Intramolecular closing

Connectivity, dimensionality, shape The ligand should be able to replace as completely as possible the solvation shell of the cation. Since the AC s and AEC s are spherical, the optimum ligand should contain an intramolecular, closed, spherical cavity thus ligands with high Lt values should be preferred. 

MALDl-TOF was used to deduce the three-dimensional structure of complex silsesquioxane polymers. These polymers have silicon coordinated with three bridging oxygen atoms in the form of [RSiOs/J and can form a wide variety of complex three-dimensional structures. Specifically, the branched-linear silsesquioxane may react with itself to form intramolecular closed loops. Four distinct levels of molecular structure were observed from the MALDl-TOF spectrum. 

The mass separation of the major cluster of peaks was foimd to be 108.25 Da as expected from the synthesis. The mass separation of 18 Da between peaks within a major cluster indicated the loss of H2O and tire formation of intramolecular closed loops. The data indicated also that the percentage of closed loops did not vary with oligomer size, and that the molecular structure was intermediate between branched-linear and a simple ladder structure, with no evidence that fully condensed structures had been formed in significant amounts. ... 

Figure 10.9 Gemcitabine prodrug hemi-p-toluenesulfonic acid hemihydrate salt cocrystal (left) and gemcitabine prodrug mono-p-toluenesulfonic acid neat Form I (right). The H atom positions (denoted by arrows) were inferred in each structure from intermolecular and intramolecular close contacts.

A closely allied reductive linking of carbonyl groups is an intramolecular version with esters, called the ac doin reaction, which again gives a 1,2-diox rgenated skeleton ... 

In order to define how the nuclei move as a reaction progresses from reactants to transition structure to products, one must choose a definition of how a reaction occurs. There are two such definitions in common use. One definition is the minimum energy path (MEP), which defines a reaction coordinate in which the absolute minimum amount of energy is necessary to reach each point on the coordinate. A second definition is a dynamical description of how molecules undergo intramolecular vibrational redistribution until the vibrational motion occurs in a direction that leads to a reaction. The MEP definition is an intuitive description of the reaction steps. The dynamical description more closely describes the true behavior molecules as seen with femtosecond spectroscopy. 

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

In a similar reaction the bis-tosylate shown below was to be converted into a bis-malonate derivative. In this cyclic derivative both groups are hold closely together and intramolecular reactions proved to be so much faster than intermolecular substitution under all ex-... 

Conformation. Neutron diffraction studies of sucrose revealed the presence of two strong intramolecular hydrogen bonds 0-2—HO-1 and 0-5—HO-6 in the crystal form (7,8). These interactions hold the molecule in a weU-ordered and rigid conformation. The two rings are disposed at an angle close to 90°, with the glucopyranosyl and fmctofuranosyl residues adapting chair and T" twist conformations, respectively. 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

The normal modes for solid Ceo can be clearly subdivided into two main categories intramolecular and intermolecular modes, because of the weak coupling between molecules. The former vibrations are often simply called molecular modes, since their frequencies and eigenvectors closely resemble those of an isolated molecule. The latter are also called lattice modes or phonons, and can be further subdivided into librational, acoustic and optic modes. The frequencies for the intermolecular modes are low, reflecting, the... 

Although these reactions are thus closely related to the acyl-alkyl diradical disproportionation to ketenes, the stereospecificity of (55) -> (56) and (57) -> (58) shows that these hydroxyketones cannot proceed through free radicals capable of rotating about single bonds prior to the intramolecular hydrogen... 

In some reactions intramolecular chalcogen nitrogen interactions may lead to stereochemical control. For example, selenenyl bromides react with C=C double bonds, providing a convenient method of introducing various functional groups. The reaction proceeds readily, but affords a racemic mixture. The modified reagent 15.22 contains a chiral amine in close interaction with the selenium atom. It reacts with olefins affording up to 97% ee of isomer A (Scheme 15.2). ... 

The currently popular approach is to compare the rate of the intramolecular reaction with the rate of an intermolecular reaction in which the reacting groups are closely similar. The intermolecular reaction will usually be overall second-order, in accordance with the rate equation... 

The molecule below has four stereoisomeric forms exoO exoCH2Br, exoO endoCH2Br, and so on. Examine electrostatic potential maps of the four ions and identify the most nucleophilic (electron-rich) atom in each. Examine the electron-acceptor orbital (the lowest-unoccuped molecular orbital or LUMO) in each and identify electrophilic sites that are in close proximity to the nucleophilic. Which isomers can undergo an intramolecular E2 reaction Draw the expected 8 2 and E2 products. Which isomers should not readily undergo intramolecular reactions Why are these inert ... 

Mechanistically, the reaction of diketosulfides and glyoxal likely proceeds via an initial aldol reaction to provide 22. A second intramolecular aldol reaction and the elimination of two equivalents of water produce the thiophene 23. The timing of the elimination reactions and the ring-closing, carbonyl condensation reaction is not completely understood. However, 2,5-disubstituted thiophenes 23 are available in good yields via this process. 

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