Metal catalysed hydroboration of vinyl sulfides, sulfoxides, sulfones, and sulfonates

Article abstract of DOI:10.1016/j.molcata.2007.05.026

The hydroboration of phenyl vinyl sulfide with catecholborane (HBcat) and pinacolborane (HBpin) has been examined with a number of rhodium complexes, all of which proceed with excellent regiocontrol in favour of the branched product PhSCH(B(OR)₂)CH₃. The corresponding linear product can be obtained exclusively in reactions employing [Cp*IrCl₂]₂ and HBcat. Catalysed hydroborations of (E)-2-(p-toluenethio)styrene with HBcat using Rh(acac)(dppp) gave predominant formation of one product while reactions using HBpin afforded several products arising from a competing C-S bond cleavage (acac = acetylacetonato, dppp = 1,3-bis(diphenylphosphino)propane). Although reactions of phenyl vinyl sulfoxide were complicated by a competing deoxygenation reaction, hydroborations of phenyl vinyl sulfone using HBcat once again gave regioselective formation of either the branched or linear products, depending on the choice of catalyst used to effect this transformation. Catalysed hydroborations of phenyl vinyl sulfonate were less chemo- and regioselective, yielding hydrogenation and diboration products in addition to the two hydroboration product isomers.

Full text of DOI:10.1016/j.molcata.2007.05.026

Journal of Molecular Catalysis A: Chemical 275 (2007) 91–100
Metal catalysed hydroboration of vinyl sulfides, sulfoxides,
sulfones, and sulfonates
Jonathan D. Webba, Daniel J. Harrisona, David W. Normana, Johanna M. Blacquierea,
Christopher M. Vogelsa, Andreas Deckenb, Craig G. Batesc, D. Venkataramanc,
R. Thomas Bakerd,∗, Stephen A. Westcotta,∗∗
a Department of Chemistry, Mount Allison University, Sackville, NB E4L 1G8, Canada
b Department of Chemistry, University of New Brunswick, Fredericton, NB E3B 5A3, Canada
c Department of Chemistry, University of Massachusetts, 710 North Pleasant Street,
Amherst, MA 01003, USA
d Chemistry Division, MS J582, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Received 24 March 2007; received in revised form 16 May 2007; accepted 18 May 2007
Available online 24 May 2007
Abstract
The hydroboration of phenyl vinyl sulfide with catecholborane (HBcat) and pinacolborane (HBpin) has been examined with a number of rhodium
complexes, all of which proceed with excellent regiocontrol in favour of the branched product PhSCH(B(OR)2)CH3. The corresponding linear
product can be obtained exclusively in reactions employing [Cp*IrCl2]2 and HBcat. Catalysed hydroborations of (E)-2-(p-toluenethio)styrene with
HBcat using Rh(acac)(dppp) gave predominant formation of one product while reactions using HBpin afforded several products arising from a
competing C–S bond cleavage (acac = acetylacetonato, dppp = 1,3-bis(diphenylphosphino)propane). Although reactions of phenyl vinyl sulfoxide
were complicated by a competing deoxygenation reaction, hydroborations of phenyl vinyl sulfone using HBcat once again gave regioselective
formation of either the branched or linear products, depending on the choice of catalyst used to effect this transformation. Catalysed hydroborations
of phenyl vinyl sulfonate were less chemo- and regioselective, yielding hydrogenation and diboration products in addition to the two hydroboration
product isomers.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Boronate esters; Catalysis; Hydroboration; Regioselectivity; Vinyl sulfides
1. Introduction
als can be used to catalyse the addition of HBcat to substrates has
become an important and well-established technique in organic
synthesis [2–6]. These reactions can have regio-, chemo-, and
stereoselectivities that are complementary, or more remark-
ably, opposite to those obtained via the uncatalysed variant.
For example, hydroboration of vinyl arenes (ArCH CH2) with
HBcat give selectively either the expected linear boronate ester
products (ArCH2CH2Bcat) or the branched boronate ester prod-
ucts (ArCH(Bcat)CH3), depending upon the choice of catalyst
used in these reactions (Scheme 1) [3]. While many transi-
tion metals can be used to catalyse these reactions, rhodium
complexes are the most common and synthetically useful. A
plausible mechanism for these reactions involves initial oxida-
tive addition of HBcat [7] followed by coordination of the alkene
to the metal centre, with subsequent insertion of the alkene
into either the Rh–H [8] or the Rh–B bond [9], and reductive
The hydroboration of alkenes and alkynes, which constitutes
the addition of a B–H bond across a carbon–carbon multiple
bond, isaremarkablyimportantreactioninorganicsynthesis [1].
Although simple boron hydride reagents such as borane (H3B·X,
where X is a Lewis base) and 9-borabicyclo [3.3.1] nonane
react readily with alkenes at room temperature, hydrobora-
tions with catecholborane (HBcat, cat = 1,2-O2C6H4) generally
require elevated temperatures. The discovery that transition met-
∗
∗∗
Corresponding author. Tel.: +1 505 667 9274; fax: +1 505 667 9905.
Corresponding author. Tel.: +1 506 364 2372; fax: +1 506 364 2313.
E-mail addresses: bakertom@lanl.gov (R.T. Baker), swestcott@mta.ca
(S.A. Westcott).
1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2007.05.026

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J.D. Webb et al. / Journal of Molecular Catalysis A: Chemical 275 (2007) 91–100
Scheme 1. The hydroboration of styrene with HBcat.
elimination to yield the desired product [10]. The branched
boronate ester product is believed to arise when the metal centre
can best stabilise a benzylic intermediate during the catalytic
cycle.
with both the 1177 injection port with a CP-8410 liquid autoin-
jector connected to an SPB-1 (Supelco) fused silica column
(30 m × 0.25 mm i.d. × 0.25 ␮m) and the 1079 solid injector
chromatoprobe, attached to a 50 cm transfer line. The GC/MS
spectrometer is controlled by the Saturn Workstation software,
Version 5.51.
Although a considerable amount of research has focused on
the catalysed hydroboration of simple unsaturated hydrocarbon
systems, much less is known about analogous reactions with
heteroatom-containing substrates [3,11–17]. For instance, catal-
ysed hydroborations of pyrrolidinyl amides with HBcat gave,
after oxidation, syn 1,3-hydroxy amides with high levels of
regio- and stereochemical control. The remarkable selectivities
observed in these reactions are believed to arise from a direct-
ing effect of the amide moiety [18]. In a similar study, Dai
and co-workers found that rhodium catalysed hydroborations
of allyl sulfones gave the branched boronate ester, where the
directing effect of the sulfone oxygen is believed to be respon-
sible for the regiocontrol in these reactions [17]. As part of our
ongoing investigation into generating biologically active boron
compounds [19], we have examined the catalysed hydrobora-
tion of vinyl sulfides, sulfoxides, sulfones, and sulfonates in
an effort to expand the scope of these reactions and to see
what effect different oxidation states of sulfur had on prod-
uct selectivities. A preliminary account of this work has been
published [16].
2.1. Preparation of 1a
A solution of HBcat (22 mg, 0.18 mmol) in C6D6 (0.5 mL)
was added dropwise to a mixture of phenyl vinyl sulfide (20 mg,
0.15 mmol) and RhCl(PPh3)3 (7 mg, 0.010 mmol) in C6D6
(0.5 mL). Thereactionwasallowedtostandfor24 hatroomtem-
perature at which point NMR spectroscopic data were collected.
1H NMR δ 1.38 (d, J = 7.2 Hz, 3H, CH3), 2.93 (q, J = 7.2 Hz, 1H,
CHB), 6.70 (m, 2H, Ar), 6.89–6.97 (ov m, 5H, Ar), 7.30 (m, 2H,
1
Ar); 13C{ H} NMR δ 16.6, 24.1 (br, CB), 112.7, 122.9, 126.9,
129.0, 131.6, 135.6, 148.3; 11B NMR δ 33.5 (br).
2. Experimental
2.2. Preparation of 2a
Reagents and solvents used were purchased from Aldrich
Chemicals and HBcat was distilled prior to use. NMR spec-
tra were recorded on a JEOL JNM-GSX270 FT spectrometer.
1H NMR chemical shifts are reported in ppm and referenced
to residual solvent protons in deuterated solvent at 270 MHz.
13C NMR chemical shifts are referenced to solvent carbon res-
onances as internal standards at 68 MHz and are reported in
ppm. 11B NMR chemical shifts are reported in ppm and are
referenced to BF3·OEt2 as an external standard at 87 MHz.
Multiplicities are reported as singlet (s), doublet (d), triplet (t),
quartet (q), multiplet (m), broad (br), and overlapping (ov). All
reactions were performed under an atmosphere of dinitrogen.
GC/MS analyses were conducted using a Varian Saturn 2000
GC/MS/MS coupled to a CP-3800 GC. The GC was equipped
A solution of HBcat (60 mg, 0.50 mmol) in C6D6 (0.5 mL)
was added dropwise to a mixture of phenyl vinyl sulfide
(62 mg, 0.46 mmol) and [Cp*IrCl2]2 (9 mg, 0.010 mmol) in
C6D6 (0.5 mL). The reaction was allowed to stand for 24 h at
room temperature at which point NMR spectroscopic data were
1
collected. H NMR δ 1.41 (t, J = 7.9 Hz, 2H, CH2B), 2.94 (t,
J = 7.9 Hz, 2H, CH2), 6.80 (m, 2H, Ar), 6.90–7.05 (ov m, 5H,
1
Ar), 7.25 (m, 2H, Ar); 13C{ H} NMR δ 11.6 (br, CB), 28.6,
112.4, 122.7, 126.0, 129.0, 129.7, 134.2, 148.5; 11B NMR δ
34.2 (br).

J.D. Webb et al. / Journal of Molecular Catalysis A: Chemical 275 (2007) 91–100
93
2.3. Preparation of 1b
A solution of HBpin (34 mg, 0.26 mmol) in C6D6 (0.5 mL)
was added dropwise to a mixture of phenyl vinyl sulfide
(30 mg, 0.22 mmol) and RhCl(PPh3)3 (9 mg, 0.010 mmol) in
C6D6 (0.5 mL). After 24 h at room temperature the solvent was
removed under reduced pressure. The residue was purified using
an alumina filter with hexane as the eluent. Yield: 40 mg (70%),
clear oil. 1H NMR δ 0.94 (s, 12H, pin), 1.41 (d, J = 7.2 Hz, 3H,
CH3), 2.85 (q, J = 7.2 Hz, 1H, CHB), 6.85–7.03 (ov m, 3H, Ar),
2.6. Preparation of 3
1
7.40 (m, 2H, Ar); 13C{ H} NMR δ 16.7, 24.1 (br, CB), 24.5,
83.6, 125.8, 128.8, 130.2, 133.5; 11B NMR δ 32.2 (br).
RhCl(PPh3)3 (0.040 g, 0.040 mmol) in toluene (1 mL) was
added to (E)-2-(p-toluenethio)styrene (200 mg, 0.88 mmol) in
toluene (3 mL). Catecholborane (160 mg, 1.33 mmol) in toluene
(1 mL) was then added dropwise and the reaction was allowed
to stand for 24 h at room temperature. Removal of solvent under
vacuum afforded an oily residue which was dissolved in hexane
(3 mL)andstoredat −30◦C. Compound3wasisolatedasawhite
solid after two recrystallizations from hexane; yield: 150 mg
1
(49%). NMR spectroscopic data (in C6D6): H NMR δ 1.98
(s, 3H, CH3), 3.20–3.42 (ov m, 3H, CH2 and CHB), 6.68–6.72
(second order m, 2H, cat), 6.77 (d, J = 7.9 Hz, 2H, Ar), 6.91–6.95
(second order m, 2H, cat), 6.97–7.05 (ov m, 3H, Ar), 7.10 (m,
2.4. General procedure for the hydroboration of ethyl vinyl
sulfide
1
2H, Ar), 7.35 (d, J = 7.9 Hz, 2H, Ar); 13C{ H} NMR δ 20.8, 32.9
A solution of HBcat (37 mg, 0.31 mmol) in C6D6 (0.5 mL)
was added dropwise to a mixture of ethyl vinyl sulfide (25 mg,
0.28 mmol) and RhCl(PPh3)3 (5 mg, 0.006 mmol) in C6D6
(0.5 mL). The reaction was allowed to stand for 24 h at room
temperature at which point NMR spectroscopic data were col-
(br, CB), 37.9, 112.5, 122.8, 126.6, 128.6, 128.9, 129.8, 131.5,
133.1, 137.5, 140.0, 148.3; 11B NMR δ 33.4 (br).
1
lected. H NMR δ 1.03 (t, J = 7.4 Hz, 3H, CH2CH3), 1.41 (d,
J = 7.4 Hz, 3H, CH(Bcat)CH3), 2.40–2.46 (ov m, 3H, CH2CH3
and CH(Bcat)CH3), 6.75–6.85 (second order m, 2H, Bcat),
1
6.95–7.05 (second order m, 2H, Bcat); 13C{ H} NMR δ 14.5,
16.1, 19.9 (br, CB), 25.4, 112.5, 122.8, 148.3; 11B NMR δ 33.4
2.7. General procedure for the hydroboration of phenyl
vinyl sulfoxide
(br).
In a typical reaction, HBcat or HBpin (0.30 mmol) was
dissolved in C6D6 (0.5 mL) and added dropwise to a mix-
ture of phenyl vinyl sulfoxide (20 mg, 0.13 mmol) and catalyst
(0.010 mmol) in C6D6 (0.5 mL). The reaction mixtures were
allowed to stand for 24 h at room temperature at which point
NMR spectroscopic data were collected.
2.5. General procedure for the hydroboration of benzyl
vinyl sulfide
2.8. Preparation of 8a
1,1ꢀ-Bis(diphenylphosphino)ferrocene (4 mg, 0.007 mmol)
was dissolved in C6D6 (0.25 mL) and added to Rh(acac)(coe)2
(3 mg, 0.007 mmol) in C6D6 (0.25 mL). This mixture was added
to a solution of phenyl vinyl sulfone (20 mg, 0.12 mmol) and
HBcat (22 mg, 0.18 mmol) in C6D6 (0.5 mL). The reaction mix-
ture was allowed to stand for 24 h at room temperature at which
point NMR spectroscopic data were collected. 1H NMR δ 1.24
(d, J = 7.4 Hz, 3H, CH3), 3.12 (q, J = 7.4 Hz, 1H, CHB), 6.69 (m,
A solution of HBcat (22 mg, 0.18 mmol) in C6D6 (0.5 mL)
was added dropwise to a mixture of benzyl vinyl sulfide (25 mg,
0.17 mmol) and RhCl(PPh3)3 (3 mg, 0.003 mmol) in C6D6
(0.5 mL). Thereactionwasallowedtostandfor24 hatroomtem-
perature at which point NMR spectroscopic data were collected.
1H NMR δ 1.34 (d, J = 7.7 Hz, 3H, CH(Bcat)CH3), 2.38 (q,
J = 7.7 Hz, 1H, CH(Bcat)CH3), 3.62 (d, J = 16.8 Hz, 2H, CH2S),
6.70–7.25 (ov m, 9H, Ar and Bcat); 13C{ H} NMR δ 15.7, 18.9
2H, Ar), 6.79–6.93 (ov m, 5H, Ar), 7.72 (m, 2H, Ar); 13C{ H}
1
1
(br, CB), 35.7, 112.5, 114.0, 122.7, 126.9, 128.4, 129.1, 138.4;
NMR δ 10.8, 49.1 (br, CB), 112.9, 123.3, 128.7, 128.8, 133.1,
139.8, 148.1; 11B NMR δ 31.4 (br).
11B NMR δ 32.9 (br).

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J.D. Webb et al. / Journal of Molecular Catalysis A: Chemical 275 (2007) 91–100
2.12. General procedure for the hydroboration of phenyl
vinyl sulfonate
In a typical reaction, HBcat or HBpin (0.18 mmol) in C6D6
(0.5 mL) was added dropwise to a mixture of phenyl vinyl sul-
fonate (20 mg, 0.11 mmol) and catalyst (0.01 mmol) in C6D6
(0.5 mL). The reaction mixtures were allowed to stand for 24 h at
room temperature at which point NMR spectroscopic data were
collected. Selected NMR data for PhOSO2CH(Bcat)CH3, 12a:
1H δ 1.62 (d, J = 7.3 Hz, 3H), 3.32 (q, J = 7.3 Hz, 1H); 13C 11.3,
43.8 ppm (br, CB); 11B 30.9 ppm. PhOSO2CH2CH2(Bcat), 13a:
2.9. Preparation of 9a
A solution of HBcat (36 mg, 0.30 mmol) in C6D6 (0.5 mL)
was added dropwise to a mixture of phenyl vinyl sulfone
(20 mg, 0.12 mmol) and [Cp*IrCl2]2 (5 mg, 0.006 mmol) in
C6D6 (0.5 mL). The reaction mixture was allowed to stand for
24 h at room temperature at which point NMR spectroscopic
data were collected. 1H NMR δ 1.26 (t, J = 7.2 Hz, 2H, CH2B),
2.88 (t, J = 7.2 Hz, 2H, CH2), 6.66–6.96 (ov m, 7H, Ar), 7.72
1H δ 1.64 (m, 2H), 3.11 (m, 2H); 13C 5.0 (br, CB), 45.7 ppm; 11
B
33.2 ppm. PhOSO2CH2CH2(Bpin), 13b:1.26(t, J = 7.8 Hz, 2H),
3.01 (t, J = 7.8 Hz, 2H). PhOSO2CH2CH(Bcat)2, 14a: 1H δ 2.58
(t, J = 8 Hz, 1H), 3.85 (d, J = 8 Hz, 2H). PhOSO2CH2CH(Bpin)2,
14b: 3.59 (d, J = 7.9 Hz, 2H). PhOSO2CH2CH3, 15: 1H δ 1.04
(t, J = 7.3 Hz, 3H), 2.61 (q, J = 7.3 Hz, 2H); 13C 7.8, 44.5 ppm.
1
(m, 2H, Ar); 13C{ H} NMR δ 4.6 (br, CB), 51.5, 112.4, 122.8,
128.4, 128.9, 133.0, 139.4, 148.3; 11B NMR δ 34.8 (br).
2.13. X-ray diffraction studies
Crystals of 9a were grown from a saturated C6D6 solution at
20 ◦C. Single crystals were coated with Paratone-N oil, mounted
using a glass fibre and frozen in the cold stream of the goniome-
ter. A hemisphere of data were collected on a Bruker AXS
P4/SMART 1000 diffractometer using ␻ and ␪ scans with a
scan width of 0.3◦ and exposure time of 30 s. The detector dis-
tance was 5 cm. The data were reduced [20] and corrected for
absorption [21]. The structures were solved by direct methods
and refined by full-matrix least squares on F2 [22]. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were located in Fourier difference maps and refined isotropically
(CCDC 642558).
2.10. General procedure for the hydroboration of phenyl
vinyl sulfone
A solution of HBcat or HBpin (0.18 mmol) in C6D6 (0.5 mL)
was added dropwise to a mixture of phenyl vinyl sulfone (30 mg,
0.18 mmol) and catalyst (3 mg, 0.003 mmol) in C6D6 (0.5 mL).
The reaction was allowed to stand for 24 h at room temperature
at which point NMR spectroscopic data were collected. Selected
NMR data for PhSO2CH2CH(Bcat)2, 10a: 1H δ 2.34 (t, J = 8 Hz,
3. Results and discussion
1
1H), 3.74 (d, J = 8 Hz, 2H). PhSO2CH2CH(Bpin)2, 10b: H δ
1.47(t, J = 7.8 Hz, 1H), 3.55(d, J = 7.8 Hz, 2H). PhSO2CH2CH3,
3.1. Vinyl sulfides
11: 1H δ 0.91 (t, J = 7.3 Hz, 3H), 2.63 (q, J = 7.3 Hz, 2H).
Boronate esters have proven to be remarkably useful inter-
mediates in organic synthesis, particularly as substrates for
metal catalysed carbon–carbon bond formation using the
Suzuki-Miyaura coupling reaction [23]. Compounds contain-
ing boronate esters, or boronic acids, have also found a number
of molecular recognition applications such as amine and sugar
sensors [24,25], saccharide transport [26,27], and as enzyme
inhibitors [28]. For instance, several research groups have shown
that ␣-aminoboron compounds are particularly selective enzyme
inhibitors for serine proteases [29–31], and related work has
shown that ␣-phosphonylboronate esters also exhibit significant
bioactivity [32]. Although some excellent chemistry has been
developed for the preparation of compounds containing both a
Lewis basic site and a Lewis acidic boronate ester group [33,34],
we have investigated the metal catalysed hydroboration of vinyl
heteroatom derivatives as a direct route to these compounds.
In this report, we have examined the hydroboration of sulfur
derivatives, owing to the wealth of bioactivity associated with
these compounds [35].
2.11. Preparation of 9b
1,4-Bis(diphenylphosphino)butane (4 mg, 0.007 mmol) was
dissolved in C6D6 (0.25 mL)and added to Rh(acac)(coe)2 (3 mg,
0.007 mmol) in C6D6 (0.25 mL). This mixture was added to a
solution of phenyl vinyl sulfone (20 mg, 0.12 mmol) and HBpin
(23 mg, 0.18 mmol) in C6D6 (0.5 mL). The reaction mixture was
allowed to stand for 24 h at room temperature at which point
1
NMR spectroscopic data were collected. H NMR δ 0.94 (s,
12H, pin), 1.14 (t, J = 7.2 Hz, 2H, CH2B), 3.02 (t, J = 7.2 Hz,
2H, CH2), 6.82–7.12 (ov m, 3H, Ar), 7.73 (m, 2H, Ar); 13C{ H}
1
NMR δ 5.0 (br, CB), 24.6, 52.1, 83.4, 128.3, 128.8, 132.7, 139.8;
11B NMR δ 33.4 (br).

J.D. Webb et al. / Journal of Molecular Catalysis A: Chemical 275 (2007) 91–100
95
Table 1
Metal catalysed hydroboration of phenyl vinyl sulfide
Entry
Catalyst precursor
Borane
1a
2a
1
2
3
4
5
6
7
8
9
RhCl(PPh3)3
HBcat
HBcat
HBcat
HBcat
HBcat
HBcat
HBpin
HBpin
HBpin
HBpin
>98
>98
>98
>98
>98
–
>98
>98
>98
–
–
–
–
–
–
>98
–
–
–
[RhCl(coe)2]2/2PPh3
Rh(acac)(coe)2/dppp
Rh(acac)(coe)2/dppb
[Rh(cod)(dppp)]BF4
[Cp*IrCl2]2
b
RhCl(PPh3)3
[RhCl(coe)2]2/2PPh3
Rh(acac)(coe)2/dppb
[Cp*IrCl2]2
10
–
a
All reactions were conducted in C6D6 at RT and product ratios were determined by 1H NMR spectroscopy and confirmed by GC/MS. Complete conversion of
starting materials was observed.
b
Reactions conducted in THF.
We have found that a number of rhodium complexes (Table 1,
entries 1–5) could be used in the hydroboration of PhSCH CH2
with HBcat to give exclusive formation of the unexpected
branched product PhSCH(Bcat)CH3 (1a). These results are par-
ticularly intriguing as this predominant regioselectivity has only
been observed in hydroborations of vinyl arenes [36–42] and
fluoroalkenes [43]. Like the fluorinated examples, the directing
effect of the sulfur heteroatom is presumably responsible for this
present selectivity. Of singular interest are reactions using the
catalyst system [RhCl(coe)2]2/2PPh3 (coe = cis-cyclooctene)
which gave the branched product selectively (entry 2), as anal-
ogous hydroborations of vinyl arenes using this catalyst are
known to give predominant formation of the corresponding lin-
ear boronate ester products [44]. It is also interesting to note
that reactions with pinacolborane (HBpin; pin = 1,2-O2C2Me4)
and a rhodium catalyst (entries 7–9) also gave selective for-
mation of the branched product PhSCH(Bpin)CH3 (1b). An
interesting paper by Crudden et al. has shown that cyclooc-
tadiene rhodium complexes can be used in the hydroboration
of vinyl arenes with HBpin to give branched products [37].
Reactions of vinyl arenes with iridium catalysts and HBpin,
however, give predominantly the corresponding linear prod-
ucts [45]. Unlike the Bcat derivatives, pinacol boronate ester
compounds are of considerable interest owing to their inher-
ent air and moisture stability [46,47]. Selective formation of
the linear boronate ester product PhSCH2CH2Bcat (2a) was
only obtained in reactions using the iridium catalyst precursor
[Cp*IrCl2]2 (Cp* = ␩5-C5Me5, entry 6). While this isomer also
slowly formed at low conversions by heating HBcat and phenyl
vinyl sulfide at 80 ◦C, extensive nucleophilic degradation of the
borane was observed as the reaction proceeded [48,49]. Previous
studies using this Ir(III) complex have shown that hydrobora-
tions with HBcat always proceed with great selectivity to give
linear boronate ester products [50]. It is possible that these
reactions may proceed via a mechanism unlike those associ-
ated with hydroborations using a rhodium catalyst. A plausible
mechanism for these iridium catalysed reactions is shown in
Scheme 2 and is similar to one reported for the lanthanum catal-
ysed hydroboration of alkenes, which also gives linear products
selectively [51]. Reactions of HBpin using [Cp*IrCl2]2, how-
ever, gave products arising from competing cleavage of the
C–S bond (entry 10). The cleavage of C–S bonds by late met-
als is an important step in the catalytic hydrodesulfurization
reaction [52].
Other primary vinyl sulfides were also investigated
(PhCH2SCH CH2 and CH3CH2SCH CH2) in the rhodium
catalysed hydroboration with HBcat, and these exam-
ples also gave selective formation of the corresponding
branched products. These results prompted us to investi-
gate the catalysed hydroboration of substituted vinyl sulfide
derivatives, prepared by coupling vinyl iodides and thiols
using a well-defined copper(I) catalyst [53]. Hydrobora-
tions of (E)-2-(p-toluenethio)styrene with HBcat using either
Scheme 2. A possible alternate mechanism for the [Cp*IrCl2]2 catalysed
hydroboration of alkenes.

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J.D. Webb et al. / Journal of Molecular Catalysis A: Chemical 275 (2007) 91–100
Table 2
Metal catalysed hydroboration of (E)-2-(p-toluenethio)styrene
Entry
Catalyst precursor
Borane
3a
4a
5a
6a
1
2
3
4
5
6
Rh(acac)(coe)2/dppp
RhCl(PPh3)3
[Cp*IrCl2]2
RhCl(PPh3)3
Rh(acac)(coe)2/dppp
[Cp*IrCl2]2
HBcat
HBcat
HBcat
HBpin
HBpin
HBpin
90
90b
45
–
–
–
–
–
35
–
–
10
10
–
80
50
60
–
–
20
20
50
40
–
a
All reactions were conducted in C6D6 at RT and product ratios were determined by 1H NMR spectroscopy and confirmed by GC/MS. Complete conversion of
starting materials was observed.
b
Compound 3 was isolated in 49% yield.
a catalytic amount of RhCl(PPh3)3 (Table 2, entry 2)
or Rh(acac)(coe)2/dppp (acac = acetylacetonato; dppp = 1,3-
bis(diphenylphosphino)propane) [44] gave significant amounts
3.3. Phenyl vinyl sulfone
We then decided to examine the catalysed hydroboration of
phenyl vinyl sulfone, where the oxygen atoms are considerably
less basic than those in related sulfoxides [59]. Interestingly,
addition of HBcat to phenyl vinyl sulfone at 80 ◦C slowly
affords the branched isomer 8a indicating that the sulfone
group is capable of functioning as a directing group in the
uncatalysed reaction. A number of rhodium and iridium com-
plexes were investigated as potential catalysts and the results
are illustrated in Table 3. Although deoxygenation was not
a problem in these reactions, generation of diborated prod-
ucts 10 and hydrogenation product 11 complicated product
distributions. Compound 10 presumably arises from initial for-
mation of the vinyl boronate esters PhSO2CH CHB(OR)2
followed by subsequent addition of either HBcat or HBpin.
Vinyl boronate esters are frequently observed as products in
catalysed ‘hydroboration’ reactions [60–64], and are believed
to form by a competing dehydrogenative borylation reaction.
This reaction is believed to involve the insertion of the alkene
into the Rh–B bond, and not the Rh–H bond, followed by
a ␤-hydride elimination step to give the corresponding vinyl
boronate ester and dihydrogen. In a previous study, high yields
of vinyl boronate esters were formed in the catalysed addition
of HBpin to vinyl arenes using a number of rhodium and ruthe-
nium cyclooctadiene complexes [60]. We were able to generate
the branched isomer PhSO2CH(Bcat)CH3 (8a) with selectivity
of up to 98% in reactions using Rh(acac)(dppf) (dppf = 1,2-
bis(diphenylphosphino)ferrocene) (entry 1). Unlike reactions
with the vinyl sulfide, other rhodium catalysts gave a mixture
of products, demonstrating that the sulfone group is a much
less effective directing group. Conversely, using the iridium cat-
alyst [Cp*IrCl2]2 with HBcat (entry 8), the linear isomer 9a
could once again be generated as the only new boron containing
product.
1
(ca. 90% by H NMR spectroscopy) of product 3 (entry 1).
Unequivocal assignment of 3 was accomplished through the
1
interpretation of H, 13C, 135◦DEPT, gHMQC, and gHMBC
NMR spectra [54]. The key long-range correlation that shows
the regiochemistry is the CH2 carbon at δ 37.9 ppm with
the aryl hydrogens of the phenyl group at δ 6.97–7.05 ppm.
Once again, however, all reactions with HBpin (entries 4–6)
were complicated by a competing C–S bond cleavage and
gave products arising from hydroboration of the liberated
styrene. Similar reactivities were observed for other 1,2-
disubstituted vinyl sulfides (trans-PhSCH CHC(O)OEt and
trans-PhSCH CHiPr).
3.2. Phenyl vinyl sulfoxide
The rhodium catalysed addition of HBcat and HBpin to
phenyl vinyl sulfoxide lead to several products (Scheme 3) aris-
ing from a competing deoxygenation reaction [55]. For instance,
the addition of HBcat proceeded to give catBOBcat, dihydrogen,
and phenyl vinyl sulfide, the latter of which reacted with either
excess HBcat to give 1a or dihydrogen to give PhSCH2CH3
(7). Although this deoxygenation reaction proceeds with the
addition of a catalyst, reactions can take days depending on the
sulfoxide [56]. The reduction of sulfoxides to the correspond-
ing sulfides is an important reaction that has found considerable
utility in organic synthesis [57] and biochemistry [58]. A pro-
posed mechanism for these catalysed deoxygenations involves
initial oxidative addition of HBcat to the metal centre, followed
by insertion of the basic oxygen (arising from the polar S–O
bond, as opposed to the more covalent S–O bonds in sulfones)
into the rhodium boron bond. Reductive extrusion would afford
the transient catBOH species, which would react with another
equivalent of HBcat to give dihydrogen and catBOBcat [55].
Unfortunately, all attempts to circumvent this pathway proved
unsuccessful.
We have found that reactions of phenyl vinyl sulfone with
HBpin also gave a considerable amount of dehydrogenative
borylation derived products (Table 3, entries 9–11). While the

J.D. Webb et al. / Journal of Molecular Catalysis A: Chemical 275 (2007) 91–100
97
Scheme 3. Metal catalysed addition of HBcat to phenyl vinyl sulfoxide.
linear boronate ester 9 could arise from a simple hydroboration
reaction, it is also plausible that formation of this compound
arises from a catalysed hydrogenation of the transient vinyl
boronate ester PhSO2CH CHBpin. Indeed, in reactions using a
deficiency of borane, this vinyl boronate ester is observed ini-
1
tially by H NMR spectroscopy. Catalysed hydrogenations of
related bis vinyl boronate esters have also been reported [64].
Formation of 11 arises from a simple hydrogenation of the start-
ing phenyl vinyl sulfone.
The molecular structure of 9a is shown in Fig. 1 and confirms
the formation of this isomer. Crystallographic data are provided
in Table 4 and selected bond distances and angles are given in
Table 5. Although this molecule contains both a Lewis acidic
(Bcat) and Lewis basic site (oxygen atoms of SO2), no appre-
ciable intramolecular or intermolecular interactions exist in the
solid state. The boron atom lies in a trigonal environment where
Fig. 1. The molecular structure of 9a showing the atomic labeling scheme and
30%probabilitydisplacementellipsoidswithhydrogenatomsomittedforclarity.
˚
the sum of the angles around boron is 359.9(2)A. The B–O dis-
dation and cleavage of the sulfonate group [68]. Unfortunately,
catalysed hydroborations of phenyl vinyl sulfonate at 20 ◦C also
gave a mixture of products regardless of the borane or metal
complex used to facilitate this reaction. For example, hydrob-
oration of phenyl vinyl sulfonate using HBcat and Wilkinson’s
catalyst (Table 6, entry 6) gave only 42% of the branched iso-
mer 12 and 29% of the corresponding linear product 13. Up to
tances of B(10)–O(11) 1.386(2) and B(10)–O(18) 1.390(2) are
similar to those reported in related structures [65–67].
3.4. Phenyl vinyl sulfonate
Uncatalysed reaction of HBcat with phenyl vinyl sulfonate
at 80 ◦C afforded multiple products derived from borane degra-
Table 3
Metal catalysed hydroboration of phenyl vinyl sulfone
Entry
Catalyst precursor
Borane
8a
9a
10a
11a
1
2
3
4
5
6
7
8
9
Rh(acac)(coe)2/dppf
Rh(acac)(coe)2/dppb
RhCl(PPh3)3
[Rh(cod)(dppp)]BF4
Rh(acac)(coe)2/dppp
Rh(acac)(coe)2/dppe
[RhCl(coe)2]2/2PPh3
[Cp*IrCl2]2
HBcat
HBcat
HBcat
HBcat
HBcat
HBcat
HBcat
HBcat
HBpin
HBpin
HBpin
98
95
80
75
70
60
20
–
2
5
10
25
20
15
55
>98
25
25
90
–
–
5
–
–
5
b
5
5
5
–
30
–
5
20
20
–
45
75c
10
RhCl(PPh3)3
[Cp*IrCl2]2
Rh(acac)(coe)2/dppb
–
–
–
10
11
–
a
All reactions were conducted in C6D6 at RT and product ratios were determined by 1H NMR spectroscopy and confirmed by GC/MS. Complete conversion of
starting materials was observed.
b
Reactions conducted in THF.
Five equivalents of HBpin were used.
c

98
J.D. Webb et al. / Journal of Molecular Catalysis A: Chemical 275 (2007) 91–100
Table 4
Table 5
◦
˚
Crystallographic data collection parameters for 9a
Selected bond lengths (A) and angles ( ) for 9a
Formula
fw
C14
288.11
H
13BO4S
Bond lengths
S(7)–O(2)
1.4408(13)
1.4438(12)
1.7781(17)
1.523(2)
1.7687(17)
1.557(2)
1.386(2)
1.390(2)
1.3872(19)
1.3879(19)
Crystal system
Space group
Monoclinic
P2(1)/c
S(7)–O(1)
S(7)–C(8)
C(8)–C(9)
C(1)–S(7)
C(9)–B(10)
B(10)–O(11)
B(10)–O(18)
O(11)–C(12)
C(17)–O(18)
˚
a (A)
6.2886(7)
9.5719(10)
22.378(2)
91.223(2)
1346.7(2)
4
˚
b (A)
˚
c (A)
β (◦)
3
˚
V (A )
Z
ρcalcd (mg m−3
)
1.421
Crystal size (mm3)
Temperature (K)
Radiation
1.00 × 0.10 × 0.10
Bond angles
198(1)
C(6)–C(1)–S(7)
C(2)–C(1)–S(7)
O(2)–S(7)–O(1)
O(2)–S(7)–C(1)
O(1)–S(7)–C(1)
O(2)–S(7)–C(8)
O(1)–S(7)–C(8)
C(1)–S(7)–C(8)
C(9)–C(8)–S(7)
O(11)–B(10)–O(18)
O(11)–B(10)–C(9)
O(18)–B(10)–C(9)
B(10)–O(11)–C(12)
C(17)–O(18)–B(10)
119.16(14)
119.73(13)
118.58(8)
107.93(8)
108.17(7)
107.46(8)
108.98(8)
104.90(8)
114.59(11)
111.63(15)
124.33(16)
123.97(15)
104.90(13)
105.02(12)
Mo K␣ (λ = 0.71073)
0.249
8938
3012
233
μ (mm−1
)
Total reflections
Total unique reflections
No. of variables
Rint
0.0235
Theta range (◦)
1.82–27.50
0.421/−0.343
1.074
0.0354
0.0941
−3
˚
Largest difference peak/hole (e A
)
S (GoF) on F2
a
R1 (I > 2σ(I))
b
wR2 (all data)
ꢀ
ꢀ
a
R1 = ||Fo| − |Fc||/ |Fo|.
ꢀ
ꢀ
wR2 = ( [w(Fo2 − Fc2)2]/ [wFo4])1/2
,
where
w = 1/[σ2(Fo2) +
b
(0.0428P)2 + (0.4406P)], where P = (max(Fo2, 0) + 2Fc2)/3.
reaction using this iridium complex and HBpin gave a compli-
cated product distribution arising from competing degradation
reactions. Excess borane had to be used (up to 5 equivalents) in
all of these reactions to effect complete reduction of the dou-
ble bond, as considerable degradation of the starting borane was
always observed. Further work is therefore needed to understand
the poor selectivities observed in these reactions, the results of
which will be published in due course.
84% of the branched product could be generated (entries 1 and
2), however, along with a considerable amount of 13, hydro-
genation product 15, and minor amounts of diborated product
14. Interestingly, the iridium catalyst [Cp*IrCl2]2 (entry 3) also
gave 60% of the branched isomer in reactions using HBcat. It
is unclear at this time why the branched isomer was favoured
in this reaction, as hydroborations using this catalyst precursor
always appear tofavour linearproducts. Attemptsto catalysethis
Table 6
Metal catalysed hydroboration of phenyl vinyl sulfonate
Entry
Catalyst precursor
Borane
12a
13a
14a
15a
1
2
3
4
5
6
7
8
9
Rh(acac)(coe)2/dppb
[Rh(cod)(dppp)]BF4
[Cp*IrCl2]2
Rh(acac)(coe)2/dppf
Rh(acac)(coe)2/dppp
RhCl(PPh3)3
Rh(acac)(coe)2/dppe
[RhCl(coe)2]2/2PPh3
RhCl(PPh3)3
HBcat
HBcat
HBcat
HBcat
HBcat
HBcat
HBcat
HBcat
HBpin
84
80
60
59
55
42
35
15
–
7
5
2
–
–
7
5
4
–
5
15
7
15
10
23
20
25
35
25
45
b
30
11
20
29
30
55
40
a
All reactions were conducted in C6D6 at RT and product ratios were determined by 1H NMR spectroscopy and confirmed by GC/MS. Complete conversion of
starting materials was observed.
b
Reactions conducted in THF.

J.D. Webb et al. / Journal of Molecular Catalysis A: Chemical 275 (2007) 91–100
99
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proceed with excellent regiocontrol in favour of the unex-
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is believed to be responsible for these observed selectivities.
The iridium(III) catalyst precursor [Cp*IrCl2]2 could be used
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1,2-disubstituted vinyl sulfides gave products arising from a
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where the boron has added ␣ to the sulfur atom. Hydroborations
of phenyl vinyl sulfoxide gave products derived from an ini-
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Catalysed hydroborations of phenyl vinyl sulfone with HBcat
generated the corresponding branched product with high selec-
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suffered from moderate selectivities, borane degradation, and in
some cases, cleavage of the sulfonate group. Of these systems,
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catalysed hydroborations to give the corresponding branched
products.
Acknowledgements
Thanks are gratefully extended to the American Chem-
ical Society-Petroleum Research Fund (Grant #37824-B1),
Canada Research Chairs Programme, Canadian Foundation for
Innovation-Atlantic Innovation Fund, Natural Science and Engi-
neering Research Council of Canada, Los Alamos National
Labs, and Mount Allison University for financial support. We
also thank Roger Smith for his expert technical assistance and
Rev. Dr. M.G.J. Lesley (SCSU) for the generous gift of dppf and
an anonymous reviewer for helpful comments. RTB thanks Los
Alamos’ Lab Directed R&D program for support.
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