Convenient ambient temperature generation of sulfonyl radicals

Article abstract of DOI:10.1071/CH12499

Presented herein is a novel method for the efficient, ambient temperature generation of sulfonyl radicals from aryl and alkyl sulfonylbromides upon autoxidation of triethylborane (Et₃B). The resultant radicals were regioselectively trapped via addition to terminal alkynes, generating a secondary vinyl radical that selectively abstracts a Br atom from RSO ₂Br, yielding the (E)-bromo vinylsulfones. Sensitivity towards Lewis basic groups was observed, presumably due to the disruptive coordination to Et3B before atom-transfer.

Full text of DOI:10.1071/CH12499

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2013, 66, 336–340
Full Paper
http://dx.doi.org/10.1071/CH12499
Convenient Ambient Temperature Generation
of Sulfonyl Radicals
A
A
A
A B
,
Kerry Gilmore, Brian Gold, Ronald J. Clark, and Igor V. Alabugin
A
Department of Chemistry and Biochemistry, Florida State University,
Tallahassee, FL 32306, USA.
B
Corresponding author. Email: alabugin@chem.fsu.edu
Presented herein is a novel method for the efficient, ambient temperature generation of sulfonyl radicals from aryl and
alkyl sulfonylbromides upon autoxidation of triethylborane (Et3B). The resultant radicals were regioselectively trapped
via addition to terminal alkynes, generating a secondary vinyl radical that selectively abstracts a Br atom from RSO2Br,
yielding the (E)-bromo vinylsulfones. Sensitivity towards Lewis basic groups was observed, presumably due to the
disruptive coordination to Et3B before atom-transfer.
Manuscript received: 7 November 2012.
Manuscript accepted: 12 December 2012.
Published online: 16 January 2013.
reductant,[13] these conditions are often high yielding.[11,13a,14]
Metal catalysts, however, limit applications due to certain
functional group incompatibility.
Introduction
Radicals offer a unique platform for synthetic organic trans-
formations as they can be generated under mild conditions and
are tolerant of many functional groups.[1,2] In particular,
sulfonyl radicals have found considerable use in synthetic
organic chemistry, specifically for their ability to add to double
and triple bonds.[3–5] We have recently utilised the chemos-
electivity of this radical addition to induce the first efficient
5-endo-dig cyclisation of a carbon centred radical.[6]
In order to investigate the mechanistic aspects of the sulfonyl
radical-induced 5-endo-dig closures more closely,[7,8] alterna-
tive modes of selective sulfonyl radical generation were
required which avoid complications caused by other approaches
(Scheme 1). In particular, photochemical homolysis of tosyl-
bromide (TsBr) led to a competing 5-endo-dig closure induced
by concomitantly generated bromine radicals. While aqueous
electrons have been used to reductively generate sulfonyl
radicals at room temperature,[9] special equipment is required
to generate g-rays for pulse radiolysis. Cu and Ru catalysts[4,10]
have been shown to produce tosyl (Ts) radicals via a redox-
transfer chain mechanism[11] from a variety of sources.[11,12]
Though sometimes requiring a stoichiometric amount of
Alternatively, sulfonyl radicals can be generated via a chain
atom-transfer process using thermal initiators. We found that
such methods resulted in low yields of the 5-endo-dig product,[6]
attributed to the unfavourable thermodynamics for addition of a
Ts radical to a terminal alkyne. Indeed, while the reaction
energies are slightly negative, the free energies of activation
for these bimolecular processes are positive (Table 1). Introduc-
tion of a radical-stabilising Ph substituent at the vinyl radical
centre has a surprisingly small effect on reaction thermo-
dynamics (4–5 kcal molꢀ1; 1 kcal molꢀ1 ¼ 4.186 kJ molꢀ1 [15]
)
and the addition step remains endergonic.
In order to alleviate the unfavourable role of entropic effects,
we sought out a method to selectively generate sulfonyl radicals
at lower temperatures.
Not only is the spontaneous reaction of triethylborane (Et3B)
with molecular oxygen (autoxidation) long known to be an
Table 1. Calculated (B3LYP/6 31Gnn) Ea and Er for sulfonyl radical
]
addition to aromatic and aliphatic alkynes
γ-Radiolysis
Photochemical
R1
O
O
O
Br
S
H
R2
γ-rays, H2O
hν
ꢃ
R2
R1
S
⋅
⋅
Br
O
O
O
S
Et3B/O2
Rꢂ
O
R
S
Br
R
S
Rꢂ
O
O
O
R
R2
R1 ¼ p-Tol
R1 ¼ Me
DEa (DGa)
[kcal molꢀ1
DEr (DGr)
[kcal molꢀ1
DEa (DGa)
[kcal molꢀ1
DEr (DGr)
[kcal molꢀ1
]
R
ΔNRꢀ ꢁ NRꢀ
MX
SO2
]
]
]
Radical addition Metal Thermal
Bu
Ph
5.4 (16.0)
3.2 (13.8)
ꢀ2.5 (10.9)
ꢀ6.5 (6.2)
4.7 (15.9)
2.3 (13.1)
ꢀ3.9 (10.0)
ꢀ7.8 (5.5)
Scheme 1. Comparison of known approaches to the generation of vinyl
radicals with the Et3B/O2-mediated approach disclosed in this work.
Journal compilation Ó CSIRO 2013
www.publish.csiro.au/journals/ajc

Sulfonyl Radicals via Triethylborane Autoxidation
337
excellent source of ethyl radicals,[16] but it can also produce
radicals efficiently at low temperatures (ꢀ788C). This method
has been utilised for reduction and cyclisation reactions
involving tributyltin hydride,[17] tris(trimethylsilyl)silane,[18]
thiols,[19] and germanes.[20] Recently, Renaud developed several
radical Et3B-mediated reactions of sulfonyl azides which should
involve the intermediate production of sulfonyl radicals.[21]
However, the reactions of R3B and sulfonyl halides have so
far received very limited attention.[22] Although an early review
by Davies and Roberts[22a] mentions the possibility of halogen
abstraction from arenesulfonyl halides by an n-butyl radical
generated from Bu3B, there is, to the best of our knowledge,
no description of this reaction in the original literature. Further-
more, Davies subsequently stated that ‘alkane- and arene-
sulfonyl halides do not react with trialkylboranes at low
temperatures in the presence of a source of free radicals.’[22b]
B3LYP/6–31G(d,p) level of theory. Reactants, products, and
intermediates were confirmed as minima and the transition state
structures were confirmed as saddle points with a single imagi-
nary frequency through frequency calculations. Coordinates,
total energies, and imaginary frequencies (for transition state
structures) can be found in the Supplementary Material.
Both p-tolyl/methyl sulfonyl bromide are readily activated
upon the addition of Et3B to a solution of monoaryl acetylene
and RSO2Br in either benzene or acetonitrile at room tempera-
ture. Under theseconditions, high yields ofE-bromovinylsulfones
are observed for both electron-rich and electron-poor aryl
acetylenes (Table 2, entries 1–5), however, the yield suffers in
the presence of Lewis basic groups on the aromatic ring,
presumably due to their coordination to the Lewis acidic borane
(vide infra).
The E-stereochemistry was established by X-ray crystallo-
graphic analysis for both the tosyl- and methylsulfonyl products
(Fig. 1). The origin of the observed stereoselectivity was
investigated by DFT calculations. For aryl alkynes, only the
Z-isomer of the intermediate vinyl radical corresponds to an
energy minimum, whereas both isomers can be optimised for the
aliphatic systems (the Z-isomer being more stable in both the
Results and Discussion
Considering the possible advantages of Et3B autoxidation,
we investigated the viability of this approach for the non-
photochemical, selective generation of sulfonyl radicals at
ambient temperatures. Terminal alkynes were chosen as a trap
for sulfonyl radicals, which regioselectively add to the external
position of the alkyne.[3a,23,24]
tosyl- and methylsulfonyl cases by 2.5 and 2.6 kcal molꢀ1
,
respectively). This geometry maximises the favourable hyper-
conjugative interaction between the radical and the s/snC-S
orbitals.[26,27] Optimised geometries of the intermediate aliphatic/
aromatic vinyl radicals show a moderate (1468, 1708) bend at the
radical position, opening the face opposite the sulfonyl to
approach by sulfonyl bromide (Fig. 2).
The equivalents of Et3B were varied with respect to
4-ethynyl toluene and TsBr (Fig. 3). Evidence that Et3B acts
as an initiator is most clearly shown by the 72 % yield obtained
with 10 mol-% Et3B loading. The high yield at this sub-
stoichiometric loading confirms the involvement of a propaga-
tion step.
Additional control experiments were run to test the radical
propagation mechanism. No bromovinylsulfones were observed
in the absence of Et3B, with or without exposure to air. Further
evidence that the reaction is initiated via a radical process is the
inhibition observed in the presence of the radical inhibitor
galvinoxyl.[28] Upon addition of Et3B (0.4equiv.) to a vigorously
stirred and open to air solution of phenylacetylene (0.07 M,
1 equiv.), TsBr (1.2 equiv.), and galvinoxyl (0.04 equiv.) in
benzene at room temperature, an inhibition period is observed
until galvinoxyl is consumed (see Supplementary Material
section). No induction period for the product formation is
observed in the absence of galvinoxyl.
Calculations were performed by using Gaussian 03
software[25] with geometries and energies obtained at the
Table 2. Yields of (E)-bromo vinyl sulfone in triethylborane-mediated
reaction of alkynes with p-tolylsulfonyl bromide or methanesulfonyl
bromide at room temperature
Entry
R
TsBr [%]A
MeSO2Br [%]B
1
Ph
99
99
99
91
98
78
49
61
43
3
p-CH3Ph
o-CH3Ph
p-OCH3Ph
p-FPh
m-Py
91
4
80
5
80
6
80
7C
8D
9D
21
87(32E)
Bu
TMS
64
A1.1 equiv. TsBr, 0.3 equiv. Et3B, 0.07 M, C6H6, 2 h.
B1.2 equiv. MeSO2Br, 0.3 equiv. Et3B, 0.07 M, CH3CN, 12 h.
C1.2 equiv. TsBr/MeSO2Br, 1.5 equiv. Et3B, 0.07 M, C6H6 (TsBr) or
CH3CN (MeSO2Br), 12 h.
D1.1 equiv. TsBr/MeSO2Br, 1.6 equiv. Et3B, 0.9 M, C6H6 (TsBr) or CH3CN
(MeSO2Br), 12 h.
E1.2 equiv. TsBr/MeSO2Br, 3.0 equiv. Et3B, 0.07 M, C6H6 (TsBr), 2 h.
Fig. 1. X-ray structures of (E)-1-(1-bromo-2-(4-methylphenylsulfonyl)vinyl)-4-methylbenzene (left) and (E)-1-(1-bromo-2-
(methylsulfonyl)vinyl)-4-methylbenzene (right).

338
K. Gilmore et al.
R1 ꢁ Bu
145.7ꢄ
R1 ꢁ Ph
169.6ꢄ
Fig. 2. Optimised geometries of b-sulfonyl vinyl radicals at the B3LYP-6–31G(d,p) level of theory.
sulfinate[29] was observed as a byproduct at 808C. Formation
of this species can be taken as evidence of a complex between
Et3B and the lone pair of either an oxygen or the bromine of
sulfonyl bromide. Presently, the role of this complex in the
reaction is unclear. However, the decrease in yield in the
presence of Lewis basic groups in the starting material may
be explained via the formation of similar complexes, which
could competitively coordinate to the borane and hinder the
formation of reactive species originating from the autoxidation
of Et3B.
100
90
80
70
60
50
40
30
20
10
0
10 mol-%, 72 % yield
Br
Et3B
ꢃ TsBr
p-Tol
0.07 M PhH
rt
p-Tol
Ts
Conclusion
In conclusion, we disclose a convenient method for the gener-
ation of sulfonyl radicals upon autoxidation of triethylborane[30]
in the presence of sulfonyl bromides. These sulfonyl radicals
can be regioselectively trapped by terminal alkynes. The sta-
bility of the intermediate vinyl radical determines the efficiency
of this process. Aromatic substituents stabilise the radical centre
and slow down the unproductive b-scission pathway. However,
even alkyl/silyl substituted alkynes can be converted into bro-
movinylsulfones in moderate yields with proper choice of
conditions. The reaction is negatively affected by the presence
of Lewis basic groups due to a competing complexation with
Et3B. While this limitation has prevented us from applying this
method in the kinetic and mechanistic studies of the diynes in
our previous work,[6] the facility of this method for sulfonyl
radical generation adds a useful tool for radical transformations.
0
10
20
30
40
50
60
70
80
90
Et3B [mol-%]
Fig. 3. The percent yield of product (diamond) and percent TsBr remaining
(triangles) as a function of equivalents Et3B. Yields were calculated relative
to a 1H NMR internal standard (benzyl phenyl ether).
Table 3. Temperature effect on the yield of the corresponding
(E)-bromovinylsulfone from 1-hexyne
EntryA
Temperature
Yield [%]B
1
2
3
ꢀ788C
08C
808C
18
38
45
AReaction conditions: 1-hexyne, 1.2 equiv. TsBr, 3 equiv. Et3B, 0.07 M in
PhCH3, 1 h.
BBased on 1H NMR using benzyl phenyl ether as an internal standard.
Supplementary Material
General procedures, NMR spectra of products, geometries and
energies of all reactants, products, and transition states are
available on the Journal’s website.
The reaction is less efficient for alkyl/silyl substituted
alkynes. For example, a significantly lower yield is observed
for R ¼ Bu compared with R ¼ Ph under identical concentra-
tions (32 and 99 %, respectively). However, the yield of bro-
mosulfone (R ¼ Bu) increases to 87 % when the concentration of
the alkyne is increased from 0.07 to 0.9 M keeping [Et3B]
constant (see Supplementary Material section). This observa-
tion confirms that the lower yields of vinyl sulfones upon
reaction with aliphatic/silyl substituted alkynes are due to an
unfavourable competition for the fate of the vinyl radical
between b-scission (loss of Ts radical) with Br atom transfer.
This experimental observation is fully consistent with the more
positive calculated DG values for the formation of alkyl substi-
tuted vinyl radicals (Table 1). Because the trapping of vinyl
radicals by reaction with RSO2Br is bimolecular, its importance
increases at the higher concentrations of the two reagents.
A moderate effect of temperature on yield was observed for
1-hexyne at 0.07 M (Table 3). The expected increase in product
formation at low temperatures, due to the entropic penalty
imposed by a bimolecular reaction, was not observed, possibly
due to enthalpic reasons (Table 1). Surprisingly, 4-methylbenzene
Acknowledgement
Financial support by NSF (Grant CHE-1152491) is gratefully
acknowledged.
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as follows from the relative energies of n-snC-S (17.8 and
9.0 kcal molꢀ1 for a and b spins, respectively) and sC-S -n (,0.5
and 6.6) interactions (for the adduct of tosyl radical and 1-hexyne). The
vinyl radical of the phenylacetylene additions displayed an unusual
Lewis structure (hypervalent carbon) which precluded the analysis of
hyperconjugative interactions.
Cyclisationsofbis-allenes:(b)S.-K. Kang, Y.-H. Ha,D.-H. Kim,Y. Lim,
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[28] G. W. Kabalka, H. C. Brown, A. Suzuki, S. Honma, A. Arase, M. Itoh,
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[30] Et3B/O2-induced thioyl radical addition to alkenes has also been
reported, see: H. Rahaman, M. Ueda, O. Miyata, T. Naito, Org. Lett.
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¨
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