Difluorinative ring expansions of benzo-fused carbocycles and heterocycles are achieved with: P-(difluoroiodo)toluene

Article abstract of DOI:10.1039/c9cc08310c

A chemoselective fluorinative ring expansion has been realized using the hypervalent iodine (HVI) reagent p-TolIF₂, which delivers β,β-difluoroalkyl arenes in yields up to 89% and allylic gem-difluorides in yields up to 78%. This rapid reaction

Full text of DOI:10.1039/c9cc08310c

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Difluorinative ring expansions of benzo-fused
carbocycles and heterocycles are achieved with
p-(difluoroiodo)toluene†
Cite this:DOI: 10.1039/c9cc08310c
Received 23rd October 2019,
Accepted 18th November 2019
Zhensheng Zhao, Avery J. To
and Graham K. Murphy
*
DOI: 10.1039/c9cc08310c
rsc.li/chemcomm
A chemoselective fluorinative ring expansion has been realized using Reactions with styrenes8a,c,d or phenylallenes11 are interrupted
the hypervalent iodine (HVI) reagent p-TolIF2, which delivers by 1,2-phenyl or 1,2-alkyl shifts, giving gem-difluorides via
b,b-difluoroalkyl arenes in yields up to 89% and allylic gem-difluorides rearrangements or ring contractions.8c,12 The state-of-the-art of
in yields up to 78%. This rapid reaction exploits the ambiphilic HVI-mediated fluorinations has been redefined, as many of
nature of alkenes and allenes, and incorporates both fluorine atoms these reactions are now possible as catalytic and/or asymmetric
of the (difluoroiodo)arene in the products. The mechanism involves processes, where chiral iodoarene catalysts13 are re-oxidized
a 1,2-phenyl shift, which provides access in one step to important in situ.14,15 Therefore, as new fluorination reactions are realized,
fluorinated building blocks for bioactive molecule synthesis.
the potential for further development is significant.
Fluorinative ring expansions mediated by 1 are surprisingly
Organofluorine compounds are critical to medicinal chemistry, unknown, but they might occur if substrates containing exo-
agrochemistry, medical imaging, and materials sciences,1 and cyclic alkenes are as viable as the related endocyclic alkenes
there exists a critical need for new fluorination strategies. As used in ring contractions. If a-exomethylene-containing benzo-
part of this effort, the past decade has seen numerous new cycloalkanes (e.g. 2) also react with 1 via a 1,2-phenyl shift
hypervalent iodine (HVI)-mediated fluorination reactions, as pathway, they would give a direct synthesis of b,b-difluoroalkyl
their inherent potential for addressing new or poorly accessible arenes 3, a motif extensively studied within many bioactive
fluorinated motifs is realized.2 HVI reagents are broadly reactive as molecules (Scheme 1a).16,17 Furthermore, if allene-containing
oxidants,3 owing to iodine’s desire to return to its natural oxidation substrates (Scheme 1b, e.g. 4) are also viable,18 it would afford a
state, and in their fluorine-transfer reactions, this results in novel, one-step synthesis of allylic gem-difluorides 5, another
an apparent reversal of polarity of one of the fluoride ligands. valuable fluorine-containing motif not currently accessible via
For example, p-(difluoroiodo)toluene (p-TolIF2, 1),4 a stable solid direct fluorination.19 We report here how indanes, tetralins and
readily prepared from iodotoluene and aqueous fluoride via related heterocyclic derivatives possessing a-exocyclic alkenes
oxidation and ligand transfer, provides an ‘‘electrophilic’’ fluorine or -allenes react with p-TolIF2 (1) to undergo rapid and chemo-
atom5 analogous to other reagents derived from fluorine gas.
(Difluoroiodo)arenes have been employed in numerous
fluorination processes, transferring one or both of their ligands.
They fluorinate nucleophiles such as silyl enol ethers6 or
b-dicarbonyls,7 and difluorinate ambiphilic functional groups
like alkenes5b,8 or diazo compounds,9 serving as fluorine gas
surrogates that deliver both ‘‘electrophilic’’ and nucleophilic
fluorine atoms. The vic-difluorination of alkenes can be inter-
rupted by various intramolecular processes, such as attack by
nucleophiles (e.g. alcohols, amines, carboxylic acid derivatives),
leading to a wide array of (hetero)cyclic fluorinated motifs.10
selective fluorinative ring expansions.
Department of Chemistry, University of Waterloo, Waterloo, Ontario, N2L3G1,
Canada. E-mail: graham.murphy@uwaterloo.ca
Scheme 1 Fluorinative ring expansions of alkenes and allenes using
p-TolIF2.
† Electronic supplementary information (ESI) available: Experimental details and
NMR spectra of new compounds. See DOI: 10.1039/c9cc08310c
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We began our investigation by reacting model indane 2a
with 1.25 equiv of p-TolIF2 in DCE at reflux, using 20 mol%
BF3ÁOEt2 as activator, from which the b,b-difluoride 3a was
observed in 64% yield by 19F NMR yield (Table 1, entry 1).
Cooling the reaction to room temperature led to an increased
yield (74%); however, further cooling to 0 1C resulted in a
significantly decreased yield (entries 2 and 3). Chlorinated,
polar and non-polar solvents were also tested, though none
proved superior to DCE (entries 4–7, also see Table S1 for
additional entries, ESI†). Of the other Lewis acidic activators
tested, none proved as effective as BF3ÁOEt2 (Table 1, entries 8–11,
also see Table S2, ESI†). A 45% yield was even realized without
an added activator, which we attribute to borosilicate activation
of 1 (entry 12).9d The loading of BF3ÁOEt2 was also studied, and
while increasing to 30 mol% offered no improvement, decreasing
it to 5 mol% was optimal, giving 4a in 78% 19F NMR yield, and
63% isolated yield after 20 minutes (entries 13 and 14). Therefore,
reacting the a-exomethylene-containing indane 2a with BF3ÁOEt2-
activated p-TolIF2 gave a rapid and highly selective difluorinative
ring-expansion, where both of the fluorine atoms derived from a
single reagent.
Scheme 2 Fluorinative ring-expansion of various exocyclic olefins.
We tested a series of a-exomethylene containing substrates
in this reaction, first probing the effect of arene substitution Methoxy substitution (2j) was moderately well-tolerated (3j, 49%
on the indane-derived scaffold (Scheme 2). Halogenation was yield); however, higher yields were achieved with the 3-bromo
universally well tolerated, with the 5-bromo derivative 2b giving derivative 2k, which gave 3k in 89% yield. gem-Dimethylation
3b in 75% yield. Halogenation at the 6-position included the adjacent to the reacting alkene inhibited the desired reaction,
bromo- (2c), fluoro- (2d) and chloro- (2e) derivatives, which gave as alkene 2l was fully consumed, and yet the reaction failed
3c–e in 72–77% yield. We were surprised that the 7-methyl to produce 3l.
derivative 2f was converted to 3f in only 43% yield, as neither
Alkenes derived from the chromane skeleton were investi-
steric or electronic biases were expected to interfere. Fluorina- gated, and the parent compound 2m reacted to give 3m in 42%
tion of the 7-bromo (2g) and 7-methoxy (2h) derivatives was also yield. Derivatives of this scaffold were prepared with substitu-
possible, giving the ring-expanded 3g and 3h in 57% and 49% ents at positions distal to exocyclic alkene, allowing us to test
yield, respectively. We then investigated the rearrangement for remote steric effects.20 Numerous substitution patterns
homologous substrates derived from the tetralin scaffold, were tolerated, such as the 2-phenyl derivative 2n, which gave
and found unsubstituted alkene 2i to give 3i in 75% yield. 3n in 67% yield. 2,2-Dialkyl scaffolds included dimethyl- (2o),
diethyl- (2p), and mixed dialkyl- (2q, 2r) derivatives, which
all gave their corresponding difluorides 3o–3r in 58–61% yield.
Table 1 Optimization of the fluorinative ring-expansion using indane 2a
2,2-Spirocyclic derivatives were also viable, with the spirocyclo-
butane (3s), -cyclopentane (3t) and -cyclohexane (3u) derivatives
undergoing the fluorinative rearrangement in 44–67% yield.
Substrates derived from the 2-oxindole scaffold (2v, 2w) failed,
which was surprising given the ease with which the related
Entry
Lewis acid (mol%)
Solvent
Temp. (1C)
Yielda (%)
acyclic cinnamides undergo fluorinative rearrangements.14c,p
Lastly, the thiochromane-derived alkene 2x was also fully con-
sumed in the reaction, but none of the desired product (3x) or
any other identifiable products were observed, presumably due
to the ease with which sulfides are oxidized by 1.21 Collectively, this
ensemble of results demonstrates that p-TolIF2 readily induces a
rapid and chemoselective difluorinative ring-expansion on benzo-
fused bicycles possessing a-exomethylene groups. While this
study was not exhaustive, we discovered that various functional
groups and substitution patterns were widely tolerated. Limita-
tions included substitution adjacent to the alkene, and aryl
methyl ethers were also low yielding, presumably due to their
instability in the reaction media.22 In any case, the ring-size
and halogen substitution patterns realized herein provide
1
2
3
4
5
6
7
8
BF3ÁOEt2 (20)
BF3ÁOEt2 (20)
BF3ÁOEt2 (20)
BF3ÁOEt2 (20)
BF3ÁOEt2 (20)
BF3ÁOEt2 (20)
BF3ÁOEt2 (20)
TiF3 (20)
TiF4 (20)
AlF3 (20)
lnF3 (20)
—
DCE
DCE
DCE
DCM
PhCl
CH3CN
THF
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Reflux
rt
0
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
64
74
35
34
25
0
Trace
31
31
9
10
11
12
13
14
27
34
45
BF3ÁEt2O (30)
57
BF3ÁEt2O (5)
rt
78 (63)b
a
b
19F NMR yield using 4-fluorotoluene as an internal standard. Iso-
lated yield.
Chem. Commun.
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À 26
where the second fluoride possibly derives from BF4
,
and
where the regioselectivity is directed by the stabilizing effect of
the existing fluorine atom.
In conclusion, we report a fluorinative ring-expansion of
benzo-fused alkene- and allene-containing substrates mediated
by p-TolIF2. This reaction exploits the iodane’s ability to deliver
both ‘‘electrophilic’’ and nucleophilic fluorides, and the substrate’s
ability to undergo 1,2-phenyl migration, providing direct access to
either b,b-difluorides or allylic gem-difluorides. The reactions were
rapid and tolerant to a variety of functional groups, though steric
hindrance and heteroatom (N,S) substitution were problematic.
This mild and operationally-simple reaction constitutes a novel
strategy for synthesizing fluorinated motifs not readily accessible
via other direct fluorination methods, and our continued efforts
will be reported in due course.
Scheme 3 Fluorinative ring-expansion of various exocyclic allenes.
a 19F NMR yield.
This work was supported by the Natural Sciences and
Engineering Research Council (NSERC) of Canada, the University
of Waterloo and the Early Researcher Award from the Province
of Ontario.
b,b-difluoroalkyl arenes with versatile handles for further
manipulation.
We tested the fluorinative ring-expansion on the related
allene-containing substrates, beginning with unsubstituted 4a
(Scheme 3). When subjected to the reaction conditions optimized
above, only a trace of 5a was observed. After screening reaction
time, temperature and loading of the Lewis acid, we discovered
that combining allene 4a, 1.25 equiv. p-TolIF2 and 20 mol%
BF3ÁOEt2 in DCE at reflux gives 5a in 74% isolated yield. We
found halogenation at any of the 5-, 6- or 7-positions on the arene
was well tolerated, providing the products 5b–5e in 56–78% yield.
Two homologous derivatives were also tested, with allene 4f giving
difluoride 5f in 44% yield, and with 7-bromo derivative 4g giving
5g in 41% yield. Oxygenation within the tether was tolerated
(5h, 29% yield), whereas the related sulfide 4i was not viable. The
tolerance for substitution patterns and functional groups dis-
played by these allenes was consistent with that of the related
alkenes; however, the yields decreased significantly for rearrange-
ments leading to the medium-ring scaffolds.
Such fluorination reactions have attracted significant interest,
and many efforts have been made to elucidate their mechanisms
both computationally and experimentally.23 The rearrangements
reported here are proposed to begin with activation of the iodane
by the Lewis acid (Fig. 1).24 The activated iodane A is then attacked
by the weakly nucleophilic p-system of the styrene (e.g. 2a) or
phenylallene derivative, via either concerted (shown) or stepwise
(not shown) processes, leading to intermediate B in which the first
C–F bond has been forged. The hypernucleofugal iodanyl leaving
group25 may be further activated by the Lewis acid, inducing
the arene to participate in expelling iodotoluene, generating
phenonium ion C. Ring expansion and rearomatization gives 3a,
Conflicts of interest
There are no conflicts to declare.
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