Borane-induced radical reduction of 1-alkenyl- and 1-alkynyl-λ3-iodanes with tetrahydrofuran

Article abstract of DOI:10.1016/S0040-4039(03)01331-5

Exposure of 1-alkenyl(phenyl)- and 1-alkynyl(phenyl)-λ³-iodanes to THF at room temperature in the presence of a catalytic amount of trialkylborane results in smooth reduction to give 1-iodo-1-alkenes and 1-iodo-1-alkynes as major products, respectively. The key step in the reductions probably involves a single-electron transfer from α-tetrahydrofuryl radical to the λ³-iodanes, which generates the labile [9-I-2] iodanyl radicals.

Full text of DOI:10.1016/S0040-4039(03)01331-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 5381–5384
Borane-induced radical reduction of 1-alkenyl- and
1-alkynyl-l3-iodanes with tetrahydrofuran
Masahito Ochiai,* Yoshimi Tsuchimoto and Takanori Hayashi
Faculty of Pharmaceutical Sciences, University of Tokushima, 1-78 Shomachi, Tokushima 770-8505, Japan
Received 9 May 2003; revised 30 May 2003; accepted 30 May 2003
Abstract—Exposure of 1-alkenyl(phenyl)- and 1-alkynyl(phenyl)-l3-iodanes to THF at room temperature in the presence of a
catalytic amount of trialkylborane results in smooth reduction to give 1-iodo-1-alkenes and 1-iodo-1-alkynes as major products,
respectively. The key step in the reductions probably involves a single-electron transfer from a-tetrahydrofuryl radical to the
l3-iodanes, which generates the labile [9-I-2] iodanyl radicals. © 2003 Elsevier Science Ltd. All rights reserved.
Unsaturated phenyl-l3-iodanes (phenyliodonium salts)
with 1-alkenyl or 1-alkynyl group as a carbon ligand on
the iodine(III) are versatile agents in organic synthesis.1
Because of the powerful electron-withdrawing nature2
and the excellent nucleofugality of the phenyl-l3-
iodanyl groups, which show a leaving group ability
about 106 times greater than triflate,3 these l3-iodanes
serve as highly activated species of the corresponding
halides in nucleophilic substitution reactions and as
excellent progenitors for generation of alkylidene
carbenes.1
with THF, which probably proceeds via a single-elec-
tron transfer from a-tetrahydrofuryl (a-THF) radical to
the l3-iodanes (Scheme 1).
Exposure of (E)-1-decenyl(phenyl)-l3-iodane 1a to
tri(sec-butyl)borane (1.3 equiv.) in THF at room tem-
perature for 1 h under argon resulted in smooth reduc-
tive fragmentation of the l3-iodane and afforded
(E)-1-iodo-1-decene (2a) as a major product in an 82%
yield, along with formation of a small amount of
1-decene (3a) (14%) and iodobenzene (17%). Compara-
ble results were obtained in the reaction under atmo-
spheric conditions (Table 1, entry 2). Nature of the
solvents used in the reaction exerts a significant effect
upon the reductive fragmentation of the l3-iodane 1a:
thus the reduction of 1a also takes place smoothly in
methanol, iso-propanol, and 1,3-dioxolane, but not in
benzene and DMF. The former solvents including THF
are known to be good hydrogen atom donors.
Although no appreciable effects of molecular dioxygen
were observed in the reduction of 1a (Table 1, entry 2),
the radical nature of the reductive fragmentation was
substantiated by inhibition of the reaction with the
added radical scavenger 2,2,6,6-tetramethylpiperidine-
N-oxyl (TEMPO) (Table 1, entry 3); in this reaction, a
large amount of 1a was recovered unchanged. Use of a
catalytic amount of tri(sec-butyl)borane gave a 75%
yield of (E)-1-iodo-1-decene (2a) (Table 1, entries 9 and
Little is known, however, for the reduction of these
unsaturated 1-alkenyl- and 1-alkynyl-l3-iodanes.
Chromium(II)-mediated reductive coupling of these
unsaturated l3-iodanes with aldehydes involves a sin-
gle-electron transfer from anhydrous chromium dichlo-
ride to the l3-iodanes generating 1-alkenyl- and
1-alkynylchromium(III) species.4,5 Diaryl-l3-iodanes
with a weakly nucleophilic heteroatom ligands (e.g.
BF4, PF6, and SbF6) are efficient photoinitiators for
cationic polymerization of three- to five-membered
cyclic ethers and vinyl ethers.6 The photocurable
cationic polymerization finds many commercial applica-
tions in areas such as photo imaging, coating, adhe-
sives, inks, and photoresists.7 The key step in this
photoinitiated polymerization involves a single-electron
reduction of diaryl-l3-iodanes by ether-derived radi-
cals.8 We report herein trialkylborane-induced reduc-
tion of 1-alkenyl- and 1-alkynyl(phenyl)-l3-iodanes
Keywords: reduction; l3-iodane; radical; borane.
* Corresponding author. Tel.: +81-(0)88-633-7281; fax: +81-(0)88-
633-9504; e-mail: mochiai@ph2.tokushima-u.ac.jp
Scheme 1.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01331-5

5382
M. Ochiai et al. / Tetrahedron Letters 44 (2003) 5381–5384
Table 1. Reduction of (E)-1-decenyl(phenyl)-l3-iodane 1aa
Entry
s-Bu3B (equiv.)
Solvent
t (h)
Product (% yieldb)
Ratio 2a:PhI
2a
3a
PhI
1
2
3
4
5
6
7
8
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
0.3
0.1
0.1h
0.1i
THF
THF
THF
MeOH
i-PrOH
1,3-Dioxolane
PhH
DMF
THF
1
82
83
–
68
54
54
5
14
16
1
17
17
11e
28f
26g
41
6
10
25
25
23
27
83:17
83:17
–
71:29
68:32
57:43
45:55
–
75:25
75:25
76:24
67:33
1c
1d
5
9
1
1
1
1
3
3
3
3
23
23
1
–
–
9
75
75
72
54
14
14
14
9
10
11
12
THF
THF
THF
a Unless otherwise noted, reduction of 1a [0.02 M] was carried out in the presence of s-Bu3B at room temperature under argon.
b GC yields.
c Under atmospheric conditions.
d The reaction was carried out in the presence of TEMPO (1.3 equiv.).
e l3-Iodane 1a (69%) was recovered unchanged.
f 1H NMR yields.
g Formation of acetone (89%) was detected by GC.
h Et3B was used.
i n-Bu3B was used.
10).9 Both triethylborane and tributylborane are found
to be effective catalysts in the reduction.
transfer from a-THF radical 7 to the l3-iodane 1a,
which generates labile [9-I-2] iodanyl radical 8 and
oxonium ion 9,11 (c) self- decomposition of the iodanyl
radical 8 to a combination of (E)-1-iodo-1-decene (2a)
and phenyl radical, which in turn abstracts an a hydro-
gen atom of THF with liberation of benzene.12 Thus,
the a-THF radical 7 is regenerated and constitutes a
(Z)-b-Acetoxyvinyl(phenyl)-l3-iodanes 1c and 1d
undergo the reductive fragmentation under our condi-
tions using a catalytic amount (0.3 equiv.) of tri(sec-
butyl)borane and afforded (Z)-2-acetoxy-1-iodo-1-
alkenes 2 stereoselectively in high yields (Table 2,
entries 2 and 3). Decreased yield of (Z)-2-chloro-1-
iodo-1-alkene 2 was obtained by the reduction of (Z)-b-
chlorovinyl-l3-iodane 1e. Entries 5 and 6 in Table 2
illustrate some substituent effects upon the regioselec-
tivity for the aryl–iodine(III) to the vinyl–iodine(III)
bond cleavage in the borane-catalyzed reductive frag-
mentation: the ratios of products, vinyl iodide 2 to aryl
iodide, changed from 75:25 in 1a (Table 1, entry 9) to
64:36 by the introduction of an electron-donating p-Me
group in 1f, while to 87:13 in the reduction of 1-decenyl
(para-chlorophenyl)-l3-iodane 1g.
Table 2. Reduction of 1-alkenyl(aryl)-l3-iodanes 1a
Entry
1
t (h)
Product (% yieldb)
Ratio
Trialkylborane-catalyzed reduction of 1-alkynyl-
(phenyl)-l3-iodanes 4 in THF at room temperature
affords 1-iodoalkynes 5 in high yields (Table 3). In the
reduction, higher selectivity of more than 90% for the
aryl–iodine(III) over the alkynyl–iodine(III) bond
cleavage was observed, compared to that of (E)-1-
decenyl-l3-iodane 1a.
2
3
ArI
2:ArI
1
2
3
4
5
6
1b
1c
1d
1ed
1f
5
1
1
2
3
2
89c
81c
85c
48
60
87
–
13
5
17
31
8
–
–
–
34
34e
13f
–
–
–
59:41
64:36
87:13
1g
As illustrated in Scheme 2, a mechanism for the reduc-
tion of (E)-1-decenyl-l3-iodane 1a might be assumed to
involve the following key steps: (a) a hydrogen atom
abstraction of THF with sec-butoxy and/or sec-butyl
radicals generated by the reaction of tri(sec-
butyl)borane with O2,10 producing a-tetrahydrofuryl
radical 7 as an initiation step, (b) a single-electron
a Unless otherwise noted, the reaction was carried out in the presence
of s-Bu3B (0.3 equiv.) in THF at room temperature under argon.
b GC yields.
c Isolated yields.
d s-Bu3B (1 equiv.) was used.
e Toluene was obtained in 66% yield (GC).
f Chlorobenzene was obtained in 83% yield (GC).

M. Ochiai et al. / Tetrahedron Letters 44 (2003) 5381–5384
Table 3. Reduction of 1-alkynyl(phenyl)-l3-iodanes 4a
5383
Scheme 3.
Entry
4
t (h)
Product (% yieldb)
Ratio
5
6
PhI
5:PhI
1
2
3
4a
4b
4c
3
3
3.5
91
94
97
9
1
3
5
8
3
94:6
92:8
97:3
a Unless otherwise noted, the reaction was carried out in the presence
of s-Bu3B (0.1 equiv.) in THF at room temperature under argon.
b GC yields.
Scheme 4.
bear electron-donating substituents.4a Our results,
shown in entry 9 in Table 1, and entries 5 and 6 in
Table 2, are in line with this observation.15 A Hammett
plot of log (k1/k2) versus | is linear with z=1.4 and
correlation coefficient=1.00. For the self-decomposi-
tion of 8, a polar transition state 10, shown in Scheme
3, with partial charge separation is in a good agreement
both with the observed cleavage aptitudes and with the
positive z value of the Hammett plots.4a,16
In the reduction of 1-alkynyl(phenyl)-l3-iodanes 4, the
observed high selectivity for the phenylꢀiodine(III)
bond cleavage yielding 1-iodoalkynes 5 (Table 3) is not
compatible with the polar transition state yielding 1-
alkynyl radicals, in which partial negative charge is
mostly developed on the sp carbon atoms. Selective
formation of 1-alkynyl radicals during decomposition
of the [9-I-2] 1-alkynyl(phenyl)iodanyl radicals seems to
be very difficult, because of the highly energetic nature
of the radicals: the stabilization energy (SE°=−15.57
’
kcal/mol) of HCꢁC is smaller than that of the phenyl
radical (−10.27 kcal/mol).17 Photochemical decomposi-
tion of (3,3-dimethyl-1-butynyl)(phenyl)-l3-iodane in
ethanol18 and reduction of 4a with tetrakis-
(dimethylamino)ethylene,4a both of which generate the
corresponding [9-I-2] iodanyl radicals, are shown to
undergo cleavage of the phenylꢀiodine(III) bond with
high selectivity. These results suggest the intervention of
1-alkynyl(phenyl)iodanyl radicals in the reduction of 4
by THF under our conditions.
Scheme 2.
catalytic cycle. As a minor process, cleavage of the
vinylic carbonꢀiodine bond in 8 produces a terminal
vinyl radical with the formation of iodobenzene. The
vinyl radical also abstracts hydrogen of THF to give
the a-THF radical 7, which creates the other catalytic
cycle.
Evidence for generation of both aryl and alkenyl radi-
cals through facile decomposition of [9-I-2] 1-alkenyl-
(aryl)iodanyl radicals, followed by abstraction of an a
hydrogen atom of THF, was obtained by the reaction
of (E)-1-decenyl(p-tolyl)-l3-iodane 1f in THF-d8 at
room temperature (5 h/Ar). GC–MS analysis showed
70 and 84% deuterium incorporations in 1-decene 3a
and toluene, respectively (Scheme 4). In the reduction
of l3-iodane 1a in methanol and iso-propanol (Table 1,
entries 4 and 5), these alcohols act as good hydrogen
atom donors toward phenyl and vinyl radicals to gener-
ate a-hydroxy carbon-centered radicals, which in turn
serve as efficient reductants for 1a. In fact, it has been
shown that 2-hydroxy-2-propyl radical undergoes a
facile single-electron transfer to diphenyl-l3-iodanes to
’
Diphenyliodanyl radical Ph2I decays very quickly to
iodobenzene and phenyl radical with the life time of 0.2
ns,13,14 and therefore the analogous [9-I-2] iodanyl radi-
cal 8 should be highly labile. Self-decomposition of the
iodanyl radical 8 constitutes a product determining step
(k1 versus k2). It has been well established that the
’
decomposition of aryl(phenyl)iodanyl radicals ArPhI
favors cleavage of the ArꢀI bond if the aryl groups
have electron-withdrawing substituents, whereas cleav-
age of the PhꢀI bond is preferred when the aryl groups

5384
M. Ochiai et al. / Tetrahedron Letters 44 (2003) 5381–5384
4. (a) Chen, D.-W.; Ochiai, M. J. Org. Chem. 1999, 64,
6804; (b) Beringer, F. M.; Bodlaender, P. J. Org. Chem.
1969, 34, 1981.
5. For reduction of diaryl-l3-iodanes with low-valent Yb
and Sm, see: Makioka, Y.; Fujiwara, Y.; Kitamura, T. J.
Organomet. Chem. 2000, 611, 509.
Scheme 5.
6. (a) Crivello, J. V.; Lam, J. H. W. Macromolecules 1977,
10, 1307; (b) Crivello, J. V.; Lam, J. H. W. J. Polym. Sci.,
Symp. 1976, 56, 383.
7. (a) Crivello, J. V. Chemtech 1980, 624; (b) Crivello, J. V.
J. Coatings Tech. 1991, 63, 35.
8. (a) Crivello, J. V. Adv. Polym. Sci. 1984, 62, 1; (b)
Kampmeier, J. A.; Nalli, T. W. J. Org. Chem. 1994, 59,
1381; (c) Ledwith, A. Makromol. Chem. Suppl. 1979, 3,
348.
generate [9-I-2] diphenyliodanyl radical.19 Formation
of a large amount of acetone (Table 1, entry 5) is
compatible with this mechanism.
Mechanism for borane-induced reduction of l3-iodane
1a in THF, shown in Scheme 2, involves generation of
THF-derived oxonium ion 9. Formation of the oxo-
nium ion 9 in the reaction was firmly established by the
detection of oxonium ion-derived acetals: for instance,
when the reduction of 1a was carried out in the pres-
ence of 1 equiv. of 2-phenylethanol as a trapping agent
of the oxonium ion 9 (room temperature/1 h/Ar), THF
adduct 11 was produced in 59% yield, along with the
formation of 2a (81%) and 3a (14%) (Scheme 5). Simi-
larly, the acetal 11 was obtained in 51% yield in the
reaction of 1-decynyl(phenyl)-l3-iodane 4a under the
conditions.
9. A typical experimental procedure is as follows (Table 1,
entry 9). To
(phenyl)(tetrafluoroborato)-l3-iodane (1a) (0.07 mmol) in
THF (3.5 mL) was added solution of tri(sec-
a stirred solution of (E)-1-decenyl-
a
butyl)borane (1 M solution in THF, 0.02 mmol) at room
temperature under argon. After 3 h, the reaction mixture
was quenched with water and extracted with
dichloromethane three times. The combined organic lay-
ers were washed with water and brine, and dried over
Na2SO4. Yields of the products were analyzed by GC
(FFS ULBON HR-1 capillary column, 0.25 mm×50 m).
10. (a) Oshima, K.; Utimoto, K. J. Synth. Org. Chem. Jpn
1989, 47, 40; (b) Suzuki, A. J. Synth. Org. Chem. Jpn
1971, 29, 995; (c) Chung, T. C.; Janvikul, W.; Lu, H. L.
J. Am. Chem. Soc. 1996, 118, 705.
11. Single-electron transfer from a-THF radical 7 to diaryl-
l3-iodanes has been reported to be a facile process with a
rate constant greater than 8×105 M−1 s−1. See Ref. 8b.
12. Phenyl radical abstracts a hydrogen atom from THF with
the rate constant k=4.8×106 M−1 s−1 at 25°C, see: Sca-
iano, J. C.; Stewart, L. C. J. Am. Chem. Soc. 1983, 105,
3609.
Thus, we have developed a method for radical-chain
reduction of 1-alkenyl(phenyl)- and 1-alkynyl(phenyl)-
l3-iodanes by THF, which involves a single-electron
transfer from a-tetrahydrofuryl radical to the l3-
iodanes.
Acknowledgements
This work was supported by a Grant-in-Aid for Scien-
tific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
13. Hennig, H.; Brede, O.; Billing, R.; Schonewerk, J. Chem.
Eur. J. 2001, 7, 2114.
14. The ab initio molecular orbital study (B3LYP/6-31G(d)
level) indicates that the diaryliodanyl radicals are transi-
tion states in the iodine-transfer reactions from aryl
iodides to aryl radicals, but not intermediates.1a
15. When the reduction of 1a using 1.3 equiv. of tri(sec-
butyl)borane was carried out in THF–cyclohexane (5:95),
the ratio of the phenylꢀiodine(III) to the vinylꢀiodine(III)
bond cleavage changed to 89:11.
16. Partial negative charge developed is more effectively sta-
bilized by a phenyl group than by a vinyl group, see:
Dessy, R. E.; Kitching, W.; Psarras, T.; Salinger, R.;
Chen, A.; Chivers, T. J. Am. Chem. Soc. 1966, 88, 460.
17. (a) Leroy, G.; Peeters, D.; Wilante, C. J. Mol. Struct.,
Theochem. 1982, 88, 217; (b) Wyatt, J. R.; Stafford, F. E.
J. Phys. Chem. 1972, 76, 1913.
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