Polyfluorinated cyclopentadienones as lewis acids

Article abstract of DOI:10.1055/s-0034-1378348

The ability of 2,3,4,5-tetrakis(trifluoromethyl)cyclopenta-2,4-dien-1-one and 2,3,4,5-tetrakis(pentafluorophenyl)cyclopenta-2,4-dien-1-one to act as organic Lewis acids in the field of frustrated Lewis pair (FLP) chemistry was evaluated. Whereas the forme

Full text of DOI:10.1055/s-0034-1378348

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Polyfluorinated Cyclopentadienones as Lewis Acids
Polyfluorinated Cyclopentadienones as Lewis Acids
Blanca Inés, Sigrid Holle, Dominique A. Bock, Manuel Alcarazo*
Max-Planck-Institut für Kohlenforschung, Kaiser Wilhelm Platz 1, 45470 Mülheim an der Ruhr, Germany
Fax +49(208)3062428; E-mail: alcarazo@mpi-muelheim.mpg.de
Received: 29.04.2014; Accepted after revision: 03.06.2014
PR3
Abstract: The ability of 2,3,4,5-tetrakis(trifluoromethyl)cyclopen-
ta-2,4-dien-1-one and 2,3,4,5-tetrakis(pentafluorophenyl)cyclopen-
ta-2,4-dien-1-one to act as organic Lewis acids in the field of
frustrated Lewis pair (FLP) chemistry was evaluated. Whereas the
former ketone formed zwitterionic adducts with all phosphines
studied, the latter did not react with bulky phosphines and, instead,
gave completely organic FLPs. Unfortunately, these did not activate
dihydrogen, even under high pressures.
O
O
O
F3C
CF3
R3P
F3C
CF3
F3C
CF3
F3C
CF3
F3C
CF3
F3C
CF3
1a
1b
2a R = Ph
2b R = Cy
2c R = Et
Key words: Lewis acids, frustrated Lewis pairs, steric hindrance,
ketones, fluorine, phosphines
Scheme 1 Resonance structures of ketone 1 and its reactivity to-
wards phosphines
The activation of small molecules such as dihydrogen,1
carbon dioxide,2 nitric oxide,3 disulfides,4 or silanes5 by
the cooperative action of an unquenched combination of a
Lewis acid and a Lewis base, known as a frustrated Lewis
pair (FLP), has been intensively explored during the last
decade. During this time, the nature of the Lewis base
partner has evolved from the original phosphines to in-
clude N-heterocyclic carbenes,6 amines,7 or pyridines,8
thereby permitting some control of the reactivity of the re-
sulting FLP.9 In sharp contrast, no such flexibility has
been achieved for the Lewis acid partner, and polyfluori-
nated boranes are generally used, with only a few excep-
tions.4b,10
(4; t-BuXPhos) or di-tert-butyl(2′,4′,6′-triisopropyl-
3,4,5,6-tetramethylbiphenyl-2-yl)phosphine (5; tetra-
methyl t-BuXPhos), might form boron-free FLPs.
However, mixtures of ketone 1 with phosphines 3–5 in
toluene at –78 °C invariably generated orange solutions
that slowly decolorized, and from which the adducts 6–
8 could be isolated in moderate to good yields (Scheme
2).
The ORTEP diagram of 8, shown in Scheme 2, is quite in-
formative (for the solid-state structures of 6 and 7 see the
Supporting Information).14 The cyclopentadienyl moiety
is completely planar and the C–C distances in the five
membered ring are nearly equal (1.399–1.424 Å), as ex-
pected for a homoaromatic system. To accommodate the
new substituent on phosphorus and to form the corre-
sponding Lewis adduct, the sterically demanding phos-
phine moiety adopts a quite disfavored conformation in
which the bulky 2,4,6-triisopropylphenyl group and one
of the tert-butyl substituents are nearly eclipsed. In addi-
tion, the tetramethylated phenyl ring in 8 shows a severe
distortion from planarity (P1–C18–C19–C24 torsion an-
gle: 40.9°). The formation of adduct 8, despite these ener-
getically destabilizing factors, demonstrates the strong
affinity between the two Lewis partners and suggested
that bulkier analogues of ketone 1 might be suitable as
Lewis acids for FLP chemistry.
Intrigued by this limitation, we recently started a program
devoted to evaluating the potential of alternative Lewis
acids in this chemistry,4d,11 and we primarily focused our
attention on 2,3,4,5-tetrakis(trifluoromethyl)cyclopenta-
2,4-dien-1-one (1).12 In this compound, the betaine-type
resonance form 1b, with reverse polarization of the car-
bonyl group and a partial positive charge on the oxygen
atom, is strongly favored due to the simultaneous occur-
rence of two factors: the natural tendency of the cyclopen-
tadienyl ring to aromatize, accepting electron density
from the C=O bond, and the additional stabilization of the
negative charge provided by the four electron-withdraw-
ing trifluoromethyl substituents. This umpolung of the
carbonyl group explains why ketone 1 undergoes attack
by phosphines at the electrophilic oxygen atom to give the
corresponding zwitterionic adducts 2a–c (Scheme 1).13
With the aim of putting this idea into practice, we rea-
soned that formal exchange of the four trifluoromethyl
substituents in ketone 1 for bulkier, but still inductively
electron-withdrawing, pentafluorophenyl groups should
increase the steric demand on the resulting ketone and
might permit the formation of FLPs in the presence of
common phosphines. We therefore identified 2,3,4,5-tet-
rakis(pentafluorophenyl)cyclopenta-2,4-dien-1-one (9) as
our next target, and we prepared this ketone by following
the known procedures.15
This unconventional reactivity prompted us to investi-
gate whether combinations of ketone 1 with bulkier
phosphines, such as tri(tert-butyl)phosphine (3), di-
tert-butyl(2′,4′,6′-triisopropylbiphenyl-2-yl)phosphine
SYNLETT 2014, 25, 1539–1541
Advanced online publication: 11.06.2014
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0034-1378348; Art ID: st-2014-d0352-c
© Georg Thieme Verlag Stuttgart · New York

1540
B. Inés et al.
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Scheme 2 Synthesis of adducts 6–8 and the molecular structure of 8
in the solid state. Thermal ellipsoids are drawn at 50% probability.
Hydrogen atoms are removed for clarity.14 (a) Reaction conditions:
toluene, –78 °C to r.t. Yields: 6, 43%; 7, 76%; 8, 81%.
When ketone 9 was mixed with triphenylphosphine,
tri(tert-butyl)phosphine, tricyclohexylphosphine, or bi-
phenyl-2-yl(dicyclohexyl)phosphine (10) in toluene at
room temperature, the corresponding classical Lewis ad-
ducts 11–14 were isolated, demonstrating that ketone 9 re-
tains Lewis-acid properties.16 Crystals of ketone 9 and the
newly prepared adducts 11–14 suitable for X-ray diffrac-
tion analysis14 were obtained from dichloromethane–
diethyl ether mixtures [see Scheme 3 (9 and 12) and Sup-
porting Information (11 and 14)]. A comparison of the
structures of ketone 9 and adduct 12 clearly shows the ar-
omatic nature of the ketone fragment after coordination to
phosphorus. Whereas in ketone 9, an alternating single–
double C–C bond pattern is observed in the cyclopentadi-
enyl ring (C1–C2, 1.512 Å; C2–C3, 1.347 Å), in adduct
12, all the C–C bonds are virtually identical and their
lengths (1.394; 1.425 Å) are intermediate between the typ-
ical values for single and double bonds. In addition, the
C=O double bond in ketone 9 (C1–O1, 1.208 Å) is elon-
gated in adduct 12 (C1–O1, 1.425 Å), approaching the ex-
pected length for a single C–O bond. Finally, the
formation of adducts 11–14 seemed to be irreversible, and
no dissociation into their constituents was detected, even
after heating to 120 °C for several hours. The strength of
the newly formed P–O bond is probably responsible for
this remarkable stability.
Scheme 3 Synthesis of adducts 11–14 and molecular structures of 9
and 12 in the solid state. Thermal ellipsoids are drawn at 50% proba-
bility. Hydrogen atoms and solvents molecules removed for clarity.14
(a) Reaction conditions: toluene, r.t: 11, 98%; 12, 86%; 13, 89%; 14,
93%.
Interestingly, when ketone 9 was mixed with trime-
sylphosphine (15) or with t-BuXPhos (4), NMR spectros-
copy indicated no interaction between the partners and,
therefore, the formation of an FLP. However, the desired
activation of dihydrogen could not be realized with these
FLPs, even under high pressures. We also obtained nega-
tive results when we attempted to activate terminal al-
kynes, ethers, or fluoroalkanes. It is likely that the limited
Lewis acidity of ketone 9, compared with that of poly-
Synlett 2014, 25, 1539–1541
© Georg Thieme Verlag Stuttgart · New York

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Polyfluorinated Cyclopentadienones as Lewis Acids
1541
(b) Chase, P.; Jurca, T.; Stephan, D. W. Chem. Commun.
2008, 1701.
(8) Geier, S. J.; Gille, A. L.; Gilgert, T. M.; Stephan, D. W.
Inorg. Chem. 2009, 48, 10466.
(9) Inés, B.; Palomas, D.; Holle, S.; Steinberg, S.; Nicasio, J. A.;
Alcarazo, M. Angew. Chem. Int. Ed. 2012, 51, 12367.
(10) Cabrera, L.; Welch, G. C.; Masuda, J. D.; Wei, P.; Stephan,
D. W. Inorg. Chim. Acta 2006, 359, 3066.
(11) Iglesias-Sigüenza, J.; Alcarazo, M. Angew. Chem. Int. Ed.
2012, 51, 1523.
(12) Dickson, R. S.; Wilkinson, G. J. Chem. Soc. 1964, 2699.
(13) It has been demonstrated that the primary attack takes place
at the carbon at the position α to the carbonyl group, but even
at low temperatures this intermediate rearranges to the
thermodynamically more stable 2, See: (a) Roundhill, D. M.;
Wilkinson, G. J. Org. Chem. 1970, 35, 3561. (b) Burk, M. J.;
Calabrese, J. C.; Davison, F.; Harlow, R. L.; Roe, D. C.
J. Am. Chem. Soc. 1991, 113, 2209.
fluorinated boranes or even with ketone 1, is responsible
for this lack of reactivity.
In conclusion, these preliminary results demonstrate that
polyfluorinated ketone 9 can form FLPs in the presence of
bulky phosphines. However, the ‘frustration level’
achieved by ketone 9 in combination with phosphines is
insufficient to promote the activation of small molecules
such as dihydrogen. The synthesis and design of new or-
ganic Lewis acids with potential applications in FLP
chemistry are currently under investigation in our labora-
tory. These species are interesting since they might create
bridges between two synthetically very powerful areas of
chemistry, namely FLPs and organocatalysis.
Acknowledgment
(14) Crystallographic data for compounds 6, 7, 8, 11, 12, and 14
have been deposited with the accession numbers CCDC
999847, 999844, 999846, 999848, 999843 , and 999845,
respectively, and can be obtained free of charge from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; Fax: +44(1223)336033;
E-mail: deposit@ccdc.cam.ac.uk; Web site:
Generous support from the Fonds der Chemischen Industrie (Do-
zentenstipendium to M.A.) and the European Research Council
(ERC Starting Grant to M.A.) is gratefully acknowledged. We also
thank Professor Alois Fürstner for his generous support, and the
NMR department of our institute for its assistance.
www.ccdc.cam.ac.uk/conts/retrieving.html.
Supporting Information for this article is available online
(15) (a) Bauer, R.; Liu, D.; Ver Heyen, A.; De Schryver, F.; De
Feyter, S.; Müllen, K. Macromolecules 2007, 40, 4753. A
new procedure has been recently reported, see: (b) Löser, P.;
Winzenburg, A.; Faust, R. Chem. Commun. 2013, 49, 9413.
(16) In a typical reaction, the appropriate phosphine was added in
one portion to a solution of ketone 9 in toluene at r.t., and the
resulting slurry was stirred at r.t. overnight. The solvent was
then removed under vacuum and the crude product washed
with pentane.
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References
(1) (a) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D.
W. Science 2006, 314, 1124. (b) Kenward, A. L.; Piers, W.
E. Angew. Chem. Int. Ed. 2008, 47, 38. For a recent review
on the chemistry of frustrated Lewis pairs see: (c) Stephan,
D. W.; Erker, G. Angew. Chem. Int. Ed. 2010, 49, 46. For
metal-free catalyzed hydrogenation see: (d) Spies, P.;
Schwendermann, S.; Lange, S.; Kehr, G.; Fröhlich, R.;
Erker, G. Angew. Chem. Int. Ed. 2008, 47, 7543. (e) Chase,
P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem.
Int. Ed. 2007, 46, 8050. (f) Greb, L.; Daniliuc, C.-G.;
Bergander, K.; Paradies, J. Angew. Chem. Int. Ed. 2013, 52,
5876. (g) Nicasio, J. A.; Steinberg, S.; Inés, B.; Alcarazo, M.
Chem. Eur. J. 2013, 19, 11016.
(2) (a) Rosorius, C.; Kehr, G.; Fröhlich, R.; Grimme, S.; Erker,
G. Organometallics 2011, 30, 4211. (b) Theuergarten, E.;
Schlösser, J.; Schluns, D.; Freytag, M.; Daniliuc, C. G.;
Jones, P. G.; Tamm, M. Dalton Trans. 2012, 41, 9101.
(3) Cárdenas, A. J. P.; Culotta, B. J.; Warren, T. H.; Grimme, S.;
Stute, A.; Fröhlich, R.; Kehr, G.; Erker, G. Angew. Chem.
Int. Ed. 2011, 50, 7567.
(4) (a) Dureen, M. A.; Welch, G. C.; Gilbert, T. M.; Stephan,
D. W. Inorg. Chem. 2009, 48, 9910. (b) Inés, B.; Holle, S.;
Goddard, R.; Alcarazo, M. Angew. Chem. Int. Ed. 2010, 49,
8389. (c) Alcarazo, M. Dalton Trans. 2011, 40, 1839.
(d) Palomas, D.; Holle, S.; Inés, B.; Bruns, H.; Goddard, R.;
Alcarazo, M. Dalton Trans. 2012, 41, 9073.
(5) (a) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118,
9440. (b) Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org.
Chem. 2000, 65, 3090. (c) Alcarazo, M.; Gomez, C.; Holle,
S.; Goddard, R. Angew. Chem. Int. Ed. 2010, 49, 5788.
(d) Chen, D.; Leich, V.; Pang, F.; Klankermayer, J. Chem.
Eur. J. 2012, 18, 5184.
12: yellow solid; yield: 66 mg (86%); mp 235 °C (decomp.);
IR (neat): 742, 856, 926, 991, 1053, 1093, 1104, 1347, 1402,
1494, 1504, 1523, 1978 cm–1; 1H NMR (400 MHz, CD2Cl2):
δ = 1.40 (d, J = 14.7 Hz, 27 H); 13C NMR (151 MHz,
CD2Cl2): δ = 29.2, 42.1 (d, J = 33.9 Hz), 77.9, 96.1, 104.2,
113.6 (m), 114.0 (m), 137.2 (dm, J = 250.4 Hz), 137.9
(dm, J = 251.8 Hz), 139.6 (dm, J = 249.0 Hz), 139.9 (dm,
J = 249.0 Hz), 144.7 (dm, J = 246.2 Hz), 145.5 (dm,
J = 243.4 Hz); 31P NMR (162 MHz, CD2Cl2): δ = 106.7;
19F NMR (282 MHz, CDCl3): δ = –(165.21–165.02) (m, 4
F), –(164.00–163.80) (m, 4 F), –(158.86–158.52) (m, 4 F),
–(140.25–140.14) (m, 4 F), –(139.00–138.85) (m, 4 F);
HRMS: m/z [M + Na]+ calcd for C41H27OF20PNa:
969.137244; found: 969.137923.
14: yellow solid; yield: 50 mg (93%); mp 229 °C (decomp.);
IR (neat): 790, 852, 864, 894, 924, 969, 991, 1060, 1094,
1106, 1287, 1358, 1403, 1475, 1490, 1501, 1522, 1535,
2862, 2933; 1H NMR (400 MHz, CD2Cl2): δ = 0.83–1.00
(m, 4 H), 1.07–1.27 (m, 6 H), 1.38–1.72 (m, 10 H), 2.01–
2.12 (m, 2 H), 7.26–7.29 (m, 1 H), 7.32–7.38 (m, 3 H), 7.44–
7.48 (m, 1 H), 7.55–7.57 (m, 3 H), 7.70–7.74 (m, 1 H); 13
C
NMR (101 MHz, CD2Cl2) (partial): δ = 25.7 (d, J = 1.4 Hz),
26.3 (d, J = 3.8 Hz), 26.9 (d, J = 13.3 Hz), 37.2 (d, J = 51.9
Hz), 83.2, 95.5, 103.5, 113.0, 114.0, 127.7 (d, J = 11.4 Hz),
129.2, 129.7, 130.3, 131.9 (d, J = 10.0 Hz), 134.6 (d,
J = 14.3 Hz), 134.7, 137.5 (dm, J = 242.7 Hz), 137.9 (dm,
J = 247.0 Hz), 139.5 (d, J = 2.4 Hz), 144.7 (dm, J = 242.7
Hz), 145.5 (dm, J = 240.3 Hz), 148.5 (d, J = 8.6 Hz); 31
P
NMR (162 MHz, CD2Cl2): δ = 80.5; 19F NMR (282 MHz,
CDCl3): δ = –(165.21–165.09) (m, 4 F), –(163.99–163.84)
(m, 4 F), –159.68 (t, J = 21.1 Hz, 2 F), –159.36 (t, J = 21.0
Hz, 2 F), –(140.52–140.34) (m, 4 F), –139.77 (dt, J = 25.0,
8.7 Hz, 4 F); HRMS: m/z [M + Na]+ calcd for
(6) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P.
G.; Tamm, M. Angew. Chem. Int. Ed. 2008, 47, 7428.
(7) (a) Sumerin, V.; Schulz, F.; Nieger, M.; Leskelä, M.; Repo,
T.; Rieger, B. Angew. Chem. Int. Ed. 2008, 47, 6001.
C53H31OF20PNa: 1117.168543; found: 1117.169092.
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Synlett 2014, 25, 1539–1541

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