Kinetic resolution of alkyne-substituted quaternary oxindoles via copper catalysed azide-alkyne cycloadditions

Article abstract of DOI:10.1039/c5cc04886a

The synthesis and kinetic resolution of quaternary oxindoles through copper catalysed azide-alkyne cycloadditions is presented. Selectivity factors (s) up to 22.1 ± 0.5 are reported. Enantioenriched alkynes and triazoles were obtained in ≥80% enantiomeric

Full text of DOI:10.1039/c5cc04886a

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Kinetic resolution of alkyne-substituted
quaternary oxindoles via copper catalysed
azide–alkyne cycloadditions†
William D. G. Brittain,a Benjamin R. Buckley‡*b and John S. Fossey‡*a
Cite this:DOI: 10.1039/c5cc04886a
Received 12th June 2015,
Accepted 12th August 2015
DOI: 10.1039/c5cc04886a
www.rsc.org/chemcomm
The synthesis and kinetic resolution of quaternary oxindoles through thus enabling recovery of non-racemic alkynes which could
copper catalysed azide–alkyne cycloadditions is presented. Selectivity be further derivatised.
factors (s) up to 22.1 Æ 0.5 are reported. Enantioenriched alkynes and
triazoles were obtained in Z80% enantiomeric excess (e.e.).
To the best of our knowledge there are no reports of successful
KRs of alkynes via the CuAAC reaction. Kinetic resolution of
terminal alkynes is therefore of importance to the overall develop-
Click chemistry was first outlined in 2001 by K. B. Sharpless ment of stereoselective click chemistry. Research into asymmetric
and co-workers, the copper catalysed azide alkyne cycloaddition click methodology thus far has been mainly focused on desym-
(CuAAC) has lived up to Sharpless’ accolade as ‘‘the cream of metrisation and only a single successful report on KR (of azides)
the crop’’ in this area.1 The CuAAC reaction predictably delivers has been published.
1,4 substituted triazoles in good yields, from the reaction of
Resolution of racemic azides via the CuAAC reaction has been
terminal alkynes with organic azides in the presence of a Cu(I) achieved by Meng et al.7 using a Cu–PyBox catalyst,8 selectivity
catalyst. Despite the ubiquity of the CuAAC reaction there are factors (s) up to s = 8 where obtained.9 Meng et al. were unable
relatively few reports of asymmetric variants.2 Yet enantio- to kinetically resolve racemic terminal alkynes (s = 1).7 A range of
enriched triazoles and alkynes are potentially important in five different terminal alkynes were tested and no enantiomeric
many areas of chemistry and biology.3
discrimination was observed whatsoever with any compounds
Kinetic resolution (KR) of a racemic mixture takes advantage tried. They accounted for this observation using a mononuclear
of a difference in the rate of the reaction of either enantiomer transition state model. However, more recently evidence has
through diastereomeric interactions.4 Fu and co-workers have emerged that Cu–PyBox systems might not always follow a mono-
championed non-enzymatic catalytic methods for the KR of nuclear arrangement. Panera et al. reported X-ray crystal structures
secondary alcohols and other substrates.5 In this area recent of binuclear phenyl and isopropyl PyBOX copper(I) chloride
advances have built on Fu’s findings and resolution of chiral complexes, and used these solid state structures to rationalise
alcohols is a relatively mature field.6 A particular advantage of the stereochemical outcome of their asymmetric catalytic
KR is that it is possible to obtain high e.e.s of both unreacted synthesis of propargylamines.14 It struck us that this binuclear
starting materials and products dependent upon the selectivity PyBox model could have synergy with the binuclear model for
factor (s) and the conversion. Kagan showed that for a given KR the CuAAC, proposed by Fokin et al.15 and Kuang et al.16 Work
it is relatively straight forwards to relate conversion and starting by Jin et al. and Makarem et al. has provided strong evidence to
material e.e. to selectivity factor. KR of chiral alkynes or azides support this catalytic model with stable dinuclear copper
through triazole formation is one strategy open to exploitation acetylide intermediates being successfully crystallised.17
to access enantioenriched triazoles and starting materials,
Our team has a long-standing interest in quaternary oxindoles,18
and a relevant report on desymmetrisation,2a prompted us to begin
investigation of their potential as substrates for catalytic kinetic
resolution. Oxindole derivatives (Fig. 1) can possess potent
biological activity as calcium channel blockers,19 anti-
angiogenics,20 antitumor agents21 and analgesics.19,22 For example,
the antitumor agent I, the natural products donaxaridine II,
dioxibrassinine III and the popular natural product target
gelsamine IV (Fig. 1). Therefore, efficient access to nonracemic
quaternary oxindoles is of considerable interest to the
a School of Chemistry, University of Birmingham, Edgbaston, Birmingham,
West Midlands, B15 2TT, UK. E-mail: j.s.fossey@bham.ac.uk
b Department of Chemistry, Loughborough University, Loughborough, Leicestershire,
LE11 3TU, UK. E-mail: b.r.buckley@lboro.ac.uk
† Electronic supplementary information (ESI) available: Experimental procedures,
HPLC traces, NMR data and detailed screening tables. See DOI: 10.1039/c5cc04886a
‡ The project was conceived equally by JSF and BRB. Experiments were performed
at the University of Birmingham hence JSF is corresponding author for experi-
mental details, but both BRB and JSF may be contacted about this manuscript.
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Table 1 KR of 1 with PhPyBox and various copper sources
Conv.a e.e. SMb e.e. 2ab Selectivity
Entry Copper Source
(%)
(%)
(%)
factor (s)c
1
2
3
4
5
6
7
8
9
CuCl
CuBr
CuI
37
14
51
34
8
26
3
0
4
1
0
0
43
8
5.3
3.2
2.1
1.8
—
2.8
—
29
16
—
57
—
—
—
Cu(OTf)Átoluene 0.5 10
Cu(OAc)
0
8
13
0
Cu(OAc)2
CuSO4, NaAsc
Cu(OTf)2
—
—
Fig. 1 Exemplar biologically active quaternary C-3 oxindoles; I: antitumor
agent;10 II: donaxaridine;11 III: dioxibrassinine;12 IV: gelsemine.13
Cu metal
0
a
b
Conversion determined by inspection of 1H NMR spectra (see ESI).
e.e. of recovered starting material (HPLC). s = ln[(1 À c)(1 À ee)]/
c
medicinal chemistry23 and veterinary science24 communities.
Not only is access to non-racemic triazoles of interest but access
to high e.e. alkynes is of importance. Access to enantioenriched all
carbon quaternary stereogenic centres bearing alkyne functionality
is demanding,25 and alkynes are a versatile synthetic handle capable
of undergoing a diverse array of transformations.26 Access to
enantioenriched alkynes therefore could lead to a plethora of
stereocontrolled derivatives. In order to test whether kinetic resolu-
tion of alkyne appended quaternary oxindoles is possible (generically
represented in Scheme 1a) we prepared compound 1. Propargylation
of 3-methyl-2-oxindole (see ESI†), followed by N-benzylation
gave racemic 1 (Scheme 1b), our substrate for KR.
To our delight, an initial probe reaction of racemic 1 with
benzyl azide, in acetone under control of a combination of
Cu(I)Cl and PhPyBOX (L1) (12.5 and 15 mol% respectively) gave
a selectivity of s = 5.3. Conversion was determined by examina-
tion of proton NMR spectrums and e.e. by HPLC analysis (see
ESI† for details). Optimisation of the reaction conditions was
then explored. A series of readily available copper sources were
probed, including Cu(I) Cu(II) and Cu(0) species, under standard
conditions of 0.6 equivalents of benzyl azide (3b), 12.5 mol% of
copper source and 15 mol% of PhPyBox ligand (L1) at 0 1C for
ln[(1 À c)(1 + ee)].
96 h (Table 1). Of copper sources tested, our first choice, Cu(I)Cl
was confirmed as the superior choice selectivity factor (Table 1,
entry 1). Cu(I)Br gave a lower conversion and selectivity factor
(Table 1, entry 2); Cu(I)I offered improved conversion but selectivity
factor was again compromised (Table 1, entry 3). Of the other
copper sources tried Cu(I)OTf, Cu(II)(OAc)2 and CuSO4 combined
with NaAsc (Table 1, entries 4, 6 and 7 respectively) gave approxi-
mately stoichiometric conversion with respect to catalyst loading.
Cu(I)OAc, Cu(II)OTf2 and Cu(0) NaAsc (Table 1, entries 5, 8 and 9
respectively) did not deliver any triazole-containing products.
Increasing the reaction temperature did not improve the reac-
tion outcomes and Cu(I)Cl was selected as the copper source of
choice for further optimisation studies.
Next, the choice of solvent was investigated (Table 2), acet-
one had already been shown to give a promising selectivity and
is restated in Table 2 (entry 1) to aid comparison. Surprisingly
2-butanone was ineffective (Table 2, entry) giving no conversion.
Since Zhou et al. had already reported dicarbonyl containing
solvents were effective for a related desymmetrisation reaction
we also included this class of solvent in our screening.2a,27 2,5-
Hexanedione (Table 2, entry 3) gave a huge jump in selectivity
factor (s = 22.1 Æ 0.5) and good conversion (42%), 2,3-
butanedione (Table 2, entry 4) gave poorer conversion and
poorer selectivity (34% and s = 2.5 respectively). A mixture of
acetone and 2,5-hexanedione (Table 2, entry 5) gave 61% con-
version (against 0.6 equiv. of azide, i.e. full conversion) but
selectivity factor was also poor (s = 3.5). THF gave a promising
selectivity factor of s = 8.7 (Table 2, entry 6), and all other solvents
tried gave inferior results (Table 2, entries 7–12). We speculated
that 2,5-hexanedione may be acting as a ligand for copper but use
in substoichiometric amounts (equivalent to catalyst loading) had
a negative effect on the selectivity. During the course of this
optimisation study the order of addition was found to be crucial
to obtaining reproducible results. Importantly, the alkyne must be
added to a solution of in situ formed catalyst at room temperature
before cooling to 0 1C (see ESI† for full details).
Scheme 1 (a) General scheme for the CuAAC KR strategy; (b) application
of KR to alkynyl C-3 quaternary oxindoles.
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Table 2 KR of 1 with PhPyBox, CuCl and various solvents
Conv.a e.e. SMb e.e. 2ab Selectivity
Entry Solvent
(%)
(%)
(%)
factor (s)c
1
2
3
4
5
Acetone
2-Butanone
2,5-Hexanedione
2,3-Butanedione
Acetone : 2,5-hexanedione 61
(10 : 1)
37
0
42
34
34
—
61
18
53
43
—
65
70
59
5.3
—
Scheme 2 Optimised conditions for the CuAAC kinetic resolution,
demonstrating Z80% e.e. starting material and product can be obtained.
22.1d
2.5
3.5
essentially unchanged but selectivity factor dropped a little
(albeit it a respectable s = 17.5). 4-Methyl benzyl azide (3c)
and 3,5-trifluoromethyl benzyl azide (3d) (Table 3, entries 3 and
4 respectively), offered the chance to judge any effects due to
electronic parameters. In both cases similar selectivity factors
(13.1 and 11.1) were obtained with a slightly lower conversion
when 3d was used.
Finally, as with any kinetic resolution, by judicious choice of
reaction time, and therefore conversion, it is possible to obtain
higher e.e. of starting material or product. Indeed, in our case
we can obtain Z80% e.e. of 2a (albeit at low conversion),
see Scheme 2.
These preliminary findings demonstrate that not only is it
possible to perform catalytic kinetic resolution with the CuAAC
reaction, but selectivity factors greater than 20 are possible
Scheme 2. Optimised conditions for the CuAAC kinetic resolu-
tion, demonstrating Z80% e.e. starting material and product
can be obtained with biologically relevant substrates. Order of
addition, choice of ligand and choice of solvent were key to
achieving reproducible results. It was shown that dependant on
reaction conversion high e.e. alkynes are recoverable and these
could be used for further derivatisation.
6
7
8
9
10
11
12
THF
36
17
19
13
6
44
14
40
6
0
59
20
—
—
—
13
13
8.7
1.9
1.0
1.0
—
1.9
1.1
1,4-Dioxane
tBuOH
tBuOH/H2O
H2O
0
—
18
1
DMSO
Acetonitrile
a
Conversion determined by inspection of 1H NMR spectra (see ESI) b e.e. of
c
recovered starting material (HPLC) s = ln[(1 À c)(1 À ee)]/ln[(1 À c)(1 + ee)]
d Average of three s = 22.1 Æ 0.5, best unique case s = 23.2.
Despite a broad range of alternative chiral ligands being
screened, in our hands, it was only PhPyBox that permitted
effective KR. Ligand classes screened that were inferior to
PhPyBox included other PyBox variants,28 PhOx ligands,29 Trost
ligands,30 phosphoramidites,31 BINAP32 and BINOL.33 For full
details of ligand screening see ESI.† Further investigation is
needed to probe the ligand requirements for this reaction.
The scope of the benzyl azide was investigated next. Initially
we ran experiments using in situ prepared azides (from sodium
azide and benzyl halide derivatives). Whilst in situ preparation
of azides offers the advantage of minimising the number of
discrete azide manipulations,34,35 conversions were very poor.
So for this communication we prepared just three more isolated
benzyl azides to compare against our benchmark reaction
(Table 3, entry 1). We probed increasing steric bulk (Table 3,
entry 2) by using 2-phenyl benzyl azide (3b), conversion was
Further expansion of substrate scope and detailed mecha-
nistic studies remain to be carried out and we look forward to
the opportunity to report on that in due course.
JSF and WDGB would like to thank the University of Birmingham
for support. BRB thanks Loughborough University and Research
Councils UK for a RCUK Fellowship. Dr Neil Spencer and Dr Chi
Tsang are thanked for helpful discussions about NMR spectroscopy
and HPLC analysis respectively. WDGB would also like to thank the
RSC and the School of Chemistry at the University of Birmingham
for travel support facilitating useful discussions via the CASE Net-
work.36 All investigators are grateful for a Royal Society Research
Grant (2012/R1) that underpins this project.
Table 3 KR using different, isolated, azides
Selectivity
Entry
R
Conv.a (%)
e.e. SMb (%)
factor (s)c
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