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N. Iranpoor et al. / Tetrahedron 58 (2002) 8689–8693
Also, in treatment of cyclohexanol and (2)-menthol as
example of monocyclic alcohols with the mixture of PPh3/
DDQ/(n-butyl) NBr (1/1.1/1.1/1.1), at room temperature,
4
the corresponding halides were obtained in low yields
together with the formation of eliminated products as the
major products.
We compared the selectivity of our method with some of the
reactions of Scheme 3 using Ph PBr as the reagent. The
reaction of binary mixture of entry 1 with Ph PBr gave
3
2
3
2
1
00% conversion for primary alcohol and 40% conversion
for the secondary one. In the case of binary mixture of entry
, in addition to the complete conversion of primary alcohol
2
into its alkyl bromide, dehydration reaction of 38 alcohol to
its corresponding alkene (50%) also occurred. Reaction of
the binary mixture of entry 3 with Ph PBr , showed a
3
Scheme 4.
2
reverse selectivity and trimethylsilyl ether was converted
into its alkyl bromide while the alcohol remained
unchanged. The binary mixtures of entries 5 and 7 were
alcohols into their corresponding alkyl halides. The results
of this study are shown in Table 2. As shown in Table 2, this
method is very suitable for the conversion of primary,
secondary, benzylic and allylic alcohols as well as diols into
alkyl halides (Table 2, entries 1–19). Although the
capability of DDQ for oxidation of benzylic alcohols has
been demonstrated, but in our reactions, no oxidative
product was observed. The present method can be applied
for the preparation of alkyl halides from alcohols having
sensitive functional groups such as carbon–carbon double
bonds (Table 2, entries 20 and 21), carbonyl groups (Table
also subjected to the reaction with Ph PBr . In the case of
3 2
entry 5, 60% of the 1,3-dithiane and in the case of entry 7,
80% of the epoxide were also consumed. This comparison
shows the high selectivity of the presented method.
1
4
Although the mechanism of the reaction is not clear, on the
basis of the reports on the reaction of Ph P and DDQ, the
3
3
a
formation of complex (1) can be assumed. Treatment of
NX (X¼Cl, Br, I) with (1) could produce (2) which later
reacts with RYH (Y¼O, S, Se) to give the intermediate (3).
Subsequently, an S 2-type displacement on the intermedi-
R
4
2
, entry 22), amino groups (Table 2, entries 23 and 24),
sulfur atom (Table 2, entries 25 and 26), and ethereal bond
Table 2, entries 28 and 29). In the conversion of (2)-
N
ate (3) by halide anion led to the formation of the desired
alkyl halide (Scheme 4). The major deriving force for this
reaction could well be due to the aromatization of DDQ ring
(
menthol to its iodide, the reaction proceeded through a SN2
reaction with the clean inversion of configuration (Table 2,
and the formation of Ph
addition of the reagents in this reaction is very important.
We observed that if alcohol is added to the mixture of Ph
and DDQ (1) before the addition of R NX, instead of the
PY (Y¼O, S, Se). The order of
3
1
6
entry 27). We then applied this method for the conversion
of thiols to their corresponding alkyl halides (Table 2,
entries 30–32). In these reactions, due to the ready
P
3
4
1
7
dimerization of thiols into disulfides by DDQ, the amount
of triphenylphosphine was slightly increased in comparison
with the reaction of alcohols and the reactions were
performed in ice bath.
formation of alkyl halide, the mono- and dialkyl ethers of
dihydro-DDQ are produced. This reaction could be some
how similar to the reaction of chloranil and trialkyl
phosphites which is reported to produce tetrachlorohydro-
3
quinone alkyl ethers.
The optimized ratio of RSH/Ph P/DDQ/R NX was found to
3
4
be 1/1.4/1.1/1.4. When we reacted selenols under the same
reaction conditions as thiols, the reactions furnished the
corresponding alkyl halides in good yield (Table 2, entries
3. Conclusion
3
3 and 34). In order to have more insight into the
In conclusion, the present investigation has demonstrated
that the use of PPh /DDQ/R NX offers a simple, novel and
applicability, selectivity and limitation of this new method,
we studied the possibility of the conversion of alcohols in
the presence of some other functional groups in binary
mixtures. The most important point about the selectivity of
this reaction is that primary alcohols can be converted to
their corresponding halides (X¼Cl, Br, I) in the presence of
secondary ones with excellent selectivity. This reagent also
converted both primary and secondary alcohols into alkyl
halides with excellent selectivity in the presence of tertiary
alcohols, thiols, epoxides, silyl ether, tetrahydropyranyl
ethers, 1,3-dithianes, disulfides and amides. The conversion
yields obtained for the selective reactions of different binary
mixtures are shown in Scheme 3. However, the conversion
of phenol or 2,4-dinitrophenol as aryl alcohols into their
corresponding halides was unsuccessful with the present
method and starting materials remained completely intact.
3
4
convenient method for the conversion of a wide varieties of
alcohols, thiols and selenols to their corresponding alkyl
halides. The method not only shows excellent selectivity
between different types of alcohols, but also between
alcohols and many other reactive functional groups.
Availability and ease of handling of the reagents, easy
work up, high yields, operation at room temperature and
especially the possibility of using the desired halide anion as
nucleophile can be considered as strong points of this
method.
4. Experimental
Chemicals were obtained from Merck and Fluka chemical
Iranpoor
