Mechanistic experiments

Trifluoromethylzinc compounds can be prepared via the direct reaction of dihaloditluoromethane with zinc powder in DMF [J5] (equation 24) In this reaction, the DMF functions both as solvent and reactant Mechanistic experiments support a difluorocarbene reaction intermediate Indeed, a mixture of zinc and difluorodibromomethane in THF has been used for the synthesis of gewi-difluo-ro-cyclopropane derivatives [34 (equaUon 25)... 

Still has also carried out mechanistic experiments from which he could deduce that the major reduction pathway is by attack of hydride ion at the sulphur atom. This conclusion was deduced from the fact that reduction with sodium borodeuteride-aluminium oxide gave a sulphoxide that had only incorporated about 25% mole equivalent of deuterium on to a methyl carbon atom bound to the sulphur atom. The mechanistic pathway for direct reduction is outlined in equation (38), whereas the pathway whereby deuterium could be incorporated is portrayed in equation (39). These reactions support the proposed mechanism for the hydride reduction of sulphones as outlined in Section III.A.l, namely that attack at sulphur by hydride ions may occur, but will be competitive with proton abstraction in cases when the attack at sulphur is not facilitated. 

Rauhut et al. [2, 3] conducted a number of mechanistic experiments aimed at elucidating the reaction mechanism of the POCL reaction. It was noted that fluorophores were not consumed in the reaction, and that the CL emission spec-... 

The hydroformylation of alkenes was accidentally discovered by Roelen while he was studying the Fischer-Tropsch reaction (syn-gas conversion to liquid fuels) with a heterogeneous cobalt catalyst in the late thirties. In a mechanistic experiment Roelen studied whether alkenes were intermediates in the "Aufbau" process of syn-gas (from coal, Germany 1938) to fuel. He found that alkenes were converted to aldehydes or alcohols containing one more carbon atom. It took more than a decade before the reaction was taken further, but now it was the conversion of petrochemical hydrocarbons into oxygenates that was desired. It was discovered that the reaction was not catalysed by the supported cobalt but in fact by HCo(CO)4 which was formed in the liquid state. 

However, mechanistic experiments have involved the apices of the cyclopropane ring as targets for the nucleophilic group of the enzyme. Only the pro-R hydrogen was usually removed by the enzyme and transferred to NAD+, and the stereochemical course of the nucleophilic ring opening of the cyclopropanes was consistent with predictions on the basis of frontier orbital theory prediction [48]. 

Trifluoromethylzinc reagent can be prepared from the direct reaction of dihalodifluoromethane and zinc in DMF [42] (Scheme 13). In this remarkable reaction, DMF functions both as a solvent and reactant, and difluorocarbene is the reactive intermediate, based on mechanistic experiments. Dolbier et al. have utilized this reaction as a difluoromethylene cyclopropanation reaction [43] (Scheme 14). 

Mechanistic Experiments. Sample solutions of [Mo -CsHs ] and [Mo(r75-C5H5)2D2] were prepared by distilling appropriate degassed solvent into Schlenk tubes that contained the complex. Tne sample solutions were subjected to short-term photolysis (<5 min) in a 350-nm Rayonet reactor. The solutions were frozen immediately after photolysis, and the mass spectrum of the gases above the frozen solutions was recorded. 

Perhaps the most significant of the numerous applications of radiotracers in chemistry has been the study of chemical reaction mechanisms. In fact, most of the proposed reaction mechanisms have been verified by means of a radiotracer study. One of the simplest mechanistic experiments using radiotracers is to test the equivalence of various atoms in molecules in chemical reactions. An example of this type of study is the work of Volpin et al. (1959) on the equivalence of the seven carbon atoms in the tropylium ring. Volpin et al. reacted labeled diazomethane with benzene and brominated the cyclohepatriene product to form a labeled tropylium bromide, as shown below ... 

Farnia and coworkers have described a novel 1,2-phenyl shift which takes place during the electrochemical reduction of the indenes 2 (equation 4)11. Mechanistic experiments demonstrated that rearrangement takes place at the dianion stage. 

The apparent rate of hydrolysis and the relative abundance of reaction products is often a function of pH because alternative reaction pathways are preferred at different pH. In the case of halogenated hydrocarbons, base-catalyzed hydrolysis will result in elimination reactions while neutral hydrolysis will take place via nucleophilic displacement reactions. An example of the pH dependence of hydrolysis is illustrated by the base-catalyzed hydrolysis of the structurally similar insecticides DDT and methoxy-chlor. Under a common range of natural pH (5 to 8) the hydrolysis rate of methoxychlor is invariant while the hydrolysis of DDT is about 15-fold faster at pH 8 compared to pH 5. Only at higher pH (>8) does the hydrolysis rate of methoxychlor increase. In addition the major product of DDT hydrolysis throughout this pH range is the same (DDE), while the methoxychlor hydrolysis product shifts from the alcohol at pH 5-8 (nucleophilic substitution) to the dehydrochlorinated DMDE at pH > 8 (elimination). This illustrates the necessity to conduct detailed mechanistic experiments as a function of pH for hydrolytic reactions. 

A Pd-catalysed method for C-H activation/C-C bond formation, with iodine(III) reagent, [Ph2I]BF4, has been reported. The reaction showed a high functional group tolerance, regioselectivity, and scope under relatively mild conditions. Preliminary mechanistic experiments have provided evidence in support of a Pd(II)/(IV) catalytic cycle for this transformation.42... 

In related chemistry, the borane/silane catalytic system can cleanly dealkylate phosphonic and phosphinic esters RPO(OR,)2 or R2PO(OR/) to give silyl esters with trialkyl silanes.259 If more active silane reagents like Ph2SiH2 or PhSiH3 are employed, catalytic reduction to primary or secondary phosphines is observed. Mechanistic experiments strongly support a silane activation pathway for this chemistry. 

Woerpel and coworkers interpreted the results of these mechanistic experiments as evidence that the insertion of silylene into the C-O bond occurs through a [1,2]-Stevens rearrangement of oxonium ylide 198 and that a competitive [2,3]-sigmatropic rearrangement of 198 could account for allylic transposition. 

The idea that N02 binding and reduction occurs at Cu-2 in NiR from A. cycloclastes is also supported by recent mechanistic experiments (24). In examinations of enzyme preparations having maximum Cu-1 content but depleted Cu-2 sites, specific activity (production of NO frc5m N02") was found to be directly proportional to the Cu-2 content. This result argues against a previous suggestion (25) that Cu-1 is the site of catalysis in the enzyme and implies that the low specific activities of other copper NiRs that are reported to contain only type 1 centers (31, 32) may be due to the fact that type 2 centers are required in these... 

For FeRu3(CO)i2(phCECPh) the equilibrium mixture contains about 90% of the axial isomer. Although detailed mechanistic experiments were not conducted for the isomerization process, it was observed that the rate of isomerization occurs more slowly in concentrated solutions than in dilute solutions and more slowly under an atmosphere of CO than under an N2 atmosphere. Furthermore, no exchange of coordinated alkyne with free alkyne occurs during the isomerization process. These various experiments indicate that the isomerization must be an intramolecular process and likely proceeds via initial CO dissociation. 

However, in most circumstances, the first reaction is an intermediate step, and hydrolysis is both acid and base catalyzed. The relative rates of the two steps also depend on the structure of the substrate. Obviously, such hydrolysis of the solvent molecules will render emf data entirely unreliable. We also feel that the reliability of many rate and mechanistic experiments done in water-acetonitrile mixed solvents may be suspect. 

Most cases, however, are probably stereoretentive by design, though quite possibly unwittingly. Mechanistic experiments are usually ar-... 

A knowledge of the stereochemistry of both exchange and carbonylation should allow some interesting mechanistic experiments to be carried out. 

S,3,)-(EBIH)Zr(=NAr)(THF)2 promotes highly enantioselective cycloaddition reactions with allenes and, in certain cases, this system allows conversion of an allene racemate into a mixture enriched in one enantiomer. Mechanistic studies about the enantioselective cycloaddition and stereoinversion of allenes mediated by imidozirconocenes have revealed that the initial [2 + 2]-cycloaddition to form the azazirconacyclobutane is stereospecific and is not involved in the racemization process.728 The reactive zirconocene imido precursor r -(EBIH)Zr(NHBut)(Me) 958 has been shown to activate a variety of hydrocarbons R-H with primary alkyl, alkenyl, and aryl G-H bonds to form the corresponding alkyl derivative r -(EBIH)Zr(NHBut)(R) 959 with concomitant elimination of methane729 (Scheme 240). Mechanistic experiments support the proposal of intramolecular elimination of methane followed by hydrocarbon G-H addition. 

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