PMA/SiO2 - An efficient recyclable heterogeneous catalyst for the synthesis of homoallyl alcohols and amines under solvent-free conditions

Article abstract of DOI:10.1139/V08-060

Aldehydes and imines can efficiently undergo nucleophilic addition reaction with allyltributylstannane in the presence of phosphomolybdic acid supported on silica (PMA/SiO₂) under solvent free-conditions to form the corresponding homoallyl alco

Full text of DOI:10.1139/V08-060

709
PMA/SiO2 — An efficient recyclable
heterogeneous catalyst for the synthesis of
homoallyl alcohols and amines under solvent-free
conditions1
Biswanath Das, Rathod Aravind Kumar, Ponnaboina Thirupathi,
Kanaparthy Suneel, and Pothkan Sunitha
Abstract: Aldehydes and imines can efficiently undergo nucleophilic addition reaction with allyltributylstannane in the
presence of phosphomolybdic acid supported on silica (PMA/SiO2) under solvent free-conditions to form the correspond-
ing homoallyl alcohols and amines respectively at room temperature in excellent yields. The catalyst can be easily recov-
ered and re-used.
Key words: homoallyl alcohol, homoallyl amine, phosphomolybdic acid, heterogeneous catalyst, allyltributylstannane,
solvent-free conditions.
Résumé : Opérant à la température ambiante et des conditions sans solvant, les aldéhydes et les imines peuvent donner
lieu à des réactions efficaces d’addition nucléophile avec l’allyltributylstannane, en présence d’acide phosphomolybdique
supporté sur de la silice (APM/SiO2) pour conduire à la formation respectivement d’alcools et d’amines homoallyliques
avec d’excellents rendements. Le catalyseur peut facilement être récupéré et réutilisé.
Mots-clés : alcool homoallylique, amine homoallylique, acide phosphomolybdique, catalyseur hétérogène, allyltributyls-
tannane, conditions sans solvant.
[Traduit par la Rédaction] Das et al. 713
Introduction
areas of organic synthesis. These catalysts are advantageous
over conventional homogeneous acid catalysts as they can
be easily recovered from the reaction mixture by simple fil-
tration and can be re-used after activation or without activa-
tion, thereby making the process more economically viable.
Here we report the utilization of a solid acid as a catalyst for
the preparation of homoallylic alcohols and amines.
Homoallylic alcohols (1) and amines (2) are important
building blocks for the construction of various biologically
active compounds, and hence the syntheses of these com-
pounds are highly useful. Lewis acid promoted nucleophilic
addition of allyltin reagent to carbonyl compounds and im-
ines is one of the straightforward methods for the synthesis
of homoallyl alcohols and amines. Various Lewis acids such
as metal halides (3), metal triflates (4), I2 (5), (bromodi-
methyl)sulfonium bromide (6), and cyanuric chloride (7) can
catalyze this reaction. However, most of the methods em-
ploying these catalysts suffer from certain disadvantages
such as high temperature, prolonged reaction times, harsh
reaction conditions, and the use of hazardous and expensive
acid catalysts. Some of the catalysts are extremely moisture
sensitive and can cause inconvenience in performing the re-
action. In recent years, the use of solid acids as heteroge-
neous catalysts has gained tremendous interest in different
Results and discussion
In continuation of our work (8) on the application of het-
erogeneous catalysts for the development of useful synthetic
methodologies, we have observed that phosphomolybdic acid
supported on silica (PMA/SiO2) (9) is very suitable to cata-
lyze the allylation of aldehydes and imines with allyltri-
butylstannane to form the corresponding homoallyl alcohols
and amines at room temperature (RT) under solvent-free
conditions (Scheme 1). This catalyst possesses excellent ac-
tivity, low toxicity, and high stability towards humidity. It
can be recovered from the reaction mixture and recycled.
Initially we studied the reaction of benzaldehyde, aniline,
and allyltributylstananne in CH3CN in the presence of
PMA/SiO2 at RT. The desired homoallylamine was obtained
in good yield (85%) within 80 min (Table 1, entry a). In
absence of the catalyst the product was not formed. We
studied the effect of different solvent systems such as THF,
Et2O, MeOH, CH3CN, CH2Cl2, and CHCl3 for the reaction,
but the conversion proceeded best in the absence of any
solvent. A wide variety of aromatic and aliphatic aldehydes
Received 19 November 2007. Accepted 13 March 2008.
Published on the NRC Research Press Web site at
canjchem.nrc.ca on 27 May 2008.
B. Das,2 R.A. Kumar, P. Thirupathi, K. Suneel, and
P. Sunitha. Organic Chemistry Division-1, Indian Institute of
Chemical Technology, Hyderabad 500 007, India.
1Part 150 in a series of studies on novel synthetic
methodologies.
2Corresponding author (e-mail: biswanathdas@yahoo.com).
Can. J. Chem. 86: 709–713 (2008)
doi:10.1139/V08-060
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Can. J. Chem. Vol. 86, 2008
Table 1. Preparation of homoallylic amines using PMA/SiO2.
Scheme 1. Preparation of homoallylic amines using PMA/SiO2.
Scheme 2. Preparation of homoallylic alcohols using PMA/SiO2.
containing electron-donating (Table 1, entries b and l) and
electron-withdrawing substituents (Table 1, entries c and d)
underwent the transformation smoothly. An acid sensitive
aldehyde such as α,β-unsaturated aldehydes (Table 1, entry
i), furfuraldehyde (Table 1, entry f), and a sterically hin-
dered aldehyde such as 2-naphthaldehyde (Table 1, entry e)
worked well to produce their corresponding homoallylic
amines. The imines (formed in situ from aldehydes and
amines in the presence of the catalyst) underwent facile re-
action with allyltributylstannane to form these compounds.
Previously, some methods were found to be unsuitable for
the preparation of homoallylic amines from enolizable
aldehydes. Interestingly the reaction of aldehydes, benzyl-
carbamate (CbzNH2), and allyltributylstannane yielded the
corresponding Cbz-protected allylamines in excellent yield
(79%–83%) (Table 1, entries k and l).
The other application of the present method is the
preparation of homoallyl alcohols from the corresponding
aldehydes on treatment with allyltributylstannane using
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Das et al.
711
Table 1 (concluded).
Note: The structures of the homoallylic amines were settled from their spectral (1H NMR and MS) data.
PMA/SiO2 under solvent- free conditions (Scheme 2). The
reaction proceeded at RT and the products were formed
within 4–5 h (Table 2). Both aromatic and aliphatic alde-
hydes were used for the preparation of homoallylic alcohols.
The aldehydes containing electron-donating (Table 2, entry
b) and electron-withdrawing groups (Table 2, entry c) in the
aromatic rings were found to undergo the conversion smoothly.
Acid-sensitive aldehydes such as furfural (Table 2, entry f)
and cinnamaldehyde (Table 2, entry e) also smoothly reacted
to afford the corresponding homoallyl alcohols.
We extended this method for the conversion of ketones
and carried out the reaction using acetophenone, cyclo-
hexanone, and β-ketoesters. None of the ketones gave
homoallylic alcohols and amines even after prolonging the
reaction times. The chemoselectivity of the present allylation
method is remarkable, providing allylation of only aldehydes
in the presence of ketones (Table 2, entries i–k). The times
required for the preparation of homoallylic alcohols (4–5 h)
are somewhat more than those required for the preparation
of homoallylic amines (30–60 min).
The mixture was stirred at RT for 6 h. Methanol was
evaporated under reduced pressure to attain the catalyst
(PMA/SiO2) as an yellowish powder.
General experimental procedure for the synthesis of
homoallyl amines
To a mixture of an aldehyde (2 mmol), amine (2 mmol),
and allyltributylstannane (2.5 mmol), PMA/SiO2 (113 mg)
was added under solvent-free conditions at RT. The mixture
was stirred and the reaction was monitored by TLC. After
completion of the reaction, the mixture was filtered and the
catalyst was recovered quantitatively after washing the resi-
due with dry methanol (2 × 5 mL). The filtrate (containing
the product in methanol) was concentrated and the residue
was subjected to column chromatography (silica gel, hex-
ane) to obtain pure homoallyl amine. The recovered catalyst
was re-used three times subsequently for the same reaction
without affecting the yield of the product.
General experimental procedure for the synthesis of
homoallyl alcohols
Conclusion
To a mixture of an aldehydes (2 mmol) and allyltributyl-
stannane (2.5 mmol), PMA/SiO2 (113 mg) was added under
solvent-free conditions at RT. The mixture was stirred and
the reaction was followed by TLC. After completion, the
mixture was filtered and the residue was washed with dry
methanol (2 × 5 mL) to recover the catalyst quantitatively.
The filtrate (containing the product in methanol) was con-
centrated and the viscous mass was subjected to column
chromatography (silica gel, 35% EtOAc and hexane) to
afford pure homoallyl alcohol. The recovered catalyst was
re-used three times subsequently in this case also for the
same reaction without affecting the yield of the product.
In conclusion, we have developed a simple and efficient
method for the allylation of aldehydes and amines using
PMA/SiO2 as a heterogeneous catalyst. The main features of
this methodology are (i) operational simplicity, (ii) short re-
action times, (iii) mild reaction conditions, (iv) excellent
yields, (v) application of an inexpensive heterogeneous cata-
lyst, (vi) high chemoselectivity, (vii) high stability towards
humidity, and (viii) reusability of the catalyst.
Experimental
Preparation of PMA/SiO2 catalyst (9)
Slica gel (100–200 mesh, 450 mg) was added slowly to a
solution of H3PMo12O40·24H2O (50 mg) in methanol (5 mL).
All the prepared compounds (except 3b, 3f, and 4b) are
known (3c, 3d, 4b, 4c, and 6). The spectral and analytical
data of representative products are given below.
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Can. J. Chem. Vol. 86, 2008
Table 2. Preparation of homoallylic alcohols using PMA/SiO2.
Note: The structures of the homoallylic alcohols were settled from their spectral (1H NMR and MS) data.
3b
4b
Oil. 1H NMR (CDCl3, 200 MHz) δ: 7.20 (2H, d, J = 8.0 Hz),
1
Oil. H NMR (CDCl3, 200 MHz) δ: 7.27 (2H, d, J = 8.0
Hz), 7.03 (2H, t, J = 8.0 Hz), 6.82 (2H, d, J = 8.0 Hz), 6.59
(1H, t, J = 8.0 Hz), 6.42 (2H, d, J = 8.0 Hz), 5.75 (1H, m),
5.21–5.09 (2H, m), 4.31 (1H, td, J = 5.5, 1.5 Hz), 4.03 (1H, m),
3.78 (3H, s), 2.61–2.40 (2H, m). FABMS m/z: 254 (M+ + 1).
Anal. calcd. for C17H19 NO: C 80.63, H 7.50, N 5.53; found:
C 80.58, H 7.54, N 5.49.
6.80 (2H, d, J = 8.0 Hz), 5.67–5.80 (1H, m), 5.08 (2H, m),
6.42 (1H, t, J = 12.5, 6.0 Hz), 3.77 (3H, s), 2.43 (2H, J =
12.5, 6.5 Hz), 2.08 (brs, OH). FABMS m/z: 179 (M+ + 1).
Anal. calcd. for C11H14O: C 74.15, H 7.86; found: C 74.22,
H 7.80.
Acknowledgements
3f
Oil. 1H NMR (CDCl3, 200 MHz) δ: 7.30 (1H, d, J = 1.8 Hz),
7.06 (2H, t, J = 8.0 Hz), 6.64 (1H, t, J = 8.0 Hz), 6.52 (2H,
d, J = 8.0 Hz), 6.23 (1H, dd, J = 2.0, 1.8 Hz), 6.09 (1H, d, J =
2.0 Hz), 5.72 (1H, m), 5.20–5.08 (2H, m), 4.52 (1H, t, J =
5.5 Hz), 3.89 (1H, brs), 2.64 (2H, t, J = 5.5 Hz). FABMS
m/z: 214 (M+ + 1). Anal. calcd. for C14H16NO: C 78.50, H
7.47; found: C 78.56, H 7.51.
The authors thank Council for Scientific and Industrial
Research (CSIR) and University Grants Commission (UGC),
New Delhi for financial assistance.
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