Exploiting C-H bond activation is difficult, although some success has been achieved using precious metal catalysts. Recently, it was reported that C-H bonds in aromatic heterocycles were converted to C-Si bonds by reaction with hydrosilanes under the catalytic action of potassium t-butoxide alone. The use of Earth-abundant potassium cation as a catalyst for C-H bond functionalization seems to be without precedent, and no mechanism for the process was established. Using ambient ionization mass spectrometry, we are able to identify crucial ionic intermediates present during the C-H silylation reaction. We propose a plausible catalytic cycle, which involves a pentacoordinate silicon intermediate consisting of silane reagent, substrate, and the t-butoxide catalyst. Heterolysis of the Si-H bond, deprotonation of the heteroarene, addition of the heteroarene carbanion to the silylether, and dissociation of t-butoxide from silicon lead to the silylated heteroarene product. The steps of the silylation mechanism may follow either an ionic route involving K + and t BuOions or a neutral heterolytic route involving the [KO t Bu] 4 tetramer. Both mechanisms are consistent with the ionic intermediates detected experimentally. We also present reasons why potassium t-butoxide is an active catalyst whereas sodium t-butoxide and lithium t-butoxide are not, and we explain the relative reactivities of different (hetero)arenes in the silylation reaction. The unique role of potassium t-butoxide is traced, in part, to the stabilization of crucial intermediates through cation-π interactions. ASSOCIATED CONTENT Supporting Information Mass spectra, NMR spectra. DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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The alkoxytriphenylphosphonium ion intermediate of the Mitsunobu reaction can be generated using the Hendrickson reagent, triphenylphosphonium anhydride trifluoromethanesulfonate, 1. Strangely, while the reagent 1 can be used in place of the Mitsunobu reagents (triphenylphosphine and a dialkylazodicarboxylate) for the esterification of primary alcohols, secondary alcohols such as menthol undergo elimination. Evidence is presented to show that this unexpected result is due to the presence of trialkylammonium triflate salts. Such salts lead to a dramatic decrease in the rate of esterification relative to competing elimination. The Mitsunobu esterification of menthol with p-nitrobenzoic acid was re-examined and the occurrence of elimination reported for the first time. The presence of traces of tetrabutylammonium triflate led to a dramatic reduction in the yield of inverted ester and a corresponding increase in the yield of anti elimination product 2-menthene. The mechanism of the Mitsunobu reaction is discussed in the light of the dramatic salt effect on both the rate and outcome of the reaction and the possible involvement of ion pair clustering. In contrast, use of the reagent 1 resulted in syn elimination to give a 1:2 mixture of 2- and 3-menthenes. Finally, 1 and sodium azide can be used to convert a primary alcohol into an azide in high yield. There was no reaction under Mitsunobu conditions.

The first 10 addition products in the free radical polymerization of acrylonitrile initiated by AIBN have been trapped and identified; the resulting yields have allowed individual values of the first eight propagation rate constants to be estimated. These are the first realistic experimentally based estimates of a reasonable sequence of early propagation rate constants in any free radical polymerization. In this experiment, the concentration of the nitroxide trapping agent was maintained at a level sufficiently high to prevent the formation of high polymer, yet low enough to allow a competitive monomer addition to form the lower members of the propagation series before being trapped to form the oligomeric addition products. These oligomers were identified and quantified by electrospray ionization mass spectrometry operated in the selected ion recording mode in series with HPLC-UV. The results have allowed the first eight individual propagation rate constants, k An (n ) 1-8), to be estimated:The values of k An at 75 °C decrease from 5.4 × 10 3 L mol -1 s -1 with n ) 1 to a minimum of 2.1 × 10 3 L mol -1 s -1 with n ) 3 before leveling off with a constant value of 4.8 × 10 3 L mol -1 s -1 at n > 5.

The aminoglycoside antibiotic neomycin B has been converted into several novel building blocks that can be used for the specific modification of three of the four ring systems. Under carefully controlled conditions, the Mitsunobu reaction can be used to selectively dehydrate the ido ring to give the talo epoxide. Subsequently however, under more forcing conditions, the 2-deoxy streptamine ring undergoes Mitsunobu dehydration to give an aziridine. An unusual remote neighboring group effect was observed. When the primary hydroxyl of the ribose ring was blocked, aziridine formation on the deoxystreptamine ring did not occur. Both the epoxide and epoxide-aziridine neomycin building blocks can be ring-opened with azide and subjected to "click" type chemistry with terminal alkynes to generate a series of new neomycin analogues. These reactions can all be carried out without recourse to O-protecting groups. A detailed conformational analysis by NMR revealed some unexpected conformer preferences in these systems.

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The reaction of tert-butyl peroxypivalate (2) with methyl methacrylate (3) has been studied by the radical trapping technique employing 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl (1) as a scavenger. Thermolysis of 2 generated tert-butoxyl, tert-butyl, and methyl radicals in the ratios of 48:50:2 at 60 °C in 3. Both alkyl radicals underwent selective tail addition to 3. tert-Butyl radicals reacted about twice as fast as methyl radicals with 3. The absolute rate constant for addition of tert-butyl radicals to 3 was estimated to be 2.3 × 106 M-1 s-1 at 60 °C. The overall ratio of addition to H abstraction in the reaction of 2 with 3 was 5:1.

Collagenous peptides containing the Ehrlich chromogen (EC) were selectively isolated from a tryptic digest of bovine tendon by coupling to a diazotized polyacrylamide support. The isolated p-phenol-azo-EC peptides were purified and characterized by amino acid and sequence analyses. EC occurred in stoichiometric amounts in trimeric cross-linked chains originating from the known cross-link regions of type-I collagen. The major locus of the EC was alpha 2(I)Hyl-933 x alpha 1(I)Lys(Hyl)-9N x alpha 2(I)Lys(Hyl)-5N but it was also shown to occur at the loci alpha 1(I)Hyl-87 x alpha 1(I)Lys(Hyl)-16C x alpha 1(I)Lys(Hyl)-16C and alpha 1(I)Hyl-930 x alpha 1(I)Lys(Hyl)-9N x alpha 2(I)Lys(Hyl)-5N. After sequence analyses of the C-terminal helical cross-link region alpha 2(I)928-963, corrections are presented for residues 927, 930, 932 and 933 of the bovine alpha 2(I) chain. The collagen-associated EC is postulated to be a trisubstituted pyrrole formed by the reaction of the aldehyde form of a telopeptidyl lysine residue with a bifunctional keto amino cross-link. It is also proposed that when the telopeptidyl lysine residue is hydroxylated the above reaction will result in pyridinoline formation.