Synthesis and redox properties of ferrocenylquinoxalines

Article abstract of DOI:10.1016/S0277-5387(98)00223-X

The ferrocenyl 1,2-diketone [ (C₅H₅)Fe (C₅H₄)COCOPh] when condensed with aromatic 1,2-diamines forms a range of ferrocenyl-substituted quinoxalines: when the diamine is unsymmetrically substituted the quinoxaline is formed as a mixture of two regio-isomers, which can in some cases be separated by careful chromatography. Electrochemical studies show that these present ferrocenylquinoxalines undergo the expected reversible one-electron oxidation centred on the ferrocenyl moiety and a two-electron reduction process centred on the quinoxalyl fragment, which is accompanied by chemical complications. The oxidation of the ferrocenyl group occurs at potentials slightly higher than that of unsubstituted ferrocene (by about 100-150 mV, depending on the substituents) indicating that the quinoxalyl substituent is slightly electron-withdrawing with respect to ferrocene.

Full text of DOI:10.1016/S0277-5387(98)00223-X

Polyhedron Vol[ 06\ No[ 12Ð13\ pp[ 3044Ð3051\ 0887
Þ 0887 Elsevier Science Ltd
Pergamon
All rights reserved[ Printed in Great Britain
9166Ð4276:87 ,08[99¦9[99
\
PII] S9166!4276"87#99112!X
Synthesis and redox properties of
ferrocenylquinoxalines
Piero Zanelloa\ Marco Fontania\ S[ Zaka Ahmedb and Christopher Glidewellbꢀ
a Dipartimento di Chimica dell|Universita di Siena\ Pian dei Mantellini\ 33\ 42099 Siena\ Italy
Á
b School of Chemistry\ University of St[ Andrews\ St[ Andrews\ Fife KY05 8ST\ U[K[
"Received 29 March 0887^ accepted 0 June 0887#
Abstract*The ferrocenyl 0\1!diketone ð"C4H4#Fe"C4H3#COCOPhŁ when condensed with aromatic 0\1!diamines
forms a range of ferrocenyl!substituted quinoxalines] when the diamine is unsymmetrically substituted the
quinoxaline is formed as a mixture of two regio!isomers\ which can in some cases be separated by careful
chromatography[ Electrochemical studies show that these present ferrocenylquinoxalines undergo the expected
reversible one!electron oxidation centred on the ferrocenyl moiety and a two!electron reduction process centred
on the quinoxalyl fragment\ which is accompanied by chemical complications[ The oxidation of the ferrocenyl
group occurs at potentials slightly higher than that of unsubstituted ferrocene "by about 099Ð049 mV\ depending
on the substituents# indicating that the quinoxalyl substituent is slightly electron!withdrawing with respect to
ferrocene[ Þ 0887 Elsevier Science Ltd[ All rights reserved
Keywords] cyclic voltammetry^ electrochemistry^ ferrocene^ quinoxaline[
———————————————————————————————————————————————
INTRODUCTION
to form the corresponding ferrocenylquinoxalines\
along with an electrochemical study of their redox
properties[ Monomeric ferrocenylquinoxalines are
themselves potentially of interest and importance] a
number of quinoxalines are powerful antibiotics and
some examples show potential as anti!cancer drugs ð6Ł
and their action in this respect is dependent on their
binding to DNA by intercalation ð7Ł[ At the same
time\ some rather simple ferricinium salts have been
shown to exhibit powerful anti!tumour activity
against a range of human tumour types ð8Ł] the intro!
duction of ferrocenyl substituents onto the quin!
oxaline framework could perhaps provide\ in a single
molecular species\ a powerful combination of these
two actions[
The reaction between 0\1!diketones and 0\1!diamines
to form quinoxalines\ well!known as a characteristic
reaction for both components\ has been successfully
exploited as a polymer!forming process when applied
to combinations of bis"0\1!diketones# and bis"0\1!
diamines# ð0Ð2Ł[ For polymer synthesis\ the reaction
can generally be performed under solvent!free "melt!
phase# conditions\ when the sole by!product is water
which distills from the reaction mixture at the melting
temperature of the components[ It is the removal of
the water by!product\ along with the in situ formation
of new heteroaromatic rings\ which drives the process
to completion[ The e}ectiveness of this reaction ð0Ð
2Ł makes it an attractive route for the synthesis of
organometallic polymers\ for example by reaction of
0\0?!ferrocenediylbis"0\1!diketones# with bis"0\1!
diamines#[ We have recently described e}ective routes
to a range of ferrocenyl 0\1!diketones ð3\ 4Ł and here
we present the results of a study of reactions of a
representative ferrocenyl diketone\ FcCOCOPh ð3\ 5Ł
ðFcꢁ"C4H4#Fe"C4H3#Ł with a range of 0\1!diamines\
RESULTS AND DISCUSSION
Syntheses of ferrocenylquinoxalines
Reaction of 0!ferrocenyl!1!phenylethanedione\
FcCOCOPh\ 0\ with 0\1!diaminobenzene\ 1\ yields the
corresponding 1!ferrocenyl!2!phenylquinoxaline\ 2[
When the reaction is conducted in the absence of
solvent by melting together the two components under
ꢀ Author to whom correspondence should be addressed[
3044

3045
P[ Zanello et al[
an inert atmosphere\ the yield of the deep!purple prod! the 0H NMR spectrum in terms of an ABX pattern
uct is generally somewhat less than 49)\ after chro! having a JAX "ꢁ4JHH# of zero[ Similarly\ with the 3!
matographic puri_cation[ However\ if the two nitrodiamine 5\ the two regio!isomers 6a and 6b of the
reactants are _rst mixed in dioxan solution and then corresponding nitroquinoxaline are formed\ with in
slowly heated _rst in solution and\ subsequently\ after this case a major]minor isomer ratio of 2]0^ again\ the
all the solvent has evaporated\ as a melt\ the yield of quinoxalyl region of the 0H NMR spectrum was ana!
puri_ed product is consistently around 54)[
lysed in terms of two ABX patterns each with a JAX
The product 2 is readily characterised by its ana! of zero[
lytical data and its 0H and 02C NMR spectra[ The
From the condensation of the diketone 0 with 1\2!
0H NMR spectrum contains\ as well as characteristic diaminotoluene\ again a mixture of two regio!isomers
signals from the monosubstituted ferrocenyl frag! 7a and 7b was formed\ with a major]minor ratio of
ment\ well!separated resonances arising from the phe! 2]1 as judged from the 0H NMR spectrum\ but on
nyl and quinoxalyl fragments] the 02C spectrum again this occasion the two isomers were separable by care!
shows the characteristic resonances from the fer! ful chromatography on silica[ On the other hand\ the
rocenyl fragment\ together with seven CH and _ve two regio!isomers 8a and 8b obtained using 1\2!diam!
quaternary resonances in the aromatic region[ A sin! inopyridine were not separable on silica\ but were
gle!crystal X!ray di}raction analysis of 2\ prepared in found to be separable on alumina provided that pure
this manner\ has been published ð09Ł[ Because of the dichloromethane was employed as eluent] even the
signi_cantly enhanced yield obtained when the reac! slightest increase in the polarity of the mobile phase
tants were _rst heated in dioxan "b[p[ 090>C# prior to caused the isomers to elute together[
the melt!phase condensation\ the reactions of 0 with
In the quinoxaline!forming condensations involv!
other aromatic 0\1!diamines were all conducted under ing substituted diamines\ the modest regio!selectivity
similar conditions^ in all these reactions\ the choice of is determined by the relative nucleophilicities of the
solvent was in~uenced by a combination of reactant two amino groups in the diamine component\ and the
solubility and solvent volatility[ As with compound 2\ relative electrophilicities of the two carbonyl carbon
nearly all the resulting quinoxalines were isolated after atoms in the diketone 0[ It is assumed that the more
chromatography on silica or alumina as hydrates\ nucleophilic amino group reacts faster with the more
either monohydrates or hemihydrates[
electrophilic carbonyl[ In diamines 3 and 5\ in which
Thus\ when the ferrocenyl diketone 0 was reacted the substituent R is electron!withdrawing\ the amino
with 2\3!diaminobenzoic acid 3\ initially in chlo! group meta to R will be the more nucleophilic\ while
robenzene "b[p[ 021>C#\ the quinoxaline 4 was the carbonyl adjacent to the phenyl ring in 0 will be
obtained in ca[ 79) yield after chromatographic puri! the more electrophilic\ because of the strong electron!
_cation[ However\ both the 0H and 02C NMR spectra donating character of the ferrocenyl fragment[ Hence\
indicated the presence of the two regio!isomers 4a and it is to be expected that 4b is the more abundant
4b\ arising from the alternative orientations of the isomer of 4^ similarly for 6\ the ferrocenyl fragment is
reactants in the condensation process] integration of preferentially para to the deactivating substituent[ In
the 0H spectrum in the ferrocenyl region indicated a 1\2!diaminotoluene\ the amino group ortho to methyl
major]minor isomer ratio of 1]0[
is the more nucleophilic\ so that 7b is expected to
No chromatographic conditions could be found predominate\ while the strong deactivation of the 1!
under which the isomers were separable and conse! amino group in 1\2!diaminopyridine leads to the
quently the assignment of the major and minor iso! expectation of 8a as the major isomer[
mers as either 4a or 4b was not possible[ However\ it
Use of the bis!0\1!diamine\ 09\ under the reaction
was possible\ despite some peak overlap\ to analyse conditions employed for the formation of quin!
completely for each isomer the quinoxalyl region of oxalines 2\ 4\ 6\ 7 and 8 and using dioxan as solvent

Synthesis and redox properties of ferrocenylquinoxalines
3046
gave\ as a mixture of regio!isomers\ the expected bis! current function ipa:v0:1 remains constant and the
product 00[ However\ when the ferrocenyl diketone 0
and the bis!diamine 09 were reacted together in alka!
line aqueous solution ð00Ł\ the predominant product
was the monoquinoxaline 01\ accompanied by only a
trace of 00[
peak!to!peak separation progressively increases from
69 to 069 mV[ Such a slight departure from the con!
stant value of 48 mV expected for an elec!
trochemically!reversible one!electron process neatly
parallels that observed for the one!electron oxidation
of unsubstituted ferrocene\ so that it may be attributed
to uncompensated solution resistances[
Electrochemistry
So far as the reduction process is concerned\ it was
not possible to perform macroelectrolysis inves!
tigations because of the closeness of the solvent
reduction[ Based on the relative heights\ we assign it
to a single two!reduction process\ which is however
complicated by subsequent reactions\ as yet uncharac!
terised[ As the inset of Fig[ 0 shows\ when the scan
rate was increased beyond 1[99 V s−0\ a reoxidation
appears directly associated to the reduction process[
This means that the rate of the chemical complication
falls within the cyclic voltammetry time window\
allowing estimation of ca[ 9[94 s as the half!life of the
electrogenerable ð2Ł1− anion ð02Ł[
Table 0 presents for all the compounds studied the
formal electrode potentials of the redox changes dis!
cussed above and\ for comparison\ the corresponding
data for the unsubstituted 1!ferrocenylquinoxaline
"02# are also reported[ The electrogenerated fer!
ricenium species ð4Ł¦\ ð8Ł¦ and ð02Ł¦ proved to be not
fully stable and to undergo slow decomposition[
In comparison with unsubstituted ferrocene itself\
the ferrocenylquinoxalines oxidise at potential values
some 099Ð049 mV higher\ thus indicating that the
Figure 0\ which refers to compound 2\ shows the
typical cyclic voltammogram exhibited by the present
compounds in dichloromethane solution[ Both an oxi!
dation process possessing features of chemical reversi!
bility and an apparently irreversible reduction process\
which is about twice greater in height than the oxi!
dation step\ are apparent[
Controlled!potential coulometric investigation of
the anodic process "Ewꢁ¦9[6 V# show the con!
sumption of one electron:molecule[ Upon exhaustive
one!electron oxidation the original ruby!red solution
of 2 turns jade!green and displays an absorption in
the visible region at lmaxꢁ581 nm[ In con_rmation of
the chemical reversibility of the ferrocene:ferricenium
oxidation\ cyclic!voltammetric tests performed on the
oxidised green solution display voltammetric pro_les
entirely complementary to that shown in Fig[ 0[
Analysis ð01Ł of the cyclic!voltammetric anodic
responses with scan rates varying from 9[91 to
0[99 V s−0 show parameters which are diagnostic of a
one!electron removal which is essentially reversible
from the electrochemical viewpoint[ As a matter of
fact\ the ipc:ipa ratio is constantly equal to unity\ the quinoxaline exerts a signi_cant electron!withdrawing

3047
P[ Zanello et al[
e}ect[ However\ the e}ects of the substituents on the compared with 2[ On the other hand\ the strongly
quinoxaline ring are always readily interpreted[ Com! electron!withdrawing NO1 group in 6 generates its
parison between 02 and 2\ indicates that the 2!phenyl expected e}ect[
substituent\ which might have been expected to make
The redox behaviour of the biferrocene complex 00
the oxidation more di.cult\ in fact makes the oxi! merits comment[ It exhibits a single oxidation process\
dation slightly easier\ suggesting that resonance e}ects but unfortunately\ because of its very low solubility
predominate over inductive e}ects[ This could also satisfactory controlled!potential coulometry was not
explain the absence of inductive e}ects exerted by possible[ However\ in view of the absence of further
the methyl group in 7\ or the carboxyl group in 4\ oxidation processes up to ¦0[1 V\ it may be specu!

Synthesis and redox properties of ferrocenylquinoxalines
3048
Fig[ 0[ Cyclic voltammogram recorded at a platinum electrode on a CH1Cl1 solution containing 2 "0[2×09−2 mol dm−2# and
ðNBu3ŁðPF5Ł "9[1 mol dm−2#[ Scan rates] main _gure\ 9[19 V s−0^ inset\ 1[99 V s−0
[
Table 0[ Formal electrode potentials "in V\ vs S[C[E[#\ peak!
to!peak separations "in mV# and spectroscopic data "in nm#
for the redox changes exhibited by the present fer!
rocenylquinoxalines in dichloromethane solution[
account for the somewhat greater peak!to!peak sep!
aration in 4 compared with the other species[
EXPERIMENTAL
a
b
Complex
E9
?
DEp
lmax
E9
?
9:1−
9:0¦
The diketone FcCOCOPh was prepared as pre!
viously described ð3Ł[ Aromatic diamines were
obtained from commercial sources and were puri_ed
using published methods ð03Ł[ NMR spectra were rec!
orded at ambient temperatures\ in CDCl2 solution
unless stated otherwise\ on a Bruker AM!299 spec!
trometer operating at 299[024 MHz for 0H and
64[358 MHz for 02C[ Diethyl ether and light pet!
roleum "b[p[ 39Ð59>C# were dried over sodium wire[
Elemental analyses were by the Microanalytical Lab!
oratory of this School[
2
4
6
7
¦9[36
¦9[38
¦9[44
¦9[38
¦9[44
¦9[35f
¦9[42
¦9[28
79
86
64
68
64
69
84
73
581
579
559
694
570
−
−0[79c
−
−0[44d\e
−0[79d
−0[40c
−
8
00
02
FcH
564
519
−0[68d
−
a Measured at 9[0 V s−0
[
b Absorption band of the oxidized species[
c Measured at 1[9 V s−0
d Peak potential value[
[
e A preceding reduction centred on the NO1 group is pre!
sent at E9?ꢁ−9[84 V[
Synthesis of 1!ferrocenyl!2!phenylquinoxaline
f Two!electron step[
Melt!phase reaction Ferrocenylphenylethanedione
0 "9[1 g\ 9[52 mmol# and 0\1!diaminobenzene 1 "9[0 g\
9[82 mmol# were taken in a Pyrex glass test tube with
a side arm and _tted with a bubbler[ The mixture
lated that this is due to a single two!electron step[ was heated to melting in an oil bath at 039>C under
Were this to be so\ it would imply that no electronic nitrogen for 29 min\ water vapours were observed con!
communication occurs between the two quinoxalyl densing on the walls of tube[ After cooling the crude
units[
material was dissolved in dichloromethane and
Finally\ as far as the presence of isomeric species is checked by TLC^ this showed one purplish!red spot
concerned\ as noted above for compounds 4\ 6\ 7\ 8 almost at the same Rf value as fer!
and 00\ it may be pointed out that only in the case of rocenylphenylethanedione[ This crude product was
compound 4 was the presence of two almost over! chromatographed on silica gel using dichloromethane
lapping oxidation processes observed\ which could as the eluting solvent\ concentrated and crystallised to

3059
P[ Zanello et al[
give dark purple crystals of 1!ferrocenyl!2!phenyl! 018[7 "d#\ 029[0 "d#\ 020[0 "d#\ 026[2 "s#\ 027[06 "s#\
quinoxaline 2 "9[00 g\ 34)#[
Reaction in dioxan
027[13 "s#\ 028[3 "s#\ 039[9 "s#\ 031[9 "s#\ 041[3 "s#\ 042[9
Ferrocenyl! "s#\ 042[5 "s# and 043[5 "s# "phenyl and quinoxalyl#^
solution
phenylethanedione 0 "9[1 g\ 9[52 mmol#\ 0\1!diamino! 055[1 "s# COOH[ The resolution of the aromatic res!
benzene 1 "9[0 g\ 9[82 mmol# were dissolved in 0\3! onances requires resolution enhancement[ Both the
dioxan "2 cm2# in a Pyrex glass test tube with a side carboxyl carbons appear at the same position\ and two
arm and _tted with a bubbler[ This mixture was heated of the quaternary carbons of the aromatic:quinoxaline
in an oil bath at 009>C under a gentle stream of nitro! moieties are superimpose on other peaks in that region
gen until dryness[ The temperature of the bath was and hence are not visible in the spectrum[
raised to 039>C and held at this temperature for
19 min[ The crude product was puri_ed as for the melt!
phase reaction[ Weight of chromatographed product
Synthesis of ferrocenylphenylnitroquinoxaline
9[05 g "54)#[
Ferrocenylphenylethanedione
0
"9[11 g\
The products from these two processes were found
to be identical by TLC and by 0H and 02C NMR[
Anal[ found C 61[1\ H 3[3\ N 5[8^ C13H07FeN1 requires
C 62[8\ H 3[6\ N 6[1)^ C13H07FeN1[9[4H1O requires
C 61[1\ H 3[7\ N 6[9)[ M[p[ 197Ð109>C "lit[ 197Ð
198>C ð3Ł#[ NMR d"H# 3[99[ "4H\ s\ C4H4#^ 3[24 "1H\
m# and 3[44 "1H\ m# "C4H3#^ 6[49Ð6[44 "4H\ m\ Ph#^
6[61 "1H\ m# and 7[97 "1H\ m# "quinoxalyl#^ d"C# 58[8
"d\ C4H4#^ 69[0 "d#\ 69[7 "d# and 71[2 "s# "C4H3#^ 017[3
"d#\ 017[7 "d#\ 017[8 "d#\ 018[9 "d#\ 018[1 "d#\ 018[2
"d#\ 018[8 "d#\ 028[7 "s#\ 039[2 "s#\ 030[8 "s#\ 043[0 "s#
and 043[0 "s# "phenyl and quinoxalyl#[
9[58 mmol#\ 3!nitro!0\1!diaminobenzene 5 "9[01 g\
9[65 mmol# and toluene "4 cm2# were re~uxed\ as
above\ under a gentle stream of nitrogen for 2 h^ the
reaction was monitored by TLC[ The solvent was
evaporated after this time[ After cooling\ the solid
residue was redissolved in dichloromethane\ _ltered\
concentrated and chromatographed over silica gel
using dichloromethane as the mobile phase to yield
ferrocenylphenylnitroquinoxaline 6 "9[11 g\ 62)# as a
dark purple solid[ The compound crystallises as a
hemi!hydrate as shown by the elemental analysis and
0
integration of the H NMR spectrum gave an isomer
ratio of 2]0[ An attempt to separate the two isomers
on silica using dichloromethane as eluent yielded a
pure fraction of the more abundant isomer\ together
with a mixture of both the isomers[ Anal[ found "iso!
mer mixture# C 54[0\ H 2[8\ N 8[0^ C13H06FeN2O1
requires C 55[1\ H 2[8\ N 8[5^ C13H06FeN2O1[9[4H1O
requires C 53[8\ H 3[0\ N\ 8[3)[ NMR "major isomer
pure\ minor isomer from spectrum of mixture# d"H#
"major isomer# 3[90 "4H\ s\ C4H4#^ 3[39 "1H\ m# and
3[46 "1H\ m# "C4H3#^ 6[4Ð6[5 "4H\ m\ Ph#^ 7[07 "0H\
d^ J\ 8[4#\ 7[33 "0H\ dd^ J\ 8[4 and 1[3# and 7[87 "0H\
d^ J\ 1[3# "quinoxalyl#^ "minor isomer# 3[99 "4H\ s\
C4H4#^ 3[32 "1H\ m# and 3[59 "1H\ m# "C4H3#^ 6[4Ð6[5
"4H\ m\ Ph#^ 7[05 "0H\ d^ J\ 8[3#\ 7[37 "0H\ dd^ J\ 8[3
and 1[3# and 7[87 "0H\ d^ J\ 1[3# "quinoxalyl#^ d"C#
"major isomer# 58[8 "d\ C4H4#^ 69[7 "d#\ 60[0 "d# and
79[7 "s# "C4H3#^ 010[7 "d#\ 013[6 "d#\ 017[3 "d#\ 017[89
"d#\ 018[4 "d#\ 029[4 "d#\ 027[8 "s#\ 039[3 "s#\ 031[9
"s#\ 036[8 "s#\ 033[7 "s# and 046[0 "s# "phenyl and
quinoxalyl#^ "minor isomer# 69[9 "d\ C4H4#^ 60[14 "d#\
60[17 "d# and 79[6 "s# "C4H3#^ 012[1 "d#\ 014[5 "d#\
017[1 "d#\ 017[76 "d#\ 018[3 "d#\ 018[5 "d#\ 027[9 "s#\
027[8 "s#\ 033[0 "s#\ 035[6 "s#\ 034[1 "s# and 047[0 "s#
"phenyl and quinoxalyl#[
Synthesis of ferrocenylphenylquinoxalylcarboxylic acid
In a similar manner\ ferrocenylphenylethanedione 0
"9[4 g\ 0[46 mmol#\ 2\3!diaminobenzoic acid 3 "9[15 g\
0[6 mmol# and chlorobenzene "4 cm2# were heated in
a silicone oil bath at 039>C under a gentle stream of
nitrogen^ after 0 h[ TLC showed that no unreacted
diketone remained[ During the reaction time the col!
our of the mixture also changed from dark red to dark
purple[ The mixture was then heated to dryness[ After
cooling\ the solid residue was dissolved in dichlo!
romethane\ _ltered\ washed with water\ dried
"Na1SO3#\ concentrated and chromatographed over
silica gel using diethyl ether as the mobile phase to
yield ferrocenylphenylquinoxalylcarboxylic acid 4
"9[44 g\ 70)# as a dark purple solid[ The 0H NMR
spectrum showed the presence of two regio!isomers[
The relative yield of the regio!isomers as calculated
from the 0H NMR integration is 1]0[ Anal[ found C
55[8\ H 3[6\ N 4[6^ C14H07FeN1O1 requires C 58[0\ H
3[1\ N 5[4^ C14H07FeN1O1[H1O requires C 55[3\ H
3[4\ N 5[1)[ NMR d"H# "major isomer# 3[90 "4H\ s\
C4H4#^ 3[27 "1H\ m# and 3[47 "1H\ m# "C4H3#^ 6[3Ð6[6
"4H\ m\ Ph#^ 7[04 "0H\ d^ J\ 7[7#\ 7[24 "0H\ dd^ J\ 7[7
and 0[5# and 7[81 "0H\ d^ J\ 0[5# "quinoxalyl#^[ "minor Synthesis of ferrocenylphenylmethylquinoxaline
isomer# 3[92 "4H\ s\ C4H4#^ 3[39 "1H\ m# and 3[59 "1H\
m# "C4H3#^ 6[3Ð6[6 "4H\ m\ Ph#^ 7[04 "0H\ d^ J\ 7[7#\
7[27 "0H\ dd^ J\ 7[7 and 0[5# and 7[80 "0H\ d^ J\ 0[5#
In a similar manner\ ferrocenylphenylethanedione
"9[4 g\ 0[46 mmol#\ 1\2!diaminotoluene "9[11 g\
0
"quinoxalyl#^ d"C# "major isomer# 57[36 "d\ C4H4#^ 58[9 0[7 mmol# and chlorobenzene "4 cm2# were re~uxed in
"d#\ 58[4 "d# and 79[0 "s# "C4H3#^ "minor isomer# 57[42 an oil bath at 049>C under a gentle stream of nitrogen[
"d\ C4H4#^ 58[2 "d#\ 58[5 "d# and 79[3 "s# "C4H3#^ "both The reaction was monitored by TLC and after 3 h the
isomers# 015[82 "d#\ 015[85 "d#\ 016[94 "d#\ 016[05 "d#\ reaction was complete[ The two regio!isomers had
016[52 "d#\ 016[61 "d#\ 016[67 "d#\ 017[2 "d#\ 018[5 "d#\ distinguishable Rf values\ but the less polar isomer

Synthesis and redox properties of ferrocenylquinoxalines
3050
had almost the same value as the diketone\ so that it m\ phenyl and quinoxalyl#\ 7[35 "0H\ d×d# and 8[04
was sometimes di.cult to di}erentiate between dike! "0H\ d# "quinoxalyl#^ d"C# "major isomer# 58[5 "d\
tone and this isomer of the product[ When the reaction C4H4#^ 69[1 "d#\ 69[7 "d# and 70[7 "s# "C4H3#^ 013[7 "d#\
was complete\ the solvent was removed and the solid 016[7 "d#\ 017[8 "d#\ 018[1 "d#\ 025[1 "s#\ 025[7 "d#\
residue was redissolved in dichloromethane\ _ltered\ 027[7 "s#\ 037[3 "s#\ 041[4 "d#\ 044[2 "s# and 044[4
concentrated and chromatographed over silica gel "s# "phenyl and quinoxalyl#^ "minor isomer# 58[7 "d\
using dichloromethane:petrol as the mobile phase to C4H4#^ 69[6 "d#\ 60[9 "d# and 79[4 "s# "C4H3#^ 012[6 "d#\
yield ferrocenylphenylmethylquinoxaline
7 "9[5 g\ 017[1 "d#\ 017[6 "d#\ 017[8 "d#\ 023[1 "s#\ 026[6 "d#\
84)# as a dark purple solid[ The relative yield of the 028[1 "s#\ 049[9 "s#\ 042[5 "d#\ 043[0 "s#\ 046[7 "s# "phe!
regioisomers as calculated from the 0H NMR inte! nyl and quinoxalyl#[
gration is 2]1[ Another column was run to separate
the two isomers from each other^ this yielded _rst
the pure less!polar isomer\ then the mixture of two
Synthesis of di" ferrocenyl#diphenylbiquinoxaline
isomers and _nally a small portion of the more!polar
In a similar manner\ ferrocenylphenylethanedione
isomer[ Anal[ found C 63[9\ H 4[9\ N 5[6^ C14H19FeN1
0 "9[29 g\ 9[83 mmol#\ 2\2?\3\3?!biphenyltetramine 09
requires C 63[2\ H 4[9\ N 5[8)[ NMR d"H# "major
"9[39 g\ 0[76 mmol# and 0\3!dioxan "1 cm2# were
isomer# 1[84 "2H\ s\ CH2#^ 3[91 "4H\ s\ C4H4#^ 3[21
heated in an oil bath at 009>C under a gentle stream
"1H\ m# and 3[44 "1H\ m# "C4H3#^ 6[3Ð6[6 "6H\ m# and
of nitrogen until all the solvent had evaporated[ The
6[83 "0H\ d# "phenyl and quinoxalyl#^ "minor isomer#
temperature of the oil bath was then raised to 049>C
1[71 "2H\ s\ CH2#^ 3[99 "4H\ s\ C4H4#^ 3[21 "1H\ m#
and the mixture was kept at this temperature for
and 3[44 "1H\ m# "C4H3#^ 6[3Ð6[6 "6H\ m# and 6[83
34 min[ After cooling\ the solid residue was dissolved
"0H\ d# "phenyl and quinoxalyl#^ d"C# "major isomer#
06[0 "q\ CH2#^ 58[6 "d\ C4H4#^ 58[6 "d#\ 69[7 "d# and
in dichloromethane[ TLC analysis showed the pres!
ence of two purple quinoxaline spots[ As three
71[8 "s# "C4H3#^ 015[7 "d#\ 017[0 "d#\ 017[49 "d#\ 017[44
regioisomers of the product 00 could be formed in this
"d#\ 018[0 "d#\ 018[4 "d#\ 025[4 "s#\ 028[5 "s#\ 028[7
"s#\ 039[7 "s#\ 041[0 "s# and 041[3 "s# "phenyl and
quinoxalyl#^ "minor isomer# 06[9 "q\ CH2#^ 58[5 "d\
reaction\ it is possible that the two of these isomers
have virtually identical Rf values[ As the Rf values of
the two distinguishable components are very closely
C4H4#^ 58[5 "d#\ 69[6 "d# and 71[6 "s# "C4H3#^ 015[1 "d#\
similar\ attempts to separate the isomers have not so
016[8 "d#\ 017[3 "d#\ 017[5 "d#\ 018[1 "d#\ 018[3 "d#\
far been successful[
026[2 "s#\ 027[7 "s#\ 039[1 "s#\ 030[3 "s#\ 040[3 "s# and
041[8 "s# "phenyl and quinoxalyl#[
Synthesis
of
ferrocenyl!phenyl!5!"2?\3?!diamino!
phenyl#quinoxaline
Synthesis of 1!ferrocenyl!2!phenyl!4!azaquinoxaline
Ferrocenylphenylethanedione
0
"9[09 g\
Ferrocenylphenylethanedione 0 "9[21 g\ 0[9 mmol#\
1\2!diaminopyridine "9[11 g\ 1[9 mmol# and 0\3!
dioxan "1 cm2# were heated\ as above\ in an oil bath
at 009>C under a slow stream of nitrogen until dryness[
The temperature was then increased to 049>C and held
for 29 min[ After cooling\ the mixture was dissolved
in dichloromethane\ _ltered\ concentrated and chro!
matographed on silica gel to give pure ferro!
cenylphenylazaquinoxaline 8 "9[15 g\ 56)# as a dark
purple solid[ The product contained both regio!iso!
mers of the azaquinoxaline\ con_rmed by 0H and
02C NMR[ The two isomers which were not separable
on silica gel were found to be separable on alumina by
very careful elution using dichloromethane as eluting
solvent[ Any slight increase in the polarity of the
mobile phase "e[g[ one drop of methanol in 099 cm2
of dichloromethane# elutes both the isomers at the
same time[ Integration of the 0H NMR spectrum gave
an isomer ratio of ca[ 2]1[ Anal[ found C 58[7\ H 3[5\
N 09[0^ C12H06FeN2 requires C 69[5\ H 3[3\ N 09[6)[
NMR d"H# "major isomer# 3[91 "4H\ s\ C4H4#^ 3[27
9[203 mmol#\ 2\2?\3\3?!biphenyltetramine "9[02 g\
9[50 mmol#\ sodium sul_te "9[95 g\ 9[37 mmol# and
potassium carbonate "9[95 g\ 9[32 mmol# were dis!
solved in 49) "v:v# aqueous ethanol "49 cm2#[ The
mixture was re~uxed with stirring for 1 h[ After this
time\ TLC of the organic extract showed two highly
polar components of very similar Rf values\ in
addition to traces of the bis!quinoxaline 00 and unre!
acted biphenyltetramine[ During the course of the
reaction some solid product precipitated out^ this was
_ltered o} and the _ltrate extracted with diethyl ether[
The combined solids were puri_ed by chro!
matography on neutral alumina using 19) "v:v#
methanolic diethyl ether as eluent to yield ferrocenyl!
phenyl!5!"2?\3?!diaminophenyl#quinoxaline
01
"9[01 g\ 66)#[ Anal[ found C 60[5\ H 3[7\ N 00[9^
C29H13FeN3 requires C 61[5\ H 3[8\ N 00[2)^
C29H13FeN3[9[4"H1O# requires C 60[2\ H 4[9\ N
00[0)[
"1H\ m# and 3[51 "1H\ m# "C4H3#^ 6[3Ð6[6 "5H\ m\ Electrochemistry
phenyl and quinoxalyl#\ 7[31 "0H\ d×d# and 8[09 "0H\
d# "quinoxalyl#^ "minor isomer# 3[95 "4H\ s\ C4H4#^
materials and apparatus for electrochemistry have
3[39 "1H\ m# and 3[57 "1H\ m# "C4H3#^ 6[3Ð6[7 "5H\ been described elsewhere ð02\ 04Ł[ All the potential

3051
P[ Zanello et al[
5[ Ferguson\ G[\ Glidewell\ C[\ Gottfried\ M[ and
values are referred to the saturated calomel electrode
"S[C[E[#[
Trotter\ J[\ Acta Cryst[ C\ 0885\ 41\ 662[
6[ Sheldrick\ G[ M[\ Heine\ A[\ Schmidt!Base\ K[\
Ã
Pohl\ E[\ Jones\ P[ G[\ Paulus\ E[ and Waring\ M[
J[\ Acta Cryst[ B\ 0884\ 40\ 876[
Acknowled`ements*P[ Z[ gratefully acknowledges the _n!
ancial support of the University of Siena "ex quota 59)#
and the technical assistance of Mrs Giuseppina Montomoli[
S[ Z[ A[ thanks the Committee of Vice!Chancellors and Prin!
cipals "U[K[# and the University of St[ Andrews for _nancial
support[
7[ Waring\ M[ J[ and Wakelin\ L[ P[ G[\ Nature
"London#\ 0863\ 141\ 542[
8[ Kopf!Maier\ P[ and Kopf\ H[\ Chem Rev[\ 0876\
Ã
Ã
76\ 0026[
09[ Ferguson\ G[\ Glidewell\ C[ and Scott\ J[ P[\ Acta
Cryst[ C\ 0885\ 41\ 669[
00[ Jones\ R[ G[ and McLaughlin\ K[ C[\ Or`[ Synth[\
0849\ 29\ 75[
01[ Brown\ E[ R[ and Sandifer\ J[\ in Physical
Methods of Chemistry] Electrochemical Methods\
eds[ B[ W[ Rossiter and J[ F[ Hamilton\ Vol[ 1\
Chap[ 3[ Wiley\ New York\ 0875[
02[ Togni\ A[\ Hobi\ M[\ Rihs\ G[\ Rist\ G[\ Albinate\
A[\ Zanello\ P[ and Zech\ D[\ Keller\ H[\ Or`an!
ometallics\ 0883\ 02\ 0113[
03[ Perrin\ D[ D[ and Armarego\ W[ L[ F[\ Puri!
_cation of Laboratory Chemicals\ 2rd edn[ Perga!
mon\ Oxford\ 0877[
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