Nucleophiles structure

Mole Fraction Moles per Mole -CH.Br Reaction ZCH.Br Nucleophile Structural Units Nucleophile TBAH Temp. Time SubStl-... 

As can be seen from Table 2 rates of reaction of nucleophiles with phenyl benzenethiolsulfinate vary markedly with nucleophile structure (Kice and Liu, 1979). The particular reactivity pattern observed will be discussed later in Section 8 in conjunction with data on the reactivity of the same nucleophiles toward phenyl benzenethiolsulfonate, PhS02SPh. Of significance at present is the fact that PhS is much more reactive than nucleophiles such as OH- or CH30. In the alkaline hydrolysis of PhS(0)SPh (Oae et al., 1977b Kice and Rogers, 1974a) this means that the thiophenoxide liberated by the initial attack of OH- on PhS(0)SPh (28) will rapidly react with a second molecule of thiolsulfinate to form disulfide and sulfenate ion (29). 

Due to direct involvement of the nucleophile in the enantiodiscriminating step, the enantioselectivity of this reaction is greatly affected by the structure of the nucleophile. In terms of the nucleophile structure, both the covalent constitution and the nature of the ion pair influence... 

Because systematic variations in selectivity with reactivity are commonly quite mild for reactions of carbocations with n-nucleophiles, and practically absent for 71-nucleophiles or hydride donors, many nucleophiles can be characterized by constant N and s values. These are valuable in correlating and predicting reactivities toward benzhydryl cations, a wide structural variety of other electrophiles and, to a good approximation, substrates reacting by an Sn2 mechanism. There are certainly failures in extending these relationships to too wide a variation of carbocation and nucleophile structures, but there is a sufficient framework of regular behavior for the influence of additional factors such as steric effects to be rationally examined as deviations from the norm. Thus comparisons between benzhydryl and trityl cations reveal quite different steric effects for reactions with hydroxylic solvents and alkenes, or even with different halide ions... 

The source of nitric oxide was diethylenetriamine/nitric oxide, DETA/ NO (Scheme 20.4) a compound that has been used in studies of the cytostatic, vasodilatory and other pharmacological properties of NO [65-67]. DETA/NO is a 1-substituted diazen-l-ium-l,2-diolate containing the [N(0)N0] functional group that has been proved to be useful for the reliable generation of nitric oxide in homogenous solutions [68]. When dissolved in blood, cell culture medium or buffer this compound dissociates to generate NO leaving the nucleophilic structure as a by-product. 

Influence of Nucleophile Structure on Reactivity. The substrate selectivity of the primary n-alkyl bromides toward the various sulfur nucleophiles listed in Tables IV through WI shows little if any dependence on the length of the alkyl chain (see below). Thus, the data in Tables IV through VII may be used to construct the following approximate order of reactivity of sulfur nucleophiles with respect to the displacement of halide from a primary n-alkyl bromide in H20 ... 

Over the years, there have been a number of reports on the kinetic conjugate addition of metallated arylacetonitriles to enones. Several proposals have been made to explain the mechanism and outcome of this reaction based on the nucleophile structure or aggregation state or on the HSAB properties of the reactants. A reexamination of these studies has now revealed that in each case the 1,4-adducts resulted from equilibration of the kinetically formed 1,2-adducts to the more stable 1,4-adducts. The 1,2-addition, retro-1,2-addition, 1,4-addition, and retro-1,4- addition of PhC=C=NLi to PhCH=CHCOMe were examined, and a free energy level diagram was constructed.159... 

Mancini PME, Fortunate G, Vottero LR (2004) Kinetics of the reactions between l-fluoro-2,4-dinitrobenzene with pyrrolidine and piperidine in binary solvent systems. Influence of the nucleophile structure. J Phys Org Chem 17 138-147... 

The acidity of these products cannot be determined by means of direct pH determination. However, the virtual acidity constant, AT l, is accessible from optical and electrochemical measurements. Comparison of the pX i values of selected Knoevenagel products and the pX values of some typical carboxylic acids in methanol establishes that they exhibit comparable acidity (Scheme 7). Using the o -values, the pX i values of benzylidene derivatives of Meldrum s acid are in good accordance with the Hammett equation. The Lewis acidity of Knoevenagel products is due to the formation of labile pseudobase adducts, the so-called anbadons (10), upon reaction with nucleophiles. Structures of the anbadons have... 

Perhaps it is worth noting in summary that the series can be extended to include X=NH and X=CH2. These analogs would be expected to be even less reactive than carboxylate toward nucleophilic addition. The latter species is an enolate. We will learn in Section 7.7 that enolates are reactive nucleophiles. Carboxylate and amide anions also are nucleophilic. Even amides are somewhat nucleophilic. Going across the periodic table from F to CHj, the carbonyl substituent X transforms the carbonyl group from an electrophile to part of an nucleophilic structure in the anionic structures by increasing electron donation. 

Table 6.2 should be helpful. It summarizes what we have said so far about the two substitution mechanisms, and it compares them with respect to two other variables, solvent and nucleophile structure, which we will discuss here. 

Sulfur ylides represent another family of nucleophiles structurally coupled with a leaving group. It is therefore to be expected that they will react with imines to give aziridines via an addition-substitution sequence. In fact, the sulfur ylid obtained by metallation c trimethylsulfonium iodide with n-butyl lithium reacts at -30 C in THF with both silylimines and p-methoxyphenyl imines to give the corresponding aziridines. Again a single isomer is obtained in the case of (S)-lactaldehyde imines (Scheme 22). 

In several of the reactions that you learned in introductory organic chemistry, and many of those that are discussed in Chapters 10 and 11, nucleophilic attack on an electron deficient center such as a carbocation or a polarized cr or -ir bond occurs. The ability of nucleophiles to participate in these reactions depends upon their molecular structure. A structural dependence suggests that there should be scales of relative nucleophilicity that depend upon the sterics, polarizability, and inductive/resonance properties of the nucleophile. Hence, changing the nucleophile becomes one of the tools that we have to study reactions. As with solvent and electronic substituent effects, LFERs provide insight into how the change in nucleophile structure affects the reaction. Similarly, the ability of a leaving group to depart... 

Different kinetic behavior was observed when secondary hydroxy-alkylic amines, methyl-2-hydroxyethylamine and butyl-2-hydroxy-ethylamine, were employed as nucleophiles. Autoacceleration appeared in dioxane for both secondary amines however, normal second order kinetics were followed in DMF when the nucleophile is methyl-2-hydroxyethylamine which has less bulky substituents. In the reaction of butyl-2-hydroxyethylamine with CMPS in DMF, rate retardation began when the conversion reached about 75% owing to the steric hindance of the bulky butyl groupThus the sensitivity of the rate profiles to reaction media and nucleophile structure complicates assessment of "polymeric effects". 

The imidazole groups in the polymer play a crucial role in creating high-affinity sites for Cu(II) ions and may also be influential in the kinetic pathway. The mechanism involves an imidazole-linked Cu(II) ion to act as an electrophile, while an adjacent free ligand imidazole functions as a nucleophile structures (50) and (51). 

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