Rhodium-NHC complexes mediate diboration versus dehydrogenative borylation of cyclic olefins: A theoretical explanation

Article abstract of DOI:10.1039/c2dt31659e

In rhodium catalysed borylation of cyclic olefins, the synergy between bidentate NHC ligands, that modify cationic Rh(i) species, and the use of non-polar solvents, such as cyclohexane, is the key factor to favour a less energetically demanding route towards the formation of diborated products versus allyl boronate esters.

Full text of DOI:10.1039/c2dt31659e

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Rhodium–NHC complexes mediate diboration versus
dehydrogenative borylation of cyclic olefins:
a theoretical explanation†
Cite this: Dalton Trans., 2013, 42, 746
Cristina Pubill-Ulldemolins,a Macarena Poyatos,b Carles Bo*a and Elena Fernández*c
Received 24th July 2012,
In rhodium catalysed borylation of cyclic olefins, the synergy between bidentate NHC ligands, that
modify cationic Rh(I) species, and the use of non-polar solvents, such as cyclohexane, is the key factor to
favour a less energetically demanding route towards the formation of diborated products versus allyl
boronate esters.
Accepted 9th October 2012
DOI: 10.1039/c2dt31659e
www.rsc.org/dalton
vinylic products although the proportion of the vinylic product
increased with prolonged heating (Scheme 1b). Miyaura and
Introduction
Dehydrogenative borylation of olefins, in which a vinylic C–H co-workers further reported the borylation of vinyl C–H bonds
bond is replaced with a C–B bond, is an efficient approach in cyclic vinyl ethers by B2pin2 catalyzed by [Ir(μ-OMe)(COD)]2
towards the synthesis of the useful vinyl boronate esters.1 and dtbpy.4 Borylation of 1,4-dioxene with 0.5 equiv. of B2pin2
However, the extension of this methodology to cyclic olefins provided the vinylboronate ester product in 81% yield. Sub-
has been much less explored and can be classified by the strates containing substituents at the γ-position in dihydropyr-
nature of the reagent (HBpin or B2pin2) or the transition metal ans reacted with higher regioselectivity where borylation
complex involved in the activation of the borane reagent. occurred solely at the α-position (Scheme 1c). In subsequent
Scheme 1 summarizes the limited examples of cyclic olefins work, Marder and co-workers explored the dehydrogenative
that underwent dehydrogenative borylation, illustrating the borylation catalysed by trans-[RhCl(CO)(PPh3)2] and the boryl-
reaction conditions and the selectivity of the reaction products. ation of indene with B2pin2.5 They observed the selective for-
When Sabo-Etienne and co-workers activated pinacolborane by mation of the vinylboronate ester with borylation occurring at
means of RuH2(H2)2(PCy3)2,2 the hydroboration of cyclohexene the 2-position, however only 19% conversion was achieved
seemed to be highly favored, whereas for cyclodecene, vinyl- after 6 days at 80 °C (Scheme 1d).5 Finally, Jamison and co-
boronate was produced with only traces of allylboronate. workers reported the benefits of xantphos on the rhodium pre-
Cyclooctene provided the vinyl- and allylboronate in a 1 : 3 cursor [Rh(μ-Cl)(COD)]2 towards the dehydrogenative boryl-
ratio, whereas only allylboronate was obtained from cyclohep- ation of cyclic alkenes with B2pin2, forming the corresponding
tene (Scheme 1a). Szabó and co-workers were the first to use cyclic 1-alkenylboronic acid pinacol esters (Scheme 1e).6
B2pin2 in the dehydrogenative borylation of cyclic olefins and
We were surprised by the fact that in all the examples where
towards the activation of the diboron reagent, the iridium pre- B2pin2 was the boron reagent used, the 1,2-diborated product
cursor [Ir(μ-Cl)(COD)]2 was selected.3 Heating at 70 °C, cyclo- was not observed, even in the presence of rhodium complexes
hexene gave a 1 : 1 ratio of allylic and vinylic borylation modified with phosphines.7 We also noticed that apart from
products while addition of 0.5 equiv. of 1,8-diazabicyclo[5.4.0]- phosphines and amines,8 no other ligands have been studied
undecane (DBU) led to an increase in the ratio of allylic to to modify the catalytic systems in order to promote the dehy-
drogenative borylation reactions. In that context, we planned
to focus our efforts on exploring the catalytic activity of
aInstitute of Chemical Research of Catalonia (ICIQ). Avda. Països Catalans,
rhodium complexes modified with N-heterocyclic carbenes
17. 43007 Tarragona, Spain
(NHC), towards the borylation of cyclic olefins in the presence
of B2pin2. But also, in view of the lack of detailed studies con-
cerning the mechanism of rhodium-mediated borylation of
cyclic alkenes, we became interested to rationalise the experi-
mental data with theoretical data and propose a plausible
mechanism based on the reaction energy profile. We selected
cyclohexene as the model substrate to undergo the borylation
bDpt. Química Inorgànica i Orgànica, Universitat Jaume I, Av Vicente Sos Baynat s/n,
12071 Castellón, Spain
cDpt. Química Física i Iorgànica, Universitat Rovira i Virgili, C/Marcel.lí Domingo
s/n, 43007 Tarragona, Spain. E-mail: mariaelena.fernandez@urv.cat;
http://www.quimica.urv.net/tecat/catalytic_organoborane_chemistry.php;
Fax: (+) 34 977559563
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c2dt31659e
746 | Dalton Trans., 2013, 42, 746–752
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Table 1 Rhodium complexes modified with NHC ligands catalysed the boryla-
tion of cyclohexenea
Convb 4/5/6/7b
Entry Rh complex
Solvent
(%)
(%)
1
2
3
4
5
6
1
2
3
2
2
2
2
2
THF
THF
THF
Cyclooctane 27
Pentane
Hexane
Cyclohexane 68
Cyclohexane 89
Cyclohexane 52
Cyclohexane 68
Cyclohexane 17
30
9
18
49/29/23/—
17/40/18/25
30/30/40/—
30/8/17/45
1/—/—/99
1/—/—99
1/—/—99
1/—/—99
14/—/—/86
28/3/3/66
64
54
7
8c
9
1
3
10
11d
12d
13d
[Rh(μ-Cl)(COD)]2
78/6/4/13
61/17/14/8
62/14/9/14
[Rh(μ-Cl)(COD)]2PPh3 (1 : 2) Cyclohexane 24
[Rh(μ-Cl)(COD)]2PPh3 (1 : 4) Cyclohexane 33
a Standard conditions: Rh–NHC complex (4 mol%), substrate
(0.2 mmol), B2pin2 (1.0 equiv.), 70 °C, 16 h. b Conversion and
selectivity calculated using 1H NMR spectroscopy and GC. c T: 120 °C.
d Rh complex (2 mol%), phosphine (4 or
(0.2 mmol), B2pin2 (1.0 equiv.), 70 °C, 16 h.
8 mol%), substrate
Scheme 1 Updated strategies on metal mediated dehydrogenative borylation
observed using complexes 2 and 3 (Table 1, entries 2 and 3),
but to our surprise, complex 2 allowed us to achieve the
1,2-diborated product in a modest percentage (25%), as the cis-
1,2-diborated product. Exploring in detail the catalytic behav-
iour of complex 2 in less polar solvents like cyclooctane, we
found that despite the fact that the conversion was still low,
the percentage of diborated product increased significantly,
becoming the major product (45%) (Table 1, entry 4). The bene-
ficial influence of this type of solvent was enhanced when the
borylation of cyclohexene took place in pentane, hexane and
cyclohexane, obtaining better conversions and quantitative for-
mation of the 1,2-diborated product (Table 1, entries 5–7).
Increasing the temperature from 70 °C to 120 °C improved the
conversion of the reaction (89%) keeping constant the quanti-
tative formation of the diborated product 7 (Table 1, entry 8).
Rhodium complexes 1 and 3 were less selective (Table 1,
entries 9 and 10) towards the diborated product relative to the
partial formation of the vinyl boronate ester 4. A direct com-
parison with an unmodified Rh complex [Rh(μ-Cl)(COD)]2 and
modified Rh complexes with phosphines confirmed the pre-
viously observed experimental trend by Jamison and co-
workers7 where dehydrogenative borylation of the cyclic
olefins is the major process (Table 1, entries 11–13). However,
under our reaction conditions, and using cyclohexane as the
solvent of choice, a representative amount of 1,2-diborated
product could be observed.
reactions of cyclic alkenes.
Scheme 2 Model borylation reaction of cyclohexene and B2pin2 with Rh–NHC
complexes.
reaction with B2pin2 and Rh(I)–NHC complexes (Scheme 2).
NHC-based rhodium complexes 1–3 are well known complexes,
first reported by Crabtree et al.,9 which have been prepared by
transmetallation from the corresponding silver carbene deriva-
tives by a two-step procedure. The first step involves the depro-
tonation of the corresponding chelating bis(imidazolium) salt
with Ag2O. The so generated Ag(I)–NHC complexes were
reacted in situ with [Rh(μ-Cl)(COD)]2 giving the desired chelat-
ing complexes in moderate yields.9
With all these data in mind we concluded at this point that
the synergy between the catalytic system 2 and the solvent
cyclohexane favoured the diboration process versus the dehy-
drogenative borylation of cyclic olefins, in the presence of
B2pin2. To generalise the methodology, we explored alternative
tetraalkoxydiboron reagents such as bis(catecholato)diboron
(B2cat2) and bis(neopentylglycolato)diboron (B2neop2) to
borylate cyclohexene with complex 2, using cyclohexane
as a solvent. In both cases, after 16 h at 70 °C, the catalytic
Results and discussion
The borylation of cyclohexene in THF, when the rhodium
complex 1 was used, provided moderate conversions of a
mixture of borylated products where the vinyl boronate ester 4
was the major one (49%) (Table 1, entry 1). In addition, certain
amounts of allyl boronate ester and hydroborated product
were also observed, but the diborated product 7 was not ident-
ified among the mixture of products. Similar results were
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 746–752 | 747

View Article Online
Paper
Dalton Transactions
1,2-diboration was the preferred borylation process with quan- shows the general mechanistic pathways that we have con-
titative formation of the corresponding 1,2-diborated product, sidered to perform the theoretical study. This proposed mech-
despite the fact that conversion values were significantly anism resembles the mechanism previously suggested by
different depending on the diboron reagent used (Scheme 3). Szabó3 and Jamison,6 where the conformational flexibility of a
When B2neop2 was involved, the conversion reached 80%. The model cyclic alkene was already considered. In our DFT calcu-
oxidative work-up of the 1,2-diborated product confirmed the lations we used Beg (eg = ethylenglycolato = OCH2CH2O) as a
cis-1,2-cyclohexanediol formation, in all the cases.
suitable model for the boryl moiety16 and [Rh(COD)(NHC)]+ (2)
The scope of the Rh–NHC mediated diboration of cyclic as the metal complex simplifying the nBu groups of the NHC
alkenes was also explored. In all the studied cases, exclusive ligand by hydrogen atoms. Cyclohexene was selected as the
formation of the 1,2-diborated product could be observed, model cyclic olefin.
with high conversion in the case of cyclopentene even at 50 °C
(Table 2, entry 2). The beneficial influence of N-heterocyclic
carbenes as ligands in the catalytic diboration of olefins has
been previously demonstrated with Pd,10 Pt,11 Ir,12 Ag13 and
Au14 complexes, with complete chemoselectivity towards the
1,2-diborated product. However, in this case where NHC
modifies rhodium complexes, it has an added value because
the dehydrogenative borylation of olefins was usually favoured
in the presence of rhodium complexes modified with
phosphines.15
In this particular case, we became interested to know more
details about the mechanism of the rhodium mediated boron
addition reaction in order to identify the elements that could
favour the 1,2-diboration versus the dehydrogenative boryl-
ation. For this purpose, using DFT based methods, we deter-
mined the reaction energy profile for the formation of the
diborated product 7 and the allyl boronate ester 5. Scheme 4
Scheme 3 1,2-Diboration of cyclohexene with bis(catecholato)diboron and bis-
Scheme 4 General proposed catalytic pathways for the formation of products
4–7.
(neopentylglycolato)diboron.
a
Table 2 Substrate scope of the Rh–NHC mediated 1,2-diboration reaction with B2pin2
Entry
1
Substrate
Product
T (°C)
Convb (%)
99
1,2-Diboratedb (%)
100
Isolated yield (%)
89
70
2
3
“
“
50
70
95
70
100
100
77
62
4
5
6
70
70
70
68
63
68
100
100
99
61
57
65
a Standard conditions: Rh–NHC complex (4 mol%), substrate (0.2 mmol), B2pin2 (1.0 equiv.), 16 h. b Conversion and selectivity calculated using
1H NMR spectroscopy and GC.
748 | Dalton Trans., 2013, 42, 746–752
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
We first focussed our study on the formation of the di-
borated product. The most accepted mechanism for the alkene
diboration reaction involves oxidative addition of the diboron
reagent to the metal centre, followed by insertion of the alkene
and reductive elimination to generate the syn-organodiboron
product. Generation of the cationic metal species [Rh-
(cyclohexene)2(NHC)]+ is an endothermic process (ΔE
=
17.8 kcal mol−1) that takes place by substitution of the labile
1,5-cyclooctadiene (COD) by two molecules of cyclohexene (see
ESI†). Concerning the oxidative addition pathway, it has exper-
imentally been demonstrated that diboron reagents such as
B2cat2 and B2pin2 can be activated by late transition metal
complexes17,18 promoting the homolytic cleavage of the B–B
bond. The resulting bisboryl Rh(III) complexes have been com-
pletely characterized and in the case of Bcat moieties the
crystal structure has also been determined.17 The oxidative
addition of diboron reagents to neutral Rh(I) complexes modi-
fied with phosphines has been studied by means of compu-
tational studies.19 However to the best of our knowledge, the
oxidative addition of diboron reagents to cationic Rh(I) com-
plexes modified with NHC ligands has not been previously
reported. In the present work, we have been able to character-
ise the structure for the resulting species of the oxidative
addition of B2eg2 to the cationic [Rh(cyclohexene)2(NHC)]+.
After a conformational study of the resulting Rh(III) diboryl
species [Rh(Beg)2(cyclohexene)(NHC)]+, we found that the most
stable conformation was characterised as a distorted square
planar pyramidal structure with one boryl moiety occupying
the apical site and the other boryl moiety in the basal site
(Fig. 1). The bond angles about the rhodium centre support
the description of the mentioned distorted square planar pyra-
midal geometry since C(1)–Rh–B(1) is 166.47° and C(2)–Rh–
C(3) is 168.61, both values are similar and reasonably close to
linear. Remarkably, angles between the basal B(1) and the
apical B(2) are 84.87°, thus the two boryl moieties adopt a
mutually perpendicular relative orientation, as was found in
Scheme 5 Comparative energy profiles for the formation of the diborated and
allyl boronate ester compounds computed at the BP86 level. Gibbs free energy
is given in kcal mol−1
.
some Rh(III) bisboryl species with B2cat2.17 Also, the B(1)–Rh–
B(2) angle results in a B⋯B separation of 2.89 Å which differs
by 1.19 Å relative to the B–B distance in B2eg2 (1.70 Å). Conse-
quently, any residual B⋯B interaction must be weak. This
newly characterized rhodium(III) complex, [Rh(Beg)2(cyclohex-
ene)(NHC)]+, has been chosen as the origin of energies in our
reaction energy profile (Scheme 5).
As has been depicted in Scheme 4, once the oxidative
addition has taken place, the second step is defined as the
insertion of the alkene in the Rh–B bond. Two plausible
mechanisms for the insertion pathway can be considered
(Scheme 5): apical insertion when the alkene binds the apical
boryl (TSA–B2) and basal insertion when the reaction occurs in
the basal plane of the distorted square-based pyramid (TSA–B1).
According to our DFT results, it is clear that basal insertion is
less energetically demanding (ΔG = 7.0 kcal mol−1 lower than
the apical one). The resulting B1 species can further undergo a
rearrangement in order to occupy the vacancy generated in the
coordination sphere of the metal. This process involves the
rotation of the B(1)–Ccyclohexene bond to afford a more stable
species C, in which an interaction of the oxygen of the boryl
unit with the metal centre has been established. Note that this
process is barrierless (TSB–C = −4.5 kcal mol−1). In the follow-
ing step, reductive elimination takes place recovering a Rh(I) D
species in which the diborated product is coordinated through
the oxygens of the Beg moiety to the metal centre (D species).
Dissociation of the diborated product is characterized as an
exothermic process (ΔG = −5.0 kcal mol−1). Next, we evaluated
the energy profile for the formation of the allyl boronate ester
(Scheme 5, blue line). Once the B intermediate was formed, an
alternative rearrangement could take place through rotation of
the Rh–Ccyclohexene bond. Although the energy barrier for this
rotation was found to be very low (ΔG = 1.0 kcal mol−1), it was
less favoured than the B(1)–Ccyclohexene rotation. The resulting
E species could favour an agostic interaction between the Rh
centre and the ortho hydrogen of the cyclohexene, promoting
the β-hydride-elimination towards the Rh–allylboronate
complex (F). Further dissociation of the allylboronate provides
Fig. 1 Model structure for the cationic bisboryl Rh(III)–NHC species [Rh(Beg)2-
(cyclohexene)(NHC)]+ (A).
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 746–752 | 749

View Article Online
Paper
Dalton Transactions
a Rh–hydride species, which is a key intermediate to obtain reported in ppm (δ) relative to the chemical shifts of tetra-
the rest of the observed organoborated byproducts (vinylic and methylsilane or residual solvent resonances. 11B {1H} NMR
hydroborated compounds). It is noteworthy to mention that chemical shifts are reported in ppm (δ) relative to BF3(CH3)2O.
the overall formation process is only slightly exothermic (ΔG = Gas chromatography was equipped with an HP-5 column with
−1.3 kcal mol−1) for the allyl boronated ester product and a FID or MS detector.
much more exothermic for the diborated one (ΔG = −6.1 kcal
mol−1). Both experimental and theoretical calculations seem
to be in agreement with the favoured formation of the di-
borated product over the β-borated one.
General procedure for the NHC–Rh catalysed diboration of
cyclohexene with B2pin2
Differences between the two reaction pathways are not very Rh complexes (1–3) (4 mol%, 0.008 mmol), or [Rh(μ-Cl)-
large, thus subtle changes in the solvent and in the ligands (COD)]2 (2 mol%, 0.004 mmol)/phosphine (4–8 mol%), and bis
can switch reactivity. Indeed, polarity and coordination ability (pinacolato)diboron (1.0 equiv., 51.0 mg, 0.2 mmol) were
of the solvent are key factors to explain the differences transferred into an oven-dried Schlenk tube under argon.
observed in selectivity (Table 1). But the nature of the biden- Dried and deoxygenated solvent (2 mL) was added. The
tate ligand NHC is of crucial importance, because when the mixture was stirred for 10 min at room temperature to dissolve
borylation of cyclohexene was performed with the analogous the diboron reagent completely. Cyclohexene (0.2 mmol) was
cationic rhodium system [Rh(COD)(BINAP)]BF4 under the added, and the reaction mixture was stirred at 70 °C oil bath
same reaction conditions as with the precursor of catalyst 2 temperature for 16 h. The reaction mixture was cooled to room
(Table 1, entry 7), less than 20% of conversion was observed temperature. An aliquot of 0.2 mL was taken from the solution.
and only 57% of selectivity towards the diborated product was It was diluted with CH2Cl2 (1 mL) and analysed by GC/GC-MS
achieved.
to determine conversion. After the GC analysis, the aliquot was
analysed by 1H-NMR to confirm the conversion previously
observed by gas chromatography. The analytical samples were
combined with the rest of the reaction mixture, all the volatiles
were removed in vacuum, and the crude product was purified
Conclusions
Although it has been well established that rhodium complexes by column chromatography (deactivated silica, petroleum
modified with phosphines favoured the dehydrogenative bory- ether–EtOAc, 15 : 1). White solid. Yield: (65%). Rf = 0.3 (silica
lation reaction in borylation of cyclic olefins, we have found TLC, petroleum ether–EtOAc, 15 : 1). 1H NMR (CDCl3,
that Rh(I) cationic complexes modified with bidentate N- 400 MHz): δ 1.67–1.50 (m, 5H), 1.48–1.34 (m, 5H), 1.23 (s, 12H,
heterocyclic carbene ligands selectively promote the diboration B(pin)), 1.22 (s, 12H, B(pin)). 13C NMR (CDCl3, 100 MHz):
reaction. The use of non-polar solvents, such as pentane, δ 82.96, 28.29, 27.07, 25.12, 25.04. 11B NMR (CDCl3, 128 MHz):
hexane or cyclohexane, is also of crucial importance to guaran- δ 34.35. HRMS (ESI+) m/z found for C18H34B2O4 [M + H]+
tee the selective organodiboronate product formation. Theor- 337.2713.
etical calculations carried out to determine the key pathways
=
that might favour the diboration mechanism versus the dehy-
drogenative borylation mechanism have demonstrated the
more energetically demanding β-H-elimination pathway versus
General procedure for the catalysed diboration of cyclohexene
with B2neop2
the reductive elimination step. We are currently evaluating the Rhodium complex 2 (4 mol%, 1.0 mg, 0.008 mmol) and bis
influence of similar ligands and solvents from a theoretical (neopentylglycolato)diboron (1.0 equiv., 0.2 mmol) were trans-
and experimental point of view. Making the methodology ferred into an oven-dried Schlenk tube under argon. Dried and
more general for non-cyclic substrates is also a matter of our deoxygenated cyclohexane (2 mL) was added. The mixture was
current interest.
stirred for 10 min at room temperature. The substrate cyclo-
hexene (21 μL, 0.2 mmol) was added, and the reaction mixture
was stirred at 70 °C oil bath temperature for 16 h. The reaction
mixture was cooled to room temperature. An aliquot of 0.2 mL
was taken from the solution. It was diluted with CH2Cl2 (1 mL)
and analysed by NMR to determine conversion of the reaction.
Experimental data
General aspects
All reactions and manipulations were carried out under an The analytical samples were combined with the rest of the
argon atmosphere by using Schlenk-type techniques. The sol- reaction mixture, all the volatiles were removed in a vacuum,
vents were distilled over dehydrating reagents and were deoxy- and the crude product was purified by column chromato-
genated before use. Bis(pinacolato)diboron was provided from graphy (deactivated silica, petroleum ether–EtOAc, 15 : 1).
Allychem. Substrates, bases and other diboron reagents were White solid. Yield: (41%). 1H NMR (CDCl3, 400 MHz): δ 0.95
used as purchased from Sigma-Aldrich and Alfa Aesar. Rh(I)– (s, 12H), 1.26–1.14 (m, 2H) 1.40–1.34 (m, 4H), 1.60–1.50 (m,
NHC complexes (1–3) were prepared following the literature 4H), 3.56 (s, 8H). 13C NMR (CDCl3, 100 MHz): δ 72.11, 31.76,
protocol.9 NMR spectra were obtained on a Varian Mercury 400 27.7, 27.06, 22.14. 11B NMR (CDCl3, 128 MHz): δ 30.9. HRMS
1
spectrometer. H NMR and 13C {1H} NMR chemical shifts are (ESI+) m/z found for C16H30B2O4 [M + H]+ = 309.236.
750 | Dalton Trans., 2013, 42, 746–752
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
General procedure for the catalysed diboration/oxidation of
cyclohexene with B2cat2
12H), 1.24 (s, 12H), 1.38–1.50 (m, 2H), 1.53–1.72 (m, 3H),
3.5–3.7 (m, 3H). 13C NMR (CDCl3, 100 MHz): 88.8, 71.9, 70.4,
28.4, 26.3, 24.7, 17.5. 11B NMR (CDCl3, 128 MHz): δ 36.2.
Rhodium complex 2 (4 mol%, 1.0 mg, 0.008 mmol) and bis-
(catecholato)diboron (1.0 equiv., 0.2 mmol) were transferred
into an oven-dried Schlenk tube under argon. Dried and deoxy-
genated cyclohexane (2 mL) was added. The mixture was
stirred for 10 min at room temperature. The substrate cyclo-
hexene (21 μL, 0.2 mmol) was added, and the reaction mixture
was stirred at 70 °C oil bath temperature for 16 h. The reaction
mixture was cooled to room temperature. An aliquot of 0.2 mL
was taken from the solution. It was diluted with CH2Cl2 (1 mL)
and analysed by NMR to determine conversion of the reac-
tion.20 The aliquot was returned to the reaction mixture and
the solvent was removed under vacuum. 1 mL of THF was
added and it was cooled at 0 °C. NaOH (2 mL, 3 M) and H2O2
(1 mL, 33%) were added to the reaction mixture and the entire
sample was stirred from 0 °C to room temperature, overnight.
Computational details
Molecular structures for all the species were optimized without
constraints by using Density Functional Theory (DFT) based
methods as implemented in the Amsterdam Density Func-
tional (ADF v2007.01) package.23,24 A triple-ζ plus polarization
Slater basis set was used on all atoms. Relativistic corrections
were introduced by scalar-relativistic zero-order regular
approximation (ZORA).25 For geometry optimizations we used
the local VWN correlation potential26 together with the Becke’s
exchange27 and the Perdew’s correlation28,29 (BP86) general-
ized gradient corrections. Stationary points in the potential
energy hypersurface were characterized either as minima or
transition states by means of harmonic vibrational frequencies
calculations. Standard corrections to Gibbs free energy at
298 K were evaluated too.
Then,
a saturated solution of Na2S2O3 was added to
the mixture and the product was extracted with ethyl acetate
(3 × 25 mL). A combination of the organic phases was gently
concentrated on a rotary evaporator and chemoselectivity of
1
the alcohol products was determined by H NMR in compari-
son with reported data.21
Acknowledgements
Research supported by the Spanish Ministerio de Economia y
Competitividad (MINECO) through project CTQ2010-16226
and CTQ2011-29054-C02-02, the Generalitat de Catalunya
(2009SGR-00462) and the ICIQ Foundation. We thank
AllyChem for the gift of bis(pinacolato)diboron.
Characterisation of the products from the NHC–Rh catalysed
diboration of cycloalkenes with B2pin2
1,2-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)cyclopentane.
This product was purified by column chromatography (deacti-
vated silica, petroleum ether–EtOAc, 15 : 1). White solid. Yield:
(89%). 1H NMR (CDCl3, 400 MHz): 1.70–1.90 (m, 2H),
1.60–1.52 (4H, m), 1.48–1.40 (2H, m), 1.21 (s, 12H), 1.20
(s, 12H). 13C NMR (CDCl3, 100 MHz): 88.8, 29.3, 32.0, 27.5,
24.7, 22.1. 11B NMR (CDCl3, 128 MHz): δ 34.0. These spectro-
scopic data are in agreement with that previously reported.22
1,2-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)cyclo-
heptane. This product was purified by column chromato-
graphy (deactivated silica, petroleum ether–EtOAc, 15 : 1).
Yellow pale solid. Yield: (80%). 1H NMR (CDCl3, 400 MHz):
δ 1.76–1.70 (m, 2H), 1.65–1.50 (m, 3H), 1.52–1.43 (m, 5H),
1.40–1.31 (m, 2H), 1.22 (s, 24H). 13C NMR (CDCl3, 100 MHz):
δ 82.4, 30.45, 28.25, 27.61, 24.89. 11B NMR (CDCl3, 128 MHz):
δ 34.6. HRMS (ESI+) m/z calculated for C19H36B2O4 [M + H]+ =
351.288.
1,2-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)cyclooctane.
The product was purified by column chromatography (deacti-
vated silica, petroleum ether–EtOAc, 15 : 1). White solid. Yield:
(72%). 1H NMR (CDCl3, 400 MHz): 1.72–1.68 (m, 2H),
1.58–1.40 (m, 10H), 1.39–1.37 (m, 2H), 1.22 (s, 24H). 13C NMR
(CDCl3, 100 MHz): δ 82.84, 28.21, 27.70, 26.58, 24.88, 24.79.
11B NMR (CDCl3, 128 MHz): δ 34.9. HRMS (ESI+) m/z calculated
for C20H38B2O4 [M + H]+ = 365.304.
Notes and references
1 I. A. Mkhalid, J. H. Barmard, T. B. Marder, J. M. Murphy
and J. F. Hartwig, Chem. Rev., 2010, 110, 890.
2 A. Caballero and S. Sabo-Etienne, Organometallics, 2007,
26, 1191.
3 V. J. Olsson and K. J. Szabó, Angew. Chem., Int. Ed., 2007,
46, 6891.
4 T. Kikuchi, J. Takagi, T. Ishiyama and N. Miyaura, Chem.
Lett., 2008, 37, 664; T. Kikuchi, J. Takagi, H. Isou,
T. Ishiyama and N. Miyaura, Chem. Asian J., 2008, 3, 2082.
5 I. A. Mkhalid, R. B. Coapes, S. N. Edes, D. N. Coventry,
F. E. S. Souza, R. Ll. Thomas, J. J. Hall, S.-W. Bi, Z. Lin and
T. B. Marder, Dalton Trans., 2008, 1055.
6 A. Kondoh and T. F. Jamison, Chem. Commun., 2010, 46,
907.
7 H. E. Burks and J. P. Morken, Chem. Commun., 2007, 4717;
J. Ramírez, V. Lillo, A. M. Segarra and E. Fernández, Comp.
Rend. Chim., 2007, 10, 138; I. Beletskaya and C. Moberg,
Chem. Rev., 2006, 106, 2320; T. Ishiyama and N. Miyaura,
Chem. Rec., 2004, 3, 271; T. B. Marder and N. C. Norman,
Top. Catal., 1998, 5, 63.
8 S. J. Geier, E. E. Chapman, D. I. McIsaac, C. M. Vogels,
A. Decken and S. A. Westcott, Inorg. Chem. Commun., 2006,
9, 788.
1,2-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-tetrahydro-
2H-pyran. The product was purified by column chromato-
graphy (deactivated silica, petroleum ether–EtOAc, 15 : 1).
1
Colorless oil. Yield: (57%). H NMR (CDCl3, 400 MHz): 1.20 (s,
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 746–752 | 751

View Article Online
Paper
Dalton Transactions
9 J. A. Mata, A. R. Chianese, J. M. Miecznikowski, M. Poyatos, 17 C. Dai, C. Stringer, T. B. Marder, A. J. Scott, W. Clegg and
E. Peris, J. W. Faller and R. H. Crabtree, Organometallics,
2004, 23, 1253.
10 V. Lillo, E. Mas-Marzá, A. M. Segarra, J. J. Carbó, C. Bo,
E. Peris and E. Fernández, Chem. Commun., 2007, 3380.
11 V. Lillo, J. Mata, J. Ramírez, E. Peris and E. Fernández,
Organometallics, 2006, 25, 5829.
N. C. Norman, Inorg. Chem., 1997, 36, 272; P. Nguyen,
G. Lesley, N. J. Taylor, T. B. Marder, N. L. Pickett, W. Clegg,
M. R. J. Elsegood and N. C. Norman, Inorg. Chem., 1994,
33, 4623; W. Clegg, F. J. Lawlor, T. B. Marder, P. Nguyen,
N. C. Norman, A. G. Orpen, A. J. Quayle, C. R. Riece,
E. G. Robins, A. J. Scott, F. E. S. Souza, G. Stringer and
G. R. Whittell, J. Chem. Soc., Dalton Trans., 1998,
301.
12 R. Corberán, V. Lillo, J. A. Mata, E. Fernández and E. Peris,
Organometallics, 2007, 26, 4350.
13 R. Corberán, J. Ramírez, M. Sanaú, E. Peris and 18 H. Braunshweig and M. Colling, Coord. Chem. Rev., 2001,
E. Fernández, Chem. Commun., 2005, 3056. 223, 1.
14 R. Corberán, J. Ramírez, M. Poyatos, E. Peris and 19 V. P. Ananikov, R. Szilagyi, K. Morokuma and
E. Fernández, Tetrahedron: Asymmetry, 2006, 1759. D. G. Musaev, Organometallics, 2005, 24, 1938.
15 J. M. Brown and G. C. Lloyd-Jones, Chem. Commun., 1992, 20 A. Bonet, C. Pubill-Ulldemolins, C. Bo, H. Gulyás
710; J. M. Brown and G. C. Lloyd-Jones, J. Am. Chem. Soc.,
1994, 116, 866; M. Murata, S. Watanabe and Y. Masuda,
and E. Fernández, Angew. Chem., Int. Ed., 2011, 50,
7158.
Tetrahedron Lett., 1999, 40, 2585; R. T. Baker, 21 S. Hanessian, R. Saladino, R. Margarita and M. Simard,
J. C. Calabrese, S. A. Westcott and T. B. Marder, J. Am. Chem.–Eur. J., 1999, 5, 2169.
Chem. Soc., 1995, 117, 8777; R. T. Baker, J. C. Calabrese, 22 T. Ishiyama, M. Yamamoto and N. Miyaura, Chem.
S. A. Westcott, P. Nguyen and T. B. Marder, J. Am. Chem. Commun., 1997, 689.
Soc., 1993, 115, 4367; R. T. Baker, P. Nguyen, T. B. Marder 23 G. T. Velde, F. M. Bickelhaupt, E. J. Baerends, C. F. Guerra,
and S. A. Westcott, Angew. Chem.,Int. Ed., 1995, 34, 1336;
S. J. A. Van Gisbergen, J. G. Snijders and T. Ziegler,
K. Burgess, W. A. van der Donk, S. A. Westcott,
J. Comput. Chem., 2001, 22, 931.
T. B. Marder, R. T. Baker and J. C. Calabrese, J. Am. Chem. 24 C. F. Guerra, J. G. Snijders, G. te Velde and E. J. Baerends,
Soc., 1992, 114, 9350; S. A. Westcott, T. B. Marder and Theor. Chem. Acc., 1998, 99, 391.
R. T. Baker, Organometallics, 1993, 12, 975; C. M. Vogels, 25 E. van Lenthe, A. Ehlers and E. J. Baerends, J. Chem. Phys.,
P. G. Hayes, M. P. Shaver and S. A. Westcott, Chem. 1999, 110, 8943.
Commun., 2000, 51; R. B. Coapes, F. E. S. Souza, 26 S. H. Vosko, L. Wilk and M. Nusair, Can. J. Phys., 1980, 58,
R. Ll. Thomas, J. J. Hall and T. B. Marder, Chem. Commun.,
1200.
2003, 614.
27 A. D. Becke, Phys. Rev. A, 1988, 38, 3098.
16 D. Li, H. Zaho and T. Marder, Organometallics, 2008, 27, 28 J. P. Perdew, Phys. Rev. B, 1986, 33, 8822.
1178.
29 J. P. Perdew, Phys. Rev. B, 1986, 34, 7406–7406.
752 | Dalton Trans., 2013, 42, 746–752
This journal is © The Royal Society of Chemistry 2013