Strain, molecular

Molecular Basis of Mechanochemical Degradation 3.1 Molecular Strain and Reactivity... 

Scheme 33 illustrates the difference in reactivity between triazolines obtained from cyclohexanone and cyclo-pentanone enamines. Thus, the reactions of azidophosphonates 239 with cyclohexanone enamines produce unstable aminotriazolines 240 that cannot be isolated due to their spontaneous elimination of amines to provide triazoles 241. Contrary to that, triazolines 242, derived from cyclopentanone enamines, are isolated in good yield (76-88%) and cannot be converted to the corresponding triazoles even by thermolysis <1995H(40)543>. Probably, introduction of a double bond between two five-membered rings would involve too much molecular strain. 

Due to molecular strain, cyclooctyne is a very reactive species. Its reactions with azides proceed rapidly even at room temperature making it a convenient tool for probing structures of unstable azides. Thus, the reaction of cyclooctyne with diazide 1061 carried out in GH2CI2 at room temperature is accomplished within 2h and provides ditriazolyl derivative 1060 in 76% yield. A similar reaction of cyclooctyne with diazide 1062 leads to ditriazolyl derivative 1063 in 90% yield (Scheme 175) <2005T8904>. 

Molecular strain typing facilitates investigation of prion disease etiology 800... 

Yet another chemical concept that has been related to EMDs is strain [21]. The classical definition of molecular strain originated in the work of Baeyer. In modem... 

Archibald and co-workers have explored the synthesis of polynitrocyclobutanes and then-derivatives. The synthesis of these compounds via the nucleophilic substitution of cyclobutyl halides with nitrite anion was ruled out at an early stage because displacement in this system is too slow for practical use. This is a consequence of the molecular strain in the cyclobutane ring, which causes carbon atoms to deviate from sp hybridization towards sp character. 

Polynitro derivatives of pentacyclo[5.4.0.0 .0 °.0 ]undecane have attracted interest as potential high-energy explosives. Molecular strain in this caged system could arise from both the constrained norbomyl moiety and the cyclobutane ring. Additional strain would be expected from nonbonding interactions if the S-endo and 1 l-endo positions were substituted with gm-dinitro groups. 

It is well-recognized that the hydrocarbons cyclopropane and cyclobutane have nearly identical strain energies, and so these microcycles have been quite naturally paired in numerous treatments of molecular strain. How similar are cyclobutylamine (12, X = NH2, 13, n = 4, X = NH2) and cyclopropylamine (2, X = NH2,13, n - 3, X = NH2) and other correspondingly monosubstituted cyclobutanes and cyclopropanes18 What about... 

To achieve a bond angle of 90°, cyclobutane must be planar. This would force the adjacent C-H bonds to be eclipsed and would also raise the energy of the system. As a compromise, cyclobutane folds diagonally by 35°. While this raises the angle strain somewhat, it decreases the eclipsing interactions so that the lowest possible energy is attained. While each methylene group is less strained than one in cyclopropane, the total molecular strain is similar to that of cyclopropane. 

For example, as shown by calicheamicin, cyclic enediynes have a lower activation barrier than acyclic enediynes. As suggested by Nicolaou in 1988, the distance between the acetylenic carbons that form the covalent bond influences the rate of cyclization. Another theory developed by Magnus and Snyder is based on the molecular strain between ground state and transition slate this seems to be more general, especially for strained cyclic systems. Often, as both the distance and the strain are not known, the development of suitable precursors remains difficult, as exemplified by the following enediyne, in which a slight change leads to a cycloaromatization ... 

The first thing that attracts attention is a stable tendency to energy decrease with an increase of the number of atoms and therefore the size of a fullerene cage (Fig. 1). It is obvious that it reflects the overall decrease of a molecular strain of spheroid molecules going from C6o to higher fullerenes. 

The synthesis and properties of the highly symmetrical hydrocarbon phanes 2 and 6 have been reported in 197018 X-ray data of 619 indicate a molecular strain exceeding that of [2.2]paracyclophane-diene (7). The latter was estimated by Gantzel and Trueblood to be about 39 kcal/mole (163.0 kJ/mole)20). 

The synthesis of strained cyclic compounds with carbon-bridges of variable length relies on two different strategies for the introduction of molecular strain. 

Magnus, P. Fortt, S. Pittana, T. Snyda, J. P. Synthetic and mechanistic studies on esperamicin A, and caUchanicin yj. Molecular strain ratha than jt-bond proximity determines the cycloaromatization rates of bicyclo[7.3.1]enediynes, J. Am. Chem. Soc. 1990,112,4986-4987. 

Stability within the IPR fiillerene family is dependent on a balance of electronic and molecular strain factors. The carbon atoms in the fullerenes are approximately sp hybridized (although curvature of the cage causes a small admixture of sp ), with the remaining p-orbitals (mixed with... 

Figure 12 Computer-generated cage structures of the nonisolable IPR fullerenes Z 6d-C72 and //j-Cgo- The experimentally elusive nature of these molecules is believed to be due to molecular strain and electronic factors, respectively...

Mercuriocyclization provides an effective approach to tetrahydrofurans and tetrahydro-2//-pyrans in high stereoselectivity. Using this method, bicyclic and polycyclic ethers have been synthesized. The high stereoselectivity in the ring formation process is determined by molecular strain or equatorial disposition of bulky substituents. Under thermodynamic conditions, the polycyclic compounds are obtained as sole isomers110-111. 

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