How to Break the Law: trans‐Hydroboration and gem‐Hydroboration of Alkynes

Abstract

Classical hydroboration is the textbook example of an organometallic reaction under rigorous frontier orbital control: a synergetic bonding mode between substrate and reagent evolves into a four‐membered transition state that enforces syn/suprafacial delivery of H−BR2 to the π‐system. This strict stereochemical “law” remained valid for decades, even after the advent of metal‐catalyzed hydroboration. During the last decade, however, numerous possibilities emerged that allow this paradigm to be challenged. Thus, direct trans‐hydroboration of terminal as well as internal alkynes was achieved through radical, ionic, organocatalytic, and metal‐catalyzed manifolds, which are summarized in this review. Among them, pathways involving either metal vinylidenes (Rh, Ir, Ru, Fe) derived from terminal alkynes or metallacyclopropenes (η2‐vinylmetal complexes) derived from an internal triple bond and a [CpXRu]‐based catalyst represent the currently most widely applicable solutions. The latter type of intermediates is also accountable for the equally perplexing gem‐hydroboration of alkynes: geminal delivery of a borane to a triple bond has no precedent in the classical canon. The only forerunner was gem‐hydrogenation, which represents an unconventional yet highly useful entry into discrete ruthenium carbene complexes. The close mechanistic ties between these transformations are outlined, and brief reference is also made to remotely related reactions such as trans‐diboration and trans‐carboboration.

Metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes

“…These two examples proved that the strict classical anti-Markovnikov hydroboration mode is apparently not unalterable either. 38 Anionic ligands ( L6 , L7 ) may play an important role in promoting unique selectivity. This novel mechanism in these two reactions that breaks traditional hydroboration rules deserves further exploration.…”

Ligand-controlled regiodivergent Ni-catalyzed trans-hydroboration/carboboration of internal alkynes with B2pin2

“… 4 With respect to the hydroboration of internal alkynes, regio- and stereoselectivity would give mixtures of α- and β-addition, cis - and trans -addition products, due to isomerization and other related processes. 5 …”

Visible-Light-Induced trans-Hydroboration of Diaryl Alkynes Utilizing Excited State of Borate Complexes

“…In general, classical (catalytic) hydroborations proceed with cis -selectivity via four-membered ring transition states, where the vacant p- or d-orbital interacts with a π-orbital of the alkyne . In contrast, since 2000, trans -selective hydroborations have also been developed with terminal alkynes as well as internal alkynes through three strategies: (i) a transition-metal-mediated process, (ii) a cation/radical-mediated process, , and (iii) a directing group-mediated process (Figure A) . For example, Fürstner reported that [Cp*Ru]-based complexes can achieve trans -hydroboration of internal alkynes .…”