Alkynes protonation

In this section, we will look at alkene, imine, enol ether and alkyne protons. It s convenient to consider the first three at this stage as they usually absorb in the 8-5 delta region and the alkyne is included here for convenience. 

Aromatic protons, such as those in benzene, have shifts at still lower fields and commonly are observed at 7-8 ppm. In contrast, alkynic protons of the type —C=CH give resonances that are upfield of alkenic or aromatic protons and come at 2-3 ppm. Another effect associated with multiple bonds is the large difference in shift between a —CH(OCH3)2 proton, which normally comes at about 5,5 ppm, and aldehyde protons, —CH=0, which are much farther downfield at 9-11 ppm. 

The rate accelerations k(X = SiMe3)/fc(X = H) and corresponding free energy differences produced by trimethylsilyl substituents in alkene and alkyne protonation reactions are shown in Table 449. 

This time a second treatment with base will be needed to make the second anion. As the OH proton in 12 is more acidic (pK 16) than the alkyne proton (p a 25), two molecules of base will be needed and the most reactive anion (alkyne) reacts first. 

Fig. 14 Zn binding to the homochiral Zn alkoxide, monitored by the 13-C shifts of the two alkyne protons and demonstrating the higher degree of association in toluene compared to thf...

Dynamic NMR studies indicate that the barrier to alkyne rotation in dithiocarbamate bisalkyne complexes is near 15 kcal/mol (87). For unsym-metrical alkyne ligands, as in Mo(PhC=CH)2(S2CNMe2)2, several isomers are possible with like substituents either adjacent ( cis ) or opposite ( trans ). Analysis of the NMR properties follows the logic presented by Faller and Murray for CpMo(RC=CR)2Cl (94). The C2 molecular symmetry dictated by the chelates can produce two different trans isomers with the two alkyne protons of PhC=CH in each isomer equivalent by C2 rotation (Fig. 26). Only one unique cis isomer is possible, but the two... 

Look at the reaction below in some ways it is quite similar to the ones we have just been discussing. Butyllithium deprotonates an sp2 hybridized carbon atom to give an aryllithium. It works because the protons attached to sp2 carbons are more acidic than protons attached to sp3 carbons (though they are a lot less acidic than alkyne protons). 

Whereas electrophiles with strong bridging tendency (e.g., halogens) react considerably faster with alkenes than with analogously substituted alkynes, protons [209,210] and carbocations [211] have been reported to attack analogously substituted double and triple bonds with similar rates [212]. 

Geoffroy and co-workers investigated the protonation of cyclopenta-dienyl dicarbonyltungsten alkylidyne complexes with HBF4 in the presence of alkynes (195,196). The outcome of the reactions depends on the nature of both the alkylidyne ligand and the alkyne. Protonation of complex 254 in the presence of diphenylacetylene affords the vinylcarbene complex 255... 

The very strong base BuLi will first remove the OH proton and then the alkyne proton. The aUr anion is more reactive (last formed, first to react) and is alkylated. Hydrogenation goes for i alkyne first and adds a molecule of hydrogen to the same side of the triple bond to give a cis alker... 

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