Communication
Although TPAP (4) is described as air-stable, and that it can
be stored indefinitely in the cold (i.e. freezer),[28] this is not our
experience, nor that of others.[29] This mild catalyst instability
most likely occurs by repetitive exposure to moisture and vari-
able temperature during transfer between the laboratory and
refrigerator over a prolonged period. Such attributes can
impact catalyst quality, in addition, to reaction reproducibility
and increased expenditure for repurchase. To address catalyst
recyclability, substantial effort has been dedicated to solid sup-
port versions of the perruthenate anion, which effectively elim-
inate stability issues. Ley was the first to investigate a polymer
bound version based on Amberlyst anion exchange resin
(IR 27), which provided pure products that avoided traditional
work-up.[30] Further developments included silicates (e.g. MCM-
41[31] or TPAP-doped[32]), and polymer supported ionic liq-
uids.[33,34] Surprisingly, however, even though substantial ad-
vances have been made to solid phase perruthenate recyclabil-
ity and stability, some of which are commercially available, the
academic (and to some extent industrial[35]) synthetic commun-
ity continue to demonstrate an insatiable desire for TPAP (4)
promoted oxidations.
Table 1. Synthesis of ammonium and phosphonium perruthenates.
In the course of our investigation into the mechanism of the
TPAP oxidation,[36] freshly prepared catalyst was required for
accurate spectroscopic measurements. This experience provid-
ed an opportunity to become familiar with the reported syn-
theses of TPAP. The ruthenium gas transfer method first pub-
lished by Ley and Griffith[1] requires the generation, and han-
dling, of toxic ruthenium tetroxide gas, but TPAP can be ob-
tained in high purity. The more commonly utilised method for
TPAP synthesis was published by Griffith in 1993,[37] which in-
volves a single pot oxidation of ruthenium trichloride by
sodium bromate in basic solution to give a perruthenate solu-
tion followed by addition of tetrapropylammonium hydroxide
solution. Although the Griffith synthesis of TPAP is practically
straightforward at the initial stage, the work up and isolation
requires precipitation with carbon tetrachloride. In our hands,
(semi-tropical conditions) only 44% yield (lit.[37] 99% yield) was
obtained. We found that replacing carcinogenic carbon tetra-
chloride with commonly reported alternatives pentane, or cy-
clohexane resulted in a 19 and 23% yield increase.
kyltriphenylphosphonium cations that are commercially avail-
able due to their use in the Wittig reaction, a wide variety of
these could be explored. Pleasingly, when methyltriphenyl-
phosphonium bromide was used in place of tetrapropylammo-
nium hydroxide, using the method of Griffith,[37] the methyltri-
phenylphoshonium perruthenate (MTP3, 16, Figure 1) readily
precipitated upon addition of the cation to a perruthenate so-
lution, in 86% yield (5.0 mmol scale) (Table 1) avoiding the ex-
traction and work up step. The crystal structure of 16 is iso-
À
morphous with the ReO4 analogue.[42] This simplified method
was expanded to include the ethyl-(17) (isomorphous with the
À
ReO4 analogue[43]) propyl-(18) and isoamyltriphenylphosphon-
With TPAP stability issues front of mind, and having experi-
ence making TPAP via both methods, we considered the
notion that perruthenates with alternative counter ions may
provide increased stability. Considering that tetraphenylphos-
phonium perruthenate (13)[38] has been reported to be a more
stable counter cation,[29] it was synthesised using a slight modi-
fication of the Griffith bromate oxidation protocol[37] (i.e. re-
placing carbon tetrachloride with pentane, cyclohexane, tolu-
ene or a,a,a-trifluorotoluene), which afforded 13 in 62–85%
isolated yields (Table 1).
ium perruthenates (ATP3, 19, Figure 1) in 86, 88 and 91%
yields, respectively (Table 1). For comparison the ruthenium
gas transfer method first published by Ley and Griffith[1] was
used as an alternative synthesis of MTP3 (i.e. see Supporting
Information for glassware), providing 48% yield of similarly
high purity material.
All of these compounds were found to be bench stable for a
period of months. In the case of MTP3 (16) and TPAP (4), ad
hoc elemental analyses (see Supporting Information) were per-
formed over a period of days and months, which revealed no
catalyst degradation at room temperature of MTP3 after
9 months, but considerable degradation of TPAP after 9–
64 hours [Note: MTP3 does degrade after 12 months of consis-
tent use]. Bridged dication diperruthenate variants 20 and 21
were isolated in 82 and 69% yield (Table 1), respectively. How-
ever, 20 decomposed when dissolved in organic solvents,
whereas benzyltriphenylphosphonium perruthenate (22, 89%)
Using this modified method tetraphenylarsonium perruthen-
ate (14)[39] and bis(triphenylphosphoranylidene) ammonium
perruthenate (15)[40] (isomorphous with the SeF3OÀ ana-
logue)[41] were synthesised in 29 and 93% yield, respectively
(Table 1, see Supporting Information for X-ray crystallographic
analysis). However, room temperature stability was only ob-
served for approximately 7 days. Considering the number of al-
&
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