Stoichiometric studies

If the balance between reactants is stoichiometric only overall orders can be obtained. However, the determination of orders in acid and in alcohol (obtained by non-stoichiometric studies) is highly desirable since it confirms the overall order and provides valuable information on the mechanism. 

In marked contrast to the reaction with (ArS)2, the l l-adduct 85 was not formed at all. The stoichiometric study also supported this fact, indicating that the formation of 85 from (ArSe)2 with 73 is a thermodynamically unfavorable process. 

The oxidation of dehydroascorbic acid to a higher oxidation product is relatively slow and could be neglected for all practical purposes in kinetic and stoichiometric studies. 

Stoichiometric studies show that the reaction is first-order with respect to the concentration of TGMDA and second-order with respect to DDS in the early stages of reaction. 

Based on a large number of stoichiometric studies, the main trends of allenylidene reactivity are presently well established [26 0]. They are governed by the electron deficient character of the and Cy carbon atoms in the cumulenic chain, the Cp being a nucleophilic center. This can also be rationalized by considering the mesomeric forms depicted in Fig. 14. However, it should be noted that, as commented on previously, only contribution from the alkynyl resonance form is supported by X-ray diffraction. 

Glass transition temperatures (Tg s) were detemined using a Dupont DSC 910 attached to a 9900 data analysis system. For off-stoichiometric studies, epoxy resin and diamine were cured in situ within a hermetically sealed DSC pan (sample tak from 25 C - 300 C at lO C/min), then cooled rapidly back to 25 C, and finally scanned from 40 C - 220 c to record the Tg. All samples were scanned under nitrogen atmosphere at a rate of 10 C/min. 

Adsorption of anions on oxides is usually accompanied by the uptake of protons (or the release of hydroxyl ions). The ratio between the number of protons that are coadsorbed and the level of anion adsorption is not usually stoichiometric. Studies of adsorption of oxyanions on goethite as a function of pH appear to indicate that, provided only one adsorbed species is present, the proton anion ratio is related to the mode of adsorption (Rietra et al., 1999). It follows that oxyanions which adsorb with a similar proton/anion stoichiometry, have a similar adsorption mechanism. 

Based on a large number of stoichiometric studies, the main trends of allenylidene reactivity are presently well established, being governed by the electron-deficient... 

Notably, the proposed stereoselectivity of a-insertion seems to run opposite to that observed for some related processes (vide supra). Such systems differ from the present system, though, because they involve an octahedral Rh(III) center, whereas intermediate 88 is a Rh(I)-square-planar complex. In Werner s original stoichiometric studies, the same mechanistic dichotomy was evident (Scheme 9.14) - square planar Rh(I)-complexes underwent cisoid insertion in the absence of other factors and transoid addition in the presence of an oxidant (HX). In intermediate 88, coordination of the alkene moiety to the rhodium center may also play a role in directing insertion. 

A thorough comparative study of the reactivity of O-, OJ, and Oj toward alkanes and alkenes on MgO has been carried out by Lunsford and co-workers (see Sections V and VI in Ref. 1 and Section VI in this article). In these stoichiometric studies, a known concentration of oxygen species on the surface was reacted with a known amount of hydrocarbon in a recirculation reactor and the products were analyzed. In all cases, a 1 1 stoichiometry was observed. 

Stoichiometrical studies together with the results of 180 experiments show that 2 atoms of molecular oxygen are incorporated into two molecules of acetone formed from 2-nitropropane. Accordingly, the enzyme is an intermolecular dioxygenase and is unique in incorporating 2 atoms of the oxygen molecule into 2 molecules of the same acceptor199. ... 

Detailed information on mechanistic aspects of the ligand oxidation reactions is limited by the fact that well-defined tractable kinetics is only found for systems so very dilute in the metal ion reactants that stoichiometric studies including isolation of reaction products have not yet been practicable. Some selected systems have, however, been studied in some detail, but at significantly higher metal ion concentrations than used for the kinetic studies. It is relevant to recall, however, that under such conditions the rate usually does not follow Eq. (1) and the stoichiometry does not conform to Eq. (2) with a value of n about 6. 

This makes spectroscopic and stoichiometric studies feasible and realistic. 

Many more recent stoichiometric studies of cobalt(III) complexes have been responsible for most of the developments in this area of research. Cobalt(III) ammine complexes effect hydrolysis of ethyl glycinate in basic conditions via intramolecular attack of a coordinated amide ion hydrolysis by external hydroxide ion attack also occurs (equation 74).341 Replacement of ammonia ligands by a quadridentate or two bidentate ligands allows the formation of aquo-hydroxo complexes and enables intramolecular hydroxide ion attack on a coordinated amino ester, amino amide... 

A mechanism (Equation (2S)) has been suggested from stoichiometric studies on cobalt complexes, but it seems (o be of limited application. 

Consequently, to narrow the definition a bit further, we will adhere to Negishi s suggestion and define cross-coupling reactions as those that follow some variation of the mechanism depicted in Scheme 1 (where Mt is a transition metal, L is an ancillary hgand see Ancillary Ligand), and n is the oxidation state of the reduced metal in the catalytic cycle). This mechanism is supported by stoichiometric studies on isolated metal complexes (mostly where Mt = Pd, n = 0, L = triphenylphosphine) thought to be the intermediates in this cycle. 

Stoichiometric study of the Ru(SiMe2Ph)Cl(CO)(PCy3)2 with vinylboronate confirms the insertion of olefin into the Ru-Si bond and supports the non-metallacarbene mechanism of this reaction [25]. 

Although the belief that steric factors influence norbomene extrusion is reasonable and supported by Catellani s studies, it is entirely possible that norbomene carbopalladation and extrusion are reversible processes. If so, a species related to 4 may be trapped as the mono-o/t/to-alkylated product. Although mono-functionalization has been observed in stoichiometric studies by Catellani [31, 42], catalytic reactions generally do not afford monoalkylated products. Interestingly, Lautens has shown that in some particular systems mono-alkylation is possible, which may occur as a result of a sterically congested system (Scheme 14) [44], This does lend some evidence to the possibility that norbomene carbopalladation and extrusion are reversible steps, and may occur between ortho functionalization steps. 

Stoichiometric studies of M-H additions to alkynes also show mixed stereochemical results. The more common cw-addition is typified by the CO-promoted transformation of Cp2Nb(H)(RCECR) to ds-Cp2Nb(CO)(Ti -CR=CHR) [90]. In mononuclear systems where frans-additions have been found, radical-type mechanisms have been implicated [91] or cis/trans isomerization of the intermediate vinyl species [92] has been found. Although the intermediacy of alkyne complexes has not been established, Schwartz s hydrozirconation of alkynes [93] by Cp2ZrHCl represents a general entry to vinyl-metal species which can be transformed stereoselectively to alkenes, vinyl halides, and/or carboxylic acids. 

Chemical polymerisation of (2a) and ( 3b ) with iron (III) chloride enabled a facile multigram scale synthesis of the polymers pBr2BT and PCI2BT. The obtainment of these polymers were corroborated by elemental analysis and surface stoichiometric studies by ESCA (Table 2). 

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