J. Kroemer et al. / Tetrahedron Letters 47 (2006) 6339–6341
6341
Rieke zinc using water/methanol, while powdered zinc
gave no reaction with this alkyne under similar condi-
tions. Thus, an attractive feature of using Rieke zinc
for organic reductions is that it can be used under con-
ditions that would less likely alter other acid sensitive
functional groups in compounds being reduced.
R
+
+
+
R
C
C R
+
+
+
Na
Na
C
C
C
C
R
R
R
R
H
R
slow
C
C
R
NH3
Na
NH2
H
C
R
R
H
C
C
C
C
C
Na
Acknowledgements
R
The authors wish to acknowledge the financial support
of the Department of Chemistry at the University of
Nebraska at Kearney and the helpful discussions with
Reuben Rieke at the University of Nebraska—Lincoln.
R
H
H
R
R
H
C
R
NH2
+
+
C
NH3
Scheme 1. Mechanism of dissolving metal reduction of alkynes.
to use Rieke zinc to reduce alkynes were propargylic
alcohols, 1,3-diynes, and 1,3-enynes; cis-alkenes were
the major products of this work done by White.2 It is
of interest to note that we obtained the trans-alkene in
contrast to the cis-isomer suggested by Olah and
obtained by White. For example, ethyl phenylpropiolate
was reduced by Rieke zinc in quantitative yield to trans-
ethyl cinnamate.
References and notes
1. Furstner, A. Angew. Chem., Int. Ed. Engl. 1993, 32, 164–
189.
2. Chou, W.-N.; Clark, D. L.; White, J. B. Tetrahedron Lett.
1991, 32, 299–302.
3. The general procedure for dissolving zinc metal reduction:
A three-necked flask, fitted with a reflux condenser and
septa, was purged with Ar for 15 min. The Rieke zinc/THF
slurry (purchased from Rieke Metals, Inc., 1001 Kingbird
Road, Lincoln, NE 68521) was then transferred to the flask
via syringe. After the Zn suspension was heated to reflux, a
methanol solution of the organic compound to be reduced
was added, followed by addition of water. The ratio of
THF:methanol:water used was 7:5:1. A representative
reduction: To 8.0 mL of a stirred suspension of 5% Rieke
zinc, 6.12 · 10À3 mol, a solution of 4.08 · 10À3 mol of
phenylacetylene dissolved in 5.70 mL of methanol was
slowly added followed by dropwise addition of 1.10 mL of
water. After refluxing for 2.5 h under argon, the solution
was cooled and flooded with 150 mL of ether. The resulting
mixture was filtered through Celite and extracted succes-
sively with 10% ammonium chloride (25 mL), 10% sodium
bicarbonate (25 mL), and saturated sodium chloride
(25 mL). After drying over anhydrous magnesium sulfate,
the solvents were stripped by rotary evaporation. The yield
and composition of the resulting crude product was
determined by proton NMR.
Of additional interest, we noted that both phenylacetyl-
ene and p-methylphenylacetylene were readily and
quantitatively reduced to their respective alkenes
(entries 10 and 12 in Table 1), while 1-phenylpropyne
showed no reaction with Rieke zinc (entry 15 in Table
1). In all three of these alkynes, the triple bond is conju-
gated with a phenyl group and so might be expected to
react similarly. It may be that the small electron releas-
ing tendency of the methyl group directly attached to the
triple bond in 1-phenylpropyne destabilizes the radical
or radical ion just enough to prevent this molecule from
being reduced. However, it might also be argued that the
para-methyl group in p-methylphenylacetylene should
have a similar effect.
While it is true that ‘regular’ powdered zinc can be used
to achieve many of the same reductions as Rieke zinc,7 it
should be noted that much milder reaction conditions
can be employed with Rieke zinc.6 For example, proton
donors as weak as water or methanol can be used with
Rieke zinc, whereas stronger acids such as hydrochloric
acid, phosphoric acid or acetic acid are needed for reg-
ular powdered zinc. For instance, ethyl phenylpropiol-
ate was quantitatively reduced to its alkene with
´
4. Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; J. Wiley
and Sons: New York, NY, 1995, 472–473.
5. House, H. O. Modern Synthetic Reactions, 2nd ed.; W.A.
Benjamin: Menlo Park, CA, 1972, 205–209.
6. Kaufman, D.; Johnson, E.; Mosher, M. D. Tetrahedron
Lett. 2005, 46, 5613–5615.
7. See, for instance; Durant, A.; Delplancke, J. L.; Libert, V.;
Reisee, J. Eur. J. Org. Chem. 1999, 2845–2851.