Not all carbon—carbon bonds are equivalent: anomeric effect of sp-hybridized carbon atom

Abstract

A possibility of manifestation of the anomeric effect of the sp-hybridized carbon atom, which is a component of triple C≡C and C≡N bonds, was studied. The effect of carbon hybridization on the acceptor capacity of the carbon—carbon σ-bonds was evaluated. A high effective electronegativity of the sp-hybridized carbon atom was found to exert a noticeable effect on the magnitude of the anomeric interactions nN→σ∗CC. The triple bond in such systems indeed can participate in anomeric interactions, which was proved for N-propargyl substituted bispidines as an example using experimental (XRD) and computational methods. This participation is expressed in the stabilization of the bisectral conformation of the N—CH2—C≡CH and N—CH2—C≡N fragments in the case of N-monosubstituted and N,N′-disubstituted bispidines. The values of charge transfer from an unshared pair of nitrogen atoms to the σ-antibonding orbital of the C≡C—CH2 bond calculated for the model Me2N—CH2—X systems by the NBO method are up to 11.0 kcal mol−1, which is relatively low for the classical anomeric effect: the corresponding value for the N≡C—CH2 fragment is 11.3 kcal mol−1. The anomeric effect manifested by the acetylene group makes it possible to stabilize certain conformations in the bispidine systems containing propargyl substituents, which can affect the reactivity of acetylenes. For the N-benzyl systems (9.4 kcal mol−1) and N-allyl (9.4 kcal mol−1) and N-propyl (10.0 kcal mol−1) derivatives, the hyperconjugation stabilization is weaker and does not play a decisive role in conformational equilibrium. The obtained data are of obvious interest for specialists working in the field of the chemistry of the triple bond of the propargyl type.