Exploiting Boron–Zinc Transmetallation for the Arylation of Benzyl Halides: What are the Reactive Species?

Abstract

One step Beyond: The transmetallation reactions of ArB(OH)2 and Ar3B3O3 with Et2Zn are far more complicated than previously supposed, with solvent‐dependent equilibria between ArB(OY)2 at one side and [RZn(solv)3][BR4] at the other (see picture). While the role of the highly reactive organozinc cation has not been implicated before, its importance for the activation of an otherwise sluggish class of electrophiles is shown.

Direct C(sp2)C(sp3) Cross‐Coupling of Diaryl Zinc Reagents with Benzylic, Primary, Secondary, and Tertiary Alkyl Halides

“…The assignment of perprotio‐ 3 a , formed from 1 and 2 , was also confirmed by GC–MS, thus ruling out any Friedel–Crafts benzylation of the [D 6 ]benzene solvent 29. This outcome was extremely surprising, as in previous investigations of the stoichiometric arylation of 3‐methoxybenzyl bromide with 1 in dioxane, essentially no cross‐coupling product (3 %) was observed after 16 h at 60 °C 30…”

Boron-Zinc Exchange in The Diastereoselective Arylation of Sugar-Based Aldehydes­: Stereoselective Synthesis of (+)-7-epi-Goniofufurone and Analogues

“…After heating the mixture to 60°C, we could reduce the reaction time to three hours with improved yields and the same diastereoselectivity. Under these conditions, good yields of the arylation products 6a-e were obtained, however with moderate diastereomeric ratios ( Table 1, entries [8][9][10][11][12]). An interesting result was obtained with the more hindered 2-tolyl derivative, that gave the corresponding product 6d in 78% yield and an excellent >20:1 dr (entry 11).…”

O2‐Mediated Oxidation of Aminoboranes through 1,2‐N Migration

“…[17] These have already been used as oxidizing agents. [19] In summary,t he following reaction sequence was devised (Scheme 1): 1. Hence, the simple delivery of O 2 after completion of the aminoborane formation should transform residual zinc organyls into peroxides and thus provide the necessary oxidizing agent.…”