This review is a guided tour along the activation modes and reactivity of B–B, B–Si, B–N, B–S, B–Se and B–P reagents, in the absence of any transition metal complex. Here are disclosed the general concepts related to the homolytic and heterolytic cleavage of B–B and B–interelement bonds, as well as the generation of new C–B and C–interelement bonds, in a selective way. The greener consequences of these novel routes facilitate the gram scale preparations of target functionalised organic compounds. Intrinsic data about the suggested mechanisms and spectroscopic evidence that supports the innovative theories are provided along the review. Since this is a stimulating area of work that has emerged within the last decade, this overview serves as the basis to understand the new trends and hopefully to generate inspiration for future discoveries in the field.

Transition-metal-free B–B and B–interelement reactions with organic molecules

A deoxygenative gem-diborylation and gem-silylborylation of aldehydes and ketones is described. The key for the success of this transformation is the base-promoted C–O bond borylation or silylation of the generated α-oxyboronates. Experimental and theoretical studies exhibit that the C–O bond functionalization proceeds via an intramolecular five-membered transition-state (9-ts) boryl migration followed by a 1,2-metalate rearrangement with OBpin as a leaving group. The transformation occurs with an inversion on the carbon center. Direct conversion of aldehydes and ketones to gem-diboron compounds was achieved by combining copper catalysis with this base-promoted C-OBpin borylation. Various aldehydes and ketones were deoxygenatively gem-diborylated. gem-Silylborylation of aldehydes and ketones were achieved by a stepwise operation, in which B2pin2 initially react with those carbonyls followed by a silylation with Bpin-SiMe2Ph.

C–O Functionalization of α-Oxyboronates: A Deoxygenative gem-Diborylation and gem-Silylborylation of Aldehydes and Ketones

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...

http://pubs.acs.org/doi/pdf/10.1021/jacs.7b00445

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

Boron-containing radical species

Single-electron transfer is an important process in organic chemistry, in which a single-electron reductant (electron donor) acts as a key component Compared with metal-based electron donors, organic electron donors have some unique advantages, such as tunable reduction ability and mild reaction conditions The development of novel organic electron donors with good reduction ability together with ease of preparation is in high demand Based on the pyridine-catalyzed radical borylation reaction developed in our laboratory, we have discovered that, the reaction system consisting of a diboron(4) compound, methoxide and a pyridine derivative could smoothly produce super electron donors in situ Two boryl–pyridine based species, the major one being a trans-2H,2′H-[2,2′-bipyridine]-1,1′-diide borate complex and the minor one being a pyridine radical anion–borate complex, were observed and carefully characterized These complexes were found to be organic super electron donors unprecedented in literature, and their formation mechanisms were studied by DFT calculations The diboron/methoxide/pyridine system enables the preparation of organic super electron donors from easily accessible starting materials under mild conditions, which has the potential to be a general and practical single-electron reducing agent in organic synthesis

/pdf/super-electron-donors-derived-from-diboron-3k2oksz6jf.pdf

Super electron donors derived from diboron

In the absence of transition metal catalysts, the unsymmetrical diborane(4), pinB-BMes2, reacted with alkynes to afford diborylalkenes. The isomer ratio of the products could be controlled via temperature, solvent, and additive(s). A reaction mechanism was proposed on the basis of two isolated intermediates, and this reaction could furthermore be applied to synthesize a luminescent molecule.

Direct and Base-Catalyzed Diboration of Alkynes Using the Unsymmetrical Diborane(4), pinB-BMes2

Transition metal-free direct and base-catalyzed 1,2-diborations of arylacetylenes using pinB-BMes2 provided a syn/anti-isomeric mixture of diborylalkenes. The kinetic analysis showed that the reaction rate and isomer ratio were affected by reaction conditions and substituents on the aryl ring. DFT calculations indicated that direct addition proceeded via the interaction of acetylene-π with the BMes2 fragment. In contrast, for the base-catalyzed diboration, the previously isolated sp2–sp3 diborane and borataallene were confirmed as stable intermediates by calculations. The whole reaction pathways can be divided into the Bpin-migration and deprotonation steps, where the borataallene should be considered as a common intermediate. It should be noted that the deprotonation step is reversible and affords the kinetically less favoured isomer under the thermodynamic conditions. As a result, the composition of isomeric products, in the base-catalyzed diboration, is attributed to the small difference of activation barriers between direct and base-catalyzed systems.

/pdf/mechanistic-study-on-the-reaction-of-pinb-bmes2-with-alkynes-9fkw3lxury.pdf

Mechanistic study on the reaction of pinB-BMes2 with alkynes based on experimental investigation and DFT calculations: gradual change of mechanism depending on the substituent

Direct and Base-Catalyzed Diboration of Alkynes Using the Unsymmetrical Diborane(4), pinB-BMes2.

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Direct and Base-Catalyzed Diboration of Alkynes Using the Unsymmetrical Diborane(4), pinB-BMes2

Mechanistic study on the reaction of pinB-BMes2 with alkynes based on experimental investigation and DFT calculations: gradual change of mechanism depending on the substituent

Direct and Base-Catalyzed Diboration of Alkynes Using the Unsymmetrical Diborane(4), pinB-BMes2.