Facile alkane functionalization in copper-[2.1.1]-(2,6)-pyridinophane- PhINTs systems

Article abstract of DOI:10.1039/b309519c

Mild catalytic dehydrogenation of cycloalkanes (cyclo-C₅H ₁₀, cyclo-C₆H₁₂, cyclo-C₈H ₁₆) and aziridination of resulting olefins is reported with PhINTs and copper-[2.1.1]-(2,6)-pyridinophane (L) complexes LCuX_n (n = 1, 2; X = Cl, OTf) "activated" with NaBAr^F₄ in dichloromethane solution.

Full text of DOI:10.1039/b309519c

Facile alkane functionalization in
copper-[2.1.1]-(2,6)-pyridinophane-PhINTs systems†
Andrei N. Vedernikov* and Kenneth G. Caulton*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Received (in West Lafayette, IN, USA) 7th August 2003, Accepted 31st October 2003
First published as an Advance Article on the web 8th December 2003
Mild catalytic dehydrogenation of cycloalkanes (cyclo-C
cyclo-C 12, cyclo-C 16) and aziridination of resulting olefins
is reported with PhINTs and copper-[2.1.1]-(2,6)-pyridino-
phane (L) complexes LCuX (n = 1, 2; X = Cl, OTf)
activated” with NaBArF
in dichloromethane solution.
5
H
10
,
Exploratory studies with cyclooctane, cyclohexane or cyclo-
pentane, introduced at 25 vol% concentration to the dichloro-
methane solution derived from LCu(OTf) , showed C–H bond
2
6
H
8
H
n
cleavage chemistry with PhINTs (eqn. (1), Table 1, entries 1–3).
The reaction solutions turned reddish in the presence of PhINTs; all
three substrates produced the corresponding olefin, in yields that
indicate mildly catalytic turnover of copper. Also produced are the
aziridines, presumably by a secondary conversion of the primary
olefinic product, as has been established independently.15 A control
experiment with cyclopentane and PhINTs but without any copper
reagent showed no alkane dehydrogenation even after 12 h. under
“
4
Mild transition metal catalyzed alkane functionalization remains
1
,2
challenging for today’s chemists. Known examples of thermal
(
non-photochemical) transformations include alkane conversion to
3
–6
7
oxygenation products,
products, dehydrogenation,10,11 and borylation,
ers. We report here preliminary observations on a new room-
temperature alkane dehydrogenation and aziridination using
PhINTs-copper-[2.1.1]-(2,6)-pyridinophane (L) systems, eqn. (1).
nitrene and carbene CH insertion
8,9
12,13
among oth-
1,4
the conditions of Table 1. No tosylamides, the products of nitrene
7
insertion into a substrate C–H bond, were observed. The copper(II
)
reagent derived from the triflato complex showed distinctly higher
activity than that produced by incomplete chloride abstraction from
LCuCl
exhibited almost equal activity (entry 4 and 5). Triflato and chloro
complexes, without the NaBArF
“activation,” showed reduced
2
(entry 4). Both copper(II) and copper(
I) cationic complexes
4
activity (entries 6–7). Addition of 30 vol% acetonitrile to the
catalyst solution showed a sharp decrease of the rate of the
cyclopentane dehydrogenation and aziridination (entry 9). Thus,
the macrocyclic structure of the tripyridine ligand and a low
coordinating environment (solvent, counterion) are equally im-
portant in developing the new systems for alkane functionaliza-
tion.
(
1)
I
n
Copper( ) and copper(II) pyridinophane complexes LCuX (X =
Cl, OTf; n = 1, 2) were prepared in benzene in almost quantitative
yield from the corresponding anhydrous copper salts and L, readily
The main limitation to catalyst efficiency is catalyst lifetime;
judging by the color change observed in the presence of PhINTs
from reddish to green, the copper complex decomposes in 2–3 h at
rt. Despite that limitation, the reactivity of some other alkanes was
studied. Isobutane introduced at low concentration (10 equiv.)
produced isobutylene and its aziridine, 2,2-dimethyl-N-tosylazir-
idine, in less than catalytic yields (entry 10). n-Butane taken under
similar conditions produced exclusively a 3 : 1 mixture of trans-
and cis-2-butenes and a 1 : 1 mixture of diastereomeric cis- and
1
4,15
available from commercial pyridines.
Pyridinophane, L, has a
3
steric constraint that prevents it from being coplanar and h to a
metal at the same time (hence no mer-geometry), even if this is not
ideal for three coordinated Cu or Cu . This “unhappy” geometry of
copper complexes makes a proposed reactive transient Cu
species16 more accessible, and that is the basis of the exceptional
reactivity of the system “LCu ”/ PhINTs towards alkanes shown
I
II
III
n+
here. This coordination geometry control idea has been independ-
8
II
17–19
ently developed for a d Pt system,
but this report represents
Table 1 Alkane dehydrogenation and aziridination with PhINTs in the
a significant advance in showing its applicability to a 3d metal.
2 4
presence of LCu(OTf) . Dichloromethane solution, 25 vol%
+ 2 NaBArF
2
LCuX
the “naked” Na in stoichiometric NaBAr
the copper( ) chloride and the triflate give pale yellow solutions
n
complexes undergo Cl removal in dichloromethane by
of alkane, rt, 1–3 h
+
F
4
, eqn. (2) (m 5 n). Both
I
#
Substrate
(amount)
Amount of
LCu/mol %
Alkene yield
(%)
Aziridine yield
(%)
stable for several days.
a
a
a
Two major peaks with m/z ratio of 350.0 and 352.0 in 2 : 1 ratio
were observed in the ESI-mass-spectrum of this solution confirm-
ing presence of isotopomeric L Cu and L Cu species. In the
case of copper(II) compounds, the color of the catalyst solution was
dependent on ligand X, being bluish-green (X = Cl) or yellow (X
1
2
3
4
5
6
7
8
9
Cyclooctane
Cyclohexane
Cyclopentane
5
6
5
15
9
26
16
16
8
8
1
4
4
5
8
18
10
10
1
1
3
3
0.3
6
3
+
65
+
b
”
”
”
”
”
”
5
c
5
=
OTf); this indicates incomplete ligand X abstraction by the
d
5
F
cation in NaBAr
do not give the same Cu product, m values, in eqn. (2)). For n =
, the resulting solutions (both X = Cl and OTf) lost their activity
in approximately 30 min.
4
for at least X = Cl (i.e. the different precursors
e
5
II
f
5
5
2
g
h
10
1
Isobutane
n-Butaneh
10
10
1
4 (3 : 1, trans- 5 (1 : 1, cis/
/
—
cis-2-butene) trans)
(2)
Cyclopropaneh
10
7
i
1
2
a
Based on the available limiting reagent PhINTs (200 mmol per 0.6 ml of
b
F
c
F
d
reaction solution); LCuCl
2
+ 2 NaBAr
4
;
LCuCl + NaBAr
30 vol% of acetonitrile as a co-
solvent, 24 h; h 10 equiv.; 2-Methyl-N-tosylaziridine.
4
; LCuCl;
e
f
F
g
4
LCu(OTf); (tpdm)CuCl + NaBAr ;
†
Electronic supplementary information (ESI) available: experimental
i
details. See http://www.rsc.org/suppdata/cc/b3/b309519c/
1
62
C h e m . C o m m u n . , 2 0 0 4 , 1 6 2 – 1 6 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

trans-2,3-dimethyl-N-tosylaziridines, also in non-catalytic yield
entry 11), thus showing selectivity for 2° vs. 1° alkane CH bonds.
In the absence of alkane substrate but in the presence of excess
II
2+
2+
(
PhINTs, both [(LCu )
2
NTs]
and LCu
species transform
Cyclopropane produced no cyclopropene, but instead an aziridine.
The aziridine formed was that derived from propene (entry 12), so
cyclopropane functionalization occurred with C–C and C–H bond
cleavage. Precedents of cyclopropane ring opening are well-known
to occur via both homolytic and heterolytic mechanisms.
While future work must focus on copper/N-tosylate complex
PhINTs to TsNH and PhI concomitant with rapid (few minutes at
rt) complex degradation, as evidenced by complete disappearance
of the initial reddish color as well as NMR signals associated with
L and growth of CHDCl (yield up to 80% on PhINTs). The latter
2
is consistent with radical D/H exchange between the solvent and
benzylic hydrogens of L.
2
2
0
identity, a few preliminary results are relevant. In the absence of
substrate, the LCu complex reacted cleanly with PhINTs in 2 : 1
Only low non-catalytic yields of olefins and aziridines were
+
formed using (tpdm)CuBArF
4
(entry 8), which employs a non-
2
2
ratio to produce diamagnetic, presumably dinuclear, deep-purple
macrocyclic (hence less strained) analog of the pyridinophane.
II
2+
+
imido complex [(LCu )
2
NTs] ( > 90% yield based on integration
The much lower stability of (tpdm)Cu species toward oxidative
degradation in the presence of PhINTs is also consistent with the
hypothesis of a homolytic mechanism of alkane CH bond cleavage.
Faster copper complex degradation here is attributed to facilitated
radical attack at the benzylic hydrogen in the (tpdm)Cu species,
which can achieve a (resonance-stabilized) planar benzyl radical
geometry. The inability of the analogous radical to achieve a planar
geometry at carbon in the coordinated macrocycle makes the
pyridinophane/copper reagent detectably longer-lived.
relative to BArF
signals), stable for at least several days in
4
dichloromethane solution at room temperature in the absence of
moisture, but alone it is completely inert towards both alkanes and
2
1
olefins. The same solution showed the presence of [(LCu)
]
2
NTs-
(BArF
) and LCu(NHTs) species in the ESI-mass-spectrum with
4
+
+
expected isotopic pattern, m/z of 1732, 1734 and 1736 for the
former and 520.0 and 522.0 for the latter. The latter species could
II
+
+
result from LCu (N·Ts) , (from LCu and PhINTs) via hydrogen
atom abstraction from the solvent. Trapping of this intermediate,
LCu (N·Ts) , by LCu could produce [(LCu ) NTs] .
2
In summary, we report a copper-pyridinophane-based system
allowing the first observation of an unprecedented mild catalytic
alkane dehydrogenation and one-pot aziridination where both low
coordinating environment (i.e. anions and solvent) and macrocyclic
ligand structure play important roles in determining the system
reactivity.
This work was supported by DOE. ANV is on leave from the
Chemical Faculty, Kazan State University, Kazan, Russia. This
work has been made possible in part due to support from Russian
Foundation for Basic Research (grant #01.03.32692).
II
+
+
II
2+
Based on these observations, DFT calculations support a
II
mechanism for alkane dehydrogenation involving Cu as a viable
catalyst precursor (Scheme 1). The main hypothesis of the Scheme
III
2+
suggests a ligand-centered radical [LCu (N·Ts)] , A, as a reactive
transient.2
1,22
DFT(PBE) calculation shows 76 % spin density localized on the
NTs nitrogen of A and a viable free energy (eqn. b) to abstract a
hydrogen atom from cyclohexane. Abstraction of the second
hydrogen atom can result from subsequent essentially thermoneu-
III
II
tral Cu ? Cu intramolecular redox isomerization, converting
Notes and references
III
2+
II
2+
singlet [LCu NHTs] , B, into the triplet [LCu (NH·Ts)] , (C,
1
2
3
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(eqn. c) by reaction of C with the cyclohexyl radical. An olefin
formed this way could then compete with alkane for reactive Cu/
NTs species, producing the corresponding aziridine. The very high
alkane : olefin ratio in the reaction mixtures makes this competition
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converted into aziridine.
4
5
6 B. Meunier, Metal Oxo and Metal-Peroxo Species in Catalytic
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7
8
9
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3
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2
+
olefin ratio), with “LCu ” catalyst under the standard conditions of
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remaining unreacted cyclohexene and aziridines derived from both
olefins (4 and 6% respectively). Thus, the reactive species
responsible for the observed hydrocarbon transformations, eqn. (1),
shows only limited selectivity towards olefins vs. alkanes, con-
sistent with the proposed radical reaction mechanism.
2
000, 122, 3063.
1
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1
1
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1
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1
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2
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Scheme 1 (DGo298, kcal mol21; DFT(PBE)-calculations)
C h e m . C o m m u n . , 2 0 0 4 , 1 6 2 – 1 6 3
163