J. Phys. Chem. A 1998, 102, 9975-9977
9975
A Four-Member Ring Hypervalent Iodine Radical
,†
,‡
†
§
Miguel A. Miranda,* Julia P e´ rez-Prieto,* Enrique Font-Sanchis, Kl a´ ra K o´ nya, and
J. C. Scaiano*,§
Instituto de Tecnolog ´ı a Qu ´ı mica, Departamento de Qu ´ı mica, UniVersidad Polit e´ cnica de Valencia,
Camino de Vera s/n, Valencia, 46071 Spain, and Departamento de Qu ´ı mica Org a´ nica, Facultad de Farmacia,
UniVersidad de Valencia, Vic e´ nt Andr e´ s Estelle s/n, Burjasot, Valencia, 46100 Spain, and
Department of Chemistry, UniVersity of Ottawa, Ottawa, Canada K1N 6N5
ReceiVed: May 4, 1998; In Final Form: August 27, 1998
A four-member ring hypervalent iodine radical has been detected in the laser flash photolysis of 1,3-diiodo-
1
,3-diphenylpropane. This species absorbs at 320 nm, has a lifetime of ∼9.5 µs in cyclohexane, and is not
quenchable by oxygen. Excitation of this radical by means of laser-drop photolysis results the formation of
trans-1,2-diphenylcyclopropane through concerted iodine extrusion.
Introduction
SCHEME 1
Photolysis of benzylic halides in nonpolar solvents is known
to produce homolysis of the carbon-halogen bond.1 In the case
of benzylic 1,2-dihalides, light-induced cleavage of the first
C-X bond gives rise to haloalkyl radicals.2 The formation of
three-membered ring bromine radicals has been proposed to
explain the unexpected stability of 1-naphthyl-2-bromoethyl
radical, which has a lifetime of 6.47 µs and is not oxygen-
quenchable.3 Moreover, photolysis of 1,5-diiodo-1,5-diphenyl-
pentane in cyclohexane leads to a 5-iodopentyl radical, which
cyclizes to a six-membered hypervalent iodine radical. This
species also has a relatively long lifetime (4.4 µs) and is
unreactive toward oxygen.4 Likewise, the stability of thio- and
seleno-alkyl radicals has been related to the participation of the
corresponding heteroatom through bridged structures.5 We now
report the detection of 3 (see Scheme 1), which is the first four-
membered ring hypervalent halogen radical described. Its
lifetime (∼9.5 µs) is even longer than that of the six-membered
ring analogue (probably reflecting the less-favorable steric
interactions in the six-member ring, where the phenyl rings,
probably equatorial, may interact with other atoms).
1
Compound 1-iodo-1,3-diphenylpropane: H NMR (250 MHz,
3
CDCl3): δ ) 2.3-2.9 (m, 4H; CH2), 5.0 (t, J(H,H) ) 7 Hz,
H; CH), 7.1-7.4 (m, 10H; arom). C NMR (75.46 MHz,
CDCl3): δ ) 143.8 (s), 140.2 (s), 128.7 (d), 128.5 (d), 127.9
d), 127.1 (d), 126.2 (d), 42.7 (d), 35.6 (t), 33.2 (t). Elemental
13
1
(
analysis for C15H15I: calcd C 55.92, H 4.69; found C 55.83, H
+
4
.65. MS (70 eV): m/z (%): 194 (100)[M - IH].
Experimental Section
Laser flash photolysis experiments were carried out with a
Compounds 1, 1-1,3-d2, and 1-iodo-1,3-diphenylpropane were
Nd:YAG laser using the fourth harmonic (266 nm, <10 ns, e20
mJ/pulse) or with a dye laser pumped with an excimer laser
operated with HCl/Xe/Ne gas mixtures (308 nm, ∼6 ns, e50
mJ/pulse). Transient signals were captured with a Tektronix
model 2440 digital oscilloscope, which was interfaced to a
computer that also controlled the experiment. The system was
operated with software written in the LabVIEW 3.1.1 environ-
ment from National Instruments. Other aspects of this instru-
prepared from the corresponding diol or alcohol by treatment
4
with iodotrimethylsilane. Data for 1 (diasteromeric mixture):
1
3
2
H NMR (250 MHz, CDCl3): δ ) 2.7 (dt, J(H,H) ) 15 Hz,
3
J(H,H) ) 8 Hz, 1H; CH2), 3.0 (t, J(H,H) ) 8 Hz, 2H; CH2),
2
3
3
.25 (dt, J(H,H) ) 15 Hz, J(H,H) ) 8 Hz, 1H; CH2), 5.0 (t,
3
3
J(H,H) ) 8 Hz, 2H; CH), 5.1 (t, J(H,H) ) 8 Hz, 2H; CH),
1
3
7
)
(
.1-7.3 (m, 20H; arom). C NMR (75.46 MHz, CDCl3): δ
142.2 (s), 141.1 (s), 128.9 (d), 128.3 (d), 127.3 (d), 127.1
d), 51.6 (d), 51.3 (d), 31.2 (t), 30.9 (t). Elemental analysis for
6,7
ment are similar to those described earlier. All experiments
were carried out with flow cells constructed from 7 × 7 mm
Suprasil quartz tubing. Samples were contained in a 100-mL
reservoir tank that was purged with a slow stream of either
nitrogen or oxygen, as required. The absorbance of the samples
pa ta tt hh .e laser wavelength was adjusted to ∼0.3 for a 7-mm optical
C15H14I2: calcd C 40.21, H 3.15; found C 40.50, H 3.20. MS
+
(
70 eV): m/z (%): 194 (100)[M - 2I].
1
Compound 1-1,3-d2: H NMR (250 MHz, CDCl3): δ ) 2.7
2
2
(
)
d, J(H,H) ) 15 Hz, 1H; CH2), 3.0 (s, 2H; CH2), 3.2 (d, J(H,H)
15 Hz, 1H; CH2), 7.1-7.3 (m, 20H; arom).
*
To whom correspondence should be addressed.
Universidad Politecnica de Valencia.
Universidad de Valencia.
For laser-drop photolysis,4,8 the beam from a Nd:YAG laser
using the fourth harmonic (266 nm, <10 ns, e20 mJ/pulse)
was focused by means of a quartz lens into a drop of the
†
‡
§
University of Ottawa.
1
0.1021/jp982103y CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/03/1998