Cycloalkynes strain energies

Since for most angle strained cycloalkynes, particularly for those which are not isolable, structural data is lacking, calculations have been carried out. The structural parameters and strain energies of cyclononyne (6)152 154,160>, cyelooctyne (14) 152,154,106,144), cycloheptyne (16) 152,154), cyclohexyne (20)1S4) and cyclopentyne (18) 154) were calculated. Some values for strain energies and C—C s C bond angles are given in Tables 3 and 4. 

Table 3. Calculated Strain Energies of Some Angle Strained Cycloalkynes...

The valence-angle constraints imposed by cyclization lead to very high strain energies (Es) in the small- and medium-ring cycloalkynes. Members of the series below ring size 6... 

In early work on the chemistry of cycloalkynes, cyclononyne was shown to react slowly with phenyl azide to give the cycloadduct in 82% yield however, the strain energy in cyclooctyne is significantly greater, calculated to be 77.5 kJ moP which accounts for the fact that a reaction of cyclooctyne with phenyl azide was reported to proceed like an explosion and resulted in the formation of the 1,2,3-triazole shown in Scheme 2 [13]. The use of cyclooctyne derivatives has been exploited in considerable detail recently, particularly because reactions can be carried out in biological systems where metal-catalysed reactions cannot be used because of toxicity problems (for reviews see [14, 15] see for example [16-19]). 

More recent developments exploit the energy content of readily accessible cycloalkynes based on phenyl-alkynyl structural motives, albeit not always with fullerene formation in mind. For example, the strained dehydrobenzoannulene 4 [19] could be converted by light, heat (145 °C), or pressure (20000 psi) in a topochemical polymerization reaction typical for butadiynes to a deeply coloured polymer. A similar thermochemical behaviour (strongly exothermic transformation around 200 °C) was observed for compounds 5 and 6 [20]. However, none of the systems 4-6 shows any tendency to produce spherical forms of carbon under the conditions investigated. 

As a consequence of the drastic lowering of the LUMO energy on C C=C bending in angle strained cycloalkynes a stronger increase is expected for nucleophilic than for electrophilic additions 178,179). This prediction could not be proved so far for isolable cycloalkynes 209,210) however, there is some, although not conclusive evidence in benzyne chemistry for such a trend 179). 

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