Diboron compounds activation

Diboron compounds with alkylamino or alkyloxy substituents are prepared from the appropriate halogenoborane derivative using active metals " ... 

Although not fitting exactly into the scope of this book, the iridium catalyzed borylation of five membered heterocycles through C-H bond activation also deserves mentioning. A recent report by Miyaura disclosed the reaction of bis(pinacolato)diboron with heteroaromatic systems, where thiophene, fiirane and pyrrole were converted to their 2-boryl derivatives with good selectivity (6.86.), The yields presented refer to the diboron compound since the heterocycles were used in excess in all cases. Indole, benzofurane and benzothiophene were monoborylated with similar efficiency.116... 

General Trends on Activation of Symmetrical Diboron Compounds 42... 

Metal Activation of Halide- or Amine-Substituted Diboron Compounds 70... 

Precise Activation of Unsymmetrical Dialkoxy-diamino-diboron Compounds 77... 

GENERAL TRENDS ON ACTIVATION OF SYMMETRICAL DIBORON COMPOUNDS... 

Scheme 10 Activation of chiral diboron compounds by Pt(0)-olefin complexes.

Since palladium complexes have proven to be efficient in activating diboron compounds throughout transmetalation, a wide range of applications have been considered in the last decade, such as palladium-catalyzed transformation of aUyhc alcohols to allylboronates, borylation of allylic hahdes or aUyhc acetates, and the P-boration of a,P-unsaturated carbonyl substrates.Interestingly, both palladium and nickel showed to be similarly efficient to activate B2pin2 and catalyze the addition to unsaturated substrates. Oshima postulated that Ni(0) species react with substrate a,P-unsaturated esters and amides to generate the r -coordinated complex, which activates the B2pin2 to favor the formation of T -coordinated... 

Moving from the Pt to Pd and Ni complexes, it can be noted a progressive change on the activation mode of the tetra(alkoxy) diboron compounds from oxidative addition of the B—B within Pt(0) complexes to o-bond metathesis with Pd or Ni. Going further to the right of the periodic table, the activation of diboron by O-bond metathesis becomes more frequent. 

In this context, an internally activated, sp -sp -hybridized diboron compound, PDIPA diboron (pinacolato diisopropanolaminato diboron) prepared by Santos et al, has shown to easily interact with CuCl salts to favor the B-B cleavage and form the corresponding Cu-Bpin species (Scheme 44) which were very active in the copper-catalyzed, P-borylation of a,P-unsaturated conjugated compounds and aUenoates. " ... 

It seems that all the efforts to activate tetra(alkoxy)diboron compounds are justified since the model CI2B—BCI2 reagent, which develops borylation without metal activation, is too difficult to handle. However, the progress in this field has opened a new window toward the activation of tetra(alkoxy) diboron compounds in a metal-free context by the formation of Lewis add—base adducts and their addition to unsaturated substrates can be performed with total chemo-, regjo-, and stereoselectivity. 

Bonet A, PubiU-UUdemolins C, Bo C, Gulyas H, Fernandez E. Transition-metal-free diboration reaction by activation of diboron compounds with simple Lewis bases. Angew Chem Int Ed Engl. 2011 50 7158-7161. 

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

---