red) can be connected to two wings of triptycene to bind a
diiron core. An additional carboxylate can be incorporated
into the ligand framework by attachment to the third wing
of the backbone to deliver the bridging ligand internally
(Figure 2, La, blue). La with a simple triptycene backbone,
however, is too compact to accommodate the desired diiron
unit, for the positions marked by “X” are separated by only
˚
4.5 A. To increase the distance between two metal-binding
arms the backbone unit, Lb, containing furan rings fused to
˚
the triptycene group, was introduced (X X = 8.1 A). By
3 3 3
Figure 1. Reduced state of the diiron core of sMMOH.
attaching two benzoxazole N,O-donor arms to Lb, the
ligand L was conceived (Figure 2). Although triptycenes
have been known since Bartlett’s first synthesis in 1942,26,27
their use to generate ligands for assembling multinuclear
metal coordination compounds has not yet been seriously
explored. We describe in the present work the synthesis of a
new class of dinucleating ligand based on triptycene and its
use in generating a carboxylate-bridged diiron complex.
Synthetic efforts to mimic the active sites of these diiron
enzymes, such as that in soluble methane monooxygenase
hydroxylase (sMMOH)5 (Figure 1), have generated a
diverse assortment of model compounds,16À21 but repro-
ducing both the structure and function of the carboxylate-
bridged diiron unit in a single biomimetic platform has not
yet been achieved. To obtain more accurate diiron protein
models, we have designed advanced ligand frameworks that
are sufficiently preorganized to afford the desired coordina-
tion geometry upon metalation with iron salts.22À25 Ulti-
mately, we wish to construct a diiron compound having two
syn N-donors, an oxygen-rich environment, and a cova-
lently tethered bridging carboxylate, and which can hydro-
xylate hydrocarbons under mild conditions using O2.
Scheme 1. Column-Free Synthesis of DHT 4
A readily scalable route with high synthetic efficiency is
desired in order to prepare gram quantities of L1. A four-
step column-free synthesis of 1,8-dihydroxytriptycene
(DHT) (4) was accomplished starting from the commer-
cially available reagent 1,8-dihydroxyanthraquinone. As
shown in Scheme 1, methylation of 1,8-dihydroxyanthra-
quinone using dimethyl sulfate (∼3.5 equiv) gave 1,8-
dimethoxyanthraquinone (1) in 95% yield. Crude 1 was
directly reduced by zinc dust in refluxing 10% aqueous
NaOH, affording 1,8-dimethoxyanthracene (2) in 96%
yield after recrystallization from DCM. To minimize the
potentially explosive benzyne intermediate generated dur-
ing the DielsÀAlder reaction in the next step, only ∼5 g of
precursor 2 was used each time in the reaction. After
several runs, the combined crude mixture was subjected
to crystallization from acetone, affording 1,8-dimethoxy-
triptycene (3) in 67% yield. Lastly, deprotection of
1,8-dimethoxytriptycene (3) using BBr3 in anhydrous
DCM, after recrystallization from acetone, gave pure
Figure 2. Ligand design.
With the aid of modeling studies using SPARTAN, we
selected the triptycene unit as a platform to approach these
goals. As depicted in Figure 2, two N,O-donor arms (La,
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publication.
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Org. Lett., Vol. 13, No. 19, 2011
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