Microwave-assisted Suzuki-Miyaura cross-coupling of 2-alkyl and 2-alkenyl-benzo-1,3,2-diazaborolanes

Article abstract of DOI:10.1016/j.tet.2011.03.095

Nitrogen-based boronate esters, such as 2-octyl-benzo-1,3,2-diazaborolane, 2-phenethyl-benzo-1,3,2-diazaborolane, and 2-{(1E)-hexenyl}-benzo-1,3,2- diazaborolane have been shown to be suitable coupling partners with arylhalides in microwave accelerated Suzuki cross-coupling reactions. Reaction yields of up to 89% were achieved. The use of a silicon group attached to the nitrogen atom, proved to enhance the reactivity of 2-octyl-benzo-1,3,2-diazaborolane.

Full text of DOI:10.1016/j.tet.2011.03.095

Tetrahedron 67 (2011) 4277e4282
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Microwave-assisted SuzukieMiyaura cross-coupling of 2-alkyl and
2-alkenyl-benzo-1,3,2-diazaborolanes
y
*
Siphamandla W. Hadebe , Siphamandla Sithebe, Ross S. Robinson
Warren Research Laboratory, School of Chemistry, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2010
Received in revised form 6 March 2011
Accepted 28 March 2011
Available online 19 April 2011
Nitrogen-based boronate esters, such as 2-octyl-benzo-1,3,2-diazaborolane, 2-phenethyl-benzo-
1,3,2-diazaborolane, and 2-{(1E)-hexenyl}-benzo-1,3,2-diazaborolane have been shown to be suitable
coupling partners with arylhalides in microwave accelerated Suzuki cross-coupling reactions. Reaction
yields of up to 89% were achieved. The use of a silicon group attached to the nitrogen atom, proved to
enhance the reactivity of 2-octyl-benzo-1,3,2-diazaborolane.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Hydroboration
2-Alkyl-1,3,2-diazaborolanes
Coupling reactions
Microwave
1. Introduction
We have recently published a new route to the synthesis of
nitrogen-based organoboranes via rhodium-catalyzed hydro-
For two decades there has been intense research into the
palladium-catalyzed cross-coupling reaction also known as the
SuzukieMiyaura cross-coupling reaction since its discovery by
Miyaura, Yanagi, and Suzuki.1 The Suzuki-coupling methodology
has proven to be extremely powerful and versatile in the formation
of carbonecarbon bonds,2aed and for the production of building
blocks of pharmaceutical importance.2eeh
boration to afford organoboranes in high yields.14
The success of our new approach prompted our research into
the synthesis of a range of nitrogen-based organoboranes (Table 1)
and to explore their respective Pd-mediated coupling reactions
with a range of arylhalides as shown in Scheme 1.
2. Results and discussion
Recently, a number of research groups have focused on the de-
velopment of new palladium catalyst systems,3e5 more effective
ligands6 and bases7 with the aim of enhancing the applicability of
the Suzuki type chemistry. Much emphasis has been on in-
vestigating the effects of different solvents, additives, ligand8,9,10a,b
and more recently the use of microwave irradiation.10c However,
literature precedent accumulated in this area has been based almost
exclusively on the utility of boronic acids and boronate esters.1,2 To
date, only a few research groups have directed their attention to-
ward expanding the scope of other potential Suzuki-coupling type
organoboranes.8,11,12 Specifically, nitrogen-based organoboranes
have not been investigated in SuzukieMiyaura chemistry. To the
best of our knowledge, only a few publications have been reported
on the synthesis of nitrogen-based organoboranes.13
Nitrogen-based organoboranes, namely 2-octyl-benzo-1,3,2-
diazaborolane 2, 2-phenethyl-benzo-1,3,2-diazaborolane 3, and
2-[2-(4-methoxyphenyl)-ethyl]benzo-1,3,2-diazaborolane 4 were
synthesized from benzo-1,3,2-diazaborolane 1, which is readily
prepared from commercially available boraneemethyl sulfide
complex and inexpensive o-phenylenediamine, as shown in Table 1.
It was interesting to note that 1 was very robust to air and moisture
when compared to the widely utilized oxygen analogues benzo-
1,3,2-dioxaborolane (catecholborane) and 4,4,6,6-tretramethyl-
1,3,2-dioxaborolane (pinacolborane). Benzo-1,3,2-diazaborlane 1
could be handled in an open vessel for several hours without no-
table oxidation to boric acid.14
We were delighted at the ease with which we were able to
prepare the novel organoboranes 2, 3, and 4 in excellent yields, via
Rh(I) catalyzed hydroboration of the corresponding alkenes. Rh(I)
catalyzed reactions of similar alkenes with catecholborane have
previously been intensively investigated and have been found to
lead to a mixture of both internal and terminal products, which is
* Corresponding author. Tel.: þ27 33 260 6363; fax: þ27 33 260 5009; e-mail
address: RobinsonR@ukzn.ac.za (R.S. Robinson).
y
Present address: Hydroprocessing and Catalysis, Sasol Technology Research and
Development, P.O. Box 1, Sasolburg 1947, South Africa.
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.03.095

4278
S.W. Hadebe et al. / Tetrahedron 67 (2011) 4277e4282
Table 2
Pd-catalyzed coupling reaction of bromobenzene with 2-octyl-benzo-1,3,2-dia-
zaborolane, optimum condition survey
Table 1
RhCl(PPh3)3-catalyzed synthesis of nitrogen-based organoboranes
H
2 mol %
CH2Cl2
Pd catalyst,
ligand, base
(CH2)7CH3
Br
N
H
RhCl(PPh3
)
3
NH2
NH2
+
(CH2)7CH3
B
40 °C
N
N
condition
BH3 ·SMe2
BH
product
+
N
H
H
5 hrs
olefin
a
> 99%
5a
6a
2
1
Entry
Catalyst
Base
Ligand
Condition
Yielda (%)
Entry Olefin
Conditions
25 ꢀC, 24 h
Product
Yield (%)
92b
1
2
3
4
5
6
7
8
9
10
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(PPh3)4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Aq Na2CO3
Aq K2CO3
K2CO3
K2CO3
K2CO3
None
None
None
None
PPh3
PPh3
PCy3
PCy3
PCy3
PCy3
A
0
0
2
2
5
2
0
30
50
88
H
N
A
Ab
B
B
6
1
N
H
Ab
B
2
K2CO3
K3PO4$H2O
K3PO4$H2O
K3PO4$H2O
K3PO4$H2O
Ab
Ac
Bd
C
H
N
B
2
40e65 ꢀC, 48 h
81b
N
H
3
Reaction conditions: (A) 1.0 equiv of 2, 1.0 equiv of bromobenzene, 3.0 equiv of base,
benzene, 4 mol % Pd(PPh3)4 or Pd(OAc)2, reflux for 24 h. (B) Same as (A) but DMF was
used and the mixture was capped in a closed vessel and irradiated with 100 W of
microwave energy for 1 h. (C) Solvent free, 4 mol % Pd(OAc)2, and 8 mol % PCy3 was
used, closed vessel 50 W microwave irradiation, 5 min.
H
N
B
3
40e65 ꢀC, 60 h
N
H
79b
78b
O
O
a
Isolated yields after flash column chromatography on silica gel.
DMF used instead.
48 h reflux in THF.
b
4
c
H
N
d
2 h reflux in THF.
10e15 ꢀC
HBBr2, 3 h
B
4
N
H
3
5
a11B NMR spectroscopy was used to monitor the formation of benzo-1,3,2-
Br
diazaborolane 1, the yields are based on 11B NMR spectroscopy.
Si
b
N
Isolated yields after flash column chromatography on silica gel. All reactions
(CH2)7CH3
were conducted under a dry nitrogen atmosphere.
B (CH2)7CH3
N
Pd (AOc)2, PCy3,
Si
Pd catalyst,
50 %
H
N
H
N
reflux, 24 hrs
ligand, base
Rh catalyst
olefin
R
BH
B
7
6a
N
H
N
H
Br
Scheme 2. Coupling reaction of silylated diazaborolane 7.
R
R = (CH2)5CH3, Ph, PhOMe
Further investigations with 2-octyl-benzo-1,3,2-diazaborolane
2 incorporating the use of K3PO4$H2O, PCy3, Pd(OAc)2 in conjunc-
tion with microwave irradiation afforded a slightly higher yield of
50% in 2 h (Table 2, entry 9) compared to conventional heating (ca.
30% in 48 h; Table 2, entry 8). The reaction yields were improved
significantly to 88% when solvent free conditions were employed,
furthermore, reaction completion was reached in only 5 min with
microwave irradiation (Table 2, entry 10).
Having achieved optimal reaction conditions (Table 2, entry 10),
we investigated the utility of diazaborolane 2, 3, and 5 with different
arylhalides (X¼Cl, Br, and I) in order to assess the scope and the
limitations of such unusual Suzuki-coupling partners. The results
obtained are summarized in Table 3. Working with optimized
reaction conditions, diazaborolane 2 afforded the coupled-product
6b in low 35% yield with an electron donating substituted sub-
strate (Table 3, entry 2), however, an appreciably high yield of 89%
was achieved with a more conjugated substrate (Table 3, entry 3). An
arylhalide bearing an electron-withdrawing substituent reacted
moderately with diazaborolane 3 in toluene affording the cross-
coupled product 6d in 57% yield (Table 3, entry 4). A solvent free
cross-coupling reaction of diazaborolane 3 with bromobenzene
furnished 6e in 79% (Table 3, entry 5).
Scheme 1. Rhodium-catalyzed hydroboration and Pd-mediated coupling reactions.
not the case in our study.15 In addition, our research group has also
successfully developed a convenient procedure for the synthesis of
2-alkenyl-benzo-1,3,2-diazaboranes from terminal alkynes without
the use of Rh(I) catalyst (Table 1, entry 4).
A model study was conducted in order to optimize the condi-
tions for SuzukieMiyaura coupling reactions. In this study 2-octyl-
benzo-1,3,2-diazaborolane 2 was reacted with bromobenzene as
a substrate (Table 2). This reaction was repeated several times
varying commonly used Suzuki-coupling reagents (Table 2, entries
1e7), however, all attempts failed to improve the coupling reaction
in yields greater than ca. 5%. Changing the solvent to THF and
allowing the reaction to reflux for 48 h gave a moderately improved
yield of 30% (Table 2, entry 8).
The low reactivity of 2 was attributed to the reduced Lewis
acidity of the boron atom, due to pronounced overlap of the ni-
trogen lone pair of electrons into the vacant pz-boron orbital. In
order to enhance the reactivity of this species, it was envisaged that
the introduction of a silicon hetero-atom
a to the nitrogen would
diminish this orbital overlap. Consequently, 2-octyl-1,3-bis-trime-
thylsilanyl-benzo-1,3,2-diazaborole 7 (Scheme 2) was synthesized.
As anticipated, 7 was more reactive than 2 giving ca. 50% of 6a
when treated similarly to entry 8 of Table 2. Despite the increased
reactivity, 7 was also more susceptible to oxidation during purifi-
cation which made it difficult to work with.
Applying the same optimal reaction conditions to couple dia-
zaborolane 5 to different arylhalides, however, failed to give the
coupled-product in excellent yields. Under these conditions, the
coupling reaction of diazaborolane 5 with 9-bromoanthracene and
p-bromonitrobenzene afforded the desired product in 34% and 64%,

S.W. Hadebe et al. / Tetrahedron 67 (2011) 4277e4282
4279
Table 3
respectively (Table 3, entries 6 and 7). As a result, this encouraged
us to screen more well known Suzuki reagents. In summary, the
Palladium-catalyzed cross-coupling reactions of 2 and 3 with arylhalides
Entry
1
Bromide
Diazaborolane
Product
Yield (%)
combination of Pd(OAc)2, PCy3, K2PO4$H2O, diazaborolane
5
H
N
(2 equiv), and arylhalide (1 equiv) in 1,4-dioxane proved to be
suitable reaction conditions for the cross-coupling of diazaborolane
5 to arylhalides.
Under these new improved reaction conditions, diazaborolane 5
was smoothly coupled to 9-bromoanthracene to provide 6f in 84%
yield (Table 3, entry 6). The cross-coupling reaction of a strongly
deactivated substrate (p-bromonitrobenzene) in the presence of
diazaborolane 5 (2 equiv) in 1,4-dioxane afforded 6g quantitatively
(Table 3, entry 7).
As shown in Table 3 (entry 8), the coupling reaction between
diazaborolane 5 and p-bromoacetophenone afforded the coupled-
product in moderate 67% yield. This yield is lower than the 86%
yield reported by Yamada et al.16 however, it is worth noting that,
our yield was achieved in 20 min compared to 9 h of reflux docu-
mented by Yamada et al.16
Under the developed optimal reaction conditions, p-bromo-
phenol and p-bromoacetophenone reacted moderately affording
the coupled-product 6i and 6j in 62% and 67% yields, respectively
(Table 3, entries 9 and 10). However, the coupling reaction of
o-iodomethylbenzoate furnished 6k in 74% yield (Table 3, entry 11).
Lastly, we investigated the coupling reaction of p-chlor-
obenzaldehyde, however, attempts failed to give any desired
product even after 40 min (Table 3, entry 12). It is not surprising
though, as it is well documented in the literature that arylchlorides
are significantly less reactive due to their slow oxidative addition to
palladium (0) catalyst.11,17
Br
B
B
(CH2
)
)
7CH3
88a
N
H
(2)
H
N
(CH2
O
)7CH3
Br
O
(CH2
7CH3
2
35a
N
H
(6b)
(2)
H
N
(CH2)7CH3
Br
B
(CH2
)
7CH3
3
4
89a
57b
N
H
(2)
(6c)
(CH2)2Ph
H
N
B
(CH2
(CH2
)
)
2Ph
2Ph
N
H
O
2N
(6d)
(3)
H
N
(CH2)2Ph
Br
B
5
6
7
79c
N
H
(6e)
(3)
(CH2)3CH3
H
N
(CH2
(CH2
)
)
3CH3
Br
(34)
84d
B
B
N
H
(5)
(5)
(6f)
(CH )3CH3
2
H
N
3CH3
(64)
81d
N
H
3. Conclusions
O2N
(6g)
In Summary, 2-alkenyl and 2-alkyl-benzo-1,3,2-diazaborolanes
are noticeably uncommon coupling partners in the Suzu-
kieMiyaura cross-coupling reactions. We have successfully de-
veloped standard reaction conditions for the cross-coupling of such
diazaborolane compounds with a wide range of arylhalides These
reactions proceed smoothly in the presence of an inexpensive and
widely used combination of Pd(OAc)2 and PCy3, affording the
coupled-products in moderate to excellent yields in only 20 min.
Our standard reaction conditions have proven to be versatile and
general tolerating a variety of functional groups, such as OMe, NO2,
OH, COOMe, and COMe.
(CH )3CH3
2
H
N
Br
(CH2
(CH2
(CH2
)
)
)
3CH3
3CH3
3CH3
B
B
B
8
67d
62d
67d
N
H
O
O
(5)
(5)
(5)
(6h)
(CH2)3CH3
H
N
Br
9
N
H
HO
HO
O
(6i)
(CH )3CH3
2
H
N
Br
10
N
H
4. Experimental
4.1. General
O
O
O
(6j)
(CH2)3CH3
All glassware was thoroughly dried in an oven at ca. 150 ꢀC
overnight. The glassware was further flame dried by heating with
a hot air gun under reduced pressure and allowed to cool 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 before use, and they were discarded after single
use. On assembling of the glassware, all joints were wrapped
with TeflonÒ tape and were subsequently sealed. 1H, 13C, and 11B
NMR spectra were recorded on a Varian Unity-Inova 500 MHz
and/or Bruker 400 MHz UltraShield spectrometers. 1H NMR and
13C NMR spectra were recorded in CDCl3 and were referenced
using the residual chloroform signal at 7.25 ppm and 77.0 ppm,
respectively. 29Si NMR spectra were referenced to TMS as an
external standard (0.0 ppm). All 11B NMR spectra were refer-
enced to BF3$OEt2 as an external standard (0.0 ppm) contained
H
N
I
(CH2
)
3CH3
B
O
O
11
74d
N
H
O
O
(5)
(6k)
(CH2)3CH3
Cl
12
NRe
O
O
(61)
a
Reaction condition C: Pd(OAc)2 (4 mol %), PCy3 (8 mol %), K3PO4$H2O (3 equiv),
solvent free, closed vessel, 100 ꢀC, 50 W microwave irradiation, 5 min.
Reaction condition D: Pd(OAc)2 (4 mol %), PCy3 (8 mol %), K3PO4$H2O (3 equiv),
toluene, 100 ꢀC, closed vessel, 200 W microwave irradiation, 15 min.
b
c
Reaction ondition E: same as condition C, however, 200 W of microwave irra-
diation was used.
d
Reaction condition F: Pd (OAc)2 (4 mol %), PCy3 (8 mol %), K3PO4$H2O (3 equiv),
closed vessel, 50 W microwave irradiation, 1,4-dioxane, 100 ꢀC, 20 min. All yields
refer to isolated material. Yields in parenthesis were obtained under condition D.
e
NR refers to no reaction.

4280
S.W. Hadebe et al. / Tetrahedron 67 (2011) 4277e4282
within a sealed capillary insert. 11B spectroscopy was utilized in
order to identify the compounds as well as to monitor the
progress of the reactions.
221.1247, calculated for C14H14N2B 221.1250. IR (nmax) neat 3385,
3364, 3027, 1621 cmꢂ1
.
GCeMS analyses were performed on a ThermofinniganÒ (GC)
coupled with a PolarisQÒ (MS) system. Thin layer chromatography
was performed on silica gel (60 F254) plates from Merck. Flash
column chromatography was performed on SP Silica Gel 60
(230e400-mesh ASTM) from Merck. HRMS were conducted on
a PerkineElmerÒ Spectrum 100 FT-IR spectrometer (with universal
ATR sampling accessory). All solvents were purified by distillation
and dried prior to use. CH2Cl2 was distilled over P2O5 under dry
nitrogen, THF, DMF, and 1-octene were all distilled over sodium
wire in the presence of an indicator benzophenone.
The solvents were distilled and transferred via cannula to a flame
dried, nitrogen flushed flask containing 0.4 nm molecular sieves (ac-
tivated in the furnace at 600 ꢀC and cooled underdrynitrogen) prior to
use. o-Phenylenediamine was obtained from Merck-Schuchardt.
BH3$SMe2 (1 M solution in CH2Cl2) and Trise(triphenylphosphine)e
rhodium(I)-chloride were obtained from SigmaeAldrich Co. All these
reagents were used without further purification.
4.2.4. 2-{2-(4-Methoxyphenyl)-ethyl}-benzo-1,3,2-diazaborolane
4. Method A was followed, benzo-1,3,2-diazaborolane (20.0 ml,
46.2 mmol) in CH2Cl2, 4-vinylanisole (5.61 ml, 46.2 mmol), and
RhCl(PPh3)3 (2 mol %, 855 mg). However, in this case the reaction
mixture was heated to ca. 60 ꢀC, and kept at that temperature for
60 h. 11B NMR analysis of the product mixture showed that the
starting material had disappeared completely. The boronate emit-
ted an intense blue color under UV light, consequently, migration of
this boronate ester during radial chromatography was followed
using a standard hand held UV lamp (350 nm). Compound 4 was
obtained as a cream-white powder (79%), mp 128e130 ꢀC. 11B NMR
(128 MHz, BF3$OEt2):
ppm 1.56 (t, J¼8.1 Hz, 2H), 2.85 (t, J¼8.0 Hz, 2H), 3.82 (s, 3H), 6.29
d
ppm 31.2 (s). 1H NMR (400 MHz, CDCl3):
d
(s, 2H, 2ꢁ NH), 6.87 (d, J¼8.5 Hz, 2H), 6.90e6.94 (m, 2H), 6.98e7.03
(m, 2H), 7.19 (d, J¼8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3):
d ppm
30.9, 55.2, 110.5, 113.5, 114.0, 118.8, 128.8, 136.0, 136.3, 157.7. HRMS
found: [Mþ] 251.1360, C15H17BN2O required 251.1356.
4.2.5. 2-(1E-1-Hexenyl)-benzo-1,3,2-diazaborolane 5. 1E-1-Hexenyl-
4.2. Synthesis
boronic acid was prepared according to known procedures.17
A
round-bottom flask fitted with Dean and Stark apparatus, a magnetic
stirrer bar and reflux condenser was charged with freshly prepared
1E-1-hexenylboronic acid (0.40 g, 0.313 mmol), o-phenylenediamine
(0.30 g, 0.313 mmol), and toluene (20 ml). The reaction mixture was
heated under reflux for 1 h followed by the removal of solvent in
vacuo. The remaining light-brown solid was purified through a radial
chromatography to afford the desired product as fluffy white plate-
like crystals (78%), mp 45e47 ꢀC. 11B NMR (128 MHz, BF3$OEt2):
4.2.1. Benzo-1,3,2-diazaborolane 1. o-Phenylenediamine (541 mg,
5.0 mmol) was dissolved in dichloromethane (5.0 ml) in a flame-
dried round-bottom flask. After complete dissolution of the solid,
boraneedimethyl sulfide complex (1 M solution in dichloro-
methane, 5.0 ml, 5.0 mmol) was introduced drop wise through the
septum. The resulting mixture was stirred under reflux for 5 h
under a dry atmosphere of nitrogen. Benzo-1,3,2-diazaborolane
was obtained as
a
clear liquid (95%). 11B NMR (160 MHz,
d
ppm 27.2 (s). 1H NMR (400 MHz, CDCl3):
d
ppm 0.93 (t, J¼7.3 Hz, 3H),
BF3$OEt2):
d
ppm 23.9 (d, J¼153.2 Hz, 1H, BH).
1.33e1.52 (m, 4H), 2.19e2.27 (m, 2H), 5.88 (dt, J¼18.1 Hz, 1H), 6.51 (dt,
J¼18.3 Hz, 1H), 6.77e6.84 (m, 2H), 6.98e7.05 (m, 2H), 7.87 (s, 2H, 2ꢁ
4.2.2. 2-Octyl-benzo-1,3,2-diazaborolane
2
(method A). Freshly
NH). 13C NMR (100 MHz, CDCl3):
d ppm 13.3, 21.9, 30.9, 35.6, 110.5,
prepared benzo-1,3,2-diazaborolane (20.0 ml, 46.2 mmol) in
dichloromethane was injected into an oven dried, nitrogen purged
two necked round-bottom flask, followed by 1-octene (7.3 ml,
46.2 mmol), with continuous stirring. To this solution was added
RhCl(PPh3)3 (2 mol %, 855 mg), which had been dissolved in
dichloromethane (5.0 ml) in a separate flame dried, nitrogen flushed
flask. The reaction mixture was allowed to stir at 25 ꢀC for 24 h. The
solvent was then removed in vacuo, and the remaining orange-
yellow waxy material was purified through flash column chroma-
tography on silica gel with hexane as an eluting solvent. Removal of
solvent in vacuo afforded 2-octyl-benzo-1,3,2-diazaborolane 2, as
low melting orange-yellow wax (92%), mp 26.2e28.9 ꢀC. 11B NMR
118.2, 137.1, 148.0. HRMS found: [Mþ] 199.1404, C12H17BN2 required
199.1407. IR (nmax) neat 3381, 3360, 2961, 2928, 2874, 2856, 1634.
4.2.6. Octylbenzene 6a. Representative procedure for solvent-free
Suzuki cross-coupling reactions (method B). To a flame dried, ni-
trogen purged microwave pressure tube (10 ml) equipped with
a magnetic stirring bar, was placed bromobenzene (0.23 ml,
2.17 mmol), K3PO4$H2O (1.0 g, 4.34 mmol), PCy3 (48.7 mg,
0.173 mmol), and Pd(OAc)2 (19.5 mg, 86.8 mmol), the vial was agi-
tated in order to mix the reagents, then purged with nitrogen. To
this heterogeneous mixture was added 2-octyl-benzo-1,3,2-dia-
zaborolane (0.50 g, 2.17 mmol), and subsequent nitrogen purging
for 5 min. The tube was sealed with a snap on cap and inserted into
the CEM DiscoveryÒ synthetic microwave reactor cavity.18 The
microwave cavity was then close with an Intellivent pressure
control system.19 The sample was irradiated with microwave en-
ergy (50 W) for 5 min, and the sample temperature was monitored
by a non-contact, infrared sensor. After 5 min, TLC analysis showed
no evidence of unreacted boronate ester or arylbromide. The
remaining material solidified at the bottom of the vial, and the
target octylbenzene was decanted as clear oil. The remaining solid
was washed with ethyl acetate (2.0 ml), which was eventually
concentrated in vacuo, combined with the decanted oil, and puri-
fied on column chromatography using hexane as an eluting solvent
to afford octylbenzene 6a (88%). 1H NMR, 13C NMR spectroscopic
analysis and GCeMS traces of this sample were identical to the
(160 MHz, BF3$OEt2): d
ppm 31.6 (s). MS (EI): m/z (%) 231 [MD] (18),
230 (100), 229 (15), 145 (15), 132 (16), 119 (17), 118 (31).
4.2.3. 2-Phenethyl-benzo-1,3,2-diazaborolane 3. Method A was fol-
lowed, benzo-1,3,2-diazaborolane (20.0 ml, 46.2 mmol) in CH2Cl2,
styrene (5.3 ml, 46.2 mmol), and RhCl(PPh3)3 (2 mol %, 855 mg).
However, in this case the reaction mixture was heated to ca. 60 ꢀC
and kept at that temperature for 48 h. 11B NMR analysis of the
product mixture showed that the starting material had disappeared
completely. The boronate emitted an intense blue color under UV
light, consequently, migration of this boronate ester during radial
chromatography was followed using a standard hand held UV lamp
(350 nm). Compound 3 was obtained as a cream powder (81%), mp
53e54 ꢀC. 11B NMR (128 MHz, BF3$OEt2):
(400 MHz, CDCl3):
ppm 1.49 (t, J¼7.9 Hz, 2H), 2.80 (t, J¼8.1 Hz,
2H), 6.18 (s, 2H, 2ꢁ NH), 6.77e6.81 (m, 2H), 6.82e6.87 (m, 2H),
d
ppm 31.2 (s). 1H NMR
d
commercial samples. 1H NMR (400 MHz, CDCl3):
d ppm 0.85
(t, J¼6.7 Hz, 3H), 1.21e1.40 (m, 10H), 1.60e1.69 (m, 2H), 3.65
7.09e7.25 (m, 5H). 13C NMR (100 MHz, CDCl3):
d ppm 31.9, 110.6,
(t, J¼7.8 Hz, 2H), 7.18e7.31 (m, 2H), 7.42e7.56 (m, 2H), 7.63e7.69
118.9,125.7,128.0,128.4,136.1, 144.3. MS (EI): m/z (%) 223 [Mþ] (16),
(m, 1H). 13C NMR (100 MHz, CDCl3):
d ppm 14.2, 22.7, 29.3, 29.5,
222 (100), 221 (27), 132 (17), 131 (44), 118 (29). HRMS found: [Mþ]
29.6, 29.7, 31.5, 31.9, 125.6, 128.3, 128.4, 143.0. MS (EI): m/z (%) 191

S.W. Hadebe et al. / Tetrahedron 67 (2011) 4277e4282
4281
[Mþ] (3), 190 (5), 133 (10), 92 (100), 91 (72), 65 (7), 41(12). All the
(100 MHz, CDCl3):
d
ppm 37.9, 125.9, 138.2, 138.3, 141.8.The
characterizations of the titled compound are consistent with the
characterizations of the titled compound are consistent with the
reported data.20,21
reported data.25,26
4.2.6.1. 1-Methoxy-2-octyl-benzene
employed, 1-bromo-2-methoxy-benzene (0.27 ml, 2.17 mmol),
K3PO4$H2O (1.00 g, 4.34 mmol), PCy3 (48.7 mg, 0.173 mmol),
Pd(OAc)2 (19.5 mg, 86.8 mmol), and 2-octyl-benzo-1,3,2-diazabor-
olane (0.50 g, 2.17 mmol). However, the reaction mixture was
irradiated with microwave energy (50 W) for 40 min to afford
1-methoxy-2-octyl-benzene as a clear oil (35%). 1H NMR (400 MHz,
6b. Method
B
was
4.2.8. Representative procedure for Suzuki cross-coupling of dia-
zaborolane 5 (method E). A microwave tube equipped with a mag-
netic stirring bar was charged with diazaborolane 5 (0.20 g,
0.10 mmol), K3PO4$H2O (0.35 g, 1.50 mmol), PCy3 (11.2 mg,
0.040 mmol), Pd(OAc)2 (4.40 mg, 0.020 mmol),1,4-dioxane (0.2 ml),
and the corresponding arylhalide (0.5 mmol). The reaction tube
was fitted with a snap on cap and irradiated with microwave en-
ergy (15 W) for 20 min at 100 ꢀC. After the completion of the re-
action, acetone was added to the tube and the content of the tube
was filtered into a round-bottom flask. The solvent was removed in
vacuo and the remaining black residue was purified by silica gel
column chromatography using hexane/ethyl acetate (9:1) mixture
as an eluting solvent.
CDCl3):
d
ppm 0.78 (t, J¼7.0 Hz, 3H), 1.10e1.47 (m, 10H), 1.55e1.62
(m, 2H), 2.50 (t, J¼7.6 Hz, 2H), 3.71 (s, 3H), 6.71e7.23 (m, 4H). The
1H NMR of 6a is consistent with the 1H NMR data reported in
literature.6
4.2.6.2. 9-Octyl-anthracene 6c. Method
B
was employed,
9-bromoanthracene (0.60 g, 2.17 mmol), K3PO4$H2O (1.00 g,
4.34 mmol), PCy3 (48.7 mg, 0.17 mmol), Pd(OAc)2 (19.50 mg,
4.2.8.1. 9-{(1E)-Hexenyl}anthracene 6f. Method E was followed
using 9-bromoanthracene (0.13 g, 0.5 mmol). Compound 6f was
obtained as a light yellow solid (84%), mp 53e54 ꢀC. 1H NMR
86.8
mmol), and 2-octyl-benzo-1,3,2-diazaborolane (0.50 g,
2.17 mmol). The solid product obtained was dissolved in hexane,
chromatographed, and afforded 9-octyl-anthracene as glassy
(400 MHz, CDCl3):
d
ppm 1.06 (t, J¼7.3 Hz, 3H), 1.52e1.62 (m, 2H),
crystals (89%), mp 60e63 ꢀC. 1H NMR (400 MHz, CDCl3):
d
ppm 0.79
1.64e1.75 (m, 2H), 2.50e2.58 (m, 2H), 6.08 (dt, J¼16.1, 6.7 Hz, 1H),
(t, J¼7.3 Hz, 3H), 1.11e1.34 (m, 8H), 1.40e1.47 (m, 2H), 1.63e1.73 (m,
7.14 (d, J¼16.0 Hz, 1H), 7.46e7.51 (m, 4H), 7.99e8.05 (m, 2H),
2H), 3.46 (t, J¼7.3 Hz, 2H), 7.28e7.40 (m, 4H), 7.82e7.89 (m, 2H),
8.32e8.39 (m, 3H). 13C NMR (100 MHz, CDCl3):
d ppm 14.12, 22.51,
8.12e8.20 (m, 2H), 8.27 (s, 1H). 13C NMR (100 MHz, CDCl3):
d
ppm
31.67, 33.41, 125.02, 125.07, 125.31, 125.33, 125.76, 126.24, 128.17,
128.56, 129.65, 131.54, 133.73, 139.62. MS (EI): m/z (%) 189.17 (6),
201.21 (10), 202.20 (66), 217.18 (100), 218.16 (71), 231.16 (63),
260.08 [Mþ] (79), 261.11 (20). HRMS: found [Mþ] 260.1565, calcu-
lated for C20H20 260.1570.
14.2, 22.8, 28.2, 29.4, 29.7, 30.5, 31.5, 32.0, 124.6, 124.8, 125.4, 128.2,
129.3, 131.8, 135.5. All the characterizations of the titled compound
are consistent with the reported data.22,23
4.2.6.3. 1-(4-Nitrophenyl)-2-phenylethane 6d. Representative
procedure for solvent-free Suzuki cross-coupling reactions (method
C). A microwave tube equipped with a magnetic stirrer bar was
charged with 4-bromonitrobenzene (0.10 g, 0.49 mmol),
K3PO4$H2O (0.34 g, 1.47 mmol), PCy3 (11 mg, 0.039 mmol),
Pd(OAc)2 (4.40 mg, 0.020 mmol), 2-phenethyl-benzo-1,3,2-dia-
zaborolane 3 (2 equiv), and toluene (0.1 ml). The reaction tube was
fitted with a snap on cap and irradiated with microwave energy
(200 W) for 15 min. After the completion of the reaction, acetone
was added to the tube and the content of the tube was filtered into
a round-bottom flask. The solvent was removed in vacuo and the
remaining black residue was purified with a flash column using
hexane/ethyl acetate (8:2) as an eluting solvent to afford the title
compound 6d as a colorless crystals (57%), mp 55e56 ꢀC. 1H NMR
4.2.8.2. 4-{(1E)-Hexenyl}nitrobenzene 6g. Method E was followed
using 4-bromonitrobenzene (0.10 g, 0.5 mmol). Compound 6g was
obtained as a clear-yellow oil (81%). 1H NMR (400 MHz, CDCl3):
d
ppm 0.95 (t, J¼7.5 Hz, 3H), 1.34e1.56 (m, 4H), 2.25e2.31 (m, 2H),
6.44e6.47 (m, 2H), 7.46 (d, J¼8.9 Hz, 2H), 8.16 (d, J¼9.0 Hz, 2H). 13C
NMR (100 MHz, CDCl3):
d ppm 13.87, 22.26, 31.11, 32.86, 123.94,
126.30, 128.09, 136.64, 144.47, 146.44. MS (EI): m/z (%) 98.11 (6),
91.14 (18), 116.15 (33), 119.08 (42), 149.01 (100), 150.02 (15), 188.01
(6), 205.02 [Mþ] (28). HRMS: found [MþþNa] 228.0999, calculated
for C12H15NO2Na 228.1000. IR (nmax) neat 2957, 2928, 1595, 1512,
1337, 1108 cmꢂ1 27
.
4.2.8.3. 4-{(1E)-Hexenyl}acetophenone 6h. Method E was fol-
lowed using 4-bromoacetophenone (0.10 g, 0.5 mmol). Compound
6h was obtained as a colorless oil (67%). 1H NMR (400 MHz, CDCl3):
(400 MHz, CDCl3):
d
ppm 2.98 (t, J¼7.7 Hz, 2H), 3.06 (t, J¼7.7 Hz,
2H), 7.15 (d, J¼7.6 Hz, 2H), 7.23 (t, J¼6.9 Hz, 1H), 7.27e7.33 (m, 4H),
8.14 (d, J¼8.6 Hz, 2H). 13C NMR (100 MHz, CDCl3):
d
ppm 37.2, 37.6,
d
ppm 0.95 (t, J¼7.3 Hz, 3H), 1.33e1.43 (m, 2H), 1.43e1.56 (m, 2H),
123.6,126.3,128.5,129.4,140.5,146.5,149.4. HRMS found: [MþþNa]
2.21e2.29 (m, 2H), 2.58 (s, 3H), 6.42 (m, 2H), 7.41 (d, J¼8.3 Hz, 2H),
250.0847, C14H13NNaO2 required 250.0844.24
7.89 (d, J¼8.5 Hz, 2H). 13C NMR (100 MHz, CDCl3):
d ppm 13.89,
22.26, 26.46, 31.27, 32.82, 125.89, 128.72, 134.49, 135.43, 142.69,
197.49. MS (EI): m/z (%) 103.16 (8), 115.15 (25), 131.10 (80), 146.02
(39), 159.11 (10), 187.14 (100), 202.03 [Mþ] (60), 203.04 (10). HRMS:
found [MþþNa] 225.1255, calculated for C14H18ONa 225.1255. IR
4.2.7. Dibenzyl 6e. Representative procedure for solvent-free Suzuki
cross-coupling reactions (method D). A microwave tube equipped
with a magnetic stirrer bar was charged with bromobenzene
(0.1 ml), K3PO4$H2O (0.34 g, 1.47 mmol), PCy3 (11 mg,
0.039 mmol), Pd(OAc)2 (4.40 mg, 0.020 mmol), and 2-phenethyl-
benzo-1,3,2-diazaborolane 3 (0.11 g, 0.49 mmol). The reaction
tube was fitted with a snap on cap and irradiated with micro-
wave energy (200 W) for 15 min. After the completion of the
reaction, acetone was added to the tube and the content of the
tube was filtered into a round-bottom flask. The solvent was
removed in vacuo and the remaining black residue was purified
with a flash column using hexane as an eluting solvent to afford
the title compound as a colorless crystals (79%), mp 52 ꢀC [lit.24
51 ꢀC]. MS (EI): m/z (%) 65.04 (24), 91.07 (100), 92.06 (7),
(
nmax) neat 2957, 2927, 1678, 1601, 1265 cmꢂ1 28
.
4.2.8.4. 4-{(1E)-Hexenyl}phenol 6i. Method E was followed using
4-bromophenol (0.086 g, 0.5 mmol). Compound 6i was obtained as
a colorless oil (62%). 1H NMR (400 MHz, CDCl3):
ppm 0.93 (t,
d
J¼7.0 Hz, 3H),1.36e1.50 (m, 4H), 2.16e2.23 (m, 2H), 6.08 (dt, J¼15.8,
6.9 Hz, 1H), 6.30 (d, J¼15.8 Hz, 1H), 6.77 (d, J¼8.6 Hz, 2H), 7.23 (d,
J¼8.5 Hz, 2H). 13C NMR (100 MHz, CDCl3):
d ppm 13.95, 22.27, 31.27,
32.66, 67.03, 115.43, 127.19, 129.01, 129.07, 154.58. MS (EI): m/z (%)
79.16 (16), 103.10 (21), 105.11 (71), 133.09 (100), 134.11 (15), 147.09
(6), 176.04 [Mþ] (57), 177.04 (9). HRMS: found [Mþ] 175.1125, cal-
culated for C12H15O 175.1123. IR (nmax) neat 3313, 2956, 2928, 1600,
104.07 (22), 182.04 [Mþ] (32). 1H NMR (400 MHz, CDCl3):
2.95 (s, 4H), 7.18e7.25 (m, 5H), 7.26e7.34 (m, 5H). 13C NMR
d ppm
1511, 1218 cmꢂ1 29
.

4282
S.W. Hadebe et al. / Tetrahedron 67 (2011) 4277e4282
4.2.8.5. 4-{(1E)-Hexenyl}methylbenzoate 6j. Method E was fol-
acknowledged for computational expertise, Mr. Craig Grimmer for
NMR, and Mrs. Caryl Janse van Rensburg for MS assistance.
lowed using 4-bromomethylbenzoate (0.11 g, 0.5 mmol). Compound
6j was obtained as a colorless oil (67%). 1H NMR (400 MHz, CDCl3):
d
ppm 0.95 (t, J¼7.2 Hz, 3H), 1.35e1.44 (m, 2H), 1.44e1.55 (m, 2H),
References and notes
2.25 (q, J¼6.6 Hz, 2H), 3.92 (s, 3H), 6.32e6.48 (m, 2H), 7.40 (d,
J¼8.1 Hz, 2H), 7.97 (d, J¼8.0 Hz, 2H). 13C NMR (100 MHz, CDCl3):
1. Miyaura, N.; Yanagi, T.; Suzuki, A. Synth. Commun. 1981, 11, 513e519.
2. (a) Shultz, A. D.; Boal, A. K.; Driscoll, D. J.; Kitchin, J. R.; Tew, G. N. J. Org. Chem.
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Tetrahedron Lett. 1999, 40, 3037e3340; (c) Kim, E.-s.; Yoo, S.-e.; Yi, K. Y.; Lee, S.;
Noh, J.-s.; Jung, Y.-S.; Kim, E.; Jeong, N. Bull. Korean Chem. Soc. 2002, 23,
1003e1009; (d) Parry, P. R.; Wang, C.; Batsonov, A. S.; Bryce, R. M.; Tarbit, B. J.
Org. Chem. 2002, 67, 7541e7543; (e) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95,
2457e2483; (f) Moleele, S. S.; Michael, J. P.; de Koning, C. B. Tetrahedron Lett.
d
ppm 13.89, 22.26, 31.30, 32.80, 51.93, 125.73, 129.02, 129.87,
134.20, 142.50, 167.00. MS (EI): m/z (%) 91.17 (25), 115.19 (31), 131.14
(77), 162.03 (100), 187.11 (14), 218.07 [Mþ] (58), 219.04 (11). HRMS:
found [MþþNa] 241.1204, calculated for C14H18O2Na 241.1204. IR
(
nmax) neat 2954, 2927, 1718, 1272, 1107 cmꢂ1 30
.
ꢀ
2006, 62, 2831e2844; (g) Torrado, A.; Lopez, S.; Alvarez, R.; de Lera, A. Synthesis
1995, 285e293; (h) Wallace, D. J.; Chen, C.-y Tetrahedron Lett. 2002, 43,
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2052e2055.
4.2.8.6. 2-{(1E)-Hexenyl}methylbenzoate 6k. Method E was fol-
lowed using 2-iodomethylbenzoate (0.13 g, 0.5 mmol). Compound
6k was obtained as a colorless oil (74%). 1H NMR (400 MHz, CDCl3):
d
ppm 0.95 (t, J¼7.2 Hz, 3H), 1.35e1.55 (m, 4H), 2.24e2.31 (m, 2H),
3.91 (s, 3H), 6.15 (dt, J¼15.6, 6.8 Hz, 1H), 7.15 (d, J¼15.9 Hz, 1H), 7.26
(dd, J¼7.6, 1.5 Hz, 1H), 7.41e7.47 (m, 1H), 7.55 (dd, J¼7.2, 0.84 Hz,
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1H), 7.87 (dd, J¼7.9, 1.4 Hz, 1H). 13C NMR (100 MHz, CDCl3):
d ppm
6. Kondolff, I.; Doucet, H.; Santelli, M. Tetrahedron 2004, 60, 3813e3818.
7. (a) Korenage, T.; Kosaki, T.; Fukumura, R.; Ema, T.; Sakai, T. Org. Lett. 2005, 7,
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13.92, 22.26, 31.41, 32.86, 51.91, 126.40, 127.16, 128.18, 128.40,
130.25, 131.85, 134.05, 139.71, 168.09. MS (EI): m/z (%) 91.19 (14),
115.21 (24), 144.15 (100), 161.11 (69), 162.08 (12), 186.08 (7), 218.00
[Mþ] (36), 219.11 (22). HRMS: found [MþþNa] 241.1205, calculated
for C14H18O2Na 241.1204. IR (nmax) neat 2954, 2927, 1720, 1247,
€
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1076 cmꢂ1 31
.
4.2.8.7. 2-Octyl-1,3-bis-trimethylsilanyl-benzo-1,3,2-diazabor-
olane 7. Freshly prepared 2-octyl-benzo-1,3,2-diazaborolane
(3.00 g, 13.04 mmol) was placed in a flame dried, nitrogen purged
two necked round-bottom flask and then dissolved in THF (15 ml).
To this solution was injected gradually, tetramethyl ethylenedi-
amine (TMEDA) (3.88 ml, 26.1 mmol) via a syringe. The mixture was
stirred for 5 min and a slight excess of butyl lithium (17.4 ml, 1.6 M)
was added drop wise for 30 min, followed by continuous stirring for
24 h at ambient temperature. 11B NMR spectroscopic analysis of the
reaction mixture showed complete conversion of the starting bor-
onate. The solvent was removed in vacuo, and the light-brown solid
was purified without quenching, using flash column chromatog-
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vacuo afforded 2-octyl-1,3-bis-tetramethylsilanyl-benzo-1,3,2-dia-
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zaborolane (83%). 11B NMR (160 MHz, BF3$OEt2):
NMR (99 MHz, TMS):
ppm 6.31 (s). 1H NMR (500 MHz, CDCl3):
ppm 0.74 (s, 18H), 1.13e1.20 (m, 13H), 1.57 (m, 4H), 7.18 (dd, 2H),
7.48 (dd, 2H). 13C NMR (125 MHz, CDCl3):
ppm 2.0, 14.2, 22.9, 29.3,
d
ppm 37.6 (s). 29Si
d
d
d
30.4, 32.2, 113.4, 118.4, 142.2. This compound was sensitive to air
and moisture, thus decomposed gradually after flash column
chromatography.
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Acknowledgements
We gratefully acknowledge NRF and the University of KwaZulu-
Natal for financial assistance, Professor Hendrik G. Kruger is also