Case formation

In both cases we could not exceed a HPLC purity of about 84-85 %. The same symptom appears in other heterocyclic series and is due to the fact that there is always formation of the allyloxy derivative and in some cases formation of the disubstituted compound. 

Further reactions of 5/6 with the sterically less bulky silanes HSiCl3, Ph2SiHCl and Ph2SiH2 give the thermo-labile /x-biscarbyne complex 14, which has been spectroscopically characterized (Table 1). In these cases, formation of the silyl hydrides 11, 13, 15 and 16 as products of an oxidative addition at the 16e -complex generated from 6 (attack at Mn) becomes important. 

A1 quantifiable evaluation documentation should be prepared so that it would conform to the nature of a facility "Safety Case" format which may be required for submittal to governmental agencies at a later date. 

Allenylsilanes 159 (Eq. 13.53) and 161 (Eq. 13.54) differ in the axial stereochemistry of the allene function. In each case, formation of the benzyl imine, followed either by treatment with tin(IV) chloride in benzene at room temperature or heating in toluene, leads to diastereomeric products 160 and 162 [63], Significantly, there is no crossover, pointing to a concerted (or fast, stepwise) process. Since the absolute stereochemistry of the allenylsilanes is easily controlled, the methodology is ideal for applications in total synthesis. Weinreb and co-workers have used the reaction for his synthesis of the marine natural product (-)-papuamine. 

LFP-Probe Method. In cases where the radicals of interest do not contain a useful chromophore, the LFP technique can be modified by incorporation of a probe radical reaction that gives a product with a chromophore. The probe reaction can be unimolecular or bimolecular, a constant concentration of probe reagent is employed in the latter case. Formation of the detectable species occurs with an observed first-order or pseudo-first-order rate constant equal to k0. In the presence of another reagent X that reacts with the original radical, the rate constant for formation of detectable species is kohs = k0 + kx [X], and the bimolecular rate constant is determined (as before) by conducting the reaction at varying concentrations of X. Note that the LFP-probe technique is a direct method even though the reactant or product of interest is not monitored. 

Mechanistically similar to the reaction of nucleophilic metallates with a-haloimines is their reaction with amides. In this case formation of the carbene complex requires treatment with a silyl chloride (Figure 2.8) [42,125,126]. 

CofNHs) also photo-cleaves DNA [117], but in this case, formation of Co (II) and oxidised ligand from the LMCT state could represent an alternative pathway to a direct oxidation, leading to strand scission. 

Figure 10. Kleitz s reaction pathway model for solid-state gas-diffusion electrodes. Traditionally, losses in reversible work at an electrochemical interface can be described as a series of contiguous drops in electrical state along a current pathway, for example. A—E—B. However, if charge transfer at point E is limited by the availability of a neutral electroactive intermediate (in this case ad (b) sorbed oxygen at the interface), a thermodynamic (Nernstian) step in electrical state [d/j) develops, related to the displacement in concentration of that intermediate from equilibrium. In this way it is possible for irreversibilities along a current-independent pathway (in this case formation and transport of electroactive oxygen) to manifest themselves as electrical resistance. This type of chemical valve , as Kleitz calls it, may also involve a significant reservoir of intermediates that appears as a capacitance in transient measurements such as impedance. Portions of this image are adapted from ref 46. (Adapted with permission from ref 46. Copyright 1993 Rise National Laboratory, Denmark.)...

Organic fungistatic and bacteriostatic additives are currently employed, but in all cases, formation of resistant strains and the toxicity of the bioactive additive must be considered. 

Dimeric products, whose structures have been confirmed by X-ray crystallography [79BSB113 80AX(B)1418], have also been isolated from the electrochemical reduction of N,N -disubstituted 6-phenyldihydrodiazepi-nium salts [79BSB113 81JCS(P2)801]. In these cases formation of pyrol-... 

I, 3,4-oxadiazole (160) occurs only when the 1,2,4-oxadiazoIe contains tautomerizable groups in the position 3 in different cases, formation of open-chain products (162) by the reaction of the solvent with the photolytic intermediate will be the predominant photoreaction (Scheme 29) (68TL2421 88JCS(P1)1313, 88JHC931). 

In systems containing two or more adsorbates, either competitive or synergistic effects may operate. The commonest synergistic effect is that of ternary adsorption (11.5.4). Competitive behaviour may involve competition for the same surface sites, indirect effects due to the change in the electrostatic properties of the oxide/water interface and in some cases, formation of non sorbing, metal-ligand complexes in solution. 

R = H, Ph) have been obtained from the anionic [Ru(N4Meg)] and ethyne or HC CPh, respectively [51], In this case, formation of an intermediate r -alkyne complex is unlikely, the probable mechanism being deprotonation of the alkyne and coordination of the alkynyl anion followed by proton transfer. 

Following are further examples of reversed reactivity order. The formation of 5-oxo-TPs is achieved by means of condensations of AT (in the presence of sodium ethoxide) with ethyl 3,3-diethoxypropionate or 3-ethoxyacrylate (64ZOB499), propiolic acid (70CB3266), and cyanoacetate (61CPB801 64CB1373, 64IZV1475), respectively. In the last case (formation of 12), the fusion of reactants without catalyst allows the isolation of intermediate amide 11 (Scheme 5). 

It was shown later that cyanodimethylsulphonium salts behave similarly upon electro-reduction, although a much more reactive ylid is formed in this case. Formation of relatively unreactive and isolable sulphur ylids has teen reported for the electrolysis of 9-fluorenylsulphonium salts . 

Anodic oxidation of JV,N-dimethyl-co-hydroxyamides (57) in CH3OH-Bu4NBF4 at a platinum anode leads to formation of N-methoxy-JV-methyl-co-hydroxyamides (58) in high yield.116 The latter could in some cases (formation of five-, six-, and seven-membered rings) easily be transformed to l,3-oxaza-4-oxo heterocyclic systems (59) by acid catalysis [Eq. (49)]. No direct formation of the 1,3-oxazaheterocycles was observed, e.g., 57++59. An intramolecular addition of the hydroxy group to the intermediate acylam-monium ion is believed to be hindered by adsorption phenomena at the anode surface. 

The condensation of an aldehyde, benzyl carbamate, and triphenyl phosphite, first described by Oleksyszyn et al., 25,26 affords a direct route to a-aminoalkylphosphonates 4 that are conveniently protected for subsequent reactions (Scheme 4). Since dealkylation of the quaternary phosphonium intermediate 3 is not possible in this case, formation of the pen-tavalent product 4 presumably involves activation of the solvent and formation of phenyl acetate. This method is useful for the synthesis of aliphatic and aromatic amino acid analogues. However, monomers with more elaborate side chains are often incompatible with the reaction conditions. The free amine can be liberated by treatment with HBr/AcOH or by hydrogenolysis after removal of the phenyl esters. The phosphonate moiety can be manipulated by ready exchange of the phenyl esters in alkaline MeOH and activation as described in Section 10.10.2.1.1. Related condensations with other trivalent phosphite derivatives have been reported. 27-30 ... 

In a detailed study of the reaction of methylene ketones with carboxylic acids in the presence of polyphosphoric acid, pyran-4-ones were obtained from l,3-diphenylpropan-2-one, phenylpropan-2-one and pentan-3-one (61JA193). In the first case, formation of 2,6-dimethylpyran-4-one ( 14 R = H) rather than the 2-benzyl-6-methyl-3-phenyI isomer... 

It seems possible that a block mechanism also operates in the assembly of the chain of polysaccharide 42, present in the cell wall of Bacillus cereus.124 In this case, formation of the polymer from a-GlcNAc-pp-Bpr, UDP-ManNAc, UDP-GalNAc, and UDP-Glc was observed.74... 

Several studies concerning D-mannosyl transfer from GDP-Man and Man-p-Pre onto unidentified acceptors in Mycobacteria have been published.67,393 The enzymic system may use methyl a-D-mannopyranoside as the exogenous acceptor for D-mannosyl transfer.394 In this case, formation of a-(l- 2) linkages was observed they are present in 3-O-methyl-D-mannose-containing polysaccharides and a D-arabino-D-mannan characteristic of mycobacterial cell-wall. 

In most cases, formation of the ethyl ester provides a satisfactory protecting group for the carboxyl function. 

In all cases, formation of a bipolaron state coincides with a complete bleaching of the original tt-tt polyene transition, which in effect moves the optical absorption "window" several hundred nm to the red. More recently we have discovered that polaronic state formation (EDG = MeO) can be preferentially controlled if the quantity of oxidizing agent is carefully monitored (25). 

Note. (1) Osmium tetroxide is extremely irritating and toxic and constitutes a severe eye injury hazard. It may be purchased in sealed ampoules, e.g. 100 mg the solution in t-butyl alcohol must be prepared and dispensed in an efficient fume cupboard, with the added protection of gloves and goggles. This solution is reasonably stable (e.g. the decomposition after one month is about 20%), provided that no 2-methylprop-l-ene arising from the t-butyl alcohol is present as impurity. In the latter case formation of... 

The position of the ASV peak on the voltage scan reflects the nature of the ion being reduced, and for complex ions the peak position moves to more negative potentials as stability increases. In some cases formation of intermediate valency states (e.g. in chloride solution, Cu2+ — Cu+ — Cu°) results in split peaks. Adsorption of species (e.g. colloidal particles, surfactants) on the mercury electrode also causes peak movement (generally in an anodic direction). 

In all cases formation of complexes was detected only on the nonsubstituted C-4- (C-6) atom of the heterocycle (contrary to many other nucleophiles). 

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