A Modular Synthesis of Multidentate S-, N- and O-Containing Meta- and Paracyclophanes

Article abstract of DOI:10.1002/ejoc.201501058

The development of a modular approach to macrocycle assembly has enabled the synthesis of a library of pyridine-based macrocycles possessing multiple donor sites where chirality was readily introduced from (R)- or (S)-alanine, a representative amino acid. The facile, regioselective, nucleophilic ring opening of aziridines by dithiols enabled the synthesis of thioether-based linkers which on subsequent alkylation provided access to optically pure macrocycles. A modular approach to macrocyclic assembly has enabled the synthesis of a library of macrocycles possessing multiple donor sites where chirality was readily introduced from (S)-alanine. Key to this approach was the facile, regioselective, nucleophilic ring opening of aziridines by dithiols followed by macrocylisation under conditions of high dilution.

Full text of DOI:10.1002/ejoc.201501058

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201501058
A Modular Synthesis of Multidentate S-, N- and O-Containing Meta- and
Paracyclophanes
Omer K. Rasheed,[a] Patrick D. Bailey,[b] Amy Lawrence,[a] Peter Quayle,*[a] and
James Raftery[a]
Keywords: Macrocycles / Aziridine / Dithiol / Chiral pool / Metacyclophanes
The development of a modular approach to macrocycle as-
sembly has enabled the synthesis of a library of pyridine-
based macrocycles possessing multiple donor sites where
chirality was readily introduced from (R)- or (S)-alanine, a
representative amino acid. The facile, regioselective, nucleo-
philic ring opening of aziridines by dithiols enabled the syn-
thesis of thioether-based linkers which on subsequent alkyl-
ation provided access to optically pure macrocycles.
Introduction
Results and Discussion
Previously Bailey et al. had reported the synthesis of a
range of pyridine-derived macrocycles with the aim of
evaluating their efficiency in a number of catalytic, asym-
metric processes.[9a] In the pursuance of this goal the syn-
thesis of ester 1 and ether 2 was also attempted. While the
synthesis of diester 1 was accomplished this system proved
to be hydrolytically unstable;[9b] the synthesis of ether 2 was
also marred by the apparent lack of regio control exhibited
during the ring opening of aziridine (S)-3 with oxygen-
centred nucleophiles such as 4 (Figure 1).[8g]
At this point the focus of our investigation switched to
the reaction between aziridine 3 with sulfur nucleophiles.
These reactions are known to proceed under milder condi-
tions than those of their oxygen counter parts.[8h,8j] Ulti-
mately, we wished to probe whether a two-directional, azir-
idine ring opening – toluenesulfonamide cyclisation strat-
egy, could be utilised in the synthesis of macrocycles pos-
sessing a variety of donor centres (Figure 2).
We were aware of only a limited number of reports con-
cerning the use of dithiols in the crucial aziridine ring-open-
ing reaction,[10] a reaction that we wished to address in the
initial phase of this investigation. Encouragingly we noted,
in a trial experiment, that reaction between (S)-3 and thiol
7 (1.0 equiv.) in methanolic triethylamine[11] (2.1 equiv.;
35 °C, 4.5 h) afforded the toluenesulfonamide 8 in 86%
yield after chromatography (Scheme 1; see the Supporting
Information for details).
Transition metals are not only vital to life (iron, copper,
zinc) but can also display accumulative and acute toxicity
(cadmium, mercury).[1] In general, the facility to detect, se-
lectively remove or attenuate the chemistry/biochemistry of
heavy metals has many potential applications ranging from
water treatment to the management of life-changing disease
states.[2] It is now apparent that mixed-donor macrocyclic
ligands, which incorporate both “soft” and “hard” donor
sets, can offer a range of co-ordination modes[3] (either exo-
or endo-) that are not available to ligand systems that pos-
sess only “hard” donor sites. The ability of thioether based
ligands to bind to metals has therefore taken on an extra
dimension leading to ligands that are capable of binding to
“soft” metal centres.[4]
The use of aziridines as chiral pool starting materials is
now well established[5] and the aziridine moiety is also a key
structural motif present in a number of natural products.[6]
Key to the use of aziridines in synthesis is their accessibility,
in optically pure form,[7] coupled with their ability to un-
dergo regioselective ring opening with a range of nucleo-
philes in a regio- and stereo-defined manner.[8] In this work
we present our findings on the use of aziridines as chiral
pool materials in the synthesis of macrocycles bearing both
soft and hard donor centres.
[a] School of Chemistry,
University of Manchester,
Analysis of the 1H NMR spectrum of the crude reaction
mixture leading to 8 indicated that the reaction was com-
pletely regioselective, with attack of the thiol taking place
at the least encumbered, methylene, carbon of (S)-3. Con-
ducting this reaction in a focused microwave reactor,[12] but
under otherwise identical conditions, had no observable ef-
fect on either the yield or rate of the reaction. The general-
Oxford Road,
Manchester M13 9PL, UK
E-mail: peter.quayle@manchester.ac.uk
[b] Department of Chemical and Petroleum Engineering,
London South Bank University,
103 Borough Road,
London SE1 0AA, UK
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201501058.
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Multidentate S-, N- and O-Containing Meta- and Paracyclophanes
Figure 1. Previous studies by Bailey et al.[9]
ity of this protocol was next evaluated by using a series of
commercially available dithiols in combination with either
(S)-3 or (R)-3. In all cases the alkylation reactions were
highly regioselective and afforded the desired bis-alkylated
products in good isolated yields, Figure 3.
The structure of the bis-toluenesulfonamide 10 was also
confirmed unambiguously by way of a single-crystal X-ray
structure determination (see Supporting Information for
details).[13]
Extension of this basic strategy to heterocyclic thiols was
also briefly investigated. Hence, reaction of bismuthiol
(pKa11.4; pKa2 7.4),[14a] a reagent commonly employed in
trace metal analysis,[14b,14c] with (S)-3 in the presence of
methanolic triethylamine, as above, afforded 11 in 82%
yield. The use of (R)-3 in these reactions also proceeded
smoothly enabling the isolation of 12 in good overall yield
(61%). The regiochemistry of these double alkylation reac-
tions (S vs. N of the thiadiazole ring) was evident from their
13C NMR spectra on the grounds of symmetry and chemi-
cal shift [C(2)–S/C(5)–S at δ = 165.59 ppm].[14d,14e]
Figure 2. Two-directional aziridine ring opening – toluenesulfon-
amide cyclisation reactions.
The synthesis of the more rigid linker, 15, was also ac-
complished via the intermediacy of dithiol 14. Reaction of
1,3-bis(bromomethyl)benzene 13 with thiourea (2 equiv.;
EtOH; 3 h) followed by hydrolysis of the of thiouronium
salt (NaOH, aq.; 4 h) afforded the dithiol 14 in a quantita-
Scheme 1. Two-directional aziridine ring-opening reaction.
Figure 3. Generality of aziridine ring-opening reaction with dithiols.
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tive yield (Scheme 2) after acidification.[15] Reaction of di-
thiol 14 with aziridine (S)-3 in methanolic triethylamine
(2.1 equiv.) afforded 15, a colourless, crystalline solid, in
61% yield. This sequence was also repeated with 1,4-bis-
(bromomethyl)benzene 16 and resulted in the isolation of
bis-toluenesulfonamide 18 in 89% yield over the three steps.
Scheme 4. Synthesis of functionalized linkers.
Having successfully prepared a small library of linkers
(8, 9–12, 15, 18, 20 and 24), their incorporation into a
macrocyclic framework was next investigated. Macrocyclis-
ation[18] of these linkers with dibromide 19 was routinely
conducted in DMF as the solvent (typically at 3 mm con-
centrations) with Cs2CO3 (5 equiv.) as the base and af-
forded a series of meta- and para-cyclophanes in yields
ranging from 39–96% (Scheme 5). In certain cases minor
quantities (3–8%) of “expanded” macrocycles[19] (such as
33–35) were also isolated from the macrocyclisation reac-
tion although in these cases purification of the major com-
ponent was readily achieved by column chromatography.
We were also fortunate to grow crystals of (S,S)-26 and
(S,S)-28 which proved suitable for X-ray crystallographic
analysis.
Although these two compounds are closely related,
analysis of their crystal structures revealed a number of
conformational subtleties in the solid state. In (S,S)-26 the
macrocyclic core adopts a conformation in which both the
linker aromatic and pyridine rings are syn-disposed, gener-
ating a “Pacman”[20] structure in which the link between S1
to S2 takes up a “boat–chair” arrangement,[21] both S1 and
S2 are exo-disposed. There is a noticeable tilt of the pyr-
idine ring towards the aromatic ring in the linker (angle
between the mean planes of each ring = 40.2°) and a rela-
tively short H26B–C33 separation of 2.98(4) Å, which may
be indicative of π–π and CH–π interactions, respectively.[22]
Finally, the C19–N1–C1–C2 and C13–N2–C12–C11 torsion
Scheme 2. Synthesis of more rigid, benzylic, linkers.
Given the malodorous nature of many of these thiols we
also demonstrated the feasibility of undertaking a one-pot,
three-step procedure for the preparation the bis-toluene-
sulfonamide 20. Hence, conversion of 2,6-bis(bromo-
methyl)pyridine 19, which is readily available from dipico-
linic acid, to its bis-thiouronium salt (thiourea, 2 equiv.;
EtOH, 78 °C; 30 min), followed by thiolate liberation
(NaOH aq., EtOH, 78 °C) and addition of (S)-3 afforded
the bis-toluenesulfonamide 20 in 89% overall yield after
chromatography (Scheme 3).[16]
Scheme 3. Synthesis of a pyridine-containing linker.
In wishing to incorporate additional functionality into angles in 26 [75.8(2)° and 78.23°, respectively] are such that
the final macrocycle the synthesis of bis-toluenesulfonamide the C27/C28 methyl groups adopt a pseudo trans-diaxial
24 was also investigated. Conversion of 5-hydroxyiso- disposition with respect to the macrocyclic core [C27–C1–
phthalic acid 21, into the allylated diester 22 was readily C2–S1 = –72.6(9)°; C28–C12–C11–S2 = –161.4(5)°] re-
accomplished in essentially quantitative yield using stan- sulting in a large C27–C28 separation of 8.0(4) Å (Fig-
dard methodology (see Supporting Information for details). ure 4).
Reduction of 22 (DIBAL-H, 4 equiv.; CH2Cl2) to the diol
In comparison to (S,S)-26, the X-ray structure of (S,S)-
and conversion to the known dibromide 23[17] (PBr3, 28 proves to be much more complex in that this macrocycle
0.3 equiv.; pyridine, 6 mol-%; 22% yield) followed by the adopts three crystallographically distinct conformations in
implementation of our in situ thiol generation/alkylation re- the solid state.[22]
action afforded bis-toluenesulfonamide 24 in 74% yield
(Scheme 4).
Two limiting conformations for this structure are de-
picted in Figure 5. Not unexpectedly, there is considerable
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Multidentate S-, N- and O-Containing Meta- and Paracyclophanes
Scheme 5. Exemplification of the macrocylisation step. Reagents and conditions: 19 (1 equiv.), Cs2CO3 (5 equiv.); spacer 8, 9, 10, 11, 15,
18, 20 or 24, (1 equiv.), DMF (300 mL mmol–1 dithiol); 20 °C, 24 h.
Figure 4. X-ray structure of (S,S)-26 (hydrogen atoms omitted for clarity).
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O. K. Rasheed, P. D. Bailey, A. Lawrence, P. Quayle, J. Raftery
SHORT COMMUNICATION
disorder about the allyloxy group in both (S,S)-28a and
(S,S)-28b. Both of these structures exhibit the “Pacman”
motif about the macrocyclic core, as observed for (S,S)-26,
and, once again, the pyridine and aromatic linker residues
are tilted towards one another.
Scheme 6. Synthesis of S,C,S-Pd pincer complexes.
Conclusions
In conclusion we have developed a modular approach to
the synthesis of a family of conformationally mobile,
mixed-donor, pyridine-based macrocycles which involves
the two-directional capping of dithiols with aziridines. The
use of readily available chiral pool starting materials derived
from either (R)- or (S)-alanine enables access to optically
pure pyridine-containing macrocycles[26] the complexation
phenomena of which are now under investigation.
Figure 5. X-ray structure of (S,S)-28a and (S,S)-28b (hydrogen
atoms omitted for clarity).
In both (S,S)-28a and (S,S)-28b the ring sulfur atoms are
exo-disposed, but in both of these cases the linker, S9 to
S10 and S1 to S2, adopt a boat–boat conformation. Rota-
tion about C83–C85 in (S,S)-28a generates (S,S)-28b with a
concomitant conformational change within the macrocycle
such that the methyl groups associated with the core now
point towards each other [(S,S)-28a: C84–C99 = 6.6(2) Å;
(S,S)-28b: C8–C23 = 3.8(6) Å]. While C85 was inward-
pointing in (S,S)-28a [C82–N8–C83–C85 = 71.5(1)°] C9,
Experimental Section
Supporting information: (see footnote on the first page of this arti-
cle): Full experimental procedures and characterisation of all new
compounds.
CCDC-1411546 (10), CCDC-1411542 (26) and CCDC-1411557
(28) contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
the analogous atom in 28b, points to the outside of the request/cif.
macrocyclic ring [C6–N2–C7–C9 = –82.9(2)°]. In solution,
1
the well-resolved H NMR spectrum of (S,S)-28 in CDCl3
Acknowledgments
at 25 °C is indicative of a rapid interchange[23] between con-
formational isomers at ambient temperatures.
We thank the EPSRC (grant number EP/K039547/1) for the provi-
sion of Bruker NMR spectrometers and an Agilent SuperNova X-
ray diffractometer. O. K. R. thanks the University of Manchester
for a research studentship.
With the synthesis of substrates such as 15 and 26 at
hand we initiated a study into their metalation chemistry.
Initial studies showed that palladation of both 15 and 26
was relatively facile [PdCl2(CH3CN)2; CH3CN, reflux; 24 h]
and afforded the S,C,S-pincer complexes 36 and 37 in essen-
tially quantitative yield.[24] Both of the pincer complexes
served as catalysts[25] in a trial Heck reaction between iodo-
benzene and styrene, affording trans-stilbene (ca. 10% iso-
lated yield after 7 h), although at a slower rate than with a
“ligandless” source of palladium [Pd(OAc)2, Et3N, DMF,
100 °C, 7 h; 91%] (Scheme 6).
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Published Online: October 8, 2015
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