Cobalt-Complexed\nAcetylenic Tetrads, a Molecular Scaffold\nfor Quadruple Ionic Functionalization Reactions

A methodology was developed for introducing nucleophiles\ninto the\nα- and α′-positions of the dicobalt hexacarbonyl-complexed\nacetylenic tetrads. A synthetic algorithm included the entry of a\ngiven nucleophile to both termini of the acetylenic tetrad <b>A</b> (α-Nu<sup>1</sup>-α′-Nu<sup>1</sup>; α-Nu<sup>2</sup>-α′-Nu<sup>2</sup>), or a pair of select nucleophiles\narranged unsymmetrically in opposing sequences (α-Nu<sup>1</sup>-α′-Nu<sup>2</sup>; α-Nu<sup>2</sup>-α′-Nu<sup>1</sup>). Thus, every substrate <b>A</b> and a pair of C-nucleophiles\ngive rise to an organometallic rectangle (<b>B</b>-<b>E</b>) and synthetic octagon (<b>B</b>-<b>I</b>). The site-selective\ntransformations that exploited the difference in thermodynamic stabilities\nof the α- and α′-cationoids, and thus in heterolytic\nbond dissociation energies (BDE) values, were coined the “quadruple\nionic functionalization reactions.” The substrate and reagent\nbases were expanded to include aromatic carbo- and heterocycles as\nα-substituents, and aliphatic and aromatic reagents as nucleophiles.\nDensity functional theory calculations allowed for identifying qualitative\ndescriptors that explained the preponderant bond formation in more\nstabilized, albeit more hindered α-propargyl positions. In mechanistic\nterms, reactions at the competing sites (α- vs α-′)\noccupy distinctly different positions in the mechanistic continuum\nspanning the classical S<sub>N</sub>1 and S<sub>N</sub>2 processes.\nOverall, quadruple functionalization methodology allows for the practically\nlimitless expansion of the acetylenic tetrads and a nucleophile base,\nand the completion of a multitude of organometallic rectangles and\nsynthetic octagons.