Michael P. Hartshorn

Making radical cations live longer

By all measures, 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) appears as a solvent with properties at the extreme. Its combination of low nucleophilicity, high hydrogen bonding donor strength, low hydrogen bonding acceptor strength, high polarity and high ionizing power makes it an ideal solvent for radical cations. Applications of HFP as a solvent for EPR spectroscopy and mechanistic studies of radical cations as intermediates in electrophilic aromatic substitution, photochemistry and spin trapping are described.

1,1,1,3,3,3-Hexafluoropropan-2-ol as a solvent for the generation of highly persistent radical cations

1,1,1,3,3,3-Hexafluoropropan-2-ol (HFP) has been found to be a superior solvent for the generation of radical cations, in many cases increasing their half-lives by a factor of 102 compared with those pertaining to trifluoroacetic acid. This study has mainly used 4-tolylthallium(III) bis(trifluoroacetate) and thallium(III) tris(trifluoroacetate) as oxidants, but DDQ could also be employed in the presence of 5% trifluoroacetic acid. Photochemical oxidation by mercury(II) bis(trifluoroacetate) also worked well. Thus room temperature recording of high quality EPR spectra of radical cations previously accessible only by flow or low-temperature procedures has become feasible.

Improved Syntheses of Chlorotricyanomethane and Carbamoylchlorodicyanomethane

Abstract Abstract. Improved procedures for the synthesis of chlorotricyanomethane by reaction of potassium tricyanomethanide with sulfuryl chloride in sulfolane and of carbamoylchlorodicyanomethane by reaction of potassium carbamoyldicyanomethanide with sulfuryl chloride in dichloromethane are described.

Reactions of epoxides—XIII : The backbone rearrangement of 3α-acetoxy-4α,5-epoxy-5α-cholestanes with lewis acid

Abstract 3α-Acetoxy-4α,5-epoxy-4β-methyl-5α-cholestane (Ia) undergoes a “backbone rearrangement” with BF3etherate to give the (enantio) 5β,14β-dimethyl-18,19-bisnorcholest-13(17)-ene derivative (IIa). One of the products obtained from reaction of 3α-acetoxy-4α,5-epoxy-5α-cholestane (Ib) with a Grignard reagent is now shown to be the related 13(17)-olefin (IIb). The epoxide Ib also undergoes a backbone rearrangement on treatment with BF3-etherate.

Reactions of propargyl alcohols. I. Lithium aluminium hydride reduction of 2,3,3-Trimethylhex-4-yn-3-ol

Lithium aluminium hydride reduction of (R)-(+)-2,3,3-trimethylhex-4-yn-3-ol (3) in diglyme gave(S)-(-)-4,5,5-trimethylhexa-2,3-diene (4) and (S)-(+)-(E)-2,3,3-trimethylhex-4-en-3-ol (5). The mechanism of the allene-forming reaction is discussed.

Nitration studies of 4-substituted toluenes, 5-substituted hemimellitenes and acetoxyprehnitene

Product distributions are reported for the nitration of 4-methoxy-, 4- bromo- and 4-acetoxy-toluenes, of 5-fluoro- and 5-acetamido- hemimellitenes, and of acetoxyprehnitene. A Hammett p value for ipso nitration is estimated from competitive nitrations with a series of 5-substituted hemimellitenes.

The addition–elimination mechanism in the photonitration of naphthalene by tetranitromethane

By the isolation and kinetic studies of an adduct (cis-1,4-dihydro-1-nitro-4-trinitromethylnaphthalene) from the photolysis of naphthalene–tetranitromethane in dichloromethane or acetonitrile, it is shown that the route to nitro substitution products proceeds via addition–elimination, the latter step being either thermal or base-catalysed.

The electronic effect of the phenylazo and t-butylazo groups

Hammett σp+-values for arylazo and t-butylazo groups have been determined by measurements of the kinetics of solvolysis of the appropriately substituted arylpropan-2-yl chlorides. They have been found to be considerably more positive than expected and differ significantly from earlier estimates based on the rates of electrophilic attack on azobenzene. An interpretation of the discrepancy has been advanced based on the differing orientations of the azo linkage with respect to the aromatic ring in the transition state. The introduction of methyl groups into positions ortho to the phenylazo and t-butylazo substituents causes a change in character from –I, –R to –I, +R. This is true not only for the solvolysis reaction but also for benzoic acid ionisation.

Formation and EPR spectra of radical species derived from theoxidation ofthe spin trap, α-phenyl-N-tert-butylnitrone(PBN),and some of its derivatives in 1,1,1,3,3,3-hexafluoropropan-2-ol.Formationof isoxazolidine radical cations

The photolysis of a solution of the spin trap, α-phenyl-N-tert-butylnitrone (PBN, 1) with 2,3-dichloro-4,5-dicyano-1,4-benzoquinone (DDQ) in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) containing 6% trifluoroacetic acid produces a persistent radical species, the six-line EPR spectrum of which has aN = 2.00 and aH = 2.78 mT. The half-life of this radical was ca. 20 min at 22 °C. A similar spectrum was obtained from PBNs substituted by an electron-withdrawing group (NO2, F) in the 4-position, whereas PBNs with an electron-donating group (4-MeO, 4-Me2N, 3,4-OCH2O) upon oxidation gave multi-line spectra which could be assigned to the corresponding radical cations.

Inverted spin trapping. Part V. 1,1,1,3,3,3-Hexafluoropropan-2-ol as a solvent for the discrimination between proper and inverted spin trapping

1,1,1,3,3,3-Hexafluoropropan-2-ol (HFP) has been tested as a solvent for spin trapping experiments. It sustained proper spin trapping of alkyl and aryl radicals generated in secondary cleavage reactions with no obvious complications, and also other neutral, stable radicals could be generated and kept stable for long periods in this solvent.The extreme persistency of radical cations in HFP was shown to be at least partly due to a strong attenuation of nucleophile reactivity. Hard nucleophiles reacted > 107 times slower with tris(4-bromophenyl)aminium ion in HFP than in acetonitrile, and also soft nucleophiles experienced large rate decreases, for example by a factor of > 103 for trinitromethanide ion. This means that the inverted spin trapping mechanism, possible under conditions in which a spin trap and a nucleophile is treated by an oxidizing agent, becomes severely impeded in HFP. This was demonstrated in a number of cases, for example, benzotriazolate ion, trinitromethamde ion and 3,5-lutidine. Even rather small proportions of HFP in dichloromethane (10–30%) had a completely inhibiting effect on the inverted spin trapping mechanism, and only triethyl phosphite and related esters underwent this reaction type in neat HFP, yielding trialkoxyphosphonio spin adducts from PBN. In addition, triphenylphosphine reacted in this way.Anions of imides become protonated in HFP, an additional factor inhibiting inverted spin trapping from potential imidyl radical sources. Thus conditions could be established for the unambiguous trapping of imidyl radicals. No radical from a ring-opened imidyl could be detected.