Observation of a Photogenerated Rh2 Nitrenoid Intermediate in C-H Amination

Article abstract of DOI:10.1021/jacs.8b05599

Rh₂-catalyzed C-H amination is a powerful method for nitrogenating organic molecules. While Rh₂ nitrenoids are often invoked as reactive intermediates in these reactions, the exquisite reactivity and fleeting lifetime of these species has precluded their observation. Here, we report the photogeneration of a transient Rh₂ nitrenoid that participates in C-H amination. The developed approach to Rh₂ nitrenoids, based on photochemical cleavage of N-Cl bonds in N-chloroamido ligands, has enabled characterization of a reactive Rh₂ nitrenoid by mass spectrometry and transient absorption spectroscopy. We anticipate that photogeneration of metal nitrenoids will contribute to the development of C-H amination catalysis by providing tools to directly study the structures of these critical intermediates.

Full text of DOI:10.1021/jacs.8b05599

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Observation of a Photogenerated Rh Nitrenoid Intermediate in C–H Amination
Anuvab Das, Andrew G Maher, Joshua Telser, and David C Powers
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Observation of a Photogenerated Rh2 Nitrenoid Intermediate in C–H
Amination
Anuvab Das,† Andrew G. Maher,^ Joshua Telser,‡ and David C. Powers†,*
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† Department of Chemistry, Texas A&M University, College Station, TX 77843, United States
^ Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, United States
‡ Department of Biological, Chemical and Physical Sciences, Roosevelt University, Chicago, IL 60605, United States
Supporting Information Placeholder
photochemistry as a tool to access authentic reactive in-
ABSTRACT: Rh2-catalyzed C–H amination is a powerful
method for nitrogenating organic molecules. While Rh2
termediates under conditions that allow their character-
ization without attenuating their reactivity.7 In contrast
nitrenoids are often invoked as reactive intermediates in
to metal oxo and nitrido complexes, which can be gener-
ated by photolysis of metal-bound oxyanions8 and azido
ligands,9 respectively, there are no readily available pho-
toprecursors to metal nitrenoid complexes. Here, we re-
port the development of a new photochemical synthesis
of metal nitrenoids, which has enabled the first spectro-
scopic observation of a Rh2 nitrenoid intermediate in C–
H amination.
these reactions, the exquisite reactivity and fleeting life-
time of these species has precluded their observation.
Here, we report the photogeneration of a transient Rh2
nitrenoid that participates in C–H amination. The devel-
oped approach to Rh2 nitrenoids, based on photochemi-
cal cleavage of N–Cl bonds in N-chloroamido ligands, has
enabled characterization of a reactive Rh2 nitrenoid by
mass spectrometry and transient absorption spectros-
copy. We anticipate that photogeneration of metal
nitrenoids will contribute to the development of C–H
amination catalysis by providing tools to directly study
the structures of these critical intermediates.
We envisioned that a Rh2 nitrenoid could be gener-
ated by photochemical homolysis of an N–X bond in an
axially bound N-haloamide ligand (Scheme 1). Such a
transformation would reduce the Rh2 fragment by one
electron, and thus a Rh2[II,III] precursor is required to ac-
cess the Rh2[II,II] nitrenoids that mediate Rh2-catalyzed
C–H amination with iminoiodinanes.10 In contrast to the
well-recognized facility of photochemical N–Cl homolysis
in N-chloroamines to generate nitrogen-centered radi-
cals (i.e. the Hofmann-Löffler-Freytag reaction),11,12 the
photochemistry of transition-metal-bound N-chloroam-
ido ligands is less well explored.13 Here we report the syn-
thesis of a Rh2 N-chloroamide complex and demonstrate
Since Kwart and Khan demonstrated that metal
nitrenoids participate in C–H amination chemistry in
1967,1 metal-catalyzed nitrogen-group-transfer catalysis
has become an important technology for the construc-
tion of C–N bonds.2 In particular, Rh2 nitrenoids — typi-
cally generated by the combination of Rh2 catalysts with
iminoiodinane reagents (e.g. PhI=NR) — have emerged
as powerful intermediates for C–H amination.1,3 The ex-
ceptional reactivity of Rh2 nitrenoids has precluded the
spectroscopic or structural characterization of these crit-
ical species.4 Thus ongoing efforts to develop new ami-
nation reactions must rely on computational descrip-
tions of the reactive nitrenoid intermediates.5
R
H
R
NHR’
X
R’
N
NR’
Rh
Rh
Rh
Rh
hν
Rh
Rh
this work
Rh2 nitrene
photoprecursor
Rh2 amination
catalyst
reactive Rh2
nitrene intermediate
Characterization of reactive intermediates is inherently
challenging due to the fleeting nature of these species.
Stable model complexes are often pursued in order to
gain insight into the structure and reactivity of reactive
intermediates.6 We have been attracted to synthetic
I
PhI
Ph
NR’
Scheme 1. Rh2-catalyzed amination chemistry proceeds through a reactive
nitrenoid intermediate generated by nitrene transfer from an iminoiodinane re-
agent. Here, we demonstrate that photogeneration of a Rh2 nitrenoid enables
direct spectroscopic observation of this critical reactive intermediate.
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Scheme 2. Synthesis of Rh2 N-chloroamide 3. Complex 3 crystallizes from
CH3CN as a salt of Rh2[II,III] ions: [Rh2(espn)2(MeCN)2][Rh2(espn)2(NTsCl)2].
that photolysis of this compound unveils a reactive Rh2
nitrenoid that participates in C–H amination.
Scheme 3. Photolysis of red-colored complex 3 (l > 400 nm) results in con-
sumption of 3 with concurrent evolution of green-colored Rh2[II,II] complex 5
and amine 6. The observed reaction products – Rh2[II,II] complex 5 and amine
6 – are consistent with amination by a photogenerated Rh2 nitrenoid.
Treatment of Rh2[II,III] chloride10b,14 1 with AgBF4 af-
fords [Rh2(espn)2]BF4 (2). Subsequent exposure of 2 to
Na[NTsCl] affords Rh2[II,III] N-chloroamido complex 3
(Scheme 2, espn = a,a,a’,a’-tetramethyl-1,3-benzenedi-
propanamidate, Ts = p-toluenesulfonate). Complex 3
crystallizes as a red-colored [Rh2L2S2][Rh2L2X2] salt from
CH3CN (L =espn, S = CH3CN, X = NTsCl); the crystal struc-
ture of 3 features a cationic Rh2[II,III] unit with two
bound acetonitrile ligands and an anionic Rh2[II,III] unit
with two N-chloroamide ligands. For comparison, Rh2
chloride 1 crystallizes from CH2Cl2 as extended (Rh–Rh–
Cl–)n chains10b but crystallizes from CH3CN as a
[Rh2(espn)2(MeCN)2][Rh2(espn)2Cl2] salt.
UV-vis analysis of the reaction mixture following photol-
ysis indicates evolution of green-colored Rh2[II,II] com-
plex 5 (~100%, Figure S6) and 1H NMR analysis of the re-
action mixture indicates evolution of 6 (47 ± 2%), the
product of THF amination (Figure S7). The balance of ni-
trogen content is accounted for by TsNH2 (45 ± 3%).
Rh2[II,II] complex 5 and tosylamide 6 are the products ex-
pected from initial N–Cl cleavage to generate a Rh2
nitrenoid and Cl•. Nitrene-transfer from the Rh2 nitrene-
oid to THF would generate Rh2[II,II] complex 5 and amine
6 and H-atom abstraction (HAA) between THF and Cl•
would generate HCl and THF radicals. We have not de-
tected products derived from THF radicals but the evolu-
tion of HCl was confirmed by the formation of Et3N·HCl
upon addition of Et3N to the photolysis reaction mixture
(Figures S8 and S9). See Supporting Information for dis-
cussion of the amination of 2-methyl-THF and toluene.
Based on EPR, IR, and electronic absorption spectros-
copies, as well as Evans method and DC susceptibility
measurements, Rh2[II,III] complexes 1, 2, and 3 share a
common S = 1/2 ground state in which the unpaired elec-
tron resides in a Rh–Rh d* orbital (Figures S1–S4).5,15 ESI-
MS analysis of a MeCN solution of 3 displays a signal at
959.168, which is consistent with [Rh2(espn)2(NTsCl)]+.
In concept, the observed amination of THF could arise
from a variety of reactive nitrogen species. For example,
initial Rh–N cleavage could generate Rh2[II,II] complex 5
and an aminyl radical that could engage in C–H amina-
tion. Alternately, photolysis could generate free tosylnit-
rene which could also engage in C–H amination. We have
carried out a series of kinetic isotope effect (KIE) experi-
ments to interrogate the nature of the reactive nitrogen
species generated during photolysis of 3 (Table 1). Pho-
tolysis of 3 in a 1:1 THF : THF-d8 mixture affords a KIE of
2.72(8). This value is well-matched to KIEs that have been
previously measured for Rh2-catalyzed C–H amination
using iminoiodinanes as nitrene sources.16 In compari-
son, photolysis of Na[NTsCl] affords tosylamide 6 with a
KIE of 4.4(3), which is consistent with amination via initial
1
The H NMR spectrum of 3 measured in MeCN displays
more
features
than
expected
for
[Rh2(espn)2(NTsCl)(MeCN)] (Figure S5). Comparison of
the 1H NMR spectrum of 3 with those of [Rh2(espn)2]BF4
(2) and Na[Rh2(espn)2(NTsCl)2] (4), obtained by treat-
ment of complex 1 with excess Na[NTsCl], indicates that
the solution structure of 3 is comprised of a ligand ex-
change equilibrium mixture of the neutral complex
Rh2(espn)2(NTsCl)(MeCN)
and
salt
[Rh2(espn)2][Rh2(espn)2(NTsCl)2] (see Supporting Infor-
mation for additional discussion of the solution-phase
structure of 3).
Photolysis of a THF solution of Rh2 complex 3 (l > 400
nm) results in a red-to-green color change (Scheme 3).
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Table 1. C–H amination KIEs for selected nitrogen sources.
1
2
3
4
D
TsHN
D
D
O
TsHN
O
O
conditions
D
+
D8/H8
D
D
D
5
6
7
8
Conditions
Rh2(espn)2Cl, PhINTs
Rh2 complex 3, hv
Chloramine-T, hv
TsN3, hv
Yield (6 + 6-d7)
(ref. 5)15
48%
kH / kD
2.6
2.72(8)
4.4(3)
1.48(2)
9
63%
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7%
HAA from THF.17 The potential intermediacy of free
nitrenes was investigated by photolysis of TsN3.18 Analy-
sis of the amination of THF under these conditions af-
fords a KIE of 1.48(2). The similarity of the KIE measured
for photochemical amination of THF with that of Rh2-cat-
alyzed amination, and the dissimilarity with the KIEs
measured for amination via aminyl radical and free
nitrene intermediates, suggests photolysis of Rh2 com-
plex 3 generates a Rh2 nitrenoid.
With a photosynthetic approach to a chemically and ki-
netically competent Rh2 nitrenoid in hand, we turned our
attention to observation of the reactive intermediate in-
volved in amination chemistry. At low laser intensity,
MALDI mass spectrometric analysis of 3 displayed an m/z
= 754.153, which corresponds to [Rh2(espn)2]+. As the la-
ser intensity was increased, we observed the emergence
of an m/z signal at 923.458, which corresponds to
nitrenoid 7 (Scheme 4a). Mass spectrometry analysis of
15N-labeled 3 displays a shift to 924.555, which is ex-
pected for replacement of the 14N with 15N. In addition
to the parent ions, m/z signals at 962.464 and 963.547
are also observed, which correspond to the mass of 14N-
and 15N labeled nitrenoid 7 with a bound MeCN ligand
(Figure S10).
Scheme 4. (a) Rh2 nitrenoid has been observed by MALDI-MS. 15N-labeled 3
provides the expected M+1 m/z shift. (b) TA spectra obtained by laser flash
photolysis of a THF solution of 3 (lpump = 280 nm). The prompt spectrum (red)
displays a transient growth centered at 580 nm that we attribute to Rh2 nitrene-
oid 7. The prompt spectrum evolves over the course of 200 µs to a spectrum
(blue) that displays a spectral bleach that corresponds to consumption of 3.
Single wavelength kinetics indicate that the disappearance of the growth at 580
nm and the appearance of the bleach at 450 nm are temporally coupled, sug-
gesting evolution of these two signals is related to the same chemical process.
(c) The low-energy feature (580 nm) observed in the spectrum of nitrenoid 7
arises from predominantly Rh–Rh p (left) to Rh–N p* (right) excitation.
We have carried out transient absorption (TA) experi-
ments to obtain spectroscopic information regarding the
reactive intermediate in amination. Laser-flash photoly-
sis (l = 280 nm or 355 nm19) of Rh2 complex 3 in CH2Cl2
reveals a transient growth centered at ~580 nm in the
prompt spectrum (red, Scheme 4b). The decay of this
lower-energy feature is accompanied by the develop-
ment of a spectral bleach centered at ~450 nm. The spec-
trum acquired after a delay of 200 µs corresponds to the
difference spectrum expected for consumption of 3 and
formation of Rh2[II,II] complex 5 (blue, Scheme 4b). The
time constants for the decay of the feature at 580 nm and
for the bleach at 450 nm are 11.6 ± 2.7 µs and 10.1 ± 3.8
µs, respectively. This similarity of temporal evolution of
the two observed transient features suggests that they
arise from the same chemical process. Time-dependent
density functional theory (TD-DFT) calculations of
Rh2[II,III] complexes 1, 2, 3, and nitrenoid 7 have been
carried out to gain insight into the TA spectra observed
(see Supporting Information for computational details).
Each of the Rh2[II,III] complexes 1, 2, and 3 share a com-
mon absorbance that is primarily derived from a Rh–Rh
p to Rh–O p* transition. The computed spectrum of
nitrenoid 7 shares the absorbance of the Rh2[II,III] struc-
tures as well as a lower-energy absorbance that predom-
inantly arises from Rh–Rh p to Rh–N p* excitation. The
computed spectra of 3 and 7 are consistent with the TA
spectra that display a prompt low-energy growth and the
subsequent evolution of a higher energy bleach.
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D. H.; Kürti, L.; Falck, J. R. Science 2014, 343, 61–65. (c) Kornecki, K. P.;
In conclusion, we have described photogeneration of a
Rh2 nitrenoid intermediate in C–H amination. Based on
the observed photochemical C–H amination, the similar-
ity of the KIE for C–H amination to that of Rh2-catalyzed
amination reactions, the observation of m/z for a molec-
ular Rh2 nitrenoid, and TA spectroscopy correlated with
TD-DFT calculations, the developed photochemistry ena-
bles the first spectroscopic observation of critical Rh2
nitrenoid (i.e. 7) implicated in amination chemistry. We
anticipate that photochemical access to reactive metal
nitrenoids will facilitate direct characterization of these
critical amination intermediates, and thus contribute to
the rational development of C–H amination chemistry.
1
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ASSOCIATED CONTENT
Supporting Information. Experimental procedures, spec-
troscopic data, optimized structure coordinates (PDF, cif).
Crystallographic data deposited in Cambridge Crystal Struc-
ture Database (1845233–1845238, 1855308). The Support-
ing Information is available free of charge on the ACS Publi-
cations website.
(7) Das, A.; Reibenspies, J. H.; Chen, Y.-S.; Powers, D. C. J. Am. Chem.
Soc. 2017, 139, 2912–2915.
(8) (a) Corcos, A. R.; Pap, J. S.; Yang, T.; Berry, J. F. J. Am. Chem. Soc.
2016, 138, 10032–10040. (b) Zhang, R.; Newcomb, M. Acc. Chem. Res.
2008, 41, 468–477. (c) Cheng, M.; Bakac, A. J. Am. Chem. Soc. 2008,
130, 5600. (d) Kunkely, H.; Vogler, A. J. Am. Chem. Soc. 1995, 117, 540–
541.
(9) (a) Smith, J. M. Prog. Inorg. Chem. 2014, 58, 417–470. (b) Berry, J.
F. Comm. Inorg. Chem. 2009, 30, 28–66.
AUTHOR INFORMATION
Corresponding Author
(10) Mixed-valent Rh2[II,III] complexes have also been observed in
amination reactions, although the role of Rh2[II,III]-derived nitrenoid
intermediates continues to be discussed. For discussion, see (a) ref. 5,
(b) Kornecki, K. P.; Berry, J. F. Chem. Eur. J. 2011, 17, 5827–5832. (c)
Zalatan, D. N.; Du Bois, J. J. Am. Chem. Soc. 2009, 131, 7558–7559.
(11) (a) Stateman, L. M.; Nakafuku, K. M. Nagib, D. A. Synthesis 2018,
50, 1569–1586. (b) Zard, S. Z. Chem. Soc. Rev. 2008, 37, 1603–1618.
(c) Neale, R. S. Synthesis 1971, 1–15. (d) Wolff, M. E. Chem. Rev. 1961,
63, 55–64.
(12) For discussion of the photochemical N–Cl homolysis in Chlora-
mine-T, see: Evans, J. C.; Jackson, S. K.; Rowlands, C. C.; Barratt, M. D.
Tetrahedron 1985, 41, 5191–5194.
(13) The only crystallographically characterized example of a metal
complex with an N-chlorosulfonamide ligand is a Sn complex reported
in: Shcherbakov, V. I.; Kuznetsova, V. P.; Chuprunov, E. V.; Ovsetsina,
T. I.; Stolyarova, N. E.; Zakharov, L. N.; Novgorod, N. Organomet.
Chem. (USSR) 1991, 4, 1350–1354.
(14) A number of isomers of Rh2[II,III] complex 3 are known. All exper-
iments reported here utilize the cis-(2,2) isomer in which each Rh cen-
ter features RhN2O2 ligation.
(15) (a) Lutterman, D. A.; Degtyareva, N. N.; Johnston, D. H.; Gal-
lucci, J. C.; Eglin, J. L.; Turro, C. Inorg. Chem. 2005, 44, 5388–5396.
(b) Kawamura, T.; Maeda, M.; Miyamoto, M.; Usami, H.; Imaeda, K.;
Ebihara, M. J. Am. Chem. Soc. 1998, 120, 8136–8142.
(16) The kh/kD = 2.9 was measured for the intramolecular amination of
3-phenylpropyl-3-d sulfamate (ref. 5). The KIE for Rh2-catalyzed ami-
nation of THF cannot be directly measured due to fast background re-
actions of PhINTs with THF to generate 6.
(17) For examples of kinetic isotope effects of HAA by aminyl radicals,
see: (a) Martínez, C.; Muñiz, K. Angew. Chem. Int. Ed. 2015, 54, 8287–
8291. (b) Corey, E. J.; Hertler, W. R. J. Am. Chem. Soc. 1960, 82, 1657–
1668.
*david.powers@chem.tamu.edu
ORCID
Anuvab Das: 0000-0002-9344-4414
Joshua Telser 0000-0003-3307-2556
David C. Powers: 0000-0003-3717-2001
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors thank Daniel G. Nocera for access to a nanosec-
ond laser apparatus, Ryan G. Hadt for helpful discussion,
and The Welch Foundation (A-1907) and Texas A&M Univer-
sity for funding. Crystal structures of 3 and 4 were deter-
mined at ChemMatCARS, which is housed at the APS, which
is principally supported by the NSF (NSF/CHE-1346572). Use
of the PILATUS3 X CdTe 1M detector is supported by the NSF
(NSF/DMR-1531283). Use of the APS was supported by the
U.S. DOE under Contract No. DE-AC02-06CH11357.
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(19) Flash photolysis at these wavelengths afforded qualitatively iden-
tical spectra. Steady-state photolysis of 3 with a 325–385 nm bandpass
filter also afforded 5 and 6.
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5
ACS Paragon Plus Environment