Photosensitized electron transfer promoted reductive activation of carbon-selenium bonds to generate carbon-centered radicals: Application for unimolecular group transfer radical reactions

Article abstract of DOI:10.1021/jo960805i

The investigation presented in this paper explores the mechanistic aspects and synthetic potentials of photosensitized electron transfer (PET) promoted reductive activation of organoselenium substrates. PET activation of substrates 1-5 is achieved through a photosystem comprised of light-absorbing 1,5-dimethoxynaphthalene (DMN) as electron donor and ascorbic acid as co-oxidant. The fluorescence quenching of ¹DMN* by organoselenium compounds 1-5, correlation of fluorescence quenching rate constant with the reduction potentials of 1-5, and the dependence of photodissociation quantum yields of 1-5 on their concentration suggests the occurrence of electron-transfer (ET) processes between ¹DMN* and 1-5. Steady state photolysis of organoselenium substrates (R₂CHSePh) in the presence of ¹DMN* and ascorbic acid leads to the cleavage of the -C-Se-bond to produce a carbon-centered radical and PhSe^- species via the intermediacy of R₂CH-SePh^-.. The mechanistic interpretation for the reductive activation of -C-Se- bonds and the synthetic utility of observed cleavage pattern is extended for the unimolecular group transfer radical sequences.

Full text of DOI:10.1021/jo960805i

J . Org. Chem. 1996, 61, 6799-6804
6799
P h otosen sitized Electr on Tr a n sfer P r om oted Red u ctive Activa tion
of Ca r bon -Selen iu m Bon d s To Gen er a te Ca r bon -Cen ter ed
Ra d ica ls: Ap p lica tion for Un im olecu la r Gr ou p Tr a n sfer Ra d ica l
Rea ction s
Ganesh Pandey,* K. S. Sesha Poleswara Rao, and K. V. Nageshwar Rao
Division of Organic Chemistry (Synthesis), National Chemical Laboratory, Pune 411 008, India
Received May 2, 1996X
The investigation presented in this paper explores the mechanistic aspects and synthetic potentials
of photosensitized electron transfer (PET) promoted reductive activation of organoselenium
substrates. PET activation of substrates 1-5 is achieved through a photosystem comprised of light-
absorbing 1,5-dimethoxynaphthalene (DMN) as electron donor and ascorbic acid as co-oxidant. The
1
fluorescence quenching of DMN* by organoselenium compounds 1-5, correlation of fluorescence
quenching rate constant with the reduction potentials of 1-5, and the dependence of photodisso-
ciation quantum yields of 1-5 on their concentration suggests the occurrence of electron-transfer
(ET) processes between 1DMN* and 1-5. Steady state photolysis of organoselenium substrates
1
(R2CHSePh) in the presence of DMN* and ascorbic acid leads to the cleavage of the -C-Se-
-•
bond to produce a carbon-centered radical and PhSe- species via the intermediacy of R2CH-SePh
.
The mechanistic interpretation for the reductive activation of -C-Se- bonds and the synthetic
utility of observed cleavage pattern is extended for the unimolecular group transfer radical
sequences.
In tr od u ction
focus has remained mainly on the chemistry originating
from the mesolysis of radical cations,5,6 despite the great
synthetic potentials of photosensitized one electron re-
ductions in organic synthesis. Fragmentation of radical
cations is generally associated with the loss of electrofu-
Addition or removal of electrons activates molecules
for mesolytic cleavage1 (fragmentation of radical ions),
providing a useful method of breaking bonds that are
otherwise strong in neutral substrates. In this context,
photopromoted exchange of electrons between two inter-
acting substrates has acquired prominence over the past
decade,2 as photoexcitation renders well-defined redox
potential differences between interacting substratessa
requisite for electron to exchange. The uniqueness of
these transformations is associated with the facile gen-
eration of potentially reactive radical ions from a neutral
molecule rather than the initially populated excited
states as is the case during the normal photochemical
reactions.3,4 In fact, the facile cleavage of these radical
ions into radicals and ions5,6 has allowed this concept to
serve as the basis for the development of many useful
organic synthetic reactions.7,8 Inspite of the remarkable
progress made in the area of PET initiated reactions, the
gal group (H+ 2c,6a,9 ) or metal cations (MR3+ 6a,10 M ) Pb,
,
Sn, Si, Ge from group 4A organometallics) leads to the
generation of carbon-centered radicals. However, a
contrasting observation was made from our group while
studying the fragmentation pattern of PET-generated R3-
+•
CSePh
where the radicals are associated with the
selenium group and the cationic charge is retained with
the carbon atom.11 This unusual cleavage pattern en-
couraged us to explore the reactivity profiles of unex-
plored PET-generated radical anions from organoseleni-
um compounds. Apart from a mechanistic interest, this
study is undertaken with a view to develop a novel
unimolecular phenylselenyl group transfer radical reac-
tion, a superior concept12 in radical chemistry, from the
organoselenium substrates of typ e-1 (7) provided the
-•
cleavage of R3CSe-
occurs as depicted in Scheme 1.
The full details of the study13 presented in this paper
unravel the generation of R3CSe-
X Abstract published in Advance ACS Abstracts, September 1, 1996.
(1) (a) Maslak, P.; Chapman, W. H.; Vallombroso, T. M., J r.; Watson,
B. A. J . Am. Chem. Soc. 1995, 117, 12380. (b) Maslak, P.; Narvaez, J .
N.; Vallombroso, T. M., J r. Ibid. 1995, 117, 12373. (c) Maslak, P. Top.
Curr. Chem. 1993, 168, 1.
(2) (a) Fox, M. A. Adv. Photochem. 1986, 13, 295. (b) Mattes, S. L.;
Farid, S. Organic Photochemistry; Padwa, A., Ed.; Marcel Dekker: New
York, 1983; Vol. 6, p 233. (c) Davidson, R. S. Adv. Phys. Org. Chem.
1983, 19, 1. (d) Kavarnos, G. J .; Turro, N. J . Chem. Rev. 1986, 86,
401. (f) J ulliard, M.; Chanon, M. Ibid. 1983, 83, 425.
(3) (a) Weller, A. Z. Phys. Chem. (Wiesbaden) 1982, 130, 129. (b)
Weller, A. Ibid. 1983, 133, 93.
(4) (a) Masuhara, H.; Mataga, N. Acc. Chem. Res. 1981, 14, 312. (b)
Mataga, N.; Okada, T.; Kanda, Y.; Shioyama, H. Tetrahedron 1986,
42, 6143.
(5) (a) Roth, H. D.; Schilling, M. L. M.; J ones, G. J . Am. Chem. Soc.
1981, 103, 1246. (b) Roth, H. D.; Schilling, M. L. M. Ibid. 1981, 103,
7210.
-•
by one-electron
transfer through a photosystem comprised of 1,5-dimeth-
oxynaphthalene (DMN) as light-harvesting electron do-
nor and ascorbic acid as co-oxidant. The synthetic
-•
potentials of the cleavage of R3CSe- from substrates
(9) Fox, M. A.; Chanon, M. Photoinduced Single Electron Transfer
Reactions; Elsevier: Amsterdam, 1988; Part C.
(10) (a) Kochi, J . K. Organometallic Mechanisms and Catalysis;
Academic Press: New York, 1978; pp 445-499. (b) Symons, M. C. R.
Chem. Soc. Rev. 1984, 13, 393. (c) Eaton, D. F. J . Am. Chem. Soc. 1981,
103, 7235. (d) Pandey. G.; Kumaraswamy, G.; Bhalerao, U. T.
Tetrahedron Lett. 1989, 30, 6059.
(11) (a) Pandey. G.; Soma Sekhar, B. B. V. J . Org. Chem. 1994, 59,
7367. (b) Pandey. G; Soma Sekhar, B. B. V.; Bhalerao, U. T. J . Am.
Chem. Soc. 1990, 112, 5650.
(12) (a) Curran, D. P.; J inyou, Xu. J . Chem. Soc., Perkin Trans. 1
1995, 24, 3061. (b) Curran, D. P.; J inyou, Xu.; Lazzarini, E. J . Chem.
Soc., Perkin Trans. 1 1995, 24, 3049. (c) Curran, D. P.; J inyou, Xu.;
Lazzarini, E. J . Am. Chem. Soc. 1995, 117, 6603.
(6) (a) Mattes, S. L.; Farid, S. J . Am. Chem. Soc. 1983, 105, 1386.
(b) Mattes, S. L.; Farid, S. J . Chem. Soc., Chem. Commun. 1980, 126.
(7) (a) Pandey, G. Top. Curr. Chem. 1993, 168, 175. (b) Mattay, J .
Angew. Chem., Int. Ed. Engl. 1987, 26, 825.
(8) (a) Mariano, P. S.; Stavinoha, J . L. Synthetic Organic Photo-
chemistry; Horspool, W. M., Ed.; Plenum Press: London, 1983; p 145.
(b) Mattay, J . Synthesis 1989, 233.
(13) Pandey, G.; Sesha Poleswara Rao, K. S.; Soma Sekhar, B. B.
V. J . Chem. Soc., Chem. Commun. 1993, 1636.
S0022-3263(96)00805-5 CCC: $12.00 © 1996 American Chemical Society

6800 J . Org. Chem., Vol. 61, No. 20, 1996
Pandey et al.
Sch em e 1
F igu r e 1. Stern-Volmer plots for fluorescence quenching of
DMN by selenides 1-5.
Further evidence of ET processes for these systems
could be obtained by studying the fluorescence quenching
of DMN (λexi ) 300 nm, λemi ) 344 nm) by 1-5 which
obeys the Stern-Volmer relation (Figure 1). From the
slopes (Kqfτ) of the straight lines (Figure 1), and singlet
lifetime (τ ) 12.6 ns)15 of DMN, the quenching rate
constants (Kqf) are calculated (correlation coefficient >
0.99, S.D. ) 0.005) and are found to be near diffusion2b
(Kdiff ) 2.30 × 1010 M-1 s-1, Table 1). Excitation and
absorption spectra of DMN remains unaffected in the
presence of the maximum concentration of 1-5. No
exciplex emission is noticed in either polar or nonpolar
solvents. Quenching due to heavy atom induced inter-
system crossing could also be suggested to be minimal,
based on logic similar to that provided by Eaton et al.10c
It is, therefore, reasonable to assume that fluorescence
quenching in these cases takes place via a charge transfer
(CT) stabilized exciplex. Convincing proof of ET in these
of typ e-1 (7) are explored, and its utility for the unimo-
lecular group transfer (UMGT) radical reactions is dem-
onstrated. Steady state analysis of the fluorescence
quenching of DMN and the dependence of R3CSePh bond
cleavage quantum yield on quencher concentration (DMN)
also form a part of the complete mechanistic study of the
PET reductive phenomenon from organoselenium sub-
strates.
Resu lts a n d Discu ssion
To generate radical anions from -C-Se- bond com-
pounds [1-5], a photosystem comprised of DMN as light-
harvesting electron donor and ascorbic acid as sacrificial
electron donor is utilized as shown in Scheme 1. The
thermodynamic feasibility of electron transfer (ET) from
1DMN* to 1-5 is established by estimating the free
energy change (∆Get) associated with ET processes by the
Weller equation14 (eq 1), where E1/2ox[D] is the oxidation
potential of DMN, E1/2red[A] is the reduction potential of
the acceptors 1-5 and E0,0 is the excitation energy of
DMN.
systems may further be obtained through a linear cor-
red
relation plot (Figure 2) of log[Kqf] vs E1/2
of 1-5.
Similarly, ET feasibility from ascorbic acid to DMN+• is
evaluated by estimating ∆Get (-4.51 kcal M-1) employing
the equation E1/2 - E1/2red. The E1/2 value (1.084 eV/
SCE) for ascorbic acid is estimated by cyclic voltamme-
try.16 The electron-donating ability of ascorbate ion and
its transformation to the dehydroascorbic acid and proton
is precedented from the literature report.17
ox
ox
Since PET reaction from a donor-acceptor pair in
solvents of high dielectric constant results in the forma-
tion of free radical ions (FRIP) via a solvent-separated
ion pair (SSIP), in competition with back electron trans-
fer,18 the ET from 1DMN* to 1-5 is envisioned to lead
the chemistry predominantly from the free radical anions
of the organoselenium substrates (1-5) where the chances
of intermolecular coupling between the donor-acceptor
∆GET ) E1/2ox[D] - E1/2red[A] - E0,0
(1)
Substituting eq 1 with the appropriate values of the
reduction potentials of 1-5 (-1.00 to -0.29 eV/SCE),
obtained by cyclic voltammetry (for details see Experi-
mental Section), the oxidation potential of DMN* (1.28
15
eV),15 and the excitation energy (87.8 kcal M-1
)
gave
(16) Since the reduction potential of ascorbic acid is irreversible,
the ∆Get value obtained is only approximate; however it agrees
generally quite well with the experimental value.
(17) (a) Brown, G. M.; Brunschwig, B. S.; Creutz, C.; Endicott, J .
F.; Sutin, N. J . Am. Chem. Soc. 1979, 101, 1298. (b) Bielski, B. H. J .;
Comstock, D. A.; Bowen, R. A. Ibid. 1971, 93, 5624.
(18) Gould, I. R.; Ege, D.; Moser, J . E.; Farid, S. J . Am. Chem. Soc.
1990, 112, 4290.
exergonic (-35.22 to -51.60 kcal M-1) values for all the
compounds (Table 1).
(14) Rhem, D.; Weller, A. Isr. J . Chem. 1970, 8, 259.
(15) Hamada, T.; Nishida, A.; Yonemitsu, O. J . Am. Chem. Soc.
1986, 108, 140.

PET-Promoted Reductive Activation of C-Se Bonds
J . Org. Chem., Vol. 61, No. 20, 1996 6801
Ta ble 1. P h ysica l Con sta n ts Eva lu a ted for P ET fr om Or ga n oselen iu m Su bstr a tes (1-5)
a,b
a,c
red d
e
Kqf
Kqr
E1/2
(eV)
∆Gee
a,f
a,f,g
selenides
(×1010 MS-1
)
(×1010 MS-1
)
(kcal M-1
)
φdisappearance
φlim
1
2
3
4
5
0.368 ( 0.001
0.303 ( 0.005
0.611 ( 0.002
0.184 ( 0.003
0.822 ( 0.005
0.361 ( 0.003
0.288 ( 0.002
0.623 ( 0.005
0.211 ( 0.004
0.782 ( 0.007
-0.70
-0.80
-0.60
-1.00
-0.29
-42.14
-39.83
-44.45
-35.22
-51.60
0.026 ( 0.004
0.023 ( 0.006
0.080 ( 0.003
0.013 ( 0.007
0.095 ( 0.004
0.083 ( 0.002
0.068 ( 0.005
0.128 ( 0.007
0.049 ( 0.003
0.130 ( 0.005
a
b
Errors are standard deviations from the average. From the fluorescence quenching Stern-Volmer plot. c From the double-reciprocal
-1
d
plot; (φdisappearance
)
vs [1-5]-1
.
Referenced to saturated calomel electrode (SCE) using tetraethylammonium perchlorate as supporting
electrolyte in dry acetonitrile. e Oxidation half-wave potential for DMN ) 1.28 eV and E0,0 ) 3.807 eV. f Light intensity was evaluated by
uranyl oxalate actinometry; for details see Experimental Section. g At infinite donor concentration; measured from the plot of (φdisappearance
)
-1
vs [1-5]-1
.
Sch em e 3
red
F igu r e 2. Correlation plot of log[Kqf] vs E1/2 of 1-5.
Sch em e 2
NMR and MS). Therefore, it is reasonable to consider
the cleavage of 5 by following path a. The other fragment
PhSe- either can dimerize directly in the presence of
oxygen to produce PhSeSePh or it may pick up a proton
to form phenylselenol and then dimerize. The quantum
yields for the disappearance of 1-5 are given in Table 1.
To correlate a common reactive species during the
photoreaction of 1-5 and the mechanism of DMN fluo-
rescence quenching by these substrates, an exciplex with
partial CT character is proposed to be formed between
1DMN* and 1-5. A quantitative description to this
aspect is derived by correlating the fluorescence quench-
ing constants (Kqf) with φdisappearence of 1-5. A double-
pair would be minimal. To probe this aspect, organose-
lenium substrate 5 (0.98 mmol) is irradiated in a 2-pro-
panol solution containing DMN (0.25 mmol) and ascorbic
acid (0.25 mmol) for 3 h, without removing dissolved
oxygen from the solution (Scheme 2). Care has been
taken that all the light under this arrangement is
absorbed by DMN only. Analysis of the photolysate
indicated the formation of deselenylated product 6 (char-
acterized by 1H NMR, 13C NMR, and MS spectral
analysis) in 83% yield along with PhSeSePh. DMN is
found almost unchanged (99%) at the end of the reaction.
A control experiment in which 5 is photolyzed under
identical conditions as mentioned above but without
DMN failed to show any product, suggesting the sensi-
tized role of DMN for this transformation. The formation
of deselenylated product 6 could be explained by the
reciprocal plot of φdisappearence vs concentration of 1-5
-1
([φdisappearence
]
vs [1-5]-1) resulted in a straight line
(Figure 3). The ratio of the intercept/slope (Kqrτ) cor-
responded almost to the identical values (within the
acceptable limits of experimental error) of Kqfτ (Table 1)
as obtained from the Stern-Volmer fluorescence quench-
ing analysis. The inverse of the intercept gave a limiting
quantum yield (φlim, Table 1) for the disappearance of
1-5.
The efficient PET cleavage of 5 to generate a carbon-
centered radical led us to envisage the utilization of -C-
Se- bonds in general as carbon radical equivalents, and
hence, we decided to explore the synthetic potentials of
such fragmentations for UMGT radical reactions, since
-•
cleavage of -CSe- following either path a or path b
(Scheme 3). However, the PET reaction of 5 in the
deuterated 2-propanol (i-PrOD) failed to indicate any
deuterium incorporation in the product (monitored by 1H

6802 J . Org. Chem., Vol. 61, No. 20, 1996
Pandey et al.
Sch em e 4
membered cyclization yields are above 70%, and the
stereochemistries of the products (19-22) are in ac-
cordance with the Beckwith rules.25 Six-membered cy-
clization yields are very poor and in case of 12b the
photolysis resulted in an uncharacterizable mixture of
products.
From the reaction (Scheme 4) it is obvious that there
will not be quantitative transformation of 11 to 17 due
to the time lag for the accumulation of PhSeSePh,
required for the termination of radical sequences. There-
fore, to quantify this aspect, quantum efficiencies for the
φdisappearance of 11 and φappearance of 17 are compared and a
difference of 11.45% is observed indicating that only
11.45% of 11 is unaccounted for in this reaction. Com-
plete conversion of 11 to 17 can be achieved by perform-
ing the PET reaction containing a trace amount of
PhSeSePh in the initial reaction mixture.
-1
F igu r e 3. Double-reciprocal plot of {(φdisap
)
vs [1-5]-1}.
PhSeSePh, a good radical trapping agent,19 is produced
in the process. Most of the existing methodologies for
radical reactions are based on tin hydride,20 despite its
inherent limitations such as toxicity and difficulty in
removing tin byproducts. The serious disadvantage21 of
the tin hydride system lies in the loss of functionality
due to the termination of radical sequence by hydrogen
abstraction and reduction of the starting radical under
normal experimental conditions. Although some partial
solutions to these problems have been addressed by
introducing rather expensive reagents,22 the atom trans-
fer approach,23 or utilizing modified tin-based reagents,21
a practical solution to this has remained elusive. In this
context, it was envisaged that PET reaction from orga-
noselenium compounds containing a proximate Π-bond
in non-hydrogen donating solvents would lead to the
formation of cyclized products through the sequences as
outlined in Scheme 1. These expectations were born out
in practice by irradiating a mixture containing 11 (1.3
mmol), DMN (0.50 mmol), and ascorbic acid (0.50 mmol)
in CH3CN-H2O (4:1) without removing dissolved oxygen
from the solution. Removal of the solvent followed by
usual workup and purification furnished 17 as a major
product in 88% yield (Scheme 4). To test the generality
of the UMGT radical reaction concept, compounds 12-
16 were irradiated, and results are listed in Table 2.
Substrates 11-12 are prepared by nucleophilic displace-
ment of in situ generated phenylselenyl anion on the
corresponding halide,24a while compounds 13-16 are
made by following the reported procedure.24b All the five-
In conclusion, we have demonstrated a mechanistically
interesting photoinduced ET activation of -C-Se- bonds
to generate carbon-centered radicals and its synthetic
utility toward UMGT radical reactions.
Exp er im en ta l Section
Gen er a l. DMN15 and PhSeSePh26 were synthesized and
purified by following the literature procedures. 1-2,24a 3,27
4,28 and 529 were prepared by following reported procedures.
Methanol and acetonitrile that are used in HPLC analysis and
fluorescence studies, respectively, were of spectroquality (E.
Merck, India) grade. Silica gel for column chromatography
(finer than 200 mesh) was obtained from Acme, India.
All nuclear magnetic resonance spectra were recorded on
either Bruker AC 200 FT NMR or Brucker MSL 300 NMR
spectrometers using CDCl3 as solvent. All chemical shifts are
reported in parts per million downfield from TMS; coupling
constants are given in hertz. IR spectra were taken on a
Perkin-Elmer model 2830 spectrometer. Mass spectra were
obtained at a voltage of 70 eV on a Finnigan MAT-1020B
instrument.
Fluorescence spectra were recorded on a Spex-Fluorolog 212
spectrofluorimeter. The excitation and emission slit widths
were maintained at 0.5 mm. The steady state emission
spectral measurements were carried out using a 1 cm × 1 cm
quartz cell. A right angle configuration for the cell holder was
utilized during the measurement of excitation and emission
spectra. HPLC analysis was performed on a Perkin-Elmer
(Model 250 binary LC pump along with LC 135C diode array
detector) liquid chromotograph using reverse phase C18 (Bonda-
pack 0.5 µm) column, eluting with a CH3CN:H2O (75:25)
solvent mixture degassed by the freeze-thaw cycle procedure.
Cyclic Volta m m etr y. The cyclic voltammetry experiments
were carried out with a three-electrode assembly on a PAR
175 Universal programmer and PAR RE0074 XY recorder. The
cell consisted of a Metro E410 hanging mercury drop electrode
(19) (a) Newcomb, M.; Marquardt, D. J .; Kumar, M. U. Tetrahedron
1990, 46, 2345. (b) Byers, J . H.; Gleason, T. G.; Knight, K. S. J . Chem.
Soc., Chem. Commun. 1991, 354.
(20) (a) Giese, B. Radicals in Organic Synthesis: Formation of
Carbon - Carbon Bonds; Pergamon Press: Oxford, 1986. (b) Curran,
D. P.; Fevig, T. L.; J asperse, C. P. Chem. Rev. 1991, 91, 1237. (c)
Curran, D. P. Synthesis 1988, 417, 489. (d) Beckwith, A. L. J .;
Westwood, S. W. Tetrahedron 1989, 45, 5269. (e) Ramaiah, M.
Tetrahedron 1987, 43, 3541.
(21) Zard, S. Z.; Boivin, J .; Camara, J . J . Am. Chem. Soc. 1992, 114,
7909.
(22) (a) J ohnston, L. J .; Lusztyk, J .; Wayner, D. D. M.; Abeywick-
reyma, A. N.; Beckwih, A. L. J .; Scaino, J . C.; Ingold, K. U. J . Am.
Chem. Soc. 1985, 107, 4594. (b) Chatgilialoglu, C.; Dickhaut, J .; Giese,
B. J . Org. Chem. 1991, 56, 6399.
(23) (a) Curran, D. P.; Chang, C.-T. J . Org. Chem. 1989, 54, 3140.
(b) Curran, D. P. Chen, M.-H.; Kim, D. J . Am. Chem. Soc. 1989, 111,
6265.
(25) (a) Beckwith, A. L. J .; Easton, C. J .; Serelis, A. K. J . Chem.
Soc., Chem. Commun. 1980, 482. (b) Beckwith, A. L. J .; Lawrence, T.;
Serelis, A. K. Ibid. 1980, 484.
(26) Sharpless, K. B.; Young, M. W. J . Org. Chem. 1975, 40, 947.
(27) Klayman, D. L.; Griffin, T. S. J . Am. Chem. Soc. 1973, 95, 197.
(28) Bergman, J .; Engman, L. Synthesis 1980, 569.
(29) Nicolaou, K. C.; Seitz, S. P.; Sipio, W. J .; Blount, J . F. J . Am.
Chem. Soc. 1979, 101, 3884.
(24) (a) Sharpless, K. B.; Lauer, R. F. J . Am. Chem Soc. 1973, 95,
2697. (b) Petrzilka, P. Helv. Chem. Acta 1978, 61, 2286.

PET-Promoted Reductive Activation of C-Se Bonds
J . Org. Chem., Vol. 61, No. 20, 1996 6803
Ta ble 2. Un im olecu la r Gr ou p Tr a n sfer Ra d ica l Rea ction s
(HMDE) and Pt wire (auxiliary electrode). The supporting
electrolyte was tetraethylammonium perchlorate, and poten-
tials are referred to SCE and are uncorrected for liquid
junction potentials.
were carried out for a short interval of time to bring about
only 8-10% of the conversion. A uranyl oxalate actinometer
was used to monitor the intensity of light.30 Quantitative loss
of the selenides 1-5 was estimated by HPLC [column C18
,
F lu or escen ce Qu en ch in g. Quenching of DMN fluores-
cence was carried out by using 1-5 as quenchers. For the
determination of Stern-Volmer constants (Kqf), the intensity
eluent CH3CN:H2O (3:1)] while loss of 4 was estimated by GC
1
(column 8 ft × /8 in., 10% OV-17) analysis using anisole as
an internal standard. The limiting quantum yield (Φlim) for
(I0) of steady state fluorescence at the maximum emission (λemi
)
each selenide (1-5) was obtained from the inverse plot of the
344 nm was measured from DMN solution (3 × 10-3 M) in
CH3CN at 25 °C, keeping the excitation wavelength (λexi) at
300 nm. Subsequently, the fluoresence quenching intensity
(I) was measured as a function of concentration [Q] of 1-5 in
the range of 3 × 10-3 to 250 × 10-3 M. Linear plots were
obtained on the basis of the equation I0/I ) 1 + Kqfτ[Q], where
I0 denotes the fluoresence intensity in the absence of quencher.
No curvature was noticed in any system, and intercepts were
1.00 ( 0.007 in all cases. Slopes were determined by least-
square fit, and coefficients were always >0.99.
variable donor concentrations (6 × 10-3 to 27 × 10
M) vs
-3
quantum yields i.e., [1-5]-1 vs Φdisappearance-1, by keeping the
DMN concentration fixed at 3 × 10-3 M.
P h otoir r a d ia tion of 5. A mixture of 5 (0.25 g, 0.98 mmol),
DMN (0.05 g, 0.27 mmol), and ascorbic acid (0.05 g, 0.27 mmol)
in 250 mL of 2-propanol was irradiated at room temperature,
without removing dissolved oxygen, with a 450-W Hanovia
medium pressure mercury vapor lamp housed in a Pyrex-
jacketed immersion well. The progress of the reaction was
monitored by GC analysis. Photolysis was discontinued after
3 h, and solvent was removed at reduced pressure. The crude
concentrate purified by column chromatography gave 0.27 mg
Qu a n tu m Yield Mea su r em en ts. Samples for the quan-
tum yield determinations were prepared by pipetting out a
noted volume from the stock solution of 1-5 into Pyrex tubes,
and the samples were irradiated in a quantum yield reactor
(Model QYR-20) using a 200-W mercury lamp. Irradiations
(30) Murov, S. L. Hand Book of Photochemistry; Marcel Dekker:
New York, 1973; p 124.

6804 J . Org. Chem., Vol. 61, No. 20, 1996
Pandey et al.
(78% yield) of deselenylated product (6). This product was
identified by comparing its spectral data with that reported
in the literature.29
145.24, 143.83, 131.55, 131.29, 131.16, 129.24, 127.73, 126.89,
126.80, 108.44, 107.59, 103.51, 69.66, 69.44, 62.79, 62.65,
40.73, 39.60, 15.26; MS m/ e (relative intensity) 284 (M+, 1),
157 (15), 129 (100), 115 (23), 91 (16). Anal. Calcd for C13H16O2-
Se: C, 55.13; H, 5.69. Found: C, 54.89; H, 5.71.
Gen er a l Meth od for P ET UMGT Ra d ica l Ch a in Rea c-
tion . A mixture of 11 (0.50 g, 1.30 mmol), DMN (0.10 g, 0.50
mmol), and ascorbic acid (0.09 g, 0.50 mmol) in 500 mL of CH3-
CN was irradiated at room temperature with a 450-W Hanoiva
medium pressure mercury vapor lamp housed in a Pyrex-
jacketed immersion well, without removing dissolved oxygen.
The progress of the reaction was monitored by HPLC analysis.
Photolysis was discontinued after 7 h, and the solvent was
removed at reduced pressure. The crude concentrate purified
by column chromatography gave cyclized product 17 (0.39 g)
1
21a : IR (neat) 3080, 1615, 1125 cm-1; H NMR (200 MHz,
CDCl3) δ 1.65-1.85 (m, 4H), 2.60 (m, 1H), 3.00 (m, 2H), 3.45
(m, 1H), 3.95 (m, 1H), 4.55 (m, 1H), 7.30 (m, 3H), 7.55 (m,
2H); 13C NMR (75.47 MHz, CDCl3) δ 132.88, 132.54, 130.25,
130.03, 129.02, 126.98, 126.84, 86.18, 85.15, 73.31, 72.17,
50.07, 47.97, 47.04, 43.37, 34.46, 34.01, 32.32, 30.73, 26.34,
25.93, 25.21, 24.05; MS m/ e (relative intensity) 282 (M+, 10),
157 (15), 124 (17), 107 (20), 95 (75), 67 (100). Anal. Calcd for
C14H18OSe: C, 59.79; H, 6.45. Found: C, 59.55; H, 6.43.
1
in 79% yield. The product 17 was characterized by H NMR,
13C NMR, and mass spectral analysis.
21b: IR (neat) 3085, 1210, 1122 cm-1; H NMR (200 MHz,
1
17: IR (neat) 3070, 2860, 1710 cm-1
;
1H NMR (200 MHz,
CDCl3) δ 1.60 (m, 4H), 2.15 (m, 1H), 2.65 (m, 1H), 2.95 (m,
2H), 3.70 (m, 3H), 4.05 (t, 1H, J ) 7.15 Hz), 5.25 (d, 1H, J )
4.3 Hz), 7.30 (m, 3H), 7.55 (m, 2H); 13C NMR (75.47 MHz,
CDCl3) δ 132.90, 129.33, 128.95, 127.04, 101.62, 69.92, 60.90,
40.93, 37.21, 25.60, 22.80, 19.05; MS m/ e (relative intensity)
298 (M+, 6), 197 (20), 157 (12), 141 (51), 116 (42), 95 (33), 77
(55), 69 (88), 55 (100). Anal. Calcd for C14H18O2Se: C, 56.57;
H, 6.10. Found: C, 56.45; H, 6.11.
CDCl3) δ 1.25 (t, 6H, J ) 9.75 Hz), 1.45 (m, 1H), 1.90 (m, 2H),
2.25 (m, 3H), 2.55 (m, 1H), 2.95 (d, 2H, J ) 4.75 Hz), 4.20 (q,
4H, J ) 9.75 Hz), 7.25 (m, 3H), 7.55 (m, 2H); 13C NMR (50.32
MHz, CDCl3) δ 172.57, 132.96, 129.27, 127.06, 61.61, 60.41,
40.94, 40.24, 34.06, 33.48, 32.64, 14.27; MS m/ e (relative
intensity) 384 (M+, 18), 339 (7), 227 (52), 119 (27), 153 (100),
79 (28). Anal. Calcd for C18H24O4Se: C, 56.39; H, 6.31.
Found: C, 56.34; H, 6.29.
1
22a : IR (neat) 3085, 1645, 1210 cm-1; H NMR (200 MHz,
An identical irradiation procedure was adopted for the
UMGT cyclization reactions of 12-16, and spectral charac-
teristics of 18-22 are given as follows.
CDCl3) δ 1.60-1.75 (m, 6H), 2.75 (m, 1H), 3.60 (m, 2H), 4.05
(t, 1H, J ) 4.3 Hz), 6.65 (m, 1H), 7.30 (m, 3H), 7.55 (m, 2H);
13C NMR (75.47 MHz, CDCl3) δ 145.75, 146.01, 133.12, 133.99,
129.75, 129.30, 129.04, 117.75, 117.23, 83.02, 82.95, 74.45,
73.82, 46.15, 45.99, 37.91, 37.72, 33.35, 32.47, 26.75, 25.21;
MS m/ e (relative intensity) 280 (M+, 3), 157 (16), 123 (100),
109 (20), 91 (15). Anal. Calcd for C14H16OSe: C, 60.22; H,
5.78. Found: C, 59.99; H, 5.76.
18a : IR (neat) 3070 cm-1; 1H NMR (200 MHz, CDCl3) δ 1.55
(m, 6H), 1.7 (m, 3H), 2.85 (d, 2H, J ) 4.5 Hz), 7.25 (m, 3H),
7.50 (m, 2H); 13C NMR (50.32 MHz, CDCl3) δ 13.15, 20.92,
28.75, 36.98, 120.15, 127.23, 129.15, 130.52, 132.84, 134.46;
MS m/ e (relative intensity) 240 (M+, 51), 171 (100), 157 (47),
83 (55).
1
22b: IR (neat) 3085, 1625, 1200, 1129 cm-1; H NMR (300
19: IR (neat) 3070, 1200, 1125 cm-1
;
1H NMR (200 MHz,
MHz, CDCl3) δ 1.65 (m, 5H), 2.75 (m, 2H), 3.50 (m, 1H), 3.80
(m, 1H), 4.50 (m, 2H), 5.35 (m, 1H), 6.40 (s, 1H), 7.25 (m, 3H),
7.55 (m, 2H); 13C NMR (50.32 MHz, CDCl3) δ 144.97, 132.85,
129.03, 128.99, 127.33, 101.55, 69.95, 61.05, 41.21, 25.06,
23.12; MS m/ e (relative intensity) 296 (M+, 7), 157 (18), 139
(100), 125 (22), 91 (28). Anal. Calcd for C14H16O2Se: C, 56.96;
H, 5.46. Found: C, 56.79; H, 5.44.
CDCl3) δ 1.20 (t, 3H, J ) 9.7 Hz), 1.75 (m, 1H), 2.20-2.50 (m,
2H), 3.00 (m, 2H), 3.45 (m, 1H), 3.75 (m, 2H), 4.05 (t, 1H, J )
9.7 Hz), 5.15 (m, 1H), 7.25 (m, 3H), 7.55 (m, 2H); 13C NMR
(50.32 MHz, CDCl3) δ 132.97, 131.60, 130.00, 129.26, 129.18,
127.79, 127.09, 104.20, 104.01, 72.04, 62.99, 62.78, 39.86,
39.64, 38.67, 37.91, 32.33, 31.54, 15.42, 15.33; MS m/ e (relative
intensity) 286 (M+, 286), 240 (15), 157 (23), 91 (42), 83 (100).
Anal. Calcd for C13H18O2Se: C, 54.74; H, 6.36. Found: C,
54.53; H, 6.38.
Ack n ow led gm en t. This work was partly supported
by BRNS, Bombay, India. One of us (K.S.S.P.R.) thank
BARC, Bombay, India, for the award of the Dr. K. S.
Krishnan fellowship.
20: IR (neat) 3085, 1620, 1200, 1129 cm-1 1H NMR (200
;
MHz, CDCl3) δ 1.25 (t, 3H, J ) 8.1 Hz), 2.75 (m, 2H), 3.50 (m,
1H), 3.80 (m, 1H), 4.50 (m, 2H), 5.35 (m, 1H), 6.40 (m,1H),
7.25 (m, 3H), 7.55 (m, 2H); 13C NMR (50.32 MHz, CDCl3) δ
J O960805I