RuV-Acylimido Intermediate in [RuIV(Por)Cl2]-Catalyzed C–N Bond Formation: Spectroscopic Characterization, Reactivity, and Catalytic Reactions

Article abstract of DOI:10.1002/anie.202100668

Metal-catalyzed C?N bond formation reactions via acylnitrene transfer have recently attracted much attention, but direct detection of the proposed acylnitrenoid/acylimido M(NCOR) (R=aryl or alkyl) species in these reactions poses a formidable challenge. H

Full text of DOI:10.1002/anie.202100668

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Accepted Article
Title: RuV-Acylimido Intermediate in [RuIV(Por)Cl2]-Catalyzed C–N
Bond Formation. Spectroscopic Characterization, Reactivity, and
Catalytic Reactions
Authors: Danyan Hong, Yungen Liu, Liangliang Wu, Vanessa Kar-Yan
Lo, Patrick H. Toy, Siu-Man Law, Jie-Sheng Huang, and Chi-
Ming Che
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10.1002/anie.202100668
Angewandte Chemie International Edition
RESEARCH ARTICLE
RuV-Acylimido Intermediate in [RuIV(Por)Cl2]-Catalyzed C–N Bond
Formation. Spectroscopic Characterization, Reactivity, and
Catalytic Reactions
[b],
[a]
[a]
[a]
Dan-Yan Hong,[a], Yungen Liu, Liangliang Wu, Vanessa Kar-Yan Lo, Patrick H. Toy, Siu-Man
†
†
Law,[a] Jie-Sheng Huang,[a] and Chi-Ming Che*[a,b,c]
[a]
[b]
Dr. D.-Y. Hong, Dr. L. Wu, Dr. V. K.-Y. Lo, Dr. P. H. Toy, Dr. S.-M. Law, Dr. J.-S. Huang, Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry and Department of Chemistry
The University of Hong Kong
Pokfulam Road, Hong Kong SAR (China)
E-mail: cmche@hku.hk
Dr. Y. Liu, Prof. Dr. C.-M. Che
Department of Chemistry
Southern University of Science and Technology
Shenzhen, Guangdong 518055 (China)
Prof. Dr. C.-M. Che
[c]
HKU Shenzhen Institute of Research and Innovation
Shenzhen 518053 (China)
[†]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Metal-catalyzed C–N bond formation reactions via acylnitrene
transfer have recently attracted much attention, but direct detection of the
proposed acylnitrenoid/acylimido M(NCOR) (R = aryl or alkyl) species in
these reactions poses a formidable challenge. Herein we report on
Ru(NCOR) intermediates in CN bond formation catalyzed by
[RuIV(Por)Cl2]/N3COR, a catalytic method applicable to aziridine/oxazoline
formation from alkenes, amination of substituted indoles, α-amino ketone
formation from silyl enol ethers, amination of C(sp3)H bonds, and
functionalization of natural products and carbohydrate derivatives (up to
99% yield). Experimental studies including HR-ESI-MS and EPR
measurements, coupled with DFT calculations, lend evidence for the
formulation of the Ru(NCOR) acylnitrenoids as a RuV-imido species.
[FeIII(TPP)Cl]/PhI=NCOCF3 (Figure 1aA).[5] In 2000, we reported
C–H amination of ethylbenzene by, e.g.,
RuIII(Me3tacn)/H2NCOPh.[6,7] Recently, there is a surge of interest
in C–H amination with acyl azides N3COR (1), dioxazolones,[8]
H2NCOCF3, or N-substituted amides H(R′O)NCOR via proposed
M(NCOR) intermediates (Figure 1aA, M = Fe,[5,9] Co,[10] Cu,[7a,11]
Ru,[6, 12 ] Rh,[ 13 , 14 ] Ir,[8,13c, 15 ] Au[7b]), as reported by Chang and
others[9,12-15] employing directing group (DG) strategy (for
intermolecular reactions) and by Warren and co-workers[11] using
CuI(diketiminate)/N3COR for hydrocarbons without DGs.
M(NCOR) (M = Cu,[16c,g] Ru,[16a,b,d,h] Ir[16e,f]) intermediates were also
proposed in the corresponding catalytic reactions to give sufimides,
oxazoles, oxazolines, lactams and amidines.[16] To the best of our
knowledge, no example of M(NCOR) intermediates in catalytic
reactions has been directly observed by spectroscopic analysis.[17]
Herein we describe spectroscopic studies including ESI-MS and
EPR, and also DFT calculations, on Ru(NCOR) species involved in
C–N bond formation reactions catalyzed by ruthenium porphyrin
(Ru(Por)) complexes; these Ru(NCOR) species, generated in
[RuIV(Por)Cl2]/N3COR(1) catalytic systems, could be formulated as
RuV-imido complexes, being different from the sulfonyl/arylnitrenoids
Introduction
Catalytic nitrogen group transfer reactions via metal-imido or -
nitrene intermediates, herein generally described as nitrenoids,
represent an appealing method for C–N bond formation.[ 1 ]
Acylnitrene (NCOR, R = aryl, alkyl) transfer is attractive for direct
installation of useful functionalities such as N-acyl groups into
hydrocarbon molecules; a hurdle to overcome is the proneness of
acylnitrenes to undergo Curtius rearrangement,[ 2 ] which also
poses challenges to direct detection of acylnitrenoid M(NCOR)
intermediates in the catalytic processes. Worthy of note are the
reports by Groves and co-worker[3] on stoichiometric aziridination
of cyclooctene with [MnV(Por)(N)]/(CF3CO)2O and by Carreira and
co-workers[4] on stoichiometric amination of electron-rich alkenes
with LnMn(N)/(CF3CO)2O, all being proposed to involve
[ 18 ]
[ 19 ]
[RuVI(Por)(NSO2Ar)2]
and [RuVI(Por)(NAr)2]
involved in
stoichiometric or [RuII(Por)(CO)]-catalyzed reactions. The
[RuIV(Por)Cl2]/N3COR systems are applicable to catalytic C–N bond
formation reactions using substrates without DGs to transform i)
alkenes 2 to aziridines 3 or oxazolines 4, ii) indoles 5 to C3-
aminated derivatives 6, iii) silyl enol ethers 7 to α-amino ketones 8,
iv) hydrocarbons 9 to C(sp3)–H amination products 10 (Figure 1b),
together with functionalization of natural products and carbohydrate
derivatives, with isolated product yields of up to 99% (a total of 75
examples). Notably, the RuV(NCOR) species are unique examples
Mn(NCOCF3)
intermediates,[3,4]
including
[MnV(TMP)(NCOCF3)(CO2CF3)] by spectroscopic evidence.[3] As
to catalytic reactions via acylnitrenoids, Mansuy and co-workers
[20,21]
of the nitrogen analogue of highly reactive RuV(O) species
involved in, e.g., [RuIV(Por)Cl2]-catalyzed oxygen atom transfer
reactions (Figure 1aB).
reported,
in
1984,
the
styrene
aziridination
by
1
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RESEARCH ARTICLE
Aziridination of Alkenes. For reaction of 1a and aliphatic alkene 2a
(5 equiv) in DCE at 50 °C, [RuIV(TDCPP)Cl2] (3 mol%) afforded N-acyl
aziridine 3aa in 80% yield, higher than that obtained for [RuIV(TTP)Cl2]
and other catalysts employed (≤45% yields, Table S1 in the
Supporting Information). Using alkenes as limiting reagent,
[RuIV(TDCPP)Cl2] (3 mol%) catalyzed the reactions of 1a–e with
aliphatic monosubstituted 1-alkenes 2a–c and p-nitro-aryl alkene 2d
giving 3aa, 3ba, 3ca–ce, 3da,dd in 60‒98% isolated yields (Figure 3).
For monosubstituted 1-alkenes 2e–h, 3ea–ha were obtained in 51–-
87% yields. Cyclic aliphatic alkene 2i (5 equiv) was converted to 3ia in
91% yield by [RuIV(TDCPP)Cl2]/1a; under similar conditions, a 99%
yield of 3da was obtained from 2d. Prior to this work, there was lack of
highly efficient metal catalyst for alkene aziridination via acylnitrene
transfer; literature examples used styrene substrate (yield of aziridine
product: 50% for [FeIII(TPP)Cl]/PhI=NCOCF3,[5] <5% for Cu-[24] and
Fe[25]-catalyzed reactions with N3COPh or N3CO-2-pyridyl).
Figure 1. a) Literature examples: A) metal-catalyzed acylnitrene transfer
reactions via proposed M(NCOR) intermediates, B) proposed RuV(O)
intermediates in [RuIV(Por)Cl2]-catalyzed oxygen atom transfer reactions. b)
RuV(NCOR) active intermediates in this work, evidenced by experimental
studies including ESI-MS and EPR measurements and DFT calculations, in
[RuIV(Por)Cl2]-catalyzed acylnitrene transfer reactions. Abbreviations: see the
Supporting Information.
IV
Figure 3. Aziridination of alkenes by [Ru (TDCPP)Cl2]/1. [a] Substrate as limiting
reagent (substrate/1 = 1:3). [b] Substrate/1 = 5:1.
Oxazoline Formation from Alkenes. Oxazoline products were
obtained by extending the [RuIV(TDCPP)Cl2](3 mol%)/N3COR
method to the following types of alkenes: i) aryl alkenes 2j–q devoid
of strongly electron-withdrawing group(s), ii) 3,4-dihydropyran 2r, and
iii) 1,1-disubstituted aliphatic alkenes 2s–u. These reactions gave
4ja,jb,jd,jf, 4kd–nd,pd, 4qa, 4sa–ua in up to 97% isolated yield (acyl
azides as limiting reagent) and 4od,rd in 87–90% isolated yields
(substrates as limiting reagent) (Figure 4); with 4od,qa,rd (from cis
alkenes) all adopting cis configuration and 4nd (from trans alkene)
adopting trans configuration. The reactions of styrenes 2j–l gave
4ja,jb,jd,jf,kd,ld in 9397% isolated yields. Previously, 4ja,jd,jf were
obtained from styrene by using stoichiometric reagents
Results
Ru(Por)-Catalyzed C–N Bond Formation
In this work, we found that [RuIV(Por)Cl2] served as active
catalysts for C–N bond formation of various substrates with N3COR by
employing
conveniently
accessible
[RuIV(TDCPP)Cl2]
or
[RuIV(TTP)Cl2] catalyst and benzoyl or naphthoyl azides 1a–g (Figures
2–7; see the Supporting Information for more details); the former
catalyst was reported to show high activity for hydrocarbon oxidation
with 2,6-Cl2pyNO[20c,22] and to catalyze cyclohexane amination with
alkoxysulfonyl, alkoxycarbonyl, or phosphoryl azide.[ 23 ] The acyl
azides 1 employed herein mainly bear electron-withdrawing para-
substituents, as such substituents enhanced nitrene-transfer-reactivity
of [RuVI(Por)(NSO2Ar)2].[18] The substrates were generally used as
limiting reagent in each type of the reactions or otherwise in selected
examples.
IV
Figure 4. Oxazoline formation from alkenes by [Ru (TDCPP)Cl2]/1. [a] Substrate
as limiting reagent (substrate/1 = 1:3). [b] Substrate/1 = 5:1.
Figure 2. Acyl azides 1 used in this work.
2
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[Mn(salen)(N)]/RCOCl (4jf: 74% yield)[26a] or RCONH2/tBuOI (4ja and
its regioisomer: dr 86:14, 78% combined yield; 4jf: 53% yield),[26b] or
by using a “Ru-Cu”/dioxazolone catalytic system (4jd: 61% yield; 4jf:
53% yield).[16a] The [RuIV(TDCPP)Cl2](3 mol%)/N3COR catalytic
system for a one-pot reaction of alkenes with acyl azides to give
oxazolines, which are valuable intermediates in organic synthesis,[27]
does not require transition metal cocatalyst, unlike the “Ru-
Cu”/dioxazolone catalytic system,[16a] which used [Ru(TTP)(CO)] (5
mol%) coupled with 15 mol% of CuCl2 cocatalyst.[16a]
N3CH2COC6H4-p-Cl produced 8bd in 30% yield[31b] (cf. product yields
by [RuIV(TTP)Cl2]/N3COR: 79% for 8ad, 81% for 8bd). The Cu-,[32a]
Ru-,[32b] and Rh[32c,d,e]-catalyzed amination of silyl enol ethers with
PhI=NSO2Ar possibly involve N-sulfonyl nitrenoids. A recently reported
Ir-catalyzed N-alkoxycarbonyl nitrene transfer reaction of
tBuMe2SiC(Ph)=CHMe with N3COOCH2CCl3 gave the amination
product in 49% yield.[32f] The [RuIV(TTP)Cl2]/N3COR transformation of
silyl enol ethers to α-amino ketones 8 (up to 81% yield, Scheme 1)
constitutes a unique method of preparing α-amino ketones featuring N-
acyl groups via metal-catalyzed acylnitrene transfer.
C(sp2)–H Amination of Indoles. Treatment of N-phenyl indole 5a
(as limiting reagent) with 1b,d and naphthyl analogue 1g (3 equiv) and
[RuIV(TTP)Cl2] (0.5 mol%) in DCE at 70 °C gave C3-amino indoles
6ab,ad,ag in 75–84% yields (Figure 5). The [RuIV(TTP)Cl2](0.5
mol%)/1b method was applied to various indoles 5b–k bearing
electron-withdrawing or -donating groups, which afforded 6bd–kd in
70–88% yields, with the structure of 6jd determined by X-ray crystal
analysis (Figure 5, inset). For N-tolyl indole, its amination by
[RuIV(TDCPP)Cl2](0.5 mol%)/1d under similar conditions led to <20%
conversion (major product: 6fd). In literature, reports on installing N-
acyl amine groups by metal-catalyzed functionalization of indole
C(sp2)–H bonds are usually focused on indoles bearing DGs for C2-
[ 28 a,c-f]
and C7[28b]-amination, except for the C2-amination of N-
methylindole
catalyzed
by
CuBr(20
Figure 6. Amination of silyl enol ethers by [RuIV(TTP)Cl2]/1. Substrate as
limiting reagent (substrate/1 =1:3).
mol%)/”H(R)NCOMe+tBuOOtBu”[29a] leading to tertiary N-acyl amines
(yields: 36–70%).[29a] The [RuIV(TTP)Cl2]/N3COR method resulted in
C3-amination of indoles without DGs (Scheme 1) to give secondary
N-acyl amines[29b] in 70–88% yields.
Amination of C(sp3)–H Bonds. Reactions of 1a,d with 1,4-
cyclohexadienes 9a,b (10 equiv), bearing allylic C–H bonds, and
[RuIV(TTP)Cl2] or [RuIV(TDCPP)Cl2] (3 mol%) in DCE at 70 °C afforded
10ad,bd,aa in 58–68% yields (Figure 7). For phthalan 9c and
isochroman 9d, bearing benzylic C–H bonds and used as limiting
reagent, their reactions with 1a and/or 1d (3 equiv) gave 10cd,dd in up
to 78% yield for [RuIV(TTP)Cl2] and 10cd,dd,da in up to 92% yield for
[RuIV(TDCPP)Cl2]. [RuIV(TDCPP)Cl2] can also catalyze reactions of
1a,b,d with indan 9e and tetralin 9f (10 equiv) giving benzylic C(sp3)–H
amination products 10ea,eb,fa,fd in 54–75% yields; no such reactions
were observed for 9e,f using [RuIV(TTP)Cl2] as catalyst. The
[RuIV(TDCPP)Cl2]/1a amination of DHA 9g and xanthene 9h furnished
10ga,ha in 60% and 96% yields. [RuIV(TDCPP)Cl2]/1a can even
catalyze C(sp3)–H amination of cyclooctane 9i to give 10ia, albeit in a
12% yield (Figure S1). Amination of C(sp3)–H bonds via
metalloporphyrin-mediated nitrene transfer was previously confined to
stoichiometric amination of indan by [RuVI(Por)(N)(OH)]/(CF3CO)2O [33]
and catalytic amination involving other types of proposed nitrenoids
such as M(NSO2Ar) and M(NAr) species.[1c,e,i,o] The C(sp3)–H
IV
Figure 5. Amination of indoles by [Ru (TTP)Cl2]/1. Substrate as limiting reagent
(substrate/1 =1:3). Inset: crystal structure of 6l (CCDC 2022367).
Amination of Silyl Enol Ethers. Reaction of trimethyl ((1-
phenylvinyl)oxy)silane (7a) and various silyl enol ethers, as limiting
reagents, with 1a,b,d,g (3 equiv) catalyzed by [RuIV(TTP)Cl2] (3 mol%)
afforded 8aa,ab,ad,ag in 67–81% yields (Figure 6). Use of
[RuIV(TDCPP)Cl2]/1d required a longer reaction time, e.g. 20 h for 7a
under similar conditions giving 8ad in 73% yield. Previously, reactions
of silyl enol ethers with stoichiometric amounts of MnV(N) complexes
30 a]
and excess (CF3CO)2O[4,
and photo-induced stoichiometric
amination of silyl enol ethers with N-acyl iminoiodinanes
ArI=NCOCF3[30b] both gave α-(N-COCF3 amino) ketones, instead of α-
(N-arylcarbonyl amino) ketones 8. The stoichiometric reaction of 4-
Figure 7. Amination of C(sp3)–H bonds by [RuIV(TDCPP)Cl2]/1. [a] Substrate
as limiting reagent (substrate/1 =1:3). [b] Substrate/1 = 10:1. [c] [RuIV(TTP)Cl2]
as catalyst.
chlorobenzoyl chloride with 1-(2-oxo-2-phenyl-ethyl) ammonium
a]
chloride furnished 8ad in 78.5% yield,[ 31
and photolysis of
3
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RESEARCH ARTICLE
amination reactions by [RuIV(Por)Cl2]/N3COR (Por = TTP, TDCPP) to
give N-acyl amines 10 in up to 96% yield (Figure 7) demonstrate the
catalytic activity of a metalloporphyrin for such acylnitrene transfer
reactions.
Application to Functionalization of Natural Products and
Carbohydrate Derivatives. The Ru(Por)/N3COR method for catalytic
acylnitrene transfer is applicable to functionalization of natural
products and carbohydrate derivatives (as limiting reagents), including
pinenes 11a,b and chromene/carbohydrate derivatives 11c–f
(Scheme 1), which are not among the substrates employed in
previous reports on metal-catalyzed acylnitrene transfer reactions.[5–16]
Using [RuIV(TDCPP)Cl2]/1a, 11a was converted to oxazoline 12aa
and 2° C(sp3)–H amination product 12aa′ in ca. 1:1 ratio with
combined yield of 61%, and 11b was functionalized to give the 1°
C(sp3)–H amination product 12ba as the major product in 60% yield
(oxazoline 12ba′: 16% yield). For 11c,d, their reactions catalyzed by
[RuIV(TTP)Cl2]/1d gave oxazolines 12cd,dd in 81% and 84% yields,
respectively. The [RuIV(TDCPP)Cl2]/1-catalyzed reactions of 11e,f led
to isolation of oxazolines 12ea,ed,fa,fb in yields of 72%98%. The
direct transformation of 11c,d,f to oxazolines (12cd,dd,fa,fb)
catalyzed by [RuIV(Por)Cl2]/N3COR is attractive in view of the previous
transformation of 11c,f to oxazoline derivatives by multistep
reactions.[34]
Scheme 2. Ring-opening reactions of 3o, in situ generated from styrene
aziridination by [RuIV(TDCPP)Cl2](0.3 mol%)/1d, with nucleophiles.
Ru(Por)/N3COR catalytic method in developing one-pot ring-opening
reactions of N-acyl aziridines directly from their alkene precursors.
Mechanistic Studies
Catalytic Activity of RuIII(Por)Cl Species. In view of the RuIII-RuV
catalytic cycle in [RuIV(Por)Cl2]-catalyzed oxygen atom transfer to
hydrocarbons,[20b,c,21] analogous phenomenon possibly occurred for
the acylnitrene transfer analogues. Indeed, [RuIII(TDCPP)(Cl)(THF)],
prepared by a procedure similar to that for crystallographically
characterized [RuIII(TDFPP)(Cl)(THF)],[21] catalyzed aziridination of 2a
with 1a to give 3aa in 82% yield and amination of 9a with 1d to afford
10ad in 65% yield, similar to the corresponding yields using catalyst
[RuIV(TDCPP)Cl2] under the same conditions (Table 1). Time course
experiments
revealed
that
both
[RuIV(TDCPP)Cl2]
and
[RuIII(TDCPP)(Cl)(THF)] catalyzed the reaction of 1a with 2a (5 equiv)
to produce 3aa in 62% yield (Figure S3), and the aziridination rate
was faster for the RuIII catalyst, consistent with pre-transformation of
[RuIV(Por)Cl2] to a RuIII species such as [RuIII(Por)Cl] before
acylnitrene transfer occurred.
Table 1: Comparison of catalytic activity of [RuIII(TDCPP)(Cl)(THF)] and
[RuIV(TDCPP)Cl2][a,b]
RuPor
Yield of 3aa [%]
Yield of 10ad [%]
[RuIII(TDCPP)(Cl)(THF)]
[RuIV(TDCPP)Cl2]
82
78
65
63
[a] Reaction conditions: same as those for the corresponding reactions in
Scheme 1. [b] [RuIV(TDCPP)Cl2] is more convenient to handle.
[RuIII(TDCPP)(Cl)(THF)], like [RuIV(Por)Cl2], also catalyzed
reaction of 1d and 2j (5 equiv) to give oxazoline 4jd; for both catalysts,
upon reducing the loading from 3 to 0.3 mol%, this reaction afforded
aziridine 3jd (Scheme 3) as the major product, with 3jd/4jd molar ratio
of ca. 10:1 (combined yield: ca. 90%) in the crude reaction mixture as
Scheme 1. Catalytic functionalization of natural products and carbohydrate
derivatives by [RuIV(Por)Cl2]/N3COR.
Application to Nucleophilic Ring-Opening of in Situ Generated N-
Acyl Aziridines. Ring-opening reactions of N-acyl aziridines by
nucleophiles give 1,2-bis-functionalized amino derivatives,[35] which are
useful building blocks in organic synthesis or fine chemical engineering.
In this work, the N-acyl aziridine 3o, in situ generated by
[RuIV(TDCPP)Cl2](0.3 mol%)/1d aziridination of styrene, underwent
ring-opening reactions with various nucleophiles (5 equiv) including
NaN3, NaCN, piperidine, indole, p-anisidine, thiophenol, and NaOAc,
affording N-acyl amines 13a–g in 65–90% yields (Scheme 2 and
Figure S2). These reactions represent useful application of the
Scheme 3. Reaction of 2j with 1d catalyzed by 0.3 mol% of [RuIV(TDCPP)Cl2]
or [RuIII(TDCPP)(Cl)(THF)] to give 3jd, and conversion of 3jd to 4jd upon
treatment with these catalysts.
4
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RESEARCH ARTICLE
determined by 1H NMR analysis. Both catalysts (3 mol%) can
promote ring expansion of 3jd to 4jd (>95% yield, Scheme 3),
reminiscent of previously reported analogues promoted by Lewis
acids.[36] By using lower-valent [RuII(TDMPP)(CO)] (3 mol%) catalyst,
the reaction of 1d and 2j (10 equiv) under similar conditions (DCE,
70 °C, 18 h) gave a crude mixture in which 4jd was not detected,
whereas 3jd was detected with a 72% yield, by 1H NMR analysis. For
the reaction of cis-stilbene (2q) with 1a catalyzed by
[RuIV(TDCPP)Cl2] (0.3 mol%), the corresponding aziridine product
adopts cis-configuration, and no trans-counterpart was detected by
NMR from the reaction mixture, indicating that the aziridination
reaction is stereospecific.
Kinetic Isotope Effect (KIE). We examined the KIE for amination
of DHA 9g with 1a by conducting competitive amination of an
equimolar mixture of 9g and 9g-d4 catalyzed by [RuIV(TDCPP)Cl2] or
[RuIII(TDCPP)(Cl)(THF)] (Scheme 4), which gave a kH/kD value of 6.0
and 6.3, respectively. These KIE values fall in the range of kH/kD ~5–
13 reported for the hydrogen atom abstraction by nitrenoids such as
Ru(NSO2Ar),[18] Fe(NAr),[37a] Fe(NSO2Ar),[37c] and Ni(NR)[37b] species.
(Figure S9a). Analogous phenomena were observed for the
treatments of [RuIV(TTP)Cl2] with 1a,h under similar conditions, which
generated new prominent signals at m/z 919.3 attributable to
[RuV(TTP)(NCOC6H4-p-NO2)]+ and at m/z 934.3 assignable to
[RuV(TTP)(NCOC6H4-p-OMe)]+, respectively, as supported by the HR-
ESI-MS and CID measurements (Figures S10–S12).
When styrene (2j, 500 equiv) was added to the reaction mixtures
of [RuIV(TTP)Cl2] with the acyl azides, the signal assignable to the
above-mentioned Ru(NCOR) species, such as the m/z 923.2 signal in
Figure S3b, was suppressed, as exemplified by the ESI-MS spectrum
of the reaction mixture of [RuIV(TTP)Cl2] with 1d in the presence of 2j
(Figure S3c), in which case the m/z 923.2 signal almost disappeared
and there appeared a new prominent signal at m/z 1027.3. The new
signal at m/z 1027.3 is assignable to [RuIII(TTP)(L)]+ (L = styrene
aziridination product 3jd or oxazoline product 4jd) by considering the
HR-ESI-MS (inset B, Figure S3c; Figure S13) and CID spectrum
(Figure S9b, showing the fragmentation attributable to loss of
coordinated 3jd or 4jd), along with the formation of 3jd or 4jd in the
reaction of 1d with 2j (5 equiv) catalyzed by [RuIV(TTP)Cl2] (0.3 mol%:
3jd/4jd = 7.6:1; 3 mol%: 4jd in 95% yield).
EPR Spectroscopy. We examined the X-band EPR spectrum of a
reaction mixture of [RuIII(TDCPP)(Cl)(THF)] with N3COC6H4-p-Cl (1d)
in CH2Cl2 at 100 K (Figure S14), which shows a new signal compared
with the EPR spectrum of [RuIII(TDCPP)(Cl)(THF)] in CH2Cl2 under
similar conditions; the absence of an EPR signal for
[RuIII(TDCPP)(Cl)(THF)], and also for previously reported
[RuIII(TDFPP)(Cl)(THF)],[21] is possibly due to fast relaxation.[21] This
new signal in the EPR spectrum, which features g values of 2.05, 1.97,
1.80 similar to those of RuV species (S = 1/2),[21,39] disappeared upon
addition of styrene to the reaction mixture (Figure S14).
Scheme 4. KIE for Ru(Por)-catalyzed amination of DHA (9g) with1a.
Radical Clock Experiment. A cyclopropyl-containing alkene, trans-
(2-vinylcyclopropyl)benzene (2v), was synthesized by the literature
procedure;[38 e] this alkene was previously employed as a radical
clock[38a] (featuring cyclopropyl ring-opening or cyclopropylcarbinyl
Density Functional Theory (DFT) Calculations
As the catalytic reactions by [RuIV(Por)Cl2]/N3COR are likely to
involve pre-generation of RuIII species (vide supra), we employed
[RuIII(TTP)Cl] as model active catalyst in the DFT calculations. The
computed binding of N3COC6H4-p-Cl (1d) by [RuIII(TTP)Cl] gives
[RuIII(TTP)(Cl)(N3COC6H4-p-Cl)] (I, Ru–NN3COR distance: 2.38 Å),
which eliminates N2 via transition state TS1 with activation free energy
(∆G⧧) of 9.9 kcal mol–1 producing doublet species
[RuV(TTP)(NCOC6H4-p-Cl)(Cl)] (II, Figure 8).
rearrangement) in Cu-,[38b,e] M(II) (M = Mn, Fe, Co, Ni)-,[38d,f,g] or Rh[38c]
-
catalyzed alkene transformations including sulfonyl nitrene transfer to
alkenes to give N-sulfonyl aziridines.[38b,f] Reaction of 2v with 1a
catalyzed by [RuIV(TDCPP)Cl2] (3 mol%) in DCE at 50 °C under argon
afforded oxazoline 4va in 89% yield (Scheme 5). Cyclopropyl ring-
opening products were not detected in the crude reaction mixture by
NMR measurements.
Scheme 5. [RuIV(TDCPP)Cl2]-catalyzed formation of oxazoline 4va from 2v
and 1a.
ESI-MS Studies. Reactions of [RuIV(TTP)Cl2] with N3COC6H4-p-X
(X = NO2, 1a; Cl, 1d, OMe, 1h) were analyzed by ESI-MS. Treatment
of [RuIV(TTP)Cl2] in CH2Cl2/MeCN (1:1 v/v) with 1d (100 equiv) at
room temperature resulted in a new prominent signal at m/z 923.2 (cf.
Figure S3a,b); this signal in the HR-ESI-MS analysis features m/z
923.1814 and is attributable to
a
Ru(NCOR) species,
[RuV(TTP)(NCOC6H4-p-Cl)]+, based on the m/z value and isotope
pattern (inset A, Figure S3b; Figure S8), coupled with its collision-
induced dissociation (CID) spectrum showing fragmentation
assignable to the loss of NCOC6H4-p-Cl ligand or COC6H4-p-Cl group
Figure 8. Computed energy profile for generation of II from I (selected bond
distances in Å). Inset: Spin density plot (contour value: 0.005) for II.
5
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10.1002/anie.202100668
Angewandte Chemie International Edition
RESEARCH ARTICLE
Figure 9. Computed energy profile for oxazoline formation from styrene (2j) and II (selected bond distances in Å; C1, N2 and O3 for V are shown).
The formulation of II as a RuV(NCOR) species, analogous to
RuV(O) species for oxidations by [RuIV(Por)Cl2]/2,6-Cl2pyNO via a RuIII
species generated in situ,[20,21] was based on the EPR studies and the
following calculation results for II: i) a doublet ground state with single
unpaired electron configuration (dxy)2(dxz)1(dyz)0 for Ru (d3, S = 1/2), ii)
a short Ru–NNCOR distance of 1.78 Å comparable to the experimental
and calculated RuVI–NNSO2Ar distances of 1.79(3) Å and 1.82–1.83 Å,
respectively, in [RuVI(TMP)(NSO2C6H4-p-OMe)2],[40] iii) a Wiberg bond
order of 1.90, iv) a spin density of 74% on Ru (only 16% on N; Figure
8, inset).
Aziridination/Oxazoline Formation from Alkene. For aziridination of
p-nitrostyrene 2d, the computed free energy profile (Figure S15)
shows that addition of 2d to II, via transition state TS2, features ∆G⧧
24.9 kcal mol–1 (lower than ∆G⧧ 29.4 kcal mol–1 for Curtius
rearrangement of II, Figure S16), generating intermediate Int1 (spin
population on carbon: 0.75) which is 6.7 kcal mol–1 above the
separate reactants. Int1 can readily undergo ring closure, with a
barrier of 2.6 kcal mol–1, to give Ru-aziridine complex III from which
the detachment of coordinated aziridine 3dd is barrierless, indicating
concomitant radical type C–N bond formation with simultaneous Ru–
N bond breakage. We then computed the energy profiles for reaction
of styrene 2j with II to give aziridine 3jd and its ring expansion to give
oxazoline 4jd (Figure 9) based on the experimental observations, i.e.,
transformation of 2j by [RuIV(TTP)Cl2](0.3 mol%)/1d to mainly afford
3jd and treatment of in situ formed 3jd with [RuIV(TTP)Cl2] (3 mol%)
(DCE, RT, ~12 h) giving 4jd in >95% yield (see also Scheme 3).
Addition of 2j to II, via transition state TS4, to form Ru-aziridine
complex IV needs to overcome a barrier of 22.2 kcal mol–1, with a
smaller barrier of 11.3 kcal mol–1 for detachment of the coordinated
3jd from IV, in accord with formation of major product 3jd from 2j and
1d catalyzed by 0.3 mol% of [RuIV(TTP)Cl2]. At a higher catalyst
loading (3 mol%), the following processes would be more likely to
occur: i) binding of 3jd to in situ generated [RuIII(TTP)Cl] species to
form adduct V, due to, for example, the electrostatic interaction
between the carbonyl oxygen of 3jd and the metal center; ii)
transformation of Ru-aziridine complex from N-bound to O-bound
form, i.e., IV (Ru–N 2.42 Å) → V (Ru–O 2.17 Å), with the latter being
13.9 kcal mol–1 more stable, unlike the corresponding process for 2d
(IV′ → V′) which is disfavored by ΔG = 2.0 kcal mol–1 (Figure S17)
presumably due to the strong electron-withdrawing p-NO2 group of 2d.
In the subsequent ring-expansion step via transition state TS6, a
barrier of 18.2 kcal mol–1 is needed to open the three-membered ring
of the O-bound aziridine 3jd.41 Taking into account the thermal
release (13.9 kcal mol–1) in the IV → V transformation, this ring-
expansion barrier can be readily overcome under the reaction
conditions employed. The C–O bond formation and the oxazoline 4jd
detachment from the catalyst can proceed in a fast concerted manner
by the proper geometry rearrangement in the transition state.
Amination of C(sp3)–H Bond. Considering the higher catalytic
activity of [RuIV(TDCPP)Cl2] relative to [RuIV(TTP)Cl2] in C(sp3)–H
amination (vide supra), we conducted DFT calculations on reaction of
DHA (9g) with [RuV(TDCPP)(NCOC6H4-p-NO2)(Cl)] (II′).42 Compared
to generation of II (Figure 10), similar formation of II′ from
[RuIII(TDCPP)(Cl)(N3COC6H4-p-NO2)] (I′, Ru–NN3COR distance: 2.60 Å)
via transition state TS9 (Figure S18a) is slightly more difficult, with ∆G⧧
of 16.1 kcal mol–1 (cf. 9.9 kcal mol–1 for generating II). Previously a
computed barrier of 18.14 kcal mol–1 was reported for the generation of
a Ir(NCOMe) species from its Ir(N3COMe) precursor.[15b] The computed
Ru–NNCOR distance in II′ is 1.777 Å, similar to that in II (1.784 Å, Figure
S18b). The spin density of II′ (Ru 54%, N 29%, O 17%, inset of Figure
S18a), like that of II, is also mainly localized at Ru. For the acylnitrene
C–H insertion of II′ with 9g, the computed pathway (Figure 9) shows
that the rate-determining step (RDS) lies in the hydrogen atom transfer
(HAT) to give species VI via transition state TS10-DHA with ∆G⧧ of
6
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10.1002/anie.202100668
Angewandte Chemie International Edition
RESEARCH ARTICLE
⧧
Figure 10. Computed energy profile for C–H amination of DHA (9g) by II′ (∆G in RDS for DAT in orange; selected bond distances in Å).
26.9 kcal mol–1. In the carbon radical rebound step giving species VII
via transition state TS11-DHA, only a barrier of 1.4 kcal mol–1 is
encountered. The product release step via transition state TS12-DHA
can also occur in a facile manner with a barrier of 2.3 kcal mol–1. For
9g-d4 (DDA), the calculated ∆G⧧ of 28.0 kcal mol–1 in the RDS, i.e.,
deuterium atom transfer (DAT, Figure 10) is 1.1 kcal mol–1 higher than
that for the HAT counterpart. Based on this difference, the
computationally determined KIE is 6.4, in good agreement with the
experimental value of 6.0.
comparable to that reported for H-atom abstraction by nitrenoids.[18,37]
v) DFT calculations provide support for formation of RuV(NCOR)
species (II and II′) with relatively small barriers (9.9 and 16.1 kcal mol–
1) and for their acylnitrene transfer to hydrocarbons with reasonable
barriers (22.2–26.9 kcal mol–1, Figures 9 and 10, Figure S15; see also
ref 42). Also, no radical intermediate is located in oxazoline formation
from styrene and II (Figure 9), in line with the radical clock experiment
(Scheme 5; note also the almost barrierless ring closure of radical
intermediate Int1 in Figure S15). In addition, the DFT calculated
doublet ground state (S = 1/2) for the RuV(NCOR) species is
consistent with the EPR measurements, and the computed KIE (kH/kD
6.4, from Figure 10) compares well with that measured experimentally
(kH/kD 6.0–6.3, Scheme 4). The intermediacy of RuV(NCOR) species
supported by porphyrin ligand can account for the effect of porphyrin
on the catalytic activity by considering steric factor and/or electronic
factor including increase in electrophilicity of RuV(NCOR) by electron-
withdrawing groups.
Discussion
The development of metal-catalyzed acylnitrene transfer (MCAT) to
organic substrates, compared with stoichiometric analogues by
LnMnV(N)/(CF3CO)2O or LnMnV(N)/RCOCl,[3,4,26a,30a, 43 ] has met with
formidable challenges[5,6] until recently;[7–16] the overwhelming majority
of the recent examples deal with C–H amination and are based on
substrates bearing directing groups (DGs) that can bind to the metal
center. Examples on the use of MCATs for other reactions such as Conclusion
alkene aziridination are sparse. The [RuIV(Por)Cl2]/N3COR method is
applicable to various MCAT reactions including alkene aziridination
described in this work and to substrates without DGs.
Mechanistic
studies
on
acylnitrene
transfer
by
[RuIV(Por)Cl2]/N3COR, including ESI-MS and EPR spectroscopic
analysis, KIE and radical clock measurements, and DFT calculations,
provide evidence for a RuV(NCOR) intermediate, a nitrogen analogue
of highly reactive RuV(O) species.20,21 The [RuIV(Por)Cl2]/N3COR
catalytic system can transform alkenes, indoles, silyl enol ethers, and
C(sp3)–H bonds, including natural products and carbohydrate
derivatives, to aziridines/oxazolines, C3-aminated indoles, α-amino
ketones, and N-acyl amines, respectively, in up to 99% yield. In view
of tunable reactivity of M(NCOR) species by altering metal center,
supporting ligand(s), and/or acyl group, coupled with accommodation
of reactivity by varying substrate types, the present work reveals a
promising potential of metalloporphyrin-catalyzed acylnitrene transfer
in synthetic applications, and, particularly, sheds light on putative
M(NCOR) intermediates widely proposed in literature.[5–16]
For the MCAT reactions in literature, the proposed M(NCOR)
intermediates[5–16] include high-valent MV(NCOR) (M = Rh, Ir) species,
which were studied by DFT calculations[13c–e,14,15b] but not directly
observed by spectroscopic means. RuV(NCOR) species are involved in
the MCAT reactions by [RuIV(Por)Cl2]/N3COR based on the following
findings: i) [RuIV(Por)Cl2] gave catalytic results similar to those for
[RuIII(Por)(Cl)(THF)] (Table 1 and Scheme 3). ii) ESI-MS analysis
revealed new signals attributable to [RuV(Por)(NCOR)]+ (Figure 8 and
Figures S9 and S10), which almost vanished in the presence of
styrene affording species assignable to 3jd or 4jd (Figure 8c and
Figure S9b). iii) Reaction of RuIII(Por)Cl complex with N3COR
generated a new EPR signal characteristic of RuV species (S = 1/2,
Figure 9),[21,39] which disappeared upon treatment with styrene (Figure
9) and could be ascribed to RuV(NCOR) species.[44] Formation of this
RuV(NCOR) species is reminiscent of RuV(O) formation by oxidizing
RuIII(Por)X (X = Cl, Ph) complex with 2,6-Cl2pyNO[20] or m-CPBA.[21] iv)
The KIE study for C–H amination revealed a kH/kD value (Scheme 4)
Acknowledgements
7
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10.1002/anie.202100668
Angewandte Chemie International Edition
RESEARCH ARTICLE
[14] A review: S. Vasquez-Céspedes, X. Wang, F. Glorius, ACS Catal. 2018, 8, 242.
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2019, 131, 7191; l) H. Wang, Y. Park, Z. Bai, S. Chang, G. He, G. Chen, J. Am.
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This work was supported by Hong Kong Research Grants Council
(HKU 17301817, 17304019) and Basic Research Program-
Shenzhen
Fund
(JCYJ20170412140251576
and
JCYJ20180508162429786). We acknowledge the funding support
from the Innovation Technology Commission of Hong Kong SAR of
the People's Republic of China for supporting the research in the
areas of Synthetic Chemistry. We thank Dr. Xiao-Yong Chang for
assistance in the X-ray crystal structure determination of compound 6l.
Keywords: Acylnitrene transfer
•
C–N bond formation
•
porphyrinoids • ruthenium • RuV-imido complex
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[41] Charge analysis of
V (Figure 9) revealed a strong electrostatic
interaction between C1 and O3. When C1–N2 breaks at TS6, the
interaction is further reinforced due to the charge accumulating and
shortening of the C1-O3 distance (3.52 Å → 3.23 Å). After structural
isomerization of N–C=O (TS6)
→ N=C–O (TS7) associated with
intramolecular charge rearrangement, the thermodynamically favorable
oxazoline product is formed.
⧧
[42] DFT calculations on the aziridination of 2d and 2j by II′ revealed ΔG
values of 17.5 and 16.2 kcal mol−1
, respectively, lower than the
corresponding values of 24.9 and 22.2 kcal mol−1 by II, in line with the
higher catalytic activity of [RuIV(TDCPP)Cl2] than [RuIV(TTP)Cl2] for the
aziridination reactions.
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[44] No hyperfine coupling with N atom(s) was observed for the EPR signal assigned
to RuV(NCOR) species in which the unpaired electron mainly resides in metal
[37c]
center, like the cases of FeV(NSO2Ar)
and M(NAr) or M(NR) complexes
reported in: a) V. M. Iluc, A. J. M. Miller, J. S. Anderson, M. J. Monreal, M. P. Mehn,
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9
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10.1002/anie.202100668
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RESEARCH ARTICLE
Entry for the Table of Contents
Evidence for M(NCOR) species in catalysis. Experimental studies including ESI-MS, EPR spectroscopy and KIE, and also DFT
calculations point to the intermediacy of a porphyrin-supported RuV(NCOR) species in [RuIV(Por)Cl2]-catalyzed C–N bond formation
reactions with acyl azides N3COR (up to 99% yield). The [RuIV(Por)Cl2]/N3COR catalytic method is applicable to various substrates
including alkenes, indoles, silyl enol ethers, and C(sp3)–H bonds.
10
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