Strained molecule

In our studies of the V t) of strained hydrocarbons, we have found that these characteristic C—C bond potentials are relatively weak, ranging from -1 to —13 kcal/mol (at the STO-5G level). The most negative values are found near the C —C bonds of three-membeted rings. For example, those in 41, 42, 44, and all but the bridgehead bound in 43 have calculated of -13 kcal/mol. 

A well-established approach for studying the bonds in strained molecules is by means of the bond path concept. A bond path corresponds to the ridge of highest electron density that links two nuclei. For many bonds, such as those between the carbons in propane, the bond path is essentially identical with the internuclear axis. In molecules with strained bonds, however, there can be a 

Although the potential has served in the past primarily as a guide to electrophilic processes, we have demonstrated that it can be used in studies of nucleophilic interactions when plotted at some distance removed from the highly positive nuclei, e.g., in planes above the molecule or on a three-dimensional surface. The 

We greatly appreciate the support of this work by the Office of Naval Research through Contract N00014-85-K-0217. 

Scrocco and J. Tomasi, in Topics in Current Chemistry, No. 42, Springer-Verlag, Berlin, 1973, p. 95. The Electrostatic Molecular Potential as a Tool for the Interpretation of Molecular Properties. 

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The reaction of the allylic acetate with a diene system 784 affords the poly-fused ring system 785 by three repeated alkene insertions[487]. An even more strained molecule of the [5.5.5.5] fenestrane 788 has been constructed by a one-pot reaction in a satisfactory yield by the Pd-catalyzed carbonylation-cycliza-tion of 786 without undergoing elimination of /3-hydrogen in the cr-alkylpalla-dium intermediate 787 owing to unfavorable stereochemistry for syn elimination[488]. 

Cycloalkane properties depend on ring size. Strained molecules, i.e., moleeules with distorted geometries, tend to be more reaetive in ring-breaking ehemieal reaetions. For example, eombustion of a strained eyeloalkane should release more energy per CH2 group than eombustion of an unstrained moleeule. 

A related reaction is the oxa-di-n-methane rearrangement, where one of the C=C double bonds is replaced by a C=0 double bond. The substrates are thus /3,y-unsaturated ketones. The rearrangement proceeds from the triplet state. This oxa-variant gives access to highly strained molecules containing small rings, as has been demonstrated by irradiation of norborn-5-ene-2-one 10 ... 

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. 

Scheme 28 The lone pair effect for the designing of less strained molecules...

The formal cyclodimer of o-iodoethynylbenzene, diyne (7), was prepared in 1974 by Sondheimer s group using analogous bromination/dehydrobromina-tion chemistry [13]. The highly strained molecule was comparatively stable, decomposing around 110°C on attempted melting. Slow decomposition of the solid was observed after 2 days, when unprotected from light and air. 

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. 

This efficient cyclobutanol forming reaction has been utilized in the synthesis of terpenes404) (4.4), medium ring compounds40s) (4.5) and strained molecules as in (4,6)406) and (4.7) 407). In the last example the tricycloP. O.O jhexane skeleton is formed. 

The fragment of 1,4-PD is also present in 1,1-divinylcyclopropane (DVC), where the central methylene group is replaced by a three-membered ring. For this strained molecule a strong interaction between cyclopropane Walsh and vinyl 7r-orbitals was expected. The photoelectron spectra of DVC5 could be best understood with the assumption of optimal... 

The intramolecular C-H insertion reaction of carbene species has been used in a number of studies for the synthesis of strained molecules and cage com-... 

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... 

Strained molecules such as cyclopropanes and cyclobutanes have emerged as important intermediates in organic synthesis. We have already demonstrated here that cyclobutane derivatives can indeed serve as starting materials for the synthesis of natural as well as unnatural products. Unlike cyclopropanes, which can be prepared asymmetrically in a number of ways 175 -182>, the asymmetric synthesis of cyclobutane derivative has received less attention, and, to our best knowledge, very few reports were recorded recently 183). Obviously, the ready availability of chiral cyclobutane derivatives would greatly enhance their usefulness in the enantioselective synthesis of natural products. The overcome of this last hurdle would allow cyclobutane derivatives to play an even more important role in synthetic organic chemistry. 

An important, implicit assumption is that the potential energy function holds only if the nuclear displacement from a position of equilibrium is not too large. Hence, highly strained molecules are not the best targets. Whereas the MM calculations give the steric energy distribution within a molecule, which no other computational method can provide, one must be aware that this distribution is correct for the model of the molecule, not necessarily for the molecule itself (110-... 

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-... 

Quadricyclane is a strained molecule which is relatively easy to synthesize. The strain energies of related structures are presented in Fig. 2.12. 

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. 

Li, Z., J. Eckwert, A. Lapicki, and S. L. Anderson. 1997. Low-energy high-resolution scattering mass spectrometry of strained molecules and their isomers. Int. J. Mass Spectrometry Ion Processes 167/168 269-79. 

In p-isosparteine (14) all rings have a chairlike shape (54). Protonation of the N-16 atom makes the distance between N-1 and N-16 equal to 2.61 A, owing to the presence of an intramolecular hydrogen bond. Molecules of sparteine stereoisomers in crystals are sterically conjugated, and in all cases the angles between atoms C-6—C-7—C-17 and C-10—C-9—C-11 were increased to 116-120° (42-50). The B and C rings are flattened at their N termini as a result of noncovalent interaction of atoms with those situated next to them. The conformation of such strained molecules is stabilized by intramolecular hydrogen bonds (46). 

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