Strain molecules with high

Other kinds of molecules besides 1 (which has unusual bond stereochemistry) for which these methods might fail to give good results are hypercoordinate molecules like NF5, molecules with noble gas atoms, particularly those of helium and neon, molecules with highly twisted C=C bonds, extraordinarily crowded molecules like hexaphenylethane, unknown dimers, trimers etc. of small familiar molecules, like C202 and N6, and very highly strained molecules. All these cases are discussed in a book on exotic molecules [4],... 

The successful inner phase stabilization of cyclobutadiene suggests that this approach may allow stabilization of other molecules with highly strained multiple bonds. An interesting class of hydrocarbons with twisted C=C bonds are anti-Bredt bridgehead olefins. These bicycloalkenes have a trani-cycloalkene and are unstable if their olefinic strain (OS) is OS > 21kcalmor. Bicyclo[2.2.2]oct-l-ene 60 and (Z)-bicyclo[3.2.1]oct-l-ene... 

The experimental evidence is suggestive, but inconclusive. Although cyclopropene is a highly strained molecule, with an estimated strain energy of 26 kcal/mol,[28] it does not dimerize spontaneously. The an e-isomer of TCH, the 6z5- em-dimethyl derivative of which has been formed by catalytic dimerization of em-dimethyl cyclopropene,[29] does not revert thermally to two monomer molecules. Instead it isomerizes to vibrationally excited cyclohexadiene [30] ... 

One of the fundamental goals of physical organic chemistry has been to establish the limits of our models for structure and energetics. How long can a C-C bond be How much angle strain can a molecule tolerate How crowded can a structure be Such questions have defined many brilliant research efforts and have produced a fantastic array of bizarre and wonderful structures. Here we present a collection of representative highly-strained molecules, with an emphasis on the structural concepts that are being tested. 

As outlined in this chapter, polymerizable cycloalkanes can be roughly divided into two families (i) highly strained, polycyclic molecules with high intrinsic polymer-izabilities (e.g. bicyclo[n.l.O]bicycloalkanes and [m.n.ljpropellanes) and (ii) monocyclic cycloalkanes, mostly cyclopropanes with a few cyclobutanes, that require further activation by judiciously placed substituents. 

Quite large elastic strains are possible with minimal stress in TPEs these are the synthetic rubbers. TPEs have two specific characteristics their glass transition temperature (7 ) is below that at which they are commonly used, and their molecules are highly kinked as in natural TS rubber (isoprene). When a stress is applied, the molecular chain uncoils and the end-to-end length can be extended several hundred percent, with minimum stresses. Some TPEs have an initial modulus of elasticity of less than 10 MPa (1,500 psi) once the molecules are extended, the modulus increases. 

Whereas the reactions of sulfones with nucleophiles via pathways A and B of equation 1 are most frequently observed, the nucleophilic substitution reaction by pathway D has been observed only in the cases where the leaving carbanion can be stabilized, or in the highly strained molecules. Chou and Chang3 has found recently that an organolithium reagent attacks the sulfur atom of the strained four-membered sulfone in 34. When this sulfone is treated with 1 equivalent methyllithium, followed by workup with water or Mel, 38 or 39 are formed in high yield. 

Viscoelasticity illustrates materials that exhibit both viscous and elastic characteristics. Viscous materials tike honey resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just as quickly return to their original state once the stress is removed. Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent strain. Viscoelasticity is the result of the diffusion of atoms or molecules inside an amorphous material. Rubber is highly elastic, but yet a viscous material. This property can be defined by the term viscoelasticity. Viscoelasticity is a combination of two separate mechanisms occurring at the same time in mbber. A spring represents the elastic portion, and a dashpot represents the viscous component (Figure 28.7). 

In molecules with more than one unpaired electron, electron-electron interactions can have a significant role in the stabilization of the system. Bond formation that results from direct overlap is highly favorable and, thus is an overriding consideration in all low-spin polyradicals, even to the extent that the system sometimes adopts a strained, closed-shell state as opposed to a polyradical. In cases in which bonding cannot occur, indirect interactions that are usually insignificant, such as electron exchange and spin-polarization, can have significant impact. The presence of these interactions is often reflected in the thermochemical properties. 

Methylene- and alkylidenecyclopropanes are highly strained molecules, but at the same time most of them are surprisingly stable to allow their use in many synthetic applications. Being multifunctional reagents with high energy, they offer enormous potential in organic syntheses that has been only partially disclosed in the last decades. 

Benzynes are highly strained molecules, which are recognized as useful intermediates in organic synthesis.44 They can be isolated by coordination to transition metals.45 Similar to the reaction of the cyclohexyne species 66, Ni-benzyne complex such as 85 reacted with C02 to give the corresponding five-membered oxanickelacyle complex 86 (Scheme 31).46... 

Some molecules with small distorted rings (of high strain energy) are explosively unstable. Individually entries are ... 

One of the most talked-about dream targets of synthesis, dodeca-hedrane (123), has been studied repeatedly by the MM method (128,235). Recent MM2 calculations (234) predict a heat of formation of 22.15 kcal/mol, incidentally equal to the average from the two previous values 45 .28 by the Allinger 1971 force field, and -0.22 kcal/mol by the Schleyer force field (26a). In the If, structure a very high strain energy is predicted by these two force fields because of the perfect eclipsed orientation of all the C—C as well as C—H bonds. Ermer calculated twisted conformations for this molecule with the CFF and confirmed that the I, structure was the GMEC (324,324a). 

In the pursuit of new high-energy-density fuels, fuels with strained molecules have been developed and their combustion characteristics studied [1]. Fuels such as dihydrobenzvalene (CeHg) and methyl cubane ((C H7)CH3) are strained dur-... 

A technique that allows rapid evaluation of molecular stability using small (20-30 mg) samples has been demonstrated and applied to three different families of strained molecules. All of the molecules studied are stable at room temperature, though most must be stored in nonmetallic containers to avoid catalytic decomposition. Of the four molecules shown in Fig. 4.1, the least thermally stable was quadricyclane, for which decomposition lifetimes drop below 10 ms at about 500 K. The other three molecules had similar stabilities, with lifetimes dropping below 10 ms above 700 K. For all systems studied, decomposition by loss of small hydrocarbon fragments (acetylene or ethene) was an important decomposition mechanism in the gas phase. For all but AEBCB, isomerization was also a significant decomposition mechanism. At high pressures, one would expect more isomerization because the very rapid collision rate should allow collisional stabilization of the isomerization products. 

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