Me2Zn-Mediated Catalytic Enantio- and Diastereoselective Addition of TosMIC to Ketones

Article abstract of DOI:10.1002/chem.201504362

The first catalytic asymmetric addition of TosMIC to unactivated ketones is presented. A combination of Me₂Zn and aminoalcohol catalyst promoted the aldol addition/cyclization reaction to render oxazolines possessing a fully substituted stereocenter with excellent yields (up to 92 %), high enantioselectivities (up to 96 %), and complete diastereoselectivity. The chiral oxazolines were then used to give, after a straightforward acid hydrolysis, enantioenriched building blocks bearing tertiary alcohol motifs such as hydroxylaldehydes, hydroxylacids, and hydroxylesters without racemization.

Full text of DOI:10.1002/chem.201504362

DOI: 10.1002/chem.201504362
Communication
&
Organic Synthesis
Me2Zn-Mediated Catalytic Enantio- and Diastereoselective
Addition of TosMIC to Ketones
Abdul Raheem Keeri,[a, b] Andrea Gualandi,*[a] Andrea Mazzanti,[c] Janusz Lewinski,[b] and
Pier Giorgio Cozzi*[a]
aldol addition/cyclization reaction of isocyanoacetate esters
Abstract: The first catalytic asymmetric addition of TosMIC
with unactivated prochiral ketones to afford functionalized ox-
to unactivated ketones is presented. A combination of
azolines with a fully substituted stereocenter.[11] Shibasaki re-
Me2Zn and aminoalcohol catalyst promoted the aldol ad-
ported a powerful yet atom-economical direct catalytic enan-
dition/cyclization reaction to render oxazolines possessing
tioselective aldol addition of a-substituted a-isothiocyanates
a fully substituted stereocenter with excellent yields (up
to simple ketones.[12] Moreover, 3-Isothiocyanato oxindoles
to 92%), high enantioselectivities (up to 96%), and com-
were also found to be suitable reagents for a direct addition to
plete diastereoselectivity. The chiral oxazolines were then
ketones, promoted by bifunctional thiourea–tertiary amine cat-
used to give, after a straightforward acid hydrolysis, enan-
alysts.[13] However, despite all the efforts dedicated to the syn-
tioenriched building blocks bearing tertiary alcohol motifs
thesis of enantioenriched tertiary alcohols by the addition of
such as hydroxylaldehydes, hydroxylacids, and hydroxyles-
nucleophiles to ketones,[14] enantioselective addition of TosMIC
ters without racemization.
to ketones is still a formidable challenge.[15] We describe herein
the first Me2Zn and chiral aminoalcohol-based catalytic system
for highly enantio- and diastereoselective addition of TosMIC
The amphoteric nature of isocyanides allows their use in the
synthesis of numerous important classes of nitrogen-contain-
ing heterocyclic compounds.[1] Recently, isocyanide-involved
multicomponent and cascade reactions have also been devel-
oped.[2] Since the pioneering work of Ito and Hayashi,[3] the ad-
dition of isocyanides, isocyanoacetate,[4] and TosMIC (p-tolue-
nesulfonylmethyl isocyanide)[5] have shown exceptional reac-
tion diversity and broad synthetic potentiality.[6] Many organo-
metallic[7] or organocatalyzed[8] asymmetric additions of isocya-
nides to different electrophiles have been described. In
principle, prochiral ketones serve as versatile substrates for the
generation of fully substituted stereocenters. As is anticipated
by their reduced reactivity and stereochemical selectivity to-
wards nucleophilic addition, ketones remain daunting sub-
strates in stereoselective addition reactions.[9] Interesting meth-
odologies for the addition of enolates to unactivated ketones
have been reported in the literature.[10] In this perspective,
Dixon described the first highly enantio- and diastereoselective
to unactivated ketones. This asymmetric aldol addition/cycliza-
tion protocol gives rise to functionalized oxazolines possessing
a fully substituted stereocenter (Scheme 1). Oxazolines are
simple heterocyclic compounds with diverse synthetic varsity
and, in addition, upon a mild hydrolysis they become promi-
nent substrates for the synthesis of many highly substituted
and enantioenriched products.
Scheme 1. Addition of TosMIC to ketones promoted by chiral zinc com-
plexes.
In the past our group has reported Reformatsky reactions[16]
and the catalytic enantioselective aldol-type addition of ethyl
diazoacetate to ketones[17] promoted by Me2Zn and chiral li-
gands. Me2Zn proved to be a suitable reagent for the deproto-
nation and activation of nucleophiles in the presence of ami-
noalcohols. We reasoned that these conditions were also suita-
ble for isocyanoacetate and TosMIC. Therefore, to prove this
supposition, we investigated the nucleophilic addition of iso-
cyanides and chose acetophenone as the model substrate.
Pleasingly, the reaction with TosMIC in a catalytic system that
comprised procatalyst (1R,2S)-1-phenyl-2-(1-pyrrolidinyl)-1-
propanol (20 mol%) and Me2Zn (1.7 equiv) was highly diaste-
reoselective and only the trans-configured oxazoline was isolat-
ed. Unfortunately, reaction with isocyanoacetate under the
same reaction conditions gave a mixture of diastereoiso-
mers.[18]
[a] A. R. Keeri, Dr. A. Gualandi, Prof. Dr. P. G. Cozzi
Dipartimento di Chimica “G: Ciamician”
ALMA MATER STUDIORUM, Università di Bologna
Via Selmi 2, 40126, Bologna (Italy)
E-mail: andrea.gualandi10@unibo.it
piergiorgio.cozzi@unibo.it
[b] A. R. Keeri, Prof. Dr. J. Lewinski
Faculty of Chemistry, Warsaw University of Technology
Noakowskiego 3, 00-664, Warsaw (Poland)
[c] Prof. Dr. A. Mazzanti
Dipartimento di Chimica “Toso Montanari”
ALMA MATER STUDIORUM, Università di Bologna
Viale del Risorgimento 4, 40136, Bologna (Italy)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504362.
Chem. Eur. J. 2015, 21, 18949 – 18952
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Communication
Inspired by this excellent diastereoselective reaction, along
with an aim of achieving enantioenriched oxazolines, we
tested several commercially available or easy to synthesize
chiral ligands in the model reaction involving TosMIC and ace-
tophenone at 08C. Although the oxazolines were formed with
complete simple diastereoselection, conversion, and facial ster-
eoselection of the reactions were discouraging (Table 1, en-
tries 1–13). Gratifyingly, enantioselectivity was increased signifi-
cantly, up to 69% ee, when the ligand 3m was employed.
Therefore, we examined other 1,2-aminoalcohols containing
a benzylic alcohol motif, under the selected reaction condi-
tions. Replacing the methyl by a phenyl at the R3-position of
ligand 3m leads to a remarkable decrease in the ee and con-
version (entry 14). Subsequently, by keeping methyl at the R3-
position constant and changing R2 substituents to more bulki-
er groups, such as N,N’-dibenzylamine, we observed the de-
crease of ee up to 22% (entry 15). Further optimization using
aliphatic substituents at the R2-position resulted in a very good
ee (72% ee, entries 16–18). Even though 3q and 3r show the
same activity and selectivity, we chose 3r for further studies
because of the ease of availability of its structural variants.
Enantioselectivity of this reaction was influenced not only by
the nature of substituents at the R1-position but also by the
relative configuration of the two stereocenters; (1R,2S)-chiral
aminoalcohol gave 72% ee, whilst the (1R,2R)-proligand result-
ed in only 48% ee (entries 18 and 20). Replacing the methyl in
the ligand 3r with an iPr resulted in a poorer ee (entry 19,
12% ee).
Table 1. Catalyst screening for the asymmetric aldol addition/cyclization
reaction of acetophenone 1a and toluenesulfonylmethyl isocyanide
(TosMIC) (2).[a]
It is worth noting that the ligand 3u (in which R1, R2 =H)
and 3v which possesses the group {(CPh2)2} did not increase
the selectivity.[19] From this observation, the commercially avail-
able (1R,2S)-1-phenyl-2-(pyrrolidin-1-yl)propan-1-ol (3r) was
found to be an optimum ligand for the asymmetric addition of
TosMIC to acetophenone.
Entry
3
Conv. [%][b] ee [%][c] Entry
3
Conv. [%][b] ee [%][c]
1
2
3
4
5
6
7
8
3a 10
3b 12
3c
5
20
14
33
0
12
13
14
15
16
17
18
19
20
21
22
3l
3m 45
3n 10
3o
3p 18
3q 45
3r
3s
3t
3u 50
3v
–
10
69
10
22
70
72
72
12
48
62
52
–
The effect of solvent on the catalytic asymmetric addition of
TosMIC to acetophenone reveals that DCM was a suitable sol-
vent for higher enantioselectivity and reactivity (Table 2, en-
tries 1–3). Reaction temperature was found to be optimum at
08C, as so significant quantities of products were isolated at
À208C whilst unwanted side reactions were observed at room
temperature.
3d 10
–
3e
3 f
3g
–
–
8
6
0
45
40
50
3h 15
15
0
0
9
10
11
3i
3j
3k
–
–
9
27
8
The limiting agent for the reaction was the TosMIC. The
amount of acetophenone was also found to be crucial; when
it was reduced in to one equivalent, the outcome of the reac-
tion resulted in a low ee and conversion (Table 2 entry 6, 30%
conv., 39% ee). In comparison, the more reactive Et2Zn in-
creased the enantioselectivity (78%), but unfortunately it also
accelerated the side reaction which generated unidentified im-
purities. A better conversion was obtained by increasing the
equivalents of Me2Zn and decreasing the concentration of
TosMIC by slow addition of a dilute solution of TosMIC in DCM
by a dropping funnel. The addition of TosMIC by syringe pump
gave rise to the formation of the impurities with a decrease in
ee and yield, probably caused by the presence of a trace of air.
Finally, a study of the catalytic loading showed that 25 mol%
was the optimum. It gave the highest enantiomeric excess
(85% ee) on the model reaction. The enantiomeric excess de-
[a] All the reactions were performed in the presence of 20 mol% 3,
1.7 equiv of Me2Zn, 0.1 mmol of TosMIC, and 0.2 mmol of acetophenone
1
in dichloromethane at 08C for 24 h. [b] Determined by H NMR analysis of
the crude reaction mixture. [c] Determined by HPLC analysis on a chiral
stationary phase.
creased slightly both by increasing (83% ee with 30 mol%) and
lowering the amount of the ligand to 10 mol% (76% ee).
The scope of the asymmetric aldol reaction of TosMIC cata-
lyzed by the chiral aminoalcohol/Me2Zn system was evaluated.
Thus a number of unactivated ketones were selected and sub-
jected to the optimized reaction conditions (Table 3). The reac-
tion was compatible with significant structural variations of ke-
tones. Notably aromatic, aliphatic, heterocyclic, and ferrocenyl
ketones were all well tolerated and resulted in only the trans-
Chem. Eur. J. 2015, 21, 18949 – 18952
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markable success was achieved when acetylferrocene was
tested. The corresponding trans-oxazoline was formed with ex-
cellent enantiomeric excess and a high yield (Table 2, entry 10,
96% ee, 90% yield).
Table 2. Me2Zn-mediated addition of TosMIC to acetophenone promoted
by chiral zinc complexes.
A possible mechanism for the reaction is reported in the
Supporting Information. In our mechanistic picture, Me2Zn is
the deprotonating reagent that is able to form the correspond-
ing MeZn–TosMIC intermediate. The ability of the chiral zinc-
alkoxide to coordinate both ketone and MeZn–TosMIC produ-
ces the stereoselective reaction. In the final stage of the cata-
lytic cycle, a MeZn–oxazoline is postulated. This hypothesis
was investigated by deuteration experiments. A selective intro-
duction of a deuterium atom on the imino carbon atom of ox-
azoline was achieved when the intermediate was quenched
with D2O, supporting our rationale (see the Supporting Infor-
mation for full details). The relative configuration of the prod-
uct trans-oxazoline 4a was assigned through Nuclear Over-
hauser effect spectroscopy (NOESY) and assumed by analogy
for all other products obtained. The absolute configuration is
determined as 4S*,5S* by simulation of the Electronic Circular
Dichroism (ECD) spectrum of compound 4c by means of TD-
DFT simulation and further confirmed by the comparison of
the hydrolyzed product 5a with reported compounds.[20]
Traditionally, TosMIC was used as a precursor in the synthesis
of heterocycles, but other interesting compounds such as a-
hydroxyaldehydes can also be obtained.[21] Van Leusen report-
ed[22] a PTC-based stereospecific method involving a diastereo-
selective reaction with enantiopure chiral TosMIC derivatives
for the synthesis of 2-hydroxy-2-phenylpropanal. Upon an acid
hydrolysis of enantioenriched 2-oxazolines, this method pro-
vided the desired products in 31% ee. The synthetic utility of
our method allows the preparation of fully substituted enan-
tioenriched a-hydroxyl aldehydes as versatile starting materials.
To illustrate the possibility offered by this methodology, we hy-
drolyzed the oxazolines 4a and 4e by using a mixture of aque-
ous 2m HCl and THF. The resulting a-hydroxyaldehydes 5a
and 5b were isolated with no racemization observed[23]
(Scheme 2). The absolute configuration for the compound 5a
was assigned based on the optical rotation of the known com-
pound.[24] This also confirmed the assignment of the absolute
configuration carried out by ECD and DFT simulations. The re-
action with phosphonate 6 under standard Horner–Wads-
worth–Emmons conditions gave the highly functionalized de-
rivatives 7a,b with no racemization. In addition, 5a,b can be
oxidized to the corresponding a-hydroxylacids bearing a fully
substituted stereocenter in a straightforward manner.
Entry
Sol.
3r [mol%]
R2Zn (equiv)
Conv. [%][a]
ee [%][b]
1
2
3
DCM
THF
20
20
20
20
20
20
20
20
20
20
20
20
10
25
30
Me2Zn (1.7)
Me2Zn (1.7)
Me2Zn (1.7)
Me2Zn (1.7)
Me2Zn (1.7)
Me2Zn (1.7)
Et2Zn (1.7)
Me2Zn (1.7)
Me2Zn (6)
45
5
35
12
5
30
57
–
50
81
100
50
100
100
100
72
50
50
40
40
39
78
76
80
83
82
82
76
85
83
toluene
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
4[c]
5[d]
6[e]
7
8[f]
9[g]
10[g]
11[g]
12[h]
13[g]
14[g]
15[g]
Me2Zn (8)
Me2Zn (10)
Me2Zn (10)
Me2Zn (10)
Me2Zn (10)
Me2Zn (10)
[a] Determined by 1H NMR analysis of the crude reaction mixture. [b] De-
termined by HPLC analysis on a chiral stationary phase. [c] Reaction at
room temperature [d] Reaction at À208C. [e] Reaction with 1 equiv aceto-
phenone. [f] Slow addition of TosMIC by using a syringe pump. [g] Addi-
tion of TosMIC in 2 mL of DCM from dropping funnel 0.5 mLhÀ1. [h] Addi-
tion of TosMIC in 2 mL of DCM from a dropping funnel 1 mLhÀ1
.
Table 3. Scope of asymmetric aldol reaction with different ketones.[a]
Entry Ketone 1
4
Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
acetophenone
4a
4b
4c
4d
4e
57
90
80
80
85
92
80
65
75
90
60
85
85
74
88
86
84
50
90
90
82
96
86
77
62
12
2-methylacetophenone
2-bromoacetophenone
4-methoxyacetophenone
2-acetonaphthone
2-bromo-2’-methoxyacetophenone 4 f
1-tetralone
4g
4h
4i
4j
4k
4l
4-methoxy-1-tetralone
1-indanone
9
10
11
12
13
14
1-acetylferrocene
1-ferrocenylethylketone
2-acetylbenzofuran
cyclohexanone
pinacolone
4m 92
4n 91
[a] All the reactions were performed with 0.1 mmol of TosMIC 2,
0.2 mmol of ketone 1, 25 mol% of 3r, and 10 equiv of Me2Zn in dichloro-
methane at 08C for 24 h. [b] Isolated yield after chromatography. [c] De-
termined by HPLC on a chiral stationary phase.
In conclusion, we have developed the first catalytic enantio-
selective addition of TosMIC to various unactivated ketones by
using Me2Zn and (1R,2S)-1-phenyl-2-(1-pyrrolidinyl)-1-propanol.
The commercially available ligand and Me2Zn make this
method highly attractive and an easy route to access many
chiral trans-oxazolines bearing a fully substituted stereocenter.
Through a mild acid hydrolysis of the product oxazolines, this
method enables the synthesis of more elaborate, valuable or-
ganic building blocks in a straightforward manner from unacti-
vated ketones without racemization. Application of this
chemistry towards the addition of other isocyanides to ketones
oxazolines with good to excellent yields and enantioselectivi-
ties (up to 92% yield and 96% ee) at 08C in 24 h. The aceto-
phenone and its derivatives gave very good yields and enan-
tioselectivities (Table 3, entries 2–4, 74–88% ee). With six-mem-
bered cyclic ketones such as a-tetralone and its derivatives,
very good enantioselectivities (90% ee) were observed. A re-
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Scheme 2. Transformation of the products 4a,e in to useful starting materi-
als. TEMPO=2,2,6,6-tetramethylpiperidine N-oxide.
and other electrophiles is under active investigation in our lab-
oratory and will be reported in due course.
Experimental Section
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In a 50 mL Schlenk flask, 3r (0.025 mmol, 5.1 mg) was dried and
dissolved in DCM (2 mL). The mixture was cooled to 08C and then
a solution of Me2Zn (1 mmol, 2m in toluene) was added. After stir-
ring for 30 min at the same temperature, acetyl ferrocene
(0.2 mmol, 45.6 mg) was added under a flow of nitrogen. Roughly
5 min later a solution of TosMIC (0.1 mmol, 19.5 mg) in DCM (5 mL)
was added to the reaction mixture from a pressure equalizing
dropping funnel (10 mL, with a Rotaflow stopcock) at a rate of
1 mLminÀ1 under anhydrous conditions. A positive pressure of ni-
trogen was maintained inside the flask throughout the reaction.
The reaction mixture was stirred magnetically for 24 h at 08C and
quenched with 10 mL of cold water. The organic layer was separat-
ed and the aqueous layer was extracted with DCM (3”5 mL). The
collected organic layers were washed with brine (5 mL), dried over
Na2SO4, and filtered through a pad of Celite and concentrated
under reduce pressure.
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Acknowledgements
We thank Fondazione Del Monte, University of Bologna
(SLAMM to A.G.) and EU-Foundation for Polish Science (MPD/
2010/13 to A.R.K) for funding. Dr. F. Benfatti is acknowledged
for preliminary investigations. A.M. thanks FARB funds, Univer-
sity of Bologna.
Keywords: addition reactions · enantioselectivity · ketones ·
Me2Zn · TosMIC
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[23] We have used batches of compounds with minor enantiomeric excess-
es for the controlled hydrolysis studies.
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Received: October 30, 2015
Published online on November 23, 2015
Chem. Eur. J. 2015, 21, 18949 – 18952
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18952
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim