Rhodium-Mediated Oxygenation of Nitriles with Dioxygen: Isolation of Rhodium Derivatives of Peroxyimidic Acids

Article abstract of DOI:10.1002/chem.201502481

Dioxygen is used as the oxygenation agent in the rhodium-mediated conversion of nitriles into amides. The characterization of intermediate species and model compounds as well as isotope-labeling studies provided an insight into the reaction mechanism. The conversions of rhodium hydroperoxido or methylperoxido complexes with nitriles into metallacyclic rhodium- κ²-(N,O)-peroxyimidate compounds represent essential key steps. The former are accessible from a rhodium(III) peroxido complex and the latter represent rhodium derivatives of Payne's reagent (peroxyimidic acids).

Full text of DOI:10.1002/chem.201502481

DOI: 10.1002/chem.201502481
Communication
&
Oxygenation
Rhodium-Mediated Oxygenation of Nitriles with Dioxygen:
Isolation of Rhodium Derivatives of Peroxyimidic Acids
Annemarie Bittner, Thomas Braun,* Roy Herrmann, and Stefan Mebs[a]
Dedicated to Professor Manfred Scheer on the occasion of his 60th birthday
ato) exhibits a five-membered peroxyimidate ring structure
(Pd{k2-OOC(CH3)N}) and is furnished by reacting the hydroper-
Abstract: Dioxygen is used as the oxygenation agent in
oxido
complex
[Pd(OOH)(TpiPr)(py)]
with
acetonitrile
the rhodium-mediated conversion of nitriles into amides.
The characterization of intermediate species and model
compounds as well as isotope-labeling studies provided
an insight into the reaction mechanism. The conversions
of rhodium hydroperoxido or methylperoxido complexes
with nitriles into metallacyclic rhodium- k2-(N,O)-peroxy-
imidate compounds represent essential key steps. The
former are accessible from a rhodium(III) peroxido com-
plex and the latter represent rhodium derivatives of
Payne’s reagent (peroxyimidic acids).
(Scheme 1).[7] It has a zwitterionic 2,3-dioxa-5-azapallada-cyclo-
penten-4-ene structure and it is additionally stabilized by cat-
ionic Pd(TpiPr)(1-C5NH5) fragment bound the b-oxygen atom. In
addition, based on microanalysis and IR spectroscopy the for-
mation of the nickel complex [Ni{k2-OOC(CH3)NH}(dl-Me6-14-
aneN4)](ClO4) (dl-Me6-14-aneN4 =dl-5,5,7,12,12,14-hexamethyl-
1,4,8,11-tetraazacyclotetradecane) was postulated after a reac-
tion of [Ni(dl-Me6-14-aneN4)](ClO4)2 in acetonitrile with hydro-
gen peroxide (Scheme 1).[8]
The use of dioxygen as oxygenation agent in transition-metal-
mediated CÀO bond formation reactions is of high interest
due to its availability and environmental sustainability.[1] Treat-
ment of late transition-metal complexes with dioxygen often
give peroxido complexes, which can act as key intermediates
in the conversion of organic compounds.[1k,2] At rhodium, the
oxygen atoms of the peroxido ligand are often nucleophil-
ic.[2a,c,3] Thus, a reaction of trans-[Rh(O2)(4-C5NF4)(CNtBu)(PEt3)2]
(1a) with formic acid yields initially the rhodium hydroperoxi-
do complex trans-[Rh(OOH){OC(O)H}(4-C5NF4)(CNtBu)(PEt3)2] at
low temperatures.[2c] Subsequently, hydrogen peroxide and the
rhodium(I) complex trans-[Rh(4-C5NF4)(CNtBu)(PEt3)2] (2) are fur-
nished.
Scheme 1. The palladium complex [Pd(TpiPr2){k2-OO{Pd(TpiPr2)(1-
C5NH5)}C(CH3)N}] and the nickel complex [Ni{k2-OOC(CH3)NH}(dl-Me6-14-
aneN4)](ClO4) exhibit the structural motif {M(k2-OOC(CH3)N)}, which repre-
sents a transition-metal-stabilized version of a peroxyimidic acid.
Herein we report on the oxygenation of nitriles with dioxy-
gen mediated by the rhodium hydroperoxido complex trans-
[Rh(OOH)(4-C5NF4)(CNtBu)(PEt3)2(1-C5NH5)]OTf (3a) to give ini-
tially metallacyclic rhodium-k2-(N,O)-peroxyimidates. Treatment
of the latter with NaBH4 led to the formation of the corre-
sponding amides and trans-[Rh(4-C5NF4)(CNtBu)(PEt3)2] (2). An
additional insight into the mechanism is provided by reactivity
studies at trans-[Rh(OOCH3)(4-C5NF4)(CNtBu)(PEt3)2(1-C5NH5)]OTf
(4).
Treatment of trans-[Rh(O2)(4-C5NF4)(CNtBu)(PEt3)2] (1a) with
trifluoromethanesulfonic acid (HOTf) in the presence of pyri-
dine gave trans-[Rh(OOH)(4-C5NF4)(CNtBu)(PEt3)2(1-C5NH5)]OTf
(3a) (Scheme 2). Compound 1a is accessible from trans-[Rh(4-
C5NF4)(CNtBu)(PEt3)2] (2) and dioxygen.[2a] 3a is stable in solu-
tion at room temperature, but decomposes under vacuum im-
mediately due to the loss of the pyridine ligand. Note that
trans-[Rh(O2)(4-C5NF4)(CNtBu)(PEt3)2] (1a) reacts with HCl or
HCOOH in a comparable manner to give the hydroperoxido
complexes trans-[Rh(OOH){X}(4-C5NF4)(CNtBu)(PEt3)2] (X=Cl,
OC(O)H), but the complexes are only stable at temperatures
below À508C.[2a,c]
The reaction of hydrogen peroxide with nitriles yields the
peroxyimidic acids (RC(NH)OOH, R=alkyl- or aryl group), which
decompose immediately to give the corresponding amides
and dioxygen.[4] Peroxyimidic acids can be used in situ as oxy-
genation agents for different substrates like alkenes or tertiary
amines (Payne’s reagent).[5] The instability of the peroxyimidic
acids hampers their identification and, so far, peroxybenzimidic
acid was in situ characterized by FT-Raman and FT-IR spectros-
copy.[6] However, there are two transition-metal-stabilized
derivatives of peroxyimidic acids reported in the literature.
The
palladium
complex
[Pd(TpiPr2){k2-OO{Pd(TpiPr2)(1-
C5NH5)}C(CH3)N}] (TpiPr2 =hydrotris(3,5-diisopropylpyrazolyl)bor-
[a] A. Bittner, Prof. Dr. T. Braun, R. Herrmann, Dr. S. Mebs
Department of Chemistry, Humboldt-Universität zu Berlin
Brook-Taylor-Straße 2, 12489 Berlin (Germany)
E-mail: thomas.braun@chemie.hu-berlin.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502481.
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Communication
9.54 ppm; 6a: d=9.46 ppm; 7a: d=9.85 ppm). The conver-
sion of trans-[Rh(OOD)(4-C5NF4)(CNtBu)(PEt3)2(1-C5NH5)]OTf (3c)
with acetonitrile led to the formation of both isotopologues
5a and 5c. This is presumably due to a H/D exchange be-
tween 3c and adventitious water in the acetonitrile. The ab-
sorption bands in the IR spectra for the isonitrile ligands in 5a,
6a, and 7a appear between 2205 and 2211 cmÀ1 which is con-
sistent with the presence of a rhodium(III) center.[9] Comparison
of the IR- and Raman spectra of complexes 5a, 6a, and 7a
with those of their isotopologues 5b, 6b, and 7b (Scheme 2)
allows an assignment of the absorption bands for the oxygen–
oxygen and metal–oxygen vibrations. Thus, the IR spectrum of
trans-[Rh{k2-OOC(CH3)NH}(4-C5NF4)(CNtBu)(PEt3)2]OTf (5a) dis-
plays absorption bands for the OÀO and for the RhO2 moiety
at 975 and 583 cmÀ1. The bands shift to lower energy at 948
and 562 cmÀ1 for the isotopologue 5b (Figure 1). The shifts are
rather small to be associated with isolated OÀO modes.[2c] The
data are in accordance with the absorption bands for the rho-
dium peroxycarbonato complex trans-[Rh{k2-OOC(O)O}(4-
Scheme 2. Synthesis of the rhodium hydroperoxido complex 3a and its iso-
topologues 3b, 3c, and 3d. Reactivity of the latter towards nitriles.
The 31P{1H} NMR spectrum of 3a shows a doublet at d=
15.8 ppm with a rhodium–phosphorus coupling constant of
85 Hz, which indicates the presence of a rhodium complex in
the oxidation state +3.[2a] The IR spectrum shows a characteris-
tic absorption band at 2199 cmÀ1 for the isonitrile ligand
bound to the rhodium(III) center.[9] The 19F NMR spectrum dis-
plays four multiplets for the tetrafluoropyridyl ligand, which in-
C5NF4)(CNtBu)(PEt3)2] (Rh{k2-OOC(O)O}: 969 cmÀ1
;
Rh{k2-
18O18OC(O)O}: 948 cmÀ1);[2c] they differ from the data for
the
rhodium
metallacycle
trans-[Rh{k2-OOB(OH)O}(4-
;
18
C5NF4)(CNtBu)(PEt3)2] (OÀO: 654 cmÀ1
18OÀ O: 644 cmÀ1).[9c]
dicates
a hindered rotation about the rhodium–carbon
1
bond.[10] Five multiplet signals are found for 3a in the H NMR
spectrum in the range from d=9.5 to 7.5 ppm, each with
a similar intensity. They can be assigned to the coordinated
pyridine. A broad singlet signal at d=8.29 ppm is assigned to
the hydroperoxido ligand.[2a,c] Treatment of rhodium peroxido
complex 1a with DOTf in the presence of pyridine led to the
formation of [Rh(OOD)(4-C5NF4)(CNtBu)(PEt3)2(1-C5NH5)]OTf
(3c). On using [D5]pyridine instead of the non-deuterated pyri-
dine, the isotopologue [Rh(OOH)(4-C5NF4)(CNtBu)(PEt3)2(1-
C5ND5)]OTf (3d) was obtained. The complexes 3c and 3d
show resonances in the 2H NMR spectra for the OOD (d=
8.43 ppm) or for the deuterated pyridine ligand (d=9.53, 9.04,
8.52, 8.29, 8.03 ppm), respectively. Synthesis of the complex
trans-[Rh(18O18OH)(4-C5NF4)(CNtBu)(PEt3)2(1-C5NH5)]OTf (3b) was
Figure 1. Part of the IR spectra of 5a (solid line) and 5b (dashed line) (atte-
nuated total reflection (ATR), diamond).
achieved
by
conversion
of
trans-[Rh(18O2)(4-
C5NF4)(CNtBu)(PEt3)2] (1b) with HOTf in the presence of pyri-
dine. The HR-ESI-MS data of both hydroperoxido complexes
3a and 3b reveal peaks at m/z ratios for [M-pyridine]+ at
605.1556 ([Rh(OOH)(4-C5NF4)(CNtBu)-(PEt3)2]+) or 609.1626
([Rh(18O18OH)(4-C5NF4)(CNtBu)(PEt3)2]+), respectively. Although
the rhodium hydroperoxido complexes 3a–d were stable at
room temperature in solution, isolation of the complexes was
not possible.
The Raman spectrum of 6a (Nd-YAG 1064 nm) shows ab-
sorption bands at 852 cmÀ1 for the OÀO vibration and at
584 cmÀ1 for the RhO2 unit. For the 18O-isotopologue 6b, the
bands shift to 815 and 561 cmÀ1. In the same manner, the ab-
sorption bands for the OÀO vibration of complexes 7a and 7b
(7a: 16OÀ O 899 cmÀ1, Rh16O2 594 cmÀ1; 7b: 18OÀ O 879 cmÀ1,
Rh18O2 549 cmÀ1) can be assigned (see the Supporting Informa-
tion). The structures of 5a and 7a in the solid state were de-
termined by X-ray crystallography (Figure 2 and the Support-
ing Information).[11] Both complexes 5a and 7a exhibit an octa-
hedral geometry. The oxygen atom of the peroxyimidate
ligand binds at the trans position to the isonitrile ligand,
whereas the nitrogen atom coordinates in the trans position to
the tetrafluoropyridyl ligand. The OÀO (1.4914(17) Š) and
RhÀO (2.0473(13) Š) bond lengths of 5a are comparable to the
corresponding distances found for other rhodium dioxametal-
16
18
In situ treatment of trans-[Rh(OOH)(4-C5NF4)(CNtBu)(PEt3)2(1-
C5NH5)]OTf (3a) with acetonitrile in a THF solution yielded the
complex trans-[Rh{k2-OOC(CH3)NH}(4-C5NF4)(CNtBu)(PEt3)2]OTf
(5a) by carbon–oxygen bond formation. In
manner, the complexes
a similar
trans-[Rh{k2-OOC(R)NH}(4-
C5NF4)(CNtBu)(PEt3)2]OTf (6a: R=CH2CH3; 7a: R=C6H5) were
synthesized using propionitrile or benzonitrile (Scheme 1). In
the 1H NMR spectra of 5a–7a the protons at the nitrogen
atom of the peroxyimidates give rise to broad singlets (5a: d=
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Figure 2. Structure of the cation in 5a in the crystal (ORTEP plot, ellipsoids
at the 50% probability level; hydrogen atoms are omitted for clarity). Select-
ed bond lengths [Š] and angles [8]: C2ÀRh1 1.9355(18), C4ÀRh1 2.0616(18),
O2ÀRh1 2.0473(13), N1ÀRh1 2.0479(16), O1ÀO2 1.4914(17), C1ÀO1 1.323(2),
C1ÀN1 1.275(2), C1ÀC15 1.494(3); N1-C1-O1 120.97 (17), N2-C2-Rh1
172.36(16), C2-Rh1-C4 93.58(7), C2-Rh1-O2 171.72(6), O2-Rh1-C4 94.70(6), C2-
Rh1-N1 92.41(7), O2-Rh1-N1 79.31(6), O1-O2-Rh1 109.61(9).
Scheme 3. Reaction of trans-[Rh(OOCH3)(4-C5NF4)(CNtBu)(PEt3)2(1-C5NH5)]OTf
(4) with acetonitrile.
though we were not able to detect an intermediate compara-
ble to 8 in the conversion of the hydroperoxido complex 3a
into 5a, the presence of such a species is very likely.
lacycles such as trans-[Rh{k2-OOB(OH)O}(4-C5NF4)(CNtBu)(PEt3)2]
(OÀO: 1.4974(14) Š; RhÀO 2.0366(10) Š)[9c] or trans-[Rh{k2-
OOC(O)O}(4-C5NF4)(CNtBu)(PEt3)2] (OÀO: 1.469(4) Š; RhÀO
2.040(3) Š).[2c] The O-O-Rh angle of trans-[Rh{k2-OOB(OH)O}(4-
C5NF4)(CNtBu)(PEt3)2] (108.29(7)8) and trans-[Rh{k2-OOC(O)O}(4-
C5NF4)(CNtBu)(PEt3)2] (107.7(2)8) are slightly smaller than that in
5a (109.61(9)8). The bond lengths and angles of 7a are compa-
rable to the data discussed for 5a.
Mechanistically, it can be assumed that a nucleophilic attack
of the b-oxygen atom of the hydroperoxido ligand at the
carbon atom of a metal-coordinated nitrile moiety occurs to
furnish the five-membered ring. A proton shift from the
oxygen atom to the nitrogen atom finally yields 5a–7a. Re-
markably, a metallaperoxyimidate can also be generated from
The compounds 5a, 6a, and 7a can be regarded as rhodium
derivatives of the peroxyimidic acids. As mentioned above,
peroxyimidic acids decompose immediately at room tempera-
ture to give the corresponding amides and dioxygen.[4] Howev-
er, treatment of 5a, 6a, or 7a with an excess NaBH4 in a THF
solution led to the formation of the amides and trans-[Rh(4-
C5NF4)(CNtBu)(PEt3)2] (2) (Scheme 4).[12] We presume that an
attack of the hydride with a concomitant cleavage of the
RhÀN bond leads to complexes trans-[Rh(H){OOC(R)NH}(4-
C5NF4)(CNtBu)(PEt3)2] bearing a k1-OOC(N)HR unit. A reductive
elimination of the latter and the metal-bound hydrogen might
give the free peroxyimidic acids, which convert into the
amides and oxygen.[4]
a
methylperoxido complex. Thus, trans-[Rh(OOCH3)(4-
The amides and the literature-known rhodium(I) complex 2
were characterized by NMR spectroscopy.[10,13] The former were
C5NF4)(CNtBu)(PEt3)2(1-C5NH5)]OTf[2a] (4) reacted in acetonitrile
to afford the complex trans-[Rh{k2-OOC(CH3)NCH3)}(4-
C5NF4)(CNtBu)(PEt3)2]OTf (9) (Scheme 3). The 31P{1H} NMR and
19F NMR data of 9 are comparable to those of 5a. Two singlet
signals in the 1H NMR spectrum are attributed to the two
methyl groups at the peroxyimidate entity.
The reaction of the methylperoxido complex 4 with acetoni-
trile was monitored by NMR spectroscopy at room tempera-
ture. The 19F and 31P{1H} NMR spectra reveal the formation of
the
intermediate
complex
trans-[Rh(OOCH3)(4-
C5NF4)(CNtBu)(PEt3)2(CH3CN)]OTf (8). After five minutes an addi-
tional doublet at d=19.7 ppm was detected in the 31P{1H}
NMR spectrum with a rhodium–phosphorus coupling constant
of 81.4 Hz. The 19F NMR spectrum showed four additional mul-
tiplets for the metal-bound tetrafluoropyridyl ligand. In the
1H NMR spectra the formation of free pyridine was observed.
Furthermore a singlet signal at 3.53 was assigned to the meth-
ylperoxido ligand in 8. After an additional 5 min the resonan-
ces for the rhodium complex 9 are also detectable in the NMR
spectra. The conversion into 9 is completed within 30 min. Al-
Scheme 4. Cyclic process for the oxygenation of nitriles with dioxygen.
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Acknowledgement
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Keywords: metallacycle · oxygenation · peroxyimidic acid ·
reaction mechanism · rhodium
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Received: June 26, 2015
Published online on July 17, 2015
Chem. Eur. J. 2015, 21, 12299 – 12302
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