Structures, Lewis Acidities, Electrophilicities, and Protecting Group Abilities of Phenylfluorenylium and Tritylium Ions

Article abstract of DOI:10.1002/chem.201603963

The isolation, characterization, and the first X-ray structures of a fluorenylium ion and its Lewis adducts with nitrogen- and phosphorus-centered Lewis bases are reported. Kinetics of the reactions of a series of fluorenylium ions with reference π-, σ-, and n-nucleophiles of various sizes and nucleophilicities allowed the interplay between electronic and structural parameters on the electrophilicities of these planarized tertiary carbenium ions to be elucidated. Structure–reactivity correlations and extensive comparisons of their reactivities with those of di- and triarylcarbenium ions are described. Quantitative determination of the electrofugalities of fluorenylium ions revealed to which extent they are complementing tritylium ions as protecting groups and how their tuning is possible. Determination of the equilibrium constants of the Lewis adducts formation between pyridines of calibrated Lewis basicities and phenylfluorenylium and tritylium ions allowed the determination of their Lewis acidities and to showcase the potential of these carbon-centered Lewis acids in catalysis.

Full text of DOI:10.1002/chem.201603963

DOI: 10.1002/chem.201603963
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Carbocations |Hot Paper|
Structures, Lewis Acidities, Electrophilicities, and Protecting
Group Abilities of Phenylfluorenylium and Tritylium Ions
Elsa Follet, Peter Mayer, and Guillaume Berionni*[a]
Dedicated to Professor FranÅois Terrier on the occasion of his retirement
Abstract: The isolation, characterization, and the first X-ray
structures of a fluorenylium ion and its Lewis adducts with
nitrogen- and phosphorus-centered Lewis bases are report-
ed. Kinetics of the reactions of a series of fluorenylium ions
with reference p-, s-, and n-nucleophiles of various sizes and
nucleophilicities allowed the interplay between electronic
and structural parameters on the electrophilicities of these
planarized tertiary carbenium ions to be elucidated. Struc-
ture–reactivity correlations and extensive comparisons of
their reactivities with those of di- and triarylcarbenium ions
are described. Quantitative determination of the electrofu-
galities of fluorenylium ions revealed to which extent they
are complementing tritylium ions as protecting groups and
how their tuning is possible. Determination of the equilibri-
um constants of the Lewis adducts formation between pyri-
dines of calibrated Lewis basicities and phenylfluorenylium
and tritylium ions allowed the determination of their Lewis
acidities and to showcase the potential of these carbon-cen-
tered Lewis acids in catalysis.
leaving groups,[7] and are complementing the use of trityl cat-
ions in organic synthesis and in polymer science.[8] The antiaro-
maticitiy of their five-membered ring has been the subject of
intense experimental and theoretical investigations.[9]
Introduction
Tritylium ions 1 are important intermediates in numerous reac-
tions,[1] co-initiators in coordination polymerizations,[2] and
strong hydride abstractors.[3] They have been extensively used
as protecting groups for oxygen and nitrogen functionalities in
amino acids, glucosides, and peptides.[4] These bulky carbon-
centered Lewis acids have been attracting increasing interest
in recent years in the fields of frustrated Lewis pairs (FLPs)[5]
and asymmetric catalysis.[6]
The reactivities of phenylfluorenylium ions 2 have been
comparatively less-studied than those of tritylium ions 1,[1–6] or
benzhydrylium ions 3,[10–12] and a systematic quantification of
the electrophilicities, Lewis acidities, and electrofugalities of
a set of 9-phenylfluorenyl ions is highly desirable for rationally
targeting their uses as hydride acceptors, protecting groups,
and as Lewis acid catalysts.
Their bridged analogues, namely the 9-phenylfluorenylium
ions 2 (Scheme 1), have been employed as tunable protecting/
Herein, we report the first X-ray structures of a 9-phenyl-
fluorenylium ion and of its Lewis adducts with nitrogen- and
phosphorus-centered Lewis bases. Kinetics of the reactions of
a series of phenylfluorenylium ions 2 with reference p-, s- and
n-nucleophiles,[10] and determination of their association con-
stants with 4-substituted pyridines of known Lewis basicities,[11]
allowed us to determine their electrophilicities and Lewis acidi-
ties. Detailed comparisons of the reactivities of 9-phenylfluore-
nylium ions 2 with those of tritylium 1[1–6,13] and benzhydrylium
ions 3[10–12] provided quantitative insights on their potential
uses as hydride acceptors, carbon-centered Lewis acids in cat-
alysis, and tunable protecting groups.
Scheme 1. Tritylium 1, 9-phenylfluorenylium 2, and benzhydrylium ions 3
used as reference electrophiles for the construction of reactivity scales of
electrophilicity,[10] Lewis acidity,[11] and electrofugality.[12]
Results and Discussion
[a] E. Follet, Dr. P. Mayer, Dr. G. Berionni
Recent spectroscopic characterizations of fluorenylium ions
have been reported,[14] and low-temperature NMR investiga-
tions of 9-substituted fluorenyliums in superacidic media were
described by Schleyer and Olah in the 1980s.[15] Earlier success-
ful isolations of stabilized fluorenylium ions[16] encouraged us
Department Chemie, Ludwig-Maximilians-Universitꢀt Mꢁnchen
Butenandtstrasse 5–13, 81377 Mꢁnchen (Germany)
E-mail: Guillaume.Berionni@cup.uni-muenchen.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201603963.
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ꢀ
ꢀ
Scheme 2. Synthesis and isolated yields of 2(a–b)-BF4 and 6(b–d)-BF4
.
to synthesize and crystalize the NMe2 and OMe substituted
ꢀ
Figure 2. ORTEP drawing (ellipsoids set at 50% probability) of anisylfluorenyl
phosphonium tetrafluoroborate 6b-BF4ꢀ (the BF4ꢀ counterion is removed
for clarity). Selected interatomic distances [ꢁ] and angles [8]: C9–P1
1.8875(17), C9–C14 1.536(2), C9–C10 1.534(2), C9–C13 1.540(2); P1-C9-C10
105.51(11), P1-C9-C13 106.10(11), P1-C9-C14 116.46(12), C10-C9-C13
101.78(14).
9-phenylfluorenylium tetrafluoroborates 2(a,b)-BF4 (Scheme 2,
top) to determine their X-ray structures.
ꢀ
Whereas all attempts to crystallize the fluorenylium 2b-BF4
were unsuccessful, storing a saturated CH2Cl2 solution of
ꢀ
2a-BF4 at 08C for several days provided dark-violet single
crystals. X-ray crystallographic data revealed that 2a-BF4 crys-
ꢀ
tallized as head-to-tail dimers (Figure 1) with interplanar distan-
ces of 3.466(4) ꢁ between the centers of gravities of the rings
1 and 2 and 3.477(4) ꢁ between rings 3 and 4. The aryl and flu-
orenyl scaffolds are nearly planar (twist angle <58).
tion of these carbenium ions by protonation of 4(c,d) with
ꢀ
HPPh3+BF4 5 (Scheme 2, bottom) and instantaneous trapping
by the released PPh3 yielded quantitatively the stable crystal-
As isolation of the highly electrophilic fluorenylium ions
line phosphonium species 6(b–d)-BF4ꢀ. The anisylfluorenyl
ꢀ
ꢀ
2(c,d)-BF4 in a pure form was not successful, in situ genera-
phosphonium 6b-BF4 was characterized by X-ray crystallogra-
phy (Figure 2).[17]
Reactions of fluorenyliums 2 with nucleophiles
The parent phenylfluorenylium ion 2c has previously been re-
ported to react instantaneously (k2 >103 mꢀ1 ꢀ1) with protic
s
solvents such as water, and alcohols.[18] Recent reports also de-
scribed its high reactivities toward arenes and heteroarenes.[19]
ꢀ
We now performed the reactions of 2(a,b)-BF4 with the
p-nucleophiles 7(a–c) and two hydrides donors 7(d–e)
(Scheme 3). In sharp contrast to the corresponding NMe2-sub-
Figure 1. ORTEP drawings (ellipsoids set at 50% probability) of the phenyl-
fluorenylium tetrafluoroborate 2a-BF4ꢀ: a) top view and b) view of the pack-
ing structure of the dimers (BF4ꢀ counterions and CH2Cl2 solvate molecules
omitted). Selected interatomic distances [ꢁ]: C9–C10 1.457(9), C10–C11
1.396(10), C11–C12 1.461(10), C12–C13 1.408(9), C13–C9 1.475(9), C9–C14
1.387(9), C17–N1 1.312(8).[17]
Scheme 3. Reactions of 2(a–b)-BF4ꢀ with the p-nucleophiles 7(a–c) and the
hydride donors 7(d–e) and isolated yields of the products (CH2Cl2, 208C,
5 min). BNAH=1-benzyl-1,4-dihydronicotinamide.
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stituted tritylium ion 1a-BF4ꢀ, which did not react with 7(a–c)
within 1 h, full conversion was reached with the corresponding
ꢀ
fluorenylium 2a-BF4 in less than 5 min, and the products
8–11 were isolated in good yields (Scheme 3).
ꢀ
The fluorenylium ion 2a-BF4 reacted reversibly with triphe-
nylphosphite (Keq =3.47ꢂ103 ꢁ3.25ꢂ102 mꢀ1), but quantitative-
ly with the strong Lewis base 4-(dimethylamino)pyridine
(DMAP, Keq @106 mꢀ1) to yield the Lewis adducts 12 and 13, re-
spectively (Scheme 4).[17] Slow evaporation of a CH2Cl2 solution
of the adduct 13 provided colorless crystals of suitable quality
for a X-ray diffraction analysis (Figure 3).
ꢀ
Scheme 5. Resonance structures of 2a-BF4 and experimental C9ꢀC14 bond
ꢀ
lengths in 2a-BF4 and 13.
served in the X-ray structures, showing that due to their plana-
rization, 9-phenylfluorenyliums 2 might not exhibit significant
steric interactions when reacting with nucleophiles.
The kinetics of the reactions of the fluorenylium ions 2 with
the nucleophiles 7(a–k) were investigated by monitoring the
disappearance of 2 at their maximum wavelength lmax by
time-resolved UV/Vis spectrophotometry, in the presence of
large excess of 7 (pseudo first-order conditions). The reactions
ꢀ
with 2a-BF4 were investigated by using a conventional
stopped-flow spectrophotometer. Laser-flash photolysis of the
phosphoniums 6(b-d) (Supporting Information) was used to
generate the fluorenylium ions 2(b–d) and to monitor the rate
of their decay at their lmax (540, 554, and 500 nm respectively)
in the presence of variable concentrations of 7.
Scheme 4. Reactions of triphenylphosphite P(OPh)3 and DMAP with
ꢀ
2a-BF4
.
In all cases, mono-exponential decays were observed, indi-
cating pseudo first-order kinetics, and plots of kobs (sꢀ1) against
the concentrations of the nucleophiles were linear with negli-
gible intercepts (see the inset of Figure 4). The second-order
rate constants k2 of all reactions were determined from the
slopes of these linear plots (see Table 1 and the Supporting In-
formation). Kinetics of the reactions of the CF3-substituted fluo-
renylium ion 2d with the nucleophiles 7 were diffusion con-
trolled (k2 >108 mꢀ1 sꢀ1) or were not exponential, preventing
further measurements.
In numerous investigations, Mayr and co-workers have
shown that the rate constants of the reactions of p-, s-, and
n-nucleophiles with several hundreds of carbenium ions (benz-
hydryliums 3, dithianylium, flavylium, acridinium, indolyl meth-
Figure 3. ORTEP drawing (ellipsoids set at 50% probability) of the fluorenyli-
um-DMAP adduct 13 (the BF4ꢀ counterion is removed for clarity). Selected
interatomic distances [ꢁ]: C9–C10 1.533(3), C10–C11 1.401(3), C11–C12
1.470(4), C12–C13 1.402(3), C13–C9 1.528(3), C9–C14 1.519(3), C17–N1
1.385(3).[17]
The much shorter C9ꢀC14 bond in the fluorenylium ion
ꢀ
2a-BF4 (1.387 ꢁ) than in its DMAP adduct 13 (1.519 ꢁ) indi-
cates that the iminium-like resonance structure B (Scheme 5) is
predominant. Calculation of the HOMA aromaticity index[20]
based on the experimental X-ray bond-lengths, confirmed that
ꢀ
the five-membered ring of 2a-BF4 is non-aromatic (HOMA=
Figure 4. Exponential decay of the absorption of 2a ([2a]0 =1.6ꢂ10ꢀ4 m) at
540 nm during the reaction with 7c ([7c]0 =1.67ꢂ10ꢀ3 m)
0.06 versus HOMA=0.98 for benzene).[20] No steric interactions
(kobs =1.46ꢂ10ꢀ1 sꢀ1). Inset: Plot of the first-order rate constants kobs versus
between the 9-aryl rings and the Lewis bases attached to the
ꢀ
flat fluorene ring (PPh3 in 6b-BF4 or DMAP in 13) are ob-
the nucleophile concentrations [7c] (k2exp =9.12ꢂ101 mꢀ1 ꢀ1) in CH2Cl2.
s
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exp
Table 1. Experimental and calculated second-order rate-constants k2
calc
and k2 (in Mꢀ1 sꢀ1, CH2Cl2, 208C) for the reactions of calibrated nucleo-
philes 7 with 2(a–c).
[a]
exp
calc[b]
calc
Nucleophiles 7
N, sN
Ions 2 k2
2b
k2
k2exp/k2
7a
6.34ꢂ104 1.12ꢂ105 5.66
2.93ꢂ106 3.53ꢂ106 0.86
4.41, 0.96 2c
7h 2a
5.21, 1.00 2b
3.10
4.78
0.65
5.78ꢂ105 7.55ꢂ105 0.77
2c
3.35ꢂ107 2.46ꢂ107 1.36
P(OPh)3
7 f
5.51, 0.76
7d
5.64, 0.59
7i
5.85, 1.03
7c
2a
2a
2a
2a
6.44
1.07ꢂ101 0.60
3.88
Ph3SnH
1.80ꢂ101 4.64
Figure 5. Plots of (logk2exp)/sN vs. N for the kinetics of the reactions of fluore-
nylium ions 2(a–c) with the nucleophiles 7(a–j) in CH2Cl2 at 208C.
N-methylpyrrole
2.01ꢂ101 2.40ꢂ101 0.84
9.12ꢂ101 8.24ꢂ101 1.11
6.57, 0.93 2b
2.35ꢂ101[c]
–
–
2b
2c
2a
9.98ꢂ106 7.95ꢂ106 1.26
2.85ꢂ108 2.25ꢂ108 1.27
2.12ꢂ103 2.29ꢂ103 0.93
2,5-dimethylpyrrole 7j
8.01, 0.96
7b
8.57, 0.84
2a
3.30ꢂ103 2.57ꢂ103 1.28
7.02ꢂ102[c]
1.27ꢂ108
–
–
–
–
2b
2a
P(p-ClC6H4)3
7k
3.94ꢂ105 1.76ꢂ105 2.24
12.58, 0.65
[a] N and sN parameters of 7 from Ref. [10]. [b] Calculated with Eq. (1),
with the N and sN parameters of 7 and E values of 2(a–c) determined in
Figure 5. [c] In CH3CN.
ylium, metal-stabilized carbenium ions, and iminium ions)
follow Eq (1),[10] whereas the rate constant of their reactions
with tritylium ions 1 strongly deviates because differential
steric effects play a major role.[3c,10a,13]
log k2 ð20 ꢂCÞ ¼ sNðN þ EÞ
ð1Þ
exp
exp
In Eq (1), k2 (Mꢀ1 sꢀ1) is the second-order rate constant, E
(electrophilicity parameter) quantifies the strength of the elec-
trophile, and N and sN characterize the nucleophilicity and the
sensitivity of the nucleophile, respectively.[10] The plots of (log
k2exp)/sN versus N were linear with slope nearly equal to
1 (Figure 5), indicating that Eq (1) applies, and that the electro-
Figure 6. Comparison of the electrophilicities of fluorenylium 2 with those
of tritylium 1,[13] and benzydrylium ions 3.[10] [a] Obtained from the Hammett
correlation in Ref. [10d].
ꢀ
philicity parameters E of the fluorenylium ions 2(a–c)-BF4 can
be determined. Negligible deviations from linearity between
predicting their reaction rates with virtually all type of nucleo-
philes, and for rationalizing kinetic data previously reported
(see the Supporting Information) within the confidence range
of Eq. (1) (factor 10) over a very wide reactivity range.
cal
carbon p-nucleophiles and triphenylphosphite 7 f (k2exp/k2
=
0.60), triarylphosphine 7k (k2exp/k2cal =2.24), or hydride donor
7d (k2exp/k2cal =3.88) were observed, with a maximum deviation
of 5.66 for the reaction of 7a with 2b (Table 1, entry 1).
Independently of the structure of the carbenium ion, intro-
duction of a NMe2 substituent on 1c and 3c to give 1a and
3a usually decreased their electrophilicities by 8 to 10 orders
of magnitude. Examination of the substituent effect of the pro-
tecting group abilities of 2 and 3 were next examined.
Kinetic data confirmed that although 2 are tertiary carbeni-
um ions, the near-planarization of their rings resulted in
a much lower steric hindrance (similar to that of diaryl carbeni-
um ions 3) than in trityliums 1. Comparisons of the substitu-
ents effects on the electrophilicities of different classes of car-
benium ions is now possible (Figure 6), and fluorenylium ions
2 can be compared with hundreds of other carbenium ions
and electrophiles reported in the Mayr reactivity scales.[10] The
E parameters for 2(a–c) (Figure 6) can be used for accurately
Fluorenylium ions as leaving/protecting groups
Due to its high robustness to hydrolysis in comparison with
the trityl group, the phenylfluorenyl scaffold 2c (R=H) has
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been widely used as protecting group.[7,22] Systematic and ex-
tended determination of the stabilities of 2(a–f) are now de-
scribed, based on the comparisons of the kinetic data reported
by Liu for the solvolysis of phenylfluorenyl-chlorides and -bro-
mides (Scheme 6)[22] with the recent data reported in the Mayr
group for the solvolysis of trityl and benzhydryl deriva-
tives.[10a,12]
+
Table 2. Electrophilicities E, electrofugalities Ef and reported pKR for the
9-phenylfluorenylium ions 2 and the tritylium ions 1.
+[a]
Carbenium ions
R
E
Ef
>1.0[b]
pKR
2a
2b
2 f
2c
2d
2e
NMe2
OMe
Me
H
ꢀ4.51
–
0.85
–
2.41
–
ꢀ1.98
ꢀ3.49
ꢀ4.50
ꢀ6.38
ꢀ4.92
ꢀ7.64
ꢀ9.43
ꢀ10.82
–
CF3
Cl
–
–
1a
1b
1 f
1c
1d
NMe2
OMe
Me
H
CF3
ꢀ7.93[c]
ꢀ1.59[c]
ꢀ0.13[c]
0.51[c]
–
–
3.88
ꢀ3.40
ꢀ5.41
ꢀ6.63
–
0.25[c]
ꢀ0.63[c]
–
1.33[c]
+
[a] pKR in H2O From Ref. [23a] for trityliums 1 and from Refs. [23b]
and [23c] for 9-phenyl fluorenyliums 2. [b] Determined in this work; see
the Supporting Information. [c] From Ref. [13b].
Scheme 6. Solvolysis of phenylfluorenyl chlorides and bromides.[22]
The nucleofugality parameters Nf and sf for the leaving
groups X were previously parametrized in a large variety of sol-
vents and solvent mixtures, according to Eq (2).[12]
1 (Table 2, Figure 7), are simultaneously 104–105 less labile
when attached to a functional group, therefore providing
a more robust protection.
log kionð25 ꢂCÞ ¼ sfðNf þ EfÞ
ð2Þ
The “tunable protecting-group strategy” proposed by Sur-
prenant and Lubell[7b] can now be rationalized on a quantitative
basis. When transforming the bromine atom of a para-bromo-
phenylfluorenyl protecting group (same Ef value as 2e consid-
ering that sp =0.23 for Br and Cl),[24] into a R2N substituent via
a nucleophilic substitution, the electrofugality of the fluorenyl
protecting group increases by a factor>106 (Ef of 2e (ꢀ4.92)
and 2a (>1.0) in Table 2). Any OH or NH2 functional groups
are therefore spontaneously deprotected by this method.
By using the reported kinetics of solvolysis (see the Support-
ing Information, part 7),[22a–c] together with the Nf and sf param-
eters for Brꢀ and Clꢀ in various solvent mixtures,[12] plots of (log
kion)/sf against Nf were constructed (Figure 7) and thus gave
access to the electrofugality parameters Ef of the five phenyl-
fluorenyls 2(b–f) (Table 2).
Comparison of the electrophilicity and electrofugality param-
eters of a large set of fluorenylium 2 and tritylium ions
1 (Table 2) now allows systematic generalization of their pro-
tecting/leaving group properties. Phenylfluorenyliums 2, which
are in average 102–103 times more electrophilic than trityliums
Lewis acidity of fluorenylium and tritylium ions
+
According to their pKR values (Table 2), phenylfluorenylium
ions 2 are stronger Lewis acids than tritylium ions 1 on aver-
age by four orders of magnitude. Since H2O is a small Lewis
+
base, the differences in pKR between 1 and 2 do not reflect
their differences in Lewis acidities with respect to larger Lewis
bases commonly employed in catalysis such as pyridines, phos-
phines, and NHCs.[25] For this purpose, we now determined the
equilibrium constants Keq of the 1:1 Lewis adducts formation
between the carbenium ions 1a and 2a with seven pyridines
16(a–g) of calibrated Lewis basicity (Scheme 7 and Table 3).[11]
Extensive UV/Vis titration experiments showed that the equi-
librium constants Keq of association of 16(a–g) with the fluore-
nylium 2a were in average 106 times greater than with the cor-
responding tritylium 1a (Table 3) (free energy difference DDG8
of almost 10 kcalmolꢀ1). The dependence of the relative Lewis
acidity strength of 1 and 2 on the steric properties of the refer-
ence Lewis base employed (H2O vs. pyridines) is similar as in
Piers landmark papers on the Lewis acidities of the triaryl
borane B(C6F5)3 17a and of the aryl borafluorene 17b
(Scheme 8),[26] where a small Lewis base such as CH3CN was in-
teracting with similar magnitude with both boranes whereas
a larger Lewis base such as THF coordinated exclusively with
17b.[26]
Figure 7. Plots of (logkion)/sf against Nf for the ionizations of para substituted
9-phenylfluorenyl chlorides and bromides in various solvents and solvent
mixtures. The kion values are taken from Ref. [22b,c] and the sf and Nf values
from Ref [12a]. See the Supporting Information for the detailed values and
the correlation equations. A=acetone, E=ethanol, W=water, M=metha-
nol.
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Scheme 7. Equilibrium between 1–3 and 16(a–g) in CH2Cl2 at 208C.
Table 3. Lewis basicity parameters for the pyridines 16 and equilibrium
constants Keq of their association with tritylium 1a and fluorenylium 2a
in CH2Cl2 at 208C.
a]
[b]
[b]
Pyridines 16, R=
LB
Keq in Mꢀ1
/
Keq in Mꢀ1
/
ꢀ
ꢀ
2a-BF4
1a-BF4
3-OAc
4-H
4-tBu
4-OMe
4-morpholino
4-NMe2
4-pyrolidino
16a 8.74
(3.45ꢁ0.10)ꢂ101 2.14ꢂ10ꢀ5[c]
16b 10.02 (1.30ꢁ0.02)ꢂ103 4.07ꢂ10ꢀ4[c]
16c 11.41 (3.68ꢁ0.03)ꢂ104 1.00ꢂ10ꢀ2[c]
16d 11.99 (1.38ꢁ0.00)ꢂ105 3.80ꢂ10ꢀ2[c]
16e 15.04 1.29ꢂ108[c]
16 f 16.42 3.09ꢂ109[c]
16g 16.82 7.76ꢂ109[c]
(5.74ꢁ0.23)ꢂ101
(8.28ꢁ0.83)ꢂ102
(2.41ꢁ0.06)ꢂ103
Figure 8. Plots of logKeq vs. Lewis basicity (LB) of 16 used for the determina-
tion of the Lewis acidities of 1a and 2a. The slopes of the correlation lines
vary from 0.9 to 1.1, but they have been forced to 1 to be able to apply
Eq (3).
[a] Lewis basicity (LB) parameters of 16 from Ref. [11a]. [b] Standard devi-
ations are shown in parentheses; see the Supporting Information for de-
tails. [c] Obtained from the correlations in Figure 8.
planar distances. Kinetic studies of their reactions of p-, s-, and
n-nucleophiles confirmed that owing to their nearly planar
structures (rather than to their antiaromaticity, cf. HOMA values
of about 0), these tertiary carbenium ions are featuring similar
steric as is benzhydrylium ions 3, and can therefore be inserted
in the Mayr reactivity scales. The electrophilicity of 2 surpasses
those of tritylium ions 1 by at least a factor of 103, which can
be useful for developing new hydride abstracting reagents, or
new coordination polymerization co-catalysts.
Phenylfluorenyls which are more robust protecting group
than trityls, can be subject to a powerful “electrofugality
tuning”. Replacement of a p-Br by a p-NMe2 substituent in-
creases their electrofugalities by more than 5 orders of magni-
tude, thereby inducing a spontaneous functional group re-
lease.
Most importantly, phenylfluorenylium ions are 106 times
stronger Lewis acids than tritylium ions with respect to para
substituted-pyridines, which may open perspectives for using
them as carbon-centered Lewis acids, or as constituent of frus-
trated Lewis pairs catalysts in combination with a highly steri-
cally hindered pyridines or phosphines. The determination of
the Lewis acidities of other tritylium and phenylfluorenylium
ions and of their association constants with bulky Lewis bases
are in progress.
Scheme 8. Triarylborane B(C6F5)3 17a and borafluorene derivative 17b.
By using seven reference pyridines 16(a–g) of variable Lewis
basicity strength, and applying Eq. (3),[11] linear correlation be-
tween logKeq a Lewis basicity (LB) were obtained (Figure 8),
giving straightforward access to the Lewis acidity parameters
LA of 2a (LA=ꢀ6.93) and 1a (LA=ꢀ13.41).
log Keq ¼ LA þ LB
ð3Þ
The phenylfluorenylium ions are thus 106 times stronger
Lewis acids than trityliums with respect to pyridines. These
very strong carbon-centered Lewis acids could therefore, in
combination with even more sterically hindered Lewis bases
(ortho-substituted pyridines or phosphines) open new avenues
for designing alternative types of frustrated Lewis pairs and
could find applications as potent Lewis acid catalysts in syn-
thesis.
Acknowledgements
The authors thank Prof. Herbert Mayr for invaluable support,
Dr. Armin R. Ofial, and Dr. Sami Lakhdar (Caen, France) for help-
ful discussions. Prof. S. Kobayashi and I. Teikmane (LMU
Munich, Germany) are acknowledged for their help for the
Conclusion
X-ray structural studies revealed that stabilized 9-phenylfluore-
nylium ions 2 are crystallizing as dimer pairs with short inter-
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Keywords: carbocations · electrophilicity scales · Lewis acid
catalysts · Lewis acidity scales · protecting groups
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Received: August 20, 2016
Published online on && &&, 0000
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FULL PAPER
&
Carbocations
E. Follet, P. Mayer, G. Berionni*
&& – &&
Structures, Lewis Acidities,
Electrophilicities, and Protecting
Group Abilities of Phenylfluorenylium
and Tritylium Ions
The reactivities of phenyl-fluorenylium
ions toward nitrogen- and phosphorus-
centered Lewis bases and the struc-
trures of their Lewis adducts are report-
ed. Kinetic and thermodynamic investi-
gations allowed to compare the Lewis
acidities, electrophilicities and protect-
ing group abilities of fluorenylium ions
with those of tritylium and other carbe-
nium ions.
Chem. Eur. J. 2016, 22, 1 – 9
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