Rhodium(I) acetylacetonato complexes containing phosphinoalkynes as catalysts for the hydroboration of vinylarenes

Article abstract of DOI:10.1139/V05-242

Three novel rhodium(I) acetylacetonato (acac) complexes bearing phosphinoalkynes (Ph₂PC≡C-t-Bu, Ph₂PC≡ CPPh₂, and Ph₂PC≡CC≡CPPh₂) have been prepared and characterized fully. Addition of B₂cat₃ (cat = 1,2-O₂C₆H₄) to Rh(acac)(Ph ₂PC≡C-t-Bu)₂ (1a) led to zwitterionic Rh(η⁶-catBcat)(Ph₂PC≡C-t-Bu)₂ (2a), the first example of this type of compound to contain monodentate phosphine ligands. All new rhodium complexes have been investigated for their ability to catalyse the hydroboration of vinylarenes.

Full text of DOI:10.1139/V05-242

146
Rhodium(I) acetylacetonato complexes containing
phosphinoalkynes as catalysts for the
hydroboration of vinylarenes1
Christopher M. Vogels, Andreas Decken, and Stephen A. Westcott
Abstract: Three novel rhodium(I) acetylacetonato (acac) complexes bearing phosphinoalkynes (Ph2PCϵC-t-Bu,
Ph2PCϵCPPh2, and Ph2PCϵCCϵCPPh2) have been prepared and characterized fully. Addition of B2cat3 (cat = 1,2-
6
O2C6H4) to Rh(acac)(Ph2PCϵC-t-Bu)2 (1a) led to zwitterionic Rh(η -catBcat)(Ph2PCϵC-t-Bu)2 (2a), the first example of
this type of compound to contain monodentate phosphine ligands. All new rhodium complexes have been investigated
for their ability to catalyse the hydroboration of vinylarenes.
Key words: catalysis, hydroboration, phosphinoalkynes, regioselectivity, rhodium.
Résumé : On a préparé et complètement caractérisé trois nouveaux complexes acétylacétonato du rhodium(I) portant
des phosphinoalcynes (Ph2PCϵC-t-Bu; Ph2PCϵCPPh2 et Ph2PCϵCCϵCPPh2). L’addition de B2cat3 (cat = 1,2-O2C6H4)
6
au Rh(acac)(Ph2PCϵC-t-Bu)2 (1a) conduit à la formation du composé zwitterionique Rh(η -catBcat)(Ph2PCϵC-t-Bu)2
(2a), le premier exemple de ce type de composé à contenir des ligands phosphines monodentates. On a examiné
l’utilité potentielle de tous les nouveaux complexes du rhodium comme catalyseurs pour l’hydroboration de vinylarènes.
Mots clés : catalyse, hydroboration, phosphinoalcynes, régiosélectivité, rhodium.
[Traduit par la Rédaction] Vogels et al. 153
Introduction
some hydroborating agents, such as polyhedral boranes, are
slow to react even at room temperature. Likewise, addition
of H3B·THF to catechol affords catecholborane (HBcat,
cat = 1,2-O2C6H4), a relatively stable hydroborating agent,
which adds to alkenes and alkynes only at elevated tempera-
tures (ca. 100 and 70 °C, respectively) (3). Catecholborane
decomposes thermally or by addition of nucleophiles to give
a number of boron-containing products (4).
That transition metals accelerate the addition of B—H
bonds to unsaturated organic moieties was initially reported
for catalysed hydroborations of alkenes and alkynes using
polyhedral boranes (5). Männig and Nöth (6) then demon-
strated that rhodium complexes could be used to catalyse the
hydroboration of alkenes with HBcat under mild conditions
and with chemoselectivity differing from that of the
uncatalysed reaction (Scheme 1). Indeed, in the catalysed
hydroboration of 5-hexen-2-one, addition of HBcat occurred
preferentially at the C=C double bond, even in the presence
of the more reactive ketone group.
One of the most important synthetic methodologies to
emerge from organic chemistry in the last century has been
the discovery that boron–hydrogen bonds add to unsaturated
organic molecules to form a class of compounds known as
organoboranes (1). Organoboranes possess an incredibly di-
verse chemistry and are remarkably useful intermediates in
organic synthesis today. Indeed, organoboranes can be trans-
formed into any number of functional groups. The simplest
boron hydride agent is borane, BH3, which reacts rapidly
with unhindered alkenes to afford initially monoalkyl-
boranes, then dialkylboranes, and finally trialkylboranes. For
sterically hindered alkenes, the second and third
hydroboration steps become increasingly sluggish. Addition
occurs in a controlled cis-fashion (syn-addition), where the
boryl (BR2) fragment adds preferentially to the least hin-
dered carbon of the unsymmetrically substituted double
bond. While borane adds rapidly to alkenes at –80 °C (2),
Since this seminal discovery, a considerable amount of re-
search has focused on investigating the mechanism and
scope of late metal catalysed hydroboration reactions (7).
Although other metals have been found to catalyse hydro-
borations with HBcat, rhodium-based catalyst systems are
usually the most effective for reactions of vinylarenes (8).
Unfortunately, these reactions often suffer from poor
selectivities or competing pathways (i.e., hydrogenation),
and as a result, a considerable amount of research has fo-
cused on designing new catalyst systems (9). We report
herein on the synthesis and hydroboration of rhodium(I)
acetylacetonato complexes containing phosphinoalkyne lig-
ands.
Received 14 June 2005. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 22 February 2006.
Dedicated to Dr. A.J. Carty in recognition of his outstanding
contributions to chemistry and for being an exceptional and
encouraging teacher and mentor.
C.M. Vogels and S.A. Westcott.2 Department of Chemistry,
Mount Allison University, Sackville, NB E4L 1G8, Canada.
A. Decken. Department of Chemistry, University of New
Brunswick, Frederiction, NB E3B 5A3, Canada.
1This article is part of a Special Issue dedicated to Professor
Arthur Carty.
2Corresponding author (e-mail: swestcott@mta.ca).
Can. J. Chem. 84: 146–153 (2006)
doi:10.1139/V05-242
© 2006 NRC Canada

Vogels et al.
147
(s, 2H, C=CH), 1.57 (s, 12H, C(O)Me). 13C{1H} δ: 184.6,
134.1 (app. quint, JC-P-Rh = 27 Hz), 133.4 (t, JC-P = 6 Hz),
128.9, 127.5 (t, JC-P = 5.2 Hz), 102.4 (td, JC-P = 109, 36 Hz,
PCC), 99.8, 26.9. 31P{1H} δ: 32.1 (d, JP-Rh = 191 Hz). Anal.
calcd. for C62H54O4P4Rh2 (%): C 62.42, H 4.57; found: C
62.26, H 4.25.
Scheme 1. Hydroboration of 5-hexen-2-one.
O
B
O
uncatalysed
O
O
O
O
+
HB
O
[{Rh(acac)}2(␮-Ph2PCϵCCϵCPPh2-␬2P)2] (1c)
O
B
HBcat
O
A solution of Ph2PCϵCCϵCPPh2 (0.22 g, 0.52 mmol) in
THF (1 mL) was added to a solution of Rh(acac)(coe)2
(0.20 g, 0.47 mmol) in THF (2 mL), and the mixture was
stirred for 1 h. The resulting orange precipitate was collected
by suction filtration and washed with hexane (3 × 3 mL) to
give 3 (0.28 g, 95%). Spectroscopic NMR data (in C6D6): 1H
δ: 7.94–7.90 (ov m, 16H, Ar), 6.98–6.90 (ov m, 24H, Ar),
5.25 (s, 2H, C=CH), 1.52 (s, 12H, C(O)Me). 31P{1H} δ: 35.5
(d, JP-Rh = 193 Hz). Anal. calcd. for C66H54O4P4Rh2 (%): C
63.87, H 4.39; found: C 63.42, H 4.49.
5 mol%
RhCl(PPh3)3
0 °C
Experimental
General
Reagents and solvents used were purchased from Aldrich
Chemicals. NMR spectra were recorded on a JEOL JNM-
GSX270 FT spectrometer. H NMR chemical shifts are re-
1
ported in parts per million (ppm) and referenced to residual
solvent protons in deuterated solvent at 270 MHz. 11B NMR
chemical shifts are reported in ppm and are referenced to
BF3·OEt2 as an external standard at 87 MHz. 13C NMR
chemical shifts are referenced to solvent carbon resonances
Rh(␩6-Bcat2)(Ph2PCϵC-t-Bu)2 (2a)
A solution of B2cat3 (0.05 g, 0.14 mmol) in toluene
(0.5 mL) was added to a solution of Rh(acac)(Ph2PCϵC-t-Bu)2
(0.10 g, 0.14 mmol) in toluene (2 mL), and the mixture was
stirred for 18 h. The precipitate that formed was removed by
suction filtration, and hexane (3 mL) was added to the fil-
trate, and the solution was stored at –25 °C for 3 days. The
resulting red precipitate was collected by suction filtration to
afford 2a (0.080 g, 66%). Spectroscopic NMR data (in
as internal standards at 68 MHz and are reported in ppm. 31
P
NMR chemical shifts are reported in ppm and are referenced
to H3PO4 as an external standard at 109 MHz. Multiplicities
are reported as singlet (s), doublet (d), triplet (t), multiplet
(m), broad (br), and overlapping (ov). All reactions were
carried out under an atmosphere of dinitrogen. Micro-
analyses for C and H were carried out at Guelph Chemical
Laboratories (Guelph, Ontario). Phosphines were prepared
as described previously and donated as a generous gift from
Dr. J.F. Corrigan (10). Rh(acac)(coe)2 was prepared as de-
scribed previously (11).
1
C6D6): H δ: 7.89–7.81 (ov m, 8H, Ar), 7.15–6.96 (ov m,
6
16H, Ar), 4.83 (m, 4H, η -cat), 0.80 (s, 18H, tert-butyl). 11B
δ: 15.0. 31P{1H} δ: 23.6 (d, JP-Rh = 213 Hz). Anal. calcd. for
C48H46BO4P2Rh (%): C 66.83, H 5.39; found: C 67.17, H
5.62.
Rh(acac)(Ph2PCϵC-t-Bu)2 (1a)
General procedure for the hydroboration of vinylarenes
Under an atmosphere of dinitrogen, 1 equiv. of catechol-
borane in 0.5 mL of C6D6 was added to a 0.5 mL C6D6 solu-
tion of the appropriate catalyst and vinylarene. The reactions
were allowed to proceed for 18 h, at which point NMR data
were collected. Product distributions were confirmed by
GC–MS.
A solution of Ph2PCϵC-t-Bu (0.25 g, 0.95 mmol) in tolu-
ene (1 mL) was added to a solution of Rh(acac)(coe)2
(0.20 g, 0.47 mmol) in toluene (2 mL), and the mixture was
stirred for 18 h. The removal of solvent under vacuum af-
forded an orange solid, which was washed with hexane (3 ×
3 mL) and collected by suction filtration to give 1a (0.31 g,
1
88%). Spectroscopic NMR data (in C6D6): H δ: 8.25–8.18
(ov m, 8H, Ar), 7.17–7.04 (ov m, 12H, Ar), 5.20 (s, 1H,
C=CH), 1.53 (s, 6H, C(O)Me), 0.93 (s, 18H, tert-butyl).
13C{1H} δ: 184.3, 137.0 (app. quint, JC-P-Rh = 26 Hz), 133.9
X-ray crystallography
Crystals of 1a and 2a were grown from satd. Et2O and to-
luene–hexane (1:1) solutions, respectively, at –30 °C. Single
crystals were coated with Paratone-N oil, mounted using a
glass fibre, and frozen in the cold stream of the goniometer.
A hemisphere of data were collected on a Bruker AXS
P4/SMART 1000 diffractometer using ω and θ scans with a
scan width of 0.3° and exposure times of 30 s (1a) and 10 s
(2a). The detector distances were 5 cm (2a) and 6 cm (1a).
Crystals of 1a were multiple twins; however, only the data
for the major isomer lead to a successful structure refine-
ment. The data were reduced (12a) and corrected for absorp-
tion (12b). The structures were solved by direct methods and
refined by full-matrix least-squares on F2 (12c). All non-
hydrogen atoms were refined anisotropically. Hydrogen at-
oms were located in Fourier difference maps and refined
isotropically.
(t, JC-P = 6 Hz), 128.7, 127.3 (t, JC-P = 5 Hz), 114.4 (t, JC-P
=
6 Hz, PCC), 99.6, 75.7 (td, JC-P = 153, 46 Hz, PCC), 29.9,
28.2, 26.8. 31P{1H} δ: 31.4 (d, JP-Rh = 196 Hz). Anal. calcd.
for C41H45O2P2Rh (%): C 67.02, H 6.19; found: C 66.79, H
6.45.
[{Rh(acac)}2(␮-Ph2PCϵCPPh2-␬2P)2] (1b)
A solution of Ph2PCϵCPPh2 (0.14 g, 0.36 mmol) in tolu-
ene (1 mL) was added to a solution of Rh(acac)(coe)2
(0.15 g, 0.36 mmol) in toluene (2 mL), and the mixture was
stirred for 18 h. The solvent was removed under vacuum,
and the resulting orange precipitate was washed with hexane
(3 × 3 mL) and collected by suction filtration to give 1b
1
(0.18 g, 82%). Spectroscopic NMR data (in C6D6): H δ:
7.95–7.88 (ov m, 16H, Ar), 7.01–6.89 (ov m, 24H, Ar), 5.26
© 2006 NRC Canada

148
Can. J. Chem. Vol. 84, 2006
Scheme 2. The rhodium-catalysed hydroboration of styrene with HBcat.
O
B
H
O
O
O
+
O
B
HB
O
5 mol%
Rh(acac)(P )
H
5 mol%
[Rh(coe) Cl] /2PPh
3
2
2
2
HBcat
Scheme 3. Synthesis of rhodium complexes 1a–1c.
PPh2
PPh2
tBu
tBu
O
O
R
R
O
O
O
O
PPh2
n = 2
n = 1
Rh
Rh
PPh2
R
+
n
Rh
Rh
- 2 coe
- 2 coe
O
O
PPh2
a: R = tBu
b: R = PPh2
c: R = CCPPh2
1b,1c
1a
Results and discussion
Fig. 1. The molecular structure of 1a showing probability ellip-
soids at 30% and hydrogen atoms omitted for clarity. Selected
bond distances (Å) and angles (°): Rh—O(1) 2.057(3), Rh—O(2)
2.061(3), Rh—P(1) 2.173(1), Rh—P(2) 2.182(1), C(18)—C(19)
1.186(6), C(36)—C(37) 1.180(6); O(1)-Rh-O(2) 88.3(1), O(1)-
Rh-P(1) 90.98(8), O(2)-Rh-P(1) 176.24(9), O(1)-Rh-P(2)
174.28(8), O(2)-Rh-P(2) 86.04(9), P(1)-Rh-P(2) 94.72(4), P(1)-
C(18)-C(19) 174.2(4), P(2)-C(36)-C(37) 178.8(4).
Rhodium phosphine complexes are versatile and efficient
catalysts for the hydroboration of alkenes using catechol-
borane (HBcat). Among this group, complexes of the type
Rh(acac)(P2) (acac = acetylacetonato; P2 = diphosphine) are
unique in that they are the only catalyst precursors to have
been used in the hydroboration of a tetrasubstituted alkene
(13a). These complexes are also active and selective cata-
lysts for the hydroboration of a wide range of vinylarenes
using HBcat, giving predominantly the corresponding
branched isomers (Scheme 2). Although a considerable
amount of research has focused on using these complexes as
catalysts for this reaction (13), all examples have used che-
lating bidentate phosphines. In this study, we have prepared
the analogous rhodium acetylacetonato complexes bearing
monodentate phosphinoalkynes and examined their ability to
catalyze the hydroboration of vinylarenes.
Rhodium complexes
The coordination chemistry of bis(diphenylphosphino)ace-
tylene and related alkynyl phosphines is an area of consider-
able interest (14–17). Although the normal mode of
coordination is via the phosphine groups, examples where
2
the alkyne unit is bound to a metal in a η -π fashion are also
known (15i). We have found that alkynyl phosphines a–c
add to Rh(acac)(coe)2 (coe = cis-cyclooctene) to give
Rh(acac)(Ph2PCϵC-t-Bu)2 (1a), [{Rh(acac)}2(µ-Ph2PCϵCP-
Ph2-κ2P)2] (1b), and [{Rh(acac)}2(µ-Ph2PCϵCCϵCPPh2-
κ2P)2] (1c), respectively (Scheme 3). All new complexes
have been characterized by a number of physical methods,
including multinuclear NMR spectroscopy. The 31P{1H}
NMR spectra all showed doublets with coupling to rhodium
with ca. JP-Rh = 195 Hz. Complex 1a has also been charac-
terized by a single crystal X-ray diffraction study (Fig. 1),3
confirming that ligand a is bound to the metal centre via the
phosphine groups. The rhodium atom lies in a distorted
square-planar geometry with typical rhodium–oxygen dis-
tances of Rh—O(1) 2.057(3) and Rh—O(2) 2.061(3) Å
found in other rhodium acetylacetonato complexes (18).
Likewise, bond distances and angles within the phosphine
ligands are similar to those reported previously, with CϵC
bond lengths of C(18)—C(19) 1.186(6) and C(36)—C(37)
1.180(6) Å (17). Crystallographic data are given in Table 1.
For complexes 1b and 1c, coordination of both phosphine
groups to rhodium is observed by 31P NMR spectroscopy.
As mentioned previously, phosphinorhodium acetyl-
acetonato complexes are active and selective catalysts for the
hydroboration of a wide range of alkenes. However, the cata-
lyst resting state in these systems is believed to be the zwit-
3 Supplementary data for this article are available on the journal Web site (http://canjchem.nrc.ca) or may be purchased from the Depository
of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 4088. For more
information on obtaining material refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 275010 and 275011 contain the crystal-
lographic data for this manuscript. These data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (Or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
© 2006 NRC Canada

Vogels et al.
149
Table 1. Crystallographic data collection parameters for 1a and 2a.
1a
2a
Formula
C41H45O2P2Rh
C48H46BO4P2Rh
fw
734.62
862.51
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Triclinic
P1
Triclinic
P1
12.151(3)
12.820(3)
13.875(3)
97.010(3)
96.656(4)
117.772(3)
1861.0(7)
2
11.749 9(9)
14.204 1(11)
14.348 2(11)
85.954(1)
70.850(1)
71.777(1)
2 147.3(3)
2
α (°)
β (°)
γ (°)
V (Å3)
Z
ρcalcd (mg m–3)
1.311
1.334
Crystal size (mm3)
Temperature (K)
Radiation
0.225×0.2×0.05
173(1)
Mo Kα (λ = 0.710 73 Å)
0.578
8835
5824
454
0.40×0.35×0.15
173(1)
Mo Kα (λ = 0.710 73 Å)
0.515
15 025
9 365
541
µ (mm–1)
Total reflections
Total unique reflections
No. of variables
Rint
0.034 1
0.015 7
θ Range (°)
1.83–24.99
2.529/–2.163
1.013
1.50–27.49
0.887/–0.239
1.067
Largest difference peak/hole (e Å–3)
S (GoF) on F2
a
R1 (I>2σ(I))
0.060 1
0.028 6
b
wR2 (all data)
0.156 4
0.077 9
aR1 = Σ||Fo| – |Fc||/Σ|Fo|.
bwR2 = (Σ[w(Fo – Fc2)2]/Σ[wFo ])1/2, where w = 1/[σ2(Fo ) + (0.1223P)2] (1a) and w = 1/[σ2(Fo ) + (0.0436P)2 +
2
4
2
2
(0.6575P)] (2a), where P = (max (Fo , 0) + 2Fc2)/3.
2
6
terionic complexes Rh(η -catBcat)(P2), arising from the
redistribution of substituents on HBcat. Indeed, hydro-
borations using these zwitterionic complexes give similar
selectivities to those observed for the acetylacetonato pre-
cursors (13). In an elegant study, Marder and co-workers
(13d) found that addition of B2cat3 to Rh(acac)(P2) led to the
potential surface for such distortions is reputably quite shal-
low. Initial attempts to prepare 1b and 1c via this route were
complicated by considerable degradation to give rhodium
metal and a number of unidentified rhodium phosphine com-
plexes. Interestingly, the reactivity of related rhodium β-
diketonate diphosphine complexes with chlorinated solvents
has been reported previously (21).
6
zwitterionic complexes Rh(η -catBcat)(P2) in high yields,
along with concomitant formation of acacBcat. The diboron
species B2cat3 is generated as an unwanted decomposition
product in nucleophilic reactions with HBcat or when HBcat
is heated to elevated temperatures (7f). We have found that
addition of B26cat3 to 1a gave the corresponding zwitterionic
complex Rh(η -catBcat)(Ph2PCϵC-t-Bu)2 (2a), which repre-
sents the first example of this type of compound to bear
monodentate phosphine ligands (13d) to be structurally char-
acterized by X-ray diffraction. The 31P{1H} NMR spectrum
shows a doublet with coupling to rhodium at JP-Rh = 213 Hz
and the 11B NMR has a sharp peak at 15 ppm, consistent
with the boron atom being four coordinate (19, 20). The mo-
lecular structure of 2a has been confirmed by a single crys-
tal X-ray diffraction study (Fig. 2) and shows that the P2Rh
fragment is bound to the catBcat– anion via one of the
Hydroboration studies
To compare complexes 1a–1c against other rhodium cata-
lysts, we have examined their ability to catalyse the hydro-
boration of a series of vinylarenes. Initial studies were done
on 4-vinylanisole in C6D6 (3, Table 2), and as with many
other rhodium catalysts, all new phosphinoalkyne complexes
gave selective formation of the branched isomer 3a. Interest-
ingly, hydroborations with isolated 2a also gave exclusive
formation of this product. In reactions of 4-fluorostyrene (4),
however, complexes 1a and 1c gave small amounts of dibor-
ation product 4c and hydrogenation product 4d. Diborated
product 4c presumably arises from hydroboration of the
transient vinyl boronate ester 4-F-C6H4CH=CHBcat, which
is generated from a competing dehydrogenative borylation
reaction (22). The vinyl boronate ester products are not ob-
6
catecholato groups in a η fashion. As with related systems
1
(19), there appears to be considerable slippage of the P2Rh
group with respect to the π-bound arene ring. Indeed, two
bond distances, Rh(1)—C(4) and Rh(1)—C(3), are consider-
ably shorter (2.254(2) and 2.275(2) Å) than the remaining
Rh—C bonds (ranging from 2.336(2) to 2.497(2) Å). The
served to any extent, via H NMR spectroscopy, in reactions
using these relatively unhindered alkenes. Dehydrogenative
borylations are generally believed to occur via initial coordi-
nation of the alkene to the metal centre followed by insertion
into the Rh—B, and not the Rh—H, bond, with subsequent
© 2006 NRC Canada

150
Can. J. Chem. Vol. 84, 2006
Fig. 2. The molecular structure of 2a showing probability ellip-
soids at 30% and hydrogen atoms omitted for clarity. Selected
bond distances (Å) and angles (°): Rh(1)—P(2) 2.2129(5),
Rh(1)—P(1) 2.2260(5), Rh(1)—C(4) 2.254(2), Rh(1)—C(3)
2.275(2), Rh(1)—C(2) 2.336(2), Rh(1)—C(1) 2.392(2), Rh(1)—
C(5) 2.429(2), Rh(1)—C(6) 2.497(2), B(1)—O(4) 1.461(3), B(1)—
O(3) 1.469(2), B(1)—O(2) 1.494(3), B(1)—O(1) 1.511(2), C(25)—
C(26) 1.193(3), C(43)—C(44) 1.185(3); P(2)-Rh(1)-P(1) 91.85(2),
O(4)-B(1)-O(3) 106.1(2), O(4)-B(1)-O(2) 112.7(2), O(3)-B(1)-O(2)
113.8(2), O(4)-B(1)-O(1) 111.5(2), O(3)-B(1)-O(1) 109.8(2),
O(2)-B(1)-O(1) 103.2(2).
Table 2. Rhodium catalysed hydroboration of vinylarenes 3–5
using 1.1HBcat.
O
O
B
O
B
O
R
R
HBcat
3-5b
3-5a
R
5 mol% 1a-1c
O
B
O
R
3: R = OMe
4: R = F
5: R = 2-naphthyl
B
R
O
O
3-5d
3-5c
Catalyst
a
b
c
d
4-Vinylanisole
1a
1b
1c
4-Fluorostyrene
1a
1b
1c
3a
3b
—
—
—
4c
—
—
—
5b
—
—
—
3c
—
—
—
4c
2
—
2
5c
—
—
—
3d
—
—
—
4d
3
—
—
5d
10
—
3
>98
>98
>98
4a
95
>98
98
5a
90
>98
97
β-H elimination to give the corresponding vinyl boronate es-
ter and an active rhodium dihydride species. This latter
dihydride is presumably responsible for the hydrogenation
of the starting alkene to give 4d. Similar selectivities were
observed in hydroborations of 2-vinylnaphthylene (5). Inter-
estingly, dirhodium complex 1b gave the best selectivities of
the three new rhodium complexes examined for these reac-
tions. Ratios of products were determined by multinuclear
NMR spectroscopy and confirmed by GC–MS (7e, 13a).
Hydroborations of electron-deficient 2,3,4,5,6-penta-
fluorostyrene (6) were also examined, and selectivities are
similar to that observed in reactions with 4-vinylanisole (Ta-
ble 3). These results suggest the electronic nature of the
arene group does not play a significant role in these
hydroborations. Increasing the steric bulk around the alkene
group by addition of fluorine groups in the 2 and 6 positions
also did not seem to affect selectivities.
2-Vinylnaphthalene
1a
1b
1c
1
Note: Ratios were determined by H NMR spectroscopy.
Table 3. Rhodium catalysed hydroboration of 6 using 1.1HBcat.
O
O
F
B
F
O
B
F
F
F
F
F
F
O
F
F
F
F
F
HBcat
6b
6a
F
5 mol% 1a-1c
We then decided to investigate reactions of 2,4,6-tri-
methylstyrene (7), where the methyl groups further increase
the steric crowding around the vinyl group. As expected,
catalysed hydroborations of this bulkier vinylarene gave a
mixture of branched (7a) and linear product (7b), along with
a considerable amount of diborated product (7c), when com-
plex 1a was used (Table 4). Minor amounts of vinyl
F
F
F
6
O
F
F
B
F
O
F
F
B
F
F
O
O
F
6d
1
boronate ester product are observed by H NMR spectros-
6c
copy (<1%). Rhodium complex 1c, containing the dialkyne
units, gave a significant amount of the linear product 7b. It
is plausible that reactions using 1b and 1c are complicated
by addition of HBcat to the sterically uncongested dialkyne
unit. Indeed, severe degradation of the starting rhodium
complex is often observed in these reactions.
Catalyst
6a
6b
6c
6d
1a
1b
1c
96
96
96
—
—
—
2
2
2
2
2
2
1
Note: Ratios were determined by H NMR spectroscopy.
While results with vinylarene 7 show that steric hindrance
caused by methyl groups ortho to the arene ring can alter
© 2006 NRC Canada

Vogels et al.
151
Table 5. Rhodium catalysed hydroboration of 9 using 1.1HBcat.
Table 4. Rhodium catalysed hydroboration of 7 using 1.1HBcat.
O
O
O
O
B
O
B
B
O
B
O
O
HBcat
HBcat
O
B
9a
9b
7b
7a
O
5 mol% 1a-1c
5 mol% 1a-1c
9
9e
7
O
O
B
O
B
O
B
B
O
O
O
O
7d
9d
7c
9c
Catalyst
7a
7b
7c
7d
Catalyst
9a
9b
9c
9d
9e
1a
1b
1c
40
40
5
40
55
85
15
—
10
5
5
—
5
5
10
5
10
5
1a
1b
1c
40
50
40
30
25
30
20
10
15
1
1
Note: Ratios were determined by H NMR spectroscopy.
Note: Ratios were determined by H NMR spectroscopy.
hydroboration selectivities, reactions with trans-β-methyl-
styrene (8), where a methyl group is substituted for a hydro-
gen in the β position of the vinyl group, gave selective
formation of C6H5CH(Bcat)CH2CH3 (8a) regardless of the
choice of catalyst used to affect this transformation. The
other isomer (C6H5CH2CH(Bcat)CH3, 8b) was not observed
to any significant extent. Reactions with α-methylstyrene
(9), however, where substitution occurs in the α position,
gave extremely complicated product distributions (Table 5).
The corresponding vinyl boronate ester (C6H5CMe=CH(Bcat),
9e) was also observed to a considerable extent in these reac-
tions, especially when complex 1b was used to catalyse this
reaction. Although linear product 9b could be generated
from a traditional catalysed hydroboration reaction, it is also
plausible that this product arises from the catalysed hydroge-
nation of vinyl boronate ester 9e.
Acknowledgements
Thanks are gratefully extended to the Natural Sciences
and Engineering Research Council of Canada (NSERC),
Canada Research Chairs Programme, Canadian Foundation
for Innovation-Atlantic Innovation Fund, and Mount Allison
University for financial support. We also thank Roger Smith
and Dan Durant for their expert technical assistance, Profes-
sor J.F. Corrigan (The University of Western Ontario, Lon-
don, Ontario) for the generous gift of phosphinoalkynes, and
anonymous reviewers for helpful comments.
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