Metal promoted asymmetry in the 1,2-diboroethylarene synthesis: Diboration versus dihydroboration

Article abstract of DOI:10.1016/j.tetasy.2005.02.016

Metal catalysed addition of diboranes to vinylarenes produces the desired 1,2-bis(boronate)ester and mono(boronate)esters as by-products. Their relative rate is a sensitive function between the nature of the catalytic system and the electronic effects of the substrate, that influences the mechanistic steps of the catalytic cycle. However, asymmetry is only induced as moderate enantiomeric excess values, providing an enantioface differentiation, between the bis- and mono(boronate)esters. Alternatively, the method based on the catalytic asymmetric dihydroboration/oxidation of alkynes as diphenylacetylene can provide 1,2-diphenyl-1,2-ethanediol (hydrobenzoin) with a selectivity of 68% mainly as the erythro isomer.

Full text of DOI:10.1016/j.tetasy.2005.02.016

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 1289–1294
Metal promoted asymmetry in the 1,2-diboroethylarene synthesis:
diboration versus dihydroboration
*
´ ´
Jesu´s Ramırez, Anna M. Segarra and Elena Fernandez
´ ´ ` ´
Dpt. Quımica Fısica i Inorganica, Universitat Rovira i Virgili, C/ Marcel.lı Domingo, 43007 Tarragona, Spain
Received 14 December 2004; accepted 18 February 2005
Abstract—Metal catalysed addition of diboranes to vinylarenes produces the desired 1,2-bis(boronate)ester and mono(boron-
ate)esters as by-products. Their relative rate is a sensitive function between the nature of the catalytic system and the electronic
effects of the substrate, that influences the mechanistic steps of the catalytic cycle. However, asymmetry is only induced as moderate
enantiomeric excess values, providing an enantioface differentiation, between the bis- and mono(boronate)esters. Alternatively, the
method based on the catalytic asymmetric dihydroboration/oxidation of alkynes as diphenylacetylene can provide 1,2-diphenyl-1,2-
ethanediol (hydrobenzoin) with a selectivity of 68% mainly as the erythro isomer.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
from both the uncatalysed4 and metal-mediated5 in situ
dihydroboration of unsaturated substrates. However,
alkenylboronic esters, previously isolated from the hyd-
roboration of terminal alkynes, can be subjected to a
second catalytic reaction to provide 1,2-diboryl deriva-
tives.6 Although they have been much less studied, enan-
tioselective routes towards the diborane adduct have
been established6,7 by applying chiral ligands on the cat-
alytic system. However, further improvements in cata-
lyst efficiency are required because yield and selectivity
are still low.
The addition of two boryl units onto adjacent carbons
is becoming increasingly important for the efficient
assembly of homo- and hetero-difunctional molecular
structures.1 The diboration (Scheme 1a) and dihydro-
boration (Scheme 1b) of alkenes and alkynes, respec-
tively, are attractive and straightforward strategies.
Whereas, non-catalysed diborane-addition reactions
are limited to the use of highly reactive tetrahalodibor-
anes,2 the mediation of transition metal catalysts allows
the use of less reactive3 and easier-to-handle tetraalk-
oxy- and tetraaryloxydiboranes. Unlike 1,2-diboro addi-
tion reactions, gem-diboro adducts are mainly produced
Herein we report our interest in determining the nature
of the side reactions and by-products produced during
RO
OR
B
B
RO
a)
b)
OR
Ar
OR
B
B
OR
Ar
Ar
Ar
OR
OR
B
OR
OR
OR
OR
OR
OR
H
B
H
B
Scheme 1.
*
Corresponding author. Tel.: +34 9 77558046; fax: +34 9 77559563; e-mail: elenaf@quimica.urv.es
0957-4166/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.02.016

´
J. Ramırez et al. / Tetrahedron: Asymmetry 16 (2005) 1289–1294
1290
the catalytic asymmetric diboration of vinylarenes and
extend our study to attempt the first example of an in
situ catalytic asymmetric dihydroboration of internal
alkynes.
reductive elimination involving the second boryl ligand
(Scheme 3, paths b–b0), the alkenyl and alkylboronate
esters could be produced as a result of a competitive
b-H-elimination (Scheme 3, paths c–c0, d–d0). Even the
addition of achiral monophosphine to block any vacant
coordination site around the rhodium, involving an
unfavourable b-H-elimination step, did not improve
selectivity (Table 1, entry 2).
2. Results and discussion
A certain level of enantioenrichment (ee = 33%), has
been described in the asymmetric diboration of styrene
1a with bis(catecholato)diboron 2 by means of the cata-
lytic system [Rh(nbd)acac]/(S)-Quinap.7 Moreover, the
reported isolated yield of purified material was moderate
(68%), and the remaining mass balance described as
unconverted starting material. When we reproduced this
reaction under identical conditions (Scheme 2), we
found that whereas the enantioselectivity was roughly
as reported, the conversion of the substrate was almost
complete and the formation of hydroborated products
became significant (Table 1, entry 1). These findings
are in agreement with the well-known studies by Baker
et al.,8 Marder and co-workers8,9 and Marder and Nor-
man9 in which, depending on the rhodium(I) catalytic
system used, a range of products from mono-, bis- and
tris-boronated derivatives, were observed. A plausible
explanation, from a mechanistic point of view, suggests
that the first step is likely to be an oxidative addition of
B–B in the diborane reagent10 to the metal, leading to
metal–diboryl complex (Scheme 3, path a). However,
the desired 1,2-bis(boronate)ester seems to arise from
alkene insertion into one M–B bond followed by B–C
At this point, we could infer two things from our initial
catalytic results with [Rh(nbd)acac]/(S)-Quinap. Firstly,
the enriched enantiomer in the 1,2-bis(boronate)ester
was R while the enriched enantiomer in the 1-phenyleth-
ylboronate ester was S. Secondly, the standard catalytic
asymmetric hydroboration of styrene provided much
higher ee values (88%)11 under similar reaction condi-
tions than as a side hydroboration reaction produced.
We should point out that the P,N-ligand (S)-Quinap
does not induce as much asymmetry in the 2-phenyleth-
ylboronate obtained as by-product in diboration as in
standard hydroboration, probably because a different
chiral metal (b-borylalkyl) species is involved in the cat-
alytic cycles.
To analyse how much the relative rates of B–C reductive
elimination versus b-hydride elimination, are sensitive
functions of the chiral ligand, new catalytic 1,2-dibor-
ations of styrene were performed with [Rh(nbd)acac]
modified with (R)-Binap and (S,S)-BDPP. Selectivity
on the 1,2-bis(boronate)ester was significantly reduced
with the ligand R-Binap, which chelates with rhodium
O
O
O
R
O
O
O
O
R
O
O
B
B
B
B
O
B
B
O
R
O
+
R
+
Rh catalyst
3
5
4
1a: R= C6H5
1b: R= p-MeO-C6H4
1c: R= p-F-C6H4
1d: R= C6H11
Scheme 2.
Table 1. Catalytic asymmetric 1,2-diboration reaction of vinylarenes with bis(catecholato)diborona
Entry
Substrate
Catalytic system
Conversionb (%)
% 3b (% ee)c
% 4b (% ee)c
% 5b
1
2
1a
1ad
1a
1a
1a
1a
1a
1b
1c
[Rh(nbd)acac]/(S)-Quinap
[Rh(nbd)acac]/(S)-Quinap
[Rh(nbd)acac]/(R)-Binap
[Rh(nbd)acac]/(S,S)-BDPP
[Rh(nbd)2]BF4/(S)-Quinap
[Rh(cod)2]BF4/(S)-Quinap
[Rh(l-Cl)(nbd)]2/(S)-Quinap
[Rh(nbd)acac]/(S)-Quinap
[Rh(nbd)acac]/(S)-Quinap
[Rh(nbd)acac]/(S)-Quinap
90
78
99
95
95
89
92
77
58
100
76 (35R)
70 (37R)
21 (21R)
17 (16R)
55 (35R)
66 (36R)
68 (35R)
82 (26R)
58 (20R)
78 (54R)
24 (34S)
27 (33S)
69 (3S)
68 (2S)
40 (47S)
34 (41S)
25 (40S)
11 (—)
—
3
3
10
14
5
4
5
6
—
7
7
8
7
9
10
36 (45S)
7 (ND)
6
15
1d
a Standard conditions: substrate/bis(catecholato)diboron/Rh complex/chiral ligand = 1/1.1/0.05/0.05; solvent: THF; T: 25 °C; t: 15 h.
b Conversion and selectivity calculated by 1H NMR.
c Enantiomeric excess determined by GC with chiral column, as derivated alcohols for 4 and 5, and derivated acetal for 3.
d Addition of 5 mol % of PPh3.

´
J. Ramırez et al. / Tetrahedron: Asymmetry 16 (2005) 1289–1294
1291
+
L
O
Rh
O
O
B
B
L
O
Ar
Ar
Solvent
H
Ar
H
+
L
S
S
d')
Rh
d)
L
L
Ar
Rh
+
L
L
O
O
O
O
O
B
Rh
+
B
B
a)
L
O
O
B
Ar
O
O
Ar
+
B
H
L
L
L
L
O
Rh
Rh
+
H
L
O
O
Rh
+
B
B
L
O
O
B
O
B
O
O
B
Ar
O
c')
O
B
Ar
c)
O
O
Ar
O
B
O
Ar
Ar
Ar
O
B
H
L
L
H
L
L
O
O
Rh
+
Rh
+
Rh
+
L
L
B
B
B
b)
b')
O
O
B
O
O
O
Ar
B
O
O
β-elimination
β-elimination
L
L
L
Ar
Rh
+
Rh
+
O
O
L
B
B
O
O
O
O
O
B
B
O
Ar
Scheme 3.
to form a seven-membered ring (Table 1, entry 3). We
observed a similar trend to that using the bidentate
ligand DPPB in the [Rh(L–L)acac]/B2cat3—catalysed
diboration of alkenes.12 However, not only does the size
of the bite angle seem to influence the selectivity of the
reaction, because the use of (S,S)-BDPP, which as
(S)-Quinap also forms six-membered ring with metal,
provided poor selectivities towards the 1,2-diborated
product (Table 1, entry 4). Asymmetry induced by both
P,P-bidentate ligands, diminishes slightly in the diborated
product and significantly in the 2-phenylethylboronate
ester. We should point out that the 1,2-diborated prod-
uct obtained from both chiral complexes modified with
(S)-Quinap and (R)-Binap, mainly provided the same
(R)-enantiomer. This contrasts substantially with the
trend observed in the hydroboration/oxidation of styr-
ene, where the Rh/(S)-Quinap catalytic system
provided the (S)-1-phenylethanol, while the Rh/(R)-Bi-
nap catalytic system favoured the (R)-enantiomer.11
to be somewhat sensitive to the nature of the precursor.
The electronic factors in the substrate usually alter as
much as the steric factors of the catalyst, so we studied
how different the aryl substituents would affect chemo-
and enantioselectivity.
The electron-rich and electron-deficient vinylarenes,
p-methoxystyrene and p-fluorostyrene, respectively, pro-
duced similar but low enantioselectivities (Table 1,
entries 8 and 9). The chemoselectivity towards the 1,2-
diborated product was the most satisfactory (82%),
when the electron-releasing aryl substituent was used
on the styrene substrate. Noting that during alkene
insertion into Rh-boryl complex, chemo- and enantiose-
lection were dependent on alkene electronics, we rea-
soned that aliphatic 1-alkenes might exhibit different
selectivity patterns from those of aromatic olefins. Table
1, entry 10 shows that vinylcyclohexane was mainly
converted into the desired 1,2-bis(boronate)ester with
moderate enantiomeric excess (54%). However the
percentage of terminal hydroborated products was twice
that of the branched hydroborated product.
We demonstrated the generality of the asymmetric di-
boration reaction by carrying out the 1,2-addition of
bis(catecholato)diboron on styrene, with cationic and
neutral precursor of rhodium catalyst modified with
(S)-Quinap (Table 1, entries 5–7). The results were sim-
ilar to those obtained with [Rh(nbd)acac]/(S)-Quinap,
although selectivity on the diborated product seemed
Attempts to inhibit the competitive b-hydride elimin-
ation process has been covered by different strategies
mainly related to the nature of the catalyst,8,12 and less
related to the nature of the diborating reagent. Miyaura

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J. Ramırez et al. / Tetrahedron: Asymmetry 16 (2005) 1289–1294
1292
and co-workers13 reported the addition of bis(pinacol-
ato)diboron to terminal alkenes using a catalytic
amount of Pt(dba)2 at 50 °C. Though cleaner 1,2-di-
boration addition was observed, the base-free Pt system
could not be modified with chiral ligands.
not modified with the (S)-Quinap (Table 2, entry 5).
As far as enantioselectivity is concerned, the chiral dibo-
rane reagent bis((+)-pinanediolato)diboron 10 showed a
similar behaviour on the 1,2-diboration of p-methoxy-
styrene and vinylcyclohexane (Table 2, entries 6–8).
These results are comparable to those of the platinum
catalysed diboration of terminal alkenes with other chi-
ral diboranes,14 although isolated yields of the 1,2-di-
borated product were higher with platinum than with
rhodium as the metal centre of the catalytic system.
Bearing these precedents in mind, we studied how tetra-
alkoxydiboranes other than bis(catecholato)diboron
(Fig. 1), influence the 1,2-diboration addition to styrene
using the catalytic system [Rh(nbd)acac]/(S)-Quinap.
Table 2 shows that chemoselectivity towards the desired
1,2-bis(boronate)ester was low and that enantioselectiv-
ity was null when bis(neopentylglycolato)diboron 6 and
bis(hexyleneglycolato)diboron 7, were used as diborat-
ing reagents (Table 2, entries 1 and 2). However, both
reagents favoured the formation of the branched hyd-
roborated by-product, but asymmetric induction was
also null. In an attempt to increase enantioselectivity
along this model reaction, we performed a double asym-
metric induction using the chiral catalytic system
[Rh(nbd)acac]/(S)-Quinap and chiral diborating
reagents.
As an alternative to the synthetic method, we became
interested in performing the dihydroboration/oxidation
of internal alkynes with bis(organo)boranes in the pres-
ence of the rhodium catalytic system modified with (S)-
Quinap, in order to explore the viability of the chiral
1,2-diol adduct formation. In particular, we focused
on the synthesis of the hydrobenzoin-type molecule,
because the enantiomerically pure hydrobenzoin has
proven to be a very useful chiral auxiliary15 and
ligand,16,17 for stereoselective organic synthesis. These
diols, which were previously accessible only through
kinetic resolution,18 can now be obtained by the dihydr-
oxylation of olefins,19 the reduction of benzyls20 or via
carbon–carbon bond formation.21 However, to the best
of our knowledge, nobody has yet studied the enantiose-
lective synthesis of hydrobenzoin through the catalytic
asymmetric dihydroboration/oxidation of the diphenyl-
acetylene with boranes such as hydroboration reagents.
As Table 2 shows, when bis(diethyl-D-tartrateglyco-
lato)diboron
lato)diboron
8
9
and bis(diisopropyl-D-tartrateglyco-
were involved in the reaction,
enantiomeric excesses were only 17% and 14%, respec-
tively (Table 2, entries 3 and 4). Note that in these cases
the enantioenriched mixture was on the (S)-enantiomer
rather than on the favoured (R)-enantiomer formed with
bis(catecholato)diboron. The change in the main enan-
tiomer must be due to the chiral diborane reagent, as
is seen by the reactivity when the catalytic system was
We therefore focused on the dihydroboration/oxidation
reaction of the internal alkyne diphenylacetylene. We
began by examining the catalytic properties of the rho-
O
O
O
O
O
O
O
O
O
O
O
O
B
B
B
B
B
B
2
6
7
COOiPr
COOiPr
CH3
PrOiOC
PrOiOC
COOEt
EtOOC
EtOOC
O
O
O
O
O
O
O
O
O
O
B
B
B
B
B
B
O
O
COOEt
CH3
8
10
9
Figure 1.
Table 2. Catalytic asymmetric 1,2-diboration reaction of vinylarenes with [Rh(nbd)acac]/(S)-Quinapa
Entry
Substrate
Diborane reagent
Conversionb (%)
% 3b (% ee)c
% 4b (% ee)c
% 5b
1
1a
1a
1a
1a
1a
1b
1b
1d
Bis(neopentylglycolato)diboron, 6
Bis(hexyleneglycolato)diboron, 7
95
36
39 (—)
22 (—)
61 (—)
78 (5S)
83 (—)
80 (—)
88 (—)
64 (—)
75 (ND)
3 (—)
—
—
—
—
—
—
—
65
2
3
Bis(diethyl-D-tartrateglycolato)diboron, 8
Bis(diisopropyl-D-tartrateglycolato)diboron, 9
Bis(diisopropyl-D-tartrateglycolato)diboron, 9
Bis((+)-pinanediolato)diboron, 10
60
17 (17S)
20 (14S)
12 (23S)
36 (14S)
25 (15S)
32 (50S)
4
100
50
5d
6
80
7d
Bis((+)-pinanediolato)diboron, 10
Bis((+)-pinanediolato)diboron, 10
100
100
8
ND: not determined.
a Standard conditions: substrate/bis(organo)diboron/Rh complex/chiral ligand = 1/1.1/0.05/0.05; solvent: THF; T: 25 °C; t: 15 h.
b Conversion and selectivity calculated by 1H NMR.
c Enantiomeric excess determined by GC with chiral column, as derivated alcohols for 4 and 5, and derivated acetal for 3.
d Catalytic system based on [Rh(nbd)acac].

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J. Ramırez et al. / Tetrahedron: Asymmetry 16 (2005) 1289–1294
1293
HO
O
OH
OH
Ph
Ph
1) [Rh(COD)(L,L)]BF4
2) EtOH, NaOH, H2O2
+
O
14
B
H
11
12
13
O
Scheme 4.
dium complex [Rh(cod)(dppp)]BF4 in THF as solvent,
with an equimolecular amount of catecholborane
(Scheme 4). Although we expected ketone 11 to be the
major product obtained, this was in fact the diol diphen-
yl-1,2-ethanediol 12 (hydrobenzoin) (Table 3, entry 1).
On the basis of these observations, we suggest that
diphenylacetylene was involved in competitive hydrobo-
ration and/or hydrogenation reactions, as illustrated in
Scheme 6. While the dihydroboration of diphenylacetyl-
ene provided the desired hydrobenzoin 12, hydrobor-
ation followed by hydrogenation of the intermediate
gave 1,2-diphenylethanol 13. Alkene 14 could also be
formed from the catalytic hydrogenation of the alkyne.
Therefore,
a double amount of borane reagent
(2.2 equiv) guaranteed a higher conversion. However,
not only did the conversion significantly improve, but
selectivity towards the diol 12 also increased to 60%
(Table 3, entry 2).
As far as the catalytic asymmetric dihydroboration/oxi-
dation of diphenylacetylene is concerned, the chiral
complex [Rh(cod)(S,S)-bdpp]BF4 with catecholborane
provided conversion and selectivity (Table 3, entry 3)
similar to those of [Rh(cod)dppb]BF4. However diphe-
nyl-1,2-ethanol 12 was characterised mainly as the ery-
thro compound, not the expected threo. Unfortunately,
even modifying the rhodium complex with other chiral
ligands, such as Quinap and Binap, did not change this
trend towards the formation of the erythro compound.
Other by-products (Scheme 4), were observed as a result
of competitive catalytic reactions, such as hydrogen-
ation. These secondary reactions may be related to the
degradation of catecholborane in the reaction media,
which depends on the nature of the phosphine ligands,
the rhodium complex and the solvent. It seems that cate-
cholborane breaks down to afford a variety of boron
products such as the diborane 15 plus metal hydro spe-
cies such as [Rh(H2)(L,L)][B(cat)2] (Scheme 5).22 The
production of the rhodium dihydro species may be
responsible for the formation of hydrogenated products.
The 11B NMR spectra determined during the dihydro-
boration reaction, showed two broad signals at
d(11B) = 33.0 ppm and d(11B) = 21.5 ppm, in agreement
with the alkylboronate products and compound 15,
respectively.
Also, the formation of diol 12 was not favoured with the
latter bidentate ligands (Table 3, entries 4 and 5). One
explanation for the formation of the syn-diol could be
that the alkenylboronate ester isomerises from the cis
to the trans isomer, because of the favoured b-H-elimi-
nation. The trans isomer could then be transformed
into the syn diboronate during the second catalytic
Table 3. Catalytic asymmetric dihydroboration of diphenylacetylene with [Rh(cod)L,L]BF4 and catecholboranea
Entry
L,L
Conversionb (%)
% 11b
% 12b (% erythro:threo)c
% 13b
% 14b
1d
2
dppp
dppp
37
98
98
78
63
17
3
50
60
25
33
26
15
31
8
3
3
(S,S)-bdpp
(S)-Quinap
(R)-Binap
4
68 (96:4)
17
31 (88:12)
2
4
68
15
—
23
5
a Standard conditions: substrate/catecholborane/Rh complex/chiral ligand = 1/2.2/0.01/0.01; solvent: THF; T: 25 °C; t: 2 h.
b Conversion and selectivity calculated by 1H NMR.
c Ratio erythro/threo determined by HPLC, in a chiral column Chiralcel OJH.
d Substrate/catecholborane/Rh complex/chiral ligand = 1/1.1/0.01/0.01.
O
COD
B
O
O
+
[Rh(H2)(L,L)]B(cat)2
+
BH3
+
B
H
[Rh(COD)(L,L)]BF4
O
O
B
O
15
Scheme 5.

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J. Ramırez et al. / Tetrahedron: Asymmetry 16 (2005) 1289–1294
1294
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Scheme 6.
hydroboration. In summary, we have shown that che-
moselectivity in catalytic diboration is sensitive to the
nature of the catalyst and to the electronics of the sub-
strate. Moderate enantioselectivity is achieved in 1,2-
bis(boronate) and 2-phenylethylboronate esters, but
with opposite enantioface. Two consecutive in situ hyd-
roboration/oxidation of alkynes, provides the syn-diol
derivative with quantitative conversion and selectivity.
17. Bruin, M. E.; Kundig, E. P. Chem. Commun. 1998, 2635;
¨
Acknowledgements
Suzuki, T.; Yamagiwa, N.; Matsuo, Y.; Sakamoto, S.;
Yamaguchi, K.; Shibasaki, M.; Noyori, R. Tetrahedron
Lett. 2001, 42, 4669.
The authors thank the ICCD for financial support and
the Generalitat de Catalunya for studentship (to J.R.)
and grant under project SGR-1999-00182 and SGR-
2001-00316 (to A.M.S.).
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