Stereospecific Asymmetric Synthesis of Tertiary Allylic Alcohol Derivatives by Catalytic [2,3]-Meisenheimer Rearrangements

Article abstract of DOI:10.1002/anie.202001725

Chiral acyclic tertiary allylic alcohols are very important synthetic building blocks, but their enantioselective synthesis is often challenging. A major limitation in catalytic asymmetric 1,2-addition approaches to ketones is the enantioface differentiation by steric distinction of both ketone residues. Herein we report the development of a catalytic asymmetric Meisenheimer rearrangement to overcome this problem, as it proceeds in a stereospecific manner. This allows for high enantioselectivity also for the formation of products in which the residues at the generated tetrasubstituted stereocenter display a similar steric demand. Low catalyst loadings were found to be sufficient and the reaction conditions were mild enough to tolerate even highly reactive functional groups, such as an enolizable aldehyde, a primary tosylate, or an epoxide. Our investigations suggest an intramolecular rearrangement pathway.

Full text of DOI:10.1002/anie.202001725

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Accepted Article
Title: Stereospecific Asymmetric Synthesis of Tertiary Allylic Alcohol
Derivatives by Catalytic [2,3]-Meisenheimer Rearrangements
Authors: Xin Yu, Nick Wannenmacher, and René Peters
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202001725
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10.1002/anie.202001725
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COMMUNICATION
Stereospecific Asymmetric Synthesis of Tertiary Allylic Alcohol
Derivatives by Catalytic [2,3]-Meisenheimer Rearrangements
Xin Yu,[a] Nick Wannenmacher,[a] and René Peters*[a]
Abstract: Chiral acyclic tertiary allylic alcohols are very important
synthetic building blocks, but their enantioselective synthesis is often
challenging. A major limitation in catalytic asymmetric 1,2-addition
approaches to ketones is the enantioface differentiation by steric
distinction of both ketone residues. In this communication we report
enantioselectivity. MeOH and m-chlorobenzoic acid (m-CBA)
were necessary to improve the enantioselectivity to ee values
>90%. Their role in this catalytic process was unknown.[7]
Herein, we report that
a planar chiral ferrocene based
bispalladacycle, which was developed in our group[8] as catalyst
for a number of different applications,[9] efficiently enables the
asymmetric formation of tertiary allylic alcohol derivatives by [2,3]-
Meisenheimer rearrangements of amine-N-oxides possessing
trisubstituted olefin moieties (Scheme 1c)). By this method acyclic
tertiary allylic alcohols are accessible with high enantioselectivity,
even when the residues R1 and R2 display a similar steric demand.
Moreover, substrates with very reactive electrophilic functional
groups such as aldehyde, epoxide, primary tosylate or ester
moieties, etc., which might be problematic in 1,2-addition
reactions, were well accommodated.
the development of
a
catalytic asymmetric Meisenheimer
rearrangement to overcome this problem, as it proceeds in a
stereospecific manner. This allows for high enantioselectivity also for
the formation of products in which the residues at the generated
tetrasubstituted stereocenter display a similar steric demand. Low
catalyst loadings were found to be sufficient and the reaction
conditions were mild enough to tolerate even highly reactive functional
groups such as an enolizable aldehyde, a primary tosylate or an
epoxide. Our investigations suggest an intramolecular rearrangement
pathway.
Chiral enantiopure allylic alcohols are highly valuable building
blocks owing to the enormous synthetic versatility of C=C double
bonds.[1] While numerous ways have been described to
synthesize secondary allylic alcohols in a highly enantioselective
fashion, there are few examples for the challenging catalytic
asymmetric synthesis of tertiary allylic alcohols.[1,2] Cyclic tertiary
allylic alcohols were, for example, enantioselectively formed from
alkynones via tandem addition/cyclization sequences.[2,3] For
acyclic products the development of asymmetric additions of
reactive vinylmetal reagents to ketones is particularly noteworthy
(Scheme 1a)).[2,4] In that case, however, high stereochemical
efficiency is limited to substrates, in which either the ketone
residues RS and RL differ considerably in terms of their steric
demand[4a-d] or where the electrophilic keto moiety experiences
further activation by a contiguous carbonyl moiety to form a
chelate complex.[4e] In addition, high catalyst loadings and large
amounts of vinyl(half)metal sources were usually required.
The [2,3]-Meisenheimer rearrangement of allylic N-oxides has
been described as an alternative approach towards highly
enantioenriched allylic alcohols.[5,6] In 2011, the so far only known
catalytic asymmetric [2,3]-Meisenheimer rearrangement was
published by Tambar et al. (Scheme 1b)).[7] Pd(OAc)2 (10 mol%)
and a chiral phosphoramidite ligand (24 mol%) were employed to
form secondary allylic alcohol derivatives within a typical reaction
time of 2-5 days. Attempts to lower the loadings of Pd(OAc)2 and
chiral ligand led to prolonged reaction times and diminished the
Scheme 1. Comparison of previous work to this work.
[a]
Dr. X. Yu, M.Sc. N. Wannenmacher, Prof. Dr. R. Peters
Universität Stuttgart, Institut für Organische Chemie
Pfaffenwaldring 55, D-70569 Stuttgart, Germany
E-mail: rene.peters@oc.uni-stuttgart.de
Allylic amine 1a equipped with a trisubstituted olefin moiety was
selected as model substrate (Table 1). N-Oxide 2a was prepared
at 20 °C by oxidation with meta-chloroperbenzoic acid (m-
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.202001725
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COMMUNICATION
CPBA). Gratifyingly, treatment of freshly prepared 2a with the
planar chiral chloride bridged ferrocenylimidazoline palladacycle
[FIP-Cl]2[10] (2.5 mol%) activated with AgOAc (5 mol%) prior to use
by chloride / acetate exchange[11] provided 3a in high yield (86%)
and with promising enantioselectivity (ee = 72%, entry 1).
Surprisingly, with the related pentaphenylferrocene containing
catalysts generated from [PPFIP-Cl]2 and [PPFOP-Cl]2, which are
excellent precatalysts for [3,3]-rearrangements of allylic imidates
and carbamates,[12-14] only poor activity and enantioselectivity was
noted (entries 2 and 3). In contrast, the readily prepared C2-
symmetric bispalladacycle [FBIP-Cl]2[8,9] (1.25 mol%) activated by
AgOAc allowed for high enantioselectivity and yield (entry 4).
Table 1. Development of the Meisenheimer rearrangement forming
a
tetrasubstituted stereocenter.
Similar results were obtained with the mixed dinuclear pallada-
[15]
/platinacycle [FBIPP-Cl]2
(1.25 mol%) activated by AgOAc
(entry 5). Because [FBIP-Cl]2 is more readily prepared and also
more robust, it was selected for further optimization. The use of
various anionic ligands by chloride exchange was investigated
next. With some of them like mesylate (entry 6), similar results as
with acetate were obtained, whereas others led to inferior results
(e.g. entry 7). Unexpectedly however, noteworthy catalytic activity
was also found with the non-activated [FBIP-Cl]2 (entry 8). For
neutral halide bridged ferrocene palladacycles the activation by
halide exchange was often crucial in other application.[16] Some
exceptions have been reported for monopalladacycles.[17] In
contrast, for the fourfold connected dimeric bismetallacycles, the
activation was so far a must to achieve satisfying activity. Next to
the practical advantage of avoiding an activation step, non-
activated [FBIP-Cl]2 also allowed for nearly identical
enantioselectivity and still displayed useful catalytic activity at
reduced catalyst loadings (entries 7-11). Thus, also with 0.25
mol% catalyst an attractive reaction outcome was noticed (entry
11).
For the investigation of the substrate scope, a catalyst loading of
0.5 mol% was chosen for most examples (Table 2). The title
reaction was found to be broadly applicable. Next to linear alkyl
groups (entries 1 & 2) also α-branched alkyl moieties were
tolerated (entries 3 & 4). Moreover, we found a high compatibility
with a number of functional groups, including quite sensitive ones.
For instance, attractive results in terms of yield and
enantioselectivity were still obtained in the presence of an
enolizable aldehyde moiety (entry 5), an unprotected alcohol
(entry 6), a silyl ether (entries 7 & 13), an electrophilic primary
tosylate (entry 8), an enolizable ester (entry 9), a benzylcarbonate
(entry 10), a benzyl ether (entries 11, 12, 17), an epoxide (entries
14 & 15) and an olefin moiety (entry 16). As a general trend it was
found, that larger residues RZ slow down the reaction and for that
reason a catalyst loading of 1.25 mol% was used in these
challenging cases (entries 11-13, 15, 17). The reaction was found
to be stereospecific, because geometric olefin substrate isomers
resulted in different optical antipodes (entries 14 & 15). For this
reason, high enantioselectivity can also be attained employing
substrates, in which the residues RE and RZ display a similar steric
demand (e.g. in entries 11 and 12). This is thus a conceptual
advantage compared to the above mentioned 1,2-addition
approach.
#
(pre)catalyst / AgX
Y; Z
t [h][a]
yield[b]
3a [%] 3a [%]
ee[c]
1
[FIP-Cl]2 / AgOAc
[PPFIP-Cl]2 / AgOAc
[PPFOP-Cl]2 / AgOAc
[FBIP-Cl]2 / AgOAc
[FBIPP-Cl]2 / AgOAc
[FBIP-Cl]2 / AgOMs
[FBIP-Cl]2 / AgTFA
[FBIP-Cl]2
2.5; 5
2.5; 5
80
80
80
80
80
80
80
90
90
24
48
86
9
72
3
2
3
2.5; 5
11
91
86
89
89
90
92
92
82
4
4
1.25; 5
1.25; 5
1.25; 5
1.25; 5
1.25; -
0.5; 2.0
0.5; -
93
93
93
78
95
95
95
95
5
6
7
8
9
[FBIP-Cl]2 / AgOMs
[FBIP-Cl]2
10
11
[FBIP-Cl]2
0.25; -
[a] Reaction time of the rearrangement step. [b] Yield of isolated product 3a. [c]
Enantiomeric excess determined by HPLC. [FIP-Cl]2: chloride bridged
The reaction type is not limited to dibenzylamine based N-oxides
as entry 17 shows, in which a diethylamino moiety was employed.
However, substrate 2o is more prone towards thermal
rearrangement and the freshly prepared isolated 2o already
contained 1.7% racemic rearrangement product, when the Pd-
catalyzed rearrangement was started. This thermal background
reaction is also the reason, why the title reaction should be
performed at 0 °C.
ferrocenylimidazoline
palladacycle;
[PPFIP-Cl]2:
chloride
bridged
pentaphenylferrocenylimidazoline palladacycle; [PPFOP-Cl]2: chloride bridged
pentaphenylferrocenyloxazoline palladacycle; [FBIP-Cl]2: chloride bridged
ferrocenediylbisimidazoline bispalladacycle; [FBIPP-Cl]2: chloride bridged
ferrocenediylbisimidazoline palladaplatinacycle; OAc: acetate; OMs: mesylate;
TFA: trifluoroacetate.
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10.1002/anie.202001725
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COMMUNICATION
The method is also partly useful in terms of enantioselectivity for
substrates with aromatic olefin substituents as shown by entry 18,
in which 74% ee were attained for RE = Ph & RZ = Me. In contrast,
which differ in both the N-oxide residues and the olefin
substituents, was treated with the palladacycle catalyst (Scheme
2). The two expected products for an intramolecular pathway were
for RE = Me & RZ = Ph, the ee was just 42% (yield 85%, not shown). formed in good and similar yields, whereas cross-over products
In order to demonstrate the practical utility, the title reaction was
also performed on a bigger scale employing (Z)-configured
substrate 2m. In that case, 522.0 mg of product 3m were isolated
(yield 82%) with an ee of 88%, which is similar to entry 13.
were not detected by 1H-NMR. Hence an intramolecular
rearrangement is the most likely scenario.
Table 2. Investigation of the substrate scope.
entry 2,
RE
RZ
R
yield[a]
ee[b]
3
3 [%]
3 [%]
1
a
b
c
d
e
f
(CH2)2Ph
nBu
Me
Me
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
92
89
90
83
86
80
85
69
93
89
80
76
83
96
95
91
Scheme 2. Cross-over experiment pointing to an intramolecular pathway.
2
1
3[c]
4[c]
5
iPr
Me
90[e]
93[e]
91[e]
89
Monitoring of the reaction up to conversions of ca. 60% by H-
NMR spectroscopy revealed a nearly linear relationship between
conversion and time (see Supporting Information) indicating a
zero order dependence on the N-oxide 2a. A substrate saturation
thus appears likely and might be the consequence of a two-point
coordination of the substrate in which both the olefin and the N-
oxide bind to either one or two metal centers. This high
coordination affinity might also explain why the usually almost
inactive chloride bridged dimer [FBIP-Cl]2 can be used as a
catalyst without prior activation by a silver salt to facilitate the
substrate coordination step.[18] Moreover, the linear relationship
indicates that product inhibition probably is negligible.
cyclo-Hex
(CH2)2CHO
(CH2)3OH
(CH2)3OTBS
(CH2)3OTs
(CH2)2CO2Et
(CH2)3OCO2Bn
nBu
Me
Me
6
Me
7
g
h
i
Me
94[e]
93
8
Me
9
Me
92
10
11[c]
12[c]
13[d]
14[d]
j
Me
93
Mass spectrometric investigations (ESI) were performed during
the course of the reaction. Palladacycle species possessing
accurate masses that would fit to coordination of either one or two
substrate molecules were detected after 10 h, but also already 5
min after the start of the reaction, in agreement with a substrate
k
l
CH2OBn
CH2OBn
(CH2)3OTIPS
Me
92
(CH2)10Me
Me
86
m
n
90
1
saturation scenario (see SI for details). Nevertheless, H-NMR
83[e]
spectra were too complex to identify the precise structure of the
resting state.
To showcase that the enantioenriched rearrangement products
are valuable precursors toward scalemic tertiary allylic alcohols,
the epoxide substituted stereoisomers 3n and 3o were
transformed into diols 5 and (ent)-5 in good yields by reductive
epoxide ring opening and subsequent cleavage of the weak N-O
bond by zinc metal (Scheme 3). Moreover, monobenzyl protected
diols 4k and 4l were prepared in 74 and 94% yield from 3k and 3l,
respectively, using the latter method. Product 3a prepared in the
model reaction was transformed to 4a in the same way to
determine the absolute configuration by comparison to reported
optical rotation data.[19] Comparison of the ee values of 3a and 4a
also revealed that cleavage of the N-O bond proceeded with only
little racemization.
15[d]
16
o
p
Me
Bn
Bn
96
89
87[e]
96
Me
17
q
r
Me
Ph
CH2OBn
Me
Et
83
87
90[f]
74
18[d]
Bn
[a] Yield of isolated product; typically 20-90 mg scale. [b] Enantiomeric excess
determined by HPLC. [c] 1.25 mol% [FBIP-Cl]2 were used. [d] 1.25 mol% [FBIP-
OAc]2 were used. [e] Determined after derivatization (see Supporting
Information). [f] The N-oxide 2o contained 1.7% of racemic rearrangement
product at the start. TBS: tert-butyldimethylsilyl; OTs: tosylate; TIPS:
triisopropylsilyl.
To get insight if the product formation proceeds via an
intramolecular rearrangement pathway, cross-over experiments
were conducted. A 1:1 mixture of allylic N-oxides 2b and 2o,
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[4]
a) H. Li, P. J. Walsh, J. Am. Chem. Soc. 2004, 126, 6538; b) H. Li, P. J.
Walsh, J. Am. Chem. Soc. 2005, 127, 8355; c) D. B. Biradar, H.-M. Gau,
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[6]
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[7]
[8]
H. Bao, X. Qi, U. K. Tambar, J. Am. Chem. Soc. 2011, 133, 1206–1208.
a) S. Jautze, P. Seiler, R. Peters, Angew. Chem. Int. Ed. 2007, 46, 1260–
1264; b) S. Jautze, S. Diethelm, W. Frey, R. Peters, Organometallics
2009, 28, 2001–2004; c) M. Weber, J. E. M. N. Klein, B. Miehlich, W.
Frey, R. Peters, Organometallics 2013, 32, 5810-5817.
[9]
Selected applications: a) S. Jautze, R. Peters, Angew. Chem. Int. Ed.
2008, 47, 9284–9288; b) M. Weber, S. Jautze, W. Frey, R. Peters, J. Am.
Chem. Soc. 2010, 132, 12222–12225; c) S. H. Eitel, S. Jautze, W. Frey,
R. Peters, Chem. Sci. 2013, 4, 2218-2233; d) M. Weber, W. Frey, R.
Peters, Angew. Chem. Int. Ed. 2013, 52, 13223-13227; e) T. Hellmuth,
S. Rieckhoff, M. Weiss, K. Dorst, W. Frey, R. Peters, ACS Catal. 2014, 4,
1850-1858; f) M. Weiss, R. Peters, ACS Catal. 2015, 5, 310-316.
[10] a) M. E. Weiss, D. F. Fischer, Z.-q. Xin, S. Jautze, W. B. Schweizer, R.
Peters, Angew. Chem. Int. Ed. 2006, 45, 5694–5699; b) D. F. Fischer, A.
Barakat, Z.-q. Xin, M. E. Weiss, R. Peters, Chem. Eur. J. 2009, 15, 8722–
8741.
[11] a) C. Schrapel, W. Frey, D. Garnier, R. Peters, Chem. Eur. J. 2017, 23,
2448–2460; b) C. Schrapel, R. Peters, Angew. Chem. Int. Ed. 2015, 54,
10289–10293; ref. 9c.
Scheme 3. Synthesis of tertiary allylic alcohols employing the rearrangement
products.
[12] a) see ref. 10; b) Z.-q. Xin, D. F. Fischer, R. Peters, Synlett 2008, 1495–
1499; c) R. Peters, Z.-q. Xin, F. Maier, Chem. Asian J. 2010, 5, 1770–
1774; d) S. H. Eitel, M. Bauer, D. Schweinfurth, N. Deibel, B. Sarkar, H.
Kelm, H.-J. Krüger, W. Frey, R. Peters, J. Am. Chem. Soc. 2012, 134,
4683–4693; e) J. M. Bauer, W. Frey, R. Peters, Angew. Chem. Int. Ed.
2014, 53, 7634-7638.
In summary, we have reported a catalytic asymmetric
Meisenheimer rearrangement as an efficient entry to acyclic
tertiary allylic alcohols. This reaction is catalyzed by the robust
ferrocene based bispalladacycle catalyst [FBIP-Cl]2 and proceeds
in an stereospecific manner. It allows for high enantioselectivity
even for the formation of products in which the residues at the
generated stereocenter display a similar steric demand. From a
practical point of view, this method is also attractive, because no
catalyst activation and no catalytic additives are required.
Moreover low catalyst loadings were sufficient and the reaction
conditions are mild enough to tolerate even highly reactive
functional groups. The experimental data suggests an
intramolecular rearrangement pathway with a substrate saturation
of the palladacycle catalyst.
[13] Selected applications of these catalysts: a) M. Weber, R. Peters, J. Org.
Chem. 2012, 77, 10846–10855; b) T. Hellmuth, W. Frey, R. Peters,
Angew. Chem. Int. Ed. 2015, 54, 2788–2791.
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Chem. Soc. 1974, 96, 597-599; b) L. E. Overman, Angew. Chem. Int. Ed.
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1740; d) M. Calter, T. K. Hollis, L. E. Overman, J. Ziller, G. G. Zipp, J.
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Acknowledgements
This work was financially supported by the Deutsche
Forschungsgemeinschaft (DFG, PE 818/4-2). Xin Yu thanks the
China Scholarship Council (CSC) for a Ph.D. scholarship.
[15] M. Weiss, W. Frey, R. Peters, Organometallics 2012, 31, 6365–6372.
[16] R. Peters, D. F. Fischer, S. Jautze, Top. Organomet. Chem. 2011, 33,
139–175.
[17] See ref. 12e, 13b.
Keywords: asymmetric catalysis • bimetallic catalyst •
palladacycle • rearrangement • tertiary alcohols
[18] In combination with the above mentioned stereospecificity, it appears
reasonable that the reaction proceeds like already suggested in allyl
imidates (see ref. 10 and 14e) and carbamate rearrangements (see ref.
12e) via a face selective coordination of the C=C double bond to a PdII
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a subsequent stereoselectivity determining C-X bond
[2]
[3]
Y.-L. Liu, X.-T. Lin, Adv. Synth. Catal. 2019, 361, 876–918.
a) Y. Li, M.-H. Xu, Org. Lett. 2014, 16, 2712–2715; b) T. Johnson, K.-L.
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10637.
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10.1002/anie.202001725
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A stereospecific catalytic asymmetric
Meisenheimer rearrangement is
reported, which allows for high
enantioselectivity even for the
formation of products in which the
residues at the generated
tetrasubstituted stereocenter display
similar steric demands. Functional
groups were well tolerated by the
robust, readily available catalyst.
Xin Yu, Nick Wannenmacher,
René Peters*
Page No. – Page No.
Stereospecific Asymmetric Synthesis
of Tertiary Allylic Alcohol Derivatives
by Catalytic [2,3]-Meisenheimer
Rearrangements
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