Cyclizations via frustrated Lewis pairs: Lewis acid induced intramolecular additions of amines to olefins and alkynes

Article abstract of DOI:10.1002/chem.200903483

(Figure Presented) Intramolecular additions of amines to olefinic and acetylenic residues in the presence of B(C₆F₅)₃ affords a "frustrated Lewis Pair" strategy to five- and six-membered-ring heterocyclic ammonium borate d

Full text of DOI:10.1002/chem.200903483

COMMUNICATION
DOI: 10.1002/chem.200903483
Cyclizations via Frustrated Lewis Pairs: Lewis Acid Induced Intramolecular
Additions of Amines to Olefins and Alkynes
Tanja Voss,[a, b] Chao Chen,[a] Gerald Kehr,[a] Elisa Nauha,[a, c] Gerhard Erker,*[a] and
Douglas W. Stephan*[b]
An emerging strategy for the activation and reaction of
small molecules is based on the concept of “frustrated Lewis
pairs” (FLPs).[1–3] Such systems exploit steric congestion that
frustrates classical Lewis acid–base adduct formation. In this
fashion the unquenched Lewis acidity and basicity is avail-
able for reaction with a third component. This concept was
first employed to effect the heterolytic cleavage of H2 by
sterically frustrated combinations of phosphines and bor-
anes.[4–7] This reactivity has subsequently been applied to de-
velop a “metal-free” approach to hydrogenation catalysis of
imines,[8,9] enamines,[10] and silylenol ethers.[11] This strategy
of FLP activation of H2 is not limited to P/B combinations.
the reactions of alkynes, the trans-olefinic products suggest
nucleophilic attack on a Lewis acid activated alkyne. Em-
À
ploying more basic phosphines, C H activation of terminal
alkynes affords phosphonium alkynyl borate salts. Recently,
the Erker group[29] has shown that the linked phosphine
borane (C6H2Me3)2PCH2CH2BACHTNUGRTNEUNG(C6F5)2, reacts with pentyne
À
to give a related C H activation product. However, this P/B
species also reacts with a vinyl ether and norbornene to give
the cyclized zwitterionic phosphonium borates. In a very
recent example of similar reactivity, Erker and co-workers[30]
have also described the addition of an amine to an intramo-
lecular olefinic residue in the presence of BACHTUNRGTNEUNG(C6F5)3, which
Indeed,
suitably
sterically
hindered
carbenes,[12,13]
affords a thermally unstable zwitterionic ferrocene ammoni-
um borate derivative. Herein, we describe reactions of steri-
cally encumbered amines with intramolecular olefin or acet-
ylene fragments in the presence of a Lewis acid. The result-
ing five- and six-membered heterocyclic ammonium borate
species demonstrate that FLP reactivity provides a facile
route to intramolecular cyclizations.
amines,[8,14,15] and pyridines[16] have been shown to generate
FLPs that react with H2. Moreover the nature of the Lewis
acid can be adjusted so as to impart reversibility to the H2
uptake.[6, 11] FLPs have also been exploited to effect the acti-
vation of a variety of small molecules including olefins,[17]
dienes,[18] alkynes,[19,20] B H bonds,[21] disulfides,[22]
À
PhNCO,[3,23,24] CO2,[25] and N2O.[26]
BACHTUNGTERNN(UG C6F5)3 was added to a solution of o-(2-propenyl)-N,N-
In the case of olefins and alkynes, Stephan and co-work-
ers[17,19,20] showed that sterically encumbered phosphines and
boranes or alanes, can effect inter- and intramolecular addi-
tions affording zwitterionic phosphonium borates. DFT stud-
ies have suggested that such additions to olefins occur via
an antarafacial asynchronous concerted addition.[25,27,28] For
dimethylaniline in CH2Cl2 and the mixture was stored at
À328C for 48 h. Subsequent solvent removal from the super-
natant gave a white powder (1) in 87% yield. The 11B NMR
spectrum of 1 showed a single resonance at d=À14.3 ppm.
The corresponding 19F NMR resonances were observed at
d=À132.0, À162.2 and À166.0 ppm. These data are consis-
tent with the quaternization of B and thus a borate
anion.[31–35] The 1H NMR spectrum reveals inequivalent
methyl resonances at d=3.25 and 3.00 ppm in addition to
broad resonances attributable to methylene protons at d=
[a] T. Voss, Dr. C. Chen, Dr. G. Kehr, E. Nauha, Prof. Dr. G. Erker
Organisch-Chemisches Institut, Westfꢀlische Wilhelms-Universitꢀt
48149 Mꢁnster, Correnstrasse 40 (Germany)
À
2.25 and 1.91 ppm. The latter are consistent with B C bond
E-mail: erker@uni-muenster.de
formation. These data together with 13C, 1H/1H-COSY,
[b] T. Voss, Prof. Dr. D. W. Stephan
1
1H/13C-HSQC, and H/13C-HMBC data were consistent with
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, ON, M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
the formulation of the product as a reduced indole deriva-
tive C6H
zwitterionic formulation of 1, comprising a five-membered,
cyclic ammonium fragment with pendant methylene
4ACHTUNGTNREN(NUG NMe2)CAHUTTGNREN(NUG CH2CHAHCUTNGERTNG(NUN CH2BACHTGUNRTEN(NUGN C6F5)3) (Figure 1). This
[c] E. Nauha
Nanoscience Center, P.O. Box 35
40014 University of Jyvꢀskylꢀ (Finland)
a
borate group, was subsequently confirmed by X-ray crystal-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903483.
lography.[36] The metric parameters were unexceptional.
Chem. Eur. J. 2010, 16, 3005 – 3008
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3005

tion of the six-membered am-
monium ring with the N atom
positioned beta to the arene
ring together with the methyl-
ene borate unit on the adjacent
carbon (Figure 1).
In each of these cases, the ad-
dition of amine and borane to
the olefinic fragment occurs
with the borane adding to the
terminal carbon and N substi-
tuting at the secondary carbon
atom. The geometry of addition
is similar to that seen for the
inter- and intramolecular addi-
tions of sterically encumbered
phosphines and boranes to ole-
fins.[17]
Such Lewis acid mediated ad-
dition reactions were extended
to involve alkyne fragments.
Reaction of o-(pentynyl)-N,N-
dimethyl toluidine with B-
AHCTNUGTREN(GUNN C6F5)3 results in a reaction on
standing at room temperature
for 24 h. The product 4, which
was isolated in 90% yield, ex-
hibited a 11B NMR signal at d=
À16.2 ppm. The 19F NMR spec-
trum comprised 15 resonances
assigned by 19F/19F-COSY to
three C6F5 units. These data
suggest the formation of
a
borate species, in which inhibit-
ed rotation of the C6F5 rings re-
sults in nonequivalent fluorine
environments. The 1H NMR
spectral data reveal nonequiva-
lent N-methyl groups. The ob-
Figure 1. Synthesis of 1–5 and POV-ray depictions of 1, 2, 3, and 5 (hydrogen atoms are omitted for clarity).
C: black, F: pink, B: yellow-green, N: aquamarine.
In a similar fashion, o-(3-butenyl)-N,N-dimethylaniline
servation of the two 13C NMR signals at d=150.7 ppm and
1
reacts with B
N
at d=142.2 ppm [q
(1:1:1:1), JCB ~55 Hz] suggest the forma-
tion of a borato-substituted olefinic fragment. Collectively
these data support the formulation of 4 as 4-methyl-N,N-di-
methyl-2-propyl-3-tris(pentafluorophenyl)boratoindolium,
lated as the zwitterionic product C6H
(NMe2)
E
C6H3Me
ACHUTGTNNERNUG(NMe2)(C(BACHTGNUTRENNUNG
A
R
of o-(phenylethynyl)-N,N-dimethyl toluidine with BACHTUNGTRENNUNG
fragment again with a pendant methylene borate unit. The
structure of 2 was also confirmed by X-ray crystallogra-
phy[36] (Figure 1).
the analogous product 5 was isolated in 93% yield. The
spectral data were analogous to those observed for 4, nota-
bly with the 13C NMR resonances at d=147.8 ppm and d=
1
A structural isomer of 2 was accessible by reaction of o-
143.7 ppm [q
A
(2-propenyl)-N,N-dimethylbenzylamine with B
(C6F5)3. The
borato-substituted olefinic subunit. The corresponding for-
mulation of 5 as the indole derivative C6H3Me(NMe2)(C(B-
product 3 was isolated in 70% yield and spectroscopic char-
acterization revealed data similar to that described above
for 2. A notable difference is the observation of the benzylic
protons at d=4.35 and 4.26 ppm, leading to the formulation
ACHTUNGTRENNUNG
AHCTNUGERTG(NNUN C6F5)3)CPh with a newly formed five-membered ring was
confirmed by X-ray crystallography[36] (Figure 1).
The formation of 4 and 5 illustrate the intramolecular ad-
dition of an amino group and borane to the alkynyl frag-
ment of the precursors. In contrast to previously reported
of
3
as C6H4ACHTUNGTRENNUNG(CH2NMe2)ACHTUNGTRNENGU(CH2CHACHTUNTREGGN(UNN CH2BACHTUNGTRENUN(NG C6F5)3). X-ray
data[36] were consistent with the connectivity and the forma-
3006
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Chem. Eur. J. 2010, 16, 3005 – 3008

Cyclizations via Frustrated Lewis Pairs
COMMUNICATION
2H, C6H4); 7.23 (m, 1H, C6H4); 3.31 (s, 3H, NCH3); 3.29 (s, 3H, NCH3);
2.97 (m, 1H, NCH); 2.79 (dm, 2J=16.9 Hz, 1H, ArCH2); 2.60 (br, 1H,
BCH2); 2.45 (m, 1H, ArCH2); 2.25 (dm, 2J=16.9 Hz, 1H, CHCH2); 2.10
(m, 1H, CHCH2); 1.59 ppm (br, 1H, BCH2); 13C NMR (101 MHz,
CD2Cl2, 298 K): d=144.6 (Cq); 131.5 (Cq); 132.0, 130.5, 128.9, 121.6
(C6H4); 81.5 (NCH); 53.9 (NCH3); 50.8 (NCH3); 27.7 (ArCH2); 22.6
(CHCH2); 20.6 ppm (br, BCH2), not assigned (C6F5); 19F NMR
(377 MHz, CD2Cl2, 298 K): d=À132.0 (m, 2F, o-C6F5); À162.2 (t, 3J=
20.6 Hz, 1F, p-C6F5); À166.0 ppm (m, 2F, m-C6F5); 11B NMR (128 MHz,
CD2Cl2, 298 K): d=À14.2 ppm (s, n1/2 =30.0 Hz); 3: 1H NMR (400 MHz,
intermolecular additions of sterically encumbered phos-
phines and BACHTUNGTRENNUNG
(C6F5)3 to acetylenes,[19] the present examples
demonstrate intramolecular addition to internal acetylenes,
affording a trans-disposition of these addenda.
The formation of 1–5 proceeds because the classical
Lewis acid–base interaction of the amino function with the
Lewis acid BACHTUNGTRENNUNG(C6F5)3 is sterically inhibited. This presumably
allows the interaction of the Lewis acid with the p-bond of
the olefin or acetylene fragment, activating it for intramo-
lecular nucleophilic attack by the amine, which results in the
cyclization. These findings illustrate the utility of the FLP
strategy in the formation of such heterocycles. It is notewor-
thy that related intramolecular phosphino-borane additions
to alkynes have been elegantly exploited by Yamaguchi and
co-workers[37] to effect the formation of heterocyclic zwitter-
ionic phosphonium borate species, which have applications
as electronic materials.
In conclusion, herein we have demonstrated that FLP re-
activity can be exploited to effect intramolecular cyclizations
of amines with olefinic and acetylenic residues. While the
present examples afford five- and six-membered heterocy-
clic derivatives, the potential for application to a wider vari-
ety of systems is evident. We continue to explore a variety
of avenues for the exploitation of the concept of “frustrated
Lewis pairs” in organic chemistry, small molecule activation,
and catalysis.
2
CD2Cl2, 298 K): d=7.30 (m, 2H, C6H4); 7.07 (m, 2H, C6H4); 4.35 (d, J=
16.0 Hz, 1H, NCH2); 4.26 (d, 2J=16.0 Hz, 1H, NCH2); 3.14 (m, 1H,
ArCH2); 3.03 (s, 3H, NCH3); 3.01 (m, 2H, ArCH2, NCH); 2.94 (s, 3H,
NCH3); 2.33 (br, 1H, BCH2); 1.70 ppm (br, 1H, BCH2); 13C NMR
1
(101 MHz, CD2Cl2, 298 K): d=148.1 (dm, J=240.5 Hz, C6F5); 138.7 (dm,
1J=237.8 Hz, C6F5); 136.7 (dm, 1J=240.6 Hz, C6F5); 131.4 (Cq); 129.6,
127.9, 129.3, 126.8 (C6H4); 125.2 (Cq); 77.0 (NCH); 66.5 (NCH2); 53.1
(NCH3); 44.0 (NCH3); 29.7 (ArCH2); 19.3 ppm (br, BCH2), not assigned
(i-C6F5); 19F NMR (377 MHz, CD2Cl2, 298 K): d=À131.7 (m, 2F, o-C6F5);
À162.2 (t, 3J=20.4 Hz, 1F, p-C6F5); À166.0 ppm (m, 2F, m-C6F5);
11B NMR (128 MHz, CD2Cl2, 298 K): d=À14.4 ppm (s, n1/2 =32.1 Hz).
Synthesis of 4 and 5: These compounds were prepared in a similar fash-
ion and thus only one preparation is detailed. In a glovebox the solution
of BACHTNURGTNE(UNG C6F5)3 (52 mg, 0.1 mmol) in pentane (5 mL) was added to the solu-
tion of alkynyl toluidine (20.1 mg, 0.1 mmol) in pentane (2 mL) in a
small glass vial. The mixture was kept at room temperature overnight.
The liquid was decanted from a precipitated solid. The solid was washed
with pentane (3ꢃ1 mL) and dried under vacuum overnight to give the
product as a white solid (63 mg, 90%) that was pure enough for analysis.
4: 1H NMR (CD2Cl2, 298 K): d= 7.29 (d, 3JHH =8.3 Hz, 1H, C6H3), 7.21
(br, 1H, C6H3), 7.17 (dm, 3JHH =8.3 Hz, 1H, C6H3), 3.41 (s, 3H, NCH3),
3.21 (s, 3H, NCH3), 2.57, 2.47 (each m, each 1H, PrCH2), 2.26 (s, 3H,
ArCH3), 1.61, 1.46 (each m, each 1H, PrCH2), 0.79 ppm (t, 3JHH =7.2 Hz,
3H, PrCH3); 13C{1H} NMR (151 MHz, CD2Cl2, 298 K): d=150.7 (br,
1
NCPr), 145.4 (NCAr), 142.2 (q
(1:1:1:1), JCB ~55 Hz, BC), 141.7 (Cq),
3
Experimental Section
140.1(Cq), 127.8 (C6H3), 125.8 (’d’, partial relaxed JCB ~6.2 Hz, C6H3),
123.3 (br, i-C6F5), 114.1 (C6H3), 54.2 (NCH3), 51.4 (NCH3), 28.1 (’d’, par-
tial relaxed JCB ~6.2 Hz, PrCH2), 22.0 (PrCH2), 21.6 (ArCH3), 14.9 ppm
3
General: All syntheses were done under an atmosphere of dry, O2-free
N2 (Toronto) or argon (Mꢁnster) using Schlenk-type glassware and a glo-
vebox. Solvents (including deuterated solvents used for NMR spectrosco-
py) were dried and distilled under protective gas prior to use. 1H, 13C,
11B, and 19F NMR spectra and the respective 2D NMR experiments were
recorded on 600 or 400 MHz Varian, 400 MHz or 300 MHz Bruker spec-
trometers. The spectra were referenced relative to SiMe4 using residual
solvent signals (1H and 13C NMR) or an external standard (11B:
(Et2O)BF3, 19F: CFCl3). Chemical shifts are reported in ppm. The o-al-
kenyl-substituted aniline derivatives, o-allyldimethylbenzylamine,[38] and
the N-dialkyl-2-(1-alkynyl)toluidines[39,40] were synthesized according to
literature procedures.
(PrCH3), not assigned (C6F5); 19F NMR (CD2Cl2, 298 K): d À129.1, À132.8
(o), À161.5 (p), À164.9, À166.40 (m) (each m, 1F, C6F5A), À130.5, À130.6
(o), À161.8 (p), À166.43, À166.8 (m) (each m, 1F, C6F5B), À131.2, À131.9
(o), À161.9 (p), À166.1, À166.9 ppm (m) (each m, 1F, C6F5C); 11B NMR
(96 MHz, CD2Cl2, 298 K): d=À16.2 ppm (n1/2 ~23 Hz). 5: 1H NMR
(CD2Cl2, 298 K): d=7.54 (br, 1H, o-Ph), 7.48 (br, 1H, m-Ph), 7.41 (tt,
3JHH =7.5 Hz, 4JHH =1.1 Hz, 1H, p-Ph), 7.36 (d, 3JHH =8.2 Hz, 1H, C6H3),
7.30 (br, m, 1H, o’-Ph), 7.28 (br, 1H, C6H3), 7.22 (dm, 3JHH =8.2 Hz, 1H,
C6H3), 7.22 (br, 1H, m’-Ph), 3.31 (s, 3H, NCH3), 3.05 (s, 3H, NCH3),
2.29 ppm (s, 3H, ArCH3); 13C{1H} NMR (151 MHz, CD2Cl2, 298 K): d=
1
147.8 (m, NCPh), 144.4 (NCAr), 143.7 (q
(1:1:1:1), JCB ~54 Hz, BC), 141.9
3
(Cq), 140.3 (Cq), 133.2 (br, o-Ph), 133.1 (’d’, partial relaxed JCB ~14 Hz,
o’-Ph), 130.4 (p-Ph), 128.6 (br, m’-Ph), 128.4 (C6H3), 128.4 (m-Ph), 127.2
(i-Ph), 126.5 (m, C6H3), 123.1 (br, i-C6F5), 114.9 (C6H3), 52.2 (NCH3),
51.5 (NCH3), 21.7 ppm (ArCH3), not assigned (C6F5); 19F NMR (564 MHz,
CD2Cl2, 298 K): d= À128.6, À133.3 (o), À161.2 (p), À166.1, À166.8 (m)
(each m, each 1F, C6F5A), À130.3, À132.3 (o), À162.1 (p), À167.2, À167.3
(m) (each m, each 1F, C6F5B), À123.7, À136.4 (o), À162.7 (p), À165.1, ,
À166.9 ppm (m) (each m, each 1F, C6F5C); 11B NMR (96 MHz, CD2Cl2,
298 K): d=À16.5 ppm (n1/2 ~22 Hz).
Synthesis of 1, 2, and 3: These compounds were prepared in a similar
fashion and thus only one preparation is detailed. A solution of o-(2-pro-
penyl)-N,N-dimethylaniline (30 mg, 0.186 mmol) and BACHTUNRGTNEUNG(C6F5)3 (95.4 mg,
0.186 mmol) in dichloromethane (2 mL) was stored at À328C for two
days. The supernatant was removed by using a syringe, and the remaining
white powder washed with pentane twice. Drying in vacuo yielded the
product (109 mg, 0.162 mmol; 87%). Single crystals suitable for X-ray
analysis were obtained from a concentrated dichloromethane solution at
room temperature after several days. 1: 1H NMR (400 MHz, CD2Cl2,
298 K): d=7.46 (m, 2H, C6H4); 7.36 (m, 2H, C6H4); 3.80 (m, 1H, NCH);
3.25 (s, 3H, NCH3); 3.00 (s, 3H, NCH3); 2.99 (m, 1H, ArCH2); 2.70 (dd,
3
2J=16.8 Hz, J=6.8 Hz, 1H, ArCH2); 2.25 (br, 1H, BCH2); 1.91 ppm (br,
1
Acknowledgements
1H, BCH2);13C NMR (101 MHz, CD2Cl2, 298 K): d=148.2 (dm, JFC
=
239.6 Hz, C6F5); 146.6 (Cq); 138.3 (dm, 1JFC =248.6 Hz, C6F5); 136.7 (dm,
1JFC =246.6 Hz, C6F5); 134.1 (Cq); 131.3, 129.2, 127.2, 116.3 (C6H4); 86.9
(NCH); 49.4 (NCH3); 48.6 (NCH3); 32.4 (ArCH2); 19.7 ppm (br, BCH2),
not assigned (i-C6F5); 19F NMR (377 MHz, CD2Cl2, 298 K): d=À132.0
(m, o-C6F5); À162.2 (t, 3J=22.8 Hz, p-C6F5); À166.0 (m, m-C6F5).
11B NMR (128 MHz, CD2Cl2, 298 K) d=À14.3 ppm (s, n1/2 =28.9 Hz); 2:
1H NMR (400 MHz, CD2Cl2, 298 K): d=7.46 (m, 1H, C6H4); 7.39 (m,
G.E. gratefully acknowledges the financial support of the Deutsche For-
schungsgemeinschaft and the Fonds der Chemischen Industrie. D.W.S.
gratefully acknowledges the financial support of NSERC of Canada, the
award of a Canada Research Chair and a Killam Research Fellowship.
Chem. Eur. J. 2010, 16, 3005 – 3008
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3007

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Keywords: alkynes · cyclic ammonium salts · cyclization ·
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P21/n, a=11.2973(8) ꢄ, b=20.4030(15) ꢄ, c=11.6555(8) ꢄ,
b
=101.277(4)8, V=2634.7(3) mm3, Z=4, data (>2s)=7212, variables
¯
426, R=0.0392, Rw =, 0.0838, GOF=1.004. 3·0.75AHCUTNGREN(NUG CH2Cl2): P1, a=
9.3247(9), b=15.7303(14), c=21.1553(19) ꢄ, a=98.742(5), b=
90.647(5), g=92.426(5)8, V=3063.8(5) mm3, Z=4, data (>2s)=
12505, variables 877, R=0.1244, Rw =, 0.3438, GOF 1.339. 5·1.5-
AHCTUNGTRENNG(NU CH2Cl2): C2/c, a=35.1995(10), b=12.4721(5), c=18.0024(5) ꢄ, b=
115.529(3)8, V=7131.6(4) mm3, Z=8, data (>2s)=6278, variables
532, R=0.0633, Rw =0.1835, GOF=1.094. CCDC-764359, CCDC-
764360, CCDC-764361, and CCDC-764362 contain the supplementa-
ry crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
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Received: December 18, 2009
Published online: February 8, 2010
3008
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 3005 – 3008