RR mechanism

Cinnamonitrile (9) and substituted analogues are easier to reduce than the simple nitriles and form less reactive radical anions. The mechanism and kinetics of the EHD of 9 has therefore been much studied. In DMF n 1, and RRDE measurements were in agreement with the RR mechanism at substrate concentrations <8 mM [55]. The rate constant for the rate-determining dimerization step (RR mechanism) has been determined by several methods in DMF (Table 3). In liquid ammonia T = —43°C), dimerization is considerably slower (Table 3), and, significantly, even in the presence of /-PrOH the coulometric n-value was small (0.05-0.27), indicating considerable polymerization [52]. 

Most—if not all—of the preparative results just presented for reduction of 12 can be rationalized within the RR mechanism when effects of hydrogen-bonding and proton transfer are taken into account. Under conditions with very low proton donor concentrations DPSCC measurements on 12a-b in DMF indicate that the initial dimerization step is reversible with a large equilibrium constant, and the rate-determining step is protonation (or cyclization) of the dimer dianion [10,11]. Assuming this mechanism, the equilibrium constants for the dimerization were determined = 109 for 12a and = 53.1... 

Under identical experimental conditions (DMF, 0.28 M H2O, 0.1 M Et4NBr), a thorough study of a large series of cinnamic acid esters led to the conclusion that the EHD reaction followed the RR mechanism with rate-determining dimerization [16]. The rate constants for dimerization were shown to correlate closely with the eP values for the reduction process the more easily the substrate is reduced, the faster is the dimerization (Table 3) [16]. The phenomenon can for this closely related series of compounds be rationalized in terms of a systemic change of the unpaired electron density at the position of dimerization [76]. 

Although no product was isolated, the results obtained by polarography, CV, RRDE, and coulometry for the reduction of l-phenyl-2-nitropropene (19g) in DMF were consistent with fast dimerization according to the RR mechanism [72] (Table 3). 

In MeCN both the water content and the substitution pattern influence the preferred coupling of 2-cyclohexen-l-ones (21) [97,98]. More 3,3 -dimer was formed in the absence of water and in the absence of substituents, whereas pinacol formation was favored by the presence of water (Table 6). The 4,4-dimethyl-2-cyclohexen-l-one only gave pinacol (35%) together with unidentified polymeric material when reduced in DMF, where the dimerization according to LSV measurements follows the RR mechanism [99]. 

On electroanalytical scales the reductions of 51 and 52 in DMF are 1-F processes, and the data obtained are consistent with dimerization of two radical anions, that is, an RR mechanism. The rate constants for dimerization have been determined by several methods (Table 11), and show a very large difference between 51 and 52, with the rate constant for dimerization of 51 being 3-4 orders of magnitude higher than that for dimerization of 52 . ... 

In dry MeCN, n = 0.6 was found for 51, but the value increased to 0.94 on addition of water up to 10% [13]. For 52a in very dry MeCN, the mechanism of the reductive dimerization was examined and the experimental results were interpreted as an RS mechanism under these conditions [125]. However, addition of water increased the rate of reaction considerably, and in the presence of water the kinetic measurements were in accord with the RR mechanism [126]. In DMF, addition of water also accelerates the dimerization process for 52 [7,10], similar to what is observed for many monoactivated alkenes (Sec. II.A.7). The accelerating effect of water in DMF on the dimerization of 52a has been studied in greater detail [15]. On the basis of a reaction order in water close to 1, it was suggested that the dimerization reaction takes place between a free radical anion and a hydrogen-bonded radical anion [15]. The involvement of hydrogen bonding between radical anions and water may also account for the low activation energies found for the reductive dimerization of 52a in MeCN [126] and in DMF [10] (see Table 11). 

The theoretical background of the above formulated principle of extremum resistance may be deduced from the following considerations. Let us examine first a one RR mechanism... 

Rando RR. Mechanism based irreversible enzyme inhibitors. Methods Enzymol 1977 46 28-41. 

Rando RR. Mechanisms of naturally occurring irreversible enzyme inhibitors. Acc Chem Res 1975 8 281-288. 

Singer TP, Ramsay RR. Mechanism of neurotoxicity of MPTP. FEBS Lett 1990 274 1-8. 

Baer L, Platman SR, Kassir S, Fieve RR. Mechanisms of renal lithium handling and their relationship to mineralocorticoids a dissociation between sodium and lithium or)s,.JPsychiatr Res 91 )%, 91-105,... 

Pisani C 1999. Software for the Quantum-mechanical Simulation of the Properties ot rr-.,sialliri Materials State of the Art and Prospects. THEOCHEM 463 125-137. 

Tschitschibabin amination of pyridine, the mechanism of which has been established as involving an intermediate charge distribution of the thiazole molecule as well as of the thiazolium ion. 

Both steps m this general mechanism are based on precedent It is called elec trophilic addition because the reaction is triggered by the attack of an acid acting as an electrophile on the rr electrons of the double bond Using the two rr electrons to form a bond to an electrophile generates a carbocation as a reactive intermediate normally this IS the rate determining step... 

The mechanism of the Diels-Alder reaction is best understood on the basis of a molecular orbital approach To understand this approach we need to take a more detailed look at the rr orbitals of alkenes and dienes... 

Resonance Raman Spectroscopy. If the excitation wavelength is chosen to correspond to an absorption maximum of the species being studied, a 10 —10 enhancement of the Raman scatter of the chromophore is observed. This effect is called resonance enhancement or resonance Raman (RR) spectroscopy. There are several mechanisms to explain this phenomenon, the most common of which is Franck-Condon enhancement. In this case, a band intensity is enhanced if some component of the vibrational motion is along one of the directions in which the molecule expands in the electronic excited state. The intensity is roughly proportional to the distortion of the molecule along this axis. RR spectroscopy has been an important biochemical tool, and it may have industrial uses in some areas of pigment chemistry. Two biological appHcations include the deterrnination of helix transitions of deoxyribonucleic acid (DNA) (18), and the elucidation of several peptide stmctures (19). A review of topics in this area has been pubHshed (20). 

In order to distinguish between kinetic and thermodynamic phenomena it is convenient to refer to the former as the 7tr/ i-effect and the latter as the tra/u-influence or static /ra/u-effect". though this nomenclature is by no means universally accepted. However, it appears that to account satisfactorily for the kinetic /rau.s-effect , both it (kinetic) and a (thermodynamic) effects must be invoked to greater or les.ser extents. Thus, for ligands which are low in the Trans series (e.g. halides), the order can be explained on the basis of a u effect whereas for ligands which arc high in the series the order is best interpreted on the basis of a jt effect. Even so, the relatively high position of H , which can have no rr-acceptor properties, seems to be a result of a a mechanism or some other interaction. 

The mechanism of this reaction is not straightforward but the crucial step appears to be nucleophilic attack by water or OH" on the eoordinated ethene to give rr-bonded —CH2CH2OH which then rearranges and is eventually eliminated as MeCHO with loss of a proton. 

Molar absorptivity. 502 Molecular ion (M+), 410 Molecular mechanics. 130 Molecular model, dopamine, 930 acetaminophen, 29 acetylene, 18 adenine, 67 adrenaline, 323 alanine, 28, 1016 alanylserine, 1028 rr helix, 1039 p-aminobenzoic acid, 25 anti periplanar geometry, 387 a recoline, 79 aspartame, 29 aspirin. 17 ball-and-stick, 61 /3-pleated sheet, 1039 p-bromoacetophenone, 449 bromocyclohexane, 121 butane, 80... 

The term vitamin E describes a family of eight antioxidants, four tocopherols, alpha (a), beta ((3), gamma (y) and delta (8), and four tocotrienols (also a, (3, y, and 8). a-Tocopherol is present in nature in only one form, RRR a-tocopherol. The chemical synthesis of a-tocopherol results in eight different forms (SRR, SSR, SRS, SSS, RSR, RRS, RSS, RRR), only one of which is RRR a-tocopherol. These forms differ in that they can be right (R) or left (S) at three different places in the a-tocopherol molecule. RRR a-tocopherol is the only form of vitamin E that is actively maintained in the human body and is therefore the form of vitamin E found in the largest quantities in the blood and tissue. A protein synthesized in the liver (a-TTP alpha-tocopherol transfer protein) preferentially selects the natural form of vitamin E (RRR a-tocopherol) for distribution to the tissues. However, the mechanisms for the regulation of vitamin E in tissues are not known... 

Detailed kinetic studies in connection with digital simulations do confirm the RR coupling mechanism postulated in older publications as well as the oxidation of the resulting dimer D to the dication D. But the surprising drop in the height of the reduction wave for the redox pair as the concentration... 

It seems likely that the mechanism of the Wurtz reaction consists of two basic steps. The first is halogen-metal exchange to give an organometallic compound (RX -(- M —+ RM), which in many cases can be isolated (12-36). Following this, the organometallic compound reacts with a second molecule of alkyl halide (RX + RM —> RR). This reaction and its mechanism are considered in the next section (10-94). 

It is necessary to postulate the solvent cage because, if the radicals were completely free, the products would be about 50% RR, 25% RR, and 25% R R. This is generally not the case in most of these reactions RR is the predominant or exclusive product. An example where an Sn2 mechanism has been demonstrated (by the finding of inversion of configuration at R) is the reaction between allylic or benzylic... 

The first step is not a free-radical process, and its actual mechanism is not known. Compound 30 is an acyl hypohalite and is presumed to be an intermediate, though it has never been isolated from the reaction mixture. Among the evidence for the mechanism is that optical activity at R is lost (except when a neighboring bromine atom is present, see p. 899) if R is neopentyl, there is no rearrangement, which would certainly happen with a carbocation and the side products, notably RR, are consistent with a free-radical mechanism. There is evidence that the Simonini... 

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