Cu-catalyzed oxidative Povarov reactions between N-alkyl N-methylanilines and saturated oxa- and thiacycles

Article abstract of DOI:10.1039/c5cc01287b

Cu-catalyzed oxidative Povarov reactions between N,N-dialkylanilines and saturated oxa- or thiacycles with tert-butyl hydroperoxide (TBHP) are described; notably, the reactions use neither [4π] nor [2π]-motifs as the initial reagents. The use of cheap alkane-based substances as building units is of mechanistic and practical interest as two inert sp³ C-H bonds are activated.

Full text of DOI:10.1039/c5cc01287b

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Cu-catalyzed oxidative Povarov reactions between
N-alkyl N-methylanilines and saturated oxa- and
thiacycles†
Cite this: Chem. Commun., 2015,
51, 6625
Received 12th February 2015,
Accepted 10th March 2015
Rahul Kisan Kawade, Deepak B. Huple, Rong-Jing Lin and Rai-Shung Liu*
DOI: 10.1039/c5cc01287b
www.rsc.org/chemcomm
Cu-catalyzed oxidative Povarov reactions between N,N-dialkyl-
anilines and saturated oxa- or thiacycles with tert-butyl hydroperoxide
(TBHP) are described; notably, the reactions use neither [4p] nor
[2p]-motifs as the initial reagents. The use of cheap alkane-based
substances as building units is of mechanistic and practical interest
as two inert sp3 C–H bonds are activated.
Metal-catalyzed [n+2C]-cycloadditions (n = 3 to 4) are powerful tools
to construct five- and six-membered carbo- and heterocyclic frame-
works.1 Typical cycloadditions involve unsaturated p-bond motifs like
dienes, nitrones, azamethine ylides, CRX, and CQX (X = C, N, O) as
the reactants. Catalytic C–H functionalizations have inspired the
development of new [n+2C]-cycloadditions (n = 2 to 3), but alkynes
and alkenes remain invariably two-carbon components.2 We envisage
that an alkane-based substance as a building block for catalytic
cycloadditions is significant and challenging in catalysis, but no
precedent has been documented in the literature.
Scheme 1 A summary of Povarov reactions.
The well known Povarov reactions3 refer to the [4+2C]-cyclo-
additions of N-aryl imines with electron-rich alkenes to afford catalysts at elevated temperatures8 rendering their Povarov
useful tetrahydroquinoline derivatives (eqn (1)); this method is reactions unfavourable (Scheme 1).
highly accessible to natural alkaloid compounds.4 Murahashi,
Table 1 presents optimized conditions for Cu-catalyzed oxidative
Miura, Seidel and Zhang reported Cu-catalyzed oxidative Povarov cycloadditions as commonly used RuCl3 and FeCl3 were ineffective
reactions5 with readily available N,N-dialkylanilines to replace catalysts (see Table S1, ESI†).7 In a typical operation, N-ethyl-N-
N-aryl imines, but electron-rich alkenes were still used as two-carbon methylaniline 1a was treated with THF (2a, 10 equiv.), Cu catalysts
units (eqn (2)). Herein, we report new Cu-catalyzed oxidative Povarov in various solvents using tert-butyl hydroperoxide (TBHP) as the
reactions involving the readily available N,N-dialkylanilines and oxidant (3 equiv.). Among the various Cu(II)X2 catalysts (10 mol%,
saturated oxa- and thiacyclic species (III) as depicted in eqn (3). X = Cl, Br, OAc and OTf, see Table S1, ESI†), only Cu(OTf)2 gave
This work represents the first achievement of a catalytic [4+2]- reasonable activity with di-tert-butyl peroxide (DTBP) in hot toluene
cycloaddition using neither [4p]- nor [2p]-motifs; herein, two (70 1C) to provide the desired cycloadduct 3a in 40% yield whereas
inert sp3 C–H bonds are activated.6 An notable application of byproducts 3b and 4 were obtained in 5% and 8% yields respec-
this work is the replacement of cyclic enol ethers (X = O, IV) tively; the conversion was ca. 66% (entry 1). The use of TBHP as the
with cheap tetrahydrofuran and tetrahydro-2H-pyran (THP);7 oxidant gave a comparable efficiency (entry 2). An attempt to increase
the ether rings of THF and THP are easily cleaved by acid the catalytic conversion with a high loading (20 mol%) of Cu(OTf)2
was unsuccessful, giving the desired 3a in 47% yield with a 21%
recovery of unreacted 1a; we noted that a red polymer was formed
National Tsing Hua University, Hsinchu, Taiwan. E-mail: rsliu@mx.nthu.edu.tw
significantly, presumably from the acid-catalyzed ring opening
† Electronic supplementary information (ESI) available. CCDC 1039307. For ESI
of THF.8 We found that with various CuX (X = Cl, Br), IPrCuCl
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5cc01287b
and Cu(CH3CN)4BF4 (see Table S1, ESI†), only Cu(OTf)(benzene)
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Table 1 Optimized conditions for oxidative cycloadditions
including R = methyl, fluoro, chloro and bromo, affording the
corresponding cycloadducts 3c–3f in satisfactory yields (67–74%,
entries 2–5). The molecular structure of product 3f has been
elucidated using X-ray diffraction studies.10 We also prepared
3-methyl and 3,5-dimethyl substituted derivatives 1g and 1h
to afford the desired products 3g and 3h in 76% and 52%
yields respectively (entries 6 and 7). We prepared additional
N-methylanilines 1i–1k bearing R0 = isopropyl, n-butyl and
2-acyloxyethyl that afforded cycloadducts 3i–3k in reasonable yields
(54–62%, entries 8–10). The use of Cu(OTf)2 (10 mol%) was also
effective to implement the cycloadditions of aniline substrate 1c with
2-methyltetrahydrofuran 2b and 2,2-disubstituted tetrahydrofurans
2c–2d, yielding the corresponding products 3l–3n in good yields
(67–73%, entries 11–13) (Table 2).
The Cu-catalyzed oxidative cycloadditions were further applied to
tetrahydro-2H-pyran 2e (THP); the results are provided in Table 3
(entries 1–6). In a typical operation, N-alkyl-N-methylanilines 1,
TBHP (2 equiv.) and Cu(OTf)2 (10 mol%) was heated with THP
(20 equiv., 90 1C) without solvents for 24–36 h. The cycloaddition of
unsubstituted aniline 1b with THP delivered cycloadduct 5a in 45%
yield together with a side product 40 in 12% yield. The reactions
worked well with their para-substituted derivatives 1d–1e and 1c
Yieldb (%)
Entry Catalysta (mol%) Oxidant Solvent Time (h) 1a 3a 3b
4
1
2
3
4
5
6
7
8
Cu(OTf)2 (10)
Cu(OTf)2 (10)
Cu(OTf)2 (20)
CuOTf.L (10)
Cu(OTf)2 (10)
Cu(OTf)2 (10)
Cu(OTf)2 (10)
Cu(s) (10)
DTBP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
Toluene 24
Toluene 24
Toluene 24
Toluene 24
34 40
32 41
21 47
30 34
28 42
—
29 43
15 53
5
6
9
6
6
—
7
8
8
13
5
10
—
5
—
—
—
—
THFc
DCE
H2Oe
H2O
H2O
24
24
24
36
28
d
—
8
9
10
11
Cu(s) (20)
Cu(s) (20)
Cu(s) (20)
— 62 10
37 28
41
DMSO 24
DMF 24
—
—
—
b
Cu(s) = copper powder, L = benzene. a [1a] = 0.37 M. Isolated yield after
c
d
silica column. 32 equiv. of THF. Complicated mixtures of products.
e
90 1C for entries 7–10.
exhibited a moderate efficiency to deliver product 3a in 34% (R = F, Cl and Me), yielding desired products 5b–5d in 69%, 67%
yield, together with unreacted 1a, 3b and 4 in 30%, 6% and 5% and 50% yields respectively (entries 2–4). For various N-alkyl sub-
recovery or yields respectively. When THF itself served as a stituted derivatives 1o and 1p (R0 = n-C5H11 and i-Pr), their resulting
solvent (32 equiv.), Cu(OTf)2 (10 mol%) afforded the desired 3a cycloadducts 5h and 5i were obtained in 61–62% yields (entries 5
in 42% yield at 72% conversion (entry 5). Dichloroethane was an and 6). To our pleasure, Cu(OTf)2 (20 mol%) enabled the cycloaddi-
inappropriate solvent, affording the products in a complicated tions of N,N-dimethylanilines 1b–1c and 1e with tetrahydrothio-
mixture (entry 6). H2O seems to be an appropriate solvent for phene 2f (20 equiv.) in water (130 equiv.), giving oxacyclic products
Cu(OTf)2 (90 1C, 10 mol%) to yield the desired 3a in 43% yield.
Notably, copper powder Cu(s) (10 mol%), TBHP (3 equiv.) and
THF (10 equiv.) in water (130 equiv.) optimized the conditions
Table 2 Cycloadditions with THF derivatives
to afford cycloadduct 3a in 53% yield at an 85% conversion. An
increasing loading of Cu(s) (20 mol%) enabled a complete con-
version to afford 3a in 62% yield together with the side product 3b
in 10% yield; herein, the weakly acidic catalyst did not form a red
insoluble species. DMF and DMSO were inappropriate solvents
for the Cu(s) catalyst, giving desired 3a in either 30% yield or a
negligible amount (entries 10 and 11). Our further analysis
indicates that the nature of this Cu(s) catalyst likely involves
homogeneous reactions.9 The use of Cu(s) catalyst (20 mol%) is
reasonably efficient because two inert sp3 C–H bonds of THF and
N,N-dimethylaniline are to be activated.
We examined these oxidative cycloadditions with various
N-alkyl-N-methylanilines (1b–1k) and tetrahydrofuran derivatives
(2a–2d) to assess the substrate scope. In instances where water-
miscible tetrahydrofuran (2a, 10 equiv.) was used, their operations
were performed with Cu powder (20 mol%) in water (130 equiv.)
as depicted in entries 1–10 (condition A). Substituted tetrahydro-
furans (2b–2d) are slightly miscible with water; their reactions
were run with aniline 1 (1 equiv.), Cu(OTf)2 (10 mol%) and 2b–2d
(20 equiv.) without a solvent, as depicted by entries 11–13 (con-
dition B). We tested the reactions of N,N-dimethylaniline 2b with
THF in hot water; its resulting cycloadduct 3b was obtained in
69% yield (entry 1). These Cu-catalyzed reactions are further
Cond. A: 20 mol% Cu(s), H2O (130 equiv.), [1] = 0.41 M, THF (2a, 10 equiv.);
B: 10 mol% Cu(OTf)2, 2b–2d (20 equiv., neat). a Product yields are reported
applicable to aniline derivatives 1c–1f bearing a para-substituent after separation from a silica column.
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Table 3 Cycloadditions with other heterocycles
tests also revealed that Cu(OTf)2 alone failed to oxidize either
tertiary amine 1b or THF in a 1 : 1 molar ratio in hot water
(90 1C, 24 h). Notably, a condensation reaction of lactol (3 equiv.),
N-methylaniline (1 equiv.) and formaldehyde (1 equiv.) in hot water
proceeded smoothly to give desired 3b with up to 69% yield (eqn (6)).
The data in eqn (4)–(6) reveal that the major species of the oxidations
in water are N-methylaniline, formaldehyde and lactol, which are
effective surrogates for replacing iminiums and 2,3-dihydrofuran.12
For the species d6-1b bearing two CD3 groups (eqn (7)), its
Cu-catalyzed condensation with THF (10 equiv.), CH2(OH)2
(3 equiv.) in water delivered d5-3b, of which the N-CH2 protons
contained only 25% deuterium content; this observation indicated a
conversion of the species d6-1b to N-methylaniline and formalde-
hyde in water as depicted in eqn (6). Notably, when TEMPO (1 equiv.)
was present in the reaction mixture, the cycloaddition was inhibited
significantly to give the desired 3b in only 7% yield together with
TEMPO-substituted tetrahydrofuran 8 (58%) and unreacted 1b
(28%) in large proportions (eqn (8)), thus identifying the inter-
mediacy of the THF radical.13
(7)
a
b
No solvents were used for all entries. Product yields were reported
after purification from a silica column.
6a–6c in 50–67% yields (entries 7–9). We also achieved the cyclo-
additions of the same anilines (1b–1c and 1e) with TBHP (3 equiv.)
and tetrahydro-2H-thiopyran (20 equiv.), giving the desired products
7a–7c in reasonable yields (46–73%).
(8)
Although Cu(s)/TBHP14 gave optimized yields for the oxidative
cycloadditions in water; the nature of their Cu(I) or Cu(II) species
in solution remains unknown. Among the Cu(I,II) catalysts (see
Table S1, ESI†), Cu(OTf)2 shows the best catalytic activity in water
(Table 1, entry 7). In hot water (90 1C, 24 h), either Cu(OTf)2 or
TBHP alone was virtually inactive to oxidize THF or tertiary amine
1b at 90 1C, but the Cu(OTf)2/TBHP mixture could oxidize both THF
and tertiary amine 1b efficiently (see eqn (4) and (5)). The working
process is summarized in Scheme 2 according to the results in
eqn (4)–(8). The measurement of the kinetic isotope effect (KIE)
reveals that kH/kD = 2.8 for the oxidation of PhN(CH3)(CD3), and
kH/kD = 5.7 for the oxidation of THF (D0 versus D8, see Scheme S1,
ESI†). The C–H bond cleavage seems to be the rate determina-
(4)
(5)
(6)
Control experiments were undertaken to probe the mechanistic tion step for the two substances.
1
insight. As monitored by H-NMR, the oxidation of THF (1 equiv.)
Although the Cu(I) species is not present at the initial stage,
with Cu(OTf)2 (10 mol%) and TBHP (1 equiv.) in hot water gave the oxidation of tertiary amine 1b with Cu(OTf)2/TBHP can still
lactol and g-lactone in 21% and 7% yields respectively at 28% generate the t-BuOꢀ radical. We envisage that the Cu(II)–TBHP
conversion (eqn (4)); the same oxidation using TBHP alone (1 equiv.) complex D becomes electron-rich to conduct a single-electron
in water gave lactol in only 3% yield.11 Cu-catalyzed oxidation of oxidation on tertiary amine 1b to produce the radical ammonium
tertiary amine (1 equiv.) (1b) with TBHP (1 equiv.) proceeded more cation F and Cu(I)–TBHP species E. For the species E, the t-BuO2H
rapidly and efficiently, furnishing N-methylaniline and CH2(OH)2, ligand oxidizes the Cu(I) center to generate Cu(OTf)(OH) and t-BuOꢀ
each in 91% yield (eqn (5)). Similarly, the use of TBHP alone radical to complete the hydrogen abstraction of the radical cation
is virtually inactive to oxidize N,N-dimethylaniline 1b, giving F to form the key iminium species A. As inferred from eqn (4),
N-methylaniline and CH2(OH)2 in negligible amounts. Our separate Cu(OTf)2/TBHP (D) also oxidizes THF to lactol and g-lactone;
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Scheme 2 A postulated mechanism.
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7 We noted a recent work of Sun and coworkers involving photo-
initiated oxidation of THF solvent under air to generate 2,3-
dihydrofuran species; but the other reactant is still a reactive
heterodiene. The product yields were moderate (40–50%) for most
instances using THF as a nucleophile. H. Guo, C. Zhu, J. Li, G. Xu
and J. Sun, Adv. Synth. Catal., 2014, 356, 2801–2806.
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the absence of oxidants were reported to give distinct products in low
yields for most aniline substrates; see ref. 8a. (a) T. Sakakibara,
S. Karasumara and I. Kawano, J. Am. Chem. Soc., 1985, 107, 6417–6419;
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its mechanism likely involves the generation of the THF radical
B and X2Cu(III)OH (X = OTf), which conduct a mutual SET
process to give lactol ultimately. TBHP in a small proportion
can also generate the t-BuOꢀ radical via thermal decomposition
(eqn (4))11 to assist the formation of a THF radical (B). One
advantage of water solvent is to provide HOTf to assist the
conversion of XCuOH to the initial CuX2; this Brønsted acid is
produced from the hydrolysis of iminium A or oxonium inter-
mediate C. The hydrated forms of aldehydes also impede their
reactions with the t-BuOꢀ radical to form carbonyl radicals,
thus quenching the reactions.15
In summary, novel oxidative Povarov reactions between
N-alkyl N-methylanilines and saturated oxa- and thiacyclic
compounds have been described; importantly, these reactions
do not involve [4p]- or [2p]-motifs. The use of cheap alkane-
based substances as four- and two-atom building units is of
mechanistic and practical interest as two inert sp3 C–H bonds
are activated. Our mechanistic study hypothesizes that tertiary
amines are oxidized by Cu(OTf)2/TBHP via two SET processes
whereas THF is oxidized by the same mixture to form the THF
radical initially. We believe that the success of this work will
inspire enthusiasm for employing alkane-based substances for
metal-catalyzed cycloaddition reactions via C–H activation.
9 We have conducted a control experiment to confirm that the
reaction of Cu(s) with TBHP (2 equiv.) in water (90 1C, 24 h) gave
water-soluble Cu complexes in 5% yield. This salt contained Cu in
28 wt% and its 5 mol% Cu loading enabled a cycloaddition of
aniline 2b (1.0 equiv.) with THF (10 equiv.) in water to yield desired
3b in 75% yield (see ESI†).
10 CCDC 1039307 (3f).
11 Thermal decomposition of TBHP without metal catalyst can proceed
at 120 1C whereas the temperature was 90 1C for eqn (1). See: T. He,
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S. Nagayama, J. Am. Chem. Soc., 1996, 118, 8977–8978; (b) C. Boglio,
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