis acids. Catalytic conversions of allyl alcohol were also examined (Scheme 3 and Table
II).

Scheme 3. Direct conversion of allylic alcohol
TABLE II. Scope of the reactions
Entry (cat.)ZY
1 (2a)Na
CO2Et
2 (2a)H
SO2Ph
3 (2b)aH
SO2Ph
4 (3b)aH
CO2Et
a

TBAF (10 mol%) was used.

Yield (%) j
53
23
40
42

The reaction with diethyl sodiomalonate proceeded as with allyl acetate, affording the productin 53% yield (Entry 1). In our case, the allylation occurred without requirement for Lewis acidhelp. The oxidative addition of allyl alcohol to molybdenum center may be followed byformation of p-allyl molybdenum hydroxide complex, and then the generated hydroxide ion
might pick up proton from active methylene compounds. Therefore, we expected that reactionsof allyl alcohol proceed under neutral conditions without addition of bases from outside.
Substitution reactions under neutral conditions are highly desirable. The test reaction that
employed allyl alcohol with diethyl malonate as the pronucleophile failed to produce significant
amounts of products. This result mirrors our earlier results in which the reactions of 2a with
dialkyl malonates afforded stable dihydrido (h1-O-enolato) molybdenum complexes.14 The
formation of the enolato complex may anticipate the oxidative addition of allyl alcohol tomolybdenum center in the reaction. The first sign of some success came in the use of reactive
ethylbenzenesulfonylacetate as the pronucleophile (Entry 2). Particularly, the reaction wasenhanced by using complex 2b along with addition of a catalytic amount of n-Bu4NF (TBAF,
Entry 3). In the present case, we may suppose that coordination of F- at silicon increases thecatalytic activity of 2b. Furthermore, we found that the combination of 3b and TBAF enables
the allylation of diethyl malonate (Entry 4).

CONCLUSION
The silyl-molybdenum complexes were found to be a new catalyst for allylic alkylation. Highly
regioselective alkylation at the unsubstituted allylic terminus was achieved. Thisregioselectivity is complementary to the Ir-catalyzed reactions, in which highly regioselective
alkylation at the substituted allylic terminus was achieved.15 We have also demonstrated thatthe silyl-molybdenum complexes possess the catalytic activity toward direct conversion ofallylic alcohol. To our knowledge, there have so far been no reports of analogous Mo-catalyzed
reactions of allylic alcohol under neutral conditions. Taking into account the lower cost ofmolybdenum compared with palladium, the prospects that it may be more generally useful will
be an important direction to pursue.
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