Catalyst-free photooxidation of triarylphosphines under aerobic conditions

AbstractA new method for the photooxidation of triarylphosphines into the corresponding oxides is developed. In this new protocol, neither a catalyst nor an additive is required. The greenest oxidant, oxygen in air atmosphere, is used. After a short period of photo irradiation at rt, stoichiometric amounts of the oxides can be easily afforded by simply recycling the solvent under vacuum. No waste is formed in the whole process of this reaction. The substrate scope of this reaction is broad, showing very good application prospects in both organic chemistry and industrial processes

Conversion of DNA methyltransferases into azidonucleosidyl transferases via synthetic cofactors

Aziridine-based cofactor mimics have been synthesized and are shown to undergo methyltransferase-dependent DNA alkylation. Notably, each cofactor mimic possesses an azide functionality, to which can be attached an assortment of unnatural groups following methyltransferase-dependent DNA delivery. DNA duplexes modified with these cofactor mimics are capable of undergoing the Staudinger ligation with phosphines tethered to biological functionalities following enzymatic modification. This methodology provides a new tool by which to selectively modify DNA in a methyltransferase-dependent way. The conversion of biological methyltransferases into azidonucleosidyl transferases demonstrated here also holds tremendous promise as a means of identifying, as yet, unknown substrates of methylation

Isomer Dependence in the Assembly and Lability of Silver(I) Trifluoromethanesulfonate Complexes of the Heteroditopic Ligands, 2-, 3-, and 4-[Di(1\u3cem\u3eH\u3c/em\u3e-pyrazolyl)methyl]phenyl(di-\u3cem\u3ep\u3c/em\u3e-tolyl)phosphine

Three isomers of a new heteroditopic ligand that contains a di(1H-pyrazolyl)methyl (−CHpz2) moiety connected to a di(p-tolyl)phosphine group via a para-, meta-, or ortho-phenylene spacer (pL, mL, and oL, respectively) have been synthesized by using a palladium(0)-catalyzed coupling reaction between HP(p-tolyl)2 and the appropriate isomer of (IC6H4)CHpz2. The 1:1 complexes of silver(I) trifluoromethanesulfonate, Ag(OTf), were prepared to examine the nature of ligand coordination and the type of supramolecular isomer (monomeric, cyclic oligomeric, or polymeric) that would be obtained. The single crystal X-ray diffraction studies showed that [Ag(pL)](OTf), 1, and [Ag(mL)](OTf), 2, possessed cyclic dimeric dications, whereas [Ag(oL)](OTf), 3, was a coordination polymer. The polymeric chain in 3 could be disrupted by reaction with triphenylphosphine, and the resulting complex, [Ag(oL)(PPh3)](OTf), 4, possessed a monometallic cation where the ligand was bound to silver in a chelating κ2P,N- coordination mode. The solution structures of 1–4 were probed via a combination of IR, variable-temperature multinuclear (1H, 13C, 31P) NMR spectroscopy, as well as by electron spray ionization (ESI)(+) mass spectrometry. A related complex [Ag(m-IC6H4CHpz2)2](OTf), 5, was also prepared, and its solid-state and solution spectroscopic properties were studied for comparison purposes. These studies suggest that the cyclic structures of 1 and 2 are likely preserved but are dynamic in solution at room temperature. Moreover, both 3 and 4 have dynamic solution structures where 3 is likely extensively dissociated in CH3CN or acetone rather than being polymeric as in the solid state

Visible light-mediated gold-catalysed carbon(sp2)-carbon(sp) cross-coupling.

A dual photoredox and gold-catalysed cross-coupling reaction of alkynyltrimethylsilanes and aryldiazonium tetrafluoroborates is described. The reaction proceeds through visible-light mediated oxidative addition of aryldiazoniums, transmetalation of alkynyltrimethylsilanes and aryl-alkynyl reductive elimination. Exclusive selectivity for silyl-substituted alkynes is observed, with no reactivity observed for terminal alkynes

Understanding the reactivity of molecular precursors to colloidal nanocrystals

Colloidal semiconductor nanocrystals are materials with intriguing properties that make them useful for a diverse array of applications such as photocatalysts, light-absorbing materials in solar cells, light emitting diodes and luminescent biological tags, to name only a few. Performance of nanomaterials in these applications is directly related to the size, shape and stoichiometry of the nanocrystals. Strategies exist to control these characteristics during colloidal synthesis, but they tend to rely on certain surfactants, additives, or multi-step procedures to achieve desirable properties. This thesis describes new directions in the synthesis of colloidal nanomaterials that use computational chemistry as a guide. Using new and efficient methods in density functional theory (DFT) to reliably calculate bond dissociation energies (BDEs) of organodichalcogenide (sulfide or selenide) precursors enables the rational synthesis of dot, rod and tetrapod morphology cadmium chalcogenide nanocrystals. Precursors with weaker C-E (E = S, Se) bonds and stronger E-E bonds yielded dot-shaped nanocrystals, while precursors with stronger C-E and weaker E-E bonds afforded rod or tetrapod shapes. This methodology readily extends to the BDE calculation of tertiary phosphine chalcogenides with substituted phenyl, alkyl, perfluoroalkyl moieties or Verkade-type cage structures. In these systems the BDE of a series of P--S or P--Se bonds increases with slightly increasing bond distance, although the BDE of P--Se bonds is significantly lower than P--S bonds. Another promising method in colloidal nanocrystal synthesis is photochemical decomposition of precursors to access unusual phases or shapes. This thesis also describes the photochemical synthesis of cobalt(III) oxyhydroxide, Co(O)OH, nanocrystals from chloropentaamminecobalt(III) salts in aqueous solution. Compared to the thermal decomposition of the starting material in the absence of light, the photochemically-synthesized material exhibits a smaller size with a lower-temperature phase transition to cobalt(II,III) oxide, Co3O4

Rapid synthesis of an electron-deficient t-BuPHOX ligand: cross-coupling of aryl bromides with secondary phosphine oxides

Herein an efficient and direct copper-catalyzed coupling of oxazoline-containing aryl bromides with electron-deficient secondary phosphine oxides is reported. The resulting tertiary phosphine oxides can be reduced to prepare a range of PHOX ligands. The presented strategy is a useful alternative to known methods for constructing PHOX derivatives

Elastomer-modified phosphorus-containing imide resins

Phosphine oxide-containing polyimide resins modified by elastomers, are disclosed which have improved mechanical properties. These products are particularly useful in the production of fiber or fabric-reinforced composites or laminates

Studies of Soluble Polymer-supported Organocatalysts

Polymer-supported reagents and catalysts have been extensively studied in the past few decades as they not only facilitate separation and isolation of products after reactions but also enable reuse of reagents/catalysts. In particular, chemistry using polymer-supported organocatalysts has the advantage of avoiding the use of sometimes toxic transition metals. Since organocatalysts are often used at high mol% loading in catalytic reactions, immobilizing organocatalysts on polymers for recycling and reusing makes chemistry using organocatalysts attractive in larger scale syntheses. Chapter II of this dissertation focuses on using variable temperature 31P NMR spectroscopy to study and compare the dynamic behavior of silver complexes prepared from soluble polymer-supported phosphines and electronically similar low molecular weight phosphine ligands. The phosphine-silver complexes supported on terminally functionalized polyisobutylene (PIB) and poly(ethylene glycol) show similar kinetic behavior compared to their low molecular weight counterparts. However, the dynamic behavior of phosphine-silver complexes supported as pendent groups on a linear polystyrene is difficult to study because of significant line-broadening on 31P NMR spectra. Chapter III of this dissertation aims at examining the recyclability and reusability of PIB-supported phosphines as organocatalysts and reagents. PIB-supported alkyldiphenyl- and aryldiphenylphosphines were prepared and used as recyclable organocatalysts in addition and allylic amination reactions. The PIB-bound phosphines were useful reagents in aza-Wittig and Mitsunobu reactions. The PIB-bound phosphine oxides formed either from adventitious oxidation or during the course of reactions can be reduce to PIB-phosphines for reuse. Chapter IV of this dissertation describes preliminary studies on soluble polymer-supported N-heterocyclic carbene (NHC) organocatalysts. PIB- and polyethylene oligomer (PE_Olig)-supported NHC adducts were synthesized and the corresponding polymer-supported NHC catalysts were generated in situ in lactide polymerization and phenyl isocyanate trimerization reactions. The PIB-bound NHC catalyst generated in situ was not recyclable in a lactide polymerization. However, PIB- and PEOlig-bound NHC precatalysts showed modest recyclability in lactide polymerization and phenyl isocyanate trimerizations