Enantioselective allylation of selected ortho-substituted benzaldehydes: A comparative study

Article abstract of DOI:10.1002/ejoc.201403034

We report a systematic study of the allylation of ortho-substituted benzaldehydes under catalysis of a Lewis base (N, Ndioxide), a Lewis acid (Keck allylation), and a Bronsted acid. ortho-Halobenzaldehydes were used as the aldehydic substrates, and specia

Full text of DOI:10.1002/ejoc.201403034

FULL PAPER
DOI: 10.1002/ejoc.201403034
Enantioselective Allylation of Selected ortho-Substituted Benzaldehydes:
A Comparative Study
[a]
[a]
[a]
[a][‡]
and Martin Kotora*[a]
ˇ
Filip Hessler, Robert Betík, Aneta Kadlcíková, Roman Belle,
Keywords: Synthetic methods / Allylation / Lewis bases / Lewis acids / Brønsted acid / Aldehydes
We report a systematic study of the allylation of ortho-substi-
tuted benzaldehydes under catalysis of a Lewis base (N,N-
dioxide), a Lewis acid (Keck allylation), and a Brønsted acid.
ortho-Halobenzaldehydes were used as the aldehydic sub-
strates, and special attention was paid to ortho-vinyl and alk-
ynyl benzaldehydes, which might serve as interesting syn-
thons for the preparation of more complex chiral compounds.
Similar enantioselectivities were achieved under catalytic
conditions. In the case of ortho-halobenzaldehydes, Lewis
base catalysis proved to be more efficient, and the highest
asymmetric induction for allylation of ortho-fluorobenz-
aldehyde reached 82% ee, which is comparable to other
used catalytic conditions. In cases of ortho-vinylbenzalde-
hyde, the Keck allylation provided the product in 88% ee. An
enantioenriched homoallylic alcohol was used as the starting
material for the synthesis of a sertraline intermediate.
Nakajima,[10] Hayashi,[11] Kocˇovský,[12] and others.[13] Our
group has also developed a series of bis(tetrahydroisoquin-
oline) based N,N-dioxides that were suitable for highly
enantioselective allylation of aryl and α,β-unsaturated
aldehydes (up to 99% ee) with a low loading of catalyst
(1 mol-%).[14]
The N-oxide-catalyzed enantioselective allylation has
been carried out predominantly with para- and, to a
lesser extent, meta-substituted benzaldehydes. Surprisingly
little attention has been paid to the systematic allylation
of ortho-substituted benzaldehydes, and only individual
Introduction
Secondary chiral homoallylic alcohols are important
synthetic building blocks that have been conveniently used
in the syntheses of various natural compounds and phar-
maceutically active substances. Out of numerous examples,
those of Fluoxetine,[1] goniothalamin,[2] argentinolactone,[3]
diospongine,[4] epicalyxin F,[5] dodonein,[6] HMG-CoA re-
ductase inhibitor FR901512,[7] and papulacandine D[8] can
be highlighted. Although there are several possible syn-
thetic approaches to chiral secondary homoallylic alcohols,
probably the most straightforward and at the same time
most flexible strategy is based on allylation of aldehydes
with a suitable allylic reagent in the presence of a chiral
catalytic system.[9]
Among these methods, the use of a Lewis-base-catalyzed
enantioselective allylation of aldehydes constitutes a vast
area for exploratory research. This methodology has
evolved over the last decade into a mature synthetic pro-
cedure that is complimentary to other allylation reactions
such as those based on catalysis by Lewis acids or transition
metals. Out of the plethora of various Lewis base catalysts,
those having the pyridine oxide activating moiety situated
within a chiral environment constitute a distinct class of
highly active catalysts that are capable of high asymmetric
induction, usually under mild reaction conditions. Various
types of N-oxide-based catalyst have been developed by
examples
have
been
occasionally
reported.[10a,11b,11c,12d,12e,14a,14d,14f,15–17] In this respect, it is
worth noting that other studies using S-oxide,[18] P-ox-
ide,[19] alcohols,[20] Brønsted acids,[21] Ti,[22] In,[23] Sn,[24]
Cr,[25] Rh,[26] Pd,[27] Bi,[28] and Ag[29] chiral catalytic systems
have also been used only superficially to address allylations
of ortho-substituted benzaldehydes. Allylations of ortho-
fluoro-, -chloro-, -bromo-, -iodo-, -methyl-, -methoxy,
-cyano, and -nitro-benzaldehydes have been reported so far.
In general, but not always, a lower asymmetric induction in
comparison with the para and meta-substituted derivatives
has been observed. As far as enantioselective allylation of
ortho-alkenyl- or alkynylbenzaldehydes is concerned, to our
knowledge, no systematic study has been undertaken, but
allylations of such compounds under racemic conditions
have been reported. These include allylation of ortho-
vinylbenzaldehyde by using a Ag-catalyzed process[30] and
allylations of ortho-alkynylbenzaldehydes with allylmagne-
sium halides.[31] A lone example of an enantioselective
methallylation of a substituted ortho-vinylbenzaldehyde was
carried out during the total synthesis of HMG-CoA re-
ductase inhibitor FR901512.[7] Recently, while working on
this project, an interesting and synthetically valuable report
[a] Department of Organic Chemistry, Faculty of Science, Charles
University in Prague,
Hlavova 8, 128 43 Praha 2, Czech Republic
E-mail: kotora@natur.cuni.cz
http://www.natur.cuni.cz/chemistry/orgchem/
kotora?set_language=en
[‡] Erasmus internship student, 2011
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403034.
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M. Kotora et al.
FULL PAPER
was published dealing with concomitant asymmetric allyl-
ation followed by ring-closing metathesis yielding cyclic
benzo-fused homoallylic alcohols with ee values up to
99%.[32]
From a synthetic point of view, this lack of interest in
broader screening is surprising, because chiral homoallylic
alcohols bearing an ortho-substituted benzene ring could be
valuable intermediates for the synthesis of interesting sub-
stances with high or at least reasonable asymmetric induc-
tion. One such molecule is the recently synthesized ana-
logue[33] of spinosine A, possessing the dihydronaphthalene
skeleton 1, as a substitute for the naturally occurring sub-
stance with the tricyclo[7.3.0.02.6]dodecane framework (Fig-
Figure 2. Catalysts (R,R)-4 and (R,S)-4.
Initially, allylation of ortho-substituted halobenzal-
ure 1). Another molecule is sertraline 2, which is an antide- dehydes was carried out. Surprisingly, one of the highest
pressant.[34]
asymmetric inductions was achieved in the case of o-fluoro-
benzaldehyde 5a: 82% ee for (R,R)-4 and 66% ee for (R,S)-
4 (Table 1, entry 1). In the case of o-chloro, and o-iodoben-
zaldehydes 5b and 5d the asymmetric induction decreased
(entries 2 and 4). Strangely, o-bromobenzaldehyde 5c did
not react at all under any conditions (entry 3). Although
speculative, the observed decrease in stereoselectivity could
be explained by the increasing atomic size of the halogen
atom. This assumption was supported by the allylation of
o-trifluoromethylbenzaldehyde 5e, which proceeded with
low enantioselectivity: 4% ee for (R,R)-4 and 22% ee for
(R,S)-4 (entry 5). Interestingly, in both cases, the same
enantiomer was obtained as the major product. In the next
step, enantioselective allylation of 2-iodo-4-methoxybenzal-
dehyde (5f), which is relevant for the synthesis of 3, was
studied. The allylation catalyzed by (R,R)-4 gave 6f in low
yield (5%) and moderate asymmetric induction (56% ee);
on the other hand, the use of (R,S)-4 proceeded with rea-
sonable yield (53%) and gave rather good enantioselectivity
(80% ee; entry 6). Attempts to increase the asymmetric in-
duction by changing the reaction conditions did not meet
with success. The reaction carried out in toluene did not
proceed (entry 7), and the reaction in dichloromethane pro-
ceeded with low enantioselectivity of 41% for (R,R)-4 and
4% for (R,S)-4 (entry 8). A change of configuration of the
product upon change of solvent was reported by us pre-
viously[14b–14h] and probably indicates a different course of
the reaction mechanism in each solvent.[35] It should also
be added that such a phenomenon is not uncommon and
has also been observed in other catalytic reactions.[36] No-
tably, in cases of allylations catalyzed by other N-oxide
bases, the reactions in tetrahydrofuran (THF) either pro-
ceeded very sluggishly or it did not proceed at all.
Figure 1. Spinosine A analogue 1 and sertraline 2.
We originally envisioned that enantioselective allylation
of a suitable ortho-substituted benzaldehyde would be a
convenient method for the introduction of a stereogenic
center and would provide a useful chiral intermediate that
would ultimately be transformed into the desired target(s).
Specifically, our goal was to synthesize the known interme-
diate 3 for the sertraline synthesis (Scheme 1). Retrosyn-
thetic analysis anticipated the following steps: (a) ring-clos-
ing metathesis of the ortho-vinyl benzyl alcohol and
(b) enantioselective allylation of ortho-vinyl benzaldehyde.
Scheme 1. Retrosynthetic analysis of sertraline 2.
Results and Discussion
The allylation of ortho-vinylbenzaldehyde (5g) in the
presence of (S,R)-4 proceeded with reasonable yields (40%)
and moderate enantioselectivity (72 and 76% ee) (entry 9).
Lewis-Base-Catalyzed Enantioselective Allylations
Given that no systematic study regarding enantioselec- Surprisingly, the reaction with 5-methoxy-2-vinylbenzalde-
tive allylation of ortho-substituted benzaldehydes 5 has been hyde (5h) did not proceed and the starting material was re-
previously undertaken, we initially screened various sub- covered unchanged (entry 10).
strates and reaction conditions. Two diastereoisomeric
Allylation of benzaldehydes 5i–n, bearing alkynyl groups
N,NЈ-dioxides, (R,R)-4 and (R,S)-4 (Figure 2), that were in the ortho position, in the presence of (R,R)-4 or (R,S)-4
able to induce a high level of asymmetric induction in the proceeded with low asymmetric induction (Table 1, entries
allylation of para-substituted benzaldehydes, were chosen as 11–16). The highest ee achieved with (R,R)-4 was 42% ee
catalysts.[14e]
for 5k and with (R,S)-4 it was 60% ee for 5m. Together,
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Enantioselective Allylation of ortho-Substituted Benzaldehydes
Table 1. Enantioselective allylation of ortho-substituted benzaldehydes with (R,R)-4 and (R,S)-4.
Entry
5
R1
R2
6
(R,R)-4[a]
Yield [%][b] er[c]
(R,S)-4[a]
ee [%][d]
Yield [%][b]
er[c]
ee [%][d]
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
5f
5f
5g
5h
5i
F
H
H
H
H
6a
6b
6c
6d
6e
6f
48
75
0
52
5
5
0
70
40
0
57
70
50
33
60
30
60
9:91
6.5:93.5
–
42:58
52:48
78:22
–
29.5:70.5
14:86
–
54:46
71:29
71:29
46:54
61:39
59:41
92.5:7.5
82 (+)-R
46 (+)-R[e]
–
16 (+)-R
4 (–)-S
56 (+)-R
–
41 (–)-S
72 (+)-R[i]
–
34
78
0
51
20
53
0
40
40
0
45
10
65
34
73
60
60
83:17
96:4
–
75.45
61.39
10.90
–
48.52
88.12
–
42.58
37.63
25.75
40.60
20.80
31.69
6.94
66 (–)-S
14 (–)-S[e]
–
30 (–)-S
22 (–)-S
80 (–)-S
Cl
Br[f]
I
CF3
I
I
I
CH=CH2
CH=CH2
CϵCTMS
CϵCH
CϵCPh
CϵC-tBu
H
OMe
OMe
OMe
H
OMe
H
OMe
OMe
OMe
OMe
7[g]
8[h]
9
6f
6f
4 (–)-R
76 (–)-S
–
16 (–)-S
27 (–)-S
50 (–)-S
20 (–)-S
60 (–)-S
38 (–)-S
88 (–)-S
6 g
6h
6i
10
11
12
13
14
15
16
17
8 (+)-R[j]
18 (+)-R[j]
42 (+)-R[j]
8 (+)-R[j]
22 (+)-R[j]
18 (+)-R[j]
85 (+)-R
5j
6j
6k
6l
6m
6n
6o
5k
5l
5m CϵCTMS
5n
5o
CϵCTBDMS OMe
H
OMe
[a] The reactions were carried out with 1 mol-% catalyst in THF at –78 °C for 20 h, unless otherwise mentioned. [b] Isolated yield.
[c] Enantiomeric ratio. The ratio numbers refer to the order of the peaks in the 19F NMR spectra. [d] The assignment of the absolute
configuration was based on the fact that it is known that compounds 6 having the sign of rotation ‘+’ possess R configuration and those
with sign ‘–’ S configuration unless otherwise mentioned. [e] These results were reported previously.[14d] [f] The reaction was attempted
several times with the purified reactants, but it did not proceed. [g] The reaction was carried out in toluene. [h] The reaction was carried
out in dichloromethane. [i] Configuration assigned by comparison with known data.[7,32] [j] The tentative assignment of configuration was
based on the results from the TRIP-PA catalyzed reactions (see the Supporting Information).
these results indicate that it is not possible to sensibly ratio- could be used as building blocks for the synthesis of struc-
nalize the influence of the substituent on the triple bond on turally more complex compounds, were chosen as model
asymmetric induction.
compounds. It should be noted that the reaction rates were
The results described above indicated that that the use of very slow in all cases and the reactions had to be carried
(R,S)-4 catalysts in allylation reactions proceeded, in gene- out for at least seven days to achieve reasonable conversion
ral, with higher asymmetric induction than those with and to obtain the corresponding homoallylic alcohols 6 in
(R,R)-4. This phenomenon has also been observed in pre- sensible isolated yields.
vious cases;[14d,14e,14g] however, exceptions have also been
In the case of allylation of aldehyde 5f to homoallylic
alcohol 6f, a mediocre asymmetric induction (60% ee) was
observed.[14d,14g] A possible, albeit speculative, explanation
could be rationalized in terms of a conformationally and
thermodynamically more favorable and more rigid transi-
tion state in which the chiral information is transferred
from the ligand into the product.
achieved under classical Keck conditions A (Table 2, en-
try 1) and also modified conditions C (entry 3).[37a–37c]
Attempts to carry out the reaction in toluene and THF
failed, as the reactions did not proceed. The use of condi-
tions B (catalyst/ligand ratio, 1:2) and C, which were re-
ported to induce higher asymmetric induction,[38] proved to
give either inferior (46% ee; Table 2, entry 2) or the same
results (60% ee, entry 3).
A similar trend was also observed during the allylation
of other aldehydes. Thus, allylation of 5g provided the cor-
responding homoallylic alcohol 6g with a nice enantio-
selectivity of 88% ee (Table 2, entry 4) under conditions A,
whereas under conditions C the enantioselectivity was only
42% ee (entry 5). In view of these results, the remaining
aldehyde 5m was allylated only using conditions A with
rather low enantioselectivity of 24% ee (entry 6).
Lewis-Acid-Catalyzed Enantioselective Allylations
Lewis-acid-catalyzed allylation is associated mainly with
the Keck protocol. The method is based on the reaction of
allyltributylstannane with carbonyl compounds catalyzed
by chiral titanium species prepared in situ from Ti(O-iPr)4
and binol. This procedure usually furnished the corre-
sponding homoallylic alcohols in high yields and enantio-
selectivities.[37]
Given that, to our knowledge, there are no reports on
enantioselective allylation of ortho-substituted benzalde-
hydes, we screened various Keck allylation protocols (Con-
The question remains how to improve the enantio-
ditions A, B, and C) to find the best conditions for high selectivity of the Keck allylation. One possible pathway
asymmetric induction. Aldehydes 5f–g, and 5m, which could rely on the use of binols possessing bulky substituents
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M. Kotora et al.
FULL PAPER
Table 2. Enantioselective allylation of ortho-substituted benzalde-
hydes by using the Keck protocol.
Table 3. Enantioselective allylation of ortho-substituted benzalde-
hydes by using (R)-TRIP PA.
Condition[a]
Yield [%][b]
er[c]
ee [%]
Entry
5
Yield [%][a] er[b]
ee [%][c]
Entry
5
1
2
3
4
5
6
7
8
5a
5d
5e
95
99
59
66
96
93
98
94
98
95
94
27:73
46 (+)-R
51 (+)-R
18 (+)-R
42 (+)-R
82 (+)-R
26 (+)-R
64 (+)-R
41 (+)-R
9 (+)-R
1
2
3
4
5
6
5f
5f
A
B
C
A
C
A
80
55
70
55
60
80
80:20
73:27
80:20
94:6
71:29
62:38
60 (+)-R
46 (+)-R
60 (+)-R
88 (+)-R
42 (+)-R
24 (+)-R
24.5:75.5
40.5:59.5
71:29
9:91
37:63
5f
5f
5g
5g
5m
5g
5i
5j
5m
5n
5o
5p[d]
82:18
[a] Reaction conditions A: Ti(OiPr)4 (10 mol-%), (S)-Binol
(10 mol-%), CH2Cl2, 4 Å MS, –20 °C, 7 d. Reaction conditions B:
Ti(OiPr)4 (10 mol-%), (S)-Binol (20 mol-%), CH2Cl2, 4 Å MS,
–20 °C, 7 d. Reaction conditions C: Ti(OiPr)4 (7.5 mol-%), TiCl4
(2.5 mol-%), (S)-Binol (10 mol-%), Ag2O (5 mol-%), 4 Å, –10 °C,
7 d. [b] Isolated yield. [c] Enantiomeric ratio. The ratio numbers re-
fer to the order of the peaks in the 19F NMR spectra.
70.5:29.5
54.5:45.5
5:95
9
10
11
90 (+)-R
65 (+)-R
82.5:17.5
[a] Isolated yield. [b] Enantiomeric ratio. The ratio numbers refer
to the order of the peaks in the 19F NMR spectra. [c] For assign-
ment of the configuration, see the Supporting Information. [d] R1
= Br, R2 = OMe.
in positions 3 and 3Ј. However, available studies that would
provide leads in this direction have, to our knowledge, not
been undertaken. Moreover, allylations of ketones under
Keck conditions with variously substituted binols suggest
that attachment of substituents in the position mentioned
above resulted in lower levels of asymmetric induction.[39]
Nevertheless, allylations of 5f were carried out under Keck
conditions A by using (R)-3,3-bis[3,5-bis(trifluormethyl)-
phenyl]binol and (R)-3,3-dibromobinol. In the former case,
the reaction did not proceed and in the latter case 30% of
the product was formed with ee of approximately 20%.
These results thus support the speculations mentioned
above on lower activity of substituted binols.
Synthesis of a Sertraline Intermediate
Given that allylation of 2-vinylbenzaldehyde 5g under
Keck conditions provided the corresponding homoallylic
alcohol 6g with a reasonable enantioselectivity (88% ee)
and isolated yield (55%), it was decided to use this substrate
for the synthesis of sertraline intermediate (Scheme 2). The
prepared compound 6g was subjected to ring-closing me-
tathesis by using Grubbs 2nd generation catalysts (G II)
(2 mol-%) under standard conditions in CH2Cl2 at 20 °C
for 48 h. The reaction proceeded smoothly and the corre-
sponding dihydronaphthalene 3 was obtained in 79% iso-
lated yield and 84% ee. Notably, a slight loss of enantiopur-
ity during ring-closing metathesis reactions have previously
been observed in this laboratory,[14e] and we do not cur-
rently have reasonable explanation for this phenomenon.
Brønsted Acid (TRIP PA) Catalyzed Enantioselective
Allylations
For comparison with the experiments described above,
allylations catalyzed by a chiral Brønsted acid, (R)-TRIP
PA,[21] were also carried out with selected benzaldehydes
(Table 3). Allylation of aldehydes 5a, 5d, 5e–g, 5i, 5j, and
5m–p, proceeded with high yields and with asymmetric in-
duction in the range of 9–90% ee with several exceptions.
In some cases these values were lower than or comparable
to those obtained with (R,R)-4 or (R,S)-4 (Table 1, entries
1, 5, 6, 15, and 16). In the case of aldehydes 5d, 5g, 5i, 5j,
and 5o, the asymmetric induction was higher than those
obtained with (R,R)-4 or (R,S)-4 (Table 1, entries 4, 9, 11,
12, 17). The lower asymmetric induction in allylation of 5g
Scheme 2. Synthesis of the sertraline intermediate 3.
Conclusions
We have shown that catalytic allylation of ortho-substi-
in comparison with previously reported results[32] might be tuted benzaldehydes under selected Lewis basic and acid
caused by the lower load of the catalyst (2.5 vs. 5 mol-%). In catalysts provides the corresponding homoallylic alcohols
this respect, the authors also observed a drop in asymmetric with reasonable enantioselectivity depending on the nature
induction. In addition, allylation of aldehyde 5p was under- of the ortho-substituent. However, it seems difficult to cor-
taken to compare this protocol with the recently reported relate the level of asymmetric induction with respect to the
Ag-catalyzed methodology.[29b] In this case, asymmetric in- steric bulk of the substituent. In addition, the obtained re-
duction was also lower (65% ee) than the reported value sults also indicate difficulties regarding the choice of a suit-
(74% ee).
able catalyst with which to achieve highly enantioselective
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Enantioselective Allylation of ortho-Substituted Benzaldehydes
raphy on silica gel afforded the title compound (13 mg, 34%) as a
colorless liquid. The spectral characteristics are in agreement with
the previously reported data.[40] 66% ee [Mosher ester: 19F NMR
(300 MHz): δ = –71.37 (83%), –71.51 (17%) ppm].
allylation. In some cases the use of N,NЈ-dioxide catalysts
gives better optical purity; in others, Keck or Brønsted acid
allylation seems to provide better results. The advantage of
the N,NЈ-dioxide catalysts is in their rather low loading,
with only 1 mol-% being sufficient to bring about the de-
sired allylation. In contrast, the Keck allylation suffers from
high catalyst loadings (usually 10 mol-%) and long reaction
times (one week). As far as TRIP PA is concerned, it has
been published that this reagent usually gives the product
with high enantiopurity, but the level of asymmetric induc-
tion depends on the catalyst loading, as has been noted by
us and by others. The optimal amount of TRIP PA to
achieve high asymmetric induction in the allylation of or-
tho-substituted benzaldehydes is 10 mol-%,[32] which can be
considered to be a high loading. In summary, although pro-
gress has been made, the development of a highly catalyti-
cally active and general system for the allylation of ortho-
substituted arylaldehydes that provides products with high
optical purity remains to be solved.
(S)-(–)-1-(2-Iodophenyl)but-3-en-1-ol (6d): Column chromatography
on silica gel afforded the title compound (32 mg, 51%) as a color-
less liquid. The spectral characteristics are in agreement with the
previously reported data.[41] 30% ee [Mosher ester: 19F NMR
(300 MHz): δ = –71.23 (75%), –71.32 (45%) ppm].
(S)-(–)-1-[2-(Trifluoromethyl)phenyl]but-3-en-1-ol (6e): Column
chromatography on silica gel afforded the title compound (10 mg,
20%) as a colorless liquid. The spectral characteristics are in agree-
ment with the previously reported data.[40] 22% ee [Mosher ester:
19F NMR (300 MHz): δ = –71.25 (61%), –71. 34 (39%) ppm].
(S)-(–)-1-(2-Iodo-5-methoxyphenyl)but-3-en-1-ol
(6f):
Column
chromatography on silica gel afforded the title compound (37 mg,
53%) as a pale solid; m.p. 28–29 °C; [α]D = –32.0 (c = 0.98, CHCl3).
1H NMR (300 MHz): δ = 2.25–2.33 (m, 2 H), 2.54–2.65 (m, 1 H),
3.79 (s, 3 H), 4.85–4.89 (m, 1 H), 5.16–5.24 (m, 2 H), 5.83–5.95 (m,
1 H), 6.58 (dd, J = 8.7, 3.0 Hz, 1 H), 7.12 (d, J = 3 Hz, 1 H), 7.65
(d, J = 8.7 Hz, 1 H) ppm. 13C NMR (75 MHz): δ = 42.3, 55.4, 76.2,
85.5, 112.7, 115.6, 118.7, 134.3, 139.7, 146.7, 160.3 ppm. IR (THF):
ν = 801, 1007, 1268, 1291, 1468, 1590, 2931, 3078 cm–1. MS: m/z
˜
Experimental Section
(%) = 123.1 (100), 283.1 (23), 301.1 (70), 341.3 (54), 368.2 (25),
418.3 (58). HRMS (ESI): m/z calcd for C11H13O2INa 326.98524;
found 326.98511. 80% ee [Mosher ester: 19F NMR (300 MHz): δ
= –71.15 (10%), –71. 27 (90%) ppm].
General: All solvents – unless otherwise stated – were used as ob-
tained. THF and Et2O were distilled from LiAlH4, DCM and Et3N
from CaH2, toluene from sodium benzophenone ketyl. All other
reagents and starting materials {e.g. (2-iodophenyl)methanol, (2-
bromo-5-methoxyphenyl)methanol, and 2-[(trimethylsilyl)ethynyl]
benzaldehyde} were obtained from commercial sources. 1H, 19F
and 13C NMR spectra were recorded with a Varian UNITY 400
(1H at 400 MHz, 13C at 100.6 MHz) and a Varian UNITY 300 (1H
and 19F at 300 MHz, 13C at 75 MHz) as solutions in CDCl3 or
C6D6 at 25 °C. Chemical shifts are given on the δ-scale, coupling
constants J are given in Hz. Melting points (uncorrected) were de-
termined by using a Kofler apparatus. Mass spectra were recorded
with a ZAB-SEQ (VG-Analytical) instrument. Infrared spectra
were recorded with a Bruker IFS 55 spectrometer as THF solutions
and are reported in wavenumbers (cm–1). Fluka 60 silica gel was
used for flash chromatography. TLC was performed on silica gel
60 F254 coated aluminum sheets (Merck). Specific rotation values
were determined with an Autopol III automatic polarimeter (Ru-
dolph Research Analytical), the samples were dissolved in CHCl3
(concentrations in g/100 mL). All reactions were carried out under
argon.
(S)-(–)-1-(2-Vinylphenyl)but-3-en-1-ol (6g): Column chromatog-
raphy on silica gel afforded the title compound (16 mg, 40%) as a
colorless liquid. The spectral characteristics are in agreement with
the previously reported data.[30] 76% ee [Mosher ester: 19F NMR
(300 MHz): δ = –71.22 (88%), –71.47 (12%) ppm].
(S)-(–)-1-{2-[2-(Trimethylsilyl)ethynyl]phenyl}but-3-en-1-ol
(6i):
Column chromatography on silica gel afforded the title compound
(24 mg, 45%) as a colorless liquid. The spectral characteristics are
in agreement with the previously reported data.[42] 16% ee [Mosher
ester: 19F NMR (300 MHz): δ = –71.22 (42%), –71.36 (58%) ppm].
(S)-(–)-1-(2-Ethynyl-5-methoxyphenyl)but-3-en-1-ol (6j): Column
chromatography on silica gel afforded the title compound (5 mg,
1
10%) as a colorless liquid. [α]D = –58 (c = 0.3, CHCl3). H NMR
(300 MHz): δ = 2.21–2.22 (m, 1 H), 2.40–2.45 (m, 1 H), 2.63–2.65
(m, 1 H), 3.26 (s, 1 H), 3.84 (s, 3 H), 5.15–5.22 (m, 3 H), 5.80–5.96
(m, 1 H), 6.77 (dd, J = 8.4, 2.7 Hz, 1 H), 7.08–7.09 (m, 1 H), 7.42
(d, J = 8.4 Hz, 1 H) ppm. 13C NMR (75 MHz): δ = 42.7, 55.3, 71.0,
80.9, 81.4, 110.7, 111.4, 113.0, 118.4, 134.3, 134.6, 148.3,
General Procedure for Allylation with N,NЈ-Dioxides: To a solution
of bis(tetrahydroisoquinoline)-N,NЈ-dioxides (R,R)-4 or (S,R)-4
(0.0023 mmol, 1 mg) in THF, toluene, or CH2Cl2 (1 mL), the corre-
sponding aldehyde (0.23 mmol) and diisopropylethylamine
(0.26 mmol, 47 μL, 35 mg) were added at –78 or –40 °C. Allyltri-
chlorosilane (0.26 mmol, 40 μL, 49 mg) was then added and the
reaction mixture was stirred for 4–20 h at the same temperature,
unless otherwise stated. The reaction was quenched with brine
(4 mL) and the mixture was extracted with EtOAc (3ϫ 5 mL), the
combined organic fractions were dried with MgSO4 and volatiles
were removed under reduced pressure. Column chromatography of
the residue on silica gel (hexane/EtOAc, 5:1) gave the product as a
yellowish liquid or colorless solid. Enantiomeric excess was deter-
mined by 19F NMR analysis of the corresponding Mosher acid
esters or by GC analysis.
160.3 ppm. IR (THF): ν = 819, 1030, 1233, 1276, 1489, 1605, 1733,
˜
2099, 2854, 2926, 3073 cm–1. MS: m/z (%) = 203.1 (100) [M+].
HRMS (ESI): m/z calcd for C13H14O2 203.10666; found 203.10660.
27% ee [Mosher ester: 19F NMR (300 MHz): δ = –71.22 (37%),
–71. 32 (63%) ppm].
(S)-(–)-1-[5-Methoxy-2-(2-phenylethynyl)phenyl]but-3-en-1-ol (6k):
Column chromatography on silica gel afforded the title compound
(34 mg, 65%) as a pale solid; m.p. 57–59 °C; [α]D = –12.6 (c = 0.33,
1
CHCl3). H NMR (300 MHz): δ = 2.25–2.27 (m, 1 H), 2.45–2.50
(m, 1 H), 2.72–2.75 (m, 1 H), 3.85 (s, 3 H), 5.16–5.28 (m, 3 H),
5.82–6.00 (m, 1 H), 6.78–6.82 (m, 1 H), 7.11–7.13 (m, 1 H), 7.32–
7.38 (m, 3 H), 7.43–7.52 (m, 3 H) ppm. 13C NMR (75 MHz): δ =
42.8, 55.4, 71.4, 87.1, 93.2, 110.7, 112.5, 113.1, 118.4, 123.5, 128.1,
128.4 (2C), 131.3 (2C), 133.6, 134.7, 147.7. 160.1 ppm. IR (THF):
Allylation of ortho-Substituted Benzaldehydes Catalyzed by (S,R)-4. ν = 686, 753, 820, 921, 1057, 1171, 1234, 1296, 1463, 1503, 1608,
˜
(S)-(–)-1-(2-Fluorophenyl)but-3-en-1-ol (6a): Column chromatog- 1895, 2210, 2837, 2924, 3048, 3276 cm–1. MS: m/z (%) = 301.1 (100)
Eur. J. Org. Chem. 2014, 7245–7252
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
7249

M. Kotora et al.
FULL PAPER
[M+]. HRMS: m/z calcd for C19H18O2Na 301.11990; found
301.11989. 50% ee [Mosher ester: 19F NMR (300 MHz): δ = –71.23
(25%), –71.29 (75%) ppm].
sure tube and heated to 50 °C for 2 h, then it was cooled to –10 °C
and the substituted benzaldehyde (1 mmol) and allyltributyltin
(360 mg, 1.1 mmol) or allenyltrimethyltin (358 mg, 1.1 mmol) were
added. The mixture was sure-sealed and left at –18 °C (freezer) for
7 d. Work-up was the same as described for Conditions A.
(S)-(–)-1-{5-Methoxy-2-[2-(tert-butyl)ethynyl]phenyl}but-3-en-1-ol
(6l): Column chromatography on silica gel afforded the title com-
pound (20 mg, 34%) as a colorless liquid. [α]D = –6.0 (c = 0.18,
(R)-(+)-1-(2-Iodo-5-methoxyphenyl)but-3-en-1-ol (6f): Condi-
tions A: yield 80%. The spectral characteristics are in agreement
with the previously reported data.[30] 60% ee [Mosher ester: 19F
NMR (300 MHz): δ = –71.17 (80%), –71.30 (20%) ppm. Condi-
tions B: yield 55%; 46% ee. Conditions C: yield 70%; 60% ee.
1
CHCl3). H NMR (300 MHz): δ = 1.24 (s, 9 H), 2.27–2.38 (m, 2
H), 2.51–2.62 (m, 1 H), 3.73 (s, 3 H), 5.00–5.14 (m, 3 H), 5.74–5.87
(m, 1 H), 5.81 (dd, J = 8.4, 2.7 Hz, 1 H), 6.95–6.96 (m, 1 H), 7.19–
7.22 (m, 1 H) ppm. 13C NMR (75 MHz): δ = 28.2, 31.1 (3C), 40.9,
42.5, 55.3, 71.4, 102.4, 110.6, 112.7, 113.2, 118.0, 133.3, 135.0,
(S)-(–)-1-(2-Vinylphenyl)but-3-en-1-ol (6g): Conditions A: yield
55%. The spectral characteristics are in agreement with the pre-
viously reported data.[30] 88 % ee [Mosher ester: 19F NMR
(300 MHz): δ = –71.24 (94%), –71.49 (6%) ppm]. Conditions C:
yield 60%; 42% ee.
147.4, 159.3 ppm. IR (THF): ν = 820, 912, 1037, 1158, 1230, 1288,
˜
1492, 1606, 2835, 2866, 2904, 2968, 3072, 3415 cm–1. MS: m/z (%)
= 281.1 (100) [M+]. HRMS: m/z calcd for C17H22O2Na 281.15120;
found 281.15108. 20% ee [Mosher ester: 19F NMR (300 MHz): δ
= –71.21 (40%), –71.23 (60%) ppm].
(R)-(+)-1-{5-Methoxy-2-[(trimethylsilyl)ethynyl]phenyl}but-3-en-1-
ol (6m): Conditions A: yield 80%. The spectral characteristics are
in agreement with the previously reported data.[42] 24% ee [Mosher
ester: 19F NMR (300 MHz): δ = –71.21 (62%), –71.26 (38%) ppm].
(S)-(–)-1-{5-Methoxy-2-[2-(trimethylsilyl)ethynyl]phenyl}but-3-en-1-
ol (6m): Column chromatography on silica gel afforded the title
compound (40 mg, 73%) as a pale solid. The spectral characteris-
tics are in agreement with the previously reported data.[42] 60% ee
[Mosher ester: 19F NMR (300 MHz): δ = –71.20 (20%), –71.25
(80%) ppm].
General Procedure for Allylboration of Aldehydes: (R)-TRIP-PA cat-
alyst (3.2 mg, 0.0042 mmol, 2.5 mol-%) and the corresponding
benzaldehyde (0.17 mmol) were dissolved in toluene (3 mL) and
cooled to –30 °C. Pinacol allylboronate (0.038 mL, 0.20 mmol) was
added dropwise and the reaction mixture was stirred for 16 h while
maintaining –30 °C, then the mixture was warmed to room tem-
perature. The solvent was evaporated under reduced pressure and
column chromatographic purification of the residue on silica gel
(hexane/EtOAc, 5:1) furnished the homoallylic alcohol as a color-
less oil. The ee values were determined by 19F NMR analysis of
the corresponding Mosher esters. The absolute configurations were
tentatively assigned primarily according to the known fact that (R)-
TRIP PA catalyzed allylation of benzaldehydes provides the corre-
sponding homoallylic alcohols with R-configuration. For other
supporting data regarding the assigned configuration, see refer-
ences in individual cases.
(S)-(–)-1-{5-Methoxy-2-[2-(tert-butyldimethylsilyl)ethynyl]phenyl}-
but-3-en-1-ol (6n): Column chromatography on silica gel afforded
the title compound (44 mg, 60%) as a colorless liquid. [α]D = –8.2
1
(c = 0.84, CHCl3). H NMR (300 MHz): δ = 0.18 (s, 6 H), 1.00 (s,
9 H), 2.25–2.27 (m, 1 H), 2.35–2.44 (m, 1 H), 2.67–2.71 (m, 1 H),
3.82 (s, 3 H), 5.15–5.30 (m, 3 H), 5.80–5.92 (m, 1 H), 6.72–6.75
(m, 1 H), 7.05–7.06 (m, 1 H), 7.37–7.40 (m, 1 H) ppm. 13C NMR
(75 MHz): δ = –4.5, 16.7, 26.1 (3C), 42.7, 55.3, 71.2, 96.3, 103.4,
110.6, 112.5, 112.9, 118.4, 134.1, 134.6, 148.2, 160.1 ppm. IR
(THF): ν = 775, 832, 1035, 1250, 1488, 1606, 2149, 2894, 2927,
˜
2951, 3077, 3406 cm–1. MS: m/z (%) = 123.1 (100), 257.0 (26), 339.2
(60) [M+]. HRMS (ESI): m/z calcd for C19H28O2NaSi: 339.17508;
found: 339.17502. 38% ee [Mosher ester: 19F NMR (300 MHz): δ
= –71.17 (31%), –71.23 (69%) ppm].
(S)-(–)-1-(3-Methoxyphenyl)but-3-en-1-ol (6o): Column chromatog-
raphy on silica gel afforded the title compound (24 mg, 60%) as a
colorless liquid. The spectral characteristics are in agreement with
the previously reported data.[43] 88% ee [GC (HP-Chiral β column;
30 mϫ0.25 mm; oven: 80 °C for 1 min, then 1 °C/min to 160 °C,
5 min at the same temperature): tR = 69.25 (+), 69.66 (–) min].
(R)-(+)-1-(2-Fluorophenyl)but-3-en-1-ol (6a): Isolated yield 95%.
The spectral[40] and optical characteristics[24] are in agreement with
the previously reported data. [α]D = +28.5 (c = 3.3, CHCl3); 46%
ee [Mosher ester: 19F NMR (300 MHz): δ = –71.38 (27%), –71.52
(73%) ppm].
(R)-(+)-1-(2-Iodophenyl)but-3-en-1-ol (6d): Isolated yield 99%. The
spectral[41] and optical[29] characteristics are in agreement with the
previously reported data. [α]D = +36.8 (c = 14.1, CHCl3); 51% ee
[Mosher ester: 19F NMR (300 MHz): δ = –71.24 (24.5%), –71.33
(75.5%) ppm].
General Procedure for Keck Allylation. Conditions A: To a solution
of Ti(OiPr)4 (0.1 mmol, 28 mg) in CH2Cl2 (5 mL), (S)-binol
(0.1 mmol, 28 mg) and 4Å molecular sieves (200 mg) were added.
The mixture was sealed in a pressure tube and heated to 50 °C for
2 h, then cooled to –10 °C and the corresponding benzaldehyde
(1 mmol) and allyltributyltin (360 mg, 1.1 mmol) or allenyltrime-
thyltin (358 mg, 1.1 mmol) were added. The pressure tube was
again sealed and left at –18 °C (freezer) for 7 d. HCl (0.5%, 20 mL)
was then added and the reaction mixture was extracted with
CH2Cl2 (3ϫ10 mL). The combined organic fractions were dried
with anhydrous Na2SO4 and volatiles were removed under reduced
pressure. Residue mixtures were purified by column chromatog-
raphy on silica gel.
(R)-(+)-1-(2-(Trifluoromethyl)phenyl)but-3-en-1-ol (6e): Isolated
yield 59%. The spectral[40] and optical characteristics are in agree-
ment with the previously reported data.[24] [α]D = +13.2 (c = 1.9,
CHCl3); 19% ee [Mosher ester: 19F NMR (300 MHz): δ = –71.25
(40.5%), –71.34 (59.5%) ppm].
(R)-(+)-1-(2-Iodo-5-methoxyphenyl)but-3-en-1-ol (6f): Isolated yield
66 %. 42 % ee [Mosher ester: 19F NMR (300 MHz): δ = –71.16
(71%), –71.29 (29%) ppm].
Conditions B: Procedure was the same as described for Condi-
tions A, but (S)-binol (0.2 mmol, 56 mg) was used.
(R)-(+)-1-(2-Vinylphenyl)but-3-en-1-ol (6g): Isolated yield 96%. The
spectral[30] and optical characteristics[32] are in agreement with the
previously reported data. Configuration was also assigned accord-
ing to analogy with a related compound.[44] [α]D = +50.2 (c = 4.3,
CHCl3); 82% ee [Mosher ester: 19F NMR (300 MHz): δ = –71.24
(9%), –71.48 (91%) ppm].
Conditions C: To a the solution of Ti(OiPr)4 (0.075 mmol, 21 mg)
and TiCl4 (0.025 mmol, 5 mg) in CH2Cl2 (5 mL), (S)-binol
(0.1 mmol, 28 mg), Ag2O (0.05 mmol, 12 mg) and 4Å molecular
sieves (200 mg) were added. The mixture was sure-sealed in a pres-
7250
www.eurjoc.org
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2014, 7245–7252

Enantioselective Allylation of ortho-Substituted Benzaldehydes
Chem. 2003, 68, 6329–6337; c) T. Shimada, A. Kina, T. Haya-
shi, Adv. Synth. Catal. 2004, 346, 1169–1174.
(R)-(+)-1-{2-[2-(Trimethylsilyl)ethynyl]phenyl}but-3-en-1-ol (6i):
Isolated yield 93%. The spectral characteristics are in agreement
with the previously reported data.[42] [α]D = +23.5 (c = 3.4, CHCl3);
26% ee [Mosher ester: 19F NMR (300 MHz): δ = –71.22 (37%),
–71.36 (63%) ppm].
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(R)-(+)-1-(2-Ethynyl-5-methoxyphenyl)but-3-en-1-ol (6j): Isolated
yield 98%. 64% ee [Mosher ester: 19F NMR (300 MHz): δ = –71.23
(82%), –71.33 (18%) ppm].
(R)-(+)-1-{5-Methoxy-2-[2-(trimethylsilyl)ethynyl]phenyl}but-3-en-
1-ol (6m): Isolated yield 94%. The spectral characteristics are in
agreement with the previously reported data.[42] 41% ee [Mosher
ester: 1 9F NMR (300 MHz): δ = –71.20 (70.5 %), –71.25
(29.5%) ppm].
[13]
[14]
(R)-(+)-1-{5-Methoxy-2-[2-(tert-butyldimethylsilyl)ethynyl]phenyl}-
but-3-en-1-ol (6n): Isolated yield 98%. 9% ee [Mosher ester: 19F
NMR (300 MHz): δ = –71.19 (54.5%), –71.24 (45.5%) ppm].
(R)-(+)-1-(3-Methoxyphenyl)but-3-en-1-ol (6o): Isolated yield 95%.
The spectral and optical[43] characteristics are in agreement with
the previously reported data. [α]D = +49.0 (c = 3.1, CHCl3); 90%
ee [Mosher ester: 19F NMR (300 MHz): δ = –71.33 (5%), –71.47
(95%) ppm].
(R)-(+)-1-(2-Bromo-5-methoxyphenyl)but-3-en-1-ol (6p): Isolated
yield 94%. Spectral and optical data are in accordance with pre-
viously published data.[29] [α]D = +48.5 (c = 2.2, CHCl3); 65% ee
[Mosher ester: 19F NMR (300 MHz): δ = –71.17 (82.5%), –71.29
(17.5%) ppm].
Supporting Information (see footnote on the first page of this arti-
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Acknowledgments
This work was supported by grants from the Czech Ministry of
Education, Youth, and Sports (MSM0021620857) and the Czech
Science Foundation (P207/11/0587).
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Received: August 5, 2014
Published Online: October 1, 2014
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