Site preference, reactive

Reactive site preference. We have already used the HSIP principle as it applies to... 

The most reactive site of the diene part is Cj of the cyclohexadienone ring with the alkoxy gronp. This corresponds to Cj of 2-alkoxybutadiene (Scheme 15), which has the largest HOMO amplitnde. The preferable frontier orbital interactions (Scheme 22) are in agreement with the reversed regioselectivities. 

The second step introduces the side chain group by nucleophilic displacement of the bromide (as a resin-bound a-bromoacetamide) with an excess of primary amine. Because there is such diversity in reactivity among candidate amine submonomers, high concentrations of the amine are typically used ( l-2 M) in a polar aprotic solvent (e.g. DMSO, NMP or DMF). This 8 2 reaction is really a mono-alkylation of a primary amine, a reaction that is typically complicated by over-alkylation when amines are alkylated with halides in solution. However, since the reactive bromoacetamide is immobilized to the solid support, any over-alkyla-tion side-products would be the result of a cross-reaction with another immobilized oligomer (slow) in preference to reaction with an amine in solution at high concentration (fast). Thus, in the sub-monomer method, the solid phase serves not only to enable a rapid reaction work-up, but also to isolate reactive sites from... 

In broad definition, regio-control over reactivity (regiochemical control) involves the demonstration of locale preferences for a chemical reaction. Generally, regio-control is thought of in terms of through bond separation of reactive sites, as in the resonance forms for the cyclohexanedionyl anion of Figure 6.1. 

Hill [263]. All of these could reasonably explain the measured reduction of the encounter pair formation rate coefficient. Being based on a more rigorous foundation, the Schmitz and Schurr model is to be preferred. From this analysis, Hasinoff [53] suggests that the heme unit in microperoxidase has a reactive site which subtends a semi-angle of 31° at the heme centre. 

Trimethylsilylacetylene (29) is preferable to ethyne for several reasons 1) It is a liquid rather than a gas 2) The metallation is more easily controlled and 3) Bisadducts are avoided, because only one reactive site is present. 

A DFT study of the reactivity of pyridine and the diazabenzenes towards electrophilic substitution, assuming frontier orbital control of the reactions, predicts their low reactivity as the HOMOs of these substrates are not n-orbitals.5 For pyridine-N-oxide, however, the HOMO is an aromatic orbital. DFT studies giving Fukui indices predict6 the preferred sites of electrophilic attack on pyrrole, furan, and thiophene and calculation of the local softness of the reactive sites rationalizes relative reactivities. 

We should not push this model too far however. Consider, for example, a 1-substituted hexadiene. In the limiting form B, the radical site at 1 is stabilized. Yet the transition state is late with the incipient 1-6 bond shorter than the partial 3-4 bond apparently, the allyl radical 1-2-3 prefers to react by its less reactive site ... 

These examples show clearly that there is no such thing as the ideal photophore . Due to the differences in reaction site preferences and the nature of the reactive intermediates of each individual moiety, the optimal photophore type needed differs from system to system. In most cases, it can be worthwhile to spend some time on the optimization of the used photophore in a certain A/BP in order to obtain the best results. In addition, interesting information can be obtained when the results derived from different photophores are combined [34],... 

This reaction is catalyzed by a platinum catalyst such as Speier s catalyst, chloroplatinic acid. Because the catalyst also isomerizes the terminal double bond, the reaction maybe run with an excess of vinyl to make sure all the reactive sites on the silicone are reacted. SiH also reacts with ROH and, for this reason, alkoxy end-capped polyethers maybe preferred. Alkoxy end-capped polyethers are also useful when the surfactant will be used in a chemically reactive system such as polyurethane foam manufacture. 

This type of reaction takes place with the preference for the loss of the largest alkyl radical at the reactive site. 

Further experiments have shown that MeAlCl2 in catalytic amounts promotes the intramolecular Diels-Alder reaction of furans because the most basic site in the system is fhe reactive site in 13 so fhat the cycloadduct does not inhibit the catalysis (Scheme 6.9). In contrast, increasing the number of equivalents of MeAlCl2 reduces fhe conversion because MeAlCh-lB interaction is the more stable of fhe complexed forms fhis might be partly because of fhe diverse aggregates preferred by MeAlCl2. 

To properly understand the fate of trace elements in soils, and particularly to comprehend their mobility with time, kinetic investigations are necessary (Sparks, 1995). Their sorption by soils is often observed to be a multistep process involving an initial fast sorption followed by slow sorption, probably by diffusion into pores of inner soil surfaces (Kinniburgh and Jackson, 1981), due to the presence of surface sites of different reactivity and site preferences (Ainsworth et al., 1994). 

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