Synthesis of olefin-oxazoline ligands (OlefOx) by rhodium(III)-catalyzed oxidative olefination

Article abstract of DOI:10.1002/adsc.201100711

The rhodium(III)-catalyzed oxidative olefination of 2-aryloxazolines is described and has been employed for the synthesis of olefin-oxazoline ligands (OlefOx). The highly modular synthesis starting from readily available 2-aryloxazolines can be performed

Full text of DOI:10.1002/adsc.201100711

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DOI: 10.1002/adsc.201100711
Synthesis of Olefin-Oxazoline Ligands (OlefOx) by
Rhodium(III)-Catalyzed Oxidative Olefination
ACHTUNGTRENNUNG
Nils Schrçder,a Tatiana Besset,b and Frank Gloriusa,*
a
Organisch-Chemisches Institut der Westfꢀlischen Wilhelms-Universitꢀt Mꢁnster, Corrensstraße 40, 48149 Mꢁnster,
Germany
Fax : (+49)-251-83-39-772; e-mail: glorius@uni-muenster.de
Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam,
b
Netherlands
Received: September 14, 2011; Revised: November 10, 2011; Published online: February 23, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100711.
Abstract: The rhodium
(III)-catalyzed oxidative ole-
binding olefin, the privileged h1-binding oxazoline
fragment taken together with the variability of both
entities provides a versatile ligand system for asym-
metric catalysis.
The disadvantages in the previously reported syn-
thesis of OlefOx ligands are the low yields for a range
of substrates and, more importantly, the lack of varia-
tion of substituents in the backbone of the ligand,
which could provide further electronic tuning of the
olefin. Therefore it was our aim to explore a new
pathway that overcomes these problems.
fination of 2-aryloxazolines is described and has
been employed for the synthesis of olefin-oxazoline
ligands (OlefOx). The highly modular synthesis
starting from readily available 2-aryloxazolines can
be performed under an atmosphere of air as the ter-
minal oxidant with catalytic amounts of copper(II)-
acetate.
À
Keywords: C H activation; olefination; olefins; ox-
azolines; rhodium
The oxidative Heck reaction, as pioneered by Fuji-
wara and Moritani,[10] has become an increasingly im-
À
portant transformation in organic chemistry for C C
coupling,[11] as it obviates prior functionalization steps
Following the recent success of chiral diene ligands,[1] and minimizes waste formation. The palladium,[12]
a range of chiral bidentate C1 symmetrical olefin li- rhodium[13] and ruthenium[14] catalyzed reactions have
gands[2] has been applied successfully to asymmetric been recently investigated intensively. A method that
catalysis.[3] The oxazoline moiety is a more common does not require prefunctionalization and that allows
and privileged[4] motif in ligands for asymmetric catal- the substitution of the backbone would represent
ysis,[5] with bisoxazolines[6] (BOX) and phosphine-oxa- a great improvement. The direct oxidative olefination,
zolines[7] (PHOX) being the most prominent exam- using the oxazoline moiety as a directing group[15]
ples.[8] In 2010 the bidentate olefin-oxazoline ligands would therefore be highly desirable. Herein we report
(OlefOx) were introduced and applied successfully in a novel, highly modular synthesis of OlefOx ligands
the rhodium-catalyzed 1,4 conjugate addition.[9] The by oxidative olefination of readily available 2-aryloxa-
unique electronic and structural behavior of the h2- zolines (Scheme 1).[16]
Scheme 1. Retrosynthethic analysis of OlefOx formation.
Adv. Synth. Catal. 2012, 354, 579 – 583
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Nils Schrçder et al.
COMMUNICATIONS
Table 1. Optimization of reaction conditions.[a]
Entry
Oxidant (equiv.)
Solvent
Yield of 3aa [%][b]
1
Cu
Cu
Cu
Cu
N
t-amyl-OH
t-amyl-OH
t-amyl-OH
DMF
t-amyl-OH
t-amyl-OH
t-amyl-OH
t-amyl-OH
t-amyl-OH
t-amyl-OH
1,4-dioxane
DMF
40
29
29
34
18
55
55
54
37
<5
50
37
47
46
2[c]
3[d]
4
5
AgOAc (2.1)
6[e]
7[e,f]
8[g]
9[e,h]
10[g,i]
11[g]
12[e]
13[e]
14[g]
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
2 ACHTUNGTNER(NUNG 0.2)
t-amyl-OH
t-amyl-OH
[a]
Reaction conditions: 1a (0.50 mmol), 2a (0.75 mmol), 2.5 mL solvent.
Isolated yields.
5 mol% [RhCp*Cl2]2, 20 mol% AgSbF6.
1408C instead of 1208C.
Reaction was run under air.
AgOTf instead of AgSbF6.
Reaction was run under oxygen.
42 h reaction time.
2.2 equiv. of Na2CO3 as additive.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
As model substrates for our reactions, we chose ox- was crucial to have a minimum of steric demands in
azoline 1a and styrene (2a) and started the optimiza- the meta-positions (e.g., Me, CF3). The use of a small-
tion (Table 1) with typical conditions from our previ- er group, like methoxy, led to an inseperable mixture
ous work.[13d] With stoichiometric amounts of Cu- of both possible mono-olefinated and di-olefinated
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
air as the terminal oxidant (Table 1, entry 6). In pros- yoxazolines (1f, 1j) to the reaction, providing the de-
pect of a possible upscaling of the reaction the re- sired olefinated products (3fa, 3ja). The scope of the
duced Cu(I) waste formation is very desirable. Even backbone could further be expanded from phenyl to
though the isolated yield was only modest with 55%, naphthyl systems (1g). The next variation was the use
it still represents a major improvement compared to of different substituents at the 4-position of the oxa-
our previous synthesis.
zoline. When switching to l-valinol-derived oxazo-
With the optimized conditions in hand we contin- lines (1h–1j), the isolated yields increased (cf. 3aa and
ued our study with a variety of different substituents 3ha, 3ba and 3ia) and to l-phenylglycinol the yield
on the OlefOx backbone (Table 2). Electron-poor decreased (cf. 3aa and 3ka). To further demonstrate
substituents (1b), electron-neutral (1a) as well as elec- the high modularity of the synthesis, a range of differ-
tron-rich ones (1f) were tolerated to give the desired ent olefins was used. With acrylates (2b) the best
products (3ba, 3aa, 3fa). For halogenated oxazolines yields could be achieved (3ab, 3ib), whereas the use
(1c, 1d) the isolated yield was only about 26–29% of 2-vinylnapththalene (2f) decreased the isolated
(3ca, 3da) and in these reactions we were able to rei- yield by a large amount. Finally, the use of different
solate large amounts of starting material. This could substituted styrenes (2c, 2d, 2e) generally led to a de-
indicate the possibility of catalyst-poisoning during crease in isolated yields (3ic, 3id, 3ie).
the reaction. For regioselective mono-olefination, it
580
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ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2012, 354, 579 – 583

Synthesis of Olefin-Oxazoline Ligands (OlefOx) by RhodiumACTHNUTRGNEU(GN III)-Catalyzed Oxidative Olefination
Table 2. Scope on the rhodium catalyzed olefination.[a]
[a]
Reaction conditions: 1 (1.0 mmol), 2 (1.5 mmol), 5 mL t-amyl-OH, isolated
yields.
[b]
Reaction was run on a 0.50 mmol scale.
To gain more insight into this transformation,
a deuteration experiment was conducted. Treatment
of 1a with a catalytic amount of [RhCp*Cl2]2 and
AgSbF6 in MeOH-d4 at 708C for 16 h, resulted in
80% deuterium incorporation in the 6-position and
25% in the 2-position. This indicates that we have
[17]
À
a reversible C H bond activation step
and, more
important, that only the ortho-positions can be func-
tionalized under the reaction conditions. In addition
the stereochemical integrity of the oxazoline moiety
stays intact, as was shown by HPLC analysis of 1a
and rac-1a.
use of air as the terminal oxidant is attractive. This
In conclusion, we have developed a new, efficient new method allows the independent tuning of all
and highly modular synthetic pathway to a broad three components, the oxazoline, the styrene and the
range of OlefOx ligands in modest to good yields. backbone benzene ring. We expect our method to be
À
The employed C H activation strategy significantly useful for preparing new OlefOx ligands with interest-
streamlines the synthesis. Furthermore, the successful ing applications in asymmetric catalysis.
Adv. Synth. Catal. 2012, 354, 579 – 583
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Nils Schrçder et al.
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Experimental Section
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General Procedure for the Olefination of 2-
Aryloxazolines
In
a
flame-dried, screw-capped tube [RhCp*Cl2]2
(OAc)2 (0.20 mmol),
(0.025 mmol), AgSbF6 (0.10 mmol), CuAHCUTNGTRENNUNG
the 2-aryloxazoline 1 (1.0 mmol scale) in t-amyl alcohol
(2.5 mL) and olefin 2 (1.5 equiv.) were united under argon.
The tube was evacuated and vented with air two times,
closed and then transferred to a pre-heated oil bath (1208C)
for 16 h. After cooling down to room temperature, NH4OAc
(approx. 1 mmol) was added and the mixture stirred for one
more hour. Then the mixture was filtered through a short
plug of silica with EtOAc/CH2Cl2, the solvent was removed
under reduced pressure, subsequent purification by flash
chromatography gave the desired product.
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We thank Dr. Frederic W. Patureau, Dr. Bjçrn T. Hahn and
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port by theDeutsche Forschungsgemeinschaft (SFB 858) is
gratefully acknowledged.
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[15] Selected examples for the oxazoline moiety as a direct-
À
ing group in C H activation, see: a) F. Kakiuchi, T.
Sato, M. Yamauchi, N. Chatani, S. Murai, Chem. Lett.
1999, 28, 19; b) R. Giri, X. Chen, J.-Q. Yu, Angew.
Chem. 2005, 117, 2150; Angew. Chem. Int. Ed. 2005, 44,
2112; c) L. Ackermann, A. Althammer, R. Born, Syn-
lett 2007, 2833.
[16] For the oxidative synthesis of oxazolines starting from
an amino alcohol and an aldehyde, see: a) K. Schwe-
kendiek, F. Glorius, Synthesis 2006, 2996; b) S. Sayama,
Synlett 2006, 1479; c) N. N. Karade, G. B. Tiwari, S. V.
Gampawar, Synlett 2007, 1921; d) B. Hahn, K. Schwe-
kendiek, F. Glorius, Org. Synth. 2008, 85, 267; e) S. Ta-
kahashi, H. Togo, Synthesis 2009, 2329.
À
[17] In this reaction, reversibility of the C H activation step
(rhodacycle formation) remains, even in the presence
of the alkene substrate; see see Supporting Informa-
tion. This is in contrast to an indole formation reported
by Fagnou,[18] in which no reversibility was observed
under standard conditions when an alkyne reaction
partner was present.
[18] D. R. Stuart, P. Alsabeh, M. Kuhn, K. Fagnou, J. Am.
Chem. Soc. 2010, 132, 18326.
Adv. Synth. Catal. 2012, 354, 579 – 583
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