Nucleophilic Addition to a Triple Bond

The double CC bond of alkenes is rather inert in regard to the nucleophiles. As opposed to it, the triple bonds of alkynes readily react with water, alcohols, amines, with trans-addition being the favored process [31]  

In the generalized scheme of Eq. (6.3), the first stage, i.e., the attack by the nucleophilic center Y(Y ) is the key step determining the stereochemistry of an addition reaction. This stage has the crucial effect on the general direction of reactions of nucleophilic substitution at the carbon atom of a triple bond. An analysis of electronic and steric demands of Eq. (6.3) may conveniently be made on the simplest model. 

Several ab initio calculations on Eq. (6.4) indicate that the hydride-ion attacks the triple bond at an obtuse angle of 110-120° [32], 127-128° [33, 34]. The planar structure of the transition state XVIII [34], calculated with the 4-3IG 

The character of deformations of the transition state structurally may readily be explained by the preferability of tram addition. Indeed, for a cis addition the stage of topomerization of the vinyl anion XIX is needed, which requires overcoming the energy berrier of the planar inversion amounting for the vinyl anion to 35 kcal/mol. The tram distortion of the C—H bonds exhibited by the triple bond when it is approached by a nucleophile is one more reason for the remarkable ease with which the nucleophilic addition reactions of this type proceed. 

The pattern of adaptation of the orbital and energy structure of the alkyne molecule to the demands of the reaction of addition to a triple bond, shown in Fig. 6.3, is, in principle, retained also in addition reactions to heteronuclear triple bonds. This conclusion follows, for example, from the structure of the transition state V in the reaction of addition of hydrogen chloride to nitriles, which at the stage of the final product formation is characterized by the making of the bond C—Cl. The angle of nucleophilic attack is in the region of the transition strueture 126°. 

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Nucleophilic addition to a triple bond serves to create the (3-al- Tips... 

Eisenstein, O., G. Procter, and J.D. Dunitz. 1978. Nucleophilic addition to a triple bond Preliminary ab initio study. Helv ChimActa 61 2538-2541. 

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