Synthesis and electro-spectroelectrochemistry of ferrocenyl naphthaquinones

DOI: 10.1016/j.tet.2010.12.057

Source and publish data:

Tetrahedron p. 1406 - 1421 (2011)

Update date:2022-08-11

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Authors:

Yucel, Baris

Sanli, Bahar

Soylemez, Huseyin

Yilmaz, Ismail

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Article abstract of DOI:10.1016/j.tet.2010.12.057

A practical approach to ferrocenyl naphthaquinone derivatives involving thermal rearrangement of variously substituted 4-aryl-4-hydroxycyclobutenones was described. The reaction of 3-ferrocenyl-4-isopropoxy-3-cyclobutene-1,2-dione with different aryl lith

Full text of DOI:10.1016/j.tet.2010.12.057

Tetrahedron 67 (20 1 1 ) 1406e1421

Contents lists available at ScienceDirect

Tetrahedron

journal homepage: w ww.elsevier.com/locate/tet

Synthesis and electro-spectroelectrochemistry of ferrocenyl naphthaquinones

Baris Yucel , Bahar Sanli, Huseyin Soylemez, Ismail Yilmaz

Istanbul Technical University, Department of Chemistry, 34469 Maslak, Istanbul, Turkey

a r t i c l e i n f o

a b s t r a c t

Article history:

A practical approach to ferrocenyl naphthaquinone derivatives involving thermal rearrangement of

variously substituted 4-aryl-4-hydroxycyclobutenones was described. The reaction of 3-ferrocenyl-4-

isopropoxy-3-cyclobutene-1,2-dione with different aryl lithiums gave the corresponding 4-aryl-4-hy-

droxycyclobutenones, which were heated in p-xylene at reﬂux open to the air to yield ferrocenyl

naphthaquinones. The redox chemistry of the ferrocenyl naphthaquinones was studied by electro-

chemical and in situ spectroelectrochemical techniques in CH₂Cl₂solution and in CH₃CN solution with

water, weak and strong acidic additives. Ferrocenyl naphthaquinones displayed reversible two reduction

processes involving semiquinone radical anion (Fcesnq^ꢀ^ꢁ), dianion (Fcenq^2ꢁ) species and a one-electron

oxidation process based on the ferrocenium/ferrocene (Fc^þenq/Fcenq) couple in CH₂Cl₂. The redox

reaction mechanism of the ferrocenyl naphthaquinones in the presence of the additives proceeded via

hydrogen bonding or proton-coupled electron transfer. Effects of the substituents on the reduction po-

tentials and intramolecular charge-transfer bands of ferrocenyl naphthaquinones were also discussed.

Received 30 September 2010

Received in revised form 27 November 2010

Accepted 20 December 2010

Available online 25 December 2010

Keywords:

Electron donoreacceptor

Electrochemisty

Spectroelectrochemistry

Electron transfer

Ferrocene

Naphthaquinones

Thermal rearrangement

1. Introduction

organic and organometallic redox chemistry.¹⁶Additionally, as re-

markable electron donoreacceptor pairs, ferrocene, and quinones

In recent years, ferrocene and its derivatives have received con-

siderableattention sincethey have found potentialapplicationsinthe

ﬁelds of asymmetric synthesis,¹bioorganometallic chemistry,²and

particularly in materials science.³Ferrocene has become one of the

most preferred components of molecular electron transfer systems in

artiﬁcial photosynthesis studies⁴and in the development of opto-

electronic devices.⁵As an electron-donor unit having chemical ver-

satility, thermal stability as well as a reversible electrochemical redox

couple, ferrocene participates in electron transfer systems with var-

ious electron acceptors and redox-active species such as fullerene,⁶

BODIPY,⁷porphyrins,⁸phthalocyanines,⁹and corroles.¹⁰Dithiafulva-

lene¹¹and tetrathiafulvalene¹²derivatives of ferrocene have been

reported as donor conducting materials for charge-transfer

complexes. Moreover, ferrocene has been combined with good elec-

tron acceptor quinones covalently or by various spacers.

provide tunable redox potentials through judicious choice of sub-

stituents. Thus, coupling ferrocene and quinones intramolecularly

would then offer interesting candidates for studying electron transfer

reactions and optical properties.¹⁷

Electron transfer plays a pivotal role not only in chemical pro-

cesses but also in biological redox processes that have tremendous

relevance to our life, such as photosynthesis and respiration.¹⁸To

understand the factors for controlling important electron-transfer

processes in biological systems, electron-transfer dynamics be-

tween donor and acceptor molecules bound to proteins have been

studied extensively.¹⁹Considering the redox chemistry of quinone-

based couples, these species are closely related to biological pro-

cesses as electron-proton transfer agents in the oxidative phos-

phorylation of ADT to ATP, or photosynthesis, which themselves are

directly affected the by acidebase properties of quinone, semi-

quinone, and hydroquinone species.²⁰

Ferroceneebenzoquinone¹³

and

ferroceneeanthraquinone¹⁴

donoreacceptor systems with or without a spacer are quite com-

mon. Both quinones and ferrocene exhibiting well-established re-

versible electrochemical redox couples are the most important and

well-studied examples of an organic/organometallic redox system.¹⁵

From a fundamental standpoint, these model molecules have

played an important role in developing our current understanding of

It has been determined that the electrochemical behavior of

quinone is strongly inﬂuenced by environmental conditions that

regulate the potentials and reaction pathways of the reduced/oxi-

dized species appearing in a proton-coupled electron trans-

fer.^15f,g,20a,21A deep insight into the redox processes of quinones

can be gained by performing detailed studies on the electro-

chemical behaviors of quinones, particularly, in non-aqueous media

using aprotic solvents, which are useful in mimicking the non-polar

environments in the cell where many biological electron-transfer

processes occur.^20aIn dry, neutral and aprotic media, quinones

* Corresponding authors. Tel.: þ90 212 2857021/2856831; fax: þ90 212 2856386;

e-mail addresses: yucel@itu.edu.tr (B. Yucel), iyilmaz@itu.edu.tr (I. Yilmaz).

doi:10.1016/j.tet.2010.12.057

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

1407

typically show two cathodic chemically reversible waves, which

2. Results and discussion

correspond to the formation of the semiquinone radical anion

(sq^ꢀ^ꢁ) and dianion (q^2ꢁ), respectively.^15fThe potentials of these

reductions depend on several parameters, such as the solvent,

supporting electrolyte and electrode material as well as the elec-

tron-withdrawing or -donating substituents appended on the

quinone unit.^15f,20a,21aThe course of electroreduction for quinones

is remarkably complex in the acidic medium as described in nu-

merous studies.^{15d,f,g,20a,21,22}Several recent studies reported hy-

drogen bonding and protonation effects on the redox behavior of

quinones through electrochemical investigations.^15d,20a,23Hydro-

gen bonds are particularly important for biological systems where

they provide essential recognition, structural, and control elements

needed to coordinate and run the complex molecular machinery

required for life.^20a,23a,24Moreover, the hydrogen bonding can be

viewed as a ﬁrst step toward proton transfer.^15d,20aThe electro-

chemical and spectroelectrochemical investigations on the quinone

redox system involving the electron transfer coupled with the hy-

drogen bonding give much information concerning the effect of

molecular structure and environment on these basic processes.^23,25

It is well-known that the combination of electrochemical and

spectroelectrochemical methods provides a powerful tool to reveal

the complementary nature of the molecular structure. Moreover,

these methods supply detailed electrochemical information to

clarify mechanism of electron transfer reactions.²⁶

2.1. Synthesis of ferrocenyl naphthaquinone derivatives

We initially attempted the synthesis of ferrocenyl-substituted 4-

hydroxy-4-phenylcyclobutenone 8a by the reaction of 3-ferrocenyl-4-

isopropoxy-3-cyclobutene-1,2-dione (6)^13c,29with phenyllithium(7a)

accordingtowell-documentedliteratureprocedures.^27,30Surprisingly,

thermolysis of the isolated ferrocenyl-substituted 4-hydroxy-4-phe-

nylcyclobutenone (8a), even under a nitrogen atmosphere, furnished

directly the oxidized product ferrocenyl naphthaquinone 10a in 51%

yield instead of a hydroquinone derivative 9a (Scheme 2). However,

isolating the 4-hydroxycyclobutenone 8a by column chromatography

was quite tedious and we noticed that the alcohol 8a was prone to

decompose slowly when it was stored, particularly at rt.

In this study, we have devoted our interests to the synthesis of

new ferrocenyl naphthaquinone derivatives via a well-established

regiospeciﬁc method, which offers an easy access to highly

substituted quinones.²⁷Generally, the method entails thermal

rearrangements of reactive 4-alkenyl-, 4-(aryl or heteroaryl)-4-

hydroxycyclobutenones, such as 1, to the naphthaquinone de-

rivative 5 after the oxidation of the initially formed hydroquinone 4

(Scheme 1). We employed this method, for the ﬁrst time, to achieve

various ferrocenyl naphthaquinones (10aek, 11g, and 13) starting

from 3-ferrocenyl-4-isopropoxy-3-cyclobutene-1,2-dione (6) and

aryl lithiums 7aek. The method has been applied before for the

synthesis of ferrocenyl benzoquinones by Zora et al.^13cHowever, to

the best of our knowledge, only three ferrocenyl naphthaquinone

derivatives have been prepared by the reaction of Fischer-type

chromium carbene complexes with ethynylferrocene.²⁸Here, we

have also presented an electrochemical approach to the mecha-

nistic study of hydrogen bonding and proton-coupled electron

transfer of ferrocenyl naphthaquinones in non-aqueous solutions in

the absence and presence of acidic additives.

Scheme 2. Synthesis of ferrocenyl naphthaquinone 10a via a thermal rearrangement

of the isolated 4-hydroxy-4-cyclobutenone intermediate 8a in p-xylene.

A more practical synthetic approach to the ferrocenyl naph-

thaquinone derivative 10a was achieved without isolation and

puriﬁcation of ferrocenyl-substituted 4-hydroxy-4-phenyl-

cyclobutenone 8a. The crude material obtained by treatment of

a THF solution of 6 at ꢁ78 ^ꢂC with phenyllithium (7a), followed by

an ammonium chloride quench, was directly dissolved in p-xylene

and the resulting solution was heated at reﬂux open to the air to

promote oxidation of intermediate hydroquinone 9a (Scheme 3).

During heating, the color of the solution slowly turned deep green

and the color change persisted with disappearance of the ferro-

cenyl-substituted 4-hydroxy-4-phenylcyclobutenone 8a in 4 h. Af-

ter evaporation of p-xylene and column chromatography, the

ferrocenyl naphthaquinone 10a was obtained in 45% overall yield

from 3-ferrocenyl-4-isopropoxy-3-cyclobutene-1,2-dione (6).

To investigate the scope and limitations of this short synthetic

approach to ferrocenyl naphthaquinone derivatives, we performed

the reaction of cyclobutenedione 6 with differently substituted aryl

lithiums 7aeg, which were in situ prepared (except for 7a) by the

reaction of a slight excess of n-BuLi with the corresponding aryl

bromides in dry THF at ꢁ78 ^ꢂC. The thermolyses of crude mixtures

of 4-aryl-4-hydroxycyclobutenones 8aeg were completed within

4 h and gave the ferrocenyl naphthaquinone derivatives 10aeg and

11g in moderate yields (40e53%). In all attempts, a ferrocenyl hy-

droquinone derivative of type 9a was not observed (Scheme 3).

Although the thermal rearrangements of 4-aryl-4-hydroxy-

cyclobutenones (8eeg) are open to give two regioisomeric naph-

thaquinones viaringclosure attwodifferent positionsofarylgroups,

for example,

a or b in 8g, only the thermolysis of the cyclobutenone

Scheme 1. Thermal rearrangement of 4-aryl-4-hydroxycyclobutenone 1 to the naph-

thaquinone derivative 5 (R¹, R²¼alkyl, aryl, heteroaryl, alkoxy, etc.).

8g gave the angularly-fused ferrocenyl naphthaquinone derivative

1408

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

Scheme 3. Synthesis of ferrocenyl naphthaquinone derivatives 10aeg and 11g via a reaction of ferrocenyl cyclobutenedione 6 with aryl lithiums 7aeg followed by open air

thermolyses of the intermediate 4-hydroxy-4-cyclobutenone intermediates 8aeg.

11g in 13% yield along with the linearly-fused derivative 10g in 40%

yield (Scheme 4).

To extend the scope of this study even further, we aimed to

synthesize structurally more complex ferrocenyl naphthaquinone

derivatives. The reaction of cyclobutenedione 6 with 2-lithio-9,9-

dibutyl-ﬂuorene (7h), followed by thermal rearrangement of the

intermediate hydroxycyclobutenone 8h via a ring closure at the

3-position of the ﬂuorenyl moiety furnished ferrocenyl naph-

thaquinone 10h in 40% yield as a single product. A possible angu-

larly-fused regioisomer 11h, which would arise upon ring closure at

the 1-position of the ﬂuorenyl group, was not observed. Therefore,

this result implies that, alkyl groups on the ﬂuorene moiety play

a pivotal role on the regioselective formation of the product 10h by

suppressing ring closure at the 1-position (Scheme 5).

In order to add another dimension to the scope of this study, we

turned our attention to the synthesis of naphthaquinones with two

ferrocenyl units since bimetallic systems containing an organic

p-conjugated backbone with redox-active sites have recently

emerged as new materials possessing unusual optoelectronic prop-

erties. For example, ferrocenyl end-capped ﬂuorene-containing

molecules have attracted much interest.^4d,31Thus, in this course, we

synthesized thelinearly-fused ferrocenyl naphthaquinones 10i albeit

in 24% yield via a reaction of 2 equiv 6 with 2,7-dilithio-9,9-diethyl-

ﬂuorene (7i), followed by heating the initially formed hydroxy-

cyclobutenone 8i. Similarly, when the related sequence was initiated

with the reaction of cyclobutenedione 6 and mono-lithiated bro-

moﬂuorene derivative 7j, ferrocenyl naphthaquinone 10j was

Scheme 4. Formation of regioisomeric ferrocenyl naphthaquinone derivatives 10g and

11g via a ring closure at two different position (a and b) of the aryl group.

Scheme 5. The reaction of cyclobutenedione 6 with 2-lithio-9,9-dibutyl-ﬂuorene (7h) followed by thermal rearrangement of the intermediate hydroxycyclobutenone 8h via a ring

closure at the 3-position; regioselective formation of linearly-fused ferrocenyl naphthaquinone 10h.

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

1409

accessed in 61% yield. Ferrocenyl naphthaquinone (2 equiv) 10j and

2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dibutyl-

ﬂuorene (12) in a mixture of dioxane and 2.0 M aqueous solution of

Na₂CO₃and tetrabutylammonium bromide (TBAB), were treated

with a palladium catalyst [PdCl₂(dppf): [1,1⁰-bis(diphenylphos-

phino)ferrocene]dichloropalla-dium(II)] for 19 h to furnish the

target product 13 containing three ﬂuorene moieties (Scheme 6).

absorptions in the visible and ultraviolet region. Examples of rep-

resentative UVevis spectra of 10b (blue line),10c (magenta line),10e

(red line), 10j (green line), and 14³²(black dash line) (used as a ref-

erence molecule in this study) were given in Fig. 1.

The spectrum of 14 was dominated by a high intensity band at

252 nm and two moderate intensity bands at 333 and 408 nm, which

were typical for naphthaquinone (n,

p/p

) transitions.³³In the

Scheme 6. Synthesis of oligocyclic ferrocenyl naphthaquinone derivatives 10iej and the formation of compound 13 by Suzuki-cross-coupling reaction of ferrocenyl naphthaquinone

10j with diboronic ester 12.

In addition to these achievements, a bis(ferrocenyl naph-

thaquinone) derivative 10k was obtained in 44% yield by the re-

action of ferrocenyl cyclobutenedione 6 with 4,4⁰-dilithiobiphenyl

(7k) and subsequent thermolysis of the intermediate hydroxy-

cylobutenones 8k in p-xylene for 4 h (Scheme 7).

spectra of the ferrocenyl naphthaquinones 10b, 10c, 10e, and 10j, the

transition bands appeared at 723, 740, 687, and 699 nm, respectively,

which indicated clearly an intramolecular charge-transfer between

the ferrocenyl donor and naphthaquinonyl acceptor centers [e_p

(HOMOeFc)/e_p(LUMOenq)] (Fig. 1 and Table 1). These transitions

were responsible for the green color of the ferrocenyl naph-

thaquinones 10aek, 11g, and 13. The assignment is analogous to the

previously well-characterized charge-transfer bands of ferroce-

neeDDQ (2,3-dichloro-5,6-dicyanobenzoquinone) [e_p(HOMO-

2.2. Electronic spectra of ferrocenyl naphthaquinone

derivatives

eFc)/e_p

(LUMOeDDQ)] and ferroceneebenzoquinone [e_p

Electronic spectra of the ferrocenyl naphthaquinone derivatives

10aek,11g, and 13 have been recorded in the 200e1100 nm range in

CH₂Cl₂solution and their corresponding data were listed in Table 1.

All ferrocenyl naphthaquinones 10aek, 11g, and 13 showed several

(HOMOeFc)/e_p

bands disappeared upon one-electron oxidation of the ferrocene

center or one-electron reduction of the naphthaquinone center, (see

(LUMOeq) donoreacceptor dyads.^17a,34These

1410

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

Scheme 7. Synthesis of bis(ferrocenyl naphthaquinone) derivative 10k via a reaction of ferrocenyl cyclobutenedione 6 with 4,4⁰-dilithiobiphenyl (7k) followed by open air ther-

molysis of the intermediate 4-hydroxy-4-cyclobutenone intermediate 8k.

Table 1

The redox properties and UVevis absorptions of the ferrocenyl naphthaquinones

Fcenq Fcesnq^ꢀ^ꢁ/Fcenq^ꢁ2Fcenq/Fcesnq^ꢀ^ꢁFc^þenq/Fcenq ^d1DE_p(V) ^e2DE_p(V) I_a/I_c

I_c/I_a

(Fc^þenq/Fcenq) (Fcenq/Fcesnq^ꢀ^ꢁ) (log

(n, p/p )

l_max/nm

/nm

max

^aE_1/2^b(V)

(log

(HOMO/LUMO)

(e_p/e_p

E_p)

(

DE_p)

(

DE_p)

)

10a

10b

10c

10d

10e

10f

10g

10h

10i

ꢁ1.26 (0.17)

ꢁ1.24 (0.18)

ꢁ1.17 (0.12)

ꢁ1.33 (0.24)

ꢁ1.36 (0.21)

ꢁ1.32 (0.31)

ꢁ1.31 (0.34)

ꢁ1.32 (0.31)

ꢁ1.41 (0.17)

ꢁ0.78 (0.12)

ꢁ0.69 (0.12)

ꢁ0.63 (0.09)

ꢁ0.84 (0.14)

ꢁ0.80 (0.17)

ꢁ0.80 (0.20)

ꢁ0.82 (0.15)

ꢁ0.76 (0.13);

ꢁ0.87 (0.13)

ꢁ0.81 (0.14)

ꢁ0.72; ꢁ0.80

ꢁ0.91 (0.12)

ꢁ0.81 (0.12)

ꢁ0.76 (0.11)

0.58 (0.11)

0.59 (0.12)

0.58 (0.09)

0.57 (0.14)

0.56 (0.13)

0.57 (0.17)

0.59 (0.19)

0.58 (0.21)

0.58 (0.19)

1.36

1.28

1.21

1.41

1.40

1.37

1.39

1.40

1.34

0.48

0.55

0.54

0.49

0.52

0.51

0.50

0.54

1.05

1.01

0.99

1.01

1.02

1.00

1.01

1.08

1.07

1.05

1.01

1.02

1.03

1.12

1.11

1.01

255 (4.44) 333 (3.90) 408 (3.63) 693 (3.36)

263 (4.56) 336 (3.99) 416 (3.74) 723 (3.51)

260 (4.58) 325 (4.09) 411 (3.88) 740 (3.56)

271 (4.44) 349 (3.99) 420 (3.48) 681 (3.28)

280 (4.45) 345 (4.13) 436 (3.1) 687 (3.24)

279 (4.52) 344 (4.13) 434 (3.04) 691 (3.28)

284 (4.66) 340 (4.31) 430 (3.56) 677 (3.42)

295 (4.51) 344 (4.02) 436 (3.47) 695 (3.16)

302 (4.83)

427 (3.94) 713 (3.61)

10j

10k

11g

ꢁ1.33 (0.22)

ꢁ1.34 (0.30)

ꢁ1.44 (0.26)

0.57 (0.14)

0.59 (0.14)

0.56 (0.12)

1.38

1.31

1.47

1.37

0.52

0.54

0.53

0.98

1.03

1.05

0.98

1.32

1.03

1.06

1.08

0.95

292 (4.63) 369 (3.98) 436 (3.67) 699 (3.36)

280 (4.77) 353 (4.35) 425 (3.96) 720 (3.62)

275 (4.45) 315 (4.25) 400 (3.94) 657 (3.36)

284 (4.19) 350 (4.49) 460 (3.85) 696 (3.22)

252 (4.71) 333 (3.85) 408 (3.44)

^f14

ꢁ1.27 (0.21)

0.51

E_1/2¼E_pcþE_pa/2.

In V versus Ag/AgCl (3 M NaCl) measured by cyclic voltammetry.

E_p¼jE_pcꢁE_paj.

¹DE_p¼E_1/2(Fc^þenq/Fcenq)ꢁE_1/2(Fcenq/Fcesnq^ꢀ^ꢁ).

²DE_p¼E_1/2(Fcenq/Fcesnq^ꢀ^ꢁ)ꢁE_1/2(Fcenq^ꢀ^ꢁ/Fcenq^2ꢁ).

Compound 14: 2,3-diisopropoxy-1,4-naphthaquinone (nq) (used as a reference molecule in this study). Conditions: 3.75ꢃ10^ꢁ3M in dichloromethane with 0.1 M n-

Bu₄NClO₄as support electrolyte; working electrode, Pt disc electrode (1.6 mm diameter); counter electrode, Pt wire; reference electrode, Ag/AgCl (3 M NaCl); For the fer-

rocene, E_1/2of Fc^þ/Fc¼0.53 V (

E_p¼0.085 V) at the scan rate of 0.10 V s^ꢁ1; c¼5ꢃ10^ꢁ5M in dichloromethane for UVevis measurement.

the electrochemistry section for more details). The existence of

intramolecular charge-transfer transitions with lower energy was

also evidenced by the solvatochromic behavior of 10b. The electronic

spectra of 10b were conducted in several solvents having different

solvatingcapabilities. Thelowerenergybandshiftedtohigherenergy

in more highly solvating solvents, whereas the higher energy bands

in the UV region showed little change in energy with solvent

[Supplementary data; Fig. S40 represents a plot of band wavelength

against Reichardt’s overall solvation scale parameter E_T(30)].³⁵As

expected, the electron-withdrawing and electron-donating sub-

stituents on the naphthaquinone moieties shifted the charge-trans-

fer bands to lower and higher energies, respectively (Fig. 1 and Table

1). The charge-transfer band was highly shifted toward lower energy

(740 nm) in the case of 10c bearing a strong electron-withdrawing

triﬂuoromethyl group. In addition, the positionof thesubstituents on

the naphthaquinone unit affected the energy of charge-transfer

transitions. For example, the related bands for the linearly-fused 10g

and the angularly-fused 11g ferrocenyl naphthaquinones (see

Scheme 3) were observed at 677 and 657 nm, respectively. The

20 nm-shift to higher energy was probably due to a close interaction

of theweak-electron donatingdioxine group andthequinone moiety

in 11g. On the other hand, the charge-transfer bands for 10i and 10k

were observed at 713 and 720 nm, respectively. The tendency of

shifting to lower energies was probably caused by increasing effec-

10b

10c

0.2

10e

10j

0.1

0.0

-0.1

600

800

1000

400

600

800

1000

Wavelength / nm

tive

p-conjugation between the ferroceneequinone redox-active

centers in these structures 10i and 10k. The maximum absorption

wavelengths of the intramolecular charge-transfer displayed a good

agreement with the difference in half-wave potentials of the

Fig. 1. UVevis spectra of 10b (blue line), 10c (magenta line), 10e (red line), 10j (green

line) and 14 (black dash line) in CH₂Cl₂at rt. The inset shows charge-transfer bands.

c¼5ꢃ10^ꢁ5M.

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

1411

oxidation and ﬁrst reduction processes [¹

E_p¼E_1/2(Fc^þenq/Fcenq)ꢁ

and displayed a chemically reversible one-electron oxidation pro-

E_1/2(Fcenq/Fcesnq^ꢀ^ꢁ)] of the ferrocenyl naphthaquinone derivatives

10aek and 11g, and is discussed in the electrochemistry section in

more detail.

cess corresponding to the ferrocenium/ferrocene (Fc^þenq/Fcenq)

couple at E_1/2¼0.58 V ( E_p¼0.11 V, I_pa/I_pc¼1.05). Scheme 8 (by blue

double arrows) summarizes the electrochemical pathways for the

ferrocenyl naphthaquinones 10aeg in non-aqueous media, which

are also applicable for 10hek, 11g, and 13.

2.3. Electrochemistry and spectroelectrochemistry of

ferrocenyl naphthaquinone derivatives

For the ferrocene, under our experimental conditions, E_1/2of Fc^þ/

Fc was calculated as E_1/2¼0.53 V ( E_p¼0.085 V) at a scan rate of

The electrochemical properties of the ferrocenyl naph-

thaquinones 10aek, 11g, 13, and the naphthaquinone 14 were in-

vestigated using cyclic voltammetry in CH₂Cl₂and CH₃CN

containing 0.1 M tetra-n-butylammonium perchlorate (n-Bu₄N-

ClO₄) as supporting electrolyte. The redox potential data obtained

in this work were gathered in Table 1. Cyclic voltammograms (CVs)

of 10a (black line) and 14 (red dash line) are shown in Fig. 2a.

The naphthaquinone 14 underwent a chemically reversible one-

electron reduction process to form the semiquinone radical anion

0.10 V^ꢁ1, which can be used as a criterion for electrochemical re-

versibility.³⁶The CVs in Fig. 2a illustrate that the anodic peak current

appearing for the second reduction process is smaller than the

current for the ﬁrst reduction process. This can be attributed to

a complexation reaction between the dianion (q^2ꢁ) and the neutral

state of the quinone (q).^15dThe redox processes of the ferrocenyl

naphthaquinone derivatives 10aek, 11g, and 13 were examined as

a function of potential scan rate in order to ascertain the mode of

mass transport. The cathodic and anodic currents for the oxidation

Fig. 2. Cyclic voltammograms of (a) 10a (black line) and 14 (red dash line); (b) 10a (black line); 10b (red line), 10c (green line), and 10e (blue line); (c) 10h (blue line) and 10i (red

line); (d) 10k (blue line) in CH₂Cl₂solution containing 0.1 M n-Bu₄NClO₄. Scan rates¼0.10 V s^ꢁ1. Inset (black line) in (d) represents Differential pulse voltammogram (DPV) of 10k.

Scan rate¼0.02 V s^ꢁ1, pulse PH/PW¼0.025 V; step height¼2.00 mV; step time: 100.0 ms. Working electrode: platinum disk electrode (1.6 mm diameter), c¼3.75ꢃ10^ꢁ3M.

(Fcesnq^ꢀ^ꢁ) at E_1/2(half-wave potential)¼ꢁ0.76 V [

E_p(the anodic

and reduction couples in CH₂Cl₂increased in a direct proportion to

the square root of the scan rates between 0.05 and 1.0 V s^ꢁ1. This

implied that these redox reactions were diffusion controlled³⁶

[Supplementary data; Fig. S41 represents CVs of 10a at various

scan rates (0.05e1.0 V s^ꢁ1)]. Fig. 2b shows CVs obtained for 10a

(black line),10b (red line),10c (green line), and 10e (blue line) under

the same experimental conditions. The general feature of the CVs for

10b, 10c and 10e was similar to that observed for 10a discussed in

detailsabove, and theircorresponding redoxpotential dataaregiven

Table 1. The E_1/2of the ﬁrst reduction of 10e, 10b, and 10c were

located at E_1/2¼ꢁ0.84, ꢁ0.69, and ꢁ0.63 V, respectively, which were

shifted to the more or less negative potentials as compared to that of

10a (E_1/2¼ꢁ0.78 V). The change in their redox potentials was caused

by the effect of strong electron-donor methoxy and electron-

to cathodic peak-to-peak separation)¼0.11 V, I_pc/I_pa(the ano-

dicecathodic peak ratio)¼0.95], which was further reduced by one

electron at more negative potential (a quasi-reversible process at

E_1/2¼ꢁ1.27 V;

E_p¼0.21 V) to form the dianion (Fcenq^2ꢁ) species at

a scan rate of 0.10 V s^ꢁ1. These results are similar to typical cyclic

voltammograms of benzoquinone and naphthaquinone derivatives

in aprotic organic solvents without acidic additives.^{15d,f,22c,e,f,23a}The

difference between the half-wave potentials of the ﬁrst and second

reduction processes [²

E_p¼E_1/2(nq/nq^ꢀ^ꢁ)ꢁE_1/2(snq^ꢀ^ꢁ/nq^2ꢁ)] for 14

was 0.51 V, which was in accord with the those of the ferrocenyl

naphthaquinones 10aek, 11g, and 13 studied in this work (see

Table 1). The ferrocenyl naphthaquinone 10a exhibited two re-

duction processes, which had almost the same waves as 14 (Fig. 2a)

1412

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

Scheme 8. The electrochemical reduction and oxidation mechanism for the ferrocenyl naphthaquinone derivatives in non-aqueous medium (the pathways indicated blue double

arrows), after addition of weak acid (AcOH), (the pathways indicated diagonal green double arrows), and water (the pathways indicated red double arrows) followed strong acid

(TFA), (the pathways indicated green double arrows) into the solution of CH₃CN.

withdrawing bromo and triﬂuoromethyl substituents on the 6,7

positions of the naphthaquinone core. Similar behavior was also

observed for the second reduction processes. However, the oxida-

tion processes corresponding to the Fc^þenq/Fcenq couple for the

ferrocenyl naphthaquinones 10aek, 11g and 13 displayed a similar

value of E_1/2(0.56e0.59 V), and a little change in E_1/2values com-

pared to that of ferrocene molecule (E_1/2of Fc^þ/Fc¼0.53 V). This

unexpected observation implied that the oxidation potential of the

ferrocene electroactive center was not shifted to the positive po-

tential by the acceptor naphthaquinone moiety. Similar behavior

was reported in the literature where the oxidation potential of the

Fc^þ/Fc couple (0.60 V) observed for 1-ferrocenylanthraquinone, di-

rectlylinked acceptoredonor units, showed onlya slight anodic shift

compared to that of the ferrocene molecule (0.55 V) in CH₂Cl₂.³⁷The

difference between the half-wave potentials of the oxidation and

Fcesnq^ꢀ^ꢁ)], relating to the energy of the intramolecular charge-

transfer transition between the ferrocenyl donor and naph-

thaquinonyl acceptor centers [e_p(HOMOeFc)/e_p

was ranged between 1.21 and 1.47 V according to substitution pat-

tern. The ¹

E_pvalues showed a good agreement with the maximum

(LUMOenq)],

absorption wavelengths of the intramolecular charge-transfer.

Fig. 2c shows the CVs of 10h (blue dot line) and 10i (red line). Similar

to the ferrocenyl naphthaquinones 10aeg, 11g, the ferrocenyl

naphthaquinone 10h bearing a conjugated aromatic unit exhibited

two chemically reversible reduction and one oxidation processes.

On the other hand, the structures involving covalently-linked two

ferrocenyl naphthaquinone units, 10i and 10k, displayed reduction

processes corresponding to the semiquinone radical anion

(Fcesnq^ꢀ^ꢁ) of the two naphthaquinone electrochemical centers as

two separated waves accompanied with two electrons. In-

terestingly, similar behavior was not determined for the oxidation

ﬁrst reduction processes [¹

E_p¼E_1/2(Fc^þenq/Fcenq)ꢁE_1/2(Fcenq/

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

1413

processes of 10i and 10k. In these oxidation processes, the unit wave

arising from the Fc^þenq/Fcenq couple occurred at the same po-

tential with the ferrocenyl naphthaquinone derivatives 10aeh, 11g.

Fig. 2d shows CV of 10k (blue line), which is similar tothat of 10i, and

the inset presents differential pulse voltammogram (DPV, black line)

for 10k where the ﬁrst reduction processes are clearly differentiated

as IIa and IIa’ waves.

The spectroelectrochemical behavior of the ferrocenyl naph-

thaquinones was investigated using an in situ spectroelec-

trochemical technique including chronoamperometry and UVevis

spectroscopy in CH₃CN solution containing 0.2 M n-Bu₄NClO₄. The

UVevis spectral changes for the reduced and oxidized species of the

corresponding derivatives were obtained in a thin-layer cell during

applied potentials. Theconvenient applied potentials values foran in

situ spectroelectrochemical experiment were determined for each

process by taking CVs of the complexes in the thin-layer cell. Time-

resolved UVevis spectral changes of 10e during the ﬁrst reduction

process at E_app(applied potential)¼ꢁ1.15 V were depicted in Fig. 3a.

The spectra of the reduced species corresponding to the semi-

quinone radical anion (Fcesnq^ꢀ^ꢁ) showed well-deﬁned isosbestic

characteristic intramolecular charge-transfer band appeared at

661 nm was almost disappeared because of the newly formed

electron-rich semiquinone center (Fcesnq^ꢀ^ꢁ). The bands at 281 and

341 nm decreased in intensity and new bands at 384 and 640 nm

appeared at low intensity. Finally, the spectrum of the reduced

species exhibited a characteristic proﬁle for the semiquinone radical

anion (Fcesnq^ꢀ^ꢁ), which was consistent with previously described

ferroceneebenzoquinone systems (Fig. 3a).^17aThe original green

color and the spectrum of 10e could be recovered upon re-oxidation

during the spectroelectrochemical measurements. The controlled

potential coulometric (CPC) study indicated that the number of

electrons transferred for the forward and reverse electrochemical

reactions of the complex was one for both the reduction and re-

ꢁ

oxidation processes, based on the Fcenq/Fcesnq^ꢀcouple. Thus,

semiquinone radical anion (Fcesnq^ꢀ^ꢁ) remained chemically stable

throughout the experiment. The ﬁrst reduction process was com-

plete within 60 s and followed by a second reduction process. In the

spectra of 10e at the more reducing applied potential

(E_app¼ꢁ1.75 V), the band at 428 nm disappeared and the intensity of

the band at 342 nm increased (Fig. 3b). After electrolysis was com-

pleted within 60 s to afford the doubly reduced forms (Fcenq^2ꢁ),

different spectral changes were monitored for the next times (the

inset in Fig. 3b). Moreover, the second reduction process could not

fully be reversed upon the applied potential (E_app¼ꢁ0.10 V). This

behavior was probably due to the complexation reaction between

the dianion (Fcenq^2ꢁ) and neutral (Fcenq) states as predicted from

the cyclic voltammetry or decomposition of doubly reduced species

(Fcenq^2ꢁ) in the thin-layer cell. Fig. 4 shows the UVevis spectral

changes during the oxidation process based on the Fc^þenq/Fcenq

couple at the applied potential (E_app¼1.30 V). As was observed for

the ﬁrst reduction process, the intensity of the characteristic intra-

molecular charge-transfer band (the inset) decreased by removing

one electron from the HOMO of the molecule during the oxidation

process, thereby forming the mono cationic species (Fc^þenq).

2,0

(a)

0.3

1,5

0.2

0.1

1,0

0.0

400

600

800

0,5

0,0

300

400

500

600

700

900

2.0

Wavelength / nm

0.10

2,0

1,5

1,0

0,5

0,0

(b)

1.5

0.05

1.0

0.5

0.00

500 600 700 800 900

0.5

0.0

400

600

800

0.0

300

400

500

600

700

800

900

Wavelength / nm

Fig. 4. Time-resolved UVevis spectral changes of 10e during the oxidation at

E_app¼1.30 V in CH₃CN solution containing 0.2 M n-Bu₄NClO₄.

300

400

500

600

700

800

900 1000

Wavelength / nm

All of the spectroscopic changes were fully reversed by re-re-

duction process at the applied potential (E_app¼ꢁ0.10 V), which in-

dicated a fully reversible oxidation process as predicted from the

cyclic voltammetry of 10e.

Fig. 3. Time-resolved UVevis spectral changes of (a) 10e during the ﬁrst reduction at

E_app¼ꢁ1.15 V (b) 10e during the second reduction at E_app¼ꢁ1.75 V in CH₃CN solution

containing 0.2 M n-Bu₄NClO₄. Arrows mark the direction of absorbance change.

points observed at 267, 312, 370, and 600 nm. This conﬁrmed that

the electrode reaction proceeded in a quantitative fashion and the

absence of any coupled chemistry (Fig. 3a).^26a,b,d,fThe original

spectrum of the neutral form (red line) showed several changes in

intensity and band position resulting from addition of one electron

into the LUMO level of the molecule. The color of the solution in the

thin-layer cell also changed from deep green to yellow. The

2.4. Effect of weak and strong acids on the electro-

spectroelectrochemistry of the ferrocenyl naphthaquinones

In the presence of acidic additives, electroreduction of quinones

is remarkably complex and depends on the effect of acid strength,

concentration, and quinone basicity.^{15d,f,g,20a,21,22}Addition of acetic

1414

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

acid (AcOH) into the solution containing the ferrocenyl naph-

thaquinones did not affect their green color and electronic spectra.

This implied that the ferrocenyl naphthaquinones were not pro-

tonated in the presence of this weak acid. Fig. 5 exhibits CVs of 10e

before (black line) and after (red line) addition of AcOH.

phenomenon was often explained by protonation of the quinone

dianion species by water,³⁸in 1971 Hayano and Fujihira³⁹suggested

that the positive shift was probably due to hydrogen bonding and

several other workers noted that this behavior was not related with

protonation.^15d,40In our case, two well-separated reduction waves,

corresponding to the formation of the semiquinone mono- and di-

anions (Fcesnq^ꢀ^ꢁand Fcenq^2ꢁ) shifted positive, with no loss of re-

versibility by the addition of water. This result was attributed to

stabilization by the hydrogen bonding of mono- and dianion re-

100

OCH₃

ꢁ

duction products (Fcesnq^ꢀand Fcenq^2ꢁ). Eventually, under our

i-PrO

OCH₃

experimental conditions, the role of hydrogen bonding could be

clearly distinguished from protonation as described by Gupta and

Linschitz.^15dSimilarly, the hydrogen bonding interactions between

water and the one- and two-electron reduced form of vitamin K₁

based on 2-methly-1,4-naphthaquinone in non-aqueous solvent

were recently described by Webster et al.^23aScheme 8 (by red

double arrows) summarizes the electrochemical pathways for the

ferrocenyl naphthaquinones in CH₃CN containing a large amount of

-50

-100

-150

water. The CV obtained by subsequent addition of 20 mL TFA (black

line in Fig. 6) was similar to that observed for the addition of AcOH.

Therefore, the reduced species stabilized by the effect of the hy-

drogen bonding of water molecules were protonated by the addition

of TFA to form the hydroquinone as the ﬁnal product. The anodic

wave appeared at E_pa¼0.36 V was assigned to the oxidation of the

hydroquinone.

The ferrocenyl naphthaquinones in CH₃CN showed different

behavior upon the addition of strong acids, such as perchloric acid

(HClO₄) and triﬂuoromethanesulfonic acid (TfOH). For example, the

color of the solutions, UVevis spectra and CVs of the ferrocenyl

naphthaquinones wereconsiderablychanged. These changes clearly

indicated a chemical association between proton and quinone

groups. Therefore, we conducted electro-spectroelectrochemical

studies to understand this proton-coupled electron transfer mech-

anism of the ferrocenyl naphthaquinones in the presence of strong

acids. Addition of HClO₄to solutions of the ferrocenyl naph-

thaquinones in CH₃CN caused an immediate color change from in-

tense green to deep yellow. In the corresponding UVevis spectra of

Fcenq, 10e, the intramolecular charge-transfer band at 695 nm al-

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Potential / V vs. Ag/AgCl

Fig. 5. Cyclic voltammograms of 10e in CH₃CN solution containing 0.1 M n-Bu₄NClO₄

before addition of AcOH (black line) and after addition of AcOH (40 mL in 4 ml solution)

(red line). Scan rates¼0.10 V s^ꢁ1, working electrode: glassy carbon disk electrode

(3.0 mm diameter).

ꢁ

The two reduction waves corresponding to Fcesnq^ꢀand

Fcenq^2ꢁspecies in acid-free solution at a scan rate of 0.10 V s^ꢁ1

(black line), are replaced by a large cathodic wave (E_pc¼ꢁ0.46 V)

with a higher peak current to form the hydroquinone product

(FcehnqH₂) by a sequence of two-electron transfer and two-pro-

tonation reactions (Fig. 5). The pathway was summarized by green

diagonal double arrows in Scheme 8. The anodic wave at more

positive potential (E_pa¼0.36 V) was attributed tothe oxidation of the

hydroquinone (FcehnqH₂) to form the quinone (Fcenq). Similar

behavior was also reported for different hydroquinones lacking the

ferrocenyl moiety under weakly acidic conditions.^15fFig. 6 exhibits

CVs of 10e before (blue line) and after addition of H₂O (red line), and

subsequent addition of triﬂuoroacetic acid (TFA) (black line).

most disappeared and the bands at 262 and 334 nm (n,

p/p

transitions) increased in intensity (Supplementary data; Fig. S42).

These changes were not entirely consistent with the formation of

Fc^þenq or Fc^þehnqH₂species. The lack of Fc^þenq or Fc^þehnqH₂

species in the presence of HClO₄did not thus support dispropor-

tionation of the semiquinone (Fc^þesnqH^ꢀ), which was previously

proposed for the ferrocenyl-1,4-benzoquinone in strong acids by

electro-spectroelectrochemical studies.^17aThe acidic solution con-

taining the semiquinone (Fc^þesnqH^ꢀ) left one green spot on the TLC

plate (SiO₂), indicating the stabilityof the species formed byaddition

of HClO₄. Additionally, the original green color and spectrum (blue

dash line) corresponding to 10e could be achieved again by the ad-

dition of SiO₂as a powder to the solution of 10e in CH₃CN where SiO₂

200

-200

OCH₃

probably acted as

a base in the presence of strong HClO₄

-400

(Supplementary data; Fig. S43). Fig. 7 exhibits CVs of 10e before

(black line) and after (red dash line) addition of HClO₄.

i-PrO

OCH₃

The two reduction waves, corresponding to the semiquinone

radical anion (Fcesnq^ꢀ^ꢁ) and the dianion (Fcenq^2ꢁ) species in

CH₃CN at a scan rate of 0.10 V s^ꢁ1completely disappeared, and

instead, one new cathodic wave at 0.08 V and a corresponding one

anodic wave at 0.92 V emerged; yet the position of the oxidation

wave of the Fc^þenq/Fcenq couple did not change. This appearance

was partly different than that described by Colbran et al.^17a

Therefore, herein, we proposed a new mechanism in Scheme 9.

According to the proposed mechanism, ﬁrstly, the naph-

thaquinone group was protonated at any o_þxygen atom to give the

ferrocenium semiquinone cation radical (Fc esnqH^ꢀ) by addition of

HClO₄. This provided an increase in the basicity of the

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Potential / V vs. Ag/AgCl

Fig. 6. Cyclic voltammograms of 10e in CH₃CN solution containing 0.1 M n-Bu₄NClO₄

(blue line), after addition of water (0.1 mL in 4 ml solution) (red line), and after ad-

dition of TFA (50

L in 4 ml solutionþ0.1 mL water) (black line). Scan rates¼0.10 V s^ꢁ1

working electrode: glassy carbon disk electrode (3.0 mm diameter).

The two reduction waves merged into one by addition of ap-

proximately 0.1 mL H₂O (Fig. 6,_ꢁred line). These results indicated that

the reduced species (Fcesnq^ꢀand Fcenq^2ꢁ) formed a hydrogen

bond with the water in the solvent. In the past, although this

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

1415

oxidation step, Fc^þehnqH₂yielded to Fc^þenq at 0.92 V by losing

two electrons decoupled two protons. The wave occurred at 0.35 V

was assigned to the corresponding cathodic wave of Fc^þehnqH₂.

The change induced by HClO₄was reversed by addition of a large

amount of SiO₂into the electrochemical cell. The partly original CV

of 10e (blue line) could be obtained except the dianionic wave

(Fcenq^2ꢁ), probably due to the presence of water to cause hydro-

gen bonding between the Fcenq^2ꢁspecies and water (see Fig. 7).

Spectral changes for the formation of Fc^þehnqH₂(the inset) and

Fc^þenq species in thin-layer cell during the applied potentials

(E_app¼0.65 and 1.3 V, respectively) are presented in Fig. 8.

100

OCH₃

i-PrO

-100

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Potential / V vs. Ag/AgCl

1.0

Fig. 7. Cyclic voltammograms of 10e in CH₃CN solution containing 0.1 M n-Bu₄NClO₄

before addition of HClO₄(black line) and after addition of 40% HClO₄(40 L in 4 mL

1.0

0.5

solution) (red line), and after addition of excess SiO₂powder into the solution (blue

line). Scan rates¼0.10 V s^ꢁ1, working electrode: glassy carbon disk electrode (3.0 mm

diameter).

0.0

naphthaquinone by electron delocalization with the donor ferro-

cene. The proton localized on positions 1 or 4 of the quinone oxygen

atom. The cathodic wave observed at 0.08 V is probably based on

the hydroquinone (FcehnqH₂) produced from Fc^þesnqH^ꢀby two-

electron reductio_þn coupled with one proton. The protonated

semiquinone (Fc esnqH^ꢀ) easily reduced than unprotonated

counterpart (FcesnqH^ꢀ^ꢁ), thereby the reduction potential of the

hydroquinone (Fc^þehnqH₂) in the presence of strong acid

appeared at more positive potential (E_pc¼0.08 V) with respect to

that in the presence of weak acid (E_pc¼ꢁ0.46 V). In the ﬁrst step of

anodic scan, FcehnqH₂probably oxidized to Fc^þehnqH₂at 0.51 V

by removing one electron from the ferrocene moiety. In the second

0.5

200

400

600

800

Wavelength / nm

0.0

300

400

500

600

700

800

900

Wavelength / nm

Fig. 8. Time-resolved UVevis spectral changes of 10e in the solution of CH₃CN con-

taining excess HClO₄during the oxidation process at E_app¼1.30 V (supporting elec-

trolyte: 0.2 M n-Bu₄NClO₄). The inset shows the UVevis spectral changes of 10e during

the oxidation process at E_app¼0.65 V in the same experimental conditions.

Scheme 9. The electrochemical reduction and oxidation mechanism for the ferrocenyl naphthaquinone derivatives in the solution of CH₃CN after addition of strong acids.

1416

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

The ﬁnal spectrum (blue line) corresponding to Fc^þenq species

while dichloromethane (CH₂Cl₂) was treated three times with

H₂SO₄and distilled over P₂O₅and CaH₂prior to use. Tetra-n-

butylammonium perchlorate used as supporting electrolyte (n-

Bu₄NClO₄, Fluka Chemical Co.) was recrystallized from ethyl alcohol

and dried in a vacuum oven at 40 ^ꢂC for at least 1 week prior to use.

Solvents for column chromatography, ethyl acetate, and hexane

were distilled in a rotary evaporator. Chromatographic separations

were performed with Merck Silica 60 (200e400 or 70e230 mesh).

TLC was performed with Merck TLC Silicagel60 F₂₅₄plates, de-

tection was under UV light at 254 nm.

in Fig. 8 was similar to that observed for Fc^þenq species during the

oxidation of 10e (Fig. 4 red line). Addition of TfOH to the solution of

the ferrocenyl naphthaquinone 10c in CH₃CN also caused the color,

spectral, and CV changes as in the case of HClO₄. One example is

given in the Supplementary data (Fig. S44, which represents

changes of CVs of 10c in CH₃CN during repetitive scan after addition

of TfOH). The ﬁnal CV (red line) exhibited general fashion in the

strong acid as mentioned above.

3. Conclusions

The following compounds were prepared according to known

literature methods: 3-isopropoxy-4-ferrocenyl-3-cyclobutene-1,2-

dione (6),^13c,292-bromo-9,9-dibutyl-ﬂuorene,⁴¹2,7-dibromo-9,9-

diethyl-ﬂuorene,⁴²2,7-dibromo-9,9-dibutyl-ﬂuorene,⁴³and 2,7-bis

(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dibutylﬂuorene

(12).⁴³

A highly efﬁcient and practical methodology has been applied for

the ﬁrst time to prepare variously substituted ferrocenyl naph-

thaquinone derivatives. In this study, molecular complexity of fer-

rocenyl naphthaquinones has been provided upon the treatment of

different aryl lithiums (7aeg) with 3-ferrocenyl-4-isopropoxy-3-

cyclobutene-1,2-dione (6). Yet, even by employing 2-lithio-9,9-

dibutyl-ﬂuorene (7h), 2,7-dilithio-9,9-diethyl-ﬂuorene (7i), and

2-bromo-7-lithio-9,9-dibutyl-ﬂuorene (7j) oligocyclic ferrocenyl

naphthaquinones 10hej have been easily achieved. Further en-

largement of the ferrocenyl naphthaquinone library is still open to

investigation through this synthetically ﬂexible methodology. Thus,

we are currently working on the synthesis of new derivatives of

ferrocenyl naphthaquinones starting from 3,4-bisferrocenyl-3-

cyclobutene-1,2-dione^13c,29and 1,1⁰-bis{3-(4-isopropoxy-3-cyclo-

butene-1,2-dioxo)}ferrocene.²⁹In this study, the redox chemistry of

the ferrocenyl naphthaquinones was examined by electrochemical

and in situ spectroelectrochemical techniques in the solution of

dichloromethane and acetonitrile. From these studies, it was con-

cluded that the ferrocenyl naphthaquinones 10aek, 11g, and 13

showed chemically reversible two reduction processes correspond-

ing to the formation of the semiquinone radical anion (Fcesnq^ꢀ^ꢁ) and

dianion (Fcenq^2ꢁ) species in non-aqueous solutions, respectively.

These complexes displayed chemically reversible one-electron oxi-

dation process based on the ferrocene/ferrocenium (Fc^þenq/Fcenq)

couple. The reduction potentials strongly depend on electron-with-

drawing or-donating substituents appended on the naphthaquinone

unit. The stability, reversibility and color changes of the reduced

[(Fcesnq^ꢀ^ꢁ) and (Fcenq^2ꢁ)] and oxidized (Fc^þenq) species were

clearlydemonstratedby thin-layerUVevisspectroelectrochemistyin

non-aqueous solutions. The electrochemistry of the ferrocenyl

naphthaquinoneswasremarkablycomplexanddependedontheacid

strength. The reaction mechanism of ferrocenyl naphthaquinones in

the presence of acidic additives proceeded via hydrogen bonding or

proton-coupled electron transfer. It was also found that the intra-

molecular charge-transfer for the ferrocenyl naphthaquinones was

sensitive to substituents on the napthaquinone center and structural

changes caused a shift in the bands to shorter or longer wavelengths.

Finally, one can anticipate that ferrocene-naphthaquinone

donoreacceptor redox systems involving the range of accessible

oxidation states may facilitate a progress in the application of redox-

based processes such as the pH-controlled electrochemical switching

from the donor centered to the acceptor centered redox chemistry.

Additionally, the ferrocenyl naphthaquinones with multiple redox-

active centers and possessing low energy charge-transfer transitions

may ﬁnd application in molecular electronics.

4.1.1. Measurements. NMR spectra were recorded with a Bruker AM

250 (250 MHz for ¹H and 62.9 MHz for ¹³C NMR), a Bruker Spec-

trospin Avance DPX400 Ultrashield (400 MHz for ¹H and

100.59 MHz for ¹³C NMR) and Varian Inova 500 (500 MHz for ¹H

and 125 MHz for ¹³C NMR) instruments. Chemical shifts

d were

given in parts per million relative to residual peaks of deuterated

solvents and coupling constants, J, were given in hertz. The fol-

lowing abbreviations are used to describe spin multiplicities in ¹H

NMR spectra: s¼singlet; br s¼broad singlet; d¼doublet; t¼triplet;

q¼quartet; dd¼doublet of doublets; m¼multiplets. Multiplicities in

¹³C NMR spectra were determined by DEPT (Distortionless

Enhancement by Polarization Transfer): þ¼primary or tertiary

(positive DEPT signal), ꢁ¼secondary (negative DEPT Signal),

C_quat¼quaternary carbon atoms or APT (Attached Proton Test)

measurements. IR spectra were recorded on a NICOLET 6700 FT IR

spectrometer. Low resolution mass spectra (API-ES, 70 eV, and

APCI) were obtained on a Agilent 1100 LCeMS instrument equip-

ped with a diode array UV-visible range detector and Thermo LCQ

Deca Ion Trap Mass Specrometer (Thermo Finnigan). High resolu-

tion mass spectra (HRMS) were obtained on a Waters Synapt

Q-TOF-MS spectrometer. Elemental analysis was carried out by the

Elementar Micro Vario CHNS Elemental Analyzer instrument in

€

Tubitak Ankara Testing and Analysis Laboratory (ATAL). UV-visible

spectra were recorded on Agillent Model 8453 diode array spec-

trophotometer. Cyclic voltammograms (CV) were carried out using

CV measurements with Princeton Applied Research Model 2263

potentiostat controlled by an external PC. A three electrode system

[BAS model (Bioanalytical System Inc.) solid cell stand] was used for

CV measurements in CH₂Cl₂and CH₃CN and it consisted of plati-

num (1.6 mm diameter) and glassy carbon (3.0 mm diameter) disc

electrodes as working electrode, a platinum wire counter electrode,

and a Ag/AgCl (3 M NaCl) reference electrode. The reference elec-

trode was separated from the bulk solution by a fritted-glass bridge

ﬁlled with the solvent/supporting electrolyte mixture. The ferro-

cene/ferrocenium couple (Fc/Fc^þ) was used as an internal standard

but all potentials in the paper are referenced to the Ag/AgCl (3 M

NaCl) reference electrode. Solutions containing the ferrocenyl

naphthaquinone derivatives were deoxygenated by a stream of

high purity nitrogen for at least 5 min before running the experi-

ment and the solution was protected from air by a blanket of

nitrogen during the experiment. Controlled potential electrolysis

(CPE) was performed with Princeton Applied Research Model 2263

potentiostat/Galvanostat. An BAS model electrolysis cell with

a fritted glass to separate the cathodic and anodic portions of the

cell was used for bulk electrolysis. The sample and solvent were

placed into the electrolysis cell under nitrogen. The platinum

4. Experimental

4.1. General considerations

All reagents were used as purchased from commercial suppliers

without further puriﬁcation unless otherwise indicated. Diethyl

ether and THF were freshly distilled from sodium/benzophenone

ketyl. Acetonitrile (CH₃CN) was distilled over P₂O₅under vacuum

electrode was polished on alumina (<10 mM) slurries on BAS felt-

pad. This was followed by rinsing with Millipore water and then

ethanol. UV-visible spectroelectrochemical experiments were

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

1417

performed with a home-built thin-layer cell that utilized a light

transparent platinum gauze working electrode.⁴⁴Potentials were

applied and monitored with a Princeton Applied Research Model

2263 potentiostat. Time-resolved UV-visible spectra were recorded

on Agillent Model 8453 diode array spectrophotometer.

chromatography on silica gel using 4:1 hexane/ethyl acetate as el-

uent to yield 10a (0.51 g, 51%, green solid).

Without isolation of 8a: To a solution of 3-ferrocenyl-4-iso-

propoxy-3-cyclobutene-1,2-dione (6,1.00 g, 3.08 mmol,1.0equiv) in

THF(10 mL)at ꢁ78 ^ꢂCunderanitrogenatmosphere, PhLi(2.57 mL of

a 1.8 M of dibutylether solution, 4.62 mmol, 1.5 equiv) was added.

After stirring for 3 h at ꢁ78 ^ꢂC, the reaction mixture was quenched

with 10% NH₄Cl (30 mL) solution at ꢁ78 ^ꢂC and then allowed to

warm to rt. The mixture was diluted with ether (100 mL) and the

organic layer was separated. The aqueous layer was extracted with

ether (2ꢃ50 mL). The combined organic layers were dried over

Na₂SO₄and ﬁltered. The solution concentrated under reduced

pressure and the remaining crude material was dissolved in p-xy-

lene (30 mL). The resulting solution was heated at reﬂux open to the

air in a preheated oil bath (165 ^ꢂC) for 4 h. After removal of the

p-xylene under reduced pressure, the residue was dissolved in ether

(50 mL) and silica gel (3.00 g) was added into the solution. The

solution again was concentrated under reduced pressure and the

greensolid residuewas subjectedtopuriﬁcation bychromatography

on silica gel using 4:1 hexane/ethyl acetate as eluent to yield 10a

(0.56 g, 45%, green solid). Mp 107e109 ^ꢂC, R_f¼0.81 (hexane/ethyl

4.2. General procedure for the synthesis of ferrocenyl

naphthaquinones (10bek)

To a solution of arylbromide (1.0 equiv) in THF at ꢁ78 ^ꢂC under

nitrogen, n-BuLi (1.1, 1.35 or 2.4 equiv of a 1.6 M of hexane solution)

was added via syringe over 15 min. The resulting mixture was

stirred at ꢁ78 ^ꢂC for 1 h and then transferred via cannula to a so-

lution of 3-ferrocenyl-4-isopropoxy-3-cyclobutene-1,2-dione (6, 1.1

or 2.2 equiv) in THF at ꢁ78 ^ꢂC under the nitrogen atmosphere. After

stirring 3 h at ꢁ78 ^ꢂC, the reaction mixture was quenched with 10%

NH₄Cl (30 mL) solution at ꢁ78 ^ꢂC and then allowed to warm to rt.

The mixture diluted with ether (100 mL) and the organic layer was

separated. The aqueous layer was extracted with ether (2ꢃ50 mL).

The combined organic layers were dried over Na₂SO₄and ﬁltered.

The solution concentrated under reduced pressure and the

remaining crude material was dissolved in p-xylene. The resulting

solution was heated at reﬂux open to the air in a preheated oil bath

(165 ^ꢂC) for 4 h. After removal of the p-xylene under reduced

pressure, the residue was dissolved in ether (50 mL) and silica gel

(3.00 g) was added into the solution. The solution was concentrated

under reduced pressure and the green solid residue was subjected

to puriﬁcation by chromatography on silica gel.

acetate 4:1); ¹H NMR (250 MHz, CDCl₃):

1.29 (d, J¼6.1 Hz, 6H, 2ꢃi-

PrO [CH₃]), 4.12 (s, 5H, Fc), 4.52 (s, 2H, Fc), 4.84e4.94 (m, 1H, i-PrO

[CH]), 5.20 (s, 2H, Fc), 7.66e7.75 (m, 2H, Ar), 8.05e8.13 (m, 2H, Ar);

¹³C NMR (62.9 MHz, CDCl₃):

Fc), 70.35 (2ꢃCH, Fc), 72.84 (2ꢃCH, Fc), 74.53 (C_quat, Fc), 76.27 (CH, i-

PrO), 125.78 (CH, Ar), 126.51 (CH, Ar), 131.58 (C_quat), 132.89 (C_quat),

133.17 (CH, Ar), 133.46 (CH, Ar), 136.27 (C_quat), 154.61 (C_quat), 181.08

22.84 (2ꢃCH₃, i-PrO), 70.04 (5ꢃCH,

(C_quat, C]O), 184.67 (C_quat, C]O); IR (ATR):

¼2975 (w), 2926 (w),

4.2.1. 2-Ferrocenyl-4-hydroxy-3-isopropoxy-4-phenylcyclobut-2-en-

1-one (8a). To a solution of 3-ferrocenyl-4-isopropoxy-3-cyclo-

butene-1,2-dione (6, 2.27 g, 7.00 mmol, 1.0 equiv) in THF (40 mL) at

ꢁ78 ^ꢂC under a nitrogen atmosphere, PhLi (5.80 mL of a 1.8 M of

dibutylether solution, 10.5 mmol, 1.5 equiv) was added. After stir-

ring for 3 h at ꢁ78 ^ꢂC, the reaction mixture was quenched with 10%

NH₄Cl (30 mL) solution at ꢁ78 ^ꢂC and then allowed to warm to rt.

The mixture diluted with ether (100 mL) and the organic layer was

separated. The aqueous layer was extracted with ether (2ꢃ50 mL).

The combined organic layers were dried over Na₂SO₄and ﬁltered.

The solution concentrated under reduced pressure and the

remaining reddish-brown oil residue was subjected to puriﬁcation

by chromatography on silica gel using 2:1 hexane/ethyl acetate as

eluent to yield 8a (1.67 g, 59%, brown solid). Mp 121 ^ꢂC, R_f¼0.23

1651 (vs), 1591 (s), 1552 (s), 1439 (m), 1384 (m), 1341 (s), 1292 (s),

1198 (s), 1099 (s), 998 (vs), 804 (vs); MS (70 eV, API-ES) m/z (%): 401

(72) [M^þþ1], 400 (100) [M^þ], 359 (33), 358 (40), 271 (17), 230 (29),

200 (14), 149 (5); HRMS [TOF MS ES^þ]: m/z [M]^þcalcd. For

C₂₃H₂₀FeO₃400.0762, found 400.0761 (ꢁ0.2 ppm).

4.2.3. 6-Bromo-3-ferrocenyl-2-isopropoxynaphthalene-1,4-dione

(10b). According to general procedure, to a solution of 1,4-dibro-

mobenzene (0.66 g, 2.80 mmol, 1.0 equiv) in THF (10 mL) at ꢁ78 ^ꢂC

under nitrogen, n-BuLi (1.92 mL of a 1.6 M of hexane solution,

3.08 mmol, 1.1 equiv) was added. The resulting mixture was stirred

at ꢁ78 ^ꢂC for 1 h and then transferred to a solution of 3-ferrocenyl-

4-isopropoxy-3-cyclobutene-1,2-dione (6, 1.00 g, 3.08 mmol,

1.1 equiv) in THF (10 mL) at ꢁ78 ^ꢂC under nitrogen atmosphere.

After stirring 3 h at ꢁ78 ^ꢂC and work-up, the crude alcohol 8b was

heated at reﬂux in p-xylene (30 mL) for 4 h. The crude product was

obtained as described in the general procedure and then subjected

to puriﬁcation by chromatography on silica gel using 4:1 hexane/

ethyl acetate as eluent to yield 10b (0.66 g, 49%, green solid). Mp

150e152 ^ꢂC, R_f¼0.70 (hexane/ethyl acetate 4:1); ¹H NMR (250 MHz,

(hexane/ethyl acetate 4:1); ¹H NMR (250 MHz, CDCl₃):

d 1.13 (d,

J¼5.9 Hz, 3H, i-PrO [CH₃]), 1.45 (d, J¼5.9 Hz, 3H, i-PrO [CH₃]), 3.46

(s, 1H, OH), 4.18 (s, 5H, Fc), 4.27 (s, 2H, Fc), 4.71 (s, 2H, Fc), 4.71e4.80

(m, 1H, i-PrO [CH]), 7.33e7.42 (m, 3H, Ar), 7.54e7.57 (m, 2H, Ar); ¹³C

NMR (125 MHz, CDCl₃):

d 22.51 (CH₃, i-PrO), 22.92 (CH₃, i-PrO),

67.67 (CH), 68.82 (CH), 69.38 (5ꢃCH, Fc), 70.47 (CH), 78.42 (C_quat),

93.60 (C_quat), 125.58 (CH, Ph), 126.90 (C_quat), 128.05 (C_quat), 128.57

(CH, Ph), 137.27 (C_quat), 177.65 (C_quat), 187.35 (C_quat, C]O); IR (ATR):

CDCl₃):

(s, 2H, Fc), 4.86e4.95 (m, 1H, i-PrO [CH]), 5.20 (s, 2H, Fc), 7.79e7.93

(m, 2H, Ar), 8.23 (s, 1H, Ar); ¹³C NMR (62.9 MHz, CDCl₃):

1.29 (d, J¼6.1 Hz, 6H, 2ꢃi-PrO [CH₃]), 4.13 (s, 5H, Fc), 4.56

¼3234 (s), 3066 (w), 2979 (w), 1738 (s), 1619 (vs), 1471 (s), 1450

d¼22.85

(m), 1344 (s), 1326 (s), 1203 (m), 1133 (w), 1093 (s), 1039 (s), 1001

(w), 987 (w), 842 (w); MS (APCI) m/z (%): 403 (19) [M^þþ1], 385

(100), 343 (60); HRMS [TOF MS ES^þ]: m/z [M]^þcalcd. For

C₂₃H₂₂FeO₃402.0918, found 402.0907 (ꢁ2.7 ppm).

(2ꢃCH₃, i-PrO), 70.12 (5ꢃCH, Fc), 70.54 (2ꢃCH, Fc), 72.87 (2ꢃCH,

Fc), 74.34 (C_quat, Fc), 76.49 (CH, i-PrO), 127.47 (CH, Ar), 128.90

(C_quat), 129.61 (CH, Ar), 130.20 (C_quat), 133.96 (C_quat), 136.13 (CH, Ar),

136.33 (C_quat), 154.57 (C_quat), 180.17 (C_quat; C]O), 183.37 (C_quat

;

C]O); IR (ATR):

¼2961 (w), 2924 (w), 1725 (w), 1645 (vs), 1580

4.2.2. 2-Ferrocenyl-3-isopropoxynaphthalene-1,4-dione

(10a). 2-

(m), 1542 (s), 1461 (w), 1382 (w), 1339 (m), 1290 (vs), 1248 (s), 1211

(s), 1196 (s), 1102 (s), 1090 (m), 1003 (vs), 908 (m), 921 (m), 810 (s),

802 (s), 744 (vs); MS (70 eV, API-ES) m/z (%): 481 (55) [M^þþ1], 480

(100) [M^þ], 479 (47), 478 (91), 476 (10), 459(21), 458 (11), 457 (14),

436 (9), 413 (5); HRMS [TOF MS ES^þ]: m/z [M]^þcalcd. For

C₂₃H₁₉BrFeO₃477.9867, found 477.9877 (2.1 ppm).

Ferrocenyl-4-hydroxy-3-isopropoxy-4-phenylcyclobut-2-en-1-one

(8a, 1.00 g, 2.49 mmol) was dissolved in p-xylene (20 mL). The

resulting solution was heated at reﬂux under a nitrogen atmo-

sphere in a preheated oil bath (165 ^ꢂC) for 3 h. After removal of the

p-xylene in a rotary evaporator, the residue was dissolved in ether

(50 mL) and silica gel (3.00 g) was added into the solution. The

solution again was concentrated under reduced pressure and the

green solid residue was subjected to puriﬁcation by

4.2.4. 3-Ferrocenyl-6-(triﬂuoromethyl)-2-isopropoxynaphthalene-

1,4-dione (10c). According to general procedure, to a solution of 4-

1418

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

bromobenzotriﬂuoride (0.63 g, 2.80 mmol, 1.0 equiv) in THF

(10 mL) at ꢁ78 ^ꢂC under nitrogen, n-BuLi (2.40 mL of a 1.6 M of

hexane solution, 3.78 mmol, 1.35 equiv) was added. The resulting

mixture was stirred at ꢁ78 ^ꢂC for 1 h and then transferred to a so-

lution of 3-ferrocenyl-4-isopropoxy-3-cyclobutene-1,2-dione (6,

1.00 g, 3.08 mmol, 1.1 equiv) in THF (20 mL) at ꢁ78 ^ꢂC under a ni-

trogen atmosphere. After stirring 3 h at ꢁ78 ^ꢂC and work-up, the

crude alcohol 8c was heated at reﬂux in p-xylene (45 mL) for 4 h.

The crude product was obtained as described in the general pro-

cedure and then subjected to puriﬁcation by chromatography on

silica gel using 4:1 hexane/ethyl acetate as eluent to yield 10c

(0.68 g, 52%, green solid). Mp 152e153 ^ꢂC, R_f¼0.78 (hexane/ethyl

using 4:1 hexane/ethyl acetate as eluent to yield 10e (0.62 g, 48%,

green solid). Mp 151e153 ^ꢂC, R_f¼0.44 (hexane/ethyl acetate 4:1); ¹H

NMR (250 MHz, CDCl₃):

1.28 (d, J¼6.1 Hz, 6H, 2ꢃi-PrO [CH₃]), 4.02

(s, 3H, OCH₃), 4.05 (s, 3H, OCH₃), 4.15 (s, 5H, Fc), 4.53 (s, 2H, Fc),

4.79e4.84 (m, 1H, i-PrO [CH]), 5.20 (s, 2H, Fc), 7.49 (s, 1H, Ar), 7.54

(s, 1H, Ar); ¹³C NMR (62.9 MHz, CDCl₃):

¼22.80 (2ꢃCH₃, i-PrO),

56.38 (2ꢃOCH₃), 69.93 (5ꢃCH, Fc), 70.09 (2ꢃCH, Fc), 72.68 (2ꢃCH,

Fc), 74.79 (C_quat, Fc), 76.19 (CH, i-PrO), 107.3 (CH, Ar), 108.2 (CH, Ar),

125.9 (C_quat), 127.5 (C_quat), 135.30 (C_quat), 153.0 (C_quat), 153.3 (C_quat),

154.4 (C_quat), 180.4 (C_quat; C]O), 184.1 (C_quat; C]O); IR (ATR):

¼2962 (w), 2936 (w), 1656 (s), 1648 (s), 1583 (s), 1567 (s), 1512 (s),

1449 (m), 1464 (s), 1370 (m), 1316 (s), 1302 (s), 1259 (m), 1242 (m),

1192 (m), 1157 (m), 1096 (s), 1057 (w), 1014 (w), 982 (w), 884 (w),

745 (s); MS (APCI) m/z (%): 461 (100) [M^þþ1], 431 (20), 418 (45), 411

(14), 371 (10), 279 (46), 117 (43), 97 (22); HRMS [TOF MS ES^þ]: m/z

[M]^þcalcd. For C₂₅H₂₄FeO₅460.0973, found 460.0988 (3.3 ppm).

acetate 4:1); ¹H NMR (250 MHz, CDCl₃):

1.30 (d, J¼6.1 Hz, 6H, 2ꢃi-

PrO [CH₃]), 4.14 (s, 5H, Fc), 4.58 (s, 2H, Fc), 4.93 (p, J¼6.1 Hz, 1H, i-

PrO [CH]), 5.22 (s, 2H, Fc), 7.92e8.21 (AB system, d_A¼8.20, d_B¼7.94,

J_AB¼7.9 Hz, 2H, Ar), 8.39 (s, 1H, Ar); ¹³C NMR (125 MHz, CDCl₃):

¼22.82 (2ꢃCH₃, i-PrO), 70.93 (5ꢃCH, Fc), 71.54 (2ꢃCH, Fc), 73.61

(2ꢃCH, Fc), 74.80 (C_quat, Fc), 76.51 (CH, i-PrO), 122.97 (q, CF₃,

4.2.7. 6-Ferrocenyl-7-isopropoxynaphtho[2,3-d][1,3]dioxole-5,8-di-

¹J_FC¼273 Hz), 123.49 (C_quat), 126.17 (C_quat), 129.27 (C_quat), 133.09

one (10f). According to general procedure, to

4-bromo-1,2-(methylenedioxy)benzene (1.10 g,

solution of

5.50 mmol,

(CH, Ar), 133.66 (CH, Ar), 134.91 (q, C_quat

²J_FC¼33 Hz), 137.52 (CH,

Ar), 154.33 (C_quat), 179.71 (C_quat; C]O), 183.01 (C_quat; C]O); IR

1.0 equiv) in THF (10 mL) at ꢁ78 ^ꢂC under nitrogen, n-BuLi (4.60 mL

of a 1.6 M of hexane solution, 7.38 mmol, 1.35 equiv) was added.

The resulting mixture was stirred at ꢁ78 ^ꢂC for 1 h and then

transferred to a solution of 3-ferrocenyl-4-isopropoxy-3-cyclo-

butene-1,2-dione (6, 1.96 g, 6.05 mmol, 1.1 equiv) in THF (20 mL) at

ꢁ78 ^ꢂC under a nitrogen atmosphere. After stirring 2.5 h at ꢁ78 ^ꢂC

and work-up, the crude alcohol 8f was heated at reﬂux in p-xylene

(45 mL) for 4 h. The crude product was obtained as described in the

general procedure and then subjected to puriﬁcation by chroma-

tography on silica gel using 4:1 hexane/ethyl acetate as eluent to

yield 10f (1.08 g, 44%, green solid). Mp 152e153 ^ꢂC, R_f¼0.57 (hex-

(ATR):

¼3086 (w), 2980 (w), 1652 (vs), 1551 (m), 1440 (w), 1324

(m), 1286 (s), 1254 (m), 1205 (m), 1165 (m), 1126 (s), 1101 (s), 1064

(m), 1005 (s), 898 (w), 814 (m), 748 (m), 710 (m); MS (APCI) m/z (%):

469 (100) [M^þþ1], 426 (77), 419 (24); HRMS [TOF MS ES^þ]: m/z

[M]^þcalcd. For C₂₄H₁₉F₃FeO₃468.0636, found 468.0638 (0.4 ppm).

4.2.5. 3-Ferrocenyl-2-isopropoxy-6-methoxynaphthalene-1,4-dione

(10d). According to general procedure, to a solution of 4-bromoa-

nisole (0.30 g, 1.60 mmol, 1.0 equiv) in THF (10 mL) at ꢁ78 ^ꢂC under

nitrogen, n-BuLi (1.35 mL of a 1.6 M of hexane solution, 2.16 mmol,

1.35 equiv) was added. The resulting mixture was stirred at ꢁ78 ^ꢂC

for 1 h and then transferred to a solution of 3-ferrocenyl-4-iso-

propoxy-3-cyclobutene-1,2-dione (6, 0.57 g, 1.76 mmol, 1.1 equiv)

in THF (10 mL) at ꢁ78 ^ꢂC under a nitrogen atmosphere. After stir-

ring 3 h at ꢁ78 ^ꢂC and work-up, the crude alcohol 8d was heated at

reﬂux in p-xylene (50 mL) for 4 h. The crude product was obtained

as described in the general procedure and then subjected to puri-

ﬁcation by chromatography on silica gel using 4:1 hexane/ethyl

acetate as eluent to yield 10d (0.36 g, 53%, green solid). Mp

133e135 ^ꢂC, R_f¼0.74 (hexane/ethyl acetate 4:1); ¹H NMR (250 MHz,

ane/ethyl acetate 4:1); ¹H NMR (250 MHz, CDCl₃):

d 1.27 (d,

J¼6.1 Hz, 6H, 2ꢃi-PrO [CH₃]), 4.12 (s, 5H, Fc), 4.51 (s, 2H, Fc),

4.79e4.84 (m, 1H, i-PrO [CH]), 5.16 (s, 2H, Fc), 6.12 (s, 2H, CH₂), 7.44

(s, 1H, Ar), 7.50 (s, 1H, Ar); ¹³C NMR (62.9 MHz, CDCl₃):

d¼22.80

(2ꢃCH₃, i-PrO), 69.99 (5ꢃCH, Fc), 70.21 (2ꢃCH, Fc), 72.72 (2ꢃCH,

Fc), 74.70 (C_quat, Fc), 76.25 (CH, i-PrO), 102.44 (CH₂), 105.28 (CH, Ar),

106.19 (CH, Ar), 128.25 (C_quat), 129.93 (C_quat), 135.34 (C_quat), 151.82

(C_quat), 152.12 (C_quat), 154.15 (C_quat), 180.0 (C_quat; C]O), 183.59

(C_quat; C]O); IR (ATR):

¼2961(w), 2929 (w), 1639 (s), 1596 (s),

1569 (s), 1508 (m), 1477 (s), 1449 (m), 1403 (m), 1380 (m), 1301 (s),

1254 (s), 1214 (w), 1141 (m), 1102 (m), 1061 (m), 1027 (s), 997 (s),

923 (m), 882 (m), 809 (s), 741 (s); MS (70 eV, ES) m/z (%): 445 (42)

[M^þþ1], 444 (100) [M^þ], 424 (29), 413 (67), 392 (23), 301 (20), 283

(39), 236 (36), 214 (19); HRMS [TOF MS ES^þ]: m/z [M]^þcalcd. For

C₂₄H₂₀FeO₅444.0660, found 444.0657 (ꢁ0.7 ppm).

CDCl₃):

¼1.28 (d, J¼6.0 Hz, 6H, i-PrO), 3.97 (s, 3H, OCH₃), 4.12 (s,

5H, Fc), 4.50 (s, 2H, Fc), 4.90 (p, J¼6.0 Hz, 1H, i-PrO), 5.17 (s, 2H, Fc),

7.16 (d, J¼8.5 Hz, 1H, Ph), 7.57 (s, 1H, Ph), 8.00 (d, J¼8.7 Hz, 1H, Ph);

¹³C NMR (62.9 MHz, CDCl₃, DEPT):

d¼22.85 (þ, i-PrO), 55.90 (þ,

OCH₃), 69.95 (þ, Fc), 70.08 (þ, Fc), 72.63 (þ, Fc), 74.72 (C_quar., Fc),

76.38 (þ,i-PrO), 109.96 (þ, Ph), 119.80 (þ, Ph), 125.05 (C_quar.), 128.25

(þ, Ph), 135.01 (C_quar.), 135.45 (C_quar.), 154.96 (C_quar.), 164.05 (C_quar.),

4.2.8. 7-Ferrocenyl-2,3-dihydro-8-isopropoxynaphtho[2,3-b][1,4]di-

180.24 (C]O), 184.61 (C]O); IR (ATR):

¼2970, 2929, 1652, 1600,

oxine-6,9-dione

(10g)

and

9-ferrocenyl-2,3-di-hydro-8-iso-

1582, 1553, 1496, 1464, 1441, 1349, 1332, 1234, 1207, 1176, 1098,

1000, 903, 881, 806 cm ^ꢁ1; MS (70 eV, EI), m/z (%): 431 (12) [M^þ],

430 (42), 388 (100), 295 (18), 281 (23), 207 (87), 121 (29), 73 (68).

C₂₄H₂₂FeO₄(430.29): calcd. C 66.99, H 5.15; found C 66.83, H 5.13.

propoxynaphtho[2,1-b][1,4]dioxine-7,10-dione (11g). According to

general procedure, to a solution of 6-bromo-1,4-benzodioxane

(0.60 g, 2.80 mmol, 1.0 equiv) in THF (10 mL) at ꢁ78 ^ꢂC under ni-

trogen, n-BuLi (2.36 mL of a 1.6 M of hexane solution, 3.78 mmol,

1.35 equiv) was added. The resulting mixture was stirred at ꢁ78 ^ꢂC

for 1 h and then transferred to a solution of 3-ferrocenyl-4-iso-

propoxy-3-cyclobutene-1,2-dione (6, 1.00 g, 3.08 mmol, 1.1 equiv)

in THF (10 mL) at ꢁ78 ^ꢂC under a nitrogen atmosphere. After stir-

ring 2.5 h at ꢁ78 ^ꢂC and work-up, the crude alcohol 8g was heated

at reﬂux in p-xylene (30 mL) for 4 h. The crude product was

obtained as described in the general procedure and then subjected

to puriﬁcation by chromatography on silica gel using 4:1 hexane/

ethyl acetate as eluent to yield 10g (0.51 g, 40%, green solid) and 11g

(0.17 g, 13%, green solid). Compound (10g): mp 133e135 ^ꢂC, R_f¼0.39

4.2.6. 2-Ferrocenyl-3-isopropoxy-6,7-dimethoxynaphthalene-1,4-di-

one (9e). According to general procedure, to a solution of 4-bro-

moveratrole (0.61 g, 2.80 mmol, 1.0 equiv) in THF (10 mL) at ꢁ78 ^ꢂC

under nitrogen, n-BuLi (2.36 mL of a 1.6 M of hexane solution,

3.78 mmol, 1.35 equiv) was added. The resulting mixture was

stirred at ꢁ78 ^ꢂC for 1 h and then transferred to a solution of

3-ferrocenyl-4-isopropoxy-3-cyclobutene-1,2-dione (6, 1.00 g,

3.08 mmol, 1.1 equiv) in THF (10 mL) at ꢁ78 ^ꢂC under a nitrogen

atmosphere. After stirring 3 h at ꢁ78 ^ꢂC and work-up, the crude

alcohol 8e was heated at reﬂux in p-xylene (30 mL) for 4 h. The

crude product was obtained as described in the general procedure

and then subjected to puriﬁcation by chromatography on silica gel

(hexane/ethyl acetate 4:1); ¹H NMR (250 MHz, CDCl₃):

J¼6.1 Hz, 6H, 2ꢃi-PrO [CH₃]), 4.12 (s, 5H, Fc), 4.35 (s, 4H, 2ꢃCH₂),

4.51 (s, 2H, Fc), 4.81e4.85 (m,1H, i-PrO [CH]), 5.19 (s, 2H, Fc), 7.52 (s,

¼1.27 (d,

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

1419

1H, Ar), 7.56 (s, 1H, Ar); ¹³C NMR (62.9 MHz, CDCl₃):

(2ꢃCH₃, i-PrO), 64.53 (CH₂), 64.59 (CH₂), 70.14 (5ꢃCH, Fc), 70.30

(2ꢃCH, Fc), 72.90 (2ꢃCH, Fc), 74.98 (C_quat, Fc), 76.15 (CH, i-PrO),

115.13 (CH, Ar), 115.95 (CH, Ar), 126.43 (C_quat), 127.99 (C_quat), 135.93

¼22.79

solution of 2,7-dibromo-9,9-diethyl-ﬂuorene (0.41 g,

1.06 mmol, 1.0 equiv) in THF (15 mL) at ꢁ78 ^ꢂC under nitrogen, n-

BuLi (1.60 mL of a 1.6 M of hexane solution, 2.54 mmol, 2.4 equiv)

was added. The resulting mixture was stirred at ꢁ78 ^ꢂC for 1 h and

then transferred to a solution of 3-ferrocenyl-4-isopropoxy-3-

cyclobutene-1,2-dione (6, 0.76 g, 2.33 mmol, 2.2 equiv) in THF

(15 mL) at ꢁ78 ^ꢂC under a nitrogen atmosphere. After stirring 2.5 h

at ꢁ78 ^ꢂC and work-up, the crude alcohol 8i was heated at reﬂux in

p-xylene (30 mL) for 4 h. The crude product was obtained as de-

scribed in the general procedure and then subjected to puriﬁcation

by chromatography on silica gel using 4:1 hexane/ethyl acetate as

eluent to yield 10i (0.22 g, 24%, green solid). Mp 130e131 ^ꢂC,

R_f¼0.67 (hexane/ethyl acetate 4:1); ¹H NMR (400 MHz, CDCl₃):

(C_quat), 147.82 (C_quat), 148.14 (C_quat), 154.69 (C_quat), 180.15 (C_quat

;

C]O), 183.70 (C_quat; C]O); IR (ATR):

¼976 (w), 1650 (s), 1595 (s),

1539 (s), 1496 (m), 1438 (m), 1382 (m), 1349 (m), 1317 (s), 1292 (s),

1255 (s), 1190 (w), 1162 (m), 1143 (w), 1102 (m), 1022 (m), 1000 (m),

925 (w), 888 (s), 734 (s); MS (70 eV, API-ES) m/z (%): 460 (28)

[M^þþ2], 459 (93) [M^þþ1], 458 (100) [M^þ], 457 (88), 417 (21), 416

(16), 314 (46), 286 (32), 258 (22), 230 (77), 219 (21), 163 (29), 149

(14); HRMS [TOF MS ES^þ]: m/z [M]^þcalcd. For C₂₅H₂₂FeO₅458.0817,

found 458.0809 (ꢁ1.7 ppm).

Compound (11g): mp 171e172 ^ꢂC, R_f¼0.25 (hexane/ethyl acetate

¼0.29 (t, J¼7.3 Hz, 6H, 2ꢃCH₃), 1.25 (d, J¼6.2 Hz, 12H, 4ꢃi-PrO

4:1); ¹H NMR (250 MHz, CDCl₃):

¼1.27 (d, J¼6.2 Hz, 6H, 2ꢃi-PrO

[CH₃]), 2.13 (q, J¼7.3 Hz, 4H, 2ꢃCH₂), 4.09 (s, 10H, 2ꢃFc [5H]), 4.49

(s, 4H, 2ꢃFc [2H]), 4.82e4.84 (m, 2H, 2ꢃi-PrO [CH]), 5.17 (s, 4H,

2ꢃFc [2H]), 8.01 (s, 2H, Ar), 8.56 (s, 2H, Ar); ¹³C NMR (100.59 MHz,

[CH₃]), 4.15 (s, 5H, Fc), 4.28e4.49 [m, 6H, Fc (2H); 2ꢃCH₂],

4.80e4.85 (m, 1H, i-PrO [CH]), 5.21 (s, 2H, Fc), 7.11 (d, J¼8.4 Hz, 1H,

Ar); ¹³C NMR (62.9 MHz, CDCl₃):

¼21.76 (2ꢃCH₃, i-PrO), 63.15

CDCl₃):

d¼7.60 (2ꢃCH₃), 21.90 (2ꢃCH₃, i-PrO [CH₃]), 31.45 (2ꢃCH₂),

(CH₂), 63.53 (CH₂), 68.96 (5ꢃCH, Fc), 69.16 (2ꢃCH, Fc), 71.61 (2ꢃCH,

Fc), 73.71 (C_quat, Fc), 74.74 (CH, i-PrO), 119.30 (CH, Ar), 120.06 (CH,

Ar), 121.13 (C_quat), 125.28 (C_quat), 136.25 (C_quat), 142.58 (C_quat),

56.90 (C_quat), 69.10 (5ꢃCH, Fc), 69.45 (2ꢃCH, Fc), 71.94 (2ꢃCH, Fc),

73.64 (C_quat, Fc), 75.42 (CH, i-PrO), 118.4 (CH, Ar), 119.47 (CH, Ar),

130.80 (C_quat), 132.20 (C_quat), 135.47 (C_quat), 145.86 (C_quat), 153.87

(C_quat), 154.93 (C_quat), 180.04 (C_quat; C]O), 183.34 (C_quat; C]O); IR

148.07 (C_quat), 151.97 (C_quat), 179.35 (C_quat; C]O), 183.44 (C_quat

;

C]O); IR (ATR):

¼2974 (w), 2929 (w), 1655 (s), 1597 (w), 1578 (s),

(ATR):

¼2964 (w), 2929 (w), 1758 (w), 1656 (s), 1602 (s), 1548 (s),

1482 (m), 1455 (m), 1429 (m), 1375 (w), 1342 (w), 1279 (s), 1211 (s),

1174 (m), 1140 (w), 1104 (s), 1092 (s), 1002 (s), 972 (w), 957 (m), 901

(s), 841 (m), 806 (s), 765 (s); MS (70 eV, API-ES) m/z (%): 460 (26)

[M^þþ2], 459 (100) [M^þþ1], 458 (55) [M^þ], 457 (25), 416 (23), 258

(63); HRMS [TOF MS ES^þ]: m/z [M]^þcalcd. For C₂₅H₂₂FeO₅458.0817,

found 458.0820 (0.7 ppm).

1440 (m), 1381 (m), 1290 (s), 1245 (m), 1213 (s), 1190 (m), 1169 (m),

1095 (s), 1084 (s), 1065 (m), 940 (w), 903 (s), 804 (s), 730 (s); MS

(APCI) m/z (%): 867 (100) [M^þþ1]; HRMS [TOF MS ES^þ]: m/z [M]^þ

calcd. For C₅₁H₄₆Fe₂O₆866.1993, found 866.2028 (4.0 ppm).

4.2.11. 2-Bromo-11,11-dibutyl-7-ferrocenyl-8-isopropoxy-11H-benzo

[b]ﬂuorene-6,9-dione (10j). According to general procedure, to

a solution of 2,7-dibromo-9,9-dibutyl-ﬂuorene (0.73 g, 1.68 mmol,

1.0 equiv) in THF (15 mL) at ꢁ78 ^ꢂC under nitrogen, n-BuLi (1.25 mL

of a 1.6 M of hexane solution, 2.02 mmol, 1.2 equiv) was added. The

resulting mixture was stirred at ꢁ78 ^ꢂC for 1 h and then transferred

4.2.9. 11,11-Dibutyl-7-ferrocenyl-8-isopropoxy-11H-benzo[b]ﬂuo-

rene-6,9-dione(10h). According to general procedure, to a solution

of 2-bromo-9,9-dibutyl-ﬂuorene (2.00 g, 5.6 mmol, 1.0 equiv) in

THF (30 mL) at ꢁ78 ^ꢂC under nitrogen, n-BuLi (4.84 mL of a 1.6 M of

hexane solution, 7.73 mmol, 1.38 equiv) was added. The resulting

mixture was stirred at ꢁ78 ^ꢂC for 1 h and then transferred to a so-

lution of 3-ferrocenyl-4-isopropoxy-3-cyclobutene-1,2-dione (6,

2.00 g, 6.17 mmol, 1.1 equiv) in THF (30 mL) at ꢁ78 ^ꢂC under a ni-

trogen atmosphere. After stirring 2.5 h at ꢁ78 ^ꢂC and work-up, the

crude alcohol 8h was heated at reﬂux in p-xylene (60 mL) for 4 h.

The crude product was obtained as described in the general pro-

cedure and then subjected to puriﬁcation by chromatography on

silica gel using 4:1 hexane/ethyl acetate as eluent to yield 10h

(1.34 g, 40%, green solid). Mp 173e175 ^ꢂC, R_f¼0.79 (hexane/ethyl

a solution of 3-ferrocenyl-4-isopropoxy-3-cyclobutene-1,2-

dione (6, 0.60 g, 1.85 mmol, 1.1 equiv) in THF (15 mL) at ꢁ78 ^ꢂC

under a nitrogen atmosphere. After stirring 2.5 h at ꢁ78 ^ꢂC and

work-up, the crude alcohol 8j was heated at reﬂux in p-xylene

(30 mL) for 4 h. The crude product was obtained as described in the

general procedure and then subjected to puriﬁcation by chroma-

tography on silica gel using 4:1 hexane/ethyl acetate as eluent to

yield 10j (0.70 g, 61%, green solid). Mp 114e116 ^ꢂC, R_f¼0.78 (hex-

ane/ethyl acetate 4:1); ¹H NMR (250 MHz, CDCl₃):

¼0.57e0.68 (m,

10H, 2ꢃCH₃, 2ꢃCH₂), 1.06e1.11 (m, 4H, 2ꢃCH₂), 1.32 (d, J¼6.1 Hz,

6H, 2ꢃi-PrO [CH₃]), 2.01e2.06 (m, 4H, 2ꢃCH₂), 4.16 (s, 5H, Fc), 4.54

(s, 2H, Fc), 4.89e4.92 (m, 1H, i-PrO [CH]), 5.21 (s, 2H, Fc), 7.53e7.76

(m, 3H, Ar), 8.02 (s, 1H, Ar), 8.39 (s, 1H, Ar); ¹³C NMR (62.9 MHz,

acetate 4:1); ¹H NMR (250 MHz, CDCl₃):

¼0.54e0.69 (m, 10H,

2ꢃCH₃, 2ꢃCH₂), 1.04e1.13 (m, 4H, 2ꢃCH₂), 1.32 (d, J¼6.1 Hz, 6H,

2ꢃi-PrO [CH₃]), 2.01e2.09 (m, 4H, 2ꢃCH₂), 4.16 (s, 5H, Fc), 4.53 (s,

2H, Fc), 4.88e4.93 (m, 1H, i-PrO [CH]), 5.21 (s, 2H, Fc), 7.41e7.43 (m,

3H, Ar), 7.87e7.90 (m, 1H, Ar), 8.03 (s, 1H, Ar), 8.42 (s, 1H, Ar); ¹³C

CDCl₃):

d¼13.71 (2ꢃCH₃), 22.90 (2ꢃCH3, i-PrO [CH₃]), 22.90

(2ꢃCH₂), 25.97 (2ꢃCH₂), 39.85 (2ꢃCH₂), 56.20 (C_quat), 70.21 (5ꢃCH,

Fc), 70.44 (2ꢃCH, Fc), 72.98 (2ꢃCH, Fc), 74.89 (C_quat, Fc), 76.42 (CH,

i-PrO), 117.89 (CH, Ar), 120.25 (CH, Ar), 122.53 (CH, Ar), 123.38

(C_quat), 126.48 (CH, Ar),130.74 (CH, Ar),130.82 (C_quat), 132.98 (C_quat),

136.04 (C_quat), 138.41 (C_quat), 145.38 (C_quat), 153.83 (C_quat), 154.97

(C_quat), 155.60 (C_quat), 181.20 (C_quat; C]O), 184.83 (C_quat; C]O) [The

CH carbon peak of i-PrO-group sits under multiplets of CDCl₃]; IR

NMR (62.9 MHz, CDCl₃):

d¼13.83 (2ꢃCH₃), 22.96 (2ꢃCH3, i-PrO

[CH₃]), 22.96 (2ꢃCH₂), 26.00 (2ꢃCH₂), 39.94 (2ꢃCH₂), 55.88 (C_quat),

70.05 (5ꢃCH, Fc), 70.23 (2ꢃCH, Fc), 72.84 (2ꢃCH, Fc), 74.83 (C_quat

Fc), 76.42 (CH, i-PrO), 117.80 (CH, Ar), 120.21 (CH, Ar), 121.28 (CH,

Ar), 123.12 (CH, Ar), 127.41 (CH, Ar), 129.06 (CH, Ar), 130.52 (C_quat),

132.85 (C_quat), 135.92 (C_quat), 139.40 (C_quat), 146.61 (C_quat), 151.72

(C_quat), 155.03 (C_quat), 156.07 (C_quat), 181.40 (C_quat; C]O), 185.08

(ATR):

¼2955 (w), 2926 (w), 2857 (w), 1652 (s), 1600 (s), 1549 (m),

(C_quat; C]O); IR (ATR):

¼2957 (w), 2927 (w), 2857 (w), 1654 (s),

1443 (m), 1407 (w), 1313 (vs), 1289 (vs), 1270 (m), 1199 (m), 1165

(m), 1098 (s), 1016 (vs), 898 (m), 819 (s), 730 (s); HRMS [TOF MS

ES^þ]: m/z [M]^þcalcd. For C₃₈H₃₉BrFeO₃678.1432, found 678.1426

(ꢁ0.9 ppm).

1602 (m), 1561 (m), 1448 (w), 1381(w), 1365 (w), 1320 (s), 1300 (s),

1284 (s), 1234 (w), 1244 (w), 1202 (m), 1184 (w), 1166 (w), 1097 (s),

1085 (s), 1054 (w), 1016 (s), 937 (w), 906 (w), 812 (m), 743 (s); MS

(APCI) m/z (%): 601 (100) [M^þþ1], 559 (40), 551 (12), 494 (10), 393

(6), 305 (5), 281 (6); HRMS [TOF MS ES^þ]: m/z [M]^þcalcd. For

C₃₈H₄₀FeO₃600.2327, found 600.2322 (ꢁ0.8 ppm).

4.2.12. 6,6⁰-Bis(3-ferrocenyl-2-isopropoxynaphthalene-1,4-dione)

(10k). According to general procedure, to a solution of 4,4⁰-dibro-

mobiphenyl (0.48 g, 1.54 mmol, 1.0 equiv) in THF (20 mL) at ꢁ78 ^ꢂC

under nitrogen, n-BuLi (1.93 mL of a 1.6 M of hexane solution,

3.08 mmol, 2 equiv) was added. The resulting mixture was stirred

4.2.10. 12,12-Diethyl-7,8-diferrocenyl-2,9-diisopropoxy-12H-dibenzo

[b,h]ﬂuorene-1,4,7,10-tetrone (10i). According to general procedure,

1420

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

at ꢁ78 ^ꢂC for 1 h and then transferred to a solution of 3-ferrocenyl-

4-isopropoxy-3-cyclobutene-1,2-dione (6, 1.00 g, 3.08 mmol,

2.0 equiv) in THF (20 mL) at ꢁ78 ^ꢂC under a nitrogen atmosphere.

After stirring 3 h at ꢁ78 ^ꢂC and work-up, the crude alcohol 8k was

heated at reﬂux in p-xylene (30 mL) for 4 h. The crude product was

obtained as described in the general procedure and then subjected

to puriﬁcation by chromatography on silica gel using 4:1 hexane/

ethyl acetate as eluent to yield 10k (0.54 g, 44%, green solid). Mp

117e119 ^ꢂC, R_f¼0.56 (hexane/ethyl acetate 4:1); ¹H NMR (250 MHz,

the reaction mixture was quenched with 10% NH₄Cl (20 mL) solu-

tion at ꢁ78 ^ꢂC and then allowed to warm to rt. The mixture diluted

with ether (100 mL) and the organic layer was separated. The

aqueous layer was extracted with ether (2ꢃ50 mL). The combined

organic layers were dried over Na₂SO₄and ﬁltered. The solution

concentrated under reduced pressure and the remaining crude

material was dissolved in p-xylene (30 mL). The resulting solution

was heated at reﬂux open to the air in a preheated oil bath (165 ^ꢂC)

for 4 h and then stirred at rt overnight. After removal of the p-xy-

lene under reduced pressure, the residue was dissolved in ether

(50 mL) and silica gel (3.00 g) was added into the solution. The

solution was concentrated under reduced pressure and the green

solid residue was subjected to puriﬁcation by chromatography on

silica gel using 4:1 hexane/ethyl acetate as eluent to yield 2,3-dii-

sopropoxy-1,4-naphthaquinone (0.83 g, 40%, yellow oil). For the

spectroscopic data of 2,3-diisopropoxy-1,4-naphthaquinone see

Ref. 32.

CDCl₃):

1.32 (d, J¼6.1 Hz, 6H, 2ꢃi-PrO [CH₃]), 4.16 (s, 10H, 2ꢃFc

[5H]), 4.57 (s, 4H, 2ꢃFc [2H]), 4.92e4.97 (m, 2H, 2ꢃi-PrO [CH]), 5.24

(s, 4H, 2ꢃFc [2H]), 8.02e8.22 (AB system, d_A¼8.20, d_B¼8.04,

J_AB¼7.9 Hz, 2H, Ar), 8.47 (s, 2H, 2ꢃAr); ¹³C NMR (62.9 MHz, CDCl₃):

¼22.88 (4ꢃCH₃, i-PrO), 70.12 (10ꢃCH, Fc), 70.52 (4ꢃCH, Fc), 72.90

(4ꢃCH, Fc), 74.53 (2ꢃC_quat, Fc), 76.45 (2ꢃCH, i-PrO), 125.28 (2ꢃCH,

Ar), 126.82 (2ꢃCH, Ar), 131.25 (2ꢃC_quat), 131.56 (2ꢃC_quat), 133.48

(2ꢃC_quat), 136.60 (2ꢃC_quat), 144.13 (2ꢃC_quat), 154.84 (2ꢃC_quat),

180.51 (2ꢃC_quat; C]O), 184.31 (2ꢃC_quat; C]O); IR (ATR): ¼2972

(w), 2928 (w),1758 (w),1649 (s), 1599 (m), 1576 (m),1542 (m),1448

(w),1383 (m),1343 (m),1285 (s),1268 (s),1198 (s),1103 (s),1087 (s),

1041 (s), 916 (m), 805 (s), 778 (m), 738 (s); MS (APCI) m/z (%): 799

(100) [M^þþ1], 757 (12), 619 (10); HRMS [TOF MS ES^þ]: m/z [M]^þ

calcd. For C₄₆H₃₈Fe₂O₆798.1367, found 798.1396 (3.6 ppm).

Acknowledgements

This work was supported by The Scientiﬁc and Technical Re-

search Council of Turkey (TBAG-107T811 and 109T025), the Re-

search Board of Istanbul Technical University (BAP-32464). B.Y. and

I.Y are also indebted to the FABED Foundation for ﬁnancial support

of this work.

4.3. Compound (13)

In a screw-cap Pyrex bottle, to a dioxane solution (5.0 mL) of

ferrocenyl naphthaquinone 10j (0.20 g, 0.29 mol) and 2,7-bis

(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dibutylﬂuorene

(12, 0.71 mg, 0.13 mmol) were added PdCl₂(dppf) (22.71 mg,

0.028 mmol), tetrabutylammonium bromide (TBAB) (26.91 mg,

0.08 mmol), and 2 M Na₂CO₃(0.5 mL) solution. The mixture was

stirred at 100 ^ꢂC for 19 h in a preheated oil bath. After cooling to rt,

the reaction mixture was taken up in diethyl ether (20 mL). The

solution was washed with water (2ꢃ20 mL), the aqueous phase was

extracted with diethyl ether (2ꢃ20 mL), and the combined organic

phases were dried (NaSO4). After removal of the solvent in a rota-

tory evaporator, the residue was subjected to puriﬁcation by

chromatography on silica gel (100 g, 3ꢃ30 cm, hexane/ethyl acetate

20:1) to yield 13 (0.16 g, 83%, green solid). ¹H NMR (250 MHz,

Supplementary data

¹H and ¹³C NMR spectra of the ferrocenyl naphthaquinones

10aek, 11g, 13, and 4-hydroxycyclobutenone 8a. Plot of Reichardt’s

overall solvation scale against band wavelength for the lowest en-

ergy transition of 10e in a variety of solvents. Cyclic voltammo-

grams of 10a at different scan rates. UVevis spectral changes of 10e

in the solution of CH₃CN upon addition of 40% HClO₄and after

addition of excess SiO₂powder. Cyclic voltammograms of 10c

recorded during repeating scans after addition of excess TfOH into

the CH₃CN solution. Supplementary data associated with this ar-

ticle can be found in the online version at, doi:10.1016/

j.tet.2010.12.057.

CDCl₃):

¼0.68e0.88 (m, 30H, 6ꢃCH₃, 6ꢃCH₂), 1.10e1.25 (m, 12H,

References and notes

6ꢃCH₂), 1.34 (d, J¼6.1 Hz, 12H, 4ꢃi-PrO [CH₃]), 2.11e2.17 (m, 12H,

6ꢃCH₂), 4.18 (s, 10H, Fc), 4.55 (s, 4H, Fc), 4.94 (p, J¼6.1 Hz, 2H, 2ꢃi-

PrO [CH]), 5.23 (s, 4H, Fc), 7.69e7.88 (m, 10H, Ar), 7.97e8.01 (m, 2H,

Ar), 8.07 (s, 2H, Ar), 8.46 (s, 2H, Ar); ¹³C NMR (100.59 MHz, CDCl₃):

1. (a) Sutcliffe, O. B.; Bryce, M. R. Tetrahedron: Asymmetry 2003, 14, 2297e2325;

(b) Colacot, T. J. Chem. Rev. 2003, 103, 3101e3118; (c) Arrayas, R. G.; Adrio, J.;

Carretero, J. C. Angew. Chem., Int. Ed. 2006, 45, 7674e7715; (d) Blaser, H.-U.;

ꢀ

Chen, W.; Camponovo, F.; Togni, A. In Ferrocenes: Ligands, Materials and Bio-

d¼13.74 (2ꢃCH₃), 13.80 (CH₃), 22.96 (2ꢃCH₂, 2ꢃCH₃i-PrO [CH₃]),

ꢁ

molecules; Stepnicka, P., Ed.; John Wiley: Chichester, UK, 2008; pp 205e235;

ꢁ

(e) Stepnicka, P.; Lamac, M. In Ferrocenes: Ligands, Materials and Biomolecules;

Stepnicka, P., Ed.; John Wiley: Chichester, UK, 2008; pp 237e277.

23.05 (CH₂), 26.08 (2ꢃCH₂), 26.11 (CH₂), 39.95 (2ꢃCH₂), 40.17

(CH₂), 55.34 (C_quat), 56.00 (C_quat), 70.38 (5ꢃCH, Fc), 70.56 (2ꢃCH,

Fc), 73.12 (2ꢃCH, Fc), 76.43 (CH, i-PrO), 77.21 (C_quat, Fc), 117.78 (CH,

Ar), 120.17 (CH, Ar), 120.43 (CH, Ar), 121.49 (CH, Ar), 121.52 (CH, Ar),

121.55 (CH, Ar), 126.29 (CH, Ar), 126.71 (CH, Ar), 130.41 (C_quat),

132.93 (C_quat), 135.92 (C_quat), 138.55 (C_quat), 140.01 (C_quat), 140.33

(C_quat), 142.43 (C_quat), 146.25 (C_quat), 151.93 (C_quat), 152.54 (C_quat),

ꢁ

2. (a) van Staveren, D. R.; Metzler-Nolte, N. Chem. Rev. 2004, 104, 5931e5985; (b)

Kraatz, H.-B. J. Inorg. Organomet. Polym. Mater. 2005,15, 83e106; (c) Metzler-Nolte,

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ꢁ

ꢂ

156.02 (C_quat), 156.26 (C_quat), 181.32 (C_quat; C]O), 185.05 (C_quat

C]O); IR (ATR):

(vs),1552 (m),1463 (m),1380 (w),1319 (vs),1292 (vs),1203 (s),1166

(m), 1100 (s), 1083 (m), 1017 (vs), 898 (m), 816 (vs), 743 (s); MS

(APCI) m/z (%): 1476 (100) [M^þþ1], 1434 (18), 1224 (10), 391 (18),

278 (31).

;

ꢁ

¼2956 (m), 2929 (s), 2859 (m), 1655 (vs), 1599

ꢁ

Materials and Biomolecules; Stepnicka, P., Ed.; John Wiley: Chichester, UK, 2008;

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4.4. 2,3-Diisopropoxy-1,4-naphthaquinone (14)

To a solution of 3,4-diisopropoxycyclobut-3-ene-1,2-dione^30b

(1.50 g, 7.57 mmol, 1.0 equiv) in THF (30 mL) at ꢁ78 ^ꢂC under a ni-

trogen atmosphere, PhLi (6.30 mL of a 1.8 M of dibutylether solu-

tion, 11.34 mmol, 1.5 equiv) was added. After stirring 3 h at ꢁ78 ^ꢂC,

ꢁ

B. Yucel et al. / Tetrahedron 67 (2011) 1406e1421

1421

ꢀ

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