C O M M U N I C A T I O N S
References
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(4) Evidence for the presence of a cationic intermediate include ring opening
reactions of cyclopropyl derivatives and racemization of chiral benzylic
alcohols. See ref 3a.
(5) As noted, alkenylboron dichlorides were used in our earlier studies to
generate carbocations (see 3a), and a stronger Lewis acid (BCl3) would
be expected to react similarly. The appearance of a dark red color upon
addition of BCl3 provides further evidence for the formation of a cation.
(6) (a) Rubin, M.; Gevorgyan, V. Org. Lett. 2001, 3, 2705. (b) Yasuda, M.;
Saito, T.; Ueba, M.; Baba, A. Angew. Chem., Int. Ed. 2004, 43, 1414.
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Formation of a cation intermediate was further supported by the
successful coupling of a lithium alkoxide with an alkyne in the
presence of boron trichloride. Contrary to the reaction of alkoxides
with alkenylboron dichloride,3a-c reaction of alkoxides with alkynes
in the presence of boron trichloride gave E-alkenyl chlorides as
major products (Scheme 2). In the literature, similar reactions using
Scheme 2. BCl3-Mediated Coupling of Lithium Alkoxides with
Phenylacetylene
(8) It was reported earlier that propargyl alcohols failed to generate cations
in the presence of a catalytic quantity of B(C6F5)3. See: Schweir, T.; Rubin,
M.; Gevorgyan, V. Org. Lett. 2004, 6, 1999.
(9) Reviews: (a) Nicholas, K. M. Acc. Chem. Res. 1987, 20, 207. (b) Teobald,
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alcohols have rarely been achieved due to the existence of a
Brønsted acid in reaction mixture.17 Our method successfully solved
the problems associated with the selective generation of a cation
from an alcohol in the presence of unsaturated carbon-carbon
bonds.
In conclusion, we have discovered that cations can be generated
from the corresponding alkoxides in the presence of boron
trichloride. The absence of a Brønsted acid makes regioselective
allylations quite straightforward. The reaction conditions make the
coupling of alcohols to alkenes and alkynes feasible. As to the
generation of cations instead of complexes B(OR)nCl3-n, we believe
there are two factors leading to weakening of the C-O bond and
its subsequent cleavage: steric hindrance between the R group and
chloride lengthens the C-O bond, and the high electronegativity
of chlorine strengthens the B-O bond. Further investigations of
this Brønsted acid free route to carbocations are under way.
(12) Georgy, M.; Boucard, V.; Campagne, J.-M. J. Am. Chem. Soc. 2005, 127,
14180.
(13) Control experiments revealed that no coupling occurs between allyltri-
methylsilane and 1,3-diphenylpropargyloxide in the absence of BCl3.
(14) Typical Experimental Procedure. A solution of propargyl alcohol (1.5
mmol) in dry dichloromethane (10 mL) was treated with n-butyllithium
(1.0 mL of a 1.6 M solution in hexanes) at 0 °C and warmed to room
temperature. After stirring at room temperature for 30 min, allyltrimeth-
ylsilane (1.8 mmol) and boron trichloride (1.5 mmol) were added. The
mixture was allowed to stir 10 h at room temperature. Water (20 mL)
was added to quench the reaction. The reaction mixture was extracted
with ethyl acetate, and the extracts were dried over anhydrous MgSO4.
The solvent was removed in vacuo, and the product was purified by silica
gel column chromatography using hexane as an eluent to provide 1a.
(15) Okajima, M.; Suga, S.; Itami, K.; Yoshida, J.-I. J. Am. Chem. Soc. 2005,
127, 6930.
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund for partial support of
this research. We also wish to thank the Robert H. Cole Foundation.
(16) Although 4 could, in theory, be formed via an E2 elimination involving
the initially formed benzyloxyboron dichloride intermediate, the absence
of a base under these acidic conditions would appear to make this unlikely.
(17) Brønsted acids react with the alkyne (or alkene) starting materials in a
competitive fashion that generally leads to more complex product mixtures.
Supporting Information Available: Experimental procedures for
synthesis and full characterization for compounds. This material is
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