Essential role of phosphines in organocatalytic β-boration reaction

Abstract

The use of phosphines to assist the organocatalytic β-boration reaction of α,β-unsaturated carbonyl compounds has been demonstrated with a selected number of substrates. The new method eludes the use of Brönsted bases to promote the catalytic active species and PR(3) becomes essential to interact with the substrate resulting in the formation of a zwitterionic phosphonium enolate. This species can further deprotonate MeOH when B(2)pin(2) is present forming eventually the ion pair [α-(H),β-(PR(3))-ketone](+)[B(2)pin(2)·MeO](-) that is responsible for the catalysis.

Mechanism of NHC-Catalyzed Conjugate Additions of Diboron and Borosilane Reagents to α,β-Unsaturated Carbonyl Compounds

“…However, in addition to the aforementioned issues vis-à-vis the need for a proton source and excess base, a number of questions regarding the details of the catalytic cycles for C–B and C–Si bond formation (Scheme 1), including those that are promoted by NHCs, remain unaddressed. 27 An appreciation of such attributes will be crucial to the success of future endeavors regarding catalyst and method development in this emerging area of research. Herein, we detail the results of studies performed aimed at finding a plausible answer for the above issues and various other questions relating to the inner workings of NHC-catalyzed boryl and silyl conjugate additions.…”

A Clear‐Cut Example of Selective BpinBdan Activation and Precise Bdan Transfer on Boron Conjugate Addition

“…The use of phosphines to assist the organocatalytic b-boration reaction of a,b-unsaturated carbonyl compounds was already demonstrated. [18] The presence of base and the use of phosphine as an additive was beneficial ( Table 1, entry 3), and the reaction was observed even at 25 8C (Table 1, entry 4). The nature of the phosphine is also important.…”

Electrophilic Amination with Nitroarenes

“…[24] Furthermore,time-resolved 11 BNMR analysis of the reaction clearly shows that ab orate species containing an sp 3 -hybridized Ba tom is formed under the reaction conditions (d = 5.0 ppm;F igure 1). [29] Its concentration first rises and is highest about 40 min into the reaction, from which point it gradually decreases,t hereby assigning it ar ole in the early stages of the reaction. Furthermore,t his coincides with cessation of the formation of EtBpin, which also occurs at around 40 min into the reaction.…”