Synthesis and Isolation of Polytrityl Cations by Utilizing Hexaphenylbenzene and Tetraphenylmethane Scaffolds

Article abstract of DOI:10.1021/jo035598i

The successful isolation of stable (and soluble) hexa- and tetratrityl cations based on hexaphenyl-benzene and tetraphenylmethane scaffold has been accomplished by using readily available starting materials. These robust polytrityl cations can be isolated in crystalline form and stored indefinitely at 0 °C. Their structures have been established by ¹H/ ¹³C NMR spectroscopy and by UV-vis spectroscopy. The structures of these polytrityl cations were further confirmed by quantitative transformations to the reduced (poly)triphenylmethyl derivatives by hydride transfer from triethylsilane, cycloheptatriene, or a borane-dimethyl sulfide complex.

Full text of DOI:10.1021/jo035598i

Syn th esis a n d Isola tion of P olytr ityl Ca tion s by Utilizin g
Hexa p h en ylben zen e a n d Tetr a p h en ylm eth a n e Sca ffold s
Rajendra Rathore,* Carrie L. Burns, and Ilia A. Guzei‡
Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, Wisconsin 53201-1881
rajendra.rathore@marquette.edu
Received October 30, 2003
The successful isolation of stable (and soluble) hexa- and tetratrityl cations based on hexaphenyl-
benzene and tetraphenylmethane scaffold has been accomplished by using readily available starting
materials. These robust polytrityl cations can be isolated in crystalline form and stored indefinitely
at 0 °C. Their structures have been established by 1H/13C NMR spectroscopy and by UV-vis
spectroscopy. The structures of these polytrityl cations were further confirmed by quantitative
transformations to the reduced (poly)triphenylmethyl derivatives by hydride transfer from
triethylsilane, cycloheptatriene, or a borane-dimethyl sulfide complex.
In tr od u ction
and its counteranion affect the catalytic activity, stability,
and, to a lesser extent, ability to stereoregulate the
polymerization reactions. A variety of nonnucleophilic
anions6 have also been engineered in an important
endeavor to improve catalytic activity of the trityl cation
in a variety of olefin polymerization reactions.5
The highly stable triphenylmethyl (or trityl) cation and
anion have been utilized in many facets of chemistry
including in organic syntheses since the beginning of the
twentieth century. The high stability of the trityl ions
can be attributed to the extensive delocalization of the
charge onto the phenyl rings via resonance. The trityl
cation and anion are also of fundamental importance in
the study of single-electron transfer (SET) mechanisms
as they are formed by transfer of 1 electron to or from
the triphenylmethyl radical, i.e. eq 1. All three stable
derivatives of trityl (i.e. cation, anion and radical) have
played important roles in organic synthesis as well as in
investigation of reaction mechanisms.1,2
Recently, Olah et al.7 prepared a tetrahedrally arrayed
tetracation that was generated from an alcoholic precur-
sor using magic acid (FSO3H-SO2ClF) and found that,
unfortunately, it was stable only at low temperatures,
i.e., eq 2. As such the polycationic system shown in eq 2
and its stable analogues could play a significantly
important role in materials science8 and/or in develop-
Possibly the most extensive usage of the trityl cation
is found in hydride abstraction reactions both in organic
syntheses and in investigation of mechanisms of organic
and organometallic transformations.3 Trityl cations have
also found widespread application as one-electron oxi-
dants,4 and as highly effective activators for olefin
polymerization reactions.5 Recently, Marder et al.5c in-
vestigated how ion-ion interactions in the trityl cation
(4) (a) Fukuzumi, S.; Ohkubo, K.; Otera, J . J . Org. Chem. 2001, 66,
1450 and references therein. (b) Hart, H.; Sulzberg, T.; Schwendeman,
R. H.; Young, R. H. Tetrahedron Lett. 1967, 15, 1337. (c) Connelly, N.
G.; Geiger, W. E. Chem. Rev. 1996, 96, 877. (d) Fukuzumi, S.; Kitano,
T.; Ishikawa, M. J . Am. Chem. Soc. 1990, 112, 5631.
(5) (a) Hou, Z.; Kaita, S.; Wakatsuki, Y. Pure Appl. Chem. 2001,
73, 291. (b) Baird, M. C.; Reid, S. J . Organometallics 2002, 21, 2325.
(c) Williams, V. C.; Irvine, G. J .; Piers, W. E.; Li, Z.; Collins, S.; Clegg,
W.; Elsegood, M. R.; Marder, T. B. Organometallics 2000, 19, 1621.
(d) Shafir, A.; Arnold, J . Organometallics 2003, 22, 567 and references
therein.
‡ Current address: Molecular Structure Laboratory, Department of
Chemistry, University of Wisconsin, Madison, WI.
(1) (a) Shchepinov, M. S.; Korshun, V. A. Chem. Soc. Rev. 2003, 32,
170. (b) Plack, V.; Schmutzler, R. Phosphorus, Sulfur Silicon Relat.
Elem. 1999, 144-146, 273. (c) Kobayashi, S. Kagaku Kogyo 1989, 42,
245. (d) Wu, J .-T. Petrochemistry 1974, 13, 19.
(2) (a) Baird, M. C.; Reid, S. J . Organometallics 2002, 21, 2325. (b)
Ru¨chardt, C. Top. Curr. Chem. 1980, 88, 1 and references therein. (c)
House, H. O.; Trost, B. M. J . Org. Chem. 1965, 30, 1341.
(3) (a) Cox, R. A. Org. React. Mech. 1991, 283. (b) Corma, A.; Garcia,
H. Top. Catal. 1998, 6, 127. (c) Bullock, R. M.; Cheng, T.-Y. Organo-
metallics 2002, 21, 2325 and references therein. (d) Dauben, H. J ., J r.;
Bertelli, D. J . J . Am. Chem. Soc. 1961, 83, 4657 and 4659.
(6) (a) Reed, C. A. Acc. Chem. Res. 1998, 31, 133. (b) Ivanov, S. V.;
Davis, J . A.; Miller, S. M.; Anderson, O. P.; Strauss, S. H. Inorg. Chem.
2003, 42, 4489.
(7) (a) Head, N. J .; Prakash, G. K.; Bashir-Hashemi, A.; Olah, G. A.
J . Am. Chem. Soc. 1995, 117, 12005. (b) Heagy, M. D.; Wang, Q.; Olah,
G. A.; Prakash, G. K. J . Org. Chem. 1995, 60, 7351.
(8) (a) MacDonald, R. A.; Whitesides, G. M. Chem. Rev. 1994, 94,
2383. (b) Desiraju, G. R. Crystal Engineering: The design of Organic
Solids; Elsevier: New York, 1989; p 54. (c) Zyss, J .; J osse, D.; Ledoux,
I. MCLC S&T, Sect. B: Nonlinear Opt. 1994, 7, 169. (d) Allen, N. S.
Polym. Degrad. Stab. 1994, 44, 357.
10.1021/jo035598i CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/31/2004
1524
J . Org. Chem. 2004, 69, 1524-1530

Synthesis and Isolation of Polytrityl Cations
SCHEME 1
ment of multidentate catalytic systems for the polymer-
ization reactions.5 Owing to the extensive utility of the
trityl cation, it was conceived to design molecules con-
taining multiple trityl sites which can serve for many of
the same purposes described above, as well as the new
usages (vide infra).
The ready availability of hexaphenylbenzene and tet-
raphenylmethane cores, and the ease of modification of
their vertexes to incorporate suitable electroactive func-
tionalities, makes them attractive starting points (or
platforms) for the construction of nanometer-sized den-
dritic materials with novel light-emitting and charge-
transport properties.9,10 Accordingly, herein we now
report the successful preparation of stable tetra- and
hexatrityl cations by making use of tetraphenylmethane
and hexaphenylbenzene platforms from readily available
starting materials. The structures of these isolated
(stable) polytrityl cations have been established by NMR
and UV-vis spectroscopy, and further confirmed by
quantitative hydride transfer reactions with cyclohep-
tatriene, borane-dimethyl sulfide, complex, or triethyl-
silane (as hydride donors) to the reduced (poly)triphen-
ylmethyl derivatives as follows.
SCHEME 2a
Resu lts a n d Discu ssion
Syn th esis of Alcoh olic P r ecu r sor s for th e P oly-
tr ityl Ca tion s Der ived fr om Hexa p h en ylben zen e
a n d Tetr a p h en ylm eth a n e Cor es. The starting mate-
rial tetrakis(4-bromophenyl)methane (2) for the synthesis
of tetrakis(4,4-diphenyl-4-hydroxymethylphenyl)methane
(3), the precursor to the tetratrityl cation, was available
from a facile bromination of readily available tetraphen-
ylmethane11 (1) using neat bromine.12 Lithiation of 2 with
n-butyllithium (or tert-butytllithium) at -78 °C in dry
diethyl ether followed by a reaction with benzophenone
afforded a mixture of desired tetraalcohol 3 (∼60%)
together with a product arising from an addition of
benzophenone to dilithiated 2 (4, ∼40%), i.e., Scheme 1.
The desired symmetrical tetraalcohol 3 was easily
isolated by chromatographic separation in greater than
>55% yield and was characterized by 1H/13C NMR
spectroscopy as well as by the presence of a characteristic
O-H stretching in its IR spectrum, and by elemental and
mass spectrometric analyses (see the Experimental Sec-
tion). The ease of the synthesis of 3 from readily available
tetrakis(4-bromophenyl)methane 2 prompted a similar
approach for the preparation of hexaalcohol 7, a precursor
to the hexatrityl cation (see Scheme 2), from the corre-
sponding hexakis(4-bromophenyl)benzene (6) as follows.
a
Reagents and conditions: (a) neat bromine, 22 °C, 96%. (b)
n-butyllithium, -78 °C, benzophone, H2O.
Thus, hexaphenylbenzene (5) treated with neat bro-
mine at room temperature followed by trituration with
cold ethanol afforded hexakis(4-bromophenyl)benzene (6)
as a colorless solid in a nearly quantitative yield.12
Lithiation of readily available 6 with n-butyllithium in
diethyl ether at -78 °C followed by the reaction with
benzophenone afforded a colorless solid. A chromato-
graphic purification and structural characterization with
1H/13C NMR spectroscopy indicated that the symmetrical
molecule obtained above was 1,3,5-tris(4-bromopheny)-
2,4,6-tris(4,4-diphenyl-4-hydroxymethylphenyl)benzene
(8) and not the expected hexaalcoholic derivative 7 (see
Scheme 2).
(9) (a) Yeh, C.-H.; Lee, R.-H.; Chan, L.-H.; Lin, T.-Y.; Chen, C.-T.;
Balasubramaniam, E.; Tao, Y.-T. Chem. Mater. 2001, 13, 2788. (b)
Zimmermann, T. J .; Freundel, O.; Gompper, R.; Muller, J . J . J . Eur.
J . Org. Chem. 2000, 3305. (c) Keegstra, M. A.; Feyter, S. D.; Schryver,
F. C. D.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 774. (d)
Oldham, W. J ., J r.; Lachicotte, R. J .; Bazan, G. C. J . Am. Chem. Soc.
1998, 120, 2987. (e) Wang, S.; Oldham, W. J ., J r.; Hudock, R. A., J r.;
Bazan, G. C. J . Am. Chem. Soc. 2000, 124, 5695 and references therein.
(f) Berresheim, A. J .; Mu¨ller, M.; Mu¨llen, K. Chem. Rev. 1999, 99, 1747.
(g) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. Rev. 2001, 101,
1267 and references therein.
(10) (a) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Lett. 2001,
3, 2887. (b) Rathore, R.; Burns, C. L.; Abdelwahed, S. A. Org. Lett.
submitted for publication.
(11) (a) Wassmundt, F. W.; Kiesman, W. F. J . Org. Chem. 1995, 6,
1713. (b) Zimmermann, T. J .; Muller, T. J . J . Synthesis 2002, 9, 1157.
(12) Burns, C. L.; Rathore, R. Org. Synth. Submitted for publication.
The surprisingly selective lithiation of the alternate
4-bromophenyl groups (i.e. 1, 3, 5 positions) in 6 is
J . Org. Chem, Vol. 69, No. 5, 2004 1525

Rathore et al.
SCHEME 3a
a
Reagents and conditions: (a) (PPh3)2PdCl2, CuI, CH3CN/
piperdine, reflux; (b) Co2(CO)8, dioxane, reflux; (c) phenyl lithium,
0 °C, H2O.
zophenone with gaseous acetylene in 65% yield, was
trimerized in refluxing dioxane in the presence of dico-
baltoctacarbonyl as a catalyst to the previously unknown
hexakis(4-benzoylphenyl)benzene (11). A reaction of ex-
cess phenyllithium with 11 afforded the hexaalcohol 7
in almost quantitative yield. The simplicity of 1H and 13C
NMR spectra together with a characteristic O-H stretch-
ing frequency at 3554 cm-1 (and the lack of absorptions
due to the carbonyl groups) in the IR spectrum readily
confirmed the structure of desired hexaalcohol 7 (see the
Experimental Section).
F IGURE 1. An ORTEP diagram of 9 confirming the selective
lithiation/alkylation of hexakis(4-bromophenyl)benzene.
hitherto unknown, and was further confirmed by the
reduction of 1,3,5-tris(4-bromophenyl)-2,4,6-tris(4,4-diphen-
yl-4-hydoxymethylphenyl)benzene (8) with triethylsi-
lane13 (as a hydride donor) in the presence of methane-
sulfonic acid in dichloromethane at 0 °C (vide infra) to 9
in excellent yield, i.e. (eq 3). The structure of the reduced
Gen er a tion a n d Sp ectr oscop ic An a lysis of Hexa -
a n d Tetr a tr ityl Ca tion s fr om Alcoh olic P r ecu r sor s
3 a n d 7 in Solu t ion . The polytrityl cations from
alcoholic precursors 3 and 7 were readily generated in
solution by reaction with a strong acid, such as trifluo-
roacetic acid, methanesulfonic acid, or tetrafluoroboric
acid, in dichloromethane at 22 °C, e.g. eq 4. The highly
9 was established by 1H/13C NMR and mass spectrometry
(see the Experimental Section) as well as being further
confirmed by X-ray crystallography14 (see Figure 1).
Further lithiation of 8 by with n-butyllithium (or tert-
butyllithium) followed by a reaction with benzophenone
yielded a complex mixture of products from which only
<10% of hexaalcohol 7 (∼90% pure as judged by 1H NMR
spectroscopy) could be isolated by tedious chromato-
graphic separation. Clearly, a new approach was neces-
sary for a successful preparation of the desired hexatrityl
precursor 7.
It was conceived that an addition of excess phenyl-
lithium to hexakis(4-benzoylphenyl)benzene 11 (see
Scheme 3) may allow direct access to 7. Thus, bis(4-
benzoylphenyl)acetylene (10), easily obtained by a Pd-
catalyzed coupling of commercially available 4-bromoben-
colored (and soluble) tetra- and hexatrityl cations (34+
and 76+) were (UV-vis) spectroscopically identical ir-
respective of the acid used (vide infra). Moreover, the
dark-colored solutions of the polytrityl cations 34+ and
7
6+ were found to be stable for prolonged periods at room
temperature. The robustness of these polytrityl cations
1
allowed their structures to be established by H and 13C
NMR spectroscopy at ambient temperatures as follows.
NMR Sp ectr oscop y. Thus, Figure 2 shows the 1H
NMR spectra of tetratrityl cation 34+, hexatrityl cation
7
6+, and for the sake of comparison, the spectrum of the
triphenylmethyl cation (12+) together with their alcoholic
precursors 3, 7, and triphenylmethanol (12). The chemi-
cal shifts of various protons in trityl cations as well as of
their alcoholic precursors are also listed in Table 1. The
(13) (a) Lambert, J . B.; Zhang, S.; Stern, C. L.; Huffman, J . C.
Science 1993, 260, 1917. (b) Also see: Badejo, I. T.; Karaman, R.; Fry,
J . L. J . Org. Chem. 1989, 54, 4591.
(14) A precise structure of 9 could not be determined due to the
disordered solvent molecules.
1526 J . Org. Chem., Vol. 69, No. 5, 2004

Synthesis and Isolation of Polytrityl Cations
F IGURE 3. The UV-vis absorption spectra of various trityl
cations in dichloromethane at 22 °C.
parent trityl cation have been assigned as follows, δ )
8.26 (triplet, the para ring protons), δ ) 7.88 (triplet, the
meta ring protons), and δ ) 7.67 (doublet, the ortho ring
protons) ppm, and were consistent with the earlier
1
assignment of Farnum.15 In a similar vein, the H NMR
chemical shifts for the hexatrityl 76+ and tetratrityl 34+
cations can be assigned as indicated in Figure 2 and
compiled in Table 1. A further structural confirmation
of these symmetrical polytrityl cations (34+ and 76+) was
obtained by observation of a single distinctive peak, for
the positively charged carbon, in their 13C NMR spectra
at 209.73 ppm, triphenylmethyl cation 12+; 208.15 ppm,
3
4+; and 208.22 ppm, 76+
UV-Vis Sp ectr a l An a lysis of Hexa - a n d Tetr a tr i-
.
tyl Ca tion s 34+ a n d 76+. The UV-vis absorption spectra
of the various cations prepared as above are shown in
Figure 3. The parent trityl cation showed a twin absorp-
tion band at λmax ) 410 and 432 nm and was the same
as reported previously.16 The UV-vis absorption spectra
of tetracation 34+ (λmax ) 430 and 474 nm) and hexacation
76+ (λmax ) 434 and 481 nm) showed red-shifted (twin)
absorption bands as compared to the parent trityl cation
(see Figure 3). The extensive distortion and the red shift
in the absorption bands of the hexacation 76+ may be
attributed to the electronic interactions between various
trityl groups via the propeller of the hexaphenyl ring
system.10b Note that one of the phenyl rings of each trityl
unit is also part of the propeller of the hexaphenylben-
zene core.
Quantitative UV-vis spectroscopic analysis of the
highly colored solutions of 34+, 76+, and 12+ allowed the
determination of the molar extinction coefficients of the
multiply charged cations in comparison to the parent
trityl cation 12+ and the values are listed in Table 2. The
measured extinction coefficient of 35 ,300 ( 200 M-1 cm-1
for the triphenylmethyl cation (12+) was found to be in
good agreement with that of the reported value.16 Inter-
estingly, a comparison of the extinction coefficient of the
monocation (12+) with that of the hexacation 76+ showed
F IGURE 2. 1H NMR spectra of various trityl tetrafluorobo-
rate salts and their alcoholic precursors in CD2Cl2 at 22 °C.
TABLE 1. Com p a r ison of th e 1H NMR Ch em ica l Sh ifts
(p p m ) of Va r iou s Tr ityl Ca tion s a n d Th eir Alcoh olic
P r ecu r sor s in CD2Cl2
alcoholic precursor
tritiyl cationa
12
3
7.20-7.31 (m, 15H)
2.80 (s, 1H, -OH)
12+
34+
7.67 (d, J ) 7.6 Hz, 6H)
7.88 (t, J ) 7.6 Hz, 6H)
8.26 (t, J ) 7.7 Hz, 3H)
7.66-7.79 (m, 24H)
7.88 (d, J ) 8.4 Hz, 16H)
8.04 (d, J ) 8.4 Hz, 8H)
8.25 (t, J ) 7.2 Hz, 8H)
7.47 (br d, J ) 8.0 Hz, 36H)
7.71 (t, J ) 7.4 Hz, 24H)
7.76 (d, J ) 8.2 Hz, 12H)
8.14 (t, J ) 7.4 Hz, 12H)
7.03 (sym m, 16H)
7.14-7.24 (m, 40H)
2.75 (s, 4H, -OH)
7
6.78 (sym m, 24H)
7.02-7.09 (m, 24H)
7.20-7.25 (m, 36H)
2.82 (s, 6H, -OH)
76+
a
The cations were generated causing excess HBF4‚OEt2.
simplicity and similarity of the proton spectra of the
tetra- and hexatrityl cations to that of the parent trityl
cation 12+ confirmed the formation of highly symmetrical
34+ and 76+. The proton NMR chemical shifts for the
(15) (a) Farnum, D. G. J . Am. Chem. Soc. 1964, 86, 934. (b) Also
see: Farnum, D. G. Adv. Phys. Org. Chem. 1975, 11, 123.
(16) Shida, T. Electronic Absorption Spectra of radical Ions; Elsevi-
er: Amsterdam, The Netherlands, 1988.
J . Org. Chem, Vol. 69, No. 5, 2004 1527

Rathore et al.
TABLE 2. UV-Vis Absor p tion Da ta for Va r iou s Tr ityl
Ca tion s in CH2Cl2 Gen er a ted fr om th e Cor r esp on d in g
Alcoh olic P r ecu r sor s w ith HBF 4 a t 22 °C
trityl cationa
λmax (nm)
ꢀmax (M-1 cm-1
)
12+
34+
76+
410, 432
430, 474
434, 481
35 300 ( 300
133 000 ( 500
177 600 ( 500
the latter to be approximately six times the value of the
model compound. Moreover, the value of the extinction
coefficient for the tetracation 34+ was roughly four times
that of the parent trityl cation. As such, the comparison
of the molar extinction coefficients of polytrityl cations
3
4+ and 76+ to that of the model trityl cation (12+) further
supports the structural identity of these polycations
arrived at by NMR spectroscopy.
Encouraged by the stability of these polytrityl cations
in solution, attempts were made for their isolation in the
solid state as follows. Thus, a solution of tetraalcohol 3
in dichloromethane was treated with a 20% excess
tetrafluoroboric acid/diethyl ether complex at 0 °C. The
resulting dark red solution was layered with dry hexanes
and was stored in a refrigerator (-25 °C) for 24 h. The
dark-colored microcrystalline precipitate of the tetra-
cation [34+ (BF4-)4] thus obtained was filtered (using a
cintered glass funnel) and washed with dry hexanes and
dried in vacuo (yield: 73%). The UV-vis absorption
spectrum as well as the 1H NMR spectrum of the
resulting crystalline tetracation in CD2Cl2 were identical
with those obtained above (see Figures 2 and 3). Simi-
larly, the hexatrityl cation [76+ (BF4-)6] can be isolated
in 78% yield; and both of these polytrityl cations were
stable for prolonged periods if stored at 0 °C. However,
repeated attempts to obtain single crystals suitable for
X-ray crystallography of [34+ (BF4-)4] and [76+ (BF4-)6]
were thus far unsuccessful.
Ch em ica l Rea ctivity of Hexa - a n d Tetr a tr ityl
Ca tion s 34+ a n d 76+. An independent confirmation of the
structures of the crystalline polytrityl cations was also
obtained by their clean and quantitative reduction to the
(poly)triphenylmethyl derivatives (see the Experimental
Section) by using triethylsilane, borane-dimethyl sulfide
complex, or cycloheptatriene as a hydride donor. For
example, treatment of a dark-red solution of microcrys-
talline hexatrityl cation [76+ (BF4-)6] (prepared as above)-
in dichloromethane at 0 °C with 6 equiv of triethylsilane
resulted in the immediate decolorization of the orange-
red solution. A standard aqueous workup of the resulting
solution afforded a white solid that was characterized by
various spectroscopic methods and by elemental analysis
(see the Experimental Section), and was shown to be
hexakis(4-diphenylmethylphenyl)benzene 13, i.e. eq 5. A
similar treatment of 76+ and 34+ with borane-dimethyl
sulfide complex or triethylsilane in dichloromethane
afforded the corresponding reduced (poly)diphenylmethyl
derivatives (13 and 14, respectively) in quantitative
yields. Moreover, reaction of a dichloromethane solution
of 76+ with cycloheptatriene afforded a colorless solution
within 10 min. Addition of hexanes precipitated tropy-
lium tetrafluoroborate salt, which was filtered, and the
filtrate was shown to contain the reduced 13 in almost
quantitative yield, i.e., eq 6. The identity of the tropylium
1
salt17 was confirmed by comparison of the H/13C NMR
spectra with that of an authentic sample.
Su m m a r y. In summary, a successful synthesis and
isolation of hexa- and tetratrityl cations has been ac-
complished by using readily available starting materials,
and their structures have been established by 1H/13C
NMR and UV-vis spectroscopy as well as by quantitative
hydride transfer from borane-dimethyl sulfide complex,
triethylsilane, or cycloheptatriene. We are now actively
exploring the cocrystallization of these polytrityl cations
with a variety of nonnucleophilic anions6 for usage as
catalysts for polymerization reactions as well as with
redox-active spherical polyoxometalate (poly)anions18 to
exploit their material properties. These polytrityl cations
also hold potential for the preparation of polytrityl
radicals, which may possess interesting magnetic proper-
ties.19
Exp er im en ta l Section
Tetr a k is(4-br om op h en yl)m eth a n e (2). A 500-mL, two-
necked flask equipped with a magnetic stirring bar and an
outlet adapter connected via rubber tubing to a pipet that is
immersed in an aqueous sodium hydroxide solution (10%, 250
mL) is charged with 18 g (50 mmol) of powdered tetraphenyl-
methane (1). Neat bromine (0.35 mol, 20 mL) is added slowly
during a 5-min period while the reaction mixture is stirred.
The reaction starts immediately as judged by an evolution of
gaseous hydrobromic acid. After the addition of bromine, the
dark-orange slurry is stirred for an additional 20 min and the
resulting slurry is poured into ethanol (250 mL) cooled in a
dry ice-acetone bath. The precipitated solid was filtered and
washed with an aqueous sodium bisulfite solution. The crude
material was recrystallized from chloroform/ethanol to afford
pure 2 as a crystalline solid in nearly quantitative yield (34.8
g, 97%); mp 312.5-313 °C (lit.20 mp 312-313 °C); 1H NMR
(17) Conrow, K. Org. Synth. 1963, 43, 101. Also see: Conrow, K. J .
Am. Chem. Soc. 1959, 81, 5461.
(18) (a) Hill, C. L. Chem. Rev. 1998, 98, 1. (b) Pope, M. T. Heteropoly
and Isopoly Oxometalates; Springer-Verlag: Berlin, Germany, 1983.
(19) Bushby, R.-J . Magnetism: Molecules to Materials; Wiley-
VCH: New York, 2001; Vol II, p 149.
1528 J . Org. Chem., Vol. 69, No. 5, 2004

Synthesis and Isolation of Polytrityl Cations
(CDCl3) δ 7.01 (d, J ) 6 Hz, 8 H), 7.39 (d, J ) 6 Hz, 8 H); 13
NMR (CDCl3) δ 65.9, 120.6, 130.8, 132.1, 144.0.
C
(m, 24 H) 6.96-7.05 (m, 12 H), 7.22-7.30 (m, 18 H); 13C NMR
(CDCl3) δ 81.7, 119.7, 126.8, 127.2, 127.86, 127.89, 130.0, 130.6,
132.9, 138.9, 139.1, 139.4, 140.2, 144.2, 146.7; IR (KBr) 3543.5
(m), 3465.6 (m), 3057.2 (m), 3028.2 (m), 1488.0 (s), 1446.2 (s),
1010.9 (s), 830.4 (m), 755.2 (s), 699.2 (s) cm-1; FAB m/z 1318
Tet r a k is(4-d ip h en yl-4-h yd r oxym et h ylp h en yl)m et h -
a n e (3). To a cooled (-78 °C) solution of tetrakis(4-bromophen-
yl)methane (2; 2.20 g, 3.46 mmol) in anhydrous diethyl ether
(35 mL) was added dropwise n-butyllithium (2.5 M in hexane,
6.92 mL, 17.3 mmol) under an argon atmosphere. The result-
ing reaction mixture was allowed to warm to -30 °C over a
1-h period, and then stirred at -30 °C for an additional 1 h
before adding solid benzophenone (3.15 g, 17.3 mmol). The
reaction mixture was stirred for 12 h and quenched by water.
The organic products were extracted with CH2Cl2 (3 × 50 mL)
and dried over anhydrous magnesium sulfate. The crude
product was purified by flash chromatography with a 10%
ethyl acetate/90% hexanes mixture to afford 3 and 4 as
colorless solids. The spectral data are given as follows. Tet-
rakis(4-diphenyl-4-hydroxymethylphenyl)methane (3): yield
58%; mp 220 °C (acetone); 1H NMR (CDCl3) δ 2.75 (s, 4H),
7.03 (sym m, 16H), 7.14-7.24 (m, 40H); 13C NMR (CDCl3) δ
64.2, 82.0, 127.0, 127.5, 127.7, 130.3, 132.5, 144.3, 144.6, 146.2;
IR (KBR) 3456.7 (m), 3055.9 (m), 3026.5 (m), 1490.0 (s), 1446.2
(s), 1010.1 (s), 821.1 (m), 759.7 (s), 699.1 (s) cm-1; FAB m/z
1049 (M+), 1049 calcd for C77H60O4. Anal. Calcd for C77H60O4:
C, 88.14; H, 5.76; O, 6.10. Found: C, 88.32; H, 5.46. Bis(4-
br om oph en yl)bis(4,4-diph en yl-4-h ydr oxym et h yl-ph en yl)-
methane (4): yield 36%; mp 290 °C (dichloromethane-hex-
anes); 1H NMR (CDCl3) δ 2.79 (s, 2H), 7.00-7.12 (m, 16H),
71.9-7.33 (m, 40H); 13C NMR (CDCl3) δ 64.1, 82.0, 126.91,
127.0, 127.5, 127.7, 130.3, 132.5, 144.2, 144.5, 146.2; IR (KBr)
3457.2 (m), 3055.8 (m), 3027.4 (m), 1490.2 (s), 1446.0 (s), 1010.6
(s), 821.3 (m), 760.0 (s), 699.4 (s) cm-1; FAB m/z 842 (M+), 842
calcd for C51H38O2Br2. Anal. Calcd for C51H38O2Br2: C, 72.69;
H, 4.55; Br, 18.96; O, 3.80. Found: C, 72.48; H, 4.36.
Hexa k is(4-br om op h en yl)ben zen e (6). A 500-mL, two-
necked flask equipped with a magnetic stirring bar and an
outlet adapter connected via rubber tubing to a pipet that is
immersed in an aqueous sodium hydroxide solution (10%, 250
mL) is charged with 26.7 g (50 mmol) of powdered hexaphen-
ylbenzene (5). Neat bromine (0.69 mol, 40 mL) is added slowly
during a 5-min period while the reaction mixture is stirred.
The reaction starts immediately as judged by an evolution of
gaseous hydrobromic acid. After the addition of bromine, the
dark-orange slurry is stirred for an additional 45 min and the
resulting slurry is poured into ethanol (500 mL) cooled in a
dry ice-acetone bath. The precipitated product was filtered
and washed with an aqueous sodium bisulfite solution. The
crude material was recrystallized from tetrahydrofuran/hex-
anes to afford pure 6 as a colorless solid in nearly quantitative
yield (96%): mp 355-357 °C; 1H NMR (CDCl3) δ 6.61 (d, J )
8.1 Hz, 12H), 7.07 (d, J ) 8.1 Hz, 12H); 13C NMR (CDCl3) δ
120.1, 130.2, 132.2, 138.0, 139.2; FAB m/z 1008 (M+), 1008
calcd for C42H24Br6. Anal. Calcd for C42H24Br6: C, 50.04; H,
2.40; Br, 47.56. Found: C, 50.25; H, 2.37.
(M+), 1318 calcd for C81H57Br3O3. Anal. Calcd for C81H57
-
Br3O3: C, 73.81; H, 4.36; Br, 18.19; O, 3.64. Found: C, 73.96;
H, 4.26.
1,3,5-Tr is(4-br om oph en yl)-2,4,6-tr is(4-diph en yl-4-m eth -
ylp h en yl)ben zen e (9). To a cooled (∼0 °C) solution of 1,3,5-
tris(4-bromophenyl)-2,4,6-tris(4-diphenyl-4-hydroxymethyl-
phenyl)benzene (0.244 g, 0.186 mmol) in anhydrous dichlo-
romethane (20 mL) was added a diethyl ether solution of
tetrafluoroboric acid (0.13 mL, 1.8 mmol) under an argon
atmosphere. The resulting dark-red solution (UV-vis, λmax
)
418 and 513 nm; ꢀ418 ) 99 000 ( 1 000 M-1 cm-1) was stirred
for 10 min before the addition of the borane-dimethyl sulfide
complex (1 M in CH2Cl2, 1 mL, 1 mmol). After being stirred
for 10 min, the colorless reaction mixture was quenched with
saturated aqueous sodium bicarbonate solution (20 mL) and
diluted with dichloromethane (50 mL). The organic layer was
separated and washed with water (3 × 25 mL) and dried over
anhydrous magnesium sulfate and evaporated. The resulting
solid was crystallized from a dichloromethane/hexanes mix-
1
ture. Yield 74%; mp 280-281 °C; H NMR (CDCl3) δ 5.44 (s,
3 H), 6.67-6.79 (m, 18 H), 6.88-7.00 (m, 12 H), 7.09-7.11
(m, 6 H), 7.20-7.28 (m, 6 H), 7.31-7.36 (m, 12 H); 13C NMR
(CDCl3) δ 56.2, 120.0, 126.6, 128.7, 130.0, 130.4, 131.5, 133.3,
138.3, 139.9, 140.8, 141.8, 144.4; FAB m/z 1270 (M+), 1270
calcd for C81H57Br3. Anal. Calcd for C81H57Br3: C, 76.60; H,
4.52; Br, 18.87. Found: C, 76.51 H, 4.48.
Bis(4-ben zoylp h en yl)a cetylen e (10). Triphenylphosphine
(3.15 g, 12 mmol), 4-bromobenzophenone (19.60 g, 75 mmol),
piperidine (30 mL, 305 mmol), and acetonitrile (30 mL, 580
mmol) were placed in a three-necked, round-bottomed flask
that was repeatedly evacuated and filled with argon. Copper
iodide (0.5 g, 2.6 mmol) and bis(triphenylphosphine)palladium-
(II) dichloride (0.53 g, 0.75 mmol) were added to the reaction
mixture and pure acetylene gas was bubbled into the yellow
mixture for 6 h while refluxing. The reaction mixture was
further refluxed for 12 h, cooled to 22 °C, and evaporated in
vacuo. The dark crude solid was triturated with dichlo-
romethane (5 × 50 mL) and the desired product precipitated
in the organic layer upon standing. The microcrystalline
precipitate of 10 was filtered and recrystallized further from
a dichloromethane/hexanes mixture. Yield 85%; mp 228-230
°C (hexanes-dichloromethane); 1H NMR (CDCl3) δ 7.51 (sym
m, 4H), 7.61 (sym m, 2H), 7.67 (sym m, 4H), 7.79-7.83 (m,
8H); 13C NMR (CDCl3) δ 91.6, 126.6, 128.1, 129.7, 129.8, 131.9,
132.4, 136.8, 136.9, 195.0; IR (KBr) 2240.2 (w), 1646.8 (s),
1603.7 (m), 1444.6 (m), 1288.1 (m), 1144.6 (w), 922.1 (w), 789.6
(w), 739.6 (m), 691.4 (s), 680.7 (m) cm-1; GCMS m/z 386 (M+),
386 calcd for C28H18O2. Anal. Calcd for C28H18O2: C, 87.02; H,
4.69; O, 8.28. Found: C, 86.89; H, 4.53.
Hexa k is(4-ben zoylp h en yl)ben zen e (11). Octacarbonyl-
dicobalt (50 mg, 0.147 mmol) and bis(4-benzoylphenyl)acety-
lene (4.0 g, 10.3 mmol) were placed in a 100-mL Schlenk flask
under an argon atmosphere. After addition of 1,4-dioxane (120
mL), the reaction was refluxed for 12 h. The solvent was
removed in vacuo and the resulting crude solid was chromato-
graphed on a short silica gel pad. Initial elution with dichlo-
romethane afforded the unreacted bis(4-benzoylphenyl)-
acetylene (0.96 g, 24%) followed by elution with a mixture of
dichloromethane (containing 1% methanol) to furnish the
desired hexakis(4-benzoylphenyl)benzene 11 (2.9 g, 73%). The
hexakis(4-benzoylphenyl)benzene was further recrystallized
from a dichloromethane/hexanes mixture to afford a pale
yellow crystalline solid: yield 95% based on the reacted bis-
(4-benzoylphenyl)acetylene.
1,3,5-Tr is(4-b r om op h en yl)-2,4,6-t r is(4-d ip h en yl-4-h y-
d r oxym eth yl-p h en yl)ben zen e (8). Anhydrous THF (40 mL)
was added to hexakis(4-bromophenyl)benzene (2.02 g, 2.00
mmol) under an argon atmosphere. The solution was brought
to -78 °C and tert-butyllithium (1.7 M solution in pentane,
8.8 mL, 14.4 mmol) was added dropwise. The reaction mixture
was allowed to slowly warm to -10 °C over 40 min and stirred
at this temperature for 30 min before adding benzophenone
(3.63 g, 15.0 mmol). The reaction was allowed to warm to 22
°C and was stirred for 2 h. The product was extracted with
CH2Cl2 (3 × 50 mL) and the organic extracts were dried over
anhydrous magnesium sulfate. The solvent was removed in
vacuo and the product was purified by flash chromatography
on silica gel with ethyl acetate/hexanes mixture as eluent to
afford 8 as a colorless solid. Yield 82; mp 281 °C (hexanes-
dichloromethane); 1H NMR (CDCl3) δ 2.77 (s, 3 H), 6.63-6.83
Alternatively, bis(benzonitrile)palladium(II) dichloride (0.250
g, 0.73 mmol) was added to a Schlenk flask (under argon
atmosphere) containing bis(4-benzoylphenyl)acetylene (1.0 g,
(20) Grimm, M.; Kirste, B.; Kurreck, H. Angew. Chem. 1986, 98,
1095.
J . Org. Chem, Vol. 69, No. 5, 2004 1529

Rathore et al.
2.6 mmol) and benzene (100 mL). The reaction was allowed
to reflux for 12 h before the solvent was removed and the
product was redissolved in dichloromethane. It was then run
through a silica gel pad and the silica gel pad was washed
with CH2Cl2, which allowed the recovery of the starting bis-
(4-benzoylphenyl)acetylene (45%). A further elution of the
silica gel column with a mixture of dichloromethane/methanol
(∼1%) afforded the desired hexakis(4-benzoylphenyl)benzene
(53%): mp 278-280 °C (hexanes-dichloromethane); 1H NMR
(CDCl3) δ 7.02-7.08 (sym m, 12H), 7.36-7.46 (m, 24H), 7.49-
7.54 (m, 6H), 7.55-7.61 (m, 12H); 13C NMR (CDCl3) δ 128.0,
128.8, 129.5, 130.7, 132.1, 134.9, 137.0, 139.5, 143.3, 195.2;
IR (KBr) 3293.3 (w), 3047.5 (w), 1652.7 (s), 1604.4 (s), 1445.6
8.04 (d, J ) 8.4 Hz, 8H), 8.25 (t, J ) 7.2 Hz, 8H); 13C NMR
(CD2Cl2) δ 62.2, 130.1, 131.8, 138.3, 142.3, 142.6, 142.9, 208.2.
Hexa tr ityl ca tion (76+): yield 78%; 1H NMR (CD2Cl2) δ 7.47
(br d, J ) 8.0 Hz, 36H), 7.71 (t, J ) 7.4 Hz, 24H), 7.76 (d, J )
8.2 Hz, 12H), 8.14 (t, J ) 7.4 Hz, 12H); 13C NMR (CD2Cl2) δ
137.8, 138.98, 139.14, 139.25, 141.60, 141.81, 142.01, 142.50,
142.67, 153.1, 208.2. [Also note that the reduction of various
polytrityl cations can also be performed with various hydride
donors without isolation of the cation according to the proce-
dure detailed for the reduction of trialcohol 8 to the corre-
sponding triphenylmethyl derivative 9 above.]
Gen er a l P r oced u r e for th e Rea ction of Cr ysta llin e
Tr ityl Ca tion s w ith Va r iou s Hyd r id e Don or s. To a solu-
tion of isolated polytrityl cation (0.05 mmol) in anhydrous
dichloromethane (10 mL) at 0 °C was added 10-20% excess
of a hydride donor (such as borane-dimethyl sulfide complex,
triethylsilane, or cycloheptatriene) under an argon atmo-
sphere. The resulting mixture was stirred for 10 min, quenched
with saturated aqueous sodium bicarbonate solution (20 mL),
and then diluted with dichloromethane (20 mL). The organic
layer was washed with water (3 × 25 mL) and dried over
anhydrous magnesium sulfate and evaporated to afford the
corresponding triphenylmethyl derivative as a colorless solid.
The spectral data for various triphenylmethyl derivatives 13
and 14 are summarized below.
(m), 1307.8 (s), 1277.6 (s), 940.5 (m), 731.9 (s), 701.5 (s) cm-1
FAB m/z 1159 (M+), 1159 calcd for C84H54O6. Anal. Calcd for
;
C
84H54O6: C, 87.02; H, 4.69; O, 8.28. Found: C, 86.92; H, 4.59.
H e xa k is(4-d ip h e n yl-4-h yd r oxym e t h ylp h e n yl)b e n -
zen e (7). Hexakis(4-benzoylphenyl)benzene (0.50 g, 0.43
mmol) and dry ether (50 mL) were placed in a Schlenk flask
under an argon atmosphere. The reaction flask was cooled to
-40 °C and a cyclohexanes-ether solution of phenyllithium
(3 mmol) was added. The reaction was allowed to warm to 22
°C while stirring was continued for 5 h and was quenched with
water. The reaction mixture was extracted with CH2Cl2 (3 ×
50 mL) and dried over anhydrous magnesium sulfate. The
crude material was purified by flash chromatography on silica
gel, using an ethyl acetate/hexane mixture as eluent to afford
Hexa k is(4-d ip h en yl-4-m eth ylp h en yl)ben zen e (13): mp
1
235-237 °C; H NMR (CDCl3) δ 5.39 (s, 6H), 6.63 (d, J ) 8.4
1
7 as a white solid. Yield 88%; mp 260 °C (acetone); H NMR
Hz, 12H), 6.71 (d, J ) 8.1 Hz, 12H), 6.90-6.96 (m, 24H), 7.17-
7.25 (m, 36H); 13C NMR (CDCl3) δ 56.6, 125.8, 127.4, 127.9,
129.1, 131.3, 138.4, 139.7, 140.1, 143.6; FAB m/z 1532 (M+),
1532 calcd for C120H90. Anal. Calcd for C120H90: C, 94.08; H,
5.92. Found: C, 94.00; H, 5.86.
(CDCl3) δ 2.82 (s, 6H), 6.78 (d, J ) 8.7 Hz, 24H), 7.02-7.09
(m, 24H), 7.20-7.25 (m, 36H); 13C NMR (CDCl3) δ 81.9, 126.1,
126.9, 127.5, 127.6, 130.8, 139.2, 139.7, 143.7, 146.4; IR (KBr)
3554.2 (m), 3452.3 (m), 3055.0 (m), 3025.9 (m), 1490.2 (m),
1445.8 (m), 1012.3 (m), 756.0 (s), 699.6 (s) cm-1; FAB m/z 1628
(M+), 1628 calcd for C120H90O6. Anal. Calcd for C120H90O6: C,
88.53; H, 5.57; O, 5.90. Found: C, 88.23; H, 5.48.
Tetr a k is(4-d ip h en yl-4-m eth ylp h en yl)m eth a n e (14): 1H
NMR (CDCl3) δ 5.47 (s, 4 H), 6.89-7.31 (m, 56 H); 13C NMR
(CDCl3)δ 56.8, 64.1, 126.0, 128.0, 128.2, 129.1, 130.6, 141.2,
143.4, 143.7; FAB m/z 984 (M+), 984 calcd for C77H60. Anal.
Calcd for C77H60: C, 93.86; H, 6.14. Found: C, 93.69; H, 6.10.
Gen er a l P r oced u r e for th e Isola tion of Cr ysta llin e
Tr ityl Ca tion s. The alcoholic precursor (0.2 mmol) was
dissolved in dichloromethane (15 mL) and was treated with
an excess solution of tetrafluoroboric acid/diethyl ether com-
plex at ∼0 °C under an argon atmosphere. The resulting dark-
colored solution was carefully layered with dry hexanes (30
mL) and placed in a refrigerator (-25 °C). During the course
of 1 to 3 days, dark microcrystalline salt was deposited that
was filtered cold with use of a cintered glass funnel, washed
with hexanes, and dried in vacuo. The yields and spectral data
for various trityl cations prepared according to this procedure
are summarized below.
Ack n ow led gm en t. We are grateful to Professor F.
A. Khan (State University of West Georgia at Carrolton)
for the mass spectral data, Dr. Ilia A. Guzei (University
of Wisconsin at Madison) for X-ray crystallography, and
the Petroleum Research Fund (AC12345) for financial
support; C.L.B. thanks the Department of Education for
a GAANN fellowship.
1
Tr ip h en ylm eth yl or tr ityl ca tion (12+): yield 42%; H
Su p p or tin g In for m a tion Ava ila ble: Tables of X-ray
crystallographic data for 9. This material is available free of
charge via the Internet at http://pubs.acs.org.
NMR (CD2Cl2) δ 7.67 (d, J ) 7.6 Hz, 6H), 7.88 (t, J ) 7.6 Hz,
6H), 8.26 (t, J ) 7.7 Hz, 3H); 13C NMR (CD2Cl2) δ 130.2, 139.4,
1
142.2, 143.0, 209.7. Tetr a tr ityl ca tion (34+): yield 73%; H
NMR (CD2Cl2) δ 7.66-7.79 (m, 24H), 7.88 (d, J ) 8.4 Hz, 16H),
J O035598I
1530 J . Org. Chem., Vol. 69, No. 5, 2004