Isotopic labelling studies for a gold-catalysed skeletal rearrangement of alkynyl aziridines

Article abstract of DOI:10.3762/bjoc.7.96

Isotopic labelling studies were performed to probe a proposed 1,2-aryl shift in the gold-catalysed cycloisomerisation of alkynyl aziridines into 2,4-disubstituted pyrroles. Two isotopomers of the expected skeletal rearrangement product were identified usi

Full text of DOI:10.3762/bjoc.7.96

Isotopic labelling studies for a gold-catalysed
skeletal rearrangement of alkynyl aziridines
Paul W. Davies*, Nicolas Martin and Neil Spencer
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 839–846.
School of Chemistry, University of Birmingham, Edgbaston,
Birmingham, B15 2TT, United Kingdom
doi:10.3762/bjoc.7.96
Received: 15 April 2011
Accepted: 24 May 2011
Published: 21 June 2011
Email:
Paul W. Davies* - p.w.davies@bham.ac.uk
* Corresponding author
This article is part of the Thematic Series "Gold catalysis for organic
synthesis".
Keywords:
1,2-aryl shift; cycloisomerisation; gold; isotopic labelling; pyrrole;
skeletal rearrangement
Guest Editor: F. D. Toste
© 2011 Davies et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Isotopic labelling studies were performed to probe a proposed 1,2-aryl shift in the gold-catalysed cycloisomerisation of alkynyl
aziridines into 2,4-disubstituted pyrroles. Two isotopomers of the expected skeletal rearrangement product were identified using
13C-labelling and led to a revised mechanism featuring two distinct skeletal rearrangements. The mechanistic proposal has been
rationalised against the reaction of a range of 13C- and deuterium-labelled substrates.
Introduction
Gold-catalysed cycloisomerisation reactions have emerged as While 1,2-alkyl migrations are well-established in gold-
powerful methods to construct a diverse array of hetero- and catalysed heterocyclic synthesis [12,13], 1,2-aryl shifts are
carbocyclic motifs under generally mild reaction conditions much less common [14-17].
using straightforward preparative procedures [1-9]. Many such
processes also incorporate a rearrangement of the hydrocarbon In relation to these processes, we recently reported the gold-
skeleton present in the starting material, which can lead to catalysed reactions of aryl substituted alkynyl aziridines, such
unexpected and synthetically interesting outcomes [10,11]. as 1. Careful choice of reaction conditions resulted in selective
cycloisomerisation to produce either the 2,5-disubstituted
One of the most common fundamental steps in these skeletal pyrrole 2 or the skeletally rearranged 2,4-disubstituted pyrrole 3
rearrangements involves C–C bond fission through 1,2-migra- (Scheme 1) [18].
tion. This step is triggered by the generation of carbocationic
character, which is generally stabilised in some form, either by The nature of both the solvent, and the counter ion to the
an adjacent gold atom or through extended delocalisation. cationic gold catalyst, proved crucial to the reaction outcome:
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Beilstein J. Org. Chem. 2011, 7, 839–846.
Path I follows proton elimination (B→C) and subsequent
protodemetallation to give 2,5-disubstituted pyrrole D, in
keeping with a previously reported furan synthesis [24,25]. The
2,4-disubstituted skeletal rearrangement product G was ratio-
nalised through Path II, where a 1,2-aryl shift (B→E) precedes
proton elimination (E→F) and protodemetallation. The
observed solvent and counter ion effects could be rationalised
by considering that the 1,2-aryl shift at B is disfavoured rela-
tive to proton elimination as its cationic character is diminished,
either by solvent stabilisation or the formation of a closer
contact ion pair, and/or by more basic solvents/counter ions
assisting proton elimination [18,26,27].
Scheme 1: Gold-catalysed cycloisomerisations of aryl–alkynyl aziri-
dine to pyrroles.
However, for the proposed skeletal rearrangement mechanism
to be valid, gold bound carbocation B must be prone to undergo
2,4-Disubstituted pyrroles were only observed in appreciable a relatively rare 1,2-aryl shift in preference to either proton
quantities when the reaction was performed with a non basic elimination or a 1,2-hydride shift. We therefore sought to vali-
solvent and counter ion combination that allows for formation date this proposal experimentally by isotopic labelling.
of a separated ion pair, such as dichloromethane and triflate
Results and Discussion
(Scheme 1). This selectivity was switched and the 2,5-disubsti-
tuted products could be achieved in quantitative yields when the A deuterium labelled precursor
counter ion was the more basic tosylate [19]. Alternatively, the Alkynyl aziridine 4 was selected as the initial probe for the
use of protic, ethereal or aromatic solvents favoured the forma- mechanism which requires fission of three bonds: The propar-
tion of the 2,5-disubstituted products regardless of the counter gylic C–N bond in ring expansion; the aryl–aziridinyl C–C
ion. Similar solvent regimes were employed in other reports bond in the 1,2 shift and the propargylic C–H bond for aromati-
into π-acid promoted alkynyl aziridine cycloisomerisations sation (Scheme 3). The positioning of a deuterium atom at the
without skeletal rearrangement [20-23].
benzylic carbon in 4 enables the carbons of the aziridine ring to
be distinguished and labels the carbon from which the aryl shift
Our working mechanism to explain this reaction divergence would occur. The tolyl and butyl groups were selected as
centred on the electrophilic activation of the alkyne in A trig- substituents in place of other aromatic units to aid analysis by
gering a ring-expansion to a common intermediate B from simplifying the key aromatic region in the resulting NMR
which both isomeric products D and G evolved (Scheme 2). spectra.
Scheme 2: Working mechanism to rationalise the formation of two regiosomeric pyrroles in the gold catalysed cycloisomerisation of alkynyl aziridines.
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Beilstein J. Org. Chem. 2011, 7, 839–846.
Scheme 3: Bond fissions featured in the proposed mechanistic hypothesis and the initial mechanism probe.
Deuterated aziridine 4 was prepared in four steps from ester 5 catalysed reactions [13], control reactions had shown that no H/
(Scheme 4). Reduction using lithium aluminium deuteride led D exchange occurred in this specific system [18,19]. There was
to the insertion of the label at the benzylic position. Oxidation also no interconversion between 2,4- and 2,5-isomers under the
to the aldehyde followed by condensation with tosylamide same reaction conditions [18]. Surprisingly, despite several
afforded the deuterated imine 6. Aziridination using the sulfo- repetitions and the use of different batches of starting materials,
nium ylide generated in situ from 7 proceeded smoothly to the reaction of 4 was always less clean than its non-deuterated
afford the desired cycloisomerisation precursor 4 [28]. The analogue, even when the substrates were tested side-by-side
aziridines were formed as a mixture of diastereomers, with the under identical conditions.
cis diastereomer predominating, and were employed as such in
the cycloisomerisation reactions.
Despite this unexplained difference, the proposed mechanism
does not account for the formation of 2,4-disubstituted pyrrole
Deuterated aziridine 4 was reacted under the standard condi- without deuterium incorporation at the C-5 position. More
tions for skeletal rearrangement (5 mol % of Ph3PAuOTf gener- information on whether a 1,2-aryl shift occurs was therefore
ated in situ in CH2Cl2), to give a mixture of 2,4- and 2,5-pyrrole sought using a non-labile 13C label at the aziridine carbon.
isomers 8c and 9 alongside the main product, 3-deutero-2,4-
disubstituted pyrrole 8b (Scheme 5). Isomer 8a, which was 13C labelling studies
predicted as the major product under the proposed mechanism 13C-Enriched benzoic acid was converted into imine 10 and
(Scheme 2), could not be indentified from the reaction mixture. subsequently coupled with 7 to give the aziridine 11, in four
Though H/D exchange in pyrroles has been noted in gold- steps, as a 1:5 13C:12C mixture (Scheme 6). Two further 13C-
Scheme 4: Preparation of D-labelled alkynyl aziridine 4. DMP = Dess–Martin periodinane.
Scheme 5: Reaction of deuterated alkynyl aziridine 4 in the skeletal rearrangement reaction.
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Beilstein J. Org. Chem. 2011, 7, 839–846.
Scheme 6: Preparation of 13C-enriched alkynyl aziridines.
enriched alkynyl aziridines, 14 and 15, with different
substituents on the alkyne, were later prepared using the sulfo-
nium salts 12 and 13.
On subjecting 11 to the conditions for skeletal rearrangement
(Scheme 7), three enhanced resonances were seen in the 13C
NMR spectra of the pyrrole products (Supporting Information
File 1). One of the enhanced resonances was associated with the
minor 2,5-disubstituted isomer 16 showing 13C enrichment at
the expected C-2 position (Scheme 2, Path I). This result was
confirmed by carrying out the control cycloisomerisation of 11
under the conditions known to give solely 2,5-disubstituted
products (Scheme 8) [19]. All other resonances were associated
with the 2,4-disubstituted pyrrole 17 and the remaining
enhancements were therefore assigned to the unexpected forma-
tion of two separate isotopomers of 17. 13C-enrichment at C-5
(117.7 ppm) corresponds to the isotopomer 17a anticipated
from the proposed 1,2-aryl shift mechanism. Isotopomer 17b
was identified by enrichment at the quaternary C-4 position
Scheme 8: Cycloisomerisation of 11 to give 2,5-disubstituted pyrrole.
Scheme 9: Cycloisomerisation of 14 in the skeletal rearrangement
reaction.
(127.0 ppm). HMBC experiments confirmed this assignment The observed inconsistency between the reactions of 11 and 14
showing clear 3J coupling between the C-4 13C-enriched was investigated using alkyl substituted alkynyl aziridine 15.
quaternary centre and the protons of the phenyl ring.
Cycloisomerisation of 15 once again afforded a mixture of
isotopomers of the 2,4-disubstituted pyrrole 21a/b alongside
Cycloisomerisation of the alkynyl aziridine 14, which bears a 2,5-disubstituted pyrrole 20 (Scheme 10).
phenyl substituent on the alkyne, resulted in a different outcome
under the same reaction conditions: Only a single isotopomer of The formation of the isotopomers 17a, 19 and 21a with 13C-
the 2,4-disubstituted pyrrole 19 was formed with 13C-enrich- enrichment at C-5 confirms C–C bond fission between the aryl
ment at the C-5 position, alongside the 2,5-disubstituted isomer group and the aziridine during the reaction. This outcome is
18 (Scheme 9).
consistent with an operative 1,2-aryl shift in the skeletal
Scheme 7: Cycloisomerisation of 11 in the skeletal rearrangement reaction.
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Beilstein J. Org. Chem. 2011, 7, 839–846.
Scheme 10: Cycloisomerisation of 15 in the skeletal rearrangement reaction.
rearrangement of alkynyl aziridines to form 2,4-disubstituted benzylic cation leads to a new 5-membered cyclic intermediate
pyrroles (Scheme 2, Path II).
J [29]. Deprotonation and protodemetallation of J results in a
2,4-disubstituted isomer G’ where the 13C-enriched carbon
However, the initial use of the alkyl substituted aziridine 11 maintains its connectivity to the aryl unit, and C–C bond fission
rather than the aryl-substituted aziridine 14 was fortuitous. A occurs between the aziridine and the alkyne fragment.
second accessible pathway must exist to account for the forma-
tion of the 2,4-disubstituted pyrrole isotopomers 17b, and 21b The observed effects of the alkyne substituent can be ratio-
by an alternate skeletal rearrangement.
nalised if the pathway is dependent on the relative stabilisation
of the developing positive charge at either carbon of the aziri-
The mechanistic proposal for the skeletal rearrangement was dine by the alkynyl aziridine intermediate H (Scheme 11). The
therefore reappraised to explain these results (Scheme 11). formation of either benzylic cation I (Path III) or the enamide
Alongside the ring expansion of metal coordinated alkynyl stabilised cation B (Path II), under a cationic ring-expansion
aziridines to the 5-membered cyclic cation B, which leads to manifold, is apparently closely balanced: Products from both
isotopomer G (Path II) and 2,5-disubstituted pyrroles D (Path pathways are observed for alkyl substituted systems such as 11
I), a competing regioisomeric ring-expansion could afford and 15. However, when the alkyne substituent is a phenyl group
4-membered ring intermediate I. Evolution of I by a ring- Path II is favoured over Path III due to the additional stabilisa-
expanding 1,2-shift of the metal stabilised vinyl unit onto the tion of B by delocalisation through an extended π-system.
Scheme 11: Revised mechanism for the formation of 2,4-isomers by skeletal rearrangement.
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Beilstein J. Org. Chem. 2011, 7, 839–846.
Scheme 12: Synthesis of alkynyl aziridines 30 and 31.
The initial report showed how the ratio of 2,5- to 2,4-disubsti- aromatic substituent to the alkyne (R = p-C6H4CF3) should
tuted pyrroles varied with the electronic properties of the disfavour Path II by destabilising B over I, whereas an electron-
aromatic unit substituent directly attached to the aziridine [18]. rich aromatic substituent on the alkyne (R = p-C6H4OMe)
While the trend was consistent with that expected for a 1,2-aryl should favour Path II by further stabilising B.
shift, the more complex mechanism also accounts for the
observed outcomes. An increasingly electron-rich aryl group
will favour both formation of benzylic cation I and 1,2-aryl
migration from B, leading to the same products from each route.
Electron-deficient aromatics would have a destabilising effect
on the formation of I and also be less prone to undergo 1,2-aryl
shift from B thus favouring Path I and formation of the 2,5-
disubstituted pyrroles.
The substrates were prepared by Sonogashira reaction between
propargyl alcohol and the appropriately substituted iodoben-
zene substrate 22, 23 (Scheme 12). Functional group intercon-
version of the alcohols by bromination and substitution results
in the sulfonium salts 28, 29 which were reacted with 13C
labelled imine 10 to yield the desired alkynyl aziridines 30 and
31.
Two further 13C labelled compounds were prepared to test this The anticipated outcomes of the catalysis were borne out under
rationalisation by predictably affecting the relative stability of the standard conditions (Scheme 13). Two isotopomers of 13C-
just one of the key intermediates: An electron-deficient enriched 2,4-disubstituted pyrrole 32 were produced from the
Scheme 13: Electronic effects on the outcome of the skeletal rearrangement processes.
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Beilstein J. Org. Chem. 2011, 7, 839–846.
reaction of 30. Qualitative analysis of the two isotopomers cycloisomerisation of aryl-substituted N-tosyl alkynyl
showed that a greater proportion of the 2,4-isomer formed from aziridines. The two competing pathways coincidentally give rise
Path III was observed than with the other cyclisation precursors to the same isomer but with different absolute connectivities, as
(Supporting Information File 1). Additionally, none of the 2,5- determined by the formation of two 13C isotopomeric products.
disubstituted pyrrole isomer, which would also result from B,
was formed.
A modified mechanistic scheme accounts for all of the results of
the labelling studies. On activation by a cationic gold source
In keeping with the previous observation that strongly electron- and in the absence of coordinating solvents and/or counter ions,
rich aromatic groups were poor substituents for this cycloiso- regioisomeric ring expansion of the alkynyl aziridine can lead
merisation [19], only a low yield of the pyrrole products was to two different cationic cyclic intermediates, featuring either a
obtained on reaction of 31. However, this was sufficient to 4-membered or a 5-membered ring. The regioselectivity, and
determine that Path II predominated over Path III, with no hence the isotopomeric reaction selectivity, is controlled by the
significant isotopomer identified from intermediate I. Isomer 34 relative ability of the substituents to stabilise the respective
was identified alongside a significant quantity of the 2,5-disub- cations in favour of a particular pathway. The observed skeletal
stituted pyrrole 33. While solvents and/or counter ions that rearrangements are consistent with either a 1,2-aryl shift or a
stabilise positive charge favour the formation of the 2,5-disub- 1,2-(metal-stabilised)-vinyl shift in the 5- or 4-membered cyclic
stituted pyrroles, likewise, reducing the localised cationic intermediates, respectively. These studies highlight some of the
charge in the 5-ring intermediate B by conjugation appears to complexities associated with gold-catalysed cycloisomerisation
have the same effect.
reactions and the continued relevance of labelling studies in this
field.
Re-evaluation of the cycloisomerisation of the deuterium-
labelled alkynyl aziridine 4 (Scheme 4) shows the results to be
consistent with both the modified mechanistic scenario and the
trends shown in the 13C-labelling study. Relative to all the other
substrates tested, excepting perhaps 30, substrate 4 would be the
most likely to favour Path III over Path II (Scheme 11). Whilst
the butyl substituent does not lend any additional stabilisation to
intermediate B, the tolyl group stabilises the benzylic cation I
better than a phenyl group. Significant deuterium incorporation
at C-5 in the product would therefore not be expected.
Following Path III, the deuterium labelled carbon ends-up at
C-4 (Scheme 14, I-4→J-4). Aromatisation results in C–D bond
cleavage with deuterodemetallation of K-4 affording the
observed major product 8b (Scheme 5). The intermolecular
Supporting Information
Supporting Information contains full experimental details
for the preparation of the cyclisation precursors and their
subsequent reactions. NMR spectra for the labelling studies
are provided.
Supporting Information File 1
Full experimental details.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-7-96-S1.pdf]
deuteron transfer allows for H/D exchange in the reaction media Acknowledgements
and hence the partial incorporation of a proton at C-3.
Financial support from the University of Birmingham is grate-
fully acknowledged. The work described here forms part of a
PhD thesis (Nicolas Martin, University of Birmingham 2010).
Conclusion
There are two operative skeletal rearrangement pathways for the The NMR and MS facilities used in this research were partly
formation of 2,4-disubstituted pyrroles by gold-catalysed supported through Birmingham Science City: Innovative Uses
Scheme 14: Mechanistic rationale for the deuterium labelling study using Ph3PAuCl/AgOTf.
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Beilstein J. Org. Chem. 2011, 7, 839–846.
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Centre for Advanced Materials Project 2), with support from
Advantage West Midlands (AWM) and partly funded by the
European Regional Development Fund (ERDF). We thank
Johnson Matthey plc for a generous loan of metal salts.
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