Cobalt-Complexed Acetylenic Tetrads, a Molecular Scaffold for Quadruple Ionic Functionalization Reactions

A methodology was developed for introducing nucleophiles into the α- and α′-positions of the dicobalt hexacarbonyl-complexed acetylenic tetrads. A synthetic algorithm included the entry of a given nucleophile to both termini of the acetylenic tetrad A (α-Nu1-α′-Nu1; α-Nu2-α′-Nu2), or a pair of select nucleophiles arranged unsymmetrically in opposing sequences (α-Nu1-α′-Nu2; α-Nu2-α′-Nu1). Thus, every substrate A and a pair of C-nucleophiles give rise to an organometallic rectangle (B-E) and synthetic octagon (B-I). The site-selective transformations that exploited the difference in thermodynamic stabilities of the α- and α′-cationoids, and thus in heterolytic bond dissociation energies (BDE) values, were coined the “quadruple ionic functionalization reactions.” The substrate and reagent bases were expanded to include aromatic carbo- and heterocycles as α-substituents, and aliphatic and aromatic reagents as nucleophiles. Density functional theory calculations allowed for identifying qualitative descriptors that explained the preponderant bond formation in more stabilized, albeit more hindered α-propargyl positions. In mechanistic terms, reactions at the competing sites (α- vs α-′) occupy distinctly different positions in the mechanistic continuum spanning the classical SN1 and SN2 processes. Overall, quadruple functionalization methodology allows for the practically limitless expansion of the acetylenic tetrads and a nucleophile base, and the completion of a multitude of organometallic rectangles and synthetic octagons.