Poly(propylene sulfide)-borane: Convenient and versatile reagent for organic synthesis

Article abstract of DOI:10.1016/j.tet.2012.07.037

Poly(trimethylene sulfide)-borane adduct has been used as an efficient borane reagent in hydroboration reactions to produce various organoboranes, which have then been used without isolation in further reactions that involve single, double and triple migrations of alkyl groups. The presence of the polymer causes no problems, but there are practical advantages associated with its use, including lack of odour and easy recoverability.

Full text of DOI:10.1016/j.tet.2012.07.037

Tetrahedron 68 (2012) 7834e7839
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Poly(propylene sulfide)eborane: convenient and versatile reagent for organic
synthesis
,
*
y
*
Keith Smith , Asim A. Balakit, Gamal A. El-Hiti
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 April 2012
Received in revised form 10 June 2012
Accepted 10 July 2012
Available online 17 July 2012
Poly(trimethylene sulfide)eborane adduct has been used as an efficient borane reagent in hydroboration
reactions to produce various organoboranes, which have then been used without isolation in further
reactions that involve single, double and triple migrations of alkyl groups. The presence of the polymer
causes no problems, but there are practical advantages associated with its use, including lack of odour
and easy recoverability.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Boraneetetrahydrofuran complex
Poly(trimethylene sulfide)
Borane adduct
Hydroboration
organoboranes
Organic synthesis
1. Introduction
therefore endeavoured to develop borane adducts that are free of
these disadvantages.
Organoboranes have multifarious applications for organic syn-
theses.1,2 Their importance arises from the fact that they can be
transformed into many different product types containing useful
functional groups (aldehydes, ketones, amines, alkenes, alkynes,
alcohols, etc.), often in a highly stereoselective manner.3,4 Such
intermediates are usually produced by hydroboration1 and the
starting material for such reactions is a complexed form of borane
(BH3). Accordingly, the borane adducts that are used to produce the
organoboranes are very important reagents.
Several alternative borane reagents have been reported, including
borane-1,4-thioxane,6 N-ethyl-N-iso-propylanilineeborane,7 tert-
butyldialkylamineeborane adducts such as tert-butylisopropylme-
thylamineeborane and tert-butylisopropylethylamineeborane,8
N,N-dialkylamineeboranes,9 borane-2-(perfluorooctyl)ethyl methyl
sulfide,10 boraneedodecyl methyl sulfide (DMS) and boraneemethyl
6-morpholinohexyl sulfide (MMS),11 but none of them have been
sufficiently advantageous to rival the two simple adducts of borane
with THF and dimethyl sulfide.
Boraneedimethyl sulfide (BH3eSMe2, BMS) and bor-
aneetetrahydrofuran (BH3eTHF) are the most commonly used
borane adducts.5 Nevertheless, both of these reagents have signif-
icant disadvantages. For example, BH3eTHF is commercially avail-
able only as a dilute solution, 1.0 M in BH3, and it is unstable over
long periods of time. It is therefore not suitable for bulk commercial
use. Although BMS is available as a neat complex,10.0 M in BH3, and
is stable and soluble in a wide range of solvents, it liberates di-
methyl sulfide, which is insoluble in water, highly volatile and
flammable and has an obnoxious odour, thereby causing environ-
mental and safety problems. Over the years researchers have
A number of polymeric borane reagents have also been repor-
ted; these are easy to handle, have minimal odour, and the poly-
meric carrier is easily recoverable.12e16 In principle, they should be
very attractive alternatives to the simple borane complexes. How-
ever, several disadvantages are associated with use of the reported
reagents, such as low borane content, the requirement for an acid
catalyst in the reduction of aldehydes and ketones, the need for use
of excess of the polymeric reagent, or limited solubility of the
complexes. Perhaps because of these reasons, such reagents have
only ever been used in simple hydroborationeoxidation reactions
and reductions of carbonyl compounds and have never been used
in more sophisticated applications in organic synthesis. They have
also not been applied at commercial scale. To be attractive as an
alternative borane source, a borane complex needs to involve
a carrier that is cheap, non-volatile, virtually odourless and easily
removed or recovered from reaction mixtures, have a borane
* Corresponding authors. E-mail addresses: smithk13@cardiff.ac.uk (K. Smith),
el-hitiga@cardiff.ac.uk (G.A. El-Hiti).
y
Permanent address: Chemistry Department, Faculty of Science, Tanta Univer-
sity, Tanta 31527, Egypt.
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.07.037

K. Smith et al. / Tetrahedron 68 (2012) 7834e7839
7835
content approaching that of BMS, and be capable of replacing BMS
across a whole range of organoboron reactions with different re-
agents and leading to different products.
(28.0 mmol) and adduct 1 (ca. 9.40 mmol), which on reaction with
DCME and lithium triethylcarboxide gave tricyclopentylmethanol
(2, Scheme 2) in 80% isolated yield.
Reduction and hydroborationeoxidation reactions place mini-
mal demands on the borane source (which can be used in excess) or
on any organoboron intermediates produced, since the organic
groups are simply individually cleaved and it does not matter if the
boron intermediates produced are monosubstituted, disubstituted
or trisubstituted with alkyl or alkoxy groups. Furthermore, these
reactions involve relatively mild reagents/conditions and give
products with a relatively unreactive functional group. By contrast,
many useful reactions of organoboranes involve more demanding
reagents, give products with more reactive functionalities (e.g.,
amines, ketones), or involve combination of two or more different
groups on the same boron atom into a single product. Such re-
actions, typically carried out in an ether solvent such as THF with
either boraneeTHF or boraneeSMe2 as the borane source, require
the clean formation of individual organoboron intermediates.
While formation of such intermediates by hydroboration reactions
involving boraneeTHF or BMS is well studied, it is not clear that the
presence of a non-volatile sulfide in a reaction mixture will be in-
nocuous or that borane supported on a polymer will lend itself
readily to clean formation and reactions of unsymmetrical orga-
noboranes, which are needed for many reactions. Alternative bo-
rane carriers have not been tested in such reactions.
As a part of our own interest in the synthesis of sulfur-
containing compounds,17 the use of solids in green methodolo-
gies18 and boron chemistry19 we have recently developed a conve-
nient procedure for the synthesis of various polymeric sulfides
from reactions of dihaloalkanes with sodium sulfide nonahydrate20
and have converted them into new polymeric sulfideeborane
complexes (e.g., adduct 1; Scheme 1). Adduct 1 is cheap to make,
has high molarity in BH3, is convenient to handle, and has minimal
odour and low volatility.21 We have also successfully used it in
simple hydroborationeoxidation and reduction reactions.
BH3
..
(CH2)3
S
n
1, LiOCEt3,Cl2CHOMe
2, H2O2, NaOH
1
B
1, 0 oC, 30 min
2, RT, 4 h
OH
2 (80%)
Scheme 2. Synthesis of tricyclopentylmethanol (2).
Isolation of 2 verified that tricyclopentylborane had indeed been
formed in substantial yield. However, Brown and Carlson22 had
reported the synthesis of 2 in 91% yield starting with a standard
(1 M) solution of tricyclopentylborane in THF on a 50 mmol scale. It
was not clear whether the lower yield obtained in the present ex-
periment with the polymereborane complex was an inherent issue
associated with the reagent or whether it was merely a problem of
the experimental procedure, caused by the use of a smaller scale or
preparation of the organoborane in situ or some other difference. In
order better to understand the situation, we repeated the reaction
represented in Scheme 2 but with BH3eMe2S instead of the poly-
mereborane adduct 1. Compound 2 was obtained in 80% yield,
which is the same as that produced using 1. Clearly, the differences
in yields between the reported literature value and that obtained
using 1 do not result from any inherent problems associated with
the reagent.
The second reaction planned was the synthesis of an internal
acetylene from an alkene and a terminal alkyne. In such reactions
a trialkylborane is reacted with a 1-lithioalkyne to form the cor-
responding lithium (1-alkynyl)trialkylborate, which is then reacted
with iodine at low temperature to give the corresponding internal
acetylene.23 The reaction involves a single alkyl migration from
boron to the carbon atom to form a new carbonecarbon bond. We
prepared tricyclopentylborane in situ from adduct 1 as described
for the previous reaction and then reacted it with 1-lithiohexyne
and then iodine as described in the literature, though on
a smaller scale. 1-Cyclopentylhexyne (3, Scheme 3) was isolated in
65% isolated yield based on 1-hexyne. This is a clear indication that
the presence of the polymeric sulfide in the reaction mixture causes
no significant problems. Suzuki et al.23 reported the synthesis of 3
in quantitative yield, but for a reaction conducted on a larger scale
(30.8 mmol R3B), using a standard solution of preformed organo-
borane rather than organoborane prepared in situ, and used GC
estimation rather than isolation of the product for determination of
the yield. It is clear, therefore, that there is no significant disad-
vantage of using the adduct 1 for in situ preparation of the tri-
alkylborane instead of preparing the organoborane using the
traditional reagents for this kind of reaction. The polymeric sulfide
was recovered (75%) by precipitation with diethyl ether and was
investigated by 1H NMR spectroscopy, which showed that the re-
covered polymer had an almost identical spectrum to that of the
polymer originally used to produce the borane complex 1.
BH3
..
..
B2H6 gas
DCM, 2 h
(CH2)3
S
(CH2)3
1
S
x
x
Scheme 1. Synthesis of poly(propylene sulfide)eborane adduct 1.
It was of interest to see whether this reagent would allow ex-
tension of the utility of a polymer-supported borane for the first
time to a range of more sophisticated applications. In order to test
the wider applications of the new borane adduct 1, we decided to
use it in various synthetic pathways leading to a varied set of
compound types, namely a tertiary alcohol, an alkyne, a primary
amine, a ketone, and a (Z)-alkene, and we now report that it can
indeed be used in such applications. It is also easy to recover the
poly(propylene sulfide), which can therefore be recycled.
2. Results and discussion
We first wanted to test whether it was possible to prepare
a single trialkylborane using the polymer adduct. In order to do so
we decided to carry out a reaction in which all three alkyl groups
would migrate from the boron atom to a single carbon atom, so that
subsequent oxidation would give a tertiary alcohol possessing all
the essential features of the original organoborane, thereby veri-
fying the organoborane structure. There are several ways to achieve
such a transformation, including carbonylation,19e cyanidation19f
BH3
..
(CH2)3
S
n
1,
1
LiC
C Bu
B
Bu
1, 0 oC, 30 min
2, RT, 4 h
2, I2
3
(65%)
Scheme 3. Synthesis of 1-cyclopentylhexyne (3).
The next reaction to be tested was the synthesis of a primary
amine via reaction of a trialkylborane with freshly prepared chlo-
ramine. Reactions of this type are reported to involve coordination
and reaction with the anion of
a,a-dichloromethyl methyl ether
(DCME).22 We decided to use the DCME reaction and conducted an
in situ preparation of tricyclopentylborane from cyclopentene

7836
K. Smith et al. / Tetrahedron 68 (2012) 7834e7839
of chloramine to the boron atom, followed by migration of an alkyl
group with displacement of chloride; a maximum of two of the
three alkyl groups migrate from boron to nitrogen, thereby limiting
the yield based on alkyl groups to 67%.24 The reactions are usually
conducted with organoboranes prepared using boraneeTHF.
The use of chloramine could provide a possible complication for
the use of the polymeric sulfide adduct, since the reagent might
oxidize the sulfide groups in the polymer. We also decided to use
tricyclohexylborane, generated in situ from cyclohexene and 1
(Scheme 4), because this would introduce a further potential
complication to use of the polymeric adduct, which produces
a turbid reaction mixture. Hydroboration of cyclohexene with
a simple borane complex results in initial precipitation of dicyclo-
hexylborane dimer, which then slowly dissolves on reaction with
further cyclohexene to give tricyclohexylborane.25 The reaction is
normally self-indicating, therefore, and can be considered complete
when no solid remains. By use of 1, however, it would not be
possible to monitor the dissolution of the dicyclohexylborane di-
mer very clearly and the hydroboration reaction time would have to
be based on literature precedent for a simple complex. A reaction
time of 4 h at room temperature was chosen by analogy with the
standard procedure reported for the in situ preparation of tri-
alkylboranes used in the original report of the generalized chlora-
mine reaction.24 Cyclohexene (36.5 mmol) and 1 (ca. 12.8 mmol
BH3) were stirred in THF for 4 h and then freshly prepared chlo-
ramine (2 mol equiv) and sodium hydroxide solution were added
(Scheme 4). Cyclohexylamine was isolated in 33% yield (based on
cyclohexene) by solvent extraction followed by distillation. Again,
75% of the polymeric sulfide was recovered.
solution, it was by no means obvious that the same conditions
would be appropriate for a polymeric borane complex, where rel-
ative rates of reaction might be different and intermediates might
have different solubilities from the starting complex, potentially
resulting in heterogeneous reaction mixtures and leading to mix-
tures of alkylboron compounds. The final two examples were
therefore designed to explore the possibility of production of un-
symmetrical organoboron compounds as well as testing out other
further reactions. In each case dicyclohexylborane was selected as
the initial target. As stated above, when prepared with boraneeTHF
this normally precipitates out as a dimer, which assists in gaining
selectivity, but it was not known whether the same would apply
with adduct 1.
The fourth test reaction was the synthesis of a ketone via re-
action of a dialkylmethoxyborane with the anion from DCME.26
This is a general reaction that involves migration of both alkyl
groups from boron to carbon, followed by oxidation with alkaline
hydrogen peroxide to give the dialkyl ketone. Adduct 1 (ca.
8.00 mmol BH3) in THF was stirred with cyclohexene (15.8 mmol)
at 0 ꢀC for 2 h and then methanol (7.80 mmol) was added. The
mixture was then further treated with DCME in the presence of
lithium triethylcarboxide (prepared in situ) followed by oxidation
with alkaline hydrogen peroxide (Scheme 5). Dicyclohexyl ketone
(dicyclohexylmethanone, 5) was isolated in 78% yield by distilla-
tion. The published procedure for this compound gave 5 in 85%
isolated yield from a 50 mmol scale reaction of dicyclohex-
ylmethoxyborane.26 The modest diminution in yield for the present
reaction is entirely understandable given the much smaller scale
used and our limited practical experience with the particular type
of reaction, so it may safely be concluded that the adduct 1 can be
used without problem to produce dicyclohexylmethoxyborane.
BH3
..
(CH2)3
S
n
2 equiv. NH2Cl
excess NaOH
1
BH3
..
NH2
B
OMe
B
1, 0 oC, 30 min
2, RT, 4 h
(CH2)3
S
H
B
n
1
MeOH
4
(33%)
0
oC, 2 h
Scheme 4. Synthesis of cyclohexylamine (4).
O
1, LiOCEt3,Cl2CHOMe
2, H2O2, NaOH
The published yield for the case of cyclohexylamine from tri-
cyclohexylborane prepared using boraneeTHF was actually 49%.24
However, the process involved use of chloramine of known con-
centration (whereas in the present reaction its concentration was
assumed based on estimated concentration of the bleach used in its
preparation) and the reaction had been applied on a significantly
larger scale (100 mmol of alkene). It is also likely that the hydro-
boration reaction to produce the organoborane had been left for
a longer period in view of the authors’ experience with such sys-
tems. In the present case, everything was carried out in one pot
without isolation of any intermediates, but synthesis of the in-
termediate tricyclohexylborane was clearly achieved successfully,
even if in somewhat lower yield as a result of the short period for
the hydroboration step. The lower yield of cyclohexylamine than by
use of boraneeTHF was not a major concern since there were many
separate steps involved and none of them were individually opti-
mized for the specific case. It is clear that the adduct 1 can be used
as a borane source even for a reaction involving chloramine as
a reagent and producing a primary amine as product.
The above examples (Schemes 2e4) showed successful pro-
duction of symmetrical trialkylboranes as intermediates by the use
of borane adduct 1. However, many reactions require either un-
symmetrical trialkylboranes or organoboranes that contain a mix-
ture of alkyl groups and other types of substituents. Inevitably, this
involves an initial hydroboration that must stop cleanly at a mon-
oalkylborane or dialkylborane stage prior to addition of a different
alkene or an alternative type of reagent. Although conditions have
been worked out for achieving this in many cases in homogeneous
5
(77%)
Scheme 5. Synthesis of dicyclohexylmethanone (5).
The final reaction was the synthesis of a (Z)-alkene by the
Zweifel reaction, which has normally been accomplished with an
alkenyldialkylborane obtained via hydroboration using BH3eTHF or
BMS.27 Such an alkenyldialkylborane is treated with a mixture of
sodium hydroxide (converts the borane into a hydroxyborate) and
iodine (attacks the alkenyl group and induces rearrangement) to
give an unstable (2-iodoalkyl)boron compound, which eliminates
an iodoboron species to give the final product, a (Z)-alkene. The
particular geometrical isomer results from the stereospecific nature
of all of the intermediate steps. The use of 1 in such a reaction
would therefore test its ability to hydroborate alkynes as well as
alkenes and its ability to form an all-carbon unsymmetrical orga-
noborane cleanly, as well as testing its performance under the
conditions of the Zweifel reaction. Adduct 1 (ca. 8.00 mmol BH3)
was therefore treated successively with cyclohexene (15.8 mmol)
and 1-hexyne (7.80 mmol) in the hope of producing dicyclohexyl-
1-hexenylborane, which was then subjected to alkaline iodination
to produce 6 (Scheme 6). Product 6 was isolated by column chro-
matography in 64% yield, based on hex-1-yne (7.80 mmol). The
reported synthesis of (Z)-1-cyclohexylhex-1-ene6 using BH3eTHF
was accomplished in 75% yield for a reaction on a three times larger
scale (25.0 mmol 1-hexyne).27 The modest diminution in yield
achieved with 1 on a smaller scale and without individual

K. Smith et al. / Tetrahedron 68 (2012) 7834e7839
7837
optimization of reaction conditions is not of significance and it can
safely be concluded that the use of 1 is entirely appropriate for such
reactions.
NMR spectrum and molecular weight, and the yields of benzyl
alcohol were consistently high. It therefore seems that the initial
polymer contains a small amount of relatively short chain oligo-
mers that are lost during the first recovery. Thereafter, the re-
covered polymer is less polydisperse and has a somewhat higher
average molecular weight, but forms a borane complex that be-
haves in a manner similar to that of the initial polymer as a re-
ducing agent.
The ability to recover the polymer increases the attractiveness of
the adduct 1 as a reagent. Its recycling would reduce the cost of the
reagent, while precipitation of the polymer after organoborane
formation but before further reaction would allow such reactions to
be conducted in the absence of the polymeric sulfide where this
was appropriate.
BH3
..
1,
2,
(CH2)3
S
n
Bu
1
0 oC, 2 h
H
1, NaOH
2, I2
H
)
2 B
HC
C Bu
H
(64%)
25 oC, 1.5 h
Bu
H
6
Scheme 6. Synthesis of (Z)-1-cyclohexylhex-1-ene (6).
At this point we had shown that adduct 1 could be used to
produce various different types of organoboranes and that a variety
of further reactions could be conducted with the poly(trimethylene
sulfide) present. In two cases we had also recovered samples of the
polymer by precipitation following addition of diethyl ether and
methanol, and their proton NMR spectra showed that both re-
covered samples were very similar to the original polymer. This
suggested that it should be easy to reuse the polymer to prepare
a fresh batch of adduct 1 from the recovered polymer or to pre-
cipitate the polymer from a reaction mixture prior to subsequent
reaction of the organoborane, which might in some cases be ad-
vantageous if the presence of the polymer is problematical.
Therefore, we decided to investigate the recovery and reuse of the
polymer in more detail. In order to do so we used reduction of
benzaldehyde with adduct 121 as a convenient reaction to study. To
a sample of adduct 1 (2.40 g, ca. 20.0 mmol borane) in THF (57 mL)
was added benzaldehyde (5.30 g, 50.0 mmol) and the mixture was
stirred for 30 min at 0 ꢀC, then for 4 h under reflux conditions,
followed by the addition of methanol (15 mL) to quench the re-
action. The polymeric material was precipitated as a white solid by
the addition of diethyl ether (100 mL) during the work up process
and was removed by filtration. The polymeric sulfide was washed
with aqueous sodium hydroxide (3 M) to remove any boric acid,
then washed with water and dried under vacuum. A sample was
withdrawn for analysis and the rest was treated with diborane gas
to reform adduct 1. The new adduct was then analyzed for borane
content and used in a further reaction with benzaldehyde in
a manner that was identical except for being on a slightly smaller
scale. The whole process of recovery and reuse was then repeated
once more. The benzyl alcohol was produced quantitatively each
time. The amount of polymer recovered and the properties of the
recovered polymer are recorded in Table 1.
3. Conclusions
In conclusion, poly(propylene sulfide)eborane adduct 1 has
proved to be a versatile, efficient and convenient reagent for in situ
generation of a range of organoboron types (trialkylboranes, alke-
nyldialkylboranes, dialkylalkoxyboranes) and their subsequent
conversion into a range of organic compound classes [tertiary al-
cohol, alkyne, (Z)-alkene, primary amine, ketone] by use of several
potentially problematical reagents (hydrogen peroxide, chlora-
mine, iodine, strong bases). All of the reactions proceeded without
problem and yields of isolated products were only marginally lower
than those reported in the literature for reactions involving orga-
noboranes prepared using boraneeTHF or boraneedimethyl sulfide
(BMS), even though the current reactions were generally conducted
on a much smaller scale and were not individually optimized. The
new polymeric borane adduct contains around 8e8.5 mmol of
borane per gram of reagent, which is much greater than that of
boraneeTHF solutions and around two-thirds of that of BMS.
However, compared to BMS it is much more stable in air and the
liberated sulfide is much less volatile, does not have an obnoxious
odour and is much less flammable. The polymeric sulfide, which is
not very expensive to produce at the outset, can be easily recovered
after reactions of the borane complex, and by recycling the material
it could further mitigate any differences in cost compared to BMS.
Adduct 1 could replace BH
3eTHF and BH3eSMe2 complexes in all
cases tried and might well be suitable for large scale industrial
application.
4. Experimental section
4.1. General experimental details
Table 1
The borane content and GPC results for the recovered polymeric sulfide
All reactions were performed under a nitrogen atmosphere.
Glassware was oven dried, assembled hot and allowed to cool
under a stream of nitrogen gas. All chemicals and reagents were
purchased from commercial sources and used without further
purification. THF was distilled from sodium benzophenone ketyl
and other solvents were purified by standard procedures.28 1H
and 13C NMR spectra were recorded on a spectrometer operating
at 400 MHz for 1H and 100 MHz for 13C measurements, re-
Polymer
First use
Second use 70
Recovery (%) Mn (Da) Mw (Da) PDIa
BH3 in 1 (mmol/g)
2.253 8.5
a
a
d
4178
6746
7736
9415
12,451
13,049
1.845 8.3
1.687 7.7
Third use
90
a
Mn¼number average molecular weight; Mw¼weight average molecular weight;
PDI¼poly dispersity index.
spectively. Chemical shifts
d are reported in parts per million
The results showed that recovery of polymer from the first run
was around 70%, but the second recovery was significantly better
(ca. 90%). The 1H NMR spectrum of the polymer recovered from
the first run was broadly similar to that of the initial polymer, but
the average molecular weight of the polymer had increased,
suggesting that the process of recovery of the polymer resulted in
loss primarily of short chain polymeric material, which was
probably more soluble in the ether used to precipitate the poly-
mer. The polymeric sample recovered from the third use was quite
similar to that recovered from the second use, in terms of both
(ppm) relative to TMS. 13C Signals corresponding to CH, CH2, or
CH3 are assigned from DEPT-90 and 135 spectra and all others
carbons are quaternary (C). Low and high-resolution mass
spectra were recorded on a time-of-fight mass spectrometer
using electron impact (EI). IR spectra were recorded on an FT-IR
spectrometer by applying droplets of the material on a NaCl
plate.
The borane adduct 1 was prepared by passing diborane gas into
a solution of the polymeric sulfide in DCM and the borane content
was measured by gas analysis.21

7838
K. Smith et al. / Tetrahedron 68 (2012) 7834e7839
4.2. Synthesis of tricyclopentylmethanol (2)
d
84.4 (C), 79.6 (C), 34.1 (CH2), 31.3 (CH2), 30.4 (CH), 24.9 (CH2), 21.9
(CH2), 18.5 (CH2), 13.6 (CH3); MS (EI) m/z (%) 150 (Mþ, 53), 137 (55),
121 (57), 137 (87), 108 (85), 93 (100), 91 (97), 67 (98); HRMS (EI)
calcd for C11H18 (Mþ) 150.1407, found 150.1409; IR (FT) nmax 2957,
2933, 2871, 2236. The Rf value and spectroscopic data of 3 were
identical with those of an authentic sample prepared according to
the literature procedure.23
Cyclopentene (1.90 g; 28.0 mmol) was added to a stirred cooled
(0 ꢀC) mixture of 1 (1.10 g, ca. 9.40 mmol BH3) and anhydrous THF
(20 mL) under nitrogen and the mixture was stirred at 0 ꢀC for
30 min and then at room temperature for 4 h. In another dried
50 mL flask 3-ethyl-3-pentanol (1.05 g, 9.04 mmol) was added to
a stirred cooled solution of n-BuLi in hexane (6.00 mL of 1.5 M,
9.00 mmol) and the mixture was stirred for 30 min at 0 ꢀC. The
tricyclopentylborane solution was cooled to 0 ꢀC and DCME (1.20 g,
10.4 mmol) was added followed by transfer of the lithium trie-
thylcarboxide by a double ended needle over a period of 10 min.
The cooling bath was removed and the reaction mixture was stirred
for 1 h. The flask was cooled to 0 ꢀC and ethanol (10 mL, 95%) was
added followed by sodium hydroxide (2.40 g, ca. 60.0 mmol) and
slow addition of hydrogen peroxide (9 mL, 35%, ca. 90 mmol). The
mixture was stirred for 1 h at 70 ꢀC and then allowed to cool.
Diethyl ether (20 mL) was added to precipitate the polymeric ma-
terial, which was filtered, and anhydrous potassium carbonate was
added to the filtrate to salt out the aqueous phase. The mixture was
filtered and the solid was washed with ether (2ꢁ10 mL). The filtrate
and washings were combined and the organic layer was separated,
washed with brine (3ꢁ20 mL) and dried over anhydrous magne-
sium sulfate. The solvent was evaporated under reduced pressure
and the crude product was purified by column chromatography
(alumina; hexane/ether; 1:1) to give 2 (1.70 g, 7.20 mmol, 80%) as
a colourless oil (lit. oil)22; Rf (10% Et2O/hexane) 0.22; 1H NMR
4.4. Synthesis of cyclohexylamine (4)
Cyclohexene (3.00 g; 36.5 mmol) was added to a stirred cooled
(0 ꢀC) solution of 1 (1.50 g, ca. 12.7 mmol BH3) in anhydrous THF
(20 mL) under nitrogen and the mixture was stirred at room tem-
perature for 4 h. An aqueous NaOH solution (3 M, 18 mL) was added
to the mixture followed by addition of a freshly prepared solution of
chloramine [ca. 25.0 mmol; prepared as follows: in a 100 mL flask
were placed water (25 mL) and aqueous ammonium hydroxide
(5.00 mL, 5 M); the mixture was cooled to 0 ꢀC after which com-
mercial bleach (aqueous sodium hypochlorite, 50.0 mL, 0.50 M)
was added and the mixture was stirred at 0 ꢀC for 1 h to produce
chloramine]. The mixture was stirred at room temperature for
a further 2 h and then acidified (checked by pH paper) with
hydrochloric acid (2 M). Diethyl ether (25 mL) was added to pre-
cipitate the polymeric material, which was removed by filtration.
Solid sodium hydroxide was used to make the solution strongly
alkaline. The organic layer was separated and dried over anhydrous
MgSO4. The solvent was removed under reduced pressure and the
product was purified by distillation under reduced pressure using
an oil pump to give pure 4 (1.20 g, 12.0 mmol, 33% based on
cyclohexene) as a colourless oil; bp 45e47 ꢀC/35 mmHg; 1H NMR
(400 MHz, CDCl3)
d
2.06 (appt. pentet, J¼9.5 Hz, 3H), 1.36e1.51 (m,
24H), 1.16 (s, exch., 1H); 13C NMR (100 MHz, CDCl3)
d
77.6 (C), 47.8
(CH), 27.9 (CH2), 25.2 (CH2); MS (EI) m/z (%) 218 ([MꢂH2O]þ, 6), 167
(90), 149 (96), 125 (19), 97 (100), 81 (52), 69 (99); HRMS (EI) calcd
for C16H26 ([MꢂH2O]þ) 218.2034, found 218.2035; IR (FT) nmax 3623,
3521, 2941, 1451. The Rf value and spectroscopic data of 2 were
identical in all respects with those of an authentic sample prepared
according to the literature procedure.22
(500 MHz, CDCl3)
d
2.58e2.52 (m, 1H),1.76e1.50 (m, 6H),1.24e0.94
50.3 (CH), 36.7 (CH2), 25.6
(m, 6H); 13C NMR (125 MHz, CDCl3)
d
(CH2), 25.0 (CH2); MS (EI) m/z (%) 99 (Mþ, 15), 82 (4), 70 (12), 67 (6),
56 (100); HRMS (EI) calcd for C6H13N (Mþ) 99.1045, found 99.1048;
IR (FT) nmax 3348, 3276, 2927, 2853. The boiling point and spec-
troscopic data of 4 were identical with those of an authentic sample
obtained from SigmaeAldrich.
4.3. Synthesis of hex-1-ynylcyclopentane (3)
Tricyclopentylborane (ca. 3.70 mmol) was prepared as described
in the synthesis of 2 from cyclopentene (0.749 g; 11.0 mmol) and
adduct 1 (0.450 g, ca. 3.80 mmol BH3). A freshly prepared solution of
hex-1-ynyllithium (ca. 3.50 mmol), prepared by addition of a solu-
tion of n-butyllithium in hexane (1.50 M, 2.40 mL) to a solution of
hex-1-yne (0.290 g, 3.50 mmol) in THF (10 mL) at 0 ꢀC, was added
slowly by syringe and the mixture was stirred for 30 min at 0 ꢀC. The
reaction mixture was stirred efficiently, cooled to ꢂ78 ꢀC, a solution
of iodine (1.00 g, 3.94 mmol) in dry ether (15 mL) was added by
syringe over 30 min and the mixture was stirred for a further 45 min
at ꢂ78 ꢀC. The reaction mixture was left to warm up to room tem-
perature and few drops of saturated aqueous sodium thiosulfate
solution were added to decompose the excess iodine. Diethyl ether
(20 mL) was added to precipitate the polymer, followed by filtration.
The solid was washed with ether (2ꢁ10 mL) and the combined fil-
trate and washings were washed with aqueous sodium hydroxide
solution (2ꢁ20 mL, 3 M). The aqueous layer was extracted with
diethyl ether (2ꢁ20 mL) and sodium hydroxide (4.00 mL, 3 M,
12.0 mmol) was added to the combined organic layers, followed by
dropwise addition of hydrogen peroxide (1.50 mL, 30%, ca.
13.0 mmol). The aqueous layer was saturated with K2CO3 and the
organic layer was separated and then dried over anhydrous MgSO4.
The solvent was removed under reduced pressure to give a yellow
oily material, which was purified by column chromatography (silica
gel, hexane) to give hex-1-ynylcyclopentane 3 (0.341 g, 2.27 mmol,
65%) as a colourless oil (lit. oil);23 Rf (hexane) 0.87; 1H NMR
4.5. Synthesis of dicyclohexylmethanone (5)
Cyclohexene (1.30 g; 15.8 mmol) was added to a stirred cooled
(0 ꢀC) solution of 1 (0.940 g, ca. 8.00 mmol BH3) in anhydrous THF
(14 mL) under nitrogen and the mixture was stirred at 0 ꢀC for 2 h.
Methanol (0.250 g, 7.80 mmol) was then added to form dicyclo-
hexylmethoxyborane. In another 50 mL flask, 3-ethyl-3-pentanol
(0.930 g, 8.00 mmol) was added to a cooled (0 ꢀC) solution of n-
BuLi in hexane (5.00 mL of 1.6 M; 8.00 mmol) and the mixture was
stirred for 30 min at 0 ꢀC to form lithium triethylcarboxide. DCME
(1.06 g, 9.22 mmol) was added to the dicyclohexylmethoxyborane
solution, followed by transfer of the lithium triethylcarboxide so-
lution by a double ended needle over a period of 10 min, then the
reaction mixture was allowed to warm up to room temperature and
stirred for 1 h. Ethanol (3.5 mL) was added followed by NaOH
(0.600 g, 15.0 mmol) then careful addition of hydrogen peroxide
(2.40 mL, 30%, ca. 21.0 mmol). The reaction mixture was stirred for
1 h at 50 ꢀC then left to cool down to room temperature. The re-
action mixture was worked up by addition of brine (20 mL) fol-
lowed by extraction with diethyl ether (2ꢁ50 mL). The combined
organic layers were dried over MgSO4 and the solvent was removed
under reduced pressure. The crude product was purified by distil-
lation under reduced pressure using an oil pump to give 5 as col-
ourless oil (1.20 g, 6.18 mmol, 77%); bp 96e98 ꢀC/1.2 mmHg; 1H
NMR (400 MHz, CDCl3)
d 2.44e2.39 (m, 2H), 1.78e1.56 (m, 10H),
1.39e1.01 (m, 10H); 13C NMR (100 MHz, CDCl3)
d 217.6 (C), 49.2
(400 MHz, CDCl3)
d
2.60e2.53 (m, 1H), 2.16 (dt, J¼2.0, 6.9 Hz, 2H),
(CH), 28.6 (CH2), 25.9 (CH2), 25.7 (CH2); MS (EI) m/z (%) 194 (Mþ,
1.92e1.35 (m,12H), 0.91 (t, J¼6.9 Hz, 3H); 13C NMR (100 MHz, CDCl3)
30),127 (12), 111 (85), 95 (38), 84 (100), 67 (35); HRMS (EI) calcd for

K. Smith et al. / Tetrahedron 68 (2012) 7834e7839
7839
C13H22O (Mþ) 194.1670, found 194.1671; IR (FT) nmax 2928, 2853,
1704. The boiling point and spectroscopic data of 5 were identical
with those of an authentic sample obtained from SigmaeAldrich.
9. Brown, H. C.; Zaidlewicz, M.; Dalvi, P. V. Organometallics 1998, 17, 4202e4205.
10. Crich, D.; Neelamkavil, S. Org. Lett. 2002, 4, 4175e4177.
11. Patra, P. K.; Nishide, K.; Fuji, K.; Node, M. Synthesis 2004, 1003e1006.
12. Crosby, G. A. U.S. Patent 3,928,293, 1975.
13. Domb, A.; Avny, Y. J. Macromol. Sci., Part A: Pure Appl. Chem. 1985, 22, 167e181.
14. Domb, A.; Avny, Y. J. Macromol. Sci., Part A: Pure Appl. Chem. 1985, 22, 183e201.
15. Domb, A.; Avny, Y. J. Appl. Polym. Sci. 1985, 30, 3589e3604.
4.6. Synthesis of (Z)-1-cyclohexylhex-1-ene (6)
16. Rajasree, K.; Devaky, K. S. J. Appl. Polym. Sci. 2001, 82, 593e600.
17. See for example: (a) El-Hiti, G. A.; Hussain, A.; Hegazy, A. S.; Alotaibi, M. A. J.
Sulfur Chem. 2011, 32, 361e395; (b) Metwally, M. A.; Khalifa, M. E.; El-Hiti, G. A.
J. Sulfur Chem. 2010, 31, 205e229; (c) Smith, K.; Barratt, M. L. J. Org. Chem. 2007,
72, 1031e1034; (d) Smith, K.; El-Hiti, G. A.; Hegazy, A. S. J. Sulfur Chem. 2005, 26,
121e131; (e) Smith, K.; El-Hiti, G. A.; Mahgoub, S. A. Synthesis 2003,
2345e2348; (f) El-Hiti, G. A. Sulfur Reports 2001, 22, 217e250; (g) Smith, K.;
Tzimas, M.; Brown, C. M.; Payne, K. Sulfur Lett. 1999, 22, 89e101; (h) Smith, K.;
Tzimas, M.; Brown, C. M.; Payne, K. Sulfur Lett. 1999, 22, 103e123; (i) Smith, K.;
Shukla, A. P.; Matthews, I. Sulfur Lett. 1996, 20, 121e137; (j) Smith, K.; Hou, D. J.
Org. Chem. 1996, 61, 1530e1532; (k) Smith, K.; Anderson, D.; Matthews, I. J. Org.
Chem. 1996, 61, 662e665; (l) Abdel-Megeed, M. F.; Aly, Y. L.; Saleh, M. A.; Abdo,
I. M.; El-Hiti, G. A.; Smith, K. Sulfur Lett. 1995, 19, 129e140; (m) Smith, K.; Tzi-
mas, M. J. Chem. Soc., Perkin Trans. 1 1995, 2381e2382; (n) Smith, K.; Anderson,
D.; Matthews, I. Sulfur Lett. 1995, 18, 79e95; (o) Smith, K.; Lindsay, C. M.; Morris,
I. K.; Matthews, I.; Pritchard, G. J. Sulfur Lett. 1994, 17, 197e216; (p) Davies, J. S.;
Smith, K.; Turner, J. R.; Gymer, G. Tetrahedron Lett. 1979, 5035e5038.
18. See for example: (a) Smith, K. Solid Supports and Catalysts in Organic Synthesis;
Ellis Harwood: Chichester, UK, 1992; (b) Smith, K. Selectivity Through the Use
of Heterogeneous Catalysts In Supported Catalysts and Their Applications;
Sherrington, D. C., Kybett, A. P., Eds.; The Royal Society of Chemistry: Cam-
bridge, 2001; pp 233e241; (c) Delaude, L.; Laszlo, P.; Smith, K. Acc. Chem. Res.
1993, 26, 607e613; (d) Smith, K.; Al-Khalaf, A. K. H.; El-Hiti, G. A.; Pattisson, S.
Green Chem. 2012, 14, 1103e1110; (e) Smith, K.; El-Hiti, G. A. Green Chem. 2011,
13, 1579e1608; (f) Smith, K.; Ajarim, M. D.; El-Hiti, G. A. Catal. Lett. 2010, 134,
270e278; (g) Smith, K.; Ajarim, M. D.; El-Hiti, G. A. Top. Catal. 2009, 52,
1696e1700; (h) Smith, K.; El-Hiti, G. A. Curr. Org. Chem. 2006, 10, 1603e1625; (i)
Smith, K.; El-Hiti, G. A. Curr. Org. Synth. 2004, 1, 253e274; (j) Smith, K.; Ewart,
G. M.; El-Hiti, G. A.; Randles, K. R. Org. Biomol. Chem. 2004, 2, 3150e3154; (k)
Smith, K.; Roberts, S. D.; El-Hiti, G. A. Org. Biomol. Chem. 2003, 1, 1552e1559; (l)
Smith, K.; El-Hiti, G. A.; Jayne, A. J.; Butters, M. Org. Biomol. Chem. 2003, 1,
2321e2325; (m) Smith, K.; El-Hiti, G. A.; Jayne, A. J.; Butters, M. Org. Biomol.
Chem. 2003, 1, 1560e1564; (n) Smith, K.; Liu, C.-H. Chem. Commun. 2002,
886e887; (o) Smith, K.; Almeer, S.; Peters, C. Chem. Commun. 2001, 2748e2749;
(p) Smith, K.; Almeer, S.; Black, S. J. Chem. Commun. 2000, 1571e1572; (q) Smith,
K.; El-Hiti, G. A.; Hammond, M. E. W.; Bahzad, D.; Li, Z.; Siquet, C. J. Chem. Soc.,
Perkin Trans. 1 2000, 2745e2752; (r) Smith, K.; Jones, D. J. Chem. Soc., Perkin
Trans. 1 1992, 407e408; (s) Smith, K.; James, D. M.; Matthews, I.; Bye, M. R. J.
Chem. Soc., Perkin Trans. 1 1992, 1877e1878.
Dicyclohexylborane (ca. 7.90 mmol) waspreparedasdescribed for
the synthesis of 5 from cyclohexene (1.30 g; 15.8 mmol) and adduct 1
(0.940 g, ca. 8.00 mmol BH3). 1-Hexyne (0.640 g, 7.79 mmol) was
added while the temperature was maintained at room temperature.
The mixture was stirred for 1.5 h at room temperature and then
cooled to ꢂ10 ꢀC and aqueous sodium hydroxide solution (6 M,
10 mL, 60 mmol) and iodine solution (2.50 g, 9.85 mmol) in THF
(15 mL) were added by syringe dropwise over 15 min. The reaction
mixture was allowed to warm up to room temperature and a small
amount of saturated aqueous sodium thiosulfate solution was added
to decompose the excess iodine. Diethyl ether (20 mL) was added
followed by filtration to remove the polymeric sulfide. The solid was
washed with ether (2ꢁ10 mL) and the filtrate and washings were
combined. Pentane (30 mL) was added and the organic layer was
separated from the aqueous layer, washed with brine (2ꢁ20 mL) and
dried over anhydrous MgSO4. The solvent was removed under re-
duced pressure and the crude product was purified by column
chromatography (silica; hexane) to give pure 6 (0.83 g, 5.00 mmol,
64%) as colourless oil (lit. oil);27 Rf (hexane) 0.76; 1H NMR (400 MHz,
CDCl3) d 5.31e5.19 (m, 2H), 2.32e2.23 (m,1H), 2.06 (m, 2H),1.76e1.61
(m, 4H),1.36e1.05 (m,10H), 0.93 (t, J¼7.0 Hz, 3H); 13C NMR (100 MHz,
CDCl3) d 136.0 (CH),128.0 (CH), 36.3 (CH), 33.4 (CH2), 32.2 (CH2), 27.2
(CH2), 26.1 (CH2), 26.0 (CH2), 22.4 (CH2), 14.0 (CH3); MS (EI) m/z (%)
166 (Mþ, 56), 154 (5), 137 (5), 124 (10), 109 (84), 96 (90), 81 (96), 67
(100); HRMS (EI) calcd for C12H22 (Mþ) 166.1721, found 166.1722; IR
(FT) nmax 2999, 2956, 2925, 2851, 1654, 1607. The Rf value and spec-
troscopic data of 6 were identical with those of an authentic sample
prepared according to the literature procedure.27
19. See for example: (a) Smith, K.; El-Hiti, G. A.; Hou, D.; DeBoos, G. A. J. Chem. Soc.,
Perkin Trans. 1 1999, 2807e2812; (b) Smith, K.; Pelter, A.; Jin, Z. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 851e853; (c) Pelter, A.; Smith, K.; Parry, D. E.; Jones, K. D.
Aust. J. Chem. 1992, 45, 57e70; (d) Pelter, A.; Smith, K.; Hutchings, M. G.; Rowe,
K. J. Chem. Soc., Perkin Trans. 1 1975, 129e138; (e) Brown, H. C.; Rathke, M. W. J.
Am. Chem. Soc. 1967, 89, 2737e2738; (f) Pelter, A.; Hutchings, M. G.; Rowe, K.;
Smith, K. J. Chem. Soc., Perkin Trans. 1 1975, 138e142; (g) Pelter, A.; Hutchings,
M. G.; Smith, K. J. Chem. Soc., Perkin Trans. 1 1975, 142e145; (h) Pelter, A.;
Hutchings, M. G.; Smith, K.; Williams, D. J. J. Chem. Soc., Perkin Trans. 1 1975,
145e150.
Acknowledgements
We thank the Iraqi Government and Cardiff University for
financial support.
References and notes
1. Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic: London, 1988.
2. Smith, K. Organoboron Chemistry In Organometallics in Synthesis: A Manual,
2nd ed.; Schlosser, M., Ed.; Wiley: Chichester, UK, 2004, Chapter III, pp
465e533.
20. Smith, K.; El-Hiti, G. A.; Al-Zuhairi, A. J. J. Sulfur Chem. 2011, 32, 521e531.
21. Smith, K.; Balakit, A. A.; Pardasani, R. T.; El-Hiti, G. A. J. Sulfur Chem. 2011, 32,
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22. Brown, H. C.; Carlson, B. A. J. Org. Chem. 1973, 38, 2422e2424.
23. Suzuki, A.; Miyaura, N.; Abiko, S.; Itoh, M.; Midland, M. M.; Sinclair, J. A.; Brown,
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3. Crudden, C. M.; Glasspoole, B. W.; Lata, C. J. Chem. Commun. 2009, 6704e6716.
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boranes; Springer: Berlin, 1995.
5. (a) Brown, H. C.; Subba Rao, B. C. J. Am. Chem. Soc. 1959, 81, 6423e6428; (b)
Brown, H. C. Organic Syntheses via Boranes; Wiley: New York, NY, 1975.
6. (a) Brown, H. C. U.S. Patent 4,298,750, 1981. (b) Brown, H. C.; Mandal, A. K. J.
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28. (a) Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook
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