Free Radical Self-Immolative 1,2-Elimination and Reductive Desulfonylation of Aryl Sulfones Promoted by Intramolecular Reactions with Ortho-Attached Carbon-Centered Radicals

Article abstract of DOI:10.1021/jo970445e

Aryl sulfones bearing an o-(bromomethyl)dimethylsilyl moiety (1), when heated with AIBN and tributyltin hydride, suffer radical elimination under mild conditions to give olefins and stannyl sulfinate 7 in high yield. The mechanism is shown to proceed via intramolecular β-sulfonyl hydrogen abstraction by o-silylmethylene radical 2. This step also shows a large deuterium isotope effect of 12:1. In contrast, radical intermediate 24, generated by tris(trimethylsilyl)silane radical addition to o-allylsilane 23, undergoes intramolecular attack on the sulfone, resulting in homolytic sulfone cleavage to afford reduced products and cyclic sulfone byproduct 25.

Full text of DOI:10.1021/jo970445e

7142
J . Org. Chem. 1997, 62, 7142-7147
F r ee Ra d ica l Self-Im m ola tive 1,2-Elim in a tion a n d Red u ctive
Desu lfon yla tion of Ar yl Su lfon es P r om oted by In tr a m olecu la r
Rea ction s w ith Or th o-Atta ch ed Ca r bon -Cen ter ed Ra d ica ls1
P. C. Van Dort and P. L. Fuchs*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Received March 11, 1997X
Aryl sulfones bearing an o-(bromomethyl)dimethylsilyl moiety (1), when heated with AIBN and
tributyltin hydride, suffer radical elimination under mild conditions to give olefins and stannyl
sulfinate 7 in high yield. The mechanism is shown to proceed via intramolecular â-sulfonyl hydrogen
abstraction by o-silylmethylene radical 2. This step also shows a large deuterium isotope effect of
12:1. In contrast, radical intermediate 24, generated by tris(trimethylsilyl)silane radical addition
to o-allylsilane 23, undergoes intramolecular attack on the sulfone, resulting in homolytic sulfone
cleavage to afford reduced products and cyclic sulfone byproduct 25.
Sch em e 1
In tr od u ction
Familiar synthetic applications which rely on the
inductive effect of the aryl sulfone functional group
include conjugate addition to vinyl sulfones and alkyla-
tion reactions of R-sulfonyl anions. Aggressive strategies
which seek further functionalization can be seen to
exploit the sulfone as a leaving group.2 Olefin-forming
reactions involving sulfinic acid elimination of aryl sul-
fones are generally promoted by R-heteroatom substitu-
tion3 or by allylic (π) activation of protons â to the
sulfone.4 In absence of activation, simple alkyl aryl
sulfones require treatment with strong base at high
temperature to afford olefin products via sulfinic acid
elimination.5 In addition, although radical mediated
elimination has been demonstrated for â-bromo and
â-nitro aryl sulfones,6 to our knowledge, there are no
examples of aryl sulfones lacking â-activation which
eliminate to olefins under standard AIBN/tributyltin
hydride free radical conditions.
We have been investigating ortho-substitution of aryl
sulfones as a means of generating activated intermedi-
ates to facilitate sulfone cleavage. It was previously
reported that o-(allyldimethylsilyl)aryl sulfones, when
converted to the silyl triflate moiety, undergo intramo-
lecular oxygen silylation forming silyl sultinium inter-
mediates which eliminate under mild conditions to afford
olefins.7 In a recent study,8 the ortho-activation strategy
was utilized to promote homolytic cleavage of allylic
sulfones via intramolecular attack of o-stannyl radicals
on the sulfonyl group.
Resu lts a n d Discu ssion
X Abstract published in Advance ACS Abstracts, September 15, 1997.
(1) Synthesis via Vinyl Sulfones. 73.
In our efforts to generate ortho-radical species capable
of activating nonallylic tertiary sulfones toward homolytic
cleavage, we examined o-(bromomethyl)silyl species 1
(Scheme 1). It was found that silylmethylene radical 2
(generated by heating 1 along with AIBN and slow
addition of tributyltin hydride, or by irradiating 1 in the
presence of bis(tributyltin)) does not attack the sulfonyl
group but instead abstracts a hydrogen â to the sulfone
via an unusual 8-membered ring transition state to
produce â-sulfonyl radical 3. The concentration of tribu-
tyltin hydride must be kept low to minimize its quenching
of silylmethylene radical 2. Intermediate 3 rapidly
suffers sulfonyl radical elimination to afford olefin 4 and
arylsulfonyl radical 5. Radical 5 is quenched with
tributyltin hydride, propagating the radical chain reac-
tion with concomitant generation of sulfinic acid 6. A
(2) Simpkins, N. S. Sulfones in Organic Synthesis; Pergamon
Press: New York, 1993.
(3) (a) Berthon, L.; Uguen, D. Tetrahedron Lett. 1985, 26, 3975. (b)
Ley, S. V.; Lygo, B.; Wonnacott, A. Tetrahedron Lett. 1985, 26, 535.
(c) Lauron, H.; J ulia, M.; Verpeaux, J . N. J . Organomet. Chem. 1990,
387, 365. (d) Gai, Y.; J in, L.; J ulia, M.; Verpeaux, J . N. J . Chem. Soc.,
Chem. Commun. 1993, 1625.
(4) (a) Conrad, P. C.; Fuchs, P. L. J . Am. Chem. Soc. 1978, 100, 346.
(b) Hamann, P. R.; Fuchs, P. L. J . Org. Chem. 1983, 48, 914. (c) Taber,
D. F.; Saleh, S. A. J . Org. Chem. 1981, 46, 4817. (d) Nicolaou, K. C.;
Claremon, D. A.; Papahatjis, D. P.; Magolda, R. L. J . Am. Chem. Soc.
1981, 103, 6969. (e) J ones, B. A.; Varma, M.; Stirling, C. J . M. J . Am.
Chem. Soc. 1986, 108, 3153.
(5) Colter, A. K.; Miller, R. E., J r. J . Org. Chem. 1971, 36, 1898.
(6) (a) Ono, N.; Tamura, R.; Hayami, J .; Kaji, A. Tetrahedron Lett.
1978, 763. (b) Ono, N.; Kamimura, A.; Kaji, A. J . Org. Chem. 1987,
52, 5111. (c) Boothe, T. E.; Greene, J . L., J r.; Shevlin, P. B. J . Org.
Chem. 1980, 45, 794.
(7) Van Dort, P. C.; Fuchs, P. L. J . Am. Chem. Soc. 1994, 116, 5657.
(8) Van Dort, P. C.; Fuchs, P. L. J . Org. Chem. 1997, 62, 7137.
S0022-3263(97)00445-3 CCC: $14.00 © 1997 American Chemical Society

Reductive Desulfonylation of Aryl Sulfones
J . Org. Chem., Vol. 62, No. 21, 1997 7143
Sch em e 2a
Ta ble 1. Con d ition s a n d Yield s for Ra d ica l-Med ia ted
Su lfon e Elim in a tion Rea ction
a
Method a: 1,3,5-trimethoxybenzene (NMR internal standard,
0.3 equiv), AIBN (0.3 equiv), slow addition of Bu3SnH (2.0 equiv)
over ∼10 h, C6D6, 80 °C, 13 h. Method b: 1,3,5-trimethoxybenzene
(NMR internal standard, 0.3 equiv), (Bu3Sn)2 (1.5 equiv), hν (350
b
nm), C6D6, 18 h. A 63% unoptimized isolated yield was obtained.
c Yield based on stannyl sulfinate byproduct due to volatility of
olefin product.
second molecule of tributyltin hydride then reacts with
6 to produce stannyl sulfinate 7 as the observed byprod-
uct along with molecular hydrogen. Two equivalents of
tributyltin hydride are therefore required to complete the
reaction. This is consistent with previous observations9
that arylsulfonyl radicals, generated in the presence of
tributyltin hydride, consumed a second equivalent of
tributyltin hydride to form stannyl sulfinate derivatives.
When bis(tributyltin) is used, a reaction between aryl-
sulfonyl radical 5 and bis(tributyltin) propagates the
radical chain producing 7 and a tributylstannyl radical.
Stannyl sulfinate 7 is unstable on silica gel and was not
isolated. However, it is clearly visible by 1H NMR during
NMR tube reactions, and mass spectra of the crude
reaction mixture display a strong and distinctive molec-
ular ion corresponding to 7. Table 1 shows products and
yields obtained from radical-mediated elimination of
unactivated tertiary o-[(bromomethyl)silyl]aryl sulfones.
It is believed that the low yield observed for entry 3 of
Table 1 is due to a competing pathway where the
o-silylmethylene radical abstracts a benzylic hydrogen
from the carbon γ to the sulfone, generating a benzylic
radical which is incapable of â-elimination.
Ortho-attachment of the (bromomethyl)dimethylsilyl
moiety was readily accomplished via directed metala-
tion10 of phenyl sulfones bearing no acidic R-protons
followed by treatment with (bromomethyl)chlorodimeth-
ylsilane to afford o-[(bromomethyl)dimethylsilyl]aryl sul-
fones 8a -d in 81-87% yield.
An NMR study of the radical-mediated sulfone elimi-
nation reaction was carried out using o-[(bromometh-
yl)dimethylsilyl]aryl sulfone 8a as the starting material
(Scheme 2). The major portion of silylmethylene radical
intermediate 9 suffered intramolecular â-sulfonyl hydro-
gen abstraction to form â-sulfonyl radical intermediate
10 which fragmented to afford eneyne 12 in 88% NMR
yield (63% unoptimized isolated yield). The remaining
a
Reagents and conditions: (a) AIBN (0.5 equiv), Bu3SnH (2.2
equiv, added over 14 h), C6D6, 80 °C, 15 h.
portion of intermediate 9 was quenched by tributyltin
hydride before â-hydrogen abstraction could take place,
thereby affording reduced trimethylsilyl product 11 in
12% NMR yield.
Although there are reports of â-sulfonyl radicals un-
dergoing intramolecular cyclizations with electron defi-
cient olefins,11 intermediate 10, which contains a good
radical acceptor with favorable regiochemistry, does not
produce cyclized product 13 in observable quantities. This
suggests that once formed, â-sulfonyl radical intermedi-
ate 10 is short-lived and that the rate-determining step
for the overall reaction is the â-sulfonyl hydrogen ab-
straction step.
In preparation for a double-labeling study to determine
whether â-hydrogen abstraction is strictly intramolecu-
lar, compound 14D with all â-sulfonyl hydrogens replaced
by deuterium was synthesized. In preliminary experi-
ments with 14D, a surprisingly large primary deuterium
isotope effect was observed for the â-sulfonyl hydrogen
abstraction step. To quantitatively determine the isotope
effect, compound 14D and its completely protonated
analog 14H were heated with AIBN and tributyltin
hydride under identical conditions (Scheme 3). A mixture
of tributyltin hydride quenched products 16 and elimina-
tion products 17 and 18 were produced in both reactions.
The rate at which intermediates 15H and 15D are
quenched with tributyltin hydride to produce products
16H and 16D should be virtually identical. Therefore,
the rate of formation of 16H and 16D can be used as a
standard for comparison with the rate of â-sulfonyl
hydrogen abstraction. The reactions were carried out in
NMR tubes, and the relative amounts of products 16 and
17 produced in each reaction were determined by 1H
NMR integration. The ratio of 16H to 17H was 1:6.7
(9) (a) Ueno, Y.; Aoki, S.; Okawara, M. J . Am. Chem. Soc. 1979,
101, 5414. (b) Watanabe, Y.; Ueno, Y.; Araki, T.; Endo, T.; Okawara,
M. Tetrahedron Lett. 1986, 27, 215.
(10) Iwao, M.; Iihama, T.; Mahalanabis, K. K.; Perrier, H.; Snieckus,
V. J . Org. Chem. 1989, 54, 24.
(11) Harvey, I. W.; Whitham, G. H. J . Chem. Soc., Perkin Trans. 1
1993, 191.

7144 J . Org. Chem., Vol. 62, No. 21, 1997
Van Dort and Fuchs
Sch em e 3a
Sch em e 5a
a
Reagents and conditions: (a) AIBN (0.4 equiv), Bu3SnH (2.4
equiv, added over 17 h), C6D6, 80 °C, 23 h.
tyltin). In this case, compound 19 is incapable of any
intramolecular reaction so any formation of products 20
and 18H would have to result from intermolecular
abstraction of a â-sulfonyl hydrogen on 19 by radical
intermediate 15D. Under these conditions, only products
17D and 18D were observed along with unreacted 19,
suggesting that no observable intermolecular â-sulfonyl
hydrogen abstraction was occurring. If no intermolecular
hydrogen abstraction is observed under such favorable
conditions, it is reasonable to conclude that under
“normal” conditions the â-sulfonyl hydrogen abstraction
step is strictly intramolecular.
a
Reagents and conditions: (a) AIBN (0.3 equiv), Bu3SnH (2.0
equiv, added over 10 h), C6D6, 80 °C, 13 h.
Sch em e 4a
In addition to o-silylmethylene radical 2, reactivities
of a number of other carbon-centered radicals with
attachments ortho to sulfones were examined. o-(2-
Bromobenzyl)aryl sulfone 21, when converted to radical
intermediate 22, also suffered intramolecular â-hydrogen
abstraction (via a 9-membered ring transition state in
this instance) followed by sulfonyl radical elimination to
afford olefin 12 in 50% NMR yield (Scheme 5). Although
addition of tributyltin hydride was slower during the
elimination reaction of sulfone 21 than for the analogous
reaction of 8a , intermediate 22 was quenched by tribu-
tyltin hydride in 30% NMR yield while intermediate 9
was only quenched in 12% NMR yield (the reactions were
carried out at the same concentration). This suggests
that the â-sulfonyl hydrogen abstraction step is slower
for 2-bromobenzyl radical 22 than for silylmethylene
radical 9.
When o-(allyldimethylsilyl)aryl sulfone13 23 was heated
with AIBN and tris(trimethylsilyl)silane (TTMSS-H),14
addition of the TTMSS radical to the allyl group produced
radical intermediate 24 (Scheme 6). Surprisingly, al-
though the secondary radical of intermediate 24 has the
same regiochemistry relative to the sulfone as the aryl
radical of intermediate 22, it did not undergo â-hydrogen
abstraction. Instead, the sulfone group was attacked
directly by the radical, causing homolytic sulfone cleavage
to afford cyclic sulfone byproduct 25 and reduced product
27 via alkyl radical intermediate 26. No trace of olefin
products was observed.
a
Reagents and conditions: (a) 14D:19, ∼1:1, (Bu3Sn)2 (1.6 equiv
based on 14D), hν (350 nm), 26 h.
while the ratio of 16D to 17D was 1:0.54. When the
product ratios from the two reactions were compared, the
relative rate of â-sulfonyl H abstraction to D abstraction
was determined to be 6.7:0.54 or ∼12:1 at 80 °C.
A
deuterium isotope effect of 12:1 has been reported previ-
ously for chlorine radical abstraction of hydrogen from
methane-d2 at 0 °C; however the effect dropped to 7:1 at
71 °C.12
A double-labeling study was then devised which took
advantage of the large deuterium isotope effect to maxi-
mize the potential for intermolecular â-sulfonyl hydrogen
abstraction by placing intramolecular deuterium abstrac-
tion in competition with intermolecular hydrogen ab-
straction (Scheme 4). Compounds 14D and 19 were
irradiated as a ∼1:1 mixture in the presence of bis(tribu-
(13) For preparation of o-allyldimethylsilyl aryl sulfones, see ref 7.
(14) Chatgilialoglu, C. Acc. Chem. Res. 1992, 25, 188.
(12) Wiberg, K. B.; Motell, E. L. Tetrahedron 1963, 19, 2009.

Reductive Desulfonylation of Aryl Sulfones
J . Org. Chem., Vol. 62, No. 21, 1997 7145
Sch em e 6a
Ta ble 2. Con d ition s a n d Yield s for Ra d ica l-Med ia ted
Red u ctive Desu lfon yla tion of o-(Allyld im eth ylsilyl)a r yl
Su lfon es
a
Yields of sulfone-cleaved products (R-H) were difficult to
determine accurately by isolation or NMR integration because of
interference from a complex mixture of TTMSS decomposition
products which formed as the reaction proceeded. Byproduct 25
b
a
Reagents and conditions: (a) AIBN (4 equiv), (TMS)3SiH (5
was isolated in 51% yield from this reaction, and formation of the
sulfone-cleaved product (R-H) was verified by GCMS analysis.c A
complex mixture of sulfone-cleaved products was produced.
equiv), toluene-d8, 111 °C.
While this reaction will probably not be synthetically
useful, due to the reagent cost and the large excess of
TTMSS-H required, the difference in reactivity observed
for intermediate 24 when compared with 2 or 22 is
intriguing. It may result from the added stability of
radical 24 imparted by two â-silyl groups.15 Steric
hindrance from the large TTMSS group is also likely to
be a factor. Products resulting from TTMSS-H reduction
of intermediate 24 were not detected even though the
concentration of TTMSS-H was always relatively high
(∼0.06 M). This suggests that â-TTMSS radical inter-
mediate 24 is sterically protected from attack by the
bulky TTMSS-H reagent.
This reaction constitutes a new mode of reductive
desulfonylation, and yields (based on formation of cyclic
sulfone by-product 25) are summarized in Table 2.
Homolytic sulfone cleavage was even observed with a
secondary sulfone (Table 2, entry 3); however, the reac-
tion proceeded at a slower rate and generated a complex
mixture of sulfone-cleaved products. Due to the tendency
of TTMSS radicals to add to carbon-carbon multiple
bonds,16 a mixture of TTMSS adducts is obtained when
unsaturated groups are present in the substrate in
addition to the allylsilane moiety such as in compounds
28 and 29. Further experiments are needed in order to
define how the variables of regiochemistry, steric hin-
drance, and reactivity of the ortho-radical species com-
bine to determine the mode of sulfone activation.
the triflic acid catalyzed elimination of o-[(allyldimeth-
yl)silyl]aryl sulfones.7 The reaction should prove to be a
useful addition to the limited methods currently available
for elimination of unactivated tertiary sulfones.
Exp er im en ta l Section
Gen er a l Meth od s. See Experimental Section of ref 7 with
the following addition: benzene-d6 and toluene-d8 were pur-
chased from Cambridge Isotope Laboratories and used without
further treatment.
Gen er a l Wor k u p P r oced u r e. The reaction mixture was
poured into a saturated aqueous solution of sodium bicarbon-
ate, and the aqueous phase was extracted three times with
CH2Cl2. The combined organic phases were dried over MgSO4
and concentrated in vacuo, providing a crude product residue.
Gen er a l P r oced u r e for R a d ica l-Med ia t ed Su lfon e
Elim in a tion (Meth od A). An NMR tube solution containing
o-[(bromomethyl)dimethylsilyl]aryl sulfone starting material
1 (0.007 M), 1,3,5-trimethoxybenzene (NMR internal standard,
0.3 equiv), and AIBN (0.3 equiv) in benzene-d6 was deoxygen-
ated by the freeze-pump-thaw method. The reaction mixture
was then heated to reflux, and a 0.25 M solution of tributyltin
hydride (2.0 equiv) in benzene was added via syringe pump
over 10 h. After an additional 3 h at reflux, the yield of the
reaction was determined by NMR integration of the olefin
product relative to the 1,3,5-trimethoxybenzene internal stan-
dard.
Gen er a l P r oced u r e for R a d ica l-Med ia t ed Su lfon e
Elim in a tion (Meth od B). A Pyrex NMR tube solution
containing o-[(bromomethyl)dimethylsilyl]aryl sulfone starting
material 1 (0.025 M), 1,3,5-trimethoxybenzene (NMR internal
standard, 0.3 equiv), and bis(tributyltin) (1.5 equiv) in benzene-
d6 was deoxygenated by the freeze-pump-thaw method. The
reaction mixture was then irradiated in a Rayonet apparatus
using 350 nm lamps, and the progress of the reaction was
monitored by NMR. Reactions were typically complete after
18-24 h of irradiation. The yield of the reaction was deter-
mined by NMR integration of the olefin product relative to
the 1,3,5-trimethoxybenzene internal standard.
Radical-mediated elimination of o-[(bromomethyl)sily-
l]aryl sulfones serves as a complementary procedure to
An a lytica l Da ta for Sta n n yl Su lfin a te 7. 1H NMR (C6D6,
as observed during NMR tube reactions): δ 0.4 (s, 9H, chemical
shift varies by (0.05 ppm), 7.07 (t, J ) 7.5, 1H), 7.25 (t, J )
7.5, 1H, chemical shift varies by (0.05 ppm), 7.40 (d, J ) 7.5,
1H), 8.27 (d, J ) 7.5, 1H, chemical shift varies by (0.1 ppm),
1H resonances from the tributylstannyl group overlapped with
peaks from other tributylstannyl species present in the reac-
(15) (a) Davidson, I. M. T.; Barton, T. J .; Hughes, K. J .; Ijadi-
Maghsoodi, S.; Revis, A.; Paul, G. C. Organometallics 1987, 6, 644. (b)
Auner, N.; Walsh, R.; Westrup, J . J . Chem. Soc., Chem. Commun. 1986,
207. (c) Hwu, J . R.; Patel, H. V. Synlett 1995, 989.
(16) Curran, D. P.; Yoo, B. Tetrahedron Lett. 1992, 33, 6931.

7146 J . Org. Chem., Vol. 62, No. 21, 1997
Van Dort and Fuchs
tion mixture and are not included. MS (CI, isobutane): m/z
505 (MH+, isotope cluster consistent with molecule bearing
one tin atom).
NaBr (3.12 g, 30.3 mmol) was added, resulting in a clear,
colorless solution after several minutes of stirring. The
reaction mixture was heated to reflux (65 °C) for 90 h, resulting
in little change in appearance, and then concentrated in vacuo
and worked up according to the general procedure to afford
212 mg (73% yield) of pure (bromomethyl)silyl product 14D:
light yellow oil; 1H NMR (CDCl3) δ 0.55 (s, 6H), 1.01-1.19 (m,
1H), 1.24-1.38 (m, 2H), 1.59-1.74 (m, 3H), 2.91 (s, 2H), 7.53-
7.65 (m, 2H), 7.82-7.92 (m, 2H) 13C NMR (CDCl3) δ -0.3 (o),
19.7 (e), 21.3 (e), 24.8 (e), 129.5 (o), 131.9 (o), 132.4 (o), 137.3
(o), 138.9 (e), 140.6 (e); MS (CI, isobutane) m/z 396/398 (MH+).
o-[(Br om om eth yl)d im eth ylsilyl]a r yl Su lfon e 14H. The
procedure outlined for the preparation of 14D was followed
using phenyl cyclohexyl sulfone as the starting material to
afford 268 mg (66% overall yield based on phenyl cyclohexyl
sulfone) of 14H: colorless oil; 1H NMR (CDCl3) δ 0.56 (s, 6H),
1.02-1.20 (m, 1H), 1.24 (s, 3H), 1.24-1.43 (m, 2H), 1.59-1.90
(m, 7H), 2.91 (s, 2H), 7.53-7.65 (m, 2H), 7.82-7.92 (m, 2H);
13C NMR (CDCl3) δ -0.3 (o), 17.3 (o), 19.7 (e), 21.5 (e), 24.9
(e), 30.1 (e), 65.2 (e), 129.5 (o), 131.9 (o), 132.4 (o), 137.3 (o),
138.9 (e), 140.6 (e); MS (CI, isobutane) m/z 389/391 (MH+).
o-(Tr im eth ylsilyl)a r yl Su lfon e 19. Compound 19 was
prepared by reducing the corresponding o-[(iodomethyl)di-
methylsilyl]aryl sulfone with tributyltin hydride. Conditions
were chosen to favor reduction over elimination via â-sulfonyl
hydrogen abstraction. A solution containing o-[(iodometh-
yl)dimethylsilyl]aryl sulfone starting material (250 mg, 0.555
mmol), AIBN (0.062 mmol), and tributyltin hydride (0.833 mL,
3.10 mmol) in benzene (5 ml) was deoxygenated by the freeze-
pump-thaw method. The reaction mixture was heated to
reflux for 6 h and then concentrated in vacuo, and the residue
was purified by flash chromatography on silica gel using 7%
ethyl acetate in hexane as the eluent to afford 152 mg (84%
yield) of 19: colorless oil; 1H NMR (CDCl3) δ 0.39 (s, 9H), 1.00-
1.20 (m, 1H), 1.22 (s, 3H), 1.25-1.43 (m, 2H), 1.57-1.75 (m,
5H), 1.77-1.89 (m, 2H), 2.41 (s, 3H), 7.30 (d, J ) 8.0, 1H),
7.60 (s, 1H), 7.75 (d, J ) 8.0, 1H); 13C NMR (CDCl3) δ 2.0 (o),
17.2 (o), 21.5 (e), 21.6 (o), 25.0 (e), 30.1 (e), 64.7 (e), 129.3 (o),
132.0, (o), 137.3 (e), 137.6 (o), 142.6 (e), 142.7 (e); MS (CI,
isobutane) m/z 309 (M+-CH3).
Gen er a l P r oced u r e for o-[(Br om om et h yl)d im et h yl-
silyl]a r yl Su lfon es 8a -d . To a stirred 0.3 M solution of aryl
sulfone in THF, cooled to -78 °C, was added n-butyllithium
(1.1 equiv), resulting in a clear, yellow-orange solution. After
the solution was stirred for 2.5 h at -78 °C, (bromomethyl)-
chlorodimethylsilane (1.3 equiv) was added and the reaction
mixture was allowed to warm to 25 °C. After the solution was
stirred at 25 °C for 12 h, the clear, light yellow reaction
mixture was worked up according to the general procedure,
leaving a light yellow oil. The crude product was purified by
flash chromatography on silica gel using 7% ethyl acetate in
hexane as the eluent.
8a (184 mg, 75% yield, 85% based on recovered starting
material): colorless oil; 1H NMR (CDCl3) δ 0.57 (s, 6H), (0.90-
1.10 (m, 1H), 1.30-2.17 (m, 13H), 2.43 (t, J ) 6.7), 2.92 (s,
2H), 7.27-7.68 (m, 7H), 7.85 (d, J ) 7.5, 1H), 7.94 (d, J ) 7.7,
1H); 13C NMR (CDCl3) δ -0.2 (o), 19.8 (e), 19.9 (e), 21.5 (e),
22.7 (e), 24.6 (e), 28.5 (e), 29.5 (e), 68.0 (e), 81.3 (e), 89.5 (e),
123.8 (e), 127.6 (o), 128.2 (o), 129.6 (o), 131.5 (o), 132.1 (o),
132.5 (o), 137.4 (o), 138.8 (e), 141.4. (e); MS (CI, isobutane)
m/z 517/519 (MH+).
1
8b (285 mg, 85% yield): white crystals, mp 47-49 °C; H
NMR (CDCl3) δ 0.56 (s, 6H), 0.86 (t, J ) 6.8, 3H), 0.90-1.08
(m, 1H), 1.20-1.42 (m, 6H), 1.45-1.70 (m, 5H), 1.81-1.92 (m,
4H), 2.91 (s, 2H), 7.54-7.65 (m, 2H), 7.83-7.93 (m, 2H); 13C
NMR (CDCl3) δ -0.3 (o), 13.9 (o), 19.8 (e), 21.6 (e), 23.3 (e),
24.6 (e), 25.5 (e), 29.1 (e), 29.5 (e), 68.2 (e), 129.5, (o), 132.0
(o), 132.3 (o), 137.3 (o), 138.7 (e), 141.8 (e); MS (CI, isobutane)
m/z 431/433 (MH+).
8c (92 mg, 81% yield): colorless oil; 1H NMR (CDCl3) δ 0.57
(s, 6H), 1.37 (s, 6H), 2.02-2.10 (m, 2H), 2.64-2.72 (m, 2H),
2.92 (s, 2H), 7.15-7.24 (m, 3H), 7.29 (t, J ) 7.5, 2H), 7.56-
7.67 (m, 2H), 7.87 (d, J ) 7.1, 1H), 7.94 (d, J ) 7.3, 1H); 13C
NMR (CDCl3) δ -0.3 (o), 19.7 (e), 21.6 (o), 30.4 (e), 37.3 (e),
64.6 (e), 126.1 (o), 128.2 (o), 128.5 (o), 129.7 (o), 131.9 (o), 132.6
(o), 137.4 (o), 138.9 (e), 141.0 (e, 2 carbons, not resolved); MS
(CI, isobutane) m/z 439/441 (MH+).
1
o-(2-Br om oben zyl)a r yl Su lfon e 21. The general proce-
dure described above for the synthesis of o-[(bromo-
methyl)dimethylsilyl]aryl sulfones 8a -d was followed except
2-bromobenzyl bromide (5 equiv) was used instead of (bromo-
methyl)chlorodimethylsilane as the electrophile. 21 (75 mg,
25% yield): colorless oil; 1H NMR (CDCl3) δ 1.00-1.18 (m, 1H),
1.35-1.55 (m, 2H), 1.59-1.85 (m, 5H), 1.87-2.12 (m, 6H), 2.46
(t, J ) 6.5, 2H), 4.65 (s, 2H), 7.03-7.14 (m, 3H), 7.21 (t, J )
7.5, 1H), 7.25-7.42 (m, 6H), 7.46 (t, J ) 7.6, 1H), 7.58 (d, J )
7.9, 1H), 7.99 (d, J ) 7.9, 1H); 13C NMR (CDCl3) δ 19.9 (e),
21.5 (e), 22.8 (e), 24.7 (e), 28.4 (e), 29.0, (e), 38.9 (e), 67.5 (e),
81.3 (e), 89.5 (e), 123.8 (e), 124.8 (e), 126.6 (o), 127.6 (o), 127.7
(o), 128.1 (o), 128.2 (o), 131.5 (o), 131.6 (o), 131.8 (o), 132.7 (o)
, 133.5 (o), 133.6 (o), 134.0 (e), 139.8 (e), 142.0 (e); MS (CI,
isobutane) m/z 535/537 (MH+).
8d (337 mg, 87% yield): white crystals, mp 58-60 °C; H
NMR (CDCl3) δ 0.55 (s, 6H), 1.32 (s, 9H), 2.91 (s, 2H), 7.55-
7.66 (m, 2H), 7.85 (d, J ) 7.1, 1H), 7.92 (d, J ) 7.3, 1H); 13C
NMR (CDCl3) δ -0.41 (o), 19.6 (e), 24.2 (o), 61.3 (e), 129.6 (o),
131.8 (o), 132.5 (o), 137.3 (o), 138.7 (e), 140.7 (e); MS (CI,
isobutane) m/z 349/351 (MH+).
Olefin P r od u ct 12. A solution containing o-[(bromo-
methyl)dimethylsilyl]aryl sulfone starting material 8a (40 mg,
0.077 mmol) and AIBN (3.8 mg, 0.023 mmol) in benzene (11
mL) was deoxygenated by the freeze-pump-thaw method.
The reaction mixture was then heated to reflux, and 0.62 mL
of a 0.25 M solution of tributyltin hydride in benzene was
added via syringe pump over 10 h. After an additional 3 h at
reflux, the reaction mixture was concentrated in vacuo and
the colorless residue was purified by flash chromatography on
silica gel using 100% hexane as the eluent to afford 11 mg
o-(Allyld im eth ylsilyl)a r yl su lfon es 23a -c. See ref 7 for
preparation and characterization of o-(allyldimethylsilyl)aryl
sulfones.
1
(63% yield) of 12: colorless oil; H NMR (CDCl3) δ 1.39-2.10
(m, 12H), 2.25 (t, J ) 6.7, 2H), 5.43 (s, 1H), 6.93-7.03 (m,
3H), 7.50 (d, J ) 7.4, 2H); MS (EI) m/z 224 (M+).
Gen er a l P r oced u r e for Ra d ica l-Med ia ted Red u ctive
Desu lfon yla tion of o-(Allyld im eth ylsilyl)a r yl Su lfon es.
An NMR tube solution containing o-(allyldimethylsilyl)aryl
sulfone starting material 23 (0.03 M), AIBN (0.5 equiv), and
tris(trimethylsilyl)silane (TTMSS) (2.0 equiv) in toluene-d8 (1
mL) was deoxygenated by the freeze-pump-thaw method and
then heated to reflux (110 °C). The progress of the reaction
was monitored by NMR, and additional aliquots of AIBN (0.5
equiv) and TTMSS (0.5 equiv) were added at 3-5 h intervals
until the reaction was complete (reaction times were typically
20-26 h for tertiary sulfones, 40-48 h for secondary sulfones).
Yields of the cyclic sulfone byproduct 25 were based on NMR
integration, and structures of desulfonylated products were
determined by GC/MS analysis.
â-Deu ter a ted o-[(Br om om eth yl)d im eth ylsilyl]a r yl Su l-
fon e 14D. To a solution of 1-(phenylsulfonyl)-2,2,6,6-tetra-
deuteriocyclohexane (0.875 g, 3.83 mmol) in THF (11 mL)
which had been cooled to -78 °C, was added n-butyllithium
(4.02 mmol). After the solution was stirred at -78 °C for 1.5
h, CD3I (Aldrich, 0.262 mL, 4.21 mmol) was added and the
reaction was allowed to warm to 25 °C. After 19 h, the reaction
mixture was cooled back to -78 °C and n-butyllithium (4.21
mmol) was added. After an additional 1.5 h at -78 °C, the
reaction was quenched with (bromomethyl)chlorodimethylsi-
lane (0.679 mL, 4.98 mmol) and allowed to warm to 25 °C.
After 2 h of stirring at 25 °C, the reaction mixture was worked
up according to the general procedure and the crude product
purified by flash chromatography on silica gel to afford 1.01 g
(65% yield) of a ∼2:1 mixture of o-(bromomethyl)silyl and
o-(iodomethyl)silyl products. The product mixture (300 mg,
0.732 mmol) was then dissolved in methanol (20 mL), and
Cyclic Su lfon e Byp r od u ct 25. The general procedure
described for radical-mediated reductive desulfonylation was
followed using 23b (17 mg, 34 µmol) as the starting material.
When the reaction was complete by NMR, the reaction mixture

Reductive Desulfonylation of Aryl Sulfones
J . Org. Chem., Vol. 62, No. 21, 1997 7147
was concentrated in vacuo and the colorless residue purified
by flash chromatography on silica gel using 100% hexane as
the eluent to afford 9 mg (51% yield) of 25: white crystals,
Ack n ow led gm en t. Support of this work was pro-
vided by NIH (GM-32693). We express our gratitude
to Arlene Rothwell for providing mass spectral data.
1
mp 65-67 °C; H NMR (CDCl3) δ 0.21 (s, 27H), 0.27 (s, 3H),
0.36 (s, 3H), 0.71 (dd, J ) 4.8, 14.5, 1H), 0.93 (dd, J ) 12.3,
14.3, 1H), 1.28 (dd, J ) 8.1, 14.5, 1H), 1.53 (dd, J ) 2.1, 14.3,
1H), 3.30 (dddd, J ) 2.1, 4.8, 8.1, 12.3, 1H), 7.21 (br,t, J )
7.3, 1H), 7.28 (br,d, J ) 7.3, 1H), 7.35 (br,t, J ) 7.3, 1H), 7.50
(br,d, J ) 7.3, 1H); 13C NMR (CDCl3) δ -1.6 (o), -0.2 (o), 1.5
(o, 9 carbons), 20.2 (e), 21.5 (e), 43.6 (o), 124.8 (o), 125.8 (o),
129.7 (o), 131.8 (o), 138.8 (e), 159.6 (e); MS (EI) m/z 349, 175,
73.
Su p p or tin g In for m a tion Ava ila ble: 1H and 13C NMR
spectra of new compounds described in the text (20 pages).
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