Catalytic reduction and intramolecular cyclization of haloalkynes in the presence of nickel(I) salen electrogenerated at carbon cathodes in dimethylformamide

Article abstract of DOI:10.1021/jo052007a

Pentylidenecyclopentane can be conveniently prepared in up to 86% yield via the catalytic reduction of 1-iodo- or 1-bromo-5-decyne by [[2,2′-[1,2- ethanediylbis(nitrilomethylidyne)]bis[phenolato]]-N,N′,O,O′] -nickelate(I) electrogenerated at a carbon cathode in dimethylformamide containing tetramethylamnionium tetrafluoroborate. This electrosynthesis can be accomplished at potentials for which the haloalkynes are electroinactive, and it can be completed within 30 min at room temperature. Attempts to synthesize pentylidenecyclobutane and pentylidenecyclohexane from 1-halo-4-nonynes and 11-halo-5-undecynes, respectively, under similar conditions afford the carbocycles in very low yields (2% and 6%, respectively), Other products derived from the various haloalkynes are dimers, alkynes, and 1-alkenynes. Dimers (alkadiynes) arising From 1-halo-4-nonynes and 11-halo-5-undecynes are formed in yields ranging from 80% to 89%, whereas icosa-5, 15-diyne (the dimer obtained from a 1-halo-5-decyne) is found in significantly lower yield (≤ 13%). Alkynes and 1-alkenynes are produced in yields of 3-10% and 2-3%, respectively. A mechanistic scheme, involving alkyn-1-yl radicals arising from niekel(I) salon catalyzed cleavage of the carbon-halogen bond of each haloalkyne, is proposed to account for the formation of all products.

Full text of DOI:10.1021/jo052007a

Catalytic Reduction and Intramolecular Cyclization of Haloalkynes
in the Presence of Nickel(I) Salen Electrogenerated at Carbon
Cathodes in Dimethylformamide
Michael A. Ischay,† Mohammad S. Mubarak,‡ and Dennis G. Peters*,†
Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405, and Department of Chemistry,
The UniVersity of Jordan, Amman 11942, Jordan
peters@indiana.edu
ReceiVed September 24, 2005
Pentylidenecyclopentane can be conveniently prepared in up to 86% yield via the catalytic reduction of
1-iodo- or 1-bromo-5-decyne by [[2,2′-[1,2-ethanediylbis(nitrilomethylidyne)]bis[phenolato]]-N,N′,O,O′]-
nickelate(I) electrogenerated at a carbon cathode in dimethylformamide containing tetramethylammonium
tetrafluoroborate. This electrosynthesis can be accomplished at potentials for which the haloalkynes are
electroinactive, and it can be completed within 30 min at room temperature. Attempts to synthesize
pentylidenecyclobutane and pentylidenecyclohexane from 1-halo-4-nonynes and 11-halo-5-undecynes,
respectively, under similar conditions afford the carbocycles in very low yields (2% and 6%, respectively).
Other products derived from the various haloalkynes are dimers, alkynes, and 1-alkenynes. Dimers
(alkadiynes) arising from 1-halo-4-nonynes and 11-halo-5-undecynes are formed in yields ranging from
80% to 89%, whereas icosa-5,15-diyne (the dimer obtained from a 1-halo-5-decyne) is found in
significantly lower yield (e13%). Alkynes and 1-alkenynes are produced in yields of 3-10% and 2-3%,
respectively. A mechanistic scheme, involving alkyn-1-yl radicals arising from nickel(I) salen catalyzed
cleavage of the carbon-halogen bond of each haloalkyne, is proposed to account for the formation of all
products.
Introduction
idenecyclobutane and pentylidenecyclohexane, respectively;
unfortunately, no intramolecular cyclization was observed for
these two haloalkynes.
Crandall and Keyton1 reported in 1969 that refluxing a
mixture of 1-bromo-5-decyne and tri-n-butyltin hydride in
benzene for 36 h, in the presence of 2,2′-azobis(2-methyl-
propionitrile) (AIBN) as a radical initiator, produces pentyl-
idenecyclopentane in 99% yield. In the same research, benzyl-
idenecyclopentane was obtained virtually quantitatively when
6-bromo-1-phenyl-1-hexyne was employed as the starting
haloalkyne. These workers also sought to promote the intra-
molecular cyclizations of 1-bromo-4-nonyne and 11-bromo-5-
undecyne, for which on the basis of other information2,3 the
resulting carbocyclic products would be expected to be pentyl-
Direct electrochemical reduction and intramolecular cycliza-
tion of haloacetylenes have been subjects of several investiga-
tions. In dimethylformamide (DMF) containing tetraalkyl-
ammonium salts, electrolyses of 6-bromo- and 6-iodo-1-phenyl-
1-hexyne4 and 1-bromo- and 1-iodo-5-decyne5 at mercury pool
cathodes do afford the expected five-membered carbocycles,
but only in relatively low yields (<25%) because the dominant
process is formation of diorganomercury compounds. When a
(2) Walling, C.; Cooley, J. H.; Ponoras, A. A.; Racah, E. J. J. Am. Chem.
Soc. 1966, 88, 5361-5363.
(3) Baldwin, J. E. J. Chem. Soc., Chem. Commun. 1976, 734-737.
(4) Willett, B. C.; Moore, W. M.; Salajegheh, A.; Peters, D. G. J. Am.
Chem. Soc. 1979, 101, 1162-1167.
* To whom correspondence should be addressed. Tel: +1-812-8559671.
Fax: +1-812-8558300.
† Indiana University.
‡ The University of Jordan.
(5) Shao, R.-L.; Cleary, J. A.; La Perriere, D. M.; Peters, D. G. J. Org.
Chem. 1983, 48, 3289-3294.
(1) Crandall, J. K.; Keyton, D. J. Tetrahedron Lett. 1969, 1653-1656.
10.1021/jo052007a CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/20/2005
J. Org. Chem. 2006, 71, 623-628
623

Ischay et al.
reticulated vitreous carbon electrode is used to carry out the
direct reductions of 1-bromo- and 1-iodo-5-decyne6 and of
6-iodo-1-phenyl-1-hexyne,7 there is a marked improvement in
the amounts of desired carbocycles; pentylidenecyclopentane
and benzylidenecyclopentane can be produced in yields of up
to approximately 60%, and there is a general tendency for a
phenyl-conjugated haloalkyne to undergo more extensive cy-
clization.
In recent years, our laboratory has become interested in the
use of electrogenerated nickel(I) complexes as catalysts to effect
the reductive intramolecular cyclizations of halogenated alkynes
and alkenes. For example, electrogenerated [[2,2′-[1,2-ethanediyl-
bis(nitrilomethylidyne)]bis[phenolato]]-N,N′,O,O′]nickelate(I),
referred to hereafter as nickel(I) salen, catalyzes the reductive
cyclization of 6-bromo- and 6-iodo-1-phenyl-1-hexyne to give
benzylidenecyclopentane in up to 95% yield.8 On the other hand,
catalytic reduction of 6-bromo-1-hexene by nickel(I) salen leads
to the formation of mixtures of methylcyclopentane, 7-cyclo-
pentylhept-1-ene, 1,2-dicyclopentylethane, 1-hexene, and 1,11-
dodecadiene.9 Several bromo propargyloxy and allyloxy esters,
e.g., ethyl 2-bromo-3-(3′,4′-methylenedioxyphenyl)-3-(propar-
gyloxy)propanoate, have been catalytically reduced by nickel(I)
tetramethylcyclam electrogenerated at a carbon cathode in DMF
to afford functionalized tetrahydrofurans in yields ranging from
80% to essentially quantitative.10,11 These last two publications
provide some comparisons between the use of electrogenerated
transition metal catalysts and of more classic techniques
involving organometallic hydrides. A recent comprehensive
review by Dun˜ach and co-workers12 provides ample evidence
for the utility and diversity of carbon-carbon bond-forming
syntheses that can be accomplished catalytically through the
use of electrogenerated low-valent nickel species.
FIGURE 1. Cyclic voltammograms recorded with a glassy carbon
electrode (area ) 0.077 cm2) at 100 mV s-1 in DMF containing 0.10
M TMABF4 and (A) 2.0 mM nickel(II) salen, (B) 2.0 mM nickel(II)
salen and 10.0 mM 1-bromo-5-decyne, and (C) 2.0 mM nickel(II) salen
and 20.0 mM 1-bromo-5-decyne.
Results and Discussion
Cyclic Voltammetric Behavior of Haloalkynes. Cyclic
voltammograms were recorded at a scan rate of 100 mV s-1
for direct reduction of a 2.0 mM solution of each of the six
haloalkynes at a glassy carbon electrode in DMF containing
0.10 M tetramethylammonium tetrafluoroborate (TMABF4)
under an argon atmosphere. For each compound, a single
irreversible peak was seen that corresponds to two-electron
reductive cleavage of the carbon-halogen bond; the cyclic
voltammograms were very similar in appearance to those
pictured in an earlier publication13 pertaining to the cathodic
behavior of alkyl halides at glassy carbon electrodes. In the
present work, the cathodic peak potentials (Epc) for 1-iodo-4-
nonyne, 1-iodo-5-decyne, and 11-iodo-5-undecyne were found
to be virtually identical (-1.36 ( 0.01 V) and in reasonable
agreement with values previously obtained for reduction of
1-iodo-5-decyne (-1.42 V)6 and 1-iododecane (-1.41 V)13
under comparable experimental conditions. Because a carbon-
iodine bond is more fragile than a carbon-bromine bond, the
latter is reduced at a more negative potential. Accordingly,
cathodic peak potentials (Epc) for 1-bromo-4-nonyne, 1-bromo-
5-decyne, and 11-bromo-5-undecyne were -1.78 ( 0.01 V,
these findings being consistent with earlier research that dealt
with the reductions of 1-bromo-5-decyne (-1.90 V)6 and
1-bromodecane (-1.85 V).13 Our results demonstrate that the
carbon-chain length of these haloalkynes has no significant
influence on the reduction potential for either the carbon-iodine
or carbon-bromine bond. This latter set of observations is
reasonable in view of the fact that at least three methylene
moieties separate the electroactive carbon-halogen bond from
the alkyne site.
In the present work, we have employed cyclic voltammetry
and controlled-potential electrolysis to examine the use of
electrogenerated nickel(I) salen as a catalyst for the reductive
intramolecular cyclization of six different halogenated alkynes
(1-bromo- and 1-iodo-4-nonyne, 1-bromo- and 1-iodo-5-decyne,
and 11-bromo- and 11-iodo-5-undecyne). No studies of the
nickel(I) salen catalyzed reductions of these haloalkynes have
been reported previously. We observed that the extent of
cyclization is greatest (at least 75%) for 1-bromo- and 1-iodo-
5-decyne. On the other hand, 1-bromo- and 1-iodo-4-nonyne
as well as 11-bromo- and 11-iodo-5-undecyne display a signifi-
cantly lesser propensity (e6%) to afford carbocycles; instead,
coupling of alkyn-1-yl radicals to form alkadiynes is dominant.
Other minor products derived from the nickel(I) salen catalyzed
reductions of the haloalkynes are alkynes and 1-alkenynes. A
mechanistic scheme involving intramolecular radical cyclization,
radical coupling and disproportionation, and hydrogen-atom
abstraction from the solvent is offered to account for the
formation of the various products.
(6) Shao, R.-L.; Peters, D. G. J. Org. Chem. 1987, 52, 652-657.
(7) Mubarak, M. S.; Nguyen, D. D.; Peters, D. G. J. Org. Chem. 1990,
55, 2648-2652.
Cyclic Voltammetric Behavior of Nickel(II) Salen in the
Absence and Presence of Haloalkynes. Shown as curve A in
both Figures 1 and 2 is a cyclic voltammogram recorded at 100
mV s-1 for reduction of 2.0 mM nickel(II) salen at a glassy
carbon electrode in DMF containing 0.10 M TMABF4 in the
absence of a haloalkyne. A reversible nickel(II) salen-nickel(I)
salen couple is seen, for which the cathodic peak potential (Epc)
(8) Mubarak, M. S.; Peters, D. G. J. Electroanal. Chem. 1992, 332, 127-
134.
(9) Fang, D. M.; Peters, D. G.; Mubarak, M. S. J. Electrochem. Soc.
2001, 148, E464-E467.
(10) Esteves, A. P.; Goken, D. M.; Klein, L. J.; Lemos, M. A.; Medeiros,
M. J.; Peters, D. G. J. Org. Chem. 2003, 68, 1024-1029.
(11) Esteves, A. P.; Goken, D. M.; Klein, L. J.; Leite, L. F. M.; Medeiros,
M. J.; Peters, D. G. Eur. J. Org. Chem. 2005, 4852-4859.
(12) Dun˜ach, E.; Franco, D.; Olivero, S. Eur. J. Org. Chem. 2003, 1605-
1622.
(13) Cleary, J. A.; Mubarak, M. S.; Vieira, K. L.; Anderson, M. R.; Peters,
D. G. J. Electroanal. Chem. 1986, 198, 107-124.
624 J. Org. Chem., Vol. 71, No. 2, 2006

Catalytic Reduction and Cyclization of Haloalkynes
TABLE 1. Coulometric Data and Product Distributions for
Catalytic Reduction of 20.0 mM Haloalkynes by 2.0 mM Nickel(I)
Salen Electrogenerated at Reticulated Vitreous Carbon Cathodes in
DMF Containing 0.10 M TMABF4
product distribution, %a
entry
haloalkyne
nb
1
2
3
4
total
1
2
3
4
5
6
1-bromo-4-nonyne
1-iodo-4-nonyne
1-bromo-5-decyne
1-iodo-5-decyne
11-bromo-5-undecyne 0.93
11-iodo-5-undecyne 0.95
0.91
0.95
0.89
1.07
2
81
85
13
5
89
80
7
10
3
4
6
tr
2
2
2
3
3
90
99
93
97
103
96
2
75
86
5
6
7
a Yield expressed as the percentage of haloalkyne incorporated into each
product; 1 ) carbocycle; 2 ) dimer; 3 ) alkyne; 4 ) 1-alkenyne. b Number
of electrons per molecule of haloalkyne.
also observed for the 1-bromo- and 1-iodo-4-nonyne and for
the 11-bromo- and 11-iodo-5-undecyne systems.
FIGURE 2. Cyclic voltammograms recorded with a glassy carbon
electrode (area ) 0.077 cm2) at 100 mV s-1 in DMF containing 0.10
M TMABF4 and (A) 2.0 mM nickel(II) salen, (B) 2.0 mM nickel(II)
salen and 5.0 mM 1-iodo-5-decyne, (C) 2.0 mM nickel(II) salen and
10.0 mM 1-iodo-5-decyne, and (D) 2.0 mM nickel(II) salen and 20.0
mM 1-iodo-5-decyne.
Controlled-Potential (Bulk) Electrolyses of Nickel(II)
Salen in the Presence of Haloalkynes. Bulk electrolyses were
performed at reticulated vitreous carbon cathodes in DMF
containing 0.10 M TMABF4, 2.0 mM nickel(II) salen, and 20.0
mM haloalkyne at -1.10 V, which is significantly more positive
than the potential for direct reduction of any haloalkyne.
Compiled in Table 1 are coulometric n values and product
distributions obtained for these experiments; each entry repre-
sents the average of two separate runs. Coulometric n values
indicate that the nickel(I) salen catalyzed reduction of a halo-
alkyne involves one-electron cleavage of the carbon-halogen
bond to give an alkyl radical intermediate. Product yields
represent the absolute amount of starting material incorporated
into each species and are accurate to (3%; none of the starting
haloalkyne remained unreduced and, within experimental error,
the total yield of products is essentially 100%. Four products
(carbocycle, dimer, alkyne, and 1-alkenyne) were separated,
identified, and quantitated with the aid of GC and GC-MS.
As seen in entries 3 and 4 of Table 1, the desired carbocyclic
species (pentylidenecyclopentane) was produced from 1-bromo-
and 1-iodo-5-decyne in yields of 75% and 86%, respectively.
These results compare reasonably well with those reported by
Crandall and Keyton,1 who observed that pentylidenecyclopen-
tane is formed in 99% yield from the tri-n-butyltin hydride
promoted intramolecular cyclization of 1-bromo-5-decyne.
However, a distinct advantage of the electrochemical synthesis
described here is that the process can be accomplished at room
temperature within 30 min. Moreover, the use, removal, and
disposal of toxic tri-n-butyltin hydride (and its byproducts) are
avoided. Although the scale of our electrochemical procedure
is relatively small (see Experimental Section), nothing should
preclude adjusting the amounts of haloalkyne and nickel(II)
salen, as well as the solution volume, to prepare the carbocycle
in larger quantities. Another product derived from the nickel(I)
salen catalyzed reductions of 1-bromo- and 1-iodo-5-decyne is
a dimer (icosa-5,15-diyne), formed in yields of 13% and 5%,
respectively, together with 5-decyne and 1-decen-5-yne, which
are obtained in small amounts (2-4%).
is -0.95 V, the anodic peak potential (Epa) is -0.86 V, and the
ratio of the peak currents (Ipa/Ipc) is close to unity. These
observations are in substantial agreement with the results of
earlier investigations.14,15
In the presence of a haloalkyne, the cyclic voltammetric
behavior of nickel(II) salen is altered dramatically. For example,
with 1-bromo- and 1-iodo-5-decyne, as seen in curves B and C
of Figure 1 and curves B-D of Figure 2, there is an increase
in the cathodic peak current for reduction of nickel(II) salen as
it is recycled catalytically; this current enhancement is more
pronounced for the iodoalkyne than for the bromoalkyne,
because the rate of the catalyst-substrate reaction is greater
for the former compound. In addition, the anodic peak for
reoxidation of nickel(I) salen disappears as a result of the
catalyst-substrate reaction. Several other features of the cyclic
voltammograms in Figures 1 and 2 deserve mention. First, there
is a positive shift in the cathodic peak potential for reduction
of nickel(II) salen that is larger with the iodoalkyne than with
the bromoalkyne, a difference again attributable to the fact that
the electron-transfer reaction between nickel(I) salen and 1-iodo-
5-decyne is faster. Second, a new cathodic peak is seen at
approximately -1.04 ( 0.03 V for curves B-D of Figure 2
(and a much less conspicuous hump is seen in the same potential
region for curves B and C of Figure 1); we now believe that
this voltammetric feature is a signal that the imino bond of the
salen ligand is attacked by an alkyl radical to afford a modified,
catalytically less efficient, and more-difficult-to-reduce nickel
salen species.16 Third, as the concentration of haloalkyne is
raised, the cathodic peak current for reduction of nickel(II) salen
does not increase in proportion to the amount of added substrate;
such behavior has been seen before and has been attributed to
sluggish regeneration of the catalyst precursor,10,11 a phenom-
enon that might be linked to the just-described formation of
modified nickel salen species. All of the preceding trends were
In comparison with earlier work,8 for which interaction of
electrogenerated nickel(I) salen with 6-halo-1-phenyl-1-hexynes
was shown to afford the phenyl-conjugated carbocycle in up to
95% yield, the highest yield of pentylidenecyclopentane for the
nickel(II) salen-1-halo-5-decyne system is 86% (Table 1, entry
4). These findings are consistent with the observation by
(14) Mubarak, M. S.; Peters, D. G. J. Electroanal. Chem. 1995, 388,
195-198.
(15) Vanalabhpatana, P.; Peters, D. G.; Karty, J. A. J. Electroanal. Chem.
2005, 580, 300-312.
(16) Goken, D. M.; Ischay, M. A.; Peters, D. G.; Tomaszewski, J. W.;
Karty, J. A.; Reilly, J. P.; Mubarak, M. S. J. Electrochem. Soc., in press.
J. Org. Chem, Vol. 71, No. 2, 2006 625

Ischay et al.
SCHEME 1
Crandall and Michaely17 that relative rates of reductive intra-
molecular cyclization of a phenyl-conjugated haloalkyne and
an alkyl-substituted haloalkyne are approximately 40:1.
intramolecular cyclization of either 1-bromo-4-nonyne or 11-
bromo-5-undecyne through the use of tri-n-butyltin hydride. As
seen in Table 1, formation of 1-nonen-5-yne from a 1-halo-4-
nonyne and of 1-undecen-5-yne from an 11-halo-5-undecyne
(via hydrogen atom abstraction from the medium by a catalyti-
cally generated primary alkyl radical) occurs to a greater extent
than intramolecular cyclization. Finally, the 1-alkenyne, arising
from disproportionation, is seen in yields lower than 3%.
Mechanistic Aspects of the Catalytic Reduction of Halo-
alkynes. Scheme 1 displays the routes by which we believe
the products are formed via the nickel(I) salen catalyzed
reactions. Earlier, in discussing the cyclic voltammetric char-
acteristics of nickel(II) salen-haloalkyne systems, we indicated
that there is evidence (Figure 2, curves B-D) for modification
(alkylation) of nickel(II) salen during the catalytic reduction of
an alkyl halide (or haloalkyne) by electrogenerated nickel(I)
salen. Indeed, several alkylated nickel(II) salen complexes have
been isolated and identified in our laboratory, and it has been
suggested that one-electron reduction of nickel(II) salen gives
rise to a species for which some electron density resides on the
imino bonds of the salen ligand (although the nickel center bears
the majority of electron density), rendering the imino bonds
susceptible to attack by an alkyl radical.16 Support for the latter
hypothesis is seen in a recent publication by Miranda, Wade,
and Little,18 where the electrocatalytic cyclization of (E)-8-oxo-
oct-2-enoic acid methyl ester to trans-(2-hydroxycyclohexyl)-
acetic acid methyl ester is reported to require a reduced form
of nickel(II) salen with electron density on the imino moieties.
In reaction 1, we represent the one-electron reduction of
nickel(II) salen (5) as both a metal-centered and ligand-centered
It is interesting to compare results for the nickel(I) salen
catalyzed reductions of 1-bromo- and 1-iodo-5-decyne with
those obtained from two earlier investigations5,6 in our labora-
tory. Direct reduction of 1-bromo- and 1-iodo-5-decyne at a
mercury pool cathode in DMF containing tetramethylammonium
perchlorate leads primarily to a diorganomercury species in
yields as high as 89%, whereas pentylidenecyclopentane is
formed in no higher than 9% yield.5 On the other hand, a
subsequent study6 of the direct reduction of 1-bromo- and
1-iodo-5-decyne at a reticulated vitreous carbon cathode in DMF
containing tetraalkylammonium perchlorates revealed that the
carbocycle is produced in up to 60% yield if 1,1,1,3,3,3-
hexafluoro-2-propanol is present as a proton donor.
As shown by entries 1 and 2 (for 1-bromo- and 1-iodo-4-
nonyne, respectively) and entries 5 and 6 (for 11-bromo- and
11-iodo-5-undecyne, respectively) in Table 1, efforts to syn-
thesize pentylidenecyclobutane and pentylidenecyclohexane
electrochemically result largely in the formation of dimers; we
obtained octadeca-5,13-diyne in yields of 81% and 85% from
1-bromo- and 1-iodo-4-nonyne, respectively, and docosa-5,17-
diyne in yields of 89% and 80% from 11-bromo- and 11-iodo-
5-undecyne, respectively. Unfortunately, the desired carbocycles
are produced in only minor amounts; pentylidenecyclobutane
was formed in only 2% yield from both 1-bromo- and 1-iodo-
4-nonyne, whereas pentylidenecyclohexane was obtained in 5%
and 6% yields, respectively, from 11-bromo- and 11-iodo-5-
undecyne. Crandall and Keyton1 did not achieve any reductive
(17) Crandall, J. K.; Michaely, W. J. J. Organomet. Chem. 1973, 51,
375-379.
(18) Miranda, J. A.; Wade, C. J.; Little, R. D. J. Org. Chem. 2005, 70,
8017-8026.
626 J. Org. Chem., Vol. 71, No. 2, 2006

Catalytic Reduction and Cyclization of Haloalkynes
Reagents. Each of the following compounds was purchased from
commercial sources and was used without further purification:
1-bromopentane (97%), 1,4-dibromobutane (99%), 1,5-dibromopen-
tane (99%), 1-hexyne (97%), 1,4-diiodobutane (99+%), 1,5-diiodo-
pentane (97%), 1,3-dibromopropane (99%), 1,3-diiodopropane
(99%), 2,2′-azobis(2-methylpropionitrile) (AIBN, 98%), tri-n-
butyltin hydride (97%), dodecane (99+%), 1.6 M n-butyllithium
in hexanes, 5-decyne (98%), and 4-nonyne (98%). Argon was used
to deaerate solutions for all electrochemical experiments. Dimethyl-
formamide (DMF, 99.9%) was used without further purification
as the solvent for cyclic voltammetry and controlled-potential
electrolysis; the concentration of residual water in the solvent is
typically 40 mM.27 Tetramethylammonium tetrafluoroborate
(TMABF4) was employed as the supporting electrolyte.
process to give species 6 and 7, respectively. However, for
purposes of the present study, the electroreductive intramolecular
cyclization of haloalkynes can be readily understood in terms
of 6 alone as the active catalyst. Accordingly, after the one-
electron reduction of 5 to 6 [reaction 1], the latter transfers an
electron to a haloalkyne, causing cleavage of the carbon-
halogen bond to give a halide ion and an alkyn-1-yl radical (8)
and to regenerate 5 [reaction 2]. As quickly as 8 appears, it can
undergo cyclization, coupling, hydrogen-atom abstraction from
the medium (SH), or disproportionation as depicted in reactions
3-6.
Experimental Section
Synthesis of Bromo- and Iodoalkynes. We prepared 1-bromo-
4-nonyne, 1-iodo-4-nonyne, 1-bromo-5-decyne, 1-iodo-5-decyne,
11-bromo-5-undecyne, and 11-iodo-5-undecyne by means of a
previously published general procedure1 that involves addition of
10 g of 1-hexyne in 80 mL of dry THF to 66 mL of 1.6 M
n-butyllithium in hexane, followed by injection of the appropriate
amount of an R,ω-dibromo- or R,ω-diiodoalkane into the solution.
After being refluxed overnight, the mixture was hydrolyzed by
addition of 2 mL of water. After drying with anhydrous sodium
sulfate, the solvent was removed under reduced pressure and the
residue was vacuum distilled to give the desired product.27
Synthesis of Pentylidenecycloalkanes. Pentylidenecyclopentane
was prepared according to a published procedure;1,5 equivalent
quantities of a 10% solution of tri-n-butyltin hydride in benzene
and a 0.07 M solution of 1-bromo-5-decyne in benzene (containing
1.8 mol % of AIBN as initiator) were mixed. Then the resulting
solution was refluxed for 36 h, after which the product was purified
by distillation: m/z (70 eV) 138, M+ (44); 95, [M - 43]+ (100);
67, [M - 71]+ (81).5
Pentylidenecyclobutane and pentylidenecyclohexane were identi-
fied as electrolysis products by a comparison of their gas chro-
matographic retention times and mass spectra with those of authentic
compounds synthesized according to a published procedure.28 For
pentylidenecyclobutane: m/z (70 eV) 124, M+ (58); 95, [M - 29]+
(76); 81, [M - 43]+ (98); 67, [M - 57]+ (100). For pentylidene-
cyclohexane: m/z 152, M+ (49); 109, [M - 43]+ (73); 81, [M -
71]+ (69); 67, [M - 85]+ (100).
Synthesis of Alkadiynes. We identified docosa-5,17-diyne as
an electrolysis product by comparison of its gas chromatographic
retention time and mass spectrum with those of an authentic sample.
This compound was prepared by overnight refluxing of 0.92 g of
sodium (0.04 mol) and 4.62 g (0.02 mol) of 11-bromo-5-undecyne
dissolved in anhydrous diethyl ether. Unreacted sodium was
destroyed by addition of methanol to the reaction mixture. Then
the ether phase was washed twice with water and dried over
anhydrous magnesium sulfate. After the ether was evaporated, the
residue was distilled to remove unreacted starting material and low-
boiling side products. Mass spectra yielded the following results:
m/z (70 eV) 302, M+ (trace); 273, [M - 29]+ (10); 259, [M -
43]+ (28); 245, [M - 57]+ (47); 147, [M - 155]+ (38); 95, [M -
207]+ (62); 81, [M - 221]+ (100); 67, [M - 235]+ (95); high-
resolution electron impact mass spectrometry (HRMS) exact mass
calculated for C22H38 (M+) 302.2974, found 302.2977.
We employed the same procedure to synthesize authentic samples
of icosa-5,15-diyne and octadeca-5,13-diyne. For icosa-5,15-
diyne: m/z (70 eV) 274, M+ (trace); 245, [M - 29]+ (8); 231, [M
- 43]+ (16); 217, [M - 57]+ (62); 175, [M - 99]+ (25); 147, [M
- 127]+ (42); 81, [M - 193]+ (100); 67, [M - 207]+ (100); high-
resolution chemical ionization mass spectrometry (HRMS) exact
mass calculated for C20H35 (M + 1)+ 275.2739, found 275.2733.
Instrumentation, Electrodes, Cells, and Procedures. Instru-
mentation used for controlled-potential electrolyses is described
elsewhere.19,20 For cyclic voltammetry, we utilized a scanning
potentiostat; experiments were performed and manipulated via a
companion computer program installed on a personal computer. A
circular, planar working electrode (area ) 0.077 cm2) was
constructed by press-fitting a 3-mm-diameter glassy carbon rod into
a Teflon shroud. Reticulated vitreous carbon disk working electrodes
(approximately 2.4 cm in diameter, 0.4 cm in thickness, and having
an approximate geometric area of 200 cm2) for controlled-potential
electrolyses were cut from material (RVC 2X1-100S) supplied by
Energy Research & Generation, Inc. (Oakland, CA); cleaning,
preparing, and handling of these electrodes are described in an
earlier publication.21 Descriptions of electrochemical cells for cyclic
voltammetry and controlled-potential electrolysis can be found
elsewhere.22,23 For each controlled-potential electrolysis, the solution
volume was approximately 40 mL, the initial concentration of
nickel(II) salen was 2.0 mM, and the concentration of haloalkyne
was 20.0 mM. All potentials reported in this study are 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 reference electrode has a potential
of -0.76 V versus an aqueous saturated calomel electrode (SCE)
at 25 °C.24,25
Separation, Identification, and Quantitation of Electrolysis
Products. Electrolysis products in DMF-0.10 TMABF4 were
partitioned between diethyl ether and water, and the ether phase
was dried over anhydrous sodium sulfate and concentrated by means
of rotary evaporation. Separation, identification, and quantitation
were performed by means of gas chromatography. Quantitation
methods have been described in a previous paper.26 Peak areas for
the various products were determined with respect to that of an
internal standard (n-dodecane) added to the solution at the beginning
of an electrolysis. Additionally, gas chromatography-mass spec-
trometry was used to confirm the identities of all products through
comparison of their retention times and mass spectra with those of
commercially available and chemically synthesized authentic
samples. High-resolution mass spectrometry employing either
chemical ionization or electron impact was performed to aid in the
identification of products.
(19) Pritts, W. A.; Peters, D. G. J. Electroanal. Chem. 1995, 380, 147-
160.
(20) Klein, L. J.; Alleman, K. S.; Peters, D. G.; Karty, J. A.; Reilly, J.
P. J. Electroanal. Chem. 2000, 481, 24-33.
(21) Cleary, J. A.; Mubarak, M. S.; Vieira, K. L.; Anderson, M. R.; Peters,
D. G. J. Electroanal. Chem. 1986, 198, 107-124.
(22) Dahm, C. E.; Peters, D. G. Anal. Chem. 1994, 66, 3117-3123.
(23) Vieira, K. L.; Peters, D. G. J. Electroanal. Chem. 1985, 196, 93-
104.
(24) Marple, L. W. Anal. Chem. 1967, 39, 844-846.
(25) Manning, C. W.; Purdy, W. C. Anal. Chim. Acta 1970, 51, 124-
126.
(26) Pritts, W. A.; Vieira, K. L.; Peters, D. G. Anal. Chem. 1993, 65,
2145-2149.
(27) La Perriere, D. M.; Carroll, W. F., Jr.; Willett, B. C.; Torp, E. C.;
Peters, D. G. J. Am. Chem. Soc. 1979, 101, 7561-7568.
(28) Michaely, W. J., Ph.D. Thesis, Indiana University, Bloomington,
1971.
J. Org. Chem, Vol. 71, No. 2, 2006 627

Ischay et al.
For octadeca-5,13-diyne: m/z (70 eV) 246, M+ (trace); 203, [M -
43]+ (46); 189, [M - 57]+ (51); 161, [M - 85]+ (43); 147, [M -
99]+ (63); 133, [M - 113]+ (65); 105, [M - 141]+ (75); 91, [M
- 155]+ (95); 79, [M - 167]+ (100); 67, [M - 179]+ (100); high-
resolution electron impact mass spectrometry (HRMS) exact mass
calculated for C18H30 (M+) 246.2348, found 246.2338.
Other Electrolysis Products. We prepared another electrolysis
product (5-undecyne) by combining 66 mL of 1.6 M n-butyllithium
in hexane with 10 g of 1-hexyne in 80 mL of dry THF. To this
solution was added 22 g of 1-bromopentane, and the mixture was
refluxed overnight. After the solvents were removed under reduced
pressure, the desired product was obtained by distillation (bp 195-
196 °C) of the residue. Mass spectral data for the product matched
those for 5-undecyne in the AIST database.29
Their retention times are very close to those of the corresponding
alkynes; for example, with our DB-5 capillary column (initial
temperature, 50 °C; initial time, 2 min; rate, 8 °C min-1), the
retention time for 5-decyne was 8.54 min and that of 1-decen-5-
yne was 8.32 min. Mass spectral data for these compounds are as
follows: (a) for 1-nonen-4-yne, m/z (70 eV) 122, M+ (50); 107,
[M - 15]+ (40); 79, [M - 43]+ (100); (b) for 1-decen-5-yne, m/z
(70 eV) 121, [M - 15]+ (22); 107, [M - 29]+ (13); 93, [M -
43]+ (35); 79, [M - 57]+ (100); 67, [M - 69]+ (33); (c) for
1-undecen-5-yne, m/z (70 eV) 135, [M - 15]+ (17); 121, [M -
29]+ (15); 93, [M - 57]+ (100); 79, [M - 71]+ (70); 67, [M -
83]+ (33).
Acknowledgment. M.A.I. thanks GlaxoSmithKline (GSK)
for an undergraduate fellowship for summer 2005 as well as
the Indiana University Science, Technology, and Research
Scholars (IU STARS) program for financial support.
Other minor electrolysis products were 1-nonen-4-yne, 1-decen-
5-yne, and 1-undecen-5-yne. These compounds were identified on
the basis of gas chromatographic retention times and mass spectra.
(29) SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/ (National Institute
of Advanced Industrial Science and Technology, September 22, 2005).
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