Topic
Intramolecular force
About: Intramolecular force is a research topic. Over the lifetime, 41618 publications have been published within this topic receiving 772261 citations.
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TL;DR: It is pointed out that translational and (overall) rotational motions provide the important entropic driving force for enzymic and intramolecular rate accelerations and the chelate effect; internal rotations and unusually severe orientational requirements are generally of secondary importance.
Abstract: It is pointed out that translational and (overall) rotational motions provide the important entropic driving force for enzymic and intramolecular rate accelerations and the chelate effect; internal rotations and unusually severe orientational requirements are generally of secondary importance. The loss of translational and (overall) rotational entropy for 2 → 1 reactions in solution is ordinarily on the order of 45 entropy units (e.u.) (standard state 1 M, 25°C); the translational entropy is much larger than 8 e.u. (corresponding to 55 M). Low-frequency motions in products and transition states, about 17 e.u. for cyclopentadiene dimerization, partially compensate for this loss, but “effective concentrations” on the order of 108 M may be accounted for without the introduction of new chemical concepts or terms.
915 citations
TL;DR: This review aims to describe the background and mechanisms of azomethine ylide formation and intramolecular cycloaddition, giving a critical account including the very first example and covering to early 2005.
Abstract: It was in 1963 that Huisgen laid out the classification of 1,3-dipoles and the concepts for 1,3-dipolar cycloaddition reactions, although it was not until 1976 that the first intramolecular 1,3-dipolar cycloaddition reaction of an azomethine ylide was reported. Since then, impressive developments have been described in this area, with the establishment of various useful methods for the formation of azomethine ylides and the determination of the requirements for a successful intramolecular cycloaddition reaction. Use of this methodology for the synthesis of pyrrolidineand pyrrole-containing natural products has been expanding rapidly. This review aims to describe the background and mechanisms of azomethine ylide formation and intramolecular cycloaddition, giving a critical account including the very first example and covering to early 2005. It is hoped that this review will be a useful resource for chemists interested in cycloaddition reactions and will inspire further exciting developments in this area. Cycloaddition reactions are one of the most important class of reactions in synthetic chemistry. Within * Corresponding author. Tel: +44 (0)114 222 9428. Fax: +44 (0)114 222 9346. E-mail: i.coldham@sheffield.ac.uk. † University of Sheffield. ‡ Tripos Discovery Research Ltd. Iain Coldham (b. 1965) is a Reader in Organic Chemistry at the University of Sheffield. He obtained his undergraduate degree and Ph.D. from the University of Cambridge, completing his Ph.D. in 1989 under the supervision of Stuart Warren. After postdoctoral studies at the University of Texas with Philip Magnus, he joined the staff in 1991 at the University of Exeter, U.K. In 2003, he moved to the University of Sheffield where he is involved in research on chiral organolithium compounds and on dipolar cycloaddition reactions in synthetic organic chemistry.
863 citations
TL;DR: The relevant mechanistic insights that enabled synthetic advances and distinguished the resulting transformations from other methods are emphasized and researchers will better understand the details of the aforementioned Rh-catalyzed C-H bond functionalization reactions, resulting in the design of more efficient and robust catalysts, expanded substrate scope, and new transformations.
Abstract: Nitrogen heterocycles are present in many compounds of enormous practical importance, ranging from pharmaceutical agents and biological probes to electroactive materials. Direct functionalization of nitrogen heterocycles through C−H bond activation constitutes a powerful means of regioselectively introducing a variety of substituents with diverse functional groups onto the heterocycle scaffold. Working together, our two groups have developed a family of Rh-catalyzed heterocycle alkylation and arylation reactions that are notable for their high level of functional-group compatibility. This Account describes our work in this area, emphasizing the relevant mechanistic insights that enabled synthetic advances and distinguished the resulting transformations from other methods. We initially discovered an intramolecular Rh-catalyzed C-2 alkylation of azoles by alkenyl groups. That reaction provided access to a number of di-, tri-, and tetracyclic azole derivatives. We then developed conditions that exploited mic...
860 citations
TL;DR: Characterization of the gelation process, from the molecular level up through the macroscopic properties of the material, suggests that by linking the intramolecular folding of small designed peptides to their ability to self-assemble, responsive materials can be prepared.
Abstract: A general peptide design is presented that links the pH-dependent intramolecular folding of β-hairpin peptides to their propensity to self-assemble, affording hydrogels rich in β-sheet. Chemical responsiveness has been specifically engineered into the material by linking intramolecular folding to changes in solution pH, and mechanical responsiveness, by linking hydrogelation to self-assembly. Circular dichroic and infrared spectroscopies show that at low pH individual peptides are unstructured, affording a low-viscosity aqueous solution. Under basic conditions, intramolecular folding takes place, affording amphiphilic β-hairpins that intermolecularly self-assemble. Rheology shows that the resulting hydrogel is rigid but is shear-thinning. However, quick mechanical strength recovery after cessation of shear is observed due to the inherent self-assembled nature of the scaffold. Characterization of the gelation process, from the molecular level up through the macroscopic properties of the material, suggests ...
812 citations
TL;DR: In this paper, the steric and pressure effects associated with the recombination of free radicals both depend on the nature of the activated complex, and are therefore intimately related, from a consideration of the reverse process of unimolecular dissociation, some equations are derived for these properties using an extension of earlier transition state and quasi-unimolecular theories.
Abstract: The steric and pressure effects associated with the recombination of free radicals both depend on the nature of the activated complex, and are therefore intimately related. From a consideration of the reverse process of unimolecular dissociation, some equations are derived for these properties using an extension of earlier transition state and quasi‐unimolecular theories. The present formalism differs from previous formulations of the latter in a number of ways, particularly in the expression used for the density of quantum states of the high energy molecules. Subsequent applications of the theory tentatively suggest that essentially all vibrational degrees of freedom of these molecules can contribute their energy to the vibrationally excited molecules. Consequently, vibrational anharmonicity would appear to be an important factor in intramolecular energy transfer. The present paper is an extension of a previously developed theory for the recombination of methyl radicals and iodine atoms.
773 citations