The D parameter (EPR zero-field splitting) of localized 1,3-cyclopentanediyl triplet diradicals as a measure of electronic substituent effects on the spin densities in para-substituted benzyl-type radicals

Article abstract of DOI:10.1021/jo960396q

The zero-field splitting parameters D of the symmetrically disubstituted and unsymmetrically monosubstituted 1,3-diaryl-1,3-cyclopentanediyl triplet diradicals 1, 2 (X = p-MeO, p-Me, p-Cl, P-NH₂, P-CO₂Me, p-CN, p-NO₂), and 5 were determined in 2-methyltetrahydrofuran glass at 77 K. The linear plot (m = 0.558, r² = 0.993) of the experimental D values for the symmetrically disubstituted derivatives versus the corresponding monosubstituted ones reveals that the electronic substituent effects are additive and implies (except for the magnetic dipolar interaction) that each benzyl-type radical site acts independently in the localized diradicals. This additivity permits us to view these triplet diradicals as a composite of the two separate monoradical components and allows us to assess valuable electronic properties of benzyl-type monoradicals from the D parameter of the triplet diradical species. A theoretical analysis shows that the D parameter is a measure of the spin density p at the benzylic positions and the inter-radical distance d in localized diradicals. A good correlation exists between the D parameter of these triplet diradicals (constant inter-radical distance d) and the EPR hyperfine coupling constants of the corresponding benzyl-type monoradicals, which establishes that the observed electronic substituent effects reflect changes in the spin densities at the radical sites. The novel AZ? scale allows us to quantify spectroscopically the para substituent effect on the spin delocalization at the benzylic position.

Full text of DOI:10.1021/jo960396q

7056
J . Org. Chem. 1996, 61, 7056-7065
Th e D P a r a m eter (EP R Zer o-F ield Sp littin g) of Loca lized
1,3-Cyclop en ta n ed iyl Tr ip let Dir a d ica ls a s a Mea su r e of Electr on ic
Su bstitu en t Effects on th e Sp in Den sities in P a r a -Su bstitu ted
Ben zyl-Typ e Ra d ica ls
Waldemar Adam,* Fumio Kita, Heinrich M. Harrer, Werner M. Nau, and Rainer Zipf†
Institute of Organic Chemistry, University of Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Received February 27, 1996X
The zero-field splitting parameters D of the symmetrically disubstituted and unsymmetrically
monosubstituted 1,3-diaryl-1,3-cyclopentanediyl triplet diradicals 1, 2 (X ) p-MeO, p-Me, p-Cl,
p-NH2, p-CO2Me, p-CN, p-NO2), and 5 were determined in 2-methyltetrahydrofuran glass at 77 K.
The linear plot (m ) 0.558, r2 ) 0.993) of the experimental D values for the symmetrically
disubstituted derivatives versus the corresponding monosubstituted ones reveals that the electronic
substituent effects are additive and implies (except for the magnetic dipolar interaction) that each
benzyl-type radical site acts independently in the localized diradicals. This additivity permits us
to view these triplet diradicals as a composite of the two separate monoradical components and
allows us to assess valuable electronic properties of benzyl-type monoradicals from the D parameter
of the triplet diradical species. A theoretical analysis shows that the D parameter is a measure of
the spin density F at the benzylic positions and the inter-radical distance d in localized diradicals.
A good correlation exists between the D parameter of these triplet diradicals (constant inter-radical
distance d) and the EPR hyperfine coupling constants of the corresponding benzyl-type monoradicals,
which establishes that the observed electronic substituent effects reflect changes in the spin densities
at the radical sites. The novel ∆D scale allows us to quantify spectroscopically the para substituent
effect on the spin delocalization at the benzylic position.
In tr od u ction
EPR spectral analysis of tailor-made triplet diradicals
may provide valuable information on the electronic
properties of monoradicals, e.g., of the aryl-substituted
cumyl radicals 4. In this context, we have defined the
The zero-field splitting parameters of triplet states,
which are readily determined by EPR spectroscopy under
matrix isolation, provide valuable information on the
electronic nature of these species through the so-called
D value and structural data through the symmetry factor
E.1 We have recently demonstrated for 1,3-diaryl-1,3-
cyclopentanediyl triplet diradicals 1 that the D parameter
varies with the spin-delocalizing ability of the aryl
substituent.2 It was previously proposed by Dougherty3
∆D scale (eq 1),2 which constitutes a quantitative mea-
sure of the spin donor (SD) and spin acceptor (SA)
∆D ) (DH - Dx)/hc
(1)
properties of substituents in localized 1,3-cyclopen-
tanediyl triplet diradicals 1 and provides information on
the electronic substituent effects in benzyl-type mono-
radicals. This novel concept for the evaluation of elec-
tronic substituent effects in monoradicals through the D
parameters of triplet diradicals comprises a new spectral
method to quantify radical delocalization and hence
radical stabilization.4c Previously, spectral scrutiny of
radical substituent effects could only be assessed through
EPR hyperfine coupling constants (aR or aâ) of monoradi-
cals.4
for the 1,3-cyclobutanediyls 3 that the D parameter for
triplet diradicals reflects the electronic substituent effects
in the corresponding monoradicals. Consequently, the
† Undergraduate research participant (Spring 1994).
X Abstract published in Advance ACS Abstracts, September 1, 1996.
(1) (a) Wassermann, E.; Snyder, L. C.; Yager, W. A. J . Chem. Phys.
1964, 41, 1763. (b) McGlynn, S. P.; Azumi, T.; Kinoshita, M. In
Molecular Spectroscopy of the Triplet State; Prentince-Hall: Englewood
Cliffs, 1969; Chapter 9. (c) Weil, J . A.; Bolton, J . R.; Wertz, J . E. In
Electron Paramagnetic Resonance Elementary Theory and Practical
Applications; J ohn Wiley & Sons: New York, 1994; Chapter 9. (d)
McWeeny, R. J . Chem. Phys. 1961, 34, 399. (e) McConnell, H. M. J .
Chem. Phys. 1958, 28, 1188.
(2) (a) Adam, W.; Fro¨hlich, L.; Nau, W. M.; Korth, H.G.; Sustmann,
R. Angew. Chem., Int. Ed. Engl. 1993, 32, 1339. (b) Nau, W. M. Ph.D.
Thesis, University of Wu¨rzburg, 1994. (c) Harrer, H. M., personal
communication.
(3) (a) J ain, R.; Sponsler, M. B.; Coms, F. D.; Dougherty, D. A. J .
Am. Chem. Soc. 1988, 110, 1356. (b) Dougherty, D. A. In Kinetics and
Spectroscopy of Carbenes and Biradicals; Platz, M. S., Ed.; Plenum
Press: New York, 1990; Chapter 5. (c) Dougherty, D. A. Acc. Chem.
Res. 1991, 24, 88.
S0022-3263(96)00396-9 CCC: $12.00 © 1996 American Chemical Society

Spin Densities in Para-Substituted Benzyl-Type Radicals
J . Org. Chem., Vol. 61, No. 20, 1996 7057
Sch em e 1. Syn th esis of th e Azoa lk a n es 9 a n d 10
a n d th e Resp ective Hou sa n es 11
The ∆D scale is based on the assumption that the D
parameter describes the magnetic dipole interaction of
two electronically independent radical centers in a triplet
state. A comparison of the ∆D values of the symmetrical
diradicals 1 with those of the unsymmetrical derivatives
2 should serve as an appropriate experimental test to
assess whether the radical centers in localized triplet
diradicals interact sufficiently weakly to be considered
as independent monoradicals. At first sight this may
seem a paradox, because it is this very interaction that
gives rise to the paramagnetic character of triplet states
and, thus, provides the opportunity to observe them
through EPR spectroscopy. However, it must be recog-
nized that such dipole interaction in triplet diradicals,
although spectroscopically significant, is extremely weak
(fraction of calories) compared to the energy changes
caused by the delocalization power of substituents in
monoradicals. The latter happenstance establishes the
general validity of the ∆D scale to quantify substituent
effects in benzyl-type radicals through the measurement
of the D parameter for 1,3-cyclopentanediyl triplet dirad-
icals.
We report herein the EPR results for the symmetrical
and unsymmetrical diradicals 1 and 2, for which, indeed,
the anticipated additivity in the ∆D values applies. It
is demonstrated in a detailed theoretical analysis that
the D parameter serves as a reliable probe of electronic
substituent effects through spin density changes. This
is confirmed through a correlation between the D pa-
rameters with known hyperfine coupling constants.
Ta ble 1. EP R D P a r a m eter sa a n d ∆D Va lu es of th e
Tr ip let Dir a d ica ls 1 a n d 2
∆D × 10-2
|D/hc| (cm-1
)
(cm-1
)
∆D × 10-2 (cm-1
2 (calculated)b
)
entry
X
1
2
1
2
1b
p-F
0.0521c
-0.17
-0.08
-0.02
0.00
0.01
0.05
0.14
0.27
0.28
0.47
Resu lts
1c/2c p-MeO
1a /2a
0.0509c 0.0509 -0.05
-0.03
0.00
0.01
0.04
0.13
0.27
0.29
0.52
H
0.0504c 0.0506
0.0502c 0.0505
0.0495c 0.0502
0.0476c 0.0493
0.00
0.02
1d /2d p-Me
1e/2e p-Cl
1f/2f p-NH2
The synthesis of the unsymmetrically substituted
azoalkanes (Scheme 1), the precursors of the triplet
diradicals 2, was conducted in analogy to the known
synthetic procedure for the symmetrical derivatives.5 The
exclusive products obtained on irradiation of the azoal-
kanes in CDCl3 solution were the corresponding housane
derivatives. For two azoalkanes, the precursors for the
diradicals 2f (p-NH2) and 2i (p-NO2), some unknown side
products (less than 10%) were detected in the NMR
spectra, which did not interfere in the analysis of the
diradical EPR signals.
The triplet diradicals 1 and 2 were generated by
irradiation of the corresponding azoalkanes with the UV
lines of an argon ion laser (333, 351, and 363 nm) in a
2-methyltetrahydrofuran (MTHF) matrix at 77 K. The
analysis of the Z signals in the EPR spectra afforded the
D parameters (Table 1), which are given by one-half of
the distance of separation between the low- and high-
field turning points; the E parameters were very small
(e0.002 cm-1). The data in Table 1 demonstrate that the
∆D parameters of the disubstituted derivatives 1 are
0.09
0.30d
0.53e
0.54
1g/2g p-CO2Me 0.0451c 0.0479
1h /2h p-CN
1i/2i p-NO2
0.0450 0.0477
0.0414c 0.0454
0.90
(0.91)d
a
EPR spectra were recorded at 77 K in a 2-methyltetrahydro-
furan matrix; the triplet diradicals 1 and 2 were generated by
direct irradiation of the corresponding azoalkanes with the 333-,
b
351-, and 363-nm lines of a CW argon ion laser. Calculated from
the symmetrically substituted triplet diradicals 1 according to eqs
1 and 6. c See ref 2a. Measured for the corresponding saturated
d
diradical, see ref 2b. e Calculated from the symmetrically substi-
tuted triplet diradical 1d and from the unsymmetrically substi-
tuted diradical (p-CO2Me/p-Me, D ) 0.0476 cm-1, ref 2b) according
to eq 6.
about twice as large as those for the monosubstituted
diradicals 2. Although the original set of EPR data
referred to the unsaturated derivatives 1,2a the hydro-
genation of the CdC double bond in the diradicals 2,
which was performed to eliminate the possibility of
regioisomers, has only a slight influence on the D
parameter as can be seen from the comparison of the D
values for 1a and 2a .
(4) (a) Wayner, D. D. M.; Arnold, D. R. Can. J . Chem. 1985, 63, 2378.
(b) Dust, J . M.; Arnold, D. R. J . Am. Chem. Soc. 1983, 105, 1221. (c)
Nicholas, A. M. de P.; Arnold, D. E. Can. J . Chem. 1986, 64, 270. (d)
Arnold, D. R.; Nicholas, A. M. de P.; Snow, M. S. Can. J . Chem. 1985,
63, 1150. (e) Fessenden, R. W.; Schuler, R. H. J . Chem. Phys. 1963,
39, 2147. (f) Korth, H. G.; Trill, H.; Sustmann, R. J . Am. Chem. Soc.
1981, 103, 4483. (g) Krusic, P. J .; Meakin, P.; Smart, B. E. J . Am.
Chem. Soc. 1974, 96, 6211. (h) Leroy, G.; Peeters, D.; Sana, M.; Wilante,
C. In Substituent Effects in Radical Chemistry; Viehe, H. G., J anousek,
Z., Mere´ny, R., Eds.; Reidel: Dordrecht, 1986; p 1.
(5) (a) Adam, W.; Harrer, H. M.; Nau, W. M.; Peters, K. J . Org.
Chem. 1994, 59, 3786. (b) Nau, W. M.; Harrer, H. M.; Adam, W. J .
Am. Chem. Soc. 1994, 116, 10972. (c) Beck, K.; Hu¨nig, S. Chem. Ber.
1987, 120, 477.
In a control experiment to assess matrix effects, the
EPR spectra of the triplet diradical 1i were recorded in
MTHF/acetonitrile (2:1) and MTHF/methanol (2:1) glasses.
In both matrices the D values (0.0414 cm-1) were the
same within experimental errors as for MTHF glass.
The EPR zero-field splitting D parameter of the nitro/
methoxy-substituted diradical 5 was determined to assess
the possible influence of donor/acceptor substitution. The
D parameter of 5 (0.0458 cm-1) falls between those of

7058 J . Org. Chem., Vol. 61, No. 20, 1996
Adam et al.
of anisotropy along the z direction is as well depreciable
in view of the large spatial separation of the spin centers
and the interacting dipoles may be regarded as point
charges. Indeed, the first term in eq 3 describes the
interaction of electrons which reside in the two given
localized MOs A and B as the origin of the D parameter.
Magnetic dipole interactions between spin-bearing carbon
centers of the particular MO A (or MO B) are not possible
due to the correlation effects of the two R spins. The
squares of the atomic orbital coefficients in the normal-
ized MOs A and B represent spin densities (F).1e In view
of the cubic distance dependence in the perturbation
term, the dominant contribution in the dipole interaction
derives from the two benzylic positions. Contributions
from other carbon atoms, i.e., between the benzylic carbon
atom of the MO A and the ortho carbon atom of the MO
B, are of minor importance. Thus, with this approxima-
tion, eq 3 transforms directly to eq 4, in which d is the
F igu r e 1. Delocalized π-type NBMOs a and b and the
localized NBMOs A and B of the benzyl radical fragments as
models for the 1,3-cyclopentanediyl diradicals 1.
the dinitro- and dimethoxy-substituted derivatives 1i
(0.0414 cm-1) and 1c (0.0509 cm-1).2
3µ0g2µ2B
16πd3
D )
F2
(4)
spatial distance and F is the spin density of the two
benzylic carbon atoms. This formula describes the D
parameter of localized diradicals, in which each radical
center bears delocalizing substituents. Equation 4, the
extended version of the usually employed eq 5,6 allows
to calculate besides interspin distances the spin densities
in localized triplet diradicals.
Discu ssion
Th eor etica l Con sid er a tion s. The zero-field splitting
parameter D of a triplet state originates from the
magnetic dipole interaction of the two uncoupled elec-
tronic spins (eq 2),1
3µ0g2µ2B
8πr3
2.604
r(Å)3
or D(cm-1) )
(5)
3µ0g2µ2B(r21,2 - 3z12,2
16πr51,2
)
D )
D ) Φk(1,2)
Φk(1,2)
(2)
|
|
Ad d itivity of th e Electr on ic Su bstitu en t Effects
in Loca lized 1,3-Dia r yl-Su bstitu ted 1,3-Cyclop en -
ta n ed iyl Tr ip let Dir a d ica ls. The electronic substitu-
ent effects on the D parameter in the localized triplet
diradicals 1 and 2 (Table 1) are the first of their kind
that have been measured.2 To assess their significance
and confirm their validity as a reliable measure of the
delocalizing propensity of substituents in such triplet
species, it is essential to correlate them with known
electronic effects in monoradicals. This condition neces-
sitates one to establish rigorously the initial premise that
the triplet diradicals 1 and 2 may be viewed as a
composite of two independently active (except for mag-
netic dipole interaction) benzyl-type (cumyl to be exact)
radicals in the rigid, planar 1,3-cyclopentanediyl molec-
ular framework. This premise implies that, for example,
in the unsymmetrical triplet diradicals 2 the electronic
substituent effect on the D parameter is about half that
of the symmetrical derivatives 1. Alternatively, the
change in the D parameter for the symmetrical triplet
diradicals 1 is the sum of the effects measured for the
corresponding monosubstituted unsymmetrical cases 2.
This anticipated additivity is, indeed, nicely corroborated
when the ∆D values for the symmetrically disubstituted
derivatives 1 are compared with the respective ones of
the unsymmetrically monosubstituted 2. A linear plot
(r2 ) 0.993) is obtained (Figure 2) with a slope m ) 0.558,
i.e., nearly one-half. Thus, the electronic effects on the
D parameter of localized triplet diradicals are additive
and this essential requisite allows us to infer the elec-
where
1
Φk(1,2) )
[a(1)b(2) - b(1)a(2)]
x
2
and
1
1
a )
(A + B) b )
(A - B)
x
2
x
2
which reside in the two fully delocalized π-type nonbond-
ing molecular orbitals (NBMO) a and b (Figure 1). Thus,
for the symmetrical diradicals 1, these electron-bearing
NBMOs are spread over both benzyl moieties. As to the
previously raised question, whether it is legitimate to
approximate the dipolar interaction of the two spins in
a localized triplet diradical by an interaction of two
independent benzyl monoradicals, these NBMOs are
transformed to the two localized benzyl-type MOs A and
B. The overlap integral SAB is nearly zero because of the
relative large spatial distance. Substitution of the two
delocalized MOs a and b by the two localized MOs A and
B in eq 2 affords eq 3, in which the second term is
3µ0g2µ2B
16πr31,2
D ) A2(1)
B2(2) -
|
|
3µ0g2µ2B
16πr31,2
A(1)B(1)
A(2)B(2) (3)
|
|
(6) Eaton, S. S.; More, K. M.; Sawant, B. M.; Eaton, G. R. J . Am.
Chem. Soc. 1983, 105, 6560.
negligible. The assumption is made that the contribution

Spin Densities in Para-Substituted Benzyl-Type Radicals
J . Org. Chem., Vol. 61, No. 20, 1996 7059
F igu r e 2. ∆D values of monosubstituted diradicals 2 versus
those of disubstituted diradicals 1.
F igu r e 3. D values of diradicals 1 versus hyperfine coupling
constants aâ of the corresponding cumyl monoradicals 4.
tronic properties of monoradicals (cumyl radicals in this
case) from the D parameter determined for the 1,3-
cyclopentanediyl triplet diradicals.
NBMO (the molecular orbitals which describe the un-
paired electron of the radical site). They are connected
to the experimental EPR hyperfine coupling constants
(a) through the McConnell equation, namely, F ) Qa,8
with the proportionality constants QR ) 23.04 G-1 and
Sp in Den sities in Tr ip let Dir a d ica ls a s a Mea su r e
of Electr on ic Su bstitu en t Effects. The general for-
mula (eq 4) reveals that the magnetic dipolar interaction
in triplet states depends on the spin density F (electronic
effects) and the distance d (geometrical effects) between
the radical sites. Since the distance d measures the
separation of the two radical carbon atoms, which is
expected to be constant in the series of localized diradi-
cals 1 and 2 (vide infra), only the F factor reflects the
electronic substituent effects on the D parameter. In
view of the spin-delocalizing properties of the phenyl
group, it is expected that the magnetic interaction will
be reduced in comparison to radical centers without
conjugating groups.3 For example, the D parameter of
Qâ ) 27.74 G-1 4h
This relation provides an opportunity
.
to test the spin density dependence of the D parameter
(eq 4) of the 1,3-cyclopentanediyl triplet diradicals 1 by
comparing them with the known experimental hyperfine
coupling constants aâ of the corresponding cumyl radicals.4d
The semiquadratic correlation (r2 ) 0.948) of the experi-
mental D parameter for diradicals 1 with the known
hyperfine coupling constants aâ of the corresponding
cumyl radicals 4 in Figure 3 is gratifying. An alternative
analysis is to employ a double-logarithmic plot in which
the slope indicates directly the exponent of a polynomial
(xn) functional dependence. Such a plot affords a slope
of 2.2, which is in good agreement with the theoretically
expected value of 2.
Although some deviation can be observed for substit-
uents with small delocalizing effects, the strongly delo-
calizing substituents (p-CN and p-CO2Me) correlate well.
Unfortunately, no aâ data are available for the p-NO2 and
p-NH2 substituents in the cumyl radicals and, conse-
quently, this correlation does not span the full range of
possible substituent effects. Inspection of the ∆D values
(Table 1) reveals that all para substituents, except p-MeO
and p-F, enhance the delocalization of the unpaired
electron into the aryl ring. For p-MeO and p-F, the
negative ∆D values signify a higher spin density at the
benzyl position than for the unsubstituted case (X ) H).
These substituents act as spin donors, which is cor-
roborated for the latter by hyperfine coupling EPR data
reported for the cumyl monoradical.4d Similar spin
localization is exercised by the spin donors p-OH and
p-OAc (∆D < 0).2c The spin density is significantly
reduced for the strongly conjugating p-NO2, p-CN,
p-CO2Me, and p-NH2 spin acceptors (∆D > 0). The
the parent 1,3-cyclopentanediyl triplet diradical (no
7a
phenyl substituents, D ) 0.084 cm-1
)
is much larger
than that of its 1,3-diphenyl-disubstituted derivative (D
) 0.045 cm-1).7b Furthermore, para substituents in the
phenyl groups, which are known4a-d to affect the spin
density at the benzylic positions through electronic
effects, are expected to alter the D parameter. Conse-
quently, for localized triplet diradicals, substituent effects
are additive, and with the same separation of the radical
sites (constant d in eq 4), the spin density F provides a
quantitative measure of electron delocalization effects by
substituents either directly bound or attached through
phenyl-bearing groups at the radical site. Spin-accepting
substituents will decrease the spin density, while spin
donors increase it. In other words, the ∆D scale (eq 1)
derived from the D parameter of 1,3-diaryl-substituted
1,3-cyclopentanediyl triplet diradicals 1 and 2 constitutes
a novel spectroscopic probe to assess substituent effects
in benzyl-type radicals.
Spin densities (F) are theoretical quantities, i.e., they
are defined as the sum of the squared coefficients in the
(7) (a) Buchwalter, S. L.; Closs, G. L. J . Am.Chem. Soc. 1975, 97,
3857. (b) Buchwalter, S. L.; Closs, G. L. Ibid. 1975, 101, 4688. (c) Coms,
F. D.; Dougherty, D. A. Tetrahedron Lett. 1988, 29, 3753. (d) Adam,
W.; Reinhard, G.; Platsch, H.; Wirz, J . J . Am. Chem. Soc. 1990, 112,
4570.
(8) (a) McConnell, H. M. J . Phys. Chem. 1956, 24, 764. (b) McCon-
nell, H. M.; Strathdee, J . Mol. Phys. 1959, 2, 129. (c) Fischer, H. In
Free Radicals; Kochi, J . K., Ed.; Wiley: New York, 1973; Vol. 2, p 435.

7060 J . Org. Chem., Vol. 61, No. 20, 1996
Adam et al.
Let us compare the spin density F calculated according
to eq 4 for the parent 1,3-diphenyl-1,3-cyclopentanediyl
triplet diradical 1a (X ) H) with those for the benzyl or
cumyl radical obtained by EPR hyperfine coupling con-
stants. For this comparison, the inter-radical distance
d is needed, which can be calculated by means of
semiempirical methods. We compute9 with the MNDO/
C10a (AM1)10b method distances of 2.44 (2.41) Å and obtain
a F value of 0.75 (0.74). From the pertinent experimental
data for the benzyl monoradical and from the McConnell
equation, the spin density is estimated as F ) 0.71 (aR )
16.25 G)4a and for the cumyl radical as F ) 0.59 (aâ
)
16.28 G).4a The latter value is smaller than that obtained
from the D parameter, which can be explained by the
fact that the dipolar interaction is not limited to only one
two-center interaction, namely, between the two benzylic
positions, an approximation that was made to derive eq
4. It is known that spin polarization effects induce R and
â spins on atoms adjacent to the radical-bearing centers.4e,8
An influence on the D parameter is only expected if these
spins are in the vicinity of both benzylic carbon atoms,
which is only the case for the gem-dimethyl bridge. The
(R/â) spin-spin interaction (with the carbon atom in R
position) lowers the D value, whereas the (R/R) spin-
spin interaction (with carbon atoms in â position) in-
creases the D value. Although these secondary effects
nearly cancel out, the (R/R) spin-spin may dominate
slightly. Nonetheless, the good correlations in Figures
3 and 4 demonstrate that the main two-center interaction
dominates and therefore the use of eq 4 to interprete
electronic substituent effects in terms of spin density
changes is justified.
F igu r e 4. D values of diradicals 3 versus hyperfine coupling
constants aR of the corresponding monoradicals.
decrease in the ∆D values is expected in view of the
delocalizing power of the substituents.4a-d
In this context the reported D parameters of the 1,3-
cyclobutanediyl triplet diradicals 33 are instructive, in
which the delocalizing substituents are directly connected
to the radical sites. Indeed, the hyperfine coupling
constants aR of the corresponding monoradicals (ethyl,4e
allyl4f,g and phenyl4b,c) comprise an excellent (r2 ) 0.990)
semiquadratic correlation with the D parameters of the
triplet species 3 (Figure 4).
Due to the additivity of substituent effects (indepen-
dent radical sites) and the substituent dependence of the
spin density F, it is now possible to predict the D
parameter of an unsymmetrically substituted diradical
(Dxy) if the two D parameters for symmetrically substi-
tuted diradicals (Dxx and Dyy) are known. For this
purpose eq 6 applies. Since the double bond in diradicals
The above results demonstrate that the D values reflect
reliably the electronic properties of substituents through
the spin density at the radical site in the triplet diradical,
a natural consequence of the dipolar interaction between
the unpaired electrons. Moreover, besides the EPR
hyperfine coupling constants (aR or aâ) as an experimental
measure of spin density in monoradicals, the D param-
eter offers this mechanistically valuable information in
triplet diradicals, provided the geometrical factor (d
dependence in eq 4) is kept constant. We assume that
all diradicals 1 and 2 possess the same geometry, which
is intuitively expected on the basis of the remote changes
in the aryl substituents. Although direct experimental
information on this requisite is difficult to acquire, the
fact that the same D parameter was obtained for the
diradical 1i in three different solvent matrices (MTHF,
acetonitrile, methanol) strongly suggests that environ-
mental changes through the matrix material have no
measurable influence. In particular, its polar and hy-
drogen-bonding properties do not play any major role in
determining the magnitude of the D parameter in such
triplet diradicals. Presumably, the similar geometry of
the diradicals 1 and 2 is dictated by the inherent
conformational preference of the triplet diradical for a
planar arrangement of the 1,3-cyclopentanediyl skeleton
and compensates the comparably small environmental
perturbations by the matrix material. Therefore, the
observed substituent effects on the D parameter are
electronic in origin and the ∆D scale2 constitutes an
experimental spectroscopic probe for electron delocaliza-
tion by substituents in localized triplet diradicals as well
as in the corresponding monoradicals.
3µ0g2µ2
16πd3
Dxy
)
BF F ) D D
(6)
x
x
y
xx yy
1 and 2 has only a slight influence on the D parameter,
the theoretically expected ∆D values of the unsym-
metrical diradicals 2 may be calculated from the set of
substituent-dependent D parameters of the symmetrical
diradicals 1 using eqs 1 and 6 (Table 1). A plot of the
experimental versus the calculated ∆D values for 2
displays an excellent linear correlation (r2 ) 0.995).
This analysis was extended to a diradical with a donor/
acceptor substitution pattern. The experimental D value
for the nitro/methoxy-substituted case 5 (0.0458 cm-1
)
was found to be in excellent agreement with the value
calculated from the symmetrical derivatives 1c and 1i
(0.0459 cm-1). This establishes the absence of any special
polar captodative effects (push-pull stabilization) in the
triplet diradicals of the 1,3-cyclopentanediyl-type. Of
course, it must be stressed that these triplet species are
localized, a necessary condition that the radical compo-
(9) Calculations were run on a Silicon Graphics Indigo workstation
employing the program VAMP 5.0. Rauhut, G.; Alex, A.; Chan-
drasekhar, J .; Steinke, T.; Clark, T. University of ErlangensNu¨rnberg,
1993.
(10) (a) Thiel, W. J . Am. Chem. Soc. 1981, 103, 1413. (b) Dewar, M.
J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J . P. J . Am. Chem. Soc.
1985, 107, 3902.

Spin Densities in Para-Substituted Benzyl-Type Radicals
J . Org. Chem., Vol. 61, No. 20, 1996 7061
poured onto 200 g of ice, and on addition of 15 mL of acetic
acid, dione 6 precipitated.
1-(4′-Meth oxyp h en yl)-3-p h en yl-1,3-p r op a n ed ion e (6c,
X ) p-MeO): 55%, pale yellow needles, mp 127-129 °C
(ethanol, lit.12,13 mp 130 °C).
1-(4′-Meth ylp h en yl)-3-p h en yl-1,3-p r op a n ed ion e (6d , X
) p-Me): 52%, colorless needles, mp 83 °C (ethanol, lit.12,14
mp 84 °C).
1-(4′-Ch lor op h en yl)-3-p h en yl-1,3-p r op a n ed ion e (6e, X
) p-Cl): 27%, colorless needles, mp 87-90 °C (ethanol, lit.12,13a
mp 87 °C).
nents are independent. In contrast, polar effects have a
strong influence on the D parameters of unsymetrically
substituted triplet carbenes.11
Con clu sion . The theoretical analysis of the magnetic
dipole interaction for triplet diradicals shows that the D
parameter is both a function of the spin densities (F) at
the radical sites and the inter-radical distance (d). Since
for the localized 1,3-cyclopentanediyl triplet diradicals the
distance factor d is constant, the D parameter is a
sensitive measure of electronic substituent effects through
dependence on the spin density factor F. The substituent
effects are additive, which implies that the electronic
effects exerted on each radical site act independent of
one another. Thus, valuable information on the delocal-
izing power of substituents in benzyl-type monoradicals
are obtained from the D parameters measured for the
1,3-diaryl-substituted 1,3-cyclopentanediyl triplet diradi-
cals 1 and 2. The fact that the D parameter of these
localized species correlates well with the known hyperfine
coupling constants of the benzyl-type monoradicals es-
tablishes that electronic substituent effects are monitored
by spin density (F) changes. Thus, a new powerful
spectroscopic method has become available to assess
quantitatively electronic substituent effects in radicals.
1-(4′-Ca r b om et h oxyp h en yl)-3-p h en yl-1,3-p r op a n ed i-
on e (6g, X ) p-CO2Me): 56%, pale yellow needles, mp 123-
125 °C (toluene). IR (KBr): ν ) 3040 cm-1, 2950, 1720, 1590,
1525, 1430, 1280, 1110, 750, 680. 1H NMR (CDCl3): δ ) 3.96
3
(s, 3H), 6.89 (s, 1H), 7.46-7.63 (m, 3H), 8.01 (dd, J ) 7.9 Hz,
3
3
4J ) 1.8 Hz, 2H), 8.04 (d, J ) 8.7 Hz, 2H), 8.15 (d, J ) 8.7
Hz, 2H), 16.77 (s, 1H). 13C NMR (CDCl3): δ ) 52.4 (q), 93.8
(d), 127.0 (d), 127.3 (d), 128.7 (d), 129.8 (d), 132.8 (d), 133.2
(s), 135.4 (s), 139.2 (s), 166.3 (s), 183.4 (s), 187.1 (s). Anal.
Calcd for C17H14O4 (282.3): C, 72.33; H, 5.00. Found: C, 72.35;
H, 4.93.
1-(4′-Cya n op h en yl)-3-p h en yl-1,3-p r op a n ed ion e (6h , X
) p-CN): 40%, yellow needles, mp 152-154 °C (ethanol). IR
(KBr): ν ) 3070 cm-1, 3040, 2210, 1580, 1535, 1280, 1215,
850, 760, 680. 1H NMR (CDCl3): 6.87 (s, 1H), 7.47-7.64 (m,
3
3
4
3H), 7.79 (d, J ) 6.8 Hz, 2H), 8.00 (dd, J ) 6.3 Hz, J )1.5
Hz, 2H), 8.07 (d, 3J ) 6.8 Hz, 2H), 16.69 (s, 1H). 13C NMR
(CDCl3): δ ) 93.9 (d), 115.4 (s), 118.1 (s), 127.3 (d), 127.5 (d),
128.8 (d, 132.4 (d), 133.0 (d), 135.1 (s), 139.3 (s), 182.1 (s), 187.5
(s). Anal. Calcd for C16H11NO2 (249.3): C, 77.10; H, 4.45; N,
5.62. Found: C, 77.06; H, 4.68; N, 5.42.
Exp er im en ta l Section
1-(4′-Nitr op h en yl)-3-p h en yl-1,3-p r op a n ed ion e (6i, X )
p-NO2): 38%, yellow needles, mp > 200 °C (lit.15 mp 160 °C).
The product was purified by being washed with chloroform
until the wash solution was colorless. Despite the different
melting points, NMR spectral data unambiguously established
sample identity. 1H NMR (d6-acetone): δ ) 6.51 (s, 1H), 7.34-
7.37 (m, 3H), 7.94 (dd, J ) 7.3 Hz, J ) 2.4 Hz, 2H), 8.14 (d,
3J ) 9.2 Hz, 2H), 8.20 (d, 3J ) 9.2 Hz, 2H). The enolic
hydrogen was not recorded (expected at 16.6 ppm).
Gen er a l P r oced u r e for th e P r ep a r a tion of th e 1-Ar yl-
2,2-d im eth yl-3-p h en yl-1,3-p r op a n ed ion e (7). The 2,2-di-
methyl-substituted diones 7 were prepared in analogy to the
literature procedure.5 The products were purified by recrys-
tallization.
Gen er a l Asp ects. NMR spectra were recorded on a Bruker
AC 200 with CDCl3 as solvent and internal standard. Infrared
spectra were measured on a Perkin-Elmer 1420 ratio recording
infrared (values in cm-1), and UV absorption spectra were
aquired on a Hitachi U 3200 spectrophotometer. Melting
points were taken on a Reichert Thermovar Kofler apparatus
and are not corrected. Combustion analyses were performed
by the Microanalytical Division of the Institute of Inorganic
Chemistry, University of Wu¨rzburg. Solvents and commer-
cially available chemicals were purified by standard proce-
dures or used as purchased. Column chromatography was
carried out on silica gel (0.032-0.063 mm, Woelm). NMR
assignments were made on the basis of known spectral data;
the specific assignements of the NMR spectra of the regioiso-
mers 9 and 9′ are based on the NMR data for the sym-
metrically substituted derivatives.5 1H NMR coupling con-
stants J HH are given with an accuracy of 0.2 Hz, and 13C NMR
multiplicities are derived from DEPT spectra.
EP R Sp ectr oscop y. The particular azoalkanes (ca. 5 ×
10-4 mmol) were dissolved in 0.5 mL of 2-methyltetrahydro-
furan and placed into an EPR sample tube. After the solution
was degassed by purging with argon gas, the sample was
sealed and the glass was prepared by freezing the sample with
liquid nitrogen. Subsequent irradiation with all UV lines (333,
351, 364 nm) of the CW argon ion laser (INNOVA 100,
Coherent Company, widened beam, 1.0 W, 2 min) at 77 K
afforded the persistent triplet diradicals 1 and 2, whose EPR
spectra were recorded (spectral accumulation by Bruker Data-
System 1620, n g 5) on a Bruker ESP-300 spectrometer. The
D values were determined by analysis of the Z signals.6
3
4
2,2-Dim et h yl-1-(4′-m et h oxyp h en yl)-3-p h en yl-1,3-p r o-
p a n ed ion e (7c, X ) p-MeO): 50%, colorless needles, mp
110-111 °C (methanol). IR (KBr): ν ) 3040 cm-1, 2970, 2900,
1660, 1630, 1580, 1250, 1160, 1010, 830. 1H NMR (CDCl3): δ
) 1.64 (s, 6H), 3.76 (s, 3H), 6.77 (d, 3J ) 9.0 Hz, 2H), 7.30 (dd,
3
3
4
3J ) 7.3 Hz, J ) 7.0 Hz, 2H), 7.41 (dt, J ) 7.3 Hz, J ) 1.3
3
3
4
Hz, 1H), 7.83 (d, J ) 9.0 Hz, 2H), 7.84 (dd, J ) 7.0 Hz, J )
1.3 Hz, 2H). 13C NMR (CDCl3): δ ) 25.5 (q), 55.3 (s), 59.2 (q),
113.8 (d), 128.5 (d), 128.5 (s), 129.2 (d), 131.6 (d), 132.9 (d),
135.5 (s), 163.2 (s), 198.6 (s), 200.7 (s). Anal. Calcd for
C18H18O3 (282.3): C, 76.57; H, 6.43. Found: C, 76.16; H, 6.61.
2,2-Dim e t h yl-1-(4′-m e t h ylp h e n yl)-3-p h e n yl-1,3-p r o-
p a n ed ion e (7d , X ) p-Me): 44%, colorless needles, mp 124-
125 °C (methanol, lit.16 mp 115 °C). Despite the higher melting
points, 1H NMR spectral15 data established sample identity.
1-(4′-Ch lor op h e n yl)-2,2-d im e t h yl-3-p h e n yl-1,3-p r o-
p a n ed ion e (7e, X ) p-Cl): 31%, colorless needles, mp 105-
107 °C (methanol). 1H NMR (CDCl3):17 δ ) 1.65 (s, 6H), 7.28
P r ep a r a tion of th e 1-Ar yl-3-p h en yl-1,3-p r op a n ed ion es
6. Diones 6c-e,g were synthesized in analogy to the literature
procedure5 and purified by recrystallization. To obtain deriva-
tives 6h and 6i, 100 mmol of acetophenone and 100 mmol of
sodium amide were added to 200 mL of dried diethyl ether,
and the formed ammonia was removed by means of a constant
nitrogen gas flow for ca. 4 h (monitored by pH indicator). The
corresponding para-substituted benzoyl chloride (30 mmol)
was added slowly, and the mixture was stirred for 30 min at
ambient temperature (ca. 25 °C). The reaction mixture was
3
3
3
(d, J ) 7.1 Hz, 2H), 7.32 (dd, J ) 6.1 Hz, J ) 5.9 Hz, 2H),
3
4
3
7.44 (tt, J ) 5.9 Hz, J ) 2.2 Hz, 1H), 7.77 (d, J ) 7.1 Hz,
2H), 7.81 (dd, J ) 6.1 Hz, J ) 2.2 Hz, 2H).
3
4
(12) (a) Ishihara, H.; Hirabayashi, Y. Chem. Lett. 1978, 1007. (b)
Bratan, S.; Strohbusch, F. Chem. Ber. 1972, 105, 2284.
(13) (a) Bradley, W.; Robinson, R. J . Chem. Soc. 1926, 23, 2356. (b)
Choshi, T.; Horimoto, S.; Wang, C. Y.; Nagase, H.; Ichikawa, M.;
Sugino, E.; Hibino, S. Chem. Pharm. Bull. 1992, 40, 1047.
(14) Weygand, C. Liebigs Ann. Chem. 1929, 472, 143.
(15) Linn, B. O.; Hauser, C. R. J . Am. Chem. Soc. 1956, 78, 6066.
(16) Barluenga, J .; J ardon, J .; Gotor, V. J . Chem. Res. 1986, 218.
(17) Saitkulova, F. G.; Abashev, G. G.; Kadyrmatova, T. P.; Lapkin,
I. I. Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Technol. 1977, 20, 669.
(11) Humphreys, R. W. R.; Arnold, D. R. Can. J . Chem. 1979, 57,
2652.

7062 J . Org. Chem., Vol. 61, No. 20, 1996
Adam et al.
1-(4′-Ca r bom eth oxyp h en yl)-2,2-d im eth yl-3-p h en yl-1,3-
p r op a n ed ion e (7g, X ) p-CO2Me): 18%, colorless needles,
mp 163-164 °C (methanol). IR (KBr): ν ) 3060 cm-1, 2980,
2930, 1710, 1640, 1425, 1270, 1100, 940, 700. 1H NMR
(CDCl3): δ ) 1.71 (s, 6H), 3.95 (s, 3H,), 7.47-7.54 (m, 3H),
3
4
8.08 (dd, J ) 5.4 Hz, J ) 3.0 Hz, 2H), 8.14 (s, 2H), 8.14 (s,
2H). 13C NMR (CDCl3): δ ) 22.6 (q), 52.4 (q), 58.8 (s), 127.8
(d), 128.0 (d), 128.9 (d), 129.7 (s), 129.9 (d), 131.1 (d), 131.8
(s), 134.0 (s), 166.4 (s), 178.2 (s), 179.5 (s). Anal. Calcd for
C19H18N2O2 (306.4): C, 74.49; H, 5.92; N, 9.14. Found: C,
74.27; H, 5.87; N, 9.13.
3
(CDCl3): δ ) 1.67 (s, 6H), 3.88 (s, 3H), 7.31 (dd, J ) 7.0 Hz,
3
4
3J ) 6.2 Hz, 2H), 7.43 (tt, J ) 6.2 Hz, J ) 1.4 Hz, 1H), 7.81
3
4
3
(dd, J ) 7.0 Hz, J ) 1.4 Hz, 2H), 7.86 (d, J ) 8.8 Hz, 2H),
7.96 (d, 3J ) 8.8 Hz, 2H). 13C NMR (CDCl3): δ ) 25.2 (q),
52.4 (q), 59.6 (s), 128.7 (d), 129.0 (d), 129.1 (d), 129.7 (d), 133.1
(d), 133.6 (s), 135.4 (s), 138.3 (s), 165.9 (s), 199.8 (s), 199.9 (s).
Anal. Calcd for C19H18O4 (310.3): C, 73.53; H, 5.85. Found:
C, 73.17; H, 5.67.
3-(4′-Cya n op h e n yl)-4,4-d im e t h yl-5-p h e n yl-4H -p yr a -
zole (8h , X ) p-CN): 85%, yellow needles, mp 184-185 °C
(cyclohexane). IR (KBr): ν ) 2960 cm-1, 2920, 2210, 1590,
1510, 1485, 1450, 1340, 1000, 835, 780, 700. 1H NMR
(CDCl3): δ ) 1.71 (s, 6H), 7.48-7.55 (m, 3H), 7.79 (d, 3J ) 8.6
3
4
3
1-(4′-Cya n op h e n yl)-2,2-d im e t h yl-3-p h e n yl-1,3-p r o-
p a n ed ion e (7h , X ) p-CN): 38%, colorless needles, mp 83-
85 °C (methanol). IR (KBr): ν ) 3050 cm-1, 2970, 2210, 1660,
1645, 1375, 1250, 1230, 940. 1H NMR (CDCl3): δ ) 1.67 (s,
Hz, 2H), 8.08 (dd, J ) 5.5 Hz, J ) 2.2 Hz, 2H), 8.19 (d, J )
8.6 Hz, 2H). 13C NMR (CDCl3): δ ) 22.5 (q), 58.6 (s), 114.0
(s), 118.2 (s), 128.0 (d), 128.2 (d), 128.9 (d), 129.3 (s), 131.2
(d), 132.5 (d), 133.9 (s), 177.3 (s), 179.9 (s). Anal. Calcd for
C18H15N3 (273.3): C, 79.10; H, 5.53; N, 15.37. Found: C, 78.84;
H, 5.84; N, 15.23.
3
3
3
6H), 7.33 (dd, J ) 7.7 Hz, J ) 7.3 Hz, 2H), 7.46 (tt, J ) 7.3
4
3
3
Hz, J ) 1.4 Hz, 1H), 7.61 (d, J ) 8.4 Hz, 2H), 7.80 (dd, J )
7.7 Hz, J ) 1.4 Hz, 2H), 7.90 (d, J ) 8.4 Hz, 2H). 13C NMR
(CDCl3): δ ) 25.0 (q), 59.6 (s), 116.2 (s), 117.6 (s), 128.7 (d),
129.0 (d), 129.4 (d), 132.4 (d), 133.3 (d), 135.2 (s), 138.4 (s) 199.0
(s), 199.4 (s). Anal. Calcd for C18H15NO2 (277.3): C, 77.96;
H, 5.45; N, 5.05. Found: C, 77.79; H, 5.33; N, 4.98.
4
3
4,4-Dim e t h yl-3-(4′-n it r op h e n yl)-5-p h e n yl-4H -p yr a -
zole (8i, X ) p-NO2): 51%, yellow needles, mp 237-238 °C.
IR (KBr): ν ) 3100 cm-1, 2970, 1585, 1520, 1505, 1330, 1100,
995, 850, 775, 700. 1H NMR (CDCl3): δ ) 1.72 (s, 6H), 7.49-
3
4
7.53 (m, 3H), 8.08 (dd, J ) 5.5 Hz, J ) 3.1 Hz, 2H), 8.25 (d,
3J ) 9.1 Hz, 2H), 8.34 (d, 3J ) 9.1 Hz, 2H). 13C NMR (CDCl3):
δ ) 22.5 (q), 58.8 (s), 123.9 (d), 128.1 (d), 128.6 (d), 128.9 (d),
129.4 (s), 131.4 (d), 135.8 (s), 148.8 (s), 177.1 (s), 180.0 (s). Anal.
Calcd for C17H15N3O2 (293.3): C, 69.61; H, 5.15; N, 14.33.
Found: C, 69.82; H, 5.11; N, 14.34.
2,2-Dim eth yl-1-(4′-n itr oph en yl)-3-ph en yl-1,3-pr opan edi-
on e (7i, X ) p-NO2): 27%, pale yellow needles, mp 140-141
°C (methanol). IR (KBr): ν ) 3060 cm-1, 2980, 2920, 1650,
1520, 1340, 1250, 1220, 940, 700. 1H NMR (CDCl3): δ ) 1.69
3
3
3
(s, 6H), 7.33 (dd, J ) 7.3 Hz, J ) 7.0 Hz, 2H), 7.46 (tt, J )
4
3
4
7.3 Hz, J ) 1.4 Hz, 1H), 7.81 (dd, J ) 7.0 Hz, J ) 1.4 Hz,
Gen er a l P r oced u r e for th e P r ep a r a tion of th e Azoa l-
k a n es 9. The azoalkanes were prepared in analogy to the
literature procedure5 and were purified by column chroma-
tography. The eluated azoalkane solution had to be filtered
through solid potassium carbonate to remove traces of acids,
since the latter catalyzes retro-Diels-Alder reactions. In all
cases a mixture of two regioisomeres (ca. 1:1) were obtained.
2H), 7.97 (d, J ) 8.1 Hz, 2H), 8.15 (d, J ) 8.1 Hz, 2H). 13C
NMR (CDCl3): δ ) 25.1 (q), 59.8 (s), 123.8 (d), 128.9 (d), 129.1
(d), 130.1 (d), 133.5 (d), 135.3 (s), 140.0 (s), 149.9 (s), 198.9 (s),
199.4 (s). Anal. Calcd for C17H15NO4 (297.3): C, 68.68; H,
5.09; N, 4.71. Found: C, 68.64; H, 4.86; N, 4.53.
3
3
Gen er a l P r oced u r e for th e P r ep a r a tion of th e 3-Ar yl-
4,4-d im eth yl-5-p h en ylisop yr a zole (8). The pyrazoles 8
were prepared in analogy to the literature procedure.5 The
products were purified by recrystallization. The derivatives
8h and 8i required heating at 130 °C and 0.5 Torr for 3 h to
quantitatively remove water.
(1a ,4a ,4a r,7a r)-4,4a ,7,7a -Tet r a h yd r o-8,8-d im et h yl-1-
(4′-m eth oxyp h en yl)-4-p h en yl- [9c, X ) p-MeO, Y ) H] a n d
(1a ,4a ,4a r,7a r)-4,4a ,7,7a -Tet r a h yd r o-8,8-d im et h yl-4-(4′-
m eth oxyph en yl)-1-ph en yl-1,4-m eth an o-1H-cyclopen ta[d]-
p yr id a zin e [9c′, X ) H, Y ) p-MeO]: 40%, colorless needles,
mp 156-157 °C dec, Rf ) 0.10 (silica gel, methylene chloride).
IR (KBr): ν ) 3020 cm-1, 2900, 1595, 1560, 1500, 1440, 1420,
1290, 1240, 1170, 1025, 1010, 810, 790. UV (benzene): λmax
(ꢀ) 329 nm (sh, 26), 352 (sh, 97), 361 (170). Anal. Calcd for
C23H24N2O (344.5): C, 80.20; H, 7.02; N, 8.13. Found: C,
79.78; H, 6.80; N, 8.00.
4,4-Dim eth yl-3-(4′-m eth oxyp h en yl)-5-p h en yl-4H-p yr a -
zole (8c, X ) p-MeO): 55%, colorless needles, mp 154-155
°C (cyclohexane). IR (KBr): ν ) 3045 cm-1, 2960, 1590, 1505,
1495, 1405, 1245, 1175, 1025, 825. 1H NMR (CDCl3): δ ) 1.69
3
(s, 6H), 3.87 (s, 3H), 7.01 (d, J ) 9.1 Hz, 2H), 7.45-7.52 (m,
3H), 8.04-8.10 (m, 4H). 13C NMR (CDCl3): δ ) 23.0 (q), 55.4
(q), 58.2 (s), 114.2 (d), 122.4 (s), 127.7 (d), 128.7 (d), 129.5 (d),
130.1 (s), 130.5 (d), 161.6 (s), 178.3 (s), 178.4 (s). Anal. Calcd
for C18H18N2O (278.4): C, 77.67; H, 6.52; N, 10.06. Found:
C, 77.37; H, 6.43; N, 9.86.
Isom er 9c. 1H NMR (CDCl3): δ ) 0.20 (s, 3H), 0.99 (s, 3H),
2.21 (d, 3J ) 7.3 Hz, 2H), 3.64 (ddd, 3J ) 3J ) 7.3 Hz, 3J ) 4.7
3
Hz, 1H), 3.87 (s, 3H), 4.06 (d, J ) 4.7 Hz, 1H), 5.50 (s, 2H,),
3
3
7.04 (d, J ) 8.8 Hz, 2H), 7.40-7.54 (m, 3H), 7.71 (d, J ) 8.8
Hz, 2H), 7.77 (dd, 3J ) 8.2 Hz, 3J ) 1.5 Hz, 2H). 13C NMR
(CDCl3): δ ) 17.0 (q), 17.3 (q), 31.6 (t), 43.1 (d), 55.2 (s), 56.6
(d), 64.0 (s), 96.6 (s), 97.7 (s), 113.7 (d), 127.1 (d), 127.2 (d),
127.8 (d), 127.8 (s), 128.3 (d), 128.7 (d), 133.4 (d), 135.9 (s),
159.1 (s).
4,4-Dim et h yl-3-(4′-m et h ylp h en yl)-5-p h en yl-4H -p yr a -
zole (8d , X ) p-Me): 75%, colorless needles, mp 135-136 °C
(cyclohexane). IR (KBr): ν ) 3045 cm-1, 2950, 1595, 1510,
1490, 1440, 1340, 1150, 1000, 820, 770, 690. 1H NMR
3
(CDCl3): δ ) 1.70 (s, 6H), 2.43 (s, 3H), 7.30 (d, J ) 8.5 Hz,
3
2H), 7.46-7.52 (m, 3H), 7.98 (d, 3J ) 8.5 Hz, 2H), 8.06 (dd, J
Isom er 9c′. 1H NMR (CDCl3): δ ) 0.20 (s, 3H), 0.99 (s,
4
) 5.6 Hz, J ) 3.0 Hz, 2H). 13C NMR (CDCl3): δ ) 21.4 (q),
3
3
3
3H), 2.22 (d, J ) 7.0 Hz, 2H), 3.60 (ddd, J ) 3J ) 7.0 Hz, J
22.9 (q), 58.5 (s), 127.1 (s), 127.8 (s), 127.8 (s), 128.7 (d), 129.5
(d), 130.1 (s), 130.6 (d), 141.2 (s), 178.7 (s), 178.9 (s). Anal.
Calcd for C18H18N2 (262.4): C, 82.41; H, 6.92; N, 10.68.
Found: C, 82.32; H, 6.61; N, 10.53.
3
) 4.7 Hz, 1H), 3.87 (s, 3H), 4.10 (d, J ) 4.7 Hz, 1H), 5.50 (s,
3
3
2H), 7.03 (d, J ) 8.8 Hz, 2H), 7.40-7.54 (m, 3H), 7.69 (d, J
) 8.8 Hz, 2H), 7.79 (dd, 3J ) 8.2 Hz, 3J ) 1.5 Hz, 2H). 13C
NMR (CDCl3): δ ) 17.0 (q), 17.3 (q), 31.6 (t), 43.0 (d), 55.2 (s),
56.6 (d), 64.0 (s), 96.6 (s), 97.7 (s), 113.7 (d), 127.2 (d), 127.5
(d), 127.7 (d), 127.9 (s), 128.3 (d), 128.3 (d), 133.5 (d), 135.9
(s), 159.2 (s).
3-(4′-Ch lor op h en yl)-4,4-d im et h yl-5-p h en yl-4H -p yr a -
zole (8e, X ) p-Cl): 51%, colorless needles, mp 155-156 °C
(cyclohexane). IR (KBr): ν ) 2950 cm-1, 2910, 1575, 1510,
1475, 1450, 1380, 1330, 1080, 995, 815, 770, 695. 1H NMR
(CDCl3): δ ) 1.69 (s, 6H), 7.47 (d, 3J ) 8.9 Hz, 2H), 7.47-
(1a ,4a ,4a r,7a r)-4,4a ,7,7a -Tet r a h yd r o-8,8-d im et h yl-1-
(4′-m eth ylp h en yl)-4-p h en yl- [9d , X ) p-Me, Y ) H] a n d
(1a,4a,4ar,7ar)-4,4a,7,7a-Tetr ah ydr o-8,8-dim eth yl-1-ph en -
yl-4-(4′-m eth ylp h en yl)-1,4-m eth a n o-1H-cyclop en ta [d ]p y-
r id a zin e [9d ′, X ) H, Y ) p-Me]: 61%, colorless needles, mp
145-147 °C dec, Rf ) 0.23 (silica gel, methylene chloride). IR
(KBr): ν ) 3010 cm-1, 2930, 2900, 1575, 1500, 1495, 1450,
1430, 1360, 1290, 1010, 805, 750, 730, 715, 695. UV (ben-
zene): λmax (ꢀ) 329 nm (sh, 45), 350 (sh, 107), 361 (179). Anal.
Calcd for C23H24N2 (328.5): C, 84.11; H, 7.36; N, 8.53.
Found: C, 83.70; H, 7.78; N, 8.21.
3
3
7.53 (m, 3H), 8.02 (d, J ) 8.9 Hz, 2H), 8.07 (dd, J ) 5.5 Hz,
4J ) 3.0 Hz, 2H). 13C NMR (CDCl3): δ ) 22.7 (q), 58.6 (s),
127.7 (d), 128.4 (s), 128.8 (d), 129.1 (d), 129.1 (d), 129.6 (s),
130.9 (d), 137.0 (s), 178.0 (s), 179.1 (s). Anal. Calcd for
C17H15N2Cl (282.8): C, 72.21; H, 5.35; N, 9.91. Found: C,
71.97; H, 5.42; N, 9.91.
3-(4′-Ca r bom eth oxyp h en yl)-4,4-d im eth yl-5-p h en yl-4H-
p yr a zole (8g, X ) p-CO2Me): 61%, colorless needles, mp
181-182 °C (cyclohexane). IR (KBr): ν ) 3040 cm-1, 2980,
2930, 1710, 1595, 1510, 1430, 1270, 1100, 770, 755. 1H NMR

Spin Densities in Para-Substituted Benzyl-Type Radicals
Isom er 9d . 1H NMR (CDCl3): δ ) 0.21 (s, 3H), 1.01 (s,
J . Org. Chem., Vol. 61, No. 20, 1996 7063
d im e t h y l-1-p h e n y l-1,4-m e t h a n o -1H -c y c lo p e n t a [d ]-
p yr id a zin e [9h ′, X ) H, Y ) p-CN ]: 84%, colorless needles,
mp 160-161 °C dec, Rf ) 0.12 (silica gel, methylene chloride).
IR (KBr): ν ) 3040 cm-1, 2960, 2930, 2210, 1595, 1490, 1455,
1365, 1010, 820, 760, 740, 730, 700. UV (benzene): λmax (ꢀ)
329 nm (sh, 33), 350 (sh, 101), 360 (177). Anal. Calcd for
C23H21N3 (339.4): C, 81.39; H, 6.24; N, 12.38. Found: C, 81.46;
H, 6.45; N, 11.92.
3
3
3H), 2.22 (d, J ) 7.8 Hz, 2H), 2.43 (s, 3H), 3.63 (ddd, J ) 3J
3
3
) 7.8 Hz, J ) 1.8 Hz, 1H), 4.08 (d, J ) 1.8 Hz, 1H), 5.51 (s,
3
3
2H), 7.32 (d, J ) 8.3 Hz, 2H), 7.40-7.55 (m, 3H), 7.69 (d, J
) 8.3 Hz, 2H), 7.78 (dd, 3J ) 8.2 Hz, 4J ) 1.5 Hz, 2H). 13C
NMR (CDCl3): δ ) 17.0 (q), 17.3 (q), 21.2 (s), 31.6 (t), 43.0 (d),
56.6 (d), 64.0 (s), 96.8 (s), 97.8 (s), 127.1 (d), 127.2 (d), 127.5
(d), 128.3 (d), 128.7 (d), 129.0 (d), 132.7 (s), 133.1 (d), 135.9
(s), 137.4 (s).
Isom er 9h . 1H NMR (CDCl3): δ ) 0.19 (s, 3H), 1.03 (s, 3H),
3
3
3
Isom er 9d ′. 1H NMR (CDCl3): δ ) 0.21 (s, 3H), 1.01 (s,
2.15-2.26 (m, 2H), 3.69 (ddd, J ) J ) 8.6 Hz, J ) 6.4 Hz,
1H), 4.03-4.17 (m, 1H), 5.50 (s, 2H), 7.39-7.55 (m, 3H), 7.74
3H), 2.23 (d, J ) 7.4 Hz, 2H), 2.43 (s, 3H), 3.67 (ddd, J ) 3J
3
3
3
3
3
4
3
) 7.4 Hz, J ) 1.8 Hz, 1H), 4.12 (d, J ) 1.8 Hz, 1H), 5.51 (s,
(dd, J ) 7.1 Hz, J ) 1.4 Hz, 2H), 7.80 (d, J ) 8.7 Hz, 2H),
7.94 (d, 3J ) 8.7 Hz, 2H). 13C NMR (CDCl3): δ ) 16.9 (q),
17.3 (q), 31.4 (t), 43.4 (d), 57.0 (d), 64.6 (s), 96.1 (s), 97.4 (s),
111.7 (s), 118.7 (s), 127.0 (d), 127.1 (d), 128.0 (d), 128.2 (d),
128.5 (d), 132.2 (d), 133.4 (d), 135.0 (s), 141.4 (s).
3
3
2H), 7.31 (d, J ) 8.3 Hz, 2H), 7.40-7.55 (m, 3H), 7.67 (d, J
) 8.3 Hz, 2H), 7.80 (dd, 3J ) 8.2 Hz, 4J ) 1.5 Hz, 2H). 13C
NMR (CDCl3): δ ) 17.0 (q), 17.3 (q), 21.2 (s), 31.6 (t), 43.0 (d),
56.6 (d), 64.0 (s), 96.7 (s), 97.9 (s), 127.1 (d), 127.1 (d), 127.6
(d), 128.3 (d), 128.8 (d), 129.0 (d), 132.7 (s), 133.4 (d), 135.9
(s), 137.5 (s).
Isom er 9h ′. 1H NMR (CDCl3): δ ) 0.19 (s, 3H), 1.03 (s,
3
3
3H), 2.15-2.26 (m, 2H), 3.62 (ddd, J ) 9.2 Hz, J ) 8.6 Hz,
3
(1a,4a,4ar,7ar)-4,4a,7,7a-Tetr ah ydr o-1-(4′-ch lor oph en yl)-
8,8-d im eth yl-4-p h en yl- [9e, X ) p-Cl, Y ) H] a n d (1a ,4a ,-
4a r,7a r)-4,4a ,7,7a -Tet r a h yd r o-4-(4′-ch lor op h en yl)-8,8-
d im e t h y l-1-p h e n y l-1,4-m e t h a n o -1H -c y c lo p e n t a [d ]-
p yr id a zin e [9e′, X ) H, Y ) p-Cl]: 42%, colorless needles,
mp 149-150 °C dec, Rf ) 0.18 (silica gel, 3:2 methylene
chloride:petroleum ether). IR (KBr): ν ) 3020 cm-1, 2940,
2900, 1575, 1480, 1450, 1360, 1290, 1080, 1000, 810, 750, 720,
695. UV (benzene): λmax (ꢀ) 328 nm (sh, 26), 350 (sh, 93), 361
(173). Anal. Calcd for C22H21N2Cl (348.9): C, 75.74; H, 6.07;
N, 8.03. Found: C, 75.29; H, 6.04; N, 7.70.
3J ) 5.3 Hz, 1H), 4.03-4.17 (m, 1H), 5.43 (ddd, J ) 5.8 Hz,
3J ) 4.3 Hz, 4J ) 2.2 Hz, 1H), 5.50 (ddd, 3J ) 5.9 Hz, 3J ) 4.2
Hz, 3J ) 2.2 Hz, 1H), 7.39-7.55 (m, 3H), 7.76 (dd, 3J ) 6.6
4
3
3
Hz, J ) 1.4 Hz, 2H), 7.79 (d, J ) 8.7 Hz, 2H), 7.92 (d, J )
8.7 Hz, 2H). 13C NMR (CDCl3): δ ) 16.9 (q), 17.3 (q), 31.5 (t),
43.1 (d), 56.7 (d), 64.6 (s), 97.1 (s), 98.5 (s), 111.8 (s), 118.7 (s),
126.3 (d), 127.5 (d), 127.9 (d), 128.1 (d), 128.5 (d), 132.2 (d),
134.3 (d), 135.0 (s), 141.5 (s).
(1a ,4a ,4a r,7a r)-4,4a ,7,7a -Tet r a h yd r o-8,8-d im et h yl-1-
(4′-n it r op h en yl)-4-p h en yl- [9i, X ) p -NO2, Y ) H] a n d
(1a,4a,4ar,7ar)-4,4a,7,7a-Tetr ah ydr o-8,8-dim eth yl-1-ph en -
yl-4-(4′-n it r op h en yl)-1,4-m et h a n o-1H -cyclop en t a [d ]p y-
r id a zin e [9i′, X ) H, Y ) p-NO2 ]: 57%, colorless needles,
mp 162-163 °C dec, Rf ) 0.21 (silica gel, methylene chloride).
IR (KBr): ν ) 3020 cm-1, 2940, 2890, 2820, 1580, 1505, 1480,
1340, 1310, 1285, 1090, 1005, 690. UV (benzene): λmax (ꢀ) 360
(sh, 440). Anal. Calcd for C22H21N3O2 (359.4): C, 73.52; H,
5.89; N, 11.69. Found; C, 73.78; H, 5.85; N, 11.26.
Isom er 9e. 1H NMR: δ ) 0.19 (s, 3H), 1.01 (s, 3H), 2.21
3
3
3
(d, J ) 8.5 Hz, 2H), 3.60 (ddd, J ) 3J ) 8.5 Hz, J ) 6.3 Hz,
1H), 4.06 (d, 3J ) 6.3 Hz, 1H), 5.50 (s, 2H), 7.41-7.55 (m, 3H),
3
3
7.47 (d, J ) 8.6 Hz, 2H), 7.75 (d, J ) 8.6 Hz, 2H), 7.77 (dd,
3J ) 8.0 Hz, J ) 1.7 Hz, 2H). 13C NMR: δ ) 16.9 (q), 17.2
4
(q), 31.5 (t), 43.0 (d), 56.8 (d), 64.1 (s), 97.0 (s), 97.3 (s), 127.0
(d), 127.1 (d), 127.9 (d), 128.4 (d), 128.6 (d), 128.9 (d), 133.4
(d), 133.7 (s), 134.3 (s), 135.4 (s).
Isom er 9i. 1H NMR (CDCl3): δ ) 0.20 (s, 3H), 1.06 (s, 3H),
3
3
3
Isom er 9e′. 1H NMR: δ ) 0.19 (s, 3H), 1.01 (s, 3H), 2.21
2.16-2.27 (m, 2H), 3.65 (ddd, J ) J ) 8.6 Hz, J ) 5.2 Hz,
1H), 4.14-4.21 (m, 1H), 5.51 (s, 2H), 7.42-7.56 (m, 3H), 7.75
3
3
3
(d, J ) 8.3 Hz, 2H), 3.60 (ddd, J ) 3J ) 8.3 Hz, J ) 6.8 Hz,
3
4
3
1H), 4.12 (d, 3J ) 6.8 Hz, 1H), 5.50 (s, 2H), 7.41-7.55 (m, 3H),
(dd, J ) 8.3 Hz, J ) 1.6 Hz, 2H), 8.01 (d, J ) 8.9 Hz, 2H),
8.37 (d, 3J ) 8.9 Hz, 2H). 13C NMR (CDCl3): δ ) 16.8 (q),
17.3 (q), 31.4 (t), 43.6 (d), 56.7 (d), 64.6 (s), 97.0 (s), 97.4 (s),
123.6 (d), 126.9 (d), 127.1 (d), 128.1 (d), 128.4 (d), 128.4 (d),
133.3 (d), 134.9 (s), 143.5 (s), 147.7 (s).
3
3
7.48 (d, J ) 8.5 Hz, 2H), 7.73 (d, J ) 8.5 Hz, 2H), 7.78 (dd,
3J ) 8.1 Hz, J ) 1.7 Hz, 2H). 13C NMR: δ ) 16.9 (q), 17.3
4
(q), 31.5 (t), 43.2 (d), 56.6 (d), 64.1 (s), 96.2 (s), 98.0 (s), 126.7
(d), 127.5 (d), 127.8 (d), 128.4 (s), 128.5 (d), 128.6 (d), 133.8
(s), 133.8 (d), 134.4 (s), 135.5 (s).
Isom er 9i′. 1H NMR (CDCl3): δ ) 0.20 (s, 3H), 1.00 (s, 3H),
3
3
3
2.16-2.27 (m, 2H), 3.70 (ddd, J ) J ) 8.4 Hz, J ) 6.3 Hz,
(1a,4a,4ar,7ar)-4,4a,7,7a-Tetr ah ydr o-1-(4′-car bom eth ox-
yp h en yl)-8,8-d im eth yl-4-p h en yl- [9g, X ) p-CO2Me, Y )
H ] a n d (1a ,4a ,4a r,7a r)-4,4a ,7,7a -Tet r a h yd r o-4-(4′-ca r -
bom eth oxyp h en yl)-8,8-d im eth yl-1-p h en yl-1,4-m eth a n o-
1H-cyclop en ta [d ]p yr id a zin e [9g′, X ) H, Y ) p-CO2Me ]:
75%, colorless needles, mp 129-130 °C dec, Rf ) 0.33 (silica
gel, 33:1 methylene chloride:tert-butyl methyl ether). IR
(KBr): ν ) 3020 cm-1, 2940, 2890, 1700, 1595, 1420, 1270,
1190, 1100, 1005, 840, 690. UV (benzene): λmax (ꢀ) 329 nm
(sh, 34), 349 (sh, 100), 360 (175). Anal. Calcd for C24H24N2O2
(372.5): C, 77.39; H, 6.49; N, 7.52. Found: C, 77.32; H, 6.16;
N, 7.09.
3
3
1H), 4.06-4.16 (m, 1H), 5.44 (ddd, J ) 5.8 Hz, J ) 4.4 Hz,
4J ) 2.2 Hz, 1H), 5.55 (ddd, 3J ) 5.8 Hz, 3J ) 4.2 Hz, 3J ) 2.1
Hz, 1H), 7.42-7.56 (m, 3H), 7.77 (dd, 3J ) 8.1 Hz, 4J ) 1.6
3
3
Hz, 2H), 7.99 (d, J ) 9.0 Hz, 2H), 8.36 (d, J ) 9.0 Hz, 2H).
13C NMR (CDCl3): δ ) 16.8 (q), 17.3 (q), 31.5 (t), 43.1 (d), 57.2
(d), 64.6 (s), 96.0 (s), 98.5 (s), 123.6 (d), 126.2 (d), 127.5 (d),
128.0 (d), 128.3 (d), 128.4 (d), 134.3 (d), 134.9 (s), 143.5 (s),
147.5 (s).
Gen er a l Hyd r ogen a tion P r oced u r e for th e Azoa lk a n es
10. (i) Azoa lk a n es 10c-e,g-i. To a solution of 1.50 mmol
of azoalkane 9c-e,g-i in 6.0 mL of methanol and 14 mL of
methylene chloride were added 5.00 g of dipotassium azodi-
carboxylate.18 After addition of 0.5 mL of acetic acid, the
reaction mixture was stirred for 4 d at ambient temperature
(ca. 20 °C) and 20 mL of 0.5 N NaOH was added. The product
was isolated by extraction with methylene chloride (3 × 15
mL), and the organic layer was dried over a mixture of
magnesium sulfate and potassium carbonate. The solvent was
removed by distillation (ca. 20 °C, 20 Torr), and the crude
product was purified by column chromatography. (ii) Azoa l-
k a n e 10f. To a solution of 1.50 mmol of azoalkane 9i in 85
mL of ethyl acetate and 45 mL of ethanol was added a catalytic
amount of palladium (on charcoal), and the mixture was
stirred for 8 h under a hydrogen gas atmosphere. The catalyst
was removed by filtration and the solvent by distillation (ca.
20 °C, 20 Torr), and the crude product was purified by column
chromatography.
Isom er 9g. 1H NMR (CDCl3): δ ) 0.19 (s, 3H), 1.02 (s,
3H), 2.14-2.19 (m, 2H), 3.57-3.73 (m, 1H), 3.94 (s, 3H), 4.08-
4.16 (m, 1H), 5.43-5.53 (m, 2H), 7.40-7.54 (m, 3H), 7.76 (dd,
4
3
3J ) 8.1 Hz, J ) 1.5 Hz, 2H), 7.89 (d, J ) 8.6 Hz, 2H), 8.17
(d, 3J ) 8.6 Hz, 2H). 13C NMR (CDCl3): δ ) 16.9 (q), 17.3 (q),
31.5 (t), 43.3 (d), 52.1 (q), 56.9 (d), 64.4 (s), 96.5 (s), 97.5 (s),
127.0 (d), 127.1 (d), 127.5 (d), 127.9 (d), 128.4 (d), 129.6 (s),
129.6 (d), 133.4 (d), 135.3 (s), 141.0 (s), 166.8 (s).
Isom er 9g′. 1H NMR (CDCl3): δ ) 0.19 (s, 3H), 1.02 (s,
3H), 2.14-2.19 (m, 2H), 3.57-3.73 (m, 1H), 3.94 (s, 3H), 4.08-
4.16 (m, 1H), 5.43-5.53 (m, 2H), 7.40-7.54 (m, 3H), 7.78 (dd,
4
3
3J ) 8.1 Hz, J ) 1.5 Hz, 2H), 7.87 (d, J ) 8.6 Hz, 2H), 8.16
(d, 3J ) 8.6 Hz, 2H). 13C NMR (CDCl3): δ ) 16.9 (q), 17.3 (q),
31.5 (t), 43.0 (d), 52.1 (q), 56.6 (d), 64.4 (s), 97.1 (s), 98.2 (s),
126.7 (d), 127.1 (d), 127.5 (d), 127.9 (d), 128.4 (d), 129.6 (s),
129.6 (d), 133.7 (d), 135.3 (s), 141.1 (s), 166.8 (s).
(1a,4a,4ar,7ar)-4,4a,7,7a-Tetr ah ydr o-1-(4′-cyan oph en yl)-
8,8-d im eth yl-4-p h en yl- [9h , X ) p -CN, Y ) H] a n d (1a ,-
4a ,4a r,7a r)-4,4a ,7,7a -Tetr a h yd r o-4-(4′-cya n op h en yl)-8,8-
(18) Berson, J . A.; Poonian, M. S.; Libbey, W. J . J . Am. Chem. Soc.
1969, 91, 5567.

7064 J . Org. Chem., Vol. 61, No. 20, 1996
Adam et al.
(1a ,4a ,4a r,7a r)-4,4a ,5,6,7,7a -Hexa h yd r o-8,8-d im eth yl-
1-(4′-m et h oxyp h en yl)-4-p h en yl-1,4-m et h a n o-1H -cyclo-
pen ta[d]pyr idazin e (10c, X ) p-MeO): 92%, colorless needles,
mp 157-158 °C dec, Rf ) 0.20 (silica gel, methylene chloride).
IR (KBr): ν ) 3030 cm-1, 2940, 1600, 1510, 1440, 1295, 1245,
1180, 1020, 810, 750, 700. UV (benzene): λmax (ꢀ) ) 330 nm
(sh, 24), 352 (sh, 75), 363 (113). 1H NMR (CDCl3): δ ) 0.16
(s, 3H), 0.95 (s, 3H), 1.37-1.67 (m, 6H), 3.44-3.55 (m, 2H),
3.86 (s, 3H), 7.02 (d, 3J ) 9.7 Hz, 2H), 7.15-7.52 (m, 3H), 7.67
(d, J ) 9.7 Hz, 2H), 7.75 (dd, J ) 8.2 Hz, J ) 1.7 Hz, 2H).
13C NMR (CDCl3): δ ) 17.0 (q), 17.8 (q), 25.5 (t), 25.5 (t), 28.5
(t), 48.8 (d), 48.8 (d), 55.2 (q), 66.0 (s), 98.2 (s), 98.2 (s), 113.7
(d), 127.4 (d), 127.6 (d), 128.3 (d), 128.3 (d), 128.6 (d), 136.3
(s), 159.1 (s). Anal. Calcd for C23H26N2O (346.5): C, 79.73;
H, 7.56; N, 8.09. Found: C, 79.29; H, 7.18; N, 7.74.
C24H26N2O2 (374.5): C, 76.98; H; 7.00; N, 7.48. Found: C,
77.08; H, 6.71; N, 7.16.
(1a,4a,4ar,7ar)-4,4a,5,6,7,7a-Hexah ydr o-1-(4′-cyan oph e-
n yl)-8,8-d im eth yl-4-p h en yl-1,4-m eth a n o-1H-cyclop en ta -
[d ]p yr id a zin e (10h , X ) p-CN): 80%, cololess needles, mp
147-149 °C dec, Rf ) 0.17 (silica gel, methylene chloride). IR
(KBr): ν ) 3030 cm-1, 2930, 2840, 2210, 1590, 1490, 1450,
1430, 1360, 1010, 650, 600. UV (benzene): λmax (ꢀ) 330 nm
(sh, 25), 351 (sh, 78), 362 (119). 1H NMR (CDCl3): δ ) 0.15
(s, 3H), 1.00 (s, 3H), 1.41-1.62 (m, 6H), 3.43-3.63 (m, 2H),
7.36-7.54 (m, 3H), 7.73 (dd, 3J ) 8.0 Hz, 4J ) 1.6 Hz, 2H),
3
3
4
3
3
7.78 (d, J ) 8.6 Hz, 2H), 7.90 (d, J ) 8.6 Hz, 2H). 13C NMR
(CDCl3): δ ) 16.9 (q), 17.8 (q), 25.4 (t), 25.4 (t), 28.5 (t), 48.9
(d), 49.2 (d), 66.6 (s), 97.6 (s), 98.9 (s), 111.6 (s), 118.8 (s), 127.4
(d), 128.0 (d), 128.1 (d), 128.4 (d), 132.2 (s), 135.4 (s), 141.8
(s). Anal. Calcd for C23H23N3 (341.5): C, 80.90; H, 6.79; N,
12.31. Found: C, 80.62; H, 7.00; N, 12.15.
(1a ,4a ,4a r,7a r)-4,4a ,5,6,7,7a -Hexa h yd r o-8,8-d im eth yl-
1-(4′-m eth ylp h en yl)-4-p h en yl-1,4-m eth a n o-1H-cyclop en -
ta [d ]p yr id a zin e (10d , X ) p-Me): 92%, colorless needles, mp
147-148 °C dec, Rf ) 0.34 (silica gel, methylene chloride). IR
(KBr): ν ) 3010 cm-1, 2940, 1480, 1455, 1430, 1290, 1015,
795, 755, 690. UV (benzene): λmax (ꢀ) ) 330 nm (sh, 22), 352
(sh, 77), 363 (118). 1H NMR (CDCl3): δ ) 0.17 (s, 3H), 0.97
(s, 3H), 1.45-1.63 (m, 6H), 2.42 (s, 3H), 3.48-3.54 (m, 2H),
(1a ,4a ,4a r,7a r)-4,4a ,5,6,7,7a -Hexa h yd r o-8,8-d im eth yl-
1-(4′-n itr op h en yl)-4-p h en yl-1,4-m eth a n o-1H-cyclop en ta -
[d ]p yr id a zin e (10i, X ) p-NO2): 90%, colorless needles, mp
144-145 °C dec, Rf ) 0.48 (silica gel, methylene chloride). IR
(KBr): ν ) 3040 cm-1, 2940, 1590, 1510, 1460, 1440, 1340,
850, 740, 690. UV (benzene): λmax (ꢀ) 360 nm (sh, 360). 1H
NMR (CDCl3): δ ) 0.16 (s, 3H), 1.03 (s, 3H), 1.41-1.62 (m,
3
3
3
7.20 (d, J ) 8.0 Hz, 2H), 7.44 (tt, J ) 9.4 Hz, J ) 8.2 Hz,
3
3
4
3
6H), 3.46-3.64 (m, 2H), 7.37-7.54 (m, 3H), 7.73 (dd, J ) 8.1
2H), 7.47 (tt, J ) 9.4 Hz, J ) 1.6 Hz, 1H), 7.64 (d, J ) 8.0
Hz, 2H), 7.76 (dd, 3J ) 8.2 Hz, 4J ) 1.6 Hz, 2H). 13C NMR
(CDCl3): δ ) 17.0 (q), 17.8 (q), 21.2 (s), 25.5 (t), 25.5 (t), 28.6
(t), 48.8 (d), 48.8 (d), 66.0 (s), 98.3 (s), 98.3 (s), 127.4 (d), 127.5
(d), 127.6 (d), 128.3 (d), 129.0 (d), 133.2 (s), 136.3 (s), 137.3
(s). Anal. Calcd for C23H26N2 (330.5): C, 83.59; H, 7.93; N,
8.48. Found: C, 83.26; H, 7.61; N, 8.11.
4
3
3
Hz, J ) 1.5 Hz, 2H), 7.97 (d, J ) 8.9 Hz, 2H), 8.35 (d, J )
8.9 Hz, 2H). 13C NMR (CDCl3): δ ) 16.9 (q), 17.9 (q), 25.4 (t),
25.4 (t), 28.5 (t), 48.9 (d), 49.5 (d), 66.7 (s), 97.5 (s), 99.0 (s),
123.6 (d), 127.5 (d), 128.0 (d), 128.3 (d), 128.4 (d), 135.4 (s),
143.9 (s), 147.5 (s). Anal. Calcd for C22H23N3O2 (361.4): C,
73.11; H, 6.41; N, 11.63. Found: C, 72.70; H, 6.37; N, 11.15.
Gen er a l P r oced u r e for th e P r ep a r a tion of Hou sa n es
11. The housanes were synthesized in analogy to literature
procedure.5 The derivatives 11a ,c-e,g,h were obtained by
direct irradiation with a CW argon ion laser (widened beam)
and 11f,i by thermolysis in refluxing toluene of the corre-
sponding azoalkanes 10.
(1a,4a,4ar,7ar)-4,4a,5,6,7,7a-Hexah ydr o-1-(4′-ch lor oph e-
n yl)-8,8-d im eth yl-4-p h en yl-1,4-m eth a n o-1H-cyclop en ta -
[d ]p yr id a zin e (10e, X ) p-Cl): 80%, colorless needles, mp
137 -138 °C dec, Rf ) 0.52 (silica gel, methylene chloride). IR
(KBr): ν ) 3040 cm-1, 2940, 1485, 1460, 1435, 1090, 1010,
810, 745, 700. UV (benzene): λmax (ꢀ) ) 329 nm (sh, 23), 351
(sh, 73), 362 (115). 1H NMR (CDCl3): δ ) 0.15 (s, 3H), 0.97
(s, 3H), 1.41-1.63 (m, 6H), 3.40-3.59 (m, 2H), 7.39-7.53 (m,
3,3-Dim e t h yl-2,4-d ip h e n yl-en d o-t r icyclo[3.3.0.02,4]-
octa n e (11a , X ) H): 99%, colorless oil, Rf ) 0.94 (silica gel,
methylene chloride). IR (film): ν ) 3140 cm-1, 3110, 2920,
2840, 1590, 1480, 1450, 1435, 1070, 1020, 770, 755, 725, 700.
1H NMR (CDCl3): δ ) 0.64 (s, 3H), 1.50 (s, 3H), 1.30-1.70
3
3
3H), 7.45 (d, J ) 8.6 Hz, 2H), 7.70 (d, J ) 8.6 Hz, 2H), 7.74
(dd, 3J ) 8.1 Hz, 4J ) 1.6 Hz, 2H). 13C NMR (CDCl3): δ )
16.9 (q), 17.8 (q), 25.4 (t), 25.4 (t), 28.5 (t), 48.8 (d), 49.0 (d),
66.2 (s), 97.7 (s), 98.5 (s), 127.5 (d), 127.8 (d), 128.3 (d), 128.6
(d), 128.8 (d), 133.6 (s), 134.8 (s), 135.9 (s). Anal. Calcd for
3
and 1.88-2.10 (m, 6H), 2.66 (d, J ) 6.0 Hz, 2H), 7.20-7.36
(m, 10H). 13C NMR (CDCl3): δ ) 14.6 (q), 23.1 (q), 25.1 (t),
28.3 (t), 29.6 (s), 43.4 (d), 44.8 (s), 125.6 (d), 127.7 (d), 129.9
(d), 138.5 (s). Anal. Calcd for C22H24 (288.4): C, 91.61; H, 8.39.
Found: C, 91.72; H, 8.08.
C
22H23ClN2 (350.9): C, 75.31; H, 6.61; N, 7.98. Found: C,
74.95; H, 7.06; N, 7.56.
(1a,4a,4ar,7ar)-4,4a,5,6,7,7a-Hexah ydr o-1-(4′-am in oph e-
n yl)-8,8-d im eth yl-4-p h en yl-1,4-m eth a n o-1H-cyclop en ta -
[d ]p yr id a zin e (10f, X ) p-NH2): 20%, colorless needles; mp
73-74 °C dec, Rf ) 0.48 (silica gel, 9:1 methylene chloride:
tert-butyl methyl ether). IR (KBr): ν ) 3440 cm-1, 3340, 3020,
2940, 1610, 1510, 1460, 1295, 1180, 700. UV (benzene): λmax
(ꢀ) ) 352 nm (sh, 74), 364 (105). 1H NMR (CDCl3): δ ) 0.16
(s, 3H), 0.93 (s, 3H), 1.38-1.58 (m, 6H), 3.39-3.52 (m, 2H),
3.70 (b s, 2H), 6.79 (d, 3J ) 8.6 Hz, 2H), 7.34-7.52 (m, 3H),
7.51 (d, J ) 8.6 Hz, 2H), 7.74 (dd, J ) 8.2 Hz, J ) 1.5 Hz,
2H). 13C NMR (CDCl3): δ ) 16.9 (q), 17.8 (q), 25.5 (t), 25.5
(t), 28.5 (t), 48.6 (d), 48.8 (d), 65.9 (s), 98.0 (s), 98.4 (s), 114.9
(d), 126.1 (s), 127.4 (d), 127.6 (d), 128.3 (d), 128.5 (d), 136.5
(s), 145.9 (s). Anal. Calcd for C22H25N3 (331.5): C, 79.72; H,
7.60; N, 12.68. Found: C, 79.51; H, 7.70; N, 12.46.
3,3-Dim e t h yl-2-(4′-m e t h oxyp h e n yl)-4-p h e n yl-en d o-
tr icyclo[3.3.0.02,4]octa n e (11c, X ) p-MeO): 95%, colorless
oil, Rf ) 0.88 (silica gel, methylene chloride). IR (film): ν )
3030 cm-1, 3000, 2920, 2840, 1590, 1500, 1445, 1430, 1235,
1165, 1035, 1020, 820, 700. 1H NMR (CDCl3): δ ) 0.63 (s,
3H), 1.48 (s, 3H), 1.34-1.71 and 1.88-2.10 (m, 6H), 2.59 (dd,
3
3
3
3J ) 6.5 Hz, J ) 4.1 Hz, 1H), 2.65 (dd, J ) 6.1 Hz, J ) 4.1
3
3
Hz, 1H), 3.81 (s, 3H), 6.86 (d, J ) 8.9 Hz, 2H), 7.19 (d, J )
8.9 Hz, 2H), 7.16-7.33 (m, 5H). 13C NMR (CDCl3): δ ) 14.6
(q), 23.2 (q), 25.1 (t), 28.3 (t), 28.3 (t), 29.4 (s), 43.2 (d), 43.3
(d), 44.1 (s), 44.5 (s), 55.1 (q), 113.2 (d), 125.4 (d), 127.6 (d),
129.7 (d), 130.3 (s), 131.0 (d), 138.7 (s), 157.6 (s). Anal. Calcd
for C23H26O (318.5): C, 86.75; H, 8.23. Found: C, 86.57; H,
8.38.
3
3
4
3,3-D im e t h y l-2-(4′-m e t h y lp h e n y l)-4-p h e n y l-en d o-
tr icyclo[3.3.0.02,4]octa n e (11d , X ) p-Me): 95%, colorless oil,
Rf ) 0.93 (silica gel, methylene chloride). IR (film): ν ) 3030
cm-1, 3000, 2940, 2840, 1585, 1500, 1480, 1430, 800, 700. 1H
NMR (CDCl3): δ ) 0.63 (s, 3H), 1.48 (s, 3H), 1.34-1.70 and
1.88-2.13 (m, 6H), 2.34 (s, 3H), 2.61 (dd, 3J ) 6.0 Hz, 3J )
4.0 Hz, 1H), 2.65 (dd, 3J ) 6.0 Hz, 3J ) 4.0 Hz, 1H), 7.09-
7.36 (m, 9H). 13C NMR (CDCl3): δ ) 14.7 (q), 21.1 (q), 23.0
(q), 25.1 (t), 28.3 (t), 28.3 (t), 29.5 (s), 43.3 (d), 43.4 (d), 44.4
(s), 44.6 (s), 125.5 (d), 127.6 (d), 128.4 (d), 129.8 (d), 129.9 (d),
135.1 (s), 135.2 (s), 138.7 (s). Anal. Calcd for C23H26 (302.5):
C, 91.34; H, 8.66. Found: C, 91.28; H, 8.64.
(1a ,4a ,4a r,7a r)-4,4a ,5,6,7,7a -H e x a h y d r o -1-(4′-c a r -
bom eth oxyp h en yl)-8,8-d im eth yl-4-p h en yl-1,4-m eth a n o-
1H-cyclop en ta [d ]p yr id a zin e (10g, X ) p-CO2Me): 85%,
colorless needles, mp 132 -133 °C dec, Rf ) 0.28 (silica gel,
methylene chloride). IR (KBr): ν ) 3040 cm-1, 2930, 1710,
1600, 1445, 1275, 1190, 1105, 1015, 750, 690. UV (benzene):
λmax (ꢀ) ) 330 nm (sh, 41), 350 (sh, 78), 362 (111). 1H NMR
(CDCl3): δ ) 0.15 (s, 3H), 0.99 (s, 3H), 1.42-1.62 (m, 6H),
3.51-3.57 (m, 2H), 3.94 (s, 3H), 7.36-7.53 (m, 3H), 7.75 (dd,
4
3
3J ) 8.1 Hz, J ) 1.4 Hz, 2H), 7.85 (d, J ) 8.4 Hz, 2H), 8.15
(d, 3J ) 8.4 Hz, 2H). 13C NMR (CDCl3): δ ) 16.9 (q), 17.8 (q),
25.4 (t), 25.4 (t), 28.5 (t), 48.8 (d), 49.1 (d), 52.1 (q), 66.5 (s),
98.0 (s), 98.7 (s), 127.4 (d), 127.4 (d), 127.8 (d), 128.3 (d), 129.5
(s), 129.6 (d), 135.7 (s), 141.2 (s), 166.9 (s). Anal. Calcd for
2-(4′-C h lo r o p h e n y l)-3,3-d im e t h y l-4-p h e n y l-en d o-
tr icyclo[3.3.0.02,4]octa n e (11e, X ) p-Cl): 95%, colorless oil,
Rf ) 0.96 (silica gel, methylene chloride). IR (film): ν ) 3030

Spin Densities in Para-Substituted Benzyl-Type Radicals
J . Org. Chem., Vol. 61, No. 20, 1996 7065
cm-1, 3000, 2920, 2840, 1585, 1475, 1080, 1005, 810, 725, 700.
) 8.6 Hz, 2H). 13C NMR (CDCl3): δ ) 14.7 (q), 22.8 (q), 25.0
(t), 28.0 (t), 28.1 (t), 30.8 (s), 43.0 (d), 43.5 (d), 44.5 (s), 45.8
(s), 109.0 (s), 119.2 (s), 126.2 (d), 127.9 (d), 129.9 (d), 130.0
(d), 131.3 (d), 137.2 (s), 145.0 (s). Anal. Calcd for C23H23N
(313.4): C, 88.14; H, 7.40; N, 4.47. Found: C, 87.91; H, 7.32;
N, 4.20.
1H NMR (CDCl3): δ ) 0.62 (s, 3H), 1.48 (s, 3H), 1.31-1.72
3
and 1.84-2.09 (m, 6H), 2.64 (d, J ) 6.2 Hz, 2H), 7.17-7.35
3
(m, 5H), 7.16 (d 3J ) 8.7 Hz, 2H), 7.27 (d, J ) 8.7 Hz, 2H).
13C NMR (CDCl3): δ ) 14.6 (q), 23.0 (q), 25.1 (t), 28.2 (t), 28.2
(t), 29.7 (s), 43.2 (d), 43.4 (d), 44.1 (s), 44.9 (s), 125.8 (d), 127.8
(d), 127.9 (d), 129.8 (d), 131.1 (d), 131.3 (s), 137.1 (s), 138.1
(s). Anal. Calcd for C22H23Cl (322.9): C, 81.84; H, 7.18.
Found: C, 81.85; H, 6.98.
3,3-Dim eth yl-2-(4′-n itr oph en yl)-4-ph en yl-en do-tr icyclo-
[3.3.0.02,4]octa n e (11i, X ) p-NO2): 95%, colorless needles,
mp 115-117 °C, Rf ) 0.94 (silica gel, methylene chloride). IR
(KBr): ν ) 2920 cm-1, 2880, 2840, 1580, 1495, 1430, 1130,
1100, 835, 705. 1H NMR (CDCl3): δ ) 0.64 (s, 3H), 1.51 (s,
2-(4′-Am in oph en yl)-3,3-dim eth yl-4-ph en yl-en do-tr icyclo-
[3.3.0.02,4]octa n e (11f, X ) p-NH2): 80%, colorless oil, Rf )
0.88 (silica gel, methylene chloride). IR (film): ν ) 3420 cm-1
,
3H), 1.36-1.74 and 1.79-2.05 (m, 6H), 2.64 (dd, J ) 6.4 Hz,
3
3340, 3000, 2910, 1600, 1500, 1430, 1330, 1260, 1170, 900, 815,
3J ) 4.0 Hz, 1H), 2.75 (dd, 3J ) 6.4 Hz, 3J ) 4.0 Hz, 1H), 7.21-
715, 700. 1H NMR (CDCl3): δ ) 0.63 (s, 3H), 1.46 (s, 3H),
7.38 (m, 5H), 7.31 (d, J ) 8.3 Hz, 2H), 8.23 (d, J ) 8.3 Hz,
2H). 13C NMR (CDCl3): δ ) 14.8 (q), 22.8 (q), 25.0 (t), 28.1
(t), 28.2 (t), 31.3 (s), 43.2 (d), 43.7 (d), 44.6 (s), 46.2 (s), 122.9
(d), 126.3 (d), 128.1 (d), 130.0 (d), 130.1 (d), 133.6 (s), 137.1
(s), 147.7 (s). Anal. Calcd for C22H23NO2 (333.4): C, 79.25;
H, 6.95; N, 4.20. Found: C, 78.96; H, 7.09; N, 4.01.
3
3
3
3
1.34-1.68 and 1.86-2.11 (m, 6H), 2.56 (dd, J ) 6.7 Hz, J )
3
3
4.0 Hz, 1H), 2.64 (dd, J ) 6.6 Hz, J ) 4.0 Hz, 1H), 6.66 (d,
3J ) 8.5 Hz, 2H), 7.07 (d, 3J ) 8.5 Hz, 2H), 7.16-7.35 (m, 5H).
13C NMR (CDCl3): δ ) 14.6 (q), 23.1 (q), 25.1 (t), 28.3 (t), 28.4
(t), 29.3 (s), 43.1 (d), 43.4 (d), 44.1 (s), 44.4 (s), 114.7 (d), 122.9
(s), 125.3 (d), 127.5 (d), 129.7 (d), 131.0 (d), 139.0 (s), 144.1
(s). Anal. Calcd for C22H25N (303.4): C, 87.08; H, 8.30; N,
4.62. Found: C, 86.81; H, 8.51; N, 4.55.
Ack n ow led gm en t. We thank the Deutsche Fors-
chungsgemeinschaft for generous financial support.
W.M.N. thanks the Fonds der Chemischen Industrie for
a Kekule´ doctoral fellowship (1992-1994).
2-(4′-Car bom eth oxyph en yl)-3,3-dim eth yl-4-ph en yl-en do-
tr icyclo[3.3.0.02,4]octa n e (11g, X ) p-CO2Me): 90%, colorless
oil, Rf ) 0.61 (silica gel, methylene chloride). IR (film): ν )
3030 cm-1, 3000, 2920, 2830, 1705, 1590, 1420, 1265, 1100,
700. 1H NMR (CDCl3): δ ) 0.63 (s, 3H), 1.50 (s, 3H), 1.37-
1.72 and 1.82-2.06 (m, 6H), 2.65 (dd, 3J ) 6.5 Hz, 3J ) 4.1
Note a d d ed in p r oof: In the customary coordinate
system, in which the D and E parameters of planar π
systems are represented, the z axis is chosen parallel to
the p orbitals (eq 3 and eq 4). However, for localized
diradicals it is more apporpriate to define the z axis along
the line which connects the two radical sites. In this case
the relation D g 3E g 0 is fulfilled and allows a
quantitative comparison of the theoretical with the
experimental zero-field splitting parameters. This coor-
dinate transformation requires multiplication of the
expressions in eq 4 and eq 6 by a distance dependent
factor (in the present case f(2.4 Å) ) 1.5); its analytical
expression is given in Gouterman M.; Moffitt W. J . Chem.
Phys. 1959, 30, 1107.
3
3
Hz, 1H), 2.71 (dd, J ) 6.2 Hz, J ) 4.1 Hz, 1H), 3.90 (s, 3H),
3
3
7.19-7.36 (m, 5H), 7.28 (d, J ) 8.6 Hz, 2H), 7.97 (d, J ) 8.6
Hz, 2H). 13C NMR (CDCl3): δ ) 14.7 (q), 22.9 (q), 25.0 (t),
28.2 (t), 28.2 (t), 30.4 (s), 43.3 (d), 43.5 (d), 44.7 (s), 45.5 (s),
51.9 (q), 126.0 (d), 127.3 (s), 127.8 (d), 128.9 (d), 129.7 (d), 130.0
(d), 137.7 (s), 144.7 (s), 167.2 (s). Anal. Calcd for C24H26O2
(346.5): C, 83.20; H, 7.56. Found: C, 83.00; H, 7.76.
2-(4′-Cyan oph en yl)-3,3-dim eth yl-4-ph en yl-en do-tr icyclo-
[3.3.0.02,4]octa n e (11h , X ) p-CN): 92%, colorless oil, Rf )
0.29 (silica gel, 20:1 petroleum ether:ethyl acetate). IR
(film): ν ) 3030 cm-1, 3000, 2920, 2830, 2210, 1585, 1480,
1430, 835, 820, 740, 700. 1H NMR (CDCl3): δ ) 0.64 (s, 3H),
3
1.52 (s, 3H), 1.37-1.75 and 1.82-2.07 (m, 6H), 2.66 (dd, J )
3
3
3
6.5 Hz, J ) 4.1 Hz, 1H), 2.74 (dd, J ) 6.5 Hz, J ) 4.1 Hz,
3
3
1H), 7.21-7.39 (m, 5H), 7.30 (d, J ) 8.6 Hz, 2H), 7.57 (d, J
J O960396Q