Rhodium-catalyzed hydroboration reactions with sulfur and nitrogen analogues of catecholborane

Article abstract of DOI:10.1002/ejoc.200600564

Rhodium-catalyzed hydroboration of 1-octene and trans-4-octene with sulfur- and nitrogen analogues of catecholborane are demonstrated with the use of in situ ¹¹B NMR spectroscopy. Our study shows that the sulfur- and nitrogen analogues are significantly less prone to disproportionation than catechol, which results in enhanced yields of the desired compounds. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejoc.200600564

FULL PAPER
DOI: 10.1002/ejoc.200600564
Rhodium-Catalyzed Hydroboration Reactions with Sulfur and Nitrogen
Analogues of Catecholborane
Siphamandla W. Hadebe[a] and Ross S. Robinson*[a]
Keywords: Rhodium / Homogeneous catalysis / Hydroboration / Boranes / Olefins
Rhodium-catalyzed hydroboration of 1-octene and trans-4-
octene with sulfur- and nitrogen analogues of catecholborane
are demonstrated with the use of in situ 11B NMR spec-
troscopy. Our study shows that the sulfur- and nitrogen ana-
logues are significantly less prone to disproportionation than
catechol, which results in enhanced yields of the desired
compounds.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
deuteriocatecholborane in the presence of Wilkinson’s cata-
lyst). This study showed that exclusive generation of the
terminal alkylborane is, in fact, due to the reversibility of
deuteride migration, coupled with a high preference for the
reductive elimination of the boron-primary alkyl moiety
rather than the boron-secondary alkyl moiety from the
metal centre.[10,11] The mechanism of rhodium(I)-catalyzed
olefin hydroboration has been further elucidated by ab intio
molecular modelling studies.[13]
Introduction
Transition-metal catalyzed hydroboration reactions have
been studied extensively by a number of research groups
because of their potential applications within organic syn-
thesis.[1–4] In the mid 1970’s, Kono et al.[5] showed that the
B–H bond of catecholborane (1) oxidatively adds directly to
the metal centre of the RhCl(PPh3)3 catalyst. RhCl(PPh3)3
was previously reported to only catalyze the hydrogena-
tion[6] and the hydrosilylation[7] of alkenes; therefore,
Kono’s work opened up the possibility of rhodium-cata-
lyzed hydroboration reactions. Kono’s work was exploited
by Männig and Nöth[8] who were able to demonstrate that
RhCl(PPh3)3 could catalyze the hydroboration of olefins. It
has been subsequently shown in the literature that the reac-
tions of multifunctional substrates can be chemoselectively
influenced by the catalyst.[1,8] That is, without a catalyst,
catecholborane adds to the carbonyl group preferentially,
whilst in the presence of the rhodium catalyst, hydrobor-
ation of the alkene is the major product (Scheme 1).[8]
Männig and Nöth proposed a mechanism for the rho-
dium-catalyzed hydroboration of olefins that involves the
oxidative addition of a B–H bond to the coordinatively un-
saturated metal centre. This is followed by alkene insertion,
then hydride migration to the coordinated alkene and sub-
sequent reductive elimination to afford the B–C bond. This
mechanism is analogous to the proposed homogeneous hy-
drogenation model.[9]
Recently, Srebnik et al.[14] successfully used RhI to cata-
lyze the hydroboration of alkenes with pinacolborane (2)
which was previously reported to be unsuccessful.[15] Ter-
minal octylpinacolboronate was obtained in high yields in
the RhCl(PPH3)3-catalyzed hydroboration of trans-4-oc-
tene.[14] It has also been reported that the regioselectivity
is also reversed in the rhodium-catalyzed hydroboration of
allylbenzene.[16] A unique reversal in the regioselectivity has
also been reported in the hydroboration of 1-octene cata-
lyzed by rhodium trichloride.[17]
Since the discovery of the transition-metal-catalyzed hy-
droboration reaction, a number of research groups have
used catecholborane (1) [1,3,2-benzodioxaborolane abbrevi-
ated as HBCat where Cat = 1,2-O2C6H4] and recently pina-
colborane (2) [4,4,6,6-tetramethyl-1,3,2-dioxaborolane ab-
breviated as HBPin where Pin = 1,2-O2C6H12] in rhodium-
catalyzed transformations with much attention focused on
the mechanistic, regio- and stereoselectivity implications of
the reaction.[1–4,10–13] More recently, attention has been fo-
cused on the enhancement of Rh-type catalysts with an em-
phasis on different phosphane ligands.[18–20] However, Bur-
gess and coworkers[21] reported evidence of the dispropor-
tionation (also known as degradation) of CatBH in the cat-
alyzed hydroboration of alkenes with RhCl(PPh3)3. In our
previous studies of the Rh-promoted hydroboration of al-
kenes with HBPin, a varying degree of disproportionation
was noted.[22a] We were intrigued by the possibility that new
heterosubstituted hydroborating agents with increased sta-
Further mechanistic investigations by Evans et al.[10–12]
were performed with the use of deuteriocatecholborane and
they revealed that there is a significant amount of deute-
rium at C1 (from the reaction between 1-decene and-
[a] Warren Research Laboratory, School of Chemistry, University
of KwaZulu-Natal,
Private Bag X01, Scottsville, Pietermaritzburg, South Africa,
3209
4898
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Rh-Catalyzed Hydroboration Reactions
FULL PAPER
Scheme 1. Uncatalyzed and catalyzed hydroboration of 5-hexen-2-one with catecholborane – altered chemoselectivity.
bility (that is, less disproportionation) over that of HBCat
or HBPin could be developed. Although Rh-catalyzed
Results and Discussion
During studies in our laboratory on hydroboration reac-
transformations are well-precedented, very few examples of
the hydroboration of sulfur- or nitrogen analogues have
been reported, which also includes the catalyzed hydrobor-
ation with (4S,5R)-ephedrineborane and (4S,5R)-pseudo-
phedrineborane.[22b] In this paper we present the first
RhCl(PPh3)3-catalyzed hydroboration of olefins with 1,3,2-
benzodithiaborolane (3) (HBThia where Thia = 1,2-
S2C6H4) (Figure 1) and 1,3,2-benzodiazaborolane (4)
(HBAza where Aza = 1,2-N2C6H6), which are new poten-
tial alternatives to HBCat and HBPin that possess en-
hanced stability towards disproportionation.
tions and the synthesis of potentially more stable mono-
functional hydroborating agents, the following observations
were noted.
1. Uncatalyzed Hydroboration of Olefins
Uncatalyzed hydroboration reactions were conducted
with a range of heterosubstituted borolanes (as indicated in
Table 1) in order to evaluate their reactivities towards ole-
fins prior to the use of Wilkinson’s catalyst. HBCat effected
the hydroboration of a terminal olefin when heated at
100 °C for 2 h (Table 1) to afford a yield of 37% of the
desired product, which is in contrast to the yield reported
by Brown et al.[23] The low yield is attributed to significant
formation of tris(catecholato)diboron (5a), (Cat)3B2 (ca.
40%), which is produced when the reaction is heated.[24]
Interestingly, the sulfur analogue, HBThia (freshly prepared
in situ from the reaction of BH3·SMe2 with 1,2-benzenedi-
thiol, Scheme 2a), effected the hydroboration of 1-octene
and trans-4-octene when heated at 150 °C for 3 h to afford
terminal octyl-1,3,2-benzodithiaboronate ester in 65% yield
(Scheme 2b) [ca. 21% of (Thia)3B2 (5b)] and 15% of in-
ternal octyl-1,3,2-benzodithiaboronate ester, which were
both characterized by a singlet at 59.6 ppm (identical chem-
ical shift for both products) in the 11B NMR spectra.
The reactions with nitrogen analogue 1,3,2-benzodiaza-
borolane (4) did not proceed at all for both 1-octene and 4-
octene even after 10 days while heated at reflux. Motry et
al.[24] noted a similar lack of reactivity in borylation-type
chemistry.
Our observations revealed a reactivity trend of these rea-
gents towards alkenes: O Ͼ S Ͼ N (Figure 2). The above
trend, though not unexpected, may be attributed to the fact
that heteroatoms such as nitrogen and sulfur are able to
back-donate electron density from the heteroatom lone pair
of electrons to the vacant pZ-orbital of the boron atom.
Thus, the Lewis acidity of the boron reagent is lowered.[25]
Because of this fact, attention was then focused on the utili-
zation of a rhodium catalyst.
Figure 1. (A).11B NMR spectrum of freshly synthesized 1,3,2-
benzodithiaborolane in CH2Cl2. (B) 11B NMR spectrum that shows
the formation of 2-octyl-1,3,2-benzodithiaboronate ester after 0.5 h
and (C) shows the product formation (65%, 21% of 5b and 4% of
3) after 3 h.
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Table 1. Rhodium-catalyzed hydroboration of 1-octene and trans-4-octene with yields for the formation of octylboronate ester (Prod.)
and the disproportionation products (Disp.) (Excluding unreacted starting material, boric acid and BH3·PPh3).
The identities of the disproportionation products were in
good agreement with those reported previously by Burgess
et al.[21] The formation of the disproportionation products
was accounted for mechanistically by Westcott et al.[26a]
(Scheme 3), who also reported the crystal structure of tris-
(catecholato)diboron.[26a] In our reactions, the yields of the
hydroboration reactions were dependent on the degree of
disproportionation of HBC, and it was found to be more
pronounced in the catalyzed hydroboration of internal ole-
Scheme 2. (a) Synthesis of 1,3,2-benzodithiaborolane (3). (b) Un-
fins, which led to significantly reduced yields. To our sur-
prise, HBPin, despite the fact that it is more stable than
HBCat, afforded the octylpinacolboronate ester in 79%
yield and a combination of disproportionation products in
a 19% yield, which was contrary to that found by Srebnik
et al.[14,27] who reported no disproportionation of HBPin.
Of the disproportionation products formed, a singlet reso-
nating at δ = 22.1 ppm in the 11B NMR spectrum can be
attributed to tris(pinacolato)diboron (15%) [also known as
2,2Ј-(2,3-dimethyl-2,3-butanediyldioxy)bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane)[26b]]. We observed no reaction
with trans-4-octene, again, unlike the observations by
Srebnik et al.[14,27] These observations are in agree-
ment with previous reports by Yamamoto et al.[28] and
Tucker et al.[15]
We attributed the discrepancy in the experimental results
to the fact that the B–H bond of the bulky pinacolborane
has to be inserted into the metal centre oxidatively, followed
by alkene insertion. Therefore, if the alkene is internal and
hindered, insertion to the metal centre would be difficult;
as a result, no reaction would occur. We have recently re-
ported that internal olefins are able to effectively undergo
hydroboration and isomerization with pinacolborane to af-
ford terminal boronate esters with the use of RhCl(PPh3)3
catalyzed hydroboration of 1-octene with 3. Yields are based on
11B NMR spectroscopy.
Figure 2. Observed reactivity trend from hydroboration of 1 and 4-
octene.
2. Rhodium-Catalyzed Hydroboration of Olefins
The rhodium catalysis study (Table 1) showed that the
reaction of 1-octene with HBCat in the presence of
RhCl(PPh3)3 afforded the desired octylboronate ester in
67% yield after a reaction time of 2 h at ambient tempera-
ture, and also that 33% of HBCat was converted into the
disproportionation products [tris(catecholato)diboron,
H3B/PPh3 and other minor fragments including boric acid
(Figure 3)].
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Rh-Catalyzed Hydroboration Reactions
FULL PAPER
Figure 3. Typical 11B NMR spectrum of the product mixture of the catalyzed hydroboration of 1-octene with CatBH, which shows a
varying distribution of products formed during catalysis.
Scheme 3. Formation of disproportionation products 5a,[26a] 5b and 5c[26b] catalyzed by PPh3.
as a catalyst in conjunction with microwave irradiation or
with the use of [O2RhCl(PPh3)2]2.[22a]
To our delight, the reaction of 1,3,2-benzodithiaborolane
(HBThia) with 1-octene and trans-4-octene catalyzed by
RhCl(PPh3)3, under the same reaction conditions, resulted
in an immediate change in colour from brick-red to dark-
green when all off the reagents were mixed. This indicates
that HBThia has spontaneously added the B–H bond into
Figure 4. 11B NMR spectrum (measured in ppm) obtained from
the metal centre. After a 24 hour period, 11B NMR spectro-
the catalyzed hydroboration of 1-octene with 1,3,2-benzodithiabor-
olane.
scopic analysis of the product mixture revealed an intense
singlet at δ = 59.6 ppm, which can be attributed to the ter-
minal (ca. 86% yield, Figure 4) and internal (ca. 60–80%,
Figure 5) 2-octyl-1,3,2-benzodithiaboronate esters, respec-
Wilkinson’s catalyst has demonstrated its usefulness in
tively. This was further corroborated by oxidative workup the promotion of the hydroboration of 1-octene with 1,3,2-
1
of the product mixtures, which were characterized by H- benzodiazaborolane at 25 °C, which has been shown from
and 13C NMR and GC–MS spectroscopic analysis and con- the early studies to be unreactive.[24] 11B NMR spectroscopy
firm the formation of 1-octanol and 4-octanol. 1,3,2-Benzo- displayed a new singlet at δ = 31.6 ppm (Figure 6) attrib-
dithiaborolane (3) proved to be a potentially superior hy- uted to the exclusive formation of the terminal octyl-1,3,2-
droborating agent in Rh-catalyzed hydroboration when benzodiazaboronate ester (ca. 70% yield) formed after a re-
compared to its oxygen analogue because of the high yield action time of 24 hours as characterized by GC–MS spec-
of the obtained octylboronate esters and the significantly troscopy. No disproportionation of 1,3,2-benzodiazaborol-
low quantity of the disproportionation products (ca. 1.8%). ane was observed.
Eur. J. Org. Chem. 2006, 4898–4904
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S.W. Hadebe, R.S. Robinson
FULL PAPER
affect the overall yield of catalyzed hydroboration reactions.
We are currently working to expand the applicability of
these hydroborating reagents in similar hydroboration reac-
tions, including aryl- and cyclic olefins. We are also investi-
gating the role of ligands in the reaction to possibly design
a Rh-type catalyst with enhanced applicability. In addition,
we are exploring the suitability of these compounds within
the realm of Suzuki-coupling-type chemistry.
Figure 5. 11B NMR spectrum (measured in ppm) obtained from
the catalyzed hydroboration of trans-4-octene with 1,3,2-benzodi-
thiaborolane.
Experimental Section
General: All glassware was thoroughly dried overnight in an oven
heated at ca. 150 °C. The glassware was further flame-dried with a
hot air gun under reduced pressure and cooled under a stream of
dry nitrogen, which was passed through a mixture of silica gel and
0.4 nm molecular sieves prior to use. Glass syringes, cannulae and
needles were oven-dried and stored in a desiccator (charged with
a mixture of silica gel and 0.4 nm molecular sieves) prior to use.
Disposable syringes and needles were stored in the desiccator be-
fore use, and they were discarded after a single use. The glassware
was assembled, and all joints were wrapped with Teflon® tape and
subsequently sealed with Parafilm “M”® to ensure a closed system.
Figure 6. 11B NMR spectrum (measured in ppm) obtained from the
catalyzed hydroboration of 1-octene with 1,3,2-benzodiazaborolane
after 24 h at 25 °C.
1
All H-, 13C- and 11B NMR spectra were recorded with a Varian
Unity-Inova 500 MHz spectrometer. 1H NMR spectra are refer-
enced to the residual chloroform signal at δ = 7.25 ppm. All 11B
NMR spectra were referenced to BF3·OEt2 as an external standard
(δ = 0 ppm) contained within a sealed capillary insert. 11B NMR
spectroscopy was utilized in order to identify the compounds as
well as to monitor the progress of the reactions. 13C- and 1H NMR
spectroscopy and GC–MS were used to identify the hydroboration
products. Quartz NMR tubes (5 cm) were used for the 11B NMR
spectroscopic experiments and were all oven-dried, flushed with dry
nitrogen and sealed with a rubber septum prior to injection of the
sample or reagents.
No product was obtained from the catalyzed hydrobor-
ation of trans-4-octene with HBAza under the same reac-
tion conditions. No hydroboration was observed even after
the reaction mixture was heated at 40 °C for 30 h in the
presence of RhCl(PPh3)3. This was attributed to the steri-
cally demanding structures of both trans-4-octene and
HBAza. As a result, it would be difficult for olefin insertion
to the metal centre to be achieved. Subsequent addition of
1-octene to the reaction mixture showed the formation of
the terminal octylboronate ester, which indicates that the
rhodium–boryl complex was already preformed. However,
interestingly, no disproportionation of HBAza was ob-
served from the analysis of the 11B NMR spectrum even
when heated at reflux in CH2Cl2 for 40 h. This is indicative
of the increased stability of 1,3,2-benzodiazaborolane (4)
over that of catecholborane (1) against catalyzed dispropor-
tionation induced by PPh3.
GC–MS analyses were performed with a Thermofinnigan® (GC)
coupled with a PolarisQ® (MS) system. Thin layer chromatography
was performed with silica gel (60 F254) plates from Merck. Flash
column chromatography was performed with SP Silica Gel 60 (230–
400-mesh ASTM) from Merck.
THF and trans-4-octene were distilled from sodium wire in the
presence of benzophenone as an indicator.[29] The solvents were
distilled and transferred by cannula to a flame-dried, nitrogen-
flushed flask containing 0.4 nm molecular sieves (activated in the
furnace at 600 °C and cooled under dry nitrogen) prior use.
Catecholborane (1), 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (pina-
colborane) (2), BH3·SMe2 in CH2Cl2, 1,2-benzenedithiol and tris-
(triphenylphosphane)rhodium(I) chloride were obtained from
Sigma–Aldrich Co; 1,2-phenylenediamine was obtained from
Merck–Schuchardt. All of these reagents were used without further
purification.
Conclusions
In conclusion, we have shown that 1,3,2-benzodithiabor-
olane and 1,3,2-benzodiazaborolane are potentially supe-
rior hydroborating agents in RhCl(PPh3)3-catalyzed hydro-
boration reactions because these derivatives are less suscep-
tible to disproportionation, catalyzed by PPh3, than cate-
cholborane. Consequently, the desired compounds are
formed in excellent yields. We have also shown that the hy-
droboration reactions can be monitored in situ with the use
of 11B NMR spectroscopy. With this technique, the forma-
tion of the desired compounds is observed, but it also re-
veals other species (that are readily destroyed upon workup)
Synthesis of 1,3,2-Benzodithiaborolane (3): 1,2-Benzenedithiol
(497 mg, 3.50 mmol) in dichloromethane (5 mL) was added to a
stirred solution of borane-dimethyl sulfide complex (1.0  solution
in dichloromethane, 3.50 mL, 3.50 mmol) at 25 °C. The reaction
was mild with no observable liberation of hydrogen gas. The reac-
tion mixture was stirred for 24 h at room temperature to afford the
desired product as a light yellow liquid (Ն 99% based on 11B
NMR). 11B NMR (160 MHz, BF3·OEt2): δ = 53.4 (d, J = 162.6 Hz,
1 H, BH) ppm. Proton noise decoupling (PND) was carried out
that result from competing side reactions that can vastly with subsequent collapse of the doublet to the expected singlet. 11
B
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Rh-Catalyzed Hydroboration Reactions
FULL PAPER
NMR (160 MHz, BF3·OEt2): PND δ = 53.4 (s) ppm. Because of
the high purity and instability of this product, it was used directly
in the hydroboration step.
RhCl(PPh3)3 (46.3 mg, 0.050 mmol) in CH2Cl2 (0.50 mL). Yield
85%. 11B NMR (160 MHz, BF3·OEt2): δ = 59.6 (s) ppm.
Synthesis of 2-(1-Propylpentyl)-1,3,2-benzodithiaborolane: 1,3,2-
Benzodithiaborolane (3; 0.40 mL, 0.248 mmol), trans-4-octene
(0.39 mL, 2.48 mmol), RhCl(PPh3)3 (46.3 mg, 0.050 mmol) in
CH2Cl2 (0.50 mL). Clear liquid (80%). 11B NMR (160 MHz,
BF3·OEt2): δ = 59.6 (s) ppm. This product was subsequently oxid-
ized by the addition of NaOH (3 , 0.83 mL) and H2O2 (0.02 mL
of 50% v/v solution) to afford the desired compound. 1H NMR
(500 MHz, CDCl3): δ = 0.95 (t, J = 7.1 Hz, 3 H, 2×CH3), 1.41 (m,
2 H, 2×CH3CH2CH2), 1.32 (m, 2 H, CH3CH2CH2CHOHCH2),
4.71 (s, 1 H, CH2CHOHCH2), 3.61 (m, 1 H, CH2CHOHCH2), 1.49
Synthesis of 1,3,2-Benzodiazaborolane (4): 1,2-Diaminobenzene
(541 mg, 5.0 mmol) was dissolved in dichloromethane (5 mL) in a
flame-dried round-bottomed flask. After complete dissolution of
the solid, borane-dimethyl sulfide complex (1  solution in dichlo-
romethane, 5.0 mL, 5.0 mmol) was introduced dropwise through
the septum. The resulting mixture was stirred and heated at reflux
for 4 h under an atmosphere of dry nitrogen to afford 4 as a clear
liquid (95%). 11B NMR (160 MHz, BF3·OEt2): δ = 23.9 (d, J =
153.2 Hz, 1 H, BH) ppm.
(m,
2
H, CH3CH2CH2CHOHCH2) 1.39 (m,
2
H,
Representative Procedure for Uncatalyzed Hydroboration (Method
A): 1,3,2-Benzodithiaborolane (3; 25.3 mg, 0.165 mmol) in diglyme
(0.4 mL) was mixed with 1-octene (0.26 mL, 1.65 mmol) in a dry
nitrogen-flushed NMR tube, capped with a rubber septum and se-
aled with Parafilm.® The tube was then inserted in an aluminium
heating block, which was immersed in a silicon oil bath heated at
150 °C. The contents of the tube were heated at reflux; the pressure
build-up in the tube was vented with a nitrogen purged syringe
every 20 min for 3 h. This afforded a clear liquid of 2-octyl-1,3,2-
benzodithiaborolane (65%) 11B NMR (160 MHz, BF3·OEt2): δ =
59.6 (s) ppm.
CH3CH2CH2CHOHCH2CH2) ppm. MS (EI): m/z (%) = 130 (20)
[M]+, 87 (100) 73 (52), 69 (36), 55 (66).
Synthesis of 2-Octyl-1,3,2-benzodiazaborolane: 1,3,2-Benzodiaza-
borolane (4; 0.40 mL, 0.20 mmol), 1-octene (0.31 mL, 2.0 mmol),
RhCl(PPh3)3 (37.0 mg, 0.040 mmol) in CH2Cl2 (0.50 mL). Orange-
yellow solution (70%). 11B NMR (160 MHz, BF3·OEt2): δ = 31.6
(s) ppm. MS (EI): m/z (%) = 231 (18) [M]+, 230 (100), 229 (15),
145 (15), 132 (16), 119 (17), 118 (31).
Acknowledgments
Synthesis of 2-(1-Propylpentyl)-1,3,2-benzodithiaborolane: Method
(A) was also employed for trans-4-octene, to afford 2-(1-propylpen-
tyl)-1,3,2-benzodithiaborolane (15%). 11B NMR (160 MHz,
BF3·OEt2): δ = 59.6 (s) ppm.
We gratefully acknowledge Sasol for financial assistance, as well as
Mr. Arno de Klerk and Dr. Hein Strauss for collaborative involve-
ment in this ongoing project.
Representative Procedure for Rhodium-Catalyzed Hydroboration
(Method B): The hydroborating agent under investigation was in-
jected into an oven-dried, nitrogen-purged and septum-capped
quartz NMR tube and then analyzed by high field 11B NMR spec-
troscopy prior to the addition of the other reagents in order to
confirm its purity. To this solution was added simultaneously the
olefin and tris(triphenylphosphane)rhodium(I) chloride (2 mol-%),
which had been dissolved in dichloromethane or THF (0.5 mL) in
a separate flame-dried, nitrogen-flushed flask. The contents of the
tube were shaken vigorously, and the tube was inserted into the
NMR spectrometer. The contents of the tube were subsequently
analyzed every 2 h for 24 h at 25 °C to monitor the progress of the
formation of the target alkylboronate ester. The amounts for each
reactant used and yields of alkylboronate esters produced for each
experiment are given in the following sections, where method B was
used.
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(s), 21.7 [s, B2(O2C6H12)3] ppm. The contents of the tube were sub-
sequently quenched by the addition of water (1 mL). The product
was extracted with ether (3×2 mL) and dried with MgSO4. Flash
chromatography on silica gel with ethyl acetate/hexane, 2:98 and
removal of solvent in vacuo afforded 4,4,5,5-tetramethyl-2-octyl-
1,3,2-dioxaborolane. 11B NMR (160 MHz, BF3·OEt2): δ = 34.3 (s)
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1
ppm. H NMR (500 MHz, CDCl3): δ = 0.71 (t, J = 7.9 Hz, 2 H),
0.82 (t, J = 5.6 Hz, 3 H), 1.19 (s, 12 H), 1.20–1.24 (m, 10 H), 1.32–
1.39 (m, 2 H) ppm. 13C NMR (125·MHz, CDCl3): δ = 14.7, 22.7,
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23.1, 24.0, 24.8, 29.7, 30.1, 32.1, 82.7 ppm. IR (neat): ν = 2960 (s),
˜
1744 (s), 1376 (s), 1234 (s), 1146 (s), 1073 (s) cm–1. MS (EI): m/z
(%) = 241 (40) [M]+, 225 (100), 224 (24), 183 (10), 127 (22), 97 (30),
69 (54).
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Synthesis of 2-Octyl-1,3,2-benzodithiaborolane: 1,3,2-Benzodithia-
borolane (3; 0.40 mL, 0.248 mmol), 1-octene (0.39 mL, 2.48 mmol),
Eur. J. Org. Chem. 2006, 4898–4904
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Received: July 1, 2006
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