Symmetrically substituted alkynes

Vollhardt and colleagues have explored the reactions between diynes and enami-nes338-341. The reactions between symmetrically substituted alkynes and alkyne tethered uracil derivatives proceeded in moderate yields, producing adducts with predominant anti configurations342-343. On the other hand, the reactions between diynes and uracil derivatives produced predominantly syn isomers. 

Selected examples are given in Table 9 for bromofluorinations of alkenes using A-bromosuccin-imide in combination with 70% hydrogen fluoride/pyridine (Method A) and hydrogen fluo-ride/polyvinylpyridine (Method B), respectively. Table 10 shows examples of the selective monoaddition of in situ produced halofluorides (70 % hydrogen fluoride/pyridine and A-iodo-. A-bromo- or /V-ehloro-succinimide) to symmetrically substituted alkynes. 

The readily available yttrocene derivative (C5Me5)2YMe(THF) has been shown to be an effective catalyst for the hydrosilylation of internal alkynes [85]. A single stereoisomer, i.e. the product of cis addition of phenylsilane to the alkyne, is formed in the reaction with symmetrically substituted alkynes. Comparable reactions with a variety of unsymmetrically substituted internal alkynes resulted in a regioselective hydrosilylation reaction in which the silane moiety is placed at the sterically less hindered carbon atom of the alkyne. Various functional groups such as halides, amines, protected alcohols, and trisubstituted... 

C=C Stretching Vibrations The weak C=C stretching band of alkyne molecules occurs in the region of 2260-2100 cm 1. Because of symmetry, no C=C band is observed in the IR for symmetrically substituted alkynes. In the IR spectra of monosubsti-tuted alkynes, the band appears at 2140-2100 cm 1. 

For example, the mesoionic 1,3-dithiolones (2) combine across the 2,5-position at 90-130 °C with symmetrically substituted alkynes as dipolarophiles with formation of non-isolable primary adducts of type (85). The latter fragment in a retro Diels-Adler type... 

CH3OCCH3. For reasons we won t discuss, symmetrically substituted alkynes such as 2-butyne do not show a OC bond absorption in the IR. This alkyne is distinguished from the other isomers in that it shows no absorptions in either the alkyne or alkene regions. 

Bands due to carbon-carbon stretching may appear at about 1500 and 1600 cm for aromatic bonds, at 1650 enr for double bonds (shifted to about 1600 cm i by conjugation), and at 2100 cm for triple bonds. These bands, however, are often unreliable. (They may disappear entirely for fairly symmetrically substituted alkynes and alkenes, because the vibrations do not cause the change in dipole moment that is essential for infrared absorption.) More generally useful bands are due to the various carbon-hydrogen vibrations. 

Hydroboration-oxidation of an alkenylborane derived from a symmetrically substituted alkyne yields a single ketone, whereas unsymmetrically disubstituted alkynes furnish mixtures of ketones. 

---