Salt effects on Diels-Alder reaction kinetics

Metal ion-catalyzed Diels-Alder and hydride transfer reactions. Catalysis of metal ions in the electron-transfer step

Rates of Diels-Alder cycloaddition of anthracenes with p-benzoquinone and its derivatives as well as rates of hydride-transfer reactions from 10-methyl-9,10-dihydroacridine to the same series of p-benzoquinones are accelerated significantly in the presence of metal ions in acetonitrile. An extensive comparison of the catalytic effects of metal ions in electron transfer from one-electron reductants (cobalt tetraphenylporphyrin and decamethylferrocene) to p-benzoquinones with those in the Diels-Alder reactions of the quinones as well as the hydride-transfer reactions has revealed that the catalysis of metal ions in each case is ascribed to the 1:1 and 1:2 complexes formed between the corresponding semiquinone radical anions and metal ions. The transient absorption and ESR spectra of the semiquinone radical anion-metal ion complexes are detected directly in the electron-transfer reduction of p-benzoquinone derivatives in the presence of metal ions. The catalytic reactivities of a variety of metal ions in each reaction are well correlated with the energy splitting values of pi(g) levels because of the complex formation between O(2)(.-) and M(n+), which are derived from the g(zz) values of the ESR spectra of the O(2)(.-)-M(n+) complex.

A method for bioconjugation of carbohydrates using Diels-Alder cycloaddition

Diels-Alder-type cycloaddition of an electronically matched pair of saccharide-linked conjugated dienes and a dienophile-equipped protein gives neoglycoproteins at ambient temperature in pure water with a reaction half-life of approximately 2 h. Uncoupled saccharides can be recovered by diafiltration with complete conservation of the diene moiety, thus allowing their repeated use. The procedure described is the first for creating a carbon-carbon covalent bond in the bioconjugation step between a saccharide and a protein.

Fluorescence maxima of 10-methylacridone-metal ion salt complexes: a convenient and quantitative measure of lewis acidity of metal ion salts

Aromatic carbonyl compounds such as 10-methylacridone form the Lewis acid complexes including paramagnetic and redox active Lewis acids. The fluorescence energies of the Lewis acid complexes are well correlated with the O2*--Lewis acid complexes derived from the gzz-values of the ESR spectra of the superoxide anion-Lewis acid complexes, thus providing a quantitative measure of the Lewis acidity.

Palladium-catalyzed cross-coupling reaction of bis(pinacolato)diboron with 1-alkenyl halides or triflates: convenient synthesis of unsymmetrical 1,3-dienes via the borylation-coupling sequence

The synthesis of 1-alkenylboronic acid pinacol esters via a palladium-catalyzed cross-coupling reaction of bis(pinacolato)diboron (pin(2)B(2), pin = Me(4)C(2)O(2)) with 1-alkenyl halides or triflates was carried out in toluene at 50 degrees C in the presence of KOPh (1.5 equiv) and PdCl(2)(PPh(3))(2)-2Ph(3)P (3 mol %). The borylation of acyclic and cyclic 1-alkenyl bromides and triflates was achieved in high yields with complete retention of configuration of the double bonds. The method was applied to the one-pot synthesis of unsymmetrical 1,3-dienes via the borylation-coupling sequence.

Photochemistry of arylidene-beta-ionones: a highly efficient route to novel tricyclic ketones through intramolecular, exoselective photochemical (4 + 2) cycloadditions, occurring only in an aqueous-organic solvent

(E,E)-Arylidene-beta-ionones (1a-f) are converted to 1,7,7-trimethyl-3-(E-2'-arylethenyl)-2-oxabicyclo[4.4.0]deca-3,5-dienes (3a-f, approximately 90%) by irradiating in anhydrous solvents. Irradiation of (3a-f) in aqueous methanol results in Z,E-arylidene-beta-ionones (2), through retro-electrocyclization, which undergoes an intramolecular, exo-selective [4 + 2] photocycloaddition leading to 11-(exo)-aryl-1,7,7-trimethyl-tricyclo[4.4.0.1(2,4)]undec-5-ene-3-ones (8a-f, 60-80%). The latter rearrange over silica gel to afford, quantitatively, 5-aryl-7,11,11-trimethyl-tricyclo[5.4.0.0(3,6)]undec-1-ene-4-ones (5a-f). Irradiation of 1a-f in aqueous methanol leads to 8a-f, except in case of 1c,f wherein formation, respectively, of tricyclic ketones 9c (55%) and 9f (80%), derived from photodeconjugation in 2, followed by intramolecular [4 + 2] cycloaddition, is observed.

Dependence of the rate of an interfacial Diels-Alder reaction on the steric environment of the immobilized dienophile: an example of enthalpy-entropy compensation

This paper describes a physical organic study of the relationship between the rate for an interfacial Diels-Alder reaction and the steric environment around the reacting molecules. The study used as a model reaction the cycloaddition of cyclopentadiene with a self-assembled monolayer (SAM) presenting benzoquinone groups surrounded by hydroxyl-terminated alkanethiolates. The accessibility of the quinone was varied by preparing monolayers from hydroquinone-terminated alkanethiols of different lengths [HS(CH(2))(n)-HQ, n = 6-14] and a hydroxyl-terminated alkanethiol [HS(CH(2))(11)-OH] of constant length. Cyclic voltammetry was used to measure the rate of the reaction by monitoring the decay of the redox-active quinone. The second-order rate constant showed a modest change as the position of quinone was varied relative to the hydroxyl groups of the monolayer. For monolayers wherein the quinone groups were extended from the interface, the rate constants oscillated near 0.20 M(-1) s(-1) with an even-odd dependence on the length of the alkanethiol. For monolayers that positioned the quinone groups below the surrounding hydroxyl groups, the rate constants decreased by approximately 8-fold. Examination of the activation parameters revealed that the quinone groups that were positioned below the interface (and in a crowded environment) reacted with an enthalpy of activation that was 4 kcal/mol greater than did the quinones that were accessible at the interface. The reaction of the buried quinone, however, proceeded with an entropy of activation that was more favorable by 13 eu, and therefore with a similar free energy of activation. The combination of SAMs for preparing model interfaces and cyclic voltammetry for measuring rates provides a new opportunity for physical organic studies of interfacial reactions.

Rate acceleration and subsequent retardation of Diels-Alder reactions in LiClO4-diethyl ether: an experimental investigation

The experimental kinetic data for several Diels-Alder reactions show why a 5 M LiClO(4)-diethyl ether (LPDE) solution offers maximum enhancement of reaction rates, endo/exo ratios, and yields. These reactions, if carried out in LPDE solutions of concentrations higher than 5 M, show a substantial decrease in these kinetic parameters. This decrease is attributed to the very high viscosity of LPDE solutions near saturation, though this interpretation is not consistent when considered in terms of sizes of the diene and dienophile. The monoetherates, dietherates, and higher other clusters in LPDE solutions and their relationship with the Lewis acid catalysis by Li+ offer a more plausible explanation of both the enhancement and the decrease of the rate of Diels-Alder reactions in this medium.

LiCB(11)Me(12): a catalyst for pericyclic rearrangements

[structure: see text] Benzene and 1,2-dichloroethane solutions of the Li(+) salt of the weakly coordinating anion CB(11)Me(12)(-) catalyze the rearrangement of cubane to cuneane, quadricyclane to norbornadiene, basketene to Nenitzescu's hydrocarbon, and diademane to triquinacene. The Claisen rearrangement of phenyl allyl ether is also strongly accelerated.