CH activation and CH2 double activation of indolines by radical translocation: Understanding the chemistry of the indolinyl radical

Article abstract of DOI:10.1039/c1ob05527e

CH activation and CH₂ double activation of indolines at C2 may be achieved efficiently through radical translocation. The fate of the C2 indolinyl radical is dictated by the substitution at C3. Fragmentation, cyclisation and tandem cyclisation reactions leading to indole, azaheterocycle and azapropellane formation, respectively, are reported. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1ob05527e

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PAPER
CH activation and CH2 double activation of indolines by radical
translocation: Understanding the chemistry of the indolinyl radical†
David C. Harrowven,*a Kerri J. Stenning,a Sally Whiting,a Toby Thompsonb and Robert Waltonc
Received 10th February 2011, Accepted 7th April 2011
DOI: 10.1039/c1ob05527e
CH activation and CH2 double activation of indolines at C2 may be achieved efficiently through radical
translocation. The fate of the C2 indolinyl radical is dictated by the substitution at C3. Fragmentation,
cyclisation and tandem cyclisation reactions leading to indole, azaheterocycle and azapropellane
formation, respectively, are reported.
with indoline 1a we were pleased to find that on treatment with
Introduction
Bu3SnH under standard radical-forming conditions the envisioned
arene to indoline radical translocation outpaced cyclisation to
C7 to give 4a in 76% yield as a 6 : 1 mixture of diastereomers
(Scheme 1).9,10 Moreover, when the same method was applied to
the analogous diiodide 1b it yielded azapropellane 5 as a 1 : 1
mixture of diastereoisomers in 90% yield, the result of a double
activation–tandem radical cyclisation at C2.
Indoles and indolines have long been considered ‘privileged
structures’ in medicinal chemistry owing to their ubiquitous
presence in natural and pharmaceutical products. Consequently,
their synthesis and functionalisation have been widely studied.1–4
In recent years the introduction of transition metal-catalysed
cross-coupling and CH bond activation strategies has provided
useful methods for the direct introduction of aryl, alkyl, vinyl
and alkynyl substituents.5 In addition, the ability to induce
intramolecular radical additions to both C2 and C3 of an indole
and to metallate indolines at C2 with strong bases are notable
advances.3,6 CH activation of indolines leading to carbon-to-
carbon bond formation is less well developed than for indoles,
having been demonstrated under both transition metal catalysis4
and, in a single example, through radical translocation and
capture of samarium(II) iodide.7 Herein we describe a detailed
examination of the chemistry of the C2 indolinyl radical in which
we exposed and exploited various fates including cyclisation,
C3-fragmentation and tandem cyclisation reactions leading to
annulation, indole and azapropellane formation, respectively.
Results and discussion
Before embarking on the study we sought an efficient method for
the generation of C2 indolinyl radical intermediates. Translocation
of an aryl radical tethered through the nitrogen seemed an
appropriate starting point as this tactic has a proven track-
record in other saturated nitrogen heterocyclic ring systems.8 Thus,
aSchool of Chemistry, University of Southampton, Highfield, Southampton,
Hampshire, UK. E-mail: dch2@soton.ac.uk; Fax: ++44 (0)23 8059 6805;
Tel: ++44 (0)23 8059 3302
Scheme
1
CH activation–cyclisation and CH2 double activation–
bAstraZeneca Charnwood, Bakewell Rd, Loughborough, Leics., UK LE11
R
tandem cyclisation reactions. VAZOꢀ–88: 1,1¢-azobis (cyclohexane-1-
5RH
carbonitrile).
cPfizer Ltd, Sandwich, Kent, UK CT13 9NJ
† Electronic supplementary information (ESI) available: Experimental
details for the procedures described herein and for the preparation of
starting materials are given, along with characterisation data and copies
of recorded 1H and 13C NMR spectra. See DOI: 10.1039/c1ob05527e
The outcome was equally clear cut with indoline 6, where the
C2 radical intermediate 8 underwent fragmentation to indole 7
rather than cyclisation to the proximal arene to form 9 (Scheme 2).
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Scheme 2 Fragmentation of a C3 benzyl substituent.
Fragmentation of C3 allyl substituents was also facile (viz. 10f →
13f) and outpaced fragmentation of a C3 benzyl in the competition
experiment 14 → 15 + 13f.11 Interestingly, no fragmentation was
observed with spirocyclopentene 16, which returned the product
of halide reduction 17 (presumably via translocation and H-atom
abstraction). Methyl, alkyl, aminoalkyl and hydrogen atoms at C3
resisted fragmentation (Scheme 3), though in the latter case this
did compete with H-atom abstraction from tributyltin hydride
when the C2 radical intermediate was stabilised by conjugation
(viz. 20 → 21 + 22).12 Fragmentation of homobenzyl substituents
at C3 was also observed as a minor pathway in the reaction
10d → 12d + 13d, which unexpectedly gave rise to a complex
product mixture.
Scheme 4 An alternative CH activation–cyclisation strategy leading to a
fused azaheterocycle.
given as a single diastereoisomer with delivery of a hydrogen atom
to the concave face of intermediate 25. It therefore seems likely
that this too involves radical translocation to 26a or 26b providing
an opportunity for further diversification.
Conclusions
In summary, access to the C2-indolinyl radical is conveniently
given by translocation of an aryl radical tethered to its nitrogen
centre. The fate of that radical intermediate is dictated by the
substitution at C3. Hydrogen, methyl, 1◦- and 2◦-alkyl and
homoallyl substituents at C3 are resistant to fragmentation,
providing an opportunity to exploit the C2-indolinyl radical in
cyclisation and tandem cyclisation reactions. By contrast, benzyl
and allyl substituents at C3 readily cleave in such circumstances
leading to the corresponding indole. That fate extends, in part, to
hydrogen atoms at C3 when the C2-indolinyl radical is stabilised
by conjugation. The CH-activation/fragmentation sequence pro-
vides further opportunities for extension, as exemplified by the
conversion of indoline 23 to the fused azaheterocycle 27.
Experimental†
General techniques
Unless specified, commercially reagents were used without fur-
ther purification. All reactions were carried out in oven-dried
glassware under an atmosphere of argon. Toluene, THF and
diethyl ether were freshly distilled from a purple solution of
sodium and benzophenone. Dichloromethane and chloroform
were freshly distilled from CaH2. Flash column chromatography
was performed using silica gel (60A Particle Size 30–70 micron)
with the stated solvent system. Chromatographic purification
of organotin-containing reaction mixtures was performed using
10% w/w anhydrous K2CO3 in silica gel.13 Melting points were
recorded on a Reichert Austria apparatus and are uncorrected.
Infrared spectra were recorded neat as a film or compressed
solid using the ATR/golden gate method and are quoted in
wavenumbers (cm-1). 1H NMR spectra were recorded on either a
Bruker AV-300 (300 MHz) or DPX-400 (400 MHz) spectrometer
Scheme 3 A study of C3 fragmentation reactions.
Finally, with the conversion of indoline 23 to the fused
azaheterocycle 27 we have been able to show how the CH-
activation/fragmentation sequence can be used to set up radical
cyclisation reactions (Scheme 4). Interestingly, the product was
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operating at 298 K. ESI mass spectra were recorded using a VG
Platform Quadrupole Electrospray Ionisation mass spectrometer,
measuring mono-isotopic masses (mode: ES+ or ES-). EI and CI
spectra were measured on a Thermoquest Trace MS.
3-Benzyl-1-(3,4-dimethoxybenzyl)-1H-indole (7). A solution
of 6 (350 mg, 0.61 mmol), tributyltin hydride (0.36 mL, 1.34 mmol)
and VAZO (0.12 mmol, 30 mg) in toluene (25 mL) was heated at
reflux for 16 h, then cooled and concentrated in vacuo. Purification
by column chromatography (10% w/w anhydrous K2CO3–silica;13
5% diethyl ether in petroleum ether) afforded the title compound
Synthetic procedures
1
as a brown oil (210 mg, 0.58 mmol, 95%). H NMR (400 MHz,
CDCl3) d 7.60 (dd, J = 7.8, 0.9 Hz, 1H), 7.38–7.31 (m, 4H), 7.26–
7.21 (m, 3H), 7.14 (ddd, J = 7.8, 6.9, 1.0 Hz, 1H), 6.93 (s, 1H),
6.86 (d, J = 8.1 Hz, 1H), 6.75 (dd, J = 8.1, 1.9 Hz, 1H), 6.71 (d,
J = 1.9 Hz, 1H), 5.27 (s, 2H), 4.20 (s, 2H), 3.92 (s, 3H), 3.83 (s,
3H). 13C NMR (100 MHz, CDCl3) d 149.3 (C), 148.5 (C), 141.4
(C), 136.9 (C), 130.2 (2 ¥ C), 128.6 (2 ¥ CH), 128.3 (2 ¥ CH),
126.4 (CH), 125.8 (CH), 121.7 (CH), 119.3 (CH), 119.2 (CH),
119.0 (CH), 114.8 (C), 111.3 (CH), 110.1 (CH), 109.6 (CH), 55.9
(CH3), 55.8 (CH3), 49.7 (CH2), 31.5 (CH2). IR (neat) nmax 3052,
3027, 2999, 2933, 2901, 2823, 1514, 1463, 1452. LRMS-ES+ (m/z,
%): 258 (100). HRMS-ES+ (m/z): [M + H]+ calcd for C24H24NO2
358.1802; found, 358.1804.
8b-But-3-enyl-4-(3,4-dimethoxybenzyl)-3-methyl-1,2,3,3a,4,8b-
hexahydrocyclopenta[b]indole (4a). A solution of 1a (200 mg,
0.40 mmol), tributyltin hydride (0.24 mL, 0.87 mmol) and VAZO
(20 mg, 0.08 mmol) in toluene (20 mL) was heated at reflux
for 4 h, then cooled and concentrated in vacuo. Purification by
column chromatography (10% w/w anhydrous K2CO3–silica;13
10% diethyl ether in petroleum ether) afforded the title compound
as a brown oil (120 mg, 0.31 mmol, 76%), as a 6 : 1 mixture of
diastereoisomers. 1H NMR (400 MHz, CDCl3) major diastereoiso-
mer d 7.48 (app. td, J = 7.9, 1.1 Hz, 1H), 7.43 (dd, J = 7.3, 1.1 Hz,
1H), 7.34–7.28 (m, 3H), 7.13 (app. td, J = 7.3, 0.6 Hz, 1H), 6.87 (d,
J = 7.9 Hz, 1H), 6.16 (ddt, J = 16.7, 10.7, 6.1 Hz, 1H), 5.39–5.31
(m, 2H), 5.04 (d, J = 15.9 Hz, 1H), 4.65 (d, J = 15.9 Hz, 1H), 4.34
(s, 3H), 4.25 (s, 3H), 4.11 (d, J = 5.8 Hz, 1H), 2.59–2.46 (m, 2H),
2.45–2.35 (m, 2H), 2.34–2.02 (m, 4H), 1.82 (m, 1H), 1.57 (d, J =
6.9 Hz, 3H). 13C NMR (100 MHz, CDCl3) major diastereoisomer
d 153.2 (C), 148.9 (C), 148.0 (C), 138.8 (CH), 136.3 (C), 131.2 (C),
127.3 (CH), 122.8 (CH), 119.8 (CH), 117.1 (CH), 113.9 (CH2),
111.0 (CH) 110.9 (CH), 107.0 (CH), 75.0 (CH), 56.9 (C), 55.8
(CH3), 55.7 (CH3), 53.3 (CH2), 41.5 (CH), 40.9 (CH2), 40.6 (CH2),
32.6 (CH2), 30.1 (CH2), 14.9 (CH3). IR (neat) nmax 3077, 3003,
2929, 2856, 2827, 1601, 1513, 1485, 1461. LRMS-CI (m/z, %): 378
(30), 151 (100). HRMS-ES+ (m/z): [M + H]+ calcd for C25H32NO2
378.2428; found, 378.2428.
Methyl 1-benzylindoline-2-carboxylate (21) and methyl 1-benzyl-
1H-indole-2-carboxylate (22). A solution of 20 (569 mg, 1.64
mmol), tributyltin hydride (0.97 mL, 3.61 mmol) and VAZO
(81 mg, 0.33 mmol) in toluene (50 mL) was heated at reflux
for 18 h, then cooled and concentrated in vacuo. Purification by
column chromatography (10% w/w anhydrous K2CO3–silica;13
2–5% diethyl ether in petroleum ether) afforded firstly 22 as a
1
colourless oil (102 mg, 0.38 mmol, 24%). H NMR (300 MHz,
CDCl3) d 7.76 (d, J = 8.1 Hz, 1H), 7.44 (s, 1H), 7.41 (d, J =
8.1 Hz, 1H), 7.36 (dd, J = 6.6, 1.1 Hz, 1H), 7.28 (d, J = 7.5 Hz,
1H), 7.33–7.17 (m, 3H), 7.10 (m, 2H), 5.89 (s, 2H), 3.91 (s, 3H).
13C NMR (75 MHz, CDCl3) d 162.3 (C), 139.5 (C), 138.2 (C),
128.5 (2 ¥ CH), 127.3 (CH), 127.1 (C), 126.2 (2 ¥ CH), 126.1
(C), 125.3 (CH), 122.7 (CH), 120.8 (CH), 111.1 (CH), 110.8 (CH),
51.6 (CH3) 47.8 (CH2). IR (neat) nmax 3062, 3031, 2857, 1706,
1605, 1518, 1248, 1191. LRMS-EI (70 eV, m/z, %): 265 (57), 233
(12), 206 (6), 188 (4), 91 (100). HRMS-ES+ (m/z): [M + Na]+
calcd for C17H15NNaO2 288.0995; found, 288.0994. Then 21 as a
2 - Aza - 2 - (3,4,5 - trimethoxybenzyl) - 8,11 - dimethylbenz[c]tri -
cyclo[3,3,3,01,5] undecane (5). A solution of 1b (300 mg, 0.46
mmol), tributyltin hydride (0.55 mL, 2.02 mmol) and VAZO
(23 mg, 0.09 mmol) in toluene (20 mL) was heated at reflux
for 16 h, then cooled and concentrated in vacuo. Purification by
column chromatography (10% w/w anhydrous K2CO3–silica;13 5–
10% diethyl ether in petroleum ether) afforded the title compound
as a white solid (170 mg, 0.41 mmol, 90%) as a 1 : 1 mixture of
diastereoisomers. 1H NMR (300 MHz, CDCl3) d 7.04 (dd, J = 7.3,
1.0 Hz, 1H), 7.05 (dd, J = 7.3, 1.0 Hz, 1H), 6.92 (app. td, J = 7.7,
1.4 Hz, 1H), 6.91 (app. td, J = 7.7, 1.4 Hz, 1H), 6.63 (app. tt, J
= 7.3, 1.0 Hz, 1H + 1H), 6.57 (br. s, 1H + 1H), 6.53 (br. s, 1H +
1H), 6.01 (d, J = 7.6 Hz, 1H), 5.95 (d, J = 7.6 Hz, 1H), 4.72 (d, J
= 16.8 Hz, 1H), 4.44 (s, 1H + 1H), 4.25 (d, J = 16.8 Hz, 1H), 3.87
(s, 3H), 3.86 (s, 3H), 3.79 (s, 6H), 3.78 (s, 6H), 2.19–1.97 (m, 4H +
4H), 1.94–1.76 (m, 2H + 2H), 1.75–1.57 (m, 2H + 2H), 1.54–1.32
(m, 2H + 2H), 1.26 (d, J = 6.9 Hz, 3H), 1.11 (d, J = 6.9 Hz, 3H),
0.92 (d, J = 7.0 Hz, 3H + 3H). 13C NMR (75 MHz, CDCl3) d
153.4 (2 ¥ C), 152.6 + 151.8 (C), 138.5 + 137.9 (C), 135.3 (C),
135.1 (C), 127.2 + 127.2 (CH), 122.7 + 122.5 (CH), 117.3 + 117.0
(CH), 107.5 (CH), 105.7 (CH), 103.8 + 103.7 (CH), 91.4 + 87.4
(C), 67.5 + 64.3 (C), 61.1 (CH3), 56.3 + 56.2 (2 ¥ CH3), 52.8 +
50.8 (CH2), 44.0 + 43.6 (2 ¥ CH), 41.5 + 41.1 + 40.1 (2 ¥ CH2),
35.0 + 34.3 + 33.8 (2 ¥ CH2), 16.4 + 15.2 + 15.1 (2 ¥ CH3). IR
(neat) nmax 2933, 2868, 2848, 1687, 1588, 1499. LRMS-ES+ (m/z,
%): 408 (50), 228 (100). HRMS-ES+ (m/z): [M + Na]+ calcd for
C26H33NNaO3 430.2353; found, 430.2349.
1
pale yellow oil (290 mg, 1.09 mmol, 67%). H NMR (300 MHz,
CDCl3) d 7.32–7.14 (m, 5H), 7.02–6.93 (m, 2H), 6.62 (app. td, J
= 7.4, 0.7 Hz, 1H), 6.39 (d, J = 7.8 Hz, 1H), 4.44 (d, J = 15.4 Hz,
1H), 4.25 (d, J = 15.4 Hz, 1H), 4.19 (dd, J = 10.3, 8.1 Hz, 1H),
3.59 (s, 3H), 3.31 (dd, J = 15.9, 10.3 Hz, 1H), 3.12 (dd, J = 15.9,
8.1 Hz, 1H). 13C NMR (75 MHz, CDCl3) d 173.3 (C), 151.3 (C),
137.7 (C), 128.4 (2 ¥ CH), 127.8 (2 ¥ CH), 127.7 (CH), 127.2 (CH),
126.8 (C), 124.1 (CH), 118.1 (CH), 107.2 (CH), 65.2 (CH), 52.1
(CH2), 52.0 (CH3), 33.4 (CH2). IR (neat) nmax 3053, 3027, 2950,
2849, 1733, 1605, 1484, 1195, 1156. LRMS-EI (70 eV, m/z, %):
267 (25), 208 (54), 117 (17), 91 (100). HRMS-ES+ (m/z): [M +
Na]+ calcd for C17H17NNaO2 290.1151; found, 290.1156.
11-Benzyl-8,9,10-trimethoxy-10b,11-dihydro-6H-isoindolo [2,1-
a]indole (27). A solution of 23 (500 mg, 0.68 mmol), tributyltin
hydride (3.01 mmol, 0.81 mL) and VAZO (0.14 mmol, 33 mg)
in toluene (40 mL) was heated at reflux for 16 h, then cooled
and concentrated in vacuo. Purification by column chromatog-
raphy (10% w/w anhydrous K2CO3–silica;13 5% diethyl ether in
petroleum ether) afforded the title compound as a colourless oil
(160 mg, 0.40 mmol, 60%). 1H NMR (400 MHz, CDCl3) d 7.42–
7.36 (m, 4H), 7.31 (m, 1H), 7.17 (app. dt, J = 7.5, 1.3 Hz, 1H),
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6.87–6.80 (m, 2H), 6.78 (ddd, J = 7.9, 7.2, 0.8 Hz, 1H), 6.51 (s,
1H), 5.06 (br. s, 1H), 4.55 (dd, J = 14.6, 1.3 Hz, 1H), 4.48 (d, J =
14.6 Hz, 1H), 4.18 (br. dt, J = 7.5, 1.9 Hz, 1H), 3.83 (s, 3H), 3.82
(s, 3H), 3.70 (s, 3H), 3.16 (dd, J = 13.3, 8.2 Hz, 1H), 3.07 (dd, J =
13.3, 7.3 Hz, 1H). 13C NMR (100 MHz, CDCl3) d 154.1 (C), 154.1
(C), 149.5 (C), 140.9 (C), 140.0 (C), 135.0 (C), 134.0 (C), 129.7 (2
¥ CH), 128.2 (2 ¥ CH), 127.8 (CH), 127.1 (C), 126.1 (CH), 124.8
(CH), 120.3 (CH), 112.0 (CH), 101.3 (CH), 74.7 (CH), 60.8 (CH3),
60.3 (CH3), 59.4 (NCH2), 56.1 (CH3), 48.1 (CH), 43.2 (CH2). IR
(neat) nmax 3032, 2995, 2938, 2856, 1597. LRMS-CI (m/z, %): 388
(80), 296 (100), 280 (15), 195 (20), 167 (20), 91 (30). HRMS-ES+
(m/z): [M + H]+ calcd for C25H26NO3 388.1907; found, 388.1901.
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Acknowledgements
The authors gratefully acknowledge the financial support of
AstraZeneca, EPSRC and Pfizer.
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Notes and references
9 The reaction can also be mediated by tris(trimethylsilyl)silane (TTMSS)
and an example related to those in Scheme 1 is presented in the ESI†.
10 The stereochemistry ascribed to 4a and 5 is based on analogous
reactions of benzo[b]furanyl radical intermediates [D. C. Harrowven,
M. C. Lucas and P. D. Howes, Tetrahedron, 2001, 57, 791–804] as
this could not be determined with rigor by NOE. The stereogenic
centre bearing the methyl substituent may therefore have the opposite
configuration.
11 J. C. Walton and A. Studer, Acc. Chem. Res., 2005, 38, 794–802.
12 Competition between H-atom abstraction from tributyltin hydride and
indole formation is also manifest in related radical cyclisations to
indoles [ref. 6]. We note that where an intermediate indolinyl radical is
stabilised by conjugation, these reactions show a greater tendency to
follow the ‘oxidative’ cyclisation pathway leading to indoles.
13 D. C. Harrowven, D. P. Curran, S. L. Kostiuk, I. L. Wallis-Guy, S.
Whiting, K. J. Stenning, B. Tang, E. Packard and L. Nanson, Chem.
Commun., 2010, 46, 6335–6337.
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2 C. W. Bird and G. W. H. Cheeseman, in Comprehensive Heterocyclic
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