Heterocyclic Non-Kekule Biradicals
J. Am. Chem. Soc., Vol. 119, No. 6, 1997 1429
conduction5 without doping. Thus, in the present work,1,6 the
tunability of a common structural motif might find eventual
application in practical devices and also in the search for
understanding of the spin interactions among very weakly bound
electrons in extended π-conjugated systems.
Scheme 1
Connectivity in Multiradicals. To preserve a non-Kekule´
structure with its NBMO-rich spectrum of energy levels, and
to prevent the formation of new π-bonds in both the high-spin
and low-spin series, the catenation of the biradicals must avoid
connectivity patterns that locate odd electrons at adjacent sites.
More generally, it is useful to be able to predict at least the
minimum number of Hu¨ckel π-NBMOs from a given con-
nectivity. In the case of alternant hydrocarbons, this has been
possible traditionally from the Coulson-Rushbrooke-Longuet-
Higgins theorem: when the compound is written in the structure
with the maximum number of bonds, the number of such
NBMOs (in the case of non-Kekule´ hydrocarbons) is equal to
or (in the case of 4m cyclopolyenes, m ) 1, 2 ...) greater than
the number of π-framework carbon atoms that bear odd
electrons.7 For tetramethyleneethane, for example, the number
of NBMOs is 2, and for cyclobutadiene, it is at least zero
(actually 2). Strictly, this rule cannot be applied to heterocyclic
or nonalternant monomers such as 1-3 or to polymers derived
by their catenation. Recently, however, a generalized form of
the theorem has been proposed by Tyutyulkov and co-workers3
to cover such cases, which are designated as “quasi-alternants”.
In quasi-alternant systems, every Hu¨ckel graph of N atoms is
characterized by a maximum subset T consisting of Nt noncon-
tiguous homonuclear π-centers. The π-centers not belonging
to T may be heteronuclear. If for a given Hu¨ckel system 2Nt
> N is fulfilled, then its energy spectrum displays at least (2Nt
- N) NBMOs. In the cases of 1-3, for example, N ) 7, Nt )
4, and the numbers of NBMOs g 1 (actually 2).
Also, in the high-spin systems, the linkages must be made
through coupling elements that favor ferromagnetic interactions
between the component triplet monomer units. Although the
theoretical basis for such choices is still in a state of
development,4c,d,f,l-r we have chosen the m-phenylene coupler,
which has a history in biradical chemistry dating back to the
studies of Schlenk and Brauns in 19158a and continuing to the
present.4,8b
To explore the electron spin properties of substances which
can serve as building blocks for the multiradical polymers 7-9,
we describe here the development of synthetic methods for the
catenation of diazene precursors 4-6, the application of these
methods to the synthesis of the bis-diazene precursors of the
set of tetraradicals 10-12, and the nature of the species
generated by deazetation of the bis-diazenes.
synthesis of diazene precursors of 3,4-dimethyleneheterocycles.
We hoped to apply analogous chemistry to the case of the
tetraradicals 10-12, and accordingly, our initial goal was a
versatile synthesis of m-phenylene-linked bis-diazenes (e.g., 13),
which would allow for variations of the heterocycle.
Palladium-Catalyzed Cross-Coupling of Pyrroles with 1,3-
Dihalobenzenes. Synthesis of Bis-Diazene 13. After indif-
ferent success (described elsewhere6b) in a few initial attempts
to prepare difuryl-, dithienyl-, and dipyrrylbenzenes by adapta-
tion of classical methods9 of 1,4-dicarbonyl cyclization, we
learned of a successful organometallic coupling of pre-formed
heterocyclic and phenyl moieties by Pelter and co-workers.10
These authors had prepared 1,3- and 1,4-difurylbenzenes by the
cross-coupling of 2 mol of 2-furylzinc chloride (obtained from
furan by lithiation and subsequent treatment with zinc chloride)
with 1,3- or 1,4-dibromobenzene in the presence of a catalytic
amount of tetrakis(triphenylphosphine)palladium. Encouraged
by this work, we sought to generalize the cross-coupling reaction
to accommodate variations in the structure and substituents of
the heterocyclic ring. We found the procedure very sensitive
to changes in the structure of the heterocycle, and separate
careful optimizations of the reaction conditions were necessary
for each heterocyclic system. Procedures for the furan and
thiophene series are described elsewhere.6 Scheme 2 outlines
our current procedure for the pyrrole series.
We chose as the heterocyclic coupling unit 3,4-bis(methoxy-
methyl)-N-p-toluenesulfonylpyrrole 14 (Scheme 2), prepared
from the known1 3,4-bis(hydroxymethyl)-N-tosylpyrrole. How-
ever, metallation of the pyrrole R positions of 14 by direct
lithium-for-hydrogen exchange seemed unlikely to be successful.
For example, N-p-toluenesulfonylpyrrole, when treated with
n-butyllithium or tert-butyllithium, has been reported11 to suffer
cleavage of the p-toluenesulfonyl (tosyl) group. One might hope
that lithium-for-halogen exchange in an R-halogenated derivative
of 14 might be sufficiently fast to compete succesfully with
tosyl cleavage. To test this idea, we prepared 2-bromo-3,4-
bis(methoxymethyl)-N-p-toluenesulfonylpyrrole (15) from 14 by
a known12 low-temperature bromination procedure (Scheme 2).
Satisfactory R-metallation in fact was achieved when 15 was
Synthesis of Catenated Bis-Diazene Precursors. Scheme
1 shows the general route we have previously used1,2 for the
(5) See inter alia: (a) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H.
Orbital Interactions in Chemistry; Wiley-Interscience: New York, 1985;
Chapter 13 and references cited therein. (b) Whangbo, M.-H. In Extended
Linear Chain Compounds; Miller, J. S., Ed.; Plenum: New York, 1982;
Vol. II. (c) Salaneck, W. R., Lundstro¨m, I., Rånby, B., Eds. Conjugated
Polymers and Related Materials; Oxford: New York, 1993. (d) Bumm, L.
A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II;
Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705 and
references cited therein.
(6) (a) Lu, H. S. M.; Berson, J. A. J. Am. Chem. Soc. 1996, 118, 265.
(b) Lu, H. S. M. Ph.D. Dissertation, Yale University, New Haven, CT,
1994.
(7) (a) Coulson, C. A.; Rushbrooke, G. S. Proc. Cambridge Phil. Soc.
1940, 36, 193. (b) Coulson, C. A.; Longuet-Higgins, H. C. Proc. R. Soc.
1947, A191, 39. (c) Ibid. 1947, A192, 16. (d) Ibid. 1947, A193, 447. (e)
Longuet-Higgins, H. C. J. Chem. Phys. 1950, 18, 265.
(9) For reviews, see: (a) Sundberg, R. J. In ComprehensiVe Heterocyclic
Chemistry; Katritzky, A., Ed.; Pergamon Press: Oxford, U.K., 1984; Chapter
3.06. (b) Jones, R. A.; Bean, G. P. The Chemistry of Pyrroles; Academic
Press: London, 1977. (c) Patterson, J. M. Synthesis 1976, 281.
(10) (a) Pelter, A.; Rowlands, M.; Clements, G. Synthesis 1987, 51. (b)
Pelter, A.; Rowlands, M.; Jenkins, I. H. Tetrahedron Lett. 1987, 28, 5213.
(11) Anderson, Jr.; Freenor, F. J. J. Org. Chem. 1972, 37, 624.
(12) Gilow, H. M.; Burton, D. E. J. Org. Chem. 1981, 46, 2221.
(8) (a) Schlenk, W.; Brauns, M. Chem. Ber. 1915, 48, 716. (b) Review:
Berson, J. A. In The Chemistry of Quinonoid Compounds, Vol II; Patai, S.,
Rappoport, Z., Eds.; Wiley-Interscience: New York, 1988; Chapter 10.