Congested C2-symmetric aryliodanes based on an anti-dimethanoanthracene backbone

Article abstract of DOI:10.1039/c2cc32280c

A family of congested aryliodanes based on an anti-dimethanoanthracene backbone has been synthesised and two diaryliodonium salts and the iodyl derivative characterised by X-ray analysis. The latter shows a rare water coordination to the iodine(v) centre in the solid state. Applications of these reagents in functional group transfer reactions are reported. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc32280c

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COMMUNICATION
Congested C2-symmetric aryliodanes based on
an anti-dimethanoanthracene backbonew
Stephen J. Murray, Helge Muller-Bunz and Hasim Ibrahim*
¨
Received 29th March 2012, Accepted 28th April 2012
DOI: 10.1039/c2cc32280c
A family of congested aryliodanes based on an anti-dimethano-
anthracene backbone has been synthesised and two diaryliodonium
salts and the iodyl derivative characterised by X-ray analysis. The
latter shows a rare water coordination to the iodine(V) centre in the
solid state. Applications of these reagents in functional group
transfer reactions are reported.
bring about in several processes showcases the potential of
chiral non-racemic aryliodanes in asymmetric synthesis and
catalysis.6
Sterically encumbered iodanes are also rare, and only a
small number have been described in the literature.7 In
addition, few systematic studies of the crowding effect on the
reactivity of iodanes have been reported to date.8 Encouraged
by the absence of investigations addressing the aforementioned
facts, we embarked on the synthesis of a family of modular,
congested aryl-l3- and l5-iodanes based on the C2h-symmetric
anti-dimethanoanthracene backbone 6 (Scheme 1). This rigid,
all carbon framework has a defined chiral space, and is robust
towards oxidising and reducing agents. Moreover, we have
recently developed a high-yielding synthetic route to this
skeleton which allows the synthesis of funtionalised and
sterically engineered analogues.9
Aryliodanes with their unique and varied reactivity profile have
become established reagents for synthesis.1 Many transformations
with these reagents generate chiral products from prochiral
substrates. This fact led to the development and application
of chiral non-racemic iodanes, either in isolated forms or
generated in situ from catalytic amounts of chiral iodoarene
precursors, which has intensified in recent years.2 Examples of
chiral aryliodanes which exert significant levels of stereocontrol
in their respective applications include the flexible chiral Koser
reagent analogue 1,3 the axially chiral diaryliodoium salt 2,4 and
the rigid C2-symmetric m-oxo-bridged bis-iodane 3.5
Iodoarene 7 was obtained by the iodination of 6 using
(diacetoxy)iodobenzene (DIB) and iodine in 74% yield. Initial
attempts to oxidise 7 to the (diacetoxy)iodane 8 proved unsuccessful
under several reaction conditions. Thus, the oxidation with
NaIO4,10 CH3CO3H,11 K2S2O812 and Selectfluort in the presence
of an acetate source failed.13 A procedure utilising sodium
perborate in acetic acid proved more promising,14 however,
initial results were inconsistent and reproducibility proved to
1
be difficult. Monitoring this reaction by H NMR spectroscopy,
we observed that 7 was initially oxidised to 8 with complete
conversion reached after 4 h; however, prolonged reaction times
resulted in the reverse reaction and only iodoarene 7 could be
detected. We were pleased to isolate 8 in 76% yield after a reaction
time of 4 h. These findings confirm previous observations by other
researchers that the isolation of congested and electron rich
aryliodanes was either difficult or not possible.3a,15
The synthesis of the iodosyl derivative was attempted using
standard basic conditions,16 however, in each case only the
starting iodoarene 7 was observed. It is possible that since
iodosylbenzene is polymeric in nature,17 the anti-dimethano-
anthracene analogue might be too bulky to adopt a zig–zag
polymeric structure. This is consistent with observations made by
Power in the attempted synthesis of the sterically encumbered
TripIO (Trip = 2,4,6-i-Pr3C6H2).7b
Monoiodanes with backbone C2-symmetry are rare, and until
recently only the Koser analogue 4 was known.3b However,
the recent development of a group of flexible C2-symmetric
aryliodanes 5, and the high levels of aysmmetric induction they
Centre for Synthesis and Chemical Biology, School of Chemistry and
Chemical Biology, University College Dublin, Belfield, Dublin 4,
Ireland. E-mail: hasim.ibrahim@ucd.ie; Fax: 353 1 7161178;
Tel: 353 1 7162978
w Electronic supplementary information (ESI) available: Experimental
procedures for Schemes 1–4 and analytical data of aryliodanes 7–12,
as well as CIF files for crystal structures 10, 11 and 12f. See DOI:
10.1039/c2cc32280c
The synthesis of the iodyl derivative 10 proved more successful.
After screening several oxidation conditions, 10 was obtained in a
high yield and purity by oxidation with DMDO.18 This material
proved to be insoluble in most organic solvents, except for DMSO,
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6268 Chem. Commun., 2012, 48, 6268–6270
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Scheme 1 (a) DIB, I2, EtOAc, rt, 12 h; (b) NaBO3, AcOH, 60 1C, 4 h; (c) PTSA, CHCl3, rt, 0.5 h; (d) DMDO, CHCl3, 0 1C-rt, 12 h; (e) mCPBA,
BF3ÁOEt2, PhB(OH)2, CH2Cl2, 80 1C to À78 1C, 1 h; (f) PhI(OH)OTs, DCM/TFE (1: 1), rt, 4 h; (g) LiX (X = Cl, Br, I, OTf) or AgX (X = BF4, PF6),
CHCl3, rt, 0.5 h.
MeOH and DMF. It is a stable solid and unlike most all-carbon
iodylarenes reported to date, has a clear melting point as
opposed to decomposing violently upon heating (vide infra).
The Koser reagent analogue 9 from the reaction of 8 with
PTSA in chloroform proved to be unstable and could not be
isolated, however, all analytical data were consistent with its
in situ formation in solution (see ESIw and Scheme 2a).
For the synthesis of diaryliodonium salts in this series, we first
attempted the phenylboronic acid/mCPBA procedure developed
by Olofsson and were able to isolate the diaryliodonium BF4 salt
12e in moderate yield.19 Fortunately, the preparation of the
diaryliodonium tosylate 11 by reacting Koser’s reagent with
6 proved high yielding.20
Fig. 1 (a) Crystal structure of 10 drawn with 50% thermal ellipsoids
showing water coordination to the iodine(V) centre. (b) A plot of the
sheet-like structure of 10 held together by hydrogen bonding and weak
intermolecular IÁ Á ÁO interactions, drawn with 50% thermal ellipsoids.
The ease in which 11 was obtained encouraged us to explore
an alternative route to other counter ions by anion metathesis.21
Thus, by stirring a solution of 11 with lithium triflate in
chloroform we were delighted to find that complete anion
exchange occurred within 30 min. The precipitated lithium
tosylate could be easily removed by filtration to yield the
diaryliodonium triflate 12d, which required no further purifica-
tion. By varying the metal salt we were able to prepare the
diaryliodonium halide salts 12a–c, the BF4 salt 12e and the PF6
salt 12f in quantitative yields and in high purity.
interaction is 2.612(5) A in length and connects the parallel
IO2Á Á ÁIO2 chains to sheets via hydrogen bonding (Fig. 1b). The
sheet-like structure is reflected by the morphology of the
crystals. The organic residues of the IO2Á Á ÁIO2 chains point
alternatingly above and below the slightly waved sheet containing
the hydrophilic interactions (Fig. 2), with the stacked sheets
held together by weak hydrophobic interactions between anti-
dimethanoanthracene units. It seems plausible that the bulk of
these units prevents the formation of a two-dimensional polymer
having only IO2ÁÁÁIO2 interactions,17 making it necessary for
water to act as a bridging ligand.
Samples of 10 were crystallised from methanol and yielded
thin transparent plates which were suitable for X-ray analysis.
The X-ray of 10 shows, to the best of our knowledge, the first
water coordination to an aryl-l5-iodane (Fig. 1a). This IÁ Á ÁOH2
Crystals of 11 and 12f, suitable for X-ray analysis, were
obtained from chloroform and revealed that both structures
are dimeric in nature due to anion bridging (Fig. 3).22
Scheme 2 (a) 8, PTSA, 3 min, benzoylacetone, MeCN, rt, 3 min,
60%; (b) 8, benzyl 2-methyl-3-oxobutanoate, Et4NCl, MeCN/H2O
(9: 1), rt, 50 min, 80%; (c) 8, benzyl 2-methyl-3-oxobutanoate, Bu4NN3,
MeCN/H2O (9: 1), rt, 1 h, 62%; (d) trans-stilbene, 8, Phth-NH2,
CH2Cl2, rt, 4 h, 95%.
Fig. 2 Side-on view of the waved sheet-like structure of 10.
Chem. Commun., 2012, 48, 6268–6270 6269
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their encumbered nature, the reported aryliodanes readily
mediated various functional group transfer reactions. Work
on the resolution and application of the reported reagents in
asymmetric reactions will be disclosed shortly.
This work was supported by Science Foundation Ireland
(07/RFP/CHEF394).
Notes and references
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Fig. 3 Thermal ellipsoid plot (50%) of the dimeric structure of (a) 11
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Scheme 3 Phenylation of diethyl methylmalonate by salts 11 or 12d.
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Scheme 4 Oxygenative DA dimerisation mediated by 10.
We have tested our congested iodanes in a series of bench
mark reactions and were delighted to find that they largely
displayed expected reactivities. First we treated 9, generated
in situ from 8 and PTSA, with benzoylacetone and obtained
the a-tosylated product in 60% yield (Scheme 2). We then
showed that 8 readily participated in DIB mediated halide23
and azide24 transfer reactions developed within our laboratory,
as well as nitrene transfer to trans-stilbene (Scheme 2).25
Interestingly, under otherwise identical reaction conditions,
the a-chlorination required 50 min as compared to 3 min with
DIB. The decrease in rate by more than one order of magnitude
may reflect the increased bulk around the iodine centre; a fact
that could prove helpful in future attempts to elucidate the
mechanism of such transformations.
14 A. McKillop and D. Kemp, Tetrahedron, 1989, 45, 3299–3306.
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sodium enolate derived from diethyl methylmalonate under
standard aryl transfer conditions in high yields (Scheme 3).26
Importantly, selective phenyl transfer was observed in an
apparent overruling of the ‘ortho-effect’, and consistent with
observations made with diaryliodonium salt 2.4,27 We believe
that our reagents represent the most sterically encumbered
diaryliodonium salts to mediate aryl transfer reported
to date.
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24 M. J. Galligan, R. Akula and H. Ibrahim, manuscript in
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25 R. D. Richardson, M. Desaize and T. Wirth, Chem.–Eur. J., 2007,
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Iodane 10 was examined in the oxygenative Diels–Alder
(DA) dimerisation of 2,6-dimethylphenol and we were delighted
to find that the anti-dimethnaoanthracene derived iodane also
mediated this reaction in 54% yield (Scheme 4).18
Furthermore, we note that in all of the above reactions, the
reduced iodoarene 7 could be recovered in high yields.
In conclusion, we have described the synthesis of a series of
congested all-carbon aryliodanes based on the C2h-symmetric
anti-dimethanoanthracene 6. The diaryliodonium salts of
the series were obtained by a novel high-yielding anion
metathesis route from tosylate 11. It was shown that despite
26 C. H. Oh, J. S. Kim and H. H. Jung, J. Org. Chem., 1999, 64,
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27 V. V. Grushin, I. I. Demkina and T. P. Tolstaya, J. Chem. Soc.,
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6270 Chem. Commun., 2012, 48, 6268–6270
This journal is The Royal Society of Chemistry 2012