Enantioselective hydroboration of olefins catalysed by cationic rhodium complexes of 2-phenylquinazolin-4-yl-2-(diphenylphosphino)naphthalene

Article abstract of DOI:10.1039/b004014m

Cationic rhodium complexes of 2-phenylquinazolin-4-yl-2- (diphenylphosphino)naphthalene catalyse the hydroboration of indene, tetrahydronaphthalene and a range of styrenes in high yields, regioselectivities and with enantiomer excesses of up to 97%.

Full text of DOI:10.1039/b004014m

Enantioselective hydroboration of olefins catalysed by cationic rhodium
complexes of 2-phenylquinazolin-4-yl-2-(diphenylphosphino)naphthalene†
Mary McCarthy,a Mark W. Hooperb and Patrick J. Guiry*a
a Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
b Dyson Perrins Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QY. E-mail: p.guiry@ucd.ie
Received (in Liverpool, UK) 9th May 2000, Accepted 7th June 2000
Cationic rhodium complexes of 2-phenylquinazolin-4-yl-
2-(diphenylphosphino)naphthalene catalyse the hydrobora-
tion of indene, tetrahydronaphthalene and a range of
styrenes in high yields, regioselectivities and with enantio-
mer excesses of up to 97%
The report by Männig and Nöth1 that Rh–phosphine complexes
successfully catalysed the hydroboration of olefins added a new
dimension to the hydroboration methodology developed by
Brown.2 With catecholborane as the borane source, the
catalysed variant offered potential advantages in terms of
chemo-, regio- and enantioselectivity.3 Burgess4 first reported
catalytic enantioselective hydroboration and Hayashi subse-
quently used Rh–BINAP complexes for the hydroboration of
styrene in ees of up to 96%.5 Togni applied Rh complexes of the
The
regioselectivity obtained using phenylethene 6 was
diphosphine Josiphos 1 (92% ee) and the related pyrazole-
found to increase using lowered reaction temperatures and the
optimised values obtained were 80 20 with an ee value of 79%
after a reaction time of 2 h. This ee value compares favourably
with PHENAP 4 (67%) but is lower than that obtained with
QUINAP 3 (88%) and our regioselectivity was poorer than both
3 and 4 (97 3 and 90 10, respectively).8a,9 The sense of
Table 1 Catalytic hydroboration of stilbenesa
Conversion Ee (%)
b
Entry
Olefin
T/°C
(%)b
(R)c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
6
6
7
7
8
8
25
0
25
0
25
0
25
25
25
0
25
0
25
25
0
68 32
80 20
75 25
77 23
78 22
83 17
91 9
92 8
88 12
89 11
93 7
91 9
—
100
100
100
91
100
100
100
100
87
72
75
65
—
63d
79d
77d
81d
46d
49d
94d
91d
88d
92d
97d
93d
—
(E)-9
(Z)-9
10
10
11
containing phosphinamine 2 (98% ee).6,7 Brown used the
axially chiral phosphinamine ligands QUINAP 3 (91% ee)8 and
PHENAP 4 (67% ee)9 and also extended the standard
hydroboration–oxidation sequence to include hydroboration–
amination.10 We have recently reported the synthesis and
resolution of the related ligand, 2-phenylquinazolin-4-yl-
2-(diphenylphosphino)naphthalene 5, and, in view of the
success of 3 and 4, we wished to test its enantiodifferentiating
ability in Rh-catalysed olefin hydroboration and we now report
our preliminary results.11
11
(E)-12
(Z)-12
(Z)-12
—
—
100
100
59e
62e
a Typical procedure: Freshly distilled catecholborane (0.5 mmol) in THF (1
ml) was added dropwise to a solution of olefin (0.5 mmol) and [(R)-
diphenyl[1-(2-phenylquinazolin-4-yl)(2-naphthyl)]phosphine]rhodium(cy-
cloocta-1,5-diene)trifluoromethanesulfonate (5 mol) in THF (1 ml). The
solution turned brown and was stirred at room temp. for 2 h. The reaction
was placed in an ice bath and quenched with EtOH (1 ml). 30% H2O2 (aq.)
(3 ml) and 1 M NaOH (aq.) (3 ml) were added and the mixture was allowed
to warm and stirred for 1 h at room temp. The mixture was transferred to a
separating funnel and Et2O was added (10 ml). The organic extracts were
washed with 1 M NaOH (aq.), brine and then dried over MgSO4. The
solvent was removed in vacuo to give the product as an oil. 1H NMR
obtained gave % conversion and regioselectivity. Enantioselectivities were
determined by either GC or HPLC. b Regioselectivities and conversions by
1H NMR. c Absolute configuration assigned by similarity in order of elution
in the GC analysis and from the sign of the optical rotation.8 d Enantiomeric
excesses were determined by GC ( -Dex®120 column, 30 m 0.25 mm,
0.25 m film thickness). e Enantiomeric excesses were determined by
HPLC (Chiralcel OD column 99.5 0.5 Hex–iPrOH.
The required cationic Rh catalyst was prepared in a standard
manner from TMS triflate and (cycloocta-1,5-diene)(pentane-
2,4-dionato)rhodium(
) and (R)-5.12 Because of its susceptibil-
I
ity to oxidation, the catalyst was freshly made in situ and we
also used freshly distilled catecholborane. We focused on
vinylarene substrates, paying particular attention to those which
would highlight the effect on reactivity and enantioselectivity of
different aryl substituents and -substitution, as exemplified by
olefins 6–12. In each of the reactions the catalysed hydro-
boration was followed by direct oxidation with H2O2 to afford
the corresponding alcohol. The results of our investigations are
given in Table 1.
DOI: 10.1039/b004014m
Chem. Commun., 2000, 1333–1334
This journal is © The Royal Society of Chemistry 2000
1333

Table 2 Catalytic hydroboration of cyclic olefins
asymmetry was identical to 3 and 4, i.e. the (R)-ligand affords
the (R)-product alcohol. The more electron-rich 4-methox-
yphenylethene 7 gave similar regioselectivities and an increased
ee of 81%, but again lower than the 94% ee value observed with
QUINAP 3.8a The electron-deficient substrate, 4-chloropheny-
lethene 8, afforded similar regioselectivities but significantly
lowered ees of 46–49%, which is in agreement with trends
observed employing QUINAP 3 and analogues.8b More promis-
ing results in terms of regio- and stereoselectivity were obtained
using -substituted arylethenes. Both the (E)- and (Z)-isomers
of propenylbenzene 9 demonstrated a preference for reaction at
the benzylic position and similarly in high ees of 94 and 91%,
respectively. Similar values were obtained with QUINAP 3 and
PHENAP 49 whereas BINAP afforded an ee of 42% for (E)-9
and 18% for (Z)-9.5 (Z)-Propenyl-4-methoxybenzene 10 was a
similarly successful substrate giving up to 92% ee in 72% yield
after reaction for 6 days at 0 °C. The combination of an even
more electron-rich arene and -substituted olefin 11 gave the
best ee of 97% using ligand 5. Again it was noted that increasing
the electron richness of the arene was beneficial in terms of
enantioselectivity but led to a retardation of the reaction as seen
by the poorer yields obtained. In order to determine the effect of
increasing the bulk at the -position we tested (E)- and (Z)-
stilbene 12 and found that (E)-12 was an unreactive substrate
whereas (Z)-12 afforded up to 62% ee in excellent yield. This
result is similar to that obtained with QUINAP 3 where high ees
(85–91%) were obtained with both isomers, although it was
noted that (E)-12 reacted at a significantly lower rate (45
turnovers after 20 h).8b The best ee reported for (Z)-12 using
BINAP was significantly lower at 16%.5
Conversion Ee (%)
(%)a
a
Entry
Olefin
T/°C
(R)b
1
2
3
4
13
13
14
14
25
0
25
0
98 2
98 2
> 99 1
> 99 1
98
99
84c
81c
89d
88d
100
> 99
a Regioselectivities and conversions by 1H NMR. b Absolute configuration
assigned by similarity in order of elution in the GC analysis and from the
sign of the optical rotation.8 c Enantiomeric excesses were determined by
GC (Supelco 2-4310 -Dex®120 column, 30 m 0.25 mm, 0.25 m film
thickness). d Enantiomeric excesses were determined by GC ( -Dex®120
column, 30 m 0.25 mm, 0.25 m film thickness).
Table 3 Variation of catalyst concentration in the hydroboration of (E)-9
Mol %
Catalyst
Conversion
(%)
Ee (%)
(R)
Entry
Time/h
1
2
3
4
0.5
0.5
0.5
0.5
0.25
1
4
17
31
75
92
94
96
95
16
100
progress and will be reported in due course from these
laboratories.14
We are indebted to Dr John Brown, University of Oxford, for
his interest in this work and for supporting the visit of M. McC.
to his laboratories where the initial hydroboration studies were
performed. Financial support from Enterprise Ireland [Basic
Research Award (SC/94/565) and a Research Scholarship (BR/
94/158) to M. McC.] is gratefully acknowledged as is the award
of the 1997 BOC Gases Postgraduate Bursary to M. McC. We
thank the EPSRC for the award of a studentship to M. W. H.
Many thanks to Shane Robinson of this department for his help
with ee determinations using HPLC.
The cyclic olefins, indene 13 and 1,2-dihydronaphthalene 14,
are two of the most challenging substrates in Rh-catalysed
hydroboration. We tested Rh complexes of 5 with these
substrates and the results are shown in Table 2. Excellent
conversions and regioselectivities were obtained and optimised
ees of 84% and 89% were obtained with 13 and 14, respectively.
The result for 13 is slightly lower than that reported for
QUINAP 3 (86%),8b but compares favourably to PHENAP 4
(64%)9 and even more so when compared to BINAP (19%).5 A
similar trend is observed for substrate 14 as our result of 89% is
again lower than that reported for QUINAP 3 (96%),8b but is
higher than the value obtained with PHENAP 4 (84%)9 and in
this case no result has been quoted for BINAP.
All the reactions noted in Table 1 were run on a 0.5 mmol
scale with 1 mol% catalyst. In a further study, the hydroboration
of (E)-propenylbenzene 9 was carried out using 0.5 mol%
catalyst and our results are given in Table 3. Using 0.5 mol%
catalyst, the ee obtained after 15 min was slightly lower than the
values obtained after 1 h (94% ee), 4 h (96% ee), and 16 h (95%
ee) which were not lowered in comparison with the value
obtained with 1 mol% catalyst (Table 1, entry 7).
In conclusion, our results demonstrate that 5, applied in Rh
catalysed hydroboration of substituted arylethenes, -substi-
tuted arylethenes and cyclic olefins, gives excellent conver-
sions, good regioselectivities and ees of up to 97%. For
substituted arylethenes our ligand, 2-phenylquinazolin-4-yl-
2-(diphenylphosphino)naphthalene 5, as with QUINAP 3 and
PHENAP 4, afforded lower ees than both BINAP and Josiphos
1. However, for -substituted arylethenes and cyclic olefins
these axially chiral phosphinamine ligands are far superior. This
can be explained by inferring that the increased steric demand of
the olefin is more easily accommodated by these less sterically
demanding ligands. Further hydroboration studies employing
structurally related quinazoline-containing ligands13 are in
Notes and references
† This compound has previously been published under the name 2-phenyl-
quinazolinap.
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1334
Chem. Commun., 2000, 1333–1334