Proximity function

The reactivity of epoxides can be modified by various proximal functionality. For example, 2,3-epoxy sulfides 118 are converted to the corresponding TMS-thiiranium species 119 upon treatment with TMS triflate. This intermediate reacts with O-silyl amides regiospecifically to form l-substituted-3-hydroxy-2-thioethers (e.g., 120). Simple primary amines undergo polyalkylation, but imines can be used as an indirect amine equivalent <96TET3609>. 

Fig. 3.7. A united-atom PE chain constituted of 100 united atoms mapped around a stream line. The bond angles as well as the bond lengths have been kept fixed. The torsion angles have been subject to a minimization considering a three-fold potential accounting for the simplified chemical structure of the chain. An additional proximity function has been used to force the chain to follow the trajectory of the stream line [114]...

The product between the ppdd and the volume of the site is termed the proximity function of that domain. Thus, we have a proximity function of energy deposition ... 

The dynamics of proton binding to the extra cellular and the cytoplasmic surfaces of the purple membranes were measured by the pH jump methods [125], The purple membranes selectively labeled by fluorescein Lys-129 of bacteri-orhodopsin were pulsed by protons released in the aqueous bulk from excited pyranine and the reaction of the protons with the indicators was measured. Kinetic analysis of the data implied that the two faces of the membrane differ in then-buffer capacities and in their rates of interaction with bulk protons. The extracellular surfaces of the purple membrane contains one anionic proton binding site per protein molecule with pA" 5.1. This site is within a Coulomb cage radius from Lys-129. The cytoplasmic surface of the purple membrane bears four to five pro-tonable moieties that, due to close proximity, function as a common proton binding site. The reaction of the proton with this cluster is at a very fast rate (3 X 1010 M-1 sec ). The proximity between the elements is sufficiently high that even in 100 mM NaCl, they still function as a cluster. Extraction of the chromophore retinal from the protein has a marked effect on the carboxylates of the cytoplasmic surface, and two to three of them assume positions that almost bar their reaction with bulk protons. Quantitative evaluation of the dynamics of proton transfer from photoactivated bacteriorhodopsin to the bulk has been done by using numerical... 

Fluorine can share three sets of lone-pair electrons with electron-deficient atoms intramo-lecularly or intermolecularly, in particular with a relatively acidic hydrogen bound to a heteroatom. In addition, as described in section 1.4, strongly electron-withdrawing per-fluoroalkyl groups increase the acidity of proximate functional groups such as alcohol, amine, amide, and carboxylic acid. 

Solid-supported radical reactions started to be exploited in carbon-carbon bond formations due to the increased ability of the chemist to control radical processes. A deeper knowledge of the thermodynamic and kinetic aspects of the radical chains allows the minimization of unwanted side reactions. Radical reactions lead mainly to kinetically determined products. Favored reactions are those involving proximal functionalities, along with reactions leading to cyclized products. In these cases, a radical generated by a selective reaction is allowed to react with a non-radical, usually a double bond. The radical character in such a reaction is not destroyed during the process therefore, only catalytic amounts of radical initiator are required. The products generated in radical reactions are mostly not diffusion-... 

The diamagnetic screening constant, involves the rotation of electrons around the nucleus and is important for proton NMR. These electrons may be immediately associated with the atom in question, or with circulating electrons associated with proximate functionalities, i.e., anisotropic effects. For the paramagnetic screening constant, cr (which makes the major contribution to the nuclei Fe, Rh, Sn, Pt,. .. etc.), the average energy approximation, for an... 

As discussed above, the efficiency of the Hpase-catalyzed acetylation heavily depends on the E factor. Thus, the purification of 2-aUcyl-l-alkanols lacking any proximal r-bonds or heterofunctional groups by Hpase-catalyzed acetylation is generally more difficult than that of their proximally functionalized derivatives, their commonly reported E factors being < 10 [316]. In more demanding (feebly chiral) cases, especially when two aUcyl groups are very similar, it is difficult to purify to > 98% ee even from enantiomerically enriched mixtures by lipasecross-coupHng sequential process as outlined in Scheme 3.100 was considered for the synthesis of various feebly chiral 2-alkyl-l-alkanols [318]. The first step involves a one-pof conversion of inexpensive allyl alcohol to various... 

The coupling of Grignard reagents and alkyl halides is dramatically enhanced when the latter possesses proximate functionality capable of com-plexation, and hence of transition-state stabilization whereas neither n-pentyl nor ethoxyethyl bromide react with iodomagnesium phenylacetylide, the bromide (196) does. ... 

Di-O-alkylated regioisomers are a potential source of calix[4]arenes with mixed ligating groups at the lower rim. Reinhoudt has reported that functionalization of the free hydroxyl groups of syn-pmximal di-O-alkylated calix[4]arenes affords a variety of cone his(syn-proximally) functionalized calix[4]arenes [13]. Cation com-plexation studies on these derivatives have shown that subtle changes in regiose-lective functionalization influences the selectivity for Na+ considerably. 

Proximally Functionalized Cavitands and Synthesis of a Flexible Hemicarcerand ... 

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