6.2. Photosystem II as a Source of Inspiration 2364 6.3. Ruthenium Complexes 2364 6.4. Manganese Complexes 2365 6.5. Concluding Remarks 2366 7. Reduction of Carbon Dioxide 2366 7.

Molecular Catalysis of Electrochemical Reactions. Mechanistic Aspects

The article discusses the mechanisms and the applications in organic synthesis, materials, supramolecular, and polymer synthesis of most organic reactions mediated by single electron transfer. Each reaction or class of reactions will be discussed by starting with the original discovery publication, followed by a summary of all or most review articles published in the field, and a discussion of the mechanism(s) and of the most important methodologic and synthetic developments since the most recent review was published. The mechanisms of most organic reactions are considered to proceed by two-electron transfer pathways, even though both biology and radical chemistry rely extensively on one-electron transfer processes. Radicals generated by homolytic cleavage at high temperature were traditionally employed in the industrial production of polymers and to a lesser extent in the synthesis of organic molecules.

Single electron transfer in radical ion and radical-mediated organic, materials and polymer synthesis.

The mechanism of methanol photorelease from 2-nitrobenzyl methyl ether (1) and 1-(2-nitrophenyl)ethyl methyl ether (2), and of ATP release from adenosine-5‘-triphosphate-[P3-(1-(2-nitrophenyl)ethyl)] ester (‘caged ATP', 3) was studied in various solvents by laser flash photolysis with UV−vis and IR detection In addition to the well-known primary aci-nitro transients (A, λmax ≈ 400 nm), two further intermediates preceding the release of methanol, namely the corresponding 1,3-dihydrobenz[c]isoxazol-1-ol derivatives (B) and 2-nitrosobenzyl hemiacetals (C), were identified The dependencies of the reaction rates of A−C on pH and buffer concentrations in aqueous solution were studied in detail Substantial revision of previously proposed reaction mechanisms for substrate release from 2-nitrobenzyl protecting groups is required:  (a) A novel reaction pathway of the aci-tautomers A prevailing in buffered aqueous solutions, eg, phosphate buffer with pH 7, was found (b) The cyclic intermediates B were identifi

/pdf/photochemical-reaction-mechanisms-of-2-nitrobenzyl-compounds-46a56gdba7.pdf

Photochemical reaction mechanisms of 2-nitrobenzyl compounds: methyl ethers and caged ATP.

Role of Organolithium Aggregates and Mixed Aggregates in Organolithium Mechanisms

The formation of novel organometallic compounds by the reaction of organic substrates with finely divided metal powders represents a powerful tool for the synthetic chemist. The direct reaction with a zerovalent metal is the only viable method of synthesis for many of these compounds. Accordingly, chemists have been actively developing new methods for increasing the reactivity of metal powders toward organic substrates. Three principal approaches have been developed in recent years: the metal vaporization method, sonochemistry, and the preparation of finely divided metal powders by the reduction of metal salts. A number of new methods of synthesis have resulted from these studies.

https://science.sciencemag.org/content/sci/246/4935/1260.full.pdf

Preparation of Organometallic Compounds from Highly Reactive Metal Powders

The rate of reaction of organic halides with magnesium is proportional to organic halide concentration and magne- sium surface area. All organic iodides examined and many secondary alkyl bromides appear to react with magnesium at mass- transport or diffusion-controlled rates in diethyl ether. The rates of less reactive organic bromides (neopentyl, phenyl, cyclo- propyl) contain contributions both from mass transport and from chemical reaction. Most organic chlorides react at rates which are not mass transport limited. Analysis of rate-structure profiles indicates that free carbanions and carbonium ions are not intermediates, and that the transition-state structure does not resemble an Sy2 reaction. Insertion of magnesium into the carbon-halogen bond also does not seem to occur, Soluble Mg(1) can be excluded as the only reactive species responsible for disappearance of alkyl halide. The two most plausible limiting rate-determining steps are halogen atom abstraction from the organic halide by the metal and electron transfer to the organic halide from the metal.

Mechanism of formation of Grignard reagents. Kinetics of reaction of alkyl halides in diethyl ether with magnesium

Preliminary studies of the mechanism of metal-halogen exchange. The kinetics of reaction of n-butyllithium with substituted bromobenzenes in hexane solution

llarlntctt constants for rcuction of'substiturcd arllbronridcs with nragncsium in scvcrillstllvcnt s lirlltlrl:dicthy lcthcr (p:l.l):di-n-but ylcthcr(r,,= 1.9):-l:lr:r'di- rr-butllethcr crclohcranc(p:1. 0):tctrahvdrol'u ran(p-0):J:lv:vt ctrahl'- drolunrn hcxanrcrirvlphos phonrnride (/r r 0):dicthrlcthcr contuining 0.lJ N{ clclopcntl'lnrrt gnesiunl bronlidc (ir - 0) In thc pollr sglvcnts. rcactlgn uppcars ttt bc nrass trrrnsport lirnitcd. The rrrtc ol'rcrtction ol'lrrll iodidcs with ntagncsiunl in dicthrl cthcr is probably also mass tnrnsport lintitcd (p : 0). Thc ratc of'rcaction ol'chlorobcnzcnc with rnagncsium (pr - 1.0) is ca. l0a slowlcr than that ol'bronrobenzenc in dicthll cthcr. and is not nrass transport lrmitcd. The rcactittn ol'rtrvl bronlidcs with tri-rr-buty'ltin hldride (AlBN. hr,) has p = l.-1. Analysis olthcsc valucs of p narrows thc choicc f or the ratc-dctermining stcp in the rcaction ol'bromobcnzenc with magncsiunr in dicthy'l cthcr to onc of threc limiting possibilitics: clcctron Lransfer from thc nrctal tp thc a11,l brontidc (3). ury'l radical tirrnration b),bronrinc abstraction from thc aryl brornidc by thc mctal (6). or. lcss probabll. inscrtion olu nugncsiunr atonr into thc carbon bronrinc bond (7).

Mechanism of formation of Grignard reagents. Kinetics of reaction of substituted aryl bromides with magnesium and with tri-n-butyltin hydride in ethereal solvents

Three observations indicate that the rate of reaction of cyclopentyl bromide wjith metallic magnesium in several ethe- real solvents is limited by the rate of its encounter with the metal. First, the rate of disappearance of cyclopentyl bromide is de- pendent on the first power of the organic halide concentration, the magnesium surface area, and rate at which the solution is stirred, and is inversely dependent on the solution viscosity (-d(RBr)/d?a (RBr)', A', @I, 7-l). Second, the activation energy for the reaction in diethyl ether (E, N 2.3 kcal/mol) is consistent with mixing and diffusion as rate limiting. Third, a number of representative alkyl halides react with magnesium in diethyl ether at very similar rates; no organic halides examined react faster than this rate. The dependence of the rate of reaction of neopentyl bromide on solution dielectric constant illustrates transition from transport-limited to non-transport-limited rates, and suggests appreciable charge separation in the chemical step which breaks the carbon-bromine bond.

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Harold R. Rogers

Papers

Mechanism of formation of Grignard reagents. Kinetics of reaction of alkyl halides in diethyl ether with magnesium

Preliminary studies of the mechanism of metal-halogen exchange. The kinetics of reaction of n-butyllithium with substituted bromobenzenes in hexane solution

Mechanism of formation of Grignard reagents. Kinetics of reaction of substituted aryl bromides with magnesium and with tri-n-butyltin hydride in ethereal solvents

Mechanism of formation of Grignard reagents. The rate of reaction of cyclopentyl bromide with magnesium is transport limited in diethyl ether

Electroorganic chemistry. II. Electroreduction of vicinal dibromides