Formation of 2-(3'-Oxocyclohexyl)-2-cyclohexen-1-one via Reduction of 2-Cyclohexen-1-one with Electrogenerated Nickel(I) Salen

Full text of DOI:10.1021/jo971538z

J . Org. Chem. 1998, 63, 1319-1322
1319
F or m a tion of
NMR data to conclude that treatment of 2-cyclohexen-
-one with aqueous sodium hydroxide leads to 1. In other
1
2
-(3′-Oxocycloh exyl)-2-cycloh exen -1-on e via
Red u ction of 2-Cycloh exen -1-on e w ith
Electr ogen er a ted Nick el(I) Sa len
1
1
electrochemical research, Tissot and co-workers found
that direct reduction of 2-cyclohexen-1-one at a mercury
cathode in acetonitrile containing tetra-n-butylammo-
nium tetrafluoroborate along with 5% water gives rise
to 3-(3′-oxocyclohexyl)cyclohexanone in essentially quan-
titative yield.
Mohammad S. Mubarak
Department of Chemistry, The University of J ordan,
Amman 11942, J ordan
In contrast to the paucity of information about the
electroreductive dimerization of 2-cyclohexen-1-one, the
Marty Pagel, Lawrence M. Marcus, and
Dennis G. Peters*
photodimerization of this species has been the subject of
12-18
numerous studies over the years,
and irradiation of
Department of Chemistry, Indiana University,
Bloomington, Indiana 47405
this compound yields principally a mixture of the head-
to-head and head-to-tail dimers, 2 and 3.
Received August 18, 1997
There have been relatively few previous reports1
concerning the electrochemistry of 2-cyclohexen-1-one.
Reduction of 2-cyclohexen-1-one by a sodium amalgam
or by magnesium or zinc in acetic acid as well as by
means of controlled-potential electrolysis has been shown
by Touboul et al.2,3 to afford mixtures of cyclohexanone,
Recently, after investigating the addition of alkyl
radicals, arising via the in situ catalytic reduction of
primary alkyl monobromides and iodides by electrogen-
erated nickel(I) salen, to activated olefins such as ethyl
acrylate and styrene, we decided to ascertain whether
3
,3′-dioxo-1,1′-bicyclohexyl, and three diethylenic ketones.
4
-7
In a series of papers by Ivcher and co-workers, various
aspects of the polarographic behavior of 2-cyclohexen-1-
one in aqueous media were probed, including the influ-
ence of pH on the protonation and reduction of the
carbonyl moiety. Denney and Mooney8 examined the
polarographic reduction of 2-cyclohexen-1-one in aqueous
buffer solutions. In an investigation of the use of
electrogenerated bases to promote Michael addition reac-
2
-cyclohexen-1-one might undergo similar alkylation
reactions. In this study, we have examined the catalytic
reduction of several alkyl iodides by nickel(I) salen
electrogenerated at a glassy carbon cathode in the
presence of 2-cyclohexen-1-one in dimethylformamide
containing tetramethylammonium perchlorate as sup-
porting electrolyte. Although we have observed the
appearance of 3-alkylcyclohexanone and 3-alkyl-2-cyclo-
hexen-1-one species in very low yields, a more significant
finding is that 1 is formed in high yield. An important
part of this research has been the use of various NMR
techniques to identify 1 unambiguously as well as to
exclude an isomer, 2-(2′-oxocyclohexyl)-2-cyclohexen-1-
one, as a possible product. We propose that the formation
of 1 is initiated by reaction of electrogenerated nickel(I)
salen with 2-cyclohexen-1-one to give the radical-anion
of the parent enone.
9
tions, Baizer et al. observed that dimerization of 2-cy-
clohexen-1-one to afford 2-(3′-oxocyclohexyl)-2-cyclohexen-
1
-one (1) in a yield of 65% can be promoted via reduction
of only 0.13% of the starting material to its radical-anion
the electrogenerated base) at a mercury cathode in
(
dimethylformamide containing tetra-n-propylammonium
tetrafluoroborate. Apparently, however, the identifica-
tion of 1 was based solely on earlier work by Leonard
and Musliner,10 who employed only one-dimensional H
Exp er im en ta l Section
1
Rea gen ts. Dimethylformamide (DMF), which served as the
solvent for all electrochemical experiments, was a Burdick and
J ackson “distilled in glass” reagent and was used as received.
Tetramethylammonium perchlorate (TMAP), from GFS Chemi-
(1) Little, R. D.; Baizer, M. M. In The Chemistry of Enones; Patai,
S., Rappoport, Z., Eds.; J ohn Wiley and Sons: Ltd.: Chichester, U.
K., 1989; pp 599-622.
(
2) Touboul, E.; Weisbuch, F.; Wiemann, J . Bull. Soc. Chim. Fr.
1
967, 4291-4294.
3) Touboul, E.; Weisbuch, F.; Wiemann, J . C. R. Acad. Sci., Paris,
Ser. C 1969, 268, 1170-1173.
4) Ivcher, T. S.; Perepletchikova, E. M.; Zil’berman, E. N. Zh. Prikl.
Khim. 1962, 35, 634-637.
5) Ivcher, T. S.; Perepletchikova, E. M.; Zil’berman, E. N. Zh. Analit.
Khim. 1962, 17, 1005-1008.
6) Ivcher, T. S.; Zil’berman, E. N.; Perepletchikova, E. M. Zh. Fiz.
Khim. 1965, 39, 749-751.
7) Ivcher, T. S.; Perepletchikova, E. M.; Zil’berman, E. N. Zh. Prikl.
(11) Tissot, P.; Surbeck, J .-P.; G u¨ la c¸ ar, F. O.; Margaretha, P. Helv.
Chim. Acta 1981, 64, 1570-1574.
(
(12) Lam, E. Y. Y.; Valentine, D.; Hammond, G. S. J . Am. Chem.
Soc. 1967, 89, 3482-3487.
(
(13) Wagner, P. J .; Bucheck, D. J . J . Am. Chem. Soc. 1969, 91,
5090-5097.
(14) Pienta, N. J .; McKimmey, J . E. J . Am. Chem. Soc. 1982, 104,
5501-5502.
(15) Pienta, N. J . J . Am. Chem. Soc. 1984, 106, 2704-2705.
(16) Schuster, D. I.; Bonneau, R.; Dunn, D. A.; Rao, J . M.; J oussot-
Dubien, J . J . Am. Chem. Soc. 1984, 106, 2706-2707.
(17) Schuster, D. I.; Insogna, A. M. J . Org. Chem. 1991, 56, 1879-
1882.
(
(
(
Khim. 1966, 39, 227-229.
(8) Denney, E. J .; Mooney, B. J . Chem. Soc., B 1968, 1410-1415.
9) Baizer, M. M.; Chruma, J . L.; White, D. A. Tetrahedron Lett.
(
1
973, 5209-5212.
10) Leonard, N. J .; Musliner, W. J . J . Org. Chem. 1966, 31, 639-
46.
(18) Schuster, D. I. In The Chemistry of Enones; Patai, S., Rappoport,
Z., Eds.; J ohn Wiley and Sons: Ltd.: Chichester, U. K., 1989; pp 623-
756.
(
6
S0022-3263(97)01538-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 01/22/1998

1
320 J . Org. Chem., Vol. 63, No. 4, 1998
Notes
cals, Inc., was employed without further purification as the
supporting electrolyte. Each of the following chemicals was used
as received: 2-cyclohexen-1-one (Lancaster, 97%), [[2,2′-[1,2-
ethanediylbis(nitrilomethylidyne)]bis[phenolato]]-N,N′,O,O′]-
nickel(II) [nickel(II) salen, Aldrich, 98%], iodoethane (Aldrich,
done by means of simple one-dimensional 1H NMR data.
Accordingly, we performed DEPT and two-dimensional HETCOR
experiments to measure the chemical shift of the aliphatic
methine hydrogen. A COSY NMR experiment revealed that at
least three hydrogens are coupled to this aliphatic methine
hydrogen; two of these methine-coupled hydrogens belong to the
same methylene group, as determined from the HETCOR
experiment. Results from one-dimensional NOE experiments
were compared with molecular models (obtained with the aid of
PC-MODEL and MMX-2 force field computations) to verify that
the three methine-coupled hydrogens must be vicinal to the
methine hydrogen. This evidence is singularly consistent with
1 and cannot arise from 2-(2′-oxocyclohexyl)-2-cyclohexen-1-one.
We identified 3-ethyl-, 3-n-propyl-, 3-n-butyl-, and 3-n-pent-
ylcyclohexanone by means of GC-MS. Their gas chromato-
graphic retention times and mass spectra were virtually identical
with authentic compounds synthesized in our laboratory; in
addition, our mass spectra for 3-ethylcyclohexanone and 3-n-
propylcyclohexanone agree well with those reported, respec-
9
1
9%), 1-iodopropane (Aldrich, 99%), 1-iodobutane (Aldrich, 99%),
-iodopentane (Aldrich, 98%), and n-undecane (Aldrich, 99%).
Deaeration procedures were carried out with Air Products zero-
grade argon.
Authentic samples of 3-ethyl-, 3-n-propyl-, 3-n-butyl-, and 3-n-
pentylcyclohexanone were synthesized according to a procedure
1
9
published by House and Snoble, which involves treatment of
a cold solution of the appropriate alkylmagnesium iodide with
cuprous chloride and then 2-cyclohexen-1-one, with diethyl ether
as solvent, followed by isolation and purification of the desired
product.
Cells, Electr od es, In str u m en ta tion , a n d P r oced u r es.
Cells, instrumentation, and procedures for cyclic voltammetry
and controlled-potential electrolysis are described in previous
2
0,21
19
26
publications.
For cyclic voltammetry, a short length of 3-mm-
tively, by House and Snoble and by House and Fischer. Mass
spectral data for the authentic compounds were acquired at 70
eV with the aid of a Hewlett-Packard 5890 Series II gas
chromatograph coupled to a Hewlett-Packard Model 5971 mass-
diameter glassy carbon rod (Grade GC-20, Tokai Electrode
Manufacturing Co., Tokyo, J apan) was press-fitted into Teflon
to give a planar, circular working electrode with a geometric area
2
+
of 0.077 cm . For controlled-potential electrolyses, the working
selective detector: (a) for 3-ethylcyclohexanone, m/z 126, M (33);
+
+
+
5
cathodes were disks of reticulated vitreous carbon (RVC
111, [M - CH3] (9); 98, [M - C H ] (18); 97, [M - C H ] (55);
2
4
2
+
+
+
9
2
X1-100S, Energy Research and Generation, Oakland, CA) with
83, [M - C H ] (100); 70, [M - C H ] (40); 69, [M - C H ]
3
7
4
8
4
2
+
+
11
surface areas of approximately 200 cm ; these electrodes were
fabricated, cleaned, and handled as described elsewhere.22 All
potentials are quoted with respect to a reference electrode
consisting of a cadmium/saturated mercury amalgam in contact
with DMF saturated with both cadmium chloride and sodium
chloride; this electrode has a potential of -0.76 V vs the aqueous
(34); 56, [M - C H
]
(22); 55, [M - C H
]
(89); (b) for 3-n-
5
10
5
+
+
4
propylcyclohexanone, m/z 140, M (10); 112, [M - C H ] (2);
2
+
+
+
7
111, [M - C H ] (2); 98, [M - C H ] (15); 97, [M - C H ]
2
5
3
6
3
+
+
11
]
(100); 81, [M - C H O] (5); 69, [M - C H
3
7
5
(15); 56, [M -
+
+
13
]
6
C H
12
]
(18); 55, [M - C H
6
(53); (c) for 3-n-butylcyclohex-
+
+
+
7
anone, m/z 154, M (11); 126, [M - C H ] (1); 111, [M - C H ]
2
4
3
2
3,24
+
+
9
saturated calomel electrode at 25 °C.
(26); 110, [M - C H ] (35); 97, [M - C H ] (100); 83, [M -
3
8
4
1
13
+
+
+
13
We acquired H and C NMR spectra in CDCl
3
, with TMS
5
C H
]
(8); 82, [M - C H
- C H14] (15); 55, [M - C H
]
(46); 69, [M - C H
]
(11); 56, [M
11
5
12
6
+
+
as an internal standard, by using 300-MHz Varian Gemini 200
15
]
(45); (d) for 3-n-pentylcyclo-
7
7
INOVA
+
+
7
and 400-MHz Varian Unity
spectrometers; chemical shifts
hexanone, m/z 168, M (4); 125, [M - C H ] (9); 111, [M -
3
+
+
+
11
are reported in parts per million (ppm).
C H ] (1); 98, [M - C H
]
(11); 97, [M - C H
]
(100); 83,
4
9
5
10
5
+
+
+
Sep a r a tion , Id en tifica tion , a n d Qu a n tita tion of P r od -
u cts. Techniques and equipment for the separation, identifica-
tion, and quantitation of electrolysis products by means of both
gas chromatography and GC-MS are presented in earlier
[M - C H
]
(7); 69, [M - C H
]
(11); 56, [M - C H16] (12);
6
13
7
15
8
+
55, [M - C H17] (40).
8
Identities of unsaturated analogues of compounds mentioned
in the preceding paragraph were established by means of GC-
MS; their gas chromatographic retention times are slightly
shorter than those for the corresponding saturated species: (a)
2
0,25
publications.
To quantitate the products, we employed
n-undecane as an electroinactive internal standard, which was
added to each solution prior to the start of an electrolysis. Gas
chromatographic response factors, relative to n-undecane, were
determined experimentally for 1 and for the various 3-alkyl-
cyclohexanones; response factors for the 3-alkyl-2-cyclohexen-
+
+
for 3-ethyl-2-cyclohexen-1-one, m/z 124, M
(47); 109, [M - CH ]
3
+
+
7
(11); 96, [M - C H ] (100); 81, [M - C H ]
2
4
3
(28); 68, [M -
+
+
+
9
C H ]
4
8
(20); 67, [M - C H ]
4
9
(54); 55, [M - C H ] (11); 53, [M
5
+
- C H11]
5
(26); (b) for 3-n-propyl-2-cyclohexen-1-one, m/z 138,
1
-ones were assumed to be the same as those for the correspond-
M
+
(83); 123, [M - CH ]
+
(41); 110, [M - C H ]
+
4
(100); 95, [M
(82); 68, [M - C H10] (24); 67,
(28); 53, [M - C H13] (72);
(67); 137, [M
3
2
ing 3-alkylcyclohexanones. All product yields reported in this
paper are absolute; yields of the alkylated cyclohexanones and
cyclohexenones are based on the initial quantity of the alkyl
iodide, whereas the yield of 1 is based on the initial quantity of
- C H ]
+
(76); 81, [M - C H ]
+
+
3
7
4
9
5
+
+
+
[M - C H
5
11
]
(73); 55, [M - C H
6
11
]
6
+
(c) for 3-n-butyl-2-cyclohexen-1-one, m/z 152, M
- CH3] (29); 124, [M - C H ] (47); 123, [M - C H ]
[M - C H ]
+
+
+
5
2
4
2
(68); 110,
2
-cyclohexen-1-one.
+
(65); 109, [M - C H ]
+
(75); 82, [M - C H10]
+
(93);
3
6
3
7
5
We isolated 1 as a yellow oil by means of silica gel column
chromatography with hexanes-ethyl acetate as the eluent; its
81, [M - C H
]
+
(100); 67, [M - C H
]
+
(93), 55, [M - C H13]
+
5
11
6
13
7
+
15
(79); 53, [M - C H
7
] (99); (d) for 3-n-pentyl-2-cyclohexen-1-
purity was confirmed with the aid of GC and TLC: IR (CHCl
3
)
one, m/z 166, M
+
(85); 151, [M - CH ]
+
(16); 137, [M - C H ]
+
5
3
2
-
1
1
+
+
7
1
2
2
707, 1670 cm ; H NMR (CDCl
.36 (m, 2H), 2.31 (m, 2H), 2.27 (m, 2H), 2.26 (m, 2H), 1.93 (m,
H), 1.90 (m, 2H), 1.84 (d, 1H), 1.56 (d, 1H); 13C NMR (CDCl
3
) δ 6.64 (t, 1H), 2.93 (m, 1H),
(98); 123, [M - C H ] (77); 111, [M - C H ] (61); 110, [M -
3
7
4
+
+
+
11
C H ]
4
8
(75); 109, [M - C H ]
4
9
(60); 95, [M - C H
5
] (87); 82,
)
[M - C H
67, [M - C H
]
+
(88); 81, [M - C H
]
+
(80); 68, [M - C H14] (30);
+
3
6
12
6
13
7
δ 211.6, 198.7, 144.4, 141.9, 46.2, 41.2, 38.6, 37.5, 30.5, 25.9,
]
+
(81); 55, [M - C H
]
+
(72); 53, [M - C H17]
+
7
15
8
15
8
+
2
1
4.9, 22.6; HRMS m/z (M ) calcd (for C12
92.1154.
H
16
O
2
) 192.1150, found
(100).
It was essential to distinguish 1 from a possible isomer,
namely 2-(2′-oxocyclohexyl)-2-cyclohexen-1-one, which cannot be
Resu lts a n d Discu ssion
Cyclic Volta m m etr y Stu d ies of In d ivid u a l Com -
p ou n d s. We have investigated the cyclic voltammetric
behavior of each electroactive species employed in this
work. For all experiments, the chosen compound was
present at a concentration of 2 mM in DMF containing
(
19) House, H. O.; Snoble, K. A. J . J . Org. Chem. 1976, 41, 3076-
083.
20) Urove, G. A.; Peters, D. G. J . Electroanal. Chem. Interfacial
Electrochem. 1993, 353, 229-242.
21) Vieira, K. L.; Peters, D. G. J . Electroanal. Chem. Interfacial
Electrochem. 1985, 196, 93-104.
22) Cleary, J . A.; Mubarak, M. S.; Vieira, K. L.; Anderson, M. R.;
Peters, D. G. J . Electroanal. Chem. Interfacial Electrochem. 1986, 198,
3
(
(
(
0
.10 M TMAP, a freshly polished glassy carbon electrode
-
1
1
07-124.
was used, and the scan rate was 100 mV s . For the
direct reduction of 2-cyclohexen-1-one, we observed a
(
(
23) Marple, L. W. Anal. Chem. 1967, 39, 844-846.
24) Manning, C. W.; Purdy, W. C. Anal. Chim. Acta 1970, 51, 124-
1
26.
(
25) Mubarak, M. S.; Nguyen, D. D.; Peters, D. G. J . Org. Chem.
(26) House, H. O.; Fischer, W. F., J r. J . Org. Chem. 1969, 34, 3615-
3618.
1
990, 55, 2648-2652.

Notes
J . Org. Chem., Vol. 63, No. 4, 1998 1321
single irreversible wave with a peak potential of -1.24
Ta ble 1. Ad d u cts Obta in ed fr om Nick el(I)
Sa len -Ca ta lyzed Red u ction of Alk yl Iod id es in th e
P r esen ce of 2-Cycloh exen -1-on e in DMF Con ta in in g 0.10
M TMAP a
V. As described in more detail in a previous paper,27 the
reduction of nickel(II) salen is a reversible process, and
the cathodic and anodic peak potentials for the nickel-
b
adducts, %
(
-
II) salen-nickel(I) salen redox couple are -0.85 and
concn of 2-cyclo-
alkyl iodide
hexen-1-one, mM
1
8
9
0.78 V, respectively. Direct reduction of each of the
alkyl iodides involved in this work gives rise to a single
irreversible wave that is attributable to cleavage of the
carbon-iodine bond, and the pertinent peak potentials
iodoethane
iodoethane
10
20
20
5
10
20
30
30
75
77
80
72
76
80
79
74
6
6
7
5
6
6
6
7
3
2
3
3
2
4
3
3
1
1
1
1
-iodopropane
-iodobutane
-iodobutane
-iodobutane
2
8
are as follows: iodoethane, -1.50 V; 1-iodopropane,
2
9
30
-
1.55 V; 1-iodobutane, -1.45 V; and 1-iodopentane,
-
1.42 V.31
1-iodobutane
-iodopentane
1
Cyclic Volta m m etr ic Beh a vior of 2-Cycloh exen -
1
-on e-Nick el(II) Sa len -Alk yl Iod id e System s. A
aFor all experiments, the initial concentrations of nickel(II)
cyclic voltammogram recorded at a scan rate of 100 mV
salen and alkyl iodide were 1.0 and 5.0 mM, respectively, and the
potential of the cathode used to electrogenerate nickel(I) salen was
-
1
s
for a solution consisting of a mixture of 1 mM nickel-
-
0.95 V. b 1 ) 2-(3′-oxocyclohexyl)-2-cyclohexen-1-one, 8 ) 3-alkyl-
(II) salen and 20 mM 2-cyclohexen-1-one in DMF con-
cyclohexanone, and 9 ) 3-alkyl-2-cyclohexen-1-one.
taining 0.10 M TMAP exhibited the reversible waves
corresponding to the nickel(II) salen-nickel(I) salen
couple (with cathodic and anodic peak potentials identical
with those reported above) along with the irreversible
wave for reduction of 2-cyclohexen-1-one (with the same
cathodic peak potential listed earlier). Thus, a redox
reaction between electrogenerated nickel(I) salen and
Con tr olled -P oten tia l Electr olytic P r od u ction of
2-(3′-Oxocycloh exyl)-2-cycloh exen -1-on e (1). A se-
ries of electrolyses was carried out in which four different
alkyl iodides were catalytically reduced by nickel(I) salen
electrogenerated at a reticulated vitreous carbon cathode
in the presence of 2-cyclohexen-1-one. Table 1 lists the
results of these experiments. Each entry represents the
average of two duplicate electrolyses; in all instances, the
coulometric n value was essentially 1, all of the alkyl
iodide was catalytically reduced, the total electrolysis
time was less than 30 min, and the yields of the products
were reproducible to (3%. As revealed in Table 1, 1 is
obtained in yields ranging from 72% to 80%, whereas
3-alkylcyclohexanones and 3-alkyl-2-cyclohexen-1-ones
are formed in small quantities. Furthermore, we ob-
served smaller amounts of other products, including
cyclohexanone along with alkanes and alkenes arising
via disproportionation, coupling, and hydrogen atom
abstraction (from DMF) of alkyl radicals.
Several experiments were carried out with systems
containing nickel(II) salen and 2-cyclohexen-1-one, but
in the absence of an alkyl iodide. First, a 1 mM solution
of nickel(II) salen in DMF containing 0.10 M TMAP was
exhaustively reduced at a reticulated vitreous carbon
cathode held at -0.95 V in order to generate nickel(I)
salen. Then, with the cathode potential kept at -0.95
V, 2-cyclohexen-1-one was added to give a concentration
of 5 mM and the solution was stirred under argon for 1
h; no current flowed, the solution retained the green color
characteristic of nickel(I) salen, and 1 was formed in 71%
yield. Second, when the preceding experiment was
repeated, except that the concentration of 2-cyclohexen-
1-one was 20 mM, 1 was produced in 64% yield. Third,
a 1 mM solution of nickel(II) salen was once again
reduced to generate nickel(I) salen. Then, the electrolysis
circuit was opened, 2-cyclohexen-1-one was introduced
to give a concentration of 10 mM, and the solution was
stirred under argon for 1 h; after only 5 min of stirring,
the color of the solution changed from green to red, and
1 was obtained in 46% yield at the end of the experiment.
Fourth, in a control experiment, in which a solution
containing 1 mM nickel(II) salen and 20 mM 2-cyclo-
hexen-1-one was stirred under argon for 1 h without any
electrolysis being performed, we observed that 1 was not
formed. In addition, it should be mentioned that direct
reduction of a 20 mM solution of 2-cyclohexen-1-one alone
in solvent-supporting electrolyte at a reticulated vitreous
carbon cathode held at -0.95 V (approximately 300 mV
2
-cyclohexen-1-one does not occur to an extent that is
detectable under the conditions of our cyclic voltammetric
measurements.
Remarkable behavior is seen when cyclic voltammo-
grams are recorded for mixtures of nickel(II) salen and
an alkyl iodide. First, there is an enhancement of the
cathodic current at -0.85 V for reduction of nickel(II)
salen as well as a disappearance of the anodic current
for reoxidation of the electrogenerated nickel(I) salen,
both features indicating catalytic reduction of the alkyl
iodide by nickel(I) salen; moreover, the cathodic current
increases in size as the concentration of alkyl iodide is
raised. Second, a new irreversible cathodic wave appears
at a potential 100-200 mV more negative than that for
reduction of nickel(II) salen; the height of this wave also
increases with the concentration of the alkyl iodide. It
is pertinent to mention here that a wave for the direct
(uncatalyzed) reduction of an alkyl iodide is observed at
a much more negative potential than these two waves.
Tentatively, we believe that the second wave might be
attributable to reduction of an alkylnickel(III) salen
intermediate, but more extensive studies must be done
to prove this hypothesis.
When increasing concentrations of 2-cyclohexen-1-one
are introduced into DMF-0.10 M TMAP solutions con-
taining 1 mM nickel(II) salen and 10 mM alkyl iodide
(either iodoethane or 1-iodopentane), the height of the
first wave (attributed to reduction of nickel(II) salen in
the presence of the alkyl iodide) is unaffected. However,
for increasing concentrations of 2-cyclohexen-1-one, the
height of the second wave decreases gradually when
iodoethane is present but increases considerably when
1
-iodopentane is present; we cannot yet explain the cause
of such behavior.
(27) Mubarak, M. S.; Peters, D. G. J . Electroanal. Chem. Interfacial
Electrochem. 1995, 388, 195-198.
(
(
28) Dahm, C. E.; Peters, D. G. Anal. Chem. 1994, 66, 3117-3123.
29) Pritts, W. A.; Peters, D. G. J . Electrochem. Soc. 1994, 141, 990-
9
95.
(
30) Pritts, W. A.; Peters, D. G. J . Electroanal. Chem. Interfacial
Electrochem. 1995, 380, 147-160.
31) Pritts, W. A.; Peters, D. G. J . Electrochem. Soc. 1994, 141,
318-3324.
(
3

1
322 J . Org. Chem., Vol. 63, No. 4, 1998
Notes
more positive than the cyclic voltammetric peak potential
for reduction of 2-cyclohexen-1-one) for 1 h led to the
formation of 1 in a yield estimated to be no higher than
reactions is identical with that described earlier and more
fully by Baizer et al.9 Although, on the basis of our cyclic
voltammetric studies, the reduction of 4 by electrogen-
erated nickel(I) salen is strongly disfavored, we believe
(as concluded by Baizer et al. ) that only a very small
amount of an electrogenerated base (radical-anion 5) is
needed to trigger the formation of 1.
Inasmuch as 6 can be formulated with a negative
charge at either its 2- or 4-position, it is conceivable that,
in addition to 1, one might observe the presence of 4-(3′-
oxocyclohexyl)-2-cyclohexen-1-one as a product. How-
ever, only a single gas chromatographic peak correspond-
ing to 1 was seen. Moreover, 4-(3′-oxocyclohexyl)-2-
cyclohexen-1-one possesses two methine hydrogen atoms,
whereas our NMR experiments revealed the existence of
only a single methine hydrogen atom, as occurs in 1.
Thus, we have no evidence for the formation of 4-(3′-
oxocyclohexyl)-2-cyclohexen-1-one or any other dimer
derived from 4, except for 1 itself.
8
%, although no current flow was observed. Thus, the
9
use of nickel(I) salen is essential to obtaining 1 in high
yields and in a short time.
Mech a n istic F ea tu r es of th e F or m a tion of 2-(3′-
Oxocycloh exyl)-2-cycloh exen -1-on e (1) a n d of th e
Alk yla t ed Cycloh exa n on es a n d 2-Cycloh exen -1-
on es. On the basis of our experimental observations, we
conclude that 1 must arise via the interaction between
2
-cyclohexen-1-one and electrogenerated nickel(I) salen.
However, it appears that this reaction only serves to
initiate the production of 1, because the experiments
described in the preceding section reveal that no signifi-
cant amount of electrogenerated nickel(I) salen is con-
sumed by reaction with 2-cyclohexen-1-one. Thus, we are
led to propose a pathway for the production of 1 that
involves an outer-sphere one-electron transfer between
nickel(I) salen and 2-cyclohexen-1-one (4) to give a
radical-anion (5).
To account for the formation of the alkylated products,
we propose that electrogenerated nickel(I) salen interacts
with an alkyl iodide (possibly via a transient alkylnickel-
(III) salen intermediate) to yield an alkyl radical which
can add to 4 to afford an alkylated radical (7).
Once formed, 5 acts as an electrogenerated base to
deprotonate unreacted 4 to give the conjugate base of the
starting material (6), and 6 can attack another molecule
In turn, 7 can abstract a hydrogen atom from the
solvent (DMF) to give 3-alkylcyclohexanone (8). Forma-
tion of 3-alkyl-2-cyclohexen-1-one (9) might take place
via disproportionation of 7 to afford both 8 and 9.
Finally, the nickel(I) salen-promoted reductive dimer-
ization of 2-cyclohexen-1-one may be unique, because our
efforts to cause an analogous reaction with 2-cyclopenten-
1
-one were unsuccessful.
of 4 to yield a dimeric species that eventually becomes
1
; this last sequence of relatively rapid self-propagating
J O971538Z