Distannoxane-Catalyzed Highly Selective Acylation of Alcohols

Full text of DOI:10.1021/jo9800412

2420
J . Org. Chem. 1998, 63, 2420-2421
Communications
Ta ble 1. Dista n n oxa n e-Ca ta lyzed Acyla tion of Alcoh olsa
Dista n n oxa n e-Ca ta lyzed High ly Selective
Acyla tion of Alcoh ols
1b
(mol %)
condns
(°C, h)
4c
entry
2
3
(% yield)
1
2
3
4
5
6
7
8
9
C8H17OH (2a )
2a
2a
2a
1
5
1
3
1
1
5
1
5
1
1
1
5
5
5
5
1
10
10
10
1
1
1
1
1
1
1
3a 30, 24
3a 30, 5
99
93
98
96
92
98
93
98
98
99
99
99
98f
98
97
96
11
97
2
Akihiro Orita, Akihiro Mitsutome, and J unzo Otera*
3a reflux, 1
3b 30, 24
3b reflux, 1
3a 30, 24
3a 30, 5
Department of Applied Chemistry, Okayama University of
Science, Ridai-cho, Okayama 700, J apan
2a
Ph(CH2)2OH (2b)
Received J anuary 12, 1998
2b
2b
2b
3a reflux, 0.5
3b 30, 24
3b reflux, 2
3a reflux, 0.5d
3a reflux, 0.5e
3c 50, 15
3d 50, 15
3e 50, 15
3f 50, 17
3a 30, 24
3a reflux, 1
3a 30, 24
3a reflux, 0.5
3a 30, 24
3a 30, 24
3a 30, 22
3a 30, 23
3a 30, 23
3a 30, 24
3a 30, 24
Acylation of alcohols has enjoyed constant renovation due
to its significance in synthetic chemistry. Acetic anhydride
is the most frequently employed reagent. Probably, the first
practical breakthrough in this technology was brought about
by the discovery of the 4-(dialkylamino)pyridine catalysts.1
Further improvements have been advanced in the relevant
studies.2 More recently, a variety of new catalysts appeared
that are either basic or acidic. Bu3P,3 MgBr2-R3N,4 and an
aminophosphine superbase5 fall in the former category,
10 2b
11 2b
12 2b
13 2a
14 2a
15 2a
16 2a
17 C6H13CH(OH)CH3 (2c)
18 2c
6
7
while Sc(OTf)3 and TiCl(OTf)3 fall in the latter. 3-Acetyl-
thiazolidine-2-thiones-NaH8 and AcCl-hindered amine9 were
employed for selective acylation of primary alcohols.
Transesterification is another important means, but
unfortunately, it is difficult to reach high conversions with
this reaction due to its reversibility.10 This drawback can
be overcome by using enol esters that are capable of escaping
from the equilibrium because of their conversion into alde-
hydes or ketones upon transesterification. This reaction is
usually conducted under acidic conditions.11 Recently, Cp*2-
Sm(thf)2 or SmI2 was found to be effective, yet this method
required the Schlenk tube technique.12
19 PhCH(OH)CH3 (2d )
20 2d
99
0
0
21 cyclohexanol
22 C6H5OH
23 geraniol
24 TBSO(CH2)4OH
25 THPO(CH2)4OH
26 CF3CO2(CH2)4OH
25 PhOC(O)CtCH2OH
98g
98h
97h
97h
97
a
b
Reaction conditions: 2 (5 mmol), 3 (3 mL). Molarity on the
basis of the monomeric formulation. c Isolated yield after column
chromatography. In 3a (2.5 mL) and toluene (2.5 mL). e In 3a
d
(2.5 mL) and THF (2.5 mL). f Determined by 1H NMR. Sc(OTf)3
g
Previously, we disclosed that 1,3-disubstituted tetraalky-
ldistannoxanes that have a dimeric formulation like 1
catalyzed transesterification under virtually neutral condi-
tions.13,14 Therefore, we expected that distannoxane-
catalyzed transesterification, when coupled with the enol
ester protocol, would give rise to a new practical method for
acylation of alcohols. This is indeed the case. We report
h
gave a complex mixture. Sc(OTf)3 afforded AcO(CH2)4OAc quan-
titatively.
herein a convenient way for the highly selective acylation
of alcohols that is difficult to achieve with other catalysts.
The operation is quite simple (eq 1). A solution of alcohol
2 (5 mmol) and a catalytic amount of 1,3-dichlorotetrabu-
tyldistannoxane (1)15 in alkenyl ester 3 (3 mL) was stirred
under conditions given in Table 1. The reaction mixture was
(1) Litvinenko, L. M.; Kirichenko, A. I. Dokl. Akad. Nauk SSSR, Ser.
Khim. 1967, 176, 97. Steglich, W.; Ho¨fle, G. Angew. Chem., Int. Ed. Engl.
1969, 8, 981. For a review: Ho¨fle, G.; Steglich, W.; Vorbru¨gen, H. Angew.
Chem., Int. Ed. Engl. 1978, 17, 569.
(2) Ho¨fle, G.; Steglich, W. Synthesis 1972, 619. Hassner, A.; Krepski, L.
R.; Alexanian, V. Tetrahedron 1978, 34, 2069. Shimizu, T.; Kobayashi, R.;
Ohmori, H.; Nakata, T. Synlett 1995, 650.
(3) Vedejs, E.; Diver, S. T. J . Am. Chem. Soc. 1993, 115, 3358. Vedejs,
E.; Bennett, N. S.; Conn, L. M.; Diver, S. T.; Gingras, M.; Lin, S.; Oliver, P.
A.; Peterson, M. J . J . Org. Chem. 1993, 58, 7268.
(4) Vedejs, E.; Daugulis, O. J . Org. Chem. 1996, 61, 5702.
(5) D’Sa, B. A.; Verkade, J . G. J . Org. Chem. 1996, 61, 2963.
(6) Ishihara, K.; Kubota, M.; Kurihara, H.; Yamamoto, H. J . Am. Chem.
Soc. 1995, 117, 4413; J . Org. Chem. 1996, 61, 4560.
(7) Izumi, J .; Shiina, I.; Mukaiyama, T. Chem. Lett. 1995, 141.
(8) Yamada, S. J . Org. Chem. 1992, 57, 1591.
(9) Ishihara, K.; Kurihara, H.; Yamamoto, H. J . Org. Chem. 1993, 58,
3791.
evaporated, and the residue was subjected to column chro-
matography to give the desired esters 4. When the substrate
is insoluble in the enol esters, a cosolvent like toluene or
THF may be used. Notably, enol esters and solvents can be
used as received without purification and no inert atmo-
sphere is necessary for the reaction because of the stability
of 1 under the ambient atmosphere. In the presence of 1 or
3 mol % of 1, both 3a and 3b acetylated octanol quantita-
tively at 30 °C in 24 h (entries 1 and 4). The reaction time
can be shortened by increasing the amount of the catalyst
to 5 mol % (entries 2) or by elevating the reaction temper-
ature (entries 3 and 5). 2-Phenylethanol reacted similarly
(entries 6-10), and the use of cosolvent gave rise to virtually
(10) Otera, J . Chem. Rev. 1993, 93, 1449.
(11) Hagemeyer, H. J ., J r.; Hull, D. C. Ind. Eng. Chem. 1949, 41, 2920.
Rothman, E.; Hecht, S.; Pfeffer, P. E.; Silbert, L. S. J . Org. Chem. 1972,
37, 3551. Kita, Y.; Maeda, H.; Takahashi, F.; Fukui, S. J . Chem. Soc., Chem.
Commun. 1993, 410. Kita, Y.; Maeda, H.; Omori, K.; Okuno, T.; Tamura,
Y. J . Chem. Soc., Perkin Trans. 1 1993, 2999.
(12) Ishii, Y.; Takeno, M.; Kawasaki, Y.; Muromachi, A.; Nishiyama, Y.;
Sakaguchi, S. J . Org. Chem. 1996, 61, 3088.
(13) Otera, J .; Dan-oh, N.; Nozaki, H. J . Org. Chem. 1991, 56, 5307.
Otera, J . In Advances in Detailed Reaction Mechanisms, Coxon, J . M., Ed.;
J AI Press, Inc.: London, 1994; Vol. 3, p 167.
(14) For another transesterification under mild conditions: Imwinkelried,
R.; Schiess, M.; Seebach, D. Org. Synth. 1987, 65, 230.
(15) Other distannoxanes such as isothiocyanato derivatives worked
similarly.
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Communications
J . Org. Chem., Vol. 63, No. 8, 1998 2421
Ta ble 2. Selective Acyla tion of th e P r im a r y Hyd r oxyl in Diols a n d 2-Mer ca p toeth a n ol
Sch em e 1a ,b
to give a quantitative yield of diacetate 15, while the Sc-
(OTf)3 and Bu3P methods resulted in the cleavage of the S-S
bond to give AcS(CH2)2OAc in 97 and 96% yields, respec-
tively.
a
Reaction conditions: alcohols, 5 mmol each, 3, 5 mL; 30 °C. b Yields
determined by GLC.
no influence (entries 11 and 12). Vinyl benzoate, acrylate,
pivalate, and chloroacetate also afforded the desired esters
quantitatively (entries 13-16). The reaction of secondary
alcohols was controlled simply by changing the reaction tem-
perature: the reaction was sluggish at 30 °C (entries 17 and
19) but proceeded quantitatively at refluxing temperature
(72 °C) (entries 18 and 20). Cyclohexanol failed to react at
30 °C, and more remarkably, phenol did not undergo
acetylation (entries 21 and 22). Acid-sensitive functional
groups remained intact (entries 23-25). The trifluoroacetyl
group, which is not tolerant of Sc(OTf)3, survived (entry 26),
and an acetylenic ester that decomposed upon exposure to
Bu3P was employable (entry 27).
With these results in hand, we turned our attention to
preferential acetylation of a primary alcohol over a secondary
analogue.16 As expected from the above results, competition
reactions between these two types of alcohols at 30 °C
resulted in selective or exclusive formation of primary
acetates (Scheme 1). The unique selectivities are high-
lighted by intramolecular versions in Table 2, where results
from some representative acidic and basic methods are also
given for comparison. Only the distannoxane method ex-
hibited the high preference for the primary hydroxyl over
the secondary one (entries 1-3, 7, and 8), whereas the other
methods afforded diacetates (entries 4-6). Marked dis-
crimination was realized between phenol and primary
hydroxyl (entry 9), while no such selectivity was attained
by other methods (entries 10-12).
In summary, a new method for acylation of alcohols has
been achieved. Its synthetic usefulness is apparent from the
simple operation and high selectivities. The precedent
efforts were directed mostly toward rendering the catalysts
applicable to hindered alcohols,2-6,11,12 and hence, the selec-
tive acylation has met with limited success.16 In this respect,
the distannoxane method is capable of complementing the
existing methods. Of more practical importance is the
stability of 1 toward hydrolysis and oxidation. Many acy-
lation catalysts such as Bu3P, Sc(OTf)3, etc. need to be
handled in an inert atmosphere. Moreover, because of its
crystallinity and high melting point (107-110 °C), 1 has low
toxicity and is readily separated from the reaction mixture
by column chromatography or distillation of volatile esters.
It is also to be noted that the catalyst 1 is prepared very
17
easily by simply mixing Bu2SnO and Bu2SnCl2 or is even
available from a commercial source.18 As such, the distan-
noxane method will find a wide variety of synthetic uses.
Su p p or tin g In for m a tion Ava ila ble: Experimental procedure
and 1H and 13C NMR spectra of esters (2 pages).
J O9800412
(16) For selective acylation of primary alcohols, see refs 8 and 9 and
refereces cited therein.
(17) Okawara, R.; Wada, M. J . Organomet. Chem. 1963, 1, 81.
(18) Aldrich Chemical Co., Inc.
Another remarkable chemoselectivity is shown in eq 2.
A disulfide linkage is completely tolerant in our procedure