the presence of an EPR silent FꢀeIII-phenoxy radical complex.
EPR of a mixture of [Fc]+BF4 and s-trioxane (60 min, rt)
similarly detected no radical species (Fig. 1b). Subsequent mW
irradiation and EPR analysis did not afford any evidence for
the formation of an s-trioxane radical complexed to an FeII
species. It did however reveal a new set of signals at high-field
(400–700 mT, Fig. 1c). These we attribute to the formation
of a new metal species, presumably formed between the
In summary, we report the unique application of [Fc]+BF4
ꢀ
for the synthesis of calixarenes via an exceptionally mild, high
yielding and time-efficient protocol that proceeds via a one-pot
or convergent strategy. A particularly exciting aspect ofꢀour
preliminary results suggests that application of [Fc+]BF4 to
previously unreactive convergent strategies to be an innova-
tive, chemoselective route capable of generating specific calix-
arenes. The minutiae of the reaction including mechanistic
features, a full investigation into the scope, the use of catalytic
quantities of ferrocenium salts and possible alternative
applications are currently under investigation.
ꢀ
s-trioxane and the FeIII metal center of [Fc]+BF4
.
Advocating innovative applications of [Fc]+BF4ꢀ (1 equiv.)
we considered its use as a mild and efficient reagent for
promoting a [3 + 1] convergent synthesis of calixarenes. In
general convergent [3 + 1] syntheses of calixarenes employ
multiple equivalents of strong Lewis acids i.e. titanium(IV)
chloride and afford very poor to moderate yields i.e. 15–38%.6
Previous work in our laboratory indicated 4 to be a ‘‘proble-
matic’’ substrate for inclusion within a tin(IV) chloride
mediated [3 + 1] coꢀnvergent synthesis.7 Thus attempting to
use 4 and [Fc]+BF4 within a convergent (chemoselective?)
calixarene synthesis it was not clear that the reaction woulꢀd
work. Gratifyingly heating a solution of 3, 4 and [Fc]+BF4
afforded an excellent 90% yield of 2[4]–2[9], (Scheme 3).
Furthermore HPLC analysis confirmed the percentage of 2[4]
had increased, as anticipated, by 36% to 19% (relative to 2[4]
generated in Scheme 1). This result is even more remarkable
when compared to the percentages of 2[4] generated within
Gutsche’s ‘‘conventional’’ tosic acid procedure,4 with an 87%
increase i.e. 2.5% to 19% of 2[4].
Notes and references
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Chemistry, Royal Society of Chemistry, Cambridge, 1989.
2 S. Shinkai, Tetrahedron, 1993, 49, 8933.
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The exciting and interesting prospect of ‘‘chemically tuning’’
ꢀ
the [Fc]+BF4 mediated protocol such that increased quan-
tities of specific calixarenes were formed merited further
investigation. The increased percentage of 2[4] arising from 3
and 4 suggested the possibility that in the absence of 4 the
reaction between 3, s-trioxane and [Fc]+BF4 may afford
ꢀ
increased quantities of 2[6] via a previously unknown self-
convergent [3 + 3] protocol (Scheme 3). Reacting 3, s-trioxane
ꢀ
and [Fc]+BF4 afforded 2[4]–2[9], in an excellent 93% yield.
Gratifyingly, HPLC analysis confirmed, as predicted, the
major product at 34% to be 2[6], (viz. Gutsche et al. 10%4
and 14% Scheme 3), whilst the amount of 2[4] present i.e. 5%
(Scheme 3) had undergone a 4-fold reduction.
4 D. R. Stewart and C. D. Gutsche, J. Am. Chem. Soc., 1999, 121,
4136.
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Scheme 3 Convergent syntheses of 2[4]–2[9], using [Fc]+BF4
.
ꢀ
ꢁc
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Chem. Commun., 2008, 5731–5733 | 5733