Transition metal-free radicaltrans-hydroboration of alkynes with NHC-boranesviavisible-light photoredox catalysis

Abstract

The development of radical hydroboration of alkyne remains an important challenge. Here we report transition metal-free trans-hydroboration of alkynes with NHC-boranes via the synergistic merger of 4CzIPN as photoredox organocatalyst...

Radical hydroboration for the synthesis of organoboron compounds

“…Recently, Zhan and co-workers reported the radical trans -hydroboration of alkynes 33 with NHC–borane 2 by employing 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as the photocatalyst and 4-(trifluoromethyl)benzenethiol as the HAT catalyst (Scheme 9). 40 While single electron transfer between the excited photocatalyst and NHC–borane allowed the formation of the NHC–boryl radical for borylation, the reaction was also found to proceed in the absence of a photocatalyst. Hence, an alternative synergistic pathway towards the generation of the NHC–boryl radical had also been proposed, whereby the coupling between NHC–borane and thiol generated boryl sulfide in situ , which then homolysed to give the corresponding NHC–boryl and thiyl radicals.…”

Photocatalytic and Electrochemical Borylation and Silylation Reactions

“…Internal alkynes were also suitable substrates for this transformation, although the photocatalyst and base needed to be switched to obtain optimal yields and diastereoselectivities (12)(13)(14)(15). Finally, using the same system as for the terminal alkynes, alkyl boronates could be obtained from the corresponding olefins (16)(17)(18)(19)(20)(21). Interestingly the transformation was effective on an extremely large range of alkenes, such as styrenes, electron-rich and electron-deficient alkenes (i. e. Michael acceptors).…”

“…[14] Similar systems were also employed to perform Giese-type borylation of α,β-unsaturated imides, [15] defluoroborylations [16] and trans-selective hydroboration of alkynes. [17] In 2021, Leonori reported the impressive milestone, i. e. the formation of boryl radicals derived from aminoboranes using the organic photocatalyst 4CzIPN in combination with persulfate as a HAT reagent. The enhanced nucleophilicity of this boryl radical compared to the one obtained from NHC-boranes enabled their addition to azines in a Minisci process for the first time.…”

BF3-promoted selective catalytic hydroboration of epoxides to primary alcohols

“…Furthermore, given the soaring prices of precious metals and heightened environmental concerns, the use of metal-free catalytic systems has been developed significantly. 41–43 Lewis acidic boron reagents have emerged as excellent catalysts for metal-free hydroboration. For instance, BArF 3 was uncovered as a catalyst with HBpin for hydroboration of various unsaturated reagents, including aldehydes, imines and alkenes.…”