The directed ortho-lithiation of lithium phenylsulfide and lithium phenylselenide plus sigma-aromaticity and the hexakis(phenylseleno)benzene dication

Loop, Cynthia Kay

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https://hdl.handle.net/2142/21941 Copy

Description

Title

The directed ortho-lithiation of lithium phenylsulfide and lithium phenylselenide plus sigma-aromaticity and the hexakis(phenylseleno)benzene dication

Author(s)

Loop, Cynthia Kay

Issue Date

1990

Doctoral Committee Chair(s)

Martin, J.C.

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Chemistry, Organic

Language

eng

Abstract

• The directed ortho-lithiation of lithium phenylselenide by reaction with $n$-butyllithium in cyclohexane with $N$,$N$,$N\sp\prime$,$N\sp\prime$-tetramethylethylenediamine gives almost quantitative conversion to lithium 2-lithiobenzeneselenolate and is estimated to be seven to eight times faster than the directed ortho-lithiation of lithium phenylsulfide to form lithium 2-lithiobenzenethiolate. Treatment of the lithium 2-lithiobenzeneselenolate with a variety of electrophiles (D$\sb2$O, carbon dioxide, selenium, methylene chloride, methyl iodide, thioxanthone, and dimethylcarbonate) form ortho-substituted products. The treatment of dilithium benzene-1,2-diselenolate with methylene chloride and 1,2-dibromoethane forms 1,3-benzodiselenole (70.4% yield) and dihydro-1,4-benzodiselenin (54.6% yield), respectively. The ortho-lithiation and $o,o\sp\prime$-dilithiation of dilithium benzene-1,2-diselenolate is presented. The adduct to thioxanthone is ring-closed to form the (1) benzoselenopyrano (2,3,4-kl) thioxanthylium cation and 13b$H$- (1) benzoselenopyrano (2,3,4-kl) thioxanthen-13b-ol (13.3% yield).
• The definition and evidence for the existence of $\sigma$-aromaticity is described in chapter two. Huckel $\sigma$-aromaticity would be 4n + 2 electrons (Mobius $\sigma$-aromaticity would be 4n electrons) delocalized in a planar, cyclic array of an even (odd) number of $\sigma$-overlapped p orbitals which impart a special stability to the compound that would not be otherwise expected. The synthesis of pentakis(phenylseleno)benzene is reported along with its conversion to hexakis(phenylseleno)benzene via ortho-lithiation of the central ring followed by treatment with phenylselenenyl chloride. Mass spectrometric investigations of fluoropentaiodobenzene, hexachlorobenzene, hexabromobenzene, hexaiodobenzene, pentakis(phenylseleno)benzene, hexakis(phenylseleno)benzene and tetraiodoethylene show the presence of dications and provide evidence for the possible formation of $\sigma$-aromatic species. The presence of the hexakis(phenylseleno)benzene dication at three times the intensity of the pentakis(phenylseleno)benzene dication in the mass spectrum of pentakis(phenylseleno)benzene suggests a special stability for the hexakis(phenylseleno)benzene dication. The C$\sb6\sp+$ cation (1.25% to 10.2% of M$\sp+$) in the mass spectra of the series of hexahalobenzenes is observed. Cyclic voltammetry shows that hexakis(phenylseleno)benzene (1.10 V) is much more easily oxidized than pentakis(phenylseleno)benzene (1.67 V). The 1,2,3,4,5-pentakis(phenylseleno)-6-(phenylseleninyl)benzene was synthesized for the preparation of hexakis(phenylseleno)benzene dication by the addition of trifluoroacetic acid and trifluoroacetic anhydride at $-$40$\sp\circ$C. The upfield shift of the central ring carbons of at least 8.8 ppm and the downfield shift of the outer phenyl carbons of at least 3.7 ppm in the $\sp{13}$C NMR upon forming the bis trifluoroacetate salt of hexakis(phenylseleno)benzene dication suggests the existence of a ring current in the selenium ring.

Type of Resource

text

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http://hdl.handle.net/2142/21941

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Copyright 1990 Loop, Cynthia Kay

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