Enantioselective rhodium-catalyzed nucleophilic allylation of cyclic imines with allylboron reagents

Article abstract of DOI:10.1002/anie.201204004

Chiral allylrhodium nucleophiles: The highly diastereo- and enantioselective title reaction of a range of cyclic imines with various potassium allyltrifluoroborates most likely proceeds via allylrhodium(I) intermediates, and represents the first rhodium-catalyzed enantioselective nucleophilic allylation of π electrophiles with allylboron compounds. Copyright

Full text of DOI:10.1002/anie.201204004

Angewandte
Chemie
DOI: 10.1002/anie.201204004
Asymmetric Allylation
Enantioselective Rhodium-Catalyzed Nucleophilic Allylation of Cyclic
Imines with Allylboron Reagents**
Yunfei Luo, Hamish B. Hepburn, Nawasit Chotsaeng, and Hon Wai Lam*
Enantioselective rhodium(I)-catalyzed additions of organo-
boron reagents to p electrophiles are a major class of
reactions for the generation of enantioenriched chiral com-
pounds.[1] Attractive features of these reactions include: (i)
the availability of a wide range of chiral ligands that impart
high enantioselectivities across several classes of electro-
philes; (ii) relatively mild reaction conditions, and (iii) the
availability, stability, functional group tolerance, and usually
low toxicity of organoboron reagents. To date, this field has
been dominated by the enantioselective addition of arylboron
reagents,[1] although there have also been reports of additions
of alkenylboron[2] and alkynylboron reagents.[3–5] Notably, the
corresponding rhodium-catalyzed enantioselective additions
of allylboron reagents have not been described,[6,7] despite the
widespread importance of nucleophilic allylations in syn-
thesis.[8,9] Herein, we describe the first enantioselective
rhodium-catalyzed additions of allylboron reagents to p elec-
trophiles in the form of asymmetric allylation reactions
involving cyclic imines and potassium allyltrifluoroborates.
Not only do aldimines undergo highly enantioselective
allylations, ketimines are also effective substrates. Further-
more, highly stereoselective additions of substituted allyltri-
fluoroborates are described.
The importance of chiral homoallylic amines as building
blocks for chemical synthesis has led to significant efforts to
develop catalytic enantioselective nucleophilic allylations of
imines and their derivatives.[8b,10,11] However, processes that
employ allylboron reagents constitute a field that is still in its
infancy,[8b,10,12,13] and we therefore selected this area in which
to develop a new rhodium-catalyzed variant that could offer
increased substrate scope and utility. Our experiments began
with attempted allylation reactions involving various benzal-
dehyde-derived imines and potassium allyltrifluoroborate
(2 equivalents)[14,15] in the presence of 2.5 mol% of [{Rh-
(cod)Cl}2] and MeOH (5 equivalents) in dioxane at 808C
(Table 1). Unfortunately, satisfactory results were not
obtained. With N-phenyl or N-diphenylphosphinoyl imines,
only starting material was recovered (Table 1, entries 1 and
2), whereas trace quantities of the allylation product were
observed using a dimethylsulfamyl imine (Table 1, entry 3).
Table 1: Rh-catalyzed allylation of benzaldehyde-derived imines.[a]
Entry
R
1 [%][b]
3 [%][b]
4 [%][b]
1
2
3
4
5
Ph
>95
>95
85
60
42
<5
<5
5
25
28
<5
<5
10
15
30
P(O)Ph2
SO2NMe2
Ts
Ns
[a] Reactions were conducted using 0.10 mmol of 1. [b] Determined by
1H NMR analysis of the unpurified reaction mixtures. cod=1,5-cyclo-
octadiene, Ns=p-nitrobenzenesulfonyl, Ts=p-toluenesulfonyl.
With more reactive N-sulfonylimines, appreciable quantities
of homoallylic sulfonamides were obtained, but significant
quantities of starting imine remained, along with benzalde-
hyde resulting from imine hydrolysis (Table 1, entries 4 and
5).
In light of our recent discovery that cyclic imines are
highly effective substrates for enantioselective rhodium-
catalyzed alkenylations,[16] the allylation of benzoxathiazine-
2,2-dioxide 5a was investigated (Table 2). Under reaction
conditions identical to those employed in Table 1, complete
consumption of 5a was observed after 3 h to give 6a in 87%
yield upon isolation (Table 2, entry 1). Next, the use of chiral
ligands was examined. Whereas the use of (R)-binap (L1) was
totally ineffective in promoting the reaction (Table 2,
entry 2), the use of chiral diene L2[17,18] provided (R)-6a in
60% conversion with a promising 67% ee (Table 2, entry 3).
However, using 2.5 mol% of the rhodium complex derived
from chiral diene L3,[16,19] (R)-6a was obtained in greater than
95% conversion and 93% ee (Table 2, entry 4).[20] The results
tabulated in Tables 1 and 2 clearly highlight the benefits of
a cyclic imine structure in facilitating efficient allylation.[21] It
should be noted that the use of potassium allyltrifluoroborate
(2a) was essential for high enantioselectivity; repeating the
reaction in Table 2, entry 4 using allylboronic acid pinacol
ester in place of 2a, with the addition of aqueous K3PO4
(0.5 equivalents), led to (R)-6a in greater than 95% con-
version, but in only 28% ee. The reasons for this lower
selectivity are not clear at the present time.
[*] Dr. Y. Luo, H. B. Hepburn, N. Chotsaeng, Dr. H. W. Lam
EaStCHEM, School of Chemistry, University of Edinburgh
Joseph Black Building, The King’s Buildings
West Mains Road, Edinburgh EH9 3JJ (UK)
E-mail: h.lam@ed.ac.uk
Homepage: http://homepages.ed.ac.uk/hlam/index.html
[**] We thank the ERC (Starting Grant No. 258580), the EPSRC
(Leadership Fellowship to H.W.L.), and the Thai government for
support of this work. We thank Dr. Gary S. Nichol (University of
Edinburgh) for X-ray crystallography, and the EPSRC National Mass
Spectrometry Service Centre at the University of Wales, Swansea, for
high-resolution mass spectra. We thank Dr. Uwe Schneider (Uni-
versity of Edinburgh) for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204004.
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Table 2: Rh-catalyzed allylation of imine 5a.[a]
Table 3: Enantioselective Rh-catalyzed allylation of cyclic imines.
Entry Product
Yield [%][a] ee [%][b]
1
2
3
6a R=H
6b R=Me
6c R=OMe
6d R=F
6e R=Cl
6 f R=Br
6g R=CN
87
93
92
86
96
79
97
96
97
91
98
96
94
98
4[c]
5[c]
6[c]
7[c]
Entry
[Rh] (2.5 mol%)
Ligand (5 mol%)
Conv [%][b]
ee [%][c]
1
2
3
4
[{Rh(cod)Cl}2]
[{Rh(C2H4)2Cl}2]
[{Rh(C2H4)2Cl}2]
[{Rh(L3)Cl}2]
none
L1
L2
>95[d]
<5
60
–
–
67
93
none
>95
8
9
6h
>95
95
98
[a] Reactions were conducted using 0.10 mmol of 5a. [b] Determined by
1H NMR analysis of the unpurified reaction mixtures. [c] Determined by
HPLC analysis on a chiral stationary phase. [d] rac-6a isolated in 87%
yield.
6i
94
With an effective ligand identified, the scope of this
process was investigated under slightly modified reaction
conditions (Table 3). Using 1.5 mol% of [{Rh(L3)Cl}2] and
MeOH (5 equivalents) in a THF/dioxane mixture at 558C,
a wide range of cyclic imines underwent highly enantioselec-
tive allylations (90–99% ee) in generally good yields. Ben-
zoxathiazine-2,2-dioxides containing various substituents
(methyl, methoxy, halogen, cyano, or dioxole) were effective
substrates (Table 3, entries 1–10), although in certain cases,
the use of iPrOH (13 equivalents) in toluene/dioxane rather
than MeOH in THF/dioxane was required to maintain high
enantioselectivities (Table 3, entries 4–7). Other cyclic aldi-
mines such as 1,2,6-thiadiazine-1,1-dioxides also underwent
allylation (Table 3, entries 11–13), although the yields were
lower in these cases. The process is not restricted to aldimines;
cyclic ketimines such as 1,2,5-thiadiazolidine-1,1-dioxides
(Table 3, entries 14 and 15) and a cyclic sulfamidate imine
(Table 3, entry 16) are also viable substrates. These results
(Table 3, entries 14–16) are notable because catalytic enan-
tioselective allylations of ketimines are rare.[9g,12a,b]
10
6j
95
93
11[d]
12[d]
13[d]
6k R=H
6l R=Cl
6m R=Br
59
65
45
95
94
90
14
6n R=Me
6o R=Ph
84
88
99[e]
97
15[f]
The allylation of cyclic imines 5a and 5o[22] with more
highly substituted potassium allyltrifluoroborates 2b—e were
also carried out (Table 4). In these reactions, clean allylic
transposition took place to form a new carbon–carbon bond
at the more substituted g carbon atom of the allyltrifluoro-
borate. E-Crotyltrifluoroborate (2b) underwent highly enan-
tioselective additions to 5a and 5o to afford anti products 7a
and 7e, respectively, with high diastereoselectivities. Impor-
tantly, Z-crotyltrifluoroborate (2c) afforded the correspond-
ing syn products 7b and 7 f with high enantioselectivities,
although the diastereomeric ratio in the case of 7b was more
modest (6:1 d.r.). Similar to 2b, E-2-hexen-1-yltrifluoroborate
(2d) afforded anti products with high levels of diastereo- and
enantioselection (products 7c and 7g). Finally, prenyltrifluor-
oborate 2e was also highly effective, reacting with aldimine
16[g]
6p
83
93[e]
[a] Yield of isolated product. [b] Determined by HPLC analysis on a chiral
stationary phase. [c] Using iPrOH (13 equiv) in toluene/dioxane for
a reaction time of 18 h. [d] Reaction time of 15 h. [e] Enantiomeric excess
determined on the N-benzyl derivative. [f] Reaction time of 12 h.
[g] Using dioxane in place of THF, at 808C, for 15 h. Bn=benzyl.
5a to deliver reverse prenylation product 7d from aldimine
5a in 97% ee, and with ketimine 5o to give 7h, which contains
two vicinal quaternary centers, in 95% ee.
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Table 4: Allylation of cyclic imines with various allyltrifluoroborates.[a]
Allyltrifluoroborate
Product using 5a
Product using 5o
Scheme 1. Deuterium-labeling experiment.
2b
7a 91%
7e 68%
15:1 d.r., 99% ee
>19:1 d.r., 97% ee
ciable configurational stability during their lifetime in the
reaction mixture despite the possibility of rapid interconver-
sion between the two s-allyl isomers, a process which could
erode E/Z stereochemistry.[24,25]
On the basis of these features, a possible catalytic cycle for
these reactions, using imine 5a for illustrative purposes, is
shown in Scheme 2. Presumably, in the presence of MeOH,
potassium allyltrifluoroborates can undergo reversible meth-
2c
2d
2e
7b 89%[b]
6:1 d.r., 99% ee
7 f 89%
>19:1 d.r., 99% ee[c]
7c 76%[b]
11:1 d.r., 99% ee
7g 61%[b]
17:1 d.r., 99% ee
7d 91%
97% ee
7h 75%
95% ee
[a] Cited yields are of isolated pure major diastereomers (where
relevant). Diastereomeric ratios were determined by 1H NMR analysis of
the unpurified reaction mixtures. Enantiomeric excess values were
determined by HPLC analysis on a chiral stationary phase. [b] Yield of an
isolated inseparable mixture of diastereomers. [c] Reaction time of 12 h.
To gain some insight into the mechanism of these
reactions, allylation of ketimine 5o with dideuterated potas-
sium allyltrifluoroborate 8 was conducted (Scheme 1).[23] This
reaction resulted in a 1:1 mixture of products 9a and 9b in
88% yield, a result, which suggests that allylation proceeds
via an allylrhodium(I) species that undergoes rapid intercon-
version between the two s-allyl haptomers, thus scrambling
the deuterium label.[24] Alternative mechanisms involving an
allylboron species as the reactive intermediate would be
expected to furnish 9b only.[8]
Because E- and Z-crotyltrifluoroborates (2b) and (2c)
provided anti- and syn-allylation products, respectively, with
good to high diastereoselectivities (Table 2), it is likely that:
(i) 6-membered cyclic transition states are operative, and (ii)
the corresponding allylrhodium(I) intermediates have appre-
Scheme 2. Possible catalytic cycle.
anolysis to generate a mixed alkoxide/fluoride boron ate
complex 10,[26] which engages in transmetalation with the
chiral diene-ligated rhodium complex 11 (where X = Cl, F, or
OMe) to form the allylrhodium species 12. Coordination of
the imine 5a then occurs in a way that minimizes unfavorable
steric interactions between the imine activating group and
a phenyl substituent of the ligand, as shown in 13; such steric
interactions would be present in the disfavored diastereo-
meric complex 14. Allylation of the bound imine in 13 via
a cyclic chairlike transition state would give 15, which is then
protonated through proton transfer from HX (X = Cl, F, or
OMe) to release the product and regenerate 11.
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In conclusion, a chiral diene-ligated rhodium complex has
been shown to be highly effective in catalyzing the addition of
potassium allyltrifluoroborates to cyclic aldimines and keti-
mines. To our knowledge, these reactions represent the first
rhodium-catalyzed enantioselective nucleophilic allylations
of p electrophiles with allylboron compounds, and proceed
with generally good yields and high levels of diastereo- and
enantioselection. Where relevant, the chiral allylrhodium(I)
intermediates have appreciable E/Z configurational stability
during the time scale of allylation, thus allowing access to
different product diastereomers through the use of either E-
or Z-allyltrifluoroborates. Further investigations of enantio-
selective rhodium-catalyzed nucleophilic allylations are
underway, and the results of these studies will be reported
in due course.
Received: May 23, 2012
Published online: && &&, &&&&
Keywords: allyltrifluoroborates · asymmetric catalysis ·
.
enantioselectivity · imines · rhodium
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[21] For application of a catalytic enantioselective addition of an
allylsilane to a cyclic imine in total synthesis, see: T. Itoh, M.
Miyazaki, H. Fukuoka, K. Nagata, A. Ohsawa, Org. Lett. 2006, 8,
1295 – 1297.
[22] Attempted allylation of cyclic imines 5k–m with more sub-
stituted allyltrifluoroborate reagents provided complex mixtures
of products, wheras imine 5p was unreactive toward these
reagents.
[23] For similar experiments, see: a) J. D. Sieber, J. P. Morken, J. Am.
Chem. Soc. 2008, 130, 4978 – 4983; b) T. J. Barker, E. R. Jarvo,
Org. Lett. 2009, 11, 1047 – 1049; c) U. Schneider, H. T. Dao, S.
Kobayashi, Org. Lett. 2010, 12, 2488 – 2491.
[24] For a discussion of the enyl (s + p) character of electrophilic
allylrhodium(III) species, see: P. A. Evans, J. D. Nelson, J. Am.
Chem. Soc. 1998, 120, 5581 – 5582.
[25] B. M. Trost, D. L. VanVranken, Chem. Rev. 1996, 96, 395 – 422.
[26] For a discussion of organotrifluoroborate hydrolysis in Suzuki–
Miyaura reactions, see: a) M. Butters, J. N. Harvey, J. Jover,
A. J. J. Lennox, G. C. Lloyd-Jones, P. M. Murray, Angew. Chem.
2010, 122, 5282 – 5286; Angew. Chem. Int. Ed. 2010, 49, 5156 –
5160; b) A. J. J. Lennox, G. C. Lloyd-Jones, J. Am. Chem. Soc.
2012, 134, 7431 – 7441.
[20] The relative and absolute configurations of the products
obtained herein were assigned by analogy with those of 6h, 6i,
and 7e, which were determined by X-ray crystallography. See
Supporting Information for details. CCDC 880900 (6h), 880901
(6i), and 880902 (7e), contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
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.
Angewandte
Communications
Communications
Asymmetric Allylation
Y. Luo, H. B. Hepburn, N. Chotsaeng,
H. W. Lam*
&&&& — &&&&
Enantioselective Rhodium-Catalyzed
Nucleophilic Allylation of Cyclic Imines
with Allylboron Reagents
Chiral allylrhodium nucleophiles: The
highly diastereo- and enantioselective
title reaction of a range of cyclic imines
with various potassium allyltrifluorobo-
rates most likely proceeds via allylrho-
dium(I) intermediates, and represents
the first rhodium-catalyzed enantioselec-
tive nucleophilic allylation of p electro-
philes with allylboron compounds.
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!