Cobalt-Catalyzed 1,1-Diboration of Terminal Alkynes: Scope, Mechanism, and Synthetic Applications

A cobalt-catalyzed
method for the 1,1-diboration of terminal alkynes
with bis­(pinacolato)­diboron (B2Pin2) is described.
The reaction proceeds efficiently at 23 °C with excellent 1,1-selectivity
and broad functional group tolerance. With the unsymmetrical diboron
reagent PinB–BDan (Dan = naphthalene-1,8-diaminato), stereoselective
1,1-diboration provided products with two boron substituents that
exhibit differential reactivity. One example prepared by diboration
of 1-octyne was crystallized, and its stereochemistry established
by X-ray crystallography. The utility and versatility of the 1,1-diborylalkene
products was demonstrated in a number of synthetic applications, including
a concise synthesis of the epilepsy medication tiagabine. In addition,
a synthesis of 1,1,1-triborylalkanes was accomplished through cobalt-catalyzed
hydroboration of 1,1-diborylalkenes with HBPin. Deuterium-labeling
and stoichiometric experiments support a mechanism involving selective
insertion of an alkynylboronate to a Co–B bond of a cobalt
boryl complex to form a vinylcobalt intermediate. The latter was isolated
and characterized by NMR spectroscopy and X-ray crystallography. A
competition experiment established that the reaction involves formation
of free alkynylboronate and the two boryl substituents are not necessarily
derived from the same diboron source.