Selenophenes

3.13 Selenophenes E. T. Pelkey Hobart and William Smith Colleges, Geneva, NY, USA ª 2008 Elsevier Ltd. All rights reserved. 3.13.1

Introduction

976

3.13.2

Theoretical Methods

976

3.13.3

Experimental Structural Methods

977

3.13.3.1 Molecular Structure

977

3.13.3.2 Molecular Spectroscopy

978

3.13.3.2.1 3.13.3.2.2 3.13.3.2.3 3.13.3.2.4 3.13.3.2.5

1

H NMR spectroscopy C NMR spectroscopy 77 Se NMR spectroscopy Ultraviolet, infrared, Raman, and photoelectron spectroscopy Mass spectrometry 13

978 978 978 978 979

3.13.4

Thermodynamic Aspects

979

3.13.5

Reactivity of Fully Conjugated Rings

980

3.13.5.1 Thermal and Photochemical Reactions

980

3.13.5.2 Substitutions at Selenium

980

3.13.5.3 Electrophilic Substitutions

981

3.13.5.4 Reactions with C-Anion Equivalents

982

3.13.5.5 Organometallic Reactions

982

3.13.5.6 Reactions of p-Metal Complexes

984

3.13.5.7 Nucleophilic Substitutions

984

3.13.5.8 Radical Reactions

985

3.13.5.9 Pericyclic Reactions

985

3.13.5.10

985

C–Se Bond Cleavage

3.13.6

Reactivity of Nonconjugated Rings

985

3.13.7

Reactivity of Substituents Attached to Ring Carbon Atoms

986

3.13.8

Reactivity of Substituents Attached to Selenium

987

3.13.9

Ring Synthesis

987

3.13.9.1 Formation of One Bond 3.13.9.1.1 3.13.9.1.2 3.13.9.1.3

988

Category Ia cyclizations Category Ib cyclizations Category Ic cyclizations

988 988 989

3.13.9.2 Formation of Two Bonds

989

3.13.9.2.1 3.13.9.2.2 3.13.9.2.3 3.13.9.2.4 3.13.9.2.5 3.13.9.2.6 3.13.9.2.7

Category IIab cyclizations Category IIac cyclizations Category IIad cyclizations Category IIae cyclizations Category IIbc cyclizations Category IIbd cyclizations Category IIbe cyclizations

989 989 990 990 993 993 993

3.13.9.3 Formation of Three Bonds

993

3.13.9.4 Synthesis of Nonconjugated Rings

994

3.13.10

995

Ring Synthesis from Another Ring

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Selenophenes

3.13.11

Selected Syntheses

996

3.13.12

Important Compounds and Applications

996

3.13.12.1

Compounds of Biological Interest

996

3.13.12.2

Compounds with Applications in Materials Science

997

3.13.13

Further Developments

References

1000 1001

3.13.1 Introduction This chapter serves as a continuation of Chapters 3.16 <1984CHEC(3)935>, 3.17 <1984CHEC(3)973>, and 2.13 <1996CHEC-II(2)731> that appeared in CHEC(1984) and CHEC-II(1996) detailing the synthesis, chemistry, and applications of selenophenes and their fused derivatives. The first two chapters covered the literature through 1982, whereas the latter covered the literature through 1993. Given the coverage of CHEC-II(1996), the present chapter herein focuses on the literature reported from 1994 through the end of 2006. During this time frame, five significant monographs have appeared that detail the synthesis and reactions of selenophenes 1 <2001SOS(9)423, 2005MI375>, benzo[b]selenophenes 2 <2001SOS(10)265>, benzo[c]selenophenes 3 <2001SOS(10)301>, and dibenzoselenophenes 4 <2001SOS(10)307>. In addition, Gronowitz has published an account that describes the work accomplished by his research group on selenophenes during a 30þ year span <1998PS59>. A periodical treatment of selenophenes (along with thiophenes and tellurophenes) has regularly appeared in Progress in Heterocyclic Chemistry <1994PHC(6)88, 1995PHC(7)82, 1996PHC(8)82, 1997PHC(9)77, 1998PHC(10)87, 1999PHC(11)102, 2000PHC(12)92, 2001PHC(13)87, 2002PHC(14)90, 2003PHC(15)116, 2004PHC(16)84, 2005PHC(17)98, 2007PHC(18)126>. All other specialized review articles are cited in the appropriate section. Finally, in terms of the organization of this chapter, selenophenes and fused derivatives will be treated together in each section.

3.13.2 Theoretical Methods Various ab inito methods have been applied to study and compare the physical properties of selenophenes with the other chalcogen homologs. These properties include linear and nonlinear polarizability, hyperpolarizability, aromaticity, molecular parameters, bond geometries, the Cotton–Mouton effect, and the Wellington elimination, with hyperpolarizability being the most studied. Calculations using the electron core potential (ECP) method showed that the polarizabilities and secondary hyperpolarizabilities of chalcogenophenes systematically increased as the heteroatom was replaced with heavy atoms, whereas the first hyperpolarizability did not <2000PCA4723>. This study also found a discrepancy between the experimental and theoretical secondary hyperpolarizabilities as the heteroatom became larger. Calculations using 6-31Gþpdd basis sets determined that the large discrepancy between theoretical and experimental second hyperpolarizabilities of the heavy chalcogenophenes could not be explained by frequency dispersion <2000SM(115)185>. Additional studies using various Hartree–Fock methods concluded that relativistic effects were not sufficient to explain the discrepancy between experimental and theoretical nonlinear polarizabilities for selenophene and tellurophene <2003JST(633)237>. Similar conclusions were obtained using time-dependent density functional theory <2002PCA10380>. Dispersion effects on hyperpolarizabilities were also studied by Millefori using the B3LYP basis set <2000CPL(332)175> and the 6-31G* , the Sadlej POL (polarized), correlation consistent Dunning and aug-CEP-31G basis sets <2000PCP2495>. In contrast, the linear and quadratic dipole polarizabilities calculated with conventional ab initio and density functional theory methods were found to increase from furan to tellurophene, and agreed well with experimental data <1998JST(431)59>. The pair density description of aromaticity of the chalcogenophenes was calculated using the atoms-in-molecules (AIM) and electron localization function (ELF) methods, with both methods yielding equal results for the formally single C–C single bond but differing for almost all other bonds <2000CPH(257)175>. The molecular parameters of the

Selenophenes

chalcogenophenes were calculated using the B3LYP basis set; the calculated selenophene bond angles agreed well with experimental data <1997JST(436)451>. Using the B3LYP/6-31G methods, the relative stability of the onium states of the chalcogens were calculated to explain the relative reactivity and positional selectivity of the chalcogenophenes and their corresponding benzannelated derivatives <2003CHE36, 2005RCB853>. Other properties that were studied for the chalcogenophenes include the Cotton–Mouton effect and the Wellington elimination <2003JCP10712, 2005CPL(416)113>. The properties of fused five-membered chalcogenophenes (e.g., selenoselenophenes) were also calculated. Using the BLYP/DZVP basis set, the selenolo[3,2-b]selenophene was found to be the most stable among the various selenoselenophene isomers while the selenolo[3,4-c]selenophene was the most stable <1997JST(398)315>. Ab initio calculations of selenophene in comparison to other heterocycles were also reported. The bandgap energies of p-conjugated polymers were calculated at the B3LYP/6-31G(d) level. In these calculations, the predicted values agreed well with experimental data, confirming that polyselenophene has the lowest bandgap energy among the unsubstituted polychalcogenophenes studied <2006OL5243>. The vibrational frequencies were calculated for selenophene, tellurophene, and the corresponding 1,2,5-seleno- and 1,2,5-tellurodiazoles using the 6-31G(p) basis set <2001JST(572)81>. Additional ab initio calculations reported on the properties of selenophenes exclusively. The molecular structure and conformational behavior of selenophene, bi-, ter-, and quarterselenophene were reported using the 6-31G* level of theory. These calculations showed that the conformational behavior of oligoselenophenes was dependent on torsional potentials between adjacent rings and predicted that polyselenophenes would likely adopt a planar conformation <1998SM(95)217>. The theoretical 77Se chemical shifts of selenium compounds were calculated at the GIAO-MP2 and GIAO-SCF levels while 13C chemical shifts of selenium compounds were calculated using the 6-311þþG* level of theory <1995JPC4000, 2002JST(616)17, 2005MI1119>. The mass fragmentation of selenoketene and selenoketyl cumulene ions was studied at the B3LYP/6–31G(d,p) and G2/G2(MP2) levels while the Renner–Teller effect of the selenoketyl radical was studied at the B3LYP level <2004JCP5801, 2005JMP796>. In addition to the many ab initio calculations, there was one report using semi-empirical calculations to study the relative geometry and charge distribution of selenophene and its azido derivatives <1994J(P2)1815>.

3.13.3 Experimental Structural Methods 3.13.3.1 Molecular Structure A review of X-ray diffraction studies of selenophene compounds and transition metal selenophene complexes has been published <2002CHE763>. X-Ray structures have also been reported for the following compounds: tetraphenylselenophene 1,1-dioxide 5 <1996CL269>, tris(selenophen-2-l)stibine 6 <2005JOM(690)3286>, benzo[c]selenophene 7 <2003OL2519>, dibenzoselenophene 8 <1995BCJ744>, fused benzoselenopheno[3,2-b][1]benzoselenophene 9 <2006JA3044>, selenophene-fused tetrathiafulvalene 10 <1995AM644>, and tetraselenafulvalenes 11 <2005PS873>.

The structures of various macrocyclic selenophenes, including selenaporphyrins, have been determined. Examples include a monoselenaporphyrin featuring an inverted pyrrole ring 12 <2000JOC8188>, a tetraselenaporphyrin in which all four heterocycles are selenophenes 13 <1997AGE2609>, a sapphyrin containing two selenophene rings 14 <2000J(P2)1788>, and an octaphyrin containing four selenophene rings <2003CEJ2282>.

977

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Selenophenes

Efforts to understand the conducting properties of the oligomeric selenophene compound quaterselenophene focused on X-ray crystallographic analysis, which revealed a unit lattice of two crystallographically independent molecules in a planar conformation with herringbone packing <1999SM(101)639>. The structure of a bis-selenoselenophene was also reported <1996T471>. X-ray absorption fine structure (XAFS) and extended X-ray absorption fine structure (EXAFS) have been used to study catalyst structure in hydrodesulfurization and hydrodeselenization reactions. Using Se K-edge XAFS, the exchange of S–Se during the hydrode–selenium reaction of selenophene was investigated over Mo sulfide clusters <2003CPL(370)813>. The hydrodeselenization of selenophene over sulfided (Co)Mo/Al2O3 catalysts was investigated using EXAFS <1997PCB11160>.

3.13.3.2 Molecular Spectroscopy 3.13.3.2.1

1

H NMR spectroscopy

1

H and 2D NMR (nuclear magnetic resonance) studies of an octaphyrin with two selenophene rings indicated a C2 symmetric conformation <2007CC43>. Sapphyrins containing two thiophene rings were found to have inverted rings at 50  C, whereas the corresponding selenophene compound did not <1999JA9053>. For rhodium rubyrin complexes containing selenophene units, the rhodium was found to bind in an 2 fashion to the bipyrrole nitrogens, as determined by 1H and 2D NMR studies and additional spectroscopic work <2001IC1637>. Dynamic NMR studies were used to examine the rotational barriers of chromium cobalt selenophene complexes <1994OM1821>.

3.13.3.2.2

13

C NMR spectroscopy

Computational 13C NMR studies using the 6-311þþG* level of theory found that the theoretical and experimental chemical shifts agreed well <2002JST(616)17>.

3.13.3.2.3

77

Se NMR spectroscopy

A comprehensive review of 77Se NMR spectroscopy that includes data for inorganic, organic, and metal complexes of selenium compounds has been published <1995MI1>. Substituent effects on the 77Se NMR shifts in selenophenes were also reviewed <1998PS59>. The 77Se NMR shifts for numerous organoselenium compounds have been measured using indirect detection methods (e.g., HSQC), an approach that could prove useful for the spectroscopic studies of selenoproteins <1995MRC191>. For transition metal complexes with an 5-selenopehene ligand, the 77Se NMR shifts were influenced by the metal and its ligands, the charge on the complex, and the number of methyl groups on the selenophene <1994OM1821, 1995OM332>. A 77Se NMR study compared free selenophenes with their ruthenium complexes and established characteristic 77Se shift ranges for 1, 2, and 5 modes of selenophene binding to transition metals <1994OM4474>. The C–Se insertion product of selenophene with Cp* Rh(PMe3)PhH displayed an upfield resonance consistent with a nonaromatic metallaselenabenzene ring <1997OM2751>. Finally, the theoretical 77Se chemical shifts of selenium compounds have been calculated at the GIAO-MP2 and GIAO-SCF levels of theory <1995JPC4000, 2005MI1119>.

3.13.3.2.4

Ultraviolet, infrared, Raman, and photoelectron spectroscopy

The ultraviolet (UV) spectroscopic data for several nonmacrocyclic selenophene compounds were reported. The photolysis products from laser irradiation of gaseous selenophene were examined using UV spectroscopy and identified as C2H2 and H2CTCH-CCH <2000JOC2759>. The UV data of donor–acceptor 2-aminoselenophenes

Selenophenes

demonstrated absorption maxima that were strongly dependent on the substitution pattern of the selenophene moiety <2002ZNB420>. Finally, studies on the unimolecular and bimolecular photoprocesses of a fused selenophene compound were reported <2001SAA1427>. The visible/electronic spectra for various selenaporphyrins have been reported. The electronic spectrum of a selenaporphyrin was found to closely resemble that of its thiaporphyrin analog, displaying a Soret band in the nearUV region and four Q bands <1996IC566>. Other selenaporphyrins for which electronic spectra were reported include octaphyrins <2003CEJ2282, 2007CC43>, sapphyrins <1999JA9053, 1999J(P2)961>, rubyrins <2001IC1637, 1999T6671>, and a monoselenaporphyrin featuring an inverted pyrrole ring <2000JOC8188>. The electronic absorption maxima of a series of oligoselenophenes systematically changed depending on the chain length of the oligomers <1997SM(84)341>. The absorption spectra of several biselenophene dyes were measured to assess their suitability for determining solvent polarity <2000AGE556>. The long-wave absorption bands of (2Z)-2(N-acetyl-N-arylaminomethylene)benzo[b]selenophene and the corresponding furan, thiophene, and tellurophene derivatives were also measured <2005ARK60>. Femtosecond optical heterodyn-detected optical Kerr effect spectroscopy and low-frequency Raman spectroscopy were used to study the molecular dynamics of selenophene <1998JCP10948>. Femtosecond Kerr effect spectroscopy was also used to examine the third-order polarizabilities of furan, thiophene, and selenophene, which was found to increase from furan to thiophene to selenophene <1996CPL(263)215>. The photoelectron spectra of bichalcogenophenes that contain a selenophene were measured, with the data indicating various interactions among the furan, thiophene, and selenophene p-orbitals <1994JPC5240>. The charge-transfer complexes of tetracyanoethylene with selenium donors were also studied using photoelectron spectra <1995JOC2891>.

3.13.3.2.5

Mass spectrometry

A combination of mass spectrometry studies and ab initio calculations was used to identify the products of dissociative electron ionization of selenophene <2005JMP796>. Cobalt–chalcogenophene ion complexes were generated in a mass spectrometer and subsequently irradiated to study their photodissociation thresholds and measure their bond energies <1997JMP475>.

3.13.4 Thermodynamic Aspects Several reviews have been reported on the aromaticity of selenophenes. Using conceptual and computational density functional theory (DFT), aromaticity has been discussed using energetic, structural or geometrical, magnetic, and reactivity-based measurements <2001CRV1451>. The HOMO–lowest unoccupied molecular orbital (LUMO) gap approximates hardness and measures stability. The highest occupied molecular orbital (HOMO) and LUMO energies of selenophene were calculated to be 8.15 and 12.28 eV, respectively. Aromaticity indices have been proposed based on polarizability; the polarizability exaltation of selenophene was calculated to be 25.06 au. Additionally, the aromatic stabilization energy of selenophene was calculated at 17.05 kcal mol1, which is less than thiophene (18.74 kcal mol1) but greater than furan (15.47 kcal mol1). This is consistent with other studies on the aromaticity of five-membered heterocycles, where aromaticity decreases as follows: thiophene > selenophene > pyrrole > tellurophene > furan <2004CRV2777>. The aromaticity of fused selenophenes has also been discussed using nuclear-independent chemical shifts (NICSs), a magnetic criterion that is a widely used aromaticity probe <2005CRV3842>. Using the BLYP/DZVP basis set, the seleno[3,4-c]selenophene was calculated to be the most aromatic among the various selenoselenophene isomers; however, this isomer was also the least stable <1997JST(398)315>. The pair density description of aromaticity of the chalcogenophenes was calculated using the atoms-in-molecules (AIMs) and electron localization function (ELF) methods, with both methods yielding equal results for the formally single C–C single bond but differing for almost all other bonds <2000CPH(257)175>. The acidities of several rhenium carbene complexes that represent derivatives of furan, thiophene, and selenophene were investigated and found to depend on aromaticity <2003JA12328>. The physiochemical properties of numerous selenophene compounds have been investigated by means of cyclic voltammetry. These compounds include benzodiselenophenes <2005JOC10569>, conjugated biselenieno-quinones <1996H(43)941, 1996JOC4784>, seleno-substituted tetrathiafulvalenes <1997H(45)1051>, seleno-fused tetrathiafulvalenes <1997CL1091>, ethylenedioxyselenophene <2001OL4283>, benzo[c]selenophene dimers

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Selenophenes

<2004OL3039>, and diarylbenzo[c]selenophenes <2005TL7201>. The reversible redox reactions of ferrocenesubstituted benzo[c]selenophenes 15 were also studied and established a new type of multistep reversible redox system <2006TL2887, 2007JOM(692)60>.

Rubyrins containing two selenophene units led to a reduced cavity size, easier oxidations and reductions potentials, and a reduction in the HOMO–LUMO energy gap <1999T6671>. The cyclic voltammagrams of a series of oligoselenophenes systematically changed depending on the chain length of the oligomers <1997SM(84)341>. The electrochemical behavior of polyselenophene films <2005JEC345>, a bis(seleninyl)ethene polymer <1998MM1221>, and a polyselenienyl thiophene polymer <1996SM(82)111> have been reported.

3.13.5 Reactivity of Fully Conjugated Rings 3.13.5.1 Thermal and Photochemical Reactions Photochemical reactions of selenophene–metal complexes (Section 3.13.5.6) and photocycloadditions of selenophenes (Section 3.13.5.9) are treated later in this subsection. The photolysis of gaseous selenophene (and also tellurophene) has been studied <2000JOC2759>. Interestingly, the cleavage of both Se–C carbons gave elemental Se in the process. Selenophene 1,1-dioxides have been found to be more thermally labile than the corresponding thiophene 1,1dioxides. While dimerization is often observed in the thermal decomposition of the latter, the neat thermolysis of selenophene 1,1-dioxides leads primarily to ring-opened products. For example, thermolysis of 2,3,4,5-tetraphenylselenophene 1,1-dioxide 5 gave a variety of a ring-opened products (Equation 1) <1998H(48)61>, whereas thermolysis of compound 5 in toluene leads to the formation of 2,3,4,5-tetraphenylfuran.

ð1Þ

3.13.5.2 Substitutions at Selenium The synthesis and chemistry of selenophene 1-oxides, selenophene 1,1-dioxides, and fused derivatives has been investigated in some detail by the Nakayama group and this work has been reviewed <1999TCC131, 2000BCJ1>. The preparation of benzo[b]selenophene-1-oxide 16 was accomplished by oxidation of benzo[b]selenophene with m-chloroperbenzoic acid (MCPBA), whereas application of the same conditions to tetraarylselenophenes led to ringopened products <1995CL485>. Alternatively, utilizing dimethyldioxirane (DMD) as an oxidant allowed for the preparation of 5, 16, benzo[b]selenophene-1,1-dioxide 17, and 2,4-di-tert-butylselenophene-1,1-dioxide 18 <1996CL269, 1996PS227>. The structure of compound 5 was confirmed by X-ray single-crystal structure analysis. Subsequently, the mono-oxidation of 2,4-disubstituted selenophene 1-oxides was reported. Notably, selenophene-1oxides (e.g., 2,4-di-tert-butylselenophene-1-oxide 19) are easier to isolate than the corresponding thiophene-1-oxides given their lower relative reactivity, which can be explained by weaker aromaticity (selenophenes < thiophenes) and lower electronegativity (Se < S) <1998JA12351>.

Selenophenes

The bromination of dihydrobenzo[c]selenophene 20 led to the formation of the corresponding 1,1-dibromo derivative 21 (Scheme 1) <2003OL2519>. Treatment of the latter with lithium hexamethyldisilazane (LiHMDS) produced benzo[c]selenophene 3 which was subsequently converted to the stable diester 23 via dilithiation. The structure of 23 was confirmed by X-ray crystallographic analysis.

Scheme 1

3.13.5.3 Electrophilic Substitutions Similar to its chalcogenic congeners and other p-excessive five-membered ring heterocycles, the electrophilic substitution of selenophenes proceeds regioselectively at free -positions. Computational methods were utilized to investigate the regioselectivity ( :  ratio) of electrophilic substitution reactions of five-membered ring heterocycles including selenophenes <2003CHE36, 2005RCB853>. The relative rates were correlated to the relative stability of the onium ions (Seþ). N-Bromosuccinimide (NBS) is the reagent of choice for the synthesis of -bromoselenophenes and other p-excessive heterocycles <1996T471, 2005TL2647, 2006JOC3786>. Recent examples include the monobromination and dibromination of selenolo[3,2-b]selenophenes <1996T471> and the dibromination of diselenophen-29-yl-2,1,3benzothiadiazole <2005MM244>. The regioselectivity of the bromination of 2-acylselenophenes with bromine in the presence of aluminium trichloride was investigated <1995JHC53>. As expected, the major products obtained were the corresponding 4-bromo-2-acylselenophenes. Vilsmeier–Haack formylation of selenophene with N-phenylN-methylformamide produced selenophene-2-carboxaldehyde which was converted into 1,2-bis(2-selenophen-29yl)ethene using a McMurry-type coupling <1998MM1221>. The latter was utilized as a building block for a novel selenophene polymer containing an ethenyl spacer. Reductive dehalogenation of perhaloselenophenes preferentially removes halogens from -positions. This method is useful for the preparation of -substituted selenophenes. This procedure was utilized to prepare 3-cyanoselenophene 26 (Scheme 2), useful in the preparation of selenophene analogs of tiazofurin, an antitumor agent

Scheme 2

981

982

Selenophenes

<1997JME1731>. Exhaustive iodination of selenophene 1 with iodine mediated by mercuric acetate gave tetraiodoselenophene 24. Reductive dehalogenation with zinc powder in acetic acid gave 3-iodoselenophene 25 which underwent a palladium-catalyzed (2 mol%) cyanation with trimethylsilyl cyanide producing 3-cyanoselenophene 26. Structure 26 was converted into ethyl selenophene-3-carboxylate which underwent Friedel–Crafts acylation reactions with -O-acetylribofuranoses to give the corresponding C-(selenophen-2-yl)glycoside antitumor agents.

3.13.5.4 Reactions with C-Anion Equivalents Treatment of selenophenes with alkyllithiums leads to selective -lithiation (e.g., 27). Lithiation of selenophene followed by quenching with iodine leads to the formation of -iodoselenophenes <1996H(43)1927, 2005TL2647, 2006JOC1552, 2006JOC3786>. The preparation of tris(selenophen-2-yl)stibine (antimony) 6 from 27 (R ¼ H) was reported <2005JOM(690)3286>. Selenophenethiols 29 have been prepared in two steps from 2-selenophenyllithiums 27 (Scheme 3) <1997CHE426>. Treatment of 27 with elemental sulfur and trimethylsilyl chloride gave the silyl thioether 28 which was converted into 29 by hydrolysis with 1 equiv of water. The reaction of 27 (R ¼ H) with 2-pyridyl 2-thienyl sulfoxide led to the formation of 2-(29-pyridyl)-selenophenes and the corresponding disulfides indicating a ligand exchange process was taking place <1994HAC223>. Treatment of -lithiated selenophenes with copper leads to the formation of 2,29-biselenophenes <1999JOC8693>, useful reagents for the preparation of selenophene-modified pyrrole macrocycles. As depicted earlier, the in situ formation and dilithiation of benzo[c]selenophene 3 provided dianion 22, which was trapped with ethyl chloroformate giving diester 23 (Scheme 1) <2003OL2519, 2005PS787>.

Scheme 3

3.13.5.5 Organometallic Reactions Organopalladium chemistry has increasingly been utilized for the preparation of highly functionalized selenophenes and the majority of this work has appeared during the past 10 years. The palladium-catalyzed cross-coupling of Grignard reagent 30 and bromo derivative 31 produced selenolo[3,2-b]selenophene dimer 32 (n ¼ 1) in good yield (Equation 2) <1996T471>. Similar chemistry was utilized to prepare trimer 32 (n ¼ 2) and tetramer 32 (n ¼ 3). The attempted synthesis of dimer 32 (n ¼ 1) using a nickel-catalyzed homocoupling reaction was very low yielding.

ð2Þ

High-yielding syntheses of biselenophenes and related chalcogenophenes were accomplished utilizing a homocoupling reaction <2006TL795>. Treatment of bromoselenophene 33 with hexabutylditin and palladium(0) led to the formation of biselenophene 34 in 92% yield (Equation 3).

ð3Þ

Selenophenes

Several examples of Suzuki [Ar–X þ Ar–B(OH)2], Stille [Ar–X þ Ar–SnR3], and Negishi [Ar–X þ Ar–ZnX] crosscoupling reactions involving selenophenes have been reported. The Suzuki reaction has received the most attention. Recently, Suzuki cross-coupling reactions of 2-haloselenophenes were examined in some detail in the context of preparing 2-arylselenophenes 35, 2,5-diarylselenophenes 36, and 2-arylselenophenyl ketones 37 <2006JOC3786>. Optimized conditions for the Suzuki cross-coupling of pentafluorophenylboronic acid 39 with thiophenes, thiophene oligomers, fused thiophenes, and selenophenes were revealed <2005S1589>. For example, treatment of 2,5-dibromoselenophene 38 with boronic acid 39 in the presence of silver(I) oxide, potassium phosphate, and palladium(0) led to the highly fluorinated 2,5-bisarylselenophene 40 (Equation 4). A double Suzuki cross-coupling of selenophene-2boronic acid with 2,5-dibromopyridine gave 2,5-bis(selenophen-2-yl)pyridine, a building block utilized for the preparation of conducting copolymers <1996CM2444>. A double Suzuki cross-coupling reaction was also utilized to prepare fluorene–biselenophene copolymers <2006MM4081>.

ð4Þ

Suzuki cross-coupling reactions of 3-halobenzo[b]chalcogenophenes were utilized to prepare tetracyclic chalcogenophenes <2006ZNB427>. In the event of the fused selenophene (Scheme 4), a Suzuki cross-coupling with 41 and boronic acid 42 gave dialdehyde 43. A McMurry cyclization of the latter gave fused selenophene 44. Suzuki and Sonogashira cross-coupling reactions of 3-iodobenzo[b]selenophenes 71 (see Equation (8), Section 3.13.9.1.1) were also recently reported <2006JOC2307>.

Scheme 4

Polymer starting materials have been prepared utilizing Stille cross-coupling reactions including 4-(selenophen-2yl)aniline <1998AM1525> and diselenophen-29-yl-2,1,3-benzothiadiazole <2005MM244>. Negishi cross-coupling reactions have also been utilized to prepare biselenophenes <1996H(43)941, 1996JOC4784>. A novel set of tris(oligoarylselenophenyl)amines were prepared utilizing a Negishi cross-coupling reaction <2004CL1266>. These compounds were investigated as novel amorphous molecular materials with interesting charge transfer properties. The synthesis of 3-arylbenzo[b]selenophenes was accomplished utilizing both Stille and Suzuki cross-coupling reactions <2000EJO1353, 2002ARK40>. Palladium cross-coupling reactions of -haloselenophenes are a useful tool for the preparation of selenophenecontaining copolymers <2005PSA823, 2005MM244>. Palladium chemistry is useful for linking selenophene and an activated carbon nucleophile (Equation 5). Treatment of 2-iodoselenophene with sodium malonitrile in the presence of a palladium(II) catalyst provided malonitrile derivative 46b formed by tautomerization of the initial adduct 46a <1996H(43)1927>.

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Selenophenes

ð5Þ

The Sonogashira cross-coupling reaction has been utilized to prepare a variety of alkynyl-substituted selenophene building blocks <2003OM3659>. A recent advance in alkynylselenophene synthesis (e.g., 47) involved a Sonogashira coupling that did not require an additive (co-catalyst free) <2006TL2179>.

A few reports involving copper-mediated substitution of selenophenes with heteroatoms (N, S) have appeared. The preparation of 2-amidoselenophenes (e.g., 48) was accomplished by treating 45 with amides in the presence of copper(I) iodide and ethylenediamine <2006JOC1552>. A similar copper-mediated reaction involving thiols was utilized to prepare sulfur-substituted selenophenes (e.g., 49) <2005TL2647>.

3.13.5.6 Reactions of p-Metal Complexes The synthesis and chemistry of an 2-selenophene osmium complex 50 has been studied <1999OM1559>. Protonation and electrophilic substitution with acetaldehyde diethyl acetal occurred at C-2. Methylation of complex 50 with methyl triflate gave 51 which upon treatment with tetrabutylammonium borohydride (TBAB) led to the selenophene ring-opened complex 52 (Scheme 5).

Scheme 5

3.13.5.7 Nucleophilic Substitutions A new solvent was investigated for the introduction of amine nucleophiles onto the selenophene nucleus via nucleophilic aromatic substitution. Treatment of 5-bromoselenophene-2-carboxaldehyde 53 with secondary amines in water produced 5-aminoselenophenes 54 (Equation 6) <1999T6511>.

ð6Þ

Selenophenes

3.13.5.8 Radical Reactions No articles regarding the radical chemistry of selenophenes were abstracted.

3.13.5.9 Pericyclic Reactions The synthesis and cycloaddition chemistry of selenolo[3,4-c]thiophenes 55 and telluro[3,4-c]thiophenes 56 was compared <2002OL1193>. The latter was claimed to be the first example of a tellurium-containing diheteropentalene.

A few reports of the [2þ2] photocycloaddition of selenophenes with various alkenes have been reported <1994M1153, 1997JPH53, 2001JPH1>. Cycloadducts that have been characterized include compounds 57 <1994M1153>, 58 <1997JPH53>, and 59 <2001JPH1>.

3.13.5.10 C–Se Bond Cleavage Removal of thiophene impurities from petroleum feedstocks is accomplished by a process called hydrodesulfurization (HDS) which involves the insertion of metals into the thiophene ring between the C–S bond. In order to better understand the mechanism of this reaction, different groups have utilized selenophene model systems due to the enhanced NMR characteristics of 77Se. Metal complexes of selenophenes that have been studied include rhodium <1997OM2751>, molybdenum <2006POL499>, manganese <2001OM3617, 1995OM332>, chromium <1994OM1821>, ruthium <1994OM4474>, and iridium <1995OM332>.

3.13.6 Reactivity of Nonconjugated Rings Only two reports involving the reactivity of nonconjugated selenophenes were uncovered. Both involved the preparation of selenoether carbohydrate derivatives. The preparation of selenonium inner salt 62 was accomplished by treatment of selenoether 60 with cyclic sulfate 61 (Equation 7) <2004CAR2205>. The salt 62 was utilized as a building block for the preparation of inhibitors of UDP-galactopyranose mutase.

985

986

Selenophenes

ð7Þ

Treatment of a selenoether-based carbohydrate derivative with ozone followed by acetic anhydride led to a mixture of Pummerer-type rearrangement products <2006JA227>.

3.13.7 Reactivity of Substituents Attached to Ring Carbon Atoms The preparation of various nitrogen-substituted selenophenes has been accomplished utilizing the Curtius rearrangement of carbonyl azides. Selenophene-2-carbonyl azide 64 was prepared by treating the corresponding carboxylic acid 63 with methyl chloroformate and trimethylsilyl azide <2000ARK58>. The thermal Curtius rearrangement of 64 in the presence of 1-methylpyrrole led to the formation of carboxamide 65 (Scheme 6). A modified Curtius rearrangement was employed in the preparation of BOC-protected 2-amino-3-iodoselenophene <1995T10323>. The latter was utilized to prepare the fused selenophene, selenolo[2,3-b]pyrrole. The mechanism of the Curtius rearrangement for a series of chalcogenophene-2-carbonyl azides has been studied by differential scanning calorimetry <2002ARK6>.

Scheme 6

A series of synthetic methods have been adapted to the preparation of quinoline-fused and naphtho-fused benzoselenophenes (C–C bond forming steps indicated by arrows). A Curtius rearrangement leading to a 2-aminobenzoselenophene followed later by a Bischler–Napieralski cyclization provided benzoselenolo[2,3-c]isoquinoline 66 <2000EJO1353>. A thermal electrocyclization of a benzoselenophenyl ketoxime produced benzoselenolo[2,3-c]quinoline 67 <2002ARK40>. Syntheses of a variety of nitrogen heterocyclic (triazole, tetrazole, pyrimidine) fused benzoselenophenes have been reported <2005RJO396>. McMurry-type coupling involving a benzoselenophene-2carboxaldehyde gave benzo[c]dibenzoselenophene 44 <2006ZNB427>.

The addition of an alkyllithium nucleophile onto a 2-acylselenophene was the key step in the preparation of a selenophene-based tamoxifen derivative 68 <1997JCM274>.

Selenophenes

A side-chain decarboxylation of a barbituric acid provided a convenient route to benzoselenophene-3-acetic acid 69 <2003CHE539>.

The kinetics of the enolization of 2-acetylselenophene was studied in the presence of a variety of metal ions <1998EJO1867>.

3.13.8 Reactivity of Substituents Attached to Selenium There were no reports of the reactivity of substituents attached to the selenium atom of selenophenes. The oxidation of selenophenes to selenophene-1-oxides and selenophene-1,1-dioxides is discussed in Section 3.13.5.2.

3.13.9 Ring Synthesis Selenophene ring synthesis has been organized in the fashion utilized by Sundberg in CHEC-II(1996) <1996CHECII(2)119> for a monograph describing pyrrole ring synthesis. Intramolecular approaches (category I) and intermolecular approaches (category II) are classified by the number and location of the new bonds that describe the selenophene ring forming step as shown below. This section then concludes with syntheses of selenophenes involving the formation of three bonds and the preparation of nonconjugated rings. The preparation of selenophenes, benzo[b]selenophenes, and other fused derivatives is treated together. Two monographs describing synthetic approaches to selenophenes have appeared <2001SOS(9)423, 2005MI375>. Thermal methods utilized for the preparation of selenophenes and thiophenes have also appeared <2000CHE1>.

Many synthetic strategies that have been developed to prepare selenophenes involve modifications (S to Se) of the corresponding route to thiophenes. Given the significant amount of synthetic attention directed to thiophenes and a comparative lack of focus on selenophenes until recently, new ‘syntheses of selenophenes’ continue to be reported.

987

988

Selenophenes

3.13.9.1 Formation of One Bond 3.13.9.1.1

Category Ia cyclizations

Electrophilic iodocyclization reactions of 1-alkynyl-(methylseleno)arenes provide a convenient route to benzo[b]selenophenes. Treatment of selenoethers 70 with iodine or iodine monochloride gives 3-iodobenzo[b]selenophenes 71 <2006JOC2307> (Equation 8). The latter were elaborated into 2,3-diarylbenzo[b]selenophenes and other functionalized benzo[b]selenophenes utilizing palladium-catalyzed cross-coupling reactions. Additional electrophiles (NBS, Br2, Hg(OAc)2, PhSeBr, PhSeCl) were also investigated in the selenophene-forming reaction. The analogous iodocyclization reaction has been adapted to the solid phase for the preparation of a small library of benzo[b]selenophene-5-carboxamides <2006JCO163>. Similar chemistry was utilized in the preparation of [1]benzoselenopheno[3,2-b][1]benzoselenophene <2006JA3044>.

ð8Þ

A novel procedure for the generation of functionalized alkyllithium reagents involved a category type Ia cyclization leading to dibenzoselenophene 4 <2001SL791>. Halogen–metal exchange of biphenyl derivative 72 followed by treatment with benzaldehyde led to the formation of 4 together with alcohol 73 (Equation 9). The latter was produced by the addition of the alkyllithium intermediate, derived from cleavage of the selenoether, to benzaldehyde.

ð9Þ

3.13.9.1.2

Category Ib cyclizations

The synthesis of the benzoselenophene analog 76 of the photochemotherapeutic psoralen was developed (Scheme 7) <1994JPH9, 1994T9315>. Benzo[b]selenophene 75 was prepared in three steps from dialdehyde 74 including an intramolecular type Ib cyclization to form the selenophene ring. The former was converted into 76 in five steps including a decarboxylation sequence and a Perkin condensation. A similar intramolecular cyclization involving a nitrile was utilized to pepare a -aminoselenophene derivative which served as an intermediate in the preparation of the pyrimido[49,59-4,5]selenolo[2,3-b]quinoline 77 <1995SC451>.

Scheme 7

Selenophenes

The effects of replacing thiophene rings with selenophene rings were studied in a series of biothiophene-type dyes (e.g., 78) <2000AGE556>. The preparation of the selenophene ring in these compounds involved a type Ib cyclization of an iminium salt. A related strategy was utilized to prepare 2,5-disubstituted donor–acceptor selenophenes from thioacrylamides <2002ZNB420, 2004ZNB439>.

3.13.9.1.3

Category Ic cyclizations

The polyphosphoric acid (PPA)-catalyzed cyclocondensation of ketone 79 gave the 3-aryl-2-benzylbenzo[b]selenophene 80 (Scheme 8) <1994JCM98>. The latter was converted into derivative 81 which was evaluated as a ligand for antiestrogenic binding sites.

Scheme 8

3.13.9.2 Formation of Two Bonds 3.13.9.2.1

Category IIab cyclizations

No IIab-type selenophene syntheses were abstracted.

3.13.9.2.2

Category IIac cyclizations

After some optimization, a novel one-pot preparation of 2-alkoxyselenophenes was developed utilizing a lithium selenolate <1995TL2807>. Treatment of selenoester 82 with lithium diisopropylamide (LDA) followed by propargyl bromide gave selenophene 84 via allenic intermediate 83 (Equation 10).

ð10Þ

The reaction between benzyne derivatives and selenium analogues of Barton’s thiopyridone esters provided a convenient entry into complex, fused benzo[b]selenophenes <2004JHC13, 2004ARK51>. For example, the generation of the benzyne 86 in the presence of selenoester 85 provided benzo[b]seleno[2,3-b]pyridine 87, presumably via a single electron transfer (SET) pathway (Equation 11). This methodology was examined utilizing a number of benzyne precursors (anthranilic acids, iodium triflates, and trimethysilyl triflates) and provided access to an impressive number of fused benzo[b]selenophenes.

989

990

Selenophenes

ð11Þ

A [3,3]-sigmatropic rearrangement reminscent of the Fischer indole synthesis was proposed as a mechanism for the formation of 2,5-diarylselenophenes from arylhydrazones <2005BCJ1121>. Treatment of arylhydrazone 88 with diselenium dibromide produced selone 89 en route to 2,5-diphenylselenophene 90 via a mechanism that included a [3,3]-sigmatropic rearrangement (Equation 12). A similar oxidative dimerization of selenothioic acid S-alkyl esters gave 2,5-bis(alkylthio)selenophenes <1996CL877>.

ð12Þ The production of selenophene via the thermolysis of dialkyl selenides in the presence of acetylene was explored <2004RJO290>. Thermal reactions leading to selenophenes and thiophenes have also been reviewed <2000CHE1>.

3.13.9.2.3

Category IIad cyclizations

No IIad-type selenophene syntheses were abstracted.

3.13.9.2.4

Category IIae cyclizations

The most utilized de novo synthetic routes to selenophenes involve reactions between selenium reagents and 4-carbon units, which are therefore category IIe cyclizations. Many of these routes have close analogs in the thiophene synthesis literature. For example, one of the more common thiophene syntheses involves the condensation of 1,4-dicarbonyl compounds with Lawesson’s reagent <2003S1929>. An equivalent reaction with selenium involved the condensation of bis(dimethylaluminium) selenide with ortho-diferrocenoylbenzene which produced compound 15 <2006TL2887, 2007JOM(692)60>. The selenium analog of Lawesson’s reagent, Woolin’s reagent 92 <2005CEJ6221>, provides another route to selenophenes from 1,4-dicarbonyl compounds and their synthetic equivalents. The synthesis of selenophene 94 was attempted by treating the unsaturated -ketoalcohol 91 with Woolin’s reagent 92 (Scheme 9) <2005TL7201>. Unexpectedly, the yield of 94 was very low and the major product obtained was furan 93. Interestingly, resubmitting 93 to Woolin’s reagent in dry dichloromethane produced selenophene 94 in 67% yield.

Scheme 9

Selenophenes

Selenolo[3,4-b]furan 97 was prepared utilizing the organic selenium transfer reagent N,N-diethylselenopropionamide 96 (Equation 13) <1998JHC71>. Treatment of bromoketone 95 with 96 gave the [c]-fused selenophene 97.

ð13Þ

The gas-phase thermal reaction of cinnamaldehyde with dimethyl diselenide at 630  C gave benzo[b]selenophene 2 <1998RCB447>. An electrophilic cyclization of allene 98 with phenylselenyl chloride led to the formation of selenophene 99 along with dihydroselenophene 100 (Equation 14) <2004PS(179)1681>.

ð14Þ

A number of different inorganic selenium reagents have been utilized to prepare selenophenes via category IIae cyclizations (4C þ Se). The central selenophene ring in the terchlcogenophene, 2,5-bis(2-tellurienyl)selenophene, was formed by combining sodium selenide (Na2S) with a butadiyne <2000H(52)159>. A group of [c]-fused selenophenes was prepared by the reaction of 1,2-(bromomethyl)arenes/heteroarenes with sodium selenide followed by an oxidation of the intermediate dihydroselenophene. For example (Scheme 10), treatment of the dibromide 101 with sodium selenide gave compound 102, and the latter was converted into 4-nitrobenzo[c]selenophene 103 utilizing two different approaches (Se bromination/elimination or a mild oxidation with phenyl iododiacetate) <2003OL2519>. Additional heterocycles that have been prepared utilizing this sequence include seleno[3,4-b]quinoxaline <2003OL4089> and 3,4-ethylenedioxyselenophene <2005PS787>.

Scheme 10

An inorganic selenium electrophile, selenium oxychloride (SeOCl2), has been utilized to prepare fused selenophenes <2002JOC2453>. Deprotonation of bis(cyanide) 104 with LDA followed by treatment with selenium oxychloride gave the benzo[c]selenophene 105 (Equation 15). This sequence also allowed for the preparation of a thieno[3,4-c]selenophene.

ð15Þ

991

992

Selenophenes

A convenient one-step synthesis of fused selenophenes has been developed <1994H(38)143, 1996T471>. For example, heating ynediol 106 in the presence of selenium metal gave selenolo[3,2-b]selenophene 107 (Equation 16).

ð16Þ

Metal-mediated category IIae cyclizations comprise the next group of selenophene syntheses to be discussed. Dibenzoselenophenes (and dibenzotellurophenes) have been prepared from 2,29-diiodobiphenyl derivatives <1995JOC5274>. Treatment of the biphenyl 108 with selenium–copper slurry, generated from disodium diselenide and copper(I) iodide, produced 3,7-dinitrodibenzoselenophene 109 (Equation 17).

ð17Þ

An approach amenable to the preparation of various fused selenophenes and benzo[b]selenophenes involves the treatment of 1,4-dilithiated intermediates with bis(phenylsulfonyl)selenide 112 <1994CPB1437>. The synthetic sequence to the selenolo[2,3-b]selenophene 113 from 3-ethynylselenophene 110 is shown in Scheme 11 <1997H(45)1891>. A stereoselective hydroalumination of 110 with diisobutylaluminium hydride (DIBAL-H) followed by bromination with NBS gave dibromo compound 111. Dilithiation of 111 followed by treatment with selenium electrophile 112 gave 113.

Scheme 11

A related lithiation approach to fused selenophenes involves o-bromoethynyl arenes. The synthesis of benzo[b]selenophene 115 utilizing this chemistry is shown in Equation 18 <1998SC713>. Lithiation of bromoarene 114 followed by treatment with selenium powder gave 115 via a 5-endo-dig cyclization. This chemistry was applied to the synthesis of a number of fused selenophenes including [1]benzoseleno[3,2-b][1]benzoselenophene <1998JHC725>, benzo[1,2-b:-4,5b’]diselenophenes <2005JOC10569, 2005PS873>, and heteroacene 166 (Section 3.13.12.2) <2005OL5301>.

ð18Þ

Additional examples of selenophene ring-formation reaction that result from the treatment of 1,4-dienes with selenium dioxide have been reported. For example, treatment of verbenone derivative 116 with selenium dioxide in the presence of pyridine gave fused selenophene 117 in 92% yield (Equation 19) <2001OL3161, 2002JOC6553>. Additional applications of this reaction include taxol analog 150 <1997BML1941> and a [1]benzopyrano[3,2-b]selenophene-9-one derivative <2001SAA1427>.

Selenophenes

ð19Þ

3.13.9.2.5

Category IIbc cyclizations

No IIbc-type selenophene syntheses were abstracted.

3.13.9.2.6

Category IIbd cyclizations

The classical Hinsberg thiophene synthesis has been adapted for the preparation of 3,4-ethylenedioxyselenophene 120, a starting material for the preparation of electron-rich selenophene polymers <2001OL4283>. The double condensation of diethyl selenodiglycolate 118 with diethyl oxalate gave 3,4-dihydroxyselenophene 119 (Scheme 12). Alkylation with dibromoethane followed by saponification and decarboxylation then provided 120. This sequence was also utilized to prepare 1,3-dicyanoseleno[3,4-b]quinoxaline <2003OL4089>. A related reaction sequence involving a type IIbd cylization was utilized to prepare a 2,5-dibenzoylselenophene, another monomer that was elaborated into selenophene polymers <2001JAP2019>.

Scheme 12

3.13.9.2.7

Category IIbe cyclizations

No IIbe-type selenophene syntheses were abstracted.

3.13.9.3 Formation of Three Bonds A few three-component reaction sequences leading to selenophenes have been reported. Lithiation of diphenylacetylene followed by treatment with elemental selenium led to 2,3,4,5-tetraphenylselenophene 121 (Equation 20) <1999JOM(573)267>.

ð20Þ

The preparation of highly functionalization selenophenes has been accomplished utilizing a three-component condensation reaction involving ketene dithioacetals, sodium selenide, and an activated carbonyl component (Scheme 13) <2003SL855, 2004S451, 2005PS939>. Ketene dithioacetal 122 was prepared from 2,4-pentanedione by condensation with carbon disulfide followed by methylation. Treatment of compound 122 with sodium selenide and ethyl bromoacetate gave selenophene 123 in modest yield.

993

994

Selenophenes

Scheme 13

An unusual four-component reaction between a phosphorus ylide (2 equiv), elemental selenium, and tetracyanoethylene (TCNE) produced a 3,4-dicyanoselenophene derivative <1995TL8813>.

3.13.9.4 Synthesis of Nonconjugated Rings The synthesis and chemistry of selenosugars (tetrahydroselenophene derivatives) has been studied <2004T2889, 2004CAR2205>. A reductive cyclization approach to tetrahydroselenophenes has been reported <1996CC1461>. Specifically, treatment of selenothioester 124 with sodium borohydride led to tetrahydroselenophene 125 (Equation 21).

ð21Þ

Phenyltelluroformates provide a new leaving group for intramolecular cyclizations <1998JOC3032>. Heating selenide 126 led to the formation of tetrahydroselenophene 127 via an intramolecular nucleophilic substitution of the telluroformate group (Equation 22).

ð22Þ

Titanium-based reductive cyclizations of dicarbonyl compounds provide a good method for preparing heteroarenes. Treatment of selenoester 128 with titanium tetrachloride in the presence of zinc produced the dihydroselenophene 129 (Equation 23) <1998CL645>. The addition of zinc proved to be crucial for this transformation.

ð23Þ

Due to their interesting electronic properties (e.g., conductors), synthetic approaches to dihydroselenophenes that are fused to tetrathiafulvenes (TTFs) have been developed <1997CL1091, 1998JOC8865>. A seleno-Claisen rearrangement was utilized to prepare dihydrobenzo[b]selenophenes <2003SC2161>. Heating selenide 130 in quinoline produced the dihydrobenzo[b]selenophene 131 after cyclization of the selenophenol Claisen product (Equation 24).

ð24Þ

Selenophenes

An alkyltelluride-mediated reductive cyclization provided a route to benzo[b]selenophenes that are potential antioxidants <1999JOC6764>. Finally, an intramolecular condensation of a 1,6-dicarbonyl derivative was utilized to prepare oxygenated benzo[b]selenophenes <2001CL826, 2001TL4899>. Treatment of 2-chloroselenoylbenzoyl chloride 132 with acetone gave 3-hydroxybenzo[b]selenophene 133 via a selenophen-3(2H)-one intermediate (Equation 25).

ð25Þ

3.13.10 Ring Synthesis from Another Ring The contraction of 1,2- and 1,4-chalcogenides (S, Se) to the corresponding chalogenophenes (S, Se) can be accomplished under photochemical or thermal conditions. 1,2-Diselenide 135 have been prepared in two steps from zincocene 134 (Scheme 14) <1999AGE1604, 2000JA5052>. Photolysis of 135 then gave selenophene 136 quantitatively. Additional methods reported for the formation of 1,4-deselenides (and subsequent conversion to selenophenes) include dimerization of bis(benzylseleno)ethene <2003SUL137> and a cycloaddition between diselenoamides and dimethyl acetylenedicarboxylate (DMAD) <1994CL77>.

Scheme 14

A Pummerer-type ring contraction of benzoselenopyrans led to the formation of benzo[b]selenophenes in modest yields <2000H(52)1021>. Another unique route to 2-cyanobenzo[b]selenophene resulted from the reaction between a selenabicyclo[3.1.0]hexene derivative with benzyne <2000H(55)465>. Mechanisms for these transformations were presented in each paper. The formation of selenophenes has also been investigated by the transformation of selenazine and selenadiazole hetereocycles. Selenazines have been prepared by utilizing cycloaddition reactions of selenoacylamidines (e.g., 137) <1995TL237, 1998T2545, 1998SC301>. The cycloaddition of 137 with butynal 138 gave selenazine 139, which was transformed into selenophene 140 in two steps (retro-cycloaddition and oxidative cyclization) (Scheme 15).

Scheme 15

The thermolysis of 1,2,3-selenadiazoles in the presence of arylacetylenes provides another route to 2,5-disubstituted selenophenes <2002TL4817>. Heating 4-phenylselenadiazole 141 in the presence of 2-pyridininylacetylene 142 gave the 2,5-diarylselenophene 143 (Equation 26). Similar radical extrusion cyclizations of 1,2,3-selenadiazoles in the presence of alkenes have been reported to give fused selenophenes and 2,3-dihydroselenophenes <1999TL6293, 2000JOM(611)488, 2002JOC1520>.

995

996

Selenophenes

ð26Þ

Finally, selenophenes have been prepared by treatment of zirconium metallocenes (e.g., 134) with diselenium dibromide (Se2Br2) <1994JA1880> or Se(SeCN)2 <1999AGE1604, 2000JA5052>.

3.13.11 Selected Syntheses No syntheses have been selected for discussion in this section because of the relative paucity of reports of selenophene synthesis which makes comparisons difficult to fully develop.

3.13.12 Important Compounds and Applications 3.13.12.1 Compounds of Biological Interest Various selenophene compounds have been found to possess anticancer activity. A tris(2-selenophenyl)stibine 6 showed selectivity for carcinogenic cell K and U growth inhibition <2005JOM(690)3286>. Selenophenfurin 144 demonstrated inhibitory activity against recombinant human inosine monophosphate dehydrogenase (IMPDH) <1997JME1731>. Structurally related selenophene diphosphate 145, an isosteric analog of a nicotinimide adenine dinucleotide, also demonstrated inhibitory activity against IMPDH <1998JME1702>. Finally, a selenophene analog of (Z)-tamoxifen 146 demonstrated a lower binding affinity for a Molt 4 cell line than the parent compound <1997JCM274>.

Selenophenes show a wide range of additional biological activity including antioxidant, antiestrogen, and antifungal. Dihydrobenzo[b]selenophene 147 was designed as a novel antioxidant <1999JOC6764>, while a 3-arylbenzo[b]selenophene 81 was designed as a selective ligand for antiestrogen-binding sites <1994JCM98>. A triazine-fused selenophene 148 possessed moderate antifungal activity against several fungi <2005RJO396>.

Selenophenes

Selenophene analogs of selected biomolecules have also been prepared. A synthesis of selenolo[3,2-b]pyrrolyl-Lalanine 149 was developed for incorporation into proteins <2002BBR257, 2001JMB925, 1999BML637>. A selenophene-fused taxol analog 150 <1997BML1941> and various indolocarbazole analogs (e.g., 151) have also been reported <1998SC1239>.

Several reviews of heteroporphyrins have been reported <2003ACR676, 2006CCR468, 2005CCR2510>. Heteroporphyrins (e.g., 152) have been identified as G-quadruplex binding agents <2005JA2944> and as sensitizers for photodynamic therapy (e.g., 153) <2000JME2403, 2002JME449, 2003JME3734>.

3.13.12.2 Compounds with Applications in Materials Science In addition to their biological applications, heteroporphyrins also serve as novel materials. Various porphyrins have been studied, including pyrrole-inverted porphyrins 12 <2000JOC8188>, tetraselenaporphyrins 13 <1997AGE2609>, sapphyrins, and rubyrins with a varying number of selenophene rings (e.g., 14) <1995IC3567, 1999JA9053, 1999JOC8693, 1999J(P2)961, 1999T6671, 2000J(P2)1788>, a meso-substituted octaphyrin featuring four selenophene rings <2003CEJ2282>, an octaphyrin 154 with a quarterthiophene unit <2007CC43>, rhodium(I) hetero-rubyrin complex 155 <2001IC1637>, and nickel(II) selenaporphyrin complexes <1996IC566>. Additional porphyrins that have been characterized include N-confused selenaporphyrins <2001JOC153>, N-confused expanded porphyrin 156 <2001JA5138>, hexaphyrins (e.g., 157) <2005JA11608, 2003OL3531, 2001TL3391>, heptaphyrins (e.g., 158) <2002JOC6309>, inverted heptaphyrins (e.g., 159) <2000OL3829>, and inverted octaphyrins (e.g., 160) <2001JA8620>.

997

998

Selenophenes

Fused selenophene ring systems have potential applications in materials science and selected syntheses highlighted in previous sections. Examples include benzo[b]seleno[2,3-b]pyridines (e.g., 87) <2004JHC13, 2004ARK51>, thienoselenophenes 161–163 <1997H(45)1891>, selenolo[3,4-b]furans 97 <1998JHC71>, selenolo[3,2-b]selenophene 164 <1994H(38)143>, selenolo[2,3-b]thiophenes 165 <2004S451>, selenophene-based heteroacene 166 <2005OL5301>, seleno[3,4-c]thiophenes <2002JOC2453>, pyrimidoselenolo[2,3-b]quinoline-4(3H)-one 167 <1995SC451>, and various other fused selenophene systems <1997CLY547>.

Selenophenes

Selenophene-containing quinones were prepared to explore their potential use as dyes and photomaterials. They were found to have good physical and chemical properties for use as dyestuffs for laser-driven high-density optical storage media <1996H(43)941, 1996JOC4784>. 2-Aminoselenophene derivatives were synthesized as indicators for measurement of solvent polarity <2002ZNB420>. Other applications include the use of selenophene-containing tris(oligoarylenyl)amines as amorphous glass materials <2004CL1266> and self-assembled monolayers of selenophene on gold <2000L4213>. A zirconocene complex featuring ligands containing a cyclopentadiene ring fused to selenophene demonstrated activity as an alkene polymerization catalyst <2005PS827>. The field of organic conducting materials has grown significantly since the initial discovery in 1977 that doped polyacetylene demonstrated excellent conducting properties. The synthesis and properties of tetrathiafulvalenes and tetraselenafulvalenes, which have generated interest as functional molecular materials and devices, have been reviewed <2004CRV5289>. Other tetrathiafulvalene (TTF) and tetraselenafulvalene (TSF) donor systems that demonstrate conducting properties include TTFs extended by selenophene (e.g., 168) and benzo[c]selenophene (e.g., 169) substitution <1997H(45)1051, 1997JMC2375>, diselenolotetrathiafulvalene 170 <1997CL1091>, bis(ethyleneseleno)tetraselenafulvalene 171 <1998JOC8865>, and a TTF 172 extended by selenophene fusion <1995AM644>. Fused benzodiselenophenes (e.g., 9) have demonstrated relatively high hole mobilities and have potential applications in field effect transistors <2006JA3044, 2005PS873, 2005JOC10569, 2004JA5084>. The field effect mobilities of quinoidal biselenophenes (e.g., 173) were comparable to or higher than those of the corresponding thiophene derivatives <2004JMC1367>. Biisoselenophene derivatives <2004OL3039>, 1,3-diarylbenzo[c]selenophenes (e.g., 94) <2005TL7201>, and other selenophene-based tetracyanoquinodimethane derivatives (e.g., 174) <2004BCJ463> have also been prepared to explore novel conducting materials.

Several selenophene oligomers have been reported. These include selenolo[3,2-b]selenophene dimers, trimers, and tetramers 175 <1996T471>, quarterselenophenes <1999SM(101)639, 2003CM6>, and alkyl-substituted oligoselenophenes 176 <1997SM(84)341>.

999

1000 Selenophenes Polyselenophenes have also generated significant interest for their potential use as conducting materials. The bandgap energy of p-conjugated polyselenophene was calculated at the B3LYP/6-31G(d) level <2006OL5243>. A high-quality polyselenophene film was prepared that showed good redox activity and high thermal stability <2005MAL1061, 2005JEC345>. The vibrational spectra of polyselenophene was calculated using the semiempirical method PM3 <1995JST(348)91>. The geometries and electronic structures of polyselenophene were compared with various polymers <1998SM(96)177>. Other polyselenophenes include polymers of 3,4-ethylenedioxyselenophene 177 <2001OL4283>, 2,5-diketoselenophene 178 <2001JAP2019>, and biselenophene 179 <2003PLM5597>.

Various copolymers of selenophene have also been reported. These include polymers of selenophene–ethene <1998MM1221>, selenophene–thiophene <1996SM(82)111>, selenophene–pyridine 180 <1996CM2444, 1996JA10389, 2005PCB10605>, selenophene–benzothiadiazole 181 and selenophene–benzoselenadiazole 182 <2005MM244>. Addition copolymers that have been investigated include: pyridine–selenophene 183 <1999SM(100)187>, selenophene–aniline 184 <1998AM1525>, selenophene-oxygenated phenylene 185 <1995MM8363>, and selenophene–tetrafluorophenylene 186 <2005CM6567>. Finally, several selenophene copolymers incorporated fluorene subunits (e.g., 181, 182, and 187) <2006MM4081, 2005PSA823, 2005MM244>.

3.13.13 Further Developments A few recent developments involving the synthesis and evaluation of funtionalized selenophenes have appeared during 2007. An electrophilic cyclization of (Z)-selenoenynes with iodine provided access to 3-iodo-2,5-disubstituted selenophenes <2007JOC6726>. The iodide moiety was later elaborated into other groups utilizing halogen-metal exchange or cross-coupling reactions. The addition of sodium hydroselenide to two equivalents of bis(diethoxyphosphoryl)acetylene followed by oxidation with m-CPBA provided access to 2,3,4,5-tetraphosphorylselenophenes

Selenophenes

<2007OL1729>. Extended heteroarenes containing selenophene rings were prepared and investigated as field-effect transistors <2007JA2224>. Finally, the mode of action of a promising anti-cancer 2,5-di(selenophen-2-yl)pyrrole has been evaluated. The compound was found to induce apoptosis through a p53-associated pathway <2007BP610> and its mode of activity was linked to DNA adduct formation <2007MI193>.

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1003

1004 Selenophenes

2003CHE36 2003CHE539 2003CM6 2003CPL(370)813 2003JA12328 2003JCP10712 2003JME3734 2003JST(633)237 2003OL2519 2003OL3531 2003OL4089 2003OM3659 2003PHC(15)116 2003PLM5597 2003SC2161 2003SL855 2003SUL137 2003S1929 2004ARK51 2004BCJ463 2004CAR2205 2004CL1266 2004CRV2777 2004CRV5289 2004JA5084 2004JCP5801 2004JHC13 2004JMC1367 2004OL3039 2004PHC(16)84 2004PS(179)1681 2004RJO290 2004S451 2004T2889 2004ZNB439 2005ARK60 2005BCJ1121 2005CCR2510 2005CEJ6221 2005CM6567 2005CPL(416)113 2005CRV3842 2005JA2944 2005JA11608 2005JEC345 2005JOC10569 2005JOM(690)3286 2005JMP796 2005MAL1061 2005MI375 2005MI1119 2005PCB10605 2005PSA823 2005MM244 2005OL5301 2005PHC(17)98 2005PS787

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Selenophenes

2005PS827 2005PS873 2005PS939 2005RCB853 2005RJO396 2005S1589 2005TL2647 2005TL7201 2006CCR468 2006JA227 2006JA3044 2006JCO163 2006JOC1552 2006JOC2307 2006JOC3786 2006MM4081 2006OL5243 2006POL499 2006TL795 2006TL2179 2006TL2887 2006ZNB427 2007BP610 2007CC43 2007JA2224 2007JOC6726 2007JOM(692)60 2007MI193

2007OL1729 2007PHC(18)126

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1005

1006 Selenophenes Biographical Sketch

Dr. Erin T. Pelkey (born in 1972) obtained his PhD degree in organic chemistry from Dartmouth College with Prof. Gordon Gribble (1998) where he invesigated the synthesis and chemistry of electron-deficient indoles. He was then an NIH postdoctoral fellow (1999–2001) at Stanford University in the laboratory of Prof. Paul Wender where he investigated the design, preparation, and evaluation of novel guanidine-rich drug delivery agents. In 2001, he joined the Chemistry Department at Hobart and William Smith Colleges located in the Finger Lakes region of upstate New York. He has been a regular contributor to Progress in Heterocyclic Chemistry (1997–present) including chapters on thiophene chemistry and pyrrole chemistry. His research interests are directed at the development of new methods for the preparation of biologically active fivemembered ring nitrogen heterocycles.