Communications
DOI: 10.1002/anie.201007315
Peroxido Complexes
A Rhodium Peroxido Complex in Mono-, Di-, and Peroxygenation
Reactions**
Gregor Meier and Thomas Braun*
Dedicated to Professor Dieter Lentz on the occasion of his 60th birthday
One of the main challenges in oxygenation chemistry is
transition-metal-mediated oxygenation using molecular
ates phosphines, dioxygenates tetrakis(dimethylamino)ethy-
lene (2), and peroxygenates 9,10-dimethylanthracene (3). The
last-named transformation can be regarded as a model
reaction for direct transfer of a dioxygen unit from a peroxido
complex to an organic substrate. According to our knowledge,
such a reaction is unprecedented. Moreover, in a photo-
catalytic experiment an anthracene endoperoxide can be
generated in stoichiometric excess.
[1]
oxygen as the primary oxidant. Peroxido transition metal
compounds have been suggested to play a significant role as
[1,2]
intermediates in oxygenation reactions with dioxygen.
In
most instances only one oxygen atom of the peroxido unit is
transferred to the organic substrate to yield mono-oxygenated
[2,3]
products.
Several of these mono-oxygenation reactions
therefore require additional oxophilic substrates such as
phosphines or sulfoxides as sacrificial agents.
Treatment of a solution of trans-[Rh(4-C F N)(CO)-
5
4
[3]
[9]
(PEt ) ] (4a) in hexane with dioxygen gave the peroxido
3 2
However, peroxido ligands can also react with various
substrates to give new peroxido entities, which remain bound
compound trans-[Rh(O )(4-C F N)(CO)(PEt ) ] (1a) over
2 5 4 3 2
1
8
two weeks (Scheme 1). trans-[Rh( O )(4-C F N)(CO)-
2
5
4
[
4,5]
to a late transition metal center.
For instance, trans-
[
Ir(O )(CH )(CO){P(p-tolyl) } ] transforms CO, CO , and
2
3
3
2
2
SO into the corresponding carbonato, peroxidocarbonato,
2
[
6]
and sulfato ligands. The rhodium peroxido complex trans-
Rh(O )(4-C F N)(CNtBu)(PEt ) ] reacts with boronic acids
[
2
5
4
3 2
RB(OH) (R = C H , C F ) to give heterocyclic compounds
2
6
5
6
5
[7]
with five-membered RhOOBO rings.
Examples of transfer of both oxygen atoms from a
transition metal peroxido complex to a substrate are
[
8]
sparse, and formation of peroxides is relatively seldom.
The complexes [M(O)(O ) (py)] (M = Cr, Mo; py = pyridine)
Scheme 1. Syntheses of 1a (sens=methylene blue).
2
2
react with electron-rich enol ethers to yield the corresponding
[
8a]
dioxetanes, but the products are formed by two consecutive
mono-oxygenation reactions and involve intermediate for-
mation of the corresponding epoxides. Apart from mono-
1
8
(PEt ) ] (1b) is available in a similar fashion by using O2.
3
2
1
3
Reaction of the C-labeled isotopomer trans-[Rh(4-C F N)-
5
4
[
2c]
13
[9]
oxygenation of 1,5-cyclooctadiene,
oxygenation of 1,5-
( CO)(PEt ) ] (4b) with dioxygen yielded trans-[Rh(O )(4-
3
2
2
1
3
cyclooctadiene with [Rh(O )(Cl)(PPh ) ], which can be
C F N)( CO)(PEt ) ] (1c). Formation of 1a, 1b, and 1c is
spin-forbidden and requires crossing between the singlet and
2
3
3
5
4
3 2
[
8b,c]
prepared from dioxygen, produces 1,4-cyclooctanedione.
[
10]
However, this rare case of transition metal mediated alkene
triplet surfaces on the reaction coordinate. Consequently,
1a can be synthesized within six hours on using singlet
dioxygen, generated in situ by irradiation of triplet dioxygen
with a tungsten halogen lamp in the presence of methylene
blue as sensitizer (Scheme 1). Alternatively, the photochem-
ical reaction conditions may provide a reaction pathway
which involves initial electron transfer from 4a to dioxy-
dioxygenation with O is rather slow and is accompanied by
2
competitive monooxygenation of the phosphine ligand.
Here we report the synthesis of the rhodium(III) peroxido
complex trans-[Rh(O )(4-C F N)(CO)(PEt ) ] (1a) using
2
5
4
3 2
triplet or singlet dioxygen. Under irradiation 1a exhibits a
variety of selective oxygenation reactions. It mono-oxygen-
[10a]
gen.
A similar method was used to synthesize trans-
[
(
Rh(O )(SC F )(CO)(PPh ) ], trans-[Rh(O )(Cl)(CO)-
2
6
5
3
2
2
[
*] G. Meier, Prof. Dr. T. Braun
PPh ) ], and trans-[Rh(O )(CO)(PPh ) (NCCH )]ClO ,
3 2 2 3 2 3 4
Humboldt-Universitꢀt zu Berlin, Department of Chemistry
Brook-Taylor-Straße 2, 12489 Berlin (Germany)
E-mail: thomas.braun@chemie.hu-berlin.de
[11]
which are unstable at room temperature. Without sensitizer
only tiny amounts of 1a were generated. When 4a was
photolyzed in the presence of triplet dioxygen with an Xe UV
[
**] We would like to acknowledge the Deutsche Forschungsgemein-
schaft for financial support. We are grateful to P. Klꢀring and A.
Penner for the crystallography. We also would like to acknowledge
Prof. C. Limberg for valuable discussions.
[
12]
lamp, phosphine oxide was produced.
The peroxido compound 1a is soluble in aromatic hydro-
carbons (benzene, toluene) and polar solvents like THF, Et O,
2
and CH Cl , and is inert to air and water. In contrast to some
2
2
[11c,13]
other rhodium peroxido species
dioxygen is not released
3
280
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 3280 –3284