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.