Brian P. Roberts

Polarity-reversal catalysis of hydrogen-atom abstraction reactions: concepts and applications in organic chemistry

The rates and selectivities of the hydrogen-atom abstraction reactions of electrically-neutral free radicals are known to depend on polar effects which operate in the transition state. Thus, an electrophilic species such as an alkoxyl radical abstracts hydrogen much more readily from an electron-rich C–H bond than from an electron-deficient one of similar strength. The basis of polarity-reversal catalysis (PRC) is to replace a single-step abstraction, that is slow because of unfavourable polar effects, with a two-step process in which the radicals and substrates are polarity-matched. This review explores the concept of PRC and describes its application in a variety of situations relevant to mechanistic and synthetic organic chemistry.

Rotational spectra and geometries of the gas-phase dimers (CH4,HF) and (CH4,HCl)

Abstract The rotational spectra of five isotopomers of the dimer (CH 4 ,HCl) and two isotopomers of the dimer (CH 4 ,HF) have been investigated by the pulsed-nozzle FT microwave technique. Detailed consideration of the spectroscopic constants allows the conclusion that the HCl complex has an effective C 3ν geometry of the type CH 4 …HCl with a single hydrogen bond and internal rotation of the CH 4 subunit while the interaction in (CH 4 ,HF) is of the two-centre type, involving bonds CH…F and C…HF, which appears effectively to quench the internal rotation.

Free radical chain reactions for the preparation of novel anomeric carbohydrate difluoromethylene -phosphonates and -phosphonothioates

Abstract The generation of phosphonyl or thiophosphonyl radicals in the presence of carbohydrate gem-difluoenol ethers provides new routes to anomeric difluoromethylenephosphonates and difluoromethylenephosphonothioates, respectively.

Metallic behaviour of boron-containing carbon nanotubes

Abstract Paramagnetism in boron-containing carbon nanotubes was detected by SQUID measurements, and a g -value of 2.0020, corresponding to the free carrier spin, was observed over a wide temperature range (108–455 K). This value implies intrinsic metallic behaviour and contrasts with thermally activated semiconduction which multi-walled carbon nanotubes usually exhibit.

An extended form of the Evans–Polanyi equation: a simple empirical relationship for the prediction of activation energies for hydrogen-atom transfer reactions

An empirical approach has been used to devise a simple relationship [eqn. (B)] between the activation energy for an elementary hydrogen-atom transfer reaction (A) and ground state properties A˙+ H–B → A–H + B˙(A), Ea=Eof+αΔH°(1–d)+βΔχAB2+γ(sA+sB)(B) of the reactants and products. The role of polar effects, which operate in the transition state, is emphasised and described quantitatively in terms of the difference in Mulliken electronegativities (ΔχAB) of the radicals A˙ and B˙. Eqn. (B) reproduces the activation energies for 65 reactions, taken from the literature, within a standard error of ±2.0 kJ mol –1 and with a correlation coefficient of 0.988. Reactions of widely differing types are included and no distinction is made between gas-phase reactions and those which take place in non-polar solvents. Examples of hydrogen-atom transfer reactions which are not treated satisfactorily by eqn. (B) are discussed.

Deoxygenation of alcohols by the reactions of their xanthate esters with triethylsilane: An alternative to tributyltin hydride in the Barton-McCombie reaction

Abstract O -Alkyl S -methyl dithiocarbonates derived from primary or secondary alcohols (ROH) react with trialkylsilanes in non-aromatic solvents to give the corresponding hydrocarbons RH in good yields; the reductions are promoted by thiols which act as polarity reversal catalysts.

Dynamic aspects of cyclodextrin host-guest inclusion as studied by an EPR spin-probe technique

Major changes in the EPR parameters of benzyl tert-butyl nitroxide and other dialkyl nitroxides on complexation by cyclodextrins (CDs) make these radicals particularly suitable probes for studying inclusion phenomena, because the signals for the free and included species are well separated. Furthermore, selective line broadening of the EPR spectra enables the kinetics of inclusion to be investigated [Eq. (a)].

Polarity-reversal catalysis by thiols of radical-chain hydrosilylation of alkenes

Abstract The radical-chain hydrosilylation of alkenes by triethylsilane under very mild conditions is promoted by thiols, which act as polarity-reversal catalysts for the abstraction of electron-rich hydrogen from the silane by nucleophilic β-silylalkyl radicals.

Enantioselective radical-chain hydrosilylation of prochiral alkenes using optically active thiol catalysts

Abstract The radical-chain hydrosilylation of alkenes of the type H2C=CR1R2, catalysed by small amounts of optically active thiols, affords functionalised organosilanes in moderate enantiomeric purity by a mechanism which involves enantioselective hydrogen-atom transfer from the thiol to a prochiral β-silylalkyl radical.

Homolytic reactions of ligated boranes. Part 11. Electron spin resonance studies of radicals derived from primary amine–boranes

Photochemically generated t-butoxyl radicals react with the primary amine–boranes RNH2→BH3(R = Me or But) to form, initially, the nucleophilic amine–boryl radical RNH2→BH2, which subsequently abstracts hydrogen from the parent amine–borane to give the more stable isomeric aminyl–borane radical RNH→BH3. The amine-boryl radicals can be intercepted by alkyl bromides or chlorides or by nitriles, with which they react by halogen-atom abstraction or by addition to the CN group to give iminyl radicals, respectively. The e.s.r. spectra of the aminyl–borane radicals show the presence of extensive hyperconjugative delocalisation of the unpaired electron onto the BH3 group [a(3BH) 64–66 G]. Monoalkylaminyl-borane radicals react readily with alkenes, with arenes, and with conjugated or cumulated dienes to transfer a β-hydrogen atom from boron to give alkyl, cyclohexadienyl, or allyl radicals, respectively. Hydrogen atom transfer to alkenes from the. Electrophilic MeNH→BH3 takes place with high regioselectivity to give the more stable alkyl radical when two adducts are possible; the rate of transfer increases as the ionisation potential of the alkene decreases along the series CH2CH2 < MeCHCH2 < Me2CCH2≈ Me2CCHMe < Me2CCMe2. The absolute rate coefficient for reduction of propene by MeNH→BH3 to give the isopropyl radical was determined to be ca. 2 × 103 dm3 mol–1 s–1 at 282 K. Competition experiments have been carried out to determine the relative rates of the various reactions undergone by RNH2→BH2 and RNH→BH3. The results are interpreted with the aid of ab initio molecular-orbital calculations at the 6-31G** level for RNH2→BH2, RNH→BH3, RNH2→BH3, and RNH[graphic omitted]BH2(R = H or Me), together with similar calculations for the isoelectronic organic counterparts in which the NB moiety is replaced by a CC grouping.