Communications
Organometallics, Vol. 20, No. 11, 2001 2131
corresponding thiol 7, but we have not yet been able to
deprotect the carbon-bound boryl group to the corre-
sponding boronic acid (Figure 3). The reactions de-
scribed here are the first examples of metal-catalyzed
boration reactions of a thiocarbonyl group.
A second potential route to thioboronate esters in-
volves the hydroboration of organosulfur-substituted
alkenes. Recent reports on hydroborations of related
enamines using 9-H-BBN generated R-boronated amine
intermediates.19 Hydroboration of phenyl vinyl sulfide
8 with HBcat at elevated temperatures gave the ex-
pected anti-Markovnikov organoboronate ester 9 along
with products arising from Lewis base-mediated deg-
radation of the borane reagent. In the presence of a
rhodium catalyst, typically 2 mol % RhCl(PPh3)3, 8
reacts with HBcat to give selective formation of the
desired R-thioboronate ester 10 resulting from an
exclusive Markovnikov addition (Figure 4; as ascer-
tained by multinuclear NMR spectroscopy).20 Prelimi-
nary results show that similar selectivities are observed
in reactions with PhCH2SCHdCH2. These results are
particularly intriguing, as Markovnikov additions have
only been observed in appreciable amounts in catalyzed
reactions of styrenes,21 fluoroalkenes,22 and allyl sul-
fones.23 A η3-benzyl intermediate has been invoked to
rationalize the styrene selectivity, and that for the
sulfones has been attributed to a directing effect of the
sulfone oxygens.
F igu r e 2. Hydroboration of thiocarbonyls 1a -d with 9-H-
BBN.
C6D6 at 60 °C gave the borylsulfides 3a -d in higher
yields (by NMR).16 Reactions carried out in THF gave
significantly more of the thiol. In contrast, catalyzed
hydroborations of aliphatic (1R)-(-)-thiocamphor (4)
with HBcat at room temperature or HBpin at elevated
temperature gave a single isomer of the corresponding
boryl sulfide 5 accompanied by minor amounts of
boronate ester degradation products B2(cat)3 and
B2(pin)3, respectively.14,17
While hydroborations of thiocarbonyl compounds give
boryl sulfides, which can be hydrolyzed to the corre-
sponding thiols, diboration reactions are unique in
delivering a boryl fragment (BR2) to the R-carbon as
well. Aqueous workup of the diborated products should
then provide a direct route to R-thioboronic acids.
Although attempts to diborate thiocarbonyl compounds
1a -d proved unsuccessful using a number of different
metal catalysts, we have found that B2(cat)2 adds to
(1R)-(-)-thiocamphor (4) in high yields using a catalytic
amount of RhCl(PPh3)3 to give a single isomer of the
diborated product 6.18 Aqueous workup provided the
Organosulfur compounds with boryl groups on both
the R- and â-carbons were obtained by catalyzed dibo-
rations of phenyl vinyl sulfide. While Wilkinson’s cata-
(18) To a benzene solution of (1R)-(-)-thiocamphor (79 mg, 0.47
mmol) was added B2(cat)2 (112 mg, 0.47 mmol) and 5 mol % RhCl-
(PPh3)3. The reaction mixture was stirred at room temperature for 16
h. Removal of the solvent yielded an amber-colored residue.
A
concentrated ether solution of this residue was cooled (-35 °C) to afford
off-white needles (160 mg, 85%). NMR (C6D6) of 6: 1H δ 7.05 (2nd order
m, 2H, cat), 6.74 (2nd order m, 2H, cat), 6.65 (2nd order m, 2H, cat),
6.52 (2nd order m, 2H, cat), 3.11 (d, J ) 14 Hz, 1H), 2.45 (ddd, J ) 14,
4, 3 Hz, 1H), 1.72 (app t, J ) 4 Hz, 1H), 1.49 (m, 3H), 1.40 (s, 3H,
CH3), 1.33 (s, 3H, CH3), 1.04 (m, 1H), 0.73 (s, 3H, CH3); 11B{1H} δ
31.7 (br); 13C{1H} δ 148.8 (C cat), 148.76 (C cat), 122.9 (CH cat), 122.7
(CH cat), 112.8 (CH cat), 112 (CH cat), 52.5, 50.5, 47.2 (CH), 46.1, 43.6
(br, C-B), 37.3, 26.9, 21.6 (CH3), 20.7 (CH3), 14.0 (CH3). To an ether
solution of 6 (97 mg, 0.24 mmol) was added water (10 µL, 0.48 mmol).
The mixture was stirred at room temperature for 0.5 h and then dried
over anhydrous MgSO4, which was removed by filtration. The filtrate
was concentrated and cooled (-35 °C) to yield off-white needles of the
corresponding thiol 7 (20 mg, 80%). NMR (C6D6): 1H δ 7.00 (2nd order
m, 2H, cat), 6.77 (2nd order m, 2H, cat), 2.57 (d, J ) 13 Hz, 1H), 2.10
(ddd, J ) 13, 6, 4 Hz, 1H), 2.03 (s, 1H, SH), 1.62 (t, J ) 4 Hz, 1H),
1.31 (s, 3H, CH3), 1.24 (s, 3H, CH3), 1.10 (m, 4H), 0.72 (s, 3H, CH3);
11B{1H} δ 32.7 (br); 13C{1H} δ 148.7 (C cat), 122.9 (CH cat), 112.7 (CH
cat), 50.3, 50.0, 46.3 (CH), 43.6 (br, C-B), 42.3, 36.8, 27.1, 21.1 (CH3),
20.9 (CH3), 13.6 (CH3).
(15) (a) Pereira, S.; Srebnik, M. Tetrahedron Lett. 1996, 37, 3283.
(b) Murata, M.; Watanabe, S.; Masuda, Y. Tetrahedron Lett. 1999, 40,
2585. (c) Ramachandran, P. V.; J ennings, M. P.; Brown, H. C. Org.
Lett. 1999, 1, 1399. (d) Crudden, C. M. Personal communication. (e)
Vogels, C. M.; Hayes, P. G.; Shaver, M. P.; Westcott, S. A. Chem.
Commun. 2000, 51.
(16) In a typical experiment, HBpin (61 mg, 0.51 mmol) in 0.5 mL
of C6D6 was added dropwise to a solution of the diarylthiocarbonyl (0.50
mmol) and 5 mol % RhCl(PPh3)3 in 0.5 mL of C6D6. The mixture was
heated to 60 °C for
2 h and then analyzed by high-field NMR
spectroscopy. Selected NMR spectroscopic data of the hydroborated
product are as follows. 3a : 1H NMR δ 7.56 (d, J ) 7 Hz, 4H, Ar), 7.16
(t, J ) 7 Hz, 4H, Ar), 7.08 (d, J ) 7 Hz, 2H, Ar), 5.99 (s, 1H, SCH),
1.02 (s, 12H, pin); 11B NMR δ 34.0 (br). 3b: 1H NMR δ 7.69 (d, J ) 5
Hz, 2H, Ar), 7.18-7.03 (ov m, 6H, Ar), 6.05 (s, 1H, SCH), 1.13 (s, 12H,
pin); 11B NMR δ 33.1 (br). 3c: 1H NMR δ 7.63 (mult, 4H, Ar), 6.66
(mult, 4H, Ar), 6.16 (s, 1H, SCH), 2.56 (s, 12H, NMe), 1.05 (s, 12H,
pin); 11B NMR δ 33.4 (br). 3d : 1H NMR δ 8.50 (s, 1H, Ar), 7.75 (d, J
) 7 Hz, 1H, Ar), 7.42 (d, J ) 7 Hz, 1H, Ar), 7.35 (d, J ) 7 Hz, 2H, Ar),
7.09 (ov m, 3H, Ar), 6.72 (t, J ) 7 Hz, 1H, Ar), 5.80 (s, 1H, SCH), 0.97
(s, 12H, pin); 11B NMR δ 33.7 (br). Boryl sulfide products decomposed
in solution over time to give the corresponding thiols and B2(pin)3.
(17) In a typical experiment, HBcat (66 mg, 0.55 mmol) in 0.5 mL
of C6D6 was added dropwise to a solution of (1R)-(-)-thiocamphor (84
mg, 0.50 mmol) and 2 mol % RhCl(PPh3)3 in 0.5 mL of C6D6. Selected
NMR spectroscopic data are as follows. 5: 1H NMR δ 7.01 (m, 2H,
cat), 6.79 (m, 2H, cat), 3.68 (t, J ) 8 Hz, 1H), 2.05 (app d, J ) 8 Hz,
3H), 1.62 (br ov m, 4H), 1.05 (s, 3H, CH3), 1.02 (s, 3H, CH3), 0.76 (s,
3H, CH3); 11B NMR δ 35.6 (br s); 13C NMR δ 149.6 (C cat), 123.1 (CH
cat), 112.7 (CH cat), 49.9, 49.5, 48.2, 46.8, 42.5, 38.7, 27.8, 20.9, 20.8,
14.5.
(19) (a) Singaram, B.; Goralski, C. T.; Fisher, G. B. J . Org. Chem.
1991, 56, 5691. (b) Fisher, G. B.; Goralski, C. T.; Nicholson, L. W.;
Singaram, B. Tetrahedron Lett. 1993, 34, 7693.
(20) In a typical experiment, HBcat (40 mg, 0.34 mmol) in 0.5 mL
of C6D6 was added to a solution of phenyl vinyl sulfide (41 mg, 0.30
mmol) in 0.5 mL of C6D6 and heated at 80 °C for 8 h to afford 9. NMR:
1H δ 7.45-6.6 (ov m, 9H, Ph and cat), 2.96 (t, J ) 8 Hz, 2H, CH2), 1.42
(t, J ) 8 Hz, 2H, CH2); 11B{1H} δ 34.3 ppm (br); 13C{1H} δ 148.9 (C
cat), 134.3, 129.3, 129.1, 122.9 (CH cat), 112.4 (CH cat), 28.7 (CH2),
27.2 (br, C-B). For the catalyzed reaction the above mixture was
stirred with 2 mol % RhCl(PPh3)3 at 20 °C for 8 h to give 10 in 95%
NMR yield. NMR: 1H δ 7.25-6.6 (ov m, 9H, Ph and cat), 3.02 (q, J )
8 Hz, 1H, CH), 1.48 (d, J ) 8 Hz, 3H, CH3); 11B{1H} δ 33.4 (br); 13C-
{1H} δ 148.4 (C cat), 131.7, 129.1, 127.0, 123.0 (CH cat), 112.7 (CH
cat), 24.0 (br, C-B), 16.7 (CH3).
(21) Beletskaya, I.; Pelter, A. Tetrahedron 2000, 53, 4957.
(22) Brown, H. C.; Chen, G.-M.; J ennings, M. P.; Ramachandran,
P. V. Angew. Chem., Int. Ed. 1999, 38, 2052.
(23) Hou, X.; Hong, D.; Rong, G.; Guo, Y.; Dai, L. Tetrahedron Lett.
1993, 34, 8513.