Catalyzed hydroboration of allyl sulfonamides

Article abstract of DOI:10.1016/S0022-328X(03)00260-2

The hydroboration of allyl sulfonamides (4-H₃CC₆ H₄SO₂NRCH₂CH=CH₂: R=H, 1; Ph, 2; Bz, 3) with catecholborane (HBcat) using different rhodium catalysts has been examined using multinuclear NMR spectroscopy. Reactions give complex product distributions, regardless of the choice of catalyst, arising from a competing isomerization reaction. This isomerization reaction can be used with N-substituted allyl sulfonamides 2 and 3 to give the corresponding enamines (4-H₃ CC₆H₄SO₂CH=CH₂ CH₃), which in turn react with HBcat to give regioselective formation of one isomer (4-H₃CC₆H₄ SO₂NRCH₂CH₂(Bcat)CH₃).

Full text of DOI:10.1016/S0022-328X(03)00260-2

Journal of Organometallic Chemistry 680 (2003) 143ꢁ147
/
www.elsevier.com/locate/jorganchem
Catalyzed hydroboration of allyl sulfonamides
Michael G. Hamilton, Catrin E. Hughes, Alison M. Irving, Christopher M. Vogels,
Stephen A. Westcott *
Department of Chemistry, Mount Allison University, Sackville, NB, Canada E4L 1G8
Received 18 February 2003; received in revised form 26 March 2003; accepted 26 March 2003
Dedicated to Professor M.F. Hawthorne on the occasion of his 75th birthday
Abstract
The hydroboration of allyl sulfonamides (4-H3CC6H4SO2NRCH2CHÄ
/
CH2: RꢀH, 1; Ph, 2; Bz, 3) with catecholborane (HBcat)
/
using different rhodium catalysts has been examined using multinuclear NMR spectroscopy. Reactions give complex product
distributions, regardless of the choice of catalyst, arising from a competing isomerization reaction. This isomerization reaction can
be used with N-substituted allyl sulfonamides 2 and 3 to give the corresponding enamines (4-H3CC6H4SO2CHÄ
/
CH2CH3), which in
turn react with HBcat to give regioselective formation of one isomer (4-H3CC6H4SO2NRCH2CH2(Bcat)CH3).
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Hydroboration; Catalysis; Isomerization; Sulfonamides; Aminoboron; Rhodium
1. Introduction
Markovnikov product (ArCH(Bcat)CH3), depending
upon the choice of rhodium catalyst used to affect this
transformation (Scheme 1) [3]. The unusual Markovni-
kov product is believed to arise when the rhodium centre
can best stabilize a benzylic intermediate during the
catalytic cycle.
Although a considerable amount of research has
focused on the catalyzed hydroboration of simple
unsaturated hydrocarbon systems, much less is known
about analogous reactions with heteroatom-containing
The hydroboration of alkenes and alkynes, which
constitutes the addition of a BÃ
carbonÃcarbon multiple bond, is an extremely impor-
tant reaction in organic synthesis [1]. Although simple
boron hydride reagents such as borane (H3B×X, where
/
H bond across a
/
/
X is a Lewis-base) and 9-borabicyclo[3.3.1]nonane react
readily with alkenes at room temperature, hydrobora-
tions with catecholborane (HBcat, catꢀ1,2-O2C6H4)
/
substrates [3,15ꢁ
hydroborations of phenyl vinyl sulfide (PhSCHÄ
and phenyl vinyl sulfone (PhSO2CHÄCH2) with HBcat
/
22]. For instance, rhodium catalyzed
generally require elevated temperatures. The discovery
that transition metals can be used to catalyze the
addition of HBcat to organic substrates has become an
/
CH2)
/
give the unusual Markovnikov addition products,
PhSCH(Bcat)CH3 and PhSO2CH(Bcat)CH3, respec-
tively [16]. Likewise, hydroborations of allyl phenyl
important and well-established technique [2ꢁ15]. These
/
reactions can have regio-, chemo-, or stereoselectivities,
complementary, or more remarkably, opposite to those
from products obtained via the uncatalyzed variant. For
sulfone (PhSO2CH2CHÄ/CH2) using Wilkinson’s cata-
lyst and HBcat were reported to give PhSO2CH2-
CH(OH)CH3 upon oxidative work-up with NaOH/
H2O2 [22]. These results, along with our interest in
generating novel biologically-active boron compounds,
prompted us to investigate the metal catalyzed hydro-
example, hydroborations of styrenes (ArCHÄ
HBcat proceed to give selectively either the expected
anti-Markovnikov product (ArCH2CH2Bcat) or the
/CH2) with
boration of allyl sulfonamides (ArSO2NRCH2CHÄ
CH2) with HBcat using multinuclear NMR spectro-
scopy, the results of which are presented herein.
/
* Corresponding author. Tel.:
3642313.
E-mail address: swestcott@mta.ca (S.A. Westcott).
ꢂ
/
1-506-3642372; fax:
ꢂ/1-506-
0022-328X/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00260-2

144
M.G. Hamilton et al. / Journal of Organometallic Chemistry 680 (2003) 143ꢁ147
/
Scheme 1. The hydroboration of styrene with HBcat.
2. Results and discussion
observed in these catalyzed reactions [3,24], an excess of
HBcat was used to ensure complete conversion of the
starting allyl sulfonamide. All attempts to improve
regioselectivities using a number of different rhodium
catalyst precursors proved unsuccessful.
The ability to make new organosulphur derivatives is
of significant importance as these compounds have
found extensive application in medicine over the past
30 years [23]. For instance, a major milestone in science
this past century has been the discovery of the anti-
bacterial properties of penicillin and other sulfonamide
drugs. We therefore decided to investigate the hydro-
boration of allyl sulfonamides as a novel route of
generating boron-containing organosulphur com-
pounds.
Interesting, however, is the observation that the NÃ
/
H
bond has reacted with an equivalent of HBcat to form a
new NÃ
/
Bcat bond and, presumably, H2. A peak at 25
ppm in the 11B{1H}-NMR spectra is attributed to this
new aminoboron species. This reaction represents an
example of the rhodium catalyzed addition of a BÃ
/H
bond to an amine [17] and suggests that product 1c
could therefore arise from a competing rhodium cata-
lyzed hydrogenation reaction of the starting allyl
sulfonamide 1.
Sulfonamides 1ꢁ
toluenesulfonyl chloride to the corresponding allyla-
mine, RHNCH2CHÄCH2 (RꢀH, Ph, CH2Ph). Hydro-
borations of 4-H3CC6H4SO2NHCH2CHÄCH2 (1) with
/
3 were prepared by the addition of p-
/
/
/
More remarkable, however, is the observation that
catalyzed reactions using a deficiency of HBcat (1
mol%) quantitatively converted 1 into 4-H3CC6H4-
HBcat proceeded at room temperature to give the
expected anti-Markovnikov product 4-H3CC6H4-
SO2NHCH2CH2CH2Bcat after 2 weeks. It is interesting
to note that, under these conditions, HBcat failed to
SO2NÄ
ably generated by a catalyzed isomerization of 1 [24,25],
using a putative RhÃH species that forms when HBcat
/
CHCH2CH3 (1d). This sulfonimine is presum-
react with the NÃ/H bond in 1 [17]. Reactions using a
/
catalytic amount (1 mol%) of RhCl(PPh3)3 and an
excess of HBcat (5 equiv) gave a mixture of both 4-
H3CC6H4SO2N(Bcat)CH2CH2CH2Bcat (1a) and 4-
H3CC6H4SO2N(Bcat)CH2CH(Bcat)CH3 (1b) along
with major amounts (ca. 80%) of hydrogenation product
1c (Scheme 2), as monitored by multinuclear NMR
spectroscopy [24]. As degradation of HBcat is often
adds to the rhodium centre during the catalytic cycle [2].
We have found that 1d could also be generated
exclusively by the addition of a catalytic amount of
RhH(PPh3)4 to 1 in the absence of HBcat. Subsequent
addition of 1 equiv of HBcat can therefore be used, in
either case, to give ‘hydrogenation’ product 1c as the
electron deficient boryl group adds to the electron rich
Scheme 2. The rhodium catalyzed hydroboration of allyl sulfonamide 1 with HBcat.

M.G. Hamilton et al. / Journal of Organometallic Chemistry 680 (2003) 143ꢁ
/147
145
Scheme 3. Isomerization/hydroboration of allyl sulfonamide 1.
imine nitrogen. It is therefore also plausible that the
unexpected hydroboration product 1b arises as a result
of a metal catalyzed addition of HBcat to the transient
enamine, which is produced in the first step of the
isomerization process (Scheme 3).
Although we were unable to control selectivities in
reactions with 1, we decided to investigate hydrobora-
tions with N-phenyl substituted allyl sulfonamide 2
where isomerization to an imine is not possible.
regioselectivities of 90% for the expected anti-Markov-
nikov product were obtained (Table 1, entry 5) when the
neutral catalyst system [RhCl(coe)2]2/2PPh3 (coeꢀcis-
/
cyclooctene) was used to affect this reaction. On the
other hand, the Markovnikov isomer could be generated
1
in 75% yield (based on H-NMR spectroscopy) if an
excess of phosphine was used with Wilkinson’s catalyst
(entry 4).
These selectivities appear to correlate with the com-
peting isomerization reaction as we were able to
generate the Markovnikov isomer 2b in high yields
using a tandem isomerization/hydroboration catalytic
Mixtures of both hydroboration products 2aꢁb were
/
once again obtained under catalytic conditions using a
variety of rhodium complexes (Table 1). For instance,
Table 1
The rhodium catalyzed hydroboration of sulfonamide 2 with HBcat
Entry
Allyl
Catalyst system a
Solvent
2a b
2b c
1
2
2
2
2
2
2
2
2
2
2
2
3
RhCl(PPh3)3
RhCl(PPh3)3
RhCl(PPh3)3
C6D6
45
50
35
25
90
85
90
85
45
80
0
55
50
65
75
10
15
10
15
55
20
100 d
10
2
THF-d8
CD2Cl2
CDCl3
CDCl3
CDCl3
THF-d8
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
3
4
RhCl(PPh3)3ꢂ
[Rh(coe)2Cl]2ꢂ
[Rh(coe)2Cl]2ꢂ
[Rh(cod)Cl]2ꢂ
Rh(acac)(coe)2
Rh(acac)(coe)2ꢂ
Rh(acac)(coe)2ꢂ
/
5PPh3
5
/
2PPh3
4PPh3
6
/
7
/AgBF4ꢂdppe
/
8
9
/
dppm
dppb
10
11
12
/
RhH(PPh3)4
RhCl(PPh3)3
90
a
acacꢀ
/
Acetylacetonato; codꢀ/cis-cyclooctadiene; coeꢀ/cis-cyclooctene; dppbꢀ/1,4-bis(diphenylphosphino)butane; dppeꢀ/1,2-bis(diphenyl-
phosphino)ethane; dppmꢀ
/
1,1?-bis(diphenylphosphino)methane.
Yields were ascertained using 1H-NMR spectroscopy.
b
c
All reactions contained minor amounts (B
/5%) hydrogenation product.
d
Tandem reaction involving complete isomerization of the allyl sulfonamide to an enamine, followed by addition of HBcat.

146
M.G. Hamilton et al. / Journal of Organometallic Chemistry 680 (2003) 143ꢁ147
/
(q), quintet (quint), sextet (sext), multiplet (m), broad
(br), and overlapping (ov).
3.2. General procedure for the hydroboration reaction
Under an atmosphere of dinitrogen, 5 equiv of HBcat
in 0.5 ml of the appropriate deuterated solvent were
added to a 0.5 ml solution of the catalyst (1 mol%) and
substrate. The reaction was allowed to proceed at room
temperature (r.t.) for 5 h, at which point NMR data
were collected.
Scheme 4. Isomerization/hydroboration of allyl sulfonamides 2ꢁ3.
/
3.3. Rhodium catalyzed hydroboration of 1 with HBcat
reaction with RhH(PPh3)4 (entry 11). In these reactions,
an initial catalyzed isomerization gave enamine inter-
mediate 2d, whereupon subsequent addition of HBcat
using the same catalyst gave selective formation of 2b
(by multinuclear NMR spectroscopy). Small amounts of
1H-NMR (CDCl3): dꢀ
(2nd order m, CH2CH(Bcat)CH3, 1b), 3.69 (t, Jꢀ
CH2CH2CH2Bcat, 1a), 3.50 (t, Jꢀ7 Hz, CH2CH2CH3,
1c), 2.40 (s, ArÃCH3), 2.38 (s, ArÃCH3), 2.37 (s, ArÃ
CH3), 2.09 (app quint, Jꢀ7 Hz, CH2CH2CH2Bcat, 1a),
1.72 (ov m, CH2CH(Bcat)CH3, 1b, and CH2CH2CH3,
1c), 1.31 (t, Jꢀ7 Hz, CH2CH2CH2Bcat, 1a), 1.23 (d,
Jꢀ7 Hz, CH2CH(Bcat)CH3, 1b), 0.91 (t, Jꢀ7 Hz,
CH2CH2CH3, 1c); 11B{1H}-NMR (CDCl3): dꢀ
29.1
/
7.94ꢁ/7.03 (ov m, Ar), 3.87
/7 Hz,
/
/
/
/
hydrogenation product (B5%) were also observed in
/
/
these tandem reactions (Scheme 4).
This tandem catalytic reaction also worked for benzyl
substituted sulfonamide 3, albeit hydrogenation was
more severe (ca. 20%) and reaction times were consider-
ably longer. For instance, isomerization of 2 was usually
complete within 1 day at room temperature but analo-
gous reactions with the benzyl derivative required times
of up to 1 week. This slow isomerization reaction is
consistent with the ‘hydroboration’ results of 3 (Table 1,
entry 12), which gave the anti-Markovnikov product 3a
in 90% yield, where isomerization/hydroboration is too
slow to give any appreciable amounts of Markovnikov
isomer 3b.
Asymmetric variants of this tandem catalytic reaction
will provide an avenue to generate enantiomerically
enriched sulfonamides containing boronate esters. This
work, along with assessing the anti-fungal and anti-
bacterial activities of the resulting organosulphur com-
plexes, is currently in progress and will be reported in
due course.
/
/
/
/
(1a), 27.9 (1b), 24.6 (NBcat, 1a and 1b), 21.3 (B2cat3).
3.4. Rhodium catalyzed hydroboration of 2 with HBcat
1H-NMR (CDCl3): dꢀ
(2nd order m, CH2CH(Bcat)CH3, 2b), 3.64 (t, Jꢀ
CH2CH2CH2Bcat, 2a), 2.41 (s, ArÃCH3), 2.40 (s, ArÃ
CH3), 1.76 (app quint, Jꢀ7 Hz, CH2CH2CH2Bcat, 2a),
1.67 (app sext, Jꢀ7 Hz, CH2CH(Bcat)CH3, 2b), 1.34 (t,
Jꢀ7 Hz, CH2CH2CH2Bcat, 2a), 1.28 (d, Jꢀ7 Hz,
CH2CH(Bcat)CH3, 2b); 11B{1H}-NMR (CDCl3): dꢀ
/
7.48ꢁ/6.95 (ov m, Ar), 3.83
/7 Hz,
/
/
/
/
/
/
/
34.8 (2a), 28.2 (2b), 21.3 (B2cat3).
3.5. Rhodium catalyzed hydroboration of 3 with HBcat
1H-NMR (CDCl3): dꢀ
NCH2Ph), 4.32 (s, NCH2Ph), 3.38 (2nd order m,
CH2CH(Bcat)CH3, 3b), 3.19 (t, Jꢀ Hz,
CH2CH2CH2Bcat, 3a), 2.43 (s, ArÃCH3), 2.41 (s, ArÃ
CH3), 1.69 (app quint, Jꢀ7 Hz, CH2CH2CH2Bcat, 3a),
1.34 (app sext, Jꢀ7 Hz, CH2CH(Bcat)CH3, 3b), 1.11 (t,
Jꢀ7 Hz, CH2CH2CH2Bcat, 3a), 1.03 (d, Jꢀ7 Hz,
CH2CH(Bcat)CH3, 3b); 11B{1H}-NMR (CDCl3): dꢀ
/
7.74-7.04 (ov m, Ar), 4.36 (s,
/7
/
/
3. Experimental
/
/
3.1. General
/
/
/
Reagents and solvents were purchased from Aldrich
Chemicals and used as received. Rhodium catalysts
RhCl(PPh3)3 [26], [RhCl(coe)2]2 [27], [RhCl(cod)]2 [28],
and Rh(acac)(coe)2 [29], were prepared by established
procedures. NMR spectra were recorded on a JEOL
JNM-GSX270 FT NMR (1H 270 and 11B 87 MHz)
spectrometer. Chemical shifts (d) are reported in ppm
34.1 (3a), 29.4 (3b), 21.3 (B2cat3).
3.6. General procedure for the isomerization/
hydroboration reaction
Under an atmosphere of dinitrogen, the allyl sulfo-
namide in 0.5 ml of the appropriate deuterated solvent
was added to a 0.5 ml solution of RhH(PPh3)4 (5 mol%).
The reaction was allowed to proceed at r.t. for 1 (2d) to
7 (3d) days, at which point NMR data were collected.
[relative to internal Me4Si (1H) or external BF3×
/
OEt2
(11B)] and coupling constants (J) in Hz. Multiplicities
are reported as singlet (s), doublet (d), triplet (t), quartet

M.G. Hamilton et al. / Journal of Organometallic Chemistry 680 (2003) 143ꢁ
/147
147
Upon conversion to the enamine, 3 equiv of HBcat in
0.5 ml of the appropriate deuterated solvent were added.
The reaction was allowed to proceed at r.t. for 5 h, at
which point NMR data were collected.
technical assistance and anonymous reviewers for help-
ful comments.
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Hz, 3H, NCHÄCHCH3).
/
7.54ꢁ
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6.87 (ov m, 10H, Ar and
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¨
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(2nd order m, 2H, CH2CH(Bcat)CH3), 2.41 (s, 3H, ArÃ
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1.28 (d, Jꢀ
7 Hz, 3H, CH2CH(Bcat)CH3); 11B{1H}-
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/
7.48ꢁ
/
6.95 (ov m, Ar), 3.83
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7.32ꢁ7.22 (ov m, 7H, Ar), 6.66 (d d, Jꢀ
NCH ÄCHCH3), 4.70 (ov d q, Jꢀ14 Hz, 1H, NCHÄ
CHCH3), 4.46 (s, 2H, NCH2Ph), 2.42 (s, 3H, ArÃCH3),
1.54 (d d, Jꢀ7, 1 Hz, 3H, NCHÄCHCH3).
/
7.68 (d, Jꢀ
/
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/
/
14, 1 Hz, 1H,
/
/
/
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1H-NMR (CDCl3): dꢀ
2H, NCH2Ph), 3.38
CH2CH(Bcat)CH3), 2.43 (s, 3H, ArÃ
sext, Jꢀ7 Hz, 1H, CH2CH(Bcat)CH3), 1.03 (d, Jꢀ
Hz, 3H, CH2CH(Bcat)CH3); 11B{1H}-NMR (CDCl3):
dꢀ29.4 (CBcat), 21.3 (B2cat3).
/
7.74ꢁ
/
7.04 (ov m, Ar), 4.32 (s,
order m, 2H,
CH3), 1.69 (app
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/
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Acknowledgements
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Thanks are gratefully extended to the Natural Science
and Engineering Research Council of Canada, Amer-
ican Chemical Society-Petroleum Research Fund (Grant
#37824-B1), Canada Research Chairs Programme, Ca-
nadian Foundation for Innovation/Atlantic Innovation
Fund, and Mount Allison University for financial
support. We also wish to thank Dan Durant, Roger
Smith, and Dr Stephen J. Duffy for their expert
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