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.
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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´,
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