2 catalyzed hydroborations of alkenes using a bulky dioxaborocine

Article abstract of DOI:10.1016/j.ica.2010.09.051

4,8-Di-tert-butyl-2,10-dimethyl-12H-dibenzo[d,g][1,3,2]dioxaborocine (1) has been prepared in high yield by the addition of H₃B· SMe₂ to 6,6′-methylene(2-tert-butyl-4-methylphenol). Dioxaborocine 1 is a relatively stable solid that reacts with a variety of aliphatic alkenes in the presence of catalytic amounts of [Cp*IrCl ₂]₂ to give the terminal hydroboration products. Analogous reactions with vinylarenes, however, afford the corresponding alkenylboronate esters along with equal amounts of the hydrogenation products. Boron products have been characterized by a number of physical and analytical methods, including single-crystal X-ray diffraction studies.

Full text of DOI:10.1016/j.ica.2010.09.051

Inorganica Chimica Acta 365 (2011) 408–413
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Inorganica Chimica Acta
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[Cp*IrCl2]2 catalyzed hydroborations of alkenes using a bulky dioxaborocine
a,
Nicole M. Hunter a, Christopher M. Vogels a, Andreas Decken b, Andrew Bell c, , Stephen A. Westcott
⇑
⇑
a Department of Chemistry and Biochemistry, Mount Allison University, Sackville, NB, Canada E4L 1G8
b Department of Chemistry, University of New Brunswick, Fredericton, NB, Canada E3B 5A3
c Promerus LLC, 9921 Brecksville Road, Brecksville, OH 44141-3289, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2010
Accepted 28 September 2010
Available online 4 November 2010
4,8-Di-tert-butyl-2,10-dimethyl-12H-dibenzo[d,g][1,3,2]dioxaborocine (1) has been prepared in high
yield by the addition of H3BꢀSMe2 to 6,60-methylene(2-tert-butyl-4-methylphenol). Dioxaborocine 1 is
a relatively stable solid that reacts with a variety of aliphatic alkenes in the presence of catalytic amounts
of [Cp*IrCl2]2 to give the terminal hydroboration products. Analogous reactions with vinylarenes, how-
ever, afford the corresponding alkenylboronate esters along with equal amounts of the hydrogenation
products. Boron products have been characterized by a number of physical and analytical methods,
including single-crystal X-ray diffraction studies.
Keywords:
Borylation
Catalysis
Dioxaborocine
Hydroboration
Iridium
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
tert-butyl-2,10-dimethyl-12H-dibenzo[d,g][1,3,2]dioxaborocine
(1, HBdd, where dd = 6,60-methylenebis(2-tert-butyl-4-methylphe-
The discovery that certain transition metal complexes catalyze
the addition of catecholborane (HBcat, cat = 1,2-O2C6H4) or
pinacolborane (HBpin, 1,2-O2C2Me4) to unsaturated substrates
has become an important strategy in organic synthesis [1–8]. Prod-
ucts obtained using a transition metal catalyzed hydroboration can
have regio-, chemo-, or stereoselectivities complementary, or more
remarkably, opposite to those from products obtained via the
uncatalyzed route. For instance, Männig and Nöth demonstrated
that addition of catecholborane to 5-hexen-2-one proceeded read-
ily at room temperature to give the expected borate, where addi-
tion of the borane has occurred at the more reactive ketone
functionality [9]. However, selective addition of HBcat to the less
reactive alkene group was achieved if the reaction was carried
out at lower temperatures in the presence of a rhodium catalyst,
e.g. RhCl(PPh3)3. Since this remarkable discovery, a considerable
amount of research has focussed on investigating the mechanism
and scope of catalyzed hydroboration reactions [6,10–13]. Much
less studied, however, is the use of alternate borane sources
[14–18]. Indeed, pinacolborane, (4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane or HBpin) is occasionally used as a replacement for HBcat
as the resulting organoborane products are stable to air and chro-
matography. Unfortunately, these reactions can suffer from poor
selectivities or competing pathways (i.e. hydrogenation and/or
dehydrogenative borylations) to give several organoboronate ester
products. As such, we have investigated the use of bulky 4,8-di-
nolato)) for its use in the catalyzed hydroboration of alkenes.
Compound 1 (Fig. 1) was prepared by modification of a known
procedure in 90% yield by the addition of 6,60-methylene(2-tert-bu-
tyl-4-methylphenol) to a solution of H3B.SMe2 in toluene heated at
reflux for 18 h and characterized by multinuclear NMR spectros-
copy [14]. Compound 1 is a white solid that can be kept indefinitely
at room temperature under a nitrogen atmosphere. A broad peak in
the 11B NMR spectrum at d 21 ppm and a broad singlet at 4.57 ppm
in the 1H NMR data are consistent with chemical shifts found in
other bulky diorganyloxyboranes.
Although the synthesis and structure of 1 have been reported in
an elegant study by Nöth and co-workers [14], its potential to act as
a hydroboration agent has not yet been investigated. We therefore
decided to examine its reactivity with a number of unsaturated
substrates and report our findings herein.
2. Experimental
2.1. General
Reagents and solvents were purchased from Aldrich Chemicals
and used as received with the exception of 1 which was synthe-
sized by modification of a known method [14]. NMR spectra were
recorded on a JEOL JNM-GSX270 FT NMR (1H 270 MHz; 11B
87 MHz; 13C 68 MHz; 19F 254 MHz) spectrometer. Chemical shifts
(d) are reported in ppm [relative to residual solvent peaks (1H
and 13C) or external BF3 OEt2 (11B) and CF3CO2H (19F)] and coupling
.
⇑
Corresponding authors.
E-mail address: swestcott@mta.ca (S.A. Westcott).
constants (J) in Hz. Multiplicities are reported as singlet (s), doublet
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.09.051

N.M. Hunter et al. / Inorganica Chimica Acta 365 (2011) 408–413
409
OCH3), 3.85 (s, OCH3), 3.84 (s, OCH3), 3.47 (d, J = 13.6 Hz), 2.80 (t,
J = 7.4 Hz), 2.52 (t, J = 7.9 Hz, CH2CH2CH2B, 4), 2.41 (m), 2.30 (s),
2.08 (m), 1.93 (d of d, J = 6.7 Hz, 1.7 Hz), 1.70 (m), 1.58 (quint,
J = 7.9 Hz, CH2CH2CH2B, 4), 1.43 (s), 1.42 (s), 0.92 (t, J = 7.9 Hz,
CH2CH2CH2B, 4); 11B @ 27 (br).
2.5. Synthesis of (E)-4,8-di-tert-butyl-6-(4-methoxystyryl)-2,10-
dimethyl-12H-dibenzo[d,g][1,3,2]dioxaborocine (5)
Fig. 1. 4,8-Di-tert-butyl-2,10-dimethyl-12H-dibenzo[d,g][1,3,2]dioxaborocine (1).
Compound 1 (250 mg, 0.71 mmol) in toluene (5 mL) was added
to
a toluene (5 mL) solution of 4-methoxystyrene (192 mg,
(d), triplet (t), quartet (q), quintet (quint), multiplet (m), broad (br),
and overlapping (ov). A CEM Discover microwave reactor was em-
ployed for all microwave reactions and the reaction temperature
was monitored by an internal IR pyrometer. Reactions were per-
formed under an atmosphere of dinitrogen.
1.43 mmol) and [Cp*IrCl2]2 (11 mg, 0.014 mmol). The reaction
was allowed to proceed for 18 h, at which point solvent was re-
moved under vacuum and the residual oily solid was dissolved in
a THF:hexane (1 mL:5 mL) mixture and stored at ꢁ30 °C. The
resulting precipitate was collected by suction filtration to afford
5 as an off-white solid. Yield: 0.27 g (79%); mp 218–220 °C. 1H
2.2. Synthesis of 4,8-di-tert-butyl-2,10-dimethyl-6-octyl-12H-
NMR (C6D6):
@ 8.02 (d, J = 18.0 Hz, 1H, CH = CHB), 7.45 (d,
dibenzo[d,g][1,3,2]dioxaborocine (2)
J = 8.6 Hz, 2H, Ar), 6.98 (s, 2H, Ar), 6.82 (s, 2H, Ar), 6.72 (d,
J = 8.6 Hz, 2H, Ar), 6.67 (d, J = 18.0 Hz, 1H, CH@CHB), 4.28 (d,
J = 13.6 Hz, 1H, CHH), 3.25 (s, 3H, OCH3), 3.24 (d, J = 13.6 Hz, 1H,
CHH), 2.07 (s, 6H, CH3), 1.51 (s, 18H, t-butyl); 11B (C6D6): @ 23
(br); 13C{1H} (C6D6): o 160.9, 150.7, 149.0, 139.2, 131.6, 131.4,
130.6, 129.0, 127.8, 126.2, 119.3 (br, CB), 114.2, 54.5, 35.1, 34.7,
Compound 1 (284 mg, 0.81 mmol) in toluene (2 mL) was added
to a toluene (3 mL) solution of 1-octene (100 mg, 0.89 mmol) and
[Cp*IrCl2]2 (13 mg, 0.016 mmol). The reaction was allowed to pro-
ceed for 18 h then passed through a small plug of alumina to re-
move the catalyst and impurities. Removal of solvent under
vacuum afforded 2 as a colorless oil. Yield: 0.25 g (67%). 1H NMR
(C6D6): @ 7.00 (s, 2H, Ar), 6.99 (s, 2H, Ar), 4.21 (d, J = 13.6 Hz, 1H,
bridging-CHH), 3.51 (d, J = 13.6 Hz, 1H, bridging-CHH), 2.33 (s,
6H, CH3), 1.79 (m, 2H, octyl), 1.52–1.29 (ov m, 30H, octyl & t-butyl),
1.00 (t, J = 8.1 Hz, 3H, octyl-CH3); 11B NMR (C6D6): @ 28 (br);
13C{1H} NMR (C6D6): @ 148.8, 139.4, 131.8, 131.1, 127.9, 126.3,
35.4, 34.9, 33.0, 32.3, 30.1, 29.9, 29.7, 25.1, 23.1, 21.3, 17.6 (br,
CB), 14.5. Anal. Calc. for C31H47BO2 (462.59): C, 80.48; H, 10.26.
Found: C, 80.00; H, 10.74%.
.
29.9, 20.8. Anal. Calc. for C32H39BO3 C4H8O2 (554.64): C, 77.95; H,
8.56. Found: C, 78.48; H, 8.90%.
2.6. Synthesis of (E)-4,8-di-tert-butyl-6-(4-fluorostyryl)-2,10-
dimethyl-12H-dibenzo[d,g][1,3,2]dioxaborocine (6)
Compound 1 (250 mg, 0.71 mmol) in toluene (5 mL) was added
to
a toluene (5 mL) solution of 4-fluorostyrene (174 mg,
1.42 mmol) and [Cp*IrCl2]2 (11 mg, 0.014 mmol). The reaction
was allowed to proceed for 18 h, at which point the solvent was re-
moved under vacuum and the resulting colorless oil was dissolved
in a THF:hexane (2 mL:10 mL) mixture and stored at ꢁ30 °C. The
resulting white precipitate was collected by suction filtration to af-
ford compound 6 as a white solid. Yield: 0.29 g (87%); mp 200–
203 °C. 1H NMR (C6D6): @ 7.83 (d, J = 18.1 Hz, 1H, CH@CHB), 7.22
(d of d, JHH = 8.7 Hz, JHF = 5.7 Hz, 2H, Ar), 6.97 (d, J = 2.0 Hz, 2H,
Ar), 6.81 (d, J = 2.0 Hz, 2H, Ar), 6.74 (ov d of d, J = 8.7 Hz, 2H, Ar),
6.57 (d, J = 18.1 Hz, 1H, CH@CHB), 4.24 (d, J = 13.6 Hz, 1H, CHH),
3.24 (d, J = 13.6 Hz, 1H, CHH), 2.07 (s, 6H, CH3), 1.49 (s, 18H, t-bu-
tyl); 11B (C6D6): @ 22 (br); 13C{1H} (C6D6): o 163.4 (d, JCF = 245.2 Hz,
CF), 149.4, 148.8, 139.2, 133.9 (d, JCF = 3.1 Hz, Ar), 131.8, 131.5,
129.2 (d, JCF = 8.2 Hz, Ar), 127.7, 126.2, 121.9 (br, CB), 115.6 (d,
JCF = 21.5 Hz, Ar), 35.0, 34.6, 29.9, 20.8; 19F{1H} (C6D6): @ꢁ112. Anal.
2.3. Synthesis of 6-(2-(bicyclo[2.2.1]heptan-2-yl)ethyl)-4,8-di-tert-
butyl-2,10-dimethyl-12H-dibenzo[d,g][1,3,2]dioxaborocine (3)
Compound 1 (930 mg, 2.65 mmol) in toluene (5 mL) was added
to a toluene (3 mL) solution of 2-norbornene (250 mg, 2.66 mmol)
and [Cp*IrCl2]2 (40 mg, 0.050 mmol). The reaction was allowed to
proceed for 18 h, at which point the solvent was removed under
vacuum and the resulting solid was dissolved in a minimum
amount of hot THF. The solution was stored at ꢁ30 °C and the
resulting precipitate was collected by suction filtration to afford
3 as an off-white solid. Yield: 0.98 g (83%); mp 174–176 °C. 1H
NMR (C6D6):
@ 6.90 (s, 2H, Ar), 6.76 (s, 2H, Ar), 4.05 (d,
.
J = 13.9 Hz, 1H, bridging-CHH), 3.23 (d, J = 13.9 Hz, 1H, bridging-
CHH), 2.79 (br s, 1H), 2.33 (br s, 1H), 2.00 (s, 6H, CH3), 1.60 (m,
2H), 1.46–1.26 (ov m, 25H, norbornyl & t-butyl); 11B (C6D6): @ 29
(br); 13C{1H} (C6D6): @ 149.1, 138.9, 131.4, 130.9, 127.6, 126.1,
39.4, 38.7, 37.2, 35.3, 34.6, 33.7, 32.6, 31.4 (br, CB), 29.8, 29.4,
Calc. for C31H36BFO2 0.5 C4H8O2 (506.54): C, 78.24; H, 7.98. Found:
C, 78.12; H, 7.91%.
2.7. Synthesis of (E)-4,8-di-tert-butyl-2,10-dimethyl-6-(2,4,6-
trimethylstyryl)-12H-dibenzo[d,g][1,3,2]dioxaborocine (7)
.
20.7. Anal. Calc. for C30H41BO2 C4H8O2 (516.64): C, 79.04; H, 9.58.
Found: C, 79.30; H, 9.56%.
Compound 1 (250 mg, 0.71 mmol) in toluene (5 mL) was added
to a toluene (5 mL) solution of 2,4,6-trimethylstyrene (208 mg,
1.42 mmol) and [Cp*IrCl2]2 (11 mg, 0.014 mmol). The reaction
was allowed to proceed for 18 h, at which point the solvent was re-
moved under vacuum and the resulting colorless oil was dissolved
in hot hexane (5 mL) and stored at ꢁ30 °C. The resulting white pre-
cipitate was collected by suction filtration to afford compound 7 as
a white solid. Yield: 0.27 g (77%); mp 159–162 °C. 1H NMR (C6D6):
o 8.10 (d, J = 18.5 Hz, 1H, CH@CHB), 6.95 (s, 2H, Ar), 6.82 (s, 2H, Ar),
6.81 (s, 2H, Ar), 6.34 (d, J = 18.5 Hz, 1H, CH = CHB), 4.30 (d,
J = 13.6 Hz, 1H, CHH), 3.22 (d, J = 13.6 Hz, 1H, CHH), 2.42 (s, 6H,
CH3), 2.17 (s, 3H, CH3), 2.06 (s, 6H, CH3), 1.47 (s, 18H, t-butyl);
11B (C6D6): @ 24 (br); 13C{1H} (CDCl3): o 149.2, 148.4, 139.5,
2.4. The [Cp*IrCl2]2 catalyzed addition of 4,8-di-tert-butyl-2,10-
dimethyl-12H-dibenzo[d,g][1,3,2]dioxaborocine to 4-allylanisole
To a stirred toluene (2 mL) solution of 4-allylanisole (140 mg,
0.94 mmol) and [Cp*IrCl2]2 (10 mg, 0.013 mmol) was added a tolu-
ene (3 mL) solution of 1 (164 mg, 0.47 mmol). The reaction was al-
lowed to proceed for 18 h at which point solvent was removed
under vacuum to afford an orange oil. Selected spectroscopic
NMR data (in CDCl3): 1H @ 7.33 (d, J = 8.9 Hz, Ar), 7.20–6.75 (ov
m, Ar), 6.52 (m), 6.43 (d of q, J = 16.1, 1.7 Hz), 6.17 (d of q, J =
16.1, 6.7 Hz), 4.24 (d, J = 13.6 Hz), 4.14 (d, J = 13.6 Hz), 3.86 (s,

410
N.M. Hunter et al. / Inorganica Chimica Acta 365 (2011) 408–413
136.9, 136.0, 135.6, 131.8, 131.2, 128.9, 128 (br, CB), 127.7, 126.1,
35.1, 34.7, 29.9, 21.1, 21.0 (2C). Anal. Calc. for C34H43BO2 C6H14
(580.78): C, 82.72; H, 9.91. Found: C, 82.36; H, 10.10%.
specific in the formation of the terminal hydroboration products
and have been postulated to proceed via metathesis of the Ir–Cl
bond with the borane B–H to give a iridium hydride species. Coor-
dination and regioselective insertion of the alkene into the Ir–H
bond, followed by a metathesis step with the borane gives the lin-
ear product and regenerates the active Ir–H catalyst (Scheme 1).
We have found that 1 (HBdd) adds to 1-octene and 2-norborn-
ene using 5 mol% [Cp*IrCl2]2 to give the expected terminal addition
products CH3(CH2)7Bdd (2) and norbornylBdd (3), respectively,
with excellent selectivity (Scheme 2). Although the corresponding
hydrogenation products are frequently observed in catalyzed hyd-
roborations [1,2], reactions using HBdd and the iridium catalyst
precursor only gave minor amounts (<2%) of the undesired satu-
rated products. Compounds 2 and 3 have been characterized by a
number of analytical methods including a single crystal X-ray dif-
fraction study for 3 (Fig. 2, Table 1), confirming that the H–B bond
of 1 has added to the alkene group in an exo fashion [23]. The bor-
on oxygen bond distances of B(1)–O(1) 1.361(2) and B(1)–O(2)
1.361(4) Å are similar to those reported previously for 1 [14] and
the B(1)–C(24) bond distance of 1.599(3) Å is within the range re-
ported for other diorganyloxyboranes [24–29]. More significantly,
the C(24)–C(29) bond length of 1.559(3) Å is typical for a single
bond. Attempts to affect the hydroboration of limonene [30] using
1 and [Cp*IrCl2]2 proved unsuccessful, even when reactions were
carried out at elevated temperatures using a microwave reactor
[18], and additions appear to be remarkably selective for unhin-
dered aliphatic alkenes.
.
2.8. Synthesis of (E)-4,8-di-tert-butyl-2,10-dimethyl-6-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)styryl)-12H-dibenzo[d,g][1,3,2]-
dioxaborocine (8)
To
a THF (10 mL) solution of 4-vinylphenylboronic acid
(300 mg, 2.03 mmol) and pinacol (240 mg, 2.03 mmol) was added
activated molecular sieves (2 g, 5 Å). The mixture was allowed to
stand at RT for 3 days at which point the sieves were removed by
suction filtration. To the resulting clear solution was added a THF
(5 mL) solution of 1 (355 mg, 1.01 mmol) and [Cp*IrCl2]2 (40 mg,
0.05 mmol). The reaction was allowed to proceed for 20 h, at which
point the solvent was removed under vacuum and the resulting
colorless oil was dissolved in hot hexane (5 mL) and stored at
ꢁ30 °C. The resulting white precipitate was collected by suction fil-
tration to afford compound 8 as a white solid. Yield: 0.40 g (68%);
mp 248–250 °C. 1H NMR (C6D6): @ 8.17 (d, J = 7.9 Hz, 2H, Ar), 7.95
(d, J = 18.1 Hz, 1H, CH@CHB), 7.55 (d, J = 7.9 Hz, 2H, Ar), 6.95 (s, 2H,
Ar), 6.80 (s, 2H, Ar), 6.78 (d, J = 18.1 Hz, 1H, CH@CHB), 4.22 (d,
J = 13.6 Hz, 1H, CHH), 3.22 (d, J = 13.6 Hz, 1H, CHH), 2.06 (s, 6H,
CH3), 1.46 (s, 18H, t-butyl), 1.12 (s, 12H, Bpin); 11B (C6D6): @ 30
(br) & 23 (br); 13C{1H} (C6D6): o 150.8, 148.8, 140.4, 139.3, 135.6,
131.7, 131.4, 130.4 (br, C-B), 127.7, 127.0, 126.2, 123.3 (br, C-B),
83.6, 35.1, 34.6, 29.9, 24.7, 20.8. Anal. Calc. for
C37H48B2O4
Conversely, reactions of 1 with 4-allylanisole 4-MeOC6H4CH2-
CH@CH2 were found to proceed at room temperature to give the
expected terminal hydroboration product 4-MeOC6H4CH2CH2-
CH2Bdd (4) along with considerable amounts of hydrogenation
product 4-MeOC6H4CH2CH2CH3 and other unidentified boron-con-
taining products. As metal catalyzed hydroborations of allylben-
zene (C6H5CH2CH@CH2) are known to give complicated product
distributions arising from a competing isomerization reaction,
where the borane can add to the resulting transient isomer
C6H5CH@CHCH3 [31], it is possible that a similar mechanism is
occurring with reactions of 1 and 4-allylanisole. To gain further in-
sight into the nature of products formed during these reactions, we
therefore investigated reactions of vinylarenes using HBdd and
[Cp*IrCl2]2 as a catalyst precursor. Considerable recent interest
has focussed on the hydroboration [32–38] and subsequent trans-
formation [39] of these important substrates. While reactions of 4-
vinylanisole (4-MeOC6H4CH@CH2) with HBcat and HBpin using a
catalytic amount of [Cp*IrCl2]2 proceed to give the terminal hyd-
(578.47): C, 76.82; H, 8.38. Found: C, 77.05; H, 8.28%.
2.9. Crystallography
Crystals of 3 were grown from a saturated toluene solution at
ꢁ30 °C and crystals of 5 were grown by slow evaporation of a
C6D6 solution at RT. Single crystals were coated with Paratone-N
oil, mounted using a polyimide MicroMount and frozen in the cold
nitrogen stream of the goniometer. A hemisphere of data was col-
lected on a Bruker AXS P4/SMART 1000 diffractometer using
h scans with a scan width of 0.3° and 20 s (3) or 10 s (5) exposure
x and
times. The detector distance was 5 cm. The data were reduced
(SAINT) [19] and corrected for absorption (SADABS) [20]. The struc-
tures were solved by direct methods and refined by full-matrix
least squares on F2 (SHELXTL) [21]. Part of the molecule for 3 was dis-
ordered over two sites and the occupancies determined using an
isotropic model as 0.85 (O(2), C(24) > C(30)) and 0.15 (O(20),
C(240) > C(300)) and fixed in subsequent refinement cycles. All
non-hydrogen atoms were refined using anisotropic displacement
parameters. Hydrogen atoms for 3 were included in calculated
positions and refined using a riding model while the hydrogen
atoms for 5 were found in Fourier difference maps and refined
using isotropic displacement parameters.
3. Results and discussion
As with other diorganyloxyboranes, no significant reaction was
observed with 1 and 1-octene at room temperature. Addition of the
B-H bond did occur at elevated temperatures but competing degra-
dation of the borane made these reactions synthetically unattrac-
tive. With rhodium complexes being the most commonly used in
catalyzed hydroborations, we decided to initiate our studies using
5 mol% RhCl(PPh3)3. Unlike other diorganyloxyboranes, however,
no reaction was observed with 1 and 1-octene using this catalyst
precursor. We then decided to examine reactions using dimeric
[Cp*IrCl2]2 as a catalyst precursor, as we have previously found this
to be remarkably active and selective for the hydroboration of a
wide range of alkenes using HBcat [22]. Reactions with HBcat are
Scheme 1. A possible mechanism for the [Cp*IrCl2]2 catalyzed hydroboration of
alkenes.

N.M. Hunter et al. / Inorganica Chimica Acta 365 (2011) 408–413
411
Scheme 2. Addition of 1 to various aliphatic alkenes.
Compound 5 has been characterized by a number of methods,
including 1H NMR spectroscopy which shows two doublets at d
8.02 and 7.45 ppm with JH–H = 18.0 Hz for the two trans alkene pro-
tons. Also consistent with this structure are two sp2 carbon peaks
for the alkene at d 150.7 and 119.3 ppm in the 13C NMR spectrum,
along with a broad peak at d 23 ppm in the 11B NMR spectrum. To
confirm the formation of 5, we have carried out a single crystal X-
ray diffraction study and the molecular structure of the alkenylbor-
onate ester is shown in Fig. 3. The short C(8)–C(9) distance of
1.3335(18) Å illustrates that the alkene has not been reduced, as
compared to the C(5)–C(8) single bond length of 1.4671(17) Å.
Bond distances and angles for the boron atom are typical of related
organoboronate esters [28,42].
The formation of alkenylboronate ester 5 in these reactions is of
particular interest as it appears to suggest that hydroborations of
vinylarenes using 1 are proceeding by a different mechanism to
the one outlined in Scheme 1. Indeed, products from this ‘addition
reaction’ presumably arise from an alternate dehydrogenative
borylation pathway, which is generally believed to occur via initial
oxidative addition of the borane to the metal center to give a hydr-
ido boryl metal intermediate, followed by coordination of the al-
kene and subsequent insertion into the M–B bond [43–46]. This
is followed by a selective b-hydride elimination to give the trans-
alkenylboronate ester along with formation of one equivalent of
dihydrogen (Scheme 3).
The formation of alkenylboronate esters in these reactions is of
significant importance as these compounds have found consider-
able utility in organic synthesis [47–51]. We therefore decided to
look at reactions with other vinylarenes and found that 1 reacts
with 4-FC6H4CH@CH2, 2,4,6-Me3C6H2CH@CH2, and 4-BpinC6H4
CH@CH2 in the presence of a catalytic amount of [Cp*IrCl2]2 to give
trans-4-FC6H4CH@CHBdd (6), trans-2,4,6-Me3C6H2CH@CHBdd (7),
and trans-4-BpinC6H4CH@CHBdd (8), respectively, as the only
new boron-containing products. Unfortunately, hydrogenation
was also significant in these reactions and one equivalent of the
Fig. 2. Molecular structure of 3 with atom labeling scheme. Thermal ellipsoids are
drawn at the 50% probability level with hydrogen atoms omitted for clarity.
Selected bond distances (Å): B(1)–O(1) 1.361(2), B(1)–O(2) 1.361(4), B(1)–C(24)
1.599(3), C(24)–C(29) 1.559(3), C(24)–C(25) 1.587(3); selected bond angles (°):
O(1)–B(1)–O(2) 128.20(19), O(1)–B(1)–C(24) 117.62(17), O(2)–B(1)-C(24)
113.81(18), B(1)–O(1)–C(1) 128.06(15), B(1)–O(2)–C(14) 139.2(3), C(29)-C(24)–
C(25) 103.0(2).
roboration products 4-MeOC6H4CH2CH2Bcat [40] and 4-MeOC6H4-
CH2CH2Bpin, respectively, reactions with HBdd gave the unusual
trans-4-MeOC6H4CH@CHBdd (5) as the only new boron-containing
product. Also observed in these reactions are equimolar amounts
of saturated product 4-MeOC6H4CH2CH3 [41].

412
N.M. Hunter et al. / Inorganica Chimica Acta 365 (2011) 408–413
Table 1
Crystallographic data collection parameters.
Complex
3
5
Formula
C
30H41BO2
C32H39BO3
482.44
monoclinic
P2(1)/c
13.6818(12)
15.4771(14)
13.4015(12)
90
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
444.44
monoclinic
C2/c
18.4499(17)
14.6402(17)
19.446(2)
90
a
(°)
b (°)
97.897(2)
90
5202.8(10)
8
96.521(1)
90
914.1(12)
4
Scheme 3. Catalyzed hydroboration of vinylarenes using 1 and 5 mol% [Cp*IrCl2]2.
c
(°)
V (Å3)
Z
esters were observed by 1H NMR spectroscopy, even when
reactions were conducted at low temperature. Further studies in
this area will focus on expanding the scope and utility of reactions
using 4,8-di-tert-butyl-2,10-dimethyl-12H-dibenzo[d,g][1,3,2]
dioxaborocine (1), the results of which will be published in due
course.
qcalc (mg mꢁ3
)
1.135
1.137
Crystal size (mm3)
Temperature (K)
Radiation
0.55 ꢂ 0.45 ꢂ 0.40
0.60 ꢂ 0.55 ꢂ 0.38
173(1)
173(1)
Mo K
a
Mo Ka
(k = 0.71073)
0.070
19294
6332
481
1.50–27.50
0.276/ꢁ0.135
(k = 0.71073)
0.068
17614
5812
330
1.78–27.50
0.677/ꢁ0.359
l
(mmꢁ1
)
Total reflections
Total unique reflections
Number of variables
Theta range (°)
4. Conclusions
Largest difference peak/hole
(e Åꢁ3
)
We
have
found
that
4,8-di-tert-butyl-2,10-dimethyl-
S Goodness-of-fit (GOF) on F2
1.039
0.0590
0.1769
1.031
0.0394
0.1139
12H-dibenzo[d,g][1,3,2]dioxaborocine (1) fails to add to alkenes
at room temperature or in the presence of a catalytic amount of
RhCl(PPh3)3. However, the iridium complex [Cp*IrCl2]2 can be used
as a precatalyst with aliphatic alkenes to give selective formation
of the corresponding terminal hydroboration products. Analogous
reactions with vinylarenes afforded alkenylboronate esters along
with equal amounts of the hydrogenation products. Unlike most
catalyzed hydroborations using the iridium precatalyst [Cp*IrCl2]2,
these reactions are unique in that they appear to be proceeding via
a competing dehydrogenative pathway. Boron products have been
characterized by a number of physical and analytical methods,
including single- crystal X-ray diffraction studies. We will continue
to expand the use of 1 in hydroboration reactions as well as inves-
tigating its potential as a bulky borane counterpart in Frustrated
Lewis Pair chemistry [52], and will report our findings in due
course.
a
R1 (I > 2
r
(I))
b
wR2 (all data)
aR1
=
R
||Fo| ꢁ |Fc||/
R
|Fo|. bwR2 = (
R
[w(F2o ꢁ F2c )2]/
R r
[wF4o ])1/2, where w = 1/[ 2(F2o) +
(0.0496 ꢂ P)2 + (0.8563 ꢂ P)] (3) and 1/[
where P = (max (F2o , 0) + 2 ꢂ Fc2)/3.
r
2(F2o ) + (0.0804 ꢂ P)2 + (6.5500 ꢂ P)] (5),
Acknowledgments
Thanks are gratefully extended to the Canada Research Chairs
Programme, Canadian Foundation for Innovation-Atlantic Innova-
tion Fund, Natural Science and Engineering Research Council of
Canada, and Mount Allison University for financial support. Roger
Smith and Dan Shrubnel Durant are thanked for their expert tech-
nical assistance.
Fig. 3. Molecular structure of 5 with atom labeling scheme. Thermal ellipsoids are
drawn at the 50% probability level with hydrogen atoms omitted for clarity.
Selected bond distances (Å): B(1)–O(2) 1.3632(16), B(1)–O(3) 1.3660(17), B(1)–C(9)
1.5521(19), C(8)–C(9) 1.3335(18), C(5)–C(8) 1.4671(17); selected bond angles(°):
O(2)–B(1)-O(3) 128.72(12), O(2)–B(1)–C(9) 116.82(11), O(3)–B(1)-C(9) 114.38(11),
C(2)–O(1)–C(1) 117.09(11), B(1)–O(2)–C(10) 130.59(10), B(1)–O(3)–C(27)
140.35(10), C(9)–C(8)–C(5) 127.25(12), C(8)–C(9)–B(1) 125.05(12).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.09.051.
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