Kinetics and mechanism of hydroboration reactions of HBBr2 · SMe2 and HBCl2 · SMe2 - Application of 11B NMR spectroscopy

Article abstract of DOI:10.1016/j.jorganchem.2006.11.021

The kinetics and mechanism of the hydroboration reactions of 1-octene with HBBr₂ · SMe₂ and HBCl₂ · SMe₂, in CH₂Cl₂ as a solvent, were studied. Rates of hydroboration were monitored using ¹¹B NMR spectroscopy. The reactions exhibited simple second-order kinetics of the form k_obs = k₂^′ [Nu]. The HBCl₂ · SMe₂ was found to be 20 times more reactive than the HBBr₂ · SMe₂. The overall activation parameters (ΔH^≠, ΔS^≠) for the reaction of HBBr₂ · SMe₂ with 1-octene were found to be 82 ± 1 kJ mol^-1, -18 ± 4 J K^-1 mol^-1 and with 1-hexyne were 78 ± 4 kJ mol^-1 -34 ± 12 J K^-1 mol^-1. For the reaction of HBCl₂ · SMe₂ with 1-octene, ΔH^≠ and ΔS^≠ were 104 ± 5 kJ mol^-1 and 43 ± 16 J K^-1 mol^-1, respectively. The activation parameters (ΔH^≠, ΔS^≠) for the dissociation of Me₂S from HBBr₂ · SMe₂ were found to be 104 ± 2 kJ mol^-1, +33 ± 8 J K^-1 mol^-1, respectively. Based on the activation parameters, it was concluded that the detaching of Me₂S from the boron centre follows a dissociative mechanism, while the hydroboration process follows an associative pathway. It was also concluded that the dissociation of Me₂S from the boron centre is the rate determining step.

Full text of DOI:10.1016/j.jorganchem.2006.11.021

Journal of Organometallic Chemistry 692 (2007) 1150–1155
www.elsevier.com/locate/jorganchem
Kinetics and mechanism of hydroboration reactions of HBBr2 Æ
11
SMe2 and HBCl2 Æ SMe2 – Application of B NMR spectroscopy
*
D. Jaganyi , N. Xaba, A. Mzinyati, C. Grimmer
School of Chemistry, University of KwaZulu Natal, Private Bag X01, Scottsville, Pietermaritzburg, 3209 KwaZulu-Natal, South Africa
Received 21 December 2005; received in revised form 11 October 2006; accepted 15 November 2006
Available online 19 November 2006
Abstract
The kinetics and mechanism of the hydroboration reactions of 1-octene with HBBr2 Æ SMe2 and HBCl2 Æ SMe2, in CH2Cl2 as a sol-
vent, were studied. Rates of hydroboration were monitored using 11B NMR spectroscopy. The reactions exhibited simple second-order
kinetics of the form kobs ¼ k02½Nuꢀ. The HBCl2 Æ SMe2 was found to be 20 times more reactive than the HBBr2 Æ SMe2. The overall acti-
¼
¼
vation parameters (DH , DS ) for the reaction of HBBr2 Æ SMe2 with 1-octene were found to be 82 1 kJ molꢁ1, ꢁ18 4 J Kꢁ1 molꢁ1
¼
¼
and with 1-hexyne were 78 4 kJ molꢁ1 ꢁ34 12 J Kꢁ1 molꢁ1. For the reaction of HBCl2 Æ SMe2 with 1-octene, DH and DS were
¼
¼
104 5 kJ molꢁ1 and 43 16 J Kꢁ1 molꢁ1, respectively. The activation parameters (DH , DS ) for the dissociation of Me2S from
HBBr2 Æ SMe2 were found to be 104 2 kJ molꢁ1, +33 8 J Kꢁ1 molꢁ1, respectively. Based on the activation parameters, it was con-
cluded that the detaching of Me2S from the boron centre follows a dissociative mechanism, while the hydroboration process follows
an associative pathway. It was also concluded that the dissociation of Me2S from the boron centre is the rate determining step.
ꢀ 2006 Elsevier B.V. All rights reserved.
1. Introduction
bond is reported to enhance the rate of hydroboration due
to electron donation while conjugating an aromatic ring
has an opposite effect. In addition to this, Brown and co-
workers have shown that hydroboration of cis-isomers pro-
ceeds faster than that of trans-isomers and that the drive to
relieve the ring strain facilitates the faster hydroboration of
cyclic alkenes [5,14].
The widely accepted mechanistic pathway for hydrobo-
ration with boron compounds attached to a Lewis base
involves a prior dissociation of the Lewis base from the
boron centre, followed by the addition of an olefin [4,8].
In the case of haloborane complexes, it is believed that
the Lewis base re-attaches itself back to the boron complex
after hydroboration [8]. However, conclusive mechanistic
studies that support this generally accepted mechanism
have not been conducted. To support and shed some light
in terms of the observed reactivities and mechanistic
behaviour, computational modelling of hydroboration
reactions have been shown to be useful [16]. Yet, simple
kinetic information in the literature dates back to the
1980s.
The importance of the hydroboration reaction in
organic chemistry has been well established in the chemical
literature [1–3]. Considerable effort has gone into delineat-
ing the mechanism of the hydroboration reaction for both
dimeric and monomeric boranes [4–10], as well as establish-
ing the relative reactivities of these compounds [11–15].
Findings indicate that structural and electronic differences
of organoboranes as well as that of olefins play an influen-
tial role on hydroboration kinetics.
Information on rates of hydroboration indicates that
hydroboration of terminal olefins is independent of the
chain length, and proceeds faster than that of internal ole-
fins, which is retarded due to steric effects around the dou-
ble bond. The retardation of the rate is also observed when
branching is introduced in the alkyl chain. Insertion of a
methyl substituent at an a-position to the terminal double
*
Corresponding author. Tel.: +27 33 260 5452; fax: +27 33 260 5009.
E-mail address: Jaganyi@ukzn.ac.za (D. Jaganyi).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.11.021

D. Jaganyi et al. / Journal of Organometallic Chemistry 692 (2007) 1150–1155
1151
Traditionally, gas chromatography has been the tech-
nique used in the kinetic investigations of hydroboration
reactions. The analysis of residual alkene or alcohol in
the products allowed for the monitoring of the reaction
progress and selectivity of the hydroboration reaction
[11–15]. Several problems arise when considering both
analysis of residual alkene or alcohol content by gas chro-
matography; not least being the tediousness of the sam-
pling and quenching process. Furthermore, this method is
particularly unsuitable for reactions whose reaction life-
time is within that of the sampling and quenching time.
The quality of the data from this type of analysis suffers
from the limit placed on the number and size of sample ali-
quots that may be removed from the reaction medium.
Modern spectroscopic set-up offers a solution to these
problems in providing non-destructive, in situ analysis
under inert atmosphere conditions. To this end, infrared
(IR) spectroscopy has been employed in kinetic investiga-
tions of various organoboranes [5–10,17,18]. 11B NMR
spectroscopy has also been shown to be an invaluable qual-
itative and analytical tool for many studies [19–25]. Yet,
despite all these applications, 11B NMR spectroscopy has
not been exploited in the direct investigation of kinetic
and mechanistic aspects of the hydroboration reaction,
up until a recent publication by Jaganyi and Mzinyati [26].
In order to test the generally accepted mechanism, we
have undertaken a detailed kinetic and mechanistic study
of HBBr2 Æ SMe2 and HBCl2 Æ SMe2 with 1-octene and 1-
hexyne. In addition, the role of the Lewis base (Me2S) in
the mechanistic route through which these haloboranes
undergo hydroboration has been investigated. We report
the kinetic data that have been obtained by exploiting the
applicability of 11B NMR spectroscopy as a tool to study
hydroboration kinetics.
The concentration of the haloboranes was kept constant at
0.05 M by diluting known volumes from stock solutions
with freshly distilled CH2Cl2, the same solvent used to pre-
pare the nucleophile solutions. The observed pseudo first-
order rate constants, kobs, were obtained by using Origin
5.0 statistical software to analyse the kinetic traces. The
equation employed was first-order exponential decay.
¼
To determine the overall activation parameters, DH
¼
and DS , rate constant values, k02, for the reaction involv-
ing the nucleophile at concentration of 0.5 M were plotted
as a function of temperature. Temperatures employed were
15 ꢁC, 20 ꢁC, 25 ꢁC and 30 ꢁC. Investigation of the effect of
the concentration of Me2S on the rate of hydroboration
was conducted, at 25 ꢁC, by varying the concentration of
the Me2S from 0.050 M to 0.085 M, whilst holding the con-
centration of HBBr2 Æ SMe2 constant at 0.05 M, and that of
1-octene at 10-fold excess to the boron complex. A temper-
ature dependence study (15 ꢁC to 30 ꢁC) was also
conducted at variable concentration of Me2S (0.050–
0.085 M) in order to determine the activation parameters
for the detaching of Me2S.
Product characterization was achieved through Varian
CP-3800 GC, employing a Teknokroma capillary column
(phase TR-FFAP, 30 m · 0.53 mm · 1 lm) in the case of
hydroboration of 1-octene, whilst 1H NMR and 13C
NMR were employed in the case of 1-hexyne. On charac-
terization of products, oxidation using NaOH/H2O2 mix-
ture was performed after completion of hydroboration.
In the case of 1-octene, the product obtained was 1-octanol
(retention time = 14.01 min). In the case of 1-hexyne, 1-
hexanal was found to be dominant product indicated by
an aldehyde hydrogen (d1H = 9.7 ppm) and an aldehyde
carbon (d13C = 203.1 ppm).
3. Results and discussion
2. Experimental
The disappearance of the 11B NMR doublet peak and the
emergence of a singlet peak, as shown in Fig. 1, is evidence
that the process monitored is that described in Eq. (1):
Standard techniques for handling of air- and moisture-
sensitive materials were used [26]. HBBr2 Æ SMe2 (1 M solu-
tion in CH2Cl2) and HBCl2 Æ SMe2 (in excess Me2S) were
purchased from Sigma–Aldrich and used as supplied.
1-Octene (provided by Sasol Ltd.) was distilled over
HBX2 ꢂ SMe2 þ R ! RBX2 ꢂ SMe2
ð1Þ
where X2 = Br2 or Cl2 and R = 1-octene or 1-hexyne.
˚
sodium wire and further dried with 3 A molecular sieves.
The CH2Cl2 (purchased from Merck) that was used as a
solvent for preparation of solutions was distilled over
P2O5 prior to use.
11B NMR spectra were recorded on Varian Unity Inova
500 spectrometer at 160 MHz in 5 mm quartz tubes.
BF3 Æ OEt2 was used as an external reference. The reactions
were monitored by following the intensity of the signal area
of the HBBr2 Æ SMe2 (doublet, d11B = ꢁ7 ppm at 25 ꢁC,
JB–H = 162 Hz) and HBCl2 Æ SMe2 (doublet, d11B = 2 ppm
at 25 ꢁC, JB–H = 158 Hz) peaks as a function of time.
In order to ensure pseudo first-order conditions with
respect to the boron complex and to force the reaction to
completion, all the kinetic runs were performed in the pres-
ence of a large excess of nucleophile (1-octene or 1-hexyne).
Fig. 1. 11B NMR kinetic array showing disappearance of HBBr2 Æ SMe2
and formation of RBBr2 Æ SMe2.

1152
D. Jaganyi et al. / Journal of Organometallic Chemistry 692 (2007) 1150–1155
4.0
The progress of the reaction of BHX2 Æ SMe2 with excess
nucleophile (1-octene or 1-hexyne) was monitored spectro-
scopically by following the change in intensity of the signal
area as a function of time in the 11B NMR spectrum, as
shown in Fig. 1. The data points for the kinetic plots were
fitted using a non-linear least-square procedure. A repre-
sentative plot of the concentration vs. time, showing an
excellent fit to a single exponential, is shown in Fig. 2.
The pseudo first-order rate constants, kobs, were plotted
against the concentration of the entering nucleophile, [Nu].
Linear dependence on [Nu] with insignificant intercepts
was observed for both 1-hexyne and 1-octene, an indication
of the absence of a back reaction as well as any parallel
reaction. Representative plots shown in Fig. 3 demonstrate
that the reactions are first-order with respect to incoming
nucleophile.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
k
1-Octene
1-Hexyne
0.0
0.5
1.0
1.5
2.0
2.5
3.0
[Nu],M
Fig. 3. Dependence of kobs on [Nu] in the hydroboration reactions of
HBBr2 Æ SMe2 in CH2Cl2 at 20 ꢁC.
The overall process can therefore be expressed by the
rate law given in Eq. (2):
kobs ¼ k02=½Nuꢀ
ð2Þ
-10.8
-11.0
-11.2
-11.4
-11.6
-11.8
This is in agreement with the practically irreversible nature
of the reaction given in Eq. (1). The composite second-
order rate constants, k02, were obtained by least-square
analysis of the plots for different temperatures, ranging
from 15 ꢁC to 30 ꢁC. The values of k02 obtained as a func-
tion of temperature, were used in plotting the Erying plots
shown in Fig. 4, whose slope and intercept were used to cal-
culate the activation parameters summarised in the upper
section of Table 1.
k
-12.0
-12.2
-12.4
-12.6
-12.8
-13.0
1-octene
1-hexyne
To rationalize our findings a modified mechanism of
that proposed by Brown [7,8], Eqs. (3)–(5), was used:
0.00330
0.00335
0.00340
0.00345
k1
1/T (K-1)
BHX2 ꢂ SMe ꢀ BHX2 þ SMe2
ð3Þ
ð4Þ
ð5Þ
k
ꢁ1
Fig. 4. Erying plots for hydroboration of 1-octene and 1-hexyne with
HBBr2 Æ SMe2 under pseudo first-order conditions.
k2
BHX2 þ R ꢀ RBX2
RBX2 þ Me2S Fꢀast RBX2 ꢂ SMe2
k
ꢁ2
The mechanism involves dissociation of the complex into
HBX2 and Me2S, followed by hydroboration of the nucle-
ophile with the free HBX2, to give uncomplexed RBX2,
which then combines with the Me2S. This leads to a rate
law given in the following equation:
0.05
0.04
0.03
0.02
0.01
0.00
k1k2½Rꢀ þ kꢁ1k ½Me2Sꢀ
ꢁ2
kobs
¼
ð6Þ
k
½Me2Sꢀ þ k2½Rꢀ
ꢁ1
At low temperatures, dehydroboration is not expected and
hence, kꢁ2 = 0. Eq. (6) then reduces to
HBX2.SMe2
RBX2.SMe2
k1k2½Rꢀ
kobs
¼
ð7Þ
k
½Me2Sꢀ þ k2½Rꢀ
ꢁ1
Since we have excess Me2S in the system,
ꢁ1[Me2S] > k2[R], therefore
k
k1k2
½Me2Sꢀ
This can be expressed as
kobs ¼ k02 ꢂ ½Rꢀ
kobs
¼
ꢂ ½Rꢀ
0
2000
4000
6000
8000
10000
k
ꢁ1
Time (s)
Fig. 2. Exponential decay curve for hydroboration of 0.5 M 1-octene with
0.05 M HBBr2 Æ SMe2.
ð8Þ

D. Jaganyi et al. / Journal of Organometallic Chemistry 692 (2007) 1150–1155
1153
Table 1
Kinetic data for hydroboration of 1-octene and 1-hexyne with HBBr2 Æ SMe2 and HBCl2 Æ SMe2 in CH2Cl2
T (ꢁC)
HBBr2 Æ SMe2
1-Octene
HBCl2 Æ SMe2
1-Hexyne
1-Octene
k02=103 ðMꢁ1 sꢁ1
Þ
k02=103 ðMꢁ1 sꢁ1
Þ
15
20
25
30
1.01 0.02
1.86 0.07
3.47 0.04
5.66 0.84
0.82 0.02
1.35 0.08
2.42 0.10
4.26 0.18
0.16 0.003
0.32 0.004
0.83 0.006
1.36 0.04
¼
¼
DH (kJ molꢁ1
)
82
1
4
78
4
104
5
DS (J Kꢁ1 molꢁ1
)
)
ꢁ18
ꢁ34 12
43 16
9
=
ꢃC
k215 ðsꢁ1
Þ
1.95 · 10ꢁ4
2
2
2
DH (kJ molꢁ1
)
104
2
¼
¼
Data for the dissociation of Me S from HBBr Æ SMe
;
DS (J Kꢁ1 molꢁ1
+33
8
which is equivalent to Eq. (2) where
-13.5
-14.0
k1k2
k02 ¼
k
½Me2Sꢀ
ꢁ1
It follows that the experimental data reported in Fig. 3 is
well described by the mechanism outlined in Eqs. (3)–(5).
To further test the proposed mechanism, Eq. (7) was re-
written in a linear form as in Eq. (9):
-14.5
-15.0
-15.5
-16.0
k
1
k
1
ꢁ1
¼
ꢂ ½Me2Sꢀ þ
ð9Þ
kobs k1k2½Rꢀ
k1
This means that if [R] is held constant while [Me2S] is var-
ied, a plot of 1/kobs against [Me2S] should be linear with
1/k1 as the y-intercept and kꢁ1/k1k2[R] as the slope. This
was found to be true as shown in Fig. 5.
0.00330
0.00335
0.00340
0.00345
1/T (K-1)
The k1 value represents the dissociation rate constant of
Me2S from the boron complex. The Erying plot for this
process, from whose slope and intercept, the activation
Fig. 6. Erying plot for the hydroboration of 1-octene with HBBr2 Æ SMe2
when the concentration of Me2S was varied.
¼
¼
parameters DH = 104 2 kJ Æ molꢁ1 and DS = +33
8
J Æ Kꢁ1 Æ molꢁ1 were calculated, is shown in Fig. 6.
HBBr2 Æ SMe2 is approximately 1.3 times faster than that
of 1-hexyne. This is contrary to Brown’s findings [8] who,
based on relative reactivities and not on absolute rate con-
stants, reported that hydroboration of 1-hexyne is approx-
imately 3 times faster than that of 1-octene. From our
results, small difference in reactivity between the two olefins
3.1. Reactivity
From analysis of the kinetic data in Table 1, it is evident
that, on average, the hydroboration of 1-octene with
¼
is not due to energy factor since their DH values are very
similar. The possible reason could be the difference in their
structure.
13000
12000
11000
To compare the reactivity between the HBBr2 Æ SMe2
and the HBCl2 Æ SMe2 an understanding of the role of the
Me2S is needed. This is because Fig. 5 clearly shows that
as the amount of excess Me2S is increased the rate of hyd-
roboration decreases, indicative of the fact that the reactiv-
ity of these complexes is dependent on the dissociation of
the Me2S from the boron atom. Consequently, when com-
paring the reactivity of these two haloboranes, it is impor-
tant to ensure that the amount of excess Me2S is the same
in both the systems.
10000
k
9000
400
200
0
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
The excess Me2S that was in the HBBr2 Æ SMe2 from the
suppliers was worked out to be 21%, which is 1/6 times the
concentration of the HBBr2 Æ SMe2. This was obtained
[Me2S] (M)
Fig. 5. Inverse relationship between [Me2S] and kobs for hydroboration of
1-octene with HBBr2 Æ SMe2 in CH2Cl2 at 25 ꢁC.
1
through the integration of the H NMR spectrum of the

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D. Jaganyi et al. / Journal of Organometallic Chemistry 692 (2007) 1150–1155
pure sample of the HBBr2 Æ SMe2, which showed two dis-
tinct signals, i.e. one from the free Me2S (d1H = 2.1 ppm)
and another from the coordinated Me2S (d1H = 5.0 ppm).
The HBCl2 Æ SMe2 was also subjected to the same treat-
ment, and was found to contain an excess of 83% Me2S,
which was five times the concentration of the HBCl2 Æ SMe2
complex.
In this investigation, the study that involved variable con-
centrations of excess Me2S was used to deduce the observed
pseudo first-order rate constant, kobs, for the HBBr2 Æ SMe2
reaction system whose excess Me2S corresponded to that
present in HBCl2 Æ SMe2 reaction system. The composite
behaviour was used to answer the above question. After
completion of hydroboration Et3N was added into the
reaction mixture in NMR tube. On analysis of the sample
a distinct change in the chemical shift from d11B =
3.67 ppm to d11B = 9.72 ppm was observed. This was
indicative of a change from RBBr2 Æ SMe2 to RBBr2 Æ NEt3,
leading to a conclusion that the Lewis base does re-attach
itself to the boron atom after hydroboration.
3.2. Activation parameters
The entropy values listed at the top of Table 1 need to be
interpreted with caution. They represent a sum of the dif-
ferent processes (i.e. detaching of Me2S from boron and
hydroboration of the nucleophile) in the mechanism as rep-
resented by k02. Evidence for the dissociation of Me2S from
boron prior to the hydroboration comes from the study
involving excess Me2S at different temperatures. In support
rate constant, at 25 ꢁC, for hydroboration with the
ꢁ5 ꢁ1
HBBr2 Æ SMe2 was calculated to be 2.05 · 10
s
while,
that of HBCl2 Æ SMe2 was found to be 4.01 · 10ꢁ4 sꢁ1. The
concentration of 1-octene in both cases was 10 times that
of the boron complex. This implies that HBCl2 Æ SMe2 is
20 times more reactive than HBBr2 Æ SMe2. This finding is
contrary to that reported in the literature, but is in support
of the fact that HBBr2 Æ SMe2 is a more stable complex than
the HBCl2 Æ SMe2 [27]. This follows the fact that BBr3 is a
stronger Lewis acid than BCl3 and hence the bromoboranes
are more acidic and more stable than the chloroboranes, in
the order [27]:
¼
is the DS value being positive (+33 8 J Kꢁ1 molꢁ1) for
the dissociation of the Me2S from HBBr2 Æ SMe2. In addi-
tion, this is supported by the retardation effect of Me2S
on hydroboration. Having shown that the entropy value
for the dissociation of Me2S is positive, the overall negative
entropy value (ꢁ18 4 J Kꢁ1 molꢁ1) for HBBr2 Æ SMe2
can indirectly be interpreted to indicate that the entropy
value from k2 (the hydroboration step) is large and nega-
tive, an indication that this process is associative in nature.
Using the same argument it can be concluded that the small
positive overall entropy (+43 16 J Kꢁ1 molꢁ1) for the
HBCl2 Æ SMe2 indicate that the entropy value for the disso-
ciation of the Me2S is bigger in magnitude than that due to
hydroboration. It is worth mentioning that the large error
limits associated with the entropy values is due to the
intrinsic extrapolation involved in their determination
[29,30]. Also, in the case of non-dissociation of Me2S the
composite nature of the rate constant used to generate
the values.
BBr3 > BCl3 > HBBr2 > HBCl2 > H2BBr > H2BCl
The strength of coordination between the boron centre and
the Lewis base is influenced by the interaction between the
boron atom and the halogen substituents attached to it.
This is because the halide attached to the boron atom af-
fects the electron density of that boron atom, which in turn
changes the strength of the other boron-substituent bonds,
in this case B–SMe2.
Covalent bond formation is of major importance in
boron chemistry. The formation of dative pp–pp bond
using pp-orbitals of the halogens and the vacant pp-orbital
of boron is particular of significance in haloboranes. This
boron–halogen p-bond strength increases with decreasing
size of the halogen. It has been shown that the p-bonding
energies of the trihalides are in the order [28]:
4. Conclusions
In conclusion, activation parameters deduced from this
study reveal that hydroboration reactions of HBBr2 Æ SMe2
and HBCl2 Æ SMe2 go through dissociation of the Me2S
from the boron centre, which is then followed by hydrobo-
ration of the nucleophile through an associative route. In
this mechanistic route, dissociation of Me2S is the rate-
determining step. The hydroboration kinetics of such
boron compounds, i.e. those attached to a Lewis base, is
controlled by the concentration of Lewis base present in
the system. As such when comparing the reactivities of
these complexes, it is of importance that the amount of
the Lewis base present in the system is taken into account.
This work has confirmed that the reactivity of halobo-
rane addition complexes is dependent on the type of halo-
gen attached to the boron atom. It has also shown through
the composite rate constants, at 25 ꢁC, for hydroboration
with the HBBr2 Æ SMe2 (2.05 · 10ꢁ5 sꢁ1) and that of
BF3 P BCl3 > BBr3 > BI3
A bigger p-bonding energy is indicative of a higher electron
density around the boron atom due to strong p-donation
from the halogen. The boron atom of HBBr2 Æ SMe2 can
be expected to have lower electron density compared to
that of HBCl2 Æ SMe2. This means that the B–SMe2 bond
will be weaker in the HBCl2 Æ SMe2 complex compared to
the HBBr2 Æ SMe2 complex. Since the reactivity depends
on the dissociation of the Me2S, the weaker the B–SMe2
bond the faster the hydroboration, hence HBCl2 Æ SMe2 is
20 times more reactive than HBBr2 Æ SMe2.
One question relating to the fate of Lewis base after
hydroboration, which needs proof, is whether the Lewis
base re-attaches itself back to the boron atom. Since
Et3N can instantaneously replace Me2S attached to boron
atom leading to a change in chemical shift of boron, this

D. Jaganyi et al. / Journal of Organometallic Chemistry 692 (2007) 1150–1155
1155
HBCl2 Æ SMe2 (4.01 · 10ꢁ4 sꢁ1), that hydroboration with
HBCl2 Æ SMe2 proceed at a rate which is 20 faster than that
of HBBr2 Æ SMe2.
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Acknowledgements
Funding of the project by Sastech R&D, DTI/Thrip, as
well as the University of KwaZulu-Natal, is gratefully
acknowledged. Our gratitude also goes to Mr. A. de Klerk
and Dr. H Strauss of Sasol Ltd. for their contribution to-
wards this work.
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