Homologation Strategy for the Generation of 1-Chloroalkyl Radicals from Organoboranes

Abstract

The generation of 1-bromo and 1-chloroalkyl radicals from organoboranes has been investigated. The direct approach involving the hydroboration of halogenated alkenes is impeded by partial dehalogenation taking place during the hydroboration process. An indirect method involving the generation of B-(1-chloroalkyl)catecholborane by homologation of B-alkylcatecholborane with dichloromethyllithium was developed. A reaction sequence involving a hydroboration reaction, a Matteson homologation, and a radical allylation process has been performed as a one-pot process that takes advantage of three different reactivities of organoboron species. Starting from styrene derivatives, it was possible to prepare B-(1-chloro-2-arylpropyl)catecholboranes that are excellent precursors to 1-chloro-2-arylpropyl radicals. A concise approach for the synthesis of an optically active α-methylene-γ-lactone from p-chlorostyrene has been developed on the basis of a two-step sequence involving an enantioselective hydroboration-homologation-cyclization reaction followed by a hydrolysis-lactonization process.

Radical‐Triggered Three‐Component Coupling Reaction of Alkenylboronates, α‐Halocarbonyl Compounds, and Organolithium Reagents: The Inverse Ylid Mechanism

“…tert-butyl methyl ether (TBME) gave enhanced results both with n-BuLi (Table 1, entry 3, 92% yield) and t-BuLi (Table 1, entry 8, 68% yield). [30] Testing the reaction with simply AIBN as the initiator and benzene as the solvent gave good results except in the case of t-BuLi complexes ( Table 1, entries 4,9). [31] Performing the reaction at room temperature (air initiation) dramatically improved the yield to 76%.…”

“…[6][7] Our long-standing interest for radical reactions involving organoboron species prompted us recently to investigate the use of a-chloroalkylboronates in radical reactions and this led to the discovery of an intramolecular cyclopropanation reaction. [8][9] In these studies, the a -chloroalkylboronates were prepared according to Matteson dichloromethyllithium to an alkylboronate. In order to find a different way to prepare ahaloalkylboronates, we decided to investigate the atom transfer mediated radical addition to alkenylboron derivatives.…”

Intramolecular Cyclopropanation of 1,4‐Dienes through Hydroboration–Homologation: Easy Access to Bicyclo[3.1.0]hexanes

“…This is in strong contrast to simple a-chloroboranes such as 1 (Scheme 1), which are stable and easily observed by 1 H-and 11 B-NMR. [22] Several possible mechanisms were envisaged. First, ar hodium-catalyzed cyclopropanation via am etallocarbene of type I (Scheme 6a)w as ruled out by running the reaction without any catalyst.…”

“…An efficient procedure for the preparation of B-(1-chloroalkyl)catecholboranes such as 1 was developed. [22] These organoboranes were used in as as ource of 1-chloroalkyl radicals in various processes,i ncluding allylation reactions involving allylsulfones (Scheme 1). Theh omologation-inter-molecular radical addition sequence is applicable to aw ide range of substrates.…”

“…[22] None of the expected chlorocyclopentane was observed, but one main product, identified as the bicyclo-[3.1.0]hexane 4a,w as obtained in modest 25 %y ield (Scheme 2). This product is already formed during the homologation sequence and all attempts to isolate the intermediate B-(1-chloroalkenyl)catecholborane 3a failed.…”