Additivity of the Electronic Meta-Substituent Effect in 3,5-Disubstituted Cumyl Radicals Assessed by the EPR D Parameter of 1,3-Arylcyclopentane-1,3-diyl Triplet Diradicals

Article abstract of DOI:10.1021/jo970756p

The D parameter (EPR zero-field splitting) of the 3,3′,5,5′-tetrasubstituted triplet diradicals 6 (X = X′ = H, NO₂, CH₃, OAc, OCH₃, NH₂, and OH) were determined in a MTHF matrix at 77 K and serves as a spectrosc

Full text of DOI:10.1021/jo970756p

J . Org. Chem. 1997, 62, 7263-7266
7263
Ad d itivity of th e Electr on ic Meta -Su bstitu en t Effect in
3,5-Disu bstitu ted Cu m yl Ra d ica ls Assessed by th e EP R D
P a r a m eter of 1,3-Ar ylcyclop en ta n e-1,3-d iyl Tr ip let Dir a d ica ls
Waldemar Adam, Heinrich M. Harrer,* and Wiebke Maas
Institute of Organic Chemistry, University of Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Received April 28, 1997X
The D parameter (EPR zero-field splitting) of the 3,3′,5,5′-tetrasubstituted triplet diradicals 6 (X
) X′ ) H, NO2, CH3, OAc, OCH3, NH2, and OH) were determined in a MTHF matrix at 77 K and
serves as a spectroscopic tool for the determination of electronic substituent effects in multiply-
substituted benzyl-type monoradicals through its spin density dependence. The linear correlation
(m ) 2.00 ( 0.01, r2 ) 0.974) of the experimental D values of these meta-disubstituted triplet
diradicals 6 versus the 3,3′-disubstituted triplet diradicals 5 demonstrates the additivity of the
electronic meta-substituent effect in the corresponding 3,5-disubstituted cumyl monoradicals 4 and
is corroborated by theoretical (PM3-AUHF) spin density calculations for the latter. Thus, the
combined use of the experimental D parameter and semiempirically calculated R spin density has
provided for the first time the unambiguous demonstration of the additivity of electronic effects
exerted by meta-substituents in the cumyl monoradicals 4.
In tr od u ction
Such localized 1,3-diaryl-substituted cyclopentane-1,3-
diyl triplet diradicals can be described as two geo-
metrically fixed but independent cumyl monoradicals.
Thus, at constant dAB, the D parameter is a sensitive
probe for electronic substituent effects through the R spin
density dependence3b and the data for the symmetrically
para- and meta-substituted 1,3-diarylcyclopentane-1,3-
diyl triplet diradicals 5 have formed the basis for the
spectroscopic ∆D scale (eq 2).3a,c,d In this context, we have
Persistent localized triplet 1,3-diradicals of the cyclo-
pentane-1,3-diyl-type are conveniently prepared under
matrix isolation at cryogenic temperatures (4-77 K), and
their electronic and structural properties are accessible
through the EPR zero-field-splitting parameters D and
E,1 which derive from the dipole-dipole interaction
between the two uncoupled spins.2 The magnitude of the
D parameter in localized triplet 1,3-diradicals such as 5
and 6 depends on the interspin distance dAB and the R
∆D ) (DX - DH)100
(2)
shown that meta-substitution in the cumyl radicals 3
destabilizes the radical at the R site by increased spin
localization relative to the parent radical 3 (X ) H), which
is expressed in the negative ∆D values for all meta-
substituents. This electronic peculiarity, which has been
a conflicting point in all other σrad scales,4-8 was cor-
roborated for the first time by semiempirical MO calcula-
tions and by a satisfactory correlation with the Swain-
Lupton resonance parameter R.3d
In contrast, only little attention has been focused on
the electronic effects caused by meta-disubstitution in the
monoradicals 2 and 4, albeit for ionic reactions it is the
consensus of opinion that electronic substituent effects
are additive, as diagnosed by the Hammett equation.9
(3) (a) Adam, W.; Fro¨hlich, L.; Nau, W. M.; Korth, H.-G.; Sustmann,
R. Angew. Chem., Int. Ed. Engl. 1993, 32, 1339. (b) Adam, W.; Kita,
F.; Harrer, H. M.; Zipf, R. J . Org. Chem. 1996, 61, 7056. (c) Adam, W.;
Harrer, H. M.; Kita, F.; Nau, W. M. Pure Appl. Chem. 1997, 69, 91.
(d) Adam, W.; Harrer, H. M.; Kita, F.; Korth, H,-G.; Nau, W. M. J .
Org. Chem. 1997, 62, 1419.
(4) (a) Fisher, T. H.; Meierhofer, A. W. J . Org. Chem. 1978, 43, 224.
(b) Fisher, T. H.; Dershem, S. M.; Prewitt, M. L. J . Org. Chem. 1990,
55, 1040.
(5) (a) Agirbas, H.; J ackson, R. A. J . Chem. Soc., Perkin Trans. 2
1983, 739. (b) J ackson, R. A. J . Organomet. Chem. 1992, 437, 77.
(6) J iang, X.-K.; J i, G.-Z. J . Org. Chem. 1992, 57, 6051.
(7) Creary, X.; Mehrsheikh-Mohammadi, M. E.; McDonald, S. J . Org.
Chem. 1987, 52, 3254.
spin densities FA and FB at the radical sites A and B
(eq 1), and both dependences have recently been con-
3µ0g2µ2B FAFB
D )
(1)
3
(
)
16π
dAB
firmed experimentally and theoretically.3
X Abstract published in Advance ACS Abstracts, September 15, 1997.
(1) Dougherty, D. A. in Kinetics and Spectroscopy of Carbenes and
Biradicals; Platz, M. S., Ed.; Plenum Press: New York, 1990; pp 117.
(2) (a) Weil, J .; Bolton, J . R.; Wertz, J . E. In Electronic Paramagnetic
Resonance: Elementary Theory and Practical Applications; J ohn Wiley
& Sons: New York, 1994. (b) McGlynn, S. P.; Azumi, T.; Kinoshita,
M. in Molecular Spectroscopy of the Triplet State; Prentice Hall:
Englewood Cliffs, NJ , 1969.
(8) (a) Dust, J . M.; Arnold, D. R. J . Am. Chem. Soc. 1983, 105, 1221.
(b) Arnold, D. R. In Substituent Effects in Radical Chemistry NATO
ASI Ser. C; Viehe, H. G., J anousek, Z., Mere´nyi, R. Ed.; Reidel &
Dordrecht: Netherlands, 1986. Vol. 189, pp 171-188. (c) J ackson, R.
A.; Sharifi, M. J . Chem. Soc., Perkin Trans. 2 1996, 775.
(9) (a) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(b) Stone, R. M.; Pearson, D. E. J . Org. Chem. 1961, 26, 257. (c) J affe´,
H. H. Chem. Rev. 1953, 53, 243.
S0022-3263(97)00756-1 CCC: $14.00 © 1997 American Chemical Society

7264 J . Org. Chem., Vol. 62, No. 21, 1997
Adam et al.
Sch em e 1
Nevertheless, the direct comparison between the EPR R
hyperfine coupling constants of the benzyl radicals 1 and
2 showed a rather poor additivity effect [aR(2) ) 1.34 (
0.23 aR(1), r2 ) 0.893, n ) 7].10 The same tendence has
been noticed earlier in the novel µSR technique for
muonated meta-disubstituted cyclohexadienyl radicals,
which have been considered to be isoelectronic with the
benzyl radicals.11 Thus, we were interested to probe
rigorously the additivity of the electronic influence of two
meta-substituents on benzyl-type monoradicals, for which
the D parameter of the corresponding triplet diradicals
6 should constitute a convenient and accurate measure.
The results on the D parameter of the triplet diradicals
6 are given in the present report and demonstrate
convincingly the additivity of electronic effects in meta-
disubstituted benzyl-type radicals.
Ta ble 1. Zer o-F ield -Sp littin g P a r a m eter D a n d ∆D
Va lu es of th e 3,3′,5,5′-Tetr a su bstitu ted Tr ip let Dir a d ica ls
6 a n d Ca lcu la ted r Sp in Den sities (Gr) for th e
3,5-Disu bstitu ted Cu m yl Mon or a d ica ls 4
X
X′
|D/hc| of 6a
F
R of 4b
∆Dc
a
b
c
d
e
f
g
h
i
H
H
H
0.0504
0.0511
0.0518d
0.0524
0.0527
0.0538
0.0525
0.0545d,e
0.0547
0.542
0.00
NO2
NO2
CH3
OAc
OCH3
NH2
NH2
OH
NO2
CH3
OAc
OCH3
H
0.546
0.549
0.551
0.558
-0.14
-0.20
-0.23
-0.34
NH2
OH
0.563
0.563
-0.39
-0.43
a
Measured in a MTHF glass matrix at 77 K, values given in
cm-1, error ( 0.0001 cm-1, for the triplet diradicals 6, |E/hc| <
b
0.001 cm-1
.
Calculated R spin densities for the cumyl monoradi-
cals; cf. the text. c Calculated according to eq 2, values given in
102 cm-1
.
Calculated according to eq 3 from the unsymmetrically
d
disubstituted diradicals; cf. the text. e The D value is 0.0506 cm-1
Resu lts a n d Discu ssion
for the parent triplet diradical without an olefinic double bond.
Syn th esis. The synthesis of the azoalkanes 10 was
conducted in analogy to previously reported pro-
cedures.3b,d,12 This synthetic route was not feasible for
the derivatives 10b (X ) H; X′ ) NO2), 10g (X ) H; X′ )
NH2), and 10i (X ) X′ ) OH). Therefore, the unsym-
metric diketone 8b was synthesized by the TiCl4-
catalyzed addition of 3,5-dinitrobenzoyl chloride to the
trimethylsilyl enol ether of isobutyrophenone (Scheme
1).13 The azoalkane 10g was obtained by catalytic
hydrogenation of the derivative 10b on PtO2 as catalyst.
In turn, saponification of the derivative 10e yielded the
hydroxy-substituted azoalkane 10i.
EP R Sp ectr oscop y. The triplet diradicals 6 were
generated in a 2-methyltetrahydrofuran (MTHF) matrix
at 77 K by means of irradiation with the 364-nm line of
an argon ion laser. The results of the low-temperature
EPR measurements are summarized in Table 1. The
smallest D value (0.0511 cm-1) is found for the unsym-
metrically disubstituted derivative 6b (X ) H; X′ ) NO2)
with the strong electron-accepting 3,5-nitro groups,
whereas the largest D value (0.0547 cm-1) is observed
for the hitherto unknown triplet diradical 6i (X ) X′ )
OH) with four strongly electron-donating meta-hydroxy
groups. In view of the recently shown3b additivity of
electronic substituent effects on radical sites, the D
parameters for the as yet unknown symmetrically tetra-
(10) J ackson, R. A.; Moosavi, R. J . Chem. Soc. Perkin Trans. 2 1992,
885.
(11) Rhodes, C. J .; Roduner, E. Tetrahedron Lett. 1988, 29, 1437.
(12) (a) Beck, K; Hu¨nig, S. Chem. Ber. 1987, 120, 477. (b) Adam,
W.; Harrer, H. M.; Nau, W. M.; Peters, K. J . Org. Chem. 1994, 59,
378(163. ) Fleming, I.; Iqbal, J .; Krebs, E. P. Tetrahedron 1983, 39, 841.
substituted derivatives 6c (X ) X′ ) NO2, 0.0518 cm-1
and 6h (X ) X′ ) NH2, 0.0545 cm-1) may be assessed.
)

Additivity of the Electronic Meta-Substituted Effect
J . Org. Chem., Vol. 62, No. 21, 1997 7265
F igu r e 1. The experimental D values of the 3,3′,5,5′-tetra-
substitute (6) versus the 3,3′-disubstituted (5) triplet diradicals.
F igu r e 2. The experimental D values of the triplet diradicals
5 and 6 versus the calculated (PM3-AUHF/CI) R spin densities
of the cumyl radicals 3 and 4 (cf. ref 3c).
For this purpose eq 3 applies and the experimental D
DH,X
)
D
x
X,XDH,H
(3)
values of the unsymmetrical derivatives 6b (X ) H; X′)
NO2) and 6g (X ) H; X′ ) NH2; cf. Table 1) were used as
input data.3d
Th e Ad d it ivit y of E lect r on ic Meta -Su b st it u en t
Effects. Table 1 reveals that all 3,3′,5,5′-tetrasubsti-
tuted triplet diradicals 6 possess large negative ∆D
values, and consequently, the R spin density in the
corresponding 3,5-disubstituted cumyl monoradicals 4 is
increased (eq 1). Thus, strong electron-donating (ED)
groups such as NH2 or OH at both meta-positions
destabilize the benzylic radical center efficiently, whereas
this effect is only moderate for strong electron-accepting
(EA) groups, e.g. NO2. The composite electronic effects
of two meta-substituents in each phenyl ring of the
diradicals 6 is destinctly larger than previously observed
for only one meta-substituent in the diradicals 53c,d and
implicates that an electronic additivity effect operates
also for two meta-substituents in the cumyl monoradicals
4. The plot of the experimental D parameter of the
3,3′,5,5′-tetrasubstituted triplet diradicals 6 versus the
D parameter of the corresponding 3,3′-disubstituted
triplet diradicals 5 (Figure 1) confirms this in the form
of the excellent linear dependence (r2 ) 0.974) and the
slope of m ) 2.00 ( 0.01. Furthermore, semiempirical
MO calculations (PM3-AUHF) corroborate this additivity
by the linear plot of the calculated R spin densities for
the monoradicals 3 and 4 against the experimental D
parameters of the corresponding triplet diradicals 5 and
F igu r e 3. Calculated (PM3-AUHF/CI) R spin densities for the
parent cumyl radical (left), the 3-monosubstituted 3 (center),
and the 3,5-disubstituted cumyl radicals 4 (right).
of the benzene ring. Thus, in these cross-conjugated
systems the electronic effects of the R spin center and
the meta-substituent compete with one another for
conjugation with the aromatic ring at the ortho- and
para-positions such that the resulting effect is spin
localization at the benzylic radical site. This interplay
may be demonstrated by the semiempirically calculated
spin densities in Figure 3, which reflect the localization
of R spin density at the benzylic position with increasing
substitution at the phenyl moiety. While the R spin
density increases only moderately (∆FR ) 0.004) for the
strongly electron-accepting NO2 group, these calculations
reveal a substantial increase (∆FR ) 0.021) for the
strongly electron-donating OH substituent in the cumyl
radicals 3 and 4. This is in line with our recent report
that the destabilizing effect of meta-substituents in the
cumyl radicals 3 depends on the electron-donating pro-
pensity of such substituents3d and even holds true for the
meta-isomers of hetaryl substituents.14
2
6 (Figure 2, |D/hc| × 100 ) 16.4FR + 0.3, n ) 13).
Additionally, the linear correlation (r2 ) 0.991) of the
calculated R spin densities of the 3,5-disubstituted cumyl
radicals 4 versus the 3-substituted radicals 33d with a
slope of m ) 2.00 (figure not shown) substantiates once
more the additivity of electronic effects in the cumyl
monoradicals exerted by meta-substituents.
Simple Hu¨ckel theory predicts that meta-substituents
in the benzyl-type radicals should exhibit no significant
effect on the R spin density. Nevertheless, it is known
that detectable spin density is found at the meta-positions
in the ground state of such radicals, i.e. ameta ) 1.70 G
for the parent benzyl 1 and ameta ) 1.65 G for the parent
cumyl radical 3.8 Evidently, the radical site and the
meta-substituents are connected through the π system
A previous EPR study of the meta-disubstituted benzyl
radicals 2 by J ackson10 reported a rather poor additivity
(r2 ) 0.893, m ) 1.34 ( 0.23) for the R hyperfine coupling
constants when compared to the meta-monosubstituted
(14) Adam, W.; Emmert, O.; Harrer, H. M. J . Chem. Soc., Perkin
Trans. 2 1997, 687.

7266 J . Org. Chem., Vol. 62, No. 21, 1997
Adam et al.
benzyl radicals 1.8 Moreover, UMINDO calculations
showed no significant changes in the R spin density for
the meta-disubstituted benzyl radicals 2.10 However, the
employed meta-substitutents, namely CF3, F, tBu, Cl, Me
and OMe, cover only a relatively small range of electronic
effects in their destabilizing ability on benzyl-type
radicals,3d which presumably is responsible for the con-
siderable scatter in the experimental data. In contrast,
the readily accessible D parameter of the triplet diradi-
cals 6 constitutes an accurate measure of electronic
effects on such radical sites (eq 1) and the featured
substituents span the whole range from strongly electron-
accepting (NO2) to strongly electron-donating (NH2)
groups, an advantage of our ∆D substituent scale which
is not offered by any of the other σrad scales.4-8
Exp er im en ta l Section
EP R Sp ectr oscop y. A sample (ca. 5 × 10-4 mmol) of the
corresponding azoalkane was dissolved in 0.3 mL of MTHF,
placed into an EPR sample tube (L ca. 2 mm) and thoroughly
degassed by purging with argon gas. The samples were sealed
and the 77-K matrix was prepared by freezing the samples in
liquid nitrogen. The triplet diradicals 6 were generated by
irradiation with the 364-nm line of an INNOVA-100 CW argon-
ion laser (widened beam, 1.5 W, 2 min) at 77 K, and their EPR
spectra were recorded on a Bruker ESP-300 spectrometer (9.43
GHz, spectra accumulation with the Bruker 1620 Data-
System, n g 5). The D values were determined by a manual
analysis of the Z signals.
Com p u ta tion s. Full geometry optimization of the cumyl
radicals 3 and 4 was carried out on the highest molecular
symmetry, i.e. C2v symmetry, for all disubstituted radicals 4
by using the semiempirical PM3 method15 and the annihilated
UHF wavefunction provided in the VAMP 5.0 program pack-
age16 and run on a IRIS INDIGO R4000 Silicon Graphics
workstation. The spin densities were determined by a single
point CI (5 × 5) calculation to afford excellent S2 values of
0.77-0.79.
Con clu sion s
A set of 3,3′,5,5′-tetrasubstituted triplet diradicals 6,
which features strong electron acceptors (NO2) as well
as strong electron donors (OH), has been used for a
detailed study of the meta-substituent additivity in the
corresponding 3,5-disubstituted cumyl radicals 4. The
slope of m ) 2.00 in the excellent correlation between
the D parameters of the triplet diradicals 6 and the 3,3′-
disubstituted triplet diradicals 5 shows unequivocally the
additivity of electronic meta-substituent effects, which is
further substantiated by semiempirical MO calculations
for the corresponding 3-substituted cumyl radicals 4.
Thus, the EPR-spectroscopic D parameter of localized
cyclopentane-1,3-diyl triplet diradicals provides for the
first time a convenient tool to probe for hitherto unknown
synergistic or antagonistic substituent effects in multiply
substituted benzyl-type monoradicals.
Ack n ow led gm en t. We thank the Deutsche Fors-
chungsgemeinschaft and and the Fonds der Chemischen
Industrie for financial support.
Su p p or tin g In for m a tion Ava ila ble: Synthetic details and
characteristic spectral data of the 1,3-propanediones 7, 2,2-
dimethyl-1,3-propanediones 8, 4,4-dimethyl-4H-pyrazoles 9,
and the azoalkanes 10 (7 pages). This material is contained
in libraries on michrofiche, immediately follows this article
in the microfilm version of the journal, and can be ordered
from the ACS; see any current masthead page for ordering
information.
(15) Stewart, J . J . P. J . Comput. Chem. 1989, 10, 209.
(16) Rauhut, G.; Alex, A.; Chandrasekhar, J .; Steinke, T.; Clark, T.
VAMP 5.0, University of Erlangen-Nu¨rnberg, 1993.
J O970756P