Novel Heteroatom-Linked Analogues of Trityl Radicals: Diaryl(benzotriazol-1-yl)methyl Radical Dimers

Article abstract of DOI:10.1021/jo9715229

The lithiation of diaryl(benzotriazol-1-yl)methanes followed by addition of iodine generates phenanthridines and dimers of hitherto unknown trityl analogues in which the radical center is directly attached to a heteroatom. These dimers differ from those of triarylmethyl radicals by not dissociating in solution, but variable temperature ESR measurements provided direct evidence for the formation of the corresponding diaryl(benzotriazol-1-yl)methyl radicals as intermediates in solution.

Full text of DOI:10.1021/jo9715229

J . Org. Chem. 1998, 63, 1467-1472
1467
Novel Heter oa tom -Lin k ed An a logu es of Tr ityl Ra d ica ls:
Dia r yl(ben zotr ia zol-1-yl)m eth yl Ra d ica l Dim er s
Alan R. Katritzky* and Baozhen Yang
Center for Heterocyclic Compounds, Department of Chemistry, University of Florida,
Gainesville, Florida 32611-7200
Naresh S. Dalal
Department of Chemistry, Florida State University, Tallahessee, Florida 32306-3006
Received August 13, 1997
The lithiation of diaryl(benzotriazol-1-yl)methanes followed by addition of iodine generates
phenanthridines and dimers of hitherto unknown trityl analogues in which the radical center is
directly attached to a heteroatom. These dimers differ from those of triarylmethyl radicals by not
dissociating in solution, but variable temperature ESR measurements provided direct evidence for
the formation of the corresponding diaryl(benzotriazol-1-yl)methyl radicals as intermediates in
solution.
In tr od u ction
intermediates.11-13 We recently reported that the lithia-
tion of diaryl(benzotriazol-1-yl)methane followed by the
addition of copper(I) iodide yielded phenanthridine de-
rivatives.14 We now report the results of our study of a
new class of radicals, Bt1C•Ar2, including the elucidation
of their dimeric structures and a comparison of their
properties with those of the analogous trityl radicals.
As is well-known,15 the class of triarylmethyl radicals
is stable because the unpaired electron can be delocalized
onto the ortho and para positions of the phenyl rings.
Dimerizations of these radicals take place at the para
ring position due to steric hindrance at the ortho position.
Lankamp et al.3 demonstrated by UV and NMR spectro-
scopic evidence in 1968 that the structure of the triphen-
ylmethyl radical dimer was 1-(diphenylmethylene)-4-
trityl-2,5-cyclohexadiene (1), and not hexaphenylethane,
as had long been assumed. Tokura et al. found that the
Free radicals are highly important and extensively
used synthetic intermediates. The triphenylmethyl or
trityl radical and its substituted derivatives are impor-
tant as the prototype class of C-centered persistent
radicals, the properties and reactivities of which have
been extensively studied.1-6 Diaryl(heteroaryl)methyl
radicals are far less well-known. Although Mangini and
co-workers et al. reported the formation and ESR spectra
of trithienylmethyl,7 thienyldiphenylmethyl, and dithi-
enylphenylmethyl8 radicals, in all of these the radical
center is still connected to three carbon atoms. No trityl
analogue, in which the radical center is connected directly
with a heteroatom of a heteroaryl ring, has previously
been reported. We were interested in such radicals and
specifically in replacing one of the three phenyl groups
in the trityl radical by a benzotriazol-1-yl substituent.
Benzotriazolyl groups possess large conjugated systems
and, like phenyl, can delocalize electrons: thus, a ben-
zotriazol-1-yl substituent stabilizes an R-carbanion center
which can then react with electrophiles to give the
expected R-substituted products.9,10 By contrast, R-depro-
tonation of N-2-alkylbenzotriazoles (Bt2CH2R) gives car-
banions which rapidly form the corresponding symmetri-
cal dimers (Bt2CHRCHRBt2), together with “tetramers”
(Bt2CHRCHRCHRCHRBt2), derived from four molecules
of the substrate, and olefins (RCHdCHR), all via radical
dimer 1 was converted into isomeric p-benzhydryltet-
raphenylmethane (2) in good yield (80%) in the presence
of protic acids, and they also undertook mechanistic
investigations using deuterated phenol.16 Interestingly,
(1) Gomberg, M. Chem. Rev. 1925, 1, 91.
(2) Ballester, M. Pure Appl. Chem. 1967, 123.
(3) Laukamp, H.; Nauta, W. Th.; MacLean, C. Tetrahedron Lett.
1968, 249.
(4) Neumann, W. P.; Uzick, W.; Zarkadis, A. K. J . Am. Chem. Soc.
1986, 108, 3762.
(5) Carilla, J .; Fajari, L.; J ulia, L.; Riera, J .; Viadel, L. Tetrahedron
Lett. 1994, 35, 6529.
(6) Neumann, W. P.; Stapel, R. Chem. Ber. 1986, 119, 2006.
(7) Mangini, A.; Pedulli, G. F.; Tiecco, M. Tetrahedron Lett. 1968,
4941.
(11) Katritzky, A. R.; Wu, J .; Kuzmierkiewicz, W.; Rachwal, S.;
Balasubramanian, M.; Steel, P. J . Liebigs Ann. Chem. 1994, 7.
(12) Dalal, N. S.; Xu, R.; Katritzky, A. R.; Wu, J .; J esorka, A. Magn.
Reson. Chem. 1994, 32, 721.
(13) Katritzky, A. R.; J esorka, A.; Wang, J .; Yang, B.; Wu, J .; Steel,
P. J . Liebigs Ann. Chem. 1996, 745.
(8) Mangini, A.; Pedulli, G. F.; Tiecco, M. J . Heterocycl. Chem. 1969,
6, 271.
(9) Katritzky, A. R.; Rachwal, S.; Hitchings, G. J . Tetrahedron 1991,
47, 2683.
(10) Katritzky, A. R.; Wu, J .; Kuzmierkiewicz, W.; Rachwal, S.
Liebigs Ann. Chem. 1994, 1.
(14) Katritzky, A. R.; Yang, B. J . Heterocycl. Chem. 1996, 33, 607.
(15) Sholle, V. D.; Rozantsev, E. G. Russ. Chem. Rev. 1973, 42, 1011.
Langhals, H.; Fischer, H. Chem. Ber. 1978, 111, 543. Kratt, G.; Beck-
haus, H.-D.; Lindner, H. J .; Ruchardt, C. Chem. Ber. 1983, 116, 3235.
(16) Takeuchi, H.; Nagai, T.; Tokura, N. Bull. Chem. Soc. J pn. 1971,
44, 753.
S0022-3263(97)01522-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/12/1998

1468 J . Org. Chem., Vol. 63, No. 5, 1998
Katritzky et al.
Sch em e 1
Staab et al. found that the dimer 1 also undergoes a [1,5]-
proton shift to form isomer 2 under basic conditions.17
These rearrangements provide further evidence of the
structure 1-(diphenylmethylene)-4-trityl-2,5-cyclohexa-
diene (1) for the dimer of Gomberg’s trityl radical. In
diaryl(benzotriazol-1-yl)methyl radicals, the unpaired
electron can also be delocalized onto the ortho or para
positions of the phenyl rings and such radicals could be
expected to dimerize in a fashion similar to that of the
trityl radical. However, such a dimer should undergo a
[1,5]-proton shift to give dimer 5 quite readily due to the
electron-withdrawing character of the benzotriazolyl
group. The unpaired electron could also go to the
benzotriazole ring and induce the ring opening by loss
of nitrogen, followed by cyclization to form phenath-
ridines 4.
was followed by the addition of an iodine solution in THF
at this temperature. After the solutions were stirred for
15 min at -78 °C, the reactions were quenched with
water. Instead of the expected diaryl(benzotriazol-1-yl)-
methyl iodides, phenanthridine derivatives 4a -c and 8
and dimers 5a -c and 9 of the desired radicals were
obtained directly (Scheme 1, Table 1). The formation of
trityl radicals by reactions of carbanions with iodine has
previously been reported by Rajca.19,20
When Ar1 and Ar2 are not identical, but are of similar
electronic character as in 3c and 3d , two isomers of
phenanthridine 4 are formed which are difficult to
separate by column chromatography. For example, when
R-(p-methylphenyl)-R-(benzotriazol-1-yl)toluene (3c) was
treated with n-butyllithium followed by I2 at -78 °C, a
mixture of 6-(p-methylphenyl)phenanthridine (4c) and
9-methyl-6-phenylphenanthridine (4c′) was obtained along
with dimer 5c. However, when Ar1 and Ar2 differ
significantly in electronic character, as in compound 3k ,
the ring closure reaction occurs only at the ring with the
lower electron density to form a single phenanthridine
(4k ). However, only one isomer of dimers 5a -d was
obtained, although Ar1 and Ar2 are different in the
starting materials 3a -d , and there should be the pos-
sibility of formation of other isomers. In every case, the
dimers 5a -d , 9 were formed, as shown in Scheme 2, via
R,para dimerization. No R,R or R,ortho dimerizations
were observed, even for the cases where there are
Resu lts a n d Discu ssion
P r ep a r a tion of Dia r yl(ben zotr ia zol-1-yl)m eth yl
Ra d ica ls, Th eir Dim er s, a n d P h en a n th r id in e Rin g
Closu r e P r od u cts. Triarylmethyl radicals are usually
prepared by the reaction of a triarylmethyl halide with
a metal.18 The starting diaryl(benzotriazol-1-yl)methanes
3a -k were prepared by the reactions of benzotriazole
with diarylmethyl halides or diarylmethanols as previ-
ously described.14 Diaryl(benzotriazol-1-yl)methanes 3a-k
were lithiated by n-BuLi at -78 °C for 10 min, and this
(17) Staab, H. A.; Bretschneider, H.; Brunner, H. Chem. Ber. 1970,
103, 1101.
(18) Gomberg, M. J . Am. Chem. Soc. 1900, 22, 757.
(19) Rajca, A. J . Am. Chem. Soc. 1990, 112, 5890.
(20) Rajca, A.; Utamapanya, S.; Xu, J . J . Am. Chem. Soc. 1991, 113,
9235.

Novel Heteroatom-Linked Analogs of Trityl Radicals
J . Org. Chem., Vol. 63, No. 5, 1998 1469
Ta ble 1. P r ep a r a tion of P h en a th r id in es 4 a n d 8 a n d Dim er s 5 a n d 9
3 and 5
4 (or 8)
5 (or 9)
starting material
Ar1
Ar2
Ar1
X
H
yield (%)
yield (%)
3a
3b
3c
Ph
Ph
Ph
Ph
Ph
Ph
25
0
44
80
59
p-ClC6H4
4-MeC6H4
9-Me
H
7-Me
H
27a
4-MeC6H4
Ph
2-MeC6H4
3d
Ph
2-MeC6H4
56a
22
3e
3f
3g
3h
3i
dibenzosuberan-5-ylb
4-MeC6H4
4-ClC6H4
8b
20
60
28
39
0
15 (9)
4-MeC6H4
4-ClC6H4
4-FC6H4
4-Me2NC6H4
4-MeOC6H4
Ph
4-MeC6H4
4-ClC6H4
4-FC6H4
9-Me
9-Cl
9-F
0
0
0
0
0
0
4-FC6H4
4-Me2NC6H4
4-MeOC6H4
4-Me2NC6H4
3j
3k
4-MeOC6H4
4-Me2NC6H4
9-MeO
H
7
35
a
b
Mixture of two isomers. See Scheme 1.
Sch em e 2
substituents in the para position. Similar behavior was
previously noted for the trityl class of radicals.3
P r oof of Str u ctu r e of Dim er s. Compounds 5a -d
and 9 were characterized by NMR and by HRMS. The
spectra showed their structures to be different from those
of the trityl radical dimers. From the NMR spectra, it
is clear that they possess two different benzotriazol-1-yl
groups. Thus, in the 13C NMR spectra, every dimer gives
two sets of benzotriazol-1-yl signals (Table 2), e.g., in
dimer 5a , (i) 146.2, 133.0, 127.4, 123.7, 120.0, 110.5 ppm
There are several noteworthy features of these reac-
tions. (i) When both aryl groups in the starting material
are substituted at the para position, the reaction gave
only the phenanthridine as product and no dimer was
obtained. The reason is probably steric hindrance by the
para substituents. However, a single para substituent
in the starting material did not inhibit dimerization. (ii)
When (2-methylphenyl)phenyl(benzotriazol-1-yl)methane
was reacted under these conditions, (2-methylphenyl)-
phenyl(benzotriazol-1-yl)methyl radical dimerized at the
2-methylphenyl group to give dimer 5d as well as the
phenanthridine 4d . (iii) Reaction of bis(4-(dimethylami-
no)phenyl)(benzotriazol-1-yl)methane (3i) under the same
conditions gave 1,1-bis(4-(dimethylamino)phenyl)pentane
and BtH, and no corresponding compound 4 was gener-
ated. Treatment of bis(4-dimethoxyphenyl)(benzotriazol-
1-yl)methane (3j) under these conditions gave 4j in only
7% yield together with a large amount of 1,1-bis(4-
methoxyphenyl)pentane and free benzotriazole. Strong
electron-donating groups (dimethylamino and methoxy)
allow the benzotriazolyl group to be displaced easily in
N-(p-substituted benzyl)benzotriazoles.9
and (ii) 146.4, 134.2, 127.7, 123.9, 120.2, 113.5 ppm (Table
2). In dimer 5d , the two methyl groups display different
chemical shifts (2.18 and 1.50 ppm). The methyl at the
higher field is shielded by a benzotriazol-1-yl or a phenyl
group. In the 1H NMR spectrum of 9, protons at the

1470 J . Org. Chem., Vol. 63, No. 5, 1998
Ta ble 2. NMR Da ta of Dim er s 5a -d a n d 9
Katritzky et al.
1H NMR
13C NMR
others
compd
H-1, H-7, H-10
others
C-11 C-12
5a
8.02-8.10 (m, 2H),
7.00-7.42 (m, 25H)
66.6 78.7 110.5, 113.5, 120.0, 123.7, 123.9, 126.7, 127.4, 127.6,
128.0, 128.2, 128.6, 128.8, 129.9, 130.8, 133.0,
134.2, 137.2, 137.3, 140.9, 142.0, 146.2, 146.4
65.7 78.1 110.0, 113.2, 120.1, 120.2, 123.8, 124.0, 127.0,
127.6, 127.7, 128.0, 128.1, 128.3, 129.0, 129.6,
130.5, 131.4, 132.8, 133.9, 134.1, 134.5, 135.7,
137.2, 139.6, 139.7, 140.2, 141.7, 146.1, 146.4
66.5 78.5 21.0, 21.1, 110.6, 113.6, 120.0, 120.1, 123.6,
123.8, 126.7, 127.3, 127.5, 127.9, 238.0, 128.2,
128.6, 129.5, 129.9, 130.6, 132.9, 134.2, 134.3,
137.5, 137.9, 138.0, 138.1, 138.4, 141.1, 142.0,
146.2, 146.4
6.39 (d, J ) 8.4 Hz, 1H)
5b
5c
8.02-8.10 (m, 2H),
6.40 (d, J ) 8.4 Hz, 1H)
7.05-7.40 (m, 23H)
8.04-8.10 (m, 2H),
6.41 (d, J ) 8.4 Hz, 1H)
7.00-7.33 (m, 22H),
7.35 (s, 1H), 2.33 (s, 6H)
5d
9
8.04-8.08 (m, 2H),
6.89-7.35 (m, 22H),
7.55 (s, 1H), 2.17 (s, 3H),
1.47 (d, J ) 3.9 Hz, 3H)
64.2 79.0 19.3, 21.7, 110.4, 113.0, 119.9, 120.1, 123.6, 123.8,
125.7, 126.3, 126.8, 127.3, 127.4, 127.8, 127.9,
128.1, 128.2, 128.7, 128.8, 129.4, 130.2, 131.0,
131.1, 132.8, 133.1, 134.5, 135.5, 136.6, 136.9,
139.1, 139.4, 139.5, 141.0, 141.7, 146.2, 146.3
70.7 78.5 31.6, 32.3, 34.4, 111.3, 114.2, 119.9, 120.0,
123.7, 123.8, 125.8, 126.6, 126.8, 127.1, 128.4,
128.6, 129.5, 130.1, 130.9, 131.1, 131.2, 133.1,
133.2, 134.1, 134.2, 134.8, 139.0, 140.1, 140.6,
141.3, 143.2, 146.3, 146.5
6.26 (d, J ) 8.5 Hz, 1H)
8.00-8.03 (m, 1H),
8.10 (d, J ) 8.4 Hz, 1H),
6.35 (d, J ) 8.4 Hz, 1H)
7.52 (d, J ) 7.1 Hz, 1H),
7.41 (d, J ) 8.0 Hz, 1H),
7.06-7.36 (m, 10H),
6.89-6.95 (m, 3H),
6.85 (s, 1H), 6.72-6.77 (m, 2H),
6.15 (d, J ) 8.4 Hz, 2H),
2.60-3.18 (m, 8H)
Ta ble 3. Meltin g P oin ts a n d HRMS of Dim er s 5a -d
Sch em e 3
a n d 9
compd mol formula mp (°C)
HRMS found (calcd) (M+)
225-227 450.2014 (450.1970) (M+ - Bt)
38H26Cl2N6 136-139 518.1019 (518.1190) (M+ - Bt)
5a
5b
5c
5d
9
C
C
C
C
C
38H28N6
40H32N6
40H32N6
42H32N6
139-141 596.2727 (596.2688)
143-145 596.2713 (596.2688)
180-183 620.2694 (620.2688)
4-position of the two benzotriazol-1-yl groups produce
signals at 8.10 and 8.00-8.03 ppm, respectively. In the
13C NMR spectrum of 9, there are two groups of benzo-
triazol-1-yl signals, 146.3, 133.1, 126.6, 123.7, 119.9,
113.3 and 146.5, 133.2, 126.8, 123.8, 120.0, 114.2 ppm.
At the high field, the signals at 78.5 and 70.7 ppm belong
to the quaternary carbon and the tertiary carbon, re-
spectively. These assignments were confirmed by 2D and
APT spectra. In comparison to the structure of the trityl
radical dimer 1, which has a large conjugated system,
the protons on the central phenyl group (protons 2, 3, 5,
and 6) are in the range of 6.01-6.40 ppm and the proton
1
Rea ction Mech a n ism . The proposed mechanism for
formation of phenanthridines 4 and 8 and dimers 5 and
9 is given in Scheme 2. Carbanion 10 (formed by
lithiation of diaryl(benzotriazol-1-yl)methane) reacts with
iodine to generate radical 11. The unpaired electron in
radical 11 is delocalized as indicated by the canonical
forms to the ortho (cf. 11b) or para positions (cf. 11c).
Dimerization involving forms 11a and 11c gives, after a
[1,5]-proton shift, a dimer of type 5. The unpaired
electron of radical 11 can also reside on the triazole ring
which opens to form 12 and looses nitrogen to give 13
which undergoes intramolecular cyclization to generate
phenanthridine. Whether 4 or 5 is formed is probably
controlled by the relative rates of the irreversible steps
12 f 13 and 14 f 5.
at position 4 is at 5.09 ppm in the H NMR spectrum. In
the 13C NMR spectrum, the carbon at position 4 is at
about 42 ppm.21 It is clear that the structures of our
dimers 5 and 9 are different from the structures of trityl
radical dimers.
In the MS spectra of compounds 5 and 9, the molecular
ion and molecular fragments are observed. The base
peak at M+ - 118 reflects the ease of loss of a benzotri-
azolyl group from the crowded molecule (Table 3).
As is well-known, the dimers of trityl radicals undergo
reversible dissociation to the corresponding radicals in
solution.3 A crossover experiment was conducted to
investigate possible dissociation of the present dimers in
solution. Dimers 5a and 5b were mixed and dissolved
in THF, but refluxing for 10 h led to the recovery of start-
ing materials, 5a and 5b, and no new dimers were gener-
ated. This also indicates that the structures of dimers 5
are different from those of the trityl radical dimer.
Rea ctivity of th e Dim er s 5. Treatment of dimer 5a
with hydrochloric acid in methylene chloride at room
temperature for 4 h gave the chloride derivative 15a with
the loss of one benzotriazol-1-yl group (Scheme 3). In
1
the H NMR spectrum of 15a , the low-field signal (8.02-
(21) Neumann, W. P.; Penenory, A.; Stewen, U.; Lehnig, M. J . Am.
Chem. Soc. 1989, 111, 5845.
8.08 ppm) integrated for one proton only existing instead

Novel Heteroatom-Linked Analogs of Trityl Radicals
J . Org. Chem., Vol. 63, No. 5, 1998 1471
Sch em e 4
F igu r e 1. ESR spectrum of 5-(benzotriazol-1-yl)dibenzosu-
berane radical.
of for two protons as for 5a . The 13C NMR spectrum of
15a confirmed that only a single benzotriazol-1-yl group
remained.
(iii) The radical’s unpaired electron is delocalized from
its central C-atom over a multitude of atomic sites.
Qualitatively, the observed spectrum is consistent with
the electron having dominant hyperfine interaction with
one strongly coupled nitrogen and two protons, which
should give rise to a roughly 1:2:1 triplet, split by a 1:1:1
triplet. The nitrogen splitting is evident from the fact
that the central peak in Figure 1 has an approximate
1:1:1 triplet structure in the model. However, the
(smaller) couplings with the remaining nitrogens and the
ring protons blur the main peaks. Computer simulation
was not possible because of the lack of resolution leading
to a large degree of redundancy in the number as well
as magnitude of the hyperfine couplings. Nevertheless,
the spectra indicate the formation of a C-centered radical
extensively delocalized with at least two nitrogen and two
or more proton couplings and is consistent with the
postulated dimerization pathway (Scheme 2).
Ra d ica l Tr a p p in g Rea ction . TEMPO is widely used
as an efficient radical trap. However, adding TEMPO
to our reaction gave only diphenyl ketone which probably
originated by decomposition of the hindered radical
intermediate 17a (Scheme 4).
In summary, the diaryl(benzotriazol-1-yl)methyl dimers
5 and 9 and the phenanthridine derivatives 4 and 8 were
obtained by lithiation of diaryl(benzotrizol-1-yl)methanes
followed by adding I2. The structure of dimers 5 and 9
is different than those of the analogous trityl dimer.
Intermediate, diphenyl(benzotriazol-1-yl)methyl radicals
of the trityl type but with the radical center directly
connected with a heteroatom have been demonstrated by
ESR and radical trapping experiments.
Treatment of 5a with LDA at -78 °C gave product 16a
(98%) in which one of the benzotriazole groups had been
eliminated (Scheme 3). Compound 16a is red due to a
1
highly conjugated system. In the H NMR spectrum of
16a , four protons, 6.18, 6.82, 6.96, 7.10 ppm, have moved
to a higher field when compared with the 1H NMR
spectrum of 5a ; the low-field signal at 8.12 ppm, inte-
grated for only one proton, which belonged to a single
benzotriazol-1-yl group. The 13C NMR spectrum of 16a
displayed only one set of benzotriazol-1-yl signals (111.0,
120.1, 124.1, 127.9, 131.9, 145.9 ppm). The structure of
16a was also further supported by HRMS. Heating
dimer 5a with methanolic sodium hydroxide at 60 °C for
12 h or treatment with MeMgBr also gave 16a .
ESR Resu lts. An important aspect of this study was
to establish definitive evidence for the formation of the
radical intermediates as postulated in Scheme 2. From
the fast formation and disappearance of coloration in the
reaction mixture, it was clear that the radicals were too
reactive to be detected from any reaction mixture above
-78 °C. However, below -78 °C, they are stable as
evidenced by strong ESR signals detected from the
reaction mixture.
Figure 1 shows a typical ESR spectrum obtained from
reaction of 5-(benzotriazol-1-yl)dibenzosuberane 3e with
n-BuLi and iodine. The spectrum is centered at a g value
of 2.0029 ( 0.0005, with a complex, only partially
resolved multiline hyperfine structure. This g value is
characteristic of a C-centered free radical with an exten-
sive delocalization of the unpaired spin density; for
example, g is 2.0026 for the triphenylmethyl radical.4
Further evidence for the delocalization is the presence
of the complex hyperfine structure. Several attempts
were made to enhance the spectral resolution by optimiz-
ing the radical concentration, microwave power, magnetic
field modulation amplitude, solvent polarity, sample
temperature, and oxygen removal, but even the best
results (shown in Figure 1) were not good enough for any
quantitative assignment of the radical structure. How-
ever, the spectra obtained do allow the following conclu-
sions:
Exp er im en ta l Section
Gen er a l. Melting points were determined with a Kofler
hot stage apparatus without correction. 1H and 13C NMR
spectra were recorded on a 300 and 75 MHz spectrometer in
CDCl3 with TMS or CDCl3 as the internal reference. High-
resolution mass spectra were measured on a Kratos/AE1-MS
30 mass spectrometer. THF was distilled from sodium/
benzophenone prior to use. Lithiation reactions were carried
out under the protection of dry nitrogen. All glassware was
oven-dried. All moisture-sensitive reagents were transferred
by means of predried syringes. Preparation of diaryl(benzo-
triazol-1-yl)methane was reported in our previous work.14
ESR Mea su r em en ts. ESR measurements were made
using a Varian E112 spectrometer operating at the X-band
(9.50 G Hz) frequencies. The g-values were measured relative
to the ESR standard 1,1-diphenyl-2-picrylhydrazyl (DPPH).
Sample temperature was varied from about 90-350 K by using
a Varian variable temperature (VT) accessory. All the re-
agents were well degassed and stored under nitrogen prior to
(i) A C-centered radical is formed as an initial step in
the reaction. This radical is highly reactive, with a
lifetime of only a few minutes at -78 °C, but more stable
at lower temperatures.
(ii) Full resolution of the hyperfine structure must
await measurements of the hyperfine couplings by the
electron-nuclear double resonance (ENDOR) technique,
as was needed for triphenylmethyl radicals.4

1472 J . Org. Chem., Vol. 63, No. 5, 1998
Katritzky et al.
ESR measurements. It was necessary to make the ESR
measurements at low temperatures, because the radical
reactivity was high in the fluid media. No radicals could be
detected at temperature above -50 °C, but they were quite
stable below -78 °C. The radical concentration was estimated
around a few mmol from signal intensities as compared with
DPPH. Unfortunately, despite our efforts at sample degassing,
temperature variation, and optimization of microwave power
saturation and modulation amplitude, the ESR spectra did not
exhibit well-resolved hyperfine splitting.
P r ep a r a tion of P h en a n th r id in es 4 a n d 8 a n d Dim er s
5 a n d 9 (Gen er a l P r oced u r e). To a solution of diaryl-
(benzotriazol-1-yl)methane (2 mmol) in dried THF (20 mL) was
added n-BuLi (1.1 mL, 2 M, 2.2 mmol) at -78 °C under
nitrogen, and the solution was stirred at this temperature for
10 min. Then, an I2 (254 mg, 1 mmol) solution of THF (5 mL)
was transferred into the reaction mixture. After the solution
was stirred for 20 min at -78 °C, water (5 mL) was added to
quench the reaction. The mixture was warmed to room
temperature, washed with sodium bisulfate solution (2 × 20
mL), and then extracted with ethyl acetate (3 × 20 mL). The
combined organic extracts were dried over MgSO4. After
removal of the solvent, the residue was purified by column
chromatography (silica gel, eluent: ethyl acetate/hexane) to
obtain phenanthridines 4 (or 8) and dimers 5 (or 9) (Tables 1,
2, 3). The NMR data of phenanthridines 4 and 8 are identical
with that obtained in our previous report.14
P r ep a r a tion of r-[4-(Dip h en ylch lor om eth yl)p h en yl]-
r-(ben zotr ia zol-1-yl)tolu en e (15a ). A mixture of dimer 5a
(568 mg, 1 mmol), methylene chloride (10 mL), and concen-
trated hydrochloric acid (2 mL) was stirred at room temper-
ature for 4 h. The mixture was neutralized by NaOH aq (2
N, 2 × 10 mL) and washed with water, before being extracted
with ethyl acetate (3 × 10 mL) and dried over MgSO4. After
removal of the solvent, the residue was purified by column
chromatography (silica gel, eluent: ethyl acetate/hexane 1:9)
to give 15a in 60% yield (290 mg): mp 80-82 °C; 1H NMR
(CDCl3) δ 8.02-8.07 (m, 1 H), 7.37 (s, 1 H), 7.05-7.35 (m, 22
H); 13C NMR (CDCl3) δ 147.2, 146.5, 145.9, 137.4, 136.3, 132.9,
128.7, 128.3, 128.2, 127.8, 127.7, 127.3, 127.2, 123.9, 119.9,
110.5, 81.7, 66.8. HRMS calcd for C32H24ClN3 485.1659 (M+),
found 485.1743.
P r ep a r a tion of 1-(Dip h en ylm eth ylen e)-4-[p h en yl(ben -
zotr ia zol-1-yl)m eth ylen e]-2,5-cycloh exa d ien e (16a ). To
a solution of 5a (568 mg, 1 mmol) in dried THF (30 mL) was
added LDA (0.72 mL, 1.5 N in hexane, 1.1 mmol) at -78 °C
under N2 and the solution was stirred for 10 min. The reaction
solution changed to red. After the reaction mixture was
warmed to 0 °C, water (2 mL) was added to quench the
reaction. The mixture was then extracted with ethyl acetate
(2 × 20 mL). The combined organic extracts were dried over
MgSO4, and the solvent was evaporated to give the product
16a as a red solid in 98% yield (440 mg): mp 220-223 °C; 1H
NMR (CDCl3) δ 8.08-8.14 (m, 1 H), 7.18-7.39 (m, 17 H), 7.05-
7.10 (m, 2 H), 6.96 (dd, J ) 10.0, 1.9 Hz, 1 H), 6.82 (dd, J )
10.0, 1.9 Hz, 1 H), 6.18 (dd, J ) 10.0, 2.0 Hz, 1 H); 13C NMR
(CDCl3) δ 145.9, 144.7, 141.4, 141.2, 135.5, 134.2, 131.9, 131.6,
131.4, 131.3, 131.0, 129.4, 128.9, 128.7, 128.1, 128.0, 127.9,
127.5, 125.5, 125.0, 124.1, 120.1, 111.0. HRMS calcd for
C32H23N3 450.1970 (M+ + 1), found 450.2018.
Ra d ica l Tr a p p in g. To a solution of 5a (283 mg, 1 mmol)
were added TEMPO (156 mg, 1 mmol) in dried THF (20 mL)
and n-BuLi (0.55 mL, 2M, 1.1 mmol) at -78 °C under nitrogen,
and the solution was stirred at this temperature for 20 min.
Then, an I2 (127 mg, 0.5 mmol) solution of THF (5 mL) was
transferred into the reaction mixture by syringe. After the
solution was stirred for 20 min at -78 °C, water (5 mL) was
added to quench the reaction. The reaction mixture was then
allowed to warm to ambient temperature before washing with
sodium bisulfite (2 × 10 mL) solution. The mixture was then
extracted with ethyl acetate (3 × 20 mL) and dried over
MgSO4. After removal of the solvent, the residue was purified
by column chromatography to give diphenyl ketone in 70%
yield, mp 48 °C.
J O9715229