Chain walking of allylrhodium species towards esters during rhodium-catalyzed nucleophilic allylations of imines

Article abstract of DOI:10.1002/anie.201508964

Allylrhodium species derived from δ-trifluoroboryl β,γ-unsaturated esters undergo chain walking towards the ester moiety. The resulting allylrhodium species react with imines to give products containing two new stereocenters and a Z-alkene. By using a chiral diene ligand, products can be obtained with high enantioselectivities, where a pronounced matched/mismatched effect with the chirality of the allyltrifluoroborate is evident. Migrate to create: Allylrhodium species from δ-trifluoroboryl β,γ-unsaturated esters undergo chain walking towards the ester moiety. The resulting allylrhodium species react with imines to give products with two new stereocenters and a Z-alkene. A chiral diene ligand leads to products with high enantioselectivities; a pronounced matched/mismatched effect with the chirality of the allyltrifluoroborate is evident.

Full text of DOI:10.1002/anie.201508964

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International Edition: DOI: 10.1002/anie.201508964
German Edition: DOI: 10.1002/ange.201508964
Chain Walking
Chain Walking of Allylrhodium Species Towards Esters During
Rhodium-Catalyzed Nucleophilic Allylations of Imines
Jose I. Martínez, Joshua J. Smith, Hamish B. Hepburn, and Hon Wai Lam*
Abstract: Allylrhodium species derived from d-trifluoroboryl
b,g-unsaturated esters undergo chain walking towards the ester
moiety. The resulting allylrhodium species react with imines to
give products containing two new stereocenters and a Z-alkene.
By using a chiral diene ligand, products can be obtained with
high enantioselectivities, where a pronounced matched/mis-
matched effect with the chirality of the allyltrifluoroborate is
evident.
T
he migration of metal centers along carbon chains occurs in
several important reactions.[1–7] Many of these migrations take
place by b-hydride elimination and hydrometalation sequen-
ces, in which the direction of travel is controlled by
thermodynamics, a ligand, or a nearby functional group.
With few exceptions,[4b–f] these migrations involve simple
alkylmetal species. The ability to chain walk a metal together
with a second functional group has significant synthetic
opportunities, but this mode of reactivity remains largely
underdeveloped. Herein, we describe, to our knowledge, the
first examples of allylrhodium chain walking, and its applica-
tion in the preparation of enantioenriched products.
Scheme 1. Discovery of allylrhodium chain walking.
Table 1: Investigation of imine scope.[a]
Entry
Product
R
Yield [%][b]
During our studies of enantioselective Rh-catalyzed
nucleophilic allylations of imines,[8] the reaction of imine 1a
with racemic allyltrifluoroborate 2a[9] in the presence of
[{Rh(cod)Cl}2] (1.5 mol%) and iPrOH (5.0 equiv) was con-
ducted (Scheme 1). Surprisingly, allylation at the a- or g-
carbon atoms relative to the boron atom of 2a was not
observed. Instead, this reaction gave homoallylic sulfamates
3a (68% yield) and 4a (6% yield), each in > 95:5 d.r.
(Scheme 1).[10–12] This result suggests the reactive intermedi-
ates are allylrhodium species 5 and 6, formed from migration
of the allylrhodium species generated initially from trans-
metalation of 2a with rhodium.
The scope of this unexpected reaction was extended to
include aldimines bearing methyl, methoxy, bromo, or
dioxozole groups, which gave products with high diastereo-
selectivities in 65–72% yield (Table 1, entries 1–5). Ketimines
containing linear alkyl groups at the imine carbon were also
1
2
3
4
3a
3b
3c
3d
Me
H
OMe
Br
68[c]
72
65[d]
65[e]
5
3e
65
6
7
8
9
3 f
3g
3h
3i
Me
Et
nBu
(CH2)3Ph
iPr
65
53
54
55
<5
10
3j
[a] Reactions were conducted using 0.30 mmol of 1. The diastereomeric
ratios were confirmed by 1H NMR analysis of the unpurified reactions.
[b] Yield of isolated products. [c] The regioisomer 4a was isolated in 6%
yield (Scheme 1). [d] In the unpurified reaction, traces of a product
derived from allylation without chain walking were detected. [e] Isolated
as an 87:13 mixture of 3d with the regioisomeric product 4d. See
Ref. [13].
[*] Dr. J. I. Martínez, J. J. Smith, Dr. H. B. Hepburn, Prof. H. W. Lam
School of Chemistry, University of Nottingham, University Park
Nottingham, NG7 2RD (UK)
E-mail: hon.lam@nottingham.ac.uk
Homepage: http://www.nottingham.ac.uk/~pczhl/
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508964.
effective (entries 6–9). However, an isopropyl-substituted
imine was recovered unchanged (entry 10). With one excep-
tion (entry 3), no products of allylation at the a- or g-carbons
relative to the boron atom of 2a were obtained. Furthermore,
except for the reactions producing 3a and 3d (entries 1 and
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Table 2: Investigation of allyltrifluoroborate scope.[a]
Entry
Product
R
Yield [%][b]
1
2
3k
3l
Me
nPr
69
70[c]
3
3m
3n
3o
(CH2)4Cl
(CH2)2Ph
CH2OBn
63
4[d]
5[d]
36 (59)[e]
(53)[e,f]
6[d]
3p
62
Scheme 2. Effect of geometrical and positional isomerism of the
allyltrifluoroborate.
7
8
3q
3r
Me
nPr
67
58
[a] Reactions were conducted using 0.30 mmol of 1. The diastereomeric
ratios were confirmed by 1H NMR analysis of the unpurified reactions.
[b] Yield of isolated products. [c] Isolated as a 95:5 mixture of 3l and the
product of allylation without allylrhodium chain walking. See Ref. [13].
[d] Using 2.5 mol% of [{Rh(cod)Cl}2]. [e] Yields in parentheses were
determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as
an internal standard. [f] Attempts to purify 3o by column chromatog-
raphy were unsuccessful. A pure sample was obtained by preparative
TLC.
Interestingly, the reaction of Z-allyltrifluoroborate 8[9]
with aldimine 1b gave 3b in 70% yield (Scheme 2, top),
which is the same product obtained from the corresponding
E-isomer 2a (Table 1, entry 2). Furthermore, despite possess-
ing a substitution pattern different to all allyltrifluoroborates
employed until this point, allyltrifluoroborate 9 reacted in the
same manner to give 3t (Scheme 2, bottom).[10] These results
suggest that regardless of the geometrical or positional
isomerism of the allyltrifluoroborate within the b to d
carbons, the reactions proceed through common types of
allylrhodium intermediates. However, homoallylic boron
reagents were unreactive.[13,15]
Because these reactions provide chiral products from
chiral substrates, we investigated whether enantioenriched
allyltrifluoroborates would give enantioenriched products.
However, the reactions of (R)-2a (94% ee)[9] with aldimine
1b and ketimine 1 f gave (S,S)-3b and (S,S)-3 f, respectively,
with low-to-moderate enantiomeric excesses (Scheme 3).
Although chain walking of alkylmetal species can proceed
with high stereospecificity,[7c] poor absolute stereochemical
transfer is observed in the reactions described herein.
4), the alternative regioisomers were difficult to detect by
1H NMR spectroscopy.
Next, the potassium allyltrifluoroborate was varied
(Table 2). As well as ethyl esters (Table 1) and benzyl esters
(Table 2, entries 1–5, 7, and 8), a 2-naphthyl ester was
accommodated (Table 2, entry 6). Regarding the substituent
a to the boron atom, alkyl (entries 1, 2, 7, and 8) and
chloroalkyl groups (entry 3) were tolerated. Product 3l was
isolated along with a product of allylation without chain
walking, in a 95:5 ratio (entry 2).[13] Alkyl substituents
containing phenyl or benzyloxy groups resulted in lower
conversions and yields (entries 4 and 5).
The reaction of 1b with allyltrifluoroborate 2h, in which
boron is bonded to a primary rather than a secondary carbon,
gave not only 3s, but also a significant quantity of product 7 in
80:20 d.r., derived from allylation without chain walking
[Eq. (1)].[14] Products 3s and 7 could not be completely
separated by column chromatography, and their yields were
Next, chiral rhodium complexes were investigated for
their ability to provide enantioenriched products from
racemic allyltrifluoroborates (Scheme 4).[8b] Although several
chiral dienes[16] gave poor conversions[13] in the reaction of
aldimine 1b with 2a, diene L1[17] gave (S,S)-3b in 72% yield
and 98% ee. Several other products (S,S)-3k, (S,S)-3m, and
(S,S)-3p were also prepared in the same manner. However,
1
determined by H NMR analysis using an internal standard.
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Scheme 3. Investigation of absolute stereochemical transfer with (R)-
2a.
Scheme 5. Investigation of matched/mismatched effects.
Scheme 4. Enantioselective allylations. Reactions were conducted
using 0.30 mmol of 1b. The diastereomeric ratios were confirmed by
1H NMR analysis of the unpurified reactions. Yields are of isolated
products. Enantiomeric excesses were determined by HPLC analysis
on a chiral stationary phase. [a] Using 2.5 equivalents of allyltrifluor-
oborate 2g.
the yields of some of these reactions were low, and the scope is
more limited than when using [{Rh(cod)Cl}2]. For example,
enantioselective additions to ketimines were unsuccessful.
Interestingly, a pronounced matched/mismatched effect
was observed with enantioenriched allyltrifluoroborates. The
reaction of 1b with (R)-2a (94% ee) using chiral diene L1
gave (S,S)-3b with results identical to the reaction using
racemic 2a (Scheme 5, top; compare with Scheme 4). How-
ever, the corresponding reaction with (S)-2a (94% ee) gave
a complex mixture; although 3b was detected in small but
Scheme 6. Proposed catalytic cycle.
A proposed mechanism, using imine 1a and allyltrifluor-
oborate 2a as representative substrates, is shown in Scheme 6.
The reaction of 2a with iPrOH can reversibly generate
a mixed alkoxide/fluoride boron ate complex 11, which
transmetalates with rhodium complex 10[18,19] to give inter-
converting allylrhodium species 12 and 13. b-Hydride elim-
ination of 13 then gives a rhodium hydride species bound to
ethyl sorbate (as in 14).[20,21] Hydrorhodation of the alkene
distal to the ester then provides interconverting allylrhodium
species 5 and 6. A possible driving force for this chain walking
migration is the formation of a more stable, more conjugated
1
unquantifiable amounts by H NMR analysis, it could not be
isolated. Currently, it is unclear which steps of the proposed
mechanism (see below) are rendered inefficient by the
stereochemical mismatch of the ligand and the allyltrifluor-
oborate.
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allylrhodium species 5. Nucleophilic allylation of 1a by 5
benzyl ester would give allylrhodium species 21, which reacts
through a chairlike conformation 15, in which the ethyl group
is pseudoaxial to avoid unfavorable interactions with the
cyclooctadiene ligand,[8c,22] gives 16.[23] Finally, protonolysis of
16 with HX (X = Cl, F, or OiPr) releases the product 3a and
regenerates 10. The minor regioisomer 4a is the result of
allylation of 1a with allylrhodium species 6.
Support for this mechanism was provided by the reaction
of aldimine 1b, allyltrifluoroborate 2c (1.5 equiv), and ethyl
sorbate (17, 1.5 equiv), using [{Rh(C2H4)2Cl}2] as a precatalyst
[Eq. (2)]. This reaction gave mostly unreacted 1b and 17,
along with unidentified products resulting from decomposi-
tion of 2c. However, by HPLC-MS, small quantities of the
expected product 3l derived from allyltrifluoroborate 2c
(0.4% yield), the crossover product 3a derived from ethyl
sorbate (17, 3.4% yield), and a,b,g,d-unsaturated benzyl ester
18 (2.7% yield) were also detected.[13]
with 1b to give the expected product 3l. Alternatively,
a structural reorganization of 20 could give 22, which can then
undergo hydrorhodation of ethyl sorbate to give allylrhodium
species 5 and the crossover product 3a.
In summary, we have reported the chain walking of
allylrhodium species derived from d-trifluoroboryl b,g-unsa-
turated esters during the rhodium-catalyzed nucleophilic
allylation of imines, which gives products with two new
stereocenters and a Z-alkene. Enantioselective catalysis is
possible using a chiral diene ligand, where a strong matched/
mismatched effect was observed. Further exploration of this
new mode of reactivity is underway in our laboratories.
Acknowledgements
We thank the ERC (Starting Grant No. 258580), the EPSRC
(Industrial CASE studentship to J.J.S. and Leadership Fellow-
ship to H.W.L.), the Basque Country government (postdoc-
toral fellowship to J.I.M.), GlaxoSmithKline, and the Uni-
versity of Nottingham for financial support. Benjamin
Pointer-Gleadhill (University of Nottingham) is gratefully
acknowledged for assistance with the crossover experiment.
We thank Dr. William Lewis (University of Nottingham) for
X-ray crystallography, and Dr. Alan Nadin (GlaxoSmithK-
line) for helpful discussions.
Keywords: allyltrifluoroborates · asymmetric catalysis · imines ·
isomerization · rhodium
How to cite: Angew. Chem. Int. Ed. 2016, 55, 1108–1112
Angew. Chem. 2016, 128, 1120–1124
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prepared by the copper-catalyzed 1,6-boration of a,b,g,d-unsa-
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assigned by comparison to those of 3a (racemic), 4a (racemic),
3 f (racemic), (S,S)-3t (resolved during crystallization), and
(S,S)-3b, which were determined by X-ray crystallography.
CCDC 1418585– 1418589 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
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hyde gave a complex mixture. The reaction of 2a with the N-
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[23] An alternative mechanism for the formation of 3a involves a 1,4-
hydrorhodation of ethyl sorbate to give a rhodium enolate,
which then reacts with the imine in a Mannich reaction (as
shown below). However, we favor the mechanism shown in
Scheme 6 on the basis that an interconverting mixture of
allylrhodium isomers
5 and 6 explains the formation of
regioisomeric products 3a and 4a in a simpler fashion.
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[13] See the Supporting Information.
[14] The relative configuration of 7 was assigned tentatively.
[15] Attempted reactions of the following homoallylic boron
reagents and imine 1b under various reaction conditions led
only to unchanged starting materials, suggesting that trans-
metalation to rhodium is difficult for these reagents.
Received: September 24, 2015
Revised: November 4, 2015
Published online: December 3, 2015
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