Planar chiral imidazolium salts in the asymmetric rhodium-catalyzed 1,2-addition of arylboronic acids to aldehydes

Article abstract of DOI:10.1055/s-2005-861800

Planar chiral imidazolium salts 8 and 13 were used as precursors for N-heterocyclic carbene ligands, which were applied in rhodium-catalyzed additions of arylboronic acids to aromatic aldehydes. With pseudo-ortho- disubstituted [2.2]paracyclophane (S,R_p)-8b bearing a phosphinoyl substituent at the paracyclophane backbone, 38% ee has been achieved in this rather unexplored reaction. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-861800

PAPER
429
Planar Chiral Imidazolium Salts in the Asymmetric Rhodium-Catalyzed
1,2-Addition of Arylboronic Acids to Aldehydes
R
hodium-Cataly
h
ze
d
1,2-Add
i
ition
o
l
f
A
rylb
o
oronic Acids to
A
l
F
dehydes ocken, Jens Rudolph, Carsten Bolm*
Institut für Organische Chemie der RWTH Aachen, Professor-Pirlet-Str. 1, 52056 Aachen, Germany
Fax +49(241)8092391; E-mail: Carsten.Bolm@oc.rwth-aachen.de
Received 15 October 2004; revised 5 November 2004
In 2001 Fürstner reported aryl transfer reactions from
arylboronic acids to aldehydes catalyzed by a rhodium
complex bearing an in situ formed N-heterocyclic carbene
(NHC).11 With a 1:1 combination of RhCl3·3H2O or
Abstract: Planar chiral imidazolium salts 8 and 13 were used as
precursors for N-heterocyclic carbene ligands, which were applied
in rhodium-catalyzed additions of arylboronic acids to aromatic al-
dehydes. With pseudo-ortho-disubstituted [2.2]paracyclophane
(S,Rp)-8b bearing a phosphinoyl substituent at the paracyclophane [Rh(OAc)2]2 and a sterically hindered imidazolium salt
backbone, 38% ee has been achieved in this rather unexplored reac-
tion.
(1–3 mol%), using NaOMe as base in aqueous DME at
80 °C, good to excellent yields of 3 were achieved.
Key words: [2.2]paracyclophane, N-heterocyclic carbene, imida-
Complexes with NHC ligands have recently attracted sig-
zolium salt, asymmetric catalysis, boronic acid, diarylmethanol
nificant attention and much progress has been made with
regard to their synthesis and application in homogenous
metal catalysis.12 One of the most prominent examples is
Chiral diarylmethanols 3 are structural core units in a
the use of NHC ruthenium complexes in olefin metathe-
number of pharmacologically active compounds.1 They
sis, where they exhibit much superior catalytic properties
are accessible by asymmetric reduction of prochiral
than their analogs bearing phosphine ligands.13 Chiral
ketones2 and enantioselective aryl transfer reactions to al-
NHCs have been used first by Herrmann and Enders,14
dehydes.3–5 Recently, we developed various approaches
and since then they found numerous applications in asym-
involving arylzinc reagents to obtain diarylmethanols in
metric metal catalysis.15 In 2002 we described the first
high yields and excellent enantioselectivities.4 By apply-
planar chiral NHC and the attempt to apply this ferrocene-
ing mixtures of arylboronic acids and diethylzinc the
based ligand in rhodium-catalyzed hydrosilylations.16
product range was significantly broadened.5
Soon after we prepared iridium complexes derived from
All these aryl transfer methods make use of air-sensitive planar-chiral imidazolium salts with pseudo-ortho-disub-
organometallic reagents, which limit their large-scale ap- stituted [2.2]paracyclophane backbones.17,18 Their use in
plication. A solution could be the application of arylbo- asymmetric hydrogenations led to products with up to
ronic acids (without any additive), which are easy to 89% ee.
handle and are commercially available or straightforward
We wondered, if [2.2]paracyclophane-based NHCs could
to prepare. Rhodium-catalyzed additions of arylboronic
also be applied as chiral ligands in rhodium-catalyzed aryl
acids to aromatic aldehydes 1 have been reported by
transfer reactions.19 Imidazolium salts 8 and 13 with
Miyaura in 1998 (Scheme 1).6–8 The corresponding di-
hemi-labile phosphinoyl and methoxy substituents, re-
spectively, became the first target compounds. Their syn-
arylmethanols 3 were obtained in good to superb yields,
and with MeO-MOP9 as chiral ligand a product with 41%
thesis
started
from
enantiopure
(Rp)-4,12-
ee was formed in the asymmetric addition of phenylbo-
ronic acid to 1-naphthaldehyde.6a A subsequent attempt
by Frost to improve this result by using sparteine or bis-
oxazolines as ligands remained unsuccessful (<10% ee).10
dibromo[2.2]paracyclophane (4)20,21 and involved a se-
quence of substitution reactions (Scheme 2).
After treatment of (Rp)-4 with 1 equivalent of n-BuLi in
the presence of TMEDA, the resulting carbanion was
trapped with diphenylphosphinic chloride affording phos-
phine oxide (Rp)-5 in 60% yield.22 Benzyl bromide 7 was
then obtained by converting phosphine oxide 5 into the
corresponding alcohol by treatment with t-BuLi and
DMF, subsequent reduction with NaBH4, and PBr3 bromi-
nation. In an analogous fashion, (Rp)-4,12-dibro-
mo[2.2]paracyclophane (4) was first converted into
methoxyparacyclophane (10) via bromine–lithium ex-
change with n-BuLi, trapping of the anion with B(OMe)3,
subsequent oxidation giving 9, and alkylation with MeI.23
Benzyl bromide 12 was then derived from 10 by reaction
with n-BuLi and paraformaldehyde followed by PBr3 bro-
mination. Benzyl bromides 7 and 12 as key intermediates
O
OH
*
5 mol% Rh/ligand
H
+
PhB(OH)2
NaOMe, 60 °C,
DME/H2O
R
R
2
3
1
Scheme 1
SYNTHESIS 2005, No. 3, pp 0429–0436
x
x
.
x
x
.2
0
0
5
Advanced online publication: 26.01.2005
DOI: 10.1055/s-2005-861800; Art ID: Z19304SS
© Georg Thieme Verlag Stuttgart · New York

430
T. Focken et al.
PAPER
O
O
O
R
PPh2
PPh2
PPh2
N
b, c
a
e
X
N
Br
Br
6 (X = OH): 78%
7 (X = Br): 52%
5: 60%
8a-d
Br
d
Br
R
OMe
X
OMe
OR
Br
N
f
h
e
4
N
Br
13a-c
9 (R = H): 93%
10 (R = Me): 87%
11(X = OH): 63%
12 (X = Br): 82%
g
i
8a: R = Me (69%); 8b: R = (S)-CHMePh (50%); 8c: R = (R)-CHMePh (83%); 8d: R = Mesityl (35%)
13a: R = Me (99%); 13b: R = (S)-CHMePh (89%); 13c: R = (R)-CHMePh (93%)
Scheme 2 Reagents and conditions: (a) n-BuLi, TMEDA, THF, –78 °C, then P(O)Ph2Cl; (b) t-BuLi, THF, –78 °C, then DMF; (c) NaBH4,
H2O, MeOH; (d) PBr3, THF; (e) imidazoles, DMF, 80–100 °C; (f) n-BuLi, THF, –78 °C, then B(OMe)3, H2O2, NaOH; (g) MeI, K2CO3, acetone,
reflux; (h) n-BuLi, THF, –78 °C, then (CH2O)x, 0 °C; (i) PBr3, pyridine, CH2Cl2.
could easily be converted into a variety of imidazolium Table 1 Ligand Screening with PhB(OH)2 and 4-Chlorobenzalde-
hyde (1a) as Substrates to Give 3aa
salts by heating with the corresponding 1-substituted imi-
dazoles in DMF in yields ranging from 35 to 99%
(Scheme 2).
Entry
Imidazolium salt
(Rp)-8a
Yield (%)b
Ee (%)c
20 (S)
29 (S)
3 (R)
1
2
3
4
5
6
7
72
84
63
90
80
75
57
Imidazolium salts 8a–d and 13a–c thus obtained were
then used as precursors for rhodium NHC complexes,
which were applied in the catalytic addition of arylboronic
acids to aromatic aldehydes (Scheme 1). In order to opti-
mize the reaction conditions a number of parameters [tem-
perature, metal to ligand ratio (1:1 to 1:3), rhodium source
([Rh(OAc)2]2, RhCl3·xH2O, [Rh(C2H4)2(acac)]), solvent
(DME–H2O, dioxane–H2O), and amount of base (1-2
equiv)] were varied using imidazolium salt (Rp)-8a, phe-
nylboronic acid (2 with Ar = Ph) and 4-chlorobenzalde-
hyde (1a) or mesitylaldehyde (1b) as model substrates.
(S,Rp)-8b
(R,Rp)-8c
(Rp)-8d
24 (S)
2 (R)
(Rp)-13a
(S,Rp)-13b
(R,Rp)-13c
6 (S)
10 (R)
a Reaction conditions: 0.4 mmol scale; for details, see Experimental
Section.
Interestingly, all changes had only a minor effect on the
enantioselectivity of the process. On the other hand, no
product was obtained by using substoichiometric amounts
of base or in the absence of ligand or water. Furthermore,
the reaction rates were significantly reduced, when the
temperature was below 60 °C. The results of a ligand
screening for the identification of the best imidazolium
salt under optimized conditions are summarized in
Table 1.
b After column chromatography.
c Enantiomer ratios were determined by HPLC using chiral columns.
Compared to published data,6,10 our results appeared to be
promising and motivated further investigations. Hence,
we tested the ability of imidazolium salts (Rp)-8a and
(S,Rp)-8b to serve as ligand precursors in the rhodium-cat-
alyzed addition of phenylboronic acid to other aldehydes.
The results are summarized in Table 2.
In general, catalysts derived from phosphinoyl imidazoli-
um salts 8 performed superior to the ones obtained from
methoxy-substituted paracyclophanes 13. Thus, use of the
latter resulted in the formation of a product with only low
ee (2–10%). In contrast, with 8 as ligand precursor the
enantioselectivity reached up to 29% ee. Catalysts derived
from the diastereomeric phosphinoyl imidazolium salts
(S,Rp)-8b and (R,Rp)-8c (Table 1, entries 2 and 3) exhibit-
ed a remarkable difference in enantioselectivity. Whereas
the heterochiral one afforded a product with 29% ee, the
enantioselectivity diminished to 3% ee, when the homo-
chiral imidazolium salt was applied. Analogous results
were obtained in a screening with mesitylaldehyde as sub-
strate.24
In most cases, imidazolium salt (S,Rp)-8b gave superior
results than (Rp)-8a in terms of enantioselectivity. Where-
as the former gave diarylmethanols with up to 38% ee, the
latter afforded products with an eemax of only 29%. An ex-
ception was mesitylaldehyde (1b). There, (Rp)-8a was
better than (S,Rp)-8b (Table 2, entry 2). In all cases, (S)-
alcohols were formed using (Rp)-configured imidazolium
salts.25 For the catalyses with (S,Rp)-8b this result indi-
cates that the planar chirality of the ligand and not the cen-
tral chirality is the stereochemistry-determining element.
The substitution pattern of the aldehyde had only a minor
effect on the enantioselectivity, and the best results were
obtained starting from aldehydes with ortho-substituents
Synthesis 2005, No. 3, 429–436 © Thieme Stuttgart · New York

PAPER
Rhodium-Catalyzed 1,2-Addition of Arylboronic Acids to Aldehydes
431
added TMEDA (0.83 mL, 5.50 mmol) followed by a solution of n-
BuLi in hexane (3.7 mL, 5.92 mmol). The reaction mixture was
stirred for 1 h at –78 °C, subsequently treated with chlorodiphe-
nylphosphine oxide (2.5 mL, 13.1 mmol), and warmed overnight to
r.t. After addition of aq sat. NH4Cl solution, phase separation and
extraction of the aqueous layer with CH2Cl2, the combined organic
phases were dried (MgSO4) and concentrated. The remaining resi-
due was purified by flash chromatography (hexane–EtOAc, 2:1 to
1:1) to give 1.6 g of the title compound as (60% yield) as a white
solid; mp 219–222 °C; [a]D25 –84 (c = 1.1, CHCl3).
such as 1-naphthaldehyde [29% and 38% ee with (Rp)-8a
and (S,Rp)-8b, respectively, Table 2, entry 7].
Table 2 Addition of PhB(OH)2 to Aldehydes 1a–g to Give the Cor-
responding Diarylmethanols 3a–g with Rhodium Catalysts Prepared
from (Rp)-8a and (S,Rp)-8ba
Entry ArCHO 1
Use of (Rp)-8a
Use of (S,Rp)-8b
Yield Ee (%)c
(%)b
Yield Ee (%)c
(%)b
Ar
IR (KBr): 1437, 1186, 1118, 711, 697, 560, 541 cm–1.
1
2
3
4
5
6
7
4-ClC6H4 (1a)
72
39
63
51
42
94
61
20 (S)
28 (S)
23 (S)
24 (S)
20 (S)
20 (S)
29 (S)
84
56
68
72
–
29 (S)
21 (S)
31 (S)
27 (S)
–
1H NMR (400 MHz, CDCl3): d = 2.72 (ddd, 1 H, J = 13.3, 10.6, 6.9
Hz, CH2), 2.79–2.86 (m, 1 H, CH2), 2.90–3.05 (m, 3 H, CH2), 3.34–
3.49 (m, 3 H, CH2), 6.48 (d, 1 H, J = 7.7 Hz, PCp-CH), 6.58 (dd, 1
H, J = 7.7, 1.7 Hz, PCp-CH), 6.58–6.61 (m, 1 H, PCp-CH), 6.65
(dd, 1 H, J = 7.7, 3.8 Hz, PCp-CH), 6.82 (dd, 1 H, J = 14.6, 2.0 Hz,
PCp-CH), 7.29 (d, 1 H, J = 1.7 Hz, PCp-CH), 7.34–7.38 [m, 2 H,
P(O)Ph2-CH], 7.42–7.47 [m, 3 H, P(O)Ph2-CH], 7.50–7.58 [m, 3 H,
P(O)Ph2-CH], 7.74–7.80 [m, 2 H, P(O)Ph2-CH].
13C NMR (100 MHz, CDCl3): d = 32.6, 34.9 (CH2), 36.1 (d,
JP,C = 3.8 Hz, CH2), 36.13 (CH2), 127.4 (qC), 128.3 [d, 2 C,
JP,C = 11.5 Hz, P(O)Ph2-CH], 128.4 [d, 2 C, JP,C = 11.4 Hz,
P(O)Ph2-CH], 130.3 (d, JP,C = 104.6 Hz, qC), 131.1 (PCp-CH),
131.6 [d, JP,C = 3.0 Hz, P(O)Ph2-CH], 131.63 [d, 2 C, JPC = 9.1 Hz,
P(O)Ph2-CH], 131.7 [d, JP,C = 2.3 Hz, P(O)Ph2-CH], 132.0 (d,
JP,C = 103.8 Hz, qC), 132.9 [d, 2 C, JP,C = 9.2 Hz, P(O)Ph2-CH],
133.4 (d, JP,C = 13.7 Hz, PCp-CH), 134.1 (PCp-CH), 135.7 (d,
JP,C = 11.5 Hz, PCp-CH), 135.6 (d, JP,C = 103.7 Hz, qC), 137.4 (d,
JP,C = 3.0 Hz, PCp-CH), 138.6 (PCp-CH), 138.64 (d, JP,C = 9.9 Hz,
qC), 139.1 (d, JP,C = 13.0 Hz, qC), 142.2 (qC), 145.6 (d, JP,C = 7.6
Hz, qC).
mesityl (1b)
2-MeOC6H4 (1c)
3-MeOC6H4 (1d)
4-MeOC6H4 (1e)
4-biphenyl (1f)
1-naphthyl (1g)
90
85
30 (S)
38 (S)
a–c See footnotes in Table 1.
In summary, new planar-chiral imidazolium salts with
hemi-labile substituents have been prepared starting from
enantiopure C2-symmetric 4,12-dibromo[2.2]paracyclo-
phane (4). Their applicability in rhodium-catalyzed addi-
tions of arylboronic acids to aromatic aldehydes has been
demonstrated, and the corresponding diarylmethanols
were obtained with up to 38% ee. Currently, we are inves-
tigating structural modifications of the [2.2]paracyclo-
phane-based NHC ligands in order to improve the catalyst
performance in terms of activity and enantioselectivity.
31P NMR (162 MHz, CDCl3): d = 27.90 (s).
MS (EI, 70 eV): m/z (%) = 488 (M+, 25), 486 (M+, 21), 305 (23),
304 (100), 303 (50), 225 (17), 178 (17).
HRMS: m/z calcd for C28H24BrOP: 486.07483 (M+); found:
486.07487.
Pseudo-ortho-dibromo[2.2]paracyclophane (4),20,21 1-mesitylimi-
dazole,26 and (S)- and (R)-1-(1-phenylethyl)imidazole27 were pre-
pared according to published procedures. All other reagents were
commercially available and used as received. THF and DME were
distilled under N2 from sodium benzophenone ketyl and CH2Cl2
from CaH2 prior to use. Anhyd DMF was purchased. All reactions
were performed under argon using standard Schlenk techniques.
The NMR spectra were recorded on a Varian Mercury 300 (1H
NMR at 300 MHz; 13C NMR at 75 MHz; 31P NMR at 121 MHz) or
( )-4-Diphenylphosphinoyl-12-hydroxymethyl[2.2]paracyclo-
phane (6)
Phosphine oxide 5 (3.4 g, 6.98 mmol) was dissolved in anhyd THF
(150 mL) and treated at –78 °C with a solution of t-BuLi in pentane
(9.9 mL, 14.65 mmol) for 30 min. Subsequently DMF (4.3 mL, 55.5
mmol) was added and the mixture was stirred overnight at –78 °C
to r.t. H2O (50 mL) was added, followed by addition of MeOH (50
mL) and NaBH4 (5.0 g, 0.132 mmol). The mixture was stirred over-
night at r.t. After acidification with HCl (10%) and extraction of the
aqueous phase with CH2Cl2, the combined organic phases were
dried (MgSO4) and concentrated. The residue was purified by flash
chromatography (hexane–EtOAc, 1:1) to give 2.39 g (78%) of 6 as
a white solid; mp 209–212 °C.
Varian Inova 400 (1H NMR at 400 MHz; 13C NMR at 100 MHz; 31
P
NMR at 162 MHz) spectrometer. Chemical shifts are given in ppm
with internal referencing to TMS (1H NMR) or the solvent peaks
(1H NMR, 13C NMR) and external to H3PO4 (31P NMR). Assign-
ments are based on 2D NMR experiments. IR spectra were mea-
sured on a Perkin-Elmer 1760 FT spectrometer. Mass spectra were
obtained by using a Varian MAT 212 (EI, ESI) and a Finnigan MAT
95 (SIMS-FAB) spectrometer. Only fragments containing the iso-
topes of the highest abundance are listed. Elemental analyses were
carried out at the Institut für Organische Chemie der RWTH
Aachen on a Heraeus CHNO-Rapid apparatus. Melting points were
measured with a Büchi B-540 melting point determination appara-
tus. Optical rotations were measured on a Perkin-Elmer 241 pola-
rimeter.
IR (KBr): 3359, 1148, 1115, 1041, 709, 653, 560, 541, 521 cm–1.
1H NMR (400 MHz, CDCl3): d = 2.74–2.85 (m, 3 H, CH2), 2.86–
3.05 (m, 4 H, CH2), 3.21–3.28 (m 1 H, CH2), 4.30 (d, 1 H, J = 15.4
Hz, CH2OH), 4.57 (d, 1 H, J = 15.1 Hz, CH2OH), 5.18 (br s, 1 H,
OH), 6.46 (d, 1 H, J = 7.4 Hz, PCp-CH), 6.56 (br d, 1 H, J = 8.2 Hz,
PCp-CH), 6.58–6.60 (m, 1 H, PCp-CH), 6.62 (dd, 1 H, J = 7.7, 4.4
Hz, PCp-CH), 6.76 (dd, 1 H, J = 14.8, 1.8 Hz, PCp-CH), 7.03 (br s,
1 H, PCp-CH), 7.38–7.53 [m, 6 H, P(O)Ph2-CH], 7.63–7.68 [m, 2
H, P(O)Ph2-CH], 7.77–7.83 [m, 2 H, P(O)Ph2-CH].
13C NMR (100 MHz, CDCl3): d = 31.3, 33.6 (CH2), 35.9 (d,
JP,C = 4.6 Hz, CH2), 35.6 (CH2), 63.2 (CH2OH), 128.7 [d, 2 C,
JP,C = 12.2 Hz, P(O)Ph2-CH], 128.8 [d, 2 C, JP,C = 12.2 Hz,
P(O)Ph2-CH], 130.3 (d, JP,C = 104.5 Hz, qC), 130.0 (PCp-CH),
131.0 (PCp-CH), 131.2 [d, 2 C, JP,C = 9.9 Hz, P(O)Ph2-CH], 131.8
(–)-(Rp)-4-Bromo-12-diphenylphosphinoyl[2.2]paracyclophane
[(Rp)-5]
To a solution of (Rp)-pseudo-ortho-dibromo-[2.2]paracyclophane
(4; 2.0 g, 5.46 mmol) in anhyd THF (60 mL) cooled to –78 °C was
Synthesis 2005, No. 3, 429–436 © Thieme Stuttgart · New York

432
T. Focken et al.
PAPER
[d, JP,C = 3.0 Hz, P(O)Ph2-CH], 131.9 [d, 2 C, JP,C = 9.2 Hz,
P(O)Ph2-CH], 132.0 [d, JP,C = 3.8 Hz, P(O)Ph2-CH], 133.0 (d,
JP,C = 104.6 Hz, qC), 132.2 (d, JP,C = 13.0 Hz, PCp-CH), 132.4
(PCp-CH), 134.4 (d, JP,C = 104.6 Hz, qC), 135.2 (d, JP,C = 12.3 Hz,
PCp-CH), 135.5 (qC), 138.1 (d, JP,C = 3.1 Hz, PCp-CH), 139.0 (d,
JP,C = 12.9 Hz, qC), 140.1, 140.7 (qC), 145.4 (d, JP,C = 8.4 Hz, qC).
31P NMR (162 MHz, CDCl3): d = 28.82 (s).
MS (EI, 70 eV): m/z (%) = 439 (23), 438 (M+, 70), 437 (13), 420
3.26–3.32 (m, 1 H, CH2), 3.64 (ddd, 1 H, J = 13.8, 10.8, 1.8 Hz,
CH2), 4.05 (s, 3 H, CH3), 4.71 (d, 1 H, J = 14.3 Hz, CH2N), 5.50 (d,
1 H, J = 14.3 Hz, CH2N), 6.54 (dd, 1 H, J = 14.3, 1.8 Hz, PCp-CH),
6.61 (d, 1 H, J = 7.6 Hz, PCp-CH), 6.62 (d, 1 H, J = 7.9 Hz, PCp-
CH), 6.56 (br d, 1 H, J = 7.9 Hz, PCp-CH), 6.68 (dd, 1 H, J = 7.8,
1.7 Hz, PCp-CH), 7.09 (d, 1 H, J = 1.5 Hz, PCp-CH), 7.37 (t, 1 H,
J = 1.9 Hz, NCH=CH), 7.40–7.43 [m, 2 H, P(O)Ph2-CH], 7.46 (t, 1
H, J = 1.7 Hz, NCH=CH), 7.48–7.56 [m, 4 H, P(O)Ph2-CH], 7.64–
7.69 [m, 4 H, P(O)Ph2-CH], 10.34 (br s, 1 H, NCHN).
13C NMR (100 MHz, CDCl3): d = 33.3, 34.1, 35.0 (CH2), 35.2 (d,
JP,C = 4.6 Hz, CH2), 37.0 (CH3), 51.3 (CH2N), 122.4, 123.7
(NCH=CH), 128.7 [d, 2 C, JP,C = 12.2 Hz, P(O)Ph2-CH], 129.0 [d,
2 C, JP,C = 11.4 Hz, P(O)Ph2-CH], 130.2 [d, JP,C = 104.5 Hz, qC],
131.0 [d, 2 C, JP,C = 9.9 Hz, P(O)Ph2-CH], 131.7 [d, 2 C, JP,C = 9.2
Hz, P(O)Ph2-CH], 131.8, 132.2 [P(O)Ph2-CH], 132.2 (d,
JP,C = 104.5 Hz, qC), 133.3 (d, JP,C = 13.0 Hz, PCp-CH), 133.7,
134.6 (PCp-CH), 135.2 (d, JP,C = 103.8 Hz, qC), 135.6 (PCp-CH),
136.2 (d, JP,C = 11.5 Hz, PCp-CH), 137.4 (2 C, NCHN, qC), 137.8
(PCp-CH), 138.6 (qC), 139.7 (d, JP,C = 12.2 Hz, qC), 141.0 (qC),
145.6 (d, JP,C = 8.4 Hz, qC).
(17), 347 (16), 305 (43), 304 (100), 303 (42), 225 (10), 178 (11).
Anal. Calcd for C29H27O2P: C, 79.43; H, 6.21. Found: C, 79.08; H,
6.50.
(–)-(Rp)-12-Bromomethyl-4-diphenylphosphinoyl[2.2]paracy-
clophane [(Rp)-7]
To a solution of PBr3 (0.70 mL, 7.37 mmol) in THF (10 mL) was
added dropwise a solution of alcohol 6 (3.2 g, 7.3 mmol) in THF
(100 mL). After stirring overnight, the mixture was treated with an
ice-cold, aq sat. NaHCO3 solution. The phases were separated, the
aqueous phase was extracted with CH2Cl2 and the combined organ-
ic phases were dried (MgSO4). Removal of the volatiles and purifi-
cation of the residue by flash chromatography (hexane–EtOAc, 2:1)
yielded 1.89 g (52%) of (Rp)-7 as a white solid; mp 240 °C (dec.);
[a]D25 –51 (c = 0.9, CHCl3).
31P NMR (162 MHz, CDCl3): d = 26.99 (s).
MS (SIMS-FAB, pos.): m/z (%) = 503 (M+, 35), 502 (100), 420
(28).
IR (KBr): 2927, 1436, 1178, 1134, 1118, 1104, 1069, 755, 748, 709,
697, 590, 560, 537, cm–1.
MS (SIMS-FAB, neg.): m/z (%) = 81 (99), 79 (100).
Anal. Calcd for C33H32BrN2OP·H2O: C, 65.89; H, 5.70; N, 4.66.
Found: C, 65.96; H, 5.68; N, 5.01.
1H NMR (400 MHz, CDCl3): d = 2.71–2.99 (m, 4 H, CH2), 3.11–
3.17 (m 1 H, CH2), 3.36–3.45 (m, 3 H, CH2), 4.09 (d, 1 H, J = 10.1
Hz, CH2Br), 4.12 (d, 1 H, J = 9.9 Hz, CH2Br), 6.30 (dd, 1 H,
J = 14.6, 1.7 Hz, PCp-CH), 6.51 (d, 1 H, J = 7.7 Hz, PCp-CH),
6.56–6.64 (m, 3 H, PCp-CH), 7.05 (d, 1 H, J = 1.6 Hz, PCp-CH),
7.34–7.38 [m, 2 H, P(O)Ph2-CH], 7.43–7.49 [m, 3 H, P(O)Ph2-CH],
7.51–7.59 [m, 3 H, P(O)Ph2-CH], 7.67–7.72 [m, 2 H, P(O)Ph2-CH].
13C NMR (100 MHz, CDCl3): d = 33.1 (CH2), 33.4 (CH2Br), 33.9,
35.0 (CH2), 35.6 (d, JP,C = 4.6 Hz, CH2), 128.56 [d, 2 C, JP,C = 11.5
Hz, P(O)Ph2-CH], 128.6 [d, 2 C, JP,C = 11.5 Hz, P(O)Ph2-CH],
130.3 (d, JP,C = 105.3 Hz, qC), 131.4 [d, 2 C, JP,C = 9.9 Hz,
P(O)Ph2-CH], 131.7 [d, JP,C = 3.0 Hz, P(O)Ph2-CH], 131.9 [d,
JP,C = 2.3 Hz, P(O)Ph2-CH], 132.2 [d, 2 C, JP,C = 9.1 Hz, P(O)Ph2-
CH], 132.6 [d, JP,C = 103.0 Hz, qC], 133.3 (PCp-CH), 133.5 (d,
JP,C = 12.2 Hz, PCp-CH), 134.6 (PCp-CH), 135.5 (d, JP,C = 103.8
Hz, qC), 136.36 (PCp-CH), 136.4 (d, JP,C = 13.0 Hz, PCp-CH),
136.8 (qC), 137.4 (d, JP,C = 2.3 Hz, PCp-CH), 137.8 (qC), 139.2 (d,
JP,C = 12.9 Hz, qC), 140.9 (qC), 146.1 (d, JP,C = 7.7 Hz, qC).
(–)-(S,Rp)-3-(4-Diphenylphosphinoyl[2.2]paracyclophan-12-yl-
methyl)-1-(1-phenylethyl)imidazolium Bromide [(S,Rp)-8b]
The compound was prepared by the method described for the syn-
thesis of imidazolium salt 8a using bromide 7 (0.7 g, 1.40 mmol),
(S)-1-(1-phenylethyl)imidazole (0.482 g, 2.8 mmol), and DMF (2
mL). The reaction mixture was concentrated and the residue washed
with Et2O and purified by flash chromatography (CH2Cl2–MeOH,
99:1 to 95:5). Thus, 0.477 g (50%) of the title compound was ob-
tained as a white solid after drying at 70–80 °C in vacuo; mp 165 °C
(dec.); [a]D25 –123 (c = 0.9, CHCl3).
IR (KBr): 3050, 2973, 2933, 1437, 1157, 1115, 709, 561, 541 cm–1.
1H NMR (300 MHz, CDCl3): d = 1.99 [s, 3 H, J = 7.2 Hz,
NCH(Ph)CH3], 2.79–2.95 (m, 3 H, CH2), 2.99–3.09 (m, 1 H, CH2),
3.14–3.33 (m, 3 H, CH2), 3.77–3.84 (m, 1 H, CH2), 4.69 (d, 1 H,
J = 13.8 Hz, CH2N), 5.69 (d, 1 H, J = 14.1 Hz, CH2N), 5.89 [q, 1 H,
J = 7.0 Hz, NCH(Ph)CH3], 6.55 (dd, 1 H, J = 14.6, 1.5 Hz, PCp-
CH), 6.59–6.31 (m, 3 H, PCp-CH), 6.68 (dd, 1 H, J = 7.8, 1.3 Hz,
PCp-CH), 7.11 (br s, 2 H, 13-H, NCH=CH), 7.34–7.55 [m, 12 H,
P(O)Ph2-CH, Ph-CH, NCH=CH], 7.61–7.70 [m, 4 H, P(O)Ph2-CH],
10.86 (br s, 1 H, N=CHN).
31P NMR (162 MHz, CDCl3): d = 26.82 (s).
MS (EI, 70 eV): m/z (%) = 503 (10), 502 (M+, 37), 501 (12), 500
(M+, 38), 422 (10), 421 (37), 420 (16), 305 (24), 304 (100), 303
(46), 225 (13), 178 (13), 115 (11).
13C NMR (100 MHz, CDCl3): d = 21.2 [NCH(Ph)CH3], 33.2, 33.8,
34.6 (CH2), 34.9 (d, JP,C = 4.6 Hz, CH2), 51.1 (CH2N), 59.8
[NCH(Ph)CH3], 120.0, 122.1 (NCH=CH), 126.9 (2 C, Ph-CH),
128.3 (d, 2 C, JP,C = 12.2 Hz, P(O)Ph2-CH), 128.7 (d, 2 C,
JP,C = 11.4 Hz, P(O)Ph2-CH), 129.17 (Ph-CH), 129.2 (2 C, Ph-CH),
129.7 (d, JP,C = 104.5 Hz, qC), 130.7 (d, 2 C, JP,C = 9.2 Hz,
P(O)Ph2-CH), 131.3 (d, 2 C, JP,C = 9.2 Hz, P(O)Ph2-CH), 131.4 (d,
JP,C = 3.0 Hz, P(O)Ph2-CH), 131.7 (d, JP,C = 103.8 Hz, qC), 131.9
(d, JP,C = 2.3 Hz, P(O)Ph2-CH), 132.0 (qC), 133.2 (d, JP,C = 13.8
Hz, PCp-CH), 133.24 (PCp-CH), 134.3 (PCp-CH), 134.4 (PCp-
CH), 135.84 (d, JP,C = 106.0 Hz, qC), 135.8 (d, JP,C = 11.5 Hz, PCp-
CH), 136.4 (NCHN), 137.4 (d, J = 3.1 Hz, PCp-CH), 137.6, 138.7
(qC), 139.6 (d, JP,C = 12.9 Hz, qC), 140.5 (qC), 145.1 (d, JP,C = 7.7
Hz, qC).
Anal. Calcd for C29H26BrOP: C, 69.47; H, 5.23. Found: C, 69.32; H,
5.23.
(–)-(Rp)-3-(4-Diphenylphosphinoyl[2.2]paracyclophan-12-yl-
methyl)-1-methylimidazolium Bromide [(Rp)-8a]; Typical Pro-
cedure
A solution of methyl bromide 7 (3.2 g, 6.38 mmol) and 1-methylim-
idazole (1.53 mL, 19.2 mmol) in DMF (20 mL) was stirred at
100 °C for 72 h. After cooling to r.t., and reduction of the volume,
the oily residue was purified recrystallization from THF–MeOH.
The white solid thus obtained was washed with THF and dried in
vacuo at 70–80 °C to give 2.58 g (69%) of the title compound; mp
272–274 °C; [a]D25 –124 (c = 1.0, CHCl3).
IR (KBr): 1438, 1175, 1121, 711, 542 cm–1.
1H NMR (400 MHz, CDCl3): d = 2.82–2.93 (m, 3 H, CH2), 2.99
(ddd, 1 H, J = 13.7, 10.2, 6.1 Hz, CH2), 3.14–3.22 (m, 2 H, CH2),
31P NMR (121 MHz, CDCl3): d = 26.12 (s).
MS (ESI, pos.): m/z (%) = 593 (M+, 17), 422 (29), 421 (100).
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Rhodium-Catalyzed 1,2-Addition of Arylboronic Acids to Aldehydes
433
MS (ESI, neg.): m/z (%) = 81 (100), 79 (82).
s, 1 H, mesityl-CH), 7.12 (d, 1 H, J = 1.1 Hz, PCp-CH), 7.10 (t, 1
H, J = 1.7 Hz, NCH=CH), 7.39–7.44 [m, 2 H, P(O)Ph2-CH], 7.46–
7.53 [m, 4 H, P(O)Ph2-CH], 7.64–7.73 [m, 4 H, P(O)Ph2-CH], 7.91
(t, 1 H, J = 1.8 Hz, NCH=CH), 10.25 (br s, 1 H, NCHN).
Anal. Calcd for C40H38BrN2OP·0.25H2O: C, 70.85; H, 5.72; N,
4.13. Found: C, 70.93; H, 5.88, N, 3.95.
(–)-(R,Rp)-3-(4-Diphenylphosphinoyl[2.2]paracyclophan-12-yl-
methyl)-1-(1-phenylethyl)imidazolium Bromide [(R,Rp)-8c]
The compound was prepared by the method described for the syn-
thesis of imidazolium salt 8a using bromide 7 (0.25 g, 0.50 mmol),
(R)-1-(1-phenylethyl)imidazole (0.172 g, 1.0 mmol), and DMF (2
mL). After cooling to r.t., a crude solid was precipitated from the re-
action mixture by addition of Et2O. It was purified by an alternate
process of dissolving in CH2Cl2 and precipitation with Et2O. After
drying in vacuo at 70–80 °C, (R,Rp)-8c (0.286 g, 83%) was obtained
as a white solid; mp 150 °C (dec.); [a]D25 –103 (c = 0.8, CHCl3).
13C NMR (100 MHz, CDCl3): d = 17.6, 17.8, 21.0 (CH3), 33.1, 33.8,
34.7 (CH2), 34.9 (d, JP,C = 4.6 Hz, CH2), 51.3 (CH2N), 122.5, 123.5
(NCH=CH), 128.3 [d, 2 C, JP,C = 12.2 Hz, P(O)Ph2-CH], 128.7 [d,
2 C, JP,C = 11.4 Hz, P(O)Ph2-CH], 129.5, 129.6 (mesityl-CH), 129.7
(d, JP,C = 104.5 Hz, qC), 130.6 (qC), 130.7 [d, 2 C, JP,C = 9.2 Hz,
P(O)Ph2-CH], 131.3 [d, 2 C, JP,C = 9.2 Hz, P(O)Ph2-CH], 131.4 [d,
JP,C = 2.3 Hz, P(O)Ph2-CH], 131.8 [d, JP,C = 2.3 Hz, P(O)Ph2-CH],
132.3, 132.5 (qC), 133.1 (PCp-CH), 133.2 (d, JP,C = 13.0 Hz, PCp-
CH), 134.0 (qC), 134.2 (PCp-CH), 134.8 (d, JP,C = 103.7 Hz, qC),
134.7 (PCp-CH), 135.7 (d, JP,C = 11.5 Hz, PCp-CH), 137.5 (d,
JP,C = 3.1 Hz, PCp-CH), 137.6 (2 C, NCHN, qC), 139.0 (qC), 139.9
(d, JP,C = 12.9 Hz, qC), 140.4, 140.9 (qC), 144.9 (d, JP,C = 8.4 Hz,
qC).
IR (KBr): 2973, 2932, 1437, 1179, 1115, 708, 560, 541 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.04 [s, 3 H, J = 6.9 Hz,
NCH(Ph)CH3], 2.80–2.96 (m, 3 H, CH2), 3.00–3.06 (m, 1 H, CH2),
3.13–3.30 (m, 3 H, CH2), 3.75–3.83 (m, 1 H, CH2), 4.71 (d, 1 H,
J = 14.1 Hz, CH2N), 5.71 (d, 1 H, J = 14.4 Hz, CH2N), 5.80 [q, 1 H,
J = 7.0 Hz, NCH(Ph)CH3], 6.55–6.62 (m, 4 H, PCp-CH), 6.67 (dd,
1 H, J = 7.9, 1.2 Hz, PCp-CH), 7.08 (d, 1 H, J = 1.0 Hz, PCp-CH),
7.10 (t, 1 H, J = 1.8 Hz, NCH=CH), 7.32–7.56 (m, 12 H, P(O)Ph2-
CH, Ph-CH, NCH=CH), 7.61–7.71 [m, 4 H, P(O)Ph2-CH], 10.79
(br s, 1 H, NCHN).
31P NMR (162 MHz, CDCl3): d = 27.27 (s).
MS (SIMS-FAB, pos.): m/z (%) = 608 (45), 607 (M+, 100), 535
(25), 534 (69), 532 (20), 420 (17), 294 (14).
MS (SIMS-FAB, neg.): m/z (%) = 81 (99), 79 (100).
Anal. Calcd for C41H40BrN2OP·2H2O: C, 68.05; H, 6.13; N, 3.87.
Found: C, 68.10; H, 6.07; N, 3.82.
13C NMR (100 MHz, CDCl3): d = 21.3 [NCH(Ph)CH3], 33.1, 33.8,
34.6 (CH2), 34.8 (d, JP,C = 4.6 Hz, CH2), 51.0 (CH2N), 60.0
[NCH(Ph)CH3], 120.2, 122.2 (NCH=CH), 126.8 (2 C, Ph-CH),
128.3 [d, 2 C, JP,C = 12.2 Hz, P(O)Ph2-CH], 128.7 [d, 2 C,
JP,C = 12.2 Hz, P(O)Ph2-CH], 129.17 (Ph-CH), 129.2 (2 C, Ph-CH),
129.8 (d, JP,C = 106.1 Hz, qC), 130.7 [d, 2 C, JP,C = 9.9 Hz,
P(O)Ph2-CH], 131.3 [d, 2 C, JP,C = 9.2 Hz, P(O)Ph2-CH], 131.4 [d,
JP,C = 2.3 Hz, P(O)Ph2-CH], 131.8 (d, JP,C = 103.8 Hz, qC), 131.84
[d, JP,C = 2.3 Hz, P(O)Ph2-CH], 132.0 (qC), 133.1 (d, JP,C = 13.0
Hz, PCp-CH), 133.2 (PCp-CH), 134.2 (PCp-CH), 134.8 (d,
JP,C = 103.8 Hz, qC), 135.2 (PCp-CH), 135.8 (d, JP,C = 12.2 Hz,
PCp-CH), 136.3 (NCHN), 137.4 (d, J = 3.1 Hz, PCp-CH), 137.8,
138.6 (qC), 139.6 (d, JP,C = 13.0 Hz, qC), 140.5 (qC), 145.0 (d,
JP,C = 8.4 Hz, qC).
(–)-(Rp)-4-Bromo-12-hydroxy[2.2]paracyclophane [(Rp)-9]
To a solution of (Rp)-pseudo-ortho-dibromo-[2.2]paracyclophane
(4; 9.0 g, 24.6 mmol) in THF (200 mL) was slowly added a solution
of n-BuLi in hexane (17.0 mL, 27.2 mmol) at –78 °C. After 30 min,
B(OMe)3 (8.3 mL, 74.05 mmol) was added and the mixture was
stirred overnight at –78 °C to r.t. Subsequently, aq 10 M NaOH (0.6
mL, 6.0 mmol) and aq 30% H2O2 (12.6 g, 0.112 mol) were added
under cooling and the mixture was stirred for 1 h. After addition of
aq sat. NH4Cl solution and extraction with CH2Cl2, the combined
organic phases were dried (MgSO4) and concentrated. Purification
of the residue by flash chromatography (pentane–CH2Cl2, 3:7)
yielded 6.905 g (93%) of (Rp)-9 as a white solid; mp 149–153 °C;
[a]D25 –23 (c = 1.2, CHCl3).
IR (KBr): 3430, 2930, 1574, 1478, 1433, 1416, 1390, 1246, 1123,
1094, 1031, 867, 708, 672, 658 cm–1.
31P NMR (121 MHz, CDCl3): d = 26.00 (s).
MS (ESI, pos.): m/z (%) = 593 (M+, 17), 422 (24), 421 (100), 304
1H NMR (400 MHz, CDCl3): d = 2.58 (ddd, 1 H, J = 13.5, 9.8, 7.3
Hz, CH2), 2.76 (ddd, 1 H, J = 13.2, 10.3, 6.2 Hz, CH2), 2.87–3.02
(m, 4 H, CH2), 3.30–3.39 (m, 2 H, CH2), 4.90 (s, 1 H, OH), 6.21 (dd,
1 H, J = 7.7, 1.7 Hz, PCp-CH), 6.24 (d, 1 H, J = 1.7 Hz, PCp-CH),
6.38 (dd, 1 H, J = 7.7, 1.7 Hz, PCp-CH), 6.43 (d, 1 H, J = 7.7 Hz,
PCp-CH), 6.49 (d, 1 H, J = 8.0 Hz, PCp-CH), 7.06 (d, 1 H, J = 1.6
Hz, PCp-CH).
13C NMR (100 MHz, CDCl3): d = 31.7, 33.3, 33.5, 35.9 (CH2),
118.0 (PCp-CH), 125.3 (PCp-CH), 125.8, 126.5 (qC), 132.3 (PCp-
CH), 132.5 (PCp-CH), 135.1 (PCp-CH), 136.0 (PCp-CH), 138.4,
142.3, 142.4, 153.9 (qC).
(12), 303 (27), 185 (13), 117 (12), 105 (14).
MS (ESI, neg.): m/z (%) = 81 (92), 79 (100).
Anal. Calcd for C40H38BrN2OP·0.5H2O: C, 70.38; H, 5.76; N, 4.10.
Found: C, 70.65; H, 6.11; N, 3.92.
(–)-(Rp)-3-(4-Diphenylphosphinoyl[2.2]paracyclophan-12-yl-
methyl)-1-(2,4,6-trimethylphenyl)imidazolium Bromide
[(Rp)-8d]
The compound was prepared by the method described for the syn-
thesis of imidazolium salt 8a using bromide 7 (0.5 g, 1.03 mmol),
mesitylimidazole (0.373 g, 2.0 mmol), and DMF (5 mL). After
cooling to r.t., a crude solid was precipitated from the reaction mix-
ture by addition of Et2O. Purification by flash chromatography
(EtOAc–EtOH, 2:1) and drying in vacuo at 70–80 °C gave 0.246 g
(35% yield) of (Rp)-8d as a white solid; mp 190–193 °C (dec.);
[a]D25 –41 (c = 1.0, CHCl3).
MS (EI, 70 eV): m/z (%) = 304 (M+, 20), 302 (M+, 22), 120 (100).
Anal. Calcd for C16H15BrO: C, 63.38; H, 4.99. Found: C, 63.66; H,
5.02.
(–)-(Rp)-4-Bromo-12-methoxy[2.2]paracyclophane [(Rp)-10]
A suspension of phenol 9 (2.58 g, 8.51 mmol), K2CO3 (4.7 g, 34.0
mmol) and MeI (5.3 mL, 85.1 mmol) in anhyd acetone (130 mL)
was refluxed for 48 h. After 24 h, an additional amount of MeI (5.3
mL) was added. The volatiles were removed in vacuo and the resi-
due was partitioned between H2O (100 mL) and CH2Cl2 (100 mL).
The aqueous phase was washed with CH2Cl2 and the combined or-
ganic phase was dried (MgSO4) and concentrated. Purification of
the residue by flash chromatography (pentane–Et2O, 10:1) yielded
IR (KBr): 3021, 2928, 2860, 1546, 1437, 1177, 1114, 709, 560, 541
cm–1.
1H NMR (400 MHz, CDCl3): d = 1.97 (s, 3 H, CH3), 2.14 (s, 3 H,
CH3), 2.32 (s, 3 H, CH3), 2.84–2.94 (m, 4 H, CH2), 3.12–3.18 (m, 3
H, CH2), 3.24–3.30 (m, 1 H, CH2), 3.89–3.95 (m, 1 H, CH2), 4.83
(d, 1 H, J = 14.2 Hz, CH2N), 6.26 (d, 1 H, J = 14.3 Hz, CH2N),
6.60–6.71 (m, 5 H, PCp-CH), 6.96 (br s, 1 H, mesityl-CH), 6.99 (br
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T. Focken et al.
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2.36 g (87%) of (Rp)-10 as a white solid; mp 181 °C; [a]D25 –13
(c = 0.011, CHCl3).
1H NMR (300 MHz, CDCl3): d = 2.57 (ddd, 1 H, J = 12.9, 10.1, 7.0
Hz, CH2), 2.83–3.17 (m, 5 H, CH2), 3.35–3.48 (m, 2 H, CH2), 3.65
(s, 3 H, CH3), 4.20 (d, 1 H, J = 10.2 Hz, CH2Br), 4.54 (d, 1 H,
J = 10.1 Hz, CH2Br), 5.78 (br s, 1 H, PCp-CH), 6.26 (dd, 1 H,
J = 7.7, 1.8 Hz, PCp-CH), 6.40 (dd, 1 H, J = 7.7, 1.8 Hz, PCp-CH),
6.44 (d, 1 H, J = 7.6 Hz, PCp-CH), 6.49 (d, 1 H, J = 7.7 Hz, PCp-
CH), 6.64 (br s, 1 H, PCp-CH).
13C NMR (100 MHz, CDCl3): d = 31.7, 33.0, 33.4, 33.5, 34.5 (CH2,
CH2Br), 54.8 (CH3), 114.1 (PCp-CH), 124.5 (PCp-CH), 127.8 (qC),
130.7, 134.3, 135.6, 135.7 (PCp-CH), 135.8, 137.7, 141.2, 142.1,
157.8 (qC).
IR (KBr): 2946, 2925, 1589, 1567, 1495, 1462, 1408, 1387, 1248,
1144, 1034, 859 cm–1.
1H NMR (400 MHz, CDCl3): d = 2.57 (ddd, 1 H, J = 13.0, 10.2, 7.0
Hz, CH2), 2.78 (ddd, 1 H, J = 13.3, 10.3, 6.7 Hz, CH2), 2.91–3.12
(m, 4 H, CH2), 3.38–3.45 (m, 2 H, CH2), 3.81 (s, 3 H, CH3), 6.23
(dd, 1 H, J = 7.7, 1.1 Hz, PCp-CH), 6.38–6.41 (m, 2 H, 7-H, PCp-
CH), 6.46 (d, 1 H, J = 7.7 Hz, PCp-CH), 6.48 (d, 1 H, J = 7.4 Hz,
PCp-CH), 6.85 (d, 1 H, J = 1.4 Hz, PCp-CH).
13C NMR (100 MHz, CDCl3): d = 32.2, 33.5, 33.6, 36.0 (CH2), 54.7
(CH3), 112.8 (PCp-CH), 124.2 (PCp-CH), 126.1, 127.3 (qC), 132.2
(PCp-CH), 132.7 (PCp-CH), 135.1, 135.2 (PCp-CH), 138.2, 142.4,
142.6, 157.7 (qC).
MS (EI, 70 eV): m/z (%) = 332 (M+, 49), 331 (11), 330 (M+, 53),
252 (15), 251 (72), 135 (10), 134 (100), 117 (14), 115 (18), 104
(38), 103(14).
Anal. Calcd for C18H19BrO: C, 65.27; H, 5.78. Found: C, 65.42; H,
6.12.
MS (EI, 70 eV): m/z (%) = 318 (M+, 22), 316 (M+, 23), 134 (100),
104 (27), 103 (11).
Anal. Calcd for C17H17BrO: C, 64.37; H, 5.40. Found: C, 64.55; H,
5.49.
(–)-(Rp)-1-(4-Methoxy[2.2]paracyclophan-12-ylmethyl)-3-
methylimidazolium Bromide [(Rp)-13a]
This compound was prepared by the method described for the syn-
thesis of imidazolium salt 8a using bromide 12 (0.33 g, 1.0 mmol),
1-methylimidazole (0.16 mL, 2.0 mmol), and DMF (1 mL). After
cooling to r.t., a crude solid was precipitated from the reaction mix-
ture upon addition of Et2O. It was purified by a recurring process of
dissolving in CH2Cl2 and precipitation with Et2O. After drying in
vacuo at 70–80 °C, (Rp)-13a (0.42 g, 99%) was obtained as an off-
white solid; mp 192–194 °C; [a]D25 –29 (c = 1.0, CHCl3).
(–)-(Rp)-12-Hydroxymethyl-4-methoxy[2.2]paracyclophane
[(Rp)-11]
To a solution of methoxyparacyclophane 10 (2.3 g, 7.25 mmol) in
THF (50 mL) was added slowly a solution of n-BuLi in hexane (9.5
mL, 15.2 mmol) at –78 °C. After stirring for 1 h at –78 °C, the so-
lution was transferred using a canula to a suspension of paraformal-
dehyde (1.1 g, 36.6 mmol) in THF (50 mL) at 0 °C. The mixture was
stirred overnight at 0 °C to r.t. After addition of aq sat. NH4Cl solu-
tion and extraction with CH2Cl2, the combined organic phases were
dried (MgSO4) and concentrated. Purification of the residue by
flash chromatography (pentane–Et2O, 4:1 to 2:1) gave 1.23 g (63%)
of alcohol (Rp)-11 as a white solid; mp 175–176 °C; [a]D25 –17
(c = 1.1, CHCl3).
IR (KBr): 2924, 1569, 1460, 1409, 1249, 1162, 1027 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.59 (ddd, 1 H, J = 12.8, 9.9, 7.1
Hz, CH2), 2.76–2.87 (m, 1 H, CH2), 2.96–3.13 (m, 3 H, CH2), 3.19–
3.31 (m, 2 H, CH2), 3.37–3.44 (m, 1 H, CH2), 3.72 (s, 3 H, OCH3),
4.03 (s, 3 H, NCH3), 5.04 (d, 1 H, J = 14.3 Hz, CH2N), 5.89 (d, 1 H,
J = 14.1 Hz, CH2N), 5.98 (br s, 1 H, PCp-CH), 6.25 (d, 1 H, J = 7.6
Hz, PCp-CH), 6.44 (d, 1 H, J = 7.6 Hz, PCp-CH), 6.49 (d, 1 H,
J = 7.7 Hz, PCp-CH), 6.55 (d, 1 H, J = 7.6 Hz, PCp-CH), 6.72 (br
s, 1 H, PCp-CH), 7.00 (br s, 1 H, NCH=CH), 7.46 (br s, 1 H,
NCH=CH), 10.32 (br s, 1 H, NCHN).
13C NMR (100 MHz, CDCl3): d = 31.6, 33.1, 33.4, 34.6 (CH2), 37.1
(NCH3), 53.0 (CH2N), 55.7 (OCH3), 114.8 (PCp-CH), 121.9, 123.7
(NCH=CH), 124.5 (PCp-CH), 127.6 (qC), 131.3 (PCp-CH), 135.2
(PCp-CH), 135.4 (PCp-CH), 136.0 (PCp-CH), 137.1 (2 C, qC,
NCHN), 138.2, 142.1, 142.3, 157.9 (qC).
IR (KBr): 3186, 2992, 2931, 2861, 2829, 1594, 1568, 1496, 1462,
1409, 1250, 1144, 1113, 1040, 865 cm–1.
1H NMR (400 MHz, CDCl3): d = 1.72 (s, 1 H, OH), 2.56 (ddd, 1 H,
J = 13.1, 10.2, 6.9 Hz, CH2), 2.76–2.91 (m, 2 H, CH2), 2.96–3.09
(m, 3 H, CH2), 3.28–3.41 (m, 2 H, CH2), 3.62 (s, 3 H, CH3), 4.32 (d,
1 H, J = 12.9 Hz, OH), 4.66 (d, 1 H, J = 12.9 Hz, OH), 5.78 (d, 1 H,
J = 0.8 Hz, PCp-CH), 6.23 (dd, 1 H, J = 7.7, 1.4 Hz, PCp-CH), 6.37
(dd, 1 H, J = 7.7, 1.1 Hz, PCp-CH), 6.43 (d, 1 H, J = 7.7 Hz, PCp-
CH), 6.46 (d, 1 H, J = 7.6 Hz, PCp-CH), 6.67 (br s, 1 H, PCp-CH).
13C NMR (100 MHz, CDCl3): d = 31.8, 32.8, 33.6, 34.4 (CH2), 54.6
(CH3), 64.6 (CH2OH), 113.6 (PCp-CH), 124.4 (PCp-CH), 127.8
(qC), 128.0 (PCp-CH), 132.7 (PCp-CH), 135.1, 135.5 (PCp-CH),
136.7, 139.4, 141.0, 142.1, 157.8 (qC).
MS (ESI, pos.): m/z (%) = 334 (12), 333 (M+, 66), 252 (16), 251
(100), 219 (31), 204 (16), 117 (66), 115 (19).
MS (ESI, neg.): m/z (%) = 81 (97), 79 (100).
MS (EI, 70 eV): m/z (%) = 269 (14), 268 (M+, 69), 135 (51), 134
(100), 119 (23), 105 (17), 104 (40), 103 (13).
Anal. Calcd for C22H25BrN2O: C, 63.93; H, 6.10; N, 6.78. Found: C,
63.95; H, 6.25; N, 6.80.
Anal. Calcd for C18H20O2: C, 80.56; H, 7.51. Found: C, 80.49; H,
7.69.
(–)-(S,Rp)-1-(4-Methoxy[2.2]paracyclophan-12-ylmethyl)-3-(1-
phenylethyl)imidazolium Bromide [(S,Rp)-13b]
This compound was prepared by the method described for the syn-
thesis of imidazolium salt 8a using bromide 12 (0.33 g, 1.0 mmol),
(S)-1-(1-phenylethyl)imidazole (0.26 g, 1.5 mmol), and DMF (1
mL). It was purified as described for imidazolium salt (S,Rp)-13b.
Thus, the product (0.45 g, 89%) was obtained as a white solid; mp
116–120 °C (dec.); [a]D25 –14 (c = 1.0, CHCl3).
(+)-(Rp)-12-Bromomethyl-4-methoxy[2.2]paracyclophane
[(Rp)-12]
PBr3 (0.4 mL, 4.21 mmol) was added dropwise to a solution of al-
cohol 11 (1.13 g, 4.21 mmol) and pyridine (0.1 mL, 1.24 mmol) in
CH2Cl2 (40 mL) at 0 °C. The mixture was stirred overnight at r.t. It
was then diluted with CH2Cl2 and washed with ice-cold, aq sat.
NaHCO3 solution and brine. After drying (MgSO4) and concentra-
tion of the organic phase, the residue was purified by flash chroma-
tography (pentane–Et2O, 8:1). Thus, 1.14 g (82%) of (Rp)-12 was
obtained as a white solid; mp 89–90 °C; [a]D25 +102 (c = 0.8,
CHCl3).
IR (KBr): 2931, 2857, 1593, 1565, 1547, 1495, 1455, 1410, 1250,
1152, 1034, 712 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.02 [d, 3 H, J = 6.9 Hz,
NCH(Ph)CH3], 2.58 (ddd, 1 H, J = 12.9, 10.0, 7.1 Hz, CH2), 2.78–
2.88 (m, 1 H, CH2), 2.95–3.14 (m, 3 H, CH2), 3.24–3.33 (m, 2 H,
CH2), 3.37–3.44 (m, 1 H, CH2), 3.71 (s, 3 H, OCH3), 5.14 (d, 1 H,
J = 14.1 Hz, CH2N), 5.80 [q, 1 H, J = 6.9 Hz, NCH(Ph)CH3], 5.93
IR (KBr): 2926, 1493, 1460, 1408, 1246, 1212, 935 cm–1.
Synthesis 2005, No. 3, 429–436 © Thieme Stuttgart · New York

PAPER
Rhodium-Catalyzed 1,2-Addition of Arylboronic Acids to Aldehydes
435
(d, 1 H, J = 14.4 Hz, CH2N), 6.01 (br s, 1 H, PCp-CH), 6.24 (d, 1 H,
J = 7.7 Hz, PCp-CH), 6.43 (br d, 1 H, J = 7.5 Hz, PCp-CH), 6.49 (br
d, 1 H, J = 7.9 Hz, PCp-CH), 6.54 (d, 1 H, J = 7.9 Hz, PCp-CH),
6.72 (br s, 1 H, PCp-CH), 6.84 (br s, 1 H, NCH=CH), 7.16 (d, 1 H,
J = 4.2 Hz, NCH=CH), 7.32–7.39 (m, 5 H, Ph-CH), 10.92 (br s, 1
H, NCHN).
ganic layers were dried (MgSO4), concentrated under reduced pres-
sure and the residue was purified by flash chromatography on silica
gel (pentane–Et2O, 4:1). The enantiomer ratios were determined by
HPLC.
(4-Chlorophenyl)phenylmethanol (3a)
Chiracel OB-H, heptane–propan-2-ol = 9:1, 0.5 mL/min, 230 nm,
30 °C; tR = 25.7 min (R), 33.6 min (S).
13C NMR (100 MHz, CDCl3): d = 21.6 [NCH(Ph)CH3], 31.6, 33.1,
33.4, 34.6 (CH2), 53.0 (CH2N), 55.6 (OCH3), 60.4 [NCH(Ph)CH3],
114.9 (PCp-CH), 120.8, 121.7 (NCH=CH), 124.4 (PCp-CH), 127.1
(2 C, Ph-CH), 127.5 (qC), 129.67 (Ph-CH), 129.7 (2 C, phenyl-CH),
131.3 (qC), 131.4 (PCp-CH), 135.1 (PCp-CH), 135.4 (PCp-CH),
136.0 (PCp-CH), 136.4 (NCHN), 138.2, 138.3, 142.1, 142.4, 158.4
(qC).
(2,4,6-Trimethylphenyl)phenylmethanol (3b)
Chiracel OD-H, heptane–propan-2-ol = 98:2, 0.4 mL/min, 254 nm,
30 °C; tR = 33.6 min (R), 37.8 min (S).
(2-Methoxyphenyl)phenylmethanol (3c)
Chiracel OD, heptane–propan-2-ol = 98:2, 0.8 mL/min, 254 nm,
20 °C; tR = 47.8 min (S), 53.2 min (R).
MS (ESI, pos.): m/z (%) = 424 (23), 423 (M+, 71), 252 (19), 251
(100), 219 (22), 204 (11), 117 (26), 105 (10).
MS (ESI, neg.): m/z (%) = 81 (82), 79 (100).
(3-Methoxyphenyl)phenylmethanol (3d)
Chiracel OD, heptane–propan-2-ol = 96:4, 0.7 mL/min, 254 nm,
20 °C; tR = 43.7 min (S), 67.5 min (R).
Anal. Calcd for C29H31BrN2O·0.25H2O): C, 68.57; H, 6.25; N, 5.51.
Found: C, 68.35; H, 6.06; N, 5.53.
(–)-(R,Rp)-1-(4-Methoxy[2.2]paracyclophan-12-ylmethyl)-3-(1-
phenylethyl)imidazolium Bromide [(R,Rp)-13c]
(4-Methoxyphenyl)phenylmethanol (3e)
Chiracel AD, heptane–propan-2-ol = 96:4, 0.7 mL/min, 254 nm,
20 °C; tR = 30.7 min (R), 33.6 min (S).
The compound was prepared by the method described for the syn-
thesis of imidazolium salt 8a using bromide 12 (0.33 g, 1.0 mmol),
(R)-1-(1-phenylethyl)imidazole (0.26 g, 1.5 mmol), and DMF (1
mL). After cooling to r.t., a crude solid was precipitated from the re-
action mixture by addition of Et2O. After purification by flash chro-
matography (CH2Cl2–MeOH, 99:1 to 95:5) and drying in vacuo at
70–80 °C, 0.47 g (93%) of (R,Rp)-13c was obtained as an off-white
solid; mp 125–128 °C (dec.); [a]D25 –23 (c = 1.0, CHCl3).
(4-Biphenyl)-phenylmethanol (3f)
Chiracel OD, heptane–propan-2-ol = 98:2, 1.0 mL/min, 254 nm,
20 °C; tR = 48.8 min (R), 53.0 min (S).
(1-Naphthyl)phenylmethanol (3g)
Chiracel OD-H, heptane–propan-2-ol = 8:2, 0.5 mL/min, 254 nm,
20 °C; tR = 17.7 min (S), 38.0 min (R).
IR (KBr): 2930, 2857, 1593, 1565, 1495, 1455, 1411, 1251, 1146,
1112, 1036, 711, cm–1.
1H NMR (300 MHz, CDCl3): d = 2.03 [d, 3 H, J = 7.1 Hz,
NCH(Ph)CH3], 2.58 (ddd, 1 H, J = 12.9, 10.0, 7.1 Hz, CH2), 2.73–
2.87 (m, 1 H, CH2), 2.95–3.10 (m, 3 H, CH2), 3.24–3.30 (m, 2 H,
CH2), 3.33–3.41 (m, 1 H, CH2), 3.70 (s, 3 H, OCH3), 5.11 (d, 1 H,
J = 14.6 Hz, CH2N), 5.79 [q, 1 H, J = 7.0 Hz, NCH(Ph)CH3], 5.97
(d, 1 H, J = 14.6 Hz, CH2N), 6.00 (br s, 1 H, PCp-CH), 6.24 (d, 1 H,
J = 7.7 Hz, PCp-CH), 6.43 (br d, 1 H, J = 7.7 Hz, PCp-CH), 6.49 (br
d, 1 H, J = 7.9 Hz, PCp-CH), 6.54 (d, 1 H, J = 7.7 Hz, PCp-CH),
6.71 (br s, 1 H, PCp-CH), 6.83 (t, 1 H, J = 1.6 Hz, NCH=CH), 7.21
(t, 1 H, J = 1.7 Hz, NCH=CH), 7.34–7.43 (m, 5 H, Ph-CH), 11.04
(br s, 1 H, NCHN).
13C NMR (100 MHz, CDCl3): d = 21.4 [NCH(Ph)CH3], 31.3, 32.9,
33.2, 34.4 (CH2), 52.7 (CH2N), 55.3 (OCH3), 60.2 [NCH(Ph)CH3],
114.6 (PCp-CH), 120.8, 121.4 (NCH=CH), 124.3 (PCp-CH), 126.9
(2 C, Ph-CH), 127.3 (qC), 129.5 (3 C, Ph-CH), 131.1 (qC), 131.2
(PCp-CH), 134.9 (PCp-CH), 135.2 (PCp-CH), 135.8 (PCp-CH),
136.1 (NCHN), 137.9, 138.1, 141.9, 142.2, 157.7 (qC).
Acknowledgment
We are grateful to the Fonds der Chemischen Industrie, the BMBF,
and the Deutsche Forschungsgemeinschaft (SFB 380) for financial
support. We also thank Umicore (formerly OMG) and Degussa AG
for the generous gift of chemicals.
References
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Addition of Arylboronic Acids to Aldehydes; General Proce-
dure
In
a
typical experiment
a
Schlenk tube was charged
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mg, 5 mol%), NaOMe (23 mg, 0.4 mmol) and anhyd DME (2 mL).
The mixture was stirred for 1 h at r.t. Then, aldehyde (0.4 mmol),
aryl boronic acid (0.8 mmol), and degassed H2O (0.5 mL) were add-
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mixture was extracted with CH2Cl2 (3 × 10 mL). The combined or-
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(22) Without addition of TMEDA, the yields were up to 10%
lower.
(23) For the preparation of 4-hydroxy[2.2]paracyclophane, see:
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(24) Preliminary attempts to use rhodium catalysts bearing
analogous planar chiral ligands obtained from oxazoline- or
phosphine-containing imidazolium salts (cf. Ref.17a or
Ref.,17b respectively) revealed a low catalytic activity of
those complexes.
(25) With the same catalyst R-configured diarylmethanols can
also be obtained. For example, starting from 1-
naphthylboronic acid and benzaldehyde, use of (S,Rp)-7b
afforded the (R)-3g with the same ee as stated in Table 2.
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G.; Herrmann, W. A.; Reisinger, C.-P.; Schwarz, J.;
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analogous to: Howarth, J.; Al-Hashimy, N. A. Tetrahedron
Lett. 2001, 42, 5777.
Synthesis 2005, No. 3, 429–436 © Thieme Stuttgart · New York