Boron trifluoride mediated allylation of aromatic α-bromoketones by allyltributyltin

Article abstract of DOI:10.1246/cl.2003.164

Replacement of bromine atom of aromatic α-brormoketones by allyltributyltin in the presence of BF₃ was described. The reaction proceeds with or without migration of the aryl group, depending on the structure of the α-bromoketone.

Full text of DOI:10.1246/cl.2003.164

164
Chemistry Letters Vol.32, No.2 (2003)
Boron Trifluoride Mediated Allylation of Aromatic ꢀ-Bromoketones by Allyltributyltin
Hideyoshi Miyake,ꢀ Ryo Hirai, Yoshie Nakajima, and Mitsuru Sasakiy
Faculty of Agriculture, Kobe University, Rokkodai, Nada, Kobe 657-8501
yCenter for Cooperative Research and Development, Kobe University, Rokkodai, Nada, Kobe 657-8501
(Received November 13, 2002; CL-020969)
Replacement of bromine atom of aromatic a-brormoketones
Table 1. Allylation of 1
by allyltributyltin in the presence of BF3 was described. The
reaction proceeds with or without migration of the aryl group,
depending on the structure of the a-bromoketone.
Allylation of carbonyl compounds by allyltributyltin is a very
useful reaction in organic synthesis.1;2 The functional groups,
adjacent to the carbonyl group, often influence the reaction to
cause somewhat complicated results. For example, a-haloketone
gives epoxide by the reaction with allyltributyltin in the presence
of Pd(0),3 Pd(II),4 and n-Bu2SnCl2-HMPT.5 Simple replacement
of halogen with allyl group occurs in the presence of radical
initiator.3 Aromatic a-haloketone gives allyl ketone or homoallyl
alcohol by the reaction with allyltributyltin, in the presence of
SnCl2, and it proceeded with migration of the aryl group.6
Formation of aldehydes in the presence of Pd(0) was also
reported.3 In this paper, we wish to report new type reactions
between aromatic a-bromoketone and allyltributyltin.
When aromatic a-bromoketone (1) was treated with allyl-
tributyltin (2) and BF3ÁOEt2, two different types of reaction
proceeded, depending on the structure of 1. The reaction pathway
is shown in Sheme 1. If R1 was an aryl group and R2 was not an
aryl group, the reaction proceeded with migration of the aryl
group to give allylketone (3). Although the reaction of tertiary
bromoketone such as 1a proceeded smoothly to give 3a, that of
secondary bromoketone such as 1d proceeded slowly, and gave
3a and considerable amount of unidentified products.7
intermediate in these reactions. When R1 is aryl group, it migrates
to the adjacent carbon during the course of the isomerization of
epoxides to ketones. It means that an aryl group migrates more
easily than an allyl group. When R1 is an alkyl group, the allyl
group migrates to give 4; an allyl group migrates more easily than
an alkyl group does.
However, these mechanisms cannot explain the allylation of
1f. If the reaction proceeds by the mechanisms shown in Scheme
2, 1f gives epoxide 6f, and the following migration does not give
Scheme 1.
When R2 was an aryl group, simple replacement of the
bromine by allyl group occurred. Results are summarized in
Table 1. When neither R1 nor R2 was aryl group the replacement
of bromine did not occur. For example, 3-bromo-3-methyl-1-
phenyl-2-butanone did not react under the similar conditions.
Allyl epoxides such as 6 in Scheme 2 seem to be a plausible
Copyright ꢀ 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.2 (2003)
165
Scheme 5.
Free radical such as 9 is another plausible intermediate in the
case of 1f. If BF3 or tin can assist the generation of 9, the
subsequent radical substitution with allyltributyltin gives 4f.
Typical procedure: The procedure of the allylation of 1a is as
follows. To a CH2Cl2 (5 ml) solution of 1a (0.57 g, 2.51 mmol)
and allyltributyltin (2) (0.99 g, 2.99 mmol), was added a CH2Cl2
(1 ml) solution of BF3ÁOEt2 (0.71 g, 5.00 mmol) at 0 ꢁC. After
stirring at room temperature for 6 h, the solution was poured into
water, and extracted with ether. The ether layer was washed with
10% aqueous solution of KF, and the solid of n-Bu3SnF was
removed. The solution was dried over MgSO4. After removing
the solvent under reduced pressure, the residue was purified by
column chromatography on silica gel. The yield of 3a was 79%
(0.37 g, 1.97 mmol).
Scheme 2.
4f, but 3f and 3f0 (Scheme 3),8{11 and neither 3f nor 3f0 was
obtained by the allylation of 1f. The consideration described
above leads us to the different mechanisms. One plausible
mechanism is as follows (Scheme 4). Initially, BF3 mediates the
formation of carbocation 7. When R1 is an aryl group and R2 is
not, the migration of the aryl group occurrs to give the acyl cation
8. On the other hand, if R2 is an aryl group, the positive charge of 7
is stabilized by the aryl group, and it is allylated by 2 without
rearrangement of the intermediate. The formation of 5 as a
byproduct of 3f is explained as the product of an E1 type reaction
of a carbocation like 7. However, the reasons why 1g did not give
the corresponding alkene are under investigation.
References and Notes
1
2
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Baba and co-workers have reported (Ref. 6) that ꢀ-haloke-
tone 1d reacted with allyltributyltin (2) in the presence of
BF3ÁOEt2 at 0 ꢁC to give 2-bromo-3-phenyl-5-hexen-3-ol.
Probably because we used 2-equivalent of BF3ÁOEt2 at higher
temperature, further reaction proceeded to give 3d.
R. C. Larock, in ‘‘Comprehensive Organic Transformations,’’
VHC, New York (1989).
Scheme 3.
8
9
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11 B. C. Ranu and U. Jana, J. Org. Chem., 63, 8212 (1998).
Scheme 4.