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Donor-Acceptor-Substituted Cyclic -Electron SystemsЧProbes for Theories and Building Blocks for New Materials.

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Donor-Acceptor-Substituted Cyclic n-Electron SystemsProbes for Theories and Building Blocks for New Materials
By Rudolf Gompper* and Hans-UIrich Wagner
Donor and acceptor substituents stabilize (4n)n-electron systems and destabilize those with
(4n + 2)n electrons. The same is true for the transition states of pericyclic reactions, which
explains the appearance of dipolar intermediates in symmetry-allowed cycloadditions and
sigmatropic rearrangements. Donor-acceptor-substituted semibullvalenes undergo rapid
Cope rearrangement, as do tetraazabarbaralanes. In contrast, tetraazasemibullvalenes cannot be isolated, since the isomeric tetrazocines always result. The usefulness of the donoracceptor concept in preparative chemistry is demonstrated by numerous stable cyclic (4n)nelectron systems, like donor-acceptor-substituted cyclobutadienes, tetraaminobenzene, and
p-benzoquinone dications, benzodiazepinyl anions, and donor-acceptor-substituted cyclopentadienyl cations and their heteroatom-containing analogues. The new compounds are of
interest in the fields of organic metals and ferromagnets, nonlinear optics, and dyestuffs,
among others.
1. Introduction
The special properties of cyclic n-electron systems, well
known to everybody from time immemorial, have always
been attracting to chemists. For the chemist it is also routine to include donor and acceptor effects in planning syntheses. The concepts of aromaticity and donor-acceptor
effect can be traced back to Kekulk‘s benzene formula
(1865)[’]and beyond and to Witt’s theory of chromophores
and auxochromic groups (1 876).[2.31Though these ideas
have been used successfully for over 100 years, their heuristic power and interpretative potential remain unexhausted. As regards the donor-acceptor concept, there are
so many examples that only a few can be mentioned here:
capto-dative-substituted radicals[41 (rnerostabilization:I5]
the concept of capto-dative stabilization has been disputed[”’), the use of donor-acceptor molecules in nonlinear optics (NLO),’6-91 TICT compounds (TICT= twisted
intramolecular charge transfer),”” and the allopoIarization
principle that describes the regioselectivity of ambident
ions.” ’1
The continuing topicality of aromaticity is to some extent surprising, since even today aromaticity has no generally applicable or accepted definiti~n.[’~-’~I
Indeed, quite a
few chemists are extremely skeptical of the usefulness of
this concept. Heilbr~nner,“~’
for example, has stated that
“aromaticity is not an observable property ... and it is not
even a concept which, in my experience, has proved very
gave an instructive article the title “Aromaticity-an Exercise in Chemical Futility”.
Nevertheless, there has been an unbroken chain of remarkable results up to the present day. Clearly, an intuitive
grasp of the phenomenon of “aromaticity” is very fruitful
for the chemist. From a theoretical point of view, aromaticity is a complex phenomenon, and its usefulness as a concept can only be maintained by applying aromaticity crite-
Prof. Dr. R. Gompper, Dr. H:U. Wagner
lnstitut fur Organische Chemie der Universitat
Karlstrasse 23, D-8000 Munchen Z (FRC)
Angew Chem. I n l . Ed. Engl. 27 (1988) 1437-1455
ria to precisely defined areas.‘”-’91 In spite of the skepticism towards aromaticity, new “aromaticities” have constantly been put forward (homoaromaticity,’20~Z11
bicycloaromaticity,”” Y-arornati~ity,[~~-~~’
“4n + 2 interstitial electron ruIe”,i191 “in-plane a r o m a t i ~ i t y ” , [ ~“trefoil
~ . ~ ~ I aromat i ~ i t y ’ ” ’ ~ .and
~ ~ ~the
) transition states of pericyclic reactions have also been i n c l ~ d e d . [ ’ ~ -On
~ * ~the other hand,
questions as fundamental as the Dhh symmetry of benzene[331and the driving force of double-bond delocalization in the “aromatic” s e ~ t e t [ ~ ~are
- ~ ’still
l open or, at least,
the classical answers have been called into question (if the
symmetrical hexagonal structure of benzene is driven by
the o framework alone, the x system favoring a localized
structure, the n-electron delocalization energy can no
longer be a criterion for the empirical resonance energ~‘~’]).
Significantly, VB theory is once again gaining importance in this field.[41‘a1
In view of the many studies on the theoretical background of aromaticity on one hand and the great significance of substituents for the course of aromatic substitutions on the other, it is amazing that the influence of substituents on aromaticity has not been systematically studied. Substituents have only been discussed in connection
with the stabilization of heterocycles (topological charge
s t a b i l i ~ a t i o n [ ~ ’and
, ~ ~ ~of) a few special n systems (tetraterr-butylcy~lobutadiene,[~~~
pental e n e ~ [ ~ ~ .such
~ ’ ] as 1,3-bis(dimethylamino)-2-azapentalene,”’I and 4,8-bis(dimethylamino)- or 1,3,5,7-tetra-tertbutyl-s-inda~ene[~~.~’])
and with distortion (Mills-Nixon eff e ~ t [ ” ~or) bending of the benzene ring.[’5,56a1
It is obvious
that any unsymmetrical distortion of annulenes will lead to
a loss of frontier orbital degeneracy. The principal distinction between the orbital schemes of aromatic and antiaromatic compounds on one hand and polyenes on the other
disappears, and the question arises which “character” substituted polyenes possess. Particularly strong substituent
effects are to be expected with acceptor groups (A) like
NOZ,CN, COR, S02R, with donor groups (Do) like NRz,
SR, OR, and with combinations of A and Do. The following discussion will pursue this question and, on the basis
0 VCH Verlagsgesellschafi mbH, D-6940 Wernheim. 1988
0570-0833/88/I1l1-1437 $ 02 50/0
of synthesized compounds, will explain the usefulness of
the concepts arising from it.
1.1. Donor- Acceptor-Substituted Poiyenes
The dramatic influence exerted by donor-acceptor
groups on the properties of cyclopolyenes can best be
demonstrated with cyclobutadiene l.f56b1
Although it is
(valence tautomerismfM1)and therefore no
longer fully antiaromatic, 1 is extremely unstable and can
only be detected in a matrix below - 196 “C. Diethyl 2,4bis(diethylamino)cyclobutadiene-1,3-dicarboxy~ate2,f6s-701
by contrast, forms yellow crystals (m.p. 56 “C), has four
equal-length ring C-C bonds corresponding to a low tendency towards second-order bond fixation (“A,,,”
= 1.24;
see below), and only dimerizes above 120°C to a red
cyclooctatetraene. ( 1,3-Bis(diethylamino)-2,4-bis(phenylt h i o ) c y ~ l o b u t a d i e n e and
~ ~ ’ ~tris(dimethyIamino)azetef7*~
also be synthesized.)
larly with polycyclic compounds (see pentalenes, Section
2.4). More dependable results can generally be obtained
from the HOMO-LUMO gap. Cyclopolyenes with
(4n -k 2)n electrons have larger HOMO-LUMO gaps than
the corresponding polyenes; in cyclopolyenes with (4n)n
electrons, degenerate frontier orbitals (SOMOs) are present (cf. Ref. [91]). How d o donor-acceptor substituents affect the HOMO-LUMO gap in these n-electron systems?
From Figure 1 it can be seen that donor and acceptor
groups reduce the HOMO-LUMO gap of 28 in benzene to
18 in a 1,4-A2-2,5-Do,-benzene, which, according to this
model, is therefore more weakly aromatic than benzene.
The corresponding substituents increase the HOMOLUMO gap of cyclobutadiene from 08 to the same value as
for the substituted benzene, 18. Resonance energies give a
similar picture. A 1 ,3-A,-2,4-Do2-cyclobutadiene
is therefore just as “aromatic” as an analogously substituted benzene derivative. The values in Figure 1 support the experimental results, and the following rule may be defined:
Donor-acceptor substituents destabilize (4n 2)n-electron
systems and can stabilize (4n)n-electron systems to such an
extent that they acquire aromatic properties.
Only on heating to > 150 “C d o cyclobutabenzenes 3 (cf.
Refs. 173,741) show noticeable valence tautomerization to
o-quinodimethanes, which can be trapped with dienophiles. The aromaticity of the annelated benzene ring
hence hinders ring opening of the cyclobutene system. Donor-acceptor-substituted o - q u i n ~ d i m e t h a n e s ~ ’like
~ ~ 4,
however, show no tendency at all to dimerize o r polymerize, nor any tendency to rearrange into cyclobutabenzenes.
Clearly, donor-acceptor groups reduce both the antiaromatic destabilization of 1 and the aromatic stabilization of
the benzene system in 3. Therefore, (4n+2)n cyclopolyenes are generally made more weakly aromatic by donor-acceptor substituents, while (4n)n cyclopolyenes become more weakly antiaromatic and may even become
“aromatic”. As explained above, the classification of cyclopolyenes as aromatic, nonaromatic, and antiaromatic
depends o n what criteria are used; it must be “defined1’5J
each time what “aromaticity” means. As a basis, the
Hiickel rule ( a n t i a r ~ m a t i c i t y l ~is~ ]used
almost exclusively
and, in addition, despite their problematic nature, the following criteria:
Resonance energy (Hii~kel,”~’
TRE,’”] PM0178b.c1
NMR data (diatropism, p a r a t r o p i ~ r n ) ~(problems
~ ~ ~ ’ ~ ~with homotropylium cationda61)
Bond a l t e r n a t i ~ n , ‘ ~ bond
~ ~ ’ ’ order189.901
HOMO-LUMO gap‘9’-931
(LUMO vs. E ; > $ ,cf. Ref. [94])
How d o the above experiments with cyclobutadienes
and cyclobutabenzenes relate to these criteria? Predictions
from resonance energy are frequently problematic, particu1438
of donorFig. 1. HOMO-LUMO gaps
and resonance energies RE
acceptor-substituted benzene and cyclobutadiene derivatives (HMO)
The same conclusion is reached if the relative HOMOLUMO gaps M E are ~onsidered,’~~]
i.e., the difference in
energy between the HOMO-LUMO gaps of the cyclopolyenes on one hand and the corresponding polyenes o n the
other (Figs. 2 and 3). The numerical value of AAE is diminished by donor substituents in cations and by acceptor
substituents in anions (Fig. 2), leading to a weakening
of the aromaticity of 2n and 6x systems (AAE positive)
and to a weakening of the antiaromaticity of 4n systems
Angew. Chem. Int. Ed. Engl. 27 (1988)1437-1455
1.2. Transition States
0 35
Fig. 2. Influence of donor or acceptor groups on the relative HOMO-LUMO
(HMO) of cyclic ?i systems.
gap M E
M E =~ E H O M O . L I I MM
( ~O, ~M
~ )O - L U M O C ~ ~ I ~ ~ ~ ~ )
( M E negative). However, when AAE is used instead of
the (4n)n systems still remain in the antiaromatic region. This is also true of systems with donor
and acceptor substituents (Fig. 3). With an identical acceptor group, the numerical value of M E falls with donors
Donor-acceptor substituents should influence the transition states of pericyclic reactions just as they do the
ground states of cyclopolyenes. In reactions which, according to the Woodward-Hoffmann rules,"01 are symmetry-forbidden as one-step processes[961
(antiaromatic transition state^),'^',^^^ this symmetry-prohibition can be circumvented by using polar reactants in two-step reactions involving 1,4-dipolar intermediate^.'^^^ It should be possible
for such two-step reactions to take place in one step, however, if substitution with sufficiently strong donor and acceptor groups gives the transition states aromatic character
and thereby reduces the activation energies. (Interpretation
by configuration interaction;198Jthe possibility of SET
processes in 12 + 2]199-i0'1
and [4+ 21 cycloadditions~'02~'071
can be only briefly mentioned here.) In polar [2+2] cycloadditions, for example, 2 would be the model for the
transition state. On the other hand, symmetry-allowed onestep reactions should be slower under the influence of
donor-acceptor groups, and two-step reactions with dipolar intermediates should gain in significance. The transition state for the Cope rearrangement of a 2-acceptor-5donor-substituted 2,5-hexadiene, for example, is comparable with a I-A-4-Do-benzene, which is more weakly aromatic than benzene (see Fig. I).
The donor-acceptor effect can be illustrated in pericyclic reactions with numerous examples (reaction rates, iso-
0 20
o on
-1 2L
Fig. 3 Influence of donor nnd acceptor groups on the relative HOMO-LUMO gap M E [BJ (HMO) of cycllc x systems.
of increasing strength. It is remarkable that the cyclopentadienyl cation is stabilized to the same extent by two amino
and two cyano groups ( M E = - 1.002 -0.56) as by two
amino groups ( M E = - 1 .OO = - 0.68). The benzoannelated diamino(dicyano)cyclopentadiene System, for example, demonstrates particularly clearly how the charcteristics of these opposing n-electron systems converge under
the influence of donor-acceptor substituents. The 6n anion
is destabilized, the 4n anion is stabilized.
Angew. Chem. Inr.
Ed. Engl. 27 (1988) 1437-1455
lation and trapping of dipolar intermediates). Several of
these will now be discussed.
1.2.1 Electroeyc& Readions
Reaction of 2 with methanol yields butadiene 6 rather
than cyclobutene 5,[108a1
apparently because 5 is unstable
as a result of donor-acceptor substitution (see also Ref.
[lOS]). The reaction of 2 with acetylenes, via the 1,4-dipoIe
7 , does not afford the Do-A-substituted Dewar benzenes
8, but the benzene derivatives 9,L108h1
even at low temperature. The symmetry-forbidden rearrangement of 8 to 9 is
hence greatly accelerated by Do-A substituents.
- 40'C
In liquid sulfur dioxide, even at - 40 " C, 15b is transformed into 15a .166,1151 (Two-step polar Diels-Alder reactions have also been observed with other dienes["'] and
heterodienes." ''-I2 'I 1
1.2.4 [3,3/-Sigmatropic Rearrangements (the Semibullvalene
Problem; Tetrazocines; Tetraazabarbaralanes)
R = Et
1.2.2. 12
+ 11 Cycloadditions; Stable Non-Kekule Systems
Singlet carbenes add to olefins stereospecifically.[llol
However, if electron-poor allenes like tetraethyl allenetetracarboxylate are reacted with nucleophilic carbenes,
the products are not cyclopropanes, but stable, crystalline
1,3-dipoles 10-12.["" An analogous compound 13 is obE
Just as with the Diels-Alder reaction, the question of the
concertedness of the Cope rearrangement of "normal" 1,5hexadienes still remains open.11221Cope and hetero-Cope
rearrangements of 2-donor-5-acceptor-substituted 1,5-(hetero)dienes like 16 and 19, for example, proceed extremely
rapidly, and 1,4-dipoles (17, 20) can be d e t e ~ t e d . [ ' ~ ~ - ' ~ ~ ~
Dipole 17, which is in equilibrium with 16, eventually affords 18 by prototropism, and 20 can be trapped with benzaldehyde o r acrylonitrile to give the tricyclic compound
21 .[lZ3l (Two-step [2,3]-sigmatropic rearrangements are also
known."3o, '3'1) The formation of 1,4-dipoles could be
caused by a slowing of the one-step Cope rearrangement
E = C0,Et
tained if the electron-rich tetrakis(dimethy1amino)allene is
combined with diazomalonic
Compounds 10-13
can also be viewed as stable trimethylenemethanes; if trimethylenemethanes, as 471-electron systems, are classified
as antiaromatic, the stabilization of destabilized n systems
is seen again. Finally, combination of electron-poor and
electron-rich allenes gives the crystalline tetramethyleneethane derivative 14.[1'31
Compounds 10-14 are of interest
because, among other things, they are stable examples of
non-Kekule systems.
1.2.3. [4 + 21 Cycloadditions
The mechanism of the Diels-Alder reaction still arouses
discussion." l4] Reactions of oppositely polarized components take place very quickly, and dipolar intermediates
can be detected. The reaction of cyclohexenylmethylenemalonodinitrile with I-( I-cyclohexeny1)pyrrolidine at
room temperature gives the colorless Diels-Alder cycloadduct 15b, but at -40 "C the crystalline, yellow, 1,6-dipole
15a is obtained, which is converted into 15b on warming.
a=b O=CH-Ph
(higher activation energy due to the weaker aromaticity of
the transition state) and an acceleration of the two-step
reaction (stabilization of the transition state leading to the
Cope rearrangement of semibullvalenes proceeds extremely rapidly.[1321
According to Dewar et al.['331and Hoflmann et al.L13411,5-didonor-2,4,6,8-tetraacceptor-substituted semibullvalenes 22 could have a bishomoaromatic
ground state 23. Till now, such systems have been sought
in vain (24a,'135124b,[136.1371
There could
be two reasons for this. Either the rearrangement of the
donor-acceptor-substituted semibullvalenes studied so far
does not go through a bishornoaromatic ground state or it
is not stabilized sufficiently for the reasons outlined above.
Possibly, the substituents used (Do, CH,; A, 4 C 0 2 R ,
2 CN, 2 ring N) were not "strong" enough.
Angew. Chem. In(. Ed. Engl. 27(1988) 1437-1455
ti 32
11 tB"0Cl
21 tBuOK
a XzC-A
X z N
a R:H
a X zC-CN RzH
b X = N R Ph
b RzOAc
E = CO,Me
Since synthesis of semibullvalene-2,4,6,8-tetracarbonit r i l e ~ " ~has
~ ' not yet been possible, we have attempted to
produce the electronically equivalent 2,4,6,8-tetraazasemii
bullvalenes 22b. Oxidation of the bisimidates 26 of the
glycouril series does not, however, lead to tetraazasemibullvalenes 29, but to the tetrazocines 28.114'1
Either the
tetraazasemibullvalenes 29 and the bishomoarenes 30 are
both destabilized relative to the "nonaromatic" tetrazocines (the X-ray crystal structure shows a boat form, as for
cyclooctatetraene) and rearrange rapidly to 28 (path b), or
a fragmentation of 27 occurs (path a). 1,5-Dimethyl-3,7diphenyl-2,6-diazasemibullvalenealso rearranges to 2,6dimethyl-4,8-diphenyl-l,5-diazocine, though only at
90 oc.I1421
To prevent conversion to tetrazocines, 3a,6a-bridgedglycourils such as 31 were synthesized (cf. cyclooctatetraenes containing (CH,), bridges between the 1,3-J143a1
l,4-,['43bland 1,5-p0sitions~'~~"~)
and transformed into 32,
which were then oxidized. However, tetrazocines['441were
produced here
(e-g., 34 and 33, prepared from an
analogue of 31). This is all the more astonishing since, although 33 is a derivative of bicyclo[3.3.3]undec-l-eneand
is therefore counted as a member of the "isolable" bridgehead ole fin^,"^^. 1461 it
. is the only "anti-Bredt" bridgehead
imine isolated to date, apart from 2-azabicyclo[3.3.l]non(cf. Ref. 11481). The synthesis of ]$bridged cyclooctatetraenes with five, six, or ten methylene groups in
Angew Chem In1 Ed Engl 27(1988) 1437-1455
p3 b O E t &4
was only possible on heating the corresponding semibullvalenes to 500-521 "C.
If 29 (and 30) are inherently unstable, then the 2,4,6,8tetraazabarbaralanes ought also to be so, at least in part.
2,6-Diphenylbarbaralanes and 2,6-barbaralanedicarbonitriles have localized double bonds and undergo faster
Cope rearrangement than the parent c ~ m p o u n d . ~ ' ~ ~ - ~ ~
(Substituents at C-3,7 have a moderate rate e f f e ~ t ; " ~ * - ' ~ ~ I
in the crystal, though not in solution, a 9-phosphoniobarbaralane is nearest to a bishomaromatic state.[1551)
The tetraazabarbaralanes 36 can be synthesized from 35 by the
same procedure as 28, 33, and 34.[l5*]
Compound 35 itself
is available from the corresponding urea derivative."s6.
The X-ray crystal structure of 36 shows a large difference
between the N2-N8 (158 pm) and N4-N6 (236 pm) distances, in accord with the given formula. However, the
N2-N8 distance is significantly greater than the N-N distance in d i a z i r i d i n e ~ [ ' ~ ~(145.3-150.6
pm), and is comparable with that in highly substituted 1,2-bis(p-bromoa,cL-dimethylbenzyl)diaziridinone~'6z1
(160.1 pm). The NMR
spectra establish that 36 in solution also has a localized
structure, for which the Cope rearrangement becomes
fast between 25 and -60 "C. A variable-temperature
NMR study showed that 36 rearranges more slowly (!)
(AG&, = 38.9 kJ mol - ') than barbaralane (AC&,= 3 1.5 kJ
mol This can be rationalized by assuming a bishomoaromatic transition state in the rearrangement of 36, resembling 3,6-diethoxy-1,2,4,5-tetrazineand therefore, according to Figure 1, being less aromatic than a benzenelike transition state. In the rearrangement of 2,6-barbaralanedicarbonitrile, however, the transition state may have a
diallyl structure (cf. Ref. [153]).
2. Novel Donor-Acceptor-Substituted Aromatic
Of all compounds synthesized recently by us and
other groups, the ones that will be presented are primarily
those examples of 8%-and 4n-electron systems which demonstrate the stabilization and double-bond delocalization
effects of donor-acceptor substituents on one hand, and
particular characteristics of the substances on the other
(color, oxidizability or reducibility, electrical conductivity).
swings to the side of the bicyclic dianions. While 37 can be
viewed as a bishomoantiaromatic1221
8n-electron system, it
is doubtful whether homoaromaticity plays any role at all
in car bani on^."^^] The formation of 37 rather than 38 demonstrates particularly clearly how a (4n)n system can be
stabilized at the expense of a ( 4 n t 2 ) n system by donoracceptor groups.
2.1. Ten n Electrons in an Eight-Membered Ring
Cyclooctatetraene (Sn, nonplanar) can be reduced to its
azocines1166.1671 and
aromatic dianion (Ion, planar);['63-1651
d i a z o c i n e ~ " ~ behave
similarly. Surprisingly, however,
tetrazocines 28 d o not afford the monocyclic dianions 38,
but the bicyclic dianions 37, which can also be obtained
from 26 with strong base.1'441This appears to contradict
the observed ring-opening of the dianionic bicycl0[3.3.0]0ctadienediide.~'~*~
In fact, when the energies of
dianionic (diaza/tetraaza)bicyclo[3.3.0]octadienediides are
compared with those of (diaza/tetraaza)cyclooctatetra-
Like benzene and pyridine, the tropylium ion and the
azatropylium ion are a pair of Hiickel-aromatic 6n-electron systems. However, while for pyridine and benzene the
effect of aza-substitution on the calculated heats of formation is negligible (AAH,= - 0.1 kcal mol- I ) (MNDO),
= Hr(PhCH=NPh)
- Hr(PhCH=CHPh) = 7.6 kcal mol-
M H f = H f ( A z a A r ) - H,(Ar) - 7.6
the azatropylium ion proves to be significantly less stable
than the tropylium ion ( M H , = 10.7 kcal mol-I). In view
of its "aromatic" electron sextet, it is nonetheless surprising that it has not yet been possible to synthesize any azatropylium salts. An attempt to oxidize 2-amino-3H-azepines 39 to 2-aminoazatropylium salts 42 with trityl tetra-
2.2. Six A Electrons in a Seven-Membered Ring
(Azatropylium Ions)
R = M e Ph
enediides, using calculated heats of formation (Table I),
the monocyclic dianions are found to be more favored. In
donor-acceptor-substituted systems, however, the balance
Table I. Heats of formation [kcal mol-'1 of bicyclic and isomeric monocyclic
L 40
ti CPh3
dianions (MNDO).
'2' ' 3
fluoroborate, where the destabilizing influence of the ring
N atom ought to be (over)compensated for by the donor
group, led to salts 43 of 2-azanorcaradienes 47, the first
salts of this type1'701(3-azanorcaradienes, see Ref. 11711).
Obviously, this occurs by initial electrophilic addition to
give 40, followed by deprotonation (-+41), reprotonation
and electrocyclization to yield 43. Treating 43 with
alkali gives 47, which suffers ring-opening to 46, and on
warming affords the 6-substituted 2-amino-3H-azepines
45. The more electron-rich 2-dialkylamino-6-methylthio3 H-azepines cannot be dehydrogenated to azatropylium
salts either.""'
The first derivatives of azatropylium salts have now
been synthesized11741via aminodibenzoazatropones 48
from morphanthridinedi~nes.~'~'~The electronic differences between the tropones and 48 can be seen in the fact
Angew. Chem. Int. Ed. Engl. 27j1988) 1437-1455
that unlike tropones, 48 are not 0-alkylated, but can be
N-alkylated to form 51. Compound 48 can only be converted into the red salt 50 with trimethylsilyl triflate; this is
then transformed in situ into the stable orange salt 49.
able. Salt 59 can be deprotonated to give the yellow N,Cbis(benz0diazepine) 61, which on heating affords the C, C
dimer 60.
Only by severely changing the azatropylium system by annelation of two benzene rings is it possible to obtain stable
derivatives. Just as for the compound types mentioned
above, the Hiickel rule is no help here. As yet, benzodiazepinium ions of type 52 have not been obtained
2.3. Eight n Electrons in a Seven-Membered Ring
a: Y = N, 2 = C-XR. X = S, NR
b: Y = C - S R . Z = N , X = S
either,"7s. '761 though, as 6( 1O)n-electron systems combined
with donor substituents (SR, NR2), they ought to be sufficiently stabilized. The low tendency to form 52 is probably
connected with the high contribution of resonance structure 52', and is illustrated by the observation that on reaction of 55 it is not the seven-membered ring but the five-
While azepinium and benzodiazepinium ions have not
yet been detected, 8n-11761
and 12n-electron
have been produced for the two classes of compounds, respectively (cf. carbon analoguesL'78-'821).The anions
and 5411741
are easy to synthesize and undergo
numerous reactions. In solution, the lithium salts of 54,
R=alkyl, and 53, RX=alkylthio, are purple and red, respectively, and therefore probably have strongly ionic
character. Compounds 53, RX = dialkylamino, with lithium as counterion, are colorless and are therefore likely to
be covalent species. Compounds 53 and 54, whose stabilities can be ascribed to resonance structures 53' and 54',
react at C-3 with electrophiles to form novel sesquifulvalenes and heptafulvalenes, among other products (Scheme
a:>s a:>N?
h e SMe
membered ring that is dehydrogenated to give 57, which
yields 56.[17s1
The reaction of silver tetrafluoroborate and
which gives the
red salt 59,["'] might involve the benzodiazepinium ion 58
as an intermediate, but other mechanisms are also conceivAngew. Chem. Int. Ed. Engl. 27 (1988) 1437- I455
Scheme I . Reactions of 1,s-benzodiazepinylanions 53 with electrophlles
2.4 Eight K Electrons in a 5/5-Ring System
((2,5-Diaza)pentalenes)and Twelve K Electrons in a
5/6/5-Ring System (2,6-Diaza-s-indacenes)
Table 2. Calculated substituent effects on double-bond delocalization in
(aza)pentalenes (A,,,, = 1.7). Numbers on the bonds, n bond order; numbers
in parentheses, n charge; numbers in square brackets, “AmdX”.
As planar analogues of the nonplanar cyclooctatetraene,
pentalenes are usually classified as a n t i a r ~ r n a t i c . ~ ~ ~ ~ - ’ * ~ - ~ ~ ~ J
However, use of sterically demanding groups makes it possible to obtain stable pentalenes like 62 and to prove that
these are compounds showing bond alternation, which undergo rapid valence tautomerization.[”. ‘881 With aza- and
diazapentalenes,‘s2,’*’I the destabilizing effect can become
~ m a l l e r . ‘ ~ The
~ . ’ ~diazapentalene
64,described as “electronically pure”,”s9J(cf. Ref. [190]) is clearly a localized system and therefore actually nonaromatic[’891(incidentally,
alkyl substituents are in no way electronically “pure”, i. e.,
without influence, as is shown, for example, by the orientation effect in aromatic electrophilic substitution, by Hammett constants, and by the structural discussion on s-inda~ene,[’*.’~~
where tert-butyl groups are not treated differently from amino groups). Bond localization also seems
H2 N
2.4.1. Donor-Substituted 2,5-Diazapentalenes
pound 67b, which can be detected as an intermediate, can
also be synthesized from tetramethylguanidine and succinyl dichloride and converted into 69 by treatment with sodium hydride in dimethylformamide
a:A. 8, X = C H ;
C: A = CH,
b:A, B = C H , X = N;
B = C-tBU, X = N;
d : A = N , B=P, X = N
dominant in 65b,c,IS11while 65arS’]and 65d1l9’]are best
characterized by dipolar resonance structures. The bond
localization in 62 agrees with the bond-localization parameter ”,2/“,
calculated according to Binsch and Heilbronner’s methodr53~87.881
(see Table 2), while that of 64 agrees
with the calculated bond orders; by calculation, 65a-c
should show approximately the same bonding situation.
1,3,4,6-Tetraamino-, 2,5-diaza-, and especially 1,3,4,6-tetraamino-2,5-diazapentalenehave ”,2/,“
values which fall
below the critical value. This, and the larger HOMOLUMO gap, leads one to expect 1,3,4,6-Do4-2,5-AZ-substituted pentalenes to be aromatically stabilized and to show
a delocalized x-bond system (cf. also Refs. 149,501).At this
point the interesting Do-A-substituted pentalene derivatives 63, R=alkyl or phenyl,”921 should also be mentioned.
The reaction of succinamide with the amidoacetals 66
is, along with the reaction of succinic acid and aromatic
nit rile^"^^^ (cf. also Ref. [1941), a simple method of producing diones such as 70,f195J
which are of great interest, not
least as pigments.“931 With 66b one obtains the colorless
compound 69r’961
in high yield. In contrast to the red 70,
compound 69 does not exist in the enamine form. Com1444
a:R =Ph; b R = NMe,
a: R ” = C I : b: R “ = SMe
T : x s N H1
c: R”= NMe,; d: R ” = rnorpholino; rnorpholino instead of NMe,
Angew. Chem. I n t . Ed. Engl. 27 (1988) 1437-1455
From 70 and the dithione derived from it,[195,197a1
can synthesize bis(trimethylsi1oxy)- and bis(alkylthi0)2,5-diazapentalenes, re~pectively.~”~~
Compound 69 is the
starting material for 1,3,4,6-tetradonor-2,5-diazapentalenes. Using triethyloxonium tetrafluoroborate one obtains
68; with Lawesson’s reagent 72 is obtained; 72 gives 71b,
which yields 71c upon treatment with dimethylamine at
140 “C. Phosphorus oxychloride gives 71a, from which
71d can be obtained. The high thermal stability of these
crystalline compounds dramatically demonstrates the effectiveness of the donor-acceptor effect (cf. Table 2). The
1,3,4,6-tetraamino-2,5-diazapentalenes71c,d are especially interesting from a theoretical point of view. They allow an investigation of the question of whether 71 has a
localized (CZjrsymmetry) or delocalized structure (& symmetry).
The use of tetradonor-substituted diazapentalenes now
makes possible the synthesis of an entire series of stable,
crystalline pentalene dication salts. The Wiberg bond indexLzoZ1
for C3a-C6a in 74 is 1.852, which corresponds almost to a CC double bond, as in the given formula (n
charges: C-1, +0.35; C-3a, +0.09; N-2, -0.46). The 6n
system formed by oxidation of 71c,d is therefore converted from a delocalized into a localized structure by firstorder bond fixation.
a R2N=morphoIino; b: R=Me
200 “C
2 HBF,
25 “c
Fig. 4. Crystal structure of 71c (ORTEP)
X-ray analysis of 71c (Fig. 4) reveals double-bond delocalization, as required by theory (as to the question of
equilibrium structure, stabilized transition state or disorder
in the crystal, cf. Ref. [197b]). Compounds 71c,d must
therefore be described as “aromatic Hiickel antiarenes”,
since they show neither strong first-order bond fixation
nor strong second-order bond f i x a t i ~ n . ‘(If
~ ~the
. ~ expres~~
sion “electronic
is adopted, then the “electronically impure” Hiickel antiarenes 71c,d represent delocalized cyclopolyenes, i. e., arenes, while the “electronically
pure” Hiickel antiarenes 63 and 64 are localized cyclopolyenes and therefore nonaromatic compounds.)
Compounds 68 and 71 provide examples of reversible
multistep redox systems (formation of radical anions and
radical cations (cf., e.g., 73), dianions and dications (cf.,
e.g., 74)). The salt 74a, X=BF,, can be obtained as reddish-purple crystals on a preparative scale by reaction of
71d with trityl tetrafluoroborate in dichloromethane; 73a,
X = BF,, which is bluish-purple in acetonitrile solution,
arises by combination of 71d and 74a, X = BF,.
With 74, dications of pentalene have been isolated and
characterized for the first time. Though the synthesis of
a hexachloropentalenediylium bis(hexach1oroantimonate)
has been reported,”981its structure was not proved. Dications of dibenzopentalene have been generated in solution
and characterized by NMR spectroscopy,[‘991and the radical cation of 1,3,5-tri-tert-butylpentalene,stable up to
-77 “C, has also only been obtained in solution.~z00~20‘1
Angew. Chem. Inr. Ed. Engl. 27 (1988) 1437-1455
Tetradonor-substituted-2,5-diazapentalenes are quite
strong bases (pK,l, pK,2 (in 50% EtOH): 71c, 6.40, 10.35;
71d, 4.28, 8.63). It is interesting to note that 71c is as basic
as 1,8-bis(dimethylamino)naphthalene (pK, = 10.40 (in
H,O, pKa = 12.34[203.2041)),
but 1,3-bis(dimethylamino)-2azapentalene 65b[205.2061
is a markedly weaker base
(pK,=6.82). Protonation of 71c,d does not take place on
the dialkylamino groups as in “proton sponge”, though,
but on the ring N atoms. Reaction of tetrafluoroboric acid
etherate with 71d in dichloromethane leads to the yellow
NH salt 75, which decomposes to the colorless CH salt 76
either thermally or on standing in acetonitrile solution.
2.4.2. 1,3,5,7-Tetraamino-2,6-diaza-s-indacenes
In 76, two azaallylium or azavinamidinium units are
connected by a CC single bond, while in 74 it is a CC
double bond that links the two. The analogous yellow dicationic salt 79dZo7]
which has a benzene ring as the linking
unit, is obtained by reaction of the hexachloro compound
77, which is available from pyromellitimide, with diethyl(trimethy2silyl)amine. (A structure corresponding to 79
has been calculated for the dication of s-indacene.’208“1)
Like 74, though at a more negative potential, 79 can be
reversibly electrochemically reduced to 78 via a stable radical cation. If the reduction is carried out with sodium in
liquid ammonia, then 78 can be isolated as air-sensitive
violet crystals. X-ray analysis (Fig. 5 ) shows that 78 has a
delocalized structure corresponding to 78’, though, like
71c it belongs to the Hiickel antiarenes. The predictions
arising from “d,,,” and R E (s-indacene, 1.57d53.881
2,6-diaza-s-indacene, 1.36, 1.341; 1,3,5,7-tetraamino-s-indacene, 1.18, 1.I51 ; 1,3,5,7-tetraamino-2,6-diaza-s-inda1445
2.4.3. Derivatives of Dibenzopentalenes
1) Me,SiNEt,
21 NaBF4
In contrast to pentalenes, dibenz~[a,e]pentalene[~*~l
its 1,9-dimethyl~z’01and 1,9-dichloro derivatives[’’ are
thermally stable. Dications and dianions of dibenzo[a,e]pentalene can be ~ b t a i n e d [ ’ ~from
~ , ~ the
~ ~ lparent
compound by oxidation and reduction, respectively,
or by metalation of 1,9-dihydrobenzo[u,e]pentalene 80,
X = H2.[”I1The ability of the dibenzopentalene system to
form both a stable 1471dication and a stable 1871dianion is
reflected in the properties of the electron-rich dihydrobenzopentalene derivatives 81a, and the corresponding electron-poor compounds 81b, X = N-CN,[2131 C(CN)2.rZ141
From 81b, X = C(CN)2, and 81a, X = 2-(3-methylbenzthiazolylidene), for example, a complex is formed that has
high electrical conductivity (a=0.74 S cm - as a powder).
By the same routes used for 81 the “phenylogous” dibenzopentalene derivatives 82 can be ~ y n t h e s i z e d , [ * ’ ~fr,om
or the corresponding d i ~ n e . ~ ” Co
~ . mpounds
~ ~ ’ ~ 81 and 82 can both be reversibly oxidized or reduced via radical cations/anions to
give the dications/dianions 83 and 84, respectively. De-
cene, 1.22, 1.367) of increasing delocalization and stabilization of s-indacene by donor and acceptor groups are
therefore fulfilled. These had already proved to be correct
with the compounds synthesized by Hujiier et al. (4,8bis(dimethylamino)-~-indacene,[~’~~~~
1,3,5,7-tetra-tert-but y l - s - i n d a ~ e n e ‘ ~ ~ ,Hexamethyl
4,8-dihydroxy-s-indacene- 1,2,3,5,6,7-hexa~arboxylate[~~~~~
is also a stable compound. Compound 79 is the phenyl analogue of 74 and
78’ is the quinodimethane homologue of 71c,d.
a: X = CHNR,, C(NR2),, etc.
b: X = (O), N-CN, C(CN),
pending on the oxidant, one obtains, for example, colored
crystalline bis(tetracyan0-p-quinodimethides) or bis(tetrafluoroborates) of 83a and 84a.
Fig. 5. Crystal structure of 78 (ORTEP)
2.5. Four n Electrons in a Six-Membered Ring (Benzene
and Quinone Dications)
Compounds 76 and 79 readily react with nucleophiles.
The formation of the dianionic salt 80 is remarkable; it
reacts with 79 to form a red dicationic dianionic salt containing a small proportion of radical ions. Compounds 79
and 78 combine to form a radical cation (Am,,(THF)=795,
875 nm).
It is worth noting that the transformation of 78‘
(A,,, (benzene) = 614 nm) to 79 (Amax(CH2Cl,) =440 nm) is
associated with a strong hypsochromic shift, while that of
(C6H6) = 483 nm) to 74a (amax
(CH3CN) = 520
71d (A,,
nm) shows a bathochromic shift.
The benzene dication, the simplest Huckel antiaromatic[219-2211 carbodication, has to date only been detected by
mass spectrometry.i2221Naturally, with increasing donor
strength, substituted benzene derivatives are more easily
Th e hexachlorobenzene dication can be
detected spectroscopically at 77 K[z251(the hexamethyl
benzene dication has a pentagonal-pyramidal struct ~ r e [ ~ ~ ~As
. ~ materials
for f e r r o m a g n e t ~ , [ ~these
~ * - ~un~~~
stable compounds are of as little use as the colorless quinone diiminium
A system with a triplet ground
state is r e q ~ i r e d , I ~ ~or
~ -one
’ ~ ‘in
~ which the triplet state is
easily accessible from the singlet ground state.r2311
Angew. Chem. Int. Ed. Engl. 27(1988) 1437-1455
The hexaaminobenzene derivative 85a[2241
and the hexacan be easily oxiaminotriphenylene derivative 86a[233,2341
dized to the stable dications 85b ( R E = +0.148!) and 86b,
respectively. These have a triplet ground state in solution
and are therefore a priori of special interest as materials
for organic ferromagnets.
a:n=O; b : n = 2
Tetraaminohydroquinone derivatives 89 are structurally
very similar to the hexaaminobenzene derivative 85a. They
are obtained by reductive acetylation (89, R=Ac) of decahydro - l ,4,6,9 - tetramethylpyrazino[2,3 -g]quinoxaline-5,10dione 88, or by reaction of tetramethoxy-p-benzoquinone
87 and dimethylethylenediamine (89, R = Me).[235,2361
They are converted into the blue benzene dicationic
salts 91 with silver or nitrosyl tetrafluoroborate
(Amax (CH3CN)= 63 1 and 680 nm, respectively). Interestingly, the donor effect of four amino groups is sufficient to
cause “umpolung” of the p-benzoquinone system; both 88
and 89 can be oxidized to the violet dicationic salt 90
= 588 nm), either electrochemically, at
very similar potentials, or with silver tetrafluoroborate (the
product of oxidation of tetrakis(dimethylamin0)-p-benzoquinone is reported to be a radical cation’2371).
The mesomeric structures 90a and 90b emphasize the benzenedicationic nature of 90 and its relationship to 91.
chromically shifted relative to that of 91. Methylation of
with trimethyloxonium tetrafluoroborate in nitromethane affords the green dicationic salt 9512361
(A,,,(CH,CN) =639 nm; salts of this type have been reported in a patent,[z401without synthetic details; a bis(Nmethylsulfate) of 94, Me instead of Ph, is also
The salt 95 is reduced by naphthalenesodium to a radical
cation, which can be isolated as a green-black tetrafluoroborate (A,,,(CH3CN)-900 nm). The radical cation is also
formed when the electronic spectrum of 95 is recorded in
DMF (A,,,=910 nm).
The dicationic salt 96 is produced from 1,2,4,5-tetrakis(dimethy1amino)benzene and iodine.[242rX-ray structural analysis shows that in 96 the two vinamidinium units
are linked by what are almost single bonds, possibly because of steric hindrance. In 91, 93, and 95 a similar distortion of the central ring is either impossible or only possible to a reduced extent, because of the additional rings.
However, the length of the CC bonds in the central ring
does not appear to have a great influence on the electronic
state of the dications, since the chemical shifts of the vinamidinium proton signals in 96 (6=5.85) and 93
(6=5.96) are not significantly different from those in 97
(6= 5.484).[2431
Only in 95 (6= 6.32) does one see a stronger
downfield shift, in agreement with the calculated charge
density (HMO charge densities: 95, -0.141; 93, -0.159;
97, -0.163). In their color, too, 91,93,95, and 96 (blue to
green) do not differ fundamentally from one another.
Me OR M e
Further derivatives of tetraaminobenzene dications can
be synthesized from pyrazino[2,3-g]quinoxalines such as
92 and quinoxalino[2,3-b]phenazines such as 94. By aIkyIation of 92Lz381
one arrives at the green, orange-yellowfluorescing dicationic salt 93‘2361(&,,,,(CF,COOH) = 572
nm), whose long-wavelength absorption maximum is bathoAngew Chem In1 Ed. Engl. 27 (1988) 1437-14S5
2.6. Four rc Electrons in a Five-Membered Ring
((Aza)cyclopentadienyl Cations)
Initially, cyclopentadienyl cations were only obtained in
solution; the cyclopentadienyl cation and its pentachloro
derivative have triplet ground states,’2281
while the pentaphenyl and pentakisb-methoxyphenyl) derivatives‘22s~2441
have singlet ground states and a low-lying triplet state. One
way of generating cyclopentadienylium salts, which are
also of interest in the production of organic ferromagnets,
among other things,‘228-23’J
is shown by the donor-acceptor
effect (cf. Figs. 1 and 2). The HMO charge densities point
in the same direction (Fig. 6). In the framework of this
model one sees no destabilization of the ?I system on moving from the ally1 cation to the 2-azaallyl cation, because of
the greater electronegativity of the N atom; in the 2-azavinamidinium cation the N atom has a stabilizing effect (cf.
electron-gas m 0 d e 1 [ ~).~Because
~ - ~ ~ of
~ ~the positive charge
on all the ring atoms of the cyclopropenylium and tropyIium ions, a n N atom in these positions destabilizes the
Ir-electron system. With the corresponding diamino derivatives this effect is smaller or not present at all. While the
azacyclopentadienyl cation is, naturally, destabilized relative to the cyclopentadienyl cation, with the diaminoazacyclopentadienyl cation one observes a dramatic stabilization. It should be stressed that as yet the “aromatic” azacyclopropenylium and azatropylium salts could not be synthesized (cf. Section 2.2), but that the “antiaromatic”
(aza)cyclopentadienyIium salts have been prepared!
@ I @
-1 5Do
2 123
2 220
0 200
-1 375
0 345
Fig. 7. HOMO-LUMO gaps (HMO) of (aza)cyclopentadienyI cations, and
long-wavelength absorption maxima of several typcial (aza)cyclopentadienylium salts.
R’ = CI, NMe,
Fig. 6. HMO charge densities of systems with allylic cation structure.
The relationship between color and structure in the compounds synthesized to date is given in Figure 7. The longwavelength absorption maxima are hypsochromically
shifted proportionally to the increases in the HOMOLUMO gap of the parent compounds.
The compounds presented in Figure 7, and related ones,
can basically all be synthesized by ( I ) substitution or (2)
dehydrogenation processes.
(1) Reaction of 101a,[2481
or 10lb,[ZSO1
with dialkyl(trimethylsi1yl)amines gives the red azacyclopentadienylium salts 99,L2s11
the benzo analogues 102b,[Z521
and the
yellow 1,3-diazacyclopentadienyliumsalt 103.[2521
Like 98
and 102a[2541
(accessible from 100a[2531),
these are stable,
crystalline compounds. (Anilino and amino derivatives of
type 102 are also k n o ~ n , [ as~ are
~ ~several
- ~ ~ other
~ ~ indenylium saIts;[258-26’1cf. also solvolyses with indenylium
cation intermediate^.^'^^.^^^]1
A variant of this method i s to synthesize enaminoketone
105b from 105a[2641
or by other routes,[z651
and from it to
obtain thioketone 1 0 5 (2-phenyl
~ ~ ~ ~
~ ~
By alkylation of 105b,c one arrives at 104, and from this
one eventually obtains 102a[2661(DMAP= 4-dimethylaminopyridine).
(2) The 1,3-diazacyclopentadienylium salt 107, the phenyl
analogue of 103 and structurally related to 98, has been
synthesized by dehydrogenation of imidazole 106.t2691
a X=CH,Y=Br
b X=N,Y=CI
I G k N E k
a’X=CH, Z=Br
b-X = N, Z = CIO,
a:X=O. Y=CI
b : X = O , Y =NR2
c ’ X = S. Y = NR2
the same principle, it has been possible to obtain 1,2-diazacyclopentadienylium salts 110 by dehydrogenation of 108
or 109[2701
(as to the parent compound, cf. Ref. [271]). The
difference in color between the colorless 6% system 109
and the blue 4x system 110 is striking.
These two routes can be used for the synthesis of numerous thermally stable, crystalline cyclopentadienylium salts.
Angew. Chem. h i . Ed. Engl. 27 (1988) 1437-1455
11 PbO,
For example, condensation of diphenylcyclopentanetrione
with N , Wdimethylethylenediamine yields the stable, red
diaminocyclopentanedienone 111. Whereas protonation of
this leads to 113, reacting it with triethyloxonium tetrafluoroborate gives the green ethoxydiaminocyclopentadienyl salt 112 in quantitative yield (Amax(Ac20)=735nm).
Analogously to the synthesis of 111, the donor-acceptor-substituted cyclopentadiene 121 is obtained by condensation of dioxocyclopentanedicarbonitrile with N , N'dimethylethylenediamine.[95.2731
If one assumes that the effects of the donor and acceptor groups cancel each other
out, then the compound ought to behave almost like a normal cyclopentadiene. This is indeed the case, permitting
formation of the yellow 122 and the synthesis of numerous
fulvenes and f ~ l v a l e n e s , ' ~ ~whose
. ~ ~ ~ UV/VIS
spectra display a bathochromic shift of 60-100 nm relative to those of
the corresponding non-Do-A-substituted compounds. The
peculiarity of 121, however, is that it can be easily converted not only into the salt 122 of a 6n-electron anion, but
also to the stable, crystalline, deep-green salt 123 of a 4nelectron ~ a t i o n ; ' ~ ' .119
~ ~ ~is] another compound of this
type.1951The fact that 122 and 123 are both stable, despite
belonging to different n-electron categories, shows the
consequence of the donor-acceptor effect particularly
graphically. In contrast to the cyclopentadienyl cation
( R E = - 0.238), the cation of 123 has a positive resonance
energy ( R E = +O.llg (HMO)).
~ - .
11 nBuLl
21 tBuOK
Further reaction with secondary amines affords crystalline blue triaminocyclopentadienylium salts like 114
(Amax(CHC13) = 624 nm).[27'1 By the same principle, one
The decan obtain cyclopentadienylium salts 115-118
pendence of the long-wavelength absorption maxima on
substituents agrees with M O calculations.
C02Et XQ
Angrn. Chem. In!.
Ed Engl. 27 (1988) 1437-1455
The UV/VIS spectra of all donor-acceptor-substituted
cyclopentadienediylium salts show very long-wavelength
absorption maxima, with medium extinction coefficients
(log E = 3). So far 123 is the record holder, with A,,, = 810
nm, and thereby comes very close to the value calculated
for the as yet unknown 120.1274-2771
Cyclopentadienylium salts 117-119, 122, and 123 are
most suitable for examining the question of the ring-current
effect in stable anti-Hiickel systems with positive RE. Figure
9 reveals that the donor-acceptor-substituted anions 122,
124, 125, and 127, the aminocyclopentadiene imine 126
and the cation 119 all fall on regression line for aromatic
compounds (6('H) = 10.4qN,+ 7.59; r = 0.988), produced by
plotting 6('H) values against calculated charge densities.
(For a detailed discussion of other regression^,[^^*-^^^' see
Ref. [95].) They can therefore be classified as arenes.
In contrast to this, and above all to the position of 119,
one finds a large upfield shift with cation 123. From the
difference in the net n charge at C-6 between 122 and 123,
a charge-induced shift Aaq = 3.45 ppm can be calculated,
which is significantly larger than that found (A6=
2.10 ppm). Though the paratropic shift for 123
(A6,=A6,-AS= 1.35 ppm) is smaller than that for the
benzocycloheptenyl anion (A6, = 3.7-6.8 pm, relative to
the corresponding cation[2s51),it is comparable to that observed for tri-tert-butylcyclobutadiene(A& = 1.04 ppm, relative to cyclopentadiene[2861)and for 63, R = alkyl or phenyl
1.10-1.64 ppm, relative to 1Jdihydropentalene).[z871According to N M R criteria (paratropism) and
130 M e 2 N & N M e 2
4 --
(jlmax(H2S04)=724nm). The salt 140 is obtained by oxidation of fulvalene 138 (/Z,,,(DMF) = 500 nm), synthesized
from 121 and 137. If one combines 138 and 140, the radical cation salt 139 is formed (/Z,,,(CF3COOH) = 819 nm).
3. Applications and Prospects
A A E H O M O - L U M O (cf. Fig. 3), 123 is classified, then, as antiaromatic (cf. Refs. [288,289]). According to RE, however,
it is aromatic (cf. the difference between absolute and relative antiar~maticity.[~*~])
In addition to the regression line for aromatic systems,
Figure 8 shows that for cyanines (S( H) = 5.36qN, 6.69 :
cf. also Ref. [290]). The open-chain compound 129
((/Z,,,(CH,CN)= 389 t~m),’~’]a model compound for 123,
fits in here. Accordingly, 123 can also be viewed as a cyanine salt without a ring-current effect, though in view of
the positions of the open-chain cyanines 129 and 130, and
of the cyclic 131[2911
and 132,[292,2931
this appears problematic. The enormous color difference between 123 and
128 (AL,,,(CH,CN)=421
nm) must also be taken into account. It is also informative that the cyanine salts 131 and
132 fit in best as (4n 2)n systems and are therefore quite
distinct from the (4n)n system 123.
The great stability of 123 is proved in the reversible formation of dication 136[951from the fulvalene 133”731(cyclic voltammetry;
M, 0.1 M KRF, in acetonitrile). It is
noteworthy that in the oxidation the stable radical cation
135 appears first, while on reduction only dianion 134 can
be detected.
Although 135 has not yet been successfully isolated,
the related dicationic salt 140[951has been made
Above, all, this review has aimed to show that antiaromatic compounds and antiaromatic transition states can be
stabilized by donor-acceptor groups. Knowledge of the
properties of donor-acceptor-substituted cyclic n-electron
systems naturally does not allow statements to be made
about the properties of the parent compounds. Indeed, the
very aim of donor-acceptor substitution is to obtain compounds that d o not have the same characteristics as the
parent compounds. This means that in the main the novel
n systems have properties that are not determined by the
outer-shell n electrons in the classical sense of “(4n)n= unstable, (4n + 2)n = stable”. Typically, donor-acceptor substitution causes a benzene-like delocalization of the ring
electrons of (4n)n systems, while that of (4n 2)n systems
brings about a localization to a certain extent (cf. crystal
structures of p - n i t r ~ a n i l i n eN,N-dimethyl-p-nitroani~~~~~
line,(2951p - n i t r ~ p h e n o l , ~and
~ ~ ~other
benzene derivatives
(cf. Ref. 12971)). We therefore suggest extending the traditional classification of cyclic n systems as aromatic and antiaromatic with the class of “aromatic (4n)n systems”:
Aromatic systems (“stable”)
(4n + 2)n electrons
weak bond fixation
Antiaromatic systems (“unstable”)
(4N)n electrons
M E = A E H o M o - L u M o-( AEHOMO
~~~~I L U M C N
strong bond fixation ( 2nonaromatic system)
~ I ~ ~ ~ ~ )
Aromatic (4n)x systems (“stable”)
(4n)n electrons
M E = A E H O M O - L U M O , ~ , ~ ~ I -LUMOipolycne)>
weak bond fixation
Scheme 2. Proposed classification of cyclic
Angew. Chem. Inr. Ed. Engl. 27(1988) 1437-1455
One can, of course, have different opinions on whether a
(4n)x system that satisfies significant criteria for aromaticity (low tendency towards bond fixation, relatively large
HOMO-LUMO gap) should be termed “aromatic”. (Heitbronner:[I5’‘“Aromaticity’, if to be used at all, should be a
purely structural concept. We should define certain molecules as ‘aromatic’, depending on the formal aspects of
their structure.” Binsch :Is8] “A conjugated n-electron system is called aromatic if it shows neither strong first-order
nor second-order double-bond fixation. ... Whether [this
criterion] is a n important one or not will have to be decided by the experimentalist.”). However, since the aromaticity criteria presented at the beginning are routinely used
with the traditional x systems, including the substituted
ones, donor-acceptor-substituted cyclic n systems should
not be made an exception.
By use of the donor-acceptor effect, classes of compounds whose members could previously only be generated at very low temperature or as very strongly hindered
derivatives have become accessible, and indeed relatively
easily so. Where can one generally make use of these DoA-substituted cyclic x systems? With the change in
HOMO-LUMO gap by donor-acceptor substitution there
is a possibility of constructing new chromophores (cf.
Refs. [3,247]). Since the novel species are commonly electron-rich or electron-poor systems, they have the ability
to become radical cations o r anions, dications or dianions.
With this, new substances can be synthesized that, among
other things, are suitable in the field of “organic metals”
and related areas of application (cf. Ref. [298]). In this respect one should recall that the electrical conductivity of
organic donor-acceptor complexes has been traced back to
a “migration of aromaticity” within molecular ~tacks.‘’~~]
Despite its not being generally a c ~ e p t e d , [ ~ ~this
’ . ~model
offers the synthetic chemist some guidelines.
Many of the compounds described are disc-shaped and
carry substituents that could be provided with long-chain
alkyl groups. Liquid-crystalline (discotic) mesophases (cf.
Ref. [302]) and monolayers should therefore also form.
The above discussions were limited to aromatic and antiaromatic systems. If the above question as to the character of substituted cyclopolyenes is to be fully answered,
then nonaromatic systems (fulvenes, fulvalenes, quinones,
quinone methides, quinodimethanes) and all those compounds whose aromaticity can be seen only in polar resonance structures (cyclopropenone, tropone, etc. ; (pseudo)oxohydrocarbons, radialenes) must be considered. p Quinodimethanes 141 ,[303-3051 for example, have a strongly
polar character, and, as has been demonstrated with mate-
N: f . .cN
ba: AA,=BN.= B
c: A = CH. B = N
d:A. B = N
143, X = S , N R
Antiaromatic compounds, which till recently have been
a curiosity, are now gaining increasingly more interest in
the search for organic f e r r o m a g n e t ~ . [ ~ ~ ’In
* ~ area
non-Kekule systems like trimethylenemethane[3’0’ and
also have a role to play, because of their triplet ground state. One possibility for stabilizing trimethylenemethane with donor and acceptor
groups has been outlined in 10-13. Another could be
the oxidation of Y-aromatic dianions of type 147a
or 147d (n=2Q), if it were possible
to suppress ring closure to methylenecyclopr~panes.‘~~~~~’~~
This could especially be the case with 147c (n = 2’) o r 147d
But even the reduction of Y-shaped dications of type 147b,c ( n = 2 @ ) (cf. Ref. [316]) could lead to
(stable?) trimethylenemethanes 147 (n =2’). A closer investigation of 147 (n=2’) would also be of interest because of the fact that no (Y-aromatic) stabilization has
been found for the parent compound or for the alkyl and
phenyl derivati~es.‘~‘’~
Trihydroxy derivatives have, however, been
and the salts of 149,13’81
and 151[3201
are stable compounds (cyclic voltammetry of
derivatives of 151 reveals two reversible reduction
Acceptor and donor groups should make it possible to
synthesize several cyclic x systems which until now have
been unknown o r have only been detected as reactive intermediates-e. g., the cyclopropenyl anion derivatives 143
and 144, the azacyclopropenylium salts 145 (cf. attempted
s y n t h e s e ~ [ ~ ~and
~ . the
~ ~ azatropylium
salts 146.
141, X = S , N R
rials based o n 141a, they are therefore predestined for
Quinone methides like 142 (synthesized from 123),1307J
are also of interest in this area.
Anqew. Chem. Int. Ed. Enql. 2 7 (1988) 1437-1455
The 13)radialenes 148 are also synthetic targets with
respect to organic metals and ferromagnets. Compound 148a (n =2Q)[32’1is known (heteroderivatives, cf.
Refs. [322-3271) and can be oxidized to the diamagnetic( !)[3zsJ 148a (n = 1 Q). Compound 148e (X = 0,
= 0,2Q)(329.3301 is
also known. Compound 148e
(X=C(CN),) would, of course, be interesting as a
tetracyanoquinodimethane (TCNQ) analogue; so too
would the [3]radiaalenes 148b,c (n = 0) as electron-rich
systems, and compounds 148d as “neutral” systems.
Compounds like 152 and 154 are further precursors of
The neighboring CS double
bonds are reminiscent of the bonding in 1,6,6a-A4-trithiapentalenes 1561332’
and in 157,1333.3341
which can be viewed
as a “sulfur-coated’’ [6]radialene or benzene. Compound
152 can be seen as a precursor of 153 (n=2°),’300’ which,
as a trimethylenemethane derivative, should have a triplet
ground state and therefore must be considered in the production of “ferromagnetic organic metals” (FOM).‘2301
Radical anions 153 (n= l o e / l e Q ) and dications or dianions 153 (n = 2@/2’) would also be interesting.’30o’Though
it has not yet been possible to synthesize 152, the closely
related salts 155 have been made.D3s1
An interesting variant of structure type 153 is compound
158. Based o n the nonbonded S-S distances it can be regarded, in accordance with formula 158’, as a “trimethylenemethane imbedded in a pseudo- 1,2-dithiolo[5,4-c][1,2]dithiole, (4n + 2) n-electron, milieu” or as “heteroaromatic trimethylenemethane”.‘3361However, the dipolar
structure 158” has to be considered as well.
Acepentalene, which is structurally related to 153, has
also not yet been s y n t h e ~ i z e d . ‘ ~ The
~ ’ . ~unfavorable
electronic structure of the parent compound (two SOMOs)
would no longer be present in 159 and 160 and, therefore,
such compounds may be synthesizable.
X = CR, N, C-CN, C-CR=NR2@;R’ = R, CN
By varying the donor and acceptor groups, donor-acceptor-substituted cyclic 71 systems can be synthesized
whose electron donatinglaccepting properties are suitable
for the application concerned. It does not need to be mentioned that such substituent effects have always been used
with success in the past, and it is not only in this respect
that the boundary between classical aromatic chemistry
and the Do-A n systems described here is a fluid one.
This article has had to omit discussion of many workers
results which have contributed signijicantly to progress in this
area. We express our thanks both to them and to those workers mentioned in the references for their commitment. The
Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie earn special thanks for their generous support of this research over many years.
Received: March 4, 1988 [A 697 IE]
German version: Angew. Chem. 100 (1988) 1492
Translated by Dr. Michael Kerfesz, Cambridge, UK.
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block, cyclic, theorie, acceptor, donor, systemsчprobes, electro, building, material, substituted, new
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