Formation and cleavage of a dicobalt complex bridged with a pentamethylene group

Article abstract of DOI:10.1246/cl.2001.346

A new dicobalt complex bridged with a pentamethylene group was prepared and characterized by electronic and ESI mass spectroscopies. This cobalt complex gives cyclopentane via radical intermediates by anaerobic photolysis.

Full text of DOI:10.1246/cl.2001.346

346
Chemistry Letters 2001
Formation and Cleavage of a Dicobalt Complex Bridged with a Pentamethylene Group
Hisashi Shimakoshi, Masaomi Koga, Takashi Hayashi, Yoshimitsu Tachi,† Yoshinori Naruta,† and Yoshio Hisaeda*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka 812-8581
†Institute for Fundamental Research of Organic Chemistry, Kyushu University, Fukuoka 812-8581
(Received January 30, 2001; CL-010090)
A new dicobalt complex bridged with a pentamethylene
group was prepared and characterized by electronic and ESI
mass spectroscopies. This cobalt complex gives cyclopentane
via radical intermediates by anaerobic photolysis.
A compound with a cobalt−carbon bond, which was found
as a key intermediate in vitamin B12 enzymic reaction, is a use-
ful reagent for the generation of an organic radical species,
because the cobalt−carbon bond cleaves readily to form a radi-
cal species by photolysis, electrolysis, or thermolysis.1
Therefore, application of a cobalt complex with a cobalt−car-
bon bond, which is a radical-forming reagent, to organic syn-
thesis in place of the conventional tin hydride method is quite
interesting from the viewpoint of green chemistry.2 In this
paper, we report a radical cyclization reaction via a dicobalt
complex bridged with a pentamethylene group as shown in
Scheme 1. Dicobalt complexes bridged by an alkyl group were
reported by several groups using natural vitamin B12 or
cobaloxime,3–5 but their reactivity and mechanism for the cleav-
age of cobalt−carbon bonds have never been reported.
mopentane (2.30 mg, 1.0 × 10–5 mol). The electronic spectrum
of the solution showed line A in Figure 1. Then, NaBH4 (3.78
mg, 1.0 × 10–4 mol) was added in portions, and the resulting
dark-blue solution was stirred for 1 h at 298 K in the dark. The
electronic spectrum of the reaction mixture was changed to line
B in Figure 1. The spectrum B is characteristic of that for the
corresponding alkylated complex with a cobalt–carbon bond
that we previously prepared.8 In addition, ESI-mass analysis
showed the positive mass spectrum with a cluster of peaks at
m/z 849 as a base peak (see Figure 2). The peaks were assigned
to [M – 2Py – I]+ ion derived from the complex 2. Dicobalt
complex 2 is stable at room temperature in the dark under
anaerobic conditions, but it is unstable in the presence of oxy-
gen. As the solution of 2 was exposed to air, the charge-trans-
New dicobalt complex 2 was prepared from the complex 1,
which has a mono-anionic ligand, as reported by Costa et al.6–8
It is well known that 1 can form the cobalt−carbon bond and is
used as a model compound of vitamin B12. A typical experi-
mental procedure is as follows: 1 (11.6 mg, 2.0 × 10–5 mol)
was added to a nitrogen-saturated methanol solution (4.5 mL)
containing pyridine (20.6 mg, 2.6 × 10–4 mol) and 1,5-dibro-
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
347
hydride under the conditions for generating the free radicals.12
fer band at 475 nm disappeared and the ESI-mass spectrum
mentioned above was no longer observed. Dioxygen insertion
into the cobalt−carbon bonds must induce the decomposition of
the complex 2.9
A proposed reaction mechanism is shown in Figure 3.
Pentamethylene-bridged dicobalt complex 2 is formed by the
reaction of the Co(I) species and 1,5-dibromopentane, and one
of the cobalt–carbon bonds in the dicobalt complex homolyti-
cally cleaves to form the intermediate A. And then, an
intramolecular homolytic substitution (SHi),13 in which the radi-
cal species tethered to the cobalt attacks the cobalt–carbon
bond, will occur, so that the expulsion of the Co(II) species is
accompanied with the cyclopentane-ring formation. The for-
mation of the Co(II) species was confirmed by ESR (g = 2.26,
g// = 2.01, A// N = 15.0 G, A// Co = 94.7 G) as well as the electron-
ic spectrum described above.
The following experiment also supported the SHi mecha-
nism. We prepared the alkylated complex 3, which has a bro-
mopentamethylene group on the cobalt, by the reaction of 1
(11.6 mg, 2.0 × 10–5 mol), pyridine (20.6 mg, 2.6 × 10–4 mol),
1,5-dibromopentane (460 mg, 2.0 × 10–3 mol), and NaBH4
(15.1 mg, 4.0 × 10–4 mol). The electronic spectrum and ESI-
mass analysis indicated the formation of the complex 3; λmax
475 nm in methanol, m/z 477 for [M – Py – I]+. The complex 3
was treated with tributyltin hydride and AIBN in the dark under
the conditions for generating the radical species as shown in
Figure 3. The products were analyzed by GC and GC–MS to
form cyclopentane as a major product. Therefore, we conclude
that the cyclization reaction mainly proceeds via the SHi mecha-
nism. Detailed mechanistic study is now in progress in our lab-
oratory.
After anaerobic irradiation of visible light with a 500-W
tungsten lamp at a distance of 30 cm, the spectrum of the solu-
tion was changed to line C in Figure 1, which is typical for the
corresponding Co(II) complex. The reaction mixture was ana-
lyzed by GLC and GC–MS after the photolysis. Cyclopentane
was obtained in 70% yield as a major product. There was little
consumption of 1,5-dibromopentane in the absence of the com-
plex 1. Peters et al. have reported the direct reduction of 1,5-
dibromopentane using an electrochemical procedure, but the
yield of cyclopentane was less than 30%.10 The yield of the
cyclic compound in this system was markedly dependent on the
length of the methylene chain in the substrate. The highest yield
for cycloalkane was obtained by using 1,5-dibromopentane, and
the yield decreased in the order of the length of the methylene
chain. When we used 1,6-dibromohexane, the major products
detected were hexane, hexene, and hexadiene, while only 3%
cyclohexane was detected. Such reactions do not proceed with-
out irradiation of visible light; therefore, the cleavage of the
cobalt–carbon bond by the photolysis must induce the reaction.
In order to clarify the reaction mechanism, the reaction was
followed by the spin-trapping technique with α-phenyl N-(t-
butyl)nitrone (PBN).11 An ESR signal attributable to the PBN
spin adducts (g = 2.006, AN = 15.3 G, AH = 3.6 G) was observed
in the presence of PBN, and the yield of cyclopentane was
decreased to 14%. This result indicates that the radical species
are generated as the reaction intermediates under the present
conditions. On the other hand, no cyclic compound was
obtained when 1,5-dibromopentane was treated with tributyltin
References and Notes
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11 Reaction conditions: Solvent, MeOH; [complex] = 4.4 ×
10–3 M, [NaBH4] = 2.2 × 10–2 M, [1,5-dibromopentane] =
2.2 × 10–3 M, [PBN] = 8.9 × 10–1 M at 298 K.
12 Reaction conditions: Solvent, benzene; [n-Bu3SnH] = 2.2 ×
10–1 M, [AIBN] = 1.4 × 10–4 M, [1,5-dibromopentane] =
2.2 × 10–3 M at room temperature with irradiation of visi-
ble light. Pentane was obtained in 86% yield.
13 J. C. Walton, Acc. Chem. Res., 31, 99 (1998).