B(C6F5)3-catalyzed allylation of secondary benzyl acetates with allylsilanes

Article abstract of DOI:10.1021/ol016300i

(Equation presented) A highly effective protocol for allylation of secondary benzylic alcohol derivatives with allylsilanes in the presence of catalytic amounts of B(C₆F₅)₃ has been developed. Some additional functionalities, such as bromo, acetoxy, and primary benzyloxy groups, were tolerated under these conditions.

Full text of DOI:10.1021/ol016300i

ORGANIC
LETTERS
2001
Vol. 3, No. 17
2705-2707
B(C F ) -Catalyzed Allylation of
Secondary Benzyl Acetates with
Allylsilanes
6 5 3
Michael Rubin and Vladimir Gevorgyan*
Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street,
Chicago, Illinois 60607-7061
Vlad@uic.edu
Received June 19, 2001
ABSTRACT
A highly effective protocol for allylation of secondary benzylic alcohol derivatives with allylsilanes in the presence of catalytic amounts of
B(C F5)3 has been developed. Some additional functionalities, such as bromo, acetoxy, and primary benzyloxy groups, were tolerated under
6
these conditions.
Lewis acid mediated nucleophilic allylation of various
substrates with allylsilanes is a powerful protocol for the
formation of a carbon-carbon bond. Besides the well-known
reaction with carbonyl compounds and acetals (Sakurai
reaction),1 the allylations of other electrophilic substrates,
such as benzylic, allylic, and propargylic halides, with
allylsilanes have been extensively investigated and well
documented.2 However, similar transformations with more
synthetically attractive alcohols and their derivatives are far
less developed.3 Most known protocols of this type require
at least stoichiometric amounts of Lewis acid. There are only
a few scattered reports on catalytic versions of this reac-
tion: allylation of allylic and benzylic alcohols with HN-
(SO2F)2,4 allyl silyl ethers with ZnCl2,5 alkyl allyl ethers with
TrClO4,6 and alkyl propargyl ethers with the SnCl4-ZnCl2
binary system.7 However, the reactions reported are not
general and are incompatible with most functional groups,
often providing moderate yields of the products. Accordingly,
development of more general, efficient, and environmentally
benign catalytic methodology for allylation of alcohols and
their derivatives is exceedingly desired. Herein we wish to
report a very simple procedure for allylation of secondary
benzylic alcohols and their derivatives with allylsilanes in
the presence of a catalytic amount of tris(pentafluorophenyl)
borane, B(C6F5)3.8
We have recently reported a novel procedure for exhaus-
tive reduction of alcohols and reductive cleavage of ethers,9
as well as direct exhaustive reduction of carboxylic acids
and their derivatives10 with hydrosilanes in the presence of
catalytic amounts of B(C6F5)3. In continuation of our study
on catalytic applications of this unusual Lewis acid in organic
(8) For recent review on use of B(C6F5)3 and analogues as cocatalysts
in the metal-catalyzed olefins polymerization, see: (a) Chen, E. Y.-X.;
Marks, T. J. Chem. ReV. 2000, 100, 1391. For the use of B(C6F5)3 as a
catalyst for 1,3-isomerization of allylstannanes, see: (b) Marshall, J. A.;
Gill, K. J. Organomet. Chem. 2001, 624, 294. For hydrosilylation of
carbonyl compounds and imines, see: (c) Blackwell, J. M.; Sonmor, E. R.;
Scoccitti, T.; Piers, W. E. Org. Lett. 2000, 2, 3921. (d) Parks, D. J.;
Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000, 65, 3090. For
allylstannylation of aldehydes, see: (e) Blackwell, J. M.; Piers, W. E. Org.
Lett. 2000, 2, 695. For silylation of alcohols, see: (f) Blackwell, J. M.;
Foster, K. L.; Beck, V. H.; Piers, W. E. J. Org. Chem. 1999, 64, 4887.
(9) (a) Gevorgyan, V.; Liu, J.-X.; Rubin, M.; Benson, S.; Yamamoto,
Y. Tetrahedron Lett. 1999, 40, 8918. (b) Gevorgyan, V.; Rubin, M.; Benson,
S.; Liu, J.-X.; Yamamoto, Y. J. Org. Chem. 2000, 65, 6179.
(1) For reviews, see: (a) Sakurai, H. Pure Appl. Chem. 1982, 54, 1. (b)
Sakurai, H. Pure Appl. Chem. 1989, 57, 1759.
(2) (a) Dau-Schmidt, J.-P.; Mayr, H. Chem. Ber. 1994, 127, 205. (b)
Mayr, H.; Pock, R. Tetrahedron 1986, 42, 4211. (c) Mayr, H.; Gorath, G.;
Bauer, B. Angew. Chem., Int. Ed. Engl. 1994, 33, 788.
(3) See, for example: Cella, J. A. J. Org. Chem. 1982, 47, 2125.
(4) Kaur, G.; Kaushik, M.; Trehan, S. Tetrahedron Lett. 1997, 38, 2521.
(5) Yokozawa, T.; Furuhashi, K.; Natsume, H. Tetrahedron Lett. 1995,
36, 5243.
(6) Murakami, M.; Kato, T.; Mukaiyama, T. Chem. Lett. 1987, 1167.
(7) Hayashi, M.; Inubushi, A.; Mukaiyama, T. Chem. Lett. 1987, 1975.
(10) Gevorgyan, V.; Rubin, M.; Liu, J.-X.; Yamamoto, Y. J. Org. Chem.
2001, 66, 1672.
10.1021/ol016300i CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/21/2001

synthesis, we attempted B(C6F5)3-catalyzed allylation of
benzylic alcohols with allylsilanes.
Bro¨nsted acid could be responsible for induction of both
undesired processes, elimination of water from benzylic
alcohols and oligomerization of forming styrenes.13 Thus,
we decided to search for a suitable leaving group, one that
would allow for easier formation of benzylic cation and
byproduct 5 (X * OH), which would not liberate water under
these conditions and thus will not initiate the above-
mentioned undesired side reactions (eq 2, Table 2).
In test experiments, both trityl alcohol (1a) and benzhydrol
(1b) smoothly underwent allylation with allyltrimethylsilane
in the presence of 5 mol % of B(C6F5)3 to give the
corresponding alkenes 2a and 2b in high yields (eq 1, Table
1, entries 1 and 2). However, allylation of secondary or
Table 2. Optimization of Leaving Group
leaving group (X)
4a , yield,a
%
tertiary benzylic alcohols possessing â-hydrogen atoms did
not proceed smoothly: the competing elimination process
took place to produce inseparable mixtures of the corre-
sponding styrenes, their oligomers, and polymers along with
the allylation products. However, in certain cases employ-
ment of slightly more nucleophilic methallylsilane made the
allylation reaction favorable (Table 1, entry 3).The observed
1
2
3
4
5
6
7
8
9
OH
(3a )
(3b)
(3c)
(3d )
(3e)
(3f)
15-20
NR
98
85
92
96
74
69
99
OTMS
OMe
OBn
OMOM
OCOCH3
OCOPh
OCOCMe3
OCOCF3
(3g)
(3h )
(3i)
a NMR yields.
Table 1. Allylation of Benzylic Alcohols
As discussed above, unprotected sec-phenethyl alcohol 3a
gave allylation product 4a in very low yields (Table 2, entry
1). TMS-protected alcohol 3b, probably due to steric reasons,
did not undergo allylation at all (entry 2). In contrast, all of
the alkyl- (3c-e) and acyl-protected (3f-i) benzylic sub-
strates tested smoothly underwent the allylation reaction to
give the allylation product 4a in good to very high yields
(entries 3-9).
Although methyl- (3c) and TFA-protected (3i) alcohols
provided the highest yields of 4a, the acetate derivatives,
for obvious synthetic reasons (being cheapest and easiest for
preparation and purification), were chosen for further inves-
tigation. Thus, a series of secondary benzylic acetates were
prepared and tested in the B(C6F5)3-catalyzed allylation
reaction (eq 3, Table 3).14 The acetate of sec-phenethyl
a Isolated yield. b Methallyltrimethylsilane was used in this reaction.
elimination process could be rationalized as follows. Ally-
lation of secondary benzylic alcohol 3a with allyltrimeth-
ylsilane produces 4a and silanol 5a (X ) OH) as a byproduct
(eq 2). Silanol 5a, in turn, undergoes dehydrocondensation
alcohol 3f gave allylation product 4a15 in 91% isolated yield
(entry 1). Allylation of 3f with methallylsilane allowed for
(12) Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, J. R.;
Friesner, R. A.; Perkin, G. J. Am. Chem. Soc. 2000, 122, 10581.
(13) Independent experiments demonstrated that treatment of certain
secondary alcohols with B(C6F5)3, preliminarily exposed to wet air, led to
the formation of notable amounts of alkenes.
(14) All reactions were carried out in the presence of 5 mol % of
B(C6F5)3. Reactions in the presence of lesser amount of B(C6F5)3 were
sluggish and incomplete.
(15) The preparation of 4a is representative. To a stirred solution of
B(C6F5)3 (26 mg, 5 mol %) in anhydrous CH2Cl2 (1 mL) was added 3f (1
mmol, 164 mg), followed by the addition of allyltrimethylsilane (1.5 mmol,
240 µL). The mixture was stirred at room temperature, and the reaction
to give siloxane11 and eliminates a molecule of water, which
is immediately caught by B(C6F5)3. The hydrates produced
by this means, B(C6F5)3‚xH2O, are known to ionize in
solution to form H3O+[B(C6F5)3OH]-, a Bro¨nsted acid which
is nearly as strong as H2SO4.12 The presence of such a strong
1
(11) The formation of hexamethyldisiloxane was detected by H NMR
analyses of the crude reaction mixtures.
2706
Org. Lett., Vol. 3, No. 17, 2001

obtaining compound 4b in 94% yield (entry 2). A somewhat
more hindered acetate 3j gave allylation products 4c and 4d
in 92% and 94% yields with allyltrimethylsilane and meth-
allyltrimethylsilane, respectively (entries 3 and 4). As
substitute a secondary benzylic acetate group in the presence
of a primary one in the diacetate 3n to produce the
monoallylation product 4h in a virtually quantitative yield
(entry 8).
We found the last result especially interesting since the
presence of an additional Lewis basic group such as primary
acetate did not deactivate B(C6F5)3 catalyst. Encouraged by
this fact we performed initial experiments on functional group
tolerance (eq 4). As expected, secondary benzylic acetates
Table 3. Allylation of Secondary Benzylic Acetates
possessing an unprotected alcohol group (6d, X ) OH) and
nitrile function (6e, X ) CN) did not undergo the allylation
reaction (eq 4).18 In contrast, diacetate 6a (X ) OAc)
smoothly underwent mono-allylation to give 7a in 91% yield.
Similarly, substrates possessing a bromide group (6b, X )
Br) or benzyl ether moiety (6c, X ) OBn) provided the
corresponding allylation products 7b and 7c in 87% and 96%
yields, respectively (eq 4).
In summary, we developed an effective protocol for
allylation of secondary benzylic alcohol derivatives with
allylsilanes in the presence of catalytic amounts of B(C6F5)3.
Various secondary benzylic acetates smoothly underwent the
allylation reaction under mild conditions to give the corre-
sponding allylation products in very high yields. Initial
experiments on functional group tolerance demonstrated that
bromo, acetoxy, and benzyloxy groups are tolerated under
these reaction conditions.
a Isolated yield.
expected, introduction of electron-donating groups, stabiliz-
ing the forming benzylic cation, essentially accelerated the
reaction:16 p-methoxy-, p-methyl-, and p-fluoro-substituted
sec-phenethyl acetates 3k, 3l, and 3m readily gave the
corresponding allylation products 4e, 4f, and 4g in excellent
yields (entries 5-7). Our test experiments indicated that
primary benzylic acetates, in contrast to secondary ones, in
the presence of catalytic amounts of B(C6F5)3 did not undergo
allylation at all.17 Accordingly, we were able to selectively
Acknowledgment. The support of the Petroleum Re-
search Fund (AC1-36370), administrated by American
Chemical Society, is gratefully acknowledged.
Supporting Information Available: Detailed experi-
mental procedures for preparation and spectroscopic data for
compounds 6a-e. Spectroscopic and analytical data for
compounds 2c, 4d,e,g,h, and 7a-c. This material is available
free of charge via Internet at http://pubs.acs.org.
course was monitored by capillary GLC analysis. After the reaction was
complete (12 h for 4a), the mixture was filtered through a short column
(silica gel) and concentrated. Purification by column chromatography (silica
gel, hexane as an eluent) gave 133 mg (91%) of 4a.
(16) Reactions, in these cases, were finished in 5-6 h, whereas normally
OL016300I
the reaction needed 12-48 h for completion.
(17) Conversion of primary benzylic acetate in the presence of a
stoichiometric amount of B(C6F5)3 did not exeed 20%.
(18) Lewis acid was passivated by complexation with these functional
groups. For binding of B(C6F5)3 with alcohols and nitriles see ref 12.
Org. Lett., Vol. 3, No. 17, 2001
2707