A new aspect of magnesium bromide-promoted enantio-selective aryl additions of triaryl(tetrahydrofuran)aluminum to ketones catalyzed by a titanium(IV) catalyst of trans-1,2-bis(hydroxycamphorsulfonylamino)cyclohexane

Article abstract of DOI:10.1002/adsc.200800169

A novel aspect of MgBr₂-promoted asymmetric triarylaluminum- tetrahydrofuran [AlAr₃ (THF)] additions to ketones catalyzed by a titanium catalyst of 20 mol% trans-1,2-bis(hydroxycamphorsulfonylamino) cyclohexane (2) is reported. The catalytic system works excellently for aromatic ketones with either an electron-withdrawing or an electrondonating substituent on the aromatic ring at the 2′-, 3′-, or 4′-positions, affording tertiary alcohols in excellent enantioselectivities of ≥90% ee, except for the cases of phenyl addition to 2′-methoxyacetophenone and 4-trimethylsilylphenyl (4-TMSC₆H₄) addition to acetopheneone.

Full text of DOI:10.1002/adsc.200800169

FULL PAPERS
DOI: 10.1002/adsc.200800169
A New Aspect of Magnesium Bromide-Promoted Enantio-
selective Aryl Additions of Triaryl(tetrahydrofuran)aluminum
to Ketones Catalyzed by a Titanium(IV) Catalyst of
trans-1,2-Bis(hydroxycamphorsulfonylamino)cyclohexane
Chien-An Chen,a Kuo-Hui Wu,a and Han-Mou Gaua,*
a
Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, ROC
Fax : +886-4-2286-2547; e-mail: hmgau@dragon.nchu.edu.tw
Received: March 19, 2008; Revised: May 8, 2008; Published online: June 23, 2008
Abstract: A novel aspect of MgBr2-promoted asym- cellent enantioselectivities of ꢀ90% ee, except for
metric
triarylaluminum-tetrahydrofuran
[AlAr3 the cases of phenyl addition to 2’-methoxyacetophe-
ACHTREUNG(THF)] additions to ketones catalyzed by a titanium none and 4-trimethylsilylphenyl (4-TMSC6H4) addi-
catalyst of 20 mol% trans-1,2-bis(hydroxycamphor- tion to acetopheneone.
sulfonylamino)cyclohexane (2) is reported. The cata-
lytic system works excellently for aromatic ketones
with either an electron-withdrawing or an electron- Keywords: asymmetric catalysis; disulfonamides; ke-
donating substituent on the aromatic ring at the 2’-, tones; magnesium bromide; titanium; triarylalumi-
3’-, or 4’-positions, affording tertiary alcohols in ex- nums
Introduction
discovered that AlAr
reagents in asymmetric aryl additions to aldehydes.
3ACHTREU(NG THF) compounds are effective
Asymmetric aryl additions to organic carbonyl com- The addition reactions catalyzed by the titanium cata-
pounds[1] have attracted considerable attention in lyst of 10 mol% commercially available (R)-H8-
recent years due to the great value of chiral diaryl al- BINOL are complete in only 10 min at 08C, and
cohols leading to bioactive compounds such as (R)-or- afford a wide variety of secondary alcohols including
phenadrine, (S)-carbinoxamine, (R)-cetirizine hydro- diarylmethanols in excellent enantioselectivities of
chloride, and (R)-clemastine fumarate.[2] Two synthet- >90% ee.[13] Furthermore, the AlAr
3ACHTRE(UGN THF) com-
ic protocols have been established for the synthesis of pounds have been proven to be highly efficient cou-
chiral diarylmethanols: (1) asymmetric reduction of
ketones[3] and (2) asymmetric aryl additions to alde-
hydes. After the first phenyl additions reported by
Seebach and a co-worker using the titanium-TAD-
DOLate catalyst and the highly reactive PhTiCAHTRE(UNG O-i-
Pr)3 reagent,[4] various zinc catalytic systems using a
variety of arylzinc reagents, such as ZnPh2,[5] mixtures
of ZnEt2/ZnPh2,[6] reagents generated from heating
[2b,c,7]
ZnEt2 and ArB(OH)2
or other arylboron com-
pounds,[8] and reagents from reactions of ZnCl2 and
aryllithium compounds,[9] have been demonstrated to
induce excellent stereocontrol of diarylmethanols. In
addition to the above-mentioned phenyltitanium and
arylzinc reagents, a few examples of direct arylboron
additions catalyzed by rhodium[10] or nickel[11] cata-
lysts have also been demonstrated. In contrast, orga-
noaluminum compounds are more reactive than the
zinc or boron reagents and have been applied to a va-
riety of asymmetric addition reactions.[12] Recently, we
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A New Aspect of Magnesium Bromide-Promoted Enantioselective Aryl Additions
FULL PAPERS
pling reagents with aryl bromides and chlorides cata- alyst of chiral phosphoramides has been established
lyzed by the economic Pd to induce excellent stereocontrol in ZnPh2/ZnEt2 ad-
(OAc)2/PCy3 system.[14]
AHCTREUNG
Due to the great success of asymmetric additions to ditions to ketones.[26]
[27]
aldehydes,[15] recent studies have shifted addition re-
To continue our efforts in asymmetric catalysis
actions to the more inert ketone substrates to con- and to compare AlAr3ACHTRE(UGN THF) and arylzinc reagents in
struct quaternary chiral carbon centers.[16] In contrast asymmetric aryl additions to ketones, we herein
to the synthesis of secondary alcohols, the formation report asymmetric AlAr3ACHTRE(UNG THF) additions to ketones
of tertiary alcohols cannot be achieved via the proto- catalyzed by titanium catalysts of chiral ligand 2,
col of reduction of ketones. The first asymmetric which have been used in titanium-catalyzed arylzinc
phenyl addition to ketones was reported by Fu and a additions to ketones. We surprisingly found that inor-
co-worker, who employed a catalytic system of ZnPh2 ganic salts such as MgBr2 and MgI2 were essential in
and 15 mol% (+)-DAIB (1) to afford tertiary alcohols promoting the addition reactions affording tertiary al-
with enantioselectivities up to 91% ee.[17] Walsh and cohols in high yields and excellent enantioselectivi-
co-workers synthesized trans-1,2-bis(hydroxycamphor- ties.
sulfonylamino)cyclohexane (2),[18] and the titanium
complex of 10 mol% 2 has been demonstrated to be
an excellent catalyst in asymmetric ZnPh2 additions Results and Discussion
to ketones with enantioselectivities up to 96% ee.[19]
Later, the titanium complex of ligand 2 was estab- Asymmetric phenyl additions of AlPh3ACHTRE(UGN THF) were
lished to catalyze phenyl additions to a,b-unsaturated first optimized on 2’-acetonaphthone [Eq. (1)] and the
ketones to give products in good to excellent enantio- results are listed in Table 1. When the reaction was
selectivities.[20] In contrast, the titanium catalytic
system of 3 was reported to give tertiary alcohols in
lower yields and lower enantioselectivities.[21] Yus and
co-workers demonstrated ZnPh2 addition reactions
using a titanium catalyst of 5 mol% 2 to furnish prod-
ucts in good to excellent stereocontrol.[22] They ex-
tended the reactions to aryl additions to ketones, em-
ploying arylzinc reagents generated by heating ZnEt2
and ArB(OH)2 compounds.[23] The catalytic reactions
added aryl groups to aliphatic and aromatic ketones,
affording products with enantioselectivities up to 93%
ee. Hayashi and co-workers reported a rhodium cata-
lyst of (R)-MeO-mop (4), which catalyzes asymmetric
aryl additions to the ketone group of isatin (5), and conducted under reaction conditions of 10 mol% 2,
produced products in enantioselectivities up to 91% 3.0 equiv. AlPh
ee.[24] We also reported on a titanium catalytic system which is the best performing condition for asymmetric
of (S)-BINOL that catalyzes AlAr3A(THF) additions to AlEt3 additions to aldehydes catalyzed by the titani-
a wide variety of ketones, and afforded tertiary alco- um catalyst of N-sulfonylated amino alcoholate,[28] the
hols in excellent stereocontrol.[25] Recently, a zinc cat- AlPh
3A(THF) addition reaction proceeded sluggishly
3ACHRTEU(NG THF), and 5.0 equiv. TiACHTRE(UGN O-i-Pr)4,
CTHREUNG
CTHREUNG
affording the tertiary alcohol in only a 21% yield
(entry 1) in a reaction time of 12 h. We then searched
for reasons for the slow reaction of the titanium cata-
lyst of 2 employing the AlPh
one occasion, the reaction using an impure sample of
AlPh3A(THF) gave the tertiary alcohol in quantitative
yield and a good 88% ee (entry 2). Since AlPh3A(THF)
3ACHTRE(UNG THF) reagent, and in
CTHREUNG
CHTREUNG
was prepared from a reaction of AlCl3 and phenyl-
Grignard reagent in THF, the impure sample likely
contained a trace amount of magnesium halide, which
might play a key role in the promotion of the asym-
metric catalytic reaction. Thus, we subsequently ex-
amined reactions with additions of various inorganic
metal salts. With the addition of 12 mol% MgBr2, the
reaction was still slow to afford the product in a
slightly higher 38% yield (entry 3). However, when
the amount of MgBr2 was increased to 24 mol%, both
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Chien-An Chen et al.
FULL PAPERS
Table 1. Optimization of asymmetric AlPh3ACHTRE(UNG THF) additions
of both yield and stereoselectivity. While keeping
ACTHREUNG ACHTREUGN
to 2’-acetonaphthone catalyzed by titanium(IV) complexes
of trans-1,2-bis(hydroxycamphorsulfonylamino)cyclohexane
(2).[a]
CTHREUNG
product decreased to 83% (84% ee, entry 11) and
Yield[b] ee[c]
23% (entry 12), respectively. When 3.0 equiv.
AHCTREUNG lPh3ACHERT(UNG THF) and 3.0 equiv. TiACHTRE(UGN O-i-Pr)4 were used, the
Entry AlPh
G
U
[equiv]
[%]
[%]
reaction gave the product in a low 42% yield
(entry 13). When the reaction was conducted at 258C,
the product was obtained in 100% yield and a lower
82% ee (entry 14). Solvent effects were examined and
the product in 38% yield was obtained using the
CH2Cl2 as solvent (entry 15). In THF, the reaction,
however, did not proceed at all (entry 16). When dou-
1
2
3
3.0
5.0
5.0
5.0
-
-
21
100
38
n.d.
88
n.d.
3.0[d]
3.0
MgBr2
(12)
MgBr2
(24)
MgBr2
(36)
MgCl2
(24)
MgI2 (24) 100
MgO (24) 15
ZnCl2
(24)
Zn
(24)
MgBr2
(24)
MgBr2
(24)
MgBr2
(24)
MgBr2
(24)
MgBr2
(24)
MgBr2
(24)
4
5
6
3.0
3.0
3.0
5.0
5.0
5.0
100
100
51
87
82
60
bling the quantities of 2, AlPh3ACHTREGNU(THF), TiCAHTRE(UGN O-i-Pr)4,
and MgBr2, the reaction gave the tertiary alcohol in a
quantitative yield and an excellent 92% ee (entry 17).
In the case of the titanium-TADDOLate complex-
7
8
9
3.0
3.0
3.0
5.0
5.0
5.0
72
n.d.
29
catalyzed PhTi
ACHTREUNG
53
lithium salt generated in the preparation of PhTi
ACHTREUNG
Pr)3 needed to be completely removed to ensure high
enantioselectivities of the phenylation products.[4] In
contrast, this study shows that the titanium catalytic
system of 2 using the AlPh3ACTHRE(UNG THF) reagent is promot-
ed by the MgBr2 additive.
10
11
12
13
3.0
2.5
1.5
3.0
5.0
5.0
5.0
3.0
5.0
5.0
5.0
10.0
(OTf)2 54
5
83
23
42
100
39
0
84
n.d.
n.d.
82
n.d.
-
Under the best performing conditions (those of the
entry 17 in Table 1), asymmetric AlAr3A(THF) addi-
CTHREUNG
tions to various ketones were examined [Eq. (2)] and
14[e] 3.0
15[f]
3.0
16[g] 3.0
17 h] 6.0
MgBr2
(48)
100
92
[a]
2’-Acetonaphthone/2=0.50/0.050 mmol; toluene, 7 mL.
Yields are based on H NMR spectra.
The ee values were determined by chiral columns from
[b]
[c]
1
Daicel.
Impure AlPh
258C.
In CH2Cl2.
In THF.
20 mol% ligand 2.
[d]
[e]
[f]
3ACHTREUNG(THF) was used.
the results are summarized in Table 2. In terms of ste-
reocontrol, the catalytic system worked equally well
for aromatic ketones bearing an electron-donating or
an electron-withdrawing substituent at 2’-, 3’-, or 4’-
position on the aromatic group, affording products
with enantioselectivities of ꢀ90% ee (entries 1–13),
[g]
[h]
yield and enantioselectivity increased dramatically to except for the 2’-methoxyacetophenone substrate that
100% and 87% ee (entry 4). This result is comparable gave only 18% ee (entry 8). In this study, a significant
to the 88% ee obtained from the reaction using the steric effect was observed in terms of substituted posi-
impure AlPh3ACHTREUNG(THF) sample. Further increasing tions of the substituent on the aromatic ring, and
MgBr2 to 36 mol% gave the product in 100% yield, longer reaction times were required for hindered aro-
but the enantioselectivity dropped to 82% ee matic ketones in order to furnish products in good
(entry 5).
yields. For example, reaction times of 36 h, 18 h, and
We then examined reactions employing other addi- 24 h were needed for 1’-acetonaphthone (entry 2), 2’-
tives such as MgCl2, MgI2, MgO, ZnCl2, and methylacetophenone (entry 3) and 2’-bromoacetophe-
ACHTREUNGZnACHTREUNG(OTf)2. Although the above additives, aside from none (entry 6), giving the corresponding tertiary alco-
MgO, promoted phenyl additions (entries 6–10), hols in satisfactory 80%, 85%, and 87% yields, re-
MgBr2 at 24 mol% remained the best choice in terms spectively. It is worth noting that the phenyl addition
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A New Aspect of Magnesium Bromide-Promoted Enantioselective Aryl Additions
FULL PAPERS
Table 2. Asymmetric aryl additions of AlAr3ACHTRE(UGN THF) to ketones catalyzed by the titanium(IV) catalytic system of 20 mol% 2
promoted by 48 mol% MgBr2.[a]
Entry
1
Ketone 6
Ar
Ph
Time [h]
12
Product 7
Yield[b] [%]
99
ee[c] [%]
92 (À)
6a
7a
2
6b
Ph
36
7b
80
93 (À)
3
6c
6d
6e
6f
6g
6h
6i
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
18
12
12
24
12
12
12
12
12
7c
7d
7e
7f
7g
7h
7i
85
95
98
87
97
99
99
92
98
96 (À)
97 (À)
92 (+)
98 (À)
92 (+)
18
4
5
6
7
8
9
92 (+)
90 (À)
97 (+)
10
11
6j
7j
6k
7k
12
13
6l
Ph
Ph
12
12
7l
97
99
93 (+)
92 (+)
6m
7m
14
15
6n
6o
Ph
Ph
12
12
7n
7o
97
82
52
83 (+)
16
17
6p
6q
Ph
Ph
12
12
7p
7q
87
95
81 (+)
75 (À)
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Chien-An Chen et al.
FULL PAPERS
Table 2. (Continued)
Entry
Ketone 6
Ar
Time [h]
12
Product 7
Yield[b] [%]
ee[c] [%]
90 (+)
18
6a
4-MeC6H4
7r
97
83
95
11
52
19
6s
6s
6a
6a
4-TMSC6H4
2-naphthyl
Ph
12
12
12
12
7s
81 (+)
91 (+)
-
20
7a’
7a
7a
21[d]
22[e]
Ph
22
[a]
Ketone/AlAr CHTREUNG(THF)/TiACHTRE(UGN O-i-Pr)4/2/MgBr2 =0.25/1.5/2.5/0.050/0.12 mmol; toluene, 7 mL.
Isolated yields.
The ee values were determined by chiral columns from Daicel, and the signs of optical rotations were indicated in paren-
3A
[b]
[c]
theses.
2’-Acetonaphthone/PhTi
2’-Acetonaphthone/PhTi(O-i-Pr)3/Ti
[d]
[e]
N
G
ACHTRE(UNG O-i-Pr)4/2/MgBr2 =0.25/1.5/2.5/0.050/0 mmol; toluene, 7 mL.
ACHTREUNG
to 2’-methoxyacetophenone afforded the product in a were conducted at room temperature for reaction
superb 99% yield but only a low 18% ee (entry 8). times of 6–24 h, affording tertiary alcohols in good to
The low enantioselectivity of the AlPh3ACHTRE(UGN THF) addi- excellent enantioselectivities for aromatic and a,b-un-
tion to 2’-methoxyacetophenone was also observed saturated ketones (88–96% ee) and for aliphatic ke-
with the titanium catalyst of (S)-BINOL, and this tones in 75 and 87% ee.[19] In the study by Yus et al.,
phenomenon is attributed to small differentiations of a titanium catalyst of 5 mol% 2 was employed at
both orientations of 2’-methoxyacetophenone chelat- 258C for 24 h on 4 examples of ZnPh2 additions to ar-
ing to the metal center. Phenyl additions to aliphatic omatic ketones, furnishing the desired products in
ketones and 1-acetyl-1-cyclohexene were also exam- enantioselectivities of 80 to 96% ee.[22,23] They also
ined. The resulting tertiary alcohols were obtained in demonstrated addition reactions of arylzinc reagents
good to excellent yields with good enantioselectivities which were generated from heating arylboronic with
of 75–83% ee (entries 15–17), except for the alcohol ZnEt2. The reactions produced tertiary diaryl alcohols
obtained from the linear 2-hexanone (52% ee, in low to moderate yields (31–65%) and in moderate
entry 14). In addition to the phenyl addition reactions, to excellent enantioselectivities from 64 to 93% ee. In
aryl additions to aromatic ketones were also conduct- this study, a wider variety of ketones was examined to
ed. The p-tolyl addition to 2’-acetonaphthone afford- produce products in good to excellent yields. With the
ed the product in excellent yield and 90% ee use of higher catalyst loadings of 20 mol%, phenyl ad-
(entry 18), and the 4-TMSC6H4 addition to acetophe- ditions to aromatic ketones were demmonstrated
none gave the desired product in good yield and good giving tertiary diaryl alcohols in excellent enantiose-
enantioselectivity of 81% ee (entry 19). When 2-naph- lectivities of ꢀ90% ee except the addition to 2’-me-
thyl was added to acetophenone, the product 7a’ was thoxyacetophenone. The phenyl addition to 1-acetyl-
obtained in excellent 91% ee but in reverse absolute 1-cyclohexene afforded the product in lower enantio-
configuration to the product 7a derived from the ad- selectivity, but the additions to aliphatic ketones gave
dition of phenyl to 2’-acetonaphthone (entry 20).
products in comparable enantioselectivities to results
Phenylzinc or arylzinc additions to ketones cata- from the work by Walsh and a co-worker.[19] For sub-
lyzed by the titanium catalyst of 2 have been estab- strates of 4’-bromoacetophenone (6g), 3’-(trifluorome-
lished. In the study by Walsh and a co-worker using thyl)acetophenone (6k), 4’-(trifluoromethyl)acetophe-
10 mol% titanium catalyst of 2, ZnPh2 additions to 5 none (6l), and 3-methyl-2-butanone (6o), 1-acetyl-1-
aromatic, 2 a,b-unsaturated, and 2 aliphatic ketones cyclohexene (6q), AlPh3ACHTRE(UNG THF) additions afforded
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A New Aspect of Magnesium Bromide-Promoted Enantioselective Aryl Additions
FULL PAPERS
(+)-7g, (+)-7k, (+)-7l, (+)-7o and (À)-7q. These terti- enantioselectivities of ꢀ90% ee, except for 2’-
ary alcohols have the same signs of optical rotations methoxyacetophenone. Third, longer reactions times
AHCTREUNG
as products obtained from the phenylzinc addition re- were required for ortho-substituted aromatic ketones
actions. This study shows a unique feature of the to furnish products in good yields. Fourth, the reac-
MgBr2-promoted aryl additions to ketones, and, in tions of PhTiACHTREU(NG O-i-Pr)3 additions to 2’-acetonaphthone
general, shorter reaction times of 12 h at 08C were catalyzed by the same catalyst gave the product in
enough to produce products in good to excellent low yield and low enantioselectivity suggesting that
yields in comparison to 24 h at 258C for arylzinc addi- the AlAr3ACHTRE(UNG THF) addition reactions might not proceed
tion reactions using 5 mol% catalyst. In summary, ad- via aryltitanium species. Further mechanistic studies
vantages of ZnPh2 additions to ketones are lower cat- of aryl additions to aldehydes and to ketones are cur-
alyst loadings of 5 to 10 mol% used and no additive rently underway.
required. A major disadvantage of this system is that
the arylzinc addition reactions do not give diaryl alco-
hols in satisfactory yields. In contrast, advantages of Experimental Section
the AlAr3ACHTREUNG(THF) reagents are shorter reaction times,
easy preparation of the reagents, and that the reac- General Remarks
tions can be extended to additions of different aryl
groups to ketones giving tertiary diaryl alcohols in
good to excellent enantioselectivities.
AlAr
cedure[13] and was stored under a dry nitrogen atmosphere.
Ti(O-i-Pr)4 was freshly distilled prior to use. Ligand 2 was
3ACHTRE(UNG THF) was synthesized according to the literature pro-
AHCTREUNG
synthesized according to the literature procedure.[18] MgBr2
was obtained from Strem. Ketones were purchased from
Acros and Lancaster. Solvents were dried by heating under
refluxing for at least 24 h over P2O5 (dichloromethane) or
sodium/benzophenone (toluene and THF). All catalytic re-
actions were carried out under a dry nitrogen atmosphere.
1H NMR (400 MHz) and 13C (100 MHz) spectra were ob-
In studies of titanium-catalyzed asymmetric organo-
zinc or organoaluminum additions to aldehydes, a re-
action mechanism via addition of organotitanium spe-
cies to the carbonyl carbon has been established.[4,13,29]
To verify if the aryl additions of AlAr3ACHTRE(UGN THF) to ke-
tones proceeded via aryltitanium species in this tri-
metallic reaction system, the catalytic reactions were
conducted under the same reaction conditions except
replacing AlPh3ACHTREUNG(THF) with PhTiACHTRE(UNG O-i-Pr)3. In the ab-
sence of MgBr2, the reaction gave the product in only
11% yield (Table 2, entry 21) which is even lower
1
tained on a Varian Mercury-400 spectrometer, and the H
and 13C NMR chemical shifts were measured relative to tet-
ramethylsilane at 0.0 ppm as the internal reference. Optical
rotations were determined on a Perkin–Elmer 241 polarime-
ter. High resolution molecular masses of tertiary alcohols
were determined by a Finnigan MAT95XL spectrometer.
Enantiomeric excesses of tertiary alcohols were performed
on a Rainin Dynamaxꢁ or an Agilent 1100 HPLC system
using appropriate chiral columns from Daicel.
than the 21% yield of the AlPh3CAHTRE(UNG THF) addition reac-
tion under conditions without the addition of MgBr2
(Table 1, entry 1). With the addition of 48 mol%
MgBr2, the yield of the product improved to 52% but
the enantioselectivity was still only 22% ee (Table 2,
entry 22). The dramatic differences in reactivities and
in stereoselectivities observed for additions of
Syntheses
General Procedure for the Asymmetric Addition of
A
lPh
3A
ACHTREU(NG O-i-Pr)3 suggest that the
A
lAr
ligand
0.0221 g) and Ti
dry toluene (1 mL) at room temperature. After stirring the
mixture for 1 h at 08C, AlAr3A(THF) (1.5 mmol, 0.496 g) in
(THF) to Ketones: Under a dry nitrogen atmosphere,
(0.050 mmol, 0.0273 g), MgBr2 (0.12 mmol,
(O-i-Pr)4 (2.5 mmol, 0.75 mL) were mixed in
AlAr
3A
2
AHCTREUNG
nium catalyst of 2 might not proceed through the
same pathway as the titanium-catalyzed organozinc or
organoaluminum additions to aldehydes.
CTHREUNG
toluene (5 mL) was added. The mixture was stirred for
30 min and the resulting solution was treated with a ketone
(0.25 mmol). The mixture was allowed to react at 08C and
quenched with 1M aqueous HCl (2 mL). The aqueous phase
was then extracted with CH2Cl2 (3”10 mL). The combined
organic phase was dried over MgSO4, filtered and concen-
trated to dryness. The residue was purified by column chro-
matography to give the tertiary alcohol. The enantiomeric
excess of the product was determined by HPLC.
Conclusions
Asymmetric AlAr
lyzed by the titanium catalyst of 20 mol% trans-1,2-
bis(hydroxycamphorsulfonylamino)cyclohexane (2)
are now reported. Several important features were
demonstrated in this study. First, a novel aspect of the
inorganic salt MgBr2 as a key additive to promote the
3ACHTREU(NG THF) additions to ketones cata-
Spectroscopic Data
aryl addition of AlAr3ACHTREU(NG THF) to ketones was demon-
7a:[19a] 1H NMR (400 MHz, CDCl3): d=7.98 (d, J=1.2 Hz,
1H), 7.84–7.75 (m, 3H), 7.49–7.25 (m, 8H), 2.27 (br, 1H),
2.06 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): d=147.70,
145.22, 132.96, 132.35, 128.22, 128.18, 127.91, 127.45, 127.00,
strated. Second, the catalytic system worked excel-
lently for aromatic ketones bearing either an elec-
tron-withdrawing or an electron-donating substituent
on the aromatic group to afford tertiary alcohols in 126.08, 125.92, 124.91, 123.70, 76.31, 30.66; [a]2D5: À16.1 (c
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1631

Chien-An Chen et al.
FULL PAPERS
1.0, CH2Cl2); HPLC analysis: Chiralcel OJ, 0.46 cm ø ”
25 cm; n-hexane/2-propanol=80:20; 1.0 mLminÀ1; major:
14.5 min, minor: 17.9 min.
25 cm, n-hexane/2-propanol=80:20, 1.0 mLminÀ1, major:
6.7 min, minor: 20.7 min.
7i: 1H NMR (400 MHz, CDCl3): d=7.43–7.41 (m, 2H),
7.34–7.22 (m, 4H), 7.02–6.95 (m, 2H), 6.79 (dd, J=2.8,
8.4 Hz, 1H), 3.78 (s, 3H), 2.18 (br, 1H), 1.95 (s, 3H);
13C{1H} NMR (100 MHz, CDCl3): d=159.32, 149.69, 147.76,
129.06, 128.07, 126.87, 125.70, 118.30, 111.91, 76.04, 55.08,
30.67; HR-MS: m/z=228.1154, calcd. for C15H16O2: 228.1150
[M+]; [a]25: +9.17 (c 3.0, CH2Cl2); HPLC analysis: Chiralcel
7b: 1H NMR (400 MHz, CDCl3): d=7.90–7.80 (m, 4H),
7.49 (dd, J=7.2, 8.0 Hz, 1H), 7.38–7.32 (m, 3H), 7.27–7.18
(m, 4H), 2.45 (br, 1H), 2.06 (s, 3H); 13C{1H} NMR
(100 MHz, CDCl3): d=148.53, 142.03, 134.88, 130.66, 129.03,
128.73, 128.27, 127.27, 126.71, 125.36, 125.20,125.13, 124.62,
124.04, 77.09, 32.77; HR-MS: m/z=248.1193, calcd. for
C18H16O: 248.1202 [M+]; [a]D25: À95.4 (c 0.94, CH2Cl2);
HPLC analysis: Chiralcel OJ, 0.46 cm ø ” 25 cm, n-hexane/
2-propanol=90:10, 1.0 mLminÀ1, major: 12.9 min, minor:
18.3 min.
OD, 0.4D6 cm
ø
”
25 cm, n-hexane/2-propanol=98:2,
1.0 mLminÀ1, major: 24.3 min, minor: 28.5 min.
7j: 1H NMR (400 MHz, CDCl3): d=7.41–7.23 (m, 7H),
6.84 (d, J=8.8 Hz, 2H), 3.78 (s, 3H), 2.19 (s, 1H), 1.93 (s,
3H); 13C{1H} NMR (100 MHz, CDCl3): d=158.36, 148.23,
140.27, 128.01, 127.09, 126.73, 125.70, 113.36, 75.82, 55.14,
30.88; HR-MS: m/z=228.1150, calcd. for C15H16O2:
228.1150 [M+]; [a]D25: À14.6 (c 0.71, CH2Cl2); HPLC analysis:
Chiralcel OJ, 0.46 cm ø ” 25 cm, n-hexane/2-propanol=
80:20, 1.0 mLminÀ1, major: 23.1 min, minor: 19.1 min.
7k:[19a] 1H NMR (400 MHz, CDCl3): d=7.76 (s, 1H), 7.53–
7.26 (m, 8H), 2.22 (br, 1H), 1.98 (s, 3H); 13C{1H} NMR
(100 MHz, CDCl3): d=149.05, 147.04, 129.42, 128.56, 128.40,
127.39, 125.79, 123.74, 123.70, 122.36, 122.33, 75.94, 30.73;
[a]2D5: +25.6 (c 3.0, CH2Cl2); HPLC analysis: Chiralcel OJ,
7c: 1H NMR (400 MHz, CDCl3): d=7.71–7.69 (m, 1H),
7.30–7.22 (m, 7H), 7.12–7.10 (m, 1H), 2.12 (br, 1H), 1.98 (s,
3H), 1.94 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): d=
147.94, 144.57, 137.10, 132.39, 128.06, 127.61, 126.54, 125.93,
125.32, 125.26, 76.73, 32.05, 21.30; HR-MS: m/z=212.1207,
calcd. for C15H16O: 212.1201; [a]2D5: À60.3 (c 1.8, CH2Cl2);
HPLC analysis: Chiralcel OJ, 0.46 cm ø ” 25 cm, n-hexane/
2-propanol=95:5, 1.0 mLminÀ1, major: 12.2 min, minor:
18.2 min.
7d: 1H NMR (400 MHz, CDCl3): d=7.80 (d, J=7.6 Hz,
1H), 7.36–7.25 (m, 8H), 3.40 (br, 1H), 1.96 (s, 3H); 13C{1H}
NMR (100 MHz, CDCl3): d=147.55, 143.63, 132.56, 131.30,
128.86, 128.12, 128.01, 126.88, 126.73, 125.24, 76.69, 29.86;
HR-MS: m/z= 232.0651, calcd. for C14H13ClO: 232.0655
[M+]; [a]D25: À44.5 (c 1.9, CH2Cl2); HPLC analysis: Chiralcel
0.46 cm
ø
”
25 cm,
n-hexane/2-propanol=99:1,
1.0 mLminÀ1, major: 21.3 min, minor: 30.4 min.
7l:[22,23] 1H NMR (400 MHz, CDCl3): d=7.56–7.54 (m,
4H), 7.42–7.26 (m, 5H), 2.21 (br, 1H), 1.97 (s, 3H); 13C{1H}
NMR (100 MHz, CDCl3): d=151.92, 147.03, 129.16, 128.84,
128.37, 127.39, 126.12, 125.81, 125.53, 125.11, 125.07, 125.04,
125.00, 122.82, 75.97, 30.56; HR-MS: m/z=266.0925, calcd.
for C15H13OF3: 266.0919 [M+]; [a]25: +24.8 (c 4.5, CH2Cl2);
HPLC analysis: Chiralcel OD-H,D 0.46 cm ø ” 15 cm, n-
hexane/2-propanol=99.5:0.5, 1.0 mLminÀ1, major: 26.8 min,
minor: 32.5 min.
OD-H, 0.46 cm ø
” 15 cm, n-hexane/2-propanol=99:1,
1.0 mLminÀ1, major: 8.2 min, minor: 9.7 min.
7e: 1H NMR (400 MHz, CDCl3): d=7.41–7.26 (m, 9H),
2.14 (br, 1H), 1.94 (s, 3H); 13C{1H} NMR (100 MHz,
CDCl3): d=147.37, 146.50, 132.64, 128.23, 128.15, 127.28,
127.14, 125.72, 75.80, 30.68; HR-MS: m/z=232.0656, calcd.
for C14H13ClO: 232.0655 [M+]; [a]D25: +8.78 (c 3.2, CH2Cl2);
HPLC analysis: Chiralcel OD-H, 0.46 cm ø ” 15 cm, n-
hexane/2-propanol=99:1, 1.0 mLminÀ1, major: 15.3 min,
minor: 17.6 min.
1
7m: H NMR (400 MHz, CDCl3): d=8.16 (d, J=8.8 Hz,
2H), 7.60 (d, J=8.8 Hz, 2H), 7.42–7.26 (m, 5H), 2.27 (br,
1H), 1.99 (s, 3H); 13C{1H} NMR (100 MHz, CDCl3): d=
155.29, 146.59, 146.41, 128.49, 127.62, 126.61, 125.72, 123.29,
75.88, 30.43; HR-MS: m/z=243.0886, calcd. for C14H13NO3:
243.0895 [M+]; [a]D25: +38.9 (c 2.6, CH2Cl2); HPLC analysis:
Chiralcel OJ, 0.46 cm ø ” 25 cm, n-hexane/2-propanol=
90:10, 1.0 mLminÀ1, major: 37.3 min, minor: 31.8 min.
7n: 1H NMR (400 MHz, CDCl3): d=7.43–7.41 (m, 2H),
7.34–7.30 (m, 2H), 7.23–7.20 (m, 1H), 1.95 (br, 1H), 1.81–
1.76 (m, 2H), 1.54 (s, 3H), 1.28–1.21 (m, 3H), 1.13–1.09 (m,
1H), 0.85–0.82 (m, 3H); 13C{1H} NMR (100 MHz, CDCl3):
d=148.09, 128.02, 126.38, 124.74, 74.63, 43.91, 30.03, 26.10,
22.96, 13.93; HPLC analysis: Chiralcel OJ, 0.46 cm ø ”
25 cm, n-hexane/2-propanol=99:1, 1.0 mLminÀ1, major:
9.3 min, minor: 13.5 min.
7f:[19a] 1H NMR (400 MHz, CDCl3): d=7.82 (dd, J=1.6,
7.6 Hz, 1H), 7.54 (dd, J=1.2, 8.0 Hz, 1H), 7.43–7.39 (m,
1H), 7.30–7.19 (m, 6H), 3.51 (br, 1H), 1.96 (s, 3H); 13C{1H}
NMR (100 MHz, CDCl3): d=147.56, 144.91, 134.88, 129.07,
128.43, 128.13, 127.27, 126.87, 125.56, 122.30, 77.43, 30.27;
[a]2D5: À43.9 (c 1.4, CH2Cl2); HPLC analysis: Chiralcel OD-
H, 0.46 cm
ø
”
15 cm, n-hexane/2-propanol=99:1,
1.0 mLminÀ1, major: 7.9 min, minor: 9.4 min.
7g:[22,23] 1H NMR (400 MHz, CDCl3): d=7.44–7.26 (m,
9H), 2.16 (br, 1H), 1.93 (s, 3H); 13C{1H} NMR (100 MHz,
CDCl3): d=147.30, 147.05, 131.13, 128.26, 127.65, 127.18,
125.72, 120.84, 75.86, 30.66; HR-MS: m/z=276.0144, calcd.
for C14H13BrO: 276.0150 [M+]; [a]D25: +7.70 (c 2.3, CH2Cl2);
HPLC analysis: Chiralcel OD-H, 0.46 cm ø ” 15 cm, n-
hexane/2-propanol=99:1, 1.0 mLminÀ1, major: 17.3 min,
minor: 20.1 min.
7o:[19a] 1H NMR (400 MHz, CDCl3): d=7.43–7.41 (m,
2H), 7.35–7.31 (m, 2H), 7.25–7.21 (m, 1H), 2.06–1.99 (m,
1H), 1.53 (s, 3H), 0.89 (d, J=6.8 Hz, 3H), 0.81 (d, J=
6.8 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): d=147.72,
127.77, 126.31, 125.20, 76.70, 38.52, 26.53, 17.35, 17.11; [a]2D5:
+3.33 (c 0.57, CH2Cl2); HPLC analysis: Chiralcel OJ,
7h: 1H NMR (400 MHz, CDCl3): d=7.45–7.42 (m, 1H),
7.32–7.17 (m, 6H), 7.04–7.00 (m, 1H), 6.87 (dd, J=1.2,
8.4 Hz, 1H), 4.67 (s, 1H), 3.56 (s, 3H), 1.84 (s, 3H); 13C{1H}
NMR (100 MHz, CDCl3): d=157.02, 149.59, 135.25, 128.73,
127.66, 126.98, 126.20, 124.80, 120.85, 112.22, 76.20, 55.52,
30.08; HR-MS: m/z=228.1158, calcd. for C15H16O2:
228.1150 [M+]; HPLC analysis: Chiralcel OJ, 0.46 cm ø ”
0.46 cm
ø
”
25 cm,
n-hexane/2-propanol=99:1,
1.0 mLminÀ1, major: 11.3 min, minor: 17.7 min.
7p:[19a] 1H NMR (400 MHz, CDCl3): d=7.42–7.39 (m,
2H), 7.35–7.31 (m, 2H), 7.25–7.21 (m, 1H), 1.75–1.56 (m,
6H), 1.53 (s, 3H), 1.20–0.95 (m, 5H); 13C{1H} NMR
1632
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Adv. Synth. Catal. 2008, 350, 1626 – 1634

A New Aspect of Magnesium Bromide-Promoted Enantioselective Aryl Additions
FULL PAPERS
(100 MHz, CDCl3): d=147.85, 127.79, 126.31, 125.28, 76.61,
48.99, 27.34, 27.17, 26.75, 26.63, 26.37; [a]2D5: +18.2 (c 0.72,
CH2Cl2); HPLC analysis: Chiralcel OJ, 0.46 cm ø ” 25 cm,
n-hexane/2-propanol=99:1, 1.0 mLminÀ1, major: 10.5 min,
minor: 12.8 min.
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4223; b) C. Bolm, N. Hermanns, J. P. Hildebrand, K.
MuÇiz, Angew. Chem. 2000, 112, 3607–3609; Angew.
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7q:[19a] 1H NMR (400 MHz, CDCl3): d=7.43–7.41 (m,
2H), 7.34–7.30 (m, 2H), 7.26–7.21 (m, 1H), 5.92–5.90 (m,
1H), 2.13–2.12 (m, 2H), 1.94–1.68 (m, 3H), 1.64 (s, 3H),
1.58–1.52 (m, 4H); 13C{1H} NMR (100 MHz, CDCl3): d=
146.80, 142.38, 127.93, 126.51, 125.25, 121.49, 76.97, 28.69,
25.15, 24.48, 22.81, 22.21; [a]25: À11.3 (c 1.1, CH2Cl2); HPLC
analysis: Chiralcel OD-H, 0D.46 cm ø ” 15 cm, n-hexane/2-
propanol=99:1, 1.0 mLminÀ1, major: 6.7 min, minor:
8.3 min.
7r: 1H NMR (400 MHz, CDCl3): d=7.96 (s, 1H), 7.83–
7.72 (m, 3H), 7.47–7.38 (m, 3H), 7.33–7.31 (m, 2H), 7.12–
7.10 (m, 2H), 2.32 (s, 3H), 2.28 (br, 1H), 2.01 (s, 3H);
13C{1H} NMR (100 MHz, CDCl3): d=145.46, 144.88, 136.68,
133.01, 132.36, 128.89, 128.24, 127.88, 127.47, 126.06, 125.89,
125.85, 124.93, 123.64, 76.22, 30.73, 20.97; HR-MS: m/z=
262.1364, calcd. for C19H18O: 262.1358 [M+]; [a]25: +0.58 (c
0.69, CH2Cl2); HPLC analysis: Chiralcel OJ, 0.D46 cm ø ”
25 cm, n-hexane/2-propanol=70:30, 1.0 mLminÀ1, major:
12.6 min, minor: 21.9 min.
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7s: 1H NMR (400 MHz, CDCl3): d=7.50–7.25 (m, 9H),
2.22 (br, 1H), 1.96 (s, 3H), 0.27 (s, 9H); 13C{1H} NMR
(100 MHz, CDCl3): d=148.48, 147.93, 138.93, 133.26, 128.15,
126.92, 125.81, 125.10, 76.17, 30.75, À1.14; HR-MS: m/z=
270.1433, calcd. for C17H22SiO: 270.1440 [M+]; [a]D25: +1.79
(c 0.28, CH2Cl2); HPLC analysis: Chiralcel OJ, 0.46 cm ø ”
25 cm, n-hexane/2-propanol=95:5, 1.0 mLminÀ1, major:
12.5 min, minor: 8.1 min.
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2007, 9, 407–414.
7a’:[19a] [a]2D5: +15.3 (c 0.73, CH2Cl2); HPLC analysis: Chir-
alcel OJ, 0.46 cm ø ” 25 cm; n-hexane/2-propanol=80:20;
1.0 mLminÀ1; major: 19.9 min, minor: 16.1 min.
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Acknowledgements
Financial support under the grant number NSC 96–2113-M-
005–007-MY3 from the National Science Council of Republic
of China is appreciated.
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