Inherent strain

The discovery of perfect geodesic dome closed structures of carbon, such as C o has led to numerous studies of so-called Buckminster fullerene. Dislocations are important features of the structures of nested fullerenes also called onion skin, multilayered or Russian doll fullerenes. A recent theoretical study [118] shows that these defects serve to relieve large inherent strains in thick-walled nested fullerenes such that they can show faceted shapes. 

Presumably, the heterolytic scission of the carbon-sulfur bond in the oxide is assisted by the hydrogen bonding, in addition to the inherent strain of the three-membered ring. 

In carbohydrate chemistry, the most described method for the preparation of saccharidic thionocarbamates involves preliminary introduction of the amine function on a partially or non-protected saccharidic template. The condensation of amino sugars with carbon disulfide or thiophosgene leads to cyclization in 1,3-oxazolidine- or l,3-oxazine-2-thiones. This reaction involves the formation of an intermediate isothiocyanate, which reacts further with a 3- or y-located hydroxyl group. The viability and facility of this process depends on the saccharidic ring size and the inherent strain. Some major rules can be put into light from the cases studied 30... 

Quantum-chemical calculations furnished very similar energies of 1,2-cyclobuta-diene (13) and the diradical 13-D (Scheme 6.5), with a small preference for the latter. Since the ring opening of 13 or 13-D was estimated to be highly exothermic (74.5 kcal mol-1) because of the inherent strain, the barrier for this step must be low [24]. 

The same hexacyclic enedione has been converted to a series of Cj -heptaquinane derivatives via the alcohol 819 which is produced by condensation with ethyl formate in base The hydroxyl group in 819 can be readily functionalized, although loss of water to arrive at 820 occurs remarkably readily, despite the inherent strain of this system (Scheme XCIX). The conjugated double bond in 820 is understandably a good Michael acceptor, a property which was utilized to prepare 821. 

As might be expected from their inherent strain, many cyclopropenes undergo rearrangement, dimerisation or even polymerisation under relatively mild conditions. The conditions required for reaction are, however, very variable and some cyclo-propenes, such as 3,3-dimethylcyclopropene, are stable at relatively high temperature (150 °C in this case). Three main reactions are described below — the ene-reaction, [2+ 2]-dimerisation, and rearrangement to vinylcarbenes. 

The reaction of 247 with DMAD in methanol results in the exclusive formation of thiazinone 24. The reaction can be catalyzed by sodium methoxide. In the absence of added base, thiazolidinone 23 is formed in addition to compound 24. This is in contrast to the flndings in the reactions of thioureas with DMAD in which thiazolidinones are the only isolable products. The difference may be due to.the enhanced stability of the 6,5,6-ring system because of pseudoaromaticity and to the inherent strain present in the 6,5,5-ring system, or it may be due to the difference in basicity between a benzimidazolyl-2-thione and thioureas. 

In examining the cycloalkenes, one must first recognize that a double bond has considerable inherent strain. For example, the dimerization of ethylene to give cyclobutane is fairly exothermic (—18 kcal mol" ) and if there were a way to readily overcome orbital symmetry restrictions, cyclobutane would be a very common reagent. However, in the following, we will take the conventional view that ethylene is unstrained. Then, in comparing cycloalkanes and cycloalkenes it is helpful to define olefinic strain (OS) as the difference in strain between the alkene and the corresponding alkane ... 

The inherent strain ( 27 kcal/mol) of epoxides makes them prone to (1) ring opening by a wide range of nucleophiles, (2) base-induced rearrangement, and (3) acid-catalyzed isomerization. 

Acrylic/wool blended fibre fabrics may be prepared according to established practice for wool, except that precautions are taken to allow for the thermoplastic properties for the acrylic fibres. Relaxation is done to remove the inherent strain. Worsted fabrics are crabbed and then scoured in either a Dolly at temperature not exceeding 40 C or in winch. 

The ready availability of achiral and racemic epoxides from simple alkene precursors renders epoxide ring-opening an appealing approach to asymmetric synthesis. The inherent strain-induced reactivity of epoxides can be enhanced by coordination of the epoxide oxygen to a Lewis acid, thereby creating the possibility for chiral Lewis acids to catalyze enantioselective ring opening events. 

As discussed above, tin(IV) enolates are readily formed from a,) -unsaturated ketones and aldehydes. The common free-radical protocol of AIBN and benzene at 80 °C can be applied to enones to generate the tin enolate in situ (Scheme 2). Moreover, the use of a-ketocyclopropanes as precursors is equally effective, as shown in Scheme 7 [5i], This protocol takes advantage of the inherent strain of the cyclopropane ring in 28 that is released via the cyclopropylcarbinyl radical when the... 

Schleyer defines strain energy somewhat differently, and uses experimental values for the compounds in question to evaluate it (Schleyer et al., 1970). However, in practice, his calculated strain energies and ours differ by trivial amounts (perhaps with a rare exception). Using our scheme, the calculated values for inherent strain are as shown in Table 6. 

Compound Inherent strain (kcal mole-1) Compound Inherent strain (kcal mole-1)... 

The normal and simple alkanes are calculated to have inherent strains of essentially zero. One can obtain from these numbers the actual strain energies, if desired, by adding the amount by which the enthalpy is raised due to admixture of the higher energy conformations. Such information is rarely of interest, however. 

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