70
M. Kuroboshi et al.
LETTER
that Zn can reduce Cr(III) and/or Ni(II) to produce the In conclusion, Cr(II) reagent can be regenerated and re-
lower valent metal species, which start the catalytic cycle. peatedly used for the alkenylation of carbonyl compounds
Presumably, commercially available CrCl3 could not be under electrolysis conditions using an Al sacrificial an-
reduced under the electrochemical conditions, whereas ode. Studies on the mechanism of the electrochemical re-
Cr(III) species generated in situ could.
cycling of Cr(II) reagent and the synthetic applications are
now progressing in our laboratories.
Representative results of the electrochemical alkenylation
of aldehydes are shown in Table 2. The alkenylation pro-
ceeded with both aromatic (Entries 1, 2, 3, 5, 6, and 7) and
aliphatic aldehydes (Entry 9): Aromatic aldehydes bear-
ing electron-donating substituents gave the corresponding
allyl alcohol derivatives in good to moderate yields,
whereas 4-cyanobenzaldehyde gave only 5% of the prod-
ucts 3 (Entry 8), due to the predominant reduction of the
aldehyde affording the corresponding pinacol.
References and Notes
1. Comprehensive Organometallic Chemistry II. Abel, E. W.;
Stone, F. G. A.; Wilkinson, G. Eds.; Pergamon Press: Oxford
1995.
2. Torii, S. Interface 1997, 46. For reviews: Torii, S. in New
Challenges in Organic Electrochemistry Osa, T. Ed.; Gordon
and Breach Science Publishers: 1998. Bersier, P. M.; Carlos-
son, L.; Bersier, J. in Top. Curr. Chem. Steckhan, E. Ed.;
Springer-Verlag: Berlin, 1994, 110, p 113. Torii, S. Yuki
Gosei Kagaku Kyokai Shi 1993, 51, 1024. Simonet, J. in Or-
ganic Electrochemistry, 3rd. Ed.; Lund, H.; Baizer, M. M.
Eds.; Marcel Dekker Inc.: NY, 1991, p 1217. Efimov, O. L.;
Strelets, V. V. Coord. Chem. Rev. 1991, 99, 15. Torii, S. Syn-
thesis 1986, 873.
3. Takai, K.; Kimura, K.; Kuroda, T.; Hiyama, T.; Nozaki, H. Te-
trahedron Lett. 1983, 24, 5281. Nicholas, A. S. Comprehensi-
ve Organic Synthesis; Trost, B. M.; Fleming, I.; Schreiber, S.
L. Eds.; Pergamon Press: Oxford, 1991; Vol. 1, Chapter 1.6.
4. Jin, H.; Uenishi, J.; Christ, W. J.; Kishi, Y. J. Am. Chem. Soc.
1986, 108, 5644. Dyer, U. C.; Kishi, Y. J. Org. Chem. 1988,
53, 3383.
trans-1-Bromo-1-propene gave only the corresponding
trans-isomer (Entry 3), whereas cis-1-bromo-1-propene
gave a complex mixture containing trans-1-(4-methox-
yphenyl)-1-buten-3-ol in 21% yield, which would be
formed by isomerization of initially formed 3 (R1 = 4-
MeOC6H4-, R2 = cis-1-propenyl) under the electrolysis
conditions (Entry 4).9
5. During our course of study, preliminary results of electroche-
mical Pd/Cr mediated vinylation in a divided cell was repor-
ted. Grigg, R.; Putnikovic, B.; Urch, C. J. Tetrahedron Lett.
1997, 38, 6307. Fürstner used Mn as a reductant of Cr(III),
wherein a large amount of Mn salts were generated as a pollu-
tive waste. Fürstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118,
12349.
6. Steckhan reported Cr-promoted electrochemical homocou-
pling of benzyl and allyl halide. Wellmann, J.; Steckhan, E.
Synthesis 1978, 901. Cr-mediated electrochemical pinacol
coupling was reported by Utley. Sopher, D. W.; Utley, J. H. P.
J. Chem. Soc., Perkin Trans. II 1984, 1361. J. Chem. Soc.,
Chem. Commun. 1979, 1087.
7. A part of the silyl ether was hydrolyzed to afford the corre-
sponding alcohol during purification by silica-gel column
chromatography.
8. The commercially available CrCl3 did not dissolve in DMF.
9. This isomerization might be caused by Lewis acidic alumini-
um salts such as AlBr3 generated during electrolysis.
Synlett 1999, No. 1, 69–70 ISSN 0936-5214 © Thieme Stuttgart · New York