Alkynyl hydrogens

Notes. (1) The progress of the reaction may be monitored by occasionally halting the stirring and removing a portion of the clear upper organic layer for infrared examination. The reaction is complete when a sample does not show hydroxyl and alkynyl hydrogen absorption at 3400 and 3300 cm-1 respectively. 

In the same vein, Schmalz has proposed a facile construction of the colchicine skeleton by a rhodium-catalyzed cyclization/cycloaddition cascade [56]. A TMS group has to be introduced on the alkyne moiety of 66 in order to avoid participation of the relatively acidic alkynyl hydrogen atom in undesired proton transfers. The resulting 6,7,7 of 67a and 67b architecture was assembled in a remarkably diastereoselective manner (14 1) and in satisfactory yield (Scheme 30). 

A The alkynyl hydrogen of propyne is acidic and hence the propyne will form an organometallic derivative. The Grignard derivative will react with carbon dioxide to give the carboxylic acid. Cis alkenes are formed by the catalytic hydrogenation of alkynes using a Lindlar catalyst. Hence a propynyl alcohol is a precursor to compound (b). This could be obtained by the addition of a propyne anion to propanone ... 

Replacement of propargylic as well as alkynyl hydrogen in the same compound can arise to give di- and polylithio products e.g., MeC=CH with n-BuLi (1 equiv) provides MeC=CLi, and with xs n-BuLi successive replacements of hydrogens occur to give CjLi finally. The structures of these anionic compounds are deduced from IR data ... 

The acidity of the alkynyl hydrogen is strongly influenced by the nature of the group on the other side of the triple bond, as shown in Table 8.3.92 When the R group is electron releasing (e.g., an alkyl group such as... 

Liu et al. reported a TpRuPPh3 (CH3CN)2-PF6-catalyzed cycloisomerization of cis-3-en-l-ynes I or their precursor alcohols 210, which afforded cyclopentadiene 211 (Scheme 80) [43]. Mechanistically, 210 undergoes ruthenium-catalyzed dehydration to afford the real substrates cis-3-en-l-ynes 1, which is converted into mthenium-vinyhdene intermediate H via [1,2]-shift of alkynyl hydrogen. A [1,5]-... 

At this point the induced magnetic fieid 6 opposes the appiied fieid Bq a hydrogen here feels" a net magnetic field, B = Bq-B. The alkynyl hydrogen is shielded, and a relatively high Bq will have to be applied to bring this shielded hydrogen into resonance... 

In Summary The characteristic hybridization scheme for the triple bond of an alkyne controls its physical and electronic features. It is responsible for strong bonds, the linear structure, and the relatively acidic alkynyl hydrogen. In addition, alkynes are highly energetic compounds, hitemal isomers are more stable than terminal ones, as shown by the relative heats of hydrogenation. 

Alkenyl hydrogens (and carbons) are deshielded and give rise to relatively low-field NMR signals compared with those in saturated alkanes (Section 11-4). In contrast, alkynyl hydrogens have chemical shifts at relatively high field, mnch closer to those in alkanes. Similarly, the sp-hybridized carbons absorb in a range between that recorded for alkenes and alkanes. Alkynes, especially terminal ones, are also readily identified by IR spectroscopy. Finally, mass spectrometry can be a useful tool for identification and structure elucidation of alkynes. 

Figure 13-3 300-MHz NMR spectrum of 3,3-dimethyl-1-butyne showing the high-field position (5 = 2.06 ppm) of the signal due to the alkynyl hydrogen.

Unlike alkenyl hydrogens, which are deshielded and give NMR signals at 5 = 4.6-5.7 ppm, protons bound to ip-hybridized carbon atoms are found at S = 1.7-3.1 ppm (Table 10-2). For example, in the NMR spectrum of 3,3-dimethyl-l-butyne, the alkynyl hydrogen resonates at 5 = 2.06 ppm (Figure 13-3). 

Figure 13-4 Electron circulation in the presence of an external magnetic field generates local magnetic fields that cause the characteristic chemical shifts of alkenyl and alkynyl hydrogens. (/ Alkenyl hydrogens are located in a region of space where hiocai reinforces Hq. Therefore, these protons are relatively deshielded. (B) Electron circulation in an alkyne generates a local field that opposes Ho in the vicinity of the all nyl hydrogen, thus causing shielding.

Infrared spectroscopy is helpful in identifying terminal alkynes. Characteristic stretching bands appear for the alkynyl hydrogen at 3260-3330 cm and for the C=C triple bond at 2100-2260 cm . There is also a diagnostic Pq-h bending absorption at 640 cm ... 

As in the previous exercise, first construct the necessary carbon-carbon bond by deprotonation of the terminal alkynyl hydrogen, followed by 8 2 reaction, and then modify the triple bond to the desired new functional group. 

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