Protonation rationale

The rationale for the predominance of linear cyclization products versus angular cyclization products has been accepted as qualitative." The mechanism of the Combes reaction has been argued. It was initially proposed that cyclization to linear products was due to initial protonation of a more reactive site on the aromatic ring (1-position of 13 corrresponding to the 10-position of 15) thus, blocking cyclization to angular products. Bom showed this not to be the case for the cyclization of 2-naphthyl amino-2-penten-4-one. No 10-deutero material was observed. 

For either experiment we can consider that irradiated protons to flip back and forth between their two spin-states so rapidly that they no longer couple with other protons in the same molecule. An alternative rationale can be couched in terms of the decoupling field equalizing the populations of the two energy levels of the irradiated protons, which is qualitatively equivalent to saturating that resonance. (Although neither of these two models is strictly correct, they do at least provide a simple rationale for the N.M.D.R. experiment.)... 

Closer examination of the mechanism for the Rh catalyzed carbonylation of ethylene provides a rationale for the poor selectivity. The mechanism for the carbonylation of ethylene (Scheme 37.1) is well known (6) and proceeds via two simultaneously operating mechanisms which generate a common EtRh(CO)2l2 intermediate which rapidly reacts with iodide (Eqn. 10) to generate EtRh(CO)2l3 . The first, and predominant, mechanism is a hydride mechanism (Eqns. 6-8 below) in which the proton required for the formation of HRh(CO)2l2 and initiation of the... 

A transition state model proposed by Marshall for the rearrangement is shown in Scheme 1. The chiral amide 63 coordinates either one of the two enantiotopic propargylic protons in the chair-like conformation (Scheme 1). Complex I should be of lower energy as it lacks the unfavorable 1,4-phenyl/alkyne interactions present in complex n, thus the pro-S proton was removed predominantly. This rationale suggests that the enantioselec-tivity is predominantly determined at the Uthiation step. 

Reaction of butyllithium with VBE tetrahydroisoquinolyl formamidine proceeds as shown in Scheme 56. The lack of a kinetic isotope effect in deuteriated substrates revealed that the slow step in the sequence is the complexation of the BuLi with the formamidine, followed by selective removal of the a proton as shown. A rationale for this selectivity is that the butyl group is oriented trans to the isopropyl of the 5-membered... 

Improving the physical selectivity. The most straightforward method to improve the efficacy of radiation therapy is to improve the physical selectivity of the irradiation, i.e., by using beams with better physical characteristics (penetration, collimation, etc.). This is the rationale for the introduction of proton beams no radiobiological advantage is expected. 

Subsequent to the work with halide anions, efforts were made to quantify the extent to which phosphate anions are bound by protonated sapphyrin. " As for chloride and fluoride, the rationale for this work was both chemical and biological. 

The rationale of classification by reaction types is that different functional groups may show the same kinds of reactions. Thus, as we have just seen, alcohols, carboxylic acids, and amines all can accept a proton from a suitably strong acid. Fortunately, there are very few different types of organic reactions — at least as far as the overall result that they produce. The most important are acid-base, substitution, addition, elimination, and rearrangement reactions. Some examples of these are given below, and you should understand that these are descriptive of the overall chemical change and nothing is implied as to how or why the reaction occurs (also see Section 1-11). 

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