Angle strain cycloalkynes

Dedicated to Prof. Dr. Dr. h. c. mult. G. Wittig who Contributed so much to the Chemistry of Angle Strained Cycloalkynes. 

Structural Effects on the Isolability of Angle Strained Cycloalkynes...202... 

Can one definitely prove the occurrence of non-isolable short-lived cycloalkynes Throughout this article the expression angle strained cycloalkynes is used. 

The methods of preparation of cycloalkynes are essentially the same as those used for their open-chain analogs, which are discussed in detail elsewhere1(. However, for special reasons connected with the nature of these compounds some methods have become more important in the preparation and generation of angle strained cycloalkynes. Some problems are ... 

The scheme on p. 194 summarizes the most important methods for the synthesis and generation of angle strained cycloalkynes (Fig. 1). 

In Tables 1 and 2 the methods of generation and experimental evidence for the intermediate occurrence of unstable cycloalkynes and methods of synthesis of isolable angle strained cycloalkynes, resp. are given. 

Table 1. Unstable Angle Strained Cycloalkyne Intermediates ...

For further angle strained cycloalkynes of larger ring size see Ref. u p. 658-662. Some isolable ten- or eleven membered cycloalkynes having additional unsaturation in the ring are not considered here, since angle deformation at the triple bond, at least according to models, should be less than 10°. 

Last not least, force field, 52 1S4) and semiempirical106 l44) quantummechanical calculations have been used to calculate structural parameters and enthalpies of formation of angle strained cycloalkynes. These calculations are at present the only way to obtain quantitative estimates of ring strain in transient cycloalkynes, such as cycloheptyne 1S2). 

In the benzoannellated compounds two opposing effects on isolability are to be expected. Annellation of a benzene ring to cyclooctyne should increase ring strain as does introduction of an additional double bond. On the other hand, the size of the benzene ring should, like a bulky substituent, impede oligomerization reactions and thereby kinetically stabilize the angle strained cycloalkyne. 

A few structural analyses of angle strained cycloalkynes have been carried out. 

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...

Table 4. Calculated C—C=C Bond Angles in Some Angle Strained Cycloalkynes [°]...

The heats of hydrogenation of some angle strained cycloalkynes have been determined in acetic acid150) or cyclohexane 151. The experimental values including the heats of hydrogenation of some open-chain reference substances are given in Table 5. 

Table 5. Enthalpies of Hydrogenation (—AH25 °c) of Angle Strained Cycloalkynes, their Corresponding Alkenes and Open-chain Reference Compounds...

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