Development of an Arylthiobismuthine Cocatalyst in Organobismuthine-Mediated Living Radical Polymerization: Applications for Synthesis of Ultrahigh Molecular Weight Polystyrenes and Polyacrylates

Diphenyl(2,6-dimesitylphenylthio)bismuthine (1a) serves as an excellent cocatalyst in organobismuthine-mediated living radical polymerization (BIRP). Both low and high molecular weight polystyrenes and poly(butyl acrylate)s (PBAs) with controlled molecular weights and low polydispersity indexes (PDIs) were synthesized by the addition of a catalytic amount of 1a to an organobismuthine chain-transfer agent, methyl 2-dimethylbismuthanyl-2-methylpropionate (3). The number-average molecular weight (Mn) of the resulting polymers increases linearly with the monomer/3 ratio. Structurally well-defined polystyrenes with Mn’s in the range from 1.0 × 104 to 2.0 × 105 and PDIs of 1.07−1.15 as well as PBAs with Mn’s in the range from 1.2 × 104 to 2.8 × 106 and PDIs of 1.06−1.43 were successfully prepared under mild thermal conditions. Control experiments suggested that 1a reversibly reacts with the polymer-end radical to generate an organobismuthine dormant species and 2,6-dimesitylphenylthiyl radical (2a). This reaction avoids the occurrence of chain termination reactions involving the polymer-end radicals and avoids undesired loss of the bismuthanyl polymer end group. The bulky 2,6-dimesitylphenyl group attached to the sulfur atom may prevent the addition of thiyl radicals to the vinyl monomers to generate new polymer chains

Synthesis and Characterization of [5]Cycloparaphenylene

The synthesis of highly strained [5]­cycloparaphenylene ([5]­CPP), a structural unit of the periphery of C60 and the shortest possible structural constituent of the sidewall of a (5,5) carbon nanotube, was achieved in nine steps in 17% overall yield. The synthesis relied on metal-mediated ring closure of a triethylsilyl (TES)-protected masked precursor 1c followed by removal of the TES groups and subsequent reductive aromatization. UV–vis and electrochemical studies revealed that the HOMO–LUMO gap of [5]­CPP is narrow and is comparable to that of C60, as predicted by theoretical calculations. The results suggest that [5]­CPP should be an excellent lead compound for molecular electronics

Selective and Gram-Scale Synthesis of [8]Cycloparaphenylene

Selective and large-scale synthesis of [8]­cycloparaphenylene (CPP) was achieved in seven steps starting from commercially available 4-bromo-4′-hydroxybiphenyl and 4,4′-dibromobiphenyl. The key unsymmetrical tetraring unit, 4-bromophenyl and 4′-bromobiphenyl-substituted cis-1,4-bis­(triethylsiloxy)-2,5-cyclohexadiene-1,4-diyl (5fA), was synthesized on an ∼50 g scale by stereoselective cis-addition of 4-bromo-4′-lithiobiphenyl to 4-(4-bromophenyl)-4-hydroxy-2,5-cyclohexadien-1-one, which was synthesized on an ∼100 g scale. Platinum-mediated selective dimerization of the four-ring unit 5fB and subsequent reductive aromatization of the cyclohexadiene-diyl by H2SnCl4 gave 2 g of [8]­CPP in 6.6% overall yield (10.2% on small scale)

Synthesis and Characterization of [<i>n</i>]CPP (<i>n</i> = 5, 6, 8, 10, and 12) Radical Cation and Dications: Size-Dependent Absorption, Spin, and Charge Delocalization

Radical cations and dications of [<i>n</i>]­cyclo-<i>p</i>-phenylenes ([<i>n</i>]­CPPs, <i>n</i> = 5, 6, 10, and 12), which are the models of those of linear oligo-<i>p</i>-phenylenes without a terminus, were synthesized as hexafluoro­antimonate salts by the one- and two-electron chemical oxidation of CPP by NOSbF₆ or SbF₅. The radical cations, [<i>n</i>]­CPP^•+, and dications, [<i>n</i>]­CPP²⁺, exhibited remarkable batho­chromic shifts in their UV–vis–NIR absorption bands, suggesting that [<i>n</i>]­CPP^•+ and larger [<i>n</i>]­CPP²⁺ exhibit longer polyene character than the shorter analogues. The larger batho­chromic shift was consistent with the narrower HOMO–SOMO and HOMO–LUMO gaps in larger [<i>n</i>]­CPP^•+ and [<i>n</i>]­CPP²⁺, respectively. In [<i>n</i>]­CPP^•+, the spins and charges were equally and fully delocalized over the <i>p</i>-phenylene rings of the CPPs, as noted by ESR. ¹H NMR revealed that the hydrogen of [<i>n</i>]­CPP²⁺ shifted to a high magnetic field from the neutral compounds due to the diamagnetic ring current derived from the in-plane aromaticity of [<i>n</i>]­CPP²⁺. The single resonances observed in all [<i>n</i>]­CPP²⁺ strongly suggest the complete delocalization of the charges over the CPPs. Furthermore, the contribution of biradical character was clarified for [10]- and [12]­CPP by VT-NMR experiment and theoretical calculation

Selective Synthesis and Crystal Structure of [10]Cycloparaphenylene

[10]Cycloparaphenylene ([10]CPP) was selectively synthesized in four steps in 13% overall yield from commercially available 4,4′-diiodobiphenyl by using mono-I–Sn exchange, Sn–Pt transmetalation, I–Pd exchange, and subsequent oxidative coupling reactions. The single-crystal X-ray structure of [10]CPP is described

Selective Synthesis and Crystal Structure of [10]Cycloparaphenylene

[10]Cycloparaphenylene ([10]CPP) was selectively synthesized in four steps in 13% overall yield from commercially available 4,4′-diiodobiphenyl by using mono-I–Sn exchange, Sn–Pt transmetalation, I–Pd exchange, and subsequent oxidative coupling reactions. The single-crystal X-ray structure of [10]CPP is described

In-Plane Aromaticity in Cycloparaphenylene Dications: A Magnetic Circular Dichroism and Theoretical Study

The electronic structures of [8]­cyclo­para­phenylene dication ([8]­CPP²⁺) and radical cation ([8]­CPP^•+) have been investigated by magnetic circular dichroism (MCD) spectroscopy, which enabled unambiguous discrimination between previously conflicting assignments of the UV–vis–NIR absorption spectral bands. Molecular orbital and nucleus-independent chemical shift (NICS) analysis revealed that [8]­CPP²⁺ shows in-plane aromaticity with a (4<i>n</i> + 2) π-electron system (<i>n</i> = 7). This aromaticity appears to be the origin of the unusual stability of the dication. Theoretical calculations further suggested that not only [8]­CPP²⁺ but also all [<i>n</i>]­CPP (<i>n</i> = 5–10) dications and dianions exhibit in-plane aromaticity

Optimization of Organotellurium Transfer Agents for Highly Controlled Living Radical Polymerization

Optimization of Organotellurium Transfer Agents for Highly Controlled Living Radical Polymerizatio

Tetracyclo(2,7-carbazole)s: Diatropicity and Paratropicity of Inner Regions of Nanohoops

Three N-substituted tetracyclo­(2,7-carbazole)­s were synthesized to investigate the inner regions of nanohoops. One compound has a 5,5-dimethylnonane bridge between two neighboring <i>anti</i>-carbazoles, which can be used as covalently bonded “methane probes”. These probes near the ring center are strongly shielded by local ring currents and exhibit a singlet at δ = −2.70 ppm in ¹H NMR. To visualize local and macrocyclic ring currents separately, we drew nucleus-independent chemical shift contour maps of tetracyclo­(9-methyl-2,7-carbazole) and [<i>n</i>]­cycloparaphenylenes (CPPs). Local ring currents make the interior diatropic, and paratropic regions exist only outside the ring. Macrocyclic ring currents in [5]­CPP to [7]­CPP generate deshielding cones, which are typical of antiaromatic [4<i>n</i>]­annulenes

Tetracyclo(2,7-carbazole)s: Diatropicity and Paratropicity of Inner Regions of Nanohoops

Three N-substituted tetracyclo­(2,7-carbazole)­s were synthesized to investigate the inner regions of nanohoops. One compound has a 5,5-dimethylnonane bridge between two neighboring <i>anti</i>-carbazoles, which can be used as covalently bonded “methane probes”. These probes near the ring center are strongly shielded by local ring currents and exhibit a singlet at δ = −2.70 ppm in ¹H NMR. To visualize local and macrocyclic ring currents separately, we drew nucleus-independent chemical shift contour maps of tetracyclo­(9-methyl-2,7-carbazole) and [<i>n</i>]­cycloparaphenylenes (CPPs). Local ring currents make the interior diatropic, and paratropic regions exist only outside the ring. Macrocyclic ring currents in [5]­CPP to [7]­CPP generate deshielding cones, which are typical of antiaromatic [4<i>n</i>]­annulenes