In quest of factors that control the enantioselective catalytic Markovnikov hydroboration/oxidation of vinylarenes

Article abstract of DOI:10.1002/chem.200400574

This study attempts to rationalise the unpredictable performance of transition metal catalysed asymmetric hydroboration of vinylarenes on varying the precursor of the catalyst from cationic to neutral species, [M(cod)(L-L)]BF₄, [M(μ-Cl)(cod)]₂/(L-L), the metal (M=Rh and Ir), and the hydroborating reagent (catecholborane, pinacolborane). The approaches are based on the agreement between experimental data provided by (R)-Binap and (R)-Quinap modified catalytic systems and computational data evidenced by DFT calculations and QM/MM strategies. Unprecedentedly high enantiomeric excesses in the hydroboration/oxidation of vinylarenes with both electron-withdrawing substituents ((R)-(+)-1-p-F-phenylethanol, ee up to 92%) and electron-releasing substituents ((R)-(+)-1-p-MeO-phenylethanol, ee up to 98%), can be attributed to a rhodium halide key intermediate.

Full text of DOI:10.1002/chem.200400574

FULL PAPER
In Quest of Factors That Control the Enantioselective Catalytic Markovnikov
Hydroboration/Oxidation of Vinylarenes
[a]
[a]
Anna M.Segarra, Elias Daura-Oller,[a] Carmen Claver,[a] Josep M.Poblet,
Carles Bo,*[b] and Elena Fernµndez*[a]
Abstract: This study attempts to ration-
alise the unpredictable performance of
transition metal catalysed asymmetric
hydroboration of vinylarenes on vary-
ing the precursor of the catalyst from
between experimental data provided
by (R)-Binap and (R)-Quinap modified
catalytic systems and computational
data evidenced by DFT calculations
and QM/MM strategies. Unprecedent-
edly high enantiomeric excesses in the
hydroboration/oxidation of vinylarenes
with both electron-withdrawing sub-
stituents ((R)-(+)-1-p-F-phenylethanol,
ee up to 92%) and electron-releasing
substituents ((R)-(+)-1-p-MeO-phenyl-
ethanol, ee up to 98%),can be attrib-
uted to a rhodium halide key inter-
mediate.
cationic
to
neutral
species,
[M(cod)(L–L)]BF4,[M(
m-Cl)(cod)]2/
Keywords: asymmetric catalysis
·
(L–L),the metal (M =Rh and Ir),and
the hydroborating reagent (catechol-
borane,pinacolborane). The ap-
proaches are based on the agreement
catecholborane · density functional
calculations
·
hydroboration
·
pinacolborane
Introduction
the temperature reaction also have
a
considerable
effect.[17,18] The fact that small electronic and steric varia-
tions can lead to unpredictable differences in the asymmet-
ric B-H addition to vinylarene substrates,in many cases
leads to an experimentally frustrating “trial and error” se-
quence. In this context and based on a previous postula-
tion,[19] we have recently used model systems from spectros-
copic postulated key intermediates in the catalytic cycle,[20]
to rationalise the role of the steric and electronic features of
the ligands and substrates. The difference in the stability of
the key intermediates,when comparing Rh complexes modi-
fied with Binap,Quinap and Pyphos ligands,agrees with its
experimental induced asymmetry. Moreover,some intermo-
lecular interactions,such as p–p stackings,seem to play an
important role in the enantiodifferentiation,when the elec-
tronic character is modified by substituted styrenes.
Transition metal catalysed asymmetric B–H addition across
the carbon carbon double bond has proved to be an effi-
À
cient means of preparing enantioselective organometallic B–
C products as useful synthetic intermediates.[1] In addition,
chiral organoboronate esters can be efficiently converted
into interesting functionalised products (HO-C,[2–8] R2N-
C,[9–11] RCO-C[12–13]) with total retention of the configura-
tion; the catalyst can be easily separated from the inter-
mediates before the work up protocol and reused. This
makes the asymmetric hydroboration reaction a recyclable
process.[14,15] A variety of chiral bidentate P–P,P–S,P–N and
P–Se ligands have been developed to induce asymmetry in
this reaction but small changes in the ligand structure result
in dramatic increases or decreases in ee.[16] However,the
nature of the chelating ligand is not the only parameter that
affects both the regio- and enantioselectivity,the alkene and
To obtain a complete picture of the sensitivity of the hy-
droboration reaction,we decided to investigate the effect of
other related aspects such as the nature of the hydroborat-
ing reagent and the metal and the counterion of the catalyst
precursor.
[a] A. M. Segarra,E. Daura-Oller,Prof. C. Claver,Prof. J. M. Poblet,
Dr. E. Fernµndez
Departament de Química Física i Inorgànica
Universitat Rovira i Virgili
Pl. Imperial Tàrraco 1,43005 Tarragona (Spain)
Fax : (+34)977-559-563
Results and Discussion
E-mail: elenaf@quimica.urv.es
The role of the hydroborating reagent: The boron atom of
the hydroborating reagent must be sufficiently Lewis acidic
to be catalytically activated for hydroboration,if we consid-
er the oxidative addition of the borane,as one of the first
[b] Dr. C. Bo
Institute of Chemical Research of Catalonia
Avgda. Països Catalans s/n,43007 Tarragona (Spain)
E-mail: cbo@iciq.es
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6456 – 6467
steps in the catalytic cycle. At this point,diorganyloxybor-
anes (RO)2BH have an extremely high degree of Lewis
acidity,particularly those which are cyclic compounds with
five-membered rings.[21] So far,catecholborane has proved
to be the most versatile diorganyloxyborane probably be-
cause B-H addition to the metal is straightforward and the
steric profile of C6H4O2B-coordinated to the metal is favora-
ble. However,it still presents several problems; for example,
its intrinsic instability and degradation give rise to complex
mixtures of highly active species responsible for a number
of side reactions during the catalytic hydroboration.[22] To
circumvent these difficulties,it has been recommended that
freshly redistilled catecholborane or alternative hydroborat-
ing reagents be used. Despite the relatively easy synthetic
protocol for preparing diorganyloxyboranes from the corre-
sponding diol or diketone and BH3,only a few have been
isolated. They have been qualitatively tested in the reactivi-
ty of transition metal-catalysed hydroboration of alkenes,al-
though to the best of our knowledge their influence on
enantioselectivity has not been reported yet.[22–24]
The present study expands the scope of the asymmetric
hydroboration/oxidation reaction of vinylarenes with pina-
colborane,because it is also a diorganyloxyborane with a
five membered ring,which is not as unstable or sensitive to
moisture as catecholborane. We wanted to determine the
extent to which the higher steric demand of pinacolborane
could affect both reactivity and selectivity. In general,we
found that the [Rh(cod)-(R)-Quinap]BF4 catalyst provided
slightly higher percentages of the enantioselective Markov-
nikov alcohol product when catecholborane is used as the
hydroborating reagent instead of pinacolborane,(Table 1,
entries 1 and 2). This trend is also observed for the hydrobo-
ration/oxidation of substituted styrenes,such as electron-rich
styrenes (Table 1,entries 3 and 4) and electron-poor styr-
enes,(Table 1,entries 5 and 6).
dium complex. Our preliminary experiments indicated that,
in the presence of pinacolborane,the percentage of the sec-
ondary alcohol formed was similar to that of the primary al-
cohol. Furthermore,the absolute configuration of the prod-
uct changed when catecholborane was changed for pinacol-
borane,which also indicate significant structural differences
in the enantioselective step,(Table 2,entries 1 and 2). On
the basis of these results,we explored the catalytic hydrobo-
ration/oxidation reaction of substituted styrenes,and we
also observed an attenuation of regioselectivity and a rever-
sal of enantioselectivity,(Table 2). Significantly,for the cor-
responding p-MeO-styrene changes were induced from ee
values around 60% on the (R)-(+)-sec-alcohol with cate-
cholborane to ee 38% on the (S)-(À)-sec-alcohol with pina-
colborane,(Table 2,entries 7 and 8). The variation in the
configuration is even greater at low temperatures,where a
more constrained metal complex is expected,(Table 2,en-
tries 9 and 10).
The catalytic cycle should be taken into account when the
hydroborating reagent on both reactivity and selectivity is to
be rationalised. Since the metal complex needs to accommo-
date both the hydroborating reagent and the alkene,it
seems evident that the more crowded the environment is,
the lower the reactivity and selectivity towards the secon-
dary alcohol will be. Thus,the comparative void provided by
the isoquinoline region in Quinap allows greater tolerance
in the accommodation of pinacolborane. The catalytic activi-
ty is then similar to that when catecholborane is used,al-
though primary alcohol is also formed in small quantities.
Taking into account that Rh-(R)-Quinap mainly provides
the Markovnikov alcohol product,(Scheme 1,path a),the
primary insertion observed in the presence of pinacolborane
could be due to a b-H elimination (Scheme 1,path b),fol-
lowed by
a primary reinsertion sequence,(Scheme 1,
path c). The pattern is different when catecholborane is in-
volved in the Rh-(R)-Quinap catalysed reaction because the
reductive elimination of the hindered secondary alkyl com-
plex may proceed at a rate
A remarkable different catalytic behavior was observed
when the chiral ancillary ligand (R)-Binap modified the rho-
Table 1. Asymmetric hydroboration/oxidation of vinylarenes towards (R)-(+)-sec-alcohol catalysed by
faster than that of b-H elimina-
tion,(Scheme 1,paths d,e).
[Rh(cod)-(R)-Quinap]BF4.[a]
Entry
1
Borane
Substrate
(R)-(+)-sec-
Alcohol
Yield
[%]
Branched
[%]
ee[b]
[%]
Similar observation is extended
to the use of catecholborane
with Rh-(R)-Binap,(Scheme 1,
paths j,k). When Rh-( R)-Binap
and pinacolborane react,the al-
kylrhodium intermediate pre-
99
95
88
2
3
4
99
98
93
75.5
97
73
89
86
sumably undergoes
a
more
facile b-H elimination
(Scheme 1,path h),because of
the highly sterically congested
secondary alkyl complex,after
a primary reinsertion sequence,
(Scheme 1,path i). However,
we can not rule out the possibil-
ity of an initial primary inser-
tion of the alkene because of
the steric hindrance of the Rh–
borane intermediate,followed
by reductive elimination which
91.5
5
6
97
93
96
93
80
83
[a] Standard conditions: substrate/borane/Rh complex 1:1.1:0.01. Solvent: THF, T = 258C, t = 1 h. [b] (R)-
Configuration determined by GC with chiral column FS-Cyclodex B-IP,50 m”0.25 mm.
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C. Bo,E. Fernµndez et al.
FULL PAPER
Table 2. Asymmetric hydroboration/oxidation of vinylarenes towards sec-alcohol catalysed by [Rh(cod)-(R)-
of the styrene and catecholbor-
ane addition to the precursor of
Binap]BF4.[a]
Entry
Borane
Substrate
sec-Alcohol
Yield
[%]
Branched
[%]
ee[b]
catalyst
[Rh(cod)(L–L)]BF4,
[%]
where L–L = (R)-Binap and
(R)-Quinap,showed evidence
of the structure of the inter-
mediates that may be involved
in the asymmetric hydrobora-
tion catalytic cycle. From this
evidence and using DFT calcu-
lations and QM/MM strategies,
we studied the origin of regio-
and stereoselectivity in the rho-
dium-catalysed hydroboration
reaction of vinylarenes and the
role of the steric and electronic
features of ligands and sub-
strates. The agreement between
the results presented in that
work and the experimental
trends was excellent. The key
intermediate was considered to
be the pentacoordinated H-Rh-
(P-N)-catecholborane-styrene
complex where styrene is coor-
dinated trans to the naphthyl-
pyridine moiety and the hy-
dride is in axial position
(Figure 1).
1
2
3
92
97
87
99
50
99
57(R)
18(S)
58(R)
4
5
6
54
91
46
41
99
46
16(S)
57(R)
16(S)
7
8
89
46
99
58
60(R)
38(S)
We defined two possible iso-
mers according to the position
of the hydride: Isomer A when
the apex of the square-based
pyramid was on the top in
Figure 1 and isomer B when it
was on the bottom. Each
isomer had four possibilities ac-
cording to the four coordina-
tion modes of styrene. In
9[c]
77
25
99
99
70(R)
77(R)
10[d]
11
12
93
71
99
45
65(R)
4(S)
Figure 1,the
modes of
coordination
monosubstituted
a
[a] Standard conditions: substrate/borane/Rh complex 1:1.1:0.01. Solvent: THF, T = 258C. t = 1 h. [b] Config-
uration determined by GC with chiral column FS-Cyclodex B-IP,50 m”0.25 mm. [c]
À788C.
alkene are labeled from A1 to
A4,and B1 to B4. Notice that
four of them lead to linear
products (A1 and A2 or B3 and
B4),two to a branched pro-S
T = 08C. [d] T =
provides the primary product in competition with the fa-
vored secondary boronate ester. The moderate reactivity of
Rh-(R)-Binap/pinacolborane with substituted styrenes,may
support this postulation.
As far as the enantioselectivity is concerned,the induced
asymmetry in the case of vinylarenes has been previously
explained by an intermolecular p–p stacking interaction be-
tween the ligand and the substrate. The reversal of enantio-
selectivity between Rh-(R)-Quinap/pinacolborane and Rh-
(R)-Binap/pinacolborane,may be due to the configuration
of the Rh–H fragment when it is transferred to the coordi-
nated alkene. However,a deeper analysis of the data is re-
quired. In a previous work,[20] an NMR spectroscopic study
product (A3 or B2) and two to a branched pro-R product
(A4 or B1).
Using the same methodology as in our previous study,[20]
we considered all eight possible isomers (A1–A4,B1–B4)
for the Quinap/styrene/catecholborane and Quinap/styrene/
pinacolborane key intermediates and for the Binap ana-
logues. Table 3 shows the relative stability of the most stable
isomeric forms (B1 and B2) for the H-Rh-(Quinap)-borane-
styrene catalytic system and (B1 and A3) for the H-Rh-
(Binap)-borane-styrene system. In the case of Quinap,the
difference between pro-R B1 and pro-S B2 isomers de-
creased,(from 4.1 to 1.9),when we compared catecholbor-
ane and pinacolborane as hydroborating reagents,respec-
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Chem. Eur. J. 2004, 10,6456 – 6467

Enantioselective Markovnikov Hydroboration
6456 – 6467
Scheme 1. Plausible catalytic pathways: oxidative addition of the hydroborating reagent to the rhodium complex followed by migratory insertion of the
À
À
alkene into M H bond (migratory insertion of the alkene into the M B bond could also be considered) and reductive elimination.
tively. This result may explain why the ee values decreased
with this hydroborating agent exchange. In our previous
work,[20] we explained the p–p stacking interactions in B1
and B2 H-Rh-(Quinap)-catecholborane-styrene complexes.
We showed that the B1 isomer (see Figure 2) was stabilized
by p2 interaction and that the B2 isomer was destabilised by
p3 an interaction between the substrate and catecholborane,
(Figure 2). In the B2 isomer of the H-Rh-(Quinap)-pinacol-
borane-styrene complex system,we observed a new interac-
Figure 3 shows the structure obtained for the A3 inter-
mediate in the case of Binap. The results obtained for the
two most stable isomers (B1 and A3) are given in Table 3.
In the case of H-Rh-(Binap)-catecholborane-styrene,the
most stable isomer was the pro-R intermediate B1 whereas
for the H-Rh-(Binap)-pinacolborane-styrene analogue,the
most stable one was the pro-S intermediate A3. The accura-
cy of the computational method to reproduce such small
energy differences is obviously questionable,but in any case
it is reassuring to be able to reproduce the reversal of enan-
tioselectivity experimentally observed when pinacolborane
was used. A detailed analysis of all the interactions of each
isomer is far beyond the scope of the present study,but we
should point out that slight changes in the catalyst structure
modify the interactions which,in turn,determine stereodif-
ferentiation (Figure 4).
2
tion,known as H(Me)-R ,(Table 3),between the substrate
and one methyl group of the pinacolborane. This kind of
stabilising interaction was calculated by Tanabe and co-
workers.[25] and in the case of the benzene–ethylene dimer,
the equilibrium distance was 3.6 Š. When we changed cate-
cholborane for pinacolborane,B2 was stabilised by this new
interaction,H(Me)-R 2. In this respect,the hydroborating
agent was responsible for stereoinduction. This is in com-
plete agreement with the experimental decrease in ee and
the regioselectivity results.
The role of the metal: As far as the catalytic system is con-
cerned,the hydroboration reaction is by no means limited
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C. Bo,E. Fernµndez et al.
FULL PAPER
Figure 1.
Table 3. Relative stability and some geometric parameters for the most stable isomeric forms of H-M ligand-borane substrate.[a]
H-Rh-Quinap-
catecholborane-
styrene
H-Ir-Quinap-
catecholborane-
styrene
H-Rh-Quinap-
pinacolborane-
styrene
H-Rh-Binap-
catecholborane-
styrene
H-Rh-Binap-
pinacolborane-
styrene
B1
B2
B1
B2
B1
B2
B1
A3
A3
B1
product
relative energy
distance
R1–R2
(R)
0.0
–
(S)
4.1
3.992
(R)
0.0
–
(S)
2.5
4.647
(R)
0.0
–
(S)
1.9
–
(R)
0.0
(S)
0.3
(S)
0.0
(R)
0.2
distance
3.555
–
3.560
–
3.410
–
–
R2–R3
distance
H(Me)–R2
angle
3.125
–
36.7
–
–
33.3
–
R1–R2
angle
24.3
24.3
R2–R3
distance
–
4.604
–
4.484
–
–
R1–R4
distance
R1–R5
5.171
5.127
[a] Energies in kcalmolÀ1,distances in Š,angles in degrees.
to rhodium complexes.[26] However,the successful enantiose-
lective transformation of vinylarenes and,to a lesser extent
norbornene,has long been restricted to the use of cationic
and neutral complexes of this particular transition metal. A
recent example of the hydroboration of meso substrates in
which the enantioselectivity between Rh and Ir systems was
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Enantioselective Markovnikov Hydroboration
6456 – 6467
Figure 3. Molecular structure of the H-Rh-Binap-pinacolborane-styrene
complex (A3). Hydrogen atoms omitted for clarity.
Table 4. Asymmetric hydroboration/oxidation of styrene towards (R)-
(+)-1-phenylethanol catalysed by [Ir(cod)-(R)-Binap]BF4.[a]
Catalytic system
Run
Yield
[%]
Branched
[%]
ee[c]
[%]
[Ir(cod)-(R)-Binap]BF4
[Ir(cod)-(R)-Binap]BF4/
montmorillonite K-10[b]
1
1
2
99
70
93
30
17
31
2
5
2
[a] Standard conditions: styrene/catecholborane/Rh complex 1:1.1:0.01.
Solvent: THF, T = 258C, t = 1 h. [b] Montmorillonite K-10 preheated at
1008C for 24 h,(0.092 mmol [Ir(cod)-( R)-Binap]BF4 per g of solid).
[c] (R)-Configuration determined by GC with chiral column FS-Cyclodex
B-IP,50 m”0.25 mm.
Figure 2. p–p Stacking interactions in the Quinap B1 and B2 isomers.
Fragments involved are highlighted. The p1 occurs between a (R5) and
the pyridine ring (R4). The p2 is the interaction between the phenyl sty-
rene (R2) and the phenyl–P ligand (R3),and the p3 corresponds to an in-
teraction between catechoborane (R1) and phenyl styrene (R2).
completely reversed,[27] led us
to wonder whether the Ir-Binap
system could also be a suitable
catalyst for the hydroboration
of vinylarenes. Thus,when the
complex
[Ir(cod)(R)-Binap]-
BF4 was used under the same
hydroboration reaction condi-
tions as the rhodium system,
conversion was complete but
enantioselectivity was almost
zero and regioselectivity only
about 30% on 1-phenylethanol
(Table 4. In our ongoing re-
search,we have found that the
low enantioselectivity provided
by ionic iridium complexes can
be improved by immobilising
the precursor of the catalyst
onto clays. This is the case of
[Ir(cod)-(S,S)-bdpp]PF6 which
efficiently catalyses the hydro-
genation of imines such as
N-(a-methyl-p-methoxybenzyl-
Figure 4.
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C. Bo,E. Fernµndez et al.
FULL PAPER
idene)benzylamine with complete conversion but zero enan-
tiomeric excess,in an homogeneous process. However,when
[Ir(cod)-(S,S)-bdpp]PF6 was immobilised onto montmoril-
lonite K-10,the catalytic system became more enantioselec-
tive on re-use,up to ee values around 60% in a third consec-
utive run.[28] To find a similar effect,we immobilised
[Ir(cod)-(R)-Binap]BF4 on montmorillonite K-10,and found
that after two consecutive runs the activities and selectivities
were similar to those provided by the analogous homogene-
ous systems (Table 4).
In an attempt to rationalise the low selectivity exhibited
by the iridium complexes,we decided to extend our theoret-
ical study to the homogenous catalytic systems [Ir(cod)-(R)-
Quinap]BF4. Once again,the question of whether the inser-
tion step or the coordination step in the catalytic cycle are
the enantiodifferentiation key steps,is the starting point of
discussion. Recently experimental[27] and theoretical stud-
ies[29] have proposed two main catalytic cycles that show
that the enantiodifferentiation step in the iridium catalysed
hydroboration involves the Ir–B migratory insertion step,
while the rhodium catalysed hydroboration involves the Rh–
H migratory insertion. However,we have calculated a con-
siderable difference in the stability of the isomers when the
substrate coordinates to the iridum(i) or rhodium(i) com-
plexes. Table 3 shows how the difference between pro-R B1
and pro-S B2 isomers decreased when the metal center is
iridium and not rhodium. These results explain the low
enantiomeric excesses that were observed experimentally.
The most significant difference between rhodium and iridi-
um structures was in the B2 isomer,where the p3 p–p stack-
ing distance interaction increased from rhodium (3.992 Š)
to iridium (4.647 Š). In our previous study,[20] we made an
energetic analysis of the p2 and p3 interactions. For the p3
interaction,the equilibrium distance between ring centroids
in a model system was found at 4.6 Š. We demonstrated
that,at this distance,the p3 interaction was less repulsive
than at the shorter distances found in the rhodium complex.
For this reason,in the case of
Figure 5.
the nature of the rhodium(i) source with a coordinated and
non-coordinated counterion. Thus we carried out a compa-
rative study of the asymmetric hydroboration of vinylarenes
with [Rh(cod)-(R)-Quinap]BF4 and [Rh(m-Cl)(cod)]2/2 equiv
(R)-Quinap. As in the case of cationic catalysts,we detected
a preferentially secondary insertion of styrene into the neu-
tral Rh complex formed from [Rh(m-Cl)(cod)]2/2 equiv (R)-
Quinap. However,the neutral catalytic system seems to
have an additional influence and favours the enantioselectiv-
ity to values up to 94% for the (R)-(+)-1-phenylethanol,
(Table 5,entries 1 and 2). The scope of the neutral catalytic
iridium,the B2 isomer is more
stable than the rhodium ana-
Table 5. Asymmetric hydroboration/oxidation of vinylarenes with catecholborane,towards ( R)-sec-alcohol,
catalysed by cationic and neutral rhodium complexes.[a]
logue,and the difference in the
relative energies of B1 and B2
is lower,(Figure 5). This result
is again in good agreement with
the observed trends,and shows
Entry
Precursor of catalytic
system
Substrate
sec-Alcohol
Yield
[%]
Branched
[%]
ee[b]
[%]
1
2
[Rh(cod)-(R)-Quinap]BF4
[Rh(m-Cl)(cod)]2/(R)Quinap
92
99
95
98
88
94
that
a change in the metal
center volume can induce
changes in the intermolecular
interactions,and therefore,
changes in ee values.
3
4
[Rh(cod)-(R)-Quinap]BF4
[Rh(m-Cl)(cod)]2/(R)-Quinap
98
92
96
99
94
96
5
6
[Rh(cod)-(R)-Quinap]BF4
[Rh(m-Cl)(cod)]2/(R)-Quinap
97
98
96
98
80
91.5
The role of the halide as coun-
terion: Since both the regio-
and the enantioselectivity of
the hydroboration of vinylar-
enes depend heavily on the
structural features of the cata-
lysts applied,we found interest-
7
8
[Rh(cod)-(R)-Quinap]BF4
[Rh(m-Cl)(cod)]2/(R)-Quinap
77
72
95
90
38
66
[a] Standard conditions: substrate/borane 1:1.1; cationic complex: 1 mol% [Rh(cod)-(R)-Quinap]BF4; neutral
complex: 0.5 mol% [Rh(m-Cl)(cod)]2/2 equiv (R)-Quinap. Solvent: THF, T = 258C, t = 1 h. [b] Configuration
ing to focus now our study on determined by GC with chiral column FS-Cyclodex B-IP,50 m”0.25 mm.
6462
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Chem. Eur. J. 2004, 10,6456 – 6467

Enantioselective Markovnikov Hydroboration
6456 – 6467
system was established for a variety of vinylarenes. In all
cases,for styrenes with electron-withdrawing and electron-
releasing substituents,the neutral catalytic system increased
enantioselectivity to values between 91.5% (p-F-styrene)
and 96% (p-MeO-styrene) (see Table 5,entries 3–6). A
clear trend is observed in the hydroboration of the electron
deficient vinylarene p-CF3-styrene,where enantioselectivity
increases from 38% with the cationic system to 66% with
the neutral system. Related literature on the hydroboration
of alkenylboronic esters,reports that enantioselectivity was
secondary alcohol has been obtained with the highest enan-
tiomeric excess reported so far as ee 98%.
In order to acquire a complete picture of the role of the
halide in the asymmetric induction of the hydroboration re-
action of vinylarenes,we first attempted to determine which
neutral metal species are involved in the first steps of the
catalytic cycle. Despite the ambiguities that remain unre-
solved for the hydroboration catalytic cycle,there is general
agreement that the oxidative addition of the hydroborating
reagent to the Rh center could be one of the first steps. The
first B-H activation from neutral rhodium complexes was
observed by Kono and Ito,who isolated the h1-boryl-rhodi-
somewhat higher for neutral than for cationic rhodium cata-
[30]
lysts,whereas the latter provided better yields.
no further explanation is given for these facts.
However,
um-hydride adduct [(PPh3)2RhClH(Bcat)],[31] (where Bcat
catecholborane),from Wilkinsonꢁs catalysts,(Figure 6,A).
=
The neutralising influence of chlorine as a coordinated
counterion was confirmed in a new experiment where differ-
ent amounts of the salt BnMe3NCl were added to the cata-
lytic system [Rh(cod)-(R)-Quinap]BF4,and the products
were distributed in a very similar way to when the neutral
system was used,(Table 6,entries 1–3). An excess of chlor-
The relative location of the hydride and boryl in these neu-
tral complexes has been confirmed by the complete structur-
al information[32] provided by the analogue complexes
[(PiPr3)2RhClH(Bcat)],[33] (where Bcat=catecholborane) and
[(PiPr3)2RhClH(Bpin)],[34]
(where
Bpin =pinacolborane),
(Figure 6,B). However,to the
best of our knowledge,little at-
tention has been paid to the
characterisation and structural
determination of h1-boryl-rho-
Table 6. Influence of the halide in asymmetric hydroboration/oxidation of vinylarenes with catecholborane,to-
wards (R)-sec-alcohol,catalysed by [Rh(cod)-( R)-Quinap]BF4.[a]
Entry
Additive
Substrate
sec-alcohol
Yield
[%]
Branched
[%]
ee[b]
[%]
dium-hydride
intermediates
modified with chelating diphos-
phines. We have recently pro-
vided,for first time,spectro-
1
BnMe3NCl (0.012 mmol)
96
95
91
2
3
BnMe3NCl (0.03 mmol)
BnMe3NCl (0.05 mmol)
98
94
96
96
91.5
93
scopic evidence of the key in-
termediate involved in the oxi-
dative addition of catecholbor-
ane to the cationic complex
[Rh(cod)(L–L)]BF4 (where L–
L=Binap and Quinap).[20] On
the basis of this evidence and
aimed to extend the factors that
govern borane additions to RhI
centers,we studied the stoichio-
metric oxidative addition of cat-
echolborane to the neutral cata-
lytic system formed from
4
5
6
BnMe3NCl (0.03 mmol)
PhMe3NBr (0.03 mmol)
PhMe3NI (0.03 mmol)
95
85
95
95
97.5
95
91.5
92
91.5
7
8
BnMe3NCl (0.03 mmol)
BnMe3NCl (0.03 mmol)
88
97
99
74
98
89.5
[a] Standard conditions: substrate/borane/Rh complex 1:1.1:0.01. Solvent: THF, T = 258C, t = 1 h. [b] (R)-
Configuration determined by GC with chiral column FS-Cyclodex B-IP,50 m”0.25 mm.
ine did not appear to be necessary. To obtain more informa-
tion about the role of the halide in the asymmetric hydrobo-
ration reaction,we used different additives containing the
À
halide ClÀ,Br and IÀ in the hydroboration of p-F-styrene,
(Table 6,entries 4–6). In the three cases,the enantioselectiv-
ity increased by the same amount,despite the different elec-
tronic and steric factors of the halide. The benefits of having
Figure 6.
the chloride in the reaction media,in terms of enantioselec-
tivity,can be clearly seen in the hydroboration of p-CF3-sty-
rene and p-MeO-styrene,(Table 6,entries 7 and 8). When
[Rh(m-Cl)(cod)]2/2 equiv L–L,,(where L–L
= Binap and
the substrate was p-CF3-styrene,the ee value achieved was
74%,which is only comparable with the asymmetric induc-
tion provided by the cationic rhodium complex modified
with the closely heterotopic (P,N)-ligand (S)-(+)-1-(2-di(2-
furyl)phosphino-1-naphthyl)isoquinoline.[17] As far as the
substrate p-MeO-styrene is concerned,their corresponding
Quinap). For the five coordinate,16-electron hydride- h1-
boryl-rhodium complex formed,we can expect at least
either a square-based pyramid (SP) or a trigonal bipyramid
(TBP) geometry. However,the resonances of the hydride
ligand were shifted upfield and centered at d = À15.82 ppm
as a double triplet (JH,Rh =31.2, JH,P =16.2 Hz) (Figure 7).
Chem. Eur. J. 2004, 10,6456 – 6467
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6463

C. Bo,E. Fernµndez et al.
FULL PAPER
Figure 7. a) Hydride signal in the 1H NMR of complex [Rh(m-Cl)(cod)]2
after the addition of Binap and catecholborane; b) 31P NMR spectrum of
the complex [Rh(m-Cl)(cod)]2 after the addition of Binap and catechol-
borane.
Figure 8. Skeleton of all the hexacoordinated H-Rh-Cl-Quinap-borane-p-
Cl-styrene isomeric forms considered.
This may indicate that the hydride position is cis to two
equivalent phosphorous nuclei. The 31P NMR spectrum
agrees with the postulated equivalence of the phosphorous
nuclei due to the single doublet centered at 44.92 ppm
(JP,Rh =197.8 Hz). This oxidative addition was confirmed by
11B NMR spectroscopy,since the initial doublet due to the B
bonded to H in free catecholborane (d=28.8 ppm, JB,H
=
197.2 Hz) was shifted to a broad signal centered at d=
23.20 ppm. The equivalence of the phosphorous in the che-
lating diphosphine could be indicative of a TBP geometry in
which the bidentate ligand is located in the equatorial plane
and the hydride ligand occupies the apical position. Since
the hydride resonances are chemically shifted upfield and
the oxidative addition of catecholborane is expected to be
Figure 9. The most stable isomeric form for the H-Rh-Cl-Quinap-borane-
p-Cl-styrene hexacoordinated complex.
III
cis,we suggest a complete arrangement around the Rh
À
center,where the halide Cl is trans to the hydride and the
boryl is in the equatorial plane (Figure 6,C). Consistent
spectroscopic values are also reported in a related work that
describes the oxidative addition of catecholborane to [IrCl-
(CO)(L–L)] complexes.[35] Taking into account that the fol-
lowing step in the catalytic cycle is the coordination of the
alkene to the hydride-h1-boryl-rhodium intermediate,we
can envisage several different isomers with octahedral ge-
ometry,which we studied theoretically.
In this section we could not consider the same approxima-
tions as in the previous cases because when ClÀ coordinates,
the new system presents a hexacoordinated structure. We
studied more than twenty different isomeric forms in order
to find the most stable structure and to explore much of the
configuration space. Figure 8 schematically shows the skele-
ton of all the intermediates considered here. We first de-
fined isomers in which styrene was trans to the naphthyl-pyr-
idine and the hydride trans to the ClÀ. Considering the four
styrene coordination possibilities and position exchange for
hydride and chloride,we took into account eight type I iso-
mers. The second type II also had the styrene trans to the
naphthyl-pyridine but the ClÀ cis to the hydride. The third
type III had the styrene trans to the ClÀ and the hydride
trans to the phenyl phosphine moiety and the last type IV
had the styrene trans to the ClÀ and the hydride trans to the
naphthyl-pyridine. The most stable form was isomer B1/
type III (see Table 7 and Figure 9),the intermediate that
leads to the (R)-product. The second and third most stable
intermediates are those that produce a linear product and
the (S)-product,respectively. Note that,in general,structur-
al types I and II are less stable than type III and IV,al-
though isomer I/A1 is the second most stable. By splitting
the relative energy in its former contribution,QM and MM,
the data in Table 7 indicates that the main difference arises
from the QM energy,that is,from the part of the molecule
that takes into account the differences in the rhodium coor-
dination sphere. In isomer I/A1,a stabilizing MM contribu-
tion was computed. A detailed analysis of this geometry sug-
gested that two p-stacking interactions may be present.
However,given the small amounts of energy involved and
the limitations of the methods used,we think that it is not
worthwhile making a detailed analysis of all the interactions
in all the isomers. Even so,it should be pointed out that the
results obtained using the present computational approach
justify,at least qualitatively,the experimentally observed re-
action outcome.
Conclusion
Experimentally we found that the efficiency of the hydrobo-
rating reagent depends heavily on the steric factors of the
6464
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Chem. Eur. J. 2004, 10,6456 – 6467

Enantioselective Markovnikov Hydroboration
6456 – 6467
Table 7. Relative stability and some geometric parameters for the most stable isomeric forms of the H-Rh-Cl-Binap-catecholborane-p-Cl-styrene system.
Isomer
type
Isomer
conformation
Hydroborated
product
Relative
energy
Contribution
Distance
R2–R3
Distance
R2–R3’
Angle
R2–R3
Angle
R2–R3’
QM
MM
III
I
III
III
IV
I
IV
IV
III
IV
I
I
I
I
II
I
B1
A1
B2
B4
A2
A3
A1
A4
B3
A3
B1
B3
A4
A2
B3
B4
B2
A3
B2
B4
B1
(R)
linear
(S)
linear
linear
(S)
linear
(R)
linear
(S)
(R)
linear
(R)
linear
linear
linear
(S)
0.0
2.0
2.3
2.5
4.1
4.4
4.4
6.4
6.6
6.7
7.7
7.8
8.3
9.2
9.7
0.0
3.3
2.1
1.2
4.7
5.4
2.9
3.9
3.9
5.1
8.5
12.4
6.4
7.0
18.1
11.7
10.9
17.5
20.6
22.8
23.5
0.0
À1.3
0.2
4.490
4.735
4.854
44.9
28.6
33.23
4.88
43.4
1.3
À0.6
À1.0
1.6
2.6
2.7
1.6
À0.8
À4.6
1.9
2.2
À8.4
12.1
12.5
17.3
17.9
19.8
20.2
0.4
I
1.6
II
II
II
II
(S)
(S)
linear
(R)
À0.2
À2.7
À3.0
À3.3
[a] Energies in kcalmolÀ1,distances in Š,angles in degrees. R 2: phenyl styrene,R 3: phenyl–P ligand,R 3’: phenyl–P ligand.
catalytic systems. So pinacolborane,which is more stable
and less sensitive to moisture,can be added to vinylarenes
and the selectivities are similar to those for catecholborane,
when the catalytic system is based on [Rh(cod)-(R)-Qui-
nap]BF4. Contrary to this trend,a significative reversal in
enantioselectivity can be achieved by using pinacolborane
or catecholborane with [Rh(cod)-(R)-Binap]BF4.
The consistently high ee values observed in the secondary
alcohols obtained from the hydroboration/oxidation of vi-
nylarenes with cationic rhodium complexes modified with
(R)-Quinap,drop sharply to an almost zero asymmetric in-
duction with the related iridium complexes. A significant
steric influence must be involved in such different catalytic
behavior provided by two catalytic systems that are based
on two transition metals with the same d shell electronic
configuration.
mers stability. For cationic complexes,we previously demon-
strated that the present computational approach was able to
discriminate ligands and substrates.[20] The results presented
here allow extending those conclusions to different hydrobo-
rating reagents and to different metals. For neutral com-
plexes,formed by chloride coordination to the catalyst,the
scene becomes more complicated since the number of possi-
ble conformational isomers is quite high. But also in this
case,the most stable structure computed is related to the
main product obtained by the experimental procedure.
We expect that those “trial-and-error” approaches could
be minimised by using a combination of experiment and
theory,as we tried to apply to the hydroboration reaction.
Nevertheless,computational modelling of homogeneous cat-
alysis can nowadays complement experimental research in
order to increase understanding,but also to provide guides
towards rational catalysts design.
In addition to the efficiently catalysed hydroboration of
vinylarenes by rhodium neutral systems,the enantioselectiv-
ity can be significantly enhanced. The coordinated chloride
has a positive neutralising effect whether it comes from the
catalyst precursor or from an additive such as R3NCl. Bene-
fits are similar when the nature of the halide is varied from
Experimental Section
General: All reactions and manipulations were conducted with standard
vacuum line techniques under an atmosphere of dry nitrogen. All rhodi-
um and iridium organometallic complexes were synthesised by using
standard Schlenk techniques. All organic solvents were purified by stan-
dard methods,stored on a molecular sieve (4 Š Aldrich),and degassed
with nitrogen flow before use. The complexes [M(m-Cl)(cod)]2,[36]
[M(cod)2]BF4,[37,38] [M(cod)-(R)-Binap]BF4,[39] (where M=Rh,Ir) and
[Rh(cod)-(R)-Quinap]BF4,[17] were prepared as previously reported. They
were characterised by elemental analysis, 1H and 31P NMR,and FTIR.
Montmorillonite K-10 was purchased from Fluka. Pre-dried montmoril-
lonite K-10 was obtained as follows: 5 g of clay was kept in a melting pot
in the oven at 1008C for 24 h. NMR spectra were recorded on a Varian
Gemini 300 and Mercury 400 spectrometer. Chemical shifts were report-
ed relative to tetramethylsilane for 1H and 13C,85% H 3PO4 for 31P and
BF3OEt2 for 11B as the external reference. Gas chromatographic analyses
were performed on a Hewlett-Packard 5890 II with a flame ionisation de-
tector equipped with a chiral column FS-Cyclodex B-IP,50 m”0.25 mm.
À
ClÀ,Br to IÀ. A more in depth analysis was made in an at-
tempt to explain the increase in ee values,to as high as 98%
for the hydroboration of p-MeO-styrene,which is the high-
est reported so far in the literature.
From this experimental evidence and applying the same
computational approach we used in a previous study,[20] the
influence of the nature of the hydroborating reagent,of the
metal,and of the counterion can be rationalised. We as-
sumed that the relative stability of the possible isomers is di-
rectly related to their population,and that the most stable
isomers are those that determine the reaction outcome. Al-
though transition states were not characterised,we found a
nice correlation between the experimental findings and iso-
Chem. Eur. J. 2004, 10,6456 – 6467
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
6465

C. Bo,E. Fernµndez et al.
FULL PAPER
Elemental analysis of organometallic complexes was carried out on a
Carlo-Erba microanalyzer. IR spectra (range 4000–400 cmÀ1) were re-
corded on a FTIR Prospect IR of Midac Corporation spectrometer in
KBr pellets.
Acknowledgements
The authors are grateful to the Ministerio de Educación y Tecnología for
financial support with reference BQU-2002-04110-C02-01 and BQU-
2001–0656 and from the CIRIT of the Generalitat de Catalunya under
Project SGR-1999-00182 and SGR-2001-00316. We thank Ramon Guer-
rero (NMR techniques) and Jose Carlos Ortiz (computational resources).
Homogeneous catalytic hydroboration/oxidation of styrene with catechol-
borane: Styrene (2 mmol) was added to a solution of catalyst (1 mol%)
in THF (2 mL) under nitrogen. The solution was stirred for 5 min and
freshly distilled catecholborane (2.2 mmol) was then added. The mixture
was stirred at ambient temperature for 1 h and then quenched with
EtOH (2 mL). Work up must be carried out carefully because of the risk
of explosion by using peroxides with Et2O and THF. Afterwards,NaOH
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2
several hours. The reaction mixture was extracted into Et2O,washed
(NaOH 2m, H2O,saturated brine) dried over MgSO 4,and the products
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were immobilised in the following manner. The dichloromethane solu-
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pared under nitrogen and added to a suspension of the solid support
Montmorillonite K-10 in deoxygenated dichloromethane (10 mL),and
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(2 mmol) was added to a suspension of supported catalyst [Ir(cod)-(R)-
Binap]BF4 in montmorillonite K-10,(2 mol% immobilised in 0.5 g of
clay),in THF (2 mL) under nitrogen. The solution was stirred for 5 min
and freshly distilled catecholborane (2 mmol) was then added. The mix-
ture was stirred at ambient temperature for 2 h. The solution was filtered
off under vacuum and the filtrates were then quenched with EtOH
(2 mL). The quenched filtrates were treated with NaOH (2m,2 mL) and
H2O2 (2 mL) and the mixture was stirred for several hours. The mixture
was finally extracted into Et2O,washed (NaOH 2 m, H2O,saturated
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2p electrons for P and Cl were treated as frozen cores. Energy differences
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À
À
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Received: June 8,2004
Published online: November 10,2004
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