Enantioselective rhodium-catalyzed addition of potassium alkenyltrifluoroborates to cyclic imines

Article abstract of DOI:10.1002/anie.201202136

Fixed: Cyclic imines, in which the C=N bond is constrained in the Z geometry, have been identified as highly effective substrates for enantioselective rhodium-catalyzed additions of potassium alkenyltrifluoroborates. Not only is the alkene in the products a useful functional handle for subsequent manipulations, products containing aryl sulfamates may be employed in nickel-catalyzed Suzuki-Miyaura and Kumada coupling reactions to generate further compounds of interest. Copyright

Full text of DOI:10.1002/anie.201202136

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DOI: 10.1002/anie.201202136
Asymmetric Catalysis
Enantioselective Rhodium-Catalyzed Addition of Potassium
Alkenyltrifluoroborates to Cyclic Imines**
Yunfei Luo, Andrew J. Carnell, and Hon Wai Lam*
Chiral a-branched allylic amines are important building
blocks for organic synthesis, and several catalytic asymmetric
methods have been developed for their synthesis. For
example, the enantioselective metal-catalyzed amination of
allylic electrophiles[1–3] and the rearrangement of allylic
imidates[4–6] have proven to be highly effective. An alternative
approach to chiral allylic amines that can be advantageous
from the viewpoint of convergency is the catalytic enantio-
selective union of an alkenyl nucleophile with an imine.[7–12] In
view of the widespread success of enantioselective RhI-
catalyzed additions of arylboron reagents to imines as
a means to access chiral a-aryl-branched amines,[13–15] the
development of the corresponding reactions of alkenylboron
reagents to prepare chiral a-branched allylic amines should
be an attractive goal. Surprisingly, however, only very limited
precedent exists for this transformation.[16] Brak and Ellman
have developed highly diastereoselective RhI-catalyzed addi-
tions of alkenylboron reagents to N-tert-butanesulfinyl aldi-
mines (Scheme 1A).[17] The only existing enantioselective
variant is that of Shintani, Hayashi, and co-workers, who, as
part of a study involving additions of potassium aryltrifluor-
oborates to N-sulfonyl ketimines, also described one example
using an alkenyltrifluoroborate (Scheme 1B).[15c] Also of
relevance is a single example of an enantioselective RhI-
catalyzed addition of an alkenylsilane to an N-sulfonyl
aldimine.[18] Therefore, a general enantioselective RhI-cata-
lyzed addition of alkenylboron reagents to imines remains
undeveloped.
Scheme 1. RhI-catalyzed additions of alkenylborons to imines.
MIDA=N-methyliminodiacetic acid.
bond is constrained in the Z geometry, appears to be
important for the success of the reactions.
This study began with the attempted alkenylation of
acyclic imines 1a–d with potassium (E)-1-hexenyltrifluoro-
borate (2 equiv) at 808C in dioxane for 24 h in the presence of
MeOH (5 equiv) and 1.5 mol% of the dimeric rhodium
complexes derived from chiral diene ligands[21,22] L1[15a] or
L2[23] (Table 1). Given that imines 1a–d are highly effective
Table 1: Attempted Rh-catalyzed alkenylation of various imines.
Herein, we demonstrate that cyclic imines are highly
effective substrates for enantioselective RhI-catalyzed addi-
tions of potassium alkenyltrifluoroborates,[19,20] providing
products in excellent enantioselectivities and generally good
=
yields. The cyclic structure of these imines, in which the C N
[*] Dr. Y. Luo, 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
Entry
Imine
Ligand
Product
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
7
8
9
10
1a
L1
L2
L1
L2
L1
L2
L1
L2
L1
L2
3a
<5
<5
11
–
–
23
7
–
Homepage: http://homepages.ed.ac.uk/hlam/index.html
1b
1c
1d
2a
3b
3c
3d
4a
80
Dr. A. J. Carnell
Department of Chemistry, University of Liverpool
Crown Street, Liverpool, L69 7ZD (UK)
<5[c]
<5[c]
45
–
43
55
96
98
[**] We thank the ERC (Starting Grant No. 258580) and the EPSRC
(Leadership Fellowship to H.W.L.) for support of this work, 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.
55
76
>95
[a] Yield determined by NMR analysis using nitromethane as an internal
standard. [b] Determined by HPLC analysis on a chiral stationary phase.
[c] Significant decomposition of 1c was observed.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202136.
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substrates for enantioselective RhI-catalyzed additions of
arylboron reagents,[14] and chiral diene L1 has provided
excellent results in these types of reactions,[15a] we were
surprised to find that imine alkenylation was far from
straightforward.
Table 2: Alkenylation of benzoxathiazine-2,2-dioxide 2a.
Tosylimine 1a and diphenylphosphinoylimine 1c were not
viable substrates, and no alkenylation was observed using L1
(Table 1, entries 1 and 5). In these reactions, imine 1a
remained largely intact, but imine 1c underwent significant
decomposition. While appreciable alkenylation was observed
using L1 with both nosylimine 1b and N,N-dimethylsulfamy-
limine 1d, the enantiomeric excesses of the corresponding
products were low (Table 1, entries 3 and 7). Similar results
were obtained using L2 as the ligand (Table 1, entries 2, 4, 6,
and 8), with the exception that alkenylation was significant
with nosylimine 1b (entry 4).
Entry
Trifluoroborate
Product
Yield [%][a]
ee [%][b]
1
2
3
4a
4b
4c
90
75
79
98
98
97
4
5
4d
4e
94
88
99
95
The results listed in Table 1, entries 1–8 clearly highlight
the difficulties of these alkenylation reactions compared with
the corresponding arylations.[13–15] The mostly poor conver-
sions into the desired products may be explained by the fact
that alkenylrhodium species are less stable than arylrhodium
species, which renders protodeboronation or other decom-
position pathways highly competitive with imine addition.[24]
However, it is more difficult to rationalize the low enantio-
selectivities obtained when alkenylation was successful
(Table 1, entries 3, 4, 7, and 8). One factor to consider in all
catalytic asymmetric additions to imines is the possibility of
E/Z isomerization of the imine during the reaction, which
usually has a negative impact upon stereoselectivity.[7a]
Although this issue does not appear to be problematic for
RhI-catalyzed imine arylation,[13–15] we surmised that it could
be important in imine alkenylation.
6
4 f
94
94
[a] Yield of isolated product. [b] Determined by HPLC analysis on a chiral
stationary phase. PMP=para-methoxyphenyl, Cy=cyclohexyl.
Table 3: Alkenylation of various benzoxathiazine-2,2-dioxides.[a]
To test this theory, the alkenylation of benzoxathiazine-
2,2-dioxide 2a, a cyclic imine in which E/Z isomerization is
precluded, was examined. Surprisingly, to our knowledge,
benzoxathiazine-2,2-dioxides have been virtually unexplored
as electrophiles in reactions with carbon nucleophiles.[25,26] We
were therefore delighted to observe that under conditions
identical to those employed for imines 1a–d, imine 2a
provided the alkenylation product 4a in high conversions
and enantioselectivities (Table 1, entries 9 and 10), with
ligand L2 giving the best results (entry 10).[27]
Under the optimized conditions, imine 2a smoothly
reacted with various alkenyltrifluoroborates[28] containing
alkyl (Table 2, entries 1, 3, and 4) or aryl (entry 5) substituents
at the b-carbon to provide alkenylation products in good
yields and high enantioselectivities (95–99% ee). In addition,
vinylation was successful (Table 2, entry 2), and substitution
at the a-carbon of the alkenyltrifluoroborate was tolerated
(entry 6). Interestingly, conducting the experiments in
Table 2, entries 2 and 3 with the corresponding alkenyl
MIDA (N-methyliminodiacetic acid) in place of the alkenyl-
trifluoroborates under the conditions described by Brak and
Ellman[17b] proceeded with less than 20% conversion into 4b
and 4c, respectively.
5a 92%, 98% ee
5d 85%, 95% ee
5g 86%, 97% ee
5b 88%, 94% ee
5e 81%, 96% ee
5h 81%, 97% ee
5c 83%,>99% ee
5 f 86%, 97% ee
5i 93%, 98% ee
Table 3 presents the alkenylation of more highly substi-
tuted benzoxathiazine-2,2-dioxides. Imines containing a range
of arene substituents (including methyl, methoxy, chloro,
bromo, and fluoro) at various positions were competent
5j 50%, 97% ee
5k 93%, 94% ee
5l 55%, 96% ee
[a] Cited yields refer to isolated material. Enantiomeric excesses were
determined by HPLC analysis on a chiral stationary phase.
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substrates, providing alkenylation products in ꢀ 81% yield
and ꢀ 94% ee (products 5a–i). However, the reaction of
potassium vinyltrifluoroborate with a benzoxathiazine-2,2-
dioxide containing the electron-donating dioxole group
provided 5j in only 50% yield, though in 97% ee. Presum-
ably, the modest yield observed here is due to the greater
propensity of potassium vinyltrifluoroborate to undergo
protodeboronation or polymerization compared with its
more sterically hindered counterparts, a problem that is
compounded by the lower electrophilicity of this imine. As
expected, a more highly substituted alkenyltrifluoroborate
provided better results, with 5k being formed in 93% yield
and 94% ee. Finally, the benzoxathiazine-2,2-dioxide derived
from 2-hydroxy-1-naphthaldehyde was also a suitable sub-
strate, though owing to the steric hindrance associated with
this imine, product 5l was formed in a modest 55% yield.
Cyclic N-sulfonyl ketimine 6 was also a viable substrate,
providing sultam 7 in 68% yield and 90% ee [Eq. (1)].[29] This
result further confirms the beneficial effect of a cyclic imine
structure, and demonstrates that the high efficiency of these
reactions is not confined to benzoxathiazine-2,2-dioxides.
activating group and one phenyl substituent of the ligand
(Figure 1). Carborhodation from the re face of the imine then
occurs to eventually provide the product.[30,31]
Figure 1. Possible stereochemical model for the formation of 4.
Scheme 2 illustrates the utility of the alkenylation prod-
ucts. Aryl sulfamates have recently been shown to be highly
effective in a range of nickel-catalyzed cross-coupling reac-
tions.[32–34] For example, Wehn and Du Bois have described
Kumada couplings of cyclic aryl sulfamates,[32b] and Garg and
co-workers have developed Suzuki–Miyaura reactions of
their acyclic counterparts.[33a,b] It was therefore of interest to
ascertain whether nickel-catalyzed Suzuki–Miyaura reactions
would be successful with cyclic aryl sulfamates derived from
the alkenylation products of this study. To this end, 4a was
converted into cyclic sulfamate 8 by alkene hydrogenation
followed by N-methylation. Gratifyingly, application of
Gargꢀs conditions[33a,b] for the Suzuki–Miyaura coupling of 8
with PhB(OH)2 smoothly delivered the biaryl compound 9 in
72% yield after acid-mediated cleavage of the sulfamic acid
intermediate.
Next, a hydroboration/oxidation sequence applied to 4b
gave alcohol 10 in 91% yield. Treatment of 8 with LiAlH4 at
reflux[35] followed by Boc2O provided carbamate 11, which
was then converted into chroman-4-amine 12 by means of
a Mitsunobu cyclization. Chroman-4-amines appear as core
scaffolds in several drug candidates, for example in the human
bradykinin B1 receptor antagonist 13.[36]
The sense of enantioinduction of these reactions[27] is
consistent with the stereochemical model proposed for the
1,4-arylation of cyclic enones.[21a] Following this model, bind-
ing of the imine to the chiral diene-ligated alkenylrhodium
species is suggested to occur in a manner that minimizes
unfavorable nonbonding interactions between the imine
Scheme 2. Further transformations of alkenylation products. 9-BBN=9-borabicyclo[3.3.1]nonane, Boc=tert-butoxycarbonyl, DEAD=diethyl
azodicarboxylate, NMO=N-methylmorpholine-N-oxide, dppp=1,3-bis(diphenylphosphino)propane.
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[8] Enantioselective metal-catalyzed reductive coupling of alkynes
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efficient ring-closing metathesis using the second-generation
Grubbs catalyst[37] to give dihydropyrrole 15. Dihydroxylation
of 15 from the least hindered face followed by acetonide
protection of the resulting diol provided 16, which was then
transformed into the biaryl-containing dihydroxylated pyrro-
lidine 17 in 84% yield by nickel-catalyzed Kumada coupling
with PhMgBr and acidic workup according to the method of
Wehn and Du Bois.[32b] Dihydroxylated 2-aryl pyrrolidines
similar to 17 are of interest as potential glycosidase inhib-
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additions of alkenylboron compounds to cyclic imines have
been described. The cyclic structure of these imines, in which
=
the C N bond is constrained in the Z geometry, appears to be
important, allowing alkenylation to proceed in generally good
yields and high enantioselectivities (ꢀ 94% ee). Moreover,
products containing aryl sulfamates may be exploited in
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Received: March 17, 2012
Published online: May 25, 2012
Keywords: alkenyltrifluoroborates · asymmetric catalysis ·
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enantioselectivity · imines · rhodium
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