Melissa A. Skidmore

Thermolysis of RAFT-synthesized poly(methyl methacrylate)

Thermolysis provides a simple and efficient way of eliminating thiocarbonylthio groups from RAFT-synthesized polymers. The course of thermolysis of poly(methyl methacrylate) (PMMA) prepared with dithiobenzoate and trithiocarbonate RAFT agents was followed by thermogravimetric analysis (TGA), 1H NMR spectroscopy, and gel permeation chromatography (GPC). The weight loss profile observed depends strongly on the RAFT agent used during polymer synthesis. PMMA with a methyl trithiocarbonate end group undergoes loss of that end group at ~180°C, at least in part, by a mechanism believed to involve homolysis of the C–CS2SCH3 bond and subsequent depropagation. In contrast, PMMA with a dithiobenzoate end appears more stable. Only the end group is lost at ~180°C and the dominant mechanism is proposed to be a concerted elimination process analogous to that involved in the Chugaev reaction.

Stannanes from Cholic Acid as Enantioselective Free-Radical Reducing Agents

7α-(Dimethylstannyl)-24-nor-5β-cholane (12) and 3β-(dimethylstannyl)-24-nor-5β-cholane (11) have been prepared from cholic and lithocholic acid, respectively, as free-radical reducing agents. Limited enantioselectivity-testing data indicate that these compounds show promise as enantioselective free-radical reducing agents. For example, reaction of ethyl (rac)-2-bromo-2-cyclopentyl-2-phenylacetate with one equivalent of either (11) or (12) in the presence of one equivalent of (S,S)-(–)-N,N’-bis(3,5-di-tert-butylsalycidene)-1,2-cyclohexanediaminomanganese(III) chloride (7) in toluene at –78˚C provides ethyl (S)-2-cyclopentyl-2-phenylacetate, obtained with 62 and 90% ee, respectively.

First Synthesis of Diindeno[1,2-g:1',2'-s]rubicene Derivatives and their Evaluation as Semiconductors

Solution-processable derivatives 4a–d of the novel diindeno[1,2-g:1′,2′-s]rubicene ring system have been prepared in three steps from 1,5-dichloroanthraquinone. Charge extraction by linearly increasing voltage measurements indicates that 4a–d have bulk mobilities <10–7 cm2 V–1 s–1. Consistent with these low mobility values, field-effect transistors fabricated from 4a–d show poor performance. X-ray crystallographic analyses indicate that in the crystalline state, molecules of compounds 4b–d pack in a manner that hinders π–π stacking, thus preventing strong electronic coupling between molecules that is essential for high charge mobility semiconductor performance.

Redox Levels of a closo-Osmaborane: A Density Functional Theory, Electron Paramagnetic Resonance and Electrochemical Study

A closo-type 11-vertex osmaborane [1-(η(6)-pcym)-1-OsB10H10] (pcym = para-cymene) has been synthesized and characterized by single-crystal X-ray diffraction and elemental analysis, as well as by (11)B and (1)H NMR, UV-visible, and mass spectrometry. The redox chemistry has been probed by dc and Fourier transformed ac voltammetry and bulk reductive electrolysis in CH3CN (0.10 M (n-Bu)4NPF6) and by voltammetry in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (Pyrr1,4-NTf2), which allows the oxidative chemistry of the osmaborane to be studied. A single-crystal X-ray diffraction analysis has shown that [1-(η(6)-pcym)-1-OsB10H10] is isostructural with other metallaborane compounds of this type. In CH3CN (0.10 M (n-Bu)4NPF6), [1-(η(6)-pcym)-1-OsB10H10] undergoes two well-resolved one-electron reduction processes with reversible potentials separated by ca. 0.63-0.64 V. Analysis based on a comparison of experimental and simulated ac voltammetric data shows that the heterogeneous electron transfer rate constant (k(0)) for the first reduction process is larger than that for the second step at GC, Pt, and Au electrodes. k(0) values for both processes are also larger at GC than metal electrodes and depend on the electrode pretreatment, implying that reductions involve specific interaction with the electrode surface. EPR spectra derived from the product formed by one-electron reduction of [1-(η(6)-pcym)-1-OsB10H10] in CH3CN (0.10 M (n-Bu)4NPF6) and electron orbital data derived from the DFT calculations are used to establish that the formal oxidation state of the metal center of the original unreduced compound is Os(II). On this basis it is concluded that the metal atom in [1-(η(6)-pcym)-1-OsB10H10] and related metallaboranes makes a 3-orbital 2-electron contribution to the borane cluster. Oxidation of [1-(η(6)-pcym)-1-OsB10H10] coupled to fast chemical transformation was observed at 1.6 V vs ferrocene(0/+) in Pyrr1,4-NTf2. A reaction scheme for the oxidation involving formation of [1-(η(6)-pcym)-1-OsB10H10](+), which rearranges to an unknown electroactive derivative, is proposed, and simulations of the voltammograms are provided.

Toward Chiral Stannane Reducing Agents Derived from Cholic Acid and Cholestanol

Molecular mechanics studies have identified 3α-(dimethylstannyl)-24-nor-5β-cholane (5α), 7α-(dimethylstannyl)-24-nor-5β-cholane (6α) and 12β-(dimethylstannyl)-24-nor-5β-cholane (7β) to be effective stannanes for use as enantioselective free-radical reducing agents. Synthetic studies toward the preparation of these stannanes as well as 3α-(dimethylstannyl)-5α-cholestane (21) are presented together with selected enantioselectivity data.

Phenylselenotris(trimethylsilyl)silane andphenyltellurotris(trimethylsilyl)silane: versatile reagents for thepreparation of phenylseleno- and phenyltelluro-formates

Phenyltellurotris(trimethylsilyl)silane 4 and (4-fluorophenyltelluro)tris(trimethylsilyl)silane 5 react with methyl, 2-methylpropyl, cyclohexyl and phenyl substituted chloroformates (7, 11, 15, 19) in benzene and in the presence of tetrakis(triphenylphosphine)palladium(0) (4 mol%) to afford the corresponding aryltelluroformate (8, 9, 12, 13, 16, 17, 20, 21) in 50–79% yield; this procedure is also applicable to the preparation of (phenylseleno)formates and seleno- and telluro-esters.

Reversibility in free-radical reactions of aryltellurides with tributylstannyl radical

119 Sn and 125Te NMR spectroscopies reveal that methyl, primary and secondary alkyl radicals, generated by reaction of phenyltelluroalkanes 4, 6, 7 with tributyltin hydride (benzene, AIBN initiator), are capable of displacing tributylstannyl radicals from (4-fluorophenyltelluro)tributylstannane to afford the 4-fluorophenyltelluroalkanes 3, 8, 9.