Unsymmetrical Diboron Reagents: Application in Borylation Reactions of Unsaturated Bonds

Abstract

In the past decades, borylation reactions have received extensive research interest and have developed into effective tools in the synthesis of versatile organoboron compounds. Boranes and symmetrical diboron compounds are commonly utilized as borylating reagents in these transformations, especially in the borylation reactions of unsaturated bonds. More recently, several types of unsymmetrical diboron reagents have been synthesized and applied in these borylation reactions, allowing for complementary chemo- and regioselectivity. This review aimed to highlight the recent development in this rising research field, focusing on new reactivity and selectivity that originates from the use of these unsymmetrical diboron reagents.

First-Row d-Block Element-Catalyzed Carbon–Boron Bond Formation and Related Processes

“…Conventionally, many organoboranes were synthesized via metal exchange reactions between highly nucleophilic organometallics (organolithium or Grignard reagents) and boron electrophiles, but synthetic application is limited due to the high reactivity of such nucleophiles and the requirement for costly protection and deprotection of many functional groups. Alternative, metal-free borylation reactions can provide milder conditions. − Frustrated Lewis pair (FLP)-catalyzed C–H borylation − and photoinduced ,,,− borylation reactions represent another powerful recent development to install boron atoms into organic molecules. Borylation reactions employing heterogeneous catalysts have also attracted attention. − However, despite these advances, transition metal-catalyzed borylation can be considered to be the most efficient method to synthesize organoboron compounds.…”

“…Catalytic addition reactions are alternative methodologies to introduce boron moieties into unsaturated organic molecules. The transformation of alkenes (refs , , , , , , , , , , , , and − ), alkynes (refs − , , , , − , , , , , , − ,, and − ), allenes (refs , , , , , , , and − ), α,β-unsaturated compounds (refs , , , , , , , and ), and carbonyl compounds (refs , , and − ) into organoboron reagents can be achieved by dehydrogenative borylation (refs , , and ), hydroboration (refs , , , , , , , − , , , − , , , and − <...…”

“…In this context, processes mediated by a copper-boryl species ,, are particularly prominent (refs − , , , , , , − , , − , , − , , , and ). However, other first-row elements are finding greater application.…”

“…However, other first-row elements are finding greater application. For example, iron ,,,,,,,, and cobalt ,,,,,,,,,, catalyst systems have been developed for the borylation of unsaturated compounds, nickel and cobalt have become more and more popular for C–H borylations, ,,,− and the borylative cleavage of C–N, C–C, and C–O bonds induced by nickel catalysts has turned out to be another powerful tool. ,,,,, …”

Boron Reagents and Catalysts for the Functionalization of Strained Heterocycles

“…More recently, a novel approach to access propargylic boron and silicon compounds starting from vinylidene cyclopropanes of type 17 has been reported by Chen and co‐workers (Scheme 21). [79] Interestingly, the introduction of boron atom was possible only using the unsymmetrical boron compound pinB‐Bdan, [80] as B‐dan is much less prone to transmetallation with copper under the basic reaction conditions to give copper‐allenyl intermediate 18 .…”