Alkynes acidic properties

The dealuminated zeolites, which possess high acidic properties, show a relatively good activity in both hydration reactions of alkynes and nitriles as already observed in the case of olefins (ref.19). Work is in progress to measure the acidity of the zeolites used, in order to gain further understanding of the different reactions. 

This observation is not related to traces of base or acid from the silver salts used since control experiments mled out this possibility. It was known from the literature that the 5-exo-dig versus 6-endo-dig cyclization mode could depend on the nature of the carbonyl group,56 57 of the alkyne substituent,58 59 and of the nature60 61 and oxidation state62 of the metallic source used. Also, work from Yamamoto25 demonstrated the importance of both a- and Jt-Lewis acidity properties of silver(I) complexes. Therefore, depending on the silver salt used, two mechanistic pathways were proposed (pathways A and B, Scheme 5.15). 

The —C = C — carbon atoms in alkynes are very close to each other and this in turn weakens the C — H bond. This enables the H atom to leave easily from the R — C = C — H molecule. As a result, allqmes with the R — C C — H formula show weak acidic properties. Because of this acidity, alkynes can react with some metal salts to form all l metal acetylene salts. If an alkyne has the structure R — C = C — R, however, there is no reaction. 

Alkynylgold compounds are readily available from reactions of the parent alkyne and gold(I) halides in the presence of even a very weak base, owing to the enhanced CH acidic properties of alkynes. The donor-free products are oligomeric, probably through a combination of a- and... 

The amide ion caimot remove a hydrogen bonded to an sp or an sp carbon. Only a hydrogen bonded to an sp carbon is sufficiently acidic to be removed by the amide ion. Consequently, a hydrogen bonded to an sp carbon sometimes is referred to as an acidic hydrogen. The acidic property of terminal alkynes is one way their reactivity differs from that of alkenes. Be careful not to misinterpret what is meant when we say that a hydrogen bonded to an sp carbon is acidic. It is more acidic than most other carbon-bound hydrogens but it is much less acidic than a hydrogen of a water molecule, and water is only a very weakly acidic compound (pA a = 15.7). 

The most distinctive aspect of the chemistry of acetylene and terminal alkynes is their acidity As a class compounds of the type RC=CH are the most acidic of all hydro carbons The structural reasons for this property as well as the ways m which it is used to advantage m chemical synthesis are important elements of this chapter... 

We have already discussed one important chemical property of alkynes the acidity of acetylene and terminal alkynes In the remaining sections of this chapter several other reactions of alkynes will be explored Most of them will be similar to reactions of alkenes Like alkenes alkynes undergo addition reactions We 11 begin with a reaction familiar to us from our study of alkenes namely catalytic hydrogenation... 

Finally, Lecomte and coworkers reported the synthesis of mikto-arm star-shaped aliphatic polyesters by implementing a strategy based on click chemistry (Fig. 36) [162]. Firstly, the polymerization of sCL was initiated by a diol bearing an alkyne function. The chain-ends were protected from any further undesired reaction by the esterification reaction with acetyl chloride. The alkyne was then reacted with 3-azidopropan-l-ol. The hydroxyl function located at the middle of the chain was then used to initiate the ROP of sCL and y-bromo-s-caprolactone. Finally, pendant bromides were reacted successfully with sodium azide and then with N, N-dimethylprop-2-yn-l-amine to obtain pendant amines. Under acidic conditions, pendant amines were protonated and the polymer turned out to exhibit amphiphilic properties. 

A series of 4-heteroaryl-5,6-di(2,5-dimethyl-3-thienyl)-2-phenyM//-l,6-thiazines 54 with photochromic properties was prepared by reacting 3-(2-(2,5-dimethylthiophen-3-yl)ethynyl)-2,5-di ethylthiophene with thiobenzamide and aldehydes <2005CHE86, 2005PS1503>. The acid-catalyzed cyclocondensation of cyclopentanone, aromatic aldehydes, ArCHO, and thiourea affords the cyclopenta[t ]l,3-thiazines 37 <2006PS1655>. Two equivalents of the aldehyde are required. The same products are isolated when 2,5-dibenzylidenecyclopentanones are treated with thiourea under the same conditions. 2-Amino-4i/-l,3-thiazines 184 are easily synthesized in one pot by the reaction of aromatic alkynes, R feCH, aromatic aldehydes, R CHO, and thiourea in the presence of TFA/acetic acid <2005OL3797>. 

Moreover, contrary to alkyne hydration where no adsorption of the carbonyl compound was detected, the problem is complicated here by the saturation of the strong acidic sites by the formed amide, the concentration of which shows a rapid stabilization against time (Fig.3). Consequently the reaction selectivity greatly depends on the ester percentage. The behaviour of the amide itself over the studied zeolites confirms this observation the conversion of the amide into ester goes faster on the HY2 g zeolite than on the Hg and on the HMg zeolites. This later point, together with the comprehension of the different mechanisms in relation with the zeolite properties, will be discussed in a further paper. 

To date, chemical properties have been reported mainly for 59 and 61. Occupancy of the silicon 3p orbital by electrons from nitrogen greatly reduces the electrophilicity of these silylenes. This, together with probable aromatic stabilization, significantly mutes the behavior of 59 and 61 as silylenes. For example, 59 does not insert into Si—H bonds, or react with alkynes such as PhC=CPh367, Moreover, 59 shows no Lewis acidic behavior, even toward bases as strong as pyridine. 

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