Transition‐Metal‐Free Borylation of Allylic and Propargylic Alcohols

Abstract

The base‐catalyzed allylic borylation of tertiary allylic alcohols allows the synthesis of 1,1‐disubstituted allyl boronates, in moderate to high yield. The unexpected tandem performance of the Lewis acid–base adduct, [Hbase]+[MeO‐B2pin2]− favored the formation of 1,2,3‐triborylated species from the tertiary allylic alcohols and 1‐propargylic cyclohexanol at 90 °C.

Tri(boryl)alkanes and Tri(boryl)alkenes: The Versatile Reagents

“…All the previous efforts devoted to introducing three vicinal C-B bonds into a one-pot sequential reaction required transition metal complexes to activate the borane reagent and facilitate the addition to the unsaturated substrates. However, Fernández, Szabó and co-workers [23] were first to observe the possibility of conducting the 1,2,3-triboration reaction in a transition-metal free context, using allylic alcohols as substrates and conducting a one-pot allylic borylation/diboration sequence (Scheme 19). The reaction takes place in the presence of catalytic amount of base Cs 2 CO 3 , that contributes to the generation of MeOfrom MeOH.…”

“…Initially the reaction proceeds through the generation of primary β-boryl radicals that undergo a rapid 1,2-boron shift to form thermodinamically favored secondary followed by a subsequent 1,2-boron shift towards tertiary radicals (Scheme 24). This protocol allows for the selective transformation of the more hindered boronic ester from the 1,2,3-triborated substrate [23], conducting its radical addition to tert-butyl-acrylate [27]. The product is obtained with high selectivity, highlighting the thermodynamic control that favors the most stabilized radical center as the site of the reaction.…”

Reductive Aromatization of Quinols with B2pin2 as Deoxidizing Agent

“…Based on the aforementioned observations and previous reports, [5][6][7][8][9] we propose a mechanism shown in Scheme 5. Nonhalogen-substituted 1 would undergo borylation on �,βunsaturated carbonyl moiety [10] assisted by neighboring hydroxyl group.…”

“…In case of deoxygenation, [4] borohydride is crucial to drive the cleavage of CÀ O bond, achieving nucleophilic substitution towards borate ester. Alternatively, deoxygenation process with diboron compounds is also available, requiring anchimeric assistance of alkene to undergo Fernádez-type diborylation/BÀ O elimation [5] or via six-membered ring [6] (followed by protodeboronation sometimes [7] ) to complete deoxygenation process (Scheme 2b) wherein the olefin migration during this transformation is inevitable. Parallelly, Song-type cascade borylation/BÀ O elimation from propynols could also afford deoxygenated structures (Scheme 2c).…”

Pyridine‐Based Organocatalysts for Regioselective syn‐1,2‐Silaboration of Terminal Alkynes and Allenes

“…N ‐Heterocyclic carbens, tertiary phosphines, and organic and inorganic bases (including alkoxide and carbonate salts) have been developed as effective catalysts for the transfer of E into α , β ‐unsaturated carbonyl compounds and imines through conjugate or direct addition fashions. The bases also catalyze substitution reactions of aryl halides, vinyl epoxides, vinyl aziridines, allylic alcohols, and α ‐(boryloxy)alkylboronates …”