An Opportunity for Mg-Catalyzed Grignard-Type Reactions: Direct Coupling of Benzylic Halides with Pinacolborane with 10 mol % of Magnesium

Abstract

Mg in catalytic amounts as the only metal permits the reductive coupling between benzyl halides and pinacolborane. HBpin acts both as an electrophile and as a reducing agent to regenerate an organomagnesium species in situ. An hydride oxidation mechanism is proposed on the basis of DFT calculations.

Transition‐Metal‐Free Synthesis of Pinacol Alkylboronates from Tosylhydrazones

“…[2] In this context, arylboron compounds have been extensively used in organic synthesis, and the corresponding alkylboron compounds are also useful building blocks. [1b,h, 3, 4] In general, alkylboron compounds can be accessed by four different methods: 1) nucleophilic reaction by Grignard [5] or organolithium reagents [6] with suitable boron compounds, such as BX 3 (X = Cl, F) or B(OR) 3 ; 2) hydroboration of alkenes; [3,7,8] 3) Miyaura borylation; [9][10][11] 4) transition-metalcatalyzed borylation of alkanes by C À H bond activation. [1b,h, 3, 4] In general, alkylboron compounds can be accessed by four different methods: 1) nucleophilic reaction by Grignard [5] or organolithium reagents [6] with suitable boron compounds, such as BX 3 (X = Cl, F) or B(OR) 3 ; 2) hydroboration of alkenes; [3,7,8] 3) Miyaura borylation; [9][10][11] 4) transition-metalcatalyzed borylation of alkanes by C À H bond activation.…”

“…We then optimized the reaction conditions by examining the effect of the base and Scheme 1. In view that the protonation process is essential for the reaction, we then introduced two equivalents of an alcohol or H 2 O, as a proton source, to the reaction ( Table 1, entries [8][9][10][11]. pin = pinacolato, Ts = p-toluenesulfonyl.…”

Highly Enantiospecific Borylation for Chiral α‐Amino Tertiary Boronic Esters

“…To facilitate the borylation, the reaction was carried out initially at À78 8 8Ca nd the temperature was slowly increased to 23 8 8C in 12 h. In these cases,t he desired borylated product 2a (DG = Boc) was obtained in 55 %and 56 %yield, but only in 80 % ee and 88 % ee,r espectively ( Table 1, entries 2a nd 4). Thel ow enantiopurities result most likely from increasing reaction temperature to enable the sluggish desired borylation of the tertiary a-N-substituted organolithium species with less nucleophilicity due to the steric hindrance and the adjacent electron-withdrawing N-DG group.T his problem could be circumvented by using less hindered and more reactive boron electrophile reagents to promote the borylation at low temperature with excellent enantiospecificity.W e were delighted to find that pinacolborane (HBpin) [17] reacted readily with a-N-substituted organolithium species in Et 2 Oat À78 8 8Ct og ive exclusively ab orylated product in 99 %y ield and > 99 % ee (Table 1, entry 6). When the Boc directing group was replaced with CON( i Pr) 2 ,t he corresponding borylated product was obtained in 80 %y ield and 97 % ee (Table 1, entry 8).…”

Transition‐Metal‐Free Synthesis of Pinacol Alkylboronates from Tosylhydrazones

“…In recent years great progress has been made in the Suzuki-Miyaura crosscoupling reactions involving alkylboron compounds. [1b,h, 3, 4] In general, alkylboron compounds can be accessed by four different methods: 1) nucleophilic reaction by Grignard [5] or organolithium reagents [6] with suitable boron compounds, such as BX 3 (X = Cl, F) or B(OR) 3 ; 2) hydroboration of alkenes; [3,7,8] 3) Miyaura borylation; [9][10][11] 4) transition-metalcatalyzed borylation of alkanes by C À H bond activation. [12,13] These powerful methods for the preparation of alkylboron compounds have been widely practiced in modern organic synthesis.…”

“…We then optimized the reaction conditions by examining the effect of the base and found that EtONa, NaOMe, and NaOH could give better results than other bases ( Table 1, entries 1-7). In view that the protonation process is essential for the reaction, we then introduced two equivalents of an alcohol or H 2 O, as a proton source, to the reaction ( Table 1, entries [8][9][10][11]. The yield of 2 a was indeed slightly improved, with MeOH affording the optimal result.…”